Janet B. Davis

Monazite-containing oxide/oxide composites

Abstract An extremely simple processing route has been used to produce LaPO 4 (La-monazite)/alumina continuous fiber-reinforced ceramic composites. In this paper, the processing, microstructure and tensile properties are reviewed. In particular, the damage tolerance and notch insensitivity of this system, which contains monazite-coated fibers, will be compared to the properties reported for other oxide composites.

Ceramic composites for thermal protection systems

Abstract Advanced coating systems based on monazite, a weak interphase for oxide composites, are being investigated as a means to increase the service temperatures of thermal protection blankets for re-entry space craft. Preliminary evaluations, including chemical compatibility, tensile strengths of coated, heat-treated fibers and fabrics, and durability in a modulated wind tunnel facility have been conducted.

Oxide Composites of Al2O3 and LaPO4

Abstract Some properties of oxide composites based on Al 2 O 3 and LaPO 4 (La-monazite) are examined. A composite consisting of woven Al 2 O 3 fibers with a porous matrix of Al 2 O 3 and LaPO 4 is shown to be damage tolerant and notch insensitive. The feasibility of achieving fiber sliding and pullout in a composite with a fully dense matrix is investigated using a small hot-pressed composite of sapphire fibers and LaPO 4 matrix.

Ceramics for future power generation technology: fiber reinforced oxide composites

Major advances have been made during the past 2 years in the development of fiber-reinforced oxide composites for long-life combustion components. These include demonstration of long-term stability of mullite-based porous-matrix composites at 1200°C, development of fiber coating and slurry infiltration methods to produce composites with weakly bonded La-monazite interphases, and development of new alumina-based fibers.

Integral Textile Ceramic Composites for Turbine Engine Combustors

Integrally formed ceramic matrix composite structures are being developed for a range of hot-structure applications involving active cooling. In this paper, some advantages of integral textile approaches are summarised and design possibilities for turbine engine combustors are suggested. Advantages of integral textile structures include: 1) joints between ceramic and other materials in hot zones can be avoided; 2) thin skins (< 1 mm) can be formed that are strong and tough, enabling tolerance of higher heat fluxes; 3) compliant structures can be designed, which can limit the development of thermal mismatch stresses; and 4) fabrication costs can be lowered by reducing part counts and steps in processing. This paper discusses some of the non-traditional design and fabrication challenges that must be met to exploit integral textile ceramic structures and offers a preliminary assessment of their viability for handling the thermomechanical loads of an advanced combustor.© 2002 ASME

High temperature creep of La-monazite

Abstract Compressive creep of La-monazite is investigated in the temperature range 1100 °C to 1500 °C. The study includes both high-purity single-phase material and material with excess phosphorus located in amorphous grain boundary phases. The results indicate that the presence of small amounts of excess P in polycrystalline LaPO4 has a large effect on microstructural stability and creep at high temperature. Materials with La/P ratio close to unity (within ∼500 ppm) show little grain growth at temperatures up to 1400 °C and deform by creep at rates similar to those of alumina and zirconia, with stress exponent ∼1. Materials containing excess P (as little as ∼1 %) show more rapid grain growth, higher creep rates, and cavitation during creep. The results are compared with creep rates of other refractory oxides and oxide fibers. Implications for the behavior of oxide composites containing La-monazite are considered.

Design Issues in Using Integral Textile Ceramic Composites in Turbine Engine Combustors

Integrally formed ceramic matrix composite structures are being developed for a range of hot-structure applications involving active cooling. Integral textile structures offer several advantages: 1) Joints between ceramicand other materials in hot zones can be avoided. 2) Thin skins (<1 mm) can be formed that are strong and tough, enabling tolerance of higher heat fluxes and the use of materials, such as oxide-oxide composites, with attractive environmental stability but relatively low conductivity and high thermal expansion. 3) Compliant structures can be designed, which can limit the development of thermal mismatch stresses. 4) Fabrication costs can be lowered by reducing part counts and steps in processing. Preliminary analyses are presented that demonstrate how these benefits might be realized for an advanced annular combustor. The possibility of a very attractive design space is indicated, based on reasonable assumptions for heat transfer coefficients for the parameter regime relevant to the new designs.

The mechanics of delocalization and energy absorption in chain composites

The mechanics of chain composites that can exhibit delocalized failure and very large values of energy absorbed per unit volume during failure are analyzed. The composites absorb energy because they are configured in such a way that chain links must displace through large distances before coming into intimate contact with one another, doing work continuously against the matrix. An approximate analytical model for the stresses within the chain links during link displacement is formulated and the results validated by finite element analyses. The condition that the maximum stress within the links be less than the link strength leads to a criterion for transition from localized failure (involving failure of the links at small strains) to delocalized failure (with distributed damage and large plastic strains occurring via link displacement). The model is also proposed as a design rule for optimizing the energy absorption of this class of composites.