Ceramurgia International最新文献

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.

The kinetics of the solid state high-temperature transformation of kyanite (Al₂SiO₅Al₂O₃·SiO₂) powders (≤40 μm) to 3:2-mullite (3Al₂O₃·2SiO₂) and silica (SiO₂ were investigated by means of quantitative X-ray diffraction techniques. The transformation interval was found to lie between about 1150 and 1350°C. The reaction law best fitting the kinetic data is: 1-α = kt^a. The transformation 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 Fe₂O₃ and TiO₂ to the starting material exerts a marked decrease of the transformation temperature, with TiO₂ having a somewhat stronger influence than Fe₂O₃. 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.

Reaction-sintered silicon nitride was investigated to determine the effect of its pore size on thermal stress resistance to fracture initiation. Samples of controlled pore structure were prepared by using an organic component to incorporate pores in the green silicon compact as well as by using silicon starting powders with different particle size. Critical temperature differences ΔT_c after water quenching is discussed in relation to changes in most important variables affecting thermal shock, such as fracture strength, Young's modulus of elasticity and thermal conductivity. The results show that when total porosity as well as other microstructural parameters are held constant, an increase in pore size leads to a decrease in ΔT_c. Moreover, the results indicate that thermal conductivity plays a significant role in the interpretation of the thermal shock behaviour of reaction-sintered Si₃N₄.

Results of an investigation of ceramics based on yttria (Y₂O₃), scandia (Sc₂O₃) and aluminum nitride (AIN) are given. Test procedures used for evaluating their mechanical behaviour as well as their thermal shock resistance on quenching and monotonic heating are described. Special emphasis was placed on yttria from which one- and two-phase materials were fabricated. It was found that, of the materials studied, ceramics of AIN possessed the highest strength and thermal shock resistance. The analysis involved also the use of data from fractographic studies.

Controlled Nucleation Thermochemical Deposition (CNTD) has emerged from classical chemical deposition (CVD) technology. This paper describes the techniques of thermochemical grain refinement. The effects of such refinement on mechanical properties of materials at room temperature and at elevated temperatures are outlined. Emphasis is given to high temperature structural ceramic materials such as SiC, Si₃N₄, AiN, and TiB₂ and ZrB₂. An example of grain refinement accompanied by improvements in mechanical properties is SiC. Grain sizes of 500 to 1000 Å have been observed in CNTD SiC with room temperature MOR of 1380 to 2070 MPa (4 pt bending) and MOR of 3450 to 4140 MPa (4 pt bending) at 1350°C. Various applications of these materials to the solution of high temperature structural problems are described.

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 Fe²⁺ to Fe³⁺. Other possible physicochemical processes, which can effect the behaviour of basalt glass fibres at high temperatures were also analysed.

The flexural strength, elastic modulus, fracture toughness (K_tc) and grain size were determined for a partially stabilized calcia-zirconia alloy (Ca-PSZ) which was progressively aged at 1300°C. Data for the same properties were obtained also for a fully stabilized cubic magnesia-zirconia alloy (Mg-CSZ) which was used as a reference material. The growth of the zirconia precipitate phase in the Ca-PSZ material was monitored. The flexural strength and fracture toughness increased smoothly to peak values of 645 MPa and 9.6 Mpa $\text{m}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, respectively, at a critical value of the ageing time and thereafter declined rapidly. The precipitate phase coarsened during ageing. Its structure was tetragonal up until the critical ageing time and thereafter the majority of the particles transformed to monoclinic. The peak strength increased three times relative to the cubic stabilized material. The grain size and elastic modulus showed only a slight dependence on ageing time. The study confirmed the hypothesis that the enhanced strength of transformation toughened zirconia alloys arises from an increase in the fracture energy. This increase is brought about by the presence of tetragonal particles, metastable at room temperature, which can be transformed by stress.

Studies on the application of hot-pressed Si₃N₄ as cutting 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 Si₃N₄ cutting tools is several times as long as that of sintered carbide tools. In threading, the Si₃N₄ 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 Si₃N₄ cutting tool can resist the shock load in milling. In cutting cast iron with high cutting speed, the Si₃N₄ cutting tool was also successful. These results indicate that hot-pressed Si₃N₄ is a cutting tool material which should be developed extensively.

Deformation mechanism maps provide a very simple and convenient method of displaying mechanical properties data at high temperatures. Several different types of maps are available, and these are discussed with reference to polycrystalline Al₂O₃ and MgO. Ambipolar diffusion is important in ceramic materials, and this leads to four different types of diffusion creep which may be illustrated directly on the deformation mechanism maps. In practice, the accuracy of the maps depends on the purity level, and this is demonstrated by comparing maps for nominally pure Al₂O₃ and Al₂O₃ doped with MgO above the solubility limit.