APPLICATION OF FRACTOGRAPHY IN ASSESSMENT OF COMPOSITE LAYERS

Size: px

Start display at page:

Download "APPLICATION OF FRACTOGRAPHY IN ASSESSMENT OF COMPOSITE LAYERS"

Susanna Watson
5 years ago
Views:

1 Acta Metall. Slovaca Conf. 219 APPLICATION OF FRACTOGRAPHY IN ASSESSMENT OF COMPOSITE LAYERS Dagmar Jakubéczyová 1)*, Marek Kočík 1), Pavol Hvizdoš 1) 1) Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovakia Received: Accepted: *Corresponding author: Tel.: , Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, Košice, Slovakia Abstract We used fractography to evaluate morphology of fractures across mono- and multi-component system coatings deposited by PVD technology to the steel substrate. Nanoindentation measurements with dynamic loading were carried out on deposited layers of TiN and AlTiCrN type in order to verify their deformation behaviour. The qualitative and quantitative analysis of metal and non-metal elements across the deposited layers was carried out by GDOES (Glow Discharge Optical Emission Spectroscopy) technique. On the basis of obtaining the results we evaluated structural, mechanical and chemical properties of the deposited types of coatings and recommended their use in practice. Keywords: layer, PVD, fractography, nanohardness, modulus of elasticity, GDOES 1 Introduction Hard coatings play an important role in industry for improving tool lifetime and performance. A variety of hard coatings are widely used in the tool industry, mainly to reduce friction and to increase wear resistance. One of the mostly used and studied coatings to date is TiN, which has beneficial properties including high hardness, low friction, and chemical inertness [1]. The statistics based on observation of German market indicate an unmistakable trend of replacement of the outdated TiN and TiCN layers by layers based on TiAlN. The increasing proportion of layers based on TiAlN is a logical consequence of their basic properties and requirements of industry. The HV hardness of the originally developed and historically oldest TiN reaches 23 GPa and their maximum working temperature is 600 C, while the TiCN layers reach a hardness of 35 GPa but their maximum working temperature is only 450 C. The layers based on TiAlN exhibit high HV hardness up to 33 GPa and their maximum working temperature may sometimes exceed even 800 C, thus fulfilling the two most important requirements for industrial applications, i.e. high abrasion resistance and chemical stability at high temperatures [2]. However, if the industrial applications require very hard coatings with increased thermal and chemical stability, then even the common layers based on TiAlN or AlTiN do not suffice. Multicomponent coatings based on different metallic and non-metallic elements combine the benefit of individual components leading to a further refinement of coating properties. Metastable TiAlN materials not only exhibit superior high temperature oxidation resistance compared to TiN and better cutting behaviour enabling the use of higher cutting speeds, but also very interesting physical properties [3,4,5]. Some TiAlN layers are supplemented with additional elements, such as Cr, Y, Zr, Hf and similar. These additives, when used in small quantities, affect positively the structure of layers, namely by improving their oxidative properties, refining

2 Acta Metall. Slovaca Conf. 220 the structure and upgrading the adhesive properties and thus contributing to better industrial properties [6,7]. This involves multilayers and gradient and nanostructure layers, each of the layers having a specific function thermal, adhesive and oxidative resistance and strength. The motivation to infiltrate into production cutting applications, involving high production of heat, and into dry applications, is based on promises of high ecological and economical contributions [8]. This is related to potential increase in service life of various substrate materials based on increased resistance to corrosion and mechanical influences of the environment. 2 Experimental procedure We compared two types of coatings deposited by Arc-PVD method to steel substrate made of machined steel (Cr-Mo-V) produced by powder metallurgy (PM). We deposited a conventional mono-component TiN layer and a multi-component AlTiCrN coating. Both coatings were subjected to fractographic analysis employing an SEM technique with EDS that was performed on the fracture surface after bending test. The surface of fractures was examined macroscopically and microscopically. An advantage of SEM compared to SM is a higher-order resolution and great-depth definition which allow one to observe surface unevenness even at high magnifications. A part of evaluation of deposited layers was determination of their hardness in the direction from the surface toward the substrate. With regard to increasing requirements for evaluation of materials surface, namely layers with decreasing thickness, it is necessary to abandon the evolution of conventional microhardness technique and instead of evaluating the size of the imprint after indentation to focus on the indentation curve. This is the reason why we evaluated the coating hardness by a dynamic nanoindentation technique which is used extensively to characterise the mechanical behaviour of materials on a small scale [9]. A nanoindenter is a testing machine with high resolution that allows one to measure properties on sub-micro and nano-scale [10,11] and serves particularly as a primary technique for determination of mechanical properties of thin layers and small construction parts [12,13]. The attractivity of this method is based on the fact that mechanical properties of very thin layers and materials with prevailing elastic deformation can be ascertained directly from the indent/imprint that cannot be observed by optical means. The loading force and position of the indenter is continuously monitored and recorded throughout the indentation cycle. The thickness of PVD layers ranges from a few nanometres to several tens of micrometers, therefore one must ensure that during the measurement of hardness the indenter tip penetrates to a maximum depth of 1/10 of the layer thickness. The nanoindentation testing of both coatings was performed using an equipment TTX-NHT from CSM Instruments, Switzerland. We used a Berkovich indenter (a three-sided pyramid) operated in a dynamic sinusoidal mode with a maximum load of 400 mn and loading rate 800 m..min -1. The results were processed by the method of Olivera and Pharr [8] which allowed us to calculate indentation hardness (H) and Young modulus of elasticity (E). The dynamic mode enabled to measure the values continuously during loading and record them as a function of the relevant indentation depth. This method is suitable for characterisation of coatings and thin layers as it enables to observe and, provided that some conditions are met, to separate values corresponding to layers and to the substrate. A minimum of 20 indentations was made on each specimen and the results were processed statistically. The qualitative and quantitative analysis of metal and non-metal elements in the cross section of deposited coatings was carried out by GDOES (Glow Discharge Optical Emission Spectroscopy) technique using an optical emission spectrometer with glow discharge GDS-750. We obtained

Due to the measurement of multi-component structures one assumes some errors in these calculations.

$3 Results and discussion The aim of the study was to use fractography to evaluate qualitatively-quantitatively the thin mono- and multilayer coatings deposited onto steel$ $1 and 2 show cross-section fractures of systems layer/steel substrate and EDS analysis of the coatings.$ The thickness of TiN layer was approximately 3000 nm and that of AlTiCrN layer approximately 2300 nm.

The thickness of TiN layer was approximately 3000 nm and that of AlTiCrN layer approximately 2300 nm.

This is confirmed by the coating/substrate interface which does not exhibit any failures or cracks in both cases.

3 Acta Metall. Slovaca Conf. 221 an in-depth concentration profile a graphic documentation of concentration changes of selected elements up to the depth of 100 µm. The accuracy of recalculation of the time axis to concentration profile axis is given by determination of accurate de-dusting rate of individual calibration standards. Due to the measurement of multi-component structures one assumes some errors in these calculations. There is no general procedure for elimination of inaccuracies and one must proceed from one case to another [14]. The selection of individual wavelengths for analytical purposes is subject to several steps that were discussed by the cited authors [14,15]. 3 Results and discussion The aim of the study was to use fractography to evaluate qualitatively-quantitatively the thin mono- and multilayer coatings deposited onto steel substrate and to verify their deformation behaviour at dynamic loading. Figs. 1 and 2 show cross-section fractures of systems layer/steel substrate and EDS analysis of the coatings. The thickness of TiN layer was approximately 3000 nm and that of AlTiCrN layer approximately 2300 nm. The figures show continuous and compact layers with excellent adhesion properties. This is confirmed by the coating/substrate interface which does not exhibit any failures or cracks in both cases. The substrate has a pit-like appearance which is typical of the structure of steel prepared by powder metallurgy while the deposited layers have columnar morphology, consisting of the so-called columnar crystals, considerably larger in the TiN layer. Spectrum 1 Element Weight % Atomic % Ti K N K Totals Fig.1 a) Cross-sectional image of TiNsubstrate Fig.1 b) Cross-sectional image of TiN-substrate and EDS analysis Fig.2 a) Cross-sectional image of AlTiCrNsubstrate Elemen Spectrum 1/2 t Weight% Atomic% Cr K 48.17/ / N K 18.49/ / Al K 14.81/ / Ti K 18.53/ / Totals Fig.2 b) Cross-sectional image of AlTiCrN-substrate and EDS analysis

4 Acta Metall. Slovaca Conf. 222 The EDS analysis was supplemented with in-depth concentration profile based on GDOES analysis, Figs. 3 and 4. The graphical relationship allows one to determine the concentration of selected elements in any thickness of the coating and to read the thickness of the deposited layers. a) b) Fig.3 GDOES analysis across TiN coating in a) weight % and b) atomic % a) b) Fig.4 GDOES analysis across AlTiCrN coating in a) weight %, b) atomic % Both types of chemical analysis corresponded very well in terms of quantification in atomic percent and weight percent values. In the monolayer, the analysis unambiguously showed presence of titanium nitride while in the multicomponent layer chromium nitride, the compound with high hardness and resistance to wear, was found at the highest concentration on the surface, followed by AlN/TiN with thermal-oxidative resistance and in depth of about 1900 nm, close to the coating/substrate interface, we observed increased concentration of Ti, i.e. the layer with good adhesive properties. Its function consisted in compensation of stress and tension at the coating/substrate interface and in acting as a barrier against development of cracks. Nanoindentation testing determined also surface hardness which reached 26.1 GPa for TiN layer and, 47.7 GPa for AlTiCrN layer and 13.6 GPa for the substrate. Fig. 5a) shows the course of hardness in the direction from the coating surface towards the substrate with stress on not exceeding 1/10 of the total coating thickness in order to eliminate the influence of substrate. This

5 Acta Metall. Slovaca Conf. 223 was in agreement with the requirements of the international standard for determination of the hardness of coatings [16]. The presence of Al and Cr in the AlTiCrN layer resulted in the expected increase in hardness compared to the TiN layer by GPa. With decreasing hardness, the modulus of elasticity decreased proportionally in both cases and it was interesting that despite lower hardness of the TiN layer the modulus of elasticity E of this layer was higher compared to the AlTiCrN layer, Fig. 5b). a) b) Fig.5 a) Hardness HV and b) modulus elasticity vs displacement into surface. With respect to the 1/10 layer thickness requirement (for TiN ~ 300 nm and for AlTiCrN ~ 230 nm), modulus of elasticity reached only 323 GPa in the AlTiCrN layer and 380 GPa in the TiN layer. The course of curves at the additional loading of the indenter was affected considerably by the substrate and approached behaviour of the base material. On the contrary, the course of hardness and elastic modulus of the base material was constant, which is indicative of respective steel structure, as described in the work [17]. DOI /amsc.v3.131 ISSN

6 Acta Metall. Slovaca Conf. 224 Many studies are available dealing with deformation mechanisms of thin layers by means of nanoindentation technique because many different factors may affect the coating and the substrate at loading. Previous studies showed that hardness and Young modulus of coatings depended on various factors, such as yield point, adhesion, layer thickness, internal structure and density of layer and, obviously, the geometry of the indenter [3,18,19]. The obtained information allowed us to state that lower values of E for AlTiCrN coating may be related to composition and internal structure of the deposited layers and morphology of grains/crystallites. Arrangement of crystallites in layers of both types is visible in fracture cross sections of systems, see Figs. 1 and 2. The cross section of TiN layer shows long columnar grains while cross section of AlTiCrN contains smaller columnar grains with the lamellar arrangement in the direction from the surface towards the substrate: layers CrN, AlN/TiN and TiN. Lower elastic modulus E of the multilayer coating most likely results from the arrangement of individual layers and concentration of elements across its thickness. Both the layers fulfilled the purpose, i.e. produced a protective layer on the surface of the steel substrate which exhibited good adhesive and mechanical properties. Due to higher hardness and thus also higher resistance to wear and thermal-oxidative influences, the AlTiCrN layer has more universal use in practical applications. 4 Conclusions Two types of coatings - TiN and AlTiCrN deposited by PVD technology to PM steel substrate were analysed and documented by fractographic and nanoindentation methods and by emission spectroscopy. Examination of cross-section fractures of the system layer/steel substrate indicated that the deposited layers were continuous and compact and had excellent adhesive properties. The interface between the layer and the substrate was free of any failures or cracks in both cases. Morphology of layers was based on columnar crystallites/grains. The grains were longer in the TiN layer in comparison with the AlTiCrN layer and their size corresponded to the thickness of lamellar layers located one above another. EDS method was used to carry out semi-quantitative analysis of the elements present. The EDS analysis of present elements corresponded well with the in-depth concentration profile always down to the substrate. GDOES analysis allowed us to determine the concentration of elements in any depth of the coating and, of course, the thickness of the deposited layer with sufficient accuracy. According to expectations, nanoindentation measurements revealed higher surface hardness of the layer AlTiCrN (47.7 GPa) resulting from the presence of Cr and Al in its composition; the TiN layer exhibited somewhat lower hardness (26.1 GPa) and the hardness of the PM substrate used reached 13.6 GPa. With decreasing hardness the elastic modulus decreased proportionally and it was interesting that that despite lower hardness of the TiN layer the modulus of elasticity E of this layer was higher compared to the AlTiCrN layer. These results allowed us to state that lower values of E in case of the AlTiCrN coating could be related to chemical composition, internal structure of deposited layers and morphology of grains/crystallites. Lower elastic modulus E of the multilayer coating most likely resulted from different arrangements of individual layers and concentration of elements across its thickness.

7 Acta Metall. Slovaca Conf. 225 The deposited types of coatings ensured increased hardness and elastic modulus of the base material and the substrate-layer interface was without failures, which confirmed excellent adhesion properties of the system. Due to their specific properties, the coatings appear suitable for use in practical operations. References [1] J. Ding, Y. Meng, S.Wen: Thin Solid Films, Vol. 371, 2000, p [2] [3] O. Nakonechna, T. Cselle, M. Morstein, A. Karimi: Thin Solid Films, Vol , 2004, p [4] C. H. Zhang et al.: Journal Applied Physics, Vol. 95, 2004, p [5] F. Kauffmann et al.: Thin Solid Films, Vol. 473, 2005, p [6] P. Holubář, M. Jílek, M. Šíma: New industrial coating technology, MM Industrial spectrum 2003 (in Czech) [7] M. Hagarová, I. Štěpánek, D. Jakubéczyová: Acta Metallurgica Slovaca, Vol. 16, 2010, No.3, p [8] [ ] [9] W. C. Oliver, G. M. Pharr: Journal of Materials Research, Vol. 19, 2004, p [10] A. C. Fischer-Cripps: Vacuum, Vol. 58, 2000, p [11] D. Newey, M. A. Wilkins, H. M. Pollock: Journal of Physics E: Sci. Instrum, Vol. 15, 1982, p [12] M. Hagarová: Journal of Metals, Materials and Minerals. Vol. 17, 2007, No. 2, p [13] R. Saha, W. D. Nix: Acta Materialia, Vol. 50, 2002, p [14] M. Vnouček: Povrchové efekty při GDOES, (Surface Effects at GDOES), PhD. study work, The University of West Bohemia, Pilsen, 2002 (in Czech) [15] L. Settineri et al.: CIRP Annals-Manufacturing Technology, Vol. 57, 2008, p [16] ISO (2007): Metallic materials - Instrumented indentation test for hardness and materials parameters -Part 4: Test method for metallic and non-metallic coatings [17] [ ] [18] S. Logothetidis, S. Kassavetis, C. Charitidis, Y. Panayiotatos, A. Laskarakis: Carbon, Vol. 42, 2004, p [19] R. Picek, P. Boháč: Materials Structure, Vol. 13, 2006, No. 2, p Acknowledgement The work was carried out within the framework of the project VEGA 2/0060/11.

INTRODUCTION. Think HSS

INTRODUCTION. Think HSS INTRODUCTION Think HSS SUMMARY METALLURGY 2 Excellent strength 3 A super sharp edge 4 Safe and reliable tools Alloy elements 6 The influence of alloy elements 7 Standard compositions of HSS 8 The HSS-PM