Indentation and Wear Damage Assessment of TiC-Stainless Steel Cermets
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Cermets, a composite of a hard ceramic phase and ductile metallic phase, are widely used in oil and gas, chemical, mining, and tooling industries, due to their high hardness, toughness, wear and corrosion resistance. A family of TiC - stainless steel cermets was produced in the current study using a simple melt infiltration method, with binder phase (steel grades 304-L, 316-L and 410-L) contents ranging from 5 to 30 vol.%. The materials were sintered at 1475°C for 15 minutes, 1500°C and 1550°C for 60 minutes, and 1550°C for 240 minutes to get fine-, intermediate- and coarse-grained cermets. The microstructural evolution arising from the sintering process was then examined. It has shown that irregular shaped grains (concave/hollow) were produced, especially when sintered at 1475°C, and was explained using the ‘instability of the solid-liquid interface’ theory. It was also demonstrated that a multi-layer, core-rim structure arose for the cermets, with selected steel constituents present in the rim of the TiC grains. The cermets were tested using both ‘sharp’ and ‘blunt’ indenters to study contact damage. Materials indented with a ‘sharp’ Vickers indenter, ranging from 1 to 30 kgf load, revealed an apparent ‘indentation size effect’, with hardness increasing with decreasing test load. Two primary indentation-cracking patterns were observed, namely median and Palmqvist cracks. Crack patterns were assessed using focused ion beam microscopy (FIB). Cermets loaded with a ‘blunt’ Hertzian indenter were tested with various WC-Co spheres, ranging from 1.19 to 3.97 mm, with applied loads from 250 to 2000 N. Indentation stress-strain curves were plotted and compared with the calculated Hertzian elastic response, and the materials were found to have a ‘quasi-plastic’ behaviour and ‘strain-hardening’ effect. The reciprocating wear response of cermets with various TiC grain sizes and binder phase contents/compositions, were evaluated using a ball-on-flat reciprocating wear tests, using a WC-Co counter face sphere and loads from 20 to 80 N. The specific wear rate of the cermets was found to increase with applied load, testing time and/or the steel binder content. The morphology of the worn surfaces was studied using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and FIB microscopy, in order to fully understand the operative wear mechanisms. A transition from two- to three-body abrasive wear was observed, with a further transition to adhesive wear identified through the formation of an oxygen-rich tribolayer.