Study On The Surface Layer Crack Damage In Diamond Wire Sawing Of Single Crystal Silicon Carbide

With the development of electric vehicles and 5G technology,the third generation semiconductor materials,represented by single crystal silicon carbide(SiC),are widely applied into the power semiconductor field due to their excellent performance.However,the high hardness and brittleness of single crystal SiC make it easy to generate crack damage in wire sawing process.Due to the lack of mechanical and mathematical tools,it is difficult to quantitatively analyze the mechanism of crack growth and to control the crack damage in wire sawing of single crystal SiC.In this thesis,the mechanism of crack propagation and the interaction of cracks in single and multiple scratching of single crystal SiC were studied,and a prediction model of crack damge depth in wire sawing of single crystal SiC was established.This research is of great significance to the high efficiency and high quality wire sawing of single crystal SiC and the development of the fabrication of its components.The main researches in the thesis are:The geometrical shape of diamond abrasives affixed on wire saw was simplified as a regular triangular pyramid with a spherical crown,and a scratching force model was established considering the elastic recovery and indentation size effect of hardness in scratching of single crystal SiC.The driving stresses for median cracks,surface lateral cracks and subsurface lateral cracks were discussed based on the scratching stress field.An experimental setup for scratching of single crystal SiC with the abrasives affixed on wire saw was established.Single scratching experiment and double scratching experiment with one abrasive were performed on(0001)4H-SiC,and the mechanisms of material removal and crack propagation in the scratching process were analyzed.A method was given to quantitatively analyze the interaction of median cracks in multiple abrasives scratching of single crystal SiC based on linear elastic fracture mechanics.The scratching-induced stress field was calculated and its effect on the propagation of median cracks was discussed.The interaction of median cracks was described by the stress intensity factors(SIFs)at the crack tip.The calculation of SIFs in multiple abrasives scratching was transformed into the calculation of SIFs under the scratching-induced stress field applying superposition principle.The SIFs at the tip of mode-I crack subjected to symmetric and anti-symmetric concentrated loads were numerically solved,respectively,with Fourier transformation and Fredolm equation.Then,the SIFs under the scratching-induced stress field were obtained applying the superposition principle and Green's function.The accuracy of the calculation method was verified by comparing with the results of isotropic materials in literatures.The effect of scratching distance on the values of SIFs in single crystal SiC was analyzed and it was found that the interaction of median cracks performs as a crack shielding effect,which means that the existence of the median crack inhibits the propagation of its adjacent median cracks.A method was given to quantitatively analyze the interaction of subsurface lateral cracks in multiple abrasives scratching of single crystal SiC based on linear elastic fracture mechanics.The scratching-induced stress field was calculated and its effect on the propagation of subsurface lateral cracks was discussed.The interaction of subsurface lateral cracks was described by the SIFs at the crack tip,and was transformed into the calculation of SIFs under the scratching-induced stress field applying the superposition principle.The SIFs at the tip of mode-I crack subjected to the scratching-induced stress field were numerically solved with Fourier transformation,gap functions of crack displacements and Schmidt method.The accuracy of the calculation method was verified by comparing with the results of isotropic materials in literatures.The effect of scratching distance on the values of SIFs in single crystal SiC was analyzed and it was found that the existence of the median crack enhances the propagation of its adjacent cracks.A model to describe the three-dimensional shape of electroplated diamond wire saw was established.Mathematical methods for the description of cone angle distribution,size distribution and density of the abrasives affixed on saw wire were provided.Based on the balance between the scratching forces of abrasives and the sawing force corresponding to the bending of wire saw,a model was established to predict the crack damage depth(SSD)in wire sawing of single crystal SiC.A wire sawing experiment of 4H-SiC was carried out on a wire sawing machine.The values of SSD under different sawing parameters were measured to verify the prediction model.Effects of wire sawing parameters and geometric parameters of wire saw on the values of SSD were analyzed and methods to decrease the values of SSD were proposed.