In situ Raman spectroscopy for examination of high temperature tribological processes

Raman spectroscopy of solid lubricant coatings during high temperature (300–700 °C) wear testing was employed for real-time correlation of sliding contact surface chemistry to the measured friction coefficient. Two coatings were investigated in this work – MoS2 and VN-Ag. Immediately prior to failure of the MoS2 coating at 350 °C, molybdenum trioxide was detected in the wear track, and an increase in friction coefficient and ultimate failure of the coating was associated with buildup of the abrasive oxide compound. For the VN-Ag nanocomposite coating, in situ Raman analysis of the contact surface during heating revealed the appearance of silver vanadate compounds at a temperature of 375 °C. At higher temperatures, competitive evolution of different silver vanadate phases (i.e., Ag3VO4, AgVO3) was observed. For the conditions examined in this work, the wear process at 700 °C inhibited formation of AgVO3 in the sliding contact, as determined by comparison of the composition of the wear track to that of the adjacent, unworn coating surface. Additionally, the composition of the wear track was significantly different after the sample had cooled sufficiently to allow handling for post-test surface characterization with conventional Raman, XRD, and SEM techniques, further illustrating the utility of in situ diagnostics for identification of active lubricant phases during wear tests. This ability to characterize surfaces during wear tests at elevated temperatures fills an important gap left by current in situ tribology techniques that are currently used to provide insight on mechanisms governing the performance of solid lubricant film materials.

Research highlights▶ Demonstration of technique for in situ measurement of wear track composition. ▶ Identified MoS2 coating failure mechanisms at 350 °C. ▶ Discovered mechanisms for adaptation for VN-Ag coating materials. ▶ Revealed effects of sliding and sample cooling on surface chemistry.