Powders of pure beta-tricalcium phosphate (beta-TCP), hydroxyapatite (HAP) and a biphasic composite mixture of HAP+beta-TCP were prepared by solid state reaction between two commercially available calcium-based precursors namely, tricalcium phosphate (TCP) and calcium hydroxide (Ca(OH)2). These reactants mixed in the molar ratios ranging from 3 : 0 to 3 : 4 (designated T0 to T4) in deionized water, milled and slip-cast into discs were heat treated in the temperature range of 600 degrees C to 1250 degrees C. The products formed were characterized by X-ray diffraction (XRD) and i.r. spectroscopic techniques for identification of phases formed and functional groups present in them. While tricalcium phosphate and calcium hydroxide taken in the molar ratio of 3 : 2 and 3 : 3 resulted in pure HAP when heat treated at 1000 degrees C for 8 h, the 3 : 1 and 3 : 1.5 molar ratio compositions resulted in a biphasic mixture of HAP+beta-TCP for similar heat treatments. Heat treatment of 3 : 4 molar ratio composition of tricalcium phosphate and calcium hydroxide at 1000 degrees C yielded HAP with free CaO as the secondary phase. Products of heat treatment at higher temperatures (1150 and 1250 degrees C) for even shorter duration (2 h) while not differing from the products obtained from T0 and T2 cases at 1000 degrees C (pure beta-TCP and pure HAP), change in the case of T1, T1.5, T3 and T4 to products with lesser percentages of HAP containing beta-TCP (in the case of T1 and T1.5) or CaO (in the case of T3 and T4) as the secondary phase.

The dispersibility in deionized water of hydroxyapatite (HA) synthesized by a high-temperature (1000°C) solid-state reaction between tricalcium phosphate and calcium hydroxide was investigated as a function of the pH of the medium and the quantity of two dispersing agents (A ‫؍‬ inorganic, B ‫؍‬ organic) added to the slips. Although pH modification had a negligible effect on dispersibility, both of the dispersing agents produced a good dispersion at considerably higher concentrations (>2 wt% of HA). At optimum amounts (2-4 wt%) of the dispersing agents, the slips showed near-Newtonian flow behavior up to 45 wt% solids loading and non-Newtonian behavior at >50 wt%. By the optimal addition of dispersing agents and conditioning by ball milling, 60 -67 wt% (32-39 vol%) solids-loaded HA slips could be cast into plaster molds to produce 50%-58% dense green bodies, which, in turn, sintered to 90%-94% density in the temperature range 1300°-1400°C. The sintered HA exhibited a three-point flexural strength of 40 -60 MPa and a homogeneous microstructure, with interspersed microporosities.

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The dispersion characteristics of commercial Si 3 N 4 powder in aqueous media (deionized water) was studied as a function of pH in the range 2-11. The slip was characterized for its dispersion quality by various experimental techniques like particle size analysis, sedimentation phenomena, viscosity and flow behaviour and zeta potential analysis. The optimum dispersion was found to be in the pH region 9-11 wherein the slurry displayed minimum sedimentation height, minimum viscosity, near Newtonian flow behaviour and maximum zeta potential. The slip is highly agglomerated in the pH range 2-8 as manifested by higher sedimentation height, higher viscosity, lower zeta potential and thixotropic non-Newtonian flow behaviour. The 72 wt% (44 vol.%) Si 3 N 4 slips made at pH = 10 resulted in green bodies having 53-59% of theoretical density after casting into plaster molds.

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A test equipment was designed to study thermal shock and thermal fatigue of ceramic materials subjected to fast heating (ascending). The equipment was designed to generate thermal stress in a test specimen by heating one surface of it by an oxy-hydrogen flame while cooling the opposite surface. The sample cracked when thermal stress exceeded its mechanical strength. The in situ crack formation was detected by an acoustic emission system coupled to the set up. The hot zone temperature was measured by an infra red pyrometer. The equipment was also designed to run thermal fatigue test cycles in automatic mode between two selected temperatures. The temperature and thermal stress distribution in the test specimen were modelled using finite element software. The effect of temperature distribution of the top and bottom surfaces on thermal stresses was studied. It was observed that the thermal stress is very sensitive to the temperature distribution on the top surface and maximum near the periphery of the top surface. This was in agreement with the experimental results in which the cracks were originated from the periphery of top surface. It was also observed that the failure temperature was higher for thicker samples. q

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