【機(jī)械類畢業(yè)論文中英文對(duì)照文獻(xiàn)翻譯】傳統(tǒng)涂料和現(xiàn)代涂料之間摩擦性質(zhì)的對(duì)比,機(jī)械類畢業(yè)論文中英文對(duì)照文獻(xiàn)翻譯,機(jī)械類,畢業(yè)論文,中英文,對(duì)照,對(duì)比,比照,文獻(xiàn),翻譯,傳統(tǒng),涂料,以及,現(xiàn)代,之間,摩擦,磨擦,性質(zhì),比較 一、 傳統(tǒng)涂料和現(xiàn)代涂料之間摩擦性質(zhì)的對(duì)比 畢章 劉小兵 鄧朝暉 和建盟都是美國CTO6292機(jī)械工業(yè)部門和各大學(xué)的領(lǐng)導(dǎo)。 電郵：zhang@enger.ucom.edu 簡介： 這篇文章對(duì)比了傳統(tǒng)材料在固定的摩擦力和空間摩擦力作用下的表面摩擦性質(zhì)。材料成型機(jī)械是和微結(jié)構(gòu)理論（材料的粒度）相聯(lián)系的。材料粒度的減少對(duì)變形材料的摩擦性質(zhì)的影響正在研究之中。 1.說明 陶瓷制品的摩擦過程影響已經(jīng)在深入的研究之中。例如:Kirchner 和Brinksmeier,1998Blake,1998)摩擦對(duì)材料微型結(jié)構(gòu)的影響同等重要。材料的微型結(jié)構(gòu)就是粒度，它將影響機(jī)械性能。例如：硬度和韌性。所以也將影響到陶瓷的摩擦性質(zhì)，在這方面幾乎還未取得研究上的突破。 Both和Tonshoff在1993年研究了不同粒度的鋁在滑動(dòng)摩擦和靜摩擦方面的摩擦性質(zhì)。對(duì)n-Al2O3/13TiO2和n-WC/12Co兩種涂料來說，其硬度和韌性比同等或粒度大的其他材料來說要髙的多。在塑性變形條件下，材料的硬度是不變的。由于固有位置的移動(dòng)，塑性變形將會(huì)被削弱。粒度越大，越會(huì)對(duì)材料的移動(dòng)和塑性變形的束縛。 和傳統(tǒng)材料不同的是，材料硬度的降低并不會(huì)導(dǎo)致材料韌度的降低。（由于更大的纖維化，更大的流動(dòng)壓力和更高的撓曲力）（Jia,1998)對(duì)傳統(tǒng)材料和現(xiàn)代材料來說，硬度和韌度的差別是涂料的摩擦性質(zhì)得的影響。大量的砂眼，裂紋和微裂紋是由于膨脹過程對(duì)材料中各成分的比率產(chǎn)生了很大的影響。對(duì)傳統(tǒng)材料和現(xiàn)代材料來說，硬度和韌度的不同并不僅僅是由于體積的不同。表1說明了傳統(tǒng)材料和現(xiàn)代材料（Wc/12Co和Al2O3/13TiO2）兩種涂料的成分比率。 一般來說，摩擦性質(zhì)可根據(jù)材料的滑移率，材料所承受的摩擦力，樣本表面的結(jié)合率作 出預(yù)測。在這篇報(bào)告中，傳統(tǒng)的摩擦力，空間摩擦能是作為傳統(tǒng)材料和現(xiàn)代材料對(duì)照。除此之外，微電子掃描技術(shù)被用來預(yù)料粒度對(duì)材料滑移機(jī)械的影響。 表1：傳統(tǒng)材料和現(xiàn)代材料（Wc/12Co和Al2O3/13TiO2）的百分比 C-Wc/12Co n-Wc/12Co C-Al2O3/13TiO2 n-Al2O3/13TiO2 邊界力：82.7 89.6 15.5 20.7 （MPa） 粒度：1.3 0.04 2.5 0.05 （um） 物體密度：14.2 14.5 3.5－4.0 3.7－4.1 （g/cm3） 維氏硬度：12.00 12.50 10.44 10.57 （GPa） 韌度：1/2 16.0 16.5 3.3 3.5 (MPa/m) 2結(jié)構(gòu)試驗(yàn) 2.1預(yù)加工特征圖表 傳統(tǒng)材料和現(xiàn)代材料都是由低碳鋼制成，其晶格大小為25×75×4mm3，晶格在熱膨脹之前將會(huì)爆炸。傳統(tǒng)材料和現(xiàn)代材料（Wc/12Co）是用高壓氧流的方法生產(chǎn)的。傳統(tǒng)材料和現(xiàn)代材料（Al2O3/13TiO2）是用等離子下熱膨脹的方法預(yù)制的。所有的涂層都有大約0.5mm厚的硬殼。材料樣本被削減為25×4×4mm3。 表1說明了用微電子掃描技術(shù)對(duì)c/n-Wc/12Co涂料的觀察結(jié)果。表1（b）揭示了小粒度的Wc在材料鈷的邊界結(jié)合在一邊。用微電子掃描技術(shù)在傳統(tǒng)材料和現(xiàn)代材料（Wc/12Co）中可觀察到大量的裂紋。 表2說明了在熱膨脹作用下，傳統(tǒng)材料和現(xiàn)代材料（Al2O3/13TiO2）的典型表面 特征：孔隙，裂紋，微裂紋和與材料微裂紋垂直相交的部分結(jié)構(gòu)。在現(xiàn)有的摩擦測試以前，為了降低材料的毀壞，外表是帶有15um的金鋼粒的砂輪。盡管這種準(zhǔn)備過程在熱膨脹的影響下非常有效，但要浪費(fèi)時(shí)間和精力。 2.2摩擦試驗(yàn) 摩擦試驗(yàn)是用計(jì)算機(jī)對(duì)精密摩擦機(jī)械（Dover Model 956-S）進(jìn)行了大量的控制而完成的。這臺(tái)機(jī)器在它的測量軸及X Y Z 坐標(biāo)方向上都有空間靜止齒輪。測量軸在軸向由0.05um的竄動(dòng)，三坐標(biāo)軸向上有0.1um-25mm的直線度誤差。機(jī)器上安裝的激光干涉儀能夠?qū)x Y Z軸向上0.07um的回路誤差進(jìn)行反饋。機(jī)器上這種回路的剛度為50N/um。 在此項(xiàng)研究中，用金剛摩擦輪（5D 600N 100V）在不同的環(huán)境下（和樣本比較而言）摩擦涂料。輪速設(shè)置為33m/s或3500r/min。為了預(yù)測在殘余應(yīng)力作用下材料滑移率的影響，切深設(shè)置為2，5，15，30 um補(bǔ)償率為1，4，8mm/s(在摩擦試驗(yàn)中)用冷卻液作為水系統(tǒng)。 2.3后摩擦預(yù)測 輪廓曲線用于測量材料在摩擦方向上的成型表面微電子探測儀（JOEL Model Jsm840）用于觀察材料表面。微電子探測儀的觀察部件能夠在材缺陷中區(qū)分出摩擦損壞。 大量的材料缺陷，例如：砂眼，熔融粒子，裂紋和微觀裂紋，在飛濺中都能被檢測出來。 因?yàn)槠渲械囊恍┤毕莺苋菀妆徽`認(rèn)為摩擦毀壞，所以微電子探測儀能夠在飛濺材料的檢測中探測出這些缺陷。熱膨脹過程中的毛孔一般呈現(xiàn)光滑的邊緣。表1和表2中，飛濺材料的裂紋和微裂紋彼此相連。依據(jù)摩擦材料，摩擦破壞將能被檢測出來。 3研究成果與發(fā)現(xiàn) 3.1普通摩擦力的對(duì)比 普通摩擦力在表征摩擦過程中非常重要。表3對(duì)c/n-Al2O3/13TiO2和c/n-Wc/12Co在相同摩擦環(huán)境下作用的普通摩擦力進(jìn)行了對(duì)比。對(duì)n-Al2O3/13TiO2來說，普通摩擦力要高于傳統(tǒng)的配對(duì)物。 人們也觀察到：對(duì)n-Al2O3/13TiO2來說，抱剎力要大的多。它表明了：n-Al2O3/13TiO2和傳統(tǒng)的配對(duì)物相比增加了機(jī)械性能（硬度和韌度），所以n-Al2O3/13TiO2得到廣泛的應(yīng)用。據(jù)觀測，摩擦中c/n-Al2O3/13TiO2具有類似的趨勢：在大的切深下，摩擦力對(duì)傳統(tǒng)材料和現(xiàn)代材料的不同影響變得非常小。這也表明在非常低的材料滑移率下，材料粒度對(duì)摩擦力的影響非常大。當(dāng)切深或材料滑移率增大時(shí)，對(duì)摩擦過程干擾儀來說，切深的影響成為次要因素。 由于在摩擦?xí)r大的負(fù)前角的從存在，切向摩擦力比普通摩擦力小的多。摩擦力如下：（a）c-Wc/12Co (b) n-Wc/12Co (a) c-AL2O3/13TiO2 (b) n-Al2O3/13TiO2 表2：微電子探測儀觀測到了飛濺的c/n-AL2O3/13TiO2。 表1：微電子探測儀觀測到了飛濺的c/n-Wc/12Co。 222222000000－mm 1200mm 表3表明了對(duì)切向摩擦力Ft來說，普通摩擦力的相對(duì)數(shù)量級(jí)為Fn，并被定義為： t n F F L=(1) 表4說明了摩擦力率和切深。摩擦力率對(duì)c/n-AL2O3/13TiO2來說要高于c/n-Wc/12Co 。c/n-AL2O3/13TiO2比c/n-Wc/12Co要硬的多。在同樣的摩擦條件下，脆性特征要比c/n-AL2O3/13TiO2明顯的多。 c/n-Wc/12Co在摩擦?xí)r，大量的可逆流導(dǎo)致切向摩擦力相對(duì)高的多，所以導(dǎo)致摩擦力率降低。據(jù)觀測：當(dāng)摩擦力率對(duì)n-AL2O3/13TiO2和n-AL2O3/13TiO2的不同不重要時(shí)，n-AL2O3/13TiO2的摩擦力率和c-AL2O3/13TiO2的摩擦力率有明顯的不同。 隨著材料的滑移率或切深降低時(shí)，這四種材料的摩擦力率也相應(yīng)的降低。另一方面，這四種材料的摩擦力率相對(duì)很窄，這說明了材料滑移機(jī)相對(duì)于給定的切深范圍并不能變動(dòng)太大?？臻g摩擦能U被定義為去除單位材料所需的能量?？臻g摩擦能是由切向摩擦能推倒出的。 F tc wdv Fv u=(2) Vc指摩擦速度，W指工件寬度，d指切深 vf指反饋率 表5表明了粒度對(duì)空間摩擦能的影響以及空間摩擦能隨著切深的變化。（a）傳統(tǒng)材料和現(xiàn)代材料AL2O3/13TiO2（b）傳統(tǒng)材料和現(xiàn)代材料Wc/12Co表3對(duì)作用了傳統(tǒng)材料和普通材料上的普通摩擦力做了對(duì)比。 0 10 20 30 0 2 4 6 8 Depth of cut, m Normal grinding force, N/mm2 Nano. Conv. Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 0 10 20 30 0 2 4 6 8 Depth of cut, m Normal grinding force, N/mm2 Nano. Conv. Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V Fig. 4 Comparison of grinding force ratio. 00 10 20 30 3 6 9 12 Depth of cut, m Grinding force ratio, l n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V Fig. 5 Comparison of specific grinding energy. 0 10 20 30 0.0 0.5 1.0 1.5 2.0 2.5 Depth of cut, m Specific grinding energy, 103 J/mm3 n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 4. 四種材料的摩擦能隨著切深漸進(jìn)的達(dá)到極限。在小的切深處，空間摩擦能非常高，這暗示了一部分摩擦能是和構(gòu)件外形有關(guān)（Malkin,1989）一般來說，一部分摩擦能是由構(gòu)件成型時(shí)儲(chǔ)存的能量upl,切削能usl ch pl sl 組成。即U=U+U+U (3)除了uch，空間摩擦能的其他部分來自在小的切深下工件和磨粒的滑動(dòng)和切削。 在比較大的切深下，滑動(dòng)變得并不重要，構(gòu)件成型也很普遍。然而，切削仍然存在，并影響到摩擦和材料表面。僅僅uch完全用于材料滑移和形成新的表面。理論上，表5中漸進(jìn)極限是uch對(duì)c/n-AL2O3/13TiO2進(jìn)行相對(duì)平坦的切割。由于材料的易脆性，切削能并不多。對(duì)現(xiàn)代材料來說，小的粒度似乎增大了材料的空間摩擦能。由于現(xiàn)代材料硬度的提高，所需的切削能增多。韌度越大，也意味著通過摩擦形成新的摩擦表面需要更多的能量。 3.3 表面粗糙度的對(duì)比 表面粗糙度是對(duì)材料的表面特征的度量。表6說明了粒度對(duì)材料的表面粗糙度的影響非常重要。和摩擦力和空間摩擦能相反，粒度越小將導(dǎo)致現(xiàn)代材料表面粗糙度的降低。這在材料滑移機(jī)械中做了很好的解釋。由微電子探測儀拍攝的圖片可觀察到（表7和表8）當(dāng)可塑流在n-AL2O3/13TiO2摩擦中其主要作用時(shí)，脆性材料在n-AL2O3/13TiO2摩擦中起支配作用。 盡管可塑流是c/n-Wc/12Co在摩擦中的主要的材料滑移成分，據(jù)觀測晶格或許也很大程度上成因于c-Wc/12Co的表面粗糙度。和摩擦力類似，在很大的切深下，傳統(tǒng)材料和現(xiàn)代材料的表面粗糙度之間非常接近。這也說明了，現(xiàn)代材料粒度的減少并不能提高材料的滑移率。 3.4微電子探測儀的表面觀測和對(duì)比 表7表明了在相同的摩擦條件下，微電子探測儀對(duì)c/n-Wc/12Co的觀測結(jié)果。c-Wc/12Co的表面分片很多，并且大量的Wc微粒能夠被觀測到。和n-Wc/12Co表面相比，n-Wc/12Co表面被一層塑性材料完全覆蓋住了。Wc微粒邊界幾乎觀測不到。表8表明了用微電子探測儀對(duì)c/n-AL2O3/13TiO2觀測結(jié)果的對(duì)比。通過聲音和平坦的表面，來自熱膨脹的缺陷將被觀測到。 表6 表面粗糙度的對(duì)比 n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 0 10 20 30 0 0.3 0.6 0.9 1.2 1.5 Depth of cut, m Surface roughness Ra, 102 nm (a) n-WC/12Co (b) c-WC/12Co Fig. 7 SEM observations of ground c/n-WC/12Co coatings. (a) n-Al2O3/13TiO2 (b) c-Al2O3/13TiO2 Fig. 8 SEM observations of ground c/n- Al2O3/13TiO2 coatings. 2121210 mm 2121210 mm 2121210 mm 2121210 mm 5. AL2O3/13TiO2表明了可塑性流是主要的滑移材料。脆性結(jié)構(gòu)導(dǎo)致c-AL2O3/13TiO2的表面粗糙。斷片和晶體組織占據(jù)了c-AL2O3/13TiO2的大部分。表面觀察結(jié)果表明了表面具有不同的粗糙度。 4.總結(jié) 從摩擦力的對(duì)比中可看出：空間摩擦能成形表面，現(xiàn)代材料和傳統(tǒng)材料的表面形態(tài)?？煽偨Y(jié)為：粒度在材料的摩擦滑移中起到了重要的作用。 當(dāng)表面粗糙度隨著粒度一起增加時(shí)，摩擦力，切削力和空間摩擦能是和粒度密切相關(guān)的。摩擦?xí)r，可塑性流和脆性結(jié)構(gòu)會(huì)發(fā)生同樣的變化。摩擦?xí)r，粒度影響著可塑性流的數(shù)量，它支配著材料的最終表面。粒度減少，邊界增大可增強(qiáng)現(xiàn)代材料的硬度和韌度。所以它將影響到這些材料的摩擦。然而，在比較高的材料滑移率的條件下，粒度的影響并不重要。 備注： [1]H.P.Kirchner和J.C.Conway“用陶瓷滑移學(xué)和材料破壞學(xué)的機(jī)理闡明了陶瓷摩擦原理”陶瓷材料的機(jī)械加工。 [2]H.K.Tonshoff和E.Brinksmeier“磨料以及對(duì)材料表面溫度的影響”。 [3]P.Blake,T.Bifano,和R.O.Scattergood“陶瓷材料的應(yīng)用前景”。 [4]P.Roth和H.K.Tonshoff“微觀組織對(duì)氧化鋁陶瓷材料性質(zhì)的影響”。 [5]K.Jia,T.E.Fischer和BGallois“現(xiàn)代材料和傳統(tǒng)材料Wc-Co的微觀組織硬度和韌度”。 [6]S.Malkin“摩擦理論，摩擦技術(shù)以及磨料的應(yīng)用”。 1 Grindiability comparison between conventional and nanostructured material coatings. Bi Zhang, Xianbing Liu, Zhaohui Deng and Jian Meng Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA Email: zhang@engr.uconn.edu ABSTRACT This paper compares the grindability of conventionaland nanostructured material coatings in terms ofnormal grinding force, specific grinding energy, surfacefinish and surface topography. Material removalmechanism is correlated with the microstructures of thematerials such as material grain size. The effect of thedecreasing material grain size in nanostructuredmaterials on the grindability is studied. 1 INTRODUCTION In grinding of ceramics, the effects of grindingprocess parameters have been extensively studied (e.g.,Kirchner and Conway, 1985;T?nshoff and Brinksmeier,1988; Blake et al., 1988). It is of equal importance tostudy the influence of material microstructure. One ofthe material microstructures is the grain size, whichinfluences the mechanical properties, such as hardnessand toughness, and therefore the grindability ofceramics. Few works have been done on this aspect. Intheir work, Roth and T?nshoff (1993) studied thegrindability of alumina with different grain sizes increep feed grinding and conventional surface grinding.Both hardness and toughness for n-Al2O3/13TiO2 andn-WC/12Co coatings are found higher than theirconventional counterparts due to the reduced grain sizeand richer binder phases. Hardness of a material is itsability to resist plastic deformation. Plastic deformationis induced by the dislocation movement. The richer binder phases in nanostructured materials constrainmaterial flow and therefore plastic deformation. Unlike in the conventional materials, the increase of hardness in nanostructured materials does not lead to thedecrease of toughness due to more bridging ligaments,higher in-situ flow stress and higher rupture strength (Jia, et al., 1998). The difference in hardness and toughness between nanostructured and conventional materials can be expected to influence the grindability of their coatings. The large quantities of voids, cracks and microcracks induced by the thermal spray process greatly influence the properties of coatings made of these materials. The difference in hardness and toughness between conventional and nanostructured material coatings are not as much as in their bulk counterparts. Table 1 shows the typical properties of conventional and nanostructured WC/12Co and Al2O3/13TiO2 coatings. Normally, grindability is evaluated based on material removal rate, grinding force, surface finish and integrity of ground samples. In this paper, normal grinding force, specific grinding energy and surface finish are compared for nanostructured and conventional coatings. In addition, the scanning electronic microscopy (SEM) is used to assess the effects of grain sizes on the material removal mechanisms. Table 1 Typical Properties of the Conventional and Nanostructured WC/12Co and Al2O3/13TiO2 Coatings. c-WC /12Co n-WC /12Co c-Al2O3 /13TiO2 n-Al2O3 /13TiO2 Bonding strength, MPa 82.7 89.6 15.5 20.7 Powder grain size, mm 1.3 0.04 2.5 0.05 Mass density, g/cm3 14.2 14.5 3.5-4.0 3.7-4.1 Vickers hardness, GPa 12.00 12.50 10.44 10.57 Toughness, MPa m1/2 16.0 16.5 3.3 3.5 2 EXPERIMENTAL CONFIGURATION 2.1 Sample preparation and characterization Conventional and nanostructured material coatings were made on low carbon steel substrates of dimensions of 25 ′ 75 ′ 4 mm3 that were cleaned and blasted before thermal spray. The conventional and nanostructured WC/12Co coatings were produced using the high velocity oxygen fuel method, and the conventional and nanostructured Al2O3/13TiO2 coatings were prepared by the plasma thermal spray method. All the coatings had a thickness of around 0.5mm. The coated samples were cut into 25 ′ 4 ′ 4 mm3 for grinding. Fig.1 shows the SEM observations of c/n-WC/12Co coatings. Fig.1 (b) indicates that the smaller grains of WC are bonded together by the binder material cobalt and a large quantity of porosities can be observed in both conventional and nanostructured WC/12Co coatings. There are no obvious cracks found in conventional and nanostructured WC/12Co coatings. Fig.2 shows the typical surface features of the thermally sprayed conventional and nanostructured Al2O3/13TiO2 coatings: pores, cracks, microcracks and segmented structures formed by the connected microcracks perpendicular to the coating surface. Prior to the formal grinding test, the coatings were preground 2 with a diamond wheel of a mean grit size of 15 mm under minimum loading to avoid damaging the coatings. This preparation process was effective in getting rid of the random influence from the thermal spray process and making the samples more uniform, although it was time-consuming and effortsdemanding. 2.2 Grinding experiments Grinding experiments were conducted on a precision grinding machine (Dover Model 956-S) with the computer numerical control (CNC). The machine had aerostatic bearings for its spindle and x, y, z slideways. The spindle had an axial run-out of 0.05 mm and the three slideways had a straightness error of 0.1 mm/25mm. A laser interferometer was equipped to the machine that formed feedback loops for the x, y, z slideways with a resolution of 0.07 mm. The loop stiffness of the machine was measured to be 50 N/mm. In this study, a diamond grinding wheel SD600N100V (600V) was used to grind the coatings under different conditions, and the ground samples were compared. The wheel speed was set to 33 m/s or 3500 rpm. In order to investigate the effect of material removal rate (MRR) on residual stresses, depths of cut were set at 2, 5, 15 and 30 mm and feedrates at 1, 4, and 8 mm/s for the grinding experiments. Water-based synthetic solution (ITW fluid products Group, Rustlick G-10066D) was used as grinding coolant. 2.3 Post-grinding evaluation A surface profilometer (Federal Products, Surfanalyzer 5000) was used to measure surface finish (Ra) of ground coatings along the directions perpendicular to the grinding direction. An SEM (JOEL, Model JSM 840) was used to observe the surfaces of the ground coatings. One issue in SEM observations was to differentiate grinding damage from the coating defects. A large quantity of defects such as voids, unmolten particles, cracks and microcracks were identified in the as-sprayed coatings (Fig.1 and Fig.2). Because some of these defects can be easily mistaken as grinding damage, SEM examinations of the assprayed coatings were conducted to identify the defects from the spray process. It can be found that the voids from the thermal spray process normally appeared with smooth edges. From Fig.1 and Fig.2 the cracks or microcracks on the as-sprayed coatings were connected to each other without obvious directionality. With the as-sprayed coatings as a reference, the grinding damage was identified. 3 RESULTS AND DISCUSSIONS 3.1 Comparison of normal grinding force The normal grinding forces are important in characterizing a grinding process. Fig.3 compares the normal grinding forces in grinding c/n-Al2O3/13TiO2 and c/n-WC/12Co coatings at the same grinding conditions. The normal grinding force is higher for n-Al2O3/13TiO2 than for its conventional counterpart. One can also observe that the break-in force for n-Al2O3/13TiO2 is larger. This shows that the resistance to wearing for n-Al2O3/13TiO2 is higher due to its enhanced mechanical properties such as hardness and toughness when compared to its conventional counterpart. A similar trend is observed in grinding c/n-WC/12Co coatings: higher grinding force and break-in force for nanostructured coatings. Fig.3 also shows that the difference between the grinding forces for the nanostructured and conventional coatings becomes smaller at a large wheel depth of cut. This means that the material grain size exerts stronger influence on the grinding force at a low material removal rate. When the wheel depth of cut or material removal rate increases, the influence of grain size becomes second to that of grinding process parameters. 3.2 Comparison of tangential grinding force and specific grinding energy Tangential grinding force is much smaller than normal grinding force due to large negative rake angles of abrasive grits in grinding. The grinding force ratio (a) c-WC/12Co (b) n-WC/12Co (a) c-Al2O3/13TiO2 (b) n-Al2O3/13TiO2 Fig. 2 SEM observations of as-sprayed c/n-Al2O3/13TiO2 coatings. Fig. 1 SEM observations of as-sprayed c/n-WC/12Co coatings. 222222000000 __mm 222222000000 _mm 1200 _ _mm 1200 __ mm 3 indicates the relative magnitude of the normal grinding force Fn to the tangential grinding force Ft and is defined as t n F F l = (1) Fig.4 shows the grinding force ratio vs. wheel depth of cut for the four coatings. The grinding force ratio is higher for c/n-Al2O3/13TiO2 coatings than for c/n-WC/12Co coatings. c/n-Al2O3/13TiO2 coatings are more brittle than c/n-WC/12Co coatings. Under the same grinding conditions, brittle fracture is more obvious for c/n-Al2O3/13TiO2 coatings. The dominant ductile flow in grinding c/n-WC/12Co coatings results in a relatively high tangential grinding force and therefore a lower grinding force ratio. It is observed that the grinding force ratio of the n-Al2O3/13TiO2 coatings is distinctly different from that of the c-Al2O3/13TiO2 coatings while the difference for the grinding force ratios of n-WC/12Co and c-WC/12Co coatings is insignificant. The grinding force ratios for the four coatings decrease with the increase of material removal rate or wheel depth of cut. On the other hand, the grinding force ratios for the four coatings change over a relatively narrow range, which suggests that the material removal mechanism does not change much for the given range of the wheel depth of cut. The specific grinding energy U is defined as the energy required to remove a unit volume of material, which is derived from the tangential grinding force, f t c W d v F v U × × × = (2) where vc the grinding speed; W the width of a workpiece; d the wheel depth of cut; vf is feedrate. Fig.5 presents the effect of grain size on the specific grinding energy and the change of specific grinding energy with wheel depth of cut. The specific grinding (a) Conventional and nanostructured Al2O3/13TiO2. (b) Conventional and nanostructured WC/12Co. Fig. 3 Comparison of normal grinding force in grinding conventional and nanostructured coatings. 0 10 20 30 0 2 4 6 8 Depth of cut, mm Normal grinding force, N/mm2 Nano. Conv. Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 0 10 20 30 0 2 4 6 8 Depth of cut, mm Normal grinding force, N/mm2 Nano. Conv. Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V Fig. 4 Comparison of grinding force ratio. 00 10 20 30 3 6 9 12 Depth of cut, mm Grinding force ratio, l n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V Fig. 5 Comparison of specific grinding energy. 0 10 20 30 0.0 0.5 1.0 1.5 2.0 2.5 Depth of cut, mm Specific grinding energy, ′103 J/mm3 n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 4 Energy for four coatings decreases with the wheel depth of cut and asymptotically reaches a limit. The high value of specific grinding energy at small depth of cut suggests that only a part of the energy is associated with the chip formation (Malkin, 1989). Generally, the specific grinding energy consists of chip-forming energy Uch, sliding energy Upl and plowing energy Usl, ch pl sl U =U +U +U (3) Except Uch, the rest of the specific grinding energy is attributed to sliding and plowing between the workpiece and abrasive grits at a small depth of cut. At a larger depth of cut, sliding becomes insignificant and chip formation commen. However, plowing still exists, which reflects by the grinding marks and material pile-up on the ground surface. Only Uch is actually used in removing material and forming new surface. Theoretically, the asymptotical limit in Fig.5 is Uch. Relative flat curves for c/n-Al2O3/13TiO2 coatings in Fig.5 suggest that the energy expended in plowing is not dominant due to their high brittleness. The reduced grain size in nanostructured coatings apparently increases the specific grinding energy. More energy is needed for plowing due to enhanced hardness in nanostructured material coatings. The higher toughness also means more energy required for new surface formation in grinding. 3.3 Comparison of surface roughness Surface roughness was measured to characterize the ground coatings. Fig.6 shows that the influence of grain size on surface roughness of both ground coatings is significant. Opposite to the grinding force and specific grinding energy, the reduced grain size results in the decrease of the surface roughness for both nanostructured coatings, which can be explained by material removal mechanism. As observed in SEM photos (Fig.7 and Fig.8), brittle fracture dominates in grinding c-Al2O3/13TiO2 coatings while ductile flow plays a main role in grinding n-Al2O3/13TiO2 coatings. Although ductile flow is the major material removal mechanism in grinding both c/n-WC/12Co coatings, the observed transgranular fracture may partially contribute to surface roughness in c-WC/12Co coatings.Similar to the grinding force, surface roughness for ground conventional and nanostructured coatings are closer to each other at a larger wheel depth of cut, which means that the effect of reduced grain size in nanostructured coatings disappears at a higher material removal rate. 3.4 SEM surface observations and comparison Fig.7 shows the SEM observations of ground c/n-WC/12Co coatings under the same grinding conditions. The ground c-WC/12Co coating surface is more segmented and larger WC grains can be observed when compared to the ground n-WC/12Co coating surface. The ground n-WC/12Co coating surface is completely covered with a layer of plastically deformed material and the WC grain boundary is hardly observable. The comparison of the SEM surface observations of ground c/n-Al2O3/13TiO2 coatings is shown in Fig.8. Although the defects from thermal spray process are observable, the sound and smooth surface of ground n- Fig. 6 Comparison of surface roughness. n-WC/12Co c-WC/12Co n-Al2O3/13TiO2 c-Al2O3/13TiO2 Wheel speed: 33 m/s Feedrate: 4 mm/s Wheel: 600V 0 10 20 30 0 0.3 0.6 0.9 1.2 1.5 Depth of cut, mm Surface roughness Ra, ′102 nm (a) n-WC/12Co (b) c-WC/12Co Fig. 7 SEM observations of ground c/n-WC/12Co coatings. (a) n-Al2O3/13TiO2 (b) c-Al2O3/13TiO2 Fig. 8 SEM observations of ground c/n- Al2O3/13TiO2 coatings. 2121210 __mm 2121210 __mm 2121210 __mm 2121210 _mm 5 Al2O3/13TiO2 coating suggests that ductile flow is a predominant material removal mechanism. Brittle fracture results in rough and fractured surface of ground c-Al2O3/13TiO2 coatings. Chipping and transgranular fracture dominate the surface of ground c-Al2O3/13TiO2 coating. The surface observations explain the above difference in roughness. 4 CONCLUSIONS From the comparisons done on grinding force, specific grinding energy, surface finish and surface topography of nanostructured and conventional coatings, it is concluded that the grain size plays a significant role in material removal for grinding. Grinding force, break-in force and specific grinding energy vary inversely with the grain size while the surface roughness increases with the grain size. Both ductile flow and brittle fracture occur during grinding. The grain size influences the extent of ductile flow in grinding, which dominates the final appearance of ground surface. The reduced grain size and richer binder phases enhance both hardness and toughness in nanostructured materials, and therefore influence the grinding of these materials. However, at a higher material removal rate, the influence of grain size becomes insignificant. 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Nanostructured Materials, Vol.10, No.5, 1998, pp. 875-891 [6] S.Malkin. “Grinding technology, theory and application of machining with abrasives”. Ellis Horwood Limited, Chichester, England, 1989