STRUCTURAL-PHASE STATE OF NANOCOMPOSITE ZrB2-MoSi2 COATINGS FOR CARBON/CARBON COMPOSITES DEPOSITED BY A NEW MULTI-CHAMBER DETONATION ACCELERATOR

The ZrB2-MoSi2 coating modified by Y2O3 and Al was prepared on a carbon/carbon composite substrate by a new multi-chamber detonation accelerator (MCDS). The thickness of deposited coatings ranged about 90-130 m. The powder containing 41 mol.% of ZrB2, 5 mol.% of MoSi2, 1 mol.% of Y2O3 and 53 mol.% of Al (1-25 μm) has been used as the feedstock material to deposit a dense layer. The structural-phase state of the coatings was characterized by X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy. The ZrB2-MoSi2 coating was well-adhered with carbon/carbon composites. The coating displayed compact microstructure with porosity lower than 1%. It was established that phase composition of ZrB2-MoSi2 powder under the influence of high temperatures and the atmosphere of detonation products changes with the formation of a complex heterogeneous structural-phase state in the coatings. As-sprayed coating consisted of t-AlZrO2, fcc-Al, m-ZrO2 and m-SiO2. ZrB2-MoSi2 coatings are characterized by the presence of nanodispersed particles. The most of the powder particles were melted and formed lamellar-like structure typical for thermally sprayed coatings. But small amount of partially-melted areas has non-uniform structure, ranging from lamella to sphere with a size range of 0.05-1.5 μm.


INTRODUCTION
Carbon-carbon composites are the most potential materials for high temperature applications due to its excellent mechanical and thermosphysical properties (low density, low coefficient of thermal expansion and improved mechanical properties at high temperatures above 2000 °C) [1][2][3]. However, in spite of its attractive mechanical and thermal properties at high temperatures, their poor resistance to an oxidizing atmosphere beyond 500ºC limits their performances as the high-temperature structural materials. One of most widely used methods for improving carbon-carbon composites oxidation resistance is to apply such coatings as MoSi2, ZrSiO4, SiO2-SiC, ZrB2-SiC [4][5][6][7]. A new multi-chamber gas-dynamic accelerator (MCDS) was proposed to spray ZrB2-MoSi2 coatings in this study [8][9][10]. ZrB2-MoSi2 nanocomposite coating containing 1 mol.% of Y2O3 and 53 mol.% of Al. Including silicide into ZrB2 can significantly promote the oxidation protection ability of ZrB2 coating because of a stable compound silicate glass layer formation [11]. Y2O3 was successfully used as a stabilizer of high temperature tetragonal and cubic modification of zirconia [12]. Aluminum was added as a bond for the oxidizing agent during of the spraying and creating "plastic" lamellae and nano-dispersed inclusions, which will relieve internal stresses. The microstructure, elemental and phase composition of coating were investigated in the present work. https://doi.org/10.37904/nanocon.2019.8657

Materials
The powder containing 41 mol.% of ZrB2, 5 mol.% of MoSi2, 1 mol.% of Y2O3 and 53 mol.% of Al (1-25 μm) (Figure 1) has been used as the feedstock material to deposit a dense layer on the carbon/carbon composites substrate.

Apparatus and Procedure
Vertically mounted, multi-chamber, gas-dynamic accelerator (MCDS) with a barrel length of 500 mm was employed to deposit the ZrB2-MoSi2 nanocomposite coatings in this study. The realized detonation regime of the combustion of the gas mixture in two chambers has a special profile in MCDS. The automated equipment (Figure 2) consists of: 1 -device for spraying, 2 -standard powder feeder with a feed rate of up to 3 kg/h, 3a standard low-pressure (max. 0.3 MPa) gas panel, 4 -an automated control system for the technological process, 5 -an automated manipulators for moving MCDS and 6 -a specimen holder.

Figure 2
Equipment for deposition of the ZrB2-MoSi2 coating.
The spray parameters used for deposition of ZrB2-MoSi2 coating are listed in Table 1. The cross-section of the coating was polished using abrasive SiC paper and diamond suspension to prepare the samples. The morphology of the polished cross-section of the coating was investigated by scanning electron microscopes (SEM) FEI Nova NanoSEM 450 and FEI Quanta 600 FEG with energy-dispersive spectroscopy (EDS). Porosity of the composite coating was measured by the metallographic method with elements of the qualitative and quantitative analysis of the pores geometry by using an optical inverted Olympus GX51 microscope. X-ray analysis was performed by the Rigaku Ultima IV diffractometer. Crystalline phases were identified by the ICDD PDF-2 (2008) powder diffraction database.

RESULTS AND DISCUSSION
The cross-sectional morphologies and elemental composition of the ZrB2-MoSi2 nanocomposite coating are illustrated in Figure 3. The thickness of deposited coatings ranged from 90 to 130 m. ZrB2-MoSi2 coating has uniform and dense microstructure with porosity lower than 1%, and has a good adhesion to the substrate. The cross-section images show two areas with different types of structure (Figure 4). The most of the powder particles were melted. It was found that the shape of the fully-melted area has typical for thermally sprayed coatings lamellar-like structure (Figure 4a). A partially-melted area consisting of unmelted non-uniform particles changes from lamella to sphere with a size ranged from 0.05 to 1.5 μm. (Figure 4b). It can be seen that the powder was consisted of hexagonal ZrB2, tetragonal MoSi2, and cubic Y2O3 and Al phases (Figure 5a). Reported powders could be melted quickly and some part of powders react with oxygen in the air during the spraying process, Tetragonal AlZrO2, cubic Al, monoclinic ZrO2 and monoclinic SiO2 were identified in the coating (Figure 5b). It was clearly shown that melted or semi-melted ZrB2 and MoSi2 particles reacted with oxygen in the surrounding environment during spraying process, led to formation of AlZrO2, ZrO2 and SiO2. It should be noted that MoO3(s) vaporizes at the same time as it is produced while MoO3(g) possesses high vapor pressures at high temperatures.

CONCLUSION
Multi-chamber detonation accelerator (MCDS) was applied for deposition of the ZrB2-MoSi2 nanocomposite coating on carbon/carbon composites in this study. MCDS has provided conditions for formation of dense and uniform coating layers. These coatings have a lamellar-like structure typical for thermally sprayed materials with a small amount of partially-melted areas and non-uniform structure ranged from lamella to sphere with an average size from 0.05 to 1.5 μm. The melted powder partialy react with oxygen in the air during spraying led to formation of AlZrO2, ZrO2 and SiO2. The results of this work open up new prospects for the further elaboration of new technologies for manufacturing protective coatings able to enhance the properties of carbon/carbon composites an oxidizing atmosphere at high temperatures.