A Study Of The Interactions Between Diamond-like Carbon Coatings And MoDTC/ZDDP Lubricating Additives

With the purpose of improving the tribological properties of W dopeddiamond-like carbon (W-DLC) coating, three series of W doped DLC coatings wereprepared using hybrid deposition techniques in combination with low temoerature ionsulfuration technique, including W-DLC coatings with different W content,W/Ti-DLC coatings and WSx-DLC coatings. Friction and wear tests of WSx-DLC,W-DLC and W/Ti-DLC coatings lubricated with MoDTC/ZDDP were performed on aMS-T3000model ball-on-disc tribo-tester and an Optimol-2SRV oscillating frictionand wear tester. Scanning Electronic Microscope (SEM) was utilized to observe thesurface morphologies of W-DLC coatings. The worn scars morphologies of W-DLCcoatings were measured by NanoMap-D profilometer. Raman was used to analyze thestructures of carbon and structural modifications in the wear tracks of DLC coatings.XPS was performed for the chemical analysis of the tribofilms formed on the wornsurfaces. The mechanisms of W-DLC, WSx-DLC and W/Ti-DLC coatings lubricatedwith MoDTC/ZDDP were studied and the mechanism models were established. Theresults are as follows:(1) The anti-friction performance of W-DLC coatings under MoDTC lubricationconditions were all improved. WS2and MoS2containing tribofilm was formed on theworn surface, which showed friction reducing effect. With the increasing of W content,the wear volume of W-DLC coating decreased firstly and then increased, which had agreat relationship with MoS2/MoOxratio. When lubricated with ZDDP, the anti-wearperformance of W-DLC coatings mainly depended on the formed tribofilm. Continoustungstate tribofilm showed good wear resistant property, on the contrary, it willexacerbate the wear of W-DLC coatings.(2) The tribological performances of WSx/W-DLC coatings were superior tothose of untreated under MoDTC lubrication condition. The main reason is theformation of WSxand high MoS2/MoOxratio. The improved anti-wear property ofWSx/W-DLC coating under ZDDP lubrication is mainly due to the generation of WSx which weakens the abrasive wear caused by phosphate tribofilm.(3) The change of Ti target current has little effect on the friction reducingproperty of W/Ti-DLC coating under MoDTC lubrication. The anti-friction behaviorimproved because MoDTC decomposed and generated a MoS2-containing tribofilm,while MoOx, another decomposition product of MoDTC, would aggravate the wearof coatings. During the friction and wear process, the decomposition of ZDDPproduced hard and rough phosphate tribofilm, leading to the poor friction reducingproperty of W/Ti-DLC coating. The formed phosphate tribofilm determined the wearresistant property of W/Ti-DLC coating. With the increasing of Ti target current,continous phosphate tribofilm formed on worn surface, thus the anti-wearperformance of W/Ti-DLC coating improved.