Quartz crystal microbalance in elevated temperature viscous liquids: Temperature effect compensation and lubricant degradation monitoring

The quartz crystal microbalance (QCM) was extended to investigate viscous liquids at elevated temperatures in both isothermal and non-isothermal systems. An analysis of the frequency–temperature behavior of the QCM resulted in a new approach to compensate for the effect of the rate of temperature rise on the theoretical QCM temperature coefficients. The temperature-dependent viscosities of a series of liquids were evaluated by measuring the damping voltage of QCM. Thermal degradation experiments on pentaerythritol tetrapelargonate based lubricants demonstrated the potential application of QCM as an in situ sensor to evaluate the thermal stability of lubricants or other viscous fluids. The solid residue deposition rates and liquid phase property changes (i.e., product of density and viscosity) were extensively investigated by monitoring variations in the QCM frequency and damping voltage during the lubricant thermally stressing over a temperature range of 150–220 °C.