The unique combination of hardness, toughness and wear resistance exhibited by heterogeneous hard materials (e.g. PcBN systems, cemented carbides, among others) has made them preeminent material choices for extremely demanding applications, such as metal cutting/forming tools or mining bits among others, where improved performance together are required. The remarkable mechanical properties of these materials result from a two-fold effectiveness associated with their intrinsic composite character. On the one hand, in terms of composite nature: combination of completely different phases (hard, brittle and soft, ductile constituents) with optimal interface properties. On the other hand, as related to composite assemblage: two interpenetrating-phase networks where toughening is optimized through different mechanisms depending on their relatively different chemical nature. After being sintered, cemented carbide parts need to go through manufacturing processes to fulfill the requirements of the final tools. In this sense, diamond grinding is a primary shaping process for these parts to achieve a particular tool geometry and/or close tolerances prescribed by its design. Similar to other hard/brittle materials, grinding of cemented carbide changes its surface integrity by inducing microcracks, compressive residual stresses and/or phase transformation of the cobalt-based metallic binder, from face-centered cubic to hexagonal close package – f.c.c. à h.c.p. In this regard, surface integrity alterations play a key role in dictating the mechanical and tribological response of these ground materials, like fracture strength, fatigue and wear, among other properties.
Since the grinding process requires repetitive abrasion movement, investigating how this affects the surface/subsurface of the materials will provide more insights into understanding tribological behavior in terms of scratch and/or wear resistance under service-like conditions. A large number of studies have been reported, mainly focused on the mechanical behavior of this composite. However, information on the small-scale mechanical response of the damage layer induced along the grinding process is relatively scarce and in particular under-service like working conditions to understand the real behavior of these materials.
Within all this information, this presentation is focused on WC-Co cemented carbide material surface integrity evolution at high/intermediate temperature. A systematic micro and nanomechanical study of several WC-Co (in terms of metallic binder content and WC particle size) ground materials is presented. In general, three different aspects are investigated to accomplish the main goal of this research: (1) assessment of the intrinsic hardness of the deformed layer from room temperature up to 600 ºC, (2) correlation of the compressive residual stresses with the hardness and elastic modulus maps by high indentation speed tests at the different temperature tested and (3) evaluation of the damage and plastic deformation mechanisms induced in the deformed layer.
It was found that for different WC-Co composites, the intrinsic hardness of the deformed layer starts to slightly decrease at temperatures ranging between 400 and 500 ºC, depending on the amount of metallic binder. This effect is attributed to different effects which take place simultaneously when the testing temperature increases: dislocation motion, reduction of compressive residual stresses and layer oxide generation mainly constituted by CoWO4, WO3 and Co3O4.