The effect of temperature and time on microstructure and friction performance of RE-borosulphurizing composite coatings

In this study, 45 carbon steel was RE(rare earth)-borosulphurized at 850 °C, 900 °C, and 950 °C for four different periods of time: 3, 5, 7 and 9 h. The microstructure examination and thickness measure of the diffusion layers were carried out by metallographic microscope and Scanning Electron Microscope (SEM). X-ray diffraction (XRD) was used to characterize the phase composition of the coatings. A ring-on-block wear tester was employed to measure the coatings friction coefficients. The results revealed a saw-tooth shaped and compact morphology of the RE-borosulphurizing layers (RBSL). The coatings thickness was found to increase with the increase of temperature and time. The maximum and the minimum values of the coating thickness were determined to be 98 μm and 32 μm, respectively. The minimum value of the FeB phase thickness was about 7.23 μm. The lowest friction coefficient value was 0.212 which occurred at 850 °C with a 10 N applied load and the maximum friction coefficient value was 0.571 which occurred at 850 °C with a 100 N applied load. The activation energy of RBSL was determined to be 138.6 kJ mol−1 using Arrhenius approach, which is lower than those of other boronizing processes.