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COMPOSITE MATERIALS OF A 99.7 % ALPHA-ALUMINA MATRIX WITH PLATELETS OF SILICON CARBIDE (SICP) HAVE BEEN PREPARED BY GELCASTING. DENSIFICATION WAS CARRIED OUT BY GAS-PRESSURE SINTERING WITHOUT FURTHER SINTERING AIDS. FIVE DIFFERENT COMPOSITIONS WITH 0, 5, 10, 15, AND 20 VOL.% SICP RESP. HAVE BEEN INVESTIGATED AND TRIBOLOGICAL PROPERTIES WERE CHARACTERIZED. INCREASING AMOUNT OF SICP AS RIGID PARTICLES IN THE FINE GRAINED MATRIX COMPLICATE THE DENSIFICATION PROCESS. WITH THE APPLICATION OF 50 BAR AR PRESSURE DURING THE FINAL SINTERING STEP AT A TEMPERATURE OF 1700 DEGREE C THE DENSITY OF THE SAMPLES COULD BE INCREASED BY 3-5%. INVESTIGATION OF THE TRIBOLOGICAL PROPERTIES AGAINST STEEL AS A COUNTERBODY UNDER UNLUBRICATED OSCILLATING SLIDING CONDITIONS SHOWED A HIGH COEFFICIENT OF FRICTION AND AN INCREASING WEAR RATE UP TO A SICP CONTENT OF 10 VOL. % . AT 15 AND 20 VOL. % SICP RESP. BOTH THE COEFFICIENT OF FRICTION AND THE COEFFICIENT OF WEAR DROP DOWN.
Influence of Slip Viscosity on the Mechanical Properties of High Purity Alumina made by Gelcasting
(1997)
The isoelectric point is a material property for the characterization of acid-base behavior of the powder surface. The IEP of two alumina powders (AKP-30, Grade A) with different particle size characteristics in aqueous suspension has been determined by colloid vibration current (CVI) and particle charge detection (PCD) methods in moderately concentrated suspensions in order to qualify these methods for a future standardization. The solid loading of the suspensions were 1 and 5 vol%. The comparison of the IEP determined by PCD and CVI was in good agreement. However, the solid loading dependence of IEP and a large IEP hysteresis (at 1 vol%) were observed, which do not allow a proper determination of the IEP. The recording of the potential curve and therfore the determination of the IEP with CVI was difficult for the suspension of Grade A powder due to formation of agglomerates at pH values near the pHIEP.
Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and Al2O3 under water
(1999)
The tribological behaviour of SiC, SiC-TiC and SiC-TiC-TiB2 was determined in oscillating sliding against SiC and -Al2O3 in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB2 differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.
The gelcasting process was used for manufacturing porous ceramic bodies from nano sized powders. 3 different powders with specific surface areas between 12 and 30 m²/g were used. Solid content of the corresponding slurries was dependent on the milling procedure and the use of a suitable dispersant. For bimodal GradeA powder solid content was as high as 58 vol%. For sintering temperatures between 1250 and 1600°C the porosity varied between 45 and 20%, whereas the pore radii varied between 50 and 91nm. The grain size of crystallites in the sintered microstructures was about 1um. The reflection ratio for visible light increased in comparison to a reference sample made from conventional powder from 0.8 to 0.95 in the wavelenght range around 400nm.