Research On The Deformation And Failure Mechanisms Of Amorphous Carbon Films

Amorphous carbons(a-C)films are a type of low-dimension amorphous materials with high strengths,low friction coefficient,good wear resistance and have been extensively deposited on vehicle parts as protecting coatings.However,the thickness of such films is too thin,which makes it difficult to measure the mechanical parameters and observe the deformation behaviors of a-C films in experiments.Such shortcoming limits the explanation for the mechanisms of a-C films in practices.This thesis combines molecular dynamics simulations(MD)and experiments to study the deformation and failure mechanisms of a-C films.It is found that the microstructures of a-C films determine their mechanical properties and the relation between their densities and ultimate tensile strengths is weakened by strain localization phenomena.At high strain rate,because of the insufficient relaxation of a-C films,the films maintain high content of diamond phase and thus own high strength.On the other hand,the high temperature induces remarkable graphitization of a-C films,hence the strength of the films is decreased.In the study of the fatigue behaviors of a-C films,the strain-localized regions own lower strength,causing severer strain localization in subsequent cycles.In the compression simulations of a-C films,their deformation and failure behaviors are related to the method of stress relief of a-C films.At the conditions of high strain rate and low temperature,a-C films are very brittle and there is relative slip on both sides of the failure zone after the film failure.In this case,a-C films have high strength and hardness,but poor ductility.With the decrease of strain rate and the increase of temperature,the expression of diffusion relaxation becomes remarkable and limits the activation of relative slip,which improves the ductility of the films.However,such relaxation promotes the graphitization of a-C films simultaneously,thus the strengths of the films are degraded.Besides,new parameters of the potential function of Coarse-grained molecular dynamics(CGMD)which are suitable for studying the deformation and failure of a-C films are proposed.This method can magnify the timestep of simulation to 40 ps,improving the comparability between MD simulations and experiments.Finally,the mechanical parameters and deformation,failure mechanisms are studied by means of experiment.The results of hardness tests show that a-C coatings can promote the hardness of GCr15 bearing steel by at least 60%.Raman analyses show that there is remarkable graphitization happens before the wear failure of a-C films due to the large tensile stress.It degrades the strength of a-C films and may cause the spalling or cracking of the films.Such phenomena validate the accuracy of MD simulation.