Study On Structure Regulation And Tribological Properties Of Cr-doped Carbon-based Coatings

The traditional design and manufacture concept to reduce friction and wear relying on the formation of continuous oil film between the friction pairs cannot satisfy the requirement of application in precision mechanical transmission system because there almost does not exist gap between the relative moving parts.Additionally,how to reduce the friction and extend the life of non-lubricated mechanical parts that serve in dry friction environment in order to avoid lubricant pollution is also a challenge.It has been recently reached the consensus that the preparation of self-lubricating coatings with friction coefficient as low as that of lubricating medium is benificial to reduce the friction and extend the lif-e of the mechanical parts in advanced manufacturing industry.Carbon-based coating is an effective way to reduce friction and prolong life of precision mechanical parts because of the simple preparation technique,low deposition temperature and superior anti-friction and wear resistance.However,the large internal stress accumulation during the deposition process of pure carbon coating not only weakens the adhesion strength between the coating and the substrate resulting in the early spalling failure of the coating but also lead to the balance of deposition and peeling.Consequently,it is difficult to prepare the pure-carbon coating with thickness larger than 1?m.In order to overcome the technique and performance defects of the above-mentioned carbon-based coating,the effects of transient current density,self-crystallization effects of Cr and CrN under different electric fields and the increment of carbon source obtained from CH4 ionization on the deposition process,microstructure and tribological properties of Cr-doped carbon-based coatings were studied systematically by regulating Cr content and introducing the reactive gas of N2 and the auxiliary carbon source of CH4 under the dc and low-frequency high-power pulses electric field conditions in the present study.The main conclusions are as follows.The incorporation of Cr in carbon-based coating can induce the microstructure changing from amorphous structure to a mixed microstructure with nanocrystalline structure covering in amorphous carbon matrix.With the increase of Cr content,the fraction of the sp2 bond and order degree of sp2 cluster increased,while the fraction of the sp2 bond and the sp3/sp2 ratio decreased.Consequently,resulting in the hardness ofthe coating reduced with the increase of Cr content.Additionally,the internal stress was decreased and the friction coefficient and the specific wear rate were decreased firstly and then increased with the increasing of Cr content.The minimum friction coefficient and specific wear rate were 0.06 and 4.6× 10-17m3/Nm respectively when the Cr content was 2.Oat.%.It indicated that small amounts of Cr incorporation could significantly reduce the internal stress and effectively improve the tribological properties of the carbon-based coating.Carbon-based coatings containing CrN nanocrystalline strengthening phase was prepared by introducing reactive gas N2.The thickness and deposition rate of coatings increased with the increase of graphite target current density.The fraction of the sp2 bond and Cr-C bond increased,while the fraction of Cr-N bond decreased with the increase of graphite target current density.The hardness and elastic modulus of the coatings increased continuously and the maximum values were 22GPa and 158GPa respectively.In addition,the internal stress increased ater a decrease and the minimum value was 0.308GPa when the graphite target current density was 0.022A/cm2.Simultaneously,the adhesion strength between the coating and matrix was reached the maximum value of 60N.Moreover,the friction coefficient and specific wear rate decreased firstly and then increased slightly with the increase of graphite target current density.The minimum friction coefficient and specific wear rate were 0.14 and 1.3 × 10-16m3/Nm respectively when the graphite target current density was 0.033A/cm2,and carbon-based coating achieved the optimal tribological properties.The hardness and adhesion strength of carbon-based coatings containing CrN nanocrystalline strengthening phase prepared by introducing reactive gas N2 was obviously improved and the load-bearing capacity of coatings has been enhanced.The increase of flow rate of CH4 gave rise to the significant increase of thickness and deposition rate.The efficient deposition of carbon-based coating was realized due to carbon source strengthened by CH4 ionization.With the increase of flow rate of CH4,the fraction of the sp2 bond decreased,while the fraction of the spa bond increased.As a result,the sp3/sp2 ratio increased with the increase of flow rate of CH4,which resulted in the increase of hardness.The internal stress increased slightly after a decrease with the increasing of flow rate of CH4.The minimum internal stress of 0.332GPa was achieved at flow rate of CH4 of 6 sccm,and the coating exhibited the highest adhesion strength of 53N.The high hardness and low friction coefficient of carbon-based coating determined the minimum specific wear rate when the flow rate of CH4 was 6 sccm.It indicated that the deposition rate and thickness of carbon-based coatings could be increased significantly and the wear resistance could be improved effectively by introducing CH4 as auxiliary carbon source.The microstructure of carbon-based coatings prepared in dc electric field was found to be a mixed microstructure consisting of nanocrystalline structure and amorphous carbon matrix.The deposition rate and the fraction of the sp2 bond of carbon-based coatings increased continuously,while the fraction of the sp3 bond was decreased slightly after an increase with the increasing of graphite target current density.The hardness of carbon-based coatings was increased with the increase of sp3/sp2 ratio.The highest hardness of 21GPa was achieved at graphite target current density of 0.033A/cm2.In addition,the internal stress,friction coefficient and specific wear rate of the carbon-based coatings decreased firstly and then increased with the increase of graphite target current density.When the graphite target current density was 0.033A/cm2,the carbon-based coating exhibited the lowest internal stress of 0.14GPa,the highest adhesion strength of 39N,the lowest friction coefficient of 0.06 and the lowest specific wear rate of 4.4 × 10-17m3/Nm.Moreover,the carbon-based coating had the highest hardness of 12GPa,the lowest internal stress of 0.12GPa,the highest adhesion strength of 52N,the lowest friction coefficient of 0.05 and the lowest specific wear rate of 4.0 × 10-17m3/Nm at the graphite target peak current density of 0.074A/cm2 in the mode of low frequency and high power pulsed electric field.It indicated that the mechanical and tribological properties of carbon-based coatings could be optimized by increasing graphite target current density.