Showing papers by "Yury Gogotsi published in 1994"

Water Effects on Corrosion Behavior of Structural Ceramics

Abstract: Environmental degradation poses a serious limitation on the utility of engineering materials such as metals, plastics, glasses, and structural ceramics. However, at least water would seem to be a harmless environment for ceramics, which are considered to be the most corrosion-resistant of all materials. When we drink water from a glass or a ceramic cup, we never think about an aqueous alteration of these materials. We cannot imagine that our ceramic mug may be dissolved in water or fail due to stress corrosion during breakfast. Even hot water which we use to make coffee or tea does not seem to be a hazardous environment for our china and earthenware. Why then are we going to discuss the effect of water on structural ceramics? The answer is simple: Under conditions of normal use of structural ceramics, which include high temperatures, high pressures, and mechanical stresses (Figure 1), even the most stable and corrosion-resistant materials may interact with water and water may become a hostile environment. It can be adsorbed on the surface of ceramics and act as a solvent for oxide ceramics or as an oxidant for non-oxide ceramics (Figure 2). However, water can cause a degradation of glasses and oxide ceramics even at room temperature if mechanical stresses are simultaneously applied (so-called stress corrosion). Water or humid air can decrease significantly the mechanical properties of certain zirconia ceramics at only slightly elevated temperatures (~200°C).

Improvement of oxidation behaviour of dense silicon nitride by surface modification

Abstract: The effect of surface modifications by ion-assisted deposition of platinum layers on the oxidation behaviour of dense silicon nitride was examined. Gas-pressure sintered silicon nitride containing neodymia or yttria and alumina as sintering aids was coated with up to 1·5 μm Pt by magnetron sputtering. The morphology, residual stresses, chemical composition and crystal parameters of the coatings were characterized by SEM, XRD and electron beam microanalysis. The oxidation behaviour was investigated at 1100°C, 1300°C and 1450°C in flowing air. In comparison with uncoated surfaces the Pt-coated surfaces were, after oxidation, less damaged by cracking, bubbles formation and spalling. The oxidation reaction was masked by the decomposition of platinum oxide and crystallization processes. Experimental results indicate that the observed improvement of oxidation resistance of the investigated materials is caused by the change of the composition and properties of the growing oxide layers, by the change of diffusion—and oxidation—processes and by promotion of the selective crystallization of silicates in the oxide layer.

Oxidation of powdered and sintered TiN

Abstract: Oxidation of a TiN powder prepared by plasma vapour phase synthesis and having the specific surface area of 46 m2g, as well as oxidation of pressureless-sintered TiN was studied under isothermal conditions and programmed heating. Noticeable oxidation of the powder starts above ∼250°C and results in the formation of TiO2 as anatase and brookite at lower temperatures, and as rutile at higher temperatures. The formation of multilayered oxide films on sintered TiN was observed above 1000°C. The growth of such rutile scales was based on the simultaneous titanium and oxygen ions' diffusion through the oxide layer in different directions.

Effect of oxidation and hydrothermal corrosion on strength of SiC fibres

Abstract: The effects of corrosion in high-temperature, high-pressure water and oxidation in air on the mechanical properties of three commercially available amorphous Si-Ti-C-O (Tyranno) fibres with different oxygen content and diameter were investigated. The fibres were exposed to elevated temperatures in various environments and subsequently tested at room temperature. Both, oxidation and hydrothermal corrosion of the fibres, resulted in a decrease of their strength and Young's modulus. The mechanisms of the environmental degradation are discussed.