Different surface integrity states by post-processing techniques applied on WC-Co cemented carbide material
RIU PERDRIX G. 1,2, LLANES PITARCH L. 1, ROA ROVIRA J. 2
1 CIEFMA - Department of Materials Science and Engineering - Universitat politécnica de Catalunya (EEBE), Barcelona, Spain; 2 Steros GPA Innovative SL, Barcelona, Spain
WC-Co is a cemented carbide material, also known as hardmetal, with a heterogeneous microstructure constituted with interlinked ceramic (WC) and metallic cobalt (Co) phases. As a result of these combinations, WC-Co presents an outstanding mechanical properties in terms of hardness and wear resistance by the ceramic WC phase, combined with high fracture toughness due to the metallic Co binder. Typically, WC-Co cutting tools are post-processed with different commercial processes technique being grinding and electrical discharges machined (EDM) the most employed in the market. However, these processes modify the surface integrity of the material in terms of microstructure and mechanical integrity at the surface level. Grinding induces a compressive residual stress, deformed layer with a huge density of defects (i.e.: microcraks, cavities) and also phase transformation from f.c.càh.c.p of the metallic Co binder at the surface level. On the other hand, EDM surface is thermally affected, producing a heat affected zone which contains voids, cracks, oxides and residual stresses. Despite of these modifications could enhance some mechanical properties, for some applications removing the damage layer induced are required. As a result, it is important to modify the superficial microstructure though a polishing step and as a consequence obtain the desired mechanical properties near the surface.
It is well known that the WC-Co grades present some local corrosion during the polishing process. In this regard, a large number of studies have been reported, mainly focused on the corrosion behaviour of the metallic Co binder in contact with acid or even neutral and/or basic media. These results show a clear tendency for the metallic Co binder to dissolve under these specific media and produce corrosion, decreasing their surface integrity in terms of mechanical properties under service like working conditions. Within this context, a new dry-electropolishing electrolyte has been developed to be employed in the DryLyte® Technology that allows to obtain corrosion-free surfaces with a roughness below 20nm. This technology uses macroporous ion exchange resins which are able to absorb the electrolytic media providing an electrical bridge between the cathode and the workpiece when an electrical current is applied. During the process the roughness is reduced layer-by-layer, since the particles only interact with the peaks of the roughness producing a selective roughness removal.
Within all this information, this study aims to compare four different post-processing techniques; grinding, EDM, chemo-mechanical polishing and recently the Drylyte® Technology. All of these techniques are employed in the industrial sector which modifies the superficial state, and as a direct consequence the surface integrity in terms of mechanical properties on WC-Co cemented carbides. In order to evaluate the microstructure as well as the mechanical properties at the micro- and sub-micrometric length scale at the surface level advanced characterization techniques have been used (Field Emission Scanning Electron Microscopy, Focused Ion beam and nanoindentation test).
The results highlight the advances provided by DryLyte® Technology in comparison with the other post-processing techniques, to achieve smooth surfaces without corrosion of the metallic Co binder, preserving the surface integrity in terms of mechanical properties.