V. V. Storozh

Effect of hydrostatic compression on the densification and sintering processes of corundum castings

ConclusionsWe studied the effect of the pressure of hydrostatic compression (HSC) on the densification of the corundum castings obtained according to the method of aqueous slip casting.It was shown that HSC of the raw castings and two-stage HSC lead to a significant densification of the castings in the range of low pressures (up to 600 N/mm2).We analyzed the interrelationship between the conditions (parameters) of HSC, the porosity of the castings, the effectiveness of sintering, and the mechanical strength of the ceramics. It was established that HSC of raw castings immediately after their structure evolution makes it possible to obtain (after sintering at 1500°C) specimens of additiveless corundum ceramics having a true porosity not exceeding 4% and a higher (1.5 times) strength as compared to the ceramics obtained without HSC.

Formation of properties of ceramics from MgO powders obtained with the use of cold isostatic pressing

Results of a study of the effect of cold isostatic pressing (CIP) of granulated MgO powder under a pressure of 0.3 and 2.0 GPa on the properties of compacts and ceramics are presented. It is shown that the density and strength of the compacts are 65 and 80%, 10 and 55 MPa after CIP conducted at a pressure of 0.3 and 2.0 GPa, respectively. The ceramics was sintered at 1600‡C for 1,2,4, and 8 h. At all sintering times the density and strength of the ceramics is higher when the powder has been compacted under a pressure of 2.0 GPa.

Hydrostatic treatment of α-aluminium oxide powders part 1. Microstructure of the particles and sintering

Transmission electron microscopy and X-ray radiography are used to show that pressing of α-aluminum oxide powders at pressures up to 2.0 GPa is accompanied by crushing of particles and the appearance of dislocations with a density of about 1011 cm−2. Sintering of these powders at temperatures up to 1550°C reduces the dislocation density 2–4-fold. When the compacts reach a critical density (about 62%) the shrinkage in sintering of nonsintered powders is shown to be independent of the compact density and the pore size. The increase in the sintering rate with increase in the hydrostatic pressure to 0.6 GPa is shown to be caused mainly by an increase in the compact density and at still higher pressures by the mechanical activation of the powders.

Hydrostatic treatment of alpha-aluminum oxide powders part 2. Evaluation of the mechanisms of sintering acceleration

The effects of different mechanisms of mechanical activation of powders on the subsequent sintering are considered. Expressions for estimating the contribution to acceleration of the sintering process by the mechanisms of dislocation sliding and climbing, fracturing of the particles, and self-diffusion in dislocation pipes are obtained. It is shown that the expansion of hydrostatically pressed specimens of alumina in the range 800–1200°C is naturally explained by the escape of some of the dislocations from the particles or by the ordering of the dislocation structure. When alumina powder with particles 2 – 3 µm in size and purity of approximately 99.8% is treated at 2.0 GPa, a substantial contribution to the acceleration of sintering is made by the mechanisms of particle fracturing and self-diffusion in dislocation pipes. The limits of applicability of the results obtained are considered in relation to the purity of the material and the size of the particles.

Influence of hydrostatic pressing of a sol-gel obtained by a powder method on the sintering and properties of ceramics in the Al2O3-SiO2-ZrO2 system

ConclusionsThe influence of hydrostatic pressing with pressures up to 2.0 GPa of powder obtained by the sol-gel method on phase transformations in sintering and the properties of the sintered material in the Al2O3-SiO2-ZrO2 system has been studied. It was shown that an increase in hydrostatic pressing pressure from 0.3 to 2.0 GPa makes it possible to significantly increase the density and strength of the sintered material. In this case the primary factor hindering development of high strength is the increased pore size in the sintered material.It was observed that an increase in hydrostatic pressing pressure of the powder leads to acceleration of decomposition of zircon and formation of mullite. These processes are accompanied by broadening of the x-ray lines of the newly formed phases.The two-stage character of the sintering process was shown on the basis of the relationships of density and strength of the investigated material to sintering temperature. The conclusion that the low-temperature stage is caused by phase transformations in the sintering process and the high-temperature by diffusion processes in the newly formed phases was drawn.The possible mechanism leading to appearance of the observed effects were considered.

Influence of hydrostatic treatment of greenware and sintering schedules on the properties of ceramics in the tialite-mullite-corundum system

ConclusionsA study was made of the effect of hydrostatic processing (HSP) of greenware with a pressure of 2.0 GPa and the rapid sintering temperature on the density and strength of ceramics in the tialite-mullite-corundum system.We detected two strength maxima in the sintered material in relation to the sintering temperature, on the basis of which we identified four temperature ranges for the sintering. We examined possible mechanisms determining the properties of the material during sintering in each of these intervals.It is shown that HSP accelerates the decomposition of tialite and increases the density and strength at sintering temperatures of 1250-1300°C; however, HSP is accompanied by a more significant reduction in strength and density for specimens sintered at 1400–1600°C. The results obtained explain the influence of the dislocations formed in the greenware during HSP.

Influence of the medium in mechanicochemical activation of powder on the formation of a reaction-bonded ceramic in the Al2O3-SiO2-ZrO2 system

ConclusionsA significant influence of the powder vibrotreatment medium on the strength of the sintered material in the Al2O3-SiO2-ZrO2 system is observed. The material after dry vibrotreatment of the powder has the highest strength while vibrotreatment of powder in water or acetone leads to a significant reduction in strength.The significant influence of powder vibrotreatment medium on the zirconium dioxide tetragonal phase content in the sintered material is shown. The tetragonal phase content is significantly higher after dry vibrotreatment than after wet.The results obtained may be explained by the different character of deformation of the particles in dry and wet vibrotreatment.

Role of hydrostatic treatment in the formation of the structure and the properties of reaction-bonded ceramics based on mullite and zirconium dioxide

ConclusionsComparative studies were made on the densification of castings during hydrostatic pressing (GSO) and the hydrostatic pressing of powders of corundum-zirconia composition, and sintering of the greenware in relation to the pressure of the hydrostatic pressing treatment. Some properties of the resulting ceramic were analyzed.It was established that the sinterability of the castings obtained by water slip casting with subsequent GSO is much higher than for the greenware obtained by hydrostatic pressing of powders at the same pressure. This is connected with the presence in the castings of unpolymerized colloidal component.A reduction in the porosity of the castings to 35–38% and less after GSO for pressures above 500 N/mm2 facilitates sintering of the ceramic to zero open porosity in a wide range of rises in temperature, and contributes to an increase in strength on account of the dissipation mechanism which is reinforced with increase in GSO pressure.