The Design And Fabrication Of Gradient Diamond-like Carbon Film And Study On Its Tribological Behavior In A Wide Temperature Range

Diamond-like carbon film,as a metastable amorphous material mainly composed of sp²and sp³hybrid carbon,is widely used as a solid lubricating film in the fields of machinery,medical treatment,automobile and microelectronics because of its high hardness,low friction coefficient and wear rate.The irreversible structure transformation of the film caused by high temperature（>200°C）will lead to a sharp deterioration of the performances,which seriously limits the application of the DLC films.In this study,DLC films with antifriction and antiwear properties from room temperature（RT）to 500°C by tailoring the film composition,structure and transition layer of the films are successfully deposited using reactive magnetron sputtering process.The effects of film-structure,chemical composition and mechanical properties on the tribological behavior of DLC films are systematically investigated.The mechanisms of antifriction and antiwear properties of DLC films in a wide temperature range from RT to 500°C are revealed.The main contents and results are as follows:（1）The Si-uniformly and Si-gradually doped（Si increases from top surface to bottom layer）DLC films with Cr interlayer are deposited.The effects of Si contents（0,1.69at.%,4.72at.%,7.06at.%,11.52at.%）and Si gradually distribution on the film structure,mechanical and tribological properties in a wide temperature range（RT～500°C）of the films are studied.The results show that with the increase of Si content from 0 to 11.52 at.%for the uniform Si-DLC films,the fraction of sp²C in the film decreases from 76.92%to 46.95%,while the fraction of sp³C increases from 11.54at.%to 30.05 at.%.The highest hardness（H）and elastic modulus（E）of 18.4 and171.0 GPa are obtained when the Si content is 4.72 at.%in the film.A small amount of Si doping（1.69 at.%～4.72 at.%）can effectively improve the tribological properties in a wide temperture range from RT to 400°C,while all the Si-uniformly doped DLC films worn out at 500°C.Thanks to the gradient distribution of mechanical properties and relative high H and E of the Si-gradually doped DLC film,the Si-gradually doped DLC film exhibits better antifriction and antiwear properties than the uniform Si-doped DLC（4.72 at.%）film in a wide temperature range from RT to 500°C.（2）The uniform W and Si co-doped and gradient W doped（W increases from top surface to bottom layer）Si-W-DLC films with Cr interlayer are deposited.The effects of W content of 4.45 at.%（Si-W_L-DLC）,13.12 at.%（Si-W_H-DLC）and gradient W distribution on the film structure,mechanical and tribological behavior between RT and500°C are investigated.For the uniform W and Si co-doped DLC films,a small amount of W doping（4.45 at.%）increases the fraction of sp²C by 0.7%,while a large amount of W doping（13.12 at.%）increases the fraction of sp²C by 6.0%.The hardness of uniform W and Si co-doped DLC film is higher than that of the Si-DLC film due to the formation hard WC phase.On the premise of similar Si content in the Si-W-DLC films,a small amount of W doping has less influence on the film graphitization,while the formed WC phase improves the film hardness.The two effects mentioned above lead to a better wear resistance of Si-W_L-DLC film than the Si-DLC film,while a large amount of W doping has a greater effect on the film graphitization,which decreases the wear resistance of the Si-W_H-DLC film at RT～400°C.For the gradient W doped Si-W-DLC film,the fraction of carbide gradually increases from the surface to the bottom layer caused by the gradient W distribution,which makes the film exhibits excellent antifriction and antiwear properties at RT～300°C.At 400～500°C,the synergistic effect of graphitization of film surface,the formed WO₃,WC and Si C phase makes the gradient W doped Si-W-DLC film excellent tribological properties at high temperatures.（3）The gradient W-doped Si-W-DLC film with W interlayer is deposited.The effects of W interlayer on the mechanical and tribological behavior of the film in a wide temperature are investigated.The results show that the W/gradient W doped Si-W-DLC film presents good antifriction and antiwear properties at RT and 300°C.The synergistic effect of WC and WO₃formed at the W/DLC interface and the graphitization in the wear track makes the W/gradient W doped Si-W-DLC film excellent antifriction and antiwear properties at 500°C.As the normal load decreases from 5 to 1 N,the ratio of actual contact area A to normal load L（A/L）increase from 1.8×10^-11to 3.1×10^-11Pa^-1and the coefficient of friction tested at 500°C increases gradually from 0.06 to 0.15and finally worn out.As the sliding speed decreases from 4 to 1 Hz,the duration of the coefficient of friction below 0.1 becomes shorter.The coefficient of friction rapidly increases to 0.4 after keeping 0.05 for just about 100 s at 1 Hz.The decrease of sliding speed will increase the chemical-adsorption time between the active species and the frictional surface at high temperature,which lead to the increase of friction force caused by the gas desorption at the sliding interface.The tribological properties of the W/gradient W-doped Si-W-DLC film deteriorates with the decrease of the sliding speed.Compared with the normal load,the sliding speed has a greater influence on the tribological properties for the W/gradient W-doped Si-W-DLC film at 500°C.