Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

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Review: liquid phase sintering

Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature (<lOOO"C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for lowtemperature (1250" to 1500°C) densification through viscous deformation. Flexural strength and creep resistS M Wiederhorn-contributing editor

Mullite for Structural, Electronic, and Optical Applications

Ceramic-matrix composites (CMCs) based on TiC–TiB2 have attracted enormous interest during recent years because, in comparison to single-phase ceramics, they exhibit superior properties including high hardness, good wear resistance and high fracture toughness. This paper begins with a review of the TiC–TiB2 equilibrium system and its possible influence on the processing and properties of the composite. The application of TiC–TiB2 composites has been limited due to the fact that they have been difficult to process. Much of the research effort has therefore focused on the synthesis, processing and fabrication of TiC–TiB2 and is based primarily on self-propagating high-temperature synthesis (SHS) and its derivatives, high-energy milling and sintering. The performance of SHS under the application of pressure has been the subject of particular investigation. These developments are the main subject of this review that also takes into account the resulting effects on the microstructure and the mechanical properties of TiC–TiB2. The influence of the reaction parameters like reactant composition, reactant particle size and green density on the microstructure and properties is also reported.

TiC-TiB2 composites : A review of phase relationships, processing and properties

A series of novel, dense, and interesting ordered mesoporous carbon (OMC)/fused silica composites with different carbon contents has been prepared by a controllable but simple sol-gel method followed by hot-pressing. In the as-sintered OMC/fused silica composites the carbon particles still exist in the form of perfectly ordered carbon nanowires. Conductivity measurements on the composites indicate that these novel composites are electrically conductive and have a typical percolation threshold of 3.5-5 vol% OMC. The electromagnetic interference (EMI) shielding efficiency (SE) of an OMC/fused silica composite containing 10 vol% OMC is as high as 40 dB in the X band which is higher than that of a carbon nanotube (CNT)/ fused silica composite with the same carbon content (∼30 dB). This indicates that these conductive OMC/fused silica composites are very suitable for an application as EMI shielding materials. Upon increasing the volume content of OMC in the composite the overall contribution as well as the increase rate of the microwave absorption are larger than those of the microwave reflection, which suggest that OMC/ fused silica composites may also be promising electromagnetic (EM) wave absorbing materials. Based on the promising properties of these composites this work will hopefully lead to the development of new low-cost and highly efficient EMI shielding or EM wave absorbing materials.

Ordered Mesoporous Carbon/Fused Silica Composites

The present paper summarizes the main results generated in a German National Science Foundation (DFG) program on projects concerned with functionally graded materials applied to optimize the thermal, wear and corrosion properties of metallic and ceramic materials. Thermal barrier coatings deposited onto Cu substrates by pulsed laser deposition showed improved spallation behavior by a graded lamella microstructure with improved interface fracture toughness. A particle-hardened graded surface structure improved the wear resistance of plasma sprayed thermal barriers. By means of evaporation techniques a graded bonding area was manufactured with a high potential of lifetime improvement. For non-oxide ceramics graded coatings based on Si 3 N 4 and mullite led to improved oxidation resistance of the substrate material. Graded TiC–TiN thin films allowed to improve the wear resistance of cutting tool alloys with good adhesion to the substrate material. On light metal alloys, the limits of grading with respect to corrosion protection as well as wear were determined. Graded layers of arc-sprayed titanium with in situ produced particles or welded alloy gradients led to improved wear characteristics. Stress profiles in graded layers were analyzed with the help of a modified X-ray diffraction analysis.

Graded coatings for thermal, wear and corrosion barriers

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.

Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

The effect of composition on mullite formation was investigated using submicrometer composite particles that consisted of α-alumina cores and amorphous silica coatings. Powders with varying alumina/silica ratios were prepared by varying the thickness of the silica coating. The mullitization behavior was monitored using differential thermal analysis, X-ray diffractometry, and scanning electron microscopy. The results were consistent with previous studies that indicated that mullite forms initially by nucleation and growth within the siliceous phase, but also that chemical interdiffusion within the mullite grains is required to complete the reaction. The reaction rate in both stages was affected by the alumina/silica ratio. Available evidence has indicated that the first stage of the reaction is controlled by the dissolution of alumina in the siliceous phase. Results for the second stage suggested that alumina diffuses more rapidly through mullite than silica.

Effect of Composition on Mullitization Behavior of α-Alumina/Silica Microcomposite Powders

An investigation was carried out concerning the effect of mullite seed particles on phase development, densification behavior, and microstructure evolution in powder compacts prepared with silica/alumina microcomposite particles. The incorporation of ∼2 wt% seed particles in the microcomposite powder compacts had relatively little effect on densification, but resulted in significant decreases in the temperature for mullite formation and the grain sizes in mullitized samples. Samples could be sintered to almost full density and subsequently converted to mullite with average grain sizes ≤0.4 μm at temperatures in the range of 1300°-1400°C. The available evidence indicated that mullite formation occurred primarily by nucleation and growth in the siliceous matrix phase.

Effect of Seeding on Phase Development, Densification Behavior, and Microstmcture Evolution in Mullite Fabricated from Microcomposite Particles

The formation of mullite in silica-coated α-alumina microcomposite powders was investigated using differential thermal analysis. An apparent activation energy of 1042 ± 32 kj/mol was determined for the mullitization reaction. This result suggests that mullite formation in microcomposite powders occurs by the same mechanism as in diphasic gels.

Activation Energy for Mullitization of α‐Alumina/Silica Microcomposite Particles

The mechanism of mullite formation was investigated using submicrometer composite particles which consisted of alpha alumina cores and amorphous silica coatings. Mullitization behavior was monitored using X-ray diffraction analysis, differential thermal analysis, and scanning electron microscopy. The transformation occurred with an incubation period which was followed by stages of rapid mullite growth (up to ∼70% conversion) and slower mullite growth. The first growth stage occurred primarily by nueleation and growth within the siliceous matrix. Available evidence indicates that the growth rate was controlled by dissolution of alumina in the siliceous phase. The second stage of mullitization occurred primarily by interdiffusion of alumina and silica through the mullite grains formed during the first stage. The transformation rate in the second stage was increased significantly by using mullite seed particles which produced smaller grain sizes during the first stage (and, thereby, decreased the interdiffusion distances needed to complete the reaction). This seeding approach allowed fabrication of bulk mullite samples with nearly 100% relative density and fine grain size (∼0.4 μm) after heat treatment at only 1400°C (2 h).

Nazim Bozkurt

Papers

Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Effect of Composition on Mullitization Behavior of α-Alumina/Silica Microcomposite Powders

Effect of Seeding on Phase Development, Densification Behavior, and Microstmcture Evolution in Mullite Fabricated from Microcomposite Particles

Activation Energy for Mullitization of α‐Alumina/Silica Microcomposite Particles

Mullitization Behavior of Alpha Alumina/Silica Microcomposite Powders