【作者】 姚旺；

【导师】 韩杰才； 张宇民；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 博士

【摘要】 为了降低加工成本,探索快速、高效加工反应烧结碳化硅(RBSiC)高性能光学材料的有效方法,本文通过材料的组成及性能测试,系统的研究了RBSiC材料的磨削去除机理和磨削表面完整性,分析了材料强度受磨削、退火和抛光处理的影响,优化了磨削加工参数,具体工作如下:对RBSiC材料进行了物相分析,并测试了材料的各项热性能和力学性能。RBSiC材料是由α-SiC和Si组成的多晶材料,Si相分散在SiC相间。性能测试结果显示,RBSiC材料较为致密,热性能良好,具有较高的室温抗弯强度和弹性模量,极高的硬度,断裂韧性较低,适合作为空间反射镜片。结合单颗磨粒临界切削深度计算,通过扫描电镜、原子力显微镜和光学显微镜等手段对磨削表面的观测,系统地研究了RBSiC材料的去除机理。RBSiC材料以脆性断裂和塑性切除两种机理去除,表现为微破碎、穿晶断裂、撕裂以及犁耕等去除方式。根据粗糙度测量与表面SEM观察,并结合磨削力及磨削比能方程,分析了加工参数对磨削表面粗糙度和表面形貌的影响。加工参数中砂轮轴向进给增加,使磨削力增加,粗糙度增大,断裂去除模式所占的比例增加,磨削比能降低;工作台转速对粗糙度的影响趋势不明显;光刀能够显著降低表面粗糙度,但也会使磨削中引入的亚表面裂纹扩展,增加粗糙度。累计去除厚度与砂轮轴向进给的交互作用对粗糙度的影响很大。通过正交试验对磨削加工工艺进行优化,得出的最佳加工参数为轴向进给0.9μm/s,工作台转速2.1转/分,光刀1分钟,累计去除厚度0.1mm后修整砂轮。根据对磨削后表面和截面的观察以及硬度的测量结果,建立了RBSiC材料磨削表面损伤层组织特征。磨削表面损伤层含有塑性变形层、破碎层和裂纹层,且随轴向进给增大,磨削力增加,损伤层厚度增加。使用X射线衍射测量以及有限元模拟方法,研究了磨削后表面残余应力状态。测量结果显示,残余应力具有很强的方向依赖性,同样轴向进给,平行工作台半径放置进行磨削,试样的残余压应力数值大,残余压应力对控制小裂纹开裂的作用较明显;抛光和退火能够释放残余压应力,800℃1h退火后残余压应力数值极低。对磨削后热应力和机械残余应力的有限元分析显示,在磨削RBSiC材料中,热载荷作用小,随轴向进给增大,机械载荷和热载荷作用降低,机械载荷降低速度快于热载荷。根据断裂力学和韦伯统计的方法,分析了磨削、抛光及退火后的裂纹和残余应力对强度的影响。轴向进给增大,磨削试样的特征强度下降,同样轴向进给,垂直工作台半径放置磨削的试样受磨削引入长直裂纹及较低残余压应力的影响,强度低。800℃1h的退火释放了表面残余压应力,但由于退火作用使裂纹尺寸减小,特征强度与磨削试样相近,都大于抛光试样。RBSiC材料性能、去除机理及强度可靠性分析对反射镜材料性能及加工工艺的研究具有重要的实用价值。更多还原

【Abstract】 RBSiC material is a promising mirror material for space optical applications. In order to reduce the damage introduced in grinding process and enhance the grinding efficiency and the mechanics reliability, grinding mechanism, grinding parameters optimization and strength of annealed, polished and ground RBSiC material are studied.The structures, composition and physical properties of RBSiC are investigated. The experiment results show that RBSiC is a polycrystalline material composing ofα-SiC and Si. The filling of residual Si to the pores makes the products compact. The properties measurement results reveal that this RBSiC is appropriate for optical applications. The material has high Young’s modulus, high bending strength and outstanding thermal properties. It also has high hardness and low fracture toughness and.According to the calculated critical grit cut depth, the removal mechanism is explored using scan electron microscope (SEM), Atom force microscope (AFM) and optical microscope technique. The removal mechanism includes brittle fracture and ductile cutting, presented as micro fracture, plowing, grains fracture and so on.According to the roughness measurement and SEM observation, the effect of grinding parameters on surface roughness and surface topography is investigated. The effect of grinding parameters on grinding force and specific energy is also analyzed. The analysis shows that the increaseing down feed leads to large grinding force, which makes surfaces rough and the fracture percents increased. There has no obvious relationship between worktable rotational speed and surface roughness. Burnishing can improve the surface quality, but it can also cause the extension of subsurface cracks induced in grinding process toward the surface. As a result, the surface gets worse. The combination of the total removal depth and the down feed has significant effect on surface roughness.The optimized grinding parameters obtained using orthogonal tests are as follow: down feed of 0.9μm/s, worktable rotational speed of 2.1 r.p.m, burnishing for 1 minute and truing/dressing the wheel after 0.1mm is removed.The surface/subsurface damage characteristics are analyzed. The subsurface damage consists of plastic distortion layer, chipping layer and crack layer. The damage layer increases with down feed.The ground surface residual stresses were determined using X-ray diffraction technique. The results show that residual stress has great direction dependency. The residual stress is high when the specimen was ground with its long axis paralleling to the worktable radius. Anneal and polishing can relax the compressive residual stress The finite element method was used to simulate the thermal and mechanical stress in grinding processing. The analysis reveals that the effects of mechanical load and thermal load decrease with the increasing down feeds.The effect of residual stresses and cracks on bending strength is assessed. The increase of down feed decreases the characteristic strength. The long straight cracks have less effect on strength of the specimens ground with their long axis paralleling to the worktable radius. The compressive residual stress has better control on the small cracks than the long cracks. After anneal at 800℃for one hour the specimens has small crack size, which makes its characteristic strength close to the ground one and larger than the polished specimens.The investigations of properties, removal mechanism and strength reliability provide valuable experimental data on material properties and grinding optimization of RBSiC optical materials.更多还原