The Damage Characterization Technique For Ultra Precision Machined Surface/Sub Surface Based On XRD

KDP crystal is an important optical components used in inertial confinement fusion,its actual laser-induced damage threshold(LIDT)is much lower than the theoretical value,which greatly influenced the performance.It is generally believed that the surface/subsurface damage plays an important role in affecting the LIDT.It is necessary to reduce the surface/subsurface damage so as to improve the LIDT of KDP crystal,the basic part of which is to characterize the damage effectively.Therefore,this thesis used a nondestructive method to detect and characterize the damage,with an aim to improve the ultra-precision machining quality.The detection method used in this experiment is coplanar grazing incidence X-ray diffraction(GIXRD),which is usually used to study film crystals.So it is an innovative application to take GIXRD to study the surface/subsurface damage.First of all,we made a study on the theoretical coplanar diffraction phenomena of both ideal single crystal and defective crystal.The structure changes of defective crystals include the spacing changes and misalignments of crystallographic planes.Then we detected and analyzed the surface/subsurface damage by means of the coplanar GIXRD method,which verified the theory proposed in the same thesis.Finally,we analyzed the coplanar diffraction of ultra-precision machining surface/subsurface structure of KDP crystal and characterized the surface/subsurface damage based on verified theory.This thesis put forward and verified an innovative method to detect the surface/subsurface damages by means of the analysis of the coplanar GIXRD phenomena about the spacing changes and misalignments of crystallographic planes as well as ideal single crystals.Based on the method described in this paper,we characterized surface/subsurface damages and discovered that the subsurface structure of KDP crystal contains polycrystalline layer,preferred orientation layer and lattice deformed layer for the first time.The thickness of polycrystalline layer and preferred orientation layer was calculated,which was obviously different between specimens.And the minimum value is more than 3?m.In addition,we found that the macroscopic stress of the first two layers is tensile stress,which increased with rising feed speed and cutting depth.Besides there is mainly tensile stress in the third layer as well as a certain compressive stress.The method described in this thesis has many excellent advantages such as high precision,high accuracy,wide applicable range,nondestructive test.It is widely suitable for the detection of ultra-precision processing single crystals,such as single crystal KDP,single crystal Si C,single crystal Si.It really plays a significant role in the improvement of ultra-precision machining quality.