Abrasive wear behaviour of Ni(Cr)–TiB2 coatings deposited by HVOF spraying of SHS-derived cermet powders

Self-propagating high temperature synthesis (SHS) was used to generate cermets consisting of TiB2 hard particles in a Ni(Cr) binder phase. Three cermets were prepared with the following target compositions namely Ni(Cr)–65 wt.% TiB2, Ni(Cr)–40 wt.% TiB2 and Ni(Cr)–40 wt.% TiB2 with 5 at.% excess boron in order to promote matrix amorphisation. The size of the TiB2 particles within the cermet depended critically on the initial composition; large particles were favoured by low binder fractions and the presence of the excess boron. The SHS products were milled and the resulting powder classified, from which coatings were deposited onto steel substrates by high velocity oxy-fuel (HVOF) thermal spraying. Coatings consisted primarily of TiB2 particles in a nickel-based binder phase. Abrasive wear behaviour was examined with both alumina and silica abradents. All the coatings had similar, low wear rates with silica abrasive whilst with alumina abrasive, significantly different wear rates were observed. Although the Ni(Cr)–65 wt.% TiB2 coating had the lowest indentation fracture toughness and the lowest hardness of all the coatings, it exhibited the greatest resistance to abrasive wear with alumina abrasive. Also, the Ni(Cr)–40 wt.% TiB2 coating with excess boron had a higher wear resistance than its stoichiometric counterpart. In light of this, it is proposed that in these coatings, the presence of the larger boride particles imparts wear resistance.