1736 Journal of the American Ceramic Sociery-Holmquisf and Lange ol Drying shrinkage yield: yp =0.03 1000 1500 Number of precursor impregnations, N Fig. 2. Linear shrinkage after heat treatments for 2 h at various temper. Fig 3. Matrix porosity for the composites measured initially, after three atures of cast mullite/alumina (70/30) powder slurry. The volume fraction and eight precursor impregnation and pyrolysis cycles, ((" indicates of solids in the slurry was 54- 1%e porosity as derived trom Eq- (4) in text. the results indicate that the chosen composition will produce a and 6 14 after eight impregnations, ared with 5.67 for the microstructurally stable matrix. However, at 1400 C the shrinkage N720 fiber. This suggests that the n720 fiber and the matrix increases more rapidly, suggesting reduced long-term stability of have similar coefficients of thermal expansion (CTEs) and the the material at temperatures above 1200C. These measurements residual stresses might be very small. The composites containing are in agreement with observations made by others made for the N610 fibers had similar crack patterns, strongly suggesting that similar materials. 2 the cracks were caused by constrained drying/densification shrink A summary of the composite panels and their fiber content and age and not differential CtE porosity is given in Table L. Fiber volume fraction did not vary a between the panels, with values Ve "40%6-42%. The porosity levels were less uniform; the first two manufactured panels(A and B)had an initial matrix porosity of -51.5%, whereas the last two panels(C and D) had a matrix porosity of -46.5%(Fig. 3).This was attributed to a processing improvement made in the vacuum bagging step: the vacuum level was reduced from -300 to-10 pyrolysis cycles is shot/ rosity with the following impregnation/ torr. The change in in Fig 3. Assuming that all the available voids in the composite are filled in each impregnation cycle, the remaining porosity Pm, after N cycles should be given by where p'm is the initial matrix porosity and yn is the volume yield of the precursor solution(measured to 3%), Equation (4) is plotted in Fig. 3 and agrees well with measurements of matrix porosity after three impregnations. The decrease in measured porosity after eight impregnations is lower than the calculations predict, suggest- ng the formation of closed pores that would prevent subsequent precursor infiltration Micrographs of the composite structure reveal that the tows were well-infiltrated and only a few large-scale pores produce from trapped air were evident, as shown in Fig 4. As expected, the large pores were more frequent in panels A and B. Cracklike flaws, (bE erpendicular to the fibers and with regular spacing, were also bserved. These were more than likely due to the constraint the densification 8.4 The matrix had an Al, O /SiO, weight ratio ot 5.4I before the multiple impregnations with the AL,O, precursor Table L. Summary of Composite Panels Panel 100pm N720 42.1 23.2 42.0 N720 NolO 38 V,= fiher volume fraction, p= composite porosity. pan Fig. 4. Microstructural views of cross sections of N720 composite HatnA po panels: (a) as processed and (b) after 1300C/( 100 h) heat treatment