Characterization of mechanical properties of tungsten carbide/carbon multilayers: Cross-sectional electron microscopy and nanoindentation observations

Multilayers of tungsten carbide/carbon (WC/C) deposited by physical vapor d eposition onto steel substrates were subjected to depth-sensing indentation testing. The investigation aimed at probing the influence of dissimilariti es between the microstructure of the multilayers and substrate on the syste m mechanical properties. The resultant load-displacement data were analyzed both by conventional load-displacement (P-delta) and load-displacement squ ared (p-delta (2)) plots. Furthermore, it was demonstrated that the occurre nce of annular through-thickness cracks around the indentation sites can be identified from the load-displacement curve. Also, analysis of the lower p art 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 nanoin dentations. The information retrieved with this technique eliminates the pr oblems, inherent in assessing at this small contact scales, whether the fra cture is by coating decohesion or by interfacial failure. In our case, it w as demonstrated that the failure mechanism was decohesion of the carbon lam ellae within the multilayers. The mechanical properties (hardness and effec tive Young's modulus) were also assessed by nanoindentation. The hysteresis loops were analyzed and discussed in terms of the method developed by Oliv er and Pharr [J. Mater. Res. 7, 1564 (1992)].