Micro Four‐Layer SiC Coating on Matrix Graphite Spheres of HTR Fuel Elements by Two‐Step Pack Cementation

Abstract

A micro four‐layer SiC coating, which includes inner transition layer, fine‐grained layer, dense bulk layer, and outer loose layer, was fabricated on the matrix graphite spheres of high‐temperature gas‐cooled reactor fuel elements to improve the oxidation‐resistant property by a two‐step pack cementation process. According to the experiment results, the micro four‐layer can be differentiated by SiC grain size and microstructure variation. The oxidation tests at 1773 K for 200 h reveal that the coating structure could effectively improve the oxidation resistance of matrix graphite spheres with a weight gain of 0.52 wt%, and the fine‐grained and dense bulk layers are evidenced as two main antioxidation layers. Although the thermal expansion coefficients of SiC and matrix graphite do not match each other so well, no obvious stress cracking was observed after thermal shocking tests from 1773 K to room temperature for 100 times.

Dual layer SiC coating on matrix graphite sphere prepared by pack cementation and fluidized‐bed chemical vapor deposition

“…Second‐order scattering of β‐SiC is in the range of 1450‐1700 cm −1 with three low‐intensity peaks . Two main absorbance bands of carbon, D band (around 1360 cm −1 ) and G band (1580‐1600 cm −1 ), are the main order peaks of crystalline carbon (main phase of matrix graphite), which represents graphite flakes in p‐ SiC layers (Figure A). Because the XRD results were detected through the surface of the coating layers and Raman spectroscopy can be relatively more sensitive to low‐level carbon, no absorption peaks of carbon or graphite can be seen in Figure .…”

“…Figure shows the cross‐section of dual layer SiC‐coated MG sphere after more than 50 times thermal shock cycle tests as mentioned in experiment section. Due to the thermal expansion coefficient (CTE) mismatch of SiC and MG, there will be a thermal stress during both the quick heating and cooling processes, which will lead to cracks and causes coating failure . As shown in Figure A, due to the p‐ SiC layer's transition area and porous structure, the dual layer SiC coating presents an excellent thermal shock resistance performance without any peeling or cracking phenomena for more than 50 times.…”

“…As described in Ref. , reactions and take place at the temperature higher than the molten point of Si powder (about 1681 K) through a solid‐liquid sintering. Surface treatment by polishing and washing, etc., can help to thinner the porous layer and remove some of the outer loose SiC particles to get a smooth surface.…”

“…And new nucleation takes place as reaction continues, and SiC grains will grow one next to one freely. After a period of time, SiC grain size increases and turns to dense packing to the dense structure of d ‐SiC layer with pores eliminated . Finally, the dual layer SiC layer coating will cover the sphere successfully and totally.…”

“…The substrates of 15 mm diameter MG spheres were polished with 300 grit SiC paper, then cleaned ultrasonically with ethanol and dried at 373 K for 8‐10 hours. MG spheres were packed in the pack mixtures in a graphite crucible and sintered at 1973‐2073 K for 2‐4 hours in an argon atmosphere, and then polished with 300 and 600 grit SiC papers and cleaned ultrasonically, etc. The d ‐SiC layer was fabricated through the fluidized‐bed chemical vapor deposition (FB‐CVD) process by the decomposition of methyltrichlorosilane (MTS, CH 3 SiCl 3 ) at 1773‐1823 K using hydrogen and argon as the fluidizing gas at atmospheric pressure.…”