Oxidation resistance in structural ceramics based on refractory materials

Abstract

Refractory oxygen-free compounds with a melting point above 2500°C show promise for development of thermostressed ceramic materials. The individual use of oxygen-free materials is limited because of their low-temperature oxidability which poses problems for manufacturing technology. Refractory oxide constituents with high thermodynamic stability make it possible to enhance the oxidation resistance of oxygen-free components of a composite system and to allow their use at higher temperatures.The ever-increasing demands placed on the stable and efficient performance of high-temperature heat powered units require the development of oxygen-free refractory materials with specifically tailored characteristics.Refractory ceramics with a melting point above 2500°C are promising candidates for the development of composite materials for use at high temperatures. In most high-melting oxygen-free compounds, the chemical bonds display a high degree of covalency which, on the one hand, predetermines their high melting point and hardness and, on the other hand, creates considerable problems for preparation of ceramic components with a high structural density [1].The use of a particular oxygen-free ceramic may be limited because of its thermodynamic instability, brittleness, and insufficient thermal resistance. Introducing high-melting oxide constituents with enhanced thermodynamic stability into a ceramic mixture makes it possible to improve the oxidative stability of oxygen-free components of a composite system.The oxide component used was a powder of partly stabilized zirconium dioxide (PSZD) prepared by chemical coprecipitation from zirconium hydroxide and 5.34 wt.% yttrium oxide [2]. Powdered zirconium nitride (TU 6-09-4050 Specifications) and zirconium boride (TU 09-03-46 Specifications) available from Donetsk Chemical Plant JSC (Ukraine) were used to prepare a composite mixture.The precursor powders of zirconium nitride and boride were analyzed by differential thermal method in air to determine the incipient oxidation temperature; for the zirconium nitride powder, it was 530°C, and for zirconium nitride, 540°C. However, the low-temperature oxidability of oxidefree ceramics presents a disadvantage for the manufacturing technology and sets a limit to the use of their high-temperature properties.The mechanical strength of zirconium nitride and boride tends to increase with temperature, as shown by the data in Table 1, which is an essential property of these compounds [1].The ZrO 2 -based ceramic materials are highly refractory and are characterized by high mechanical strength that is retained at high temperatures. The mechanism of strengthening of PSZD materials involves the buildup of an ultradisperse structure in ceramics with a high percentage of the tetragonal phase that is capable of converting to a monoclinic phase; this conversion occurs near the crack tip and is accompanied by an increase in volume under the action of mechanical stresses, which finally results in crack closure. For the crack to propagate further, addi...