TRIBOLOGICAL CHARACTERISTICS OF DLC FILMS AND DUPLEX PLASMA NITRIDINGDLC COATING TREATMENTS/

An innovative approach to improving tribological behavior of surfaces and meeting long-term durability requirements of engineering devices i s to design and develop novel systems incorporating multilayers and/or duplex diffusion/plasma coating treatments. In the present work, the wear and friction characteristics of diamond-like carbon (DLC) films a nd composite surface layers were studied by conducting pin-on-disc exp eriments. M 50 steel and Ti-6Al-4V alloy were used as substrate materi als. The composite layers consisted of a N-diffusion zone obtained by ion nitriding, followed by a 500 Angstrom vapor-deposited Si bond laye r and a 0.4 mu m DLC him. The purpose of the bond layer was to enhance adhesion between the substrate and the DLC films. An ion-beam method was used for the deposition of the DLC films. The pin-on-disc results showed that for both materials the DLC coating produced a reduction in the coefficient of friction of about one order of magnitude. The redu ction in the coefficient of friction was found to be consistent with t he formation of a carbon-rich transfer film on the contact surfaces. W ear scar profiling and weight loss calculations showed that the wear r esistance of the DLC-coated materials was dramatically improved. Compa risons between duplex N-diffusion layer/DLC coating and single DLC coa ting on Ti-6Al-4V alloy substrates showed that the duplex treatments p ossessed a significantly higher wear resistance. Nitriding was found t o cause substrate hardening that reduces subsurface deformation, thus improving coating support and extending considerably DLC film lifetime .