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The resonant beam technique (RBT) allows the simultaneous determination of the Young's and the shear modulus. In the present paper this method is extended to materials covering an extremely wide range of porosities from nearly dense to the limit of stability, which is shown for ZrO 2 -toughened reaction-bonded Al 2 O 3 (RBAO) ceramics as an example. Additional characterisation of the pore structure and the material's composition was performed to exclude an influence of structural parameters other than porosity.

The reactions that take place during the formation of ceramic matrix composites that are based on α-Al2O3–(Al–Si)3Ti interpenetrating networks were analysed. A reactive preform was pressure infiltrated with an Al–Si alloy. After pressure infiltration, the composite did not react in a full manner and further thermal annealing was required. The reduction of TiO2 by the liquid Al–Si alloy results in the formation of (Al-Si)3Ti (Al60Si12Ti28). The formation of (Al–Si)3Ti is governed by the consumption of TiO that is formed as an intermediate phase during the reduction of TiO2.

WOS: 000232525200005, Three-layer alumina-based composites reinforced with iron in the inner layer and with chromium in the outer layers were fabricated by first uniaxially pressing the three-layer assembly, followed by cold isostatic pressing at 300 MPa and sintering in a graphite vacuum furnace at 1500 degrees C for 1 h. The residual compressive stresses in the outer Cr-Al/Al2O3 layers and the residual tensile stresses in the Fe-Al/Al2O3 inner layer were predicted as a function of composition and the thickness ratio of the outer and innerlayers. Theoretical calculations showed that the compressive stresses in the outer layers increased while the tensile stresses in the inner layer decreased with decreasing outer layer thickness. The existence of compressive stresses was verified by microscopic evidence, which showed that propagation of cracks perpendicular to the interface is suppressed in the outer layer, but promoted in the inner layer. Indentation and subsequent strength testing showed that these layered composites exhibited improved damage tolerance. Three-layer composites showed four-point bend strengths exceeding the bend strength of unindented monolithic Al2O3 even after indentation at 300 N

Porous Si 3 N 4 ceramics with a fibrous interlocking microstructure were synthesized directly by carbothermal nitridation (CT/N) of SiO 2 . Carbon black was used as the carbon source and α-Si 3 N 4 was used as seed. The cold-pressed samples of powder mixtures were heated at nitrogen pressures above 0.6 MPa and temperatures exceeding 1600 °C. The addition of the α-Si 3 N 4 to the initial powder mixtures had important effects on the microstructure and mechanical properties of the porous Si 3 N 4 . Fine elongated fibrous β-Si 3 N 4 grains were developed in the seeded samples with stoichiometric C–SiO 2 ratio and Y 2 O 3 as the sintering additive, when sintered at 1700–1750 °C. A sample of outstanding strength resulted, five times stronger than the seed-free samples. Such a technique offers the possibility of synthesizing highly porous and strong Si 3 N 4 materials at considerably lower cost than at present.

Different Fe-Al2O3 and FeAl-Al2O3 composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al2O3, Fe, and Al mixtures. Low Al contents (

Niobium-alumina (Nb-Al 2 O 3 ) composites have been fabricated via the pressureless sintering of compacts of intensively milled Nb-Al 2 O 3 mixtures. Strength and fracture toughness of the composites increase as the niobium content increases. For a composite that contains 50 vol% niobium, strengths of up to 680 MPa, with a corresponding fracture toughness of 6.3±0.3 MPa.m 1/2 and hardness of 6.4 GPa, have been obtained.

A reactive sintering process has been used to produce almost fully dense composites with interpenetrating networks of NbAl 3 and Al 2 O 3 . The process involves the reaction synthesis of niobium aluminides and Al 2 O 3 from compacts of intensively milled aluminum and Nb 2 O 5 powder mixtures. During carefully controlled heating under an inert atmosphere, the oxide reduction by aluminum to form niobium aluminides and Al 2 O 3 , proceeds at temperatures below the melting point of aluminum. At temperatures of >1000°C, the reaction-formed niobium aluminides and Al 2 O 3 sinter. The present paper discusses processing parameters, such as attrition milling, the heating cycle, and the metal:ceramic ratio in the starting mixture, that control microstructure development and mechanical properties.

Two-phase layered composites were prepared by the repeated rolling and folding of paraffin-wax-based Cr-Al 2 O 3 and Al 2 O 3 suspensions at room temperature. The electrical resistivity of sintered samples was characterized in three mutually perpendicular directions. The ac resistivity of the material is governed by the position of the percolation threshold of this highly anisotropic system composed of an insulating Al 2 O 3 matrix and Cr-Al 2 O 3 conducting clusters. The resisitivity values differed by up to seven orders of magnitude between different directions, depending on the number of foldings and the mismatch in the plastic behavior of the starting suspensions. Above the percolation threshold the anisotropy in the resistivity strongly depends on the mismatch in the plastic behavior of the starting suspensions: it is small for a small mismatch and significantly larger for a large mismatch.

Mullitization temperatures and mechanical properties of reaction-bonded mullite composites were investigated using silicon carbide (SiC) of two different particle sizes (180 nm and 2.5 μm) as one of the starting components. The smaller SiC particle size resulted in earlier mullitization, lower final densities, and lower strength of these composites. The sintering shrinkage of these composites was investigated. Low-to-zero shrinkage was rendered possible via volume expansions that were associated with the oxidation of SiC and aluminum in the green material. Green bodies that contained 55 vol% aluminum were compacted to 70% of the theoretical density. The materials showed a linear sintering shrinkage of

Wet milling of Al2O3-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe2O3/Al/Al2O3 powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al2O3 powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.

Dense alumina/5 vol% SiC nanocomposites were prepared by sol-gel processing using nanosized (180 nm) precoated SiC powders and a commercial boehmite sol. The SiC powder was precoated with boehmite by a controlled heterogeneous precipitation from an aluminum nitrate solution. The coated SiC powder was then dispersed in a boehmite sol, gelled, calcined, and densified by gas pressure sintering under argon atmosphere at 7-8 MPa pressure. The dependence of the calcination conditions on densification, the effect of seeding on the microstructural development, as well as the mechanical behavior of the sintered specimens, are presented and discussed.

Short distance infiltration (sdi) was used to fabricate metal matrix composites (MMC) with very fine ceramic reinforcements. This ‘sdi-MMC’ process was used to synthesize a dense composite comprised of Fe–Cr–Ni/Al2O3. Fe2O3/Fe/Ni/Cr/Al powder compacts were rapidly heated to 900 °C and held at this temperature and a pressure of 20 MPa in air for 5 min. This thermomechanical treatment triggered the highly-exothermic thermite reaction between starting powders that, in turn, resulted in rapid in-situ formation of a composite comprised predominantly of γ(fcc) Fe–Cr–Ni, along with smaller amounts of α(bcc) Cr–Fe–Ni and α-alumina. SEM and TEM analyses revealed a microstructure consisting of a uniformly dispersed network of very fine (a few hundred nanometers) Al2O3 grains in a matrix of a fine-grained (=5 microns) metallic alloy. The dense composites exhibit average bend strength and toughness values of 1100 MPa and 18 MPa (m)1/2, respectively.

Al2O3/ZrO2/Ag composites were obtained by intense attrition milling of silver and alumina powders with TZP balls. The characteristics of mixtures and the mechanical properties of sintered samples were investigated. Milling in acetone for 7 h results in a homogeneous submicron mixture. Ag composites (10 vol.%) CIPed at 300 MPa contain 8.1 vol.% tetragonal zirconia and 8.0 vol.% silver after sintering in air for 2 h at 1550 °C. Fracture toughness and bending strength as high as 5.44 MPa m and 586 MPa were obtained (645 MPa by post-HIPing). The incorporation of more silver (20 vol.%) is detrimental to microstructure quality and strength, but fracture toughness remains high thanks to crack bridging effects developed by ductile metallic inclusions.

The thermomechanical behavior of metal-ceramic composites with interpenetrating networks is dominated by the ceramic matrix, i.e. good wear and oxidation resistance as well as high-temperature strength, while the metal phase can improve the fracture toughness [1, 2]. A novel pressureless reaction sintering process was presented for the fabrication of alumina-aluminide alloys (3A) [3–6]. Densification was achieved by inert-atmosphere heat-treatment of compacts prepared from high-intensity milled mixtures of metal oxide (MeaOb) and aluminum powders. The process takes advantage of the higher oxygen affinity of aluminum compared to other metals, described by the general reaction scheme

Hypothesis Zirconia microparticles produced by sol–gel synthesis have great potential for photonic applications. To this end, identifying synthetic methods that yield reproducible control over size uniformity is important. Phase transformations during thermal cycling can disintegrate the particles. Therefore, understanding the parameters driving these transformations is essential for enabling high-temperature applications. Particle morphology is expected to influence particle processability and stability. Yttria-doping should improve the thermal stability of the particles, as it does in bulk zirconia. Experiments Zirconia and YSZ particles were synthesized by improved sol–gel approaches using fatty acid stabilizers. The particles were heated to 1500 °C, and structural and morphological changes were monitored by SEM, ex situ XRD and high-energy in situ XRD. Findings Zirconia particles (0.4–4.3 μm in diameter, 5–10% standard deviation) synthesized according to the modified sol–gel approaches yielded significantly improved monodispersities. As-synthesized amorphous particles transformed to the tetragonal phase at ∼450 °C with a volume decrease of up to ∼75% and then to monoclinic after heating from ∼650 to 850 °C. Submicron particles disintegrated at ∼850 °C and microparticles at ∼1200 °C due to grain growth. In situ XRD revealed that the transition from the amorphous to tetragonal phase was accompanied by relief in microstrain and the transition from tetragonal to monoclinic was correlated with the tetragonal grain size. Early crystallization and smaller initial grain sizes, which depend on the precursors used for particle synthesis, coincided with higher stability. Yttria-doping reduced grain growth, stabilized the tetragonal phase, and significantly improved the thermal stability of the particles.

For advanced ceramic composites, affordable manufacturing is still the most essential shortcoming with respect to successful commercial use. This holds particularly for components made out of composites with complex hierarchical structures and high demands of mechanical performance and reliability at the same time, e.g. fiber-reinforced ceramic matrix composites (FRCMCs). Therefore, a new processing route is presented here, which is based on the lamination of thermoplastic prepregs. This route allows not only affordable manufacturing, but also advanced mechanical reliability. Powder metallurgy techniques are combined here with concepts from the prepreg technology in a route consisting of the following steps (a) manufacturing of 2 D prepregs using commercial fiber fabrics which are infiltrated with compounds of ceramic particles embedded in an organic matrix, (b) followed by respective stacking and joining, (c) burn out of the organic matrix and (d) sintering to consolidate the matrix. Composites consisting of a porous Al2O3/ZrO2 matrix, reinforced by 8 layers of NextelTM 610 fiber fabric exhibit a bending strength of ~440 MPa, with graceful failure behavior, e.g. a stepwise stress reduction after peak nominal stress. The fracture of these composites is controlled by a series of interfacial delamination events, which enhance energy dissipation during failure.

Fe/Al2O3 composites with metal contents between 23 and 35 vol% have been fabricated via Al-assisted pressureless reaction sintering. The effect of variation of oxygen partial pressure during sintering on phase development, microstructure and mechanical properties has been investigated. The formation of the spinel phase FeO·Al2O3 is found to occur at elevated temperatures if the oxygen partial pressure during sintering exceeds a critical value. Microstructural observations and image analysis reveal that the composites exhibit a microstructure with the ceramic and the metal phase forming interpenetrating networks. The fracture toughness increases with increasing metal content and strongly depends on the phase content of the sintered specimens. In composites containing a small amount of FeO·Al2O3, the maximum toughness is 7·1 MPa m . A significant toughness enhancement up to 10·2 MPa m was achieved by avoiding the spinel formation to obtain composites consisting only of Fe and Al2O3.

Wet milling of powders containing metallic and ceramic particles in different organic milling media has been investigated. These powders are used as precursors for the reaction bonding of aluminum oxide (RBAO) and for the reaction sintering of alumina–aluminide alloys (3A). An attempt is made to explain effects on the milling efficiency in terms of interactions between the organic medium and the particle surfaces. The effects of oxide particle and crystal size have also been examined. The milling efficiency can be optimized by choosing proper milling medium and a suitable particle and crystal size of the abrasive components. Highest milling efficiency was obtained when the powder mixture was milled in a nonpolar organic medium (cyclohexane). The abrasion contribution of the oxide particles is more significant in slurries of higher stability, i.e., in acetone. The area of new Al surfaces formed per milling time is discussed in terms of an in situ determination of the water consumption, where an ethanol–water mixture was used as milling medium, and in terms of the fraction of Al oxidized per time of milling.

The recently developed Al 2 O 3 -aluminide alloys (i-3A) technique for fabricating near net-shape components was applied to manufacture graded bodies. During the i-3A process, Al is pressure infiltrated into porous preforms consisting of Al 2 O 3 and metal oxide. The metal oxide is reduced by the liquid aluminum, and Al 2 O 3 and the respective aluminides are formed. The process is characterized by the use of low-cost raw materials (e.g. ilmenite), low processing temperatures (600-800 °C) and short processing times when die pressure (∼1 s) or squeeze casting (> 1 min) are used as infiltration methods. In the present case, Al 2 O 3 /TiO 2 graded bodies were gas-pressure infiltrated and converted into Al 2 O 3 /TiAl 3 composites with aluminide contents varying from 0 vol% to 40 vol% TiAl 3 . The green bodies were produced by dry pressing of Al 2 O 3 /TiO 2 powder mixtures. Microstructure and interfaces between different layers of the graded composites were investigated by light microscopy and SEM. Additionally, all individual layers of the graded composite were characterized separately by hardness measurements. Basic principles for microstructural design and technical applications of graded i-3A components are discussed.

It is well known that the mechanical properties of ceramic components for structural applications can be enhanced by the introduction of surface compressive stresses during fabrication. For alumina ceramics, this goal can be attained by incorporating a second phase such as mullite into the surface region. Due to the mismatch of thermal expansion coefficient between mullite and alumina, a residual surface compressive stress will be created during cool-down after heat-treatment. Recently, an increase of 50--150% in fracture toughness has been obtained in the experiment by making a three-layer alumina composite in which the outer layers contain 40 vol% mullite. In other experiments where mullite was formed in the surface region by heating alumina in a bed of SiC powder or by annealing an Si-deposited alumina in air, the strength increases of > 25% have also been reported. These increases are attributed to the presence of surface residual compressive stresses. The existence of residual surface stresses can be confirmed by X-ray diffraction. The purpose of this work is to show that the X-ray diffraction technique can be used to determine the residual stresses in the layers of three-layered composites, and that the mechanical properties are closely related to the magnitudes of more » the residual stresses. « less

High temperature forming processes offer encouraging routes for shaping ceramic components. Particularly, pressure assisted sintering techniques in combination with ductile deformation at high temperatures allow the fabrication of components of complex shape and advanced performance in one step. In the present paper first the background of densification and forming at elevated temperatures is reviewed. In the second part, the potential of high temperature forming will be exemplified by reaction bonded aluminum oxide.

The distribution of residual stresses in three-layer reaction bonded alumina (RBAO) composites with 17.5 vol % zirconia in the inner layer and 40 vol % mullite in the outer layers was determined by an indentation technique. It could be shown that the outer layers are subject to a residual compressive stress while the inner layer is under a tensile stress. With increasing outer layer thickness, the residual compressive stress decreases, and the residual tensile stress increases. Compressive stresses up to 550 MPa were measured in this study. It was found that the residual stresses are not uniform throughout the layers. The magnitude of the stresses decreases with increasing distance from the interfaces. This is attributed to creep during cooling from the sintering temperature.

Reaction-bonded aluminium niobate-based composites with tailored microstructures were fabricated through controlled nucleation and growth of AlNbO4 needle-like grains. Using heating rates of 0.5 to < 160 °C min−1, the effects of heat-up period on microstructure development, and the volume content, grain-size distribution as well as aspect ratio of AlNbO4 grains has been investigated. The morphology of the final microstructure is principally determined by the grain growth during heat-up. Equiaxed to elongated grains were obtained by using different heating rates and thermal ageing at 1320 °C in air. SEM observations revealed that increasing the heating rate promotes the development of large and homogeneously distributed needle-like grains. The results presented show that tailoring of final AlNbO4 microstructures become possible by the control of heating rate.

Three-layer alumina-based composites with Al2O3-containing mullite as the outer layer and ZrO2-containing alumina as the inner layer were fabricated by die-pressing and reaction bonding. The effects of the outer-layer compositions on the densification behavior of three-layer composites were investigated. The existence of residual stresses in the layers was verified using indentation methods. Compared with single-layer ceramics, three-layer composites exhibit an improved fracture toughness and an excellent damage resistance due to the presence of the compressive stresses in the outer layers.

A promising approach to develop composites with long time stability at high temperatures even in oxidising atmospheres is the use of oxide materials both for the matrix and for the reinforcing elements. Ceramic composites of alumina and mullite can be formed through conventional powder metallurgical processing using the reaction bonding technique, developed at TUHH. Continuous filaments of single crystal alumina (sapphire fibres) serve as model reinforcement. Single fibre samples are used to investigate the basic interfacial behaviour in these composite systems by observations of crack deflection and crack propagation, and by high temperature push-out measurements. Despite the interesting results of these tests, the micromechanical mechanisms in these pure oxide composites are not fully resolved yet. It is demonstrated that the most commonly accepted models for the description of the interfacial behaviour do not seem to be applicable to this class of material. In the following, the most recent experience in pure oxide composites based on Reaction Bonded Aluminium Oxide (RBAO) is described with special emphasis on the requirements for further modelling.

Abstraet-Intermetallics-reinforced Al 2 O 3 composites have been fabricated by reaction sintering compacts of intensively milled powder mixtures containing either metal oxides and Al or elemental metals, Al and Al 2 O 3 . During carefully controlled heat-treatment in non-oxidizing atmosphere. the metal oxides, for instance TiO 2 Fe 2 O 3 and Nb 2 O 5 , are reduced by Al to form the respective aluminides (Ti x Al y Nb x Al y ), Alternatively, aluminides can also be formed through reaction between Al and elemental metals (e.g. Fe), By adding Al 2 O 3 to the starting mixture, the intermetallic ceramic ratio of these alumina-alsiminide alloys (3A) can be adjusted within a wide range. At intermetallic volume fractions> ∼20 vol.%, both phases are continuous, exhibiting a microstructure similar to that of products of directed metal oxidation or reactive metal penetration processes. In this paper, systems based on Ti x Al y , FeAl and NbAl 3 are emphasized. The principles of the reaction synthesis process and the influence of the processing parameters on microstructural development and mechanical properties are described.

CrAl2O3 and NbAl2O3 composites containing 50 vol% metal have been fabricated by pressureless sintering of compacts of attrition milled powder mixtures. Successful fabrication of high-strength and high toughness composites requires fine and homogeneous powders. Strength and fracture toughness of the composites increase with increasing milling time. Short milling times do not lead to the required particle fineness and powder homogeneity. For a composite containing 50 vol% Nb, strengths of up to 690 MPa with corresponding fracture toughness of 6.6 ± 0.4 M Pa m 1 2 and hardness of 11.2 GPa (Hv20) have been obtained, whereas strengths of 592 MPa, fracture toughness of 6.6 ± 0.3 M Pa m 1 2 and hardness of 9.3 GPa have been obtained for CrAl2O3 composites.

The breakdown strength as well as the mechanical strength of ceramic materials decreases with increasing volume. The volume-effect of the mechanical strength can be explained by the Weibull theory. For the breakdown strength the same explanation has been often assumed. In order to validate this assumption breakdown strength and mechanical strength of alumina samples with defined porosities were compared. Differences in the Weibull moduli of breakdown and mechanical strength distributions indicate that the volume-effect cannot explain the thickness-dependence of the breakdown strength. In particular, the thickness-dependence of the breakdown strength always leads to a Weibull modulus of two which is not in agreement with the measured Weibull moduli for samples with constant thickness. It can be concluded that the thickness-dependence of the breakdown strength cannot be explained by the Weibull concept. A recently developed breakdown model which is based on space charge injection is able to explain the experimental results.

The deformation characteristics of dense, reaction bonded, zirconia dispersed alumina (RBAO) have been investigated in uniaxial compression and biaxial tension at temperatures of 1350–1500 °C. Microstructural observations demonstrate that grain boundary sliding is the dominant deformation mechanism. In the lower temperature range the stress exponent has a value near 1 and increases slightly at higher temperatures. Diffusional accommodation is most likely the rate limiting step during grain switching. The superplastic formability was assessed by deep drawing tests and it was found that high temperatures (1500°C) and low punch rates are needed to obtain crack-free specimens. Sinter forging tests were performed to analyze the potential of this pressure-assisted sintering technique to produce fine-grained, high-strength RBAO. Strengths of more than 1 GPa have been obtained by this process. The sinter forging process has been analysed in depth at 1320°C.

Changes in microstructure and phase composition of ceria stabilized tetragonal zirconia polycrystals (Ce-TZP), magnesia and yttria partially stabilized zirconia [(Mg,Y)-PSZ] and magnesia partially stabilized zirconia (Mg-PSZ) were studied in diluted aqueous HCl, H2SO4 or H3PO4 solutions at a temperature of 390° C and a pressure of 27 MPa. Ce-TZP is corrosion resistant under these conditions in HCl, while Mg-PSZ is attacked severely and (Mg,Y)-PSZ undergoes a surface tetragonal to monoclinic phase transformation. All investigated zirconia ceramics suffer severe weight losses and transformation to the monoclinic phase on the surface in H2SO4. Only a small weight gain and a slight increase of m-phase on the surface of the ceramics is found in H3PO4.

The corrosion of various ceramic materials in simulated supercritical water oxidation (SCWO) environment was measured. Supercritical water with 0.44 mol kg −1 oxygen and 0.05 mol kg −1 hydrochloric acid was used to simulate typical SCWO conditions after the decomposition of the organic material. The experimental temperature was 465 °C and the pressure 25 MPa. The experiments were performed within a reactor with an inner surface made of alumina. In this very corrosive fluid only a few Al 2 O 3 - and ZrO 2 -based materials did not corrode severely. Homogeneous surface attack and grain boundary diffusion were observed. HIP-BN, B 4 C, TiB 2 , Y 2 O 3 and Y-TZP disintegrated. SiC and Si 3 N 4 -based materials showed a large weight loss, up to above 90%.

Reaction-bonded alumina was fabricated using standard powder preparation methods and the low-pressure injection moulding (LPIM) forming technique, followed by reaction sintering. The feasibility of LPIM was investigated in terms of the compounding ability of a highly agglomerated mechanically alloyed powder in a non-polar organic vehicle, and the microstructural homogeneity and resulting reliability of sintered LPIM parts. The green density of LPIM parts after debinding, roughly corresponding to the solids loading in the LPIM feedstock, was in the range of fractional density achieved by dry pressing, although the powder packing and aluminium particle deformation during forming were not the same. LPIM forming and debinding induced microstructural inhomogeneities (i.e. larger voids due to trapped air and density fluctuations) which were reflected in a slightly lower Weibull modulus, while the average strength did not differ significantly from the values obtained with dry pressed samples. The microstructure and mechanical properties of sintered parts were also related to the purity of the starting powders. The presence of impurities in the starting aluminium powder resulted in a somewhat coarser microstructure, characterized by a broader Al2O3 grain-size distribution, as well as in the presence of a thin glassy phase on the grain boundaries and in partial destabilization of dispersed tetragonal (Y2O3-stabilized) ZrO2 particles. In spite of a less favourable microstructure, the room-temperature strength and Weibull modulus were still comparable to those obtained from high-purity starting powder.

Reaction-bonded Si3N4 toughened by oriented SiC platelets was fabricated via low pressure injection moulding (LPIM). Initially, the rheology of ceramic suspensions was optimized with respect to solid content, SiC platelet loading, particle surface properties and binder composition. Surface active additives were used to modify the particle–polymeric binder interphase in order to prevent particle reagglomeration, to reduce the viscosity and/or to increase the solid content. The relationship between LPIM processing variables and platelet orientation in injection moulded reaction bonded silicon nitride ceramics was studied and the resultant mechanical properties were compared to composites containing randomly dispersed platelets.

A new method for preparing porous silicon nitride ceramics with high porosity had been developed by carbothermal reduction of die-pressed green bodies composed of silicon dioxide, carbon, sintering additives, and seeds. The resultant porous silicon nitride ceramics showed fine microstructure and uniform pore structure. The influence of SiO2 particle size and sintering process (sintering temperature and retaining time) on the microstructure of sintering bodies was analyzed. X-ray diffractometry demonstrated the formation of single-phase β-Si3N4 via the reaction between silicon dioxide and carbon at high temperature. SEM analysis showed that pores were formed by the banding up of rod-like β-Si3N4 grains. Porous Si3N4 ceramics with a porosity of 70–75%, and a strength of 5–8 MPa, were obtained.

Alumina/zirconia matrix composites with platinum-coated sapphire fibers were fabricated using the technology of reaction bonding of aluminum oxide (RBAO) as the synthesis route. The interfacial behavior of both sintered and annealed specimens was investigated by crack path observations and fiber pushout tests. The results show that precious metals like platinum or related alloys can be used as high-temperature interphases for oxide/oxide composites. Weak interfaces were obtained both between fiber and coating as well as between matrix and coating, which should be suitable to achieve a damage-tolerant behavior of the composite.

Three-layer alumina-based composites with Al 2 O 3 -containing mullite as an outer layer and ZrO 2 -containing alumina as an inner layer were fabricated by die-pressing and reaction-bonding. The strength of samples with Vickers indents made at loads up to 300 N was investigated. When compared to monolithic ceramics, three-layer composites exhibit excellent damage resistance due to compressive stresses in the outer layer. Hot isostatically pressed samples show a four-point bend strength of 800 MPa after indentation at 300 N.

A novel pressureless reaction sintering process is presented for the fabrication of Al2O3-aluminide alloys (3A). Compacts of intensively milled metal oxide-aluminum mixtures are heat-treated in vacuum or inert atmosphere such that the exothermic reactions take place in a controlled manner essentially at temperatures below the melting point of Al. Dense, homogeneous microstructures were obtained with a variety of Al2O3-matrix systems with interpenetrating networks of aluminides of Ti, Fe, Nb, Mo, Zr, Ni, etc. By adding modifiers in the form of oxides or metals, volume and phase composition as well as properties can be tailored in a wide range.

Reaction-bonded aluminum niobate-based (AlNbO{sub 4}-based) composites with tailorable microstructures were fabricated through controlled nucleation and growth of needlelike AlNbO{sub 4} grains. Equiaxed to highly elongated grains were obtained by different heat treatments at 1,340 and 1,400 C in air. Scanning electron microscopy observations revealed that increasing the heating rate promoted the development of large, homogeneously distributed needlelike grains. Because of the elongated nature of the AlNbO{sub 4} grains and the high residual stresses at the interface, enhanced crack wake bridging occurred. This bridging accounted for the observed increase of fracture toughness.

International Workshop on Cellular Materials (I.Wo.C.Mat) -- DEC 15, 2011 -- Turin, ITALY, WOS: 000318325700004, In this study, porous beta-Si3N4 ceramics containing limited amount of Sm2O3 and CaO as sintering aids were produced by addition of potato starch (10 and 20 vol.%) and partial sintering. Two different Si3N4 powders, alpha- and beta-, were used as starting materials. Scanning electron microscopy investigations revealed that development of elongated beta-Si3N4 grains were much more pronounced when alpha-Si3N4 starting powder was used. Even though porosity values of the compositions prepared by using alpha-Si3N4 (similar to 57.0-58.4%) is significantly higher than the samples produced by beta-Si3N4 (42.6%), no significant change was observed for the bending strength, fracture toughness and Weibull modulus. This indicates that microstructural features have a significant contribution to the mechanical properties of the porous materials in terms of bending strength and fracture toughness

Al2O3/ZrO2 composites have been prepared by fast firing of oxidized Al/Al2O3/ZrO2 precursors produced by the reaction-bonded aluminium oxide (RBAO) technique. This fabrication route results in high-strength ceramics at relatively low densities. For example, after fast firing for 20 min at 1550 °C, RBAO containing 20 vol% ZrO2 shows four-point bending strengths of > 600 MPa at a density of ∼95% theoretical which is comparable to conventionally sintered RBAO.

Densification behavior and mechanical performance of alumina fast fired at relatively low temperatures were evaluated and compared to those of conventionally sintered alumina. A density of about 99% of the theoretical value and a grain size of 1.2 μm were achieved by firing pure alumina powder compacts at 1350 °C for 20 min. Fast fired alumina exhibits room temperature strengths (520 MPa) and Weibull moduli (10.4) nearly comparable to those of conventionally sintered alumina although very high heating/cooling rates were used. A qualitative densification model is presented based on the beneficial effects of transient mismatch stresses.

A new technology to fabricate low-to-zero shrinkage Al2O3-based composites, Reaction Bonding of Aluminum Oxide (RBAO), is presented: Al and Al2O3 precursor powder mixtures are intensively milled, compacted, and heat-treated in air, such that Al fully oxidizes to Al2O3. The volume expansion associated with the oxidation partially compensates for the sintering shrinkage. The process can be modified by incorporating other metals or ceramic phases to change the final composition, to enhance the mechanical properites, and to further compensate for the sintering shrinkage. Interesting characteristics of this technology are easy green machining without binders, superior mechanical properties of reaction-bonded bodies without final machining (also when green machined), superplastic formability, near-net-shape capability, and broad microstructural and compositional versatility.

Thermal shock data are presented for two types of Al/Al2O3 composites (35 vol% Al) with different microstructural scale and compared to a monolithic alumina. The fine-scale Al/A12O3 material exhibits high room-temperature strength (765 MPa) and a gradual decrease in retained strength with increasing quenching temperature difference, δT, and improved thermal shock resistance—as compared to monolithic alumina—even at δT= 600°C. The coarse-scale Al/Al2O3 composite shows high long-crack fracture toughness (10.5 MPa·m1/2) and almost no strength degradation up to δT= 500°C. A qualitative picture for the effect of microstructural scale on thermal shock resistance emerges by employing a simple fracture mechanics description contrasting the crack driving force for transient stress fields with R-curve behavior.