Ceramic materials for aviation technology

Abstract

666.762.11: 629.73 Improving the potential and reliability of components in gas-turbine engines operating in conditions of high thermal and aerodynamic stress is closely linked with the need to improve ceramic parts used in the production of complex articles from nickel, heat-resistant alloys.The Obninsk Enterprise "Tekhnologiya" has carried out a comprehensive investigation aimed at developing thermalshock resistant refractories based on corundum-mullite materials possessing high metal resistance to heat-resistant nickel alloys, and which are used for making crucibles for precision casting of the blades of gas-turbine engines (GTE), the production of measured blanks of heat-resistant alloys, and saggers for firing rods* [1][2][3][4][5][6].These investigations established that it is possible in theory to increase the thermal-shock resistance of high-alumina ceramics with a low porosity and sufficiently high mechanical strength. This was achieved by forming a microcracked, fragmented structure by introducing coarse-grained additions of sintered multite, electrocorundum, silicon carbide, and electrofused zirconium dioxide. It was shown that the thermal-shock resistance depends on the concentration of microcracks and the size of the fragments; and, moreover, the increase in the thermal-shock resistance is greater the more the temperature coefficient of linear expansion (TCLE) of the additive differs from that of the high-alumina matrix.The increase in the labor productivity and the quality of the engine blades, cast in vacuum induction furnaces (designate UPPF), are mainly determined by the thermal-shock resistance of the ceramic crucible and the degree of contamination, from the molten metal. Rammed crucibles that were used in this work, and which lasted (with intermediate repairs) up to 400 castings, are eroded by the chemically active alloys, and are a source of dirt and blisters, as a result of which the total loss of blades due to metal contamination reaches 60-80 %.We therefore developed corundum-mullite material TSM-323 (see Table 1) and a production technology for changeable ceramic crucibles, height 320 mm, internal diameter about 135 ram, and wall thickness 18 mm for use in UPPF furnaces, and VIP-3 furnaces (Fig. 1). The enhanced thermal-shock resistance of TSM-323 is due to the formation of a fragmented structure. During a final firing at 1580~ the high-alumina matrix undergoes shrinkage, and the grains of mullite prevent this. As a result, the matrix is broken into several fragments that are capable of being mutually displaced and thereby resisting thermal shock.It is found that the highest increase in thermal-shock resistance is attained by introducing into the ceramic composition sintered mullite grade with grain sizes of 0.4-0.63 mm (Fig, 2). The phase composition of the ceramic consists of corundum (a-A1203), mullite (3A1203-2SiO2), strontium anorthite (Sr2A12Si2Os), and strontium titanate (SrTiO3). The ceramic has a high refractoriness --1740~The crucibles were made by hot vibration-forming fro...