Journal of the American Ceramic Sociery-Cinibulk et al. Vol. 85. No. 1I are thicker than 150 nm (YAG grain size), some porosity is still present, but the amount of porosity has decreased to "10 vol% This work shows that a porous fiber coating increase the strength of composites produced with small-diameter fibers in a polycrystalline oxide matrix. However, the improve- ment in strength diminishes when the composites are exposed to air at high temperatures for extended (100 h) periods. After 100 h at 1200C in air, the composites containing the porous YAG coating have the same strengths as those composites produced without a fiber coating From previous work on microcomposites with porous matrixes, we find that a pore volume fraction of at least "15% is needed to give tough, composite-like behavior 200 nm likely penetrate the fibers without being deflected or trapped The densified yag coatings now can introduce flaws that most within the porous coating itself. The dramatic loss in strength of the porous Y AG-containing composites after 100 h at 1200C in Matrix air when the coating has densified suggests that the higher strength after only short heat treatments is due to the presence of a porous coating. The fact that both the control minicomposite and the minicomposite with the porous YAG fiber coating yield the same strength indicates that once the fiber coating has densified the twe composites behave the same, and there is no apparent benefit to simply having a dense YAG fiber coating Pores trapped within a dense solid are constrained, and it is generally very difficult to remove them. Similarly a porous fiber coating, trapped between a dense fiber and dense matrix, would be equally constrained, and little densification of the porous coating would be expected to occur. Because the porous coating in Fig. 7. TEM images of minicomposite A heated at 1200C for 2 h in air. a dense matrix is constrained from densifying, only pore coarser Where coating is thin, porosity is reduced to zero as grains grow to equal ing is expected to occur. However, the minicomposites in the film thickness and sinter to full density. In thicker coatings, coarsening of present study contain a porous alumina matrix that cannot be rains and porosity occul expected to constrain the porous fiber coating from sintering. A Figs. 7-9 show, in addition to grain growth and pore coarsening, there is a significant amount of densification that occurs. In fact, the rate of densification approaches that of the fiber coatings in the nicomposite D(Fig. 9), the grain size of Y AG(100-150 nm) absence of a matrix(Fig. 4). We estimate that the final porosity in has approached the coating thickness in some cases, leaving either a dense or discontinuous coating behind. When the fiber coatings where grain size is comparable to coating thickness to -10 volo for the thickest coatings. If a critical volume fraction of pores is required to encourage crack deflection, a reduction in porosity Matrix atrix p-YAG YAG Fiber Matrix fibe Fig. 8. TEM images of fiber coatings in m osite C heated at I 100oC for 2 h in air. porosity retained after short exposures to high Fig. 9. TEM images of fiber coatings in minicomposite D heated at 1200C for 100 h in air. Porosity is reduced to <10 vol% after 100 h.minicomposite D (Fig. 9), the grain size of YAG (100–150 nm) has approached the coating thickness in some cases, leaving either a dense or discontinuous coating behind. When the fiber coatings are thicker than 150 nm (YAG grain size), some porosity is still present, but the amount of porosity has decreased to
10 vol%. This work shows that a porous fiber coating can be used to increase the strength of composites produced with small-diameter fibers in a polycrystalline oxide matrix. However, the improve￾ment in strength diminishes when the composites are exposed to air at high temperatures for extended (100 h) periods. After 100 h at 1200°C in air, the composites containing the porous YAG coating have the same strengths as those composites produced without a fiber coating. From previous work on microcomposites with porous matrixes, we find that a pore volume fraction of at least
15% is needed to give tough, composite-like behavior.35 The densified YAG coatings now can introduce flaws that most likely penetrate the fibers without being deflected or trapped within the porous coating itself. The dramatic loss in strength of the porous YAG-containing composites after 100 h at 1200°C in air when the coating has densified suggests that the higher strength after only short heat treatments is due to the presence of a porous coating. The fact that both the control minicomposite and the minicomposite with the porous YAG fiber coating yield the same strength indicates that once the fiber coating has densified, the two composites behave the same, and there is no apparent benefit to simply having a dense YAG fiber coating. Pores trapped within a dense solid are constrained, and it is generally very difficult to remove them. Similarly a porous fiber coating, trapped between a dense fiber and dense matrix, would be equally constrained, and very little densification of the porous coating would be expected to occur. Because the porous coating in a dense matrix is constrained from densifying, only pore coarsen￾ing is expected to occur. However, the minicomposites in the present study contain a porous alumina matrix that cannot be expected to constrain the porous fiber coating from sintering. As Figs. 7–9 show, in addition to grain growth and pore coarsening, there is a significant amount of densification that occurs. In fact, the rate of densification approaches that of the fiber coatings in the absence of a matrix (Fig. 4). We estimate that the final porosity in the YAG coatings ranges from zero, for the thinnest coatings where grain size is comparable to coating thickness, to
10 vol%, for the thickest coatings. If a critical volume fraction of pores is required to encourage crack deflection, a reduction in porosity Fig. 7. TEM images of minicomposite A heated at 1200°C for 2 h in air. Where coating is thin, porosity is reduced to zero as grains grow to equal film thickness and sinter to full density. In thicker coatings, coarsening of grains and porosity occur. Fig. 8. TEM images of fiber coatings in minicomposite C heated at 1100°C for 2 h in air. Porosity is retained after short exposures to high temperatures. Fig. 9. TEM images of fiber coatings in minicomposite D heated at 1200°C for 100 h in air. Porosity is reduced to 10 vol% after 100 h. 2708 Journal of the American Ceramic Society—Cinibulk et al. Vol. 85, No. 11