Research On The Dynamic Contact Behavior Of Amorphous Carbon Films At Micro/Nano Scale

Due to their high hardness,low friction coefficient,excellent wear resistance and good chemical stability,amorphous carbon films?a-C?have shown great application prospect in the aerospace,mechanical engineering,electronic information,medical devices and other fileds as protective films and wear-resistant coatings.Due to the complex service conditions,a-C films often bear dynamic contact behavior at micro/nano scale,such as nano-friction,fretting wear and impact-erosion,etc.which is easy to lead to the local damage and failure of the film,and the loss of the protective effect on the parts,even the failure of the parts in serious cases.Therefore,it is of great theoretical significance and application value to investigate the dynamic damage behavior nad mechanism of a-C film at the mico/nano scale.However,it's difficult to take into account both of the requirements of micro/nano scale and dynamic contact in current common experimental methods.For example,the evaluation of the tribological behavior of a-C films are mainly based on the sliding friction with large load and stroke.The assessment of the mechanical properties of a-C films are also based on the nanoindentation experiments under quasi-static conditions.It leads relatively weak understanding to the wear mechanism and dynamic indentation of a-C film at micro/nano scale.In recent years,fretting wear and nano-impact characterization methods have shown outstanding advantages in the study of micro/nano scale dynamic contact behavior of thin films.Among them,fretting wear,which can be used to evaluate the material loss under the condition of small amplitude relative motion?generally<300?m?,has got more and more applications.Nano-impact technology has the advantages of nano displacement resolution and high strain rate.It is widely used to evaluate the performance of films under impact conditions,especially in the evaluation of film impact fatigue.However,the understanding of the physical process and mechanism of nano impact is still weak.Therefore,aiming at the dynamic contact behavior of amorphous carbon films at micro/nano scale in the actual service environment,a series of amorphous carbon films with different thickness,structure and composition were prepared on different substrate by using physical vapor deposition technology.The effects of load,atmosphere and other experimental conditions on the nano-friction and fretting wear properties were systematically studied,and the wear failure mechanism was discussed.In addition,the repeated contact fatigue damage and failure mechanism of a-C films under high strain rate were studied by using multiple nano-impact testing based on the nanoindentation technology.Through the analysis of high-precision single nano-impact curves,the evaluation method of dynamic properties?dynamic hardness and dynamic toughness?of hard films is proposed.This study provids theoretical guidance and technical indicators for optimizing the film process and screening the film-based system.The specific research contents are as follows:?1?Nano-friction behavior.The graphite-like carbon films?GLC?with different thickness were prepared by closed field unbalanced magnetron sputtering.The effects of thickness on the microstructure,mechanical properties and nano-friction behavior of GLC films were studied systematically.The results show that the content of sp²carbon atoms and the surface roughness of the GLC films increase with the increase of the film thickness,which leads to the decrease of the hardness and the increase of the internal stress of the films.The curves of friction coefficient show that the process of sliding wear of GLC films can be divided into three different stages:the initial stage,the fluctuation stage and the stable wear stage,and the main wear mechanism is abrasive wear.Due to the influence of intrinsic hardness,the friction coefficient of the films in the stable stage increases with the increase of film thickness,while the friction coefficient and specific wear rate of the film are decreasing with the increase of normal load.?2?Fretting wear behavior.Based on the method of bias-graded deposition,GLC films with gradient change of composition were designed,and their fretting wear behavior was compared with that of GLC films with constant bias.Compared with the films prepared with constant bias,the bias-graded GLC films have comparable surface hardness and better bonding properties.The fretting wear results show that the unique microstructure and excellent mechanical properties of the GLC film result in the lowest friction coefficient and the longest fatigue life.The mechanism analysis shows that the fretting wear process of GLC film can be divided into three different stages:surface working area,intermediate layer transition area and film failure area,and each stage corresponds to different wear mechanism.?3?Impact fatigue behavior.The impact fatigue and fracture mechanism of ultra-thin tetrahedral amorphous carbon films?ta-C,?80nm?and high-thickness a-C and a-C:H films?².8?m?were studied by using multiple nano-impact and micro-impact technology,respectively.The results of nano-impact of ta-C films show that the delamination only occurred on the surface of 80nm ta-C film,and the Si substrate remains intact under low impact load.However,the impact depth of 5nm thick ta-C film is higher than the uncoated substrate,and the substrate was broken,which maybe caused by the lower uniformity of the ultra-thin film and the stress concentration caused by the aggregation of pariticles.Under high impact load,the Si substrate under both of the ta-C films were broken,the impact resisistance is much higher than that of Si substate,which shows that the high hardness and bearing capacity of ta-C film can delay the phase transformation and crack initiation of Si substrate,providing a good impact protection,and the thicker the film,the more significant the protective effect.For thick a-C and a-C:H films,the results of multiple micro-impact experiments with high impact energy show that the sp³ carbon atom in a-C:H film will change to sp2under micro-impact load,while the structure of a-C film will remain basically unchanged.Compared with a-C:H film,a-C film has the higher impact cracking resistance,which may be attributed to its high structure stablility and interface bonding strength.?4?Dynamic mechanical properties.The dynamic mechanical properties of hard films were evaluated by a single nano-impact technique with high data acquisition rate.It is found that the whole impact process should include four stages:acceleration stage,indentation stage,rebound stage and deceleration stage.The dynamic hardness of GLC film calculated by energy method is higher than that of quasi-static mechanical properties,which may be related to overestimation of absorption energy by plastic deformation and the strain rate sensitivity.During the impact process of CrN coating,it is found that the impact curve shows a short"platform"stage,which reflects the fracture phenomenon of the coating.The dynamic fracture toughness of the coating obtained by the combination of impact energy analysis and indentation fracture toughness calculation method is between 2.75-7.74 MPa·m^1/2,which is comparable with the results reported in the literature.