High-temperature sliding wear performance of HVOF-sprayed TiC-HEA coatings

Hardmetal coatings for high-temperature applications are usually based on Cr₃C₂-NiCr; however, the limited hardness and wear resistance of these systems are a drawback which is sometimes overcome through the addition of controlled amount of WC. However, W is a critical raw material subjected to significant supply insecurity and price volatility. Hence, in order to find potential alternatives, we aim to investigate TiC-based hardmetals with High-Entropy Alloy (HEA) matrices, since HEAs can maintain good mechanical strength and oxidation resistance up to elevated temperatures, and can be formulated without critical elements like W and/or Co.

In this work, we therefore studied the ball-on-disc sliding wear resistance of HVOF-sprayed coatings with 60 vol.% TiC and four different HEA coatings based on the AlCrFeNi alloys added with Cu and/or Mo, since these elements can specifically provide favorable tribo-oxidation behavior. The compositions were chosen to have a mainly FCC matrix, since HEAs with FCC lattices are typically tougher than those with BCC or hexagonal structure, which is desirable in the matrix of a hardmetal.

Especially the formulations containing simultaneously Mo and Cu exhibit stably low specific wear rates of 10^-6 mm³/(N·m) from room temperature to 800 °C under high contact pressure conditions, one order of magnitude lower than for a Cr₃C₂–25 wt%NiCr benchmark. SEM, XRD and micro-Raman analyses of the worn samples indicated that this performance was related to a tribo-oxidation mechanism based on the formation of a uniform film of molybdates. Accordingly, increasingly more continuous coverage by the oxide tribofilm resulted in a drop of the friction coefficient from 0.8 at room temperature to 0.3 at 800 °C. The results demonstrate that TiC-AlCrCuFeMoNi coatings are promising candidates for industrial applications requiring stable wear performance over a wide temperature range.