Deformation mechanism maps for applications at high temperatures

CERAMICS

INTERNATIONAL,

THE CORROSION GLAZES G. Bagnasco,

141

Vol. 11, n. 4, 1985

OF TIN OXIDE SEMICONDUCTING

P. Pernice and P. Giordano Orsini

FRACTUREMECHANICSSTUDYOFA TRANSFORMATION TOUGHENED THE CaO-ZrOz SYSTEM

ZIRCONIA

ALLOY IN

R.C. Garvie. 13.H.J. Hannink and C. Urbani An SnOz-SbzOs glaze, which seems the most promising among semiconducting porcelain glazes for high voltage applications, has been operated as electrode in Na2S04 solution, to study under controlled conditions its corrosion behaviour. The morphology of the corrosion has been observed by microscopy. Metallic tin produced by electrochemical reduction has been identified by X-ray diffraction.

YOUNG’S ALUMINA

MODULUS-POROSITY SUBSTRATES

RELATIONSHIP

FOR

G. De Portu and P. Vincenzini Short communication.

SOME OBSERVATIONS ON OPACIFICATION CONTAINING BOROSILICATE GLASSES

IN KzO-

H. Takashima

The flexural strength, elastic modulus, fracture toughness (KI,) and grain size were determined for a partially stabilized calcia-zirconia alloy (Ca-PSZ) which was progressively aged at 1300%. 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 m”2, 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.

Short communication. EFFECT OF POROSITY ON THE THERMAL SHOCK BEHAVIOUR OF REACTION-SINTERED SILICON NITRIDE G. Ziegler and J. Heinrich

Vol. VI, January/December, 1980 HIGH TEMPERATURE STRUCTURAL CERAMIC MATERIALS MANUFACTURED BY THE CNTD PROCESS J. J. Stiglich,

Jr., D.G. 5hat and R.A. Holzl

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 TiBz and ZrBz. An example of grain refinement accompanied by improvements in mechanical properties is SIC. Grain sizes of 500 to 1000 A 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%. Various applications of these materials to the solution of high temperature structural problems are described.

DEFORMATION APPLICATIONS

MECHANISM MAPS FOR AT HIGH TEMPERATURES

Terence G. Langdon 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 A&O3 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 AlzOJ and Al203 doped with MgO above the solubility limit.

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 AT, after water quenching is temperature differences 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 AT,. Moreover, the results indicate that thermal conductivity plays a significant role in the interpretation of the thermal shock behaviour of reaction-sintered SiJN4.

THERMAL STRESS BEHAVIOUR AND AIN CERAMICS

OF YTTRIA, SCANDIA

G.A. Gogotsi Results of an investigation of ceramics based on yttria (YzO~), scandia (Sc201) 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.

STUDIES ON THE APPLICATION OF HOT-PRESSED SILICON NITRIDE CERAMICS AS CUTTING-TOOLS Miao Ho-Cho, Chow Chia-Baa, Liu Wan-Ho, Chiang TsoChao, Liu Kuo-Liang and Tang Chen-Huei Studies

on the application

of hot-pressed

SilNj

as cutting