2608 Journal of the American Ceramic Society-Kerans et al. Vol. 85. No. I1 were relat gh for both coatings, considering the fiber the nature of the process. Although the desired Al,O, phase strength degradation during processing: the strengths were greater remains difficult to process, work has been reported where almost e matrix-cracking stresses (Fig. 10). However, the mean 85%-dense ZrO, has been deposited around woven preforms. re not significantly different from that of the fiber/ However, the stability of the interface coating during CVD control specimens, although coated- fiber composites had higher Weibull moduli. The lack of difference in strength is processing is unknown and likely to be a major issue because of the use of gaseous hydrogen and CO in the attributed to the porosity in the matrix; porous-matrix composites are known to perform well without interface treatments(see next This has led to the use of glass matrices to test coating concepts. section). The results imply that the matrix density needs to be Preliminary work using Blackglas"(Allied Signal, Inc.(now Honeywell), Morristown. NJ) polymer-derived glass as matrix The processing of even minicomposites having a dense oxide shows some promise, although the matrices remain far from ideal matrix can be challenging. Use of chemical vapor deposition CVD) to deposit oxides remains in the developmental stage array of shrinkage microcracks. Oxide-fiber-reinforced minicom CVD-deposited Al, O, matrices are amorphous and do not bond posites having a dense but microcracked glassy matrix of Black readily to coated or uncoated fiber tows, which causes debonding as have been used in two studies to test oxidation-resistant even in control specimens. o There is no known work on oatings. In one study, the technique was used to evaluate Nextel polycrystalline-oxide-matrix composites with high enough matrix 610(3M Corp. St. Paul, MN)/Blackglas composites with and densities to definitively suppress the mechanism of debonding via without porous lanthanum hexaluminate fiber coatings. 06The matrix cracking(say 90%). CVI of dense stable polycrystalline minicomposites with the fiber coatings had significantly higher oxides is made difficult by the formation of amorphous or ultimate strengths than the uncoated control specimens. In a metastable oxides( which later crystallize or transform, introducing another study, porous oxide(ZrO - SiO, mixture)and monazite significant stresses and cracking) and by the ity to reach were evaluated in Nextel 720-reinforced Blackglas. 7BN- porosity levels below the permeation threshold (-15)because of oated and uncoated fibers were used as controls for comparison. (a Control(uncoated) (b BN 2 CMC- Control CMC. Control (uncoated) 265MPa;m=8.8 265MPa;m=8.8 00 90 2 TOw: Contro uncoated) cMc·BN 742MPa;m=6.5 383MPa;m=64 5.5 6.5 7 5 5.5 6 Ln[ Stress, MPa] Ln[ Stress, MPa] Porous Zro2-sio (d Monazite CMC- Control CMC. Control (uncoated) (uncoated) 265MPa;m=8.8 265MPa;m=8.8 与 cMc· porous Zro-sie CMC. Monazite 356MPa;m=6.0 353MPa;m=8.8 5.56 6.5 6.5 Ln Stress, MPa] Ln Stress, MPa Fig. 11 plots of the strengths of minicomposites using dense Blackglas as the matrix show that porous (c) ZrO - SiO, and (d) monazite coatings ers are as effective as the(b) BN-coated fibers. (a) Control is significantly weaker than the fiber, showing that Blackglas might be a good model ma alate interface coatings