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 the preferred lightweight optical materials 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 therefore the imaging performance of the corresponding optical systems employing the SiC primary mirror. In this paper, 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 were developed to describe the defect-free and defect regions of a SiC bulk material. By analyzing the frequency dependence of PTR phase of the SiC bulk material with deferent defect depths, an empirical formula for estimating the defect depth via a characteristic frequency (appeared at the minimum of the PTR phase-frequency curve) defined thermal diffusion length is proposed, and simulation results showed reasonably good agreement between the estimated and simulated defect depths in the depth range of 0.05mm to 0.5mm. Experimentally, a SiC bulk sample with a subsurface defect region was tested by PTR via position scanning and modulation frequency scanning to obtain the position and frequency dependences of the PTR amplitude and phase. From the spatial distributions of PTR amplitude and phase measured at different frequencies and the phase difference frequency curve of measurement positions in the defect region, the depth and shape of the defect region were estimated and found to be in good agreement with the actual shape of the defect region, which was destructively measured via a depth profiler. The experimental and calculated results demonstrated that 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 could be determined accurately by the developed empirical formula.