Kinetics Investigation of the Formation of a Gas-Resistant Glass-Forming Layer during the Oxidation of ZrB₂-MoSi₂-Y₂O₃-Al Coatings in the Air Atmosphere

Abstract

In this article, the coatings of ZrB₂-xMoSi₂-Y₂O₃-yAl (x = 24, 35, 45 wt %; y = 10, 15, 20 wt %) were applied to the surface of a carbon/carbon composite to protect against high-temperature oxidation using a multi-chamber detonation accelerator. The kinetic analysis of the formation processes of a glass-forming layer during the oxidation of the initial components of the system ZrB₂-MoSi₂-Y₂O₃-Al in an air atmosphere at a temperature of 1400 °C was carried out and the kinetically significant stages of the heterogeneous reaction were determined. It is shown that the speed and density of the formation of a glassy matrix can be adjusted by fine-tuning the ratio of components in the initial powder.

1. Introduction

The widespread use of composite carbon-containing materials is still significantly hindered by their extremely low or insufficient heat resistance in an oxygenated environment [1]. In vacuum and inert media, carbon materials are operable up to 3000 °C; however, in an air atmosphere, they oxidize or burn out at temperatures of 350–500 °C. Reliable protection against high-temperature oxidation can significantly expand the temperature and time intervals of the use of carbon-containing materials. In most cases, it is the only possible way to realize their heat-resistant characteristics and functional properties. It is possible to prevent the destruction of materials by creating a protective heat-resistant gas-resistant coating on the surface of carbon-containing materials which prevents the access of oxygen to carbon [2,3,4].

One of the most common approaches to choosing the composition of the main layer of a high-temperature coating is the selection of the composition of refractory compounds that are in a viscous plastic state during the operation of the product. This state is achieved by using refractory boron- or aluminum-borosilicate glasses as a matrix in which reinforcing particles of silicides, carbides, and borides (less often oxides and nitrides) of metals from groups IV–VI are additionally introduced to increase the heat resistance. The use of precursors of the ZrB₂–MoSi₂ system is promising for this class of coatings [5,6,7].

Molybdenum disilicide has good resistance at temperatures of 1000–1700 °C due to the formation of a protective layer based on silicon dioxide. According to prior research conducted by Soo-Jin et al. [8], the introduction of MoSi₂ (4–20 wt %) into carbon/carbon compositions has an inhibitory effect on the oxidation of materials. The paper by McKee [9] reports that by adding silica to molybdenum disilicide, elements with low deformation during heating and elements not prone to short circuits due to a solid glassy layer were obtained. Zirconium diboride has a high corrosion resistance, and, as a result of its oxidation, zirconium dioxide is formed—a chemically stable refractory compound. The introduction of ZrB₂ (26, 34 vol %) into the coating makes it possible to increase the resistance of carbon materials to oxidation at temperatures up to 1500 °C [9]. However, the kinetics of reactions of the formation of a protective layer is usually not considered in the literature or is of an estimated, unsystematic nature.

In the present work, a multi-chamber detonation accelerator has been used for preparing ZrB₂-xMoSi₂-Y₂O₃-yAl (x = 24, 35, 45 wt %; y = 10, 15, 20 wt %) coatings on the surface of C/C composites without a sublayer. The kinetic analysis of the processes of formation of the glass-forming layer during the oxidation of the initial components of the coating in the air atmosphere at a temperature of 1400 °C was carried out and the kinetically significant stages of the heterogeneous reaction were determined.

2. Materials and Methods

ZrB₂, MoSi₂, and Y₂O₃ micro-powders (MP “Complex”, Izhevsk, Russia,) were used for applying composite coatings. The powders were sieved, and the working fractions (<20 microns) were selected. Then, to obtain composite powders, ZrB₂ + xMoSi₂ + 5 wt % Y₂O₃ (x = 24, 35, 45 wt %) wet mechanical mixing and homogenization of the initial powders was carried out in a Pulverisette 5 laboratory planetary mill (Fritsch, Idar-Oberstein, Germany) with balls and grinding glasses made of zirconium oxide in alcohol for 24 h at a drum rotation speed of 150 rpm. Then, the charge was removed from the grinding glasses, dried in a drying cabinet (100 °C, 6 h) and wiped through a sieve. All operations were performed in air.

Based on previous work [10,11,12] describing optimal technical conditions for the formation of coatings (

Table 1. Spraying parameters of the coating.

Barrel Length, mm	Barrel Diameter, mm	Powder Feed Rate, g/h	Flow Rate of Fuel Mixture Components, m3/h			Oxygen/Fuel Ratio
			Oxygen	C3H8 + C4H10	Air
500	16	600	4.00 */ 3.60 **	0.75 */ 0.68 **	0.12 */ 0.12 **	5.28 */5.38 **

* Cylindrical form combustion chamber. ** Combustion chamber in the form of a disk.

), coatings from composite ZrB₂-MoSi₂-Y₂O₃-Al micro-powders were formed on the surface of carbon-containing materials using a multi-chamber detonation accelerator (MCDS, IntelMashin LLC, Moscow, Russia) [13,14,15]. Coatings were formed on the substrate surface without intermediate layers in the “carbon-containing materials–coating” system that perform barrier-compensation functions. To increase the wettability of the carbon-containing materials surface, aluminum powder (10, 15 and 20 wt %) was added to the initial powder mixture. Acting as an amorphizer, aluminum oxide reduces the ability to crystallize, and the structure of the formed alloyed glass makes the coating more stable, especially at high temperatures; and, in the technologically advanced formation, it binds to the oxidizer promoting the formation of low-melting and mobile aluminum compounds (stress compensators) when heated [15,16,17].

The phase composition of the coatings was studied using a Rigaku SmartLab diffractometer equipped with a high-temperature Rigaku SHT-1500 (XFA) (Rigaku, Tokyo, Japan) prefix. The prefix is intended for the study of phase transitions and structural changes at high temperatures (up to 1500 °C). The samples were heated to a predetermined temperature at a rate of 10 °C/min, and then the system was thermostated for 30 min to stabilize the temperature and the course of the processes. The range of the diffraction angle 2θ was from 20 to 80° with a step width of 0.02°, a speed of 2 °/min, CuKα radiation, and Ni filter. Phase identification was carried out using the database of powder radiographic standards PDF (JCPDS ICDD), and PDF-2 [18]. The grid was indexed by graphical analysis methods, using PDXL (RIGAKU) programs. The analysis of the kinetics of oxidation of materials was carried out using a combined TGA/DSC/DTA analyzer SDT Q600 (TA Instruments, New Castle, DE, USA) on the change in mass (Δm, %) for a certain time interval of heat treatment, at a temperature of 1400 °C, i.e., Δm = f(t). The change in mass is caused by the formation of non-volatile (Al₂O₃, ZrO₂, SiO₂) and volatile oxides (B₂O₃, MoO₃) as a result of high-temperature oxidation of the initial components.

3. Results

Figure 1 shows the study results of the microstructure of the cross-section of composite coatings ZrB₂-MoSi₂-Y₂O₃-Al before and after high-temperature oxidation.

The coating of ZrB₂-MoSi₂-Y₂O₃-Al after processing was dense. A large number of white particles are distributed in the matrix (Figure 1). The porosity of the coating was about 1.0%. Porosity was determined by the metallographic method with elements of the qualitative and quantitative analysis of the geometry of the pores using an optical inverted Olympus GX51 microscope (Olympus Corporation, Tokyo, Japan). Ten arbitrarily selected micrographs were registered with an optic microscope. Mechanical interlocking is the main adhesion mechanism of the ZrB₂-MoSi₂-Y₂O₃-Al coating to the surface of C/C composites. Fragile materials on the surface of C/C composites (carbon matrix) are destroyed, which leads to the formation of voids and cavities. Discrete ceramic particles are broken and fixed in cavities, and they also provide the creation of a ceramic coating material [19].

As a result of the study using X-ray diffraction analysis (Figure 2) and point microstructural analysis of the ZrB₂-MoSi₂-Y₂O₃-Al coating after high-temperature oxidation (Figure 1c), it was defined that the white particles (globular grains) were ZrO₂, and the light gray vitreous layer was SiO₂ with mullite and zircon in small quantities. The ZrO₂ particles were randomly distributed in a glassy matrix.

Glassy SiO₂ has a low oxygen diffusion coefficient at 1200 °C and can reduce the rate of oxygen penetration through the coating [20,21,22,23].

As a result of the oxidation of coatings, the surface layer is sealed due to the formation of a glass matrix and the distribution of refractory particles (oxides, silicates) in it which contribute to the high heat resistance of coatings. The glassy film formed on the coating surface makes it difficult for oxygen to enter the coating volume, protecting it from further oxidation.

Based on the obtained data on the study of the reactionary high-temperature oxidation in air of composite ZrB₂-MoSi₂-Y₂O₃-Al coatings, it can be assumed that the following processes occur during the high-temperature oxidation.

Oxidation of the initial components with air oxygen by reactions:

$$\begin{gathered} {{\displaystyle ZrB}}_2(s)+{{\displaystyle 2.5O}}_2\to {{\displaystyle ZrO}}_2(s)+{{\displaystyle B}}_2{{\displaystyle O}}_3(l) \\ {{\displaystyle MoSi}}_2(s)+{{\displaystyle 3.5O}}_2\to {{\displaystyle MoO}}_3\uparrow +{{\displaystyle 2SiO}}_2(s) \\ 2Al(s)+{{\displaystyle 1.5O}}_2\uparrow \to {{\displaystyle Al}}_2{{\displaystyle O}}_3(s) \end{gathered}$$

(1)

In this case, there is a partial volatilization of boron and molybdenum oxides.

Sequential dissolution of zirconium dioxide, aluminum oxide and silicon dioxide in liquid boron anhydride to form a single anionic matrix of the melt:

$${{\displaystyle Al}}_2{{\displaystyle O}}_3(s)+{{\displaystyle B}}_2{{\displaystyle O}}_3(l)+{{\displaystyle 2ZrO}}_2(s)+{{\displaystyle 2SiO}}_2(s)$$

$$\to {{\displaystyle kAl}}_2{{\displaystyle O}}_3(s)\cdot {{\displaystyle mB}}_2{{\displaystyle O}}_3(l)\cdot {{\displaystyle nZrO}}_2\cdot {{\displaystyle cSiO}}_2(s)$$

Crystallization of the melt with the release of the phase of zircon ZrSiO₄, zirconium dioxide ZrO₂, silicon dioxide SiO₂ and mullite Al_2.35Si_0.64O_4.82.

The concept of degree transformation of a substance was used as a characteristic of kinetic processes:

$$\alpha \left(t\right)=\raisebox{1ex}{$\Delta m\left(t\right)$}\!\left/ \!\raisebox{-1ex}{$m_0$}\right.$$

(2)

where Δm(t) is the change in the mass of the material of the coating under study during high-temperature oxidation at time t, and m₀ is the mass of the initial material of the coating under study.

Figure 3 and Figure 4 show the characteristic kinetic curves of changes in the mass of the ZrB₂-MoSi₂-Y₂O₃-Al coating material in the coordinates $\alpha =f\left(t\right)$ and $\alpha =f\left(t_{1/2}\right)$. Figure 3 shows that the degree of dependency of transformation on time for all compositions has the same character. Between 100 and 190 min of heat treatment the transformation degree becomes almost unchanged. For all series of samples with a constant Al content, an increase in the MoSi₂ content leads to an increase in the degree of transformation of the substance.

In parabolic coordinates, two independent kinetic sections can be distinguished (Figure 4).

Firstly, the initial components of the coating are oxidized under the conditions of intensive oxygen supply. The glass melt is formed locally around the particles of the initial components. The system is characterized by the biggest contribution of surface energy. The stage of intensive oxidation is completed in the first 15 min of heat treatment. During further heat treatment, the oxidation rate decreases due to the spreading of the oxide melt and the closing of the surface layer. This stage of the process is characterized by a parabolic dependence of the kinetic curves of oxidation and is completed after 100–190 min of heat treatment (Figure 4, Section 1 of the curves). It can be noticed that the completion time of the formation of a dense, gas-tight layer depends on the content of both molybdenum disilicide and aluminum in the initial system. However, this dependence is not linear in relation to the aluminum content. The reason for this behavior of the kinetic characteristics of the formation of a single glassy matrix may be different rates of crystallization of the melt with the release of phases of zircon, zirconium dioxide, silicon dioxide and mullite. Zircon and zirconium dioxide are mainly formed in the samples with minimum aluminum content. The proportion of mullite increases with an increase in the aluminum content in the system.

Furthermore, the oxidation reaction slows down sharply and becomes limited by the rate of diffusive mass transfer of oxygen through a dense ceramic layer that has formed a solid gas-tight coating (Figure 4, Section 2 of the curves). A slight change in the mass of the sample during high-temperature oxidation in air is due to the slow evaporation of molybdenum oxide, which is indirectly confirmed by the results of the study of the evolution of the phase composition during high-temperature annealing in an oxidizing environment and the results of synchronous thermographic analysis (TGA) or differential scanning calorimetry (DSC).

The oxidation reaction rate constants which allow us to describe the first stage of the process within the framework of the model on Figure 4 are:

$$\alpha \left(t\right)={\alpha }_0+\kappa \cdot t^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$

(3)

where α(t) is the transformation degree at a time t, α₀ is the transformation degree in the initial period of formation of a gas-tight layer, and k is the reaction rate constant characterizing the first stage of the process.

It can be seen that the first stage of the process is well described by the parabolic dependence. With an increase in the content of MoSi₂ in the samples of ZrB₂-xMoSi₂-5Y₂O₃-yAl (where x = 24, 35, 45 wt %) with a constant content of Al, the rate constant of the chemical reaction of the formation of a gas-tight coating increases. There are minimal and contradictory data in the literature on the kinetics of spreading of an oxide melt [17]. Additionally, in the case of the formation of coatings, the processes occurring are complex and multicomponent. The wetting ability of the melt is a complex physical and chemical characteristic, to a certain extent, depending on the concentration of glass-forming components, the structure and morphology of the starting material.

4. Conclusions

This study described the kinetics of the formation of a gas-tight glass-forming layer during the oxidation of the components of a ZrB₂-xMoSi₂-5Y₂O₃-yAl system (where x = 24, 35, 45 wt %; y = 10, 15, 20 wt %) in an air atmosphere. The processes of formation of a single vitreous matrix are fundamental in the formation of dense, gas-tight coatings. It was established that the rate constant of the chemical reaction of the formation of a gas-tight coating increases with an increase in the MoSi₂ content, at a constant Al content, which is accompanied by an increase in the degree of transformation of the substance. Thus, the speed and density of the formation of a vitreous matrix can be adjusted by fine-tuning the ratio of components in the initial powder.