ZrSi₂-SiC/SiC Gradient Coating of Micro-Structure and Anti-Oxidation Property on C/C Composites Prepared by SAPS

Abstract

A ZrSi₂-SiC/SiC gradient coating system was designed to reduce the thermal stress of anti-oxidation coatings for C/C composites and prolong their anti-oxidation time at a high temperature. The SiC transition layer was prepared by pack cementation and the gradient ZrSi₂-SiC outer coatings with different ZrSi₂ contents added were deposited by supersonic air plasma spraying (SAPS). The micro-morphologies and phase compositions of the coatings were studied by SEM, EDS, XRD, and TG/DSC, and their anti-oxidation performances were tested by a static oxidation experiment. The findings suggested that the gradient coating with 30 wt.% ZrSi₂ content displayed the optimum and dense microstructure without obvious pores and microcracks, compared with the other three proportional coatings. During the oxidation test, because of the oxidation reaction of ZrSi₂ and SiC phases, a large amount of silica was formed in the coating to fill the pores and microcracks and densify the coating further. Oxidation products ZrO₂ and ZrSiO₄, having a high melting point and outstanding anti-oxidation property, were embedded in the SiO₂ glass layer to reduce the layer volatilization rate and improve the ability to block oxygen. Therefore, the specimen with 30 wt.% ZrSi₂ still kept mass gain after 188 h oxidation time at 1500 °C. However, when the oxidation time was increased to 198 h, it had a mass loss of 0.1%, because the coating compactness was destroyed by the escape of the oxidation gases.

1. Introduction

Outer coating technology is an effective method to improve the oxidation resistance of C/C composites in a high temperature aerobic environment in air [1,2,3]. The core of the technology is to choose reasonable coating materials, which generally must have excellent anti-oxidation property, mechanical capacity, and low oxygen permeability at a high temperature [4,5]. Among them, SiC ceramic has been successfully applied to the anti-oxidation coating for C/C composites, because it can not only meet the above-stated requirements, but also generate dense oxidation film SiO₂ at middle and high temperatures on the coating surface [6,7,8]. SiO₂ film could effectively prevent oxygen from entering C/C composites for a long time and then prolong their high-temperature service life [9,10,11]. However, with the rapid development of aerospace technology, an SiC single coating has not been able to protect the C/C composites from being oxidized in extreme environments [12,13,14]. In order to further improve the oxidation resistance of the C/C composites, some materials possessing low volatility, high melting point, and thermal stability, such as ZrB₂ [15], ZrC [16], TaC [17], YSZ [18], and so on, are added into the SiC coating to optimize its microstructure and enhance the interfacial bonding strength between anti-oxidation coatings and the C/C composites. Related studies show that the anti-oxidation abilities of these coating systems still gradually lead to failure; the major reasons involve two aspects [19,20,21]. First, the mismatched values of the coefficient of thermal expansion (CTE) between C/C substrates and coating could lead the generation of microcracks in the coating or even debonding from the substrate, especially when the coating experiences sudden temperature changes. Second, slow evaporation of an inadequate SiO₂ dense layer formed on the coating surface at a high temperature might destroy the compactness of the whole coating as the service time of C/C composites is added, which provides the opportunity for oxygen to access the substrate through the defects of the coating [22]. Therefore, the key to solving the two problems is that CTE values of coating materials agree well with the values of SiC and C/C composites, and that they could produce more SiO₂.

Silicon-based ceramic materials, such as MoSi₂, CrSi₂, ZrSi₂, and so on, have high strength, moderate CTE values, and excellent anti-oxidation properties. They are widely applied to the oxidation resistance coatings for C/C composites because of the formation of metallic oxide with a high melting point and SiO₂ glass phase with a low oxygen permeability coefficient [23,24,25]. Researchers found in the study of the oxidation behavior of ZrSi₂ that it started to oxidize at 500 °C, and its mass growth rate was near to 45% because of the formation of oxidation products ZrO₂, SiO₂, and Si, when the temperature was increased to 900 °C. Furthermore, as the temperature was increased to 1420 °C, obvious mass growth appeared again, attributed to the production of ZrSiO₄ from the reaction of ZrO₂ and SiO₂ [26]. That is to say, if ZrSi₂ is chosen as the anti-oxidation coating for C/C composites, flowing SiO₂ and Si oxidation products could fill the pores and microcracks of the coating to prevent oxygen diffusing inward to the substrate at a low temperature (500–1000 °C). Meanwhile, flowing SiO₂ and Si can be used as a penetration enhancer to bring ZrO₂ into the internal coating, together with ZrSiO₄ (${\alpha }_{{\mathrm{ZrSiO}}_4}=4.9\times {10}^{-6}{\mathrm{K}}^{-1}$), which has a high melting point and a low CTE value close to that of SiC (${\alpha }_{\mathrm{SiC}}=4.5\times {10}^{-6}{\mathrm{K}}^{-1}$), which could be able to form a “nail” structure that inhibits the further grow of microcracks and relieves the thermal mismatch of the coating [27,28].

Therefore, in this paper, the ZrSi₂-SiC/SiC gradient coating system was designed to improve the oxidation resistance of the C/C composites, in order to tackle the poor dense and thermal mismatch of the existing coatings’ problems. The research contents were as follows. First, an SiC transition layer was prepared on the surface of C/C composites by pack cementation. Second, different proportional ZrSi₂-SiC outer coatings were deposited onto the surface of the SiC coating by supersonic air plasma spraying (SAPS), which could form a coating with denser and higher interfacial bonding strength compared with the traditional plasma spraying technology, as the central temperature of plasma arc of SAPS can be up to 10,000 °C and its spraying velocity can reach 600 m/s [29]. Third, the oxidation resistance and mechanism of the ZrSi₂-SiC/SiC coatings at 1500 °C were discussed. The microstructures, phase compositions, and distributions before and after oxidation tests were also studied.

2. Materials and Methods

2.1. Anti-Oxidation Coating Preparation

Cubic blocks (10 mm × 10 mm) used as substrates were cut from bulk 2D C/C composites, the density of which was 1.72 g/cm³. These blocks needed to be polished first by 400 grit SiC papers, then impurities were removed by means of ultrasonic cleaning in ethanol and they were finally dried at 80 °C for 2–3 h in an oven. The ZrSi₂-SiC/SiC gradient coating was made in two steps. Firstly, the SiC transition coating was formed on the C/C substrate surface by pack cementation. The related mixed powders and their proportion were 65%–85% Si, 10%–20% graphite, and 5%–15% Al₂O₃, and the detailed preparation processes are reported in the literature [29]. Secondly, the ZrSi₂-SiC outer coatings with different ZrSi₂ ratios (10 wt.%, 20 wt.%, 30 wt.%, and 40 wt.%) were deposited onto the SiC transition coating surface by SAPS. Before preparation, four kinds of mixed ZrSi₂-SiC powders were blended with PVA aqueous solution to form slurry, and then were carried into a centrifugal spray dryer to form ZrSi₂-SiC particles with fine fluidity for spraying. The related plasma spraying parameters are listed in

Table 1. Spraying parameters of the ZrSi₂-SiC coatings prepared by SAPS.

Parameters	Values
Spraying power (kW)	40
Main gas flow (Ar), L/min	75
Carrier gas (Ar), L/min	10
Secondary gas (H2), L/min	5
Power feed rate (g/min)	20
Spraying distance (mm)	100
Nozzle diameter (mm)	5.5

and [29] can provide a more detailed preparation process.

2.2. Anti-Oxidation Property Test

The oxidation resistance experiments were carried out in an electrical furnace in air at 1500 °C. Before the test, the furnace was heated up to 1500 °C until the temperature remained stable. The specimens were divided into four groups and placed into the furnace respectively according to the different proportion of ZrSi₂. After a given time, they were taken out directly and cooled from 1500 °C to RT in air. Meanwhile, the specimens also underwent thermal shock resistance evaluation. Their mass changes were measured by an electronic balance with a sensitivity of ±0.1 mg. The mass loss and oxidation time curves of the four kinds of specimens were used to evaluate their anti-oxidation behaviors. The weight loss rates could be calculated according to Formula (1):

Δw = (m₀ − m₁)/m₀

(1)

where Δw is the oxidation weight loss rate of the specimen; m₀ is the mass of the specimen before oxidation; and m₁ is the mass of the specimen after oxidation.

2.3. Microstructure and Phase Composition Analysis

The crystalline structures of specimens were obtained by X-ray diffraction (XRD, Bruker D2 Phaser, BRUKER, Karlsruhe, Germany with Cu Kα radiation in the range of 0–90° [30]. A scanning electron microscope (SEM, JSM-6460, JEOL Ltd., Mitaka, Japan) equipped with energy dispersive spectroscopy (EDS) was used to study the morphologies and element distributions of the specimens [31]. The thermogravimetric (TG, METTLER, Zurich, Switzerland) test was carried out on a Metter Toledo Star TGA/SDTA 851 thermal analyzer in air from room temperature to 1500 °C with a heating rate of 10 K/min.

3. Results and Discussion

The XRD pattern of the 20 wt.% ZrSi₂-80 wt.% SiC mixed powders is shown in Figure 1. The result indicates that the powders did not have any other impurity. Figure 2 shows the SEM images and EDS results of the 20 wt.% ZrSi₂-80 wt.% SiC particles prepared by the centrifugal spray dryer. The particles with a spherical shape with good mobility were able to be uniformly melted during the spraying process to form a denser coating on the SiC surface. Particles included mainly C, Zr, and Si elements, and were consistent with preformed coating components. O element was incorporated into the particles in the process of centrifugal spray granulation.

Figure 3 shows the SEM surface images of the four ZrSi₂-SiC coatings with different proportions of ZrSi₂. The distinct morphology features in every image are obvious. From Figure 3a–d, the coatings changed from porous to dense and then to porous, accompanied by the change in visible pores. The porosities of the ZrSi₂-SiC outer coating with different ZrSi₂ contents were analyzed by means of Image-Pro Plus software. Their porosities were 18.9%, 11.7%, 10.6%, and 15.4%, respectively. In Figure 3a, some big grooves can be seen in the coating surface, indicating that most of the flying ZrSi₂-SiC mixed particles were blown away by plasma jet and not deposited onto the substrate. The main reason was that, because the density of SiC was lower than that of ZrSi₂ ($\mathsf{\rho }$ = 4.88 g·cm⁻³), the flying velocity of SiC particles was higher than that of ZrSi₂ after ejecting from the nozzle. Therefore, the mixed particles with a greater SiC content did not melt sufficiently and produced a splash upon impact with the substrate surface [26]. With the increase in the ZrSi₂ content in the ZrSi₂-SiC particles, their flying velocity could decline and their deposition rate could increase. Therefore, the ZrSi₂-SiC coating presented a dense and lamellar structure in Figure 3c. However, when the content of ZrSi₂ was increased to 40 wt.%, it could be excessively oxidized by oxygen to form more ZrO₂ and ZrSiO₄ compared with other three samples, according to the XRD results of Figure 4. These oxidation products were embedded in the coating in the form of granules, leading to the decrease in the interfacial compatibility between the coating and the products and the formation of more pores in the coating in Figure 3d. The related oxidation reactions are as follows:

ZrSi₂ (s) + O₂ (g) = ZrO₂ (s) + SiO₂ (s)

(2)

ZrO₂ (s) + SiO₂ (s) = ZrSiO₄ (s)

(3)

In order to prove the above conclusions, Figure 5 shows the cross-section micro-topographies of the four kinds of the ZrSi₂-SiC coatings with different ZrSi₂ contents. In Figure 5a,b, it can be seen that the particle interfaces were clearly visible, proving that the particles in these two coatings were not well melted. The widths of the thin areas in the two images were only about 2 μm and 4 μm, respectively. Fortunately, when the content of ZrSi₂ was up to 30 wt.%, the coating with an average width of 25 μm featured a lamellar and dense structure owing to fully melted ZrSi₂-SiC particles. However, resulting from having more refractory oxidation products, the grainy structure appeared again in the coating, accompanied by decreases in the coating thickness, as shown in Figure 5d. In addition, obvious microcracks were not found in the four images, signaling that the gradient coating structure was helpful to form a thermal gradient in the coating to alleviate thermal stress coming from the cooling process from a high temperature to RT. The EDS results of line scanning for the whole cross-section of the sample in the Figure 6 agree with the XRD analysis in Figure 4. The main elements included C, O, Si, and Zr, with no other impurity.

Figure 7a illustrates the static oxidation curves of the four samples at 1500 °C. It can be seen that, as the oxidation time increased, the curves of the four samples all sharply decreased at first, and then slowly increased. In the initial stage, named as the mass gain phase, when the whole coating surfaces were exposed to a high temperature oxidizing environment, they could quickly oxidize with oxygen to form many oxidation products. According to the XRD result of the coating with 30 wt.% ZrSi₂ content in Figure 7b, after 21 h oxidation at 1500 °C, it could be confirmed that the increasing mass of the four kinds of the samples was attributed to the formation of ZrO₂, SiO₂, and ZrSiO₄, and the related reactions are (2), (3), (4), and (5). According to the previous relevant results, because of quickly being oxidized and generating Si, ZrO₂, and SiO₂, the mass growth rate of pure ZrSi₂ was up to near 45% at 900 °C, and it further increased owing to the formation of ZrSiO₄ when the oxidation temperature increased to 1420 °C [26]. That is to say, the greater the ZrSi₂ content in the coating, the more oxidation it produces. Therefore, as the oxidation time increased to 21 h, the mass gain of the specimen with 40 wt.% ZrSi₂ content was 2.34%, which was more than that of the other three specimens.

SiC (s) + 2O₂ (g) = SiO₂ (s) + CO₂ (g)

(4)

2SiC (s) + 3O₂ (g) = 2SiO₂ (s) + 2CO (g)

(5)

With the increase in oxidation time, the mass losses of all specimens remained negative for a while, among which the sample with 30 wt.% ZrSi₂ content maintained negative when the oxidation time reached 188 h. In order to explain this phenomenon, the surface morphology and elementary compositions of the sample with 30 wt.% ZrSi₂ content after 92 h oxidation were analyzed by SEM and EDS, and the results are shown in Figure 8. It can be seen that the smooth gray phase could properly cover the whole surface, fill the pores of the coating, and effectively prevent oxygen from entering the coating, which was the main reason to improve the oxidation resistance of the sample. The gray phase marked as Spot B was made of O; Si; and a small amount of Zr, Al, and Au elements, thus it could be concluded that the gray phase was mainly SiO₂ phase. Al from the SiC transition layer and Au from the SEM test could be ignored. The light gray phase marked as Spot A contained O, Zr, and Si elements, which had ZrO₂ and ZrSiO₄, and a small amount of SiO₂ covering ZrO₂. Therefore, first, the SiO₂ phase could dense the outer coating to keep oxygen out by means of its good fluidity and low viscosity at 1500 °C; second, although SiO₂ had a certain volatility at a high temperature, granular ZrO₂ and ZrSiO₄ were embedded, which could greatly reduce its losses; third, the gradient coating system could effectively reduce CTE mismatch between the SiC layer and outer layer to avoid cracking. The effective combination of the above three factors was able to substantially protect the specimen for a long time from being oxidized. However, CO₂ and CO gases were produced by oxidation in the coating in the initial stage, and could break through the silica layer and leave varying pores, as seen in Figure 9. These pores provided the opportunity for oxygen to enter the coating to react with the substrate, finally leading to the anti-oxidation failure of the coating. When the oxidation time was increased to 198 h, the weight loss of the sample with 30 wt.% ZrSi₂ content was 0.1%. In addition, the coating still appeared dense and smooth without obvious microcracks after the oxidation test. The XRD results of the gray phase and light gray phase in Figure 9, marked as Spot C and Spot D, respectively, are consistent with those of Figure 8. It is demonstrated that the main reason for anti-oxidation failure was the formation of the pores to provide channels to oxygen into the coating.

In order to further research the physical and chemical reactions of the ZrSi₂-SiC coating at a high temperature, the DSC/TG results are shown in Figure 10. From the TG curve in Figure 10, it can be found that the weight of the coating decreased by 1.65% from 180 °C to 430 °C. Corresponding to the DSC curve, there was an obvious absorption peak at 273 °C. The main reason for this was the volatilization of the residual binder like PVA coming from the preparation process of the ZrSi₂-SiC particles. With the increase in the oxidation temperature, the DSC curve tended to be stable. When the temperature was increased to about 519 °C, ZrSi₂ started to oxidize at a low rate, which could be proved by a slightly ascendant peak in the TG curve and a weak exothermic peak in the DSC curve. At the same time, SiC was oxidized to generate CO₂ and CO gases. When the oxidation rate of SiC was higher than that of ZrSi₂ and it had a larger proportion in the ZrSi₂-SiC coating, the sample would experience weight loss. Therefore, the TG curve had a slight decrease with the weight loss of 0.07% at 730 °C. With the temperature increase, as the oxidation reaction of ZrSi₂ was dominant, the production rate of its products was higher than the volatilization rate of the gases. Therefore, the sample experienced weight gain of 5.89%, corresponding to the increase in the TG curve. DSC shows an absorption peak at 1205 °C, which could be identified as the formation of ZrSiO₄.

Figure 11 illustrates the cross-section morphology and element distribution of the specimen after oxidation for 198 h at 1500 °C. Although the sample had a 0.1% weight loss, its cross section presented a dense microstructure and had no obvious microcracks and pores. Compared with its thickness before the oxidation test in Figure 5c, it can be found that the ZrSi₂-SiC outer coating was thicker after oxidation up to about 100 μm. The reason may be that the SiO₂ layer and oxidation products were formed. Meanwhile, the outer coating and SiC coating had a strong interfacial combination without microcracks and disbonding, suggesting that the gradient anti-oxidation coating system largely alleviated the thermal stress to improve its thermal shock resistance. According to the EDS results, the SiC transition coating had a low O element content, proving that SiO₂ layer consumption mainly destroyed the compactness of the outer coating, resulting in the failure of its oxidation resistance. Meanwhile, from the XRD result of the outer coating surface in Figure 12, it can be seen that the surface contained mainly ZrO₂, further proving the above conclusion.

4. Conclusions

Four kinds of compositional gradient ZrSi₂-SiC/SiC coating systems with different ZrSi₂ contents were prepared by combined pack cementation and supersonic air plasma spraying. The ZrSi₂-SiC coating with 30 wt.% ZrSi₂ content possessed an optimum microstructure compared with the other three contents. Its morphologies of the surface and cross section appeared to be denser and had no obvious defects. This illustrated that ZrSi₂-SiC mixed particles in this proportion were able to be melted fully and by evenly deposited during the process of deposition, which could generate a compact lamellar structure owing to their flattening when impacting the SiC coating surface. Meanwhile, the gradient coating system could effectively reduce the CTE mismatch between the SiC coating and the outer coating to improve their interfacial bonding strength. The compositions included ZrSi₂, SiC, and a small amount of oxidation products, proving that the particles were oxidized slightly in the process of plasma spraying. In the static oxidation test at 1500 °C, a large amount of glass phase SiO₂ was produced on the coating surfaces and filled with their defects, which could fundamentally prevent oxygen from entering the coating. Granular ZrO₂ and ZrSiO₄ oxidation products pinned in the SiO₂ layer could slow down its volatilization rate. The gradient coating was designed to visibly enhance its thermal shock resistance to avoid cracking. Therefore, the coating with 30 wt.% ZrSi₂ content still kept gaining weight after oxidation for 188 h. However, because of the inevitable consumption of SiO₂ at 1500 °C and the detached film resulting from the escape of the oxidation gases, the coating failed to protect the sample from oxidizing. Finally, it had a weight loss of 0.1% when the oxidation time was increased to 198 h.