Unlubricated sliding wear of B4C composites spark-plasma sintered with Si aids and of their reference B4C monoliths

Two fully-dense B4C–SiC composites were fabricated by spark-plasma sintering (SPS) from B4C+Si powders, one superhard (i.e., ∼28.7(8) GPa) with abundant SiC by SPS of B4C+20vol%Si at 1400 °C and the other ultrahard (i.e., ∼35.1(4) GPa) with little SiC by SPS of B4C+4.28vol%Si at 1800 °C, and their unlubricated sliding wear was investigated and compared with those of the reference B4C monoliths. It was found that the two B4C–SiC composites underwent mild tribo-oxidative wear with preferential removal of the oxide tribolayer, with the one SPS-ed at 1400 °C from B4C+20vol%Si being, despite its lower hardness and greater proneness to form oxide tribolayer, only slightly less wear resistant than the one SPS-ed at 1800 °C from B4C+4.28vol%Si (i.e., ∼1.0(5)·107 vs 1.37(8)·107 (N⋅m)/mm3). The reference B4C monolith SPS-ed at 1400 °C is comparatively two orders of magnitude less wear resistant (i.e., ∼1.70(6)·105 (N⋅m)/mm3), attributable to its undergoing severe purely mechanical wear by microfracture-dominated three-body abrasion due to its very poor sintering (i.e., high porosity of ∼33.5 %), poor grain cohesion, and low hardness (i.e., ∼3.1(5) GPa). The reference B4C monolith SPS-ed at 1800 °C, while equally or less hard (i.e., ∼28.4(9) GPa) and slightly porous (i.e., ∼5.3 %), is somewhat more wear resistant (∼1.8(3)·107 (N⋅m)/mm3) than the B4C–SiC composites, attributable to its undergoing only mild purely mechanical wear by plasticity-dominated two-body abrasion without porosity-induced grain pull-out, but it requires SPS temperatures well above 1400 °C. Finally, relevant implications for the ceramics and hard-materials communities with interests in tribological applications are discussed.