THE EFFECT OF ANNEALING ON THE HARDNESS, INTERFACIAL ADHESION AND WEAR BEHAVIOR OF PLASMA-VAPOR-DEPOSITED TIN COATINGS ON STEELS

The effect of annealing on the wear behaviour of three separate samples of plasma-vapour-deposited TiN coatings on M2 tool steels has been investigated. Samples were annealed in air in the temperature range 20-700 °C. Subsequent room temperature indentation hardness and scratch experiments were used to simulate the response of the composite surfaces to sharp and blunt asperity contacts, while X-ray diffraction analysis and scanning electron microscopy techniques were used to monitor both deformation behaviour and microstructural changes. In addition, the effect of temperature cycling on (hot) indentation hardness and indentation deformation was studied in vacuum (about 10-6 Torr) in the temperature range 20-700 °C. Annealing in air was found to cause oxidation of TiN at temperatures greater than about 450 °C, with evolution of nitrogen resulting in coating degradation by the formation of porosity. Thermal modification of the stresses within the coating was also observed to contribute to coating degradation at the higher annealing temperatures. Closely associated with the microstructural degradation, a drastic reduction in both indentation hardness and coating-substrate interfacial adhesion was observed. Further, the indentation hardness of the composites in vacuum was found not only to decrease dramatically with increasing temperature, but also to exhibit hysteresis in temperature-hardness response. Based on this, a simple model relating the hardness hysteresis to residual stress has been evaluated. The results are explainable in terms of the parameters affecting coating integrity and interfacial adhesion, and the implications for wear performance discussed. © 1990.