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 aluminosilica te fiber-reinforced dense mullite-matrix composites, suitable fiber/matrix- interfaces are an important requirement in order to obtain sufficiently wea k bondings between fibers and matrices. Two types of chemical vapor deposit ed (CVD) fiber/matrix-interfaces have been studied in the present work poro us 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). Poro us ZrO2 coatings were produced by an optimized CVD-process with Zr-acetylac etonate as a precursor. The constancy of the layer thickness depended on th e deposition temperature. It was found that at a temperature of approximate ly 300 degrees C and a pressure of 5 hPa, suitably uniform layers with thic kness ranging between 100 and 300 nm were achieved. The coatings contained approximately 15 wt% carbon which produced, after annealing in air, a porou s structure. The deposition kinetics can be described by a first order reac tion. The carbon layer in C/ZrO2-double layers was produced by using propan e. The thickness of carbon layer was 10 and 100 nm, respectively. Aluminosi licate fiber/mullite matrix composite prepegs were fabricated by infiltrati on of coated and unidirectionally oriented fiber (0 degrees) with a slurry, containing a pre-mullite powder, calcined at 1100 degrees C. Uniaxial hot pressing of dried prepegs was carried out at < 1250 degrees C for 15 min, a t 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 ai r (1000 degrees C) in order to burn out the C-layer (fugitive coating). The se composites yielded a controlled fracture with a high deflection rate and a favorable fracture strength of about 200 MPa, due to crack-deflection an d fiber pull-out. Composites with ZrO2-interfaces, on the contrary yielded no crack deflection or pull-out. Therefore, they are less damage tolerant t han those having C/ZrO2 double layer systems. (C) 2000 Elsevier Science Ltd . All rights reserved.