Wear Mechanisms and Methods of Controlling the Wear Resistance of TiN/AlN Nanocomposite Coatings for Tribological Purposes

The aim of this work was to analyze the wear resistance of composite ion-plasma coatings and substantiate methodological approaches for its prediction. The object of the study was multilayer 2D-nanocomposite coatings of the TiN/AlN system with a total thickness of 0.8–4.0 μm. The coatings were applied using vacuum ion-plasma technology onto a substrate made of 40CrNiMo structural steel with a sorbite structure and a hardness of HRC 28–30. Within each nitride layer, the composition of the components was close to equiatomic, and for the coating as a whole it was Ti : Al : N = 1 : 1 : 2 (at %). The structure of the coatings, the morphology of the friction tracks and the wear mechanisms were studied using high-resolution scanning electron microscopy. The mechanical characteristics of the coatings (hardness H and elastic modulus E) were determined using standard continuous indentation techniques. The tribological characteristics (friction coefficient μ and volume wear J) were determined using the ball-on-disk sliding friction test with a circular trajectory of the indenter ball. The analysis of the obtained experimental data made it possible to construct a dependence of the coating hardness on their elemental composition and showed that the mechanical characteristics and wear resistance of the coatings sprayed at a lower temperature of 300–350°C are consistently higher than those sprayed at 400–450°C. In addition, it was found that the highest wear values J were observed for the coatings with the smallest thickness. In order to analyze the effect of coating thickness δ on their wear resistance, a computational and analytical model for predicting coating wear during tribological tests was used. On its basis, diagrams of critical states of ion-plasma composite TiN/AlN coatings were constructed, which made it possible to calculate the critical value of coating thickness δmin, at which the mechanism of coating wear during friction changes. With a relatively thin coating (δ < δmin), located on a “softer” (plastic) substrate, during tribotesting, accelerated wear of the coating occurs by the abrasive-mechanical mechanism due to the deflection of the coating and its subsequent cracking, peeling, and chipping. With a relatively thick coating (δ ≥ δmin), wear occurs by the abrasion mechanism, which, with high hardness of the coating, is characterized by its significant wear resistance and significantly extends the service life.