SPARK PLASMA SYNTHESIS AND TRIBOLOGY OF MAX PHASE AND CERAMIC COMPOSITES

Synthesis and tribological behaviour of metallo-ceramics and a particulate ceramic composite have been investigated. The microscale deformation mechanisms and effects of intrinsic lubricity of Ti-based MAX phases on their friction and wear behaviour has been investigated. The role of phase transformation toughening and thermal expansion mismatch on the microstructure and wear properties of a TiC particulate-reinforced ceramic composite have been investigated.
The transition in friction and wear observed in MAX phases from low friction and low wear regimes to high friction and high wear regimes was found to the linked to the formation of tribofilms consisting of rutile (TiO2), titanium oxycarbide (TiOXCY) and graphitic carbon. The low friction and wear regime prior to transition is due to the evolution of easy-shear graphitic material formed at the sliding surface intrinsically. The spallation of the non-adherent frictional heating-induced tribofilms (rutile and oxycarbide) generated over time at the sliding surface led to the transition in friction and wear. The spalled tribofilm constituted an abrasive third-body that led to the removal of the lubricious graphitic layer. The contact between the tribocouple following the removal of the graphitic layer led to the initiation of a series of energy absorbing microscale deformation mechanisms followed by the eventual grain pull-outs and fracture, thus the high friction and wear post transition. An intriguing feature also observed was that the fracture surface can self-heal once the fractured grains becomes pulverized, oxidized and smeared along the sliding direction over time. The healed sliding surface creates a smooth surface necessary for graphitization to bring about a further transition in friction from a high friction regime to a low friction regime. For the Al-based MAX phases, due to their superior oxidation resistance as compared to the Si-based MAX phases, the initiation of graphitization and/or oxidation of the sliding surface took longer and as such the transition in friction and wear proceeded from an initially high friction and wear regime to a low friction and wear regime. Further, due to the larger grain size of the Al-based MAX phases as compared to their Si-based counterpart, this makes them much more susceptible to sliding-induced deformation.
Ancillary TiC formed in situ and/or intentionally added to the Ti3SiC2 matrix was found to be beneficial to the tribological behaviour of the Ti3SiC2 MAX phase. The presence of TiC as particles in the Ti3SiC2 matrix enhances the tribological behaviour of Ti3SiC2 MAX phase by improving its oxidation resistance thereby delaying the deleterious formation of non-adherent rutile as well as acting as load bearing element to further shield the surrounding Ti3SiC2 matrix. Also, TiC particles in the matrix acts as reinforcement for the weak Ti3SiC2 grain boundaries by impeding grain pull-out as well as inhibiting the propagation of kink-bands (KB) - a deleterious grain-scale deformation mechanism.
The low friction and wear of the monolithic titanium carbide disc was found to be linked to frictional heating-induced tribo-oxidation product preventing direct ball-to-disc contact. As tribo-oxidation product rutile (TiO2) is non-adherent, it was easily scraped-off by the Al2O3 ball, leading to the eventual contact between the Al2O3 ball and the TiC sliding surface as evident by the evolution of wear grooves and surface fracture. A wear mechanism involving “oxidation-scrape-re-oxidation” is proposed for the TiC/Al2O3 tribocouple.
TiC containing 30 and 50 mol.% SiC particulates exhibited high friction and wear rates against alumina albeit superior fracture toughness in comparison to monolithic TiC. The lower wear resistance is believed to be a result of the incipient stress relaxation caused by the superposition of tensile stress during the sliding contact unto the inherent residual stresses in the composite system. Stress relaxation led to Ti–C bond degradation followed by a systematic mechanical exfoliation of carbon from TiC. A wear mechanism involving the nucleation and propagation of ripplocations – a deformation micromechanism – is proposed for the first time for the SiC-particulate/TiC-matrix ceramic composite.