Preparation And Load-bearing Capacity Of DLC Film On Titanium Alloy

In this paper,the research object is TC4 alloy.Firstly,the diamond-like carbon(DLC)films with Cr,Ti and Si transition layers were prepared by unbalanced magnetron sputtering.The influence of the transition layer on the structure and property of the DLC films was investigated.The relationship between reactive sputtering parameters(substrate bias voltage,deposition pressure,methane flow and sputtering power)and the structure,mechanical and tribological property of DLC films was also studied.Secondly,based on the DLC films prepared under different bias voltage,the influence of different film order(bias controlling soft and hard film),modulation period and modulation ratio on the load-bearing capacity of multilayer DLC films was analyzed by the results of finite element modeling.Finally,according to the results of finite element modeling,multilayer DLC films were prepared by experiments.The load-bearing capacity of multilayer films was studied by testing the mechanical and tribological property of the films.The main conclusions are as follows:a.The structure of DLC films is not changed by transition layers.The compressive internal stress of DLC films in different transition layers is related to the coefficient of linear expansion between the film and substrate.The Si/DLC film has the lowest internal stress(~0.44GPa)and the Cr/DLC film has the highest critical load(~25.42N).The effect of substrate bias voltage on the structure,mechanical and tribological properties of DLC films is the most significant.With the increase of the substrate bias,the degree of graphitization of the film increased.In contrast,the hardness,the Young’s modulus and the compressive internal stress gradually reduced.The film prepared at the bias of-800 V has the highest critical load(~25.32N)and exhibits a lower coefficient of friction(0.05)and a longer wear life.b.The stress/strain distribution of double-layer DLC film systems with different film order is similar.Maximum radial compressive stress appears on the surface of films at the bottom of the indenter.The maximum shear stress appears in the film away from the symmetry axis,and the plastic strain only appears in the substrate.SOFT-1000V/HARD-800 V and SOFT-1000V/HARD-600 V film systems have the lower radial stress,shear stress,equivalent stress and elastic/plastic strain.The increase in the modulation period not only changes the stress/strain distribution of the system,but also greatly decreases the maximum value of the stress,and the maximum stress of more than 2 periods increasesgradually.With the increase of the modulation ratio(the thickness of the hard film),the maximum radial stress decreases slightly,the maximum shear stress reaches a maximum value and then decreases,and the maximum equivalent stress increases gradually.Based on the analysis of the finite element modeling results,the film order(SOFT-1000V/HARD-600 V,SOFT-1000V/HARD-800 V and SOFT-1000V/HARD-400V),the modulation period(2,3 and 4 periods)and the modulation ratio(1:1,1:2 and 1:3)film systems have higher load-bearing capacity.c.Introduction of multilayer structure improves the load-bearing capacity of DLC film.Compared with single-layer DLC films,multilayer DLC films have higher critical load and load-bearing capacity.Experimental study of multilayer DLC films with different structures is consistent with which of finite element modeling.The friction failure load of SOFT-1000V/HARD-600 V,SOFT-1000V/HARD-800 V and SOFT-1000V/HARD-400 V film systems is11,17 and 10 N,respectively.The increase of the modulation period greatly improved the adhesion strength between the film and substrate(~ 31N),and the 3 period system has the highest hardness(~ 18.68GPa).The friction failure load 2,3 and 4 period film systems is14,17 and 10 N,respectively.The modulation ratio has little effect on the hardness and critical load of the multilayer film and the soft and hard film thickness ratio 1:2 system has the lowest compressive internal stress(~0.86GPa)and exhibits a stable and low coefficient of friction.The friction failure load of 1:1,1:2 and 1:3 film systems is 17,14 and 10 N,respectively.