urna Am. Ceran.Soc.87151881-887(2004 Effect of Yttrium Aluminum Garnet Additions on Alumina-Fiber Reinforced Porous-Alumina-Matrix Composites Michael K. Cinibulk, *Kristin A Keller, and Tai-Il Mah"+ Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio 45433-7817 The effects of incorporating yttrium aluminum garnet (YAG) the matrix with the fibers that further degrades fiber strength or degradation of the fiber by volatile byproducts of the matrix fibers were investigated. Composites with various precursors during processing. The presence of silica as a matrix of YAG added to the matrix were prepared to phase accelerates this strength degradation at elevated tempera to 1100-and 1200 C Strengths of YAG-containing composites in atmospheres of high H,o partial pressures. ia-1on temperatures were slightly lower than those of an all-alumina- matrix com- posite after heating for 5 h to 1100 C. However, after heating Nextel 610 fibers were found to retain the strength of the fibers displayed greater strengths and greater strains to failure than the all-alumina composite. At the higher temperature, the 1200 C. 1.I7 However, after heating at 1200C for prolonged presence of YAG is believed to inhibit the densification of the riods(100 h), the porous coating, with initially -20-nm grain matrix, which helps to maintain higher levels of porosity and and pore sizes, densified and the composite strength degraded to weaker interparticle bonding that allows for crack-energy that of a composite without the fiber coating. Subsequently dissipation within the matrix. A reduction in grain growth of initial work with 10 vol% YAG in an alumina matrix showed the fibers by the presence of segregated Y was also observe improved retained strength of both Nextel 610 and 650 fiber which may also contribute to higher fiber strength, thereby reinforced composites compared with similar all-alumina-matrix increasing the retained strengths of the YAG-containing composites after a 1200C heat treatment. The solution-derived composites YAG was originally incorporated during matrix processing with the intent of forming an in situ porous fiber coating in addition to I. Introduction filling particle interstices in the matrix with a fine(20-nm grain and pore size) grained porous YAG phase. However, the formation O Xine composites with poro ts matrices have b en shown to be necessary to provide enhanced strength. Our objective in the distribution of porosity in the matrix is controlled. -Rather than present study was to extend the effort of our initial attempts at relying on a fiber coating to deflect matrix cracks at a weak producing a YAG-alumina-matrix composite by studying the fiber-matrix interface, as in conventional ceramic-matrix compos effects of the Yag volume fraction in the matrix on the micro- ites, porous matrices rely on their inherent weakness to prevent controlled study of the effect of yttrium and YAG on the sintering stress concentrations on the fiber sufficient to fracture it. Matrix and grain growth of alumina at 1200-1300 C to better understand crack energy is dissipated through multiple microfractures in tension and shear in the porous matrix . These materials can their influence on matrices of identical compositions in fiber retain their mechanical properties after long-term exposure to reinforced composites is presented in another paper. 2 1 100C during cyclic tension and fatigue Long-term heat treatments at temperatures above 1 C can n. Experimental Procedure degrade the strength of NextelTM 610 fiber-reinforced porous- alumina-matrix composites. The observed strength degradation is Nextel 610 tows(3M Corp, Minneapolis, MN) were used to more than what would be obtained if the fibers alone were exposed reinforce all composites in this study. Matrices consisted of to a similar thermal treatment. The degree of matrix densifi alumina powder (0. 1-0. 2 um. AKP-53. Sumitomo Chemical Ce cation that occurs in a porous matrix is expected to be limited at Sumitomo, Japan) and/or YAG powder(-I um, Cerelox, Condea emperatures of 1200.C and below and as such would not seem Vista Corp, Tucson. AZ). with a binder of either alumina derived dramatically reduced composite strengths. However, recent work MA)or YAG derived from a polymeric solution. .The matrix has shown that a reduction in matrix porosity, via multiple matrix compositions of each composite, identified as A. YO. 1, AY50 precursor reinfiltrations, of only -17%(29%-24% pore volume YY50. and Y 100. are given in Table L. Composite YO. I was Other reasons forthe th reduction of-64%(220 to 80 MPa). produced using 3000 wt ppm yttria(derived from yttrium nitrate). suIt in with alumina powder to form YAG. This method produced a more omogeneous distribution of a low volume fraction of YA than could be obtained by using the solution-derived YAG. Nextel 610 F. w. Zok--contributing editor tows were filament-wound after passing through a water-based slurry of the appropriate matrix composition. During the filament winding process, the number of tows within a given width was counted to estimate the fiber volume fraction in the final compo pt No. 10272. Received June 9, 2013: approved December 10, 2003 e, which ranged from. 2 to 0.3. After winding, the tape was cut Day m oH es supported by Contract No, F33615-01-C-5214 through and stacked uniaxially in an aluminum mold for warm-press CA under rough vacuum. The green cor e was subsequently d