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48HEAT-RESISTANT COATINGS FORMED FROM SHS POWDER OF THE ZrSi2-MoSi2-ZrB2 SYSTEM FOR CARBON COMPOSITES
A. N. Astapov", E. A. Levashovfi, I. P. Lifanov", Yu. S. PogozhevA, A. Yu. PotaninA, and M. V. Prokofiev"
aMoscow Aviation Institute (National Research University), Moscow, 125993 Russia bNational University of Science and Technology MISiS, Moscow, 119049 Russia e-mail: Lexxa1985@inbox.ru
DOI: 10.24411/9999-0014A-2019-10014
Composite materials based on Cf/C and Cf/SiC classes are among the most promising heat-resistant materials with unique properties [1], the combination of which determines the priority of their use in heat-stressed structural elements of rocket and space technology products. However, in oxygen-containing environments, their use is limited by the tendency of carbon to oxidize, starting at temperatures of 400-450°C, and the insufficient heat resistance of the SiC matrix, which causes the materials to lose mechanical properties. In high-velocity, high-enthalpy flows of oxygen-containing gases, the degradation of the structure of composites is significantly aggravated as a result of the simultaneous course of thermochemical processes (oxidation, catalysis) and mechanical entrainment (erosion). The expansion of the temperature-time intervals of operability of Cf/C and Cf/SiC composites is the exceedingly actual task of modern materials science. The research is carried out in several directions [2, 3]: modifying of composites matrixes, applying of protective coatings to reinforcing fibers, but the most effective is the application of heat-resistant coatings on the working surfaces of parts in contact with oxidizing media.
Currently the main objects of interest are coatings with the main structural component of ultra-high-temperature ceramics based on super-high-melting transition metal borides (ZrB2, HfB2, TiB2) with the addition of carbides (SiC, ZrC, HfC, TiC, TaC) and silicides (MoSi2, TiSi2, ZrSi2, TaSi2, WSi2). The report presents the current results of ongoing theoretical and applied research in the direction of creating heat-resistant coatings based on the ZrSi2-MoSi2-ZrB2 system. Materials of this system fully meet the requirements of the physicochemical model of the operation of a heat-resistant coating as a part of the structural wall of heat-resistant materials in hypersonic high-enthalpy flows of oxidizing gases [4]. The model provides the formation of a branched microcomposite structure in the form of a refractory dendritic-cellular type of heat-resistant phases (MoSi2, ZrB2) with the presence of relatively low-melting phases (ZrSi2) in its cells. The framework provides material resistance to erosion entrainment, and the low-melting phase provides self-healing effect and accelerated formation of a protective oxide film through the liquid phase. The compositions of the studied material in the ZrSi2-MoSi2-ZrB2 system were selected based on the results of oxidation in air at 1650°C for 5 h of compacts obtained from the corresponding powders by hot pressing [5].
To obtain the composite ceramic powders based on the ZrSi2-MoSi2-ZrB2 system, a magnesium thermal synthesis scheme with a reduction stage was used [6]. The starting components for the preparation of the reaction mixtures were analytically pure powders of silica SiO2, zirconia ZrO2, and molybdenum oxide MoO3 with a content of the main substance of 98.0-99.5% in its composition. As a source of boron, granulated coarse-grained powder of boric anhydride B2O3 with a content of the main substance of not less than 99.0% was used, which was preliminarily milled in a ball rotating mill (BRM) (ISMAN, Chernogolovka) with a mass ratio of powder and grinding bodies of 1:5, and then sieved to separate the fraction with a particle size less than 100 |im. Pure magnesium powder MPF-3 with an active component
content of 98.5-99.5% and an average particle size of 100-150 p,m was used as a reducing agent, which was introduced into the reaction mixtures with some excess, which provided the maximum degree of reduction of oxide components.
The powders were mixed in BRM in sealed steel drums using cylindrical carbide grinding bodies at a ratio of 1:6. The synthesis of final reaction mixtures was carried out in a universal SHS reactor of the brand SHS-8 (ISMAN, Chernogolovka) with a reaction chamber volume of 8 liters. The reaction mixture was loaded into a graphite container by free filling. The combustion process was initiated using a tungsten coil, which was placed in such a way that it touched the surface of the mixture. The synthesis was carried out in argon with pressure excess of 3 MPa. The combustion process was monitored by a manometer recording the pressure inside the reaction chamber of the SHS reactor.
This intermediate product consisting of composite particles containing the target phase and magnesium oxide MgO was crushed in a jaw crusher, and then crushed in BRM. To remove MgO, as well as excess of unreacted magnesium and extraction of ceramic powders, chemical (acid) enrichment in an aqueous solution of hydrochloric acid (HCl of chemical purity) was used. The resulting precipitate of the target product was filtered, washed with water and dried at 100°C, and then grinded in BRM and sieved through a sieve with a cell size of 63 p,m. To deagglomerate the particles of the obtained powder, the powder was subjected to grinding in a planetary centrifugal mill (PCM) of the MPP-1 brand (Technics and Disintegration Technology LLC, St. Petersburg). X-ray phase analysis (XRD) was performed on an ARL X'tra diffractometer (Thermo Fisher Scientific, Switzerland) with a Cu Ka copper anode. The phase composition of the synthesized powders includes zirconium diboride ZrB2, disilicides ZrSi2, MoSi2, as well as minor amounts of elemental Si and monosilicide ZrSi.
Coatings were formed by the method of slip-firing fusion. Square samples with a side of 20 mm and a thickness of 8 mm made of Cf/C and Cf/SiC composites were used as substrates. A solution of colloxylin in amyl acetate and diethyl oxalate was used as a binder in the slip suspension and powders of the ZrSi2-MoSi2-ZrB2 system with a dispersion rate of < 10 p,m were used as the filler. The ratio of binder and powder in the composition was 1:1. Slip layers were applied with a brush on all surfaces and edges of the samples. Drying was performed in an oven at 80°C for 30 min. The firing was carried out in a shaft-type SSHVE-1.2.5/25 I2 vacuum furnace (LLC «OZ VNIIETO», Russia) in a high purity argon (grade 4.8) atmosphere at an operating pressure in the chamber of 0.1-0.2 Pa. A rational temperature-time firing parameters was established: heating to 180°C at a rate of 10°C/min, then to 800°C at a rate of 30°C/min with intermediate 10-min isothermal exposures at 180 and 700°C, and further up to 1660-1680°C with a rate of 50-55°C/min.
Microstructural studies were performed using a scanning electron microscope (SEM) EVO-40 (Carl Zeiss, Germany) equipped with an X-ray energy dispersive spectrometer (EDS) (Oxford Instruments, United Kingdom). Samples for metallography were obtained with precision equipment of the Struers company (Denmark).
By means of XRD, SEM and EDS, it was established that the structure of the formed coatings is represented mainly by the same components as in the initial powder - the ZrSi2 matrix with uniformly distributed MoSi2 and ZrB2 particles in it. In addition to these compounds, there is also a significant amount of highly dispersed ZrC particles with a linear size in the range of 1-3 p,m (the result of the reduction of silicon from ZrSi2 by the reaction C + ZrSi2 ^ ZrC + 2Si) and a small amount of SiC particles (the result of the interaction of silicon with carbon formed during binder decomposition in the process of coating firing). The formation of an intermediate layer between the substrate and the main coating layer with a thickness of 2-3 p,m represented by SiC is observed during the formation of coatings on samples of Cf/C composite. Typical microstructures of coatings on Cf/SiC and Cf/C samples are shown in Fig. 1. The ability of the liquid phase to heal defects in the protected material in particular discontinuities and very deep cracks in the SiC layer has been established.
To compensate the loss of silicon as a result of sublimation under conditions of high-temperature vacuum firing, additional studies have been carried out on applying a thin layer of slurry made of elemental silicon powder over the layer of the base material slip. A typical microstructure of slip coating obtained with the use of an additional layer of silicon slip is shown in Fig. 2. A characteristic distinguishing feature of these coatings is the presence in their structure, along with the above-mentioned constituent elements of elemental silicon and interlayers of highly dispersed ZrSi2 particles in a matrix of silicon, resembling a structure of eutectic type. According to the data of high-quality XRD, the coating includes four phases: ZrB2, MoSi2, ZrSi2, and Si.
(a) (b)
Fig. 1. Microstructure of slip coating formed from ZrSi2-MoSi2-ZrB2 powders on (a) Cf/SiC and (b) Cf/C composite samples.
Fig. 2. Microstructure of slip coating formed from ZrSi2-MoSi2-ZrB2 powders with an additional layer of silicon slip on samples of Cf/SiC composite.
At present, experimental samples with coatings for heat resistance tests under static oxidation conditions in chamber furnaces and for gas-dynamic bench tests under conditions of thermochemical interaction with high-velocity, high-enthalpy oxygen-containing plasma flows have been prepared.
This work was carried out with the financial support from the Ministry of Science and Higher Education of the Russian Federation within the State Assignment (no. 9.1077.2017/PCh).
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