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金相
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金相试样切割机的设计,金相,试样,切割机,设计
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河南科技学院2009届本科毕业设计（论文）,设计题目：金相试样切割机的设计学生姓名：张静所在院系：机电学院所学专业：机电技术教育导师姓名：刘贯军,【摘要】：金相试样切割机主要用于金相试样的截取和各种材料的下料、切口等，在冶金、汽车、航空航天等制造业中应用极为广泛。20世纪90年代后，金相制样技术发展极为迅速，金相试样切割机作为金相取样设备也取得了很大的进步。本设计通过对金相试样切割机的整体造型、机械结构和控制系统进行了分析，完成了切割机主体结构的设计，控制系统采用了铣床导轨原理，实现了低成本和手动化。最后确定切割机的装配总图。通过此次设计，掌握了相关设计的主要步骤，并对于ProE软件应用方面有了进一步的提高。,1引言金属零件的力学性能不仅与它的化学成分有关，也与它的金相组织密切有关。金相检验是控制和评定产品质量不可缺少的重要手段，是科学研究中研究新材料、新工艺和提高金属制品内在质量的重要方法。要进行金相分析，就必须制备能用于微观检验的样品金相试样。通常，金相试样的制备要经过取样、镶嵌、磨光和抛光几个步骤。每个步骤都应该细心操作，因为任何阶段上的失误都可能影响最后的结果，因为这可能会造成组织假象，从而得出错误的结论。金相试样的制备是通过切割机、镶嵌机、磨抛光机来完成。金相试样的截取是金相试样制备过程中一个重要环节。截取试样的方法有手锯、锯床、砂轮切割机和线切割机等等。根据零件的形状和材料，选择适当的方法来切割。,目前砂轮切割机广泛应用于金相试样的截取上，主要原因是其适应性强，树脂砂轮片可切割软的金属零件如铜、铝及合金和硬的金属零件如淬火后的碳钢、高速钢；金刚石切割机可切割超硬材料如硬质合金、陶瓷等。另外其切割速度快、劳动强度低、操作简便和切割成本低。选择可靠性高的金相试样切割机，可以提高制样效率和质量，降低成本，提高经济效益。,金相试样切割机主要特点：本切割机的切割砂轮直接固定在与电动机的轴同轴线相连接的轴上，利用滑板箱的横向和纵向的移动来切割固定在钳口中的试样电动机固定在底座上，轴套套在电动机的轴上，砂轮片由螺母和夹片加以固定。固定在电动机的前面的滑板箱上装有可沿纵向移动的加紧装置，由手柄的转动来移动钳口把试样夹紧在钳座中，当转动手柄时，就可以进行试样切割了。机器工作时，由罩壳将砂轮片等档住，以防冷却液飞溅和砂轮片碎裂时飞出伤人,2设计要求金相试样切割机的具体设计要求为：（1）利用Pro-E软件设计（2）确定结构的尺寸（3）绘制相应的零件图、实体图及总装配图,3切割机的总体设计过程3.1电动机的选择,3.2传动机构的设计3.2.1轴的计算3.2.2轴的结构设计,3.3控制系统的设计3.3.1夹具的主要结构与使用,3.3.2进给机构的设计,上滑板,中滑板,下滑板,进给装置,4用ProE软件对切割机进行实体造型和装配4.1切割机各主要零件的实体造型,轴的实体图,上滑板的实体图,中滑板的实体图,下滑板的实体图,进给系统的实体图,4.2切割机的装配,切割机的内部实体图,5结束语在此次设计的过程中，培养了我的综合运用所学知识的能力，分析和解决实际中所遇到问题的能力，并且能巩固和深化我所学的专业知识，使我在调查研究和收集资料等方面有了显著的提高，同时在理解分析能力、制定设计计算和绘图能力方面有较大的进步；另外我的技术分析和组织工作的能力也有一定程度的提高。,致谢非常感谢学院领导和老师给我提供了这次良好的深入学习的机会和宽松的学习环境。通过这次毕业设计，不但使我将大学期间所学的专业知识再次回顾学习，而且也使我学到了专业领域中一些前沿的知识。非常感谢在本次设计中曾给予我耐心指导和亲切关怀的老师及帮助过我的同学，正是由于他们的帮助和鼓励才使我能够在毕业设计过程中克服种种困难，最终顺利完成论文，他们的学识和为人也深深地影响着我。在此，请允许我再次向曾直接给予我多次指导的导师表示最忠诚的敬意！同时也感谢百忙之中前来参加答辩的各位老师、专家和教授!,敬请指导批正!,谢谢!答辩人：张静,河南科技学院本科生毕业论文（设计）课题审核表院（系）名称机电学院专业名称机电技术教育042指导教师姓名刘贯军课题名称金相试样切割机设计课题来源自拟课题立题理由和所具备的条件 金相试样切割机种类很多，但适合本院专业实验室条件的用于制做透射电镜样品精密切割的切割机却很少见，国外有符合要求的此类产品，但价格昂贵。随着科研工作的深入，设计制做一种高精度低成本的金相试样切割机很有必要，而且设计条件已经具备。教研室审批意见教研室主任签字： 年 月 日毕业论文（设计）工作领导小组审批意见组长签字： 年 月 日注：本表存院（系）备查。学生姓名张静班级机教042指导教师刘贯军论文（设计）题目金相试样切割机的设计目前已完成任务1.制定毕业设计计划2.查找相关文献3.完成毕业论文开题报告是否符合任务书要求进度：符合尚需完成的任务1.继续对论文材料进行组织和整理；2.按照论文提纲，有步骤有计划的开展论文工作，存在问题要及时与老师沟通；3.对已完成的论文内容进行检查审核，力求把问题降到最少；4.到规定的时间完成论文初稿；5.根据指导老师的指导意见和全部材料完成论文；能否按期完成论文（设计）：能存在问题和解决办法存在问题阅读资料不足，对论文主题的研究不够透彻，且相关的理论知识还不够全面；与指导老师的交流不够充分。拟采取的办法继续查找资料，加强对相关理论知识的理解和掌握，应多和老师交流，在老师的指导下更好完成设计。指导教师签 字日期 年 月 日教学院长（系主任）意 见 签字： 年 月 日河南科技学院本科毕业论文（设计）中期进展情况检查表河南科技学院本科生毕业论文（设计）任务书题目名称 金相试样切割机的设计学生姓名张静所学专业机教学号 20040315049指导教师姓名 刘贯军所学专业 机械设计职称 教授完成期限2008 年 11 月 01 日 至 2009 年 05 月 24 日一、论文（设计）主要内容及主要技术指标 1、连接金相试样切割机的主要用途，国内外研究及使用状况（包括选择国内市场上此类产品的性能及不足）； 2、研究制定设计方案； 3、对受力构件进行受力分析并有必要计算后方可进行设计制图；二、 毕业论文（设计）的基本要求1、 通过互联网、校内期刊数据库等途径了解切割机的工作原理、分析存在问题，提出改进方案；2、 学习并熟练使用ProE绘图软件，并用其进行零件和产品设计（重要部件应有受力分析），提交任务内的全部零件图及部件总成图。3、 完成不少于2000字（单词）的专业英文资料翻译。三、毕业论文（设计）进度安排 2008年11月1日12月30日 查找相关专业资料，熟悉ProE绘图软件的使用，提交开题报告，论证设计方案、完成不少于2000单词英文资料翻译稿。 2009年2月16日5月16日 基本完成毕业设计规定的绘图任务。 2009年5月17日5月24日 撰写毕业论文（设计说明书）。 2009年5月24日交齐全部毕业设计资料。 毕业设计（论文）开题报告题目名称： 金相试样切割机学生姓名张静专业机电技术教育班级042一、选题的目的意义 目前,正处在科学技术飞速发展的信息时代,自动化、最优化、集成化、智能化和精密化等使现代机械制造行业正经历着巨大的变化,也是其今后发展的必然趋势.金相取样设备作为其中一个重要分支,正在由原来的手工操作逐渐走向半自动化和自动化.为此,我设计了对金相切割的半自动化控制系统.与此同时在设计的过程中，能培养我综合运用所学知识，分析和解决实际中所遇到的问题，并且能巩固和深化我所学的专业知识，使我在调查研究和收集资料等方面有了显著的提高，同时在理解分析能力、制定设计或试验方案能力、设计计算和绘图能力方面有较大的进步；另外我的技术分析和组织工作的能力也有一定程度的提高。希望在此次毕业设计中，充分发挥出我们的创新能力，树立良好的学术思想和工作作风，牢牢把握住这次走上岗位之前的实践机会，充分锻炼出自己的工作能力。二、国内外研究综述 金属零件的力学性能不仅与它的化学成分有关，也与它的金相组织密切有关。金相检验是控制和评定产品质量不可缺少的重要手段，是科学研究中研究新材料、新工艺和提高金属制品内在质量的重要方法。要进行金相分析，就必须制备能用于微观检验的样品 金相试样。通常，金相试样的制备要经过取样、镶嵌、磨光和抛光几个步骤。每个步骤都应该细心操作，因为任何阶段上的失误都可能影响最后的结果，因为这可能会造成组织假象，从而得出错误的结论。金相试样的制备是通过切割机、镶嵌机、磨抛光机来完成。金相试样的截取是金相试样制备过程中一个重要环节。截取试样的方法有手锯、锯床、砂轮切割机和线切割机等等。根据零件的形状和材料，选择适当的方法来切割。目前砂轮切割机广泛应用于金相试样的截取上，主要原因是其适应性强，树脂砂轮片可切割软的金属零件如铜、铝及合金和硬的金属零件如淬火后的碳钢、高速钢；金刚石切割机可切割超硬材料如硬质合金、陶瓷等。另外其切割速度快、劳动强度低、操作简便和切割成本低。选择可靠性高的金相试样切割机，可以提高制样效率和质量，降低成本，提高经济效益。金相试样切割机主要特点：本切割机的切割砂轮直接固定在与电动机的轴同轴线相连接的轴上，利用滑板箱的横向和纵向的移动来切割固定在钳口中的试样 电动机固定在底座上，轴套套在电动机的轴上，砂轮片由螺母和夹片加以固定。固定在电动机的前面的滑板箱上装有可沿纵向移动的加紧装置，由手柄的转动来移动钳口把试样夹紧在钳座中，当转动手柄时，就可以进行试样切割了。机器工作时，由罩壳将砂轮片等档住，以防冷却液飞溅和砂轮片碎裂时飞出伤人。三、毕业设计主要研究内容 1、研究切割机的切割原理；2、利用Pre-E软件绘制切割机模型；3、绘制相应的零件图及总装配图四、毕业设计（论文）的研究方法和技术路线 1采用理论和实际操作相结合的方式再结合现代设计理念的基础上，利用现有的条件来进研究。2结合指导教师的教学经验来重新完善和提高自己新的认识和研究。3大量查阅有关书籍和资料来扩充自己视野与认识，提高理论成果的技术含量。4充分利用互连网来查找最新技术成果，提高自身的创新意识。 五、主要参考文献与资料获得情况 1成大先。机械设计手册。北京：化学工业出版社，2004 2成大先。机械设计手册 第四版。北京：化学工业出版社，2002 3毛谦德,李振清。袖珍机械设计手册 第三版。北京：机械工业出版社，2007 4机械设计实用手册编委会。 机械设计实用手册 。北京：机械工业出版社，20085陈立德。 机械设计基础课程设计。北京：高等教育出版社，20066濮良贵，纪名刚。机械设计 第八版。北京：高等教育出版社，20077朱金波。ProE 3.0 工业产品设计完全掌握。北京：兵器工业出版社，20078金鑫，陈雪梅，贾长治。ProE 3.0中文版机械设计专家指导教程。 北京：机械工业出版社，20079 曹岩。ProE 3.0 机械设计实例精解。北京：机械工业出版社，200710朱文坚，黄平，吴昌林。机械设计。北京：机械工业出版社，200511朱龙根。机械设计。北京：机械工业出版社，200612吴克坚，于晓红，钱瑞明。机械设计。北京：高等教育出版社，2003六、指导教师审批意见 年 月 日The electroless nickel-plating on magnesium alloy using NiSO4d6H2Oas the main saltJianzhong Lia,*, Zhongcai Shaob, Xin Zhanga, Yanwen TianaaSchool of materials and metallurgy, Northeastern University, Shenyang 110004, ChinabFaculty of Environment and Chemical Engineering, Shenyang Institute of Technology, Shenyang 110168, ChinaReceived 23 July 2004; accepted in revised form 19 December 2004Available online 26 January 2005AbstractIn this paper, the electroless nickel-plating on magnesium alloy was studied, using NiSO4d 6H2O as the main salt in the electroless platingalkaline solutions. The effects of the buffer agent and plating parameters on the properties and structures of the plating coatings onmagnesium alloy were investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Xraydiffraction (XRD). In addition, the weight loss/gain of the specimens immersed in the test solution and plating bath was measured byusing the electro-balance, to evaluate the erosion of the alloy in the plating solutions. The adhesion between the electroless plating coatingsand the substrates was also evaluated. The compositions of the non-fluoride and environmentally friendly plating bath were optimizedthrough Latin orthogonal experiment. The buffer agent (Na2CO3) added to the plating bath was found to be useful in increasing the growthrate of the plating coating, adjusting the adhesion between the electroless plating coatings and the substrates, and maintaining the pH valuewithin the range of 8.511.5, which is required for the successful electroless nickel-plating on magnesium alloy with NiSO4d 6H2O as themain salt. Trisodium citrate dihydrate was found to be an essential component of the plating bath to plate magnesium alloy, with an optimumconcentration of 30 g L_1. The obtained plating coatings are crystalline with preferential orientation of (111), having advantages such as lowphosphoruscontent, high density, low-porosity, good corrosion resistance and strengthened adhesion.D 2004 Elsevier B.V. All rights reserved.Keywords: Magnesium alloy; Electroless plating; Buffer; Corrosion resistance; Adhesion1. IntroductionThe use of magnesium alloys in a variety of applications,particularly in aerospace, automobiles, and mechanical andelectronic components, has increased steadily in recent yearsas magnesium alloys exhibit an attractive combination oflow density, high strength-to-weight ratio, excellent castability,and good mechanical and damping characteristics.However, magnesium is intrinsically highly reactive and itsalloys usually have relatively poor corrosion resistance,which is actually one of the main obstacles to theapplication of magnesium alloys in practical environments13.Hence, the application of a surface engineering techniqueis the most appropriate method to further enhance thecorrosion resistance. Among the various surface engineeringtechniques that are available for this purpose, coating byelectroless nickel is of special interest especially in theelectronic industry, due to the possession of a combinationof properties, such as good corrosion and wear resistance,deposit uniformity, electrical and thermal conductivity, andsolderability etc. As far as magnesium alloys are concerned,the main salts of electroless plating solutions mostly focusattentions on basic nickel carbonate or nickel acetate 49,which result in high-cost, low-efficiency, instability ofelectroless plating solutions and little applications. Inaddition, the basic nickel carbonate or nickel acetate ofplating solutions yet including fluoride, are harmful to theenvironment, therefore, it is urgently needed to develop newenvironmentally friendly plating bath. It is difficult to carry0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2004.12.009* Corresponding author. Tel.: +86 24 8368 7731; fax: +86 24 2398 1731.E-mail address: (J. Li).Surface & Coatings Technology 200 (2006) 3010 3015/locate/surfcoatout electroless plating on magnesium alloys due to the highcorrosionrate of magnesium alloys in the plating bath withNiSO4d 6H2O or NiCl2d 6H2O as the main salt. It is reported10 that the corrosion rate of magnesium and its alloys inNaCl solutions solely depends on the pH of the bufferedchloride solutions. The objective of this study was to find abuffer agent and determine how the buffer agent affects thedissolution of magnesium alloy in NiSO4d 6H2O alkalinesolutions, and the non-fluoride plating solutions for magnesiumalloy with NiSO4d 6H2O as the main salt. Themicrostructure, compositions and corrosion behavior ofthe coatings were investigated in detail.2. ExperimentalThe substrate material used in the research was AZ91Dingot-cast alloy. The chemical composition of the alloy isgiven in Table 1.Substrates with a size of 50 mm_40 mm_20 mm wereused in the research. The substrates were mechanicallypolished with emery papers up to 1000 grit to ensure similarsurface roughness. The polished substrates were thoroughlywashed with distilled water before passing through the precleaningprocedure as shown in Table 2.The substrates were air-dried after the fluoride activation(the last step in the pre-cleaning procedure). In a typicalexperiment, the initial weight of a air-dried substrate wasmeasured and then quickly transferred to the plating bath(1000 mL) in a glass container placed in a water bath with aconstant temperature of 80 8C. A fresh bath was used for eachexperiment to avoid any change in concentration of bathspecies. The bath compositions and other parameters used inthese experiments are given through Latin orthogonalexperiment in Table 3.Final weights of the specimens were determined and thecoating rates in micrometer per hour were calculated fromthe weight gain. At the same time, in order to study the eachbuffers influence on the substrates and find a bufferappropriate for the electroless plating on magnesium alloy,test solutions with compositions similar to those of theplating bath except that sodium hypophosphite was notadded, were prepared to simulate the corrosion rates ofmagnesium alloy in plating bath and the behaviors of thebuffers. Duplicate experiments were conducted in each case,and the coating rate reported is the average of twoexperiments. The growth rates of the plating coating weremeasured using the electro-balance made in America, whichis the 0.1 mg precision. In the research, the pH value ofplating bath was monitored by a pHS-25C model ofprecision pH/mV meter. Morphology of the coatings wasanalyzed using a scanning electron microscope. The energydispersive X-ray spectroscopy analysis was used fordetermining the content of phosphorus in the coatings.Crystallinity of the coatings was investigated by Rigaku D/max-rA X-ray diffractometer with Cu K-alpha radiation.The adhesion strength of the electrolessly deposited nickelcoatings to the magnesium alloy substrates was determinedby scratch test. During the scratch test, the specimen wasmoved at a constant speed of approximately 11.4 mm/min.Scratches were generated on the specimen using a diamondindenter with a spherical tip of 300 Am in diameter.Corrosion potential measurement in 3.5 wt.% NaCl solutionwas carried out to comparatively investigate the corrosionbehaviors of the bare substrate and the nickel-platedsubstrates. The electrochemical cell used for corrosionpotential measurement consisted of a bare substrate or anickel-plated substrate as the working electrode (exposedarea: 1 cm2), a saturated calomel electrode (SCE), and aplatinum-foil counter electrode.Table 1Chemical composition of the AZ91D alloy (in wt.%)Al Mn Ni Cu Zn Ca Si K Fe Mg9.1 0.17 0.001 0.001 0.64 b0.01 b0.01 b0.01 b0.001 BalTable 2Optimized pre-cleaning procedureTable 3Optimized bath composition and parametersBath species and parameters QuantityNiSO4d 6H2O 25 g/LNaH2PO2d H2O 30 g/LC6H5Na3O7d 2H2O 30 g/LNa2CO3 30 g/LNH3d H2O Adjusting pHpH value 11Temperature 80F2 8CJ. Li et al. / Surface & Coatings Technology 200 (2006) 30103015 30113. Results and discussion3.1. The buffers behaviors in the test NiSO4 solutions andthe choice of an appropriate bufferFig. 1 shows the variation of weight loss of magnesiumalloy as a function of the immersion time with differentbuffers in the test solutions. The compositions and thecontrolled temperature of the test solutions were similar tothose of the plating bath except that sodium hypophosphitewas not included. The pH values of the test solutions wereadjusted by NH3d H2O to fix at 11. The weight loss increaseslinearly with the immersion time increasing of magnesiumalloys in the Na2CO3, Na2B4O7, and CH3COONa testsolutions. It is revealed in Fig. 1 that the corrosion rateswere constant throughout the examined immersion time.As recognized from the slope of each solid line in Fig. 1,corrosion rate in the test solution containing Na2CO3buffer is the lowest among the three tested buffers. Theobtained slopes are 0.015, 0.022 and 0.056 mg cm_2min_1 for Na2CO3, Na2B4O7 and CH3COONa buffers,respectively. These results can be explained in terms ofdissociation constants of the corresponding acids, whichare k 2=4.7_10_11 ( k 1=4.4_10_7 ) , k 2=1_10_9(k1=1_10_4), and k=1.75_10_5 for H2CO3, H2B4O7 andCH3COOH, respectively. The second dissociation constantof a binary acid decides the buffer capability of the buffer.Obviously, the Na2CO3 buffer has the lowest cost and bestbuffer capability among the tested buffers.Fig. 2 shows the weight loss of the substrates versusimmersion time in the test solutions with pH values at 9, 10and 11, using Na2CO3 as the buffer. Corrosion of thespecimens in non-buffered test solutions with pH values at9, 10 and 11 was also investigated. The correspondingweight loss curves are shown in Fig. 2. All test solutionsused for these experiments had compositions similar tothose in the plating bath except that sodium hypophosphitewas not included. The weight loss linearly changes with theincrease of the immersion time in all cases shown in Fig. 2.Under the same pH value, the corrosion rate of thesubstrates in the buffer solution is obviously lower thanthat of the substrates in the non-buffered solution, as shownby the slopes of the curves in Fig. 2. This suggests that thebuffer solution has a considerable effect on the corrosionrate of magnesium alloy. In both the Na2CO3 buffered andnon-buffered test solutions, the corrosion rates of magnesiumalloy decrease with the increase of the pH value. Thisindicates the weight-loss of the substrates is related to thereaction between the substrate metal and the hydrogen ions.But the corrosion reaction between the substrate metal andthe hydrogen ions goes gradually on, because the lowconcentration of hydrogen ions is presented in the platingalkaline solutions. And then, the concentration of hydrogenions is weakly decreased during the test progress. This leadsto the constant corrosion rates in the short test time, which isshown in Figs. 1 and 2. At the same time, knowing that forMg(OH)2 Ksp at 25 8C=8.9_10_12 at pH 9, OH_=10_5 M,most Mg2+ diffused into plating solution to form up to 10_2M. At pH 11, OH_=10_3 M, the Mg2+ couldnt exceed10_6 M, thus most Mg2+ formed Mg(OH)2 and stayed nearthe substrate. Mg(OH)2 could increase the adsorptionenergy barrier and reduce the corrosion rate. Therefore,higher pH resulted in lower corrosion rate. As to theNa2CO3 buffered solutions, for MgCO3 Ksp at 25 8C=10_15,in test solutions, Na2CO3N0.1 M, thus the possibleMg2+b10_14 M. This means that the driving force forMg to form Mg2+ was very low. Instead of dissolving Mg,the CO32_ ion would bond or be adsorbed to the substratesurface to form local MgCO32_. In this case, the substratesurface area exposed to H2O or H+ was reduced a lot,0 5 10 15 20 25 30 35-0.20.00.81.01.8Na(CH3COO)Na2B4O7Na2CO3Weight loss/mg.cm-2Time/minFig. 1. The variation of weight loss of magnesium alloy in test solutionswith different buffers.0 5 10 15 20 25 30 35012345solutionpH=9pH=10pH=11pH=9pH=10pH=11Weight loss/mg.cm-2Time/minin non-buffered solutionin Na2CO3 bufferedFig. 2. The variation of weight loss of magnesium alloy in test solutionswith different pH values.3012 J. Li et al. / Surface & Coatings Technology 200 (2006) 30103015leading to lower corrosion rates. The pKa2 for Na2CO3 is10.33, at pH lower than 10.33 some CO32_ ions formedHCO3_. Reaction Mg+2HCO3_=MgCO3+H2 potentiallyexisted. At pH higher than 10.33, HCO3_ is negligible.Therefore in Fig. 2, we can see that the corrosion rate at pH11 was not reduced as much, compared the rate at 10.H2B4O7 and CH3COOH dont have such advantages.3.2. The effects of plating parameters on coatingsThe coating rate, surface appearance, and adhesion of thecoatings at different concentrations of Na2CO3 buffer arelisted in Table 4. The critical load (LC) was measured underprogressive loading conditions, which can be used toaccurately characterize the adhesion strength of the deposit/substrate system 13. The adhesion between the coatings andsubstrates decreases obviously with the increase of theconcentration of Na2CO3. Surface appearance of the platingcoatings becomes gradually shining with the increase of theNa2CO3 concentration. Grave corrosion of the substrates wasfound in the non-buffered plating bath. The growth rate of thecoatings noticeably increases with the increase of the Na2CO3concentration. Considering the combination of growth rate,surface appearance, and adhesion of the coatings, theoptimum concentration of the Na2CO3 buffer was determinedto be 30 g L_1.With this concentration, the purpose of addingNa2CO3 in plating bath is commendably achieved.In the research, it was found that the pH value of platingbath had a considerable effect on the growth rate and thesurface appearance of the coatings. The hydrogen ions inplating bath were not only astricted by the CO32_ ionsdissociated from the buffer Na2CO3, but linked with the OH_ions. When the pH value of the plating bath was below 8.5,point corrosion or dark gray coatings were obtained and thecoating growth rate was low. When the pH value of theplating bath was above 11.5, the adhesion between coatingsand substrates were deteriorated, although the growth rateand the surface appearance of the coatings were satisfying.During the electroless plating, the pH value of the plating bathwas monitored with a pHS-25C model of precision pH/mVmeter. In this research, the preferred pH range of the platingbath for electroless plating on magnesium alloy is 8.511.5.Table 4Coating rate, surface appearance and adhesion of the coatings obtainedfrom the plating bath with different amounts of Na2CO3Concentration ofNa2CO3 (g L_1)Coating rate(Am/h)Surface appearance LC (N)0 Grave corrosion 10 12.32 Point corrosion 8120 16.41 Dark gray 7630 18.32 Shining 7340 18.91 Shining 6150 19.26 Shining 5120 30 40 50 60 701314151617181920The coating thickness/mThe trisodium citrate dihydrate content/g.L-1Fig. 3. Relationship between the coating thickness and the trisodium citratedihydrate concentration.30 40 50 6010002000300040005000600070008000Intensity2 /( )Fig. 4. XRD patterns of the electroless plating coating.Fig. 5. Surface morphology of a plating coating.J. Li et al. / Surface & Coatings Technology 200 (2006) 30103015 3013Fig. 3 shows the variation of coating thickness onmagnesium alloy at same plating time as a function of thetrisodium citrate dihydrate concentration at constant temperatureand pH. The coating thickness decreases with theincrease of the trisodium citrate dihydrate concentration.According to De Minjer and Brenners explanation 11, atlow concentrations the low adsorption of ligand on thecatalytic surface of the substrate accelerates the platingreaction. At higher concentration, there is a high adsorptionof ligand on the surface, which slows down the platingreaction. But when the concentration was below 20 g L_1,the plating bath became destabilized and nickel precipitatewas observed.3.3. Properties of the plating coatings from nickel sulfateThe coating obtained under optimized bath compositionwas probably preferentially crystallized (see Fig. 4). The onlyand strong diffraction observed in the XRD spectrumcorresponds to the (111) peak of nickel. Fig. 5 shows thesurface morphology of the plating coating. The surface isoptically smooth and of low porosity. No obvious surfacedamage was observed. The compositions of the platingcoating were determined to be 5.39 wt.% P and 94.61 wt.%Ni by energy dispersive X-ray spectroscopy. Fig. 6 shows thecross section of an electroless plating coating. The coatinghas a good adhesion to the substrate and no cracks or holeswere observed.Fig. 7 shows the curve of the NiP coating free corrosionpotential with time. After the sample was immersed in 3.5wt.% NaCl solution at room temperature for 2 h, the freecorrosion potential of the coated magnesium alloyapproached to about _0.4 V. The steady-state workingpotential of magnesium electrode is generally about _1.50V, although its standard potential is _2.43 V 14. Thisindicates the improved corrosion resistance of the platingcoatings prepared in this research, compared with the barealloy.The adhesion between the coatings and the substrateswas evaluated by means of quenching and the scratch test.The plated specimens were heated at a temperature of 2508CF10 8C for 1 h, and then quenched in the cold water. Thisprocess was repeated for 20 times on each specimen. Nodiscoloration, cracks, blisters, or peeling was observed 12.For the scratch test, the critical load (LC) of 73 N was foundfor the coatings obtained in the optimized bath compositionand parameters. These results suggest the excellent adhesionof the plating coating to the substrate.3.4. Proposed mechanism of the electroless plating nickelEven under the same pH value, the magnesium alloyexhibits better corrosion resistance in the Na2CO3 bufferedplating solution than in the non-buffered plating solution.Fig. 6. Cross section view of an electroless plating coating.0 1 2 3 4 5 6 7 8-0.46-0.44-0.42-0.40-0.38-0.36-0.34-0.32-0.30ESCE/V 103, time/sFig. 7. Curve of the NiP coating free corrosion potential with time.3014 J. Li et al. / Surface & Coatings Technology 200 (2006) 30103015Fig. 8 gives a simple model to explain this phenomenon.Large amount of H2 gas is produced in the electrolessplating process. Most of the H+ ions are taken out by the H2gas bubbles and combine with the CO32_, to form HCO3_.Therefore, a very thin layer of dilute H+ solution is formednear the surface of substrate. The Ni2+ ions react with themagnesium atoms to form the autocatalysis nickel, whichleads to the deposition of the NiP coating. If theconcentration of the CO32_ ions is low, more H+ ions willbe free and erode the thin NiP coating and the substrate. Ifthe concentration of the CO32_ ions is much higher, the H+ions concentration in the thin dilute H+ solution layer nearthe substrate surface will be much lower. Therefore almostno corrosion process will exist in the interface between theNiP coating and the substrate,
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