Oxidation of 3D‐printed SiC in air and steam environments

• Published in: Journal of the American Ceramic Society Vol. 104; no. 5; pp. 2225 - 2237
• Main Authors: Kane, Kenneth, Stack, Padraig, Schappel, Danny, Montoya, Katherine, Mouche, Peter, Sooby, Elizabeth, Terrani, Kurt
• Format: Journal Article
• Language: English
• Published: Columbus Wiley Subscription Services, Inc 01.05.2021
• Subjects: Chemical vapor deposition; Cracks; Cristobalite; Finite element method; modeling/model; Oxidation; Scale (corrosion); Silicon carbide; Silicon dioxide; Silicon substrates; Stresses; Three dimensional printing; Transformations
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.17632

The high‐temperature oxidation of additively manufactured and chemically vapor infiltrated (3D‐printed SiC) has been compared to chemical vapor deposited (CVD) SiC. 100‐h isothermal exposures were conducted at 1425° and 1300°C at 1 atm under both dry air and steam environments. A SiC reaction tube was utilized to reduce silica volatility. After steam oxidation at 1425° and 1300°C, on the 3D‐printed SiC surface, which was intrinsically rougher than the CVD surface, scales were 70%–90% thicker at the convex regions compared to concave/flat regions. In the convex regions, large cracks perpendicular to the oxidizing interface were observed. After dry air oxidation, scale thicknesses were comparable between 3D‐printed SiC and CVD SiC, regardless of geometry. Finite element modeling, conducted to elucidate the relationship between SiC geometry and ß‐ to α‐cristobalite transformation stress, determined cristobalite transformation tensile stresses to be on the order of 103 MPa during cool down, assuming a 6 vol% reduction. Compared to flat SiC substrates, tensile transformation stresses were elevated at concave regions and relaxed at convex regions. Combined with specimen mass gain (accounting for the rougher surface) of 3D‐printed SiC being 15%–32% higher for 3D‐printed SiC after 1300°C and 1425°C steam oxidation, the work presented concludes that the increased oxidation of 3D‐printed SiC is primarily caused by tensile hoop stresses driven by oxidation volume expansion. Lastly, the efficacy of the 3D‐printing method is demonstrated through the production of tristructural isotropic imbedded 3D‐printed SiC fuel forms.