Research On Radial Nanofretting Of Chromium Nitride Films And Carbon Nitride Films

Due to their excellent mechanical properties,chromium nitride(CrN_x)and amorphous carbon nitride(a-CN_x)films have been widely employed as protective hard coatings in the areas of cutting tools,biomedical enginerring and microelectromechanical systems(MEMS).On the other hand,because of the variation of temperature and mechanical vibration,nanofretting widely exists in MEMS and has become a key tribological problem of concern.Therefore,it is important to understand the nanofretting properties of the CrN_x and a-CN_x films.With a nanoindenter,the nanofretting behaviors of CrN_x film and a-CN_x film were studied by using a spherical diamond indenter under various applied normal load.The nanofretting damage of the films was analyzed by scanning electron microscopy(SEM)and laser confocal scanning microscopy(LCSM).Finally,the effects of film thickness and curvature radius of indenter on the nanofretting damage of film were also discussed.The main conclusions of the research can be summaried as following:1.The CrN_x film can effectively improve the anti-pressure ability of 40Cr substrate.Under the same conditions,the CrN_x film exhibited larger contact stiffness,but smaller residual indentation depth and less energy dissipation than 40Cr substrate.The radial nanofretting damage in 40Cr was mainly identified as the pileup of the plastic deformation around indents.However, the radial nanofretting damage in CrN_x film was characterized as the initiation and propagation of the ring and radial cracks,which were raised from the radial and hoop tensile stress on the interface of CrN_x film and substrate.2.The nanofatigue damage of the a-CN_x film on Si(100)substrate was found to start from the buckling of the film,follow by the initiation and propagation of the ring-like crack at the edge of the buckled film,and finish at the detachment of the film from the substrate with the increase in the number of the nanofretting cycle.3.The contact stiffness is very sensitive to the deformation and damage of the film.Therefore,an indentation method could be developed to characterize the nanofatigue behavior of ultrathin films from the variation of their contact stiffness.4.Film-substrate system will exhibit better deformation compatibility and in turn better unti-nanofretting ability when the elastic modulus of the film is similar as that of the substrate.Therefore,it is possible to improve the service life of the MEMS by adjusting the elastic modulus of films.5.The film thickness effect on the nanofretting behaviors of the films was very complicated,which may be affected by the indentation stress of indenter,the residual stresses induced in film by the deposition process and the deformation state of the film.With the decrese in the curvature radius of indenter,the maximum contact stress will increase sharply at the same indentation force,which leads to the decrease in the critical load of nanofretting damage.