Showing papers on "Ceramic matrix composite published in 1982"

Carbon fibre-reinforced silicon nitride composite

Abstract: The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m−2 for unreinforced Si3N4, to 4770 J m−2; its fracture toughness,K lc , increased from 3.7 MN m−3/2 for unreinforced material, to 15.6 MN m−3/2. The strength remains about the same as unreinforced Si3N4 and the thermal expansion coefficient is only 2.51×10−6 ° C−1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.

Determination of the thermal conductivity and diffusivity of thin fibres by the composite method

Abstract: It is suggested that the thermal conductivity of very fine fibres can be evaluated indirectly with the aid of composite theory using the experimental data for the heat transport properties of an appropriate composite which contains the fibres. The feasibility of this approach was investigated by determining the thermal conductivity and diffusivity of fibres of amorphous silicon carbide from 25° C to 1000° C contained within a lithium aluminosilicate glass-ceramic using the laser-flash technique for measurement of the thermal diffusivity of the composite. Due to the amorphous nature of the fibres, values for their thermal conductivity and diffusivity were found to be far less than the corresponding data for crystalline silicon carbide. The positive temperature dependence of the thermal conductivity, coupled with the independent observation of an increase in thermal conductivity with specimen thickness, suggests that radiative heat transfer makes a significant contribution to the total heat transferred. A number of advantages and limitations of the composite method for the evaluation of thermal transport properties of fibres are discussed.

Plasma coatings comprised of sprayed fibers

Abstract: A process for adhering small metal fibers (24), to a surface (23) by plasma spraying the fibers on a workpiece (20), and articles made using the process. The process is especially useful for improving the strength of plasma arc coatings, as well as for improving the bonding of plasma arc coatings to substrates. To make an improved ceramic faced metal article, fibers (24) are sprayed onto the workpiece (20, 34) by injecting fibers (44) into the plasma stream (36) external to the plasma gun nozzle (38). Then, plasma sprayed ceramic particles are caused to surround the fibers as a matrix (26). The optional interposition of a removable polymer material (3) on the workpiece surface (23), after the fibers (24) are sprayed but before the ceramic matrix (26') is sprayed, provides an effective way of providing a low stiffness connector (30') between a low thermal expansion coefficient ceramic material (26') and a high expansion coefficient metal substrate (20'). The connector alleviates strains from thermal expansion differences.

Discontinuous silicon carbide fiber reinforced ceramic composites

Abstract: Discontinuous silicon carbide fiber reinforced ceramic composites are disclosed having high strength, fracture toughness, and oxidative stability, even in high temperature oxidative environments. Disclosed composites include silicon carbide fiber reinforced aluminosilicate, lithium aluminosilicate, barium aluminosilicate, magnesium aluminosilicate, and combinations thereof. Flexural strengths in excess of 20,000 psi (138 MPa) at temperatures in excess of 1000° C. are attainable with such composites. The composite is formed by starting with the ceramic matrix material in the glassy state and converting it from the glassy state to the ceramic state after densification of the composite.

Ceramic materials by sol-gel route

Abstract: Sintered materials are known comprising a ceramic matrix (e.g. of Al 2 O 3 ) and, distributed therein, a refractory material in a metastable, high temperature enantiotropic form of higher density than its room temperature enantiotropic form such as ZrO 2 which has a metastable tetragonal form and a room temperature monoclinic form. The sintered materials have enhanced fracture and bending strength. The invention provides a method of making such materials from a mixture where the refractory material and optionally the ceramic material are in the form of a sol. The mixture is dried and subsequently sintered to give a product where the refractory material is distributed very uniformly and in its metastable form. Improved properties have been obtained by including a stabilizing agent (e.g. Y 2 O 3 ) in the final product.

Ceramic matrix material

Abstract: Sintered materials comprising a ceramic matrix (e.g. of Al2O3) and, distributed therein, a refractory material in a metastable, high temperature enantiotropic form of higher density than its room temperature enantiotropic form such as ZrO2 which has a metastable tetragonal form and a room temperature monoclinic form, are prepared from a mixture where the refractory material and optionally the ceramic material are in the form of a sol. The mixture is dried and subsequently sintered to give a product where the refractory material is distributed very uniformly and in its metastable form. Improved properties have been obtained by including a stabilising agent (e.g. Y2O3) in the final product.

Method of fabricating an improved ceramic radome

Abstract: Ceramic radomes are fabricated using a method which reduces the dielectric losses of the ceramic material. A Si3 N4 ceramic powder is mixed with a suitable densification aid and then sintered to form a dense ceramic having a glassy phase. Silicon dioxide is then provided on the surface of the ceramic by packing it in silicon dioxide powder or by heating it in air to oxidize its surface. The ceramic and silicon dioxide are heated at a temperature sufficient to cause diffusion of impurities and additive cations from the glassy phase into the silicon dioxide. The surface of the ceramic is then ground to remove pits and to shape the ceramic into a radome.

Research on ultra-high-temperature materials, monolithic ceramics, ceramic matrix composites and carbon/carbon composites

Abstract: Research on three classes of materials that show potential for allowing significant increases in operating temperatures in gas turbine engines is discussed. Monolithic ceramics, ceramic matrix composites, and carbon-carbon composites are discussed. Sintering, hot pressing, and densification are discussed.

Development and testing of matrices for the encapsulation of glass and ceramic nuclear waste forms.

Abstract: This report details the results of research on the matrix encapsulation of high level wastes at PML over the past few years. The demonstrations and tests described were designed to illustrate how the waste materials are effected when encapsulated in an inert matrix. Candidate materials evaluated for potential use as matrices for encapslation of pelletized ceramics or glass marbles were categorized into four groups: metals, glasses, ceramics, and graphite. Two processing techniques, casting and hot pressing, were investigated as the most promising methods of formation or densification of the matrices. The major results reported deal with the development aspects. However, chemical durability tests (leach tests) of the matrix materials themselves and matrix-waste form composites are also reported. Matrix waste forms can provide a low porosity, waste-free barrier resulting in increased leach protection, higher impact strength and improved thermal conductivity compared to unencapsulated glass or ceramic waste materials. Glass marbles encapsulated in a lead matrix offer the most significant improvement in waste form stability of all combinations evaluated. This form represents a readily demonstrable process that provides high thermal conductivity, mechanical shock resistance, radiation shielding and increased chemical durability through both a chemical passivation mechanism and as a physical barrier. Othermore » durable matrix waste forms evaluated, applicable primarily to ceramic pellets, involved hot-pressed titanium or TiO/sub 2/ materials. In the processing of these forms, near 100% dense matrices were obtained. The matrix materials had excellent compatibility with the waste materials and superior potential chemical durability. Cracking of the hot-pressed ceramic matrix forms, in general, prevented the realization of their optimum properties.« less

Carbon fibre-reinforced silicon composite nitride

Abstract: The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by lowtemperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m -2 for unreinforced Si3N4, to 4770J m-2; its fracture toughness, Kic, increased from 3.7 MN m -3/2 for unreinforced material, to 15.6 MN m -3/2. The strength remains about the same as unreinforced Si3N4 and the thermal expansion coefficient is only 2.51 x 10-6~ -1 (RT to 1000 ~ C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.