郭忠诚
论文题目：电沉积RE-Ni-W-P-SiC多功能复合材料镀层的理论研究与应用
作者简介：郭忠诚，男，1965年12月出生，1997年09月师从于昆明理工大学杨显万教授，于2001年06月获博士学位。
摘 要
本文研究了电沉积RE-Ni-W-P-SiC多功能复合材料镀层的工艺和理论，主要包括以下一些内容：
根据热力学分析，计算并绘制了Ni-P-H2O系和Ni-C-H2O系E-pH图。结果表明，Ni与P能以Ni3P的形式在阴极上沉积。镀层中有Ni3C生成，C来源于加入镀液中的有机物和溶于水中的空气中的CO2。
工艺条件试验结果表明，采用适宜的复合镀工艺，可得到一系列各种不同成分的复合材料镀层，其成分范围为：SiC 5~30wt%、Ni 50~60wt%、W 10~25wt%、P 5~15wt%、RE 5~10wt% 。 氯化稀土、氧化稀土和硫酸盐稀土的加入，有利于提高复合材料中W、P和SiC的含量。次亚磷酸钠的添加量应适中，添加过多会降低镀层中W和SiC的含量，一般应控制在10~15g/L之间。电流密度、温度和PH值对复合材料中的W、P和SiC含量影响较大，综合考虑它们的影响，Dk应为5~10A/dm2、pH应控制在6.0~6.5之间、温度应控制在55~65℃之间。此外，Dk和PH对复合镀层的表面形貌影响也较大，Dk和PH 高时，复合材料结晶粗；相反，DK和PH 低时，结晶细。搅拌间歇时间对复合材料中的Ni、W、P含量影响不大，但对SiC含量的影响较大，搅拌时间长和间歇时间长都会降低复合材料中SiC的含量，所以，一般间歇时间为3min左右，搅拌时间4~5min。
阴极过程试验结果显示，当镀液中加入SiC 微粒和稀土后，复合材料的阴极沉积电流密度增加，有利于Ni-W-P 合金在阴极沉积，并形成Ni-W-P-SiC或RE-Ni-W-P-SiC复合材料。而在镀液中加入PTFE后，却降低复合材料镀层的阴极沉积电流密度。当稀土的添加量为7~9g/l时，复合材料镀层的阴极沉积电流密度增加并不明显；随着稀土添加量的增加，复合材料镀层的阴极沉积电流密度增加较明显，当添加量达到11~13g/l时，镀层的阴极沉积电流密度增加达到最大值；若进一步增加稀土用量，则阴极沉积电流密度有所下降。SiC 微粒与Ni-W-P合金共沉积的机理是：SiC 微粒本身带负电荷，当加入到镀液中，它会吸附周围的正电荷，在流体动力学和电场力的作用下，迁移到阴极表面形成弱吸附；其次，到达阴极表面的SiC微粒在静电场力的作用下脱去水化膜与阴极直接接触而形成强吸附；第三，吸附到阴极表面的SiC微粒被Ni-W-P合金捕获一起沉积到镀层中。
RE-Ni-W-P-SiC复合镀层在不同浓度的硫酸、盐酸、磷酸和氯化铁等溶液中的腐蚀试验结果表明，以Ni-W-P合金为基体的复合材料镀层在镀态或热处理条件下在硫酸,磷酸,盐酸和氯化铁溶液中具有较好的耐蚀性,其耐蚀性优于316L不锈钢;
Ni-W-P-SiC复合镀层在盐酸、硫酸和氯化铁溶液中的耐蚀性优于Ni-W-P 合金镀层和RE-Ni-W-P-SiC复合镀层，而RE-Ni-W-P-SiC复合镀层在磷酸溶液中的耐蚀性又优于Ni-W-P-SiC和Ni-W-P等镀层。RE-Ni-W-P-SiC复合镀层在硫酸和磷酸溶液中的腐蚀机理为缝隙腐蚀和晶间腐蚀，而在盐酸和氯化铁溶液中的腐蚀机理为点蚀和晶间腐蚀。
复合材料镀层的硬度和耐磨性试验结果表明，随着加热温度的提高，复合材料镀层的硬度增加，在400℃时达峰值，加热温度继续升高，镀层硬度呈下降趋势；此外，阴阳极相互垂直所得复合镀层的硬度高于阴阳极相互平行的硬度；镀态时复合镀层的磨损率均最高，随着热处理温度的提高，磨损率呈下降趋势，在400℃时磨损率最低，耐磨性最好。继续升高温度，磨损率有所上升。另外，随着镀层中磷含量的增加，其耐磨性改善。在400℃热处理条件下，随着热处理时间的延长，复合镀层的硬度和耐磨性增加，当热处理时间达到3小时时，镀层硬度和耐磨性达到最佳值；若继续延长时间，镀层的硬度和耐磨性将降低。随着镀液中SiC浓度的增加，RE-Ni-W-P-SiC复合材料镀层的硬度增加，同时镀层的耐磨性能也增强。镀液中钨酸钠浓度对RE-Ni-W-P-SiC多功能复合材料硬度及耐磨性的影响规律与镀液中SiC浓度对RE-Ni-W-P-SiC复合材料硬度及耐磨性的影响规律基本一致，即随着钨酸钠浓度的升高，RE-Ni-W-P-SiC复合镀层的硬度和耐磨性均提高。随着镀液中次亚磷酸钠浓度的升高，镀层的硬度和耐磨性均降低。
RE-Ni-W-P-SiC复合材料镀层的组织与结构分析表明，复合镀层在镀态下为非晶态，当热处理温度升到200℃时，镀层开始晶化并析出Ni3P相；当温度达到500℃时，镀层晶化完毕，产生新相g-(FeNi)。因此，整个镀层的显微结构随温度的变化过程是：非晶态→混晶态→晶态；稀土元素对复合镀层的显微组织无影响，但可以提高复合镀层中SiC微的含量；镀液中钨酸钠和柠檬酸的浓度对复合材料镀层的结构影响不大，复合材料中的磷含量是镀层非晶化的主要决定因素；镀液中钨酸钠和柠檬酸的浓度对复合材料镀层的表面形貌影响较大，当钨酸钠的浓度为90—150g/L
和柠檬酸的浓度为150—170g/L
时，复合材料镀层表面颗粒细小，而且平整光滑。
复合材料镀层的抗高温氧化试验结果表明，在高温氧化过程中,纯镍镀层、Ni-W-P、Ni-W-P-SiC和RE-Ni-W-P-SiC复合镀层的氧化膜重量和氧化时间的关系,在氧化时间小于60min时，氧化膜的增长规律近似于直线方程；而在氧化时间大于60min后,它的增长规律可以用幂函数方程表示。四种镀层氧化速率的大小顺序是Ni > Ni-W-P > Ni-W-P-SiC > RE-Ni-W-P-SiC。在高温氧化过程中, Ni-W-P、Ni-W-P-SiC和RE-Ni-W-P-三种镀层的氧化膜重量随着氧化温度的升高而呈指数型增加。RE-Ni-W-P-SiC复合镀层与Ni-W-P合金层相比,它的高温抗氧化性能可以提高2-3倍。镀层的截面形貌表明,经500℃下氧化，Ni-W-P合金已向基体扩散,与基体之间没有明显的分界线；Ni-W-P-SiC复合镀层与基体之间有分界线,但不明显；而RE-Ni-W-P-SiC复合镀层在此温度下有明显的分界线。经800℃下氧化后,除RE-Ni-W-P-SiC复合镀层与基体有明显分界线外,其它两种镀层均无分界线。Ni-W-P、Ni-W-P-SiC和RE-Ni-W-P-SiC三种镀层经800℃下氧化后的X-射线衍射图同样显示,
RE-Ni-W-P-SiC复合镀层具有更好的抗高温氧化能力。
对电沉积RE-Ni-W-P-SiC多功能复合材料进行了日处理5平方米的中试试验，结果表明，用该工艺处理的多种零部件在磷化工、制糖和卷烟等工业中应用，其寿命明显高于国产零部件，接近或超过进口件的水平，并取得了一定的经济效益。
关键词：电沉积，RE-Ni-W-P-SiC复合材料镀层，硬度与耐磨性，耐蚀性，抗高温氧化性，组织与结构，应用。
Abstract
Process and basic theory of electrodeposited RE-Ni-W-P-SiC multifunctional composite composite coating have been studied，including some contents as follows：
E-pH patterns of Ni-P-H2O and Ni-C-H2O systems were drown out on the basis of thermodynamic analysis，and results show that Ni and P can co-deposit on the cathode in the form of Ni3P；there are some Ni3C phases in the coating，and C comes from organic compound which was added in the bath and CO2 in the air that was dissolved in water.
Experimental results of process parameters indicate that a series of RE-Ni-W-P-SiC composite coatings with different contents were obtained by means of appropriate parameters，and component scopes of the coatings are SiC 5~30wt%，Ni 50~60wt%，W 10~25wt%，P 5~15wt% and RE 5~10wt%. The contents of W ，P and SiC in the composite coatings were increased when chloride，oxide and sulfate rare earth was added in the bath respectively or together. The addition of sodium hypophosphate must be appropriate，if added much，W and SiC contents in the coatings were decreased ，So the addition of sodium hypophosphate must be controlled 10~15 g/l. Current density，temperature and pH value have considerable effects on the contents of W，P and SiC in the composite coatings. Generally，current density（Dk）must be controlled 5~10 A/dm2，pH 6.0~6.5 and temperature 55~65 oC. Besides，Dk and pH have a great effect on the surface morphologies of the coatings，and the composite coatings get coarse when Dk and pH were high，on the contrary，the composites gain fine crystal when Dk and pH were low. The agitating interval time has little effect on Ni，W and P contents，while it has considerable effect on SiC content in the deposits. Extension of agitation and interval time will decrease SiC content in the composites，so interval and agitating time must be controlled 3 mins and 4~5 mins respectively.
Curves of cathodic polarization display that when SiC particles and rare earth （RE）were added in the bath，cathodic depositing current density of the composites increases，and it is profitable for Ni-W-P coating to deposit in the cathode，forming Ni-W-P-SiC and RE-Ni-W-P-SiC composites. On the contrary，the addition of PTFE in the bath decreases cathodic depositing current density of the coatings. The current density increases a little when the amount of rare earth（RE）is 7~9 g/l；however，the density increases greatly with increasing amount of RE，and it reaches peak value when the amount of RE is 11~13 g/l. But if the amount of RE is raised further，the current density decreases.Mechanism of SiC particles and Ni-W-P coating co-deposition is: SiC particles carry about negative electric charge itself, and they may adsorb positive electric charge around when SiC particles are added in the bath. They move to the surface of cathode and form weak adsorption under the effects of fluid dynamics and electric field force; secondly, SiC particles on the cathodic surface dehydrate under the effect of the electrostatic field force and form strong adsorption; thirdly, SiC particles adsorbed on the cathodic surface are captured by Ni-W-P coating and deposited in the composite.
Corrosion experiments of RE-Ni-W-P-SiC composite coating in H2SO4，HCl，H3PO4 and FeCl3 solutions show that the composite coatings on the base of Ni-W-P coatings have better corrosion resistance in H2SO4，HCl，H3PO4 and FeCl3 solutions at as-deposited or heat treatment，and their corrosion resistance is superior to that of 316L stainless steel；the corrosion resistance of Ni-W-P-SiC coating in HCl，H2SO4 and FeCl3 solutions is much better than that of Ni-W-P and RE-Ni-W-P-SiC coatings；however，the corrosion resistance of the RE-Ni-W-P-SiC composite in H3PO4 solution is superior to that of Ni-W-P-SiC and Ni-W-P coatings. The corrosion mechanism of RE-Ni-W-P-SiC composite coating in H2SO4 and H3PO4 solutions is gap and intergranular corrosion，and in HCl and FeCl3 solutions is point and intergranular corrosion.
Results of hardness and wear resistance of the composite coatings show that the hardness of the coatings increases with increasing heat treatment temperature，and it reaches peak value at 400oC.But it decreases with increasing heat treatment temperature continually.Besides，the hardness of the coating by vertical hanging between cathode and anode is much higher than that of one by parallel hanging between cathode and anode .The abrasion rate of the composites is highest at as-deposited，while it decreases with the rise of temperature，it cuts down lowest at 400 oC. But the abrasion rate increases with continuing raising heat treatment temperature. The wear resistance of the coating is raised with increasing the phosphorus content in the deposit. The hardness and wear resistance of the composite increase with extension of heat treated time at 400 oC，and they reach their peak values at 3 hours respectively. However，the hardness and wear resistance of the coating decrease with the rise of the heat treated time. The hardness and wear resistance of the RE-Ni-W-P-SiC composite coating increase with the rise of SiC and sodium tungstate concentrations in the bath respectively，while the hardness and wear resistance of the RE-Ni-W-P-SiC coating decrease with increasing sodium hypophosphate concentration in the bath.
Microstructure of electrodeposited RE-Ni-W-P-SiC composite coating was studied，and results show that the RE-Ni-W-P-SiC composite coating is amorphous as-deposited. While the coating changes into the crystal and Ni3P phases precipitate when heat treated temperature is raised to 200 oC；The crystal process of the coating is finished and new γ-（FeNi）phase is produced when temperature rises to 500 oC. Therefore ，the microstructure changing process of the coating is amorphous → mixture → crystal；the rare earth has no effect on the microstructure of the coating，while it can increase the SiC content in the deposits. Sodium tungstate and citric acid concentrations in the bath have no effects on the microstructure of the composite，however they have considerable effects on the surface morphologies of the coatings. The coatings with fine crystal and smooth surface will be obtained when sodium tungstate concentration is 90~150 g/l and citric acid concentration is 150~170 g/l. The phosphorus content in the composite is a decisive factor that enables the coating to change into amorphous state.
The resistance to oxidation at high temperature of
the multifunctional electrodeposited RE-Ni-W-P-SiC composites is investigated.
The results show that during high temperature oxidation the relationship
between the oxidized film weight of pure Ni , Ni-W-P, Ni-W-P-SiC or
RE-Ni-W-P-SiC coating and the oxidation time is a mixed curve, i.e. it is
approximately a linear relationship when the oxidation time is less than 60
mins while it is a power function relationship when the oxidation time is over
60 mins. The oxidation rate order of the four coatings is Ni> Ni-W-P> Ni-W-P-SiC>RE-Ni-W-P-SiC.
The oxidized film weight of Ni-W-P, Ni-W-P-SiC or RE-Ni-W-P-SiC coating
increases exponentially with a rise in oxidation temperature. The high
temperature-oxidation resistance of RE-Ni-W-P-SiC composite coating is 3~4
times that of Ni-W-P alloy coating. The cross section morphologies show that
after heat-treatment at 500oC the Ni-W-P coating diffuses into the
matrix , and there is no obvious boundary between Ni-W-P coating and matrix; there
is a boundary between the Ni-W-P-SiC and matrix, but it is not clear; there is
an obvious boundary between RE-Ni-W-P-SiC coating and matrix. After heat
treatment at 800oC there is no boundary between Ni-W-P or Ni-W-P-SiC
coating and matrix while there is a boundary between RE-Ni-W-P-SiC coating and
matrix. X-ray diffraction patterns of Ni-W-P，Ni-W-P-SiC and RE-Ni-W-P-SiC
coatings show clearly that RE-Ni-W-P-SiC coating has better resistance to high
temperature oxidation.
Scale-up experiment of electrodeposited RE-Ni-W-P-SiC multifunctional composite material coating with 5 m2/d products have been finished. Results show that the lifespan of various products electrodeposited RE-Ni-W-P-SiC multifunctional composite material coating that were applied in petrochemicals，manufacturing sugar，cigarette and textile industries is obviously superior to that of the products produced in China and close to or surpasses that of the products imported from foreign countries .The some economic benefit has been gained.
Keywords:electrodeposition , RE-Ni-W-P-SiC composite coating , hardness
and wear resistance ,corrosion resistance , resistance to high temperature
oxidation , microstructure , application .
