【作者】 鲁春朋；

【导师】 高航；

【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】 磷酸二氢钾(KDP)晶体是一种优质非线性光学晶体,具有较大的电光及非线性光学系数、高的激光损伤阈值、低的光学吸收系数、良好的光学均匀性等特点。大口径高质量的KDP晶体是目前唯一可用于惯性约束核聚变(ICF)、强激光武器等光路系统中的激光倍频、电光调制和光电开关器件的非线性光学材料,在高新技术和国防尖端技术领域具有重要的应用前景。但是,KDP晶体具有各向异性、软脆、易潮解等特点使其成为极难加工材料,而高精度高表面完整性的加工要求使精密和超精密加工技术面临新的挑战。针对目前人们对KDP晶体精密和超精密加工困难的科学本质认识有限,本文以延性加工为目标,系统的研究了KDP晶体的纳米力学行为、弹塑性变形、断裂韧性、微裂纹扩展、动态摩擦学特性、材料去除机理,以及结合KDP晶体材料特性的延性加工理论分析和试验验证。针对上述问题进行了一些探索性的研究工作,主要内容与学术贡献如下：通过纳米压痕技术对比研究了KDP晶体三个典型晶面的纳米力学特性。应用MPSR模型和应变梯度理论能够很好的解释和说明KDP晶体纳米压痕尺寸效应现象是一种载荷和压痕深度的非线性比例阻尼现象,并且非线性程度和试件的表面加工残余应力密切相关。指出滑移是KDP晶体产生塑性变形的主要模式,塑性变形初始是pop-in现象发生的原因,卸载曲线末端发生变化是由于压头和KDP晶体表面的粘附作用所致。进一步分析结果表明,KDP晶体(001)晶面、二倍频晶面和三倍频晶面纳米硬度和弹性模量随压痕方向变化呈现周期性。这是由晶面结构和压头形状对称性及压头下等效剪切应力随晶向变化决定的。(001)晶面、二倍频晶面和三倍频晶面硬度和弹性模量周期分别为90°、120°和120°。此外,(001)晶面硬度各向异性程度为46.2%,弹性模量各向异性程度为63.9%；二倍频晶面的纳米硬度各向异性程度为32.9%,弹性模量各向异性程度为75.6%；三倍频晶面硬度各向异性程度为36.3%,弹性模量各向异性程度为51.2%。借助划痕测试手段,研究了KDP晶体材料的去除过程和摩擦学特性。结果表明,KDP晶体去除过程可分为如下4个阶段：弹塑性变形,塑性耕犁,微切屑生成和表面损伤。由于KDP晶体具有软脆特性,弹塑性变形阶段和塑性耕犁阶段的界限区分并不明显,并且变形机理受晶向影响强烈。(001)晶面、二倍频晶面和三倍频晶面划痕摩擦系数变化周期分别为90°,180°和180°,各向异性程度分别为50%,43.8%和43.8%。(001)晶面摩擦系数变化范围为0.09±0.01～0.18±0.01；二倍频晶面摩擦系数变化范围为0.09±0.02～0.16±0.01；二倍频晶面摩擦系数变化范围为0.09±0.02～0.16±0.02。同时,研究了单颗粒划痕对KDP晶体表面和亚表面产生的加工损伤,分析了各向异性对微裂纹产生的影响,探讨了晶体表面和亚表面损伤行为及形成机理。重点研究了KDP晶体不同晶面的压痕断裂特征。研究发现KDP晶体在进行低载压痕断裂韧性测试时三个常用典型晶面的裂纹长度和压痕载荷均呈现P/c1.5=常数的关系,且断裂韧性值对载荷增加依赖性不大。(001)晶面、二倍频晶面和三倍频晶面的断裂韧性值受晶向影响较大,各向异性程度分别为63.8%,64.3%和34.1%。断裂韧性值的范围分别为0.096～0.265 MPa·m0.5,0.096～0.269 MPa·m0.5和0.122～0.185 MPa·m0.5。结合上述KDP材料力学行为的研究结果和理论分析,以磨削加工为例,理论上给出了KDP晶体不同晶面的最佳磨削方向和临界延性域切削深度,得到KDP晶体(001)晶面最佳磨削方向为平行于[100]或[010]晶向,最大延性域切削深度为0.22±0.05μm；二倍频晶面最佳磨削方向为和[110]晶向成135°方向,最大延性域切削深度为0.52±0.06μm；三倍频晶面最佳磨削方向为平行于[100]晶向,最大延性域切削深度为0.23±0.05μm。使用两种平面磨床进行了KDP晶体延性加工试验验证和分析,得到了磨削方向对表面质量影响规律,为KDP晶体超精密加工技术研究提供了理论依据。更多还原

【Abstract】 Potassium Dihydrogen Phosphate (KH2PO4, KDP) is a nonlinear optical crystal material with larger opto-electronic and nonlinear optical coefficient, higher laser induced damage threshold, lower linear absorption coefficient, and better optical homogeneity, etc. Only KDP crystal with large aperature and high quality can be applied in the ICF and high power laser weapon as laser frequency multiplier, electro-optic modulator and photoelectric switches. Furthermore, it has very important position in the field of high technology and the leading technology of national defense. However, KDP crystal is an extremely difficult-to-machine material due to the nature of anisotropy, softness, brittleness, easy deliquescence, and so on. Perfect surface quality and surface integrality of KDP crystal part make ultraprecision machining technologies encounter a challenge. Thus it is distinctly significant to understand the ultrasmooth surface processing technology of KDP crystal. In this work, mechanical behaviors at the nanoscale, elastic-plastic deformation, fracture toughness, micro cracks growth, dynamic friction characteristics, materials removal mechanism, and ductile-regime machining technology of KDP crystal are systematically studied and estimated. The results and the main contributions of this study are followed as:Mechanical behaviors on the different crystal planes of KDP crystals at the nanoscale are investigated using nanoindentation approaches. Indentation size effect of KDP crystal is analyzed in detail in terms of strain gradient theory and the modified proportional specimen resistance (MPSR) model, and the results show that indentation size effect of KDP crystal is a nonlinear proportional specimen resistance phenomena between the load and indentation depth and the nonlinear amplitude of them is influnced greatly by the surface reisdual stress of processed sample. In addition slip, the initiation of which appears to cause pop-in events, is concluded to be the major mode of plastic deformation, and the variation of unloading curve end is the contribution of the adehesion between the tip and the contact surface. Moreover, the nanohardness and elastic modulus of the (001) plane, the doubler plane and the tripler plane were varied periodically with crystallographic orientations, which is correlated to the symmetry of the crystal structure, the tip shape, and the variation of the effective resolved shear stress under the tip. The periods of nanohardness and elastic modulus on the (001) plane, the doubler plane and the tripler plane are 90°,120°and 120°, respectively. The relative anisotropy of nanohardnes is 46.2%,32.9% and 36.3%, respectively. Material removal process of KDP crystal is investigated using scratching technology. In the nanoscratch test, the deformation and removing process of the crystal materials consist of four parts, namely elastic-plastic deformation, the ploughing, micro chips formation, and surface damage. The difference between the elastic-plastic stage and the plowing stage is not very clear due to the soft and brittle nature of KDP crystal, and the material deformation greatly depends on the crystallographic orientations. Furthermore, the periods of the coefficients of friction on the (001) plane, the doubler plane and the tripler plane are 90°,180°and 180°, and their relative anisotropy are 50%,43.8% and 43.8%, respectively. The friction coefficient of the (001) plane is in the range of 0.09±0.01 to 0.18±0.01; the friction coefficient of the doubler plane is in the range of 0.09±0.02 to 0.16±0.01; the friction coefficient of the tripler plane is in the range of 0.09±0.02 to 0.16±0.02. Meanwhile, the effect of the anisotropy on surface/subsurface damage also was studied in micro scratching test.The characteristics of the fracture toughness of different crystal planes are measured with micro indentation technique. The relation between the crack length and indentation load for the measured crystal planes is P/c1.5=constant and the effect of the indentation load on the fracture toughness values is no apparent. The fracture toughness dramatically depends on the crystallographic orientation for the (001) pane, the doubler plane and the tripler plane, their relative anisotropy are 63.8%,64.3% and 34.1%, and their fracture toughness values are in the range of 0.096 to 0.265 MPa·m0.5,0.096 to 0.269 MPa·m0.5 and 0.122 to 0.185 MPa·m0.5, respectively.According to mechanical behaviors of KDP crystal obtained above, the critical ductile depth of cut and optimum machining direction on different crystal plane are given. The maximum ductile depth of cut on the (001) plane is 0.22±0.05μm in which its direction is paralleled to [100] or [010]; the maximum ductile depth of cut on the doubler plane is 0.53±0.06μm in which the angle between its direction and [110] is 135°; the maximum ductile depth of cut on the tripler plane is 0.23±0.05μm in which its direction is paralleled to [100]. The relation between the processed surface quality and grinding directions is clarified and validated experimentally with two kinds of surface grinders, which provides a theoretical reference for ultraprecision machining technology of KDP crystal.更多还原