Effect of temperature on tribo-oxide formation and the fretting wear and friction behavior of zirconium and nickel-based alloys

Fretting wear damage in nuclear power plants is a safety and economic concern. Therefore, understanding the effect of operating conditions on the wear mechanism is essential for optimum design. Fretting tests were performed on a nickel-based alloy 718 against a zirconium alloy in the range 25 degrees C < T < 315 degrees C. The effect of temperature was investigated by examining the tribo-surface oxides, using SEM, EDX, and Raman spectroscopy. Compared to room temperature, results showed that the wear coefficient of zirconium alloy increases significantly at a transition temperature Ttr similar to 250-260 degrees C, which is consistent with published data. A similar trend was observed for the friction coefficient. At Ttr, a thick layer of zirconium was transferred and adhered to the mating Ni alloy. In addition to adhesive wear, the generation of subsurface micro-cracks caused delamination and fatigue disruption. The high hardness of the fragmented wear debris caused abrasive wear, promoting the observed high wear coefficient. At T = 315 degrees C, which is above the glazing temperature of the mating Ni alloy, the oxidized surface layer ZrO2 was thinner and adherent to the substrate, acting as solid lubricant. On the mating Ni alloy surface, the detected zirconium oxide (ZrO2), nickel oxide (NiO) glaze, iron and chromium oxide (CrFeO3) contributed to the lower fretting wear and friction coefficients. The wear coefficient of the Ni alloy was found to decrease monotonically with temperature. At room temperatures, NiO oxide particles acted as loose abrasives. The presence of iron oxide (Fe2O3) contributed also to the reduced tribological performance. At T = 315 degrees C, the formation of a protective compact oxide glaze acted as a self-repairing solid lubricant, contributing to the relatively low fretting wear coefficient, which is further enhanced by the presence of CrFeO3 oxide.