基于纳米划痕的熔石英玻璃去除机制与亚表面裂纹研究

Material Removal Mechanism and Sub-surface Cracking in Quartz Glass Based on Nano-scratch

目的研究熔石英玻璃在不同动态载荷作用下的材料去除机制和亚表面裂纹形成与扩展机理。方法对熔石英玻璃试样进行纳米划痕试验,分别从划痕轮廓、划痕力2个方面分析不同载荷下熔石英玻璃的力学行为,通过扫描电镜和光学显微镜观察划痕形貌,分析材料去除机制;采用逐层截面显微法对划痕截面的亚表面裂纹形貌进行观测,研究动态载荷下工件亚表面裂纹的形成与扩展机理。结果当动态载荷小于118mN时,材料发生塑性变形,亚表面未产生裂纹;当动态载荷大于118mN且小于245mN时,材料处于塑脆转变阶段,亚表面裂纹以动态载荷加载点为起点,向试样内部扩展形成赫兹锥形裂纹并伴有横向裂纹的存在;当动态载荷超过245mN时,材料进入完全脆性断裂阶段,亚表面裂纹不断扩展至表面导致材料破碎。结论随着动态载荷的不断增大,熔石英玻璃的材料去除经历了弹塑性变形、塑脆转变、脆性断裂3个阶段;亚表面裂纹受到动态载荷的影响,裂纹从载荷加载点形成,并沿着应力最大方向不断扩展,导致材料表面发生脆性断裂;熔石英玻璃的临界切深与其动态弹性模量成正比,与其流动应力成反比,动态冲击载荷使熔石英玻璃的临界切削深度下降,亚表面裂纹更易扩展,材料去除更快进入塑脆转变阶段。

Nano-scratch experiment is a commonly employed method for studying the behavior of materials,specifically focusing on material properties and mechanical responses at the nanoscale.The work aims to conduct nano-scratch experiments on fused silica glass,so as to gain deeper insights into the removal behavior of individual abrasive particles and investigate the mechanisms of material removal and the propagation of subsurface cracks under dynamic loading conditions.Circular samples of fused silica glass with dimensions ofϕ30 mm×6 mm were prepared for the scratch experiments.Prior to experiments,the samples underwent a polishing pretreatment to reduce surface roughness to approximately 3 nm.The experiments were carried out with a Nano Indenter G200 nano-scratch instrument,at a constant scratch speed of 30μm/s and a scratch length of 1 mm.The loads ranging from 0 to 300 mN were gradually applied to the samples,and the profiles of the scratches were recorded.To investigate the subsurface crack morphology,samples were sectioned near the scratch location.Cross-sectional views of the scratches were obtained through successive polishing steps,followed by etching with a 2%HF solution.Scanning electron microscopy(SEM)was employed to observe the surface topography of the scratches and the morphology of subsurface cracks at various stages along the scratch.Comprehensive analysis of the scratch profile curves and cross-sectional views at different stages revealed the following:when the dynamic load was less than 118 mN,the scratch exhibited a uniform and smooth appearance with plastic deformation of the material.Subsurface cracks were not observed at this stage.For dynamic loads greater than 118 mN but less than 245 mN,the scratch showed noticeable material pile-up on the right side and slight brittle damage on the left side.The scratch profile exhibited significant fluctuations,indicating a transition from plastic to brittle behavior.Subsurface cracks began to form and propagate,primarily in the form of"eight"-shaped radial cracks and transverse cracks.When the dynamic load exceeded 245 mN,the scratch exhibited extensive fragmentation and spalling.The scratch profile displayed severe fluctuations,and the material reached a state of complete brittle fracture.Subsurface cracks continued to expand,forming multiple intersecting radial cracks,leading to material failure.This resulted in the gradual formation of central cracks and a"claw"morphology with multiple coexisting radial cracks.The material removal process of fused silica glass underwent three distinct phases with increasing dynamic loads:elastic-plastic deformation,plastic-brittle transition,and complete brittle fracture.Subsurface cracks,affected by dynamic impact loads,initiated from the point of loading and continually propagated along the direction of maximum stress,ultimately extending to the material surface and leading to extensive brittle fracture.The mechanical properties of fused silica glass were affected by the impact loads,with dynamic fracture toughness and dynamic hardness being directly proportional to the flow stress during loading.By combining the JC constitutive equation for fused silica glass,a new formula for the critical cutting depth of plastic-brittle transition was established.It was found that the critical cutting depth was directly proportional to the elastic modulus and inversely proportional to the flow stress of the material.Under dynamic loading conditions,the dynamic elastic modulus of the material experienced a slight decrease,while the flow stress significantly increased.Consequently,fused silica glass develops subsurface cracks more rapidly under dynamic loads,resulting in a reduced critical cutting depth.This effect accelerates the transition of the material into the plastic-brittle transition stage.