等离子体密度调控CrN薄膜结构改性Ti6Al4V双极板

Modification of Ti6Al4V Bipolar Plate with CrN Film Structure by Plasma Density Control

目的满足质子交换膜燃料电池双极板的使用要求。方法采用热丝增强等离子体磁控溅射技术,通过改变热丝放电电流调控溅射等离子体密度,在Ti6Al4V(TC4)合金表面制备了氮化铬(CrN)薄膜。结果随着热丝放电电流从0 A增加至32 A,真空腔内等离子体密度增强,‒50 V偏压下基体偏流密度从0.07 mA/cm^(2)增至0.7 mA/cm^(2)。CrN薄膜择优取向从低应变能的(111)转变成表面能更低的(200)择优取向。薄膜表面形貌由较疏松的四棱锥型转变成致密球形;无热丝时,CrN薄膜显示有铬的(110)衍射峰且铬原子数分数为52.16%,为富金属薄膜。热丝放电电流为16 A和32 A时,CrN薄膜中的铬原子数分数分别降至50.79%和49.82%,且无Cr的衍射峰,即逐渐转变为贫铬。采用热丝辅助磁控溅射,将使氮气离化率增大,活性增强,引起薄膜贫铬。模拟双极板工作环境下,与TC4腐蚀电流密度1.5×10^(‒8) A/cm^(2)相比,CrN薄膜的腐蚀电流密度由无热丝的3×10^(‒5) A/cm^(2)降至使用热丝的9×10^(‒9) A/cm^(2)。对电化学阻抗谱拟合等效电路表明,无热丝放电电流条件下制备的CrN薄膜等效电路中出现了基体与涂层间的电阻,说明疏松涂层为腐蚀液提供了通道,在基体和涂层间形成了腐蚀。16 A和32 A热丝放电电流条件下制备的CrN薄膜与表面无涂层的钛合金等效电路相同,说明致密涂层能有效阻碍腐蚀介质的渗入,具有最佳腐蚀抗性。无热丝放电电流时接触电阻为7.95 mΩ·cm^(2),热丝放电电流16 A时接触电阻增至15.65 mΩ·cm^(2),32 A时接触电阻大幅增加。结论在质子交换膜燃料电池双极板备选材料钛合金表面制备致密CrN薄膜,增强了基体的耐蚀性,但贫铬组分导致薄膜电阻增大。在钛合金电极板表面制备致密且略富金属或化学剂量比相当的CrN薄膜,将满足其作为燃料电池双极板的使用条件。

This paper aims to meet the requirements of using bipolar plates for proton exchange membrane fuel cells.In this paper,the hot-wire enhanced plasma magnetron sputtering technology is used to control the sputtering plasma density by changing the hot-wire discharge current,and a chromium nitride(CrN)film is prepared on the surface of Ti6Al4V(TC4)alloy.As the discharge current of the hot filament increased from 0 A to 32 A,the plasma density in the vacuum chamber increased,and the bias current density of the substrate increased from 0.07 mA/cm^(2) to 0.7 mA/cm^(2) under‒50 V bias;the preferred orientation of the CrN film changed from that of low strain energy.(111)is transformed into the(200)preferred orientation with lower surface energy;the surface morphology of the film changes from a loose quadrangular pyramid to a dense spherical shape;when there is no heating wire,the CrN film shows the(110)diffraction peak of chromium and the chromium.The content is 52.16at.%,which is a metal-rich film;when the hot filament discharge current is 16 A and 32 A,the chromium content of the CrN film drops to 50.79at.%and 49.82at.%,and there is no diffraction peak of Cr,that is,it gradually changes to chromiumpoor.The use of hot-wire assisted magnetron sputtering will increase the nitrogen ionization rate and increase the activity,causing the thin film to be depleted in chromium.In the simulated bipolar plate working environment,compared with the TC4 corrosion current density of 1.5×10^(‒8) A/cm^(2),the corrosion current density of CrN film decreased from 3×10^(‒5) A/cm^(2) without hot wire to 9×10^(‒9) A/cm^(2) with hot wire.Fitting equivalent circuit of the electrochemical impedance spectroscopy shows that the resistance between the substrate and the coating appears in the equivalent circuit of the CrN thin film prepared under the condition of no hot wire discharge current,indicating that the loose coating provides a channel for the corrosive liquid.Corrosion is formed between the substrate and the coating;the CrN film prepared under the conditions of 16 A and 32 A hot wire discharge current is the same as the equivalent circuit of the uncoated titanium alloy,indicating that the dense coating can effectively prevent the penetration of corrosive media and has the best corrosion resistance.The contact resistance is 7.95 mΩ·cm^(2) when there is no hot wire discharge current,the film contact resistance increases to 15.65 mΩ·cm^(2) when the hot wire discharge current is 16 A,and the film contact resistance increases significantly at 32 A.The preparation of dense CrN film on the surface of titanium alloy,which is a candidate material for proton exchange membrane fuel cell bipolar plate,enhances the corrosion resistance of the substrate;but the chromium-depleted component leads to an increase in film resistance.Preparing a dense and slightly rich CrN film on the surface of a titanium alloy electrode plate or a stoichiometric ratio of equivalent CrN film will meet its use conditions as a fuel cell bipolar plate.