CERAMIC COMPOSITE INTERFACES 513 1.0 Debonding Alumina to Debonding Monazite to -1.0 0.5 0.5 10 C=(E2-E1)(E2+E1) Figure 7 Comparison of the measured fracture energies for the monazite-alumina system with the debonding criterion of He& Hutchinson (7, 8)(from 60) (described below) formed near the alumina-monazite interface, but this proved to be only a surface effect. Its formation is not expected to influence the long term eficacy of monazite coatings due to the refractoriness of the B-alumina An alternative to lanthanum phosphate is yttrium phosphate(YPO4), as has been explored by Xu& Kriven for yttrium-aluminum-garnet Y3AlsO12 systems (62). YPO4 has been found to be a better match for yttrium-aluminum-garnet (YAG fibers and/or matrices, although its effectiveness as a brittle composite interface is not equivalent to lanthanum-monazite TIN OXIDE SnO, has been explored as an interface coating in alumina-glass systems because of its low solubility in silica and non-existent solubility in alumina up to 1600C(63, 64). Although no quantitative fracture measure ments are available for this system, qualitative fracture experiments show that large-scale debonding can occur provided that the fiber surface is sufficiently smooth. A sapphire fiber provides a relatively smooth surface, whereas a sec- ond polycrystalline alumina fiber(PrD-166) is rough enough to preclude any pullout(see Figure 9; Reference 64). Roughnesses were quantified by Chawla Xu using atomic force microscopy(15)P1: ARK/MBL/rkc P2: MBL/vks QC: MBL/agr T1: MBL May 16, 1997 13:47 Annual Reviews AR034-16 CERAMIC COMPOSITE INTERFACES 513 Figure 7 Comparison of the measured fracture energies for the monazite-alumina system with the debonding criterion of He & Hutchinson (7, 8) (from 60). (described below) formed near the alumina-monazite interface, but this proved to be only a surface effect. Its formation is not expected to influence the long￾term efficacy of monazite coatings due to the refractoriness of the β-alumina magnetoplumbite. An alternative to lanthanum phosphate is yttrium phosphate (YPO4), as has been explored by Xu & Kriven for yttrium-aluminum-garnet Y3Al5O12 systems (62). YPO4 has been found to be a better match for yttrium-aluminum-garnet (YAG) fibers and/or matrices, although its effectiveness as a brittle composite interface is not equivalent to lanthanum-monazite. TIN OXIDE SnO2 has been explored as an interface coating in alumina-glass systems because of its low solubility in silica and non-existent solubility in alumina up to 1600◦C (63, 64). Although no quantitative fracture measure￾ments are available for this system, qualitative fracture experiments show that large-scale debonding can occur provided that the fiber surface is sufficiently smooth. A sapphire fiber provides a relatively smooth surface, whereas a sec￾ond polycrystalline alumina fiber (PRD-166) is rough enough to preclude any pullout (see Figure 9; Reference 64). Roughnesses were quantified by Chawla & Xu using atomic force microscopy (15)