【作者】 张礼平；

【导师】 李国卿；

【作者基本信息】 大连理工大学 ， 材料学， 2009， 硕士

【摘要】 2024铝合金具有密度小、比强度和比刚度高、导电、导热性能好,易加工成形等特性,在汽车工业、航空航天和军工工业等领域具有广阔的应用前景。然而其表面硬度低、摩擦磨损大和耐腐蚀性能差等缺点难以满足应用高要求。多弧离子镀具有靶材离化率高、绕镀性好和结合力大等优点,在现代材料表面改性工艺中占有重要的地位,在2024铝合金表面镀CrN_X硬质膜可以充分发挥硬质涂层和铝合金结构件各自的优良性能。但是铝的化学性质十分活泼,在其表面易于形成一层硬而脆的陶瓷相氧化铝薄膜（20～30A）,严重阻碍了镀层与基体的紧密结合。铝的标准电极电位约为-1.67V,很容易失去电子,当它浸入电解液中能和多种金属离子发生置换反应,在铝表面形成接触置换层,这层膜粗糙、疏松,与铝基体的结合强度很差。同时铝合金在酸碱溶液中很不稳定,给电化学镀膜造成麻烦和困难;再者铝的膨胀系数(22.9^*10^-6/K)与CrN涂层(9.4^*10^-6/K)相差很大,环境温度发生变化时,镀层易脱落,这些都严重影响膜基之间的结合力,影响实施硬质涂层表面沉积改性技术,需要经过特殊的预处理和加镀过渡层。目前,浸锌/各种镀层是工业上铝合金电镀预处理应用最广的方法,该方法简单,结合力好。本文采用了二次浸锌预处理、化学预镀镍、电镀铜作为中间过渡层,再利用磁过滤脉冲电弧离子镀技术镀Cr/CrNx调制周期膜。利用扫描电子显微镜（SEM）、X射线衍射仪（XRD）、电子探针（EPMA）、显微硬度仪等测试手段分析薄膜的成分、结构、形貌、显微硬度、摩擦系数和阳极极化曲线等特性:结合热震法和划痕法测试薄膜的结合力。实验结果表明:（1）采用二次浸锌预处理,然后化学镀镍的前处理技术,有效解决了铝合金表面的氧化铝薄膜的结合力差的难题,从而将难镀基材转化为易镀基材。选择薄铜层来作为Ni-P镀层和Cr/CrN_x层间的粘结层,缓和应力。先碱性化学预镀镍,再采用酸性化学镀镍的二步化学镀镍技术能够有效避免锌对酸性镀镍液的毒化作用,同时也起到镍磷层中磷含量的一个成分梯度分布。形成的Zn/Ni/Cu/Cr/CrN_X多层膜,其成分和性质呈现梯度分布,结构交替变化,减少膜基之间的应力,有利于提高铝合金上CrN_X涂层的结合力。（2）磁过滤脉冲偏压电弧离子镀Cr/CrNx膜表面光滑致密,主要由Cr、CrN和Cr₂N组成,以CrN（220）为择优取向。镍磷过渡层硬度高,对表面氮化物膜起强烈的支撑作用,表面显微硬度达26GPa。（3）磁过滤脉冲偏压技术大大减少了大颗粒的数目和尺寸,表面质量较大幅度的改善,对削弱点蚀起到一定作用:交替分布的韧性金属Cr层和CrN_x层界面,能够打断离子镀过程中容易形成的柱状结构,对腐蚀性能起到很好的阻隔作用;镍磷层致密,对孔蚀有阻隔作用;同时多层膜的低应力和电化学性质的梯度变化,也大大削弱了膜层之间的相位腐蚀速率,电极电位逐渐变正,表面自腐蚀电位正移了400mV,这些都大大提高了2024铝合金的耐腐蚀性能。（4）多层膜的摩擦系数为0.40,耐磨性能良好,主要得力于氮化铬的良好性能和镍磷层的硬度支撑作用。（5）采用热震法和划痕法相结合,将铝合金试样经过渡层处理后经划痕以及在220℃保温20min,然后在冷水中骤冷5min,循环12次都未见起皮,鼓包和剥落现象,结合力良好。结合良好的CrN_X薄膜能使2024铝合金表面具有高硬度,高耐磨性和良好的耐腐蚀性等优良性能,对轻合金的表面改性技术有一定的应用价值。更多还原

【Abstract】 Zn/Ni/Cu/Cr/CrN_X coatings deposited on 2024 Aluminium Alloys by electrochemistry and magnetic filter pulse bias arc ion plating（PBAIP） technology are investigated.Al alloys are the most lightest materials with excellent properties such as high compare intensity、good heat conduction and conductance and easily machining etc,which are applied in many fields chiefly in automobile industry,aerospace industry and war industry.PBAIP has been extensively used to deposit various hard coatings especially on high-speed steel due to its higher ionization,higher deposition rate and stronger adhesion with substrate.But Aluminium Alloys’surface modification is hard to carry out due to its extreme lively chemical property,which are surrounded by a hard and crisp Oxide film quickly（20～30A）. Moreover,the difference of expansion coefficient between CrN(22.9^*10^-6/K) and Al alloys (9.4^*10^-6/K) is very large.So a particular pretreatment and proper transitional layers are needed.Based on some scholars’ research,a reasonable process is adopted to improve coatings-substrate adhesion that is composed of double dipping zinc、pre-electroless nickel and electroplate copper、followed by Cr/CrN_X coatings by PBAIP.In this paper,Scan electron microscope（SEM）、X-ray diffraction（XRD）、Electronic probe microanalysis（EPMA） and hardness tester are used to characterize the morphology、composition、structure and micro-hardness of the films respectively,The results show that:（1） Pretreatment technique of double dipping zinc and electroless nickel can resolve Al₂O₃ thin film completely and turn to easy plating material.Electroplate copper which stick Ni-P film with Cr/CrNx films relieve stress.Two-step electroless nickel can protect solution from poisoning by dissolving Zn and form a gradient P content.Such multilayer films of Zn/Ni/Cu/Cr/CrN_X decreased in compositions and properties and structure distributed by turns are beneficial to relieve stress and enhance adhesion.（2） Cr/CrNx coatings produced by PBAIP is consist of Cr、CrN and Cr₂N phases with CrN（220） major orientation,own glazed and compact performance.Ni-P film which possess high hardness strongly hold nitride film and contribute to surface hardness of 26 Gpa.（3） PBAIP with magnetic filter which reduce number and size of big grain with a good performance can weaken spot corrosion;toughness Cr and hard CrN_x coatings distributed by turns interrupt column structure and thick Ni-P film are good for bore corrosion;low stress and gradient electrochemistry property of Zn/Ni/Cu/Cr/CrN_X multilayer films also weaken phase corrosion speed,self-corrosion potential value shift positive 400mV,those improve 2024Al alloys corrosion resistance largerly.（4） frictional coefficient value of multilayer films is 0.4,CrNx performances and high hardness hold of Ni-P film are beneficial to good wear resistence.（5） Heat shaking law at 220℃for 20min and then cooling for 5min as a cycle,after 12 cycles without crack and flake,multilayer films have better film-substrate adhesion.Perfect CrNx coatings with higher hardness,higher wear resistance and good corrosion resistance can protect Aluminium Alloys and obtain industry applications widely in the future.更多还原