Effect of Hardness Ratio on the Wear Performance and Subsurface Evolution of Ni₃Al Matrix Composites

A favorable hardness ratio (H_disk/H_pin = H_d/H_p) could lead to a transition to mild wear during sliding contact. To determine a more appropriate H_d/H_p value for the sliding wear, the dry sliding pin-on-disk wear tests of Ni₃Al matrix composites (NMCs) with multilayer graphene (MLG) are undertaken at H_d/H_p values of 0.99, 0.83, 0.42, and 0.35 at sliding speeds of 0.1, 0.3, 0.5, and 0.7 m/s. It is found that the tribological properties of NMCs are strongly affected by the various hardness ratios. At 0.1 m/s, the friction coefficient decreases with a decrease in H_d/H_p value. The low friction coefficient is 0.14 and the wear rate is 0.9 × 10⁻⁵ mm³ N⁻¹m⁻¹ under the ceramic counterpart with H_d/H_p of 0.35. At 0.7 m/s, the tribological properties show the opposite trend with a decrease in H_d/H_p. At an H_d/H_p of 0.35, the smooth compact layer on the worn surface could decrease the friction at 0.1 m/s, and the improved hardness in the subsurface by strain hardening would play an important role in the improvement of wear resistance. Under the metal counterpart with H_d/H_p of 0.99, plastic deformation only occurs on the contact surface and the MLG could suppress further shear deformation in the subsurface, leading to a low wear rate (2.4 × 10⁻⁵ mm³ N⁻¹m⁻¹) and friction coefficient (0.15) at 0.7 m/s.