Effect of Annealing Temperature on Microstructure and Tribological Properties of DLC and Si-DLC Coatings

Diamond-like carbon (DLC) coatings have excellent tribological properties, high chemical stability, and corrosion resistance, and they are often used as solid lubricants in the industrial field. However, the service lives of DLC coatings are shortened by their shortcomings of low thermal stability and high internal stress. Currently, defects in DLC coatings can be optimized via element doping, multilayer film design, and heat treatment. The carbon bond structure inside DLC coatings can be changed by Si doping, which reduces the internal stress of the coatings. Annealing treatments not only maintain the structural characteristics of the coating but also release its internal stress. However, the structure of the coating is destroyed when the annealing temperature is high. To study the effect of annealing temperature on the microstructure and tribological properties of DLC and Si-DLC coatings, DLC and Si-DLC coatings were prepared via plasma enhanced chemical vapor deposition and arc ion plating. The coating was composed of a surface layer and a CrN transition layer. The Cr element came from the Cr target, the Si element of the Si-DLC coatings was derived from tetramethylsilane, and the C element was derived from acetylene gas. Then, the coatings were placed in a muffle furnace and annealed at 200, 300, 400 and 500 degrees C, and the annealed coatings were tested. The microstructure, mechanical properties, and tribological properties of the coatings at different annealing temperatures were studied to analyze the relationships between them and their annealing temperatures. The morphologies and structures of the coatings were characterized via scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The mechanical and tribological properties of the coatings were tested using nanoindentation and friction and wear testing machine. The results showed that the coatings had both transition layer and surface layer structures and that the coating surface was smooth and dense. Both DLC and Si-DLC coatings had amorphous structures. The annealing temperature had an important effect on the microstructure and tribological properties of the coatings. With the increase of annealing temperature from room temperature to 500 degrees C, owing to the fracture of the C-H bond and the clustering of the C network, the sp3 content of the coating first increased and then decreased. After annealing at 200 degrees C, the structure and mechanical properties of DLC and Si-DLC coatings did not change significantly. The coating structure was changed by the precipitation of H atoms. The DLC coating heat-treated at 300 degrees C achieved the maximum sp3 content, and the hardness of the DLC coating reached a maximum value of 20.66 GPa. The sp3 content of the Si-DLC coating was the largest at 400 degrees C, and the maximum hardness of the Si-DLC coating was 24.28 GPa. The thermal stability of the Si-DLC coating was improved due to the doping of the Si element. When the temperature exceeded 400 degrees C, the coatings failed. After annealing at 500 degrees C, the ID / IG value of the DLC and Si-DLC coatings increased, and the coatings were graphitized, which resulted in a sudden drop in the mechanical properties of the coatings. The tribological test results showed that the wear mechanism of the coatings against the ZrO2 ball was abrasive wear. The wear rate of the coatings was related to the mechanical properties, showing the trend of first decreasing and then increasing. The optimal heat treatment temperatures of DLC and Si-DLC coatings were 300 and 400 degrees C, respectively. In summary, the microstructure and tribological properties of the coating can be controlled by the heat treatment temperature, and the coatings have excellent comprehensive properties at the optimal heat treatment temperature. The results of this study provide theoretical support for the effect of annealing treatments on the properties of DLC and Si-DLC coatings, and have theoretical and practical value for the application of heat treatments to DLC coatings.