| Single crystal silicon carbide(SiC)is characterized by a high hardness,a high melting point,a high thermal conductivity,a very high chemical and thermal stability and excellent semiconductor properties.It is the third-generation semiconductor material and has a wide range of applications and development prospects in the semiconductor industry.However,single crystal SiC is a typical difficult-to-machine and hard-brittle material.To improve the machining efficiency and quality,this dissertation systematically investigated the near-damage-free micromachining mechanisms of single crystal SiC wafer using hybrid laser-waterjet.The research results of this dissertation can be applied to near-damage-free micromachining of similar hard-brittle materials.Formability of single crystal SiC wafer using hybrid laser-waterjet micromachining was experimentally studied.The effect significance of process parameters on the microgroove depth,microgroove width and material removal rate was studied.The results show that the waterjet inclination angle,waterjet offset distance,nozzle stand-off distance,scanning speed,average laser power and water pressure have a very significant effect on the microgroove depth,microgroove width and material removal rate.The laser focus height has a very significant effect on both the microgroove depth and the material removal rate,but has an insignificant effect on the microgroove width.The laser pulse frequency is very significant for the microgroove depth,and moderately significant for both the microgroove width and the material removal rate.Regression prediction models for microgroove depth,microgroove width and material removal rate were established.The average errors of the models for the prediction of microgroove depth,microgroove width and material removal rate are less than 10.0%.The effect of single process parameters on the microgroove depth,microgroove width,and material removal rate was studied.The results show that the microgroove depth,microgroove width and material removal rate increase with an increase in the average laser power.The microgroove depth,microgroove width and material removal rate increase firstly and then decrease with an increase in the waterjet ofifset distance.The microgroove depth,microgroove width and material removal rate decrease with an increase in the nozzle stand-off distance and water pressure.With an increase in the scanning speed,the microgroove depth and microgroove width decrease and the material removal rate increases firstly and then decreases.The effect of interaction between process parameters on the microgroove depth,microgroove width and material removal rate was studied.The results show that the critical waterjet offset distance that maximizes the microgroove depth,microgroove width and material removal rate,increases with an increase in the water pressure,but hardly changes with a change in the nozzle stand-off distance.The surface quality of single crystal SiC wafers machined by hybrid laser-water micromachining was evaluated.The results show that the hybrid laser-waterjet micromachining technology derived a high surface quality.On the surface machined by hybrid laser-waterjet micromachining,the boundaries between the machined area and the unmachined area are clear,the edges of the cut are straight,and the material surface on both sides of the cut is clean.The obtained V-shaped microgroove has a good three-dimensional shape,a good cross-sectional profile and a smooth transition between microgroove edges and the unmachined surface.The obvious heat-affected zone was not observed on both sides of the cut.The surface texture of the cut sidewall was clear,which is a feature of plastic slip.Besides,there are not recast layers on the material surface.The effect mechanisms and rules of multi-field coupling between laser,water and workpiece material during hybrid laser-waterjet micromachining were studied.The heating effect of the laser on the workpiece material was studied,and the distribution model of the laser heat source in both space and time domains was established.The results show that the energy of the laser heat source is exponentially distributed on the z-axis and gradually weakens along the negative direction of the z-axis.The distribution of the laser heat source in time domain is pulsed and periodical.The laser intensity is approximately a normal distribution during the pulse period.The impinging effect of the waterjet on the workpiece material was studied.The results show that the maximum wall shear stress decreases with an increase in the nozzle stand-off distance.With an increase in the nozzle stand-off distance,the maximum wall shear stress point gradually moves away from the stagnation point.The cooling effect of the waterjet on the workpiece material was studied.The results show that the average convection heat transfer coefficient in the impingement region is two orders in magnitude higher than that in the wall jet region.The forced convection heat transfer of waterjet mainly occurs in the impingement region.The average convection heat transfer coefficients in both the impingement region and the wall jet region increase with an increase in the water pressure.The interference of the waterjet on the laser was studied.The results show that the laser reflectivity of water is about 2%.The laser absorptivity of water is less than 1.4%in the impingement region and less than 0.3%in the wall jet region.After refraction,the laser focus plane moves down,and the focused laser diameter increases.The refractive index slightly decreases with an increase in temperature,and the change of water pressure has little effect on the refractive index.The turbulence and air entrainment of water can reduce the laser beam quality when the laser passes through the water.The increase of the water pressure enhances the water turbulence and hence further reduces the laser beam quality.Plasma is not formed in the water during machining.The temperature field distribution inside the workpiece material during hybrid laser-waterjet micromachining was studied.The initial conditions and boundary conditions of the temperature field model of the hybrid laser-waterjet micromachining of single crystal SiC were determined.The relationship between thermal properties of single crystal 4H-SiC and temperature was studied.A temperature field model of hybrid laser-waterjet micromachining of single crystal SiC was established by considering the change of the thermal properties of the workpiece material at high temperatures.The results show that within the time range of 0-200 ns,the internal temperature of the workpiece material gradually increases,and the heated area of the workpiece material gradually expands.The isotherm distribution law inside the workpiece material is consistent with the energy distribution law of the laser heat source acting inside the workpiece material.The material removal mechanism of hybrid laser-waterjet micromachining of single crystal SiC was uncovered.When the temperature is higher than the brittle-ductile transition temperature,the yield stress of single crystal SiC is lower than the fracture stress,and the material shows plastic behavior.When the applied resolved shear stress exerted on the primary slip system,(0001)<1120>,is greater than the critical resolved shear stress(CRSS)of the workpiece material,the workpiece material undergoes lattice slip and deformation,and is finally removed.With an increase in temperature,the CRSS of single crystal 4H-SiC decreases significantly.When the temperature is 1650 K,the CRSS of single crystal 4H-SiC is less than 5 MPa.A finite difference model for workpiece material removal profile in hybrid laser-waterjet micromachining of single crystal 4H-SiC was established.The validity of the model was experimentally verified and the results show that the model can predict the microgroove depth,microgroove width and material removal rate with a good accuracy.The evolution process of temperature and material removal profile in one and multiple laser pulse cycles were respectively studied.The results show that during one laser pulse cycle,the material is heated throughout the pulse period(0-350 ns),and is removed approximately in the 200-350 ns time range.When t ≈ 350 ns,the heating and removal of the material are suspended,and the workpiece material gradually cools under the forced convection heat transfer effect of the waterjet.As the number of laser pulses increases,the microgroove depth gradually increases but the microgroove width increases insignificantly until the final cut contour is formed.The higher the average laser power,the more the material is softened and removed.However,the maximum temperatures of the workpiece material under different laser average powers are approximately same. |
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