| Because of the high optical homogeneity and the stable physical and chemicalproperties, optical glasses are widely used in the fields of laser technology,optoelectronics, communications, aerospace and defense industry. Grinding process isone of the most important processes which determine the quality and performance ofoptical glasses. Due to the hardness and brittleness of optical glasses, however, surfaceand subsurface damages often occur during grinding process and strongly influence bothoptical quality and mechanical strength of optical glasses. Scratch process in nano/microscale and grinding process for optical glass BK7were investigated theoretically andexperimentally. Material removal and surface formation mechanisms in grinding ofoptical glass were analyzed systematically. A method to optimize grinding parameterswas proposed. The main contents of this paper are as follows:Firstly, the compositions, elastic modulus, hardness and fracture toughness of opticalglass BK7and fused silica were investigated by means of Energy dispersive X-raySpectroscopy (EDS) and nano/micro indentation tests. Compared with fused silica,optical glass BK7has larger modulus, smaller indentation hardness and smaller fracturetoughness. By means of multi-indentation tests, it was found that optical glass BK7ismore likely to be removed through brittle fracture during abrasive processes, e.g.,grinding, lapping and polishing.Secondly, by considering the effect of elastic recovery on contact area during scratchprocess, the model of scratch hardness for optical glass was proposed. Based on theindentation fracture mechanisms as well as the analogy between indentation and scratch process, the relationship between subsurface crack depth and scratch depth was predicted.By means of3D finite element analysis, the simulation model of single scratch processwas established. The relationships between scratch stresses and three kinds of cracks(median cracks, lateral cracks and radial cracks) were analyzed. The drive forces,initiation positions and initiation consequences of different cracks were studied.Thridly, by conducting double scratch tests on optical glass, the influence of scratchload and scratch interaction on material removal mechanisms and surface/subsurfacecrack depths were analyzed. It was found that the interaction between lateral cracks is themajor type of material removal mode. Based on double scratch tests and high-speedmulti-scratch tests, the predition models of surface and subsurface crack depth, whichconsidered the effect of scratch interaction and scratch speed, were established.Fourthly, based on the kinematic analysis of horizontal surface grinding as well asthe features of grinding-induced cracks, four grinding modes were proposed. They arebrittle mode, semi-brittle mode, semi-ductile mode and ductile mode. The criticalconditions of different grinding modes were proposed. The four different grinding modeswere investigated with the characteristics of surface morphologies, subsurface cracks andindentation hardness.Fifthly, the effects of grinding parameters (grit size, feed rate, wheel speed andgrinding depth) on surface roughness and subsurface crack depth were investigated andanalyzed. The quantitative relationship between surface roughness and subsurface crackdepth was established and provided valuable insights into a quick and non-destructiveway for the measurement of subsurface crack depth.The choices of grinding parametersunder different grinding processes were analyzed.The research results are helpful to:(1) explain the fundamental interaction betweengrit and specimen as well as material removal mechanism during grinding;(2) providevaluable insights into the optimization of grinding parameters;(3) develop the grindingtechnology of optical glass. |
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