December 1997 Control of Interfacial Properties through Fiber Coatings: Monazite Coatings Alo, fiber/Lapo4 coating /Al, O, matrix Average Coating Thickness: 6.5 um Embedded Fiber Length, L (mm) 000 20 YAG fiber/ LaPO, coaling Average Coating Thickness: 2 um LLLLL Embedded Fiber Length, L(mm) Fig 4. Variation of the oad(Pp)during pushout as a function of the embedded fiber length(L) for(a)the Al,O, fiber system and(b)the YAG fiber system. The term sd is given by tion(u), and the roughness-induced sliding resistance (To)are 2μB1d the key factors that affect the maximum pushout stress, Pp tha is measured immediately before complete interfacial debond- ng occurs. For the composite systems that have been evaluated and d is the length of fiber that remains embedded in the in this study, it is postulated that Ti, u, and the fiber/coating interface roughness remain constant as the coating thickness Considering Eq.(1), the residual stresses(PR and NR, the changes. Thus, the key outcome of changing the interphase debonding fracture energy( the coefficient of interface fric hickness is the effect on the residual stresses and hence, on the ensuing debonding and sliding properties Using the LH model (Eqs. (1)and(4), it is clear that the magnitude of the applied stress that is required to initiate and propagate debonding and subsequent fiber sliding is dependent on the residual stress state. If the applied stress, Pp(positive), YAG fiber/Laro, coating/Al o, matrix and the residual axial stress, PR, generate shear stresses that act in the same direction, then PR assists debond crack propaga- Embedded Fiber Length, L=1.12 +0.03 mm tion. That is, an axial tensile stress(negative) in the fiber gen- erates an interfacial shear stress that has the same effect as that which is induced by the application of a pushout stress. With regard to the post-debond sliding stress(Eq.(4)), the effect of PR is negligible for several reasons: (i) the uB, PR term is small,(ii) the axial residual stress is being relieved during the sliding process e roug dominate the post-debond sliding The values Al, O, fiber/LalO, coating /AlO, matrix re shown in table ll Embedded Fiber length L-076+0.02 mm The residual radial stress across the interface, NR, also af fects fiber debonding and subsequent sliding. A compressiv ILLIL radial pressure of NR(positive in the Lh equations)increases the Coulombic frictional sliding resistance of the fib LaPo, Coating Thickness, t(um) the matrⅸx sets up an "effective"Mode II bridging stress behind the debond crack front during progressive interfacial Fig. 5. ation of the pl load with the thickness of the LaPO debond propagation. If the residual stress that acts across the Al,O,fiber-reinforced and(O)YAG-fiber-reinforced fiber/coating interface is tensile(negative), it reduces the ef fective roughness contribution by subtracting from To(Eq (4))The term zd is given by zd = 2mB1d Rf and d is the length of fiber that remains embedded in the matrix. Considering Eq. (1), the residual stresses (PR and NR, the debonding fracture energy (Gi ), the coefficient of interface fric￾tion (m), and the roughness-induced sliding resistance (t0) are the key factors that affect the maximum pushout stress, PP, that is measured immediately before complete interfacial debond￾ing occurs. For the composite systems that have been evaluated in this study, it is postulated that Gi , m, and the fiber/coating interface roughness remain constant as the coating thickness changes. Thus, the key outcome of changing the interphase thickness is the effect on the residual stresses and, hence, on the ensuing debonding and sliding properties. Using the LH model (Eqs. (1) and (4)), it is clear that the magnitude of the applied stress that is required to initiate and propagate debonding and subsequent fiber sliding is dependent on the residual stress state. If the applied stress, PP (positive), and the residual axial stress, PR, generate shear stresses that act in the same direction, then PR assists debond crack propaga￾tion. That is, an axial tensile stress (negative) in the fiber gen￾erates an interfacial shear stress that has the same effect as that which is induced by the application of a pushout stress. With regard to the post-debond sliding stress (Eq. (4)), the effect of PR is negligible for several reasons: (i) the mB1PR term is small, (ii) the axial residual stress is being relieved during the sliding process, and, finally (iii) the interface roughness term may dominate the post-debond sliding. The values of m and B1 are shown in Table II. The residual radial stress across the interface, NR, also af￾fects fiber debonding and subsequent sliding. A compressive radial pressure of NR (positive in the LH equations) increases the Coulombic frictional sliding resistance of the fiber within the matrix. This sets up an ‘‘effective’’ Mode II bridging stress behind the debond crack front during progressive interfacial debond propagation. If the residual stress that acts across the fiber/coating interface is tensile (negative), it reduces the ef￾fective roughness contribution by subtracting from t0 (Eq. (4)). Fig. 5. Variation of the pushout load with the thickness of the LaPO4 coating in (d) Al2O3-fiber-reinforced and (s) YAG-fiber-reinforced systems. Fig. 4. Variation of the peak load ( pP) during pushout as a function of the embedded fiber length (L) for (a) the Al2O3 fiber system and (b) the YAG fiber system. December 1997 Control of Interfacial Properties through Fiber Coatings: Monazite Coatings 2991