K K. Chawla et al. Journal of the European Ceramic Society 20(2000)551-559 Alumina Matrix (a) 25μm SEM SEI Fiber Coating Matrix Fig. 5. Interfacial debonding at the(10-dip) monazite/fiber interfac th cracks produced by two 98N(1 s)indentations in the matrix (a) low magnification SEM image and (b) high magnification SEM 50m image. Note the roughness of the debonded fracture surface. In brittle systems, Mode II fracture typically occurs by Fig. 6. Fiber pushout test r Saphikon/five-dip LaPO4/Al2O3 er and 134 um specimen thickness e coalescence of microcracks in a layer In the case of (a)SEM image showing tl ed fiber is pushed out of the alu- a thick coating, the 10 dip coating in the present case, mina matrix,(b)SEM image showing the debonded monazite coating this layer coincides with the coating itself such that is stuck to the matrix debonding involves a diffuse zone of microcrack damage. In other cases, the layer is very thin and the matic of the interfacial debonding models for thin and thick interphase is shown in Fig 8. The 10-dip coating may have some defects in it, making it easier to form microcracks in the coating itself and debonding may occur by the coalescence of microcracks within the monazite coating. The coefficient of friction in the frac- Fig. 7 D4A1 0, cor stress-displacement curve for a Saphikon/ five-dip tured monazite is likely to be higher than that of mon- LaPo4 mposite with 163 um fiber diameter and 145 um spe- azite/Saphikon interface with a thinner coating(five clmen Note a sinusoidal variation in the curve which corre. sponds to the asperities on the as received fiber surface. dip), where initiation of debonding occurs by a single crack along the Saphikon fiber/monazite interface when the matrix failed. The monolithic alumina failed The stress-displacement curves in three-point bend catastrophically; its bend strength was very low (140 tests at room temperature of five-dip fiber coated com- MPa). Comparatively, the work of fracture of monazite posites and monolithic alumina are shown in Fig 9. The coated Saphikon fiber/alumina matrix composites is composite failed in a non-brittle manner. The load much higher than that of monolithic alumina. Note that increased until a stress of about 180 MPa. where the he fiber volume fraction in the composite was very fibers started to debond and pullout from the matrix. small, about 0.01. The fracture surfaces of five-dip spe- After debonding and pullout, the matrix still could cimens fiber pullout was observed and the average transfer some load. Finally, the ultimate stress (230 length of pullout fiber was about 130 um. In most cases, MPa)was reached. There, the stress suddenly decreased the monazite coating was largely peeled off the fiberIn brittle systems, Mode II fracture typically occurs by the coalescence of microcracks in a layer. In the case of a thick coating, the 10 dip coating in the present case, this layer coincides with the coating itself such that debonding involves a di€use zone of microcrack damage. In other cases, the layer is very thin and the debond has the appearance of a single crack. A sche￾matic of the interfacial debonding models for thin and thick interphase is shown in Fig. 8. The 10-dip coating may have some defects in it, making it easier to form microcracks in the coating itself and debonding may occur by the coalescence of microcracks within the monazite coating. The coecient of friction in the frac￾tured monazite is likely to be higher than that of mon￾azite/Saphikon interface with a thinner coating (®ve￾dip), where initiation of debonding occurs by a single crack along the Saphikon ®ber/monazite interface. The stress±displacement curves in three-point bend tests at room temperature of ®ve-dip ®ber coated com￾posites and monolithic alumina are shown in Fig. 9. The composite failed in a non-brittle manner. The load increased until a stress of about 180 MPa, where the ®bers started to debond and pullout from the matrix. After debonding and pullout, the matrix still could transfer some load. Finally, the ultimate stress (230 MPa) was reached. There, the stress suddenly decreased when the matrix failed. The monolithic alumina failed catastrophically; its bend strength was very low (140 MPa). Comparatively, the work of fracture of monazite coated Saphikon ®ber/alumina matrix composites is much higher than that of monolithic alumina. Note that the ®ber volume fraction in the composite was very small, about 0.01. The fracture surfaces of ®ve-dip spe￾cimens ®ber pullout was observed and the average length of pullout ®ber was about 130 mm. In most cases, the monazite coating was largely peeled o€ the ®ber. Fig. 7. Shear stress±displacement curve for a Saphikon/®ve-dip LaPO4/Al2O3 composite with 163 mm ®ber diameter and 145 mm spe￾cimen thickness. Note a sinusoidal variation in the curve which corre￾sponds to the asperities on the as-received ®ber surface. Fig. 6. Fiber pushout test results on a Saphikon/®ve-dip LaPO4/Al2O3 composite with 122 mm ®ber diameter and 134 mm specimen thickness: (a) SEM image showing the debonded ®ber is pushed out of the alu￾mina matrix, (b) SEM image showing the debonded monazite coating is stuck to the matrix. Fig. 5. Interfacial debonding at the (10-dip) monazite/®ber interface with cracks produced by two 98 N (1 5 s) indentations in the matrix; (a) low magni®cation SEM image and (b) high magni®cation SEM image. Note the roughness of the debonded fracture surface. K.K. Chawla et al. / Journal of the European Ceramic Society 20 (2000) 551±559 555