Correlation between surface damage and mechanical properties at micro- and nanometric length scale for WC-Co hardmetals

Abstract

Cemented carbides (WC-Co) are ceramic-metal composite materials made by hard tungsten carbide particles bonded through a metallic binder matrix, mainly of cobalt. It is a hard material characterized by an exceptional combination of strength, toughness and wear resistance. As result, cemented carbides are first choice materials for cutting tools and wear parts. However, final shaping of these components usually require diamond grinding. During this hard machining, surface integrity may became altered, particulary in terms of compression stresses and/or microcracking. Such defects can locally affect the mechanical properties. The aim of this investigation is to analyze the influence of the surface finish quality on the mechanical properties at the surface level for WC-Co materials as well as the influence over the properties of a TiN coating deposited on hardmetal substrates. The study has been done at micrometric (to analyze the general properties) and nanometric scale (local properties aiming to capture residual stress state effects) by using nanoindentation and nanoscratch testing. Tests done in plain view prove that roughness plays an important role in the assessment of mechanical properties at the surface, as it induces significant scatter, as compared to results determined on cross-sections. Finally, a sequential polishing process has been done in order to extract the polishing rate for cemented materials as well as study how roughness affects the mechanical properties measured. This process points out that roughness can mask surface damage, like cracks or chipping, among others. As a final conclusion, an optimized protocol is proposed to study the mechanical properties of the samples with high roughness and exhibiting a compressive residual stress state.