Investigation of mechanical and tribological properties of super-thick DLC films with different modulation ratios prepared by PECVD

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Thick films can effectively improve the load-carrying capacity, but the thickness of films often fails to meet the requirements due to internal stress and other factors. The design of the multilayer film achieves an increase in film thickness by effectively reducing internal stress. The performance and structure of the super-thick DLC film can be structurally optimized by changing the modulation ratio. The super-thick DLC films consisting of Six-DLC layer (low Si content) and Siy-DLC layer (high Si content) with different modulation ratios (deposition time ratio of the Six-DLC to Siy-DLC) were successfully deposited on SUS304 stainless steel substrates by plasma enhanced chemical vapor deposition. The effect of modulation ratio on mechanical and tribological properties of the DLC-coating stainless steel was investigated by nano-indentation, scratcher and ball-on-disc reciprocating friction test. The results showed that hardening and toughening have been achieved through the modulation ratio design, and particularly remarkable when the modulation ratio ranges from is 1:5 to 1:1. The super-thick DLC film with a modulation ratio of 2:1 shows the best adhesion strength. Moreover, a film with a modulation ratio of 1:2 exhibits low COF about 0.03 at a load of 5 N due to the Si-rich transfer layer between the sliding interfaces; once the load reaches 20 N, the friction coefficient cannot establish such a low friction interface between the tribopairs, which is related to the peeling of the Si-rich transfer layer caused by high contact stress.