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 degrees-1400 degrees 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 similar to 0.5 Pa at 1200 degrees C to similar to 50 Pa at 1400 degrees 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 degrees C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300 degrees 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.