Journal of the American Ceramic Sociery--Cinibulk et al Vol. 87. No. 5 Grain Y 100m Energy [ke V] (a) Grain Grain Boundary Grain Boundary g Energy [ke V Energy ke V Y. While the signaka trix grain boundary, and (d) Nextel 610 fiber grain boundary near fiber-matrix interface in YO. 1, showing the presence of segregated spectra obtained af e boise ratio is low in the spectra in(d)due to the low amount of Fe, Si, and Y present, these peaks were always discernible in several and a much more fibrous fracture surface for the YAG-alumina Nextel 650 had a tow strength of <200 MPa versus 1 100 MPa for matrix composite after heating for 5 h at 1200C Nextel 610 after heating at 1200 C for 100 h, so a lower strength for the Nextel 650 fiber-reinforced composite would be expecte as well. In the study using Nextel 610 fibers with a porous YAG IV. Discussion fiber coating. it was found that after 100 h at 1200.C, the The results are consistent with previous work that showed that 20-nm-sized pores in the coating had coarsened and then had been climinated when the coating sintered to -90%0 relative YAG-containing alumina-based composites retained greater strengths than an equivalent all-alumina-mat density. This resulted in composites having the same strength shor regardless of whether the YAG fiber coating was present. The In the present study, the strengths were retained even after extended times(100 conclusion was that the dense YAG fiber coating no longer h) at 1200C. Approximately 75% of the strength was retained protected the fiber by deflecting matrix cracks and acted as if no fter the 5-h hold at 1200oC. which is three times greater in ber coating was present, which resulted in decreased strength These observations suggest that the effect of Yag in the matrix in magnitude than that of the control composite A. Two reasons for the strength improvement during longer exposure times compared he present study was to increase the retention of fiber strengt with the earlier studies are the use of Nextel 610 fibers versus the use of an early version of Nextel 650 in one of the prior studies, and the use of a dispersed YAG-matrix phase rather than only a ent version of Nextel 650 had tensile strengths of 1070, 760, and 650 MPa porous YAG fiber coating in another study. The early version of ating at 1200"C for 0. 5, and 100 h, respectively