Some phenomena occurring in basalt glass fibres at high temperatures

Some phenomena occurring in basalt glass fibres at high temperatures

142 CERAMICS tool were carried out and satisfactory results achieved. In cutting hardened tool steels, chilled cast iron, pure molybdenum and pyrol...

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142

CERAMICS

tool were

carried out and satisfactory results achieved. In cutting hardened tool steels, chilled cast iron, pure molybdenum and pyrolytic graphite, the tool life of S&N4 cut-

ting tools is several times as long as that of sintered carbide tools. In threading, the S&N4 cutting tool also performs satisfactorily. In order to test its cutting property under shock load, a milling test was made. The results show that the S&N4 cutting tool can resist the shock load in milling. In cutting cast iron with high cutting speed, the S&N4 cutting tool was also successful. These results indicate that hot-pressed SilNs is a cutting tool material which should be developed extensively.

SOME PHENOMENA OCCURRING IN BASALT FIBRES AT HIGH TEMPERATURES

GLASS

EXPANSION PTZ

INTERNATIONAL,

DURING THE REACTION

Yoshikazu Nakamura, M. Fulrath

Vol

SINTERING

Sudhir S. Chadratreya,

11. n 4. 1985

OF

and Richard

Expansion during the reaction sintering of PTZ from PT and PZ constituents was studied at different temperatures: compositional and particle size effects were observed. The expansion is related to the size of the PT particles present and exhibits a maximum at 50 mole O/oPT for equal sizes of PT and PZ.

KINETICS KYANITE

OF THE THERMAL

14. Schneider

DECOMPOSITION

OF

and A. Majdie

J. Rzechula and M. Grylicki

It is well known that at high temperatures

basalt glass wool looses its elasticity; crystalline phases appear and the fibres eventually disintegrate. Experiments reported here show that one of the factors responsible for this processes is oxidation of Fe’+ to Fe3 ’ Other possible physicochemical processes, which can effect the behaviour of basalt glass fibres at high temperatures were also analysed.

INTERACTION BETWEEN CREEP, OXIDATION AND MICROPOROSITY IN REACTION-BONDED SILICON NITRIDE G. Grathwohl

and F. Thiimmler

Kinetic studies have been performed on primary and secondary (minimal) creep of reaction-bonded silicon nitride in 4-pt-bending tests up to 1500%. The creep deformation depends strongly on the extent of internal oxidation. In spite of the very marked dependence of the creep rate on material and pretreatment parameters, the stress exponents (n I 1.7-l .8) and activation energies (360-390 kJ/mole) are hardly influenced, suggesting similar creep mechanism. Creep deformation is provided by relative motion and separation of grain boundaries. Oxidation and deformation lead to remarkable changes of the pore size distribution; the creep processes are accompanied by deformation of the pores. The creep rupture strain is very limited in highly creep resistant materials and vice versa. Methods for the determination of oxidation products and oxide profiles along sample cross section have been developed and the chemical changes which the material can undergo during creep are outlined.

ON THE CI TO fl PHASE TRANSFORMATION AND GRAIN GROWTH DURING HOT-PRESSING OF S&N4 CONTAINING MgO H. Knoch and G.E. Gazza The cr/p phase transformation and correlated grain growth during hot-pressing of silicon nitride containing MgO has been studied. The reconstructive transformation appears to be reaction controlled with the rate dependent on the amount of pre-existing p phase and cylliquidlp interface formed during hot-pressing. The microstructure of the dense, hotpressed product, i.e. grain size, shape, and distribution, is influenced by the starting u/p ratio and nucleation rate of the p phase. Different growth rates perpendicular and parallel to the crystallographic c-axis of p result in an elongated microstructure. increasing the hot-pressing pressure increases the transformation rate as a consequence of the enhanced nucleation rate of p. This produces a fine, uniform microstructure with good mechanical strenght. If hotpressing time is extended beyond that required for CY/Pconversion, coarsening of the p grains will degrade mechanical properties.

The kinetics of the solid state high-temperature transformation of kyanite (A&i05 = Al20J-Si02) powders (5 40 pm) to 3:2-mullite (3Alr03-2Si02) and silica (Si02) were investigated by means of quantitative X-ray diffraction techniques. The transformation interval was found to lie between about 1150 and 1350%. The reaction law best fitting the kinetic data is: 1-a = kt”. The trasformation is believed to be reconstructive with decomposition of the kyanite structure, solid-state atom diffusion, and (epitactic) rearrangement of mullite and cristobalite. Cristobalite represents part of the <(free* Silica, the rest being present as a glassy phase. Addition of Fez03 and Ti02 to the starting material exerts a marked decrease Of the transformation temperature, with Ti02 having a somewhat stronger influence than Fe203. The reason may be an oxide-catalyzed reaction; the decomposition begins at nuclei formed at the surfaces of the kyanite particles, which are coated with thin layers of hematite and rutile respectively. ELECTRON MICROSCOPY BASED PORCELAINS

OF SOME WOLLASTONITE

C. Coma Diaz, J. M a Gonztilez Peiia and D. Alvarez-Estrada Electron microscopy studies were made on five calcium porcelains whose starting batches consisted of kaolinite clay, wollastonite and a lead borate frit. Transmission and scanning electron microscopy, as well as X-ray energy dispersive microanalysis were used. The mineral composition of the resulting porcelains was found to be primarily anorthite, wollastonite and quartz. Anorthite, the most abundant crystal phase, was found to be present in a wide range of sizes and shapes, suggesting that it is formed in these porcelains by more than one mechanism. This may be related to the presence of a vitreous phase of special characteristics. It seems possible to direct the growth of the anorthite crystals, not only towards different sizes but also towards specific shapes for the purpose of inducing convenient microstructures.

THERMAL INSULATIONS AGRICULTURAL WASTE

FROM RICE HUSK ASH, AN

P.C. Kapur Rice husk ash, an agricultural waste material, is available in large quantitaties in the rice paddy growing countries of the world at little or no cost. This ash is highly porous, mostly silica and possesses refractory and thermal insulation properties. It is therefore an attractive starting raw material for the manufacture of low to moderate cost thermal insulations for dryers, ovens, kilns and furnaces, including those employed in the ceramic industry. This paper deals with manufacture, properties and usage of a spectrum of low to high temperature thermal insulations and insulating refractories that can be made from rice husk ash, namely; (i) Calcium ferrite bonded porous silica refractory; (ii) Sodium