Microstructure And Tribological Properties Of Plasma-sprayed Titanium-based Non-oxide Ceramic Coatings

In this work,TiN,TiC and TiB₂ coatings were fabricated by atmospheric plasma spray（APS）technique.The phase composition,microstructure,mechanical and tribological properties of the three kinds of coatings were investigated and compared in detail.On this basis,the tribological performances of the TiC coating sliding against different counterparts（304 stainless steel,Si₃N₄ and WC-Co）were evaluated.At last,in order to improve the tribological properties of the TiC coating,the TiC-Graphite composite coatings were fabricated by APS technique.The phase composition and microstructure of the TiC-Graphite coatings were discussed,and effects of graphite addition on mechanical and tribological properties of the TiC-Graphite coatings were also investigated.The objective of this study is to develop wear resistant coatings with low friction coefficient,wear rate and long service life.The main results obtained in this study are described as follows:1.TiN,TiC and TiB₂ coatings were fabricated by APS technique.The oxidation of the as-received powders occurred during the spray processes.The TiC coating exhibited the higher hardness values among the three kinds of coatings,which was related to the lower porosity（8.0±1.1%）and content of titanium oxide phases（13 wt.%）of the TiC coating,and the higher intrinsic hardness of TiC.2.The tribological performances of the TiN,TiC and TiB₂ coatings against WC-Co ball were evaluated by sliding wear tests under 20 N and 50 N.Results revealed that the TiC coating exhibited the best tribological properties with the lower friction coefficients and wear rates under different loads.For the three kinds of coatings,the fatigue spalling and tribo-oxidation occurred,and the transfer layer was formed during the friction process.For the TiN and TiB₂ coatings,the material transfer took place during the friction process,which was an indication of adhesion wear.3.The tribological performances of the TiC coating against different counterparts （304 stainless steel,Si₃N₄ and WC-Co）were evaluated under 50 N.Results revealed that the rank of friction coefficients wasμ_TiC/Si3N4<μ_TiC/WC-Co<μ_TiC/seel.However,μ_TiC/Si3N4 showed a significant fluctuation,which was mainly ascribed to the serious abrasive wear of Si₃N₄ ball.The TiC/steel and TiC/Si₃N₄ tribopairs exhibited the lower coating wear rates,which was corresponding to the lower hardness of steel and a good lubrication effect of the tribo-oxidation products（SiO₂）,respectively.4.When sliding against steel and Si₃N₄ counterparts,large amounts of adhesions were observed on the wear tracks of the TiC coating due to the high plastic deformation of steel and the tribo-oxidation reaction of Si₃N₄ ball,respectively,and the wear mechanism of the TiC coating was primarily fatigue,tribo-oxidation and adhesion.While the wear mechanism of the TiC coating sliding against WC-Co was only fatigue and tribo-oxidation.5.The TiC-Graphite composite coatings were fabricated by APS.With increasing graphite content,more microcracks were found within the coatings,the roughness of the coatings increased and the hardness of the coatings decreased.6.The tribological performances of the TiC-Graphite coatings against WC-Co ball were evaluated under 50 N.Results revealed that the friction coefficients of the TiC-Graphite coatings decreased with the increase of graphite content,and the wear rate first decreased then increased.The TiC-2.5 wt.%Graphite coating exhibited the lowest wear rate of 0.67×10^-5 mm³/（N.m）,and the friction coefficient was 0.354,which were reduced by 72.41%and 27.76%compared with the TiC coating,respectively.7.Less cavities and more transfer layers were detected on the wear tracks of the TiC-Graphite coatings with increasing graphite content,which led to the decrease of friction coefficients and wear rates of the TiC-Graphite coatings.With further increasing graphite content,more microcracks within the coatings and less density resulted in the increase of the wear rates.In addition,transfer layer contained more oxygen element,and tribo-oxidation occurred.The main wear mechanism of the TiC-Graphite coatings was fatigue and tribo-oxidation.