STUDY OF THE STABILITY OF PHASE COMPOSITION AND STRUCTURAL PARAMETERS OF NITRIDE CERAMICS TO THERMAL EFFECTS

In the work, using the method of in-situ measurements of X-ray diffraction patterns, the stability of the phase composition, as well as the structural parameters of Si 3 N 4 ceramics, was studied in a wide temperature range from 25 to 1400 ℃, covering the range of operating temperatures during the use of ceramics in extreme conditions. The main aim of this study is to determine the phase and structural changes in Si 3 N 4 ceramics as a result of thermal effects, as well as to determine the role of thermal effects on the structural features of ceramics associated with thermal expansion. During the experiments it was established that thermal heating of samples in a vacuum does not lead to the initialization of oxidation processes characteristic of heat treatment of Si 3 N 4 ceramics in air, which are accompanied by the formation of the oxide phase SiO 2 . At the same time, the assessment of the weight contributions of the established phases in the composition of ceramics revealed the absence of any significant alterations in the phase ratio in the entire measured range of annealing temperatures. Based on changes in the volumes of the crystal lattice of both phases in the composition of Si 3 N 4 ceramics, the value of the thermal volume expansion coefficient was determined depending on the exposure temperature. It was found that the average value of the β V ( T ) coefficient is about 8.5–10·10 −6 K −1 for both established phases in the composition, while in the case of the α-Si 3 N 4 phase, the change in the β V ( T ) value has a clearly expressed dependence on the heating temperature.