| In order to improve fuel efficiency and reduce CO2 emissions,the industry has developed a variety of wear-resistant and anti-friction hard coatings to protect mechanical parts and reduce friction and wear.In the past decade,boron-rich boride thin film coatings are becoming an attractive research subject due to their excellent mechanical and tribological properties,especially AlMgB14-TiB2 and B4C.Originally developed by Ames Laboratory of USDOE,AlMgB14-TiB2 coatings has demonstrated great potential as a wear resistant coating for mechanical parts,particularly under lubricated conditions.Furthermore,compared to uncoated mechanical parts,B4C coating parts with full formulation oil lubrication exhibited enhanced scuffing and macro-pitting resistance.Currently,B4C coatings have been applied to the surface of automotive transmission components.However,the tribochemical interaction between the boron-rich boride coating and the lubricant additive has not been systematically studied.In this paper,the effects of pure PAO base oil and friction modifying additive molybdenum dialkyldithiocarbamate(MoDTC)on the tribological behavior and tribological mechanism of as-deposited and annealed AlMgB14-TiB2 coatings and B4C coatings were investigated,using pin-on-disk unidirectional and reciprocating sliding test,Raman spectroscopy,Scanning Electron Microscopy(SEM),Energy Dispersive X-ray Spectroscopy(EDX)and X-ray Photoelectron Spectroscopy(XPS),Laser Scanning Confocal Microscopy(LSCM),Three-dimensional white light interferometer surface topography instrument.The results are as follows:(1)Under unidirectional rotational sliding with PAO lubrication,annealed AlMgB14-TiB2 coating and as-deposited B4C coating have the lowest friction coefficient,about 0.1.Annealed B4C coating has the highest friction coefficient of about 0.15.Under reciprocating sliding with PAO lubrication,AlMgB14-TiB2 and B4C have the lowest friction coefficient of 0.12,while annealed AlMgB14-TiB2 coating and annealed B4C coating have a higher friction coefficient,about 0.14-0.15.SEM indicates that coating has adhesive wear.Annealed B4C has local spalling and large wear loss.EDS indicates that there is a large amount of Fe,O element covering the wear track and other elements such as B,Ti are covered by the transfer-layer.(2)Under the condition of unidirectional rotational sliding with PAO/MoDTC lubrication,the friction coefficient of the boride coating exhibited unstable and fluctuating behavior and gradually reached a stable value of 0.08-0.12.In contrast,The friction coefficient of these coatings keep at a very stable low level between 0.05 and 0.07 in the reciprocating sliding friction.SEM examinations show that no severe wear occurred to the boride coatings,regardless of the sliding contact configurations.However,in the case of unidirectional rotating contact,the wear of steel ball is relatively significant,while mild wear showed on the steel balls under reciprocating sliding contact.EDX results and XPS analysis indicate in the case of unidirectional sliding,MoDTC at the wear track of the coating is not completely decompressed,and no MoS2 friction film is formed on the surface of steel counterfaces,while the AlMgB14-TiB2 coating forms an obvious MoS2 friction film on the surface under the condition of reciprocating sliding.Under the condition of MoDTC lubrication,the surface friction chemical process of AlMgB14-TiB2 coating and B4C coating is closely related to the sliding contact configurations and the surface chemical states of the boride coatings.(3)By comprehensive comparison,the MoS2 friction film degraded by MoDTC has obvious lubrication effect,and the matching between MoDTC and AlMgB14-TiB2 coating is better than that of B4C coating... |
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