The processing, mechanical analysis, and performance of functionally graded materials containing a metallic phase is reviewed in two parts. This first part focuses on processing, and distinguishes two principal process Classes in graded materials production. The first class, termed ‘constructive processes’, produces gradients by stacking selectively two or more starting materials, allowing full and potentially automated control of compositional gradients. The second class is termed ‘transport based processes’ and utilises natural transport phenomena to create compositional and microstructural gradients during production of a component. Timescales for relevant transport phenomena are evaluated and compared to usual processing times to assess their role and importance in both functionally graded materials process classes. Constructive processes comprise chiefly powder consolidation processes, such as solid state powder metallurgy or reactive sintering, and coating processes, such as plasma spraying ...

Functionally graded metals and metal-ceramic composites: Part 1 Processing

Processing of cellular ceramics with anisotropic pore structures by infiltration of liquid Silicon into carbonized wood and subsequent reaction to SiC was investigated. Natural wood of different pore size distribution and composition was carbonized at 800–1800°C in inert atmosphere resulting in a one-to-one reproduction of the original wood structure. The carbon template was converted to SiC by a rapid liquid infiltration-reaction process at 1600°C. Spontaneous infiltration was achieved by using the continous tracheidal cells in wood as a transportation path for liquid silicon. β-SiC formed by solid–liquid reaction at the pore surface exhibits a crystalline texture which may be related to the initial microfibril orientation in the cell walls of wood. Depending on the initial cellular microstructure of the various kinds of wood (ebony, beech, oak, maple, pine, balsa) ceramic materials of different anisotropic pore structures in the form of pseudomorphs of the original wood were obtained.

Biomorphic Cellular Silicon Carbide Ceramics from Wood: I. Processing and Microstructure

Inspection of the scientific literature reveals that intermetallic compounds have, in recent years, attracted considerable interest as a result of their unique elevated temperature characteristics. Among the wide range of intermetallic compounds that are actively being studied, MoSi2 has been singled out as a result of its unique combination of properties, which include an excellent oxidation resistance, a high modulus of elasticity, and an elevated melting point (2030°C). In view of this interest, the present work was undertaken with the objective of providing the reader with a comprehensive review of the mechanical and oxidation behaviour of MoSi2, paying particular attention to the synergism between processing and microstructure. Accordingly, synthesis techniques, including powder metallurgy, self-propagating hightemperature synthesis, spray processing, solid-state displacement reactions, and exothermic dispersion, are critically reviewed and discussed. In addition, recent efforts aimed at using MoSi2 as a matrix material in metal-matrix composites are also critically reviewed and discussed.

Processing of molybdenum disilicide

Structural ceramics for high-temperature applications should embody the following properties: oxidation resistance, chemical stability, low volatility, resistance to creep deformation, resistance to creep cavitation at interfaces, sufficient toughness at ambient temperature, and thermalshock resistance. These criteria lead to five themes for fundamental research and design of high-temperature structural ceramics: chemical and environmental stability, grain-boundary sliding and cavitation, single-crystal microstructure design, room-temperature mechanical properties, and thermal shock. It is recommended that research that is confined to any one of these five areas takes into consideration the broader implications of research results. For example, microstructure designs that require weak interfaces for obtaining toughness at room temperature directly or indirectly conflict with creep and cavitation resistance needed for long-term service at high temperatures. New research should be directed at mechanisms that can simultaneously achieve good mechanical properties over a wide range of temperatures. This paper addresses the following recommendations: (i) although non-oxide systems can be viable for structural applications below 1500°C, oxidebased ceramics are necessary for service above 1500°C; (ii) microstructure designs based on acicular grain morphologies and/or single-crystal fiber reinforcements have the potential for meeting the mechanical property requirements from room temperature up to very high temperatures; (iii) for fundamental studies of mechanical properties at high temperatures, simple uniaxial tension experiments should be used in tandem with four-point bending and uniaxial compression experiments; (iv) the study of the reinforcement phase should center on very pure, highly stoichiometric materials in the case of non-oxides, and on mixed and alloyed single crystals of cubic symmetry, or crystals having isotropic properties, and large unit cells in the case of oxides; (v) the study of interfaces in non-oxides should focus on the chemistry of the intergranular glass phase, particularly the control of the oxygen content and the crystallization of this phase for improvement of high-temperature properties; (vi) the study of interfaces in oxides is best directed at the relationship between interface structure, defect chemistry, and interfacial mechanical properties over a wide range of temperature; (vii) the understanding of the micromechanisms of thermal-shock failure and the application of this understanding for designing graded interfaces that may be able to cope with thermal-expansion stresses without leading to microfracture and cavitation is important in all classes of ceramic materials, and is of critical importance in the development of oxides for very-high-temperature applications; and (viii) research in processing science should emphasize the study of basic mechanisms that lead to in-situ growth of acicular and fibrous microstructures.

Fundamental Research in Structural Ceramics for Service Near 2000°C

Reaction kinetics and ablation properties of C/C–ZrC composites fabricated by reactive melt infiltration

The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

Reaction-formed silicon carbide

The authors have investigated reaction-forming of silicon carbide by the infiltration of carbonaceous preforms using alloyed silicon melts, in order to synthesize composite materials free of the residual silicon phase that has previously limited mechanical properties and upper use temperatures. In this approach, rejection of the alloying component(s) from the primary silicon carbide phase into the remaining melt results in the formation of a secondary refractory phase, such as a silicide, in place of residual free silicon. Experiments conducted in the Si-Mo melt system show that relatively dense ({gt}90%) silicon carbide-molybdenum silicide materials free of residual silicon and residual carbon can be obtained. A model for reactive infiltration based on time-dependent permeabilities is proposed. Processing variables important for control of the reaction rate relative to the infiltration rate, and associated processing flaws, are discussed.

Liquid‐Phase Reaction‐Bonding of Silicon Carbide Using Alloyed Silicon‐Molybdenum Melts

A carbonaceous preform is infiltrated with a silicon melt alloyed with at least one element which is substantially insoluble in silicon carbide and which forms refractory phases. The resulting silicon carbide composite has substantially no free silicon. Silicides of the alloying elements are a suitable refractory phase. Suitable alloying elements which form refractory silicides include Mo, W, Re, Hf, Zr, Cr, B, and Ti. The carbonaceous preform may include graphite particulates, carbon particulates, porous carbon skeletons, carbon-carbon composites, carbon fibers, and graphite fibers. Particulates may be incorporated into the preform for the purpose of improving mechanical properties. Suitable particulates include refractory metals, carbides, nitrides, borides, silicides, and oxides. Fiber-reinforced composites can be produced by incorporating whiskers, chopped fibers, or continuous fibers of a refractory material into the preform. The refractory phase in the composite has a beneficial effect on the high temperature mechanical properties, such as the hardness, strength, toughness, and creep resistance of the silicon carbide composites.

Method of preparing refractory silicon carbide composites and coatings

Reaction-bonded silicon carbide free of the residual silicon phase that has previously limited upper use temperatures and mechanical properties has been synthesized by the infiltration of carbonaceous preforms using alloyed silicon melts. In this approach, rejection of the alloying components from the primary reacted silicon carbide phase into the remaining liquid forms a secondary refractory silicide, in a potentially broad and controllable range of volume fractions. Dense SiC-MoSi/sub 2/ materials free of residual silicon and residual carbon have been synthesized. In addition to a fully-infiltrated monolith, surface coatings on a carbon body have been synthesized by control of processing parameters. In each, the absence of free silicon and incorporation of the secondary phase is expected to improve mechanical properties. 14 references.

Robert P. Messner

Papers

Reaction-formed silicon carbide

Liquid‐Phase Reaction‐Bonding of Silicon Carbide Using Alloyed Silicon‐Molybdenum Melts

Method of preparing refractory silicon carbide composites and coatings

Processing of Reaction-Bonded Silicon Carbide Without Residual Silicon Phase