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

Abstract

The stability of lanthanum orthophosphate (LaPO4) on SiC was investigated using a LaPO4‐coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO4 is reduced in the presence of SiC and reacts to form La2O3 or La2Si2O7 and SiO2 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 SiO2 film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO4 coating. Continuous LaPO4 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 LaPO4 coating becomes morphologically unstable due to free‐energy minimization as the grain size reaches the coating thickness, which allows the SiO2 oxidation product to penetrate the coating.

Accurate determination of fiber strength distributions

“…Comparisons of results from the two methods led to the conclusion that the Weibull modulus m obtained using nominal strengths is the same as that obtained using true strengths. That study has since served as the principal basis for justifying continued use of nominal fiber strengths …”

In situ Y2Si2O7 coatings on Hi‐Nicalon‐S SiC fibers: Phase formation and fiber strength

“…The YPO 4 ‐coated fibers were oxidized in laboratory air at 1000°C for up to 100 hours or 1200°C for 1 hour to form a silica layer on SiC. In prior work, good surface and grain boundary wetting of monazite (LaPO 4 ) by SiO 2 was observed during oxidation of monazite‐coated SiC fibers . It was therefore anticipated that during oxidation, silica might infiltrate porous YPO 4 coatings.…”

Evaluation of SiC/SiC minicomposites with yttrium disilicate fiber coating

“…In our prior work, we showed that La 2 Si 2 O 7 formed when LaPO 4 ‐coated SiC fibers were heat treated in argon/low pO 2 mixtures . We also developed a process to apply continuous YPO 4 coatings on Nextel 720 fiber tows using a hetero‐precipitation method .…”

“…We also developed a process to apply continuous YPO 4 coatings on Nextel 720 fiber tows using a hetero‐precipitation method . Based on the formation of La 2 Si 2 O 7 , a thermodynamic analysis of reaction was conducted and experimental conditions for the formation of a single‐phase Y 2 Si 2 O 7 from YPO 4 on SiC fibers were determined . The details of the thermodynamic analysis and relevant experiments will be published elsewhere.…”

“…In prior studies, LaPO 4 monazite was shown to be an effective interface coating in oxide‐oxide CMCs . However, it was found to be thermodynamically incompatible with SiC . We recently showed that rare‐earth disilicates are promising oxidation‐resistant replacements for oxidation‐prone BN and C coatings in SiC/SiC composites .…”