High-temperature sliding wear behaviour of HVOF-sprayed TiC-based hardmetals with HEA matrices

The present research aims to develop coating materials for sliding wear protection over a wide temperature range. Indeed, while hardmetal coatings deposited by thermal spraying are widely used to mitigate sliding wear, the conventional formulations face limitations. WC-based materials are not applicable above 400 °C, while Cr3C2-NiCr, despite its oxidation resistance, has good but not optimal wear resistance. Thus, in this work we explored TiC-based hardmetals with Cu- and/or Mo-containing high-entropy alloy (HEA) matrices as an alternative. We coated AISI 304 stainless steel plates with High Velocity Oxy-Fuel (HVOF)-sprayed hardmetals containing 60 vol% TiC in four different AlCuCrFeNi or Al(Cu)CrFeMoNi HEA matrices. All the coatings were 250–300 μm thick and very dense (porosity <1 vol%), with ∼800–860 HV hardness and a largely FCC matrix. In dry sliding ball-on-disc tests against Al2O3, TiC-HEA coatings showed lower wear than a Cr3C2-NiCr benchmark at all temperatures (room temperature, 400 °C, 600 °C, and 800 °C). In particular, the specific wear rates of Mo-containing HEAs exhibited rather stable values ≤3 × 10−6 mm3/(N·m), whilst those of Cr3C2-NiCr increased gradually from ∼2 × 10−5 at room temperature to ∼4 × 10−5 mm3/(N·m) at 800 °C. The wear resistance of the Mo-based HEAs relied on the formation of continuous titanate- and molybdate-rich films by tribo-oxidation at 400 and 600 °C. At 800 °C, the TiC-HEA coatings formed a thick oxide scale and wear proceeded by abrasion and spallation of the scale itself, without affecting the underlying hardmetal. Overall, TiC-HEA hardmetal coatings emerge as interesting options for high-temperature sliding wear resistance.