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国际杂志机床制造43 ( 2003 ) 161.167磨削纳米陶瓷涂料:损害评价柳现炳.张碧机械工程系,康涅狄格大学,斯托斯, 06269CT美国于2002年6月27日收到; 接受2002年8月21日摘要： 本文的损害, 主要是表面和内部裂缝,地面 n-wc/12co和涂料. 表面裂缝下形成的一些特殊的条件. 存在和形成的表面裂缝的影响. 表层破坏,尤其是裂缝,内部成分大大影响其使用性能和寿命的服务. 影响磨削条件,如材料去除率, 砂轮粒度和粘结材料对地表的破坏,特别是地下裂缝的影响. 不同材质两种涂料也影响了表面裂缝. 不同于散装样品 大量的缺陷遗留下来在热喷涂过程中扮演一个重要角色。目前裂纹配置较复杂,较典型的体系中,如横向裂缝观测地面散装 陶瓷. Elsevier2002科技有限公司保留所有权利. 关键词:磨削; 纳米陶瓷涂料; 表层破坏 . 1. 介绍虽然引进的提高, 与金属材料相比断裂韧性纳米陶瓷仍然偏低. 较低的断裂韧性,使纳米陶瓷敏感的破坏,尤其是裂缝在受磨情况下. 据裂缝的表面采样, 他们的存在可分为两类:表面和内部裂缝. 一般来说,无论是磨削条件和材料性能的决定性因素的形成,存在和损害程度. 在热喷涂过程中有大量的缺陷,如孔洞, 吸湿性和unmolten颗粒以及残余应力1所形成的涂层. 缺陷和残余应力,也极大地影响了平整度和深度损害. 无损和破坏性方法被用来评估损害地面陶瓷样品. 无损方法 超声波技术已被用来探测地下横向裂缝,地面氮化硅等. 2 . photothermics是另一个技术探测近地表的财物,包括损伤的不同生产工序的3 . X-射线衍射和拉曼光谱也被用来测量表层损坏零件加工4 . 虽然这无损方法有很大的优势,例如有能力被集成在一条生产线, 和保存样品检验,但有其局限性,如间接观察表层性能和有限的观察深度. 在另一方面,这些方法都不能提供任何资料,对裂缝配置 这是重要的评价和预测性能的部件地面服务. 破坏性方法提供直接观察表层损坏. 分层,腐蚀 骨折和锥度抛光技术已被用来评估损伤单一砂粒金刚石yoshikawa】. 5张碧和howes 6,7 和63】. 8陶瓷基金刚石砂轮. 徐和jahanmir 9用保税接口技术研究表层损坏,其中两边工件加工后分别观察损伤. 本研究采用锥形研磨与抛光技术和电子显微镜直接观察表层损坏表面涂料. 全谱表层裂缝配置得到. 观察表面裂纹是比较简单的,可直接用SEM观察，在某些情况下帮助一个蚀刻技术。 更重要的是损害与磨削条件,材料性能和内部缺陷遗留下来的热喷涂加工. 形成机加工的裂缝探讨. 2 . 实验程序流毁伤评估 每个样品制备研磨锥度沿试样长度(垂直于磨削方向)的夹角 相对地面,如图1 ： 研磨过程进行了抛光Struers公司系统共分五个阶段逐步较小的金刚石磨具. 平均磨料尺寸为每一阶段分别为第40 ,第20 ,第9 ,第3和第1米,分别. 该磨料40和20米固定在抛光光盘和其他磨料是免费的. 整个研磨过程中的润滑. 特制夹具是用来举行样品,使样品研磨成角 相对的表面. 介绍研磨样品这一进程是为了这样减少了损害 ,并产生之间实现平稳过渡内部及表面研磨. 图1说明搭接样本及如何表层是用SEM观察. 放大了1000倍是用来提供一个框架约110米宽度. 遍历整个长度的样品沿过渡接口逐帧, 最高损伤深度在每一帧录同一个尺度上镜画面. 这些最大破坏深度均超过平均值的整个长度的样本,以获取平均损伤深度样品. 在此期间,配置内部裂缝系统得以确定. 在损伤深度测量,它是重要的搭接工件及SEM观测区. 确定接口和表面搭接面. 在高放大倍率的界面似乎接近三个特点,在样品用来帮助识别接口. 接口本身是明确的低倍率和有益的决定大致位置的接口. 由于不同的角度,在地面和水面搭接相对入射电子束, 有一个对比的两个面. 最后,两端的表面特征如打磨痕迹,有利于确定接口. 类似的表面观察, 另一个问题从内部缺陷的涂层,在扫描电镜的观察损害的是区分磨破坏. 据发现该缺陷,如气泡,裂纹遗留下来的热喷涂过程中通常会出现边缘光滑. 出现裂纹或微裂纹等离子喷涂涂料无明显方向性. 与等离子喷涂作为参考,对磨削加工损伤进行了鉴定. 3 . 结果与讨论3.1 . 车轮在一定条件下表面裂缝已观察到的表面均和涂料地上铺着10kc. 10架KC车轮是铸铁纤维去骨(积分层叠反馈) 而平均粒度1.5米,且浓度为100 .图 2显示地表裂缝涂料地面的车轮10kc深度削减2和5 米,分别. 基本上表面裂纹垂直于磨削方向. 不过有些裂缝平行于磨削方向或其他方向,也是观察涂料. 2 . 表面裂缝的地面上的10架KC车轮. ,张碧/国际期刊机床制造43 ( 2003 ) 161.167 163. 3 表面裂缝的n-wc/12co地面上的10架KC车轮. Web结构的地面. 部分的表面被分为独立区隔的互联裂缝. 典型的表面裂纹(图3 )观察到的表面涂层地面有10架KC砂轮 深度削减等于或大于2.5米 从图 3 , 其中可以发现,长期表面裂纹几乎是相互平行,垂直于磨削方向. 不同于地面涂料,在其他方向观察表面涂料有没有裂缝. 有裂缝的表面,地面涂料所引起的机械相互作用磨削 车轮表面和工件材料. 为袋鼠10轮的平均粒度1.5米, 铸铁纤维保证金蹭涂层表面时,砂轮切削深度是够大. 如观察 2号和3号揉搓行动造成了非常光滑的地面. 在另一方面这反映在组件切磨削力在磨削方向,擦形成一个庞大的摩擦力.图 4显示了两种成分的磨削力在磨削 , 从中可以看出,组成切磨削无花纹. 4 . 比较切(一)正常( b )组成磨削力磨削时,与10架KC车轮. 方向是接近正常组成部分. 这次大切组成磨削力将诱发大的拉应力,在磨裂面出发 从薄弱部位的表面涂层. 3.2 . 表层裂缝配置位数及横向裂缝已报告作为主要地表裂缝类型 10.12 加工脆性材料. 如上所述,存在着一些缺陷,等离子喷涂涂料. 这些缺点在表层裂纹的萌生和发展中发挥了重大作用,. 不同配置的表层裂缝观测地面涂料. 3.2.1 . 地面涂料. 5显示配置裂缝出现在表层2表面涂料. 地表裂缝主要是微颗粒. 双方位数及横向裂缝都是看得见的. 其中部分有助于粗糙地表n涂料相比,地表氮-精密 涂料. 表面轮廓13 实测图.5 . 观察地表裂缝,地面涂料.用原子力显微镜( AFM )和扫描 观察14地面涂膜显示打开的表层裂缝. 3.2.2 . 地面涂料无裂纹 图6显示不同结构的表层裂缝观察地面涂料. 双方位数和横向裂缝的观察. 但是,由于受缺陷的影响,从热喷涂到配置表层裂缝的地面N-二精密涂层的过程中,非晶配置也可以观察到. 6 ( a )列出了一个典型的中位数裂缝无任何附带横向裂缝. 6 ( b )项, ( c )项和( d )相类似: 位数的裂缝从地面,而横向裂纹平行于地面.这些配置长度的位数和横向裂缝也不尽相同. 第6 ( e )显示了裂缝,从地面 发展提供了锐利的角度,以相对地面,而止于深入地下一层. 另一个特殊的配置一个美船形裂纹是列图. 6 ( f )项,其中从地表, 发展到深度的地下一层,最后结束在地面. 6还显示效果的缺陷,在涂层形成的地表裂缝. 一方面,其中包含缺陷区,是最有可能的领域开展裂缝. 此外,裂纹倾向于扩大沿晶界,微裂纹和微孔, 其中被视为主要的原因形成的裂缝如图 6 ( e )和( f ) . 但在另一方面,当发展中裂纹会见了一些缺陷,如失效。 这一缺陷可以作为缓解能源和防止裂缝的进一步发展, 这是典型的列图 6 ( b )项. 应当指出,裂缝的发展可以通过孔洞如观察图 6(d) , ( e )和( f ) . 3.3 . 影响磨削条件的深度表层损坏完全因子进行实验研究的影响 磨削参数对深度的表层损坏. 调查磨削条件砂轮切削深度,进表,轮式债券型和砂粒大小. 3.3.1 . 地面涂料不同于地面涂料 特色表层裂缝的地面N-二涂层不是很明显, 使得测量的深度损伤困难. 图7和8显示效果的砂轮切削深度和进给表的意思,并 最大水深破坏涂层与地面600第五轮. 平均深度的损害日益增加,砂轮切削深度或进. 但是,由于受影响的缺陷的最大深度损害并不完全遵循这一趋势，较大的砂粒大小导致更深的损害. 图9和10显示效果车轮型和砂粒大小的均值和最大深度表层损坏2地面涂料.图 7 影响切削深度损伤地面涂料.图 8 影响进深度损坏地面2涂料. 不过, 相比从砂轮切削深度和进表，影响车轮型和砂轮砂粒大小对最大或平均深度损害是微不足道. 3.3.2 . 地面涂料 图9 . 影响车轮债券型深度损伤地面N-二涂料. 图10 . 影响砂轮粒度深度损伤地面N-二涂料. 率(切削深度下15m,进小于4毫米/秒) , 深度损伤增长缓慢增加,切削深度也是如此. 随着进一步增加切削深度和进给速度,深度的损害就会大大提高. 最大水深达毁坏近50m在砂轮切削深度30m或表面进给 8毫米/秒. SEM照片显示一些大型裂缝的发展. 如图13 ,平均和最高深的损害较大。图11 影响切削深度损伤地面N-二精密涂层. 图12 影响进深度损伤表面N-二精密涂层. 金属结合剂砂轮,比起在地上的玻璃和树脂的轮子并无很大差异。这些涂层与地面600 V和1000二轮. 大型轮式砂粒大小( 120第五轮)的增幅平均和最大深度损伤(图14 ) 。当其他条件不变,更大的车轮砂粒大小结果深入砂砾。图13 . 影响车轮型深度损伤地面涂料. 图14 影响砂轮粒度深度损伤表面N-二精密涂层.因此,高速磨削力每砂粒和更深的损害. 3.4 . 影响材料性能的深度内部损害，人们普遍认为的深度损伤，是与磨削脆性材料密切相关性能的地面材料,特别是脆性材料,他们指按比例硬度，韧性. 研究结果显示,高脆性结果,是由于较小的深度破坏. 张碧howes 7提出一个模式来形容双方的关系深度损伤和脆性 ( 200Dmax ) ( L (小时/尺寸) ( 1 )。最高的砂粒切削深度是一个常数. ( 1 )表明,深度的损害减小增加了脆性材料. 但是,应该指出,这种关系是不相称的. 至于目前的两种材料,脆性的N-二是结构高于中. 对比图. 7和11,人们可以发现,在小切深,即少于15米，深处的损害涂料双方是互相接近的. 随着切削深度增加,深度损坏的N急剧增加. 类似趋势存在时,改变进给. 实验表明,大量的微裂纹分布在近表面层,表面涂料个别呈大裂缝,可以发现,在biaomian涂料. 这种现象可以解释为不同机制的能量耗散在不同的脆性材料,在磨削时， 装载在研磨可以轻易发起新的裂缝,材料的脆性有很高的价值.能源消耗是由形成新的裂缝,而不是由裂缝的发展. 4 . 结论不同结构的表层裂缝观测地面涂料. 由于影响的缺陷,在涂料中遗留下来的热喷涂过程中, 大批微观察到表层地面涂料. 不同于裂缝的地面N-二精密涂层,这些微裂纹是很难给以分类. 双方 和涂料较大的材料去除率。以及其平均深度以下损坏的趋势, 车轮较大砂粒大小和较高的车轮硬度产生较大的平均深度损害. 然而,由于随机因素的涂料的最大深度损害不严格遵循这一趋势。在有些情况下, 有专门为表面N-二涂层. 影响车轮从债券和砂粒大小是微不足道相比,材料去除率. 较高的脆值的涂料解释较低的深度破坏. 不同的耗能机制落后的差异及裂纹深度损害. 鸣谢作者衷心感谢财政支持来自康涅狄格创新公司和办事处美海军研究和 供应涂料样品inframat公司. 还要感谢蔡秀&有线电视公司和东京金刚石工具有限公司品牌为他们捐一些 金刚石砂轮. 参考文献： 1 X. Liu ， B. Zhang,效果评价微粉碎过程中的残余应力在热喷涂纳米涂层 学报材料科学37 ( 2002 ) 3229.3239 . 2 H. Ahn ， L. Wei ， S. Jahanmir, ASME规范期刊工程材料与技术118( 3 ) ( 1996 ) 402 . 3 G. Goch ， B. Schmitz ， B. Karpuschewski ， J. Geerkens ， M. Reigl,P. Sprongl ， R. Ritter, 钢筋 审查非破坏性测量方法来评估表面完整性:一项调查,新的测量方法,涂料, 层状结构和表面处理,精密工程23 ( 1999 ) 9.33 .4 E. Brinksmeier, 国际先进的非破坏性检测表层材料性质和损害,精密工程11 ( 1989 ) 211.224 . 5 M. Yoshikawa ， B. Zhang ， H. Tokura,观察陶瓷表面裂缝的新建议的方法, 日刊陶瓷协会,日本国际教育署. 95 ( 1987 ) 911.918 . 6 B. Zhang,T.D. ,材料去除机加工磨削陶瓷 ,CIRP 43(1994)305 308 的编年史.7 B. Zhang,T.D. 潜流评价陶瓷表面,CIRP 44(1995)263 266 的编年史.8 T.M.A. Maksoud,A.A. Mokbel,J.E.穆克比勒,理e.摩根学报材料加工技术88 ( 1999 ) 222.243 9 h.k.k. S jahanmir ,简单技术观察表层损伤加工陶瓷, 日刊美国陶瓷学会第77 ( 5 ) ( 1994 ) 1388.1390 . 10 S. Malkin,T.W. Hwang,窑业磨削机加工,CIRP 45(2)(1996)569 580 的编年史.11 B.R. Lawn， A. Wilshaw,期刊材料科学12 ( 1977 ) 2195.2199 .12 B.R. Lawn,A.G. 埃文,D.B.马歇尔杂志美国陶瓷协会63 ( 1980 ) 574.581.13 B. Zhang ， X. Liu,C.A. 布朗,T.S. Bergstrom,Microgrinding微纳米材料涂层， CIRP 51 的编年史 (1)(2002) 251 254.14 X. Liu ， B. Zhang ， Z. Deng,研磨纳米金属和陶瓷涂层: 地面观测和材料去除机制,国际学报机床与制造42 ( 15 ) ( 2002 ) 1665.1676 .9International Journal of Machine Tools & Manufacture 43 (2003) 161167Grinding of nanostructural ceramic coatings: damage evaluationXianbing Liu, Bi ZhangDepartment of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USAReceived 27 June 2002; accepted 21 August 2002AbstractThis paper investigates the damage, mainly surface and subsurface cracks, in ground n-Al2O3/13TiO2and n-WC/12Co coatings.Surface cracks are formed under some special conditions. The presence and formation of the surface cracks are studied.Subsurface damage, especially cracks, of the ground components greatly influences their performance and life in service. Theeffects of the grinding conditions such as material removal rate, wheel grit size and bond materials on the subsurface damage,especially subsurface cracks, are investigated. The difference of material properties of these two coatings also influences the subsur-face cracks. Different from bulk samples, large quantities of defects inherited from the thermal spray process play a significant rolein the initiation and development of the cracks, present the crack configurations more complex than the typically reported systemof median and lateral cracks observed in ground bulk ceramics. This complexity is analyzed. 2002 Elsevier Science Ltd. All rights reserved.Keywords: Grinding; Nanostructured ceramic coating; Subsurface damage1. IntroductionAlthough being enhanced, the fracture toughness fornanostructured ceramics is still low compared with met-als. The relatively low fracture toughness makes nanos-tructured ceramics sensitive to damage, especially crack-ing, when subjected to grinding. According to theirpresence, the cracks in a ground sample can be classifiedinto two categories: surface and subsurface cracks. Gen-erally, both the grinding conditions and material proper-ties are the deterministic factors for the formation, pres-ence and magnitude of damage. During thermal sprayprocess, large quantities of defects, such as voids,microcracks and unmolten particles as well as residualstresses 1 are formed in the coatings. The defects andresidual stresses also greatly influence the formation anddepth of damage.Nondestructive and destructive approaches have beenutilized to assess damage in ground ceramic samples. Innondestructive methods, the ultrasonic technique hasbeen used to detect subsurface lateral cracks in groundsilicon nitride by Ahn et al. 2. Photothermics is anotherCorresponding author.E-mail address: xianbing (X. Liu).0890-6955/03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0890-6955(02)00157-8techniquefor detectingthe near-surfacepropertiesincluding damage induced by different manufacturingprocesses 3. X-ray diffraction and Raman spectroscopyare also used in measuring subsurface damage ofmachined components 4. Although these nondestruc-tive methods have great advantages, e.g., capability ofbeing integrated in a production line, and preservation ofinspected samples, they have limitations such as indirectobservation of subsurface properties and limited obser-vation depth. On the other hand, these methods cannotprovide any information on crack configurations, whichare important for evaluating and predicting the perform-ance of ground components in service.Destructive methods provide direct observation ofsubsurface damage. The slicing, etching, fracture andtaper polishing techniques have been employed to assessdamage induced by single grit diamond by Yoshikawaet al. 5 and Zhang and Howes 6,7 and by Maksoudet al. 8 for ceramics ground with diamond wheels. Xuand Jahanmir 9 used the bonded interface techniqueto study subsurface damage in which two halves of aworkpiece were bonded into one. After machining, thetwo halves were separated to observe for damage.This study uses the taper lapping and polishing tech-nique and SEM to directly observe subsurface damageof ground coatings. A full spectrum of subsurface crack162X. Liu, B. Zhang / International Journal of Machine Tools & Manufacture 43 (2003) 161167configurations is obtained. The observation of a surfacecrack is relatively simple and can be directly observedwith SEM, helped by an etching technique in somecases. More important, damage is correlated with grind-ing conditions, material properties and internal defectsinherited from the thermal spray process. The formationmechanism of the cracks is discussed.2. Experimental proceduresFor subsurface damage evaluation, each sample wasprepared by taper lapping along the sample length(perpendicular to the grinding direction) at an angle of45 relative to the ground surface as shown in Fig. 1.The lapping process was performed on a Struers pol-ishing system and consisted of five stages with progress-ively smaller diamond abrasives. The mean abrasive gritsizes for each stage were 40, 20, 9, 3 and 1 m, respect-ively. The abrasive grits of 40 and 20 m were fixed onpolishing discs and other abrasive grits were free. Thewhole lapping process was lubricated. A specially madefixture was used to hold the samples in order that thesamples were lapped at an angle of 45 relative to theground surface. Lapping the samples in this way minim-ized the damage introduced by this process and gener-ated a smooth transition between the ground surface andlapped surface.Fig. 1 schematically illustrates the lapped sample andhow the subsurface was observed with SEM. A magni-fication of 1000 was used to provide a frame of about110 m in width. Traversing the whole length of thesample along the transition interface frame by frame, themaximum damage depth in each frame was recordedwith a scale on the SEM screen. These maximum dam-age depths were averaged over the whole length of thesample to obtain the mean damage depth for the sample.In the meantime, the configurations of subsurface cracksystems are identified.During the damage depth measurement, it is importantFig. 1.Schematic of lapped workpiece and SEM observation area.to determine the interface between the ground surfaceand lapped surface. Under high magnification, the inter-face appeared ambiguous. Three characteristics on thesample were used in helping the identification of theinterface. The interface itself was clear under low magni-fication and useful in roughly deciding the position ofthe interface. Due to the different angles of the groundsurface and lapped surface relative to the incident elec-tron beam, there was a contrast between the two sur-faces. Finally, the ends of the ground surface character-istics,suchasgrindingmarks,werehelpfulindetermining the interface.Similar to the surface observations, another issue inSEM observations of damage was to differentiate grind-ing damage from the internal defects of the coatings. Itwas found that the defects such as voids and cracksinherited from the thermal spray process normallyappeared with smooth edges. The cracks or microcracksin the as-sprayed coatings were connected to each otherwithout obvious directionality. With the as-sprayed coat-ings as a reference, the grinding damage was identified.3. Results and discussions3.1. Surface cracksSurface cracks have been observed on the surfaces ofboth n-Al2O3/13TiO2and n-WC/12Co coatings groundwith one 10 kC wheel under certain conditions. The 10kC wheel is cast iron fiber boned (CIFB), and has a meangrit size of 1.5 m and a concentration of 100. Fig. 2showssurfacecracksonn-Al2O3/13TiO2coatingsground by the 10kC wheel at depths of cut of 2 and 5m, respectively. Basically the surface cracks are per-pendicular to the grinding direction. However, somecracks, parallel to the grinding direction or in otherdirections, are also observed on n-Al2O3/13TiO2coat-ings. These cracks intersect with each other to form aFig. 2.Surface cracks on the n-Al2-O3/13TiO2surface ground by 10kC wheel.163X. Liu, B. Zhang / International Journal of Machine Tools & Manufacture 43 (2003) 161167Fig. 3.Surface cracks on the n-WC/12Co surface ground by 10 kCwheel.web structure on the ground surface. Some parts of thesurface are separated into independent segmentations bythe interconnected cracks.The typical surface cracks (Fig. 3) are observed onthe surfaces of n-WC/12Co coatings ground with 10 kCwheel at depths of cut equal to or larger than 2.5 m.From Fig. 3, one can find that the long surface cracksare almost parallel to each other and perpendicular tothe grinding direction. Unlike ground n-Al2O3/13TiO2coatings, there are no cracks in the other directionsobserved on the surfaces of n-WC/12Co coatings.The surface cracks on the surfaces of both groundcoatings have been caused by the mechanical interactionbetween the grinding wheel surface and workpiecematerial. For the 10 kC wheel with a mean grit size of1.5 m, the cast iron fiber bond would rub against thecoating surface when the wheel depth of cut was largeenough. As observed in Figs. 2 and 3, the rubbing actioncreated a very smooth ground surface. On the other hand,the rubbing resulted in a large friction force, which wasreflected in tangential component of grinding force inthe grinding direction. Fig. 4 shows the two componentsof grinding force in grinding n-WC/12Co, from whichone can see that the tangential component in grindingFig. 4.Comparison of tangential (a) and normal (b) components ofgrinding force when grinding with the 10 kC wheel.direction is close to the normal component. This largetangential component of grinding force would induce alarge tensile stress during grinding and split the surfacestarting from the weak parts on the coating surfaces.3.2. Subsurface crack configurationsMedian and lateral cracks have been reported as majorsubsurface crack types 1012 in machining brittlematerials. As discussed above, there exist a number ofdefects in the as-sprayed coatings. These defects playa significant role in the subsurface crack initiation andpropagation.Differentconfigurationsofsubsurfacecracks are observed in ground coatings.3.2.1. Ground n-Al2O3/13TiO2coatingsFig. 5 shows the configuration of cracks observed inthe subsurface of ground n-Al2O3/13TiO2coatings. Thesubsurface cracks are mainly the microcracks in grains.Both median and lateral cracks are observable. Thesecracks open to the ground surface, which partially con-tributes to the rougher surface of ground n-Al2O3/13TiO2coatings when compared with the surface of ground n-WC/12Co coatings. The surface profile 13 measuredFig. 5.Observation of subsurface cracks in ground n-Al2-O3/13TiO2coatings.164X. Liu, B. Zhang / International Journal of Machine Tools & Manufacture 43 (2003) 161167by atomic force microscope (AFM) and SEM obser-vation 14 of ground n-Al2O3/13TiO2coating surfacesshow the opened subsurface crack.3.2.2. Ground n-WC/12Co coatingsFig. 6 shows different configurations of subsurfacecracks observed in ground n-WC/12Co coatings. Bothmedian and lateral cracks are observed. However, dueto the influence of the defects from the thermal sprayFig. 6.Configurationsofsubsurfacecracksinthegroundn-WC/12Co cess, amorphous configurations can also be observed.Fig. 6(a) shows a typical median crack without anyaccompanying lateral crack. Fig. 6(b), (c) and (d) aresimilar to each other: the median crack starts from theground surface while the lateral crack parallel to theground surface develops at the end of the median crack.The difference among these configurations is that thelengths of the median and lateral cracks vary. Fig. 6(e)shows a crack that starts from the ground surface,develops with a sharp angle relative to the ground sur-face, and ends at the depth of subsurface layer. Anotherspecial configuration of a U shape-like crack is shownin Fig. 6(f), which starts from the ground surface,develops to the depth of the subsurface layer and thento the ground surface at some point, and then ends atthe ground surface.Fig. 6 also shows the effects of the defect in the coat-ings on the formation of subsurface cracks. On one hand,the area that contains the defects is the most possiblearea to initiate crack. In addition, cracks tend to expandalong grain boundaries, microcracks and microvoids,which are considered as the main reason for the forma-tion of the cracks in Fig. 6(e) and (f). On the other hand,when the tip of a developing crack meets some defectsuch as a void, this defect can function as energy relieverand prevent the crack from further developing, which istypically shown in Fig. 6(b). It should be pointed outthat the cracks can develop through voids as observedin Fig. 6(d), (e) and (f).3.3. Effects of grinding conditions on the depth ofsubsurface damageFull factorial experiments were conducted to study theeffects of grinding parameters on the depth of subsurfacedamage. The investigated grinding conditions are wheeldepth of cut, table feedrate, wheel bond type and gritsize.3.3.1. Ground n-Al2O3/13TiO2coatingsUnlike the ground n-WC/12Co coatings, the charac-teristicsofsubsurfacecracksforthegroundn-Al2O3/13TiO2coatings are not very obvious, whichmade the measurement of the depth of damage difficult.Figs. 7 and 8 show the effects of the wheel depth of cutand the table feedrate on the mean and maximum depthsof damage in the coatings ground with a 600 V wheel.The mean depth of damage grows with the increase ofwheel depth of cut or feedrate. However, due to theinfluence of the defects in the coatings or other randomfactors, the maximum depth of damage does not com-pletely follow this trend.Figs. 9 and 10 show the effect of wheel bond type andgrit size on the mean and maximum depth of subsurfacedamage in ground n-Al2O3/13TiO2coatings. Harderwheel bond and larger grit size result in deeper damage.165X. Liu, B. Zhang / International Journal of Machine Tools & Manufacture 43 (2003) 161167Fig. 7.Effects of depth of cut on depth of damage in ground n-Al2-O3/13TiO2coatings.Fig. 8.Effects of feedrate on depth of damage of ground n-Al2-O3/13TiO2coatings.However, the effects of wheel bond type and wheel gritsize on the maximum or mean depth of damage areinsignificant when compared with those from wheeldepth of cut and table feedrate.3.3.2. Ground n-WC/12Co coatingsFigs. 11 and 12 show that both mean and maximumdepths of damage increases with the increase of wheeldepth of cut or table feedrate. At small material removalFig. 9.Effects of wheel bond type on depth of damage in ground n-Al2-O3/13TiO2coatings.Fig. 10.Effects of wheel grit size on depth of damage in ground n-Al2-O3/13TiO2coatings.rate (depth of cut under 15 m and feedrate less than 4mm/sec), the depth of damage grows slowly with theincrease of depth of cut or feedrate. With the furtherincrease of depth of cut or feedrate, the depth of damageincreases dramatically. The maximum depth of damagereaches nearly 50 m at wheel depth of cut of 30 mor table feedrate of 8 mm/sec. SEM observations showthat some large scale cracks have developed.As shown in Fig. 13, the mean and maximum depthsof damage are larger in the coatings ground with the166X. Liu, B. Zhang / International Journal of Machine Tools & Manufacture 43 (2003) 161167Fig. 11.Effects of depth of cut on depth of damage in ground n-WC/12Co coatings.Fig. 12.Effects of feedrate on depth of damage in ground n-WC/12Co coatings.metal bond wheel than in those ground with the vitrifiedand resin bond wheels. There is no great differenceamong those coatings ground with the 600 V and 1000B wheels. The large wheel grit size (120 V wheel)increases both mean and maximum depths of damage(Fig. 14). When other conditions remain the same, alarger wheel grit size results in larger actual grit depthFig. 13.Effects of wheel bond type on depth of damage in groundn-WC/12Co coatings.Fig. 14.Effects of wheel grit size on depth of damage in ground n-WC/12Co coatings.of cut, and therefore higher normal grinding force pergrit and deeper damage.3.4. Effect of material properties on the depth ofsubsurface damageIt is generally believed that the depth of damage ingrinding brittle materials is closely related with thepropertiesof groundmaterials, especiallymaterialbrittleness that is defined as the ratio of hardness to167X. Liu, B. Zhang / International Journal of Machine Tools & Manufacture 43 (2003) 161167toughness. Researches have shown that higher brittlenessresults in smaller depth of damage. Zhang and Howes7 proposed a model to describe the relationshipbetween the depth of damage and brittleness,d ? (200dmax)1/log(l(H/KIC)(1)where dmaxis the maximum grit depth of cut and is aconstant. Eq. (1) shows that the depth of damagedecreases with an increase of material brittleness. How-ever, it should be noted that this relation is not pro-portional.For the current two materials, the brittleness of n-Al2O3/13TiO2is higher than that of n-WC/12Co. Com-paring Figs. 7 and 11, one can find that at small depthof cut, that is, less than 15 m, the depths of damagefor both coatings are close to each other. With theincrease of depth of cut, the depth of damage for n-WC/12Co increases dramatically. A similar trend existswhen changing the feedrate. The experiment indicatesthat a large number of microcracks distribute in the nearsurface layer of ground n-Al2O3/13TiO2coatings whileindividual scattered major cracks can be found in theground n-WC/12Co coatings. This phenomenon can beexplained by the different mechanisms of energy dissi-pation in the materials with different brittleness duringgrinding. Loading during grinding can easily initiate newcracks in a material with a high brittleness value. Mostgrinding energy is consumed by the formation of newcracks rather than by the development of existing cracks.4. ConclusionsDifferent configurations of subsurface cracks areobserved in ground n-WC/12Co coatings. Due to theinfluence of the defects in the coatings inherited fromthe thermal spray process, amorphous configurationshave been observed. A large quantity of microcracks isobserved in the subsurface of ground n-Al2O3/13TiO2coatings. Different from the cracks in the ground n-WC/12Co coatings, these microcracks are difficult tobe classified.For both n-Al2O3/13TiO2and n-WC/12Co coatings,the mean depth of damage follows the trend that largermaterial removal rate, larger wheel grit size and higherwheel bond hardness result in larger mean depth of dam-age. However, due to the random factors in the coatings,the maximum depth of damage does not strictly followthis trend in some c