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碳纳米管增强镍基复合镀层的性能及影响因素分析

The Effect of Deposition Parameters on the Properties of Nickel-carbon Nanotubes Composite Coatings

【作者】 郭超

【导师】 左禹;

【作者基本信息】 北京化工大学 , 材料学, 2007, 博士

【摘要】 复合电镀是指在电镀溶液中加入一种或几种不溶性固态增强体,在金属离子电沉积的同时将不溶性的固态增强体颗粒或纤维均匀牢固地结合到金属镀层中的方法。复合镀层是一类以基质金属为均匀连续相和以不溶性增强体为分散相的金属基复合材料。碳纳米管增强镍基复合镀层由于综合了碳纳米管的高强度、高模量、良好的自润滑效果和镍金属的耐高温、耐腐蚀等方面的作用而日益受到人们的关注。但已有文献仅对碳纳米管增强镍基复合镀层的部分性能有所简单描述,本文系统研究了各种工艺参数对碳纳米管增强镍基复合镀层性能的影响。在本文的工作中首次发现了一种电沉积过程镍金属的管状生长现象。本文在传统Watts镀液的基础上添加了经过表面处理的碳纳米管,通过阴极E-i曲线、表面观察、硬度测试、拉伸试验、极化曲线和交流阻抗等手段综合研究了各种因素对复合镀层性能的影响。研究发现通过前处理,本试验所采用的碳纳米管被有效的提纯、裁短、活化,碳纳米管在复合镀层内均匀分布,分散状态的碳纳米管与镍基体结合紧密。所得到复合镀层结构致密,厚度均匀。与纯镍镀层相比这种复合镀层具有更高的硬度和更好的结合力。镀液中碳纳米管浓度的增加有助于提高镀速,提高镀层的抗开裂性能、与基体的结合力,但CNTS浓度的增加降低了镀层硬度,同时却损害了镀层表面平整度,降低了镀层阻抗值,增大了阳极电流。随电流密度的增加复合镀层表面粗糙度增加,镀层硬度、抗开裂性、与基体附着力、以及耐蚀性都随电流的升高而增加,在8A/dm~2左右达到峰值后又开始下降。升高槽液温度可明显提高镀速和复合镀层硬度,较低温度和较高温度都可获得相对比较平整的Ni-CNTs复合镀层。搅拌速度对复合镀层性能影响不显著,提高搅拌速度可以轻微提高镀速、硬度,增加复合镀层表面粗糙度。本文首次将双向脉冲方法与碳纳米管增强复合材料结合起来,研究了脉冲参数与复合镀层性能之间的联系,结果表明负向工作比和脉冲频率的增加都可以通过反向工作过程中尖端放电的影响优先溶解复合镀层上的突起部分,使复合镀层表面粗糙度降低。负向工作比和脉冲频率的增加分别通过增加镍基体溶解过程持续的时间和迅速成核捕获碳纳米管来增加复合镀层内碳纳米管的含量。复合镀层的硬度主要受到镍基体的影响随负向工作比和脉冲频率的增加分别呈现上升和下降的趋势。在30%的负向工作比和100Hz的脉冲频率条件下获得的Ni-CNTs复合材料由于碳纳米管起到很好的增强增韧效果而具有最好的抗开裂性能和与基体的结合性力。脉冲参数对镍基体的影响和镀层中碳纳米管的影响共同作用下使得在30%的负向工作比和100Hz的脉冲频率条件下获得的复合镀层在3.5%中性NaCl溶液中具有最好的耐蚀性。综合研究了阴离子表面活性剂和阳离子表面活性剂对碳纳米管增强镍基复合镀层的碳纳米管含量、表面形貌、硬度、附着力、耐蚀性等性能并与未添加活性剂的镀层进行比较,发现阴离子活性剂能有效提高镀层的力学性能而阳离子的作用使得复合镀层的各种性能都有所下降。由于阴离子表面活性剂和阳离子表面活性剂能分别使碳纳米管表面带负电荷和正电荷,因此导致了它们与带负电的电极表面分别产生静电排斥和吸引,因此阴离子表面活性剂降低了镀层内碳纳米管的含量而阳离子表面活性剂提高了镀层内碳纳米管的含量。阴离子活性剂在碳纳米管表面的吸附会加速镍离子的还原从而导致镀层粗糙程度的增加,而吸附有阳离子活性剂的碳纳米更容易吸附在阴极表面造成团聚,也使复合镀层表面变得更加粗糙。阴离子表面活性剂和阳离子表面活性剂都能减小复合镀层晶粒的尺寸,它们也引起了镀层的晶面取向发生了改变。由于阴离子表面活性剂使得金属基体与镀层之间和碳纳米管增强体与镍基体之间的结合更加紧密所以提高了复合镀层的硬度和与基体之间的附着力。而阳离子表面活性剂破坏了金属基体与镀层之间和碳纳米与镍基体之间的结合所以使得镀层硬度和附着力都急剧下降。存在阴离子表面活性剂没有破坏碳纳米管增强镍基电沉积复合材料的耐蚀性能,而阳离子表面活性剂使得膜层致密性变差,失去对基体的保护效果。本文首次发现了一种碳纳米管增强镍基金属材料管状生长现象,这种生长方式与目前金属微管制备方法中的条件复杂、对模板的依赖性强的特点相比有一定优势。同时探讨了这种复合金属管的生长机理。研究过程中发现碳纳米管增强镍基金属管状复合材料内壁均匀,管壁结构致密,大部分金属管在生长过程中直径保持一致,金属管外径尺寸分布在200-300μm之间,管壁的厚度约为10-20μm左右。在复合金属管的生长过程中阴极表面析氢反应形成的氢气泡是复合金属管生长的重要原因。碳纳米管在氢气-镀液两相界面的吸附起到了稳定氢气和构建金属管生长基础框架的关键作用。

【Abstract】 Composite electrodeposition is a process that introduces some solid reinforcers into a plating bath, and the reinforcers will combine into the metal matrix when the metal ions reduce on the cathode. So composite deposits is a metal matrix composite material, which include a metal continuous phase and reinforcer discontinuous phase. The Nickel-carbon nanotubes composite synthesize the high-intensity, high modulus and excellent self-lubricating properties of carbon nanotubes and the pyrolytic stability and corrosion resistance of nickel, so this composite has been paid more attention in these years. But the existed references only gave some partial and simple researches on this composite. So, a systematic research about the effect of each factor on the composite electrodeposit was performed in this thesis. Base on this research, the tubular growth of nickel-carbon nanotubes was firstly found during the electrodeposition.A traditional Watts bath with treated carbon nanotubes was used as the plating cell. The cathodic E-i curve, surface morphology, elements analysis, hardness test, tensile test, polarization curve and electrochemical impedance spectrum (EIS) were performed to find out the relationship between the factors and the composite coating’s properties. The results showed that the carbon nanotubes had been purified, shortened and actived after the treatment. The carbon nanotubes uniformly distributed in composite coatings and the dispersed carbon nanotubes combined with nickel matrix tightly. The resultant composite coating has a compact structure and uniform thickness. This composite coating also keeps higher hardness and cohesion ability than pure nickel coating. In the following experiments it has been found that the increasing of carbon nanotubes concentration is helpful to improve the sedimentation velocity, cohesion of the composite coatings but decreases its hardness and surface smoothness. The increasing of deposit current density makes the coating’s surface coarser. The hardness, cohesion and corrosion resistance increase firstly with the increasing of current density, and decrease after reaching their peak values at 8A/dm~2. Higher bath temperature can improve the sedimentation velocity and coating’s hardness. A smooth surface can be performed in either higher temperature or lower temperature bath. The agitation keeps weak effects on composite coating’s properties, and the increasing of agitation can improve the coating’s sedimentation velocity and hardness slightly.The pulse reverse process and Ni-CNTs composite was firstly utilized together to gain a perfect composite in this thesis. The relationship between pulse reverse parameters and the composite coating’s properties was studied. The results showed that the increasing of both reverse ratio and frequency made the coating’s surface smoother because the selective solution of nickel on the bumps. The carbon nanotubes content in composite coatings increases with higher reverse ratio and frequency because the increasing of reverse ratio can make more nickel dissolved and higher frequency give more opportunities to combine carbon nanotubes in composite coatings. Harder composite coatings can be gained at higher reverse ratio and lower frequency, which is mainly effected by the nickel matrix. The coating formed at 30% reverse ratio and 100Hz keeps the best cohesion to the carbon steel because the carbon nanotubes’ reinforce. The effect of nickel matrix and carbon nanotubes make the coating formed at 30% reverse ratio and 100Hz keep the best corrosion resistance.The effect of anion surfactant and cation surfactant on the carbon nanotubes content, surface morphology, hardness, cohesion and corrosion resistance of composite coatings was studied and compared with the coatings without the assistance of surfactant. The results showed the addition of anion surfactant improved the composite coatings’ mechanical properties but the cation surfactant made the coatings worse. The anion surfactant makes the carbon nanotubes charged with anions so it decrease the carbon nanotubes content in coatings because of the electrostatic repulsion, and the cation surfactant makes the carbon nanotubes charged with cations so it increase the carbon nanotubes content because of the electrostatic attraction. The nanotubes with anion surfactant can accelerate the nickel ions reduction on the cathode, which make the composite coatings coarser, and the cation surfactant can also make composite surfaces coarser by the more tangled carbon nanotubes. Both anion surfactant and cation surfactant can decrease the nickel grain size and change the texture coefficient of crystal plane. The anion can increase the cohesion between the and subsequently improve coating’s hardness and the cohesion between the composite coating and the carbon steel. The cation makes the cohesion between carbon nanotubes and nickel deposits worse, so the composite coatings keep low hardness and poor cohesion to the carbon steel. Corrosion resistance of coatings with anion surfactant is not decreased but coatings with the assistance of cation surfactant loss its corrosion resistance and can not protect the parent metal effectively.The tubular growth of nickel-carbon nanotubes was firstly found during the electrodeposition in this thesis. This process is different from the existed template auxiliary deposition process about its complex procedures and the dependence on template material. The deposition mechanics of this nickel matrix composite was also discussed in this thesis. The research on this metal tube shows that the inner surface of this tube is smooth and the tube wall is compact. The outside diameter of this tubule is about 200-300μm with a wall thickness of about 10-20μm. The hydrogen bubbles play a great part in the metal tube formation. The absorption of carbon nanotubes on the interface between the bubble and electrolyte is the key procedure to stabilize the bubble on the electrode surface and act as the frame construction of nickel ions reduce.

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