【作者】 董志刚；

【导师】 康仁科；

【作者基本信息】 大连理工大学 ， 机械制造及其自动化， 2010， 博士

【摘要】 单晶MgO在耐温性、透光率、热导率、电绝缘性、化学稳定性和机械强度等方面性能优异,被广泛用作高温超导等薄膜生长的基片材料,同时也是一种重要的光学材料。基片是薄膜材料生长的载体,其超精密加工表面质量直接影响薄膜性能。目前,由于对单晶MgO的微观力学特性和加工损伤机制以及MgO基片的精密加工技术缺乏系统研究,影响了单晶MgO的应用。本文深入研究了单晶MgO的纳米力学性能、微观变形和加工损伤特征以及MgO基片的磨粒加工工艺,对于推动单晶MgO在光电领域的应用具有重要的理论意义和实用价值。主要研究内容和结论如下：通过纳米压痕、微压痕和微划痕试验,研究了单晶MgO不同晶面的的纳米力学性能以及微观变形和损伤特征。根据加载条件的不同,单晶MgO会发生如下变形：(1)弹性变形, (2)弹塑性变形, (3)蠕变变形, (4)微裂纹和微破碎。弹性变形时的纳米压痕力-位移曲线符合赫兹弹性接触理论,其变形在卸载后可完全恢复；塑性变形时MgO在{110}易滑移晶面系内位错形核和滑移的结果。材料由弹性变形转变为弹塑性变形时,纳米压痕力-位移曲线上出现pop-in现象,并对应有特征明显的声发射(Acoustic emission, AE)信号,pop-in弹性释放能和AE信号能量成线形比例关系。计算得到单晶MgO(001)和(110)晶面纳米压痕导致位错形核的平均临界剪应力分别为14.85 GPa和12.61 Gpa,这与理论剪切应力13.82 GPa接近。增加纳米压痕保载时间导致蠕变变形,该蠕变变形存在瞬态蠕变和稳态蠕变两个阶段。稳态蠕变时单晶MgO (001)、(110)和(111)晶面的蠕变应力指数分别为36.5,53.7和22.4。当单晶MgO (001)晶面微压痕和微划痕试验的载荷足够大时,表面和亚表面会形成微裂纹,包括放射状裂纹、亚表面水平裂纹和中位裂纹。通过对压痕和划痕的截面进行观察和分析发现,根据裂纹成因不同,亚表面微裂纹主要有两种：一种是两相邻互成120°角的{110}45°滑移面内的位错滑移相交积塞,导致沿MgO (110)晶面开裂,形成与表面垂直的{110}90°裂纹,以及与表面成45°的倾斜裂纹；另一种是应力集中导致沿MgO(100)解理面解理,形成垂直于表面的中位裂纹(又称{100}90°裂纹)和平行于表面的水平裂纹。通过对锯切、研磨和磨削三种磨粒加工方法加工的单晶MgO基片表面和截面的显微观察,结合MgO微观力学行为的分析结果,研究了单晶MgO基片的表面和亚表面损伤。不同方法加工的单晶MgO基片截面均存在两个位错滑移系统,即位错在{110}90°滑移面内滑移形成的垂直滑移系统和位错在{110}45°晶面内滑移形成的倾斜滑移系统,滑移系统与压痕划痕截面观察到的位错滑移线一致。在粗加工单晶MgO基片的亚表面存在横向、垂直和倾斜三种基本形状的微裂纹,分别对应于微压痕和微划痕中的的水平裂纹,放射状裂纹和倾斜裂纹。钩形、“人”字形、“之”字形、伞形等形状的微裂纹是由这三种基本形状的微裂纹构成的。通过观察和分析单晶MgO基片磨粒加工后的表面和亚表面损伤,建立了单晶MgO亚表面损伤模型,揭示了磨粒加工单晶MgO的材料去除机理。粗加工单晶Mg0基片时,表面和亚表面产生微裂纹并扩展相交,材料以微破碎形式去除,表面形成微台阶结构并残留微裂纹,亚表面产生由表面破碎层、位错滑移层和弹性畸变层组成的损伤层。精加工单晶MgO基片时,表面产生塑性变形并形成高应变区,材料以疲劳破坏形式去除,表面没有微裂纹和微破碎出现,亚表面层损伤仅包括位错滑移和弹性畸变。进行了单晶MgO基片研磨工艺试验,研究了不同加工条件下的材料去除率、表面粗糙度和亚表面损伤,确定了包括粗研磨、半精研磨和精研磨工序的研磨工艺路线和合理的工艺参数。进一步探讨了金刚石砂轮超精密磨削单晶MgO基片的加工工艺,和研磨加工相比,在表面质量和亚表面损伤相当的情况下,超精密磨削可达到很高的加工效率,在单晶MgO基片加工中具有很好的应用前景。更多还原

【Abstract】 Single crystal MgO has good thermal stability, light transmittance, electrical insulativity, chemical stability and mechanical properties. It is mainly used as substrate for high temperature superconductor (HTS) thin films and also a kind of important optical material. The surface quality of the ultra-precision machined substrates greatly affects the performance of the thin films grown on it. However, the micromechanical characteristics, the damage mechanism and the precision processing technology of MgO substrates have not been systematically studied, the applications of single crystal MgO have been limited. In this dissertation, the nano-mechanical behhavior, microscopic deformation and surface layer damage mechanism of single crystal MgO were studied, and abrasive processing technology were investigated. It has great theoretical significance and applied value to promot the application of single crystal MgO in the optoelectrionics field.The main research contents and conclusions are as follows:The nano-mechanical properties, microscopic deformation and surface layer damage characteristic were investigated by nanoindentation, micro-indentation and micro-scratch on different crystallographic planes of single crystal MgO. Under different loading conditions, single crystal MgO present different deformation, including elastic deformation, plastoelastic deformation, creep deformation, micro cracks and micro fractures. The load-displacement curve of elastic deformation fits well with the curve of Herz elastic contact, the elastic deformation can recovery after unloading. The plastic deformation is the result of the dislocations nucleation and glidding in the {110} easy slip planes of MgO. There is pop-in phenomenon on load-displacement curve of nanoindentation at the transition point from elastic to plastoelastic deformation, which is associated with the acoustic emission (AE) signal with distinct waveform, and the elastic released energy of pop-in is in proportion to AE energy. The average critical shear stress of dislocation nucleation of nanoindentation on MgO (001) and MgO (110) planes were calculated to be 14.85 GPa and 12.61 GPa, which are very close to the theory critical shear stress of 13.82 GPa. Increasing the holding time in nanoindentation induces creep deformation, including transient stage creep and steady stage creep. The creep stress exponents of MgO (001), (110) and (111) planes in steady state creep were calculated to be 36.5,53.7 and 22.4, respectively. When increasing the load of micro-indentation and micro-scratch to a certain extent, micro cracks including radial cracks, lateral cracks and median cracks generate in surface and subsurface of MgO. Through the cross-section observation of the micro-indentation and micro-scratch on MgO, two types of subsurface cracks were confirmed according to the different forming mechanism. One is the cracks in (110) planes, including the {110}90°cracks and inclined cracks, intersecting the top (001) surface at 90°and 45°, respectively, which result from the intersection and blocking of dislocations gliding in two easy slip planes intersecting each other at 120°; the other is the median cracks (vertical to the top (001) surface) or lateral cracks (parriable to the top (001) surface), which result from the cleavage in the (100) easy cleavage planes.Based on the analysis of micromechanical behavior, the surface and subsurface damage of MgO substrates were studied through the observation of the surface and cross-section of sawed, lapped and ground MgO substrates. The vertical apnd inclined dislocation glidding systems were observed in the cross-section of MgO substrates machined by different abrasive processing methods. The vertical slip lines come from the dislocations glidding in the {110}90°easy slip planes, and the inclined slip lines result from the dislocation glidding in the {110}45°slip planes. Three basic shape cracks including lateral cracks, vertical cracks and inclined cracks were found in the cross-section of the rough machined MgO substrates. Other cracks with complicated shapes, such as the "hook" crack, the "zigzag" crack, the "chevron" crack and the "umbrella" crack, consist of the above mentioned three basic cracks.Based on the observation and analysis of the surface and subsurface damages of MgO substrates, the material removal mechanism of rough and fine processing MgO substrates was analyzed and the model of subsurface damage induced in MgO substrates after rough and fine processing were proposed. In rough processing MgO substrate, the material is removed in fracture mode by the extension and intersection of large numbers of median cracks, radial cracks and lateral cracks, the formed surface have micro-steps structures, and the subsurface damage layer consists of the fracture layer, the dislocation glidding layer and the elastic deformation layer.In fine processing MgO substrate, the material is removed in fatigure failure mode by the high strain result from the continuous plastic deformation and the subsurface damage layer is composed of the dislocation glidding layer and the elastic deformation layer.The systematic precision lapping experiments of MgO substrates wconducted, the influences of processing parameters on the surface roughness, material removal rate and surface damage were studied, the processing route composed of rough lapping, semi-fine lapping and fine lapping and resonable processing paramaters were obtained. Furthermore, the ultra-precision grinding technology of MgO substrates with diamond wheel was investigated. Compared with the lapping technology, the grinding has much higher machining efficiency under the condition of obtaining comparative surface quality and subsurface damage, and shows bright application prospect.更多还原