High‐Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO 4 ) on SiC was investigated using a LaPO 4 ‐coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO 4 is reduced in the presence of SiC and reacts to form La 2 O 3 or La 2 Si 2 O 7 and SiO 2 as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO 2 film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO 4 coating. Continuous LaPO 4 coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO 4 coating becomes morphologically unstable due to free‐energy minimization as the grain size reaches the coating thickness, which allows the SiO 2 oxidation product to penetrate the coating.