Chemical vapor deposition of ZrO2 and C/ZrO2 on mullite fibers for interfaces in mullite/aluminosilicate fiber-reinforced composites

For the realization of crack deflection and fiber pull-out in aluminosilicate fiber-reinforced dense mullite-matrix composites, suitable fiber/matrix-interfaces are an important requirement in order to obtain sufficiently weak bondings between fibers and matrices. Two types of chemical vapor deposited (CVD) fiber/matrix-interfaces have been studied in the present work porous ZrO2 and C/ZrO2-double layers. In the latter case, carbon was burned out to form a gap during the processing of composites (fugitive coating). Porous ZrO2 coatings were produced by an optimized CVD-process with Zr-acetylacetonate as a precursor. The constancy of the layer thickness depended on the deposition temperature. It was found that at a temperature of approximately 300°C and a pressure of 5 hPa, suitably uniform layers with thickness ranging between 100 and 300 nm were achieved. The coatings contained approximately 15 wt% carbon which produced, after annealing in air, a porous structure. The deposition kinetics can be described by a first order reaction. The carbon layer in C/ZrO2-double layers was produced by using propane. The thickness of carbon layer was 10 and 100 nm, respectively. Aluminosilicate fiber/mullite matrix composite prepegs were fabricated by infiltration of coated and unidirectionally oriented fiber (0°) with a slurry, containing a pre-mullite powder, calcined at 1100°C. Uniaxial hot-pressing of dried prepegs was carried out at <1250°C for 15 min, at 20 MPa. Prepegs with ZrO2 fiber/matrix-interfaces were hot-pressed in air, while the samples with C/ZrO2-interfaces were processed in flowing argon. After hot-pressing, samples with C/ZrO2-interfaces were heat-treated in air (1000°C) in order to burn out the C-layer (fugitive coating). These composites yielded a controlled fracture with a high deflection rate and a favorable fracture strength of about 200 MPa, due to crack-deflection and fiber pull-out. Composites with ZrO2-interfaces, on the contrary yielded no crack deflection or pull-out. Therefore, they are less damage tolerant than those having C/ZrO2 double layer systems.