Preparation And Tribological Properties Explorations Of A-C/WC Films For The Heavy Duty And High Speed Gears

Diamond-like carbon(DLC) films have been widely used as solid lubricating protective coatings because of its high hardness, chemical stability and well corrosion resistance and excellent tribological properties. However, it prone to cracking, peeling and a higher friction coefficient which make the film lubrication and protection failure under harsh working conditions such as heavy load and high temperature. The mechanical and tribological properties of DLC films can be significantly improved by compositing the films with WC which possess high hardness, good red hardness and wear resistance. But, it is not clear how the WC phase content in the composite films influence it’s mechanical properties, phase composition, microstructure and tribological properties, especially elevated temperature tribological performance. So, we need further research to reveal the truth. In this study, unbalanced magnetron sputtering technology was used to prepare the composite films by sputtering two graphite targets and a WC target. At the same time, WC phase content in the conposite films was controled by changing the sputtering power of WC target. The effect of the WC phase content on the microstructure, mechanical properties and tribological performance of the conposite films under heavy duty and high speed conditions was systemic investigated, and possible anti-wear and friction reduction mechanism of a-C/WC films at room temperature and elevated temperature was also discussed. The results showed that:1. An alternate multilayer structure of “superlattice coatings” with grey a-C and dark WC layer in nano-scale was observed in the composite films and the WC phase embedded in an amorphous carbon matrix with nanocrystalline WC1-X particles. It was due to this kind of multilayer structure that the growth of columnar structures was inhibited and the composite films change from a obvious columnar structure to the disordered "glass-like" structure with the increasing of WC phase content.2. Eventhough, hardness of the DLC film was reduced by mixing WC phase, the superlattice structure with two phases could effectively reduce the internal stress of thin film and improve the toughness. Therefore,with the increase of WC phase content, the internal stress of composite film was reduced and the adhesion strength was improved. While the value of H/E reached the maximum once the W content was 5.43 at.% in the film. So, the film toughness achieved optimal state at this point.3. At 25℃ under dry friction condition, a-C/WC films showed a low coefficient of friction when W content was less than 5.43 at.%, and the minimum value of friction coefficient(0.05) and wear rate was achieved when W content was 5.43 at.%. The increasing of friction and wear with W content in the films more than 5.43 at.% indicated that too much of WC phase was not conducive to the improvement of tribology performance. Under the condition of 25℃ oil lubrication, the best tribology performance was still obtained on the a-C/WC film with 5.43 at.% W content.4. At 200℃ under dry friction condition, when W content was less than 5.43 at.%, a-C/WC film showed a abnormally high friction coefficient, they completely had no function of lubrication.With the increase of WC phase content, its friction coefficient was improved significantly, and the minimum value of 0.28 was achieved when the W content was 5.43 at.%. Too much of WC phase may not be useful for friction reduction, but it could make the friction coefficient more stablely. However, the wear rate of a-C/WC film was gradually increased with the increase of WC phase content; Under the condition of 200 ℃ oil lubrication, the friction coefficient of a-C/WC film had no obvious regularity with the change of WC phase content, but the wear rate still ahcieved the minimum value with the W content of 5.43 at.%.5. The low friction of a-C/WC film with 5.43 at.% W content may mainly derived from the transfer film with low shear strength which formed on the counterpart during the friction process. At 25℃, the transfer film was dominated by the graphitized amorphous carbon and the friction triggered WO3 only played the auxiliary role. At 200℃, the transfer film contained quite a number of WO3, and the WO3 thin film formed on the friction interface was the key to leading the tribological behavior of a-C/WC film.