February 2003 Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monacite 309 by atomic force microscopy (ATM) have been reported else- fiber surface. In some areas, this smeared layer was overlaid with where28,29 The cusp heights were typically 50 nm, and the a less dense, coarser grained agglomeration of angular monazite ular distortions of the surface were small(=20%). The cusps or particles of diameter 50-100 m(Fig. 7), suggestive f cata- the mullite surfaces were very similar clastic flow, a process involving repeated microfracture and Some insight into the effect of interfacial roughness on fiber fine-particle transport. Similar features(intense deformation sliding and pullout can be gained from the observations of Figs. fine crystallites, and agglomerates of angular particles)were 2-4. As the debond grows around the circumference of the fiber, observed in monazite debris (irregularly shaped balls, -100-500 the loading on the crack tip due to the indentation stress field is nm diameter) in the debond crack. initially mostly shear(although the loading eventually changes to C) Mullite Fibers: Sliding of the mullite fibers occurred tension if the crack grows sufficiently ) Because fiber pullout also predominantly at the fiber-coating interface SEM observations of involves shear loading of a debond crack, the initial region of separated interfaces showed plastic deformation of the LaPOa growth of the deflected cracks in Figs 2-4 should be representa- coating where the varying fiber diameter caused compression tive of the behavior during the corresponding stage of pullout. the coating during sliding, as depicted in region B of Fig. 9.(Note In all cases. the initial debond crack followed the fiber-matrn that the sliding displacements are smaller than the period of the interface, even when the interface was rough. For the mullite fibers diameter fluctuations and larger than the spacing of cusps associ- (Fig. 4)the sliding displacement of the debond crack surfaces ated with grain boundaries in the LaPO, coating )Where sliding the coating-fiber interface(region A in Fi is smaller than the average spacing between the interfacial cusps 9), the separated interface was similar to that of the sapphire fiber, (-600 nm). Sliding caused separation of the debonded surfaces to with grain-boundary cusps, clean separation, and no damage in the ccommodate their misfit (Fig. 4(b)), despite the constrained fiber or the coating configuration with large residual compressive normal stress(-700 MPa, Table D). The misfit was apparently accommodated by elastic diameter fluctuation by TEM was difficult, because only limited strains. with no irreversible deformation of the mullite fiber or the areas were observed. Nevertheless. some trends are evident. LaPO4 coating discernable by SEM. In contrast, sliding of the Deformation was distributed, ofte eutectic fibers caused extensive damage in the LaPO coating(Fig. entire coating thickness(Figs. 10 and 11), rather than being 2(c), without discernable damage in the fibers. The dama localized in a thin layer adjacent to the fiber as for the Al,O3/YAG included cracks across the full width of the coating. aligned at 45 to the interface on planes of maximum tension, similar to undeformed, whereas in others plastic deformation was confined to enVious o pservations of cracking in layers of LaPOa sandwiched an isolated grain(Fig. 10). The region of Fig. 10 was thought to between polycrystalline Al 2O3. More intense local damage is have experienced tension during sliding(as in Region A, Fig 9) evident at individual asperities, as in Fig. 2(c). The damage although the correlation with fiber diameter is uncertain because included cracking and fine lamellar features, which could be some of the fiber adjacent to the debond crack was removed during racks or twins ion milling. Extensive microcracking was distributed throughout the coating, often at x+45 to the fiber surface(Figs. 10 and 11) 3) Fiber pushout In regions of coating inferred to have been compressed during sliding(as in region B of Fig. 9), almost the entire coating was All the fibers debonded during the pushout experiments. Sliding microcracked and plastically deformed (Fig. 11). Extensive dislo- curred unstably over distances of "5-10 um at a critical load between 10-20 N. The average shear stress (load divided by fiber cation plasticity was evident, with variations in density from grai urface area)at the critical load was 130 10 MPa for the to grain. Some grains were twinned parallel to the fiber/matrix sapphire fibers, 200+ 20 MPa for the mullite fibers, 190+20 interface(Fig. ID), the orientation of maximum shear stress due to MPa for the Al2O3/YAG fibers, and 255 t 30 MPa for the pushout of the fiber. Microcracking at -45 to the fiber surface AL,O/ZrO, fibers was extensive, with crack spacings as small as "50 nm and ar (A) Sapphire Fibers: Sliding of the sapphire fiber occurred abundance of planar segments consistent with cleavage on, (100), at the fiber-coating interface, as reported previously. Grain (010), and (001), as reported previously. There was some tendency for cracks oriented normal to the maximum tensile stress boundary cusps were observed al he separated interfaces by SEM and AFM, although no damage was visible in either the fiber (northwest to southeast in Fig. 11)to be longer and have greater opening displacements than other cracks; however, the trends are subjective and the possibility of a sample preparation artifact B)Al2O,YAG and AlyOyzrO, Fibers: Extensive wear cannot be ruled out tracks were observed in the ApoA coatings on both eutectic fibers, indicating that sliding involved plastic deformation (Fig. 5). Sliding occurred mostly adjacent to the fiber-matrix interfac although smeared fragments of the LaPO coating remained on the (1) Effects of Residual Stress ber surface. In some regions(such as Fig. 5), sliding occurred near the matrix-coating interface The residual stresses noted in Table I might be expected to TEM observations from a typical specimen containing influence interfacial debonding. Therefore, it is necessary to ut Al,O3/YAG fiber are shown in Figs. 6-8. Slidi establish whether the fracture behavior in the model experiments along a debond crack between the ApoA coating and the fiber eported here is representative of that in real composites, given the most regions, a thin layer of the lapA coating within -100-300 differences in residual stress states and crack orientation nm of the fiber was heavily deforme In the analysis of He et al, 2 the presence of residual stresses igs 6-8). The intensity of shifts the debond criterion by an amount that depends on the deformation decreased with distance from the debond crack, with ions more than-500 nm from the fiber being undeformed. The parameters mp and nd Deformation in the ApoA consisted of tangled dislocation K lamellar features resembling twins, microcracks, and regions of densely packed fine crystallites(diameter as small as 10 nm) that where o and od are the residual stresses normal to potential crack resemble recrystallized microstructures(Fig. 6). The density of paths along the interface or into the fiber, K is the applied stress dislocations varied from grain to grain and generally decreased intensity factor for the incident crack, and a is a defect size. For a with distance from the debond crack. The nano-crystalline regions crack approaching the fiber on a radial plane, as in the indentation were within -50-100 nm of the debond crack. In one region, there cracking experiments of Section Il(2), the residual stresses o and was no deformation on the monazite side of the debond crack, but ca (radial and hoop stresses at the fiber surface) are equal, so the a thin layer of dense nano-crystalline monazite was smeared on the debond condition is not affected by the residual stressesby atomic force microscopy (ATM) have been reported else￾where.28,29 The cusp heights were typically 50 nm, and the angular distortions of the surface were small (
20°). The cusps on the mullite surfaces were very similar. Some insight into the effect of interfacial roughness on fiber sliding and pullout can be gained from the observations of Figs. 2–4. As the debond grows around the circumference of the fiber, the loading on the crack tip due to the indentation stress field is initially mostly shear (although the loading eventually changes to tension if the crack grows sufficiently). Because fiber pullout also involves shear loading of a debond crack, the initial region of growth of the deflected cracks in Figs. 2–4 should be representa￾tive of the behavior during the corresponding stage of pullout. In all cases, the initial debond crack followed the fiber-matrix interface, even when the interface was rough. For the mullite fibers (Fig. 4) the sliding displacement of the debond crack surfaces (250 nm, i.e., opening displacement of initial indentation crack) is smaller than the average spacing between the interfacial cusps (600 nm). Sliding caused separation of the debonded surfaces to accommodate their misfit (Fig. 4(b)), despite the constrained configuration with large residual compressive normal stress (700 MPa; Table I). The misfit was apparently accommodated by elastic strains, with no irreversible deformation of the mullite fiber or the LaPO4 coating discernable by SEM. In contrast, sliding of the eutectic fibers caused extensive damage in the LaPO4 coating (Fig. 2 (c)), without discernable damage in the fibers. The damage included cracks across the full width of the coating, aligned at 45° to the interface on planes of maximum tension, similar to previous observations of cracking in layers of LaPO4 sandwiched between polycrystalline Al2O3. 1 More intense local damage is evident at individual asperities, as in Fig. 2(c). The damage included cracking and fine lamellar features, which could be cracks or twins. (3) Fiber Pushout All the fibers debonded during the pushout experiments. Sliding occurred unstably over distances of 5–10 m at a critical load between 10–20 N. The average shear stress (load divided by fiber surface area) at the critical load was 130  10 MPa for the sapphire fibers, 200  20 MPa for the mullite fibers, 190  20 MPa for the Al2O3/YAG fibers, and 255  30 MPa for the Al2O3/ZrO2 fibers. (A) Sapphire Fibers: Sliding of the sapphire fiber occurred at the fiber-coating interface, as reported previously.1 Grain￾boundary cusps were observed along the separated interfaces by SEM and AFM, although no damage was visible in either the fiber or the coating. (B) Al2O3/YAG and Al2O3/ZrO2 Fibers: Extensive wear tracks were observed in the LaPO4 coatings on both eutectic fibers, indicating that sliding involved plastic deformation (Fig. 5). Sliding occurred mostly adjacent to the fiber-matrix interface, although smeared fragments of the LaPO4 coating remained on the fiber surface. In some regions (such as Fig. 5), sliding occurred near the matrix-coating interface. TEM observations from a typical specimen containing a pushed out Al2O3/YAG fiber are shown in Figs. 6–8. Sliding occurred along a debond crack between the LaPO4 coating and the fiber. In most regions, a thin layer of the LaPO4 coating within 100–300 nm of the fiber was heavily deformed (Figs. 6–8). The intensity of deformation decreased with distance from the debond crack, with regions more than 500 nm from the fiber being undeformed. The Al2O3/YAG fiber was also undamaged. Deformation in the LaPO4 consisted of tangled dislocations, lamellar features resembling twins, microcracks, and regions of densely packed fine crystallites (diameter as small as 10 nm) that resemble recrystallized microstructures (Fig. 6). The density of dislocations varied from grain to grain and generally decreased with distance from the debond crack. The nano-crystalline regions were within 50–100 nm of the debond crack. In one region, there was no deformation on the monazite side of the debond crack, but a thin layer of dense nano-crystalline monazite was smeared on the fiber surface. In some areas, this smeared layer was overlaid with a less dense, coarser grained agglomeration of angular monazite particles of diameter 50–100 nm (Fig. 7), suggestive of cata￾clastic flow, a process involving repeated microfracture and fine-particle transport.30 Similar features (intense deformation, fine crystallites, and agglomerates of angular particles) were observed in monazite debris (irregularly shaped balls, 100–500 nm diameter) in the debond crack. (C) Mullite Fibers: Sliding of the mullite fibers occurred predominantly at the fiber-coating interface. SEM observations of separated interfaces showed plastic deformation of the LaPO4 coating where the varying fiber diameter caused compression of the coating during sliding, as depicted in region B of Fig. 9. (Note that the sliding displacements are smaller than the period of the diameter fluctuations and larger than the spacing of cusps associ￾ated with grain boundaries in the LaPO4 coating.) Where sliding caused tension across the coating-fiber interface (region A in Fig. 9), the separated interface was similar to that of the sapphire fiber, with grain-boundary cusps, clean separation, and no damage in the fiber or the coating. Direct correlation of the changes in coating damage with fiber diameter fluctuation by TEM was difficult, because only limited areas were observed. Nevertheless, some trends are evident. Deformation was distributed, often nonuniformly, through the entire coating thickness (Figs. 10 and 11), rather than being localized in a thin layer adjacent to the fiber as for the Al2O3/YAG fiber. In some places, the monazite adjacent to the fiber was undeformed, whereas in others plastic deformation was confined to an isolated grain (Fig. 10). The region of Fig. 10 was thought to have experienced tension during sliding (as in Region A, Fig. 9), although the correlation with fiber diameter is uncertain because some of the fiber adjacent to the debond crack was removed during ion milling. Extensive microcracking was distributed throughout the coating, often at 45° to the fiber surface (Figs. 10 and 11). In regions of coating inferred to have been compressed during sliding (as in region B of Fig. 9), almost the entire coating was microcracked and plastically deformed (Fig. 11). Extensive dislo￾cation plasticity was evident, with variations in density from grain to grain. Some grains were twinned parallel to the fiber/matrix interface (Fig. 11), the orientation of maximum shear stress due to pushout of the fiber. Microcracking at 45° to the fiber surface was extensive, with crack spacings as small as 50 nm and an abundance of planar segments consistent with cleavage on, (100), (010), and (001), as reported previously.31 There was some tendency for cracks oriented normal to the maximum tensile stress (northwest to southeast in Fig. 11) to be longer and have greater opening displacements than other cracks; however, the trends are subjective and the possibility of a sample preparation artifact cannot be ruled out. IV. Discussion (1) Effects of Residual Stress The residual stresses noted in Table I might be expected to influence interfacial debonding. Therefore, it is necessary to establish whether the fracture behavior in the model experiments reported here is representative of that in real composites, given the differences in residual stress states and crack orientations. In the analysis of He et al., 32 the presence of residual stresses shifts the debond criterion by an amount that depends on the parameters p and d: p pa1/ 2 K , d da1/ 2 K (1) where p and d are the residual stresses normal to potential crack paths along the interface or into the fiber, K is the applied stress intensity factor for the incident crack, and a is a defect size. For a crack approaching the fiber on a radial plane, as in the indentation cracking experiments of Section II(2), the residual stresses p and d (radial and hoop stresses at the fiber surface) are equal, so the debond condition is not affected by the residual stresses. February 2003 Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite 309