Monazite Coatings on SiC Fibers I: Fiber Strength and Thermal Stability

Abstract

Commercially available SiC fibers were coated with monazite (LaPO4) using a continuous vertical coater at 1100°C. Coated fibers were heat treated in dry air, argon, and laboratory air at 1200°C for 1–20 h. The tensile strengths of uncoated and coated fibers were measured and evaluated before and after heat treatment. Fiber coating did not degrade SiC fiber strength, but heat treatment afterwards caused significant degradation that correlated with silica scale thickness. Possible strength degradation mechanisms for the coated fibers are discussed. Coating morphology, microstructure, and SiC oxidation were observed with scanning electron microscopy and transmission electron microscopy. Monazite reacted with SiC to form lanthanum silicate (La2Si2O7) in argon, but was stable with SiC in air. Despite the large coefficient of thermal expansion difference between monazite and SiC, micron thick monazite coatings did not debond from most types of SiC fibers. Possible explanations for the thermomechanical stability of the monazite fiber coatings are discussed.

Processing of 0.7BaTiO3–0.3BiScO3 Solid‐Solution Coatings

“…Figure is the TGA/DTA plot for the BaTiO 3 –BiScO 3 precursor. The gradual TGA weight loss from 25 to 200°C was attributed to water of hydration . Weight loss from ~300 to 350°C was attributed to combustion and loss of citric acid, consistent with the DTA exothermic peak at 325°C .…”

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 …”

Mechanical and microstructural characterisation of SiC- and SiBNC-fibre reinforced CMCs manufactured via PIP method before and after exposure to air

“…Silica formation on SiC-fibres is associated with fibre strength degradation. [49][50][51][52][53][54] Thus, the decrease of bending and tensile strength of SiC pyc /SiCN after exposure to air is mainly caused by fibre strength degradation as well as increase of fibre/matrix bonding due to silica formation.…”