On Performance Evaluation & Wearing Behaviors Of Doped Graphite-like Amorphous Carbon Films Under Thermal Wearing Conditions

Ternary Mo-Cr-C & Si-Cr-C a-C films with different Mo/Si content and contrasting binary Cr-C films with similar gradient structure and thickness scale are prepared by magnetron sputtering method to accurately evaluate thermal wearing performance of obtained graphite-like a-C films and effectiveness of Non-Chromium Mo/Si dopant on retarding oxidation.This research also tries to determine the deviation level in evaluation results output by different thermal wearing performance evaluation methods that obviously have different approaching degree to realistic thermal wearing process of coated engineering parts,and to clarify the wearing behavior and affecting factors & primary failure mechanism of the multi-component doped graphite-like a-C films.Through the direct thermal wearing test which is supported to be more closed to the wearing condition for real coated parts,and the more conventional room temperature wearing test which is preceded right after a pre-oxidation heat-treatment,the thermal wearing performance of prepared three kinds of sample films are experimentally measured at different testing temperature respectively by using same contacting load,wearing time,and sliding velocity.Combined the testing results and the wearing morphology obtained under different wearing conditions,the effects of evaluation method and Mo/Si dopant on thermal wearing performance & critical servicing temperature of different kind of doped a-C films are analyzed,while their wearing behaviors and failure mechanism are also discussed.The results show that,(1)The thermal wearing resistance and critical servicing temperature of graphite-like a-C films could be significantly improved by introducing Non-Chromium Mo/Si dopant in their working layer.Compared to the contrasting Cr-C binary film,the ternary Mo-Cr-C and Si-Cr-C films not only have higher critical servicing temperature which was increased from about 300 Celsius degree to the range of 350-400 Celsius degree,but also reveal better lubricating & wearing properties including lower specific wearing rate and lower coefficient of friction level decreased from the range of 0.4-0.55 to the range of 0.1-0.4.(2)Compared to the direct thermal wearing test in which the films suffered dynamic & harsh oxidation-wearing competitive process,the conventional room temperature wearing test preceded after pre-oxidation heat-treatment provides a wilder condition in which the films experienced a static oxidation process and then a wearing plus heat-up process.The latter procedure is beneficial to the formation of oxide layer on film surfaces.However,the former procedure is more closed to the wearing condition for real coated parts.As the result,the evaluation results of thermal wearing resistance and critical servicing temperature,and even the effectiveness of Mo/Si dopant output by the latter testing procedure will be overestimated on various testing conditions.Under same testing temperature,this kind of over-evaluation appears as an approximately 0.3-0.4 higher level of COF and 1-2 order of magnitude higher level of specific volume wearing rate of Mo-Cr-C and Si-Cr-C films obtained by the direct thermal wearing test than the conventional room temperature wearing test preceded after a pre-oxidation heat-treatment.Consequently,the critical servicing temperature of the corresponding films will be also overestimated 50-100 oC.However,it is more credible to adopt the evaluation results output by the direct thermal wearing test.(3)Along with the increase of wearing temperature,the oxidation of graphite-like a-C films gets more severe and their wearing process will sequentially experience three wearing stages of a)stable wearing stage with non-oxidation,b)stable wearing stage with light oxidation and c)exceptional wearing stage with severe oxidation.However,the Mo/Si dopant could retard this three-stage evolution and increase the critical temperature for starting the two latter wearing stages.The critical temperature for Mo-Cr-C and Si-Cr-C films to entering the 3rd 'exceptional wearing stage with severe oxidation' could be increased from 300 oC of Cr-C films to above 350 oC and to above 400 oC,respectively.That is the reason why Mo-Cr-C & Si-Cr-C films not only have lower specific wearing rates at all the three wearing stages,but also higher critical servicing temperatures than Cr-C films.Combined the above research results,it can be concluded that Mo/Si dopant could effectively improves the critical servicing temperature and the thermal wearing performance of graphite-like a-C films.Compared to the direct thermal wearing test,the evaluation results of the conventional RT wearing test preceded after pre-oxidation heat-treatment have relative over-evaluation on thermal wearing performance of graphite-like a-C films.However,owing to its trend is quite closed to the results obtained by the costlier thermal wearing tests,it might be adopt as a low-cost convenient evaluation method after necessary correction according to the testing results provided by the more credible direct thermal wearing tests.Along with the increase of wearing temperature,the a-C films will sequentially experience stable wearing stage with non-oxidation,stable wearing stage with light oxidation and exceptional wearing stage with severe oxidation.The primary acting mechanism of Mo/Si dopant is to retard the three-stage evolution and thus to cause the different level of increase in the critical temperature for starting the two latter wearing stages.