【作者】 王炜；

【导师】 万珍平； 周运岐；

【作者基本信息】 华南理工大学 ， 工程硕士（机械工程）（专业学位）， 2020， 硕士

【摘要】 光学玻璃具有良好的物理化学性能和独特的光学性能,广泛应用于航空航天、微机电、显示与触控等精密领域。光学玻璃是典型的硬脆性材料,在机械加工过程中极易产生表面裂纹和亚表面损伤,给加工制造带来极大挑战。织构砂轮已被广泛用于硬脆性材料的精密及其表面微结构加工,为探究织构砂轮磨粒的形状及其排布方向对磨削质量的影响,本文系统研究不同形状压头、不同刻划方向微纳刻划BK7光学玻璃时的力--位响应(P～h)、弹性回复率、摩擦特性、致密化、抗裂性、脆塑转变及脆性去除等力学行为。通过纳米压痕实验,测量分析BK7光学玻璃的弹性模量、微纳硬度等材料特性;以单晶硅为参照对象,分析、计算BK7光学玻璃的断裂韧性,其在相同条件下抵抗接触载荷的能力和抵抗裂纹扩展的能力比较弱。推导了不同形状压头(Vickers、Berkovich/Cube cornor压头)不同刻划方向(面朝前刻划、棱朝前刻划)刻划时的P～h响应关系模型,构建不同刻划深度下的考虑压头几何形状、材料参数的微纳刻划硬度模型和脆塑转变临界深度模型。实验研究Vickers、Berkovich压头在相同刻划载荷下,面朝前和棱朝前刻划时BK7光学玻璃的P～h响应、弹性回复率和材料去除量,刻划深度及材料去除量的理论预测值与实验结果能很好的吻合。在此基础上,建立不同刻划方向微纳刻划摩擦系数理论模型并与实验结果对比,理论模型预测值与刻划实验摩擦系数值相对误差在9%以内。刻划过程中材料致密化受到刻划方向的影响明显,而致密化对材料抗裂性有重要影响。利用Vickers、Berkovich压头不同刻划方向等载荷刻划BK7光学玻璃,将刻划后的玻璃高温退火处理,测定退后前后划痕沟槽的宽度、深度、下塌体积、塑性隆起的高度和体积,用划痕退火回复的体积比定量评价致密化。通过理论预测不同刻划方向下材料的致密化、残余应力场及抗裂性之间的关系。实验结果表明,相同刻划载荷下,Vickers压头刻划时的致密化大于Berkovich压头引起的致密化,棱朝前刻划的致密化大于面朝前刻划导致的致密化。致密化越大,残余拉应力越小,越不容易导致亚表面裂纹的萌生及扩展,从而有利于提高材料的抗裂性。在材料致密化与抗裂性研究的基础上,深入研究压头形状及刻划方向对光学玻璃脆塑转变及脆性去除的影响。利用FIB-SEM(聚焦离子束—扫描电镜)研究Berkovich压头和Vickers压头面朝前、棱朝前以及圆锥形压头线性加载刻划BK7光学玻璃时的脆塑转变临界深度。实验结果表明,Vickers压头刻划时光学玻璃的裂纹产生的临界载荷值比Berkovich压头更大,棱朝前刻划时光学玻璃的脆塑转变临界深度值和临界载荷更大,与前述不同刻划方向下材料的致密化对抗裂性的影响规律一致。此外,圆锥形压头刻划时脆塑转变临界深度比Vickers压头和Berkovich压头面朝前刻划时脆塑转变临界深度大,但小于Vickers压头棱朝前刻划时的脆塑转变临界深度。实验结果与前述考虑压头几何形状、材料参数的脆塑转变临界深度模型计算值进行了对比,预测结果的平均误差为7.29%。研究了维氏压头在较大载荷作用下不同刻划方向刻划BK7光学玻璃的材料去除,并通过有限元分析得到不同刻划方向下刻划的应力场。结果表明,面朝前刻划时更容易导致横向裂纹和径向裂纹的萌生和扩展,棱朝前刻划时由于较大的摩擦力,材料以切屑的形式从沟槽两侧排出,有限元应力场能很好地解释实验结果。更多还原

【Abstract】 Optical glass is widely used in precision fields such as aerospace,micro-electro-mechanics,display and touch fields with good physical chemical characteristics and unique optical properties.Surface cracks and subsurface damage are extremely easy to occur during machining since optical glass is a typical hard-brittle material,which is a great challenge for processing and manufacturing.Textured grinding wheels have been widely used in the precision and microstructure processing of hard-brittle materials.In order to explore the influence of the shape and arrangement direction of the abrasive grains in textured grinding wheels on the grinding quality,mechanical behaviors of BK7 optical glass during nano/micro scratching under different shaped indenters and different scratch directions were systematically researched in this paper,including force and displacement response(P～h),elastic recovery rate,friction characteristic,densification,crack resistance,ductile-to-brittle transition(DBT)and brittle removal.The characteristics of BK7 optical glass including the elastic modulus,the micro-nano hardness,and the fracture toughness were systematically measured and analyzed based on the nano-indentation test.Compared with single crystal silicon,the ability to resist contact load and crack propagation is relatively weaker for BK7 optical glass under the same conditions.Based on micro-nano scratch of optical glass,the mechanical response and micro-nano hardness models of different shapes(including Vickers and Berkovich indenter)and different scratch directions(including face-forward and edge-forward)were established.A predictive model of critical depth for ductile-to-brittle transition was established considering the indenter geometry,scratch directions and the elastic recovery rate of the material.Experimental research on the P～h response,elastic recovery rate,material removal volume of BK7 optical glass under face-forward and edge-forward scratching using Vickers indenter and Berkovich indenter was investigated.The theoretical prediction values of the scratching depth and material removal volume were well matched with the experimental results.On this basis,the theoretical model of micro-nano scratching friction coefficient under different scratch directions was established and compared with the experimental results.The relative error between the predicted value of the theoretical model and the experimental friction coefficient value is within 9%.During the scratching process,the scratch direction has a profound effect on the densification of the material,and the densification has an important effect on the crack resistance of the material.Using Vickers and Berkovich indenters under different scratch directions with the same normal load to scratch BK7 optical glass,the samples after scratching were annealed at high temperature.The width,depth,collapse volume,pile-up height and volume of the grooves before and after annealing were measured,and the densification was quantitatively evaluated by the volume ratio of annealing recovery V_R.The relationships between densification deformation,residual stress field and crack resistance of optical glass under different scratch directions were theoretically speculated.The experimental results showed that under the same scratch load,the densification of the optical glass during scratching by Vickers indenter was greater than that caused by Berkovich indenter,and the densification of edge-forward scratching is greater than that caused by face-forward scratching.The greater the densification,the smaller the residual tensile stress,and the less likely it is to cause the initiation and propagation of subsurface cracks,thereby helping to improve the crack resistance of the material.Based on the study of material densification and crack resistance,the effects of indenter shape and scratch direction on ductile-to-brittle transition and brittle removal of optical glass were thoroughly investigated.The critical depth of ductile-to-brittle transition for BK7 optical glass was obtained by the focused ion beam(FIB)-scanning electron microscope(SEM)method,and then the effects of different shapes of indenters and different scratch directions with the same were explored under linear loading scratch mode.The experimental results showed that the critical load of crack generated by Vickers indenter was larger than that of Berkovich indenter,and the critical depth and load of ductile-to-brittle transition for optical glass under edge-forward scratching was larger than that under face-forward scratching,which was consistent with the aforementioned effect of densification on crack resistance.In addition,the critical depth of ductile-to-brittle transition by conical indenter is larger than that by Vickers indenter and Berkovich indenter under face-forward scratching,but it is smaller than that by the Vickers indenter under edge-forward scratching.The measured value of the critical depth for ductile-to-brittle transition was compared with the predicted value of the theoretical model.The average error of the predicted result is 7.29%.The material removal of the BK7 optical glass scratched by Vickers indenter under a large load and different scratch directions was studied,and the stress field of the BK7 optical glass under different scratch directions was obtained by finite element analysis.The face-forward scratch is more likely to introduce the initiation and propagation of lateral cracks to the surface due to the larger residual stress,while the edge-forward scratch is more likely to cause the chip to discharge from both sides of the groove due to the larger COF.The experimental results matched the theoretical COF model and FEM simulation well.更多还原