Study On Thermal Stability Of N And Si Doping In Hydrogenated Diamond-like Carbon Coatings

Diamond-like carbon（DLC）films have excellent physical and chemical properties,and they have wide industrial potential application.However,DLC coatings have obvious disadvantages,i.e.,high internal stress and poor thermal stability,which limit extensive application of DLC coatings.In this study,the DLC hydrogenated coatings were deposited by plasma-enhanced chemical vapor.By means of controlling process parameters,e.g.,adjusting the N₂ flow,Si/N co-doping and adjusting the bias voltage,the DLC coatings with different composition and microstructure were deposited.Using Raman,FTIR,XPS,nano indentor system,residual stress rester,and so on,the compostion,chemical structurebond,microstructure,nanohardness,residual stress were characterized for those coatings as deposited and as annealed.The experimental results gives out the following conclusion.（1）Increasing N₂ flow in the work gas results in,the N content in DLC coatings increase,and sp³ content rising first and then dropping at the critical inflection point of 0.12 at%N content.The CN bond formed in the studied coating,which is the term for all kinds of structure bonds between C and N,is the C=N bond dominantly.The C=N bond breakage initiates only after annealing above 490°C.There is a critical temperature 430°C during the annealing process.The carbon sp³ transforms to carbon sp² in a-CN:H coatings at about 330°C,and this transformation is slow when annealed below 430°C.In contrast,a pure a-C:H coating initiates this transformation at a lower rate when annealed below 430°C.When annealed above 430°C,the sp³→sp² transition rate of nitrogen-doped DLC coatings is lower than that of nitrogen-free coating.As far as the DLC coatings with low nitrogen doping is concerned,its thermal stability increases with nitrogen content rising in it.When annealed at 430°C,it can secure good combined properties,i.e.,with high hardness,high conductivity,low residual stress and low friction coefficient.（2）With increasing the negative bias voltage,the sp³ content of a-CN:H coatings decreases,the amount of the CH_n bonds in the coating varies with bias applied,it decreases rapidly when the bias is lower than-500 V,and then drops slowly,or even almost disappears at-740 V.When the bias voltage is-420 V,the CH_n bonds in a coating deposited under bias voltage-420 V begin to break between 330°C and 430°C,resulting in structure transformation of sp³→sp² in the coating.Rising the bias voltage to-500 V,CH_n bond content of the coating decreases significantly,and the sp³→sp² take places only above 430°C annealing.When rising the bias voltage more than-500 V,sp³ content decreases further,and the defect density in the as deposited a-CN:H coating increases.The sp³→sp² transition rate of this coating slightly increase with raising annealing temperature.In this study,the thermal stability of the a-CN:H coatings varies with applied bias voltag during fabrication,it firstly increase with bias rising,and then decrease with the further increase the bias voltage.A coating prepared at bias voltage-500 V has excellent performance,which is with:nanohardness of 22.5 GPa,residual stress of0.57 GPa,friction coefficient of 0.13,and wear rate of 8.1×10-17 m3/（N·m）.Even after annealled at 590°C,this coating is still with nanohardness of 14.8 GPa,and friction coefficient of 0.08.（3）In this study,the coatings with Si/N doping has a high sp² content,probably because of those processing parameters employed.Si in the as deposited coating for dominantly a Si-C bond,while N forms mainly a C=N bond.It has critical temperature 430°C during annealing.When annealing temperature is below this temperature,all chemical bondst of the coatings are relatively stable,in spite that only a small amount of unstable carbon atoms are re-arranged,coating structure is stable.When annealed above 430°C,thermal activation will induce structural transformation in the coatings.Especially when the annealing temperature is greater than 490°C,Si-H bond and Si=N bond begin to break partially,to promote the transformation of the coatings structure.The Si-N-DLC coatings prepared in this study has a lower structural transformation rate under high temperature annealing than that of a pure a-C:H coating.The more the Si content of the coating’s,the more is thermal stability of the Si-N-DLC coating’s.Among all the studied Si-N-DLC coatings,the Si₁₄-N₄-DLC coating have the best thermal stability.