Effect of Chromium on Structure and Tribological Properties of Hydrogenated Cr/a-C:H Films Prepared via a Reactive Magnetron Sputtering System

Hydrogenated Cr-incorporated carbon films （Cr/a-C：H） are deposited successfully by using a dc reactive mag- netron sputtering system. The structure and mechanical properties of the as-deposited Cr/a-C：H films are characterized systematically by field-emission scanning electron microscope, x-ray diffraction, Raman spectra, nanoindentation and scratch. It is shown that optimal Cr metal forms nanocrystalline carbide to improve the hardness, toughness and adhesion strength in the amorphous carbon matrix, which possesses relatively higher nano-hardness of 15. 7 CPa, elastic modulus of 126.8 GPa and best adhesion strength with critical load （Lc） of 36 N for the Cr/a-C：H film deposited at CH4 flow rate of 20sccm. The friction and wear behaviors of as-deposited Cr/a-C：H films are evaluated under both the ambient air and deionized water conditions. The results reveal that it can achieve superior low friction and anti-wear performance for the Cr/a-C：H film deposited at CH4 flow rate of 20sccm under the ambient air condition, and the friction coetllcient and wear rate tested in deionized water condition are relatively lower compared with those tested under the ambient air condition for each film. Superior combination of mechanical and tribological properties for the Cr/a-C：H film should be a good candidate for engineering applications.

Synergistic tribological effect between polyisobutylene succinimide-modified molybdenum oxide nanoparticle and zinc dialkyldithiophosphate for reducing friction and wear of diamond-like carbon coating under boundary lubrication

Organic molybdenum lubricant additive like molybdenum dialkyl dithiocarbamate(MoDTC)can cause wear acceleration of diamond-like carbon(DLC)coating coupled with steel under boundary lubrication,which hinders its industrial application.Therefore,polyisobutylene succinimide(PIBS),an organo molybdenum amide,was adopted to modify molybdenum oxide affording molybdenum polyisobutylene succinimidemolybdenum oxide nanoparticles(MPIBS-MONPs)with potential to prevent the wear acceleration of DLC coating.The thermal stability of MPIBS-MONPs was evaluated by thermogravimetric analysis.Their tribological properties as the additives in di-isooctyl sebacate(DIOS)were evaluated with MoDTC as a control;and their tribomechanism was investigated in relation to their tribochemical reactions and synergistic tribological effect with zinc dialkyldithiophosphate(ZDDP)as well as worn surface characterizations.Findings indicate that MPIBS-MONPs/ZDDP added in DIOS can significantly reduce the friction and wear of DLC coating,being much superior to MoDTC.This is because MPIBS-MONPs and ZDDP jointly take part in tribochemical reactions to form a composite tribofilm that can increase the wear resistance of DLC coating.Namely,the molybdenum amide on MPIBS-MONPs surface can react with ZDDP to form MoS2 film with excellent friction-reducing ability;and MPIBS-MONPs can release molybdenum oxide nanoparticle to form deposited lubrication layer on worn surfaces.The as-formed composite tribofilm consisting of molybdenum oxide nanocrystal,amorphous polyphosphate,and molybdenum disulfide as well as a small amount of Mo2C accounts for the increase in the wear resistance of DLC coating under boundary lubrication.