Characterization of mechanical properties of tungsten carbide/carbon multilayers: Cross-sectional electron microscopy an
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Multilayers of tungsten carbide/carbon (WC/C) deposited by physical vapor deposition onto steel substrates were subjected to depth-sensing indentation testing. The investigation aimed at probing the influence of dissimilarities between the microstructure of the multilayers and substrate on the system mechanical properties. The resultant load-displacement data were analyzed both by conventional load-displacement (P-␦) and load-displacement squared (P-␦2) plots. Furthermore, it was demonstrated that the occurrence of annular through-thickness cracks around the indentation sites can be identified from the load-displacement curve. Also, analysis of the lower part of the unloading curve permitted us to identify whether the coating had popped up by localized fracture. The cracking mechanism was characterized using a new technique for cross-sectional electron microscopy of the nanoindentations. The information retrieved with this technique eliminates the problems, inherent in assessing at this small contact scales, whether the fracture is by coating decohesion or by interfacial failure. In our case, it was demonstrated that the failure mechanism was decohesion of the carbon lamellae within the multilayers. The mechanical properties (hardness and effective Young’s modulus) were also assessed by nanoindentation. The hysteresis loops were analyzed and discussed in terms of the method developed by Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)].
I. INTRODUCTION
Tungsten carbide/carbon (WC/C) deposited by physical vapor deposition (PVD) has received considerable attention in recent years, owing to its low friction coefficient and high wear resistance1,2 in combination with substrate temperatures of 200 to 400 °C achieved during deposition. Furthermore, the fact that adhesion between the coating and the substrate has been improved by depositing an interlayer of chromium3 has helped to promote the deposition of such coatings onto machine components subjected to a high shear stress component. However, if the advantages are to be fully employed in improving the surface properties of engineering components, it is necessary to understand more thoroughly the detailed mechanisms by which surface properties are improved. Ultralow load indentation (nanoindentation) experiments have become increasingly widespread to gauge the mechanical properties of coated systems. The appeal of a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 8, Aug 2001
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this technique is that very low loads and displacements are available, enabling characterization on a submicrometer scale. These features permit mechanical property data to be obtained from thin films not only in the microstructural and residual stress state in which they exist as coatings while on a substrate, but also in a regime where their properties should dominate.4,5 The combination of load-displacement data obtained from nanoindentation cycle with post mortem observation of the ind
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