Mechanics of nanomaterials and nanotechnology

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2 Series in Applied Mathematics and Mechanics INSTITUTE OF MECHANICS Vol. 3 Mechanics of nanomaterials and nanotechnology Thematic collection BULGARIAN ACADEMY OF SCIENCES

3 Series in Applied Mathematics and Mechanics, Volume 3 ISSN: Vassil Kаvardzhikov, Editor-in-chief Ludmila Parashkevova, Anguel Baltov, Editors Mechanics of nanomaterials and nanotechnology, Series in Applied Mathematics and Mechanics, Volume 3, Institute of Mechanics - BAS, Sofia, 2012, 326 pp. This collection is issued with the financial support of Bulgarian Science Fund, Ministry of Education, Youth, and Science. The jubilee collection of original works of the authors team presented is aimed to acquaint the scientific community, the doctoral students, and the high-tech industry with the achievements of scientists from the Institute of Mechanics (IMech), BAS, in international research collaboration. The book is dedicated to the 50th anniversary of the organized research in mechanics and the 35th anniversary of IMech. The purpose of this collection is to demonstrate the research level in IMech in the field of mechanics of nanomaterials (NM) within the context of the recent world achievements and the capabilities of the modern equipment available in IMech and the Open Laboratory of Experimental Mechanics of Micro- and Nanomaterials. This book is based on the results of 26 autors from different generations with significant contribution from young researchers. It consists of 11 chapters distributed into 4 sections: Nanomaterials based on metals; Nanomaterials based on polymers; Modeling of nanomaterials and nanostructurs; and Experimental nano- and micromechanics. The chapters describe the most recent technologies, theoretical methods, and experimental techniques and their application in NM studies. Each chapter contains rich overview and analysis in the specific sub-field. Total are cited 624 publications of which 58 belong to the authors. The book is illustrated with 13 tables, 181 photos, charts and diagrams, of them ~ 14% colored. Referees: V. Kamburov, Assoc. Prof., PhD ( Chapters I.1, ІІІ.4 ) M. Popova, Prof., PhD ( Chapters I.2, III.1, III.2 ) A. Topliyska, Assoc. Prof., PhD ( Chapters II.1, IV.3 ) N. K. Vitanov, Prof., DS ( Chapter III.3 ) J. Ivanova, Prof., DS ( Chapter III.5 ) Tz. Baeva, Assoc. Prof., PhD ( Chapter IV.1 ) M. Jordanov, Assoc. Prof., PhD ( Chapter IV.2 ) 2012, Institute of Mechanics, Bulgarian Academy of Sciences

4 Contents Preface... vii Historical notes... ix I. Nanomaterials based on metals... 1 I.1. Technology of nanocrystalline-structured materials obtained through glass-transition of amorphous metallic alloys Nikola Nikolov... 3 I.2. Severe plastic deformation (SPD) technology for bulk nanocrystalline metals and layers Anguel Baltov II. Nanomaterials based on polymers II.1. Polymer nanocomposites of epoxy resin and multiwall carbon nanotubes: processing-structure-properties relationships Rumiana Kotsilkova, Evgeni Ivanov, Ekaterina Krusteva III. Modeling of nanomaterials and nanostructurs III.1. Analytical modeling of nanocomposites stiffness Ludmila Parashkevova, Nikolina Bontcheva, Vitali Babakov III.2. Continuum approach in nanomechanics Anguel Baltov, Ana Yanakieva III.3. Equilibrium shapes of fluid membranes and carbon nano-structures Vassil Vassilev, Petar Djondjorov, Mariana Hadzhilazova, Ivailo Mladenov, Jan J. Slawianowski ІІІ.4. Numerical modeling of nanocrystalline metallic materials obtained by glass-transition or severe plastic deformation Nikola Nikolov, Anguel Baltov ІІІ.5. Hierarchical modeling of biological nanocomposites Svetoslav Nikolov, Helge Fabritius, Martin Friák, Dierk Raabe IV. Experimental nano- and micromechanics IV.1. Micro- and nanometrology Vassil Kavardzhikov, Dessislava Pashkouleva IV.2. Mechanical characterization of layers and thin films via nanoindentation and numerical simulations Roumen Iankov, Sabina Cherneva, Maria Dacheva, Dimitar Stoychev IV.3. Experimental nano and micro mechanics of nanostructured materials Evgeni Ivanov, Irena Borovanska, Boryana Milosheva, Rumiana Kotsilkova

5 Preface Today, nanomaterials are being increasingly used to improve well-known products and to create new ones. From recent drugs to sunscreens, and from water cleaning systems to high-strength turbine elements, nowadays nanomaterials are becoming a part of every industry. It is well-known that this very active research area is among the most advanced fields of science that are being rapidly developed recently all over the world. As a whole, the study of nanoscience differs in nature from the similar problems posed on a larger scale. On the nanometer scale physics is different and properties not elicited on macro level now become important, caused by quantum mechanical and thermodynamic interactions. Some specific properties of molecules and atoms can be combined in non-trivial ways to produce new materials with novel and surprising characteristics. The tendency for intensive development of nanoscience dates back to the 70s of 20th century when the physics-chemical and structural principles for manufacture of those materials were built up and the manufacturing technologies were elaborated on their basis. In addition to the enabling technologies, scientists have realized the future potential of this research. The efforts of scientific community around the world have instituted initiatives to promote nanoscience and nanotechnology in their organizations and labs. Many scientists are fascinated by this research and the amount of discoveries and innovations has increased dramatically. The mechanics of nanomaterials and nanotechnologies can be considered as the indispensably mediator between the unique samples invented in the physics-chemical labs and the real industry with serial production. The Institute of Mechanics became a natural centre for the investigations in this field in Bulgaria. This collection is structured following two interconnected streams. First stream: Mechanics and manufacturing technologies for nanomaterials of different nature; Experimental methods for establishing the physics-mechanical properties of manufactured and experimentally tested nanomaterials; Process modelling for creation and usage of new nanomaterials; Second stream: solving the above defined problems for nanomaterials with metal and polymer matrices and finding effective and competitive new applications. This collection reveals the ideas of the research team from Institute of Mechanics to Bulgarian Academy of Sciences in the field of mechanics of nanomaterials and nanotechnology. We hope that the results presented in this issue give a clear picture for the achievements and research capacity of the scientists in the Institute of Mechanics in this field and can become a reasonable basis for future studies and research collaboration. Sofia, May 2012 From the Editors

6 Part IV Experimental nano- and micromechanics

7 Part IV, Chapter Experimental nano and micro mechanics of nanostructured materials Evgeni Ivanov* ivanov_evgeni@imbm.bas.bg Irena Borovanska* reniboro@imbm.bas.bg Boryana Milosheva* milosheva@imbm.bas.bg Rumiana Kotsilkova* kotsilkova@imbm.bas.bg *Open Laboratory for Experimental Mechanics of Micro- and Nanomaterials (OLEM), Department of Physico-Chemical Mechanics, Institute of Mechanics, Bulgarian Academy of Sciences, Abstract This chapter presents an overview of the micro/nanomechanical and micro/nanotribological characterization methodologies applied in recent developments for nanostructured materials characterization. Various nanometrology techniques and characterization tools used in nano/micro mechanics and tribology are described in details. The chapter summarizes the methodology and author s research activities and experience in the areas of a) Nano and microindentation, b) Atomic force microscopy (AFM), c) Profilometry, d) Wear, friction and scratch resistance techniques, to characterize and evaluate the performance of nanocomposite structures. Examples and experimental results are obtained from nanoindentation tests and AFM on thermosetting and thermoplastic polymers and their nanocomposites containing multiwall carbon nanotubes (MWCNT), and studied as an example of soft materials, these having different structure and homogeneity, while investigations on metal alloys Zr-Hf, Zr-Mo are studied and presented as hard materials. AFM scans on biological samples and pharmaceutical drug carriers are performed, in particular on red blood cells and solution of poly(vinyl acetate) latex in water. Microindentation and microtribological investigations on polypropylene/mwcnt and polypropylene/rice flakes nanocomposites, as well as on epoxy/mwcnt and epoxy/alumina nanocomposites are obtained in OLEM Research Laboratory and discussed in details. The given examples justify the need of various analytical tools and in depth analysis to describe the complex nature of nanostructured materials. Keywords: nanoindentation, nanoscratch, AFM, microindentation, microtribology tests, friction and wear, microscratch test, adhesion at scratch, integrated high resolution imaging, 3D profilometry Introduction Nanoscience and nanotechnology are broad domains of research and innovation, which therefore necessitate special research techniques and methodologies for fine

8 288 analysis. It is known that materials and structures with nano-scale dimensions do not behave in the same manner as their macro-scale counterparts. Unfortunately, most of our knowledge is based on macro-scale material behaviour, which often fails to describe material response in small- scale dimensions because of the dominance of surface and interface effects, finite number of grains in a given structure, etc. In general, mechanics and nanomaterials are essential elements in all of the transcendent technologies. New measurement tools, which can be integrated into high-resolution imaging instruments, are necessary in order to make further advances in the mechanics of nanomaterials [1]. Micro and nanomechanics are concerned with experimentation, modelling and application of three-dimensional structures and systems with dimensions of the order of micrometers and lower. In last decades experimental mechanics gained increasing importance in nanomaterials characterization. On one hand it is believed that a significant improvement in the comprehension of mechanics theories can be achieved through experimental validation; on the other hand the challenge of performing complex mechanical measurements for which multidisciplinary engineering problems have to be solved is very important [2]. Hence, novel methods and experimental techniques have been recently proposed for use in micro- and nanomechanical characterization of nanostructured materials. There are several important modern developments in nanomechanical characterization tools and methods. Recently, the nanoindentation method has widely been adopted and used in the characterization of mechanical behaviour of materials at small scales, so called nanoindentation [3]. The attractiveness of this method stems largely from the possibility to determine the mechanical properties directly from indentation load and displacement measurements without the need to image the hardness impression. The method was well established for elastic-plastic materials, such as metals [4], or ceramics [5], and for characterization of polymeric [6,7] and other materials response depending on time. Various indentation techniques have been used to measure matrix, fiber, and interphase characteristics in polymers [8]. Nanoindentation can also be a powerful tool for testing biological samples, which have highly hierarchical microstructures as well as limited volumes [9,10]. It also offers a favorable means for the investigation of the mechanical behavior of thin films and coatings, which is essential for many technical applications, e.g. microelectronic devices and protective coatings [11,12]. Atomic force microscopy (AFM) is increasingly used as a nanoindentation tool to measure local elastic properties of surfaces, especially in medicine. In particular, there is an opportunity to estimate the local elastic modulus of red blood cells area with an accuracy of localization and size of the minimum area impact indenter up to a few nanometers [13]. On the other hand, there is an increasing demand of using these techniques for characterization of synthetic bone and hard tissue replacement materials that require improved mechanical properties. In advanced composite materials, it is now increasingly important to assess the quality of the interface between the reinforcement and the matrix, which facilitates improvements to composite predictive models [14]. In published literature [15-18], either instrumented indentation (IIT), or atomic force microscopy are often used to characterize the interface region, in which interphase layers of various thicknesses (1 5 µm) are expected depending on the type of sizing applied to the reinforcement. However, tests generally suffer from either lack of resolution (i.e. size

9 Part IV, Chapter of the probe in IIT), or surface artefacts (i.e. sharp probe interaction with surface features in AFM) and cannot probe the properties of the interphase region conclusively. Another application that requires high lateral resolution methods is the study of intermetallic compounds with a thickness of 1 2 µm. High resolution imaging techniques, such as Atomic Force Microscopy, is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale [19]. Information is gathered by "feeling" the surface with a mechanical probe. Contact mode AFM maps the nanoscale surface by monitoring the deflection of a cantilever ending with an ultra-sharp tip probe and scanned while in contact with the surface. Noncontact mode uses an oscillating cantilever and the detection scheme is based on measuring changes to the resonant frequency or amplitude of the cantilever. The 3D and 2D Profilometers are the industry standardised methods of measuring surface roughness and topography on micro- and nanoscale. Micromechanical devices and methods are widely used for characterization of nanostructured materials. For example, microhardness measurements quantify the resistance of the material to plastic deformation on a microscopic scale. Microindentation hardness tests compose the majority of processes used to determine material hardness at forces less than 2 N. Consequently, ASTM Committee E-4 on Metallography recommends the term "microindentation hardness testing [20]. Hardness, however, cannot be considered to be a fundamental material property; instead it is used to provide a comparative idea of the material's resistance to plastic deformation [21]. It was found that when testing metals, indentation hardness correlates linearly with tensile strength [22]. Adding of finely dispersed nanoscale filler can modify the tribological performance, the friction and wear behavior of the polymers and the good understanding of the role of the filler will facilitate the formulation of optimal criteria for the design and selection of proper material subjected to specific tribological applications. Wear is normally a very complex phenomenon involving many parameters, making wear prediction extremely difficult [23]. Hardness and morphology of nanoparticles have great influence in tribology of nanocomposites, as well as the resulting surface roughness and surface microstructure. The triboengineering properties appear rather sensitive to the nature and size of nanoparticles, their concentration, the type of matrix polymer, as well as the operation regime. The size and the shape of the nanoparticles may exert great influence on the friction and wear behavior of various polymer matrices [24]. A lot of experimental studies have demonstrated that nanofibers or nanoparticles can result in remarkable improvements in the friction and wear properties of both bulk materials and coatings [23, 25-30]. Scratch test is one of the most used techniques for evaluating a wide range of surface mechanical properties of nanomaterials. Some of the areas where microscratch has been successfully used in the engineering field, both by research and industry, include the determination of the relative hardness of nanomaterials, characterization of coatings on different substrates (including coating adhesion), the wear resistance of nanomaterials and the estimation of different material deformation characteristics when they are subjected to hard asperity damage [31,32]. This paper has, as its principal objective, the description of methodologies and testing techniques applied in recent developments in the nano- and micromechanics of

10 290 nanostructured materials and used in our research Laboratory OLEM. It also serves to narrow the gaps between conventional macro-/meso- systems and the emerging micro- /nanomaterials technologies. Experimental Techniques The nano- and micromechanical investigations presented in this study are performed at the Open Laboratory for Experimental Mechanics of Micro- and Nanomaterials (OLEM), Department of Physico-Chemical Mechanics at the Institute of Mechanics, Bulgarian Academy of Sciences. Nanomechanical fine analyses are performed using Universal Nanomechanical Tester (UNMT), equipped with Nanoindenter & Integrated Imaging, these including Atomic Force Microscopy (AFM, Ambios Technology), Digital Optical Microscopy and PRO500-3D Profilometer produced by the Center for Tribology (CETR, USA). The Nanoindenter has a possibility for installing Berkovich type tip, and Cube-corner and conical tip indenters. The nanoindentation tests of fine materials are also possible by using the tip of the Atomic Force Microscope in contact mode. Micromechanical investigations are made using Mechanical and Tribology Tester (UMT-2M) produced by the Center for Tribology (CETR), USA. This advanced module type equipment is built of several exchangeable test modules allowing variety of measurements, such as: (i) microindentation (Vickers and Rockwell indenters); (ii) tribology, friction, wear and lubricity (ball/pin-on-disk and ball/pin-on-flat reciprocating modes); (iii) scratch and adhesion tests (with integrated acoustic emission and electrical contact resistance sensors); and (iv) macromechanical tests (tensile, 3 point bending, compression, torsion, tensile & torsion, fracture, fatigue and creep in controllable environmental test chamber, where the temperature reaches up to 150 C). 1. Nanoindentation of Nanostructured Materials 1.1. Basic Principles of Nanoindentation Method The nanoindentation tests involve surface finding indentation using an indenter (often Berkovich triangular pyramid) contacting a material surface and penetrating to a maximum load, holding at maximum load, uploading up to usually 10% of the maximum load, holding and complete uploading. Load is measured as a function of penetration depth. The test is repeated to have statistical data. Each indentation is done on a new location on the specimen. Calculation methods to determine hardness and modulus are typically based on the method of Oliver and Pharr [3, 33]. The method was developed to measure the hardness and elastic modulus of a material from indentation loaddisplacement data obtained during one cycle of loading and unloading. Figure 1 shows a typical load penetration depth curve. In this case, penetration depth is the displacement into the surface of the specimen. Hardness, H is defined as the mean contact pressure calculated by dividing the indenter load, P by the projected contact area, A at that load:

11 Part IV, Chapter P H = (1) A The projected contact area is dependent on the geometry of the indenter and is obtained from the contact depth of the indent, h c calculated from the total penetration depth h, indenter load P and contact stiffness S at the beginning of unloading (Figure 1). The stiffness S, defined as S = dp/dh (indicated in Figure 1), is determined from the slope of the initial portion of the unloading curve, fitted by a regression function, usually in the form of Oliver and Pharr [3,33]: m ( h h f P = c ) (2) with regression constants c, h f and m; h f corresponds to the final depth of the permanent imprint after unloading. The contact stiffness and contact area also serve for determination of reduced modulus Er: E r π S = (3) 2β A where β is a constant that depends on the geometry of the indenter. The reduced modulus accounts for the fact that the measured displacement includes contributions from both the specimen and the indenter. The elastic modulus for the test material, E is then calculated using the Poisson s ratio of the test material, ν, the modulus of the indenter, E i, the Poisson s ratio of the indenter, ν i, and the reduced modulus E r : ν 1 ν i = + (4) E E E r i Figure 1. Representative load-displacement curve of one cycle indentation test. P load; h total depth of penetration; h c contact depth; h f depth of imprint after unloading

12 292 For a diamond-tipped indenter, E i = 1141 GPa and ν i = 0.07 GPa. The Poisson s ratio of the test material must come from tests on the bulk material or estimation. The aforementioned procedure measures hardness and modulus at the maximum penetration depth of a single load unload indent cycle Nanoindentation with Berkovich and Conical Indenters Nanoindentation tests are performed using Nanoindenter embedded in UNMT (CETR) and equipped with integrated Atomic Force Microscope and digital optical microscope. Three indenters are available, as follows: type Berkovich Diamond with Tip Radius 70 nm, Cube-corner Indenter with Tip Radius nm and Conical Indenter with Spherical Tip Radius 2.5 µm. The high-resolution imaging instruments (AFM and 3D profilometer) integrated with the indentation technique allow in depth understanding of the mechanics of nanomaterials. Thus, immediately after the nanoindentation, the obtained imprint is visualized by Atomic Force Microscopy. The nanohead of the UNMT Nanoindenter performs indentation tests, where the applied load and displacement are continuously monitored, thus the load versus displacement data is generated for a test specimen. Young s modulus and hardness are derived from the unload data segments through in-situ monitoring of the load vs. displacement plot and automatic calculations by utilizing the Oliver-Pharr method (Figure 1). The method provides possibilities to apply single and multiple indents for measuring hardness, Young s modulus, contact stiffness, etc. of thin and ultra-thin films, and bulk materials. This nanometrology test fully corresponds to ISO standard on instrumented nanoindentation. The device is isolated from air currents, acoustic noise, and mechanical vibrations by thermal & acoustic enclosure and vibration isolating table. AFM (Ambios Technology) allows in-line imaging option. The images of indentation are generated automatically after the test without removing the sample from the tool. The stage with the sample allows for tilt check and levelling. The device allows programmable load, displacement, velocity, motion control routines; automated multi-channel data acquisition recording and displaying; intuitive, flexible & powerful programmable test recipes (scripts) and unlimited test duration. The device allows load-control nanoindentation in the range of 0.1 µn to 500 mn. The maximum displacement in any test is usually not known a priori, but there is a possibility to run test with multiple indents on different places on the examined sample with increasing force (for example from 0.1 µn to 500 mn) and to choose correct displacement range after analyzing the load-displacement curves Single and Multiple Nanoindentations The nanoindentation tests involve surface finding, indentation by loading up to a maximum load, holding at maximum load, unloading up to usually 10% of the maximum load, holding, and complete unloading. The test is repeated to have statistical data. Depending upon the material requirements, the tests are repeated with a relevant test script, which applies a single maximum specified load (Single Load Type) or several

13 Part IV, Chapter loads (Multiple Loads Type). Each time an indentation is performed in a new location of the specimen. The specimen is moved with the linear drive and slider. We summarize herewith the methodology and a part of our experimental results obtained from nanoindentation tests of soft materials (polymers, polymer nanocomposites) and hard materials (metal alloys). Thermoset and thermoplastic polymers and their nanocomposites containing multiwall carbon nanotubes (MWCNT) are studied as example of soft materials, these having different structure and homogeneity, as follows: neat epoxy resin, carbon nanotube/epoxy composites, isotactic polypropylene, and multiwall carbon nanotube/polypropylene composites. Hard materials investigated are metal alloys of Zr-Hf and Zr-Mo. (a) (b) Figure 2(a,b). Multicycle Graph in the left (a) and Summary Graph on the right (b) for series of 96 indentations (8 lines x 12 indentations) with increasing force in the range from 3 to 50 mn for a pure bulk epoxy resin sample. The standard deviation for the hardness and Young s Modulus is and GPa, respectively Figures 2(a,b) present examples of: (a) Multicycle Graph and (b) Summary Graph for series of 96 indentations (8x12; spacing between indents is 80 µm) with increasing force in the range from 3 to 50 mn for a pure bulk epoxy resin sample. The sample surface was polished by means of Leica RM2245 microtome with stereo microscope Leica A60S and diamond knife, Leica Microsystems, Germany. Before indentation tests the roughness of the samples is measured with either AFM, or 3D Profilometer. The prepared software program (script) for this experiment consists of 8 lines with 12 indentations each and spacing of 80 µm. Each subsequent indentation from one line is done with increasing force in the range from 3 to 50 mn (3, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50 mn). A typical indentation experiment consists of subsequent steps: (i) approaching the surface; (ii) loading to the peak load (for example 5 mn) for 15 sec; (iii) holding the indenter at peak load for 10 s; (iv) unloading from maximum force (for example 5 mn) to 10 % for 15 sec; (v) holding at 10 % of max force for 15 sec; (vi) final complete unloading for 1 sec (load function 15s-10s-15s trapezoid). The hold step was

14 294 included to avoid the influence of creep on the unloading characteristics since the unloading curve was used to obtain the elastic modulus of the material. The experiment lasts about 5 hours. The powerful statistical analysis software of the UNMT equipment, called Viewer, allows viewing, editing and analyzing the experimental data. Viewer opens the Analysis window, which shows two graphical windows: Multicycle Graph (Fig. 2(a)) and Summary Graph (Fig. 2(b)). The ordinate (Y-axis) allows to choose variety of paired parameters, such as: Hardness-Young s Modulus, Contact Stiffness-Contact Depth, Contact Depth-Contact Area, maximum Indentation Depth-Maximum Load, and Fit Coefficient a-fit Coefficient m. These paired parameters can be plotted on the abscissa (X-axis) of the summary graph with one of the following parameters: Cycle Number, Contact Depth, Indentation Depth, Maximum Load, Contact Area, Effective Contact Radius, and Contact Stiffness. The hardness and elastic modulus in Fig. 2(b) were calculated from the recorded load-displacement curves. In this case, there are four options for visualization of impressions: (1) AFM; (2) optical microscope of the AFM; (3) optical microscope of the nanohead; and (4) PRO500 3D Profilometer. Figures 3(a,b) present optical micrographs made with the optical microscope of AFM (a) and the optical microscope of nanohead (b) of this series of 96 indentations (8 lines x 12 indentations) with increasing force in the range from 3 to 50 mn for a pure bulk epoxy resin sample. As seen in Fig. 3(b), the optical microscope of the nanohead allows verification of the fact that the indentations are at a distance of 80 µm from each other. (a) (b) Figure 3(a,b). Optical micrographs made with: (a) optical microscope of AFM, and (b) optical microscope of nanohead, for the series of 96 indentations (8 lines x 12 indentations) with increasing force in the range from 3 to 50 mn for a pure bulk epoxy resin sample.

15 Part IV, Chapter (a) (b) Figure 4(a,b). AFM images (AFM, Ambios Technology) of print made on a pure bulk epoxy resin sample with Berkovich nanoindenter tip at loads of: (a) 5 mn (4th row, 2nd indent from the optical image in Fig. 3(a)) and (b) 45 mn (8th row, 11th indent from the optical image in Fig. 3(a)) Figures 4(a,b) present the AFM (Ambios Technology) image of imprints made with Berkovich nanoindenter on the aforementioned bulk epoxy resin sample at load of: (a) 5 mn and (b) 45 mn, (4th row, 2nd indent and 8th row, 11th indent, respectively, from the optical image in Figure 3(a)). This technique allows mapping the materials with different features on the surface and analyzing the obtained results for the specific local nanomechanical properties (for example Young s modulus and hardness for a certain interfacial surface). With the Image Stitching utility AFM software allows several images of adjacent surface features to be combined together to form a single large-view image. (a) (b) Figure 5(a,b). Representative load-displacement curves (a), and values of hardness ( ) and Young s Modulus ( ) (b), obtained after nanoindentation of non-functionalized 0.03 wt% epoxy resin/carbon nanotube composites. Standard deviation for the hardness and Young s Modulus is and GPa, respectively.

16 296 (a) (b) Figure 6(a,b). Representative load-displacement curves (a) and values of hardness ( ) and Young s Modulus ( ) (b), obtained after nanoindentation of aminofunctionalized 0.03 wt% epoxy resin/carbon nanotube composites. Standard deviation for the hardness and Young s Modulus are and GPa, respectively. Figures 5(a,b) and 6(a,b) show representative load-displacement curves (a) and values of hardness and Young s modulus (b) obtained after nanoindentation of nonfunctionalized (Fig. 5(a,b)) and amino-functionalized (Fig. 6(a,b)) 0.03 wt% epoxy resin/multiwall carbon nanotube (MWCNT) composites studied elsewhere [34-38]. Indenter type Berkovich Diamond with Tip Radius 70 nm was used for indentations in force control mode of 5 mn. A series of 48 indentations was performed for each sample. These case nanoindentation studies clearly demonstrate that the aminofunctionalization of the nanotube surfaces improves significantly the nanomechanical properties of epoxy/carbon nanotube composites where the hardness and the Young s modulus have values of GPa and GPa, respectively, compared to the values of a non-functionalized sample that are equal to GPa and 3.39 GPa, respectively. Figures 7(a,b) and 8(a,b) present our results obtained for polypropylene and 3 wt% polypropylene/mwcnt nanocomposites?, respectively, as follows: (a) representative load-displacement curves, and (b) the values of hardness ( ) and Young s Modulus ( ) [39,40]. Indenter type Berkovich Diamond with Tip Radius 70 nm was used for indentations in force control mode of 5 mn. The addition of 3 wt% of MWCNT displaced the curves to lower penetration depths, i.e. the nanocomposite material has higher resistance to penetration compared to the pristine polypropylene. All values measured show experimental dispersion, which is higher in the case of nanoreinforced materials than for the neat resin. The level of deviation of the experimental results demonstrates the sensibility of the indenter to the inhomogeneity of the composite structure, produced by the presence of MWCNT agglomerates on the surface, when compared to the pristine polymer. The results obtained allow studying the competition between the reinforcing effect of the MWCNTs and the mobility of the polymer molecules. While the reinforcing increased the elastic modulus, larger mobility resulted in lower modulus.

17 Part IV, Chapter (a) (b) Figure 7(a,b). Representative load-displacement curves (a) and values of hardness ( ) and Young s Modulus ( ) (b), obtained after nanoindentation of isotactic polypropylene. Standard deviation for the hardness and Young s Modulus is and 0.05 GPa, respectively (a) (b) Figure 8(a,b). Representative load-displacement curves (a) and values of hardness ( ) and Young s Modulus ( ) (b), obtained after nanoindentation of 3 wt% polypropylene/ MWCNT nanocomposite. Standard deviation for the hardness and Young s Modulus is and GPa, respectively. Figures 9(a,b) and 10(a,b) give representative load-displacement curves (a) and values of hardness and Young s modulus (b) obtained after nanoindentation of two metal alloys Zr-Hf and Zr-Mo. Indenter type Berkovich Diamond with Tip Radius 70 nm was used for indentations in force control mode of 50 mn. A series of 12 indentations were performed for each sample. Importantly, the nanoindentation techniques allow concluding that the Zr-Hf alloys have much better performance regarding hardness, Young s modulus and homogeneity of the structures compared to the Zr-Mo alloy. These results

18 298 were obtained in our Lab OLEM during the training of master degree student in collaboration with Assoc. Prof. T. Avdjieva (Sofia University St. Kliment Ohridski, Department of Physics) and published in the master thesis [41]. (a) (b) Figure 9(a,b). Representative load-displacement curves (a) and values of hardness ( ) and apparent elastic modulus ( ) (b) formed from all nanoindentation tests of a Zr-Hf alloy. (a) (b) Figure 10(a,b). Representative load-displacement curves (a) and values of hardness ( ) and apparent elastic modulus ( ) (b) formed from all nanoindentation tests of a Zr-Mo alloy Multiple Partial Unloading Nanoindentation Multilayer coatings with layer thickness in the nanometric range exhibit very interesting specific structural and mechanical properties. The nanometric multilayered materials demonstrated improvements in their hardness, yield strength and toughness and they can also lead to stabilization of metastable structures.

19 Part IV, Chapter Multiple partial unloading nanoindentation tests using the UNMT techniques enable to generate rapidly a large amount of mechanical properties data vs. indentation depth usable for nanomechanical characterization of multilayer coatings [73]. Such tests are very useful when the purpose is to know the variations of hardness and modulus values of a specimen when the indenter penetrates through different layers or reaches the substrate, respectively. Importantly, the multiple partial unloading tests may throw some light on the coating thickness comprised of multilayer materials. If the film layer is composed of different materials, the multiple partial unloading tests are ideal to identify these coating layers. The indentation in multiple partial unloading is performed at a single point. The whole series of tests of loading and partial unloading are performed at the same point without completely lifting the tip, i.e. loading up to a maximum load, then unloading partially (say 10% of the maximum load), then loading again to a different higher load level, and unloading partially (say 10% of the new load). Each time the indentation is performed with a higher load level than the previous loading step. It is necessary to repeat such loading and partial unloading several times to cover the entire load range. It is possible to modify the load levels based on the specific needs. The total number of steps may vary depending upon the requirements. The UNMT Nanoindenter allows repeating the step as many times as required at different locations automatically to generate statistical data. Some materials are susceptible to strain hardening, and multiple partial unloading indentation data might be affected by the strain-hardening phenomenon. Therefore, care should be taken during analysis and interpretation of test data on such specimens. In the Summary Graph window software, hardness and modulus profiles are displayed as a function of contact depth. When a coating with film thickness comparable to the contact depth is measured, it shows a change in the profile due to the inherent measurement of the hardness and modulus properties of the substrate. At a contact depth greater than the film thickness, the hardness and Young s modulus values are mostly these of the substrate, whereas at very low contact depth these values are mostly of the same order as the film [73]. 2. Nanomechanical Measurements with AFM 2.1. Advances in AFM Nanoindentation and Nanoscratch Nanomechanical measurements using Atomic Force Microscopy can be performed to identify materials at nanoscale and to determine some of their mechanical properties. For quantitative results and single molecule pulling, one needs to determine the spring constant of the AFM cantilever. There are works on improved methods and also efforts are made for the standardisation of these techniques via ISO [42-44]. With AFM indentation, smaller tips (radius < 100 nm) can be used to indent the sample surface and it is possible to produce arrays of low force indentations (force curves) over the desired region with well-controlled position accuracy [45]. However, quantitative test methods based on AFM are complicated by surface preparation and a number of assumptions about the tip geometry and applied forces [46].

20 300 A variety of measurements can be performed with the Surface Force software of the AFM (Ambios Technology). Besides the basic function of exploring the interaction force between the cantilever tip and the surface, the Surface Force software may also be used to perform nanomechanical indentation or scratch measurements, to set the scanning force for contact mode scans, to determine the amplitude of vibration in intermittentcontact scans, and to do simple forms of lithography involving indentation and scratching Nanoindentation There are some steps for elastic indentations where the purpose is to measure the interaction force between the probe tip and the surface, and inelastic indentations, where the purpose is to permanently deform the surface. Operationally, the only difference is how far the probe is pushed against the surface. The steps that have to be followed are [74]: (1) Selecting a probe which is appropriate for the type of measurement; (2) Calibrating of the cantilever optics. A measurement of the photodetector T-B deflection signal as a function of probe height Z is used to generate a calibration factor to convert the T-B voltage signal into an equivalent deflection of the end of the cantilever in microns. This procedure should be followed prior to Deflection (µm) and Force (µn) indentation measurements. It should be repeated whenever the cantilever is changed or repositioned, or the laser or photodetector position knobs are moved; (3) Setting the cantilever spring force constant. An approximate value for the spring force constant may be obtained from the manufacturer s data sheet, or estimated; (4) Selecting which Data signal to measure: probe deflection, normal probe force, or lateral probe force; (5) If the purpose is to perform an indent to a specific downward Z position (displacement control nanoindentation), it is necessary to set the required value in the Z Range text box and to leave the Limit box empty. If the purpose is to perform the indent to a specific maximum Force (force controlled nanoindentation), it is necessary to set the value in the Limit text box, and to put a sufficiently large entry in Z Range so that the limit (for the force) will be reached; (6) Setting the loading Speed, the number of Points/Segment, and the number of indent Cycles to perform; (7) Indentation. Figures 11(a,b) present experimental results obtained on erythrocytes after z- controlled (a) and force-controlled (b) loading-unloading curve of nanoindentation with applied force of 50 nn. Figure 12(a,b) shows AFM scans of the indented samples. First the tip approaches the sample. Since the tip is not in contact with the sample yet, the force (or cantilever deflection) is constant. At the moment when the tip touches the sample, the force increases (or the cantilever starts to bend). Then the tip indents the sample, and the force (or the bending of the cantilever) follows an inward course. The tip is then retracted, as indicated by the curve, which shows the force (or deflection) during retraction. However, if there is adhesion between the sample and the tip, the tip will stick to the sample beyond the point of contact until it finally breaks free again and the force (or

21 Part IV, Chapter deflection) returns to zero. AFM gives an opportunity not only to study the individual cell s morphology, but also to evaluate the surface topography of the membrane and its elasticity [13]. The above results are obtained in OLEM in collaboration with the visiting researcher Dr. Lizaveta Drozd. A part of the results are published in the master thesis [47] with supervisor Assoc. Prof. Dr. N. Antonova (IMech- BAS). (a) (b) Figure 11(a,b). Z-controlled (a) and force-controlled (b) loading-unloading curve obtained after nanoindentation with applied displacement of 1000 nm and force of 50 nn on erythrocytes (b) (a) Figure 12(a,b). AMF scans of the indented red blood cells samples at different magnification One of the techniques for calculating the Young s modulus is using the Hertz model describing the elastic deformation of the two bodies in contact under loading [48]. It is considered that the indented sample is assumed to be extremely thick in comparison with the indentation depth. In this case the elastic modulus can be calculated as [49, 50]:

22 k Z defl E = ( 1 ν ) (5) 4 R 1/ 2 ( Z Z ) 3/ 2 pos defl where ν sample is Poisson s ratio, R is the tip radius of curvature, k is the spring constant of the cantilever, Z pos position of the probe relatively to the initial position and Z defl the value of the cantilever bend Nanoscratch For the scratch measurements using AFM tip, it is necessary to select the scratch type, either to a controlled loading force or along a fixed scratch profile [74]. This has to be done by selecting either Force (µn) or Depth (µm). Next step is defining Scratch Rate, scratch Length and Angle of the scratch with respect to the x-axis of the scan image and selecting the data to record during the scratch: probe load Force, L-R lateral force, or Scanner Z movement Series Nanoscratch or Nanoindentation Measurements The Surface Force software of AFM allows series scratch or indentation measurements [74]. If the objective is to produce a series of surface indentations, first it is necessary to perform a single indent of the surface to ensure that the parameters are appropriate for the kind of measurement required. Similarly, if the objective is to produce a series of surface scratches, first it is necessary to perform a single scratch on the surface to ensure that the scratch parameters are set appropriately. It is necessary to enter the X,Y coordinates for the series. This would be the center point for indentations, or the starting coordinates for scratches. It is necessary to select the scratch or indent to a specified load force F (µn) (load-controlled), or indentation distance Z (nm) (displacement-controlled). If indenting mode is chosen, when Z (nm) is selected the indents are performed up to these distances. When F (µn) is selected, each indent is performed up to the specified load force F. If scratching mode is chosen, when F (µn) is selected each scratch is performed with a controlled force between the probe tip and the surface (this is the initial force for each scratch and it can be changed over the course of the scratch), when Z (nm) is selected the probe follows the programmed path along the sample surface (this is the depth parameter for the initial indentation of the probe and it can be changed over the course of scratch). 3. Nanoscale Scanning of Surfaces with AFM The Atomic Force Microscope has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and biological samples. Binnig, Quate, and Gerber invented the atomic force microscope in 1986 [19]. The purpose of their invention was to be able to acquire sample images in the atomic level of practically conductive and non-conductive samples with a better lateral and vertical resolution. The AFM is widely used for observing and analysing sample surfaces in the atomic scale. In general, the AFM tip makes a soft physical contact with the surface of the sample,

23 Part IV, Chapter without damaging it [51]. Piezoelectric elements that facilitate the tiny but accurate and precise movements on electronic command enable the very precise scanning. In some variations, electric potential can also be scanned using conducting cantilevers Experimental Details The Atomic Force Microscope used in OLEM is an AFM Q-Scope (Ambios Technology), where scanner XY Range is 80x80 µm and Z Range is 8 µm. The basic operation with AFM is contact and intermittent contact imaging. Usually contact-type cantilevers work well in situations where the material to be imaged is reasonably hard (e.g. metals, ceramics, most polymers) and the surface topography does not have abrupt edges or tall, steep features. Intermittent-contact cantilevers generally work well on all surfaces. Intermittent contact cantilevers also have the ability to detect the elastic and adhesive properties of surface materials, when this type of measurement is desired. The contact modes of operation of the microscope are: Z-Height, Broadband, Lateral Force and BiLateral Force. All intermittent-contact modes of operation of the microscope this includes Wavemode, BB Wavemode, Phase, and ME use stiff cantilevers. AFM is capable of imaging a surface in two different modes labelled as Metrology and Standard. In Metrology mode in addition to the usual piezoelectric scan tube which moves the probe along the x, y, and z axes, there are three capacitive position sensors measuring the exact probe position, which provides greater precision and repeatability when measuring surface feature dimensions. There are four real-time tilt removal methods built into the AFM software: RT Parabolic, RT Edge, RT L/L Parabolic, and RT Topology Map. The Image Graphics window has tools for presenting image data in 2D or 3D format, either with "true color" palette Z scaling, or one of three different shading representations. The powerful software allows refining the image through removing tilt and image streaks and spots. There is an option for matrix and Fourier transform filtering. Imaging analyses include histogram analysis, dimension analysis, section detail and image math AFM Scan Types Because of AFM s versatility, it has been applied to a large number of research topics. The Atomic Force Microscope has also gone through many modifications for specific application requirements. Scan Type selects between different types of data which may be measured as the probe is rastering. The following scan types can be applied using the AFM (Ambios Technology) [74] Z-Height Scan This is a contact-mode topology scan based on maintaining a constant force between the probe tip and the surface. Figure 13(a,b) presents experimental results obtained in OLEM for a 3D AFM image of red blood cells (a) and 2D AFM image of nanoindentation imprint on 1 wt% polypropylene/mwcnt nanocomposite (b) scanned in Z-Height mode.

24 Broadband Scan As in Z Height topology scanning, the system attempts to maintain a constant contact force between the tip and the surface during a Broadband scan. The difference is that any deviations from the nominal constant force condition between the probe and the surface are corrected during rastering. (a) (b) Figure 13(a,b). 3D AFM image of red blood cells (a) and 2D AFM image of nanoindentation imprint on 1 wt% polypropylene/mwcnt nanocomposite (b) scanned in Z-Height mode Lateral Force Mode This is the recording of the twist of the end of the cantilever as it is scanning over the surface. During a contact-mode scan there will be a friction-induced shear force applied to the probe tip, which will cause the entire cantilever structure to twist. The degree of twisting is, in part, a reflection of the strength of the frictional force between the probe tip and the surface. The frictional force may vary as the probe passes over different materials in the sample surface. Under favorable circumstances this can be used to extract information about where different materials are located in the surface topography BiLateral Force Mode The measurements that can be performed are the same as in Lateral Force mode, except that the data are recorded both for the forward and reverse raster of each scan line Wavemode Scan This scan type produces a topological view of the sample surface. The cantilever is set into vibration with amplitude of the order of 100 nm. Then the probe is lowered to the

25 Part IV, Chapter surface where it makes intermittent contact with the surface, damping the oscillation amplitude. Figure 14(a-d) presents examples of 3D AFM images of: (a) red blood cells; (b) MEMS; (c) MWCNTs and (d) solution of poly(vinyl acetate) latex in water for medical applications scanned in Wavemode. All images were obtained in OLEM. (a) (b) (c) (d) Figure 14 (a-d). 3D AFM images of: (a) red blood cells; (b) MEMS; (c) MWCNTs and (d) solution of poly(vinyl acetate) latex in water for medical applications scanned in Wavemode BB Wavemode This is another topological imaging mode. Similarly to the Broadband mode, BBWavemode attempts to correct for any deviations from the ideal constant damping condition in a Wavemode-type scan Phase Scan During a Wavemode surface scan, not only the amplitude of the cantilever s motion is dampened as it bumps into the surface, but the phase of the motion is also shifted. Contact with different materials on the surface may cause this phase shift to differ. For

26 306 example, a hard surface will advance the phase, while a soft or sticky surface will retard the phase. Under favorable circumstances this can be used to extract information about where different materials are located in the surface topography. Figure 15 presents an example of 3D AFM image of a core-shell hydrophobic PNIPAM PEO nanoparticles loaded with a hydrophobic drug scanned in Phase mode. Figure 15. 3D AFM image of core-shell hydrophobic PNIPAM PEO nanoparticles loaded with a hydrophobic drug scanned in Phase mode BiPhase The measurement performed in this mode is the same as in Phase mode, except that now data are recorded for both the forward and reverse raster of each scan line ME Camp/Cphase/Tamp/Tphase These four modes are used when applying the AFM as both Magnetic Force Microscopy (MFM) and Electrostatic Force Microscopy (EFM). MFM maps the magnetic field gradients above a surface; EFM maps the electric field gradients above a surface. Magnetic Force Microscopy requires a special cantilever with a magnetized tip. While the cantilever is vibrated near its resonance frequency it is rastered just out of contact with the sample surface in order to detect the fields near the surface through changes in the cantilever vibration amplitude or phase. At Electrostatic Force Microscopy the probe is vibrated with small amplitude above the sample to interact with the stray electric field above the surface, and what is detected by the probe is essentially an electrostatic force gradient. The purpose of EFM is to infer from a set of force gradient measurements information about the surface potential and surface charge distribution in materials, such as piezoelectrics, ferroelectrics, and dielectrics, or man-made structures such as semiconductor circuitry. EFM requires a Wavemode cantilever coated on the front surface with a conductive film, and the cantilever must be attached to the stainless steel premount with conductive glue.

27 Part IV, Chapter C-AFM This mode produces a map of the current contact between a conducting probe and the surface. 4. Microindentation Testing 4.1. Advantages of the Method Microindentation hardness testing (or microhardness testing) is a method for measuring the hardness of a material on a microscopic scale. The standard way of microindentation hardness measuring uses static penetration of a specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness is calculated by dividing the peak contact load, P by the projected area of impression, A [52]. The general advantages of indentation testing in relation to other procedures for mechanical characterization are the possibility of testing the mechanical properties of a device in its original assembly, and the ability to spatially map the surface mechanical properties in the micron or sub-micron range. There are three principal standard test methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Vickers and Rockwell. For practical and calibration reasons, each of these methods is divided into a range of scales, defined by a combination of applied load and indenter geometry. The hardness value determined for the selected test load using conventional hardness test methods according to Brinell, Vickers or Rockwell does not take into account the elastic deformation of the indentation. These values always characterize, as an integral characteristic, the material resistance of the entire indentation area. An alternative procedure for the study of indentation response of a given material is in-situ continuous force-depth monitoring during loading and unloading parts of an indentation cycle, which allows hardness and Young s modulus evaluation using the Oliver-Pharr theory [53, 54]. Depth-sensing indentation devices make use of small loads in the µn range and are capable of producing penetration depths in the sub-micron scale, hence, opening up the possibility of investigating the mechanical properties of thin films and at the near surface of polymers. This is particularly useful for small indents required for hardness measurements of extremely thin films. The main drawback of this technique, however, is that the analysis of the unloading cycle, which is needed to obtain hardness and elastic modulus values, is based on elasticity considerations [55]. Hence, its application to polymers encounters great difficulties as a consequence of the visco-elastoplastic character of these materials [56-58]. So far, there is still not a complete and sound methodology for the derivation of mechanical properties of polymers from depth-sensing data. The method offers considerable advantages over the traditional microindentation hardness techniques due to the lack of geometry measurement of the indentation and the resulting substantial experimental error. The continuous acquisition of the test load and the indentation depth under load has the advantage that the elastic and plastic deformations are measured [59]. In addition, the microhardness can be measured

28 308 quantitatively for every indentation point and presented as a function of the indentation depth, the test load, or the effective load duration. This results in additional capabilities, for example the analysis of coatings of either hardened, or softened surfaces. Aside from the hardness under load, additional material characteristics that allow for an in-depth evaluation of the specimen can be derived from the load/indentation depth curve. Compared to the methods using optical evaluation, the measurement of the indentation depth offers the additional advantage of being performed automatically (operatorindependent) Microindentation Hardness Testing using Mechanical and Tribology Tester (UMT-2M) Microindentation hardness testing by UMT-2M is performed applying force or displacement using standard Vickers indenter diamond squared pyramid with face angle of 136, or standard conical Rockwell indenter, 120 o angle, tip radius 200 microns. The microindentation module of the UMT-2M tester is shown schematically in Figure 16. Figure 16. Microindentation UMT-2M CETR setup. The model system consists of upper load sensor with tip holder, and lower stationed part with specimen holder. The motorized vertical carriage supplies a loading force for micro-scale tests of 5mN to 1kN. The depth of indentation is measured by capacitance sensor (from 0.05 to 250 µm). Multiple indents on specific sample areas are obtained automatically with a lateral resolution of 0.5 µm for reproducible results. The instrumented indentation may be applied on macro-scale (for loads greater than 2 N) and micro-scale (loads under 2 N) to calculate hardness, Young s modulus, contact stiffness, etc. of coatings and bulk materials. When the standard Vickers indenter is embedded in the UMT-2M tester, the load during the microindentation hardness test is applied smoothly, without impact, and held in place for 10 or 15 seconds. The physical quality of the indenter and the accuracy of the

29 Part IV, Chapter applied load must be controlled to get the correct results. Vickers indenter is the preferred tip geometry because it gives better reproducibility due to its sharpness. Using the UMT-2M software, the calculation method to determine hardness and modulus is based on the Oliver and Pharr approaches, (Eqs.1-4) [3,33]. The visualization of imprints of the instrumented indentation, however, is possible using either the embedded digital optical microscope, or the 3D Profilometer. Figures 17(a,b) present examples of properly formed Vickers indents on Cu alloy, visualized by using: (a) optical microscopy and (b) 3D Profilometer. (a) (b) Figure 17(a-b). Optical micrograph (a) and 3D Profilometer image (b) of impressions left behind by avickers indenter on the surface of Cu alloy. The software of the UMT-2M equipment allows measurement of a full loadunloading cycle, from which a number of micromechanical parameters, such as: microhardness, Young s modulus, contact stiffness and contact depth can be obtained. As an example, Figures 18(a,b) present the microindentation results of polypropylene nanocomposite with 2 wt% of carbon nanotubes, using Vickers indenter tip, as follows: (a) load-depth curves, and (b) calculated hardness and Young s modulus. The program (script) compiled for this example consists of several steps: 1) preloading, 2) loading for 40 sec until 2 N target load, 3) holding at 2 N maximal load for 30 sec, 4) unloading until 10 % from maximal load is reached for 40 sec, 5) holding at 10 % from maximal load, 6) full unloading. For each indentation step, load, displacement (penetration depth or indentation depth), and real time are recorded. These raw voltage data are converted to load vs. displacement data by using load and displacement calibration constants. From the displacement data, the contact depth is calculated for calculations of the hardness. The slope of the unloading curve is used to calculate the modulus of elasticity. The data obtained are from 7 measurements on the same sample. The software automatically calculates the following parameter values: Hardness - mean GPa with a standard deviation of 0.001, and Young s modulus GPa with a standard deviation of at set value for Poison ratio of 0.35.

30 310 (a) b) Figure 18(a,b). Load-depth curves in left side (a) and values of hardness ( ) and Young s modulus ( ) in right side (b), of polypropylene nanocomposite modified with 2 wt% of carbon nanotubes using Vickers indenter tip. The standard conical Rockwell indenter can be also embedded in the UMT-2M tester. The Rockwell test determines the hardness by measuring the depth of penetration of a conical indenter under a large load compared to the depth of penetration made by a preload [60]. As an example, Figure 19 presents an optical micrograph of the Rockwell microindentation on Cu alloy. Figure 19. Optical micrograph of impression left behind by a Rockwell indenter on the surface of Cu alloy.

31 Part IV, Chapter Tribology Testing Polymer nanocomposites and nanostructured materials have been used increasingly for tribological applications in recent years. Tribology is a study of the interaction between surfaces in contact and spans many disciplines from physics and chemistry to mechanical engineering and material science [61,62]. The purpose of research in tribology is the minimization and elimination of losses resulting from friction and wear at all levels of technology where the friction of surfaces is involved. During the sliding process between two surfaces, interfacial forces responsible for friction and wear are generated at the contact points. The size of contact points ranges from nanometers to micrometers, making tribology a multiscale phenomenon [63]. The friction coefficient is a strategic parameter to perform mechanical calculations of nanomaterials with the purpose to analyze the damage of surfaces observed after tribological treatment. The damage depends on the kind of sample used and may be variable: fissure, crack, rupture or chipping of thin layers [64] Microtribological Measurements using Ball/Pin-on-Disk UMT-2M System Setup The CETR UMT-2M micro-tribometer is a single measuring system with multimotion tribo-metrology. Figure 20 shows the scheme of the wear module of the UMT- 2M. The tribological properties of two types of sliding wear - the rotational motion (ball/pin-on-disc) and reciprocating motion (ball/pin-on-flat) can be achieved on this system using rotational motion drive and reciprocating motor drive, correspondingly [25,65]. The system allows continuous monitoring and recording of the following parameters: the actual dynamic normal load, friction force, friction coefficient, critical test load, wear/wear rate and depth of wear. Optionally, one can measure additional parameters of contact acoustic emission and electrical contact resistance [66]. The Pin-on-Disk test configuration below is an example of one of the numerous possible combinations of friction/load sensors with shaped tip holder and rotational drive with claimed specimen. This test method includes a ball/pin shaped tip of the testing device that slides against a rotating disk as a lower specimen under a set of conditions. The ball used in this equipment can be made of stainless steel or chrome steel with diameter 6.35 mm and the pin used is of stainless steel with the same diameter. The force applied is in the range of 5 mn to 1kN, and precision spindle of rotary drive can rotate the lower specimen at speed ranging from 10 rpm up to 1000 rpm. The combination of loading force and friction is used to calculate the coefficient of friction. A suspension can be mounted to the force sensor which allows the upper specimen to follow variations in the height of the lower specimen as it rotates. After the wear test, the morphologies of each wear scar can be observed by optical microscopy or profilometer.

32 Microtribological Measurements using Ball/Pin-on-Flat Reciprocating UMT-2M System Setup This test method utilizes a pin or ball shaped upper tip that slides against a flat lower specimen in a linear, back and forth sliding motion, under a prescribed set of conditions. The UMT-2M applies the load vertically downward through a motor driven carriage that uses a force/load sensor for feedback to maintain a constant load. Furthermore, the control of the test parameters such as velocity, frequency, contact force, load, time and environmental parameters (temperature, humidity and lubricant) allows simulation of the real life conditions of a practical wear situation. Figure 20. Scheme of the wear module of the UMT-2M. On the UMT 2M micro-tribometer, the actual dynamic normal load, friction force, friction coefficient and depth of wear as function of the testing time are measured. In addition, it measures parameters of contact acoustic emission and electrical contact resistance. The electrical contact option is a useful measurement when variations of conductivity could be observed in a coating/substrate system. For example, the difference of conductivity between a coating and a substrate can be detected and allows determination of the rupture of the coating during a wear test. The Viewer software program is used to plot test results. The recorded parameters are displayed by checking the appropriate boxes such as WR for wear and COF for coefficient of friction under Parameters on the Viewer screen. The frictional coefficient (COF) is recorded and calculated by the ratio between tangential force and normal load. After the wear test, the mass loss of the specimen can be measured for calculation of the specific wear rate (w s ) by the following equation,

33 Part IV, Chapter ω s m ρlf = [mm 3 /mn] (6) N in which F N is the normal load applied on the specimen during sliding, m is the specimen s mass loss, ρ is the density of the specimen, and L is the total sliding distance. In order to evaluate the wear behaviour of materials under different wear conditions, the time related depth wear rate (w t ) was also introduced, h ω t = = ρvw s [m/s] (7) t in which h is the height reduction of the specimen during the test and t is the test time. Based on (8), the pv factor could be considered as a tribological criterion of the load carrying capacity for bearing materials [67]. As an example, stainless steel metallic ball with diameter of 6.35 mm was used to perform a ball on flat reciprocating wear test on a thin film of epoxy nanocomposite containing 3 wt% of alumina Disperal OS1, deposited on a conductive substrate. A linearly increasing load from 20 to 60 N, velocity of 10 mm/s and distance of 10 mm were applied. In Figure 21, the friction (Fx) and normal (Fz) force data, together with electric contact resistance (ECR) data were continuously monitored and the coefficient of friction values were continuously calculated during the test. The epoxy/alumina nanocomposite thin film was progressively cut at about 640 s, corresponding to a critical load of 42 N. At the same moment a sharp increase in the coefficient of friction value could be observed, while the ECR dropped to zero, which indicates that the tip has reached the conductive subrstrate. The afore-mentioned result is obtained in OLEM and is a part of a master degree thesis [68] with supervisor Assoc. Professor PhD Antonia Topliyska (IMech - BAS). Such microtribological measurements can be used to study failure mechanisms on the microscale and evaluate the mechanical integrity (wear resistance) of ultrathin films at low loads. 6. Scratch Testing Scratching is an alternative to conventional wear testing to evaluate the tribological properties of nanomaterials [69]. The scratch is a mechanical deformation process where controlled force or displacement is exerted on a hard tip to indent onto material, the tip moving relative to the material and the latter s scratch resistance is define by its ability to withstand mechanically induced surface damage under defined conditions. Unlike standard indentation process where the normal load is uniformly distributed beneath the indenter, scratching involves a high-friction-induced sliding process [70]. It should be noted that the scratch tip geometry, tip material, substrate thickness, surface characteristics and rate of testing, can all significantly affect the scratch performance of the nanomaterials. Extensive research efforts have been dedicated to the development of an objective test methodology for polymer nanomaterials scratch. This has led to the establishment of ASTM D 7027, ISO 15184, ISO standards for scratch testing of polymers in which more in-depth description of the standardized polymer scratch testing

34 314 can be found. Generally speaking, there are two main types of damage found in polymer nanomaterials: ductile damage (e.g., shear yielding and ironing) and brittle damage (e.g., crazing and cracking); their occurrence depends on the material characteristics and applied stress state and magnitude. In addition, debonding and voiding can take place if the polymer nanocomposites contain different phases [71]. Figure 21. Coefficient of friction (COF), friction force (Fx) and normal force (Fz) vs. time plot of epoxy/alumina nanocomposite tested with embedded electric contact resistance (ECR) sensor Principle of the Method Microscratch test may be performed with either constant load, or linearly increasing load with several tip geometries. The micro-blade geometry was found to be the most effective counter surface for accelerated wear tests for hard films or coatings on different substrates, as compared to a ball or pin geometry. The advantage of using microblade to evaluate delamination of film coatings is based on the contact stress analysis [32]. For spherical or cylindrical contact geometry, the contact stress is distributed well beyond a few nanometers (film thickness) deep. The contact stress should be concentrated within or near the surface film that will be studied, rather than distributed well into the substrate/underlay, as in the case of ball or pin contact geometry. Any damage to the material surface as a result of scratching at a critical increasing load, results in an abrupt and gradual increase in friction. The material may be deformed either by plastic deformation or fracture. Ductile materials deform primarily by plastic deformation, resulting in significant plowing during scratching. Ductile deformation results in plowing, whereas brittle deformation aids in debris generation. Hard overcoats generally consist of significant compressive residual stresses. It is these compressive stresses that allow ductile deformation with little cracking [63]. After the scratch test, the morphology of the scratch region including debris can be observed by different techniques as profilometer, SEM and optical microscope [72].

35 Part IV, Chapter Microscratch Measurements using Mechanical and Tribology Tester UMT-2M The configuration in Figure 22 of Micro-Scratch UMT (CETR-USA) Test System Setup is an example of how a scratch test can be performed. The configuration consists of upper load sensor with tip holder and lower drive with lower specimen holder. The applied load can vary in the range of 5 mn to 1000 N. The microscratch test is using a linear or rotary drive, providing linear motions with defined speeds for moving the sample during scratching. The tip is micro-cutting blade made of composite diamond with tip radiuses available of 0.4 and 0.8 mm. The parameters which can be measured and recorded are: friction force (Fx), normal load (Fz), electric contact resistance (ECR), acoustic emission (AE), COF coefficient of friction, z - depth of penetration. Acoustic emission and Electrical contact resistance are additional sensors used with the UMT-2M. The electrical contact option is a useful measurement when variations of conductivity could be observed in a coating/substrate system. For example, the difference of conductivity between a coating and a substrate can be detected, which allows determination of the rupture of the coating during the test. These sensors measure the electrical resistance in ohms between the tip and the surface of the sample. Acoustic Emission is a phenomenon of sound and ultrasound wave radiation in materials which undergo deformation and fracture processes. Since the acoustic emission sensor converts the mechanical energy carried by the elastic wave into an electrical signal, the sensor is more properly termed as a transducer. The extent of the damage in a scratch test is estimated by the width and the depth of the scratch debris generated toward the end of the scratch. The changes in the friction force as a function of normal load also can help to determine the critical load of film destruction. Figure 23(a,b) presents an example of scratch tests conducted on a pure polypropylene (a) and polypropylene/ rice flakes microcomposite (b) with constant force of 0.5 N for 120 sec with micro-blade with tip radius of 0.4 mm. During the tests the penetration of the micro-blade increased evenly up to 0.13 mm for the bulk sample of pure polypropylene, while for the polypropylene microcomposite with rice flakes reached a value of 0.1 mm. The polypropylene microcomposite demonstrates better scratch resistance according to the test results Adhesion at Scratch One of the most important properties of thin film coatings is the adhesion (interfacial forces between two surfaces) between the coating and the substrate. The most common method of accurate measurement of thin film coating adhesion is the scratch test. The typical scratch test used to detect coating adhesion includes a constant or linearly increasing load to measure the change in friction, acoustic emission which detects crack initiation and propagation, depth measurement, and after the test is completed, the scratch channel could be viewed using an optical microscope. The intensity of the acoustic emission is dependent on the type of thin film coating failure during the adhesion test, e.g. cracking, chipping (cohesive failure) or delamination (adhesive failure). It is

36 316 therefore a good practice to view always the coating failure after the adhesion test using an optical microscope to confirm the critical load. Figure 22. Micro-Scratch UMT-2M (CETR-USA) Test System Setup. The adhesion at scratch experimental technique is used to measure different types of failure cohesive or adhesive, as well as adhesion strength, critical load, friction force. It finds application in semiconductors industry, hard disk industry (disc and head overcoats, DLC coatings), optical components (window glass, lenses and optical coatings), automotive and aerospace industry when investigating paints and intermediate layers, engine components, etc Adhesion at Scratch Test with UMT 2M Micro-Scratch Setup The UMT-2M tester enables multiple or single micro-scratch measurements at constant, increasing or user defined programmable load (5 mn up to 1kN) using rotational, translational or reciprocating motions with speeds ranging from 0.1 µm/s up to 10 m/s in order to evaluate scratch adhesion of hard and soft coatings, thin films and bulk materials (metals, plastics, ceramics, silicates, etc.). The high frequency Acoustic emission sensor detects the crack initiation and the propagation in hard coatings. The Electrical contact resistance changes as the conductive tip reaches the substrate, which is also conductive. Friction force, normal load, wear depth, AE and ECR are measured and recorded. Scratch tests using micro-blade with tip radius of 0.4 mm have been performed on a film coating of thermoset nanocomposite modified with multiwall carbon nanotubes deposited on a metal substrate at gradually increasing normal load in the range of 0 to 20 N. Figure 24 presents the parameters plotted as a function of the testing time in seconds. During the scratch test, as the micro-blade moving slowly against the film coating, progressive material s removal occured. The thermoset nanocomposite coating was

37 Part IV, Chapter progressively cut by the micro-blade at about 104 seconds and a well-defined failure event occurred at the same moment corresponding to a critical load of about 11 N. The critical load can be used as a qualitative measure of coating/substrate adhesion. At this critical load, the character of the friction force curve changed and shifted to a higher value. At exactly the same time, ECR dropped to practically zero, due to the fact that the micro-blade made contact with the conductive substrate after cutting the coating. Meanwhile, the AE signal started to fluctuate significantly after destruction of the coating. a) b) Figure 23(a,b). Microscratch testing of a pure polypropylene (a) and polypropylene microcomposite modified with 1 vol.% rice flakes 7. 3D Profilometry A range of industries together with material research require measurements of surface topography to either control their processes or research new material

38 318 characteristics. Typical surface measurement parameters include flatness, roughness, curvature, peak-to-valley, asperity, texture, thickness, slope and distance. Profilometers are the industry standard method of measuring surface topography on micro- and nanoscale. It is a measuring instrument used for two different requirements surface measurements and contour measurements on any kind of materials, e.g. transparent, opaque, specular, diffusive, polished, rough, etc. The method finds particular application in scanning metal etch uniformity on wafers, solar cell finger width and height, microlens height/curvature and V-groove depth analyses, aspheric lens characterization, surface quality and defect review, high aspect ratio trench depth measurements, etc. Figure 24. Microscratch testing of coating of a epoxy resin modified with multiwall carbon nanotubes on metal substrate Profilometry Measurements using 3D Stylus Surface Profilomter PRO500 The stylus based contact Surface Profilometer PRO500 is a seamless module of the UNMT tester with integrated optical camera. It generates both 2D and 3D images from 10x10 µm up to 500X500 µm scanning area and utilizes superior repeatable, ultra low force that allows it to measure surface topography and contour for any kind of materials. The diamond stylus is moved vertically in contact with the sample and then moved laterally across the sample for a specified distance and specified contact force. The PRO500 profilometer measures small vertical features ranging in height from 500 nm to 500 um. The height position of the diamond stylus generates an analog signal which is converted into a digital signal stored, analyzed and displayed by the PRO500 Software. The radius of the diamond stylus is standard 5 µm according to ISO 3274 standard, and the horizontal resolution is controlled by the scan speed and data signal sampling rate. The stylus tracking force ranges from less than 0.1 to 100 milligrams.

39 Part IV, Chapter Figure 25 represents the optical image of part of the scratch performed with microblade on a brass alloy. Figure 26 is a 2D profilometry scan on the same scratch, which gives the width and the depth at a certain location on the scratch, µm and 6.23 µm, respectively. The roughness of the scratch is 1.82 µm. The 2D scan generates a line profile along the horizontal (x) direction and the data is plotted as height values (z) vs. (x). During a 3D scan, the stylus goes through a raster type of scanning from the bottom to the top of the field of view. The data from a 3D scan is displayed in different colours containing the information of Z heights vs. X and Y positions. Figure 25. Optical image of a scratch on brass alloy. Figure 26. 2D Profilometer scan of a scratch on brass alloy. Figure 27. 3D Topography scan of a scratch on brass alloy Example of a 3D topographic scan in a perspective view on the same scratch is given in Figure 27. Figure 28 is another example of a 3D scan of a thin film of tin alloy giving the roughness (Ra) of the chosen area, in particular µm and the peak to peak value of a µm. Figure 29 presents a 3D scan of epoxy nanocomposite filled

40 320 with 3wt% of Disperal OS1. The average roughness value is 1.29 µm as estimated by the PRO500 Software. Figure 28. 3D Roughness scan of tin alloy. Figure 29. 3D Roughness scan of epoxy/alumina nanocomposite. Conclusions In the present review the potential capacity of nano and microindentation, AFM, profilometry, scratching, adhesion and wear techniques on investigating nanostructured materials are presented and discussed in details. Nanoindentation tests of bulk polymer materials like epoxy resin and nanocomposites of epoxy resin and MWCNT, polypropylene and nanocomposites of polypropylene and MWCNT as well as metal alloys, were performed using nanoindenter UNMT (CETR) equipped with integrated atomic force microscope and digital optical microscope. Young s modulus and hardness are derived from the unload data segments through in-situ monitoring of the load displacement curves and automatic calculations by utilizing the Oliver-Pharr method. The methods provide possibility to apply single and multiple indentations. Multiple partial unloading nanoindentation at a single point is used when the purpose is to know the variation of hardness, modulus values and coating thickness of multilayer materials. When applying AFM for indentation measurements the tip is used to indent the sample surface and it is possible to produce indentations over the desired region with well-controlled position accuracy. A variety of measurements can be performed with the Surface Force software of the AFM (Ambios Technology) including exploring the interaction force between the cantilever tip and the surface, performing a nanomechanical single/series of indentations or scratch measurements. In the chapter are presented example results obtained after z-controlled and force-controlled nanoindentations with AFM on erythrocytes, as well as the possibility to investigate the individual red blood cell s morphology, surface topography and calculation of its elasticity using a model. Different AFM scan types including Z-Height, Broadband, Lateral Force, BiLateral Force, Wavemode, BB Wavemode, Phase, and ME are selected in relation with

41 Part IV, Chapter the different types of data which may be measured as the probe is rastering on materials like red blood cells, polypropylene/mwcnt nanocomposite, MEMS, MWCNT, solution of poly(vinyl acetate) latex in water for medical applications, etc. The microindentation hardness testing technique (UMT-2M, CETR) is implying force-depth monitoring during loading and unloading parts of indentation cycle, which allows hardness and Young s modulus evaluation using the Oliver-Pharr theory. This is particularly useful for small indents required for hardness measurements of extremely thin films and coatings in the micro- and nanosystems range. In micro/nanotribology techniques (scratch, wear, friction, adhesion), measurements are made on materials with relatively small mass under lightly loaded conditions, as presented in the examples obtained in OLEM Research Laboratory and discussed in details. These techniques have been used to study scratching, wear, friction, scratch adhesion of epoxy/alumina and epoxy/mwcnt nanocomposites, bulk polypropylene and polypropylene filled with micron sized rice flakes. In this situation, negligible wear occurs and the surface properties dominate the tribological performance. The micro/nanotribological investigations are needed to develop fundamental understanding of interfacial phenomena on a small scale and to study the friction and wear processes of ultrathin films and micro/nanostructures. Although micro/nanotribological studies are critical to study ultrathin films and micro/nanostructures, these studies are also valuable in fundamental understanding of interfacial phenomena in macrostructures to provide a bridge between science and engineering. Surface profilometry is applied in order to quantify the surface roughness and to study the topographical and contour features of different metal alloys and polymer nanocomposites. The chapter provides detailed insight into the micro/nanomechanical and micro/nanotribological characterization methodologies and serves to narrow the gaps between conventional macro systems and the emerging micro/nano materials technologies. Acknowledgements This work was supported by the Research Infrastructure Project DO at National Science Fund of Bulgaria, the EC-FP7-BY Nano ERA project, and Theme 1(1) at PCM-IMech, BAS. Authors thanks Dr. Victoria Michaylova (MU, Sofia), Dr. N. Antonova (IMech, BAS), Dr. A. Topliyska (IMech, BAS), Dr. T. Avdjieva (SU St. Kl. Ohridski ) and Dr. V. Andonova (MU, Plovdiv) for the fruitful collaboration. References: 1. B. C. Prorok, Zhu Yong, H. D. Espinosa, Guo Zaoyang, Z. P. Bazant, Zhao Yufeng, B. I. Yakobson. Micro-and Nanomechanics. In: Encyclopedia of Nanoscience and Nanotechnology 4. American Scientific Publishers, Yi-Zhang Tong, Kim Jang-Kyo, editors. Macro-, Meso-, Micro- and Nano- Mechanics of Materials. Advanced Materials Research, 9, 2005.

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43 Part IV, Chapter J. K. Kim, M. L. Sham, J. Wu Nanoscale characterisation of interphase in silane treated glass fibre composites. Composites A, 32: , G. Binnig, C. F. Quate, Ch. Gerber. Atomic Force Microscopy. Phys. Rev. Let., 56: , P.J. Blau, R. Brian Lawn. Microindentation techniques in materials science and engineering, ASTM Committee E-4 on Metallography, International Metallography Society, M. A. Meyers, K.K. Chawla. Mechanical Behavior of Materials, Prentice-Hall, E. J. Pavlina, C. J. Van Tyne. Correlation of Yield Strength and Tensile Strength with Hardness for Steels. J. Mater. Eng. Perform., 17(6): , Friedrich K., Schlarb A. K. Tribology of Polymeric Nanocomposites: Friction and Wear of Bulk Materials and Coatings. Elsevier Ltd, Q. Wang, X. Pei. The influence of nanoparticle fillers on the friction and wear behaviour of polymer matrices. Tribology of Polymeric Nanocomposites, Elsevier Ltd, G. Yang, Zh.-g. Geng, H. Ma, Zh. Wu, P. Zhang. Preparation and tribological behavior of Cu-nanoparticle polyelectrolyte multilayers obtained by spin-assisted layer-by-layer assembly. Thin Solid Films, 517: , Y. Xue, W. Wu, O. Jacobs, B. Schädel Tribological behaviour of UHMWPE/HDPE blends reinforced with multi-wall carbon nanotubes. Polym. Test., 25: , L.-H. Sun, Zh.-G. Yang, X.-H. Li. Study on the friction and wear behavior of POM/Al 2 O 3 nanocomposites. Wear, 264: , G. Suia, W. H. Zhonga, X. Renc, X. Q. Wangc, X.P. Yang. Structure, mechanical properties and friction behavior of UHMWPE/HDPE/carbon nanofibers. Mater. Chem. Phys, 115: , G. Biresaw, K. L. Mittal. Surfactants in tribology. Taylor & Francis Group, LLC, Z. Rymuza. Tribology of Polymers. Archives of Civil and Mechanical Engineering,VII (4): , B. J. Briscoe, S. K Sinha. Scratch Resistance and Localised Damage Characteristics of Polymer Surfaces. Mat Wiss. Werkst., 34(10/11): , N. Gitis, A. Daugela,J. Xiao. Integrated tribo-spm for nano-tribology. In Proc.Viennano '05, , Vienna, Austria, March W.C. Oliver, G. M. Pharr. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res., 7(6): , R. Kotsilkova. Thermoset nanocomposites for engineering applications. Smithers Rapra Technology, Shawbury, Shrewsbury, UK-USA, 2007.

44 E. Ivanov, D. Nesheva, E. Krusteva, T. Dobreva, R. Kotsilkova. Rheological and electrical properties of epoxy nanocomposies filled with multiwall carbon nanotubes. Nanoscience & Nanotechnology 9:40 43, E. Krusteva, R. Kotsilkova, E.Valcheva, V. Donchev, E. Ivanov, T. Dobreva. Effect of processing and external magnetic field on the structure of epoxy/multiwall carbon nanotube composites. Nanoscience & Nanotechnology, 9: , E. Ivanov, R. Kotsilkova, E. Krusteva, C. Silvestre, D. Duraccio, M. Pezzuto, E. Logakis, A. Kyritsis, P. Pissis. Effects of Processing Conditions on Rheological, Thermal, and Electrical Properties of Multiwall Carbon Nanotube/Epoxy Resin Composites. J. Polym. Sci. Pol. Phys., 49: , E. Ivanov, R. Kotsilkova, E. Krusteva. Effect of processing on rheological properties and structure development of Epoxy/MWCNT nanocomposites. J Nanopart Res., 13: , R. Kotsilkova, E. Ivanov, E. Krusteva, C. Silvestre, S. Cimmino, D. Duraccio Isotactic Polypropylene Composites Reinforced with Multiwall Carbon Nanotubes. Part 2. Enhancement in Thermal and Mechanical Properties Related to the Structure. J. Appl. Polym., 115: , E. Ivanov, E. Krusteva, S. Djoumalijsky, R. Kotsilkova, R. Krastev, D. Duracio, C. Silvestre, S. Cimmino. Effect of Processing on Rheology and Structure of Polypropylene/Carbon Nanotube Composites. Nanoscience & Nanotechnology, 8:89-92, A. Erinin. Structure and properties of Zr-Hf alloys. Master Thesis, Sofia University St. Kliment Ohridski, Sofia, C. A. Clifford, M. P. Seah. Improved methods and uncertainty analysis in the calibration of the spring constant of an atomic force microscope cantilever using static experimental method. Meas. Sci. Technol., 20:125501, C. A. Clifford, M. P. Seah. Nanoindentation measurement of Young's modulus for compliant layers on stiffer substrates including the effect of Poisson's ratios. Nanotech, 20, , M. Munz. Force calibration in lateral force microscopy - A review of the experimental methods, J. Phys. D 34: , D. C. Lin, E. K Dimitriadis., F. R. Horkay. Strategies for Automated AFM Force Curve Analysis-I. Non-adhesive Indentation of Soft Inhomogeneous Materials. J. Biomech. Eng, 129(3): , H. J. Butt, B. Capella, M. Kappl. Force measurements with the atomic force microscope: Technique, interpretation and applications. Surf. Sci. Rep., 59(1-6):1-152, R. Krustev. Investigation of morphological and mechanical characteristics of the erythrocyte membrane. Dependance with rheological properties of blood. Master thesis, Sofia University St. Kliment Ohridski, Sofia H. Hertz Über die Berührung fester elastischer Körper. J. Reine Angew. Math. 92: , 1881.

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46 K. Friedrich. Wear of reinforced polymers by different abrasive counterparts. Friction and wear of polymer composites. Amsterdam: Elsevier Science Publishers, Angelkova E. Mechanical properties of polymer nanocomposite coatings. Master Degree Thesis. UCTM-Sofia, S. Tao, D. Y. Li. Tribological, mechanical and electrochemical properties of nanocrystalline copper deposits produced by pulse electrodeposition, Nanotechnology 17:65 78, A. Dasari, Zh.-Zh. Yu, Y.-W. Mai. Fundamental aspects and recent progress on wear/scratch damage in polymer Nanocomposites. Mater Sci Eng, R63:31 80, H. Jiang, R. Browning, H.-J. Sue. Understanding of scratch-induced damage mechanisms in polymers. Polymer, 50: , L.-P. Sung, P.L. Drzal, M.R. VanLandingham, T.Y. Wu, S.-H. Chang. Metrology for Characterizing Scratch Resistance of Polymer Coatings. JCT Research, 2(8): , CETR Nanohead (NH) Manual. Center For Tribology, Campbell CA 95008, USA, Ambios Technology Corporation Operators Manual Q-Scope TM 250/400 Nomad TM, 2009.

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