Study On Mechanical Properties Of Diamond Films Using Nanoindentation And Finite Element Method

Diamond films are ideal tool coating material due to the excellent properties of high hardness,high elastic modulus,low friction coefficient and outstanding wear resistance.However,the design and application of diamond films have always been limited by the following problems.First,the works reported the mechanical properties of diamond films lacked discussion on the validity of nanoindentation test results,thus the results obtained are questionable.Second,only the elastic properties such as hardness and elastic modulus of diamond films were obtained,the plastic properties and stress-strain characteristics of diamond films are still unknown.Third,the method of film structure design is simple,time-consuming and unpredictable.Based on these issues,in this paper,the mechanical properties of diamond films were studied further using nanoindentation and finite element method(FEM),and the structure of multilayer diamond films was also designed and simulated by FEM.Firstly,diamond films were deposited by hot filament chemical vapor deposition with different deposition pressure.With the increase of deposition pressure,the deposited diamond film transformed from nanocrystalline diamond(NCD)film to microcrystalline diamond(NCD)film,and the film crystalline quality improved(FWHM reduced from 13.2 cm-1 to 9.1 cm-1).Hardness(H)gradually increases from 75.5±1.4 GPa to 87.3±5.0 GPa,and elastic modulus(E)increases from 890.2±24.9 GPa to 1138.5±161.4 GPa,which is attributed to the increase of sp3 carbon ratio in diamond film.It was found that polishing helps improve the repeatability and reliability of nanoindentation tests.A series of nanoindentation experiments adopting various loads obeying the well-known 10%rule of thumb shows that both H and E decrease as the indentation depth/load increases,especially when the film thickness is small.Based on the massive data,empirical criteria were proposed which is helpful to precisely evaluate diamond films by nano indentation.That is diamond films should be polished until the roughness(Ra)is less than 20 nm,the maximum indentation depth should be limited within 5%of the film thickness to avoid the substrate effect and the film thickness greater than 2 μm is preferred.Secondly,diamond films were prepared with various CH4/H2 ratios by microwave plasma chemical vapor deposition.The elastic-plastic properties and stress-strain characteristics of diamond films were determined by nanoindentation experiment,dimensional analysis and FEM.The results were input into the FEM model and simulated again,the simulated load-displacement curves were consistent with the experimental results,thus proving the correctness of the results.Nanoindentation experiments show that with the increase of CH4/H2 ratio,H of the deposited diamond films decreases from 97.5±2.5 GPa to 72.5±0.8 GPa,and the reduced elastic modulus(Er)decreases from 590.5±15.8 GPa to 461.8±7.2 GPa.It was found that the yield strength(σy)of diamond films decreases synchronously with Er,from 184.5±3.1 GPa decreased to 111.6±3.9 GPa,and the yield strain εy of the diamond film is near constant.Finally,FEM was utilized to simulate multi-layer diamond films.The influence of indentation depth on the measured mechanical properties of multi-layer diamond films was discussed,and the effect of structure sequence,modulation period and modulation ratio on the mechanical properties of multi-layer diamond films was investigated.The measured mechanical properties mainly depend on the surface structure when the indentation depth is extremely small.With the increase of the indentation depth,the obtained mechanical properties of the multi-layer diamond films with MCD as the top layer decrease significantly,while the obtained mechanical properties of ones with NCD as the top layer gradually increase until the substrate deformed.When MCD is the top layer,crack formation and propagation are much easier and the toughness is poor.Therefore,NCD is more suitable as the top layer of diamond multi-layer films.Taking NCD as the top layer,with the decrease of modulation period,the mechanical properties of multi-layer diamond films are improved.As the ratio of indentation depth divided by film thickness increases,the influence of the modulation period on the measure mechanical properties decreases and finally depends on the component properties and modulation ratio.With the increase of the modulation ratio of MCD/NCD,the mechanical properties of films are also improved and kept stable before being influenced by the substrate,but the toughness becomes worse.Most importantly,the structure sequence,modulation period and modulation ratio of multilayer diamond films can be optimized by FEM,which helps to customize the mechanical properties of diamond film.