Variation analysis of wideband emissivity on friction pair interface under grease lubrication based on infrared temperature measurement

Abstract: Unstable surface states of the sliding interface lead to fluctuant emissivity, causing inaccurate infrared temperature measurement. In this study, wideband emissivity on end-face sliding contact interface, which was composed of a 45# steel upper sample and a tin bronze lower sample, was researched under grease lubrication by an infrared temperature measurement system, which was designed and built using a thermal imager and 3 infrared thermometers. Lithium base grease contains molybdenum disulfide of 10% (mass fraction). Three through-holes were drilled on the lower sample, right below the contact area. Through these holes, the infrared thermometers aimed at the contact area, and the brightness temperature of the contact area (BTCA) could be obtained with the constant emissivity of 1. All the lateral surfaces of the samples and the fixture were sprayed with matt black paint, which had a constant emissivity of 0.95. Then the temperature distribution of the lateral surface was obtained using the thermal imager which was placed 400 mm away from the samples. The temperature of the point P (TP), which was located in the outermost of the contact area and whose temperature was measured by the thermal imager, was considered as the actual real average temperature of the contact area. A series of experiments were carried out with different load and rotational speed, and the instantaneous data, such as friction coefficient were recorded. And the wideband emissivity was calculated with the TP and BTCA. Through the comparison and analysis, it was found that the distribution of the lateral surface of lower sample tended to be homogeneous after 1 800 s friction. However, on the upper sample's lateral surface, the nearer from the contact area, the higher the temperature. The TP and BTCA showed a similar variation trend during the friction process, which steeply rose in the beginning and then slowed down. With the rotation speed of 400 r/min, the temperature rise under 100 N was higher than that under 200 N, because the temperature rise was not just affected by the load and sliding speed, but also by the friction coefficient. The BTCA was obviously less than the TP, because the preset emissivity for the thermometers exceeded the real wideband emissivity of the interface. The wideband emissivity of the interface was 0.8 because the grease film and the surface states changed. According to the changing curve of friction coefficient and wideband emissivity, the friction process was divided into running-in stage and steady wear stage. In the running-in stage, the wideband emissivity slumped from 0.96 to 0.86 (slightly different under different test conditions) due to the radical changes of surface states caused by the radical surface morphology and temperature changes. In the steady wear stage, in contrast to the previous, the wideband emissivity grew slowly and trended to be stable. This experimental result reveals the wideband emissivity change regularity on the sliding contact interface under grease lubrication during the friction, and the study may also provide a reference for improving the precision of infrared temperature measurement.