Fabrication and characterization of TiB2-TiC-Co wear-resistant coatings on AZ91D magnesium alloy

TiB2-TiC-Co cermet coatings were fabricated on AZ91D Mg alloy substrate by atmospheric plasma spraying (APS) using the powders produced by self-propagating high-temperature synthesis (SHS) from Co-Ti-B4C system. In order to improve the bonding stability between cermet coating and substrate, the interlayer of Ni-P/NiCrAlY was prepared by the combination of electroless and APS methods. Microstructure and phase composition of the SHS products and coatings were investigated. Correlation between microstructure and wear properties of the cermet coatings was discussed. The results showed that the interlayer prevented Mg alloy substrate from oxidation or ablation, and bonded tightly to both the substrate and cermet coating. The tensile failure of triple-layer coatings was mainly located at the interface between the NiCrAlY layer and Ni-P layer, with the bonding strength of 21.4 +/- 3.2 MPa. The two cermet coatings with different grain sizes of TiB2-TiC provided good wear resistance for substrates owing to their high hardness of about 1059 and 1018 HV, respectively. Moreover, the cermet coating composed of finer ceramic particles exhibited a more compact and uniform structure, which led to a significant reduction in wear rate and friction coefficient. The wear mechanisms of cermet coatings against alumina-ball counterpart were a mix of exfoliation of hard phases and abrasive wear by micro-cutting.