Rolf Janssen

REACTION SINTERING OF ALUMINA-ALUMINIDE ALLOYS (3A)

A novel pressureless reaction sintering process is presented for the fabrication of Al{sub 2}O{sub 3}{endash}aluminide alloys (3A). Compacts of intensively milled metal oxide{endash}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 Al{sub 2}O{sub 3}-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. {copyright} {ital 1996 Materials Research Society.}

Reaction synthesized Al2O3-based intermetallic composites

Abstract 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-aluminide 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.

Corrosion screening tests of high-performance ceramics in supercritical water containing oxygen and hydrochloric acid

Abstract 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%.

Microstructure and mechanical properties of Al-assisted Sintered Fe/Al2O3 Cermets

Abstract 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.

Volumetric analysis of osteoclastic bioresorption of calcium phosphate ceramics with different solubilities

Commonly, to determine osteoclastic resorption of biomaterials only the resorbed area is measured. The depth of the resorption pit, however, may also be important for the performance of a material. To generate such data we used two calcium phosphate ceramics (Ca(10) and Ca(2)). The solubility of the materials was determined according to DIN EN ISO 10993-14. They were scanned three-dimensionally using infinite focus microscopy and subsequently cultivated for 4 weeks in simulated body fluid without (control) or with human osteoclasts. After this cultivation period osteoclasts number was determined and surface changes were evaluated two- and three-dimensionally. Ca(10) and Ca(2) showed solubilities of 11.0+/-0.5 and 23.0+/-2.2 mgg(-1), respectively. Both materials induced a significant increase in osteoclast number. While Ca(10) did not show osteoclastic resorption, Ca(2) showed an increased pit area and pit volume due to osteoclastic action. This was caused by an increased average pit depth and an increased number of pits, while the average area of single pits did not change significantly. The deduced volumetric osteoclastic resorption rate (vORR) of Ca(2) (0.01-0.02 microm(3)microm(-2)day(-1)) was lower than the remodelling speed observed in vivo (0.08 microm(3)microm(-2)day(-1)), which is in line with the observation that implanted resorbable materials remain in the body longer than originally expected. Determination of volumetric indices of osteoclastic resorption might be valuable in obtaining additional information about cellular resorption of bone substitute materials. This may help facilitate the development of novel materials for bone substitution.

Fast firing of alumina

Abstract 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.

Synthesis and thermal stability of zirconia and yttria-stabilized zirconia microspheres

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.

Nb and CrAl2O3 composites with interpenetrating networks

Abstract 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.

Abrasive wear of ceramic-matrix composites

Abstract The abrasive wear of various experimental and commercial ceramic-matrix composites was measured using a pin-on disc technique with SiC paper. The results are presented in a comparative way.

Wet milling of Al-containing powder mixtures as precursor materials for reaction bonding of alumina (RBAO) and reaction sintering of alumina–aluminide alloys (3A)

Abstract 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.