SiC光学材料亚表面缺陷的光热辐射检测

Detection of subsurface defects in silicon carbide bulk materials with photothermal radiometry

SiC以优异的物理性能和良好的工艺性能,逐渐成为大型空间成像光学系统主镜的首选轻量化光学材料.SiC镜坯制备及加工过程中引入的亚表面缺陷会严重影响最终的镜面质量以及光学系统的成像品质.针对SiC材料亚表面缺陷的检测问题,本文采用光热辐射技术进行分析:分别建立均匀样品的单层理论模型和含空气层缺陷的三层理论模型,用于计算无缺陷和存在缺陷区域的光热辐射信号.通过对三层理论模型信号的相位仿真分析,提出利用相位差-频率曲线的特征频率估算缺陷深度的经验公式;利用光热辐射装置测量存在亚表面缺陷的SiC样品,分析缺陷区域的光热辐射信号分布,利用经验公式计算缺陷深度,并与缺陷实际深度分布进行对比分析.实验与计算结果显示,光热辐射技术能有效探测SiC镜坯的亚表面缺陷及其形貌,并且对于界面与样品相对平行且较为平缓的亚表面缺陷,其缺陷深度可通过经验公式准确确定.

With excellent physical,mechanical and processing properties,silicon carbide(SiC)has gradually become a preferred lightweight optical material for primary mirrors of large space optical systems.The subsurface defects generated during the preparation and processing procedures of SiC will affect the optical quality of the primary mirrors and the imaging performance of the corresponding optical systems employing the SiC primary mirror as well.In this work,photothermal radiation(PTR),a powerful nondestructive testing technique for detecting sub-surface defects of solid materials,is employed to characterize the subsurface defects of bulk SiC material for primary mirrors.Theoretically,three-dimensional one-layer and three-layer PTR theoretical models are developed to describe the defect-free and defect regions of an SiC bulk material.By analyzing the frequency dependence of PTR phase of the SiC bulk material with different defect depths,an empirical formula for estimating the defect depth via a characteristic frequency(appearing at the minimum of the PTR phase-frequency curve)defined thermal diffusion length is proposed,and simulation results show reasonably good agreement between the estimated and simulated defect depths in a depth range of 0.05–0.50 mm.Experimentally,an SiC bulk sample with a subsurface defect region is tested by the PTR via position scanning and modulation frequency scanning to obtain the position and frequency dependent PTR amplitude and phase.From the spatial distributions of PTR amplitude and phase measured at different frequencies and the phase difference frequency curves of measurement positions in the defect region,the depth and shape of the defect region are estimated and found to be in good agreement with the actual shape of the defect region,which is destructively measured via a depth profiler.The experimental and calculated results demonstrate that the PTR is capable of detecting non-destructively the subsurface defects of SiC bulk material.In addition,for subsurface defects with relatively flat interface,the defect depth can be determined accurately by the developed empirical formula.