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

Abstract: 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 fine-grained 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 phase transformation of binder regions, down to 5 microns in depth; and large compressive residual stresses, gradually decreasing from the surface to baseline values at depths of about 10-12 microns.Additional changes in surface integrity are induced during subsequent ion etching and coating deposition. In general, removal of material from the surface during sputter cleaning and extended low-temperature (during film deposition) treatment resulted in a significant residual stresses decrease (about half its original value). However, damage induced by grinding was not completely removed, and some microcracks were still left on the substrate surface (close to the interface). On the other hand, and as expected, high temperature annealing (GTT condition) resulted in a complete relief of the referred residual stresses, but without inducing any additional change in terms of existing microcracks and depth of damaged layer. This was not the case for the metallic binder phase where thermal treatment induced an unexpected microporosity, development of a recrystallized subgrain structure, and reversion of grinding-induced phase transformation.Flexural strength was measured on both uncoated and coated hardmetals, and complemented with extensive fractographic analysis. It was found that grinding significantly enhances the strength of hardmetals, as compared to AS and P conditions. However, such beneficial effect was partly lost in the corresponding coated specimens. On the other hand, film deposition increases strength measured for GTT surface variant. These findings were analyzed on the basis of the changes on nature and location of critical flaws, induced by the effective residual stress field resulting at the surface and subsurface after each manufacturing stage.The influence of substrate surface finish on scratch resistance of coated hardmetals and associated failure mechanisms was investigated. It was found that coated AS, G and P samples exhibit similar critical load for initial substrate exposure and the same brittle adhesive failure mode. However, damage scenario was discerned to be different. Substrate exposure was discrete and localized to the scratch tracks for G samples, while a more pronounced and continuous decohesion was seen for AS and P ones. Relieving of the substrate compressive residual stresses (GTT condition) yielded lower critical loads and changes in the mechanisms for the scratch-related failure, the latter depending on the relative orientation between scratching and grinding directions.The cracking and delamination of TiN-coated hardmetals when subjected to Brale indentation was studied while varying the microstructure and surface finish of the substrate. In this case, another fine-grained WC-Co cemented carbide with lower binder content (6 wt.%Co) was included in the investigation. It was found that polished and coated hardmetals exhibit more brittleness (radial cracking) and lower adhesion strength (coating delamination) with decreasing binder content. Such a response is postulated on the basis of the influence of intrinsic hardness/brittleness of the hardmetal substrate on both cracking at the subsurface level and effective stress state, particularly regarding changes in shear stress component. On the other hand, grinding was discerned to promote delamination, compared to the polished condition, but strongly inhibits radial cracking. This was the result of the interaction between elastic-plastic deformation imposed during indentation and several grinding-induced effects: remnant compressive stress field, pronounced surface texture, and microcracking within a thin microcracked subsurface layer. It is then concluded that coating spallation prevails over radial cracking as the main mechanism for energy dissipation in ground and coated hardmetals.Contact damage resistance of coated hardmetals with different substrate surface finish conditions was investigated by means of spherical indentation under increasing monotonic loads. It was found that grinding enhanced resistance against both crack nucleation at the coating surface and subsequent propagation into the hardmetal substrate. Hence, crack emergence and damage evolution was effectively delayed for the coated G condition, as compared to the reference P one. The observed system response was discussed on the basis of the beneficial effects associated with compressive residual stresses remnant at the subsurface level after grinding, ion-etching and coating. The influence of the stress state was further corroborated by the lower contact damage resistance exhibited by the coated GTT specimens. Finally, differences observed on the interaction between indentation-induced damage and failure mode under flexural testing pointed in the direction that substrate grinding also enhances damage tolerance of the coated system when exposed to contact loads.