Process Development And Performance Research Of Diamond-like Carbon Films With High Interfacial Strength

Aiming at the application of diamond-like carbon（DLC）films as wear-resistant protective coatings,a new deposition process for thicker DLC films with low residual stress,high bonding force with metal and strong wear resistance was studied.In this paper,several composite DLC films under different bias voltages were prepared,the structure and performance characteristics of each film were compared through characterization tests,and the deposition conditions with the best comprehensive performance were explored.A new type of DLC film deposition equipment was proposed for the application of large aspect ratio workpiece,which provided guidance for subsequent industrial production.The main research contents and conclusions include:DC plasma enhanced chemical vapor deposition（DC-PECVD）technology and self-developed deposition system were used,a new deposition process with a-Si:C:H bonding layer and H-DLC transition layer was taken,with 45 steel as the substrate material,sample#0 with only bonding layer and transition layer was prepared,and four composite DLC film samples numbered#1,#2,#3,#4 with an additional top layer deposited at bias voltages of-600 V,-800 V,-1000 V,-1200 V,respectively,were also prepared.AFM,SEM and surface roughness tester were used to observe the surface morphology,cross-sectional structure,film thickness and surface roughness of each composite DLC film sample:when the preparation bias voltage of top film was changed from-600 V to-1200 V,the micro-regions on the film surface tended to be flat,but the overall surface roughness increased gradually,with values of 0.451μm,0.731μm,0.835μm and 0.875μm,respectively.The thicknesses of the films with three-layer structure were all more than 10μm,and the maximum thickness of films reached 16.3μm.The structural components of each sample were characterized by Raman spectrometer,and the Raman spectrum was fitted by the full Gaussian five-peak fitting method:when the bias voltage of preparation was changed from-600V to-1200 V,the relative content of sp³ phase in the film first increased and then decreased,the hydrogen content decreased,the relative graphite content tended to increase.The residual stress and film-substrate bonding force of each composite DLC film were measured by the curvature method and the scratch method,respectively.On the whole,the residual stress increased with the increase of the negative bias for the deposition of the top film,and the change of the bonding force was on the contrary.The binding force values of all samples exceeded 30 N,indicating that the film and the substrate were well bonded.When the negative bias voltage for the deposition of the top film was-600 V,the binding force was the largest,reaching 54.6 N.The tribological properties of each sample were evaluated by a Ball-on-disk friction and wear tester:when the preparation bias of the top film was changed from-600 V to-1200 V,the average wear rate gradually increased,indicating that the wear resistance of films decreased sequentially,the average wear rates were about 1.5×10^-16 m³/（N m）,2.3×10^-16 m³/（N m）,4.1×10^-16 m³/（N m）,and 8.6×10^-16 m³/（N m）,respectively.When the negative bias voltage for the deposition of the top film was-600 V,the average wear rate was the smallest,which improved the wear resistance of 45 steel by about30 times.Therefore,within the experimental scope of this paper,the overall performance of the film is optimal when the negative bias voltage of the top film deposition is-600V.Under the deposition conditions,the total thickness of the film is 11.2μm,and the bonding force with 45 steel is as high as 54.6 N,but the average wear rate of the film is only 1.5×10^-16 m³/（N m）.In addition,the reasons for the difficulty in depositing DLC films on the inner wall of workpieces with large aspect ratios were also analyzed.The traditional anode cylinder/anode rod was replaced by cylindrical wire mesh to improve the gas circulation in the deposition equipment in this case.A new type of deposition equipment suitable for cylindrical or tubular workpieces with a large aspect ratio was proposed,and a high-quality DLC film was successfully deposited on the inner surface of the internal combustion engine cylinder liner.