Journal of the American Ceramic SocieryDavis et al. VoL 86. No. 2 ( Interfacial Debonding LaPO, interface(-4.5 J/m). It is noteworthy that the fibers were cted all the fibers from penetration of protected from cracking even when the contact area of the Vickers indentation cracks, whereas uncoated fibers were always pene indentation was close enough to the fiber to overlap the coating AL,O,/YAG, AL, O,ZrO, and (Fig 3(b). In that case, the residual stress from the indentatio mullite fibers in Figs. 2-4. The indentation cracks generally (compressive normal to the interface, tensile on the prospective xtended from the matrix into the lapo, coatings then arrested crack plane into the fiber) would tend to inhibit debonding and nd caused debonding at the coating/fiber interface. In a few cases favor fiber penetration. with the AL,O,/Zro, fibers, debonding occurred at both interfaces The interfacial roughnesses for both of the eutectic fibers were (matrix/ coating and coating/fiber). The former response was ob- similar to the surface roughnesses of the as-formed fibers, with served previously with coated sapphire fibers and was shown to be amplitude"100-300 nm and period 500 nm(Figs. 2(a) and consistent with the debond criterion of He and Hutchinson- and 3(a)). This roughness amplitude is greater than that of the inter the measured fracture toughnesses of the fibers, coating, and faces at the single-crystal mullite and sapphire fibers. The initially interface.Although the fracture toughnesses of the YAG/ApoA smooth single crystal fibers developed cusps during hot pressing d mullite/LaPO4 interfaces have not been measured, the present where grain boundaries of the monazite coating intersected the observations suggest that they are similar to that of the alumina/ fiber surface. Measurements of the cusp profiles on sapphire fibers Indentation ALO / ZrO. lber Al2O3 Al2O3/Zro2 Al2o 2 um 20 um Fig. 3. SEM micrographs showing interactions of indentation cracks with Al,O, eutectic fibers:(a)uncoated fiber in alumina matrix(indentation located below region shown);(b) fiber coated with LapO4(indentation located at top of field of view) LaPo (b) Al2O3 LaPO Mullite Mullite 21 Fig. 4. SEM micrographs showing interaction of indentation crack with single-crystal mullite fiber(coated with LaPO4, in alumina matrix):(a) of indentation crack with interface and debonding(indentation located above region shown), (b) same fiber as in(a) but showing region further to the right along the debonded interface (arrows indicate magnitude of sliding displacement across debond crack).(2) Interfacial Debonding The LaPO4 coatings protected all the fibers from penetration of indentation cracks, whereas uncoated fibers were always pene￾trated. Examples are shown for the Al2O3/YAG, Al2O3/ZrO2 and mullite fibers in Figs. 2–4. The indentation cracks generally extended from the matrix into the LaPO4 coatings then arrested and caused debonding at the coating/fiber interface. In a few cases with the Al2O3/ZrO2 fibers, debonding occurred at both interfaces (matrix/coating and coating/fiber). The former response was ob￾served previously with coated sapphire fibers and was shown to be consistent with the debond criterion of He and Hutchinson27 and the measured fracture toughnesses of the fibers, coating, and interface.1 Although the fracture toughnesses of the YAG/LaPO4 and mullite/LaPO4 interfaces have not been measured, the present observations suggest that they are similar to that of the alumina/ LaPO4 interface (4.5 J/m2 ). It is noteworthy that the fibers were protected from cracking even when the contact area of the Vickers indentation was close enough to the fiber to overlap the coating (Fig. 3(b)). In that case, the residual stress from the indentation (compressive normal to the interface, tensile on the prospective crack plane into the fiber) would tend to inhibit debonding and favor fiber penetration. The interfacial roughnesses for both of the eutectic fibers were similar to the surface roughnesses of the as-formed fibers, with amplitude 100–300 nm and period 500 nm (Figs. 2(a) and 3(a)). This roughness amplitude is greater than that of the inter￾faces at the single-crystal mullite and sapphire fibers. The initially smooth single crystal fibers developed cusps during hot pressing where grain boundaries of the monazite coating intersected the fiber surface. Measurements of the cusp profiles on sapphire fibers Fig. 3. SEM micrographs showing interactions of indentation cracks with Al2O3/ZrO2 eutectic fibers: (a) uncoated fiber in alumina matrix (indentation located below region shown); (b) fiber coated with LaPO4 (indentation located at top of field of view). Fig. 4. SEM micrographs showing interaction of indentation crack with single-crystal mullite fiber (coated with LaPO4, in alumina matrix): (a) intersection of indentation crack with interface and debonding (indentation located above region shown); (b) same fiber as in (a) but showing region further to the right along the debonded interface (arrows indicate magnitude of sliding displacement across debond crack). 308 Journal of the American Ceramic Society—Davis et al. Vol. 86, No. 2