Detection of Subsurface Defects in Silicon Carbide Bulk Materials with Photothermal Radiometry

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.