512 FABER The CVD amorphous coatings demonstrated greater oxidation resistance than pure pyrolytic boron nitride and enhanced resistance to moisture attack. The oxidation resistance was improved by three orders of magnitude at 1500C with a slight increase in debonding and sliding resistance measured through push-out Both carbon and boron nitride serve another function in ceramic-matrix compositesthat of a compliant layer. Such compliance is useful for ac- commodating thermal mismatch stresses between the reinforcement and the matrix. Furthermore, and just as important, is the role of the compliant layer in accommodating misfit stresses due to interfacial fracture surface roughness (described above) The Oxide Interfaces The failure of carbon and boron nitride to meet chemical stability and oxidation resistance for oxide-oxide composites led to exploration of a series of oxide interface materials. These include the highly stable oxides of monazite and tin oxide and the highly anisotropic layered oxides MONAZITE The oxide receiving the greatest attention for brittle matrix com- posites is monazite, M"PO4, where M is a trivalent cation. Morgan& marshall (59)first proposed the use of LapO4 as an oxide fiber coating for alumina alumina composites. The first criterion, compatibility with alumina, is possible ith some mixed oxides, particularly when the oxide is formed from an element on the basic side of the periodic table with one from the acidic side(60).How- ever, a second criterion requires that a weak interface be formed. Characteristic fracture experiments performed by morgan Marshall(60) show interfacial fracture energies(Gis)of 4.5 J/m, sufficiently low to allow crack deflection during crack propagation through monazite to alumina [Ge(Al2O3)=20J/m-1 but a crack propagating in the reverse direction, alumina to monazite, would penetrate, as shown in Figure 7. This was demonstrated in the fracture exper- iments shown in Figure 8, where an indentation crack penetrates the monazite layer but is arrested at the monazite-alumina interface. Additional microscopy shows evidence of debonding Because fracture is confined to the interface, the question remains as to why onazite bonds so poorly to alumina. Morgan& Marshall speculate that(a)the open irregular oxygen crystal planes of monazite show no epitaxial relationship to the close-packed oxygens of alumina, and(b)the oxygen surface of the monazite is fully satisfied with the lanthanum and phosphorus ions of high bilit The lanthanum-monazites have been found to retain their stability at elevated temperatures(61). After heat treatment at 1600oC, B-alumina-magnetoplumbiteP1: ARK/MBL/rkc P2: MBL/vks QC: MBL/agr T1: MBL May 16, 1997 13:47 Annual Reviews AR034-16 512 FABER The CVD amorphous coatings demonstrated greater oxidation resistance than pure pyrolytic boron nitride and enhanced resistance to moisture attack. The oxidation resistance was improved by three orders of magnitude at 1500◦C with a slight increase in debonding and sliding resistance measured through push-out experiments. Both carbon and boron nitride serve another function in ceramic-matrix composites—that of a compliant layer. Such compliance is useful for ac￾commodating thermal mismatch stresses between the reinforcement and the matrix. Furthermore, and just as important, is the role of the compliant layer in accommodating misfit stresses due to interfacial fracture surface roughness (described above). The Oxide Interfaces The failure of carbon and boron nitride to meet chemical stability and oxidation resistance for oxide-oxide composites led to exploration of a series of oxide interface materials. These include the highly stable oxides of monazite and tin oxide and the highly anisotropic layered oxides. MONAZITE The oxide receiving the greatest attention for brittle matrix com￾posites is monazite, MIIIPO4, where M is a trivalent cation. Morgan & Marshall (59) first proposed the use of LaPO4 as an oxide fiber coating for alumina￾alumina composites. The first criterion, compatibility with alumina, is possible with some mixed oxides, particularly when the oxide is formed from an element on the basic side of the periodic table with one from the acidic side (60). How￾ever, a second criterion requires that a weak interface be formed. Characteristic fracture experiments performed by Morgan & Marshall (60) show interfacial fracture energies (Gic) of 4.5 J/m2 ; sufficiently low to allow crack deflection during crack propagation through monazite to alumina [Gc (Al2O3) = 20 J/m2 ], but a crack propagating in the reverse direction, alumina to monazite, would penetrate, as shown in Figure 7. This was demonstrated in the fracture exper￾iments shown in Figure 8, where an indentation crack penetrates the monazite layer but is arrested at the monazite-alumina interface. Additional microscopy shows evidence of debonding. Because fracture is confined to the interface, the question remains as to why monazite bonds so poorly to alumina. Morgan & Marshall speculate that (a) the open irregular oxygen crystal planes of monazite show no epitaxial relationship to the close-packed oxygens of alumina, and (b) the oxygen surface of the monazite is fully satisfied with the lanthanum and phosphorus ions of high polarizability. The lanthanum-monazites have been found to retain their stability at elevated temperatures (61). After heat treatment at 1600◦C,β-alumina-magnetoplumbite