The present invention is generally directed to methods of making diester-based lubricant compositions, wherein such diester-based lubricant compositions generally comprise diester species prepared by reacting vicinal diol species with monoester of one or more fatty acids. The amount of retained austenite of the surface layer of the roller bearing is increased by about 30%. The processing device is adapted to identify a pre-determined characteristic of the material from the recorded resonance andor vibration measurement.