Grinding effects on surface integrity, flexural strength and contact damage resistance of coated hardmetals

The tribological and mechanical behavior of coated tools depends not only on intrinsic properties of the deposited film but also on substrate surface and subsurface properties – such as topography and residual stress state – as well as on interface adhesion strength. It is particularly true in the case of coated tools based on WC-Co cemented carbides (backbone materials of the tool manufacturing industry, and simply referred to as hardmetals in practice) as substrates. Manufacturing of hardmetals often involves grinding, and in the case of cutting tools also edge preparation, etching and coating. The quality of the shaped components is influenced by how the surface integrity evolves through the different process steps. In this regard, substrate grinding and coating deposition represent key steps, as they are critical for defining the final performance and relative tool manufacturing cost. Within this framework, it is the main objective of this thesis to assess the influence of substrate surface integrity on different mechanical (flexural strength and contact damage resistance under spherical indentation) and tribological (scratch resistance as well as cracking and delamination response under Brale indentation) properties for a TiN-coated finegrained hardmetal grade (WC-13 wt.%Co). In doing so, three different surface finish conditions are studied: as-sintered (AS), ground (G), and mirror-like polished (P). Moreover, aiming for an in-depth analysis of surface integrity evolution from grinding to coating, a relevant part of the work is devoted to document and understand changes induced by grinding in nude hardmetal substrates. The study is also extended to a fourth surface finish variant (GTT), corresponding to a ground substrate which is thermal annealed before being ion etched and coated. Because residual stress induced by grinding are effectively relieved after this high temperature thermal treatment, GTT condition allows to separate grinding-induced effects associated with surface texture and surface/subsurface damage changes (inherited from the G surface finish) from those related to the referred residual stresses. Surface integrity was characterized in terms of roughness, residual stresses (prior and after coating deposition), and damage at the subsurface level. It was found that grinding induces significant alterations in the surface integrity of cemented carbides. Main changes included relevant roughness variations; emergence of a topographic texture; anisotropic distribution of microcracks within a subsurface layer of about 1 micron in depth; severe deformation, microstructure refinement and