No abstract

A new family of high-temperature, oxidation-resistant, ceramic composites, with (La)-monazite (LaPO,) serving as a weakly bonded interphase, is proposed. Monazite is stable and phase-compatible with alumina at temperatures at least as high as 1750°C in air. Especially important for use in high-toughness composites, the monazite-alumina interface is shown to be sufficiently weak that interfacial debonding prevents cracks from growing from monazite into alumina. Observations of fracture responses of fibrous and laminar reinforcements are presented.

No abstract

No abstract

No abstract

Two-phase composites consisting of LaPO 4 or CePO 4 and alumina, mullite, or zirconia were found to be machinable; i.e., they can be cut and drilled using conventional tungsten carbide metal-working tools. Single-phase LaPO 4 was also machinable. Measurements of drilling rates, grinding rates, and normal forces are used to compare the ease of machining in these materials and in a conventional machinable glass-ceramic material, and to provide preliminary information on the relation between microstructure and machining properties. In Hertzian contact experiments these materials showed extensive nonlinear behavior associated with a damage zone beneath the contact site, similar to other machinable ceramics. Mechanisms of material removal are discussed.

Room temperature debonding and sliding of La-monazite coated fibers is assessed using a composite with a polycrystalline alumina matrix and fibers of several different single crystal (mullite, sapphire) and directionally solidified eutectic (Ab03 /Y3AlsO 12 and Ab03/Y-Zr02) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface rouglmess were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations and occasional twinning, whereas the fibers were undamaged . Dense, fine-grained (lO nm) microstructures suggesti ve of dynamic recrystallization were also observed in the coatings. Frictional heating during sliding is assessed. The possibility of low temperature recrystallization is discussed in the light of the known resistance of monazite to radiation damage. The ability of Lamonazite to undergo plastic deformation relatively easily at low temperatures may be enabling for its use as a composite interface.

No abstract