Friction and wear behaviors of Ta-based nanocrystalline composite coating under extreme sliding conditions

Although a great interest of researchers is focused on Tantalum (Ta) due to its unique physicochemical properties, the tribological investigation on Ta-based materials has been rarely reported. In this work, the friction and wear behaviors of a Ta-based nanocrystalline composite coating fabricated by plasma spray were studied under extreme sliding conditions via a SRV friction tester and an extremely low wear rate (4.14 × 10−8 mm3/N·m) was achieved under extreme friction condition (i.e., reciprocating friction at 50 N and 60 Hz with hard Al2O3 ball as the counterpart). The results show that with the increase of the severity of test condition, the dominant wear mechanism of the coating gradually changed from mild abrasive wear to fatigue wear, and then to noteworthy plastic deformation and a special delamination wear, which is characterized by the peeling of the whole thermal sprayed splat along its formation boundary and occurred under the most extreme sliding condition (100 N, 120 Hz) investigated. However, under the same extreme test conditions the Ta-based coating with high hardness and toughness (as well as the superb ductility of metal Ta) revealed much improved anti-wear and friction reducing performance than the widely-used hard chromium (Cr) coating. Furthermore, three factors related to the Ta-based coating, namely, the slightly higher content of oxides, the enhancement of (110) plane preferred orientation of α-Ta, and the increased ordering degree of amorphous carbon tribofilm generated at the sliding surfaces, seem to have some correlation with its better tribological behaviors under extreme conditions. This research provides a reference and basis for the application of Ta-based coating under harsh operating situation.