Papers by Keyword: Alumina Ceramic

Abstract: The present work studies the corrosion of three most widely used types of structural ceramics – silicon nitride, solid state sintered alumina and liquid phase sintered alumina – in 3 % aqueous solutions of sodium chloride at temperatures up to 290 °C and pressures up to 7 MPa. The corrosion of silicon nitride was controlled by attack of Si3N4 matrix grains, while yttrium oxynitride amorphous grain boundary phase was corrosion resistant. Corrosion of Si3N4 in reference media -distilled water - at 290 °C was characteristic by formation of passivation layer, which hindered further dissolution of silicon nitride matrix. The presence of sodium chloride resulted in formation of discontinuous layer of corrosion products, resulting in more severe corrosion than in distilled water. The corrosion of liquid phase sintered alumina was mainly attributed to congruent dissolution of SiO2 and CaO from grain-boundary amorphous film, which was accelerated at higher temperature, and accompanied by precipitation of siliceous phases from oversaturated solution at 200 °C. Pure polycrystalline alumina corroded by loss of alumina grains, which did not dissolve in the corrosion media. The corrosion impaired significantly the fracture strength of silicon nitride, creating new, corrosion related defects at the surface, while the influence of corrosion on fracture strength of polycrystalline aluminas was negligible.

Abstract: The relationship of KR versus crack length c (R curve) for Al2O3-30wt.% Ti(C,N).and for comparison alumina ceramics has been examined. The R-curve has been evaluated using pronounced long-crack formed during the three point bending (3PB) of the double edge notched beam. A combination of in situ microscopic crack growth observation and mechanical testing enabled measurement of crack growth resistance curves. The special device consisting of light microscope coupled with CCD camera, was fitted to Zwick 1446 testing machine. These observations reveal the existence of flat R-curve for Al2O3-30wt.% Ti(CN) and increasing R-curve for pure alumina. A study of slow-crack-growth (SCG) in tested materials was carried. The load-relaxation technique was used for observation at slow-crack-growth. The crack length was evaluated by linear-elastic analysis from the compliance of single-edge-notched specimen in three-point bending test. Parameters of stable crack growth n and logA, work-of fracture (WOF), stress intensity factor at the moment of crack initiation KI0 and maximum values of stress intensity factor KImax were determined. Mechanism of grain bridging responsible for occurrence of R-curve was observed by SEM and TEM.

Abstract: Mullite fiber reinforced alumina ceramic matrix composites (MFACC) were prepared using CaO-MgO-SiO2 (CMS) and TiO2 as sintering aids. The effects of the contents of sintering aids and mullite fiber on the density and sintering temperature of MFACC are studied. The results showed that when the CMS content is 8.0% and the TiO2 content is 1.0%, the density of the as-sintered MFACC is 98.9%. When the mullite fiber content is 15.0% and the sintering temperature is 1450 °C, the flexural strength of the resultant composite increases to 504.5MPa, 70.7% higher than the original matrix, and the relative density of the composites reaches 98.4%. The reinforcement mechanisms are fibers pull-out and sticky point.

Abstract: Al2O3 ceramics samples were prepared by changing the ingredient and sintering conditions. The resistance experiment of the Al2O3 ceramics to hydrochloride corrosion and thermal shock were carried out, and then identified by means of scanning electron microscopy and energy dispersive spectral analysis. The results indicated the optimal Al2O3 and sintering condition for the dechlorinating reactor.

Abstract: This paper investigated the influence of sintering temperature on the dielectric performance of alumina ceramic. Three different kinds of alumina ceramic samples were prepared using the same formula with three different sintering temperatures. The flashover voltage and surface charge distribution of the test samples were measured with a finger type electrode system under a positive pulse voltage application in a 1×10-4Pa vacuum circumstance. The experimental results showed that the samples with higher sintering temperature have worse dielectric performance. Based on the SEM observation, the change of dielectric performance could be attributed to the change of microstructure of alumina ceramics.

Abstract: The goal of hip arthroplasty is to restore the patient to an active and pain free life for ever. Unfortunately, prosthetic implants fail because of loosening, wear or infection. This study sets out to ascertain whether or not Hydroxyapatite hip arthroplasty (HA) will produce long-lasting function of at least ten years in unselected patients of all ages and all diagnoses. Cemented implants used in hip arthroplasty often loosen after ten or more years of use. This is partly mechanical but also associated with debris disease arising from plastic wear products. Cementing techniques have been improved and the Swedish Hip Register [1] demonstrates that the revision rates have been reduced with improved cementing techniques. Revision surgery after cemented arthroplasty is difficult and may be complicated by fracture of the host bone Bioactive materials are being used to secure joint implants [2,3] and this paper presents the results of a consecutive series of hip arthroplasties carried out over a total period of over 19 years using the Furlong Hydroxyapatite Ceramic Coated hip prosthesis.[4, 5]

Abstract: This paper describes the clinical application of alumina ceramic and, more recently, Zirconia Toughened Alumina (ZTA), to the bearing surfaces of hip implants with a maximum follow up of fifteen years. Patients are assessed pre- and post-operatively using the Harris Hip Score. [1] X-rays are taken annually and, where appropriate, post mortem histological examination is carried out. Results are discussed. Some failures of alumina ceramic components have been seen and measures to prevent such incidents are described.

Abstract: High-purity alumina ceramics was prepared using high-purity α-Al2O3 powder as raw material, nitrates or oxides of magnesium, chromium and copper as additives by a wet ball milling with a later dry pressing forming and normal pressure sintering process. The influence of additives on the sintering temperature, microstructure and bending strength of the prepared alumina ceramics was studies. The results showed that the additive doped with nitrate can be dispersed uniformly in the body with molecule scale, and the oxides obtained by decomposing of nitrates have the higher reactivity. Thus, the nitrate additives have better capacity than oxide additives in reducing the sintering temperature and inhibiting the abnormal grain growth, and the alumina ceramics prepared by adding of nitrate additives have higher density and bending strength.

Abstract: The objective of this paper is to investigate the applicability of ultrasonic wave technique to monitor the progress of the thermal shock damage on alumina ceramic. For this purpose, alumina ceramic specimen was heated in the furnace and then was quenched into the water tank. The initiation, growth behaviors of surface micro-cracks as a function of the number of thermal shock cycle have been discussed by taking into account the change of ultrasonic wave velocity and attenuation. The change of both velocity and attenuation of ultrasonic wave showed good relation with the surface crack density changing due to the number of thermal shock cycle. Measuring the change of attenuation gives more effective information to evaluate thermal shock damage than that of velocity nondestructively. The flexural strength was also measured for the thermal shocked specimen. The flexural strength was decreased rapidly at the point of time of observation of microcracks on the surface of specimen, and the flexural strength decline by the crack growth caused thermal shock cycles was slight.

Abstract: In this work, was proposed to produce nanosized α-Al2O3 powder from precipitation reactions, in order to reduce the alumina ceramics sintering temperature. For this an addition of nanosized powder in the commercial microsized powders was done. The nano and microsized powder particle mixtures were obtained by precipitation of nanosized aluminum hydroxide in microsized Al2O3 in liquid suspension. Several compositions of nano and microsized powders were carried out. These powder compositions were based on data of simulated particle packing. The nano and microsized powder mixtures was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric and differential thermal analysis (TGA and DTG). The powder mixtures were compacted by uniaxial (75MPa) and isostatic (300MPa) pressing, producing cylindrical samples. The powder compacted samples were sintered at 1400oC. Sintered ceramics were characterized by XRD and SEM. The results showed that the higher density of sintered ceramic was obtained for 65% of microsized powder and 35% of nanosized powder mixture. This composition of size of particles of the powder was in agreement with the theoretical simulation.