Research On Theory And Key Technology In Precision Grinding Of KDP Crystal

Potassium dihydrogen phosphate(KH₂PO₄,abbreviated as KDP)crystal possesses good transparency,high nonlinear optical coefficient,high laser damage threshold and excellent electro-optic coefficients from near infrared to ultraviolet domain.It is widely used in optical components such as electro-optic modulators,Q-switching switches and shutters for high-speed photography.Meanwhile,KDP crystal is an irreplaceable material for making the crucial optical components in laser nuclear fusion ignition systems as large size KDPs(>400mm)are producible using artificial crystal growth method.The production cycle for making KDP components is rather long,as cumbersome manufacturing processes are required starting from crystal growth up to various precision and ultra-precision machining steps.The present work studies the feasibility using grinding method,as an alternative approach to the single point diamond(SPD)cutting technique,to improve the process efficiency of KDP crystals at the precision machining stage.The main research contents and results are as follows:Based on mechanics of anisotropic material,micro-indentation test and Anstis fracture toughness calculation model,the anisotropy of elastic modulus E,micro Vickers hardness H_vand fracture toughness KIC of KDP crystal were analyzed and[100]or[010]was determined as the best direction for precision grinding.Suitable grinding wheel and process parameters for the precision grinding of KDP were determined by analyzing the influence of average size of abrasive grains and the grinding parameters on the surface roughness.Grindability assessment tests were carried out,using surface roughness,surface topography,sub-surface damage(SSD)depth and process efficiency as evaluation indexes.Surface roughness about R_a0.25?m and SSD depth?6?m were achieved,which are similar to that obtained in a single-point diamond turning process,whilst the process efficiency was improved by nearly 10 times.The grindability assessment verified that grinding method was able to achieve high efficiency machining of KDP crystal with good surface quality and low damage.An analytical temperature field model for anisotropic workpiece material under cup-wheel surface grinding mode was derived.Considering the material removal characteristics of the cup-wheel,a quadratic heat source distribution was proposed to derive the grinding temperature distribution.The grinding process along the workpiece length was divided into three parts:cutting-in,stable grinding and cutting-out,taking account of the variation of wheel-work contact along grinding direction,and a transient grinding temperature model was established.The assumption of quadratic heat source distribution was verified by comparing the measured temperature signal curves with those calculated using uniform,triangular and quadratic heat source distributions respectively.Verification tests of the grinding temperature model were conducted by comparing the variation of measured and theoretically calculated grinding temperatures under different grinding parameters.It was proved that the grinding temperatures calculated using the transient temperature model,which considers the grinding wheel cutting-in and cutting-out,were more accurate.Through microscopic observation and Raman spectroscopy measurements on the ground samples obtained under different grinding temperature rises,it is qualitatively proved that there is no thermal damage crack,new material or new phase on the crystal surfaces after grinding using the resin bonded grinding wheel with an average grain size of 36?m.To solve the problems of inevitable binder and abrasives embedding onto the KDP crystal surfaces when using a conventional grinding wheel,an innovative wheel design with no bonding agent was proposed.After detailed comparison of the characteristics of different diamonds,CVD diamond was selected as the abrasive layer material of the grinding wheel.Femtosecond laser was determined as an effective means to produce micro-cutting edges on a large-diameter CVD diamond surface,through assessment tests comparing the ablation ability of ultraviolet laser and femtosecond laser for diamond.Comparison tests between a traditional resin bond wheel and the newly developed binderless wheel,were conducted.The grinding performance indexes include wheel wear,grinding ratio,grinding force,specific grinding energy,surface roughness,surface topography and subsurface damage.It shows that the binderless wheel not only has a grinding ratio up to 1000,but also a single wear pattern,which avoids the introduction of binder and abrasives embedding onto the crystal surfaces.In order to further improve the surface quality of KDP crystals ground by the binderless wheel,another binderless wheel with positive rake angles on the cutting edges was designed and manufactured,based on the analysis of feed and rotational motions of the wheel.The cutting edges on the grinding wheel surface are divided into a main cutting zone and a minor cutting zone,and the rake angle of the cutting edges in the main cutting zone is 5°.Compared with the previous wheel with 0°rake angles in the main cutting zone,it showed that the grinding wheel with positive rake angles effectively reduced the grinding force,specific grinding energy and surface roughness.The wheel with positive rake angle design shows a good potential to realize high-efficiency and low-damage precision grinding of KDP crystals.