金相試樣切割機(jī)的設(shè)計(jì)

金相試樣切割機(jī)的設(shè)計(jì),金相,試樣,切割機(jī),設(shè)計(jì) 河南科技學(xué)院 2009屆本科畢業(yè)設(shè)計(jì)（論文）,設(shè)計(jì)題目：金相試樣切割機(jī)的設(shè)計(jì) 學(xué)生姓名： 張靜 所在院系： 機(jī)電學(xué)院 所學(xué)專(zhuān)業(yè)： 機(jī)電技術(shù)教育 導(dǎo)師姓名： 劉貫軍,【摘 要】： 金相試樣切割機(jī)主要用于金相試樣的截取和各種材料的下料、切口等，在冶金、汽車(chē)、航空航天等制造業(yè)中應(yīng)用極為廣泛。20世紀(jì)90年代后，金相制樣技術(shù)發(fā)展極為迅速，金相試樣切割機(jī)作為金相取樣設(shè)備也取得了很大的進(jìn)步。 本設(shè)計(jì)通過(guò)對(duì)金相試樣切割機(jī)的整體造型、機(jī)械結(jié)構(gòu)和控制系統(tǒng)進(jìn)行了分析，完成了切割機(jī)主體結(jié)構(gòu)的設(shè)計(jì)，控制系統(tǒng)采用了銑床導(dǎo)軌原理，實(shí)現(xiàn)了低成本和手動(dòng)化。 最后確定切割機(jī)的裝配總圖。通過(guò)此次設(shè)計(jì)，掌握了相關(guān)設(shè)計(jì)的主要步驟，并對(duì)于ProE軟件應(yīng)用方面有了進(jìn)一步的提高。,1 引言 金屬零件的力學(xué)性能不僅與它的化學(xué)成分有關(guān)，也與它的金相組織密切有關(guān)。金相檢驗(yàn)是控制和評(píng)定產(chǎn)品質(zhì)量不可缺少的重要手段，是科學(xué)研究中研究新材料、新工藝和提高金屬制品內(nèi)在質(zhì)量的重要方法。 要進(jìn)行金相分析，就必須制備能用于微觀檢驗(yàn)的樣品 金相試樣。通常，金相試樣的制備要經(jīng)過(guò)取樣、鑲嵌、磨光和拋光幾個(gè)步驟。每個(gè)步驟都應(yīng)該細(xì)心操作，因?yàn)槿魏坞A段上的失誤都可能影響最后的結(jié)果，因?yàn)檫@可能會(huì)造成組織假象，從而得出錯(cuò)誤的結(jié)論。金相試樣的制備是通過(guò)切割機(jī)、鑲嵌機(jī)、磨拋光機(jī)來(lái)完成。金相試樣的截取是金相試樣制備過(guò)程中一個(gè)重要環(huán)節(jié)。截取試樣的方法有手鋸、鋸床、砂輪切割機(jī)和線(xiàn)切割機(jī)等等。根據(jù)零件的形狀和材料，選擇適當(dāng)?shù)姆椒▉?lái)切割。,目前砂輪切割機(jī)廣泛應(yīng)用于金相試樣的截取上，主要原因是其適應(yīng)性強(qiáng)，樹(shù)脂砂輪片可切割軟的金屬零件如銅、鋁及合金和硬的金屬零件如淬火后的碳鋼、高速鋼；金剛石切割機(jī)可切割超硬材料如硬質(zhì)合金、陶瓷等。另外其切割速度快、勞動(dòng)強(qiáng)度低、操作簡(jiǎn)便和切割成本低。選擇可靠性高的金相試樣切割機(jī)，可以提高制樣效率和質(zhì)量，降低成本，提高經(jīng)濟(jì)效益。,金相試樣切割機(jī)主要特點(diǎn)：本切割機(jī)的切割砂輪直接固定在與電動(dòng)機(jī)的軸同軸線(xiàn)相連接的軸上，利用滑板箱的橫向和縱向的移動(dòng)來(lái)切割固定在鉗口中的試樣 電動(dòng)機(jī)固定在底座上，軸套套在電動(dòng)機(jī)的軸上，砂輪片由螺母和夾片加以固定。固定在電動(dòng)機(jī)的前面的滑板箱上裝有可沿縱向移動(dòng)的加緊裝置，由手柄的轉(zhuǎn)動(dòng)來(lái)移動(dòng)鉗口把試樣夾緊在鉗座中，當(dāng)轉(zhuǎn)動(dòng)手柄時(shí)，就可以進(jìn)行試樣切割了。機(jī)器工作時(shí)，由罩殼將砂輪片等檔住，以防冷卻液飛濺和砂輪片碎裂時(shí)飛出傷人,2 設(shè)計(jì)要求 金相試樣切割機(jī)的具體設(shè)計(jì)要求為： （1）利用Pro-E軟件設(shè)計(jì) （2）確定結(jié)構(gòu)的尺寸 （3）繪制相應(yīng)的零件圖、實(shí)體圖及總裝配圖,3 切割機(jī)的總體設(shè)計(jì)過(guò)程 3.1 電動(dòng)機(jī)的選擇,3.2 傳動(dòng)機(jī)構(gòu)的設(shè)計(jì) 3.2.1 軸的計(jì)算 3.2.2 軸的結(jié)構(gòu)設(shè)計(jì),3.3 控制系統(tǒng)的設(shè)計(jì) 3.3.1夾具的主要結(jié)構(gòu)與使用,3.3.2 進(jìn)給機(jī)構(gòu)的設(shè)計(jì),上 滑 板,中 滑 板,下 滑 板,進(jìn) 給 裝 置,4 用ProE軟件對(duì)切割機(jī)進(jìn)行實(shí) 體造型和裝配 4.1 切割機(jī)各主要零件的實(shí)體造型,軸 的 實(shí) 體 圖,上 滑 板 的 實(shí) 體 圖,中 滑 板 的 實(shí) 體 圖,下 滑 板 的 實(shí)體 圖,進(jìn) 給 系 統(tǒng) 的 實(shí)體 圖,4. 2 切割機(jī)的裝配,切 割 機(jī) 的 內(nèi) 部 實(shí) 體 圖,5 結(jié)束語(yǔ) 在此次設(shè)計(jì)的過(guò)程中，培養(yǎng)了我的綜合運(yùn)用所學(xué)知識(shí)的能力，分析和解決實(shí)際中所遇到問(wèn)題的能力，并且能鞏固和深化我所學(xué)的專(zhuān)業(yè)知識(shí)，使我在調(diào)查研究和收集資料等方面有了顯著的提高，同時(shí)在理解分析能力、制定設(shè)計(jì)計(jì)算和繪圖能力方面有較大的進(jìn)步；另外我的技術(shù)分析和組織工作的能力也有一定程度的提高。,致謝 非常感謝學(xué)院領(lǐng)導(dǎo)和老師給我提供了這次良好的深入學(xué)習(xí)的機(jī)會(huì)和寬松的學(xué)習(xí)環(huán)境。通過(guò)這次畢業(yè)設(shè)計(jì)，不但使我將大學(xué)期間所學(xué)的專(zhuān)業(yè)知識(shí)再次回顧學(xué)習(xí)，而且也使我學(xué)到了專(zhuān)業(yè)領(lǐng)域中一些前沿的知識(shí)。非常感謝在本次設(shè)計(jì)中曾給予我耐心指導(dǎo)和親切關(guān)懷的老師及幫助過(guò)我的同學(xué)，正是由于他們的幫助和鼓勵(lì)才使我能夠在畢業(yè)設(shè)計(jì)過(guò)程中克服種種困難，最終順利完成論文，他們的學(xué)識(shí)和為人也深深地影響著我。在此，請(qǐng)?jiān)试S我再次向曾直接給予我多次指導(dǎo)的導(dǎo)師表示最忠誠(chéng)的敬意！同時(shí)也感謝百忙之中前來(lái)參加答辯的各位老師、專(zhuān)家和教授!,敬請(qǐng)指導(dǎo)批正!,謝謝! 答辯人：張靜,河南科技學(xué)院本科生畢業(yè)論文（設(shè)計(jì)）課題審核表院（系）名稱(chēng)機(jī)電學(xué)院專(zhuān)業(yè)名稱(chēng)機(jī)電技術(shù)教育042指導(dǎo)教師姓名劉貫軍課題名稱(chēng)金相試樣切割機(jī)設(shè)計(jì)課題來(lái)源自擬課題立題理由和所具備的條件 金相試樣切割機(jī)種類(lèi)很多，但適合本院專(zhuān)業(yè)實(shí)驗(yàn)室條件的用于制做透射電鏡樣品精密切割的切割機(jī)卻很少見(jiàn)，國(guó)外有符合要求的此類(lèi)產(chǎn)品，但價(jià)格昂貴。隨著科研工作的深入，設(shè)計(jì)制做一種高精度低成本的金相試樣切割機(jī)很有必要，而且設(shè)計(jì)條件已經(jīng)具備。教研室審批意見(jiàn)教研室主任簽字： 年 月 日畢業(yè)論文（設(shè)計(jì)）工作領(lǐng)導(dǎo)小組審批意見(jiàn)組長(zhǎng)簽字： 年 月 日注：本表存院（系）備查。學(xué)生姓名張靜班級(jí)機(jī)教042指導(dǎo)教師劉貫軍論文（設(shè)計(jì)）題目金相試樣切割機(jī)的設(shè)計(jì)目前已完成任務(wù)1.制定畢業(yè)設(shè)計(jì)計(jì)劃2.查找相關(guān)文獻(xiàn)3.完成畢業(yè)論文開(kāi)題報(bào)告是否符合任務(wù)書(shū)要求進(jìn)度：符合尚需完成的任務(wù)1.繼續(xù)對(duì)論文材料進(jìn)行組織和整理；2.按照論文提綱，有步驟有計(jì)劃的開(kāi)展論文工作，存在問(wèn)題要及時(shí)與老師溝通；3.對(duì)已完成的論文內(nèi)容進(jìn)行檢查審核，力求把問(wèn)題降到最少；4.到規(guī)定的時(shí)間完成論文初稿；5.根據(jù)指導(dǎo)老師的指導(dǎo)意見(jiàn)和全部材料完成論文；能否按期完成論文（設(shè)計(jì)）：能存在問(wèn)題和解決辦法存在問(wèn)題閱讀資料不足，對(duì)論文主題的研究不夠透徹，且相關(guān)的理論知識(shí)還不夠全面；與指導(dǎo)老師的交流不夠充分。擬采取的辦法繼續(xù)查找資料，加強(qiáng)對(duì)相關(guān)理論知識(shí)的理解和掌握，應(yīng)多和老師交流，在老師的指導(dǎo)下更好完成設(shè)計(jì)。指導(dǎo)教師簽 字日期 年 月 日教學(xué)院長(zhǎng)（系主任）意 見(jiàn) 簽字： 年 月 日河南科技學(xué)院本科畢業(yè)論文（設(shè)計(jì)）中期進(jìn)展情況檢查表河南科技學(xué)院本科生畢業(yè)論文（設(shè)計(jì)）任務(wù)書(shū)題目名稱(chēng) 金相試樣切割機(jī)的設(shè)計(jì)學(xué)生姓名張靜所學(xué)專(zhuān)業(yè)機(jī)教學(xué)號(hào) 20040315049指導(dǎo)教師姓名 劉貫軍所學(xué)專(zhuān)業(yè) 機(jī)械設(shè)計(jì)職稱(chēng) 教授完成期限2008 年 11 月 01 日 至 2009 年 05 月 24 日一、論文（設(shè)計(jì)）主要內(nèi)容及主要技術(shù)指標(biāo) 1、連接金相試樣切割機(jī)的主要用途，國(guó)內(nèi)外研究及使用狀況（包括選擇國(guó)內(nèi)市場(chǎng)上此類(lèi)產(chǎn)品的性能及不足）； 2、研究制定設(shè)計(jì)方案； 3、對(duì)受力構(gòu)件進(jìn)行受力分析并有必要計(jì)算后方可進(jìn)行設(shè)計(jì)制圖；二、 畢業(yè)論文（設(shè)計(jì)）的基本要求1、 通過(guò)互聯(lián)網(wǎng)、校內(nèi)期刊數(shù)據(jù)庫(kù)等途徑了解切割機(jī)的工作原理、分析存在問(wèn)題，提出改進(jìn)方案；2、 學(xué)習(xí)并熟練使用ProE繪圖軟件，并用其進(jìn)行零件和產(chǎn)品設(shè)計(jì)（重要部件應(yīng)有受力分析），提交任務(wù)內(nèi)的全部零件圖及部件總成圖。3、 完成不少于2000字（單詞）的專(zhuān)業(yè)英文資料翻譯。三、畢業(yè)論文（設(shè)計(jì)）進(jìn)度安排 2008年11月1日12月30日 查找相關(guān)專(zhuān)業(yè)資料，熟悉ProE繪圖軟件的使用，提交開(kāi)題報(bào)告，論證設(shè)計(jì)方案、完成不少于2000單詞英文資料翻譯稿。 2009年2月16日5月16日 基本完成畢業(yè)設(shè)計(jì)規(guī)定的繪圖任務(wù)。 2009年5月17日5月24日 撰寫(xiě)畢業(yè)論文（設(shè)計(jì)說(shuō)明書(shū)）。 2009年5月24日交齊全部畢業(yè)設(shè)計(jì)資料。 畢業(yè)設(shè)計(jì)（論文）開(kāi)題報(bào)告題目名稱(chēng)： 金相試樣切割機(jī)學(xué)生姓名張靜專(zhuān)業(yè)機(jī)電技術(shù)教育班級(jí)042一、選題的目的意義 目前,正處在科學(xué)技術(shù)飛速發(fā)展的信息時(shí)代,自動(dòng)化、最優(yōu)化、集成化、智能化和精密化等使現(xiàn)代機(jī)械制造行業(yè)正經(jīng)歷著巨大的變化,也是其今后發(fā)展的必然趨勢(shì).金相取樣設(shè)備作為其中一個(gè)重要分支,正在由原來(lái)的手工操作逐漸走向半自動(dòng)化和自動(dòng)化.為此,我設(shè)計(jì)了對(duì)金相切割的半自動(dòng)化控制系統(tǒng).與此同時(shí)在設(shè)計(jì)的過(guò)程中，能培養(yǎng)我綜合運(yùn)用所學(xué)知識(shí)，分析和解決實(shí)際中所遇到的問(wèn)題，并且能鞏固和深化我所學(xué)的專(zhuān)業(yè)知識(shí)，使我在調(diào)查研究和收集資料等方面有了顯著的提高，同時(shí)在理解分析能力、制定設(shè)計(jì)或試驗(yàn)方案能力、設(shè)計(jì)計(jì)算和繪圖能力方面有較大的進(jìn)步；另外我的技術(shù)分析和組織工作的能力也有一定程度的提高。希望在此次畢業(yè)設(shè)計(jì)中，充分發(fā)揮出我們的創(chuàng)新能力，樹(shù)立良好的學(xué)術(shù)思想和工作作風(fēng)，牢牢把握住這次走上崗位之前的實(shí)踐機(jī)會(huì)，充分鍛煉出自己的工作能力。二、國(guó)內(nèi)外研究綜述 金屬零件的力學(xué)性能不僅與它的化學(xué)成分有關(guān)，也與它的金相組織密切有關(guān)。金相檢驗(yàn)是控制和評(píng)定產(chǎn)品質(zhì)量不可缺少的重要手段，是科學(xué)研究中研究新材料、新工藝和提高金屬制品內(nèi)在質(zhì)量的重要方法。要進(jìn)行金相分析，就必須制備能用于微觀檢驗(yàn)的樣品 金相試樣。通常，金相試樣的制備要經(jīng)過(guò)取樣、鑲嵌、磨光和拋光幾個(gè)步驟。每個(gè)步驟都應(yīng)該細(xì)心操作，因?yàn)槿魏坞A段上的失誤都可能影響最后的結(jié)果，因?yàn)檫@可能會(huì)造成組織假象，從而得出錯(cuò)誤的結(jié)論。金相試樣的制備是通過(guò)切割機(jī)、鑲嵌機(jī)、磨拋光機(jī)來(lái)完成。金相試樣的截取是金相試樣制備過(guò)程中一個(gè)重要環(huán)節(jié)。截取試樣的方法有手鋸、鋸床、砂輪切割機(jī)和線(xiàn)切割機(jī)等等。根據(jù)零件的形狀和材料，選擇適當(dāng)?shù)姆椒▉?lái)切割。目前砂輪切割機(jī)廣泛應(yīng)用于金相試樣的截取上，主要原因是其適應(yīng)性強(qiáng)，樹(shù)脂砂輪片可切割軟的金屬零件如銅、鋁及合金和硬的金屬零件如淬火后的碳鋼、高速鋼；金剛石切割機(jī)可切割超硬材料如硬質(zhì)合金、陶瓷等。另外其切割速度快、勞動(dòng)強(qiáng)度低、操作簡(jiǎn)便和切割成本低。選擇可靠性高的金相試樣切割機(jī)，可以提高制樣效率和質(zhì)量，降低成本，提高經(jīng)濟(jì)效益。金相試樣切割機(jī)主要特點(diǎn)：本切割機(jī)的切割砂輪直接固定在與電動(dòng)機(jī)的軸同軸線(xiàn)相連接的軸上，利用滑板箱的橫向和縱向的移動(dòng)來(lái)切割固定在鉗口中的試樣 電動(dòng)機(jī)固定在底座上，軸套套在電動(dòng)機(jī)的軸上，砂輪片由螺母和夾片加以固定。固定在電動(dòng)機(jī)的前面的滑板箱上裝有可沿縱向移動(dòng)的加緊裝置，由手柄的轉(zhuǎn)動(dòng)來(lái)移動(dòng)鉗口把試樣夾緊在鉗座中，當(dāng)轉(zhuǎn)動(dòng)手柄時(shí)，就可以進(jìn)行試樣切割了。機(jī)器工作時(shí)，由罩殼將砂輪片等檔住，以防冷卻液飛濺和砂輪片碎裂時(shí)飛出傷人。三、畢業(yè)設(shè)計(jì)主要研究?jī)?nèi)容 1、研究切割機(jī)的切割原理；2、利用Pre-E軟件繪制切割機(jī)模型；3、繪制相應(yīng)的零件圖及總裝配圖四、畢業(yè)設(shè)計(jì)（論文）的研究方法和技術(shù)路線(xiàn) 1采用理論和實(shí)際操作相結(jié)合的方式再結(jié)合現(xiàn)代設(shè)計(jì)理念的基礎(chǔ)上，利用現(xiàn)有的條件來(lái)進(jìn)研究。2結(jié)合指導(dǎo)教師的教學(xué)經(jīng)驗(yàn)來(lái)重新完善和提高自己新的認(rèn)識(shí)和研究。3大量查閱有關(guān)書(shū)籍和資料來(lái)擴(kuò)充自己視野與認(rèn)識(shí)，提高理論成果的技術(shù)含量。4充分利用互連網(wǎng)來(lái)查找最新技術(shù)成果，提高自身的創(chuàng)新意識(shí)。 五、主要參考文獻(xiàn)與資料獲得情況 1成大先。機(jī)械設(shè)計(jì)手冊(cè)。北京：化學(xué)工業(yè)出版社，2004 2成大先。機(jī)械設(shè)計(jì)手冊(cè) 第四版。北京：化學(xué)工業(yè)出版社，2002 3毛謙德,李振清。袖珍機(jī)械設(shè)計(jì)手冊(cè) 第三版。北京：機(jī)械工業(yè)出版社，2007 4機(jī)械設(shè)計(jì)實(shí)用手冊(cè)編委會(huì)。 機(jī)械設(shè)計(jì)實(shí)用手冊(cè) 。北京：機(jī)械工業(yè)出版社，20085陳立德。 機(jī)械設(shè)計(jì)基礎(chǔ)課程設(shè)計(jì)。北京：高等教育出版社，20066濮良貴，紀(jì)名剛。機(jī)械設(shè)計(jì) 第八版。北京：高等教育出版社，20077朱金波。ProE 3.0 工業(yè)產(chǎn)品設(shè)計(jì)完全掌握。北京：兵器工業(yè)出版社，20078金鑫，陳雪梅，賈長(zhǎng)治。ProE 3.0中文版機(jī)械設(shè)計(jì)專(zhuān)家指導(dǎo)教程。 北京：機(jī)械工業(yè)出版社，20079 曹巖。ProE 3.0 機(jī)械設(shè)計(jì)實(shí)例精解。北京：機(jī)械工業(yè)出版社，200710朱文堅(jiān)，黃平，吳昌林。機(jī)械設(shè)計(jì)。北京：機(jī)械工業(yè)出版社，200511朱龍根。機(jī)械設(shè)計(jì)。北京：機(jī)械工業(yè)出版社，200612吳克堅(jiān)，于曉紅，錢(qián)瑞明。機(jī)械設(shè)計(jì)。北京：高等教育出版社，2003六、指導(dǎo)教師審批意見(jiàn) 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: mengsuo66163.com (J. Li).Surface & Coatings Technology 200 (2006) 3010 3015www.elsevier.com/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 35-0.20.00.20.40.60.81.01.21.41.61.8Na(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 su