Nano and microhardness testing of heterogeneous structures

Transcription

1 85 ISSN MECANIKA Volume 22(2): Nano and mirohardness testing of heterogeneous strutures S. Baskutis*, V. Vasauskas**, A. Žunda*** *Kaunas University of Tehnology, Studentų str. 56, Kaunas, Lithuania, **Kaunas University of Tehnology, Studentų str. 56, Kaunas, Lithuania ***Aleksandras Stulginskis University, Studentų str. 11, Kaunas region, Lithuania, Introdution Indentation methods, as remarkably flexible mehanial tests, are finding inreasing use in the study of mehanial properties of bulk and thin film nonhomogeneous materials over a wide range of size sales [1]. Most engineering materials are of heterogeneous struture, different phase properties and faing dynami hanges. Therefore, the ontrol of the loal mirostruture is espeially important aiming to attain the maro sale properties. Instrumented indentation is an important tool that an be used to evaluate the mehanial properties of a wide range of engineering materials aross the nano and miro length sales. By using the different shapes of indenters tip geometry and varying the applied load, indentation tehniques an be used to probe different volumes of materials [2]. Correlation between the hardness and the mirostruture in weld joints has been established for engineering materials [3]. The nanoindentation tehnique allows rather small regions in grains to be investigated and different phase strutures are distinguished using this tehnique [4]. For low load nano and miroindentation, the area of ontat between the indenter and material varies during testing and is indiretly dependent on the measured depth of penetration. Miro and nanoindentation tests, i.e. indentation depths of 0.1 to 100 µm and less than 0.1 µm, respetively, proved to be the most ost-effiient, as well as fast, preise and non-destrutive insert. owever, there are numerous indentation tests at sales on the order of a miron or a submiron that have shown that the measured hardness inreases signifiantly with dereasing the indentation size [5, 6]. This phenomenon is ommonly referred to as the indentation size effet (ISE) [7]. The behaviour of the ISE in single rystals for nano and miroindentation was investigated by Manika et.al. [8]. Assoiation of the ISE with frition and with strain hardening was onfirmed by M. Atkinson [9]. Aording to [10], ranges of hardness testing are defined: maro sale test fore diapazon 2 N 30 kn, miro sale test fore less than 2 N, indentation depth more than 0.2 µm, nano sale test fore less than 0.3 N, indentation depth less than 0.2 µm. The miro sale distinguished by the test fore in relation to the indentation depth. For the nano sale, the mehanial deformation strongly depends on the real shape of indenter tip. omogeneity in the material ould be an important issue partiularly in omposite, where the indenter tip may be or may not be hit the partile and this ould hange the values of material properties. The effets on the loalization of deformation at various sales ranging from the mirosales down to the nanosales are disussed for heterogeneous strutures. At the nanosale, a dominant mehanism of deformation is the rearrangement of free nano volume and exhange of momentum between bulk and grain boundary spae. At the mirosale, a most ommon mehanism of deformation is disloation motion [11]. Investigations developed by L.Qian et.al. [12] showed that nanoindentation and miroindentation hardness tests have similar load effet and their differenes are between 10% and 30%. Correlation aspets between nanoindentatrion hardness tests results and Vikers hardness was desribed by T. Sawa [13]. It should be noted, that nanoindentation hardness and orresponding Vikers hardness values are different. This differene sale is beause of the different manner the hardness values are defined. Nanoindentation hardness is determined using the indenter under load, while Vikers hardness is alulated after the load is removed. The presented work deals with relatively more reliable and aurate hardness analysis in heterogeneous strutures engineering. The nano and miroindentation of polyrystalline opper thin films of different thikness and approximately the same grain size and arbon steel speimens, welded by gas tungsten ar welding (TIG), were used for the experiments. 2. Bakground Nano and miroindentation methods often are presented together and it seems to be simple and similar. In fat, proess of indentation is very ompliated regarding the deformation mehanism as well as the hanges in the material struture under the indenter. The trend of nanohardness and mirohardness profiles in the intermediate area between the inner layers with onstant hardness and abnormal area gives information on strutural heterogeneity [14]. This is the area where the ISE in indentation measurements an be demonstrated [15, 16]. The idea, whih enables the assessment of the homogeneity of the samples, was proposed and desribed in this artile. Furthermore, many materials and espeially strutural ones exhibit phase heterogeneity and mehanial differenes of the phases on different length sales. In order to model heterogeneous material systems, multi-sale approah that allows for separation of sales based on some harateristi dimension of a material mirosopi feature for eah level is often utilized. Averaged (effetive) omposite properties an be found if the indentation depth is muh larger than the harateristi phase dimension (h >> D). In this ase averaged properties are obtained. If the (h << D), intrinsi properties of the distint phase are obtained. Sine Vikers tips ause

2 86 relatively negligible strain in the material during indentation, the hardness results obtained from test using the different tip geometries an be diretly ompared. owever, are must be taken to orretly perform the onversion of the results. There is a definition hange between the hardness measured using miroindentation to the hardness used in nanoindentation. While both hardness values are alulated as the peak fore divided by the area of ontat, the definition of the ontat area differs between the test tehniques [15]. For miroindentation, the ontat area is the surfae area of the tip that area in ontat with the sample. While, for nanoindentation the ontat area defined as the projeted area between the sample and tip. In nanoindentation the hardness is determined as the mean ontat pressure. The nanoindentation hardness of the speimen an be analysed by the urves with the Oliver-Pharr method [1]. The onventional hardness: h F h (1) A h and the differential hardness: d df h, (2) da where F is load, A is indentation area, are alulated as ontinuous funtions of the depth h and ompare to eah other in this paper. It turns out that d desribes the momentary material resistane to deformation, whereas integrates our deformation states from first tip sample ontat to urrent penetration h. This differene is important for materials not homogeneous in depth, e.g. layer systems. Fig. 1 Depth-fore orrelation for an indent at maximum fore of 250 mn with harateristi quantities: maximum indentation depth h max, final indentation depth h f and ontat indentation depth h The nanoindentation hardness ( nano) an be alulated using following formula: nano F (3) A max h, where in equation F max is the peak indentation load, A is the ross-setional area and h represents the ontat indentation depth between the indenter and the speimen (Fig. 1). The determination of the ontat indentation depth h is obtained from equation: Fmax h h max 0. 75, (4) S where S is the ontat stiffness and an be obtained by: df S, dh where df / dh is the stiffness at the upper portion of the unloading data alulated from the slope of the depth-fore urve. The miroindentation of tested samples had a larger satter due to the influene of several fators: hardness of grains ( nano), ISE, mirostruture and grain boundary phase and at higher loads by mix-phase volume below the indenter. 3. Experimental Two types of heterogeneous materials were used in present study. At first, arbon steel was welded by using gas tungsten ar welding method, in the Ar/CO 2 protetive gas environment, with the non-fusible eletrode from tungsten, and seond, opper (Cu) films were made by eletrospark deposition (ESD) on steel substrates. The speimens were deposited using 3A urrent and sparking voltage of 60 V. Film thikness was varied by hanging the deposition time. In this paper we examined opper films of thikness 60 µm and 120 µm. Welding experiments were arried out under different welding regimes: urrent and voltage. The welding with non-fusible eletrode required relatively large density welding urrent, therefore, small diameter (0.8 mm) welding wire, whih was fed into eletri ar by a relatively high feed rate. The welds were ross setioned for miroexamination under the mirosope. The proess of preparation of mirosetions involved three steps: utting, mehanial polishing and ething. The abrasive water jet tehnology, whih provides the possibility to eliminate the heat load, was used for the speimens preparation. Vikers mirohardness and depth sensing indentation tests were performed using the prepared metallographi speimens. The properties in both nanolevels and mirolevels were haraterized using Nano-ardness Tester developed by CSM Instruments, Switzerland. Indents were performed over a range of loads from 7 to 250 mn. We used a onstant strain rate setting 1 / P (dp / dt) to be 2 min - 1 and a pause of 10 s was set at maximum load. The layered substrate film system is not ompletely equivalent to the disordered strutural multiphase materials but it an be suessfully used as the first estimation. Mirostruture was analysed using optial mirosope Nion Elipse 1000 with magnifiation from 50 to For mirohardness and nanohardness investigations, idential speimens and areas have been used. owever, it should be noted, that exatly the same grains of heterogeneous strutures an t be investigated, but similar properties in adjusted regions of the mirostruture are assumed. Mirohardness has been measured on the fine and oarse-grained strutures of both samples whih were ompared to the nanohardness measurements. Srath test was made by CSM Nano-Srath Tester using ontinuous- (5)

3 87 ly inreasing load from 5 to 2500 mn. A srath length of 4 mm has been used. The speimen showed high plastiity and exellent adhesion (Fig. 2) with no adhesion raks. diretion Fig. 2 Total view of the srath-trak of opper oating on steel substrate Neither failure to a bigger extent nor omplete exposure of substrate ourred within the predetermined range of normal load. 4. Results and disussion Sliding The area of heat affeted zone (AZ) of welded joints basially depends on heat input and thermal ondutivity of base metal. The boundary between the fuse zone and AZ is lear to identify (Fig. 3) as the rystals in the first are dendrites and in the seond globular. Due to rerystallization proess, two different zones of AZ an be observed, viz., AZ1 and AZ2. AZ1 has oarse grains while AZ2 has fine grains. Size and shape differene of grains is due to the distane from the weld. Cross setion image of Cu eletro-sparking oating with thikness of 60 µm on steel substrate, layers zones as well as indents of hardness testing are shown in Fig. 4. Different indentation data may be obtained analysing different perpendiular ross setions of the weld. Approximately 10% variation of the nano and miroindentation hardness data measured on the longitudinal ross setion was observed. This flutuation of the hardness values along the weld struture entreline is not uniform. Mirohardness values were influened by elasti reovery whereas nanohardness readings were not affeted by plastiity. Mirohardness was also more affeted by grown of grains, than nanohardness. Table 1, Fig. 5, Fig. 7 and Fig. 8 show the relationship between mirohardness and nanohardness. The linear regression equation for this relationship is nano = 1.22 V, where nano is the nanohardness value in MPa and V is the mirohardness value. Table 1 Summary of the average values of nanohardness and mirohardness in different zones of the speimens Cladding (ESD) Speimen Wel-ding (TIG) Zones Nanohardness Relation 7 mn 50 mn 250 mn nano/ V50 Base metal AZ AZ Fuse zone nano/ V250 Cu oating Outer layer Inner layer Substrate As follows from Table 1, the fuse zone has a smaller indentation depth, i.e. higher hardness beause it ontains larger amounts of alloying elements suh as silion, arbon, and manganese. Fig. 3 Cross-setional marographs of the different zones of the TIG weld Fig. 5 ardness values aross the welded region vertially from the surfae of fuse zone Fig. 4 Cross setion of eletro-spark Cu oating with the thikness of 60 m AZ2 zone also distinguished by the mirohardness values, thus onfirming that the mirohardness redution was influened by self-tempering. Fig. 5 shows a nano and mirohardness profiles obtained aross the welded region from the fuse zone through the unaffeted base metal. Tests were arried on three speifi loations: fuse zone at A, base metal at B and AZ regions at C with referene to Fig. 5. A redution in hardness (softening) with respet to base metal (avg.

4 V) was learly revealed in the sub-ritial AZ. Apparent Vikers mirohardness values V were alulated at eah load using onventional approah The hardness inreases while dereasing penetration depth at both miro and nano-sales. owever, the extent of the inrease is muh more dramati in nano-sale regime than in miro-sale regime. ene, as the indentation depth dereases from about 1600 to 200 nm, the hardness inreases by a fator of about Independent zones also have quite similar hardness values for indentation depths between the miro-sale to the nano-sale. Mirohardness, as well, was measured as the funtion of the residual area (ontated area) after load removal. owever, the area funtion is not onsidered to be ideal beause of blunting in the nanoindentation experiments and suh a orretion is not onsidered here. This is suffiient for a relative omparison of the data. The miroindentation with a diagonal length about 14 µm always rosses a grain boundary. It was found in both mirostrutures that an indent, whih rosses several boundaries, shows lower hardness values. Indents, whih ross grain boundaries and preipitations as well as grain boundary of arbides, indiate higher hardness values, orresponding to the fat that an interfae inreases the hardness. Therefore, the oarse-grained struture (existing among other phases of needle-shaped ferrite) has a higher hardness than the fine-grained struture. Measurements revealed smaller hardness in the entre and higher towards the grain boundary. Typial load progression (load-displaement or P-h) urves during indentation of Cu oating, outer and inner layers and steel substrate are shown in Fig. 7. In the ase of ladded materials, the elasti part is reovered upon load removal ausing a derease in the size of the residual area. Consequently, the resulting hardness value (Fig. 8) will appear larger. The nanoindentation trials were therefore speifially designed to investigate tempering indued of fats on individual phases-zones. The indentation measurements along the oating ross-setion showed that hardness values were lower at the oating side than at the oating-substrate interfae (Fig. 7). V P 2, (6) d where d is the average diagonal length of the Vikers indentation impression and P is the indentation test load. Clear ISE was observed in all of the samples that were tested. The ISE means the inrease of indentation hardness with the derease of indentation depth, whih is lose related with load. Partiularly it was lear for determining hardness values of base metal as homogeneous struture (Fig. 6). Fig. 7 Depth-fore urves of Cu oating on steel substrate At a low load of 7 mn, the oating demonstrated a hardness value of about 94 MPa at a depth of about 170 nm whih dropped by 15%, e.g., near 10 MPa at a depth of about 2000 nm for a higher load of 250 mn (Fig. 8). Fig. 6 ardness of base metal measured by onventional method Fig. 8 ardness testing results of opper oating zones on steel substrate under different loads Obtained data showed the presene of a strong ISE on the nanohardness behaviour of the oatings. At low indentation load (7 mn), the size of indent was relatively small, whereas the measured hardness values were higher. Under the higher load (250 mn), indentation size was bigger and hardness values smaller, while measuring in the idential areas. It was defined experimentally that nanoindentation hardness and miroindentation hardness have relationship desribed using a oeffiient k 1: nano k V (7) 1, where k 1 = owever, the interpretation of the data is very diffiult due to ISE. The ISE is learly present in single grains, but is absent in fine-grained volumes as indentation size exeeds or is omparable to the grain size. 5. Conlusions 1. It was found that for Cu oating samples the

5 89 nanohardness values were highest for the lowest indenter load of 7 mn. At higher indenter loads in the range of mn, the hardness values dereased. The main differene onerns the omparison between the size of the indentation surfae and the size of the heterogeneities of the material. The hetero ratio of the nanohardness ( nano) to the mirohardness (V) was smaller in the AZ, indiating that there is a signifiant grain-boundary effet. 2. It was found that nanoindentation hardness and miroindentation hardnesses are related by the oeffiient k 1. By measuring nanohardness and mirohardness in automati regimes the ISE was not learly expressed if ompared to the onventional hardness testing method. 3. Mirohardness is also more sensitive to hanges in the mirostruture, e.g. rerystallization, than nanohardness. This sale of testing minimizes the potential variations aused by loal heterogeneity in submiron strutural features. The results show that hardness an no longer be simply ompared based solely on the same indenter type and indentation load. 4. Problems assoiated within the indentation proess in nonhomogeneous materials remain a problem that is still under investigation. Referenes 1. Oliver, W.C.; Pharr, G.M An improved tehnique for determining hardness and elasti modulus using load and displaement sensing indentation experiments, J. Mater. Res. 7(6): Gouldstone, A.; Chollaoop, N.; Dao, M.; Li, J.; Minor, A.M.; Shen, Y.L Indentation aross size sales and disiplines: Reent developments in experimentation and modelling, J. Ata Mater. 55: Lan, L.; Qiu, Ch.; Zhao, D Analysis of the hardness and elasti modulus distribution in a high strength steel welded joint by nanoindentation, Advan. Materials Researh : Maier, P.; Rihter, A.; Faulkner, R.G.; Ries, R Appliation of nanoindentation tehnique for strutural haraterisation of weld materials, Materials Charaterization 48(4): Abu Al-Rub, R.K; Faruk, N.M Predition of miro and nano indentation size effets from spherial indenters, Mehanis of Advaned Materials and Strutures 19: Bull, S.J Nanoindentation of oatings, J. of Physis D: Applied Physis 38(24): R393-R Voyiadjis, G.Z.; Peters, R Size effets in nanoindentation: an experimental and analytial study, Ata Meh. 211: Manika, I.; Maniks, J Size effets in miro- and nanosale indentation, Ata Materialia 54(8): Atkinson, M Further analysis of the size effet in indentation hardness tests of some metals, Journal of Materials Researh 10(11): BS EN ISO : Metalli materials Instrumented indentation test for hardness and materials parameters Part 1: Test method, Int. Standardisation Org., Geneva, Switzerland, 46 p. 11. Aifantis, E.C Gradient effets at maro, miro, and nano sales, J. of the Mehanial Behavior of Materials 5(3): Qian, L.; Li, M.; Zhou, Z.; Yang,.; Shi, X Comparison of nano-indentation hardness to mirohardness, Surfae and Coatings Tehnology 195(2-3): Sawa, T Correlation between nanoindentation test results and Vikers hardness, IMECO TC3, TC5 and TC22 Conf., Metrology in Modern Context, Pattaya, Chonburi, Thailand, Zamfirova, G.; Gaydarov, V Influene of the surfae effet and sale fator on the mirohardness investigation, Pro. of the 10th Int. Conf. on Mehanis and Tehnology of Composite Materials. Bulgaria. 15. Fisher-Cripps, A.C Nanoindentation, 2nd ed., Springer-Verlag LLC, New York, 264 p Swadener, J.G.; George, E.P.; Pharr, G.M The orrelation of the indentation size effet measured with indenters of various shapes, J. Meh. Phys. Solids 50(4): S. Baskutis, V. Vasauskas, A. Žunda NANO AND MICROARDNESS TESTING OF ETEROGENEOUS STRUCTURES S u m m a r y The objetive of the paper is to ompare different hardness measuring methods for the evaluation of mehanial properties in a wide load range (7-250 mn) for heterogeneous strutures. Vikers nano and miroindentation hardness test was used for mehanial properties assay of hosen samples. Relationships between nanohardness and mirohardness were obtained for heat affeted zones and fusion zone of the welds and the opper oatings on steel substrates. The experimental results of nano and miroindentation tests show orrelation with indentation size effet for heterogeneous strutures. Keywords: nanohardness, mirohardness, indenter, oating, indentation size effet. Reeived November 23, 2015 Aepted Marh 15, 2016