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silicide coatings in air at 1800 and 1900°C. Metallographic, x-ray diffraction, electron microprobe, and chemical analyses show that the introduction of alumina into molybdenum and tungsten disilicides improves their hardness and produces fine-grained silicides with high corrosion resistance. The composition with fine alumina particles (Ø < 0.1 μm) with the total alumina concentration from 3 to 15 wt.% in the coating is most favorable for improving the oxidation resistance of disilicides. It is shown that the high oxidation resistance of MoSi 2 −Al 2 O 3 and WSi 2 −Al 2 O 3 coatings is due to the presence of alumina in disilicides, which contributes to the formation of an external high-temperature oxide layer. This layer consists of α-Al 2 O 3 and a mullite sublayer in which Al 2 O 3 crystals are introduced into SiO 2 .
Articles made of carbon materials which exhibit unique physicochemical and mechanical properties are used in various branches of science and technology, i.e., from structural elements for space equipment to everyday articles. With the aim of expanding the range of application for carbon materials, particularly in the field of high-temperature technology, it is necessary to improve their heat resistance. As a rule this problem is resolved either by creating heat-resistant composites (for this purpose corrosion-resistant additions are made to the composition of the carbon matrix) or by applying protective coatings. The second path is assumed to be more promising.Information about multilayer coatings on carbon materials is presented in [1]. A substrate of titanium carbide with a thickness of the order of ten microns was precipitated from a mixture of titanium chloride and methane gases, and a protective coating of molybdenum disilicide with a thickness of 100 #m was applied by plasma deposition followed by annealing. The heat resistance of these coatings in air at 1600°C was of the order of tens of hours. The most promising are multilayer coatings of silicon carbides and nitndes, molybdenum and tungsten silicides, and also zirconium and hafnium oxides [2]. Thesecoatings arecapable ofprotecting carbon-carbonmaterials in oxygen-containing atmospheres at temperaturesup to 1800°C.The aim of the present study is to develop high-temperature and anti-corrosion coatings for carbon materials based on transition metal borocarbides and silicides.Considering the comparatively low linear expansion coefficients for carbon materials, the nonuniformity of their structure, high porosity, and high vapor pressures of carbon oxides with their greater chemical activity, in order to obtain high-temperature and heat-resistant coatings the methods of diffusion impregnation, deposition from the gas phase, and impregnation through the liquid phase and fusion were used. Experience in applying high-temperature (up to 2000°C) protective coatings on refractory metals and their alloys [3-5] was used.The original specimens made of carbon material, in particular graphite grade ARV and graphite bonded with pyrocarbon (GBP), were cylindrical in shape 8 mm in diameter 8 mm and 70 mm high. The carbon-borosilicides of refractory metals were selected as protective coatings. Titanium, niobium, and zirconium for subsequent preparation of a carbide substrate were applied by thermal decomposition from chlorides and iodides in the gas phase. A high-frequency generator or a furnace with an electric heater was used in order to heat the specimens. The temperature was measured by an optical pyrometer of the 'Promin' type or a tungsten-rhenium thermocouple. Titanium (zirconium) iodide was prepared directly in the device for applying coatings by passing iodine vapor through titanium (zirconium) turnings. The layout of the device and the procedure for applying coatings from the gas phase are described in [6]. In order to provide good coating adhesion with the base c...
The paper studies the kinetics of the diffusion redistribution of phases in the MoSi 2 -W system when tungsten samples with molybdenum silicide coatings are heated in air at 1500-1800°C. It is established that the (Mo x ,W y ) 5 Si 3 phase, which represents a molecular solid solution of lower molybdenum and tungsten silicides, forms in an exchange reaction between molybdenum and tungsten at the MoSi 2 -W 5 Si 3 interface. The MoSi 2 -W system is much more stable than the WSi 2 -W and MoSi 2 -Mo systems.It is of current importance to develop structural materials for components and mechanisms designed to operate in oxidizing and reducing media above 1500°C used in, for example, aerospace and electrothermal engineering. Refractory metals and alloys with protective silicide coatings are used as such materials. Protective coatings based on boron silicides have proved to be especially effective. The effectiveness of coatings depends on how stable the metal-coating system is. At high temperatures, the coating material actively interacts with the base metal to form intermediate compounds. In particular, the formation of lower silicide phases leads to substantial changes in the specific volume of interacting materials. The resulting stresses and worse oxidation resistance of lower silicides deteriorate the performance of the coatings. Therefore, to increase the stability of the system, the rate at which the coating interacts with the base needs to be decreased.The diffusion of silicon in silicides of refractory metals and the stability of silicide coatings at high temperatures was examined in [1][2][3][4][5][6]. It was established there that the diffusion mobility of silicon decreases in the following sequence: Mo 5 Si 3 , W 5 Si 3 , Nb 5 Si 3 , Ta 5 Si 3 . It was also pointed out that the diffusion mobility of silicon was especially low in mixed lower phases because of the exchange diffusion of metals when metal Me 1 was coated with a silicide of another metal Me 2 .It was established [2,3] in examining the WSi 2 -W system that excess silicon that was mainly concentrated in the near-surface layer of WSi 2 coating on tungsten greatly influenced the kinetics of phase redistribution in the system, thus increasing the stability and oxidation resistance of the coating. To increase the concentration of silicon in the near-surface layer, a complex coating was examined, which was produced by siliconizing a molybdenum layer preliminary gas-sprayed onto tungsten [3]. As compared with the WSi 2 -W system, the rate at which the higher
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