Fracture Resistance of Ceramics upon Edge Chipping

Modern methods of determining fracture resistance are analyzed. The necessity of developing a crucially new method based on edge chipping of a brittle material is shown. The results of experimental studies are presented. The applicability of the method to the comparative fracture resistance evaluation of ceramics is substantiated.     

The effect of SiO2 on high-temperature deformation and strength of zirconia-toughened alumina

At room temperature, SiO₂ additions may increase the fracture toughness, K _IC, by diminishing the tetragonal phase contents to about 50%, but with ground surfaces the influence on strength is small. A pronounced decrease in strength is observed with rising temperature in the high toughness region from 20°C to M _s, the starting temperature for martensitic transformation. Beyond M _s at lower toughness, the strength behaviour is very similar to nontransforming alumina ceramics, and an even modest increase of the silicate concentration intensively promotes propagation-controlled failure in the brittle creep region (> 900°C) and inelastic deformation. With less than 1% amorphous grain boundary phases, damage-free superplasticity is restricted to small strains of less than 10%. The significance of high-temperature data for tool applications is considered by cutting tests with high feeding rates.     

Mechanical Properties and Oxidation Behavior of Al2O3–AlN–TiN Composites

The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al₂O₃–42 wt% TiN, differing in the AIN/Al₂O₃ ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al₂O₃ and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al₂O₃-rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters.     

Formation of Porous SiC Ceramics by Pyrolysis of Wood Impregnated with Silica

Biomorphous β-SiC ceramics were produced at 1400°C from pine wood impregnated with silica. This one-step carbothermal reduction process decreases the cost of manufacturing of SiC ceramics compared with siliconization of carbonized wood in silicon vapor. The synthesized sample exhibits a 14 m²/g surface area and has a hybrid pore structure with large 5–20 μm tubular macropores and small (<50 nm) slit-shaped mesopores. SiC whiskers of 20–400 nm in diameter and 5–20 μm in length formed within the tubular pores. These whiskers are expected to improve the filtration by removing dust particles that could otherwise penetrate through large pores. After ultrasonic milling, the powdered sample showed an average particle size of ∼30 nm. The SiC nanopowder produced in this process may be used for manufacturing SiC ceramics for structural, tribological, and other applications.     

Effect of boron nitride addition on some properties of aluminosilicate refractories

Conclusions An addition of 10–60 wt.% of boron nitride significantly alters the thermal conductivity, thermal expansion, and the elastic modulus of aluminosilicate refractories.As the boron nitride content is increased from 1–60% the strength of specimens heated in a nitrogen atmosphere with an oxygen content of 0.02% decreases.During cyclic heat treatment in an oxidizing atmosphere between 900 and 20°C additional bonds develop between the particles of the aluminosilicate and the oxygen-free additive. As a result the strength of the specimen increases.The thermal shock resistance of the specimens increases with an increase in the amount of boron nitride addition. Specimens with 40–50% boron nitride addition are in practice insensitive to temperature drops in the range 20–2400°C.Aluminosilicate refractories with the addition of 30–40 wt.% of boron nitride can be used as lining material in high temperature systems with brief nonsteady or cyclic work schedules.Translated from Ogneupory, No. 4, pp. 36–39, April, 1968.     

Study of thermal shock resistance of fired and unfired zirconia refractories

Conclusions Comparative studies were made of fired and unfired refractories based on zirconium dioxide. The fired refractories are more resistant to cracking. However, the unfired materials possess a higher resistance to the development of cracking which is noted during the registration of surface deformations in the process of destruction of the specimen; and this is confirmed by analytical criterial evaluation.The determination of the destructive drop in temperatures before the development of the first crack in specimens based on granular bodies should not be considered as a reliable evaluation of the thermal shock resistance of the refractories.A new method was developed for observing the nature of the failure in specimens during thermal loading.Translated from Ogneupory No. 1, pp. 52–56, January, 1973.