Effect of In Situ Generated TiNx on Structure and Tribological Behavior of Al2O3-13wt.% TiO2 Composite Coatings Produced via Reactive Plasma Spraying

Microstructure, mechanical properties, and tribological behavior of Al2O3-13wt.% TiO2 (AT13)-based coatings with different TiNx (x = 0.3 or 1) contents were investigated. Herein, TiNx complexes were generated from Ti powder via reactive plasma spraying. The morphology and microstructure of AT13-TiNx multiphase ceramic coatings were analyzed by scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction. The presence of TiNx complexes improved microhardness of coatings. At the same time, excessive TiNx content led to uneven microhardness distribution in coatings. Moreover, with the increase in TiNx concentration, wear mechanisms of coatings changed from adhesive and abrasive wear (coating A) or abrasive wear with lubricating phase (coating D) to severe brittle fracture and abrasive wear (coating E). Besides, the coefficient of friction (COF) of coating D reached its lowest value. This was because TiO2 and TiN0.3 with the smallest hardness were predominant in the coating, playing the role of lubricating phases in friction process. As a result, the pinning effect of hard TiN particles prevented plastic deformation of the coating, thus reducing COF and wear quality loss of coating D. In turn, excessive TiNx particles led to the formation of uneven coating (coating F), in which stress concentration during friction testing increased and abrasive wear was aggravated, causing an increase in COF. At low friction speed (100 rpm), wear mechanism of AT13-TiNx composite coating remained unchanged, and COF reached its maximum. At high friction speed (300 rpm), wear mechanism was transformed into adhesive wear and oxidation wear, and COF achieved its lowest value. At last, under the condition of low load (5 N), the wear mass loss of AT13-TiNx composite coating was the minimum, but COF increased.