Journal of the American Ceramic Sociery-Kuo et al. Vol. 80. No. 12 ∽05苏品 20 LL1,11L,L1I1111 Lapo Coating Thickness (um ig. 8. Variation of average sliding stress as a function of interfacial coating thickness for(O)the Al2O, and (O)the YAG fiber systems (LaAl1O18)2,34 was observed at both the fiber and the matrix length and 1-2 um in width. Some porosity existed at the sides of the (Fig 9(a)). The reaction product was thin reaction layer; however, the specimen was dense elsewhere with elongated ix at the fiber side but formed elongated This LaAl, O1s reaction product affected the residual stress, I um) and saigo at the mat ide. This B-Al2O3 layer was -8 um thick the interfacial bonding at both sides of the coating, and the grains that had dimensions of 10-15 um subsequent debond crack propagation. Additionally, the forma- tion of a reaction product occurred at the expense of the Al2O3 fiber and is expected to have an effect on the in-situ strength (a statistics of the Al2O, fibers. 35 In this study, this reaction prod- uct at the coating/matrix interface penetrated to a constant depth for each coating thickness, thereby representing a larger volume fraction at a small coating thickness. It is postulated that the high r values that are calculated for thinner coatings are due to some amount of debond crack propagation along the LaPO,/LaAl,Oug interface For a thin coating thickness, the crack might wander to each side of the coating, which would result in crack jumping. " Similar crack propagation has been noted in the four-point flexure tests that were conducted by Morgan and Marshall.13 Such a crack-propagation mechanism LaAlo is expected to increase the Ti value. As the coating thickness increases. the debond crack is confined to the interface that is nearest the fiber, which results in a distinctly different crack propagation path (2) Debonding and Sliding in the YAGlLaPO,/Al,O, Fiber System The residual stresses for the Y AG fiber system are shown in Fig. 1, which illustrates that both the axial (PR)and radial (NR) stresses are tensile. The axial residual stress decreases as the coating thickness increases, the effect of which would increase the required peak pushout stress. The radial tensile stress was small and increases only slightly as the coating thickness in- ceases, the result of which would be only a slight decrease in he peak pushout stress. Combining these effects, the stress that is required to debond the fiber should increase slightly as the coating thickness increases. This trend is illustrated by the experimental results that are summarized in Fig. 5. In regard to the radial tensile stress, such a stress would effectively recoil o Thus, the YAG fiber system must generate sliding resistance face. The high asperity value in the YAG fiber system is a factor of -2-4 greater than that for the AlO, fiber system and is postulated to result entirely from surface-roughness effects ( Fig. 3). For this reason, the sliding resistance after debonding is not a strong function of the residual stresses for the yag fiber system; i.e., the sliding resistance was not strongly de- Fig 9. SEM mi of the interface region for(a) the Al,O endent on the coating thickness. This behavior is observed product at the f oating interface) and experimentally, where the average sliding stress(p/(2TrL)), as (b)the YAG fiber reactio t is obvious at the a function of the coating thickness, is plotted in Fig.8 and coating/matrix interface for both fiber systems exhibits a constant sliding stress at all coating thicknesses(LaAl11O18) 12,34 was observed at both the fiber and the matrix sides of the coating (Fig. 9(a)). The reaction product was thin (1 mm) and uniform at the fiber side but formed elongated grains at the matrix side. This b-Al2O3 layer was ∼8 mm thick with elongated grains that had dimensions of 10–15 mm in length and 1–2 mm in width. Some porosity existed at the reaction layer; however, the specimen was dense elsewhere. This LaAl11O18 reaction product affected the residual stress, the interfacial bonding at both sides of the coating, and the subsequent debond crack propagation. Additionally, the forma￾tion of a reaction product occurred at the expense of the Al2O3 fiber and is expected to have an effect on the in-situ strength statistics of the Al2O3 fibers.35 In this study, this reaction prod￾uct at the coating/matrix interface penetrated to a constant depth for each coating thickness, thereby representing a larger volume fraction at a small coating thickness. It is postulated that the high Gi values that are calculated for thinner coatings are due to some amount of debond crack propagation along the LaPO4/LaAl11O18 interface. For a thin coating thickness, the crack might wander to each side of the coating, which would result in ‘‘crack jumping.’’ Similar crack propagation has been noted in the four-point flexure tests that were conducted by Morgan and Marshall.13 Such a crack-propagation mechanism is expected to increase the Gi value. As the coating thickness increases, the debond crack is confined to the interface that is nearest the fiber, which results in a distinctly different crack￾propagation path. (2) Debonding and Sliding in the YAG/LaPO4 /Al2O3 Fiber System The residual stresses for the YAG fiber system are shown in Fig. 1, which illustrates that both the axial (PR) and radial (NR) stresses are tensile. The axial residual stress decreases as the coating thickness increases, the effect of which would increase the required peak pushout stress. The radial tensile stress was small and increases only slightly as the coating thickness in￾creases, the result of which would be only a slight decrease in the peak pushout stress. Combining these effects, the stress that is required to debond the fiber should increase slightly as the coating thickness increases. This trend is illustrated by the experimental results that are summarized in Fig. 5. In regard to the radial tensile stress, such a stress would effectively recoil the coating from the interface (neglecting elastic expansion). Thus, the YAG fiber system must generate sliding resistance from the asperity pressure sasperity along the debonded inter￾face. The high sasperity value in the YAG fiber system is a factor of ∼2–4 greater than that for the Al2O3 fiber system and is postulated to result entirely from surface-roughness effects (Fig. 3). For this reason, the sliding resistance after debonding is not a strong function of the residual stresses for the YAG fiber system; i.e., the sliding resistance was not strongly de￾pendent on the coating thickness. This behavior is observed experimentally, where the average sliding stress (pl /(2prL)), as a function of the coating thickness, is plotted in Fig. 8 and exhibits a constant sliding stress at all coating thicknesses. Fig. 8. Variation of average sliding stress as a function of interfacial coating thickness for (d) the Al2O3 and (s) the YAG fiber systems. Fig. 9. SEM micrographs of the interface region for (a) the Al2O3 (arrows mark the reaction product at the fiber/coating interface) and (b) the YAG fiber systems; the reaction product is obvious at the coating/matrix interface for both fiber systems. 2994 Journal of the American Ceramic Society—Kuo et al. Vol. 80, No. 12