The Residual Stress Control And Mechanical Properties Of Multilayer Diamond-Like Carbon Films

Diamond-like carbon (DLC) films have great potential as protective coating and wear resistance material because of their unusual and promising properties such as high hardness, high elastic modulus, low friction coefficient, excellent wear resistance, good corrosion resistance and excellent chemical stability etc. However, the major drawback of DLC film is its high internal compressive stress. And this high stress frequently leads to debonding, cracking, or delamination of the film from the Ti6A14V alloy substrate, which limits the DLC film practical application. Multilayer films can be structured by deposited with several different materials by order. The multilayer films have wide application prospect since their have a lot of interface, usually will increase the toughness of materials and prevent crack propagation. The multilayer films have lower compressive stress, better wear resistance and corrosion resistant than the monolithic film. Considering the urgent problems of DLC film and industrial appilcations, we designed soft/hard DLC multilayer system, in which soft layer operate as shearing bonds to release the internal stress and interface stress.This paper synthesized a series of soft/hard DLC multilayer films and TiC/DLC multilayer films with different modulation parameters on Si(100) and Ti6A14V alloy by filtered cathodic vacuum arc(FCVA)technology in order to solve the problem of the internal compressive stress in the DLC film, increase hardness and enhance tribological properties of titanium alloy. The effect of modulation periods and modulation ratios on the micro structure and properties of the DLC multilayer films including surface morphology, residual stress, composition, structure, mechanical properties and wear resistance were studied. ANSYS finite element software was used to simulate the residual stress generation during filtered cathodic vacuum arc deposition in DLC multilayer films. In order to make a good transition between the film and further increase the adhesion strength. The effect of Ti/TiC gradient transition layer on mechanical proptery and tribological proptery of DLC multilayer films were studied. The main results of this paper are as follows:(1) The soft/hard DLC multilayer films and TiC/DLC multilayer films with compact structure, low residual stress, high hardness and wear resistance have been fabricated on Ti64alloys and Si (100) wafer using FCVA technique.(2) The soft/hard DLC multilayer films have synthesized by FCVA technique. Within constant modulation period of140nm, the sp3content increases with modulation ratios (thickness ratios, hard DLC:soft DLC) increasing. Within constant modulation ratio of1:1, the sp3content decreases with modulation periods decreasing. The soft/hard DLC multilayer films have lower compressive stress than the monolithic hard DLC layers, and the mechanical properties are superior to monolithic films. The soft/hard DLC multilayer film with modulation ratio of1:1and modulation period of140nm shows the most excellent wear resistance.(3) The finite element model (FEM) with an axial symmetric was used to simulate residual stress in DLC film.The finite element simulation results of residual stress are in reasonably good agreement with experimental results. Finite element calculation results show that the DLC films delamination from the outside origin is due to the largest shear stress and the first principal stress. The residual stress strongly affects the mechanical properties of multilayer DLC films with different modulation periods. The DLC multilayer film with modulation period of280nm has the bad adhesion strength owing to high residual stress and shear stress. The DLC multilayer film with modulation period of140nm has the most excellent wear resistance owing to better adhesion strength and the minimum tensile stress.(4) The bias voltage and acetylene partial pressure strongly affects the microstructure and mechanical properties of the TiC films. The composition and structure were measured by XPS and XRD.The carbon exist as hydrogenate amorphous phase (a-C:H) besides combine with the titanium and format TiC phase.The titanium atomic content decrease with increase of acetylene partial pressure, and the a-C:H phase concentration increase with decrease of titanium content in films. Hardness of the TiC films increases firstly and then decreases with decreasing titanium content. The TiC film with substrate bias of AC1000V (20KHz,50%) and acetylene partial pressure of0.15Pa has the minimum compressive stress4.17GPa.With the introduction of Ti/TiC gradient transition layer, the hardness and adhesion strength of DLC multilayer film on Ti6A14V alloys can be increased, and the wear resistance of Ti6A14V alloys can be further improved.(5) The TiC/DLC multilayer films have synthesized by FCVA technique. The multilayered modulation structure and clear interface were confirmed by TEM. Raman’s spectra of TiC/DLC multilayer films indicate that the sp3content decreases with modulation ratios (thickness ratios, TiC:DLC) increasing. Within constant modulation period of106nm, the residual stress decreases with increasing of modulation ratio. The TiC/DLC multilayer film with modulation ratio of4:1has the best adhesion strength and the lowest compressive stress (4.81GPa). Within constant modulation ratio of1:1, with modulation period decreasing, the residual stress decreases. The TiC/DLC multilayer film with modulation period of80nm has the highest hardness、best adhesion strength and the lowest compressive stress. With the optimal modulation ratio of1:1and modulation period of106nm, the TiC/DLC multilayer film has the most excellent wear resistance. The TiC/DLC multilayer films have lower compressive stress than the monolithic hard DLC film; the multilayer films exhibited better adhesion strength and wear resistance than monolithic DLC film.All results above demonstrated that soft/hard DLC multilayer films and TiC/DLC multilayer films prepared by filtered cathodic vacuum arc technology can effectively reduce the residual stress of DLC film and increase adhesion strength, while keeping the high hardness and wear resistance of DLC film.