C composites via plasma spraying technology

Journal Pre-proof Ablation mechanism and properties of SiO2 modified ZrB2-SiC coatings fabricated on C/C composites via plasma spraying technology

Sun Shijie, Ma Zhuang, Liu Yanbo, Liu Ling, Wang Fuchi, Luan Xingtao PII:

S0257-8972(19)31123-5

DOI:

https://doi.org/10.1016/j.surfcoat.2019.125132

Reference:

SCT 125132

To appear in:

Surface & Coatings Technology

Received date:

6 December 2018

Revised date:

2 April 2019

Accepted date:

2 November 2019

Please cite this article as: S. Shijie, M. Zhuang, L. Yanbo, et al., Ablation mechanism and properties of SiO2 modified ZrB2-SiC coatings fabricated on C/C composites via plasma spraying technology, Surface & Coatings Technology (2019), https://doi.org/10.1016/ j.surfcoat.2019.125132

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© 2019 Published by Elsevier.

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Ablation mechanism and properties of SiO2 modified ZrB2-SiC coatings fabricated on C/C composites via plasma spraying technology Sun Shijie1, Ma Zhuang1*, Liu Yanbo1*, Liu Ling1, Wang Fuchi1, Luan Xingtao2 1

School of Materials Science and Engineering, National Key Laboratory of Science and Technology on Materials

under Shock and Impact, Beijing Institute of Technology, Beijing 100081, China 2

China ordnance industrial standardization research institute, Beijing 100089, China

*Authors to whom correspondence should be addressed: E-mail address: [email protected] (Ma Zhuang); [email protected] (Liu Yanbo) Telephone: +86-10-68911144

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Fax number: +86-10-68911144

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Abstract: To improve the ablation resistance performance of carbon/carbon (C/C) composites at

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high temperature, SiO2 modified ZrB2-SiC (ZSS) coating with 4 different SiO2 contents was prepared on SiC-Al2O3-coated C/C composites by plasma spraying technology. And the ablation behaviors of ZSS protective coatings was investigated at 1400°C, 1700°C and 1800°C using oxyacetylene torch. The results show that the adding 30 vol.% SiO2 could visibly improve the densification of ZrB2-SiC coatings. However, the ZSS coating with 10 vol.% SiO2 can effectively protect C/C composites for 600s at 1800 C with the ablation mass rate of 3.4×10-4 g/s. The ZSS coating with 30 vol.% SiO2 cracked at the center of ablation region after the same ablation condition because of breakthrough of gaseous by-products inside coating. It is revealed that SiO2 could successfully prevent the active oxidation of SiC by breaking the interconnected SiC network and reducing its chances of contact with oxygen, and a discontinuous SiC consumption layer formed after ablation at 1700°C. Keywords: SiO2; ZrB2-SiC coating; ablation; plasma spraying technology

1. Introduction

Applying the ultra-high temperature ceramics (UHTC) coating on C/C composites is an effective method to improve the ablation resistance of C/C composites in high-temperature oxidizing working environments [1-6]. In recent years, various UHTCs have been tried like ZrC, HfC, TaC and ZrB2[7-10]. Among them, Zirconium diboride (ZrB2) is a promising thermal protection material owing to its high melting temperature (>3000K), low theoretical density (6.09 g/cm3), high thermal conductivity (65–135 W/mK), and good resistance to oxidation [11,12]. At temperatures above 1400 C, however, the oxidation of ZrB2 is very poor due to the volatilization of B2O3 and a porous, non-protective ZrO2 layer [13,14]. Studies have shown that adding SiC can significantly improve the oxidation resistance of ZrB2-based ceramics in the moderately high temperature range by promoting the formation of borosilicate glass. ZrB2-SiC coating exhibits good ablation resistance at high temperature to protect C/C composites [15,16]. Many methods, including plasma spraying[11,17,18], vapor silicon infiltration (VSI) [19], precursor infiltration and pyrolysis (PIP)[20] and pack cementation [21-23] have been explored to produce ZrB2-based coatings. Compared with other processes, plasma spraying is a versatile and efficient technique for the substrates with different sizes and shapes. Moreover, plasma spraying has enough energy to 1

Journal Pre-proof process a broad variety of coating materials, especially for refractory ceramics. Therefore, plasma spraying is a suitable mean of preparing ultra-high temperature ceramic coatings. Currently, the plasma-sprayed ZrB2-SiC coating has three challenges. Firstly, the mismatch of thermal expansion coefficient between coating and C/C composites. The coefficient of thermal expansion (CTE) of ZrB2 (5.9×10-6/C) is much higher than that of C/C composites (1× 10-6/C)[24,25]. Although the addition of SiC (4.5×10-6/C) [26] to some extent can mitigate the mismatch problem, the mismatch of CTE still exists between coating and C/C composites, which may cause cracks, or even peel-off. Secondly, the ZrB2-SiC coating fabricated by plasma spraying has relatively poor densification, relatively loose structure, which reduces the anti-oxidant effect (formation of oxygen channels) and the erosion resistance [27]. Lastly, active oxidation of SiC

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occurred inside the coating during ablation. When temperature is higher than 1600C, most of the borosilicate evaporated resulting in porous ZrO2 layer. This porous ZrO2 layer could act as thermal barrier to preserve SiO2 melted phase in the subsurface layer and further form a dense ZrO2-SiO2 layer. The dense ZrO2-SiO2 layer could prevent the inward diffusion of oxygen. And the low partial pressure of oxygen under the ZrO2-SiO2 layer causes the active oxidation of SiC resulting in the appearance of a SiC-depleted layer under this dense ZrO2-SiO2 layer. [28-30]. It should be noted that SiC particles are homogenously dispersed within the coating after preparation by plasma spraying, which will form an interconnected network in three dimensions in the coating. And the degree of network increases with increasing SiC content. Consequently, a high degree of open pore channels are formed due to active oxidation of SiC, which will cause the failure of the ZrB2-SiC coating[30,31]. Therefore, it expects that solving these challenges could improve the anti-ablation performance of the ZrB2-SiC coating. In the previous works, many available methods has been attempted to solve those problems, such as preparing a suitable transition layer, seeking a proper coating deposit technology, adjusting the coating materials[22,23,31,32]. However, most of the methods just can solve partial above problems. So in order to comprehensively solve these problems, the addition of SiO2, as third component, is tried in present study to promote densification of the coating, reduce the mismatch of CTE, and prevent the active oxidation of SiC. Because SiO2 is one of the ablation resistance ceramics with low density, low coefficient of thermal expansion (0.5×10-6/C), slow oxygen diffusion coefficient and excellent high-temperature properties [33,34]. And raw material SiO2 could melt directly to seal cracks effectively and reduce holes and bubbles generated in the coating in the intermediate temperature range. And SiO2 glassy phase can act as a bonding phase to improve the plastic deformation capability of ZrB2-SiC powders during plasma spraying deposition. In addition, the ZrO2 formed by the oxidation of ZrB2 can react with SiO2 to form highly thermal stable ZrSiO4, which could reduce the consumption of SiO2 and improve the oxidation resistance[35,36]. In this paper, the ablation behaviors of the as-sprayed ZrB2–SiC–SiO2 coatings were evaluated using oxyacetylene torch. The effects of SiO2 contents on the ablation behavior was determined by investigating the microstructure as well as the variation of weight after ablation. Meanwhile, the effects of different ablation temperatures on the ZrB2-SiC-SiO2 coatings have been deeply studied from 1400 C to 1800 C as well.

2. Experimental 2.1 Preparation of ZrB2-SiC-SiO2 coating for SiC-Al2O3 coated C/C composites 2

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Two-dimensional C/C composites with a density of about 1.7 g/cm3. (Hunan Jiuhua Carbon Hi-Tech Co., Ltd.) were used as substrate with 10mm×10mm×5mm and Φ25.4mm×6mm for coating analysis and ablation testing, respectively. The SiC-Al2O3 bonding layer (SA) was prepared by plasma spray and the details have been reported in our previous literature[37].The outer ZrB2-SiC-SiO2 coatings (ZSS) were prepared on the SiC-Al2O3-coated C/C composites by Praxair GTS-5500 atmospheric plasma spraying system, and the operating parameters are summarized in Table 1. Commercially available ZrB2 powder (1～3m; purity>99.9%; China New Metal Materials Technology Co., Ltd., China), SiC powder (0.5～1.5m; purity>99.9%; China New Metal Materials Technology Co., Ltd., China) and amorphous SiO2 powder (1～3m; purity>99.9%; China New Metal Materials Technology Co., Ltd., China) were chosen as raw materials to agglomerate 4 kinds of ZrB2-SiC-SiO2 powers in this work (Table 2). The volume ratios of ZrB2 to SiC in these powders remained constant. Before spraying process, the agglomerated ZrB2-SiC-SiO2 powers were treated using induction plasma spheroidization system (IPS, PL-35, TEKNA Plasma Systems Inc., Canada) to ensure flow ability and reduce defects. Those IPS-treated particles with particle size of 30～80 m were sieved and used as the feedstock powders for APS. 2.2 Ablation test Ablation behavior of the ZSS coatings was tested using an oxyacetylene torch rig (FP-73, PRAXAIR-TAFA Inc., USA). The pressure of O2 and acetylene was 0.8 MPa and 0.05 MPa, the flux of O2 and acetylene was 25 L/min and 50 L/min, respectively. Each sample was held by a graphite fixture vertically placed to the flame during testing. A double colorimetric infrared thermometer (MR1S, Raytex Marathon Inc., USA) was used to measure the temperatures of the front coating surface and was connected to a data logger showing real time temperatures as a function of time. Specimens were exposed to the flame for 600 s, starting from when temperatures

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of the specimen surface reach to 1400C, 1700C and 1800C, respectively. And mass ablation rates of the samples were calculated by the mass changes before and after ablation test of each specimen. 2.3 Characterization A BrukerD8 Advance X-ray diffractometer using Cu radiation was employed to analyze the phase constitution of the powders, as-obtained coating, and ablated coating. X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi; Al Kα radiation) was used to semi-quantify the presence of SiO2 present on the sample surface. The observation of microstructure, morphology and analysis of element distribution, were performed by an S-4800 scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS). The percentage of void of the coatings was evaluated by SEM micrograph analysis using an Image Pro Plus software. The weights of specimens were measured before and after ablation test by an electrical balance with the accuracy of 0.1 mg.

3. Results and discussion 3.1 Microstructure of agglomerated and IPS-treated ZSS powders The detected phase of 4 kinds of agglomerated ZSS powders after spray drying are shown in Fig.1. It is obvious that XRD pattern mainly identifies ZrB2 and SiC diffraction peaks. A little ZrO2 also was detected because ZrO2 ball debris mixed into ZSS powders during ball milling. The surface micrographs of the spray dried ZSS powders are displayed in Fig.2. All Kinds of the ZSS 3

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particles are nearly spherical with rough surfaces. According to the dimensions and colors, three types of granules can be distinguished: coarse white ZrB2 granules with a size range from 1 μm to 3μm and coarse grey SiO2 granules with a size range from 1μm to 3μm fine grey SiC granules with submicron size. According to EDS results, with the increase of SiO2 content, the at.% of Zr element decrease and the at.% of O element increase. XPS spectrums of 4 kinds of aggregated ZSS powders by spray dying have given in Fig. 3. The identified binding energies of SiC and SiO2 are according to standards established by Onneby and Pantano[38], where they defined specific energy level of SiC(99.5-100.1 eV) and SiO2(102.5-102.9 eV). So the results of the Si2p spectrums reveal the presence of SiO2(amorphous) and SiC in the agglomerated ZSS powders. And it was evident that the SiO2 peak intensity increase and the SiC peak intensity decrease from ZSS1 to ZSS4, which is agreement with the trend of original composition ratios.

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Fig.1. XRD pattern of agglomerated ZSS powder after spray drying.

Fig.2. Surface micrographs of the agglomerated ZSS powders: (a) ZSS1; (b) ZSS1; (c) ZSS3; (d) ZSS4.

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Fig. 3. Si2p XPS spectrums of agglomerated ZSS powders.

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XRD pattern of IPS-treated ZSS powder after IPS treatment is shown in Fig.4. Combined with XRD pattern of agglomerated ZSS powders, there are not phase change, mainly including ZrB2, SiC and alittle ZrO2. The surface micrographs of the 4 kinds of ZSS powders after the IPS treatment are showed in Fig.5. Though the IPS-treated ZSS powders have the same subglobular shape as the agglomerated powders, different surface morphology of the IPS-treated ZSS powders can be distinguished. ZSS1, ZSS2 and ZSS3 powders show dense and sintered morphologies after IPS treatment, ZSS4 powder presents coarse morphology. Due to high temperature and high enthalpy of plasma field, ZSS powders would melt and tend to densification during IPS treatment. However, excessive silica causes the serious volatilization problem absorbing a large amount of heat, which causes poor sinter for ZSS4 powder. Compared with agglomerated ZSS powder, all kinds of IPS-treated ZSS powders suffered different degrees of “Si” loss according to EDS results. XPS spectrums of 4 kinds of IPS-treated ZSS powders have shown in Fig. 6. The analysis of the XPS results reveals that C and O exist in the IPS-treated powders in the forms of SiC and SiO2 after IPS treatment. However, compared with agglomerated ZSS powders, SiC on the all kinds of ZSS powder surfaces significantly reduce. In order to further investigating the situation of “Si” inside the powder, we observed the cross sections of ZSS powders after IPS treatment, as shown in Fig.7. It clearly indicates that there is obvious sintering inside the IPS-treated ZSS powder, which causes the number and size of pores and interfaces decrease. It is worth noting that the Si elements in the four powders are evenly distributed rather than the gradient powder of “Si”, so there is no loss of “Si” inside the powder.

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Fig.4. XRD pattern of IPS-treated ZSS powder after IPS treatment.

Fig.5. Surface micrographs of the 4 kinds of ZSS powders after the IPS treatment: (a) ZSS1; (b) ZSS2; (c) ZSS3;

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(d) ZSS4.

Fig. 6. Si2p XPS spectrums of IPS-treated ZSS powders.

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Fig.7. The cross sections of 4 kinds of ZSS powders after IPS treatment: (a) ZSS1; (b) ZSS2; (c) ZSS3; (d) ZSS4.

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3.2 Phase composition and microstructure of the sprayed ZSS coatings

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The XRD patterns (Fig. 8) show that the 4 kinds of ZrB2-SiC-SiO2 coatings prepared by plasma spray technology are composed of the relatively large ZrB2 and SiC phase as well as a small amount of the ZrO2 phase due to the erosion of the ZrO2 milling media during ball milling. This phenomenon also appears in ref. [39]. And another reason for ZrO2 production is that a fraction of the ZrB2 was converted to ZrO2 phase due to the oxidation of ZrB2 in air during the spraying process. Because B2O3 evaporates rapidly at high temperature, only ZrO2 phase was observed in the coating. It is worth noticing that with the increase of SiO2 content, the intensity values of SiC peak are getting weaker. The results of the Si2p XPS spectrums of the plasma-sprayed ZSS coatings are shown in Fig. 9. It is evident that the exterior surface of the plasma-sprayed ZSS coatings are covered by the amorphous SiO2 and no SiC was found. During plasma spraying, SiC on the outermost coating surface was decomposed due to the moving heat of the plasma spray gun. XPS is extremely surface sensitive revealing the formation of species on the surface to 10 nm depth meaning that the SiC inside the coatings are not detected.

Fig. 8. XRD patterns of the 4 kinds of as-sprayed ZrB2-SiC-SiO2 coatings.

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Fig.9. The Si2p XPS spectrums of the plasma-sprayed ZSS coating surfaces. Polished cross-sectional morphologies of the as-deposited ZSS coatings are exhibited in Fig. 10, which shows that all kinds of ZSS coatings are dense and uniform, presenting a lamellar structure. No obvious cracks could be observed in the coatings. Consequently, introduction of SiO2 is evidently favorable to toughen ZSS coatings. The thickness values of the ZSS coatings are

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about 180 m. From fig. 10a to 10c, the increase of SiO2 is beneficial to improving the densification of the composite coatings, which could further verify by the decrease of corresponding void content (Table 3). The melting point of SiO2 (1650 °C[40]) is much lower than that of ZrB2 (3245 °C[41]) and SiC (2700 °C[41]), so the SiO2 phase in the powders was easily melted compared to the ZrB2 phase and SiC under the same spray parameters, which is good for the deformation adhering of semi-melted or un-melted particles and could offset the voids caused by overlap among splats. The higher magnification image inserted in Fig. 6c is BSE image of dense region of ZSS3 coating. The white is the ZrB2, dark gray is SiC, and they are surrounded by gray SiO2 phase. Therefore, characters of SiO2 would improve the densification of the composite coatings and release the residual stresses accumulated during the deposition process, simultaneously. Logically, compared with the other coatings, the ZSS4 coating with more SiO2 content would display more compact structure. However, the structure of ZSS4 coatings actually is unconsolidated and more porous, which is result from the evaporation of SiO2. This evaporation in ZSS4 coating deposition is more serious than the others and would absorb too much heat from plasma jet resulting in insufficiently melted ZrB2 and SiC particles.

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Journal Pre-proof Fig. 10. Polished cross-sectional morphologies of the as-deposited ZSS coatings: (a) ZSS1, (b) ZSS2, (c) ZSS3, (d) ZSS4.

3.3 Ablation resistance of the ZSS coatings The macrographs of the coated samples carried out in oxyacetylene torch for 600s under

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1800 C are shown in Fig. 11. After ablation test, the surface colors of the ZSS coatings visibly change from gray black to white due to the oxidation of ZrB2. It is worthily noted that the sprayed ZSS3 coating with minimum voids has a small bump on which it can see many divergent cracks. And the reason why there is an evident discrepancy between ZSS3 and other coatings will be explained later. It is clear that ZSS2 and ZSS4 of the ablated samples have depressions in the ablation center, while ZSS1 coating shows flat surface. And no spallation or hole occurred in these coatings. The existence of a depression in the samples (ZSS2 and ZSS4 coating) can be attributed to the following four reasons: firstly, the evaporation of SiO2 by the reaction: SiO2(l)→SiO(g)+1/2O2(g) (1) The volatility diagram for the oxidation of SiC (Fig.12), shows that SiO(g) becomes the predominant vapor phase in equilibrium with the condensed oxide above about 2073 K [39]. Therefore, the SiO2 liquid in the ablation center zone transfers into the SiO(g) and evaporates causing a depression on the surface of the sample. Secondly, it should be concerned that there is water vapor generation during oxyacetylene torch testing, which cause additional volatile species (e.g. Si(OH)4) in ZSS coating[42,43]. CH4(g)+2O2(g)→CO2(g)+2H2O(g) (2) Thirdly, during the ablation test using oxyacetylene torch, the ZSS coatings were also subjected to the large aerodynamic loads, such as shearing force and high pressure, which is adverse to remained configurational stability. When the content of ZrB2 was large enough to support a ZrO2 skeleton, configuration of the ZSS coatings could not occur change and vice versa. Fourthly, compared with compact sample prepared by hot press technology, the existence of voids in ZSS coatings prepared by plasma spraying is another reason for depression. The oxides at the ablation center region would be blown away under the conditions coupled with mechanical loads and high temperature. The mass ablation rates of ZSS coated C/C composites are listed in Table 4. The ZSS coated samples present a good ablation resistance property. After ablation at 1800 C, the mass ablation rate of the ZSS1 coating are 3.4×10-4 g/s, which is better than others samples with mass ablation rates of 6.67×10-4 g/s, 8.3×10-4 g/s, 6.67×10-4 g/s, respectively.

Fig. 11. The macrographs of the coated samples after ablation at 1800 C for 600s.

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Fig. 12. The volatility diagram for the oxidation of SiC at 1800 C[39].

3.4 Effect of SiO2 contents to ablation resistance of ZSS Coatings

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The XRD patterns of ZSS14 coatings ablated at 1800 °C for 600s are presented in Fig. 13. The XRD patterns are similar to each other, and the coatings are mainly composed of ZrO2 after the ablation test, while the formation of boron oxide and silica glass cannot be identified. Compared with other coatings, new zircon phase was formed in the ZSS3 coating under the same oxidation condition according to reaction (8) and (9). It should be noted that the ablated samples were not quenched so that some reactions yielding zircon formation must be taken into account. To some extent, the formation of ZrSiO4 is beneficial to improving the ablation resistance of the ZSS coatings.

Fig. 13. The XRD patterns of ZSS14 coatings ablated at 1800 °C for 600s.

The center surface morphologies of ZSS14 coatings after ablation for 600s are presented in Fig. 14. In the center of the ablation region, oxidation and mechanical erosion were the main ablation behaviors for ZSS coatings. EDS results reveal that the elements are primarily composed of Zr, Si, and O, so the main phases are confirmed as ZrO2 and SiO2. The bright phase is ZrO2 and the dark phase is SiO2. The ZrO2 phase can improve the ablation resistance of ZSS coating because it could play a thermal barrier role, reducing the temperature in the coating. Another function of ZrO2 is to remain the coating’s configurational stability. There are many ZrO2 conglomerations surrounded by the glass phase with some small pores dispersed on the surface of ZSS1 coating (Fig.14a), which causes oxygen throughdiffuse into the interior during ablation. The inset in Fig. 14a reveals the presence ZrO2 with a dendritic morphology at the edge of the conglomerations showing the grow mechanism of conglomerations. Although the ablation 10

Journal Pre-proof temperature (2073K) was lower than the melting temperature of pure ZrO2 (2953 K), the fusion of ZrO2 is most likely due to the solution of ZrO2 in SiO2 and ZrO2 phase separate out with the decrease of temperature during cooling period according to ZrO2-SiO2 binary phase diagram[44]. Some pores appeared on the ZSS1 surface due to the formation gas byproduct (such as SiO, CO, and B2O3) beneath the outermost scale and then escaped from the surface during ablation. With the increase ofSiO2 content, the resistance of erosion is decrease, which indicates that the coating is easier to suffer by great shearing force during ablation process. Therefore, the microstructure of

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ZSS24 are more smooth and no conglomerations in fig. 14b to 14d. For further observation, it can be seen that ZrO2 exhibiting island-like structure is matched distribution, and the glassy SiO2 connects ZrO2 to a whole area in ablation center in the higher magnification image inserted in Fig. 14c and 6d. Both the island-like ZrO2 and glassy phase can protect the C/C composites for further ablation. In addition, there is no obvious crack or spall in the ablation center of ZSS1, ZSS2 and ZSS4 coating, which may be attributed to the possibility that the glassy phase effectively seals the microcracks. And the introduction of SiO2 from raw material can directly melt to promote the sintering of ceramic coating, which is more efficient than oxidation of SiC. Therefore, the coatings containing some content SiO2 may exhibit better long time oxidation protection ability.

Fig. 14. The center surface morphologies of ZSS14 coatings after ablation at 1800C for 600 s: (a) ZSS1, (b) ZSS2, (c) ZSS3, (d) ZSS4.

During the ablation test, oxygen diffused to the reaction interface, and reacted with ZrB2 and SiC to produce ZrO2 and other products like gas byproducts. The main reactions that might occur during the ablation process are described as follows [23,31]: ZrB2(s)+O2(g)→ZrO2(s)+B2O3(g) (3) SiC(s)+3O2(g)→2SiO2(l)+2CO(g) (4) SiC(s)+2O2(g)→SiO2(l)+CO2(g) (5) SiC(s)+O2(g)→SiO(g)+CO(g) (6) B2O3(l)→B2O3(g) (7) 2ZrO2(s)+2SiO(g)+O2→2ZrSiO4(s) (8) ZrO2(s)+SiO2(s)→ZrSiO4(s) (9) Corresponding polished cross-section SEM micrographs in the center region of the ZSS14 coatings after ablation at 1800 °C for 600s are presented in Fig. 15. Apparent layered 11

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structure is found in the cross-sections for both of ZSS1 and ZSS3. The surfaces of the ablated ZS1and ZSS3 are covered by a thick silica glass layer with some small pores (fig. 15a and fig. 15c). The formation of an external glassy layer is very effective in limiting the inward diffusion of oxygen into the inner coating. Beneath the continuous glassy layer, a SiC/SiO2-depleted layer is formed due to active oxidation of SiC. It can be confirmed that those gaseous by-products will cause the high pressure increase inner coating with the increase the ablation time, which is probable to break through the topmost glassy layer and make coating cracking or spalling. On the ZSS3 coating surface, many pores formed around the cracks, as shown in Fig. 16. In this layer, a big horizontal gap appeared in ZSS1 coating, and a similar discontinuous holes can be found in ZSS3, which is quite different from the good compact structure in the as-sprayed coatings. One of the reasons is that the coefficient of thermal expansion of ZrO2 is larger than that of ZrB2. Furthermore, when the sample was cooled from 2073K to room temperature, the t-m phase transformation could result in the volume expansion of the coating, and easily lead to the formation of gap. In addition, the holes left by SiC and SiO2 consumption and formed during the coating preparation by plasma spraying usually exist as a stress center, which will also cause the generation and extension of cracks in the coating. Compared with ZSS1, ZSS3 coating possessed an unaffected layer at the same condition. As for the relatively loose structure, the consumption of the SiC and SiO2 inner ZSS2 and ZSS4 coatings were more severe. As shown in Fig. 15b and 15d, the SiC and SiO2 in the coating have almost consumed and the porous ZrO2 phase left behind after ablation for 600 s, because oxygen could easily infiltrate inner coating by the interconnected holes, which is a bad cycle for the ablation resistance. The holes growth and gap propagation have occurred with the increase of ablation time. Therefore, there is a larger gap emerging between the sprayed ZrB2-SiC-SiO2 coating and SA inner coating, and a similar gap can be found in Fig. 16b. Although the coating remains integrated, the results highlight the fact that the addition of 20 and 40 vol.% SiO2 could not continue to protect the C/C composites for longer time in air at 1800 °C.

Fig. 15. Cross-section SEM micrographs of the ZSS14 coatings after ablation at 1800 °C for 600s: (a) ZSS1, (b) ZSS2, (c) ZSS3, (d) ZSS4.

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Fig. 16. Morphologies of ZSS3 around the cracks.

3.3 Effect of ablation temperatures to characters and ablation mechanisms of ZSS3

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coating In order to analyze of the effect of temperature on ZSS coating and role of SiO2 additives at different temperatures, the ZSS3 coatings also were ablated at 1400 C and 1700 C for 600s, respectively. Fig. 17 shows the XRD patterns of ZSS3 coatings after ablation at different temperatures. No SiC and ZrB2 phase were detected after 600s ablation at those temperatures. Apparently, the results of phase analysis indicate that the ZSS3 coating ablated at 1400 C are composed of ZrO2 and Zr2SiO4, and the intensity value of ZrSiO4 at 1800 C is lower than that of ZrSiO4 at 1400 C because of severe consumption of SiO2 at 1800 C. It should be noted the oxides were mainly of ZrO2 after ablation at 1700 C. The reason is that the formation of liquid SiO2 was covered the coating surface when temperature is about 1700 C, which may cause the XRD deviation of ZSS3 coating after ablation at 1700 C.

Fig.17. The XRD patterns of ZSS3 coatings after ablation at 1400 C, 1700 C and 1800 C for 600s, respectively.

The center surface morphologies of ZSS3 coatings ablated at different temperatures are showed in Fig. 18. Although the surface of ablation center suffered mechanical erosion, the formed ZrO2 apparently has not changed the initial framework of ZrB2 at 1400 C, and the morphology of the ablated surface is similar to that of the as-sprayed coating on which ZrB2, SiC and SiO2 were replaced by ZrO2 and SiO2, respectively. With the increase of ablation temperature, the mobility of ZrO2 at temperatures of 1700 C is evident which has large effect on the oxidation resistance at low temperatures. Ping Hu el. present that the oxidation process of ZrB2 is not only a process where ZrB2 is transferred into ZrO2 and B2O3, but one that also includes migration, agglomeration and growth of ZrO2[28]. And the formed ZrO2 conglomerations could increase configurational stability of coating. And due to original SiO2 participate in melting process, the surface ablated at 1700 C covers a silica glass, which is the reason for XRD deviation at 1700 C. When the temperature of ablation center region was around 1800 C, the coating was severely 13

Journal Pre-proof oxidized because of the high temperature and high-pressure gas flow. Also, the oxides blown away by the shearing action of the oxyacetylene flame and rapid volatilization of SiO2 resulted in the smooth ZrO2 layer. The amounts of holes increased significantly, which meant ZrB2, SiC and SiO2 in inner coating underwent the severe oxidation during ablation.

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Fig. 18. The center surface morphology of ZSS3 coatings after ablation tests for 600s: (a) 1400 C, (b) 1700 C, (c)

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The cross-sectional morphologies of ZSS3 coatings after 600s ablation at 1400 C and 1700 C are illustrated in Fig.19. A glassy SiO2-rich phase embedding ZrO2 grains and original SiO2 grains was observed in the outer layer (Fig. 19b) after ablation at 1400 C. However, the oxide scale formed at 1400 C shows a discontinuous protective layer with some porous regions in which some open pores channels could be observed (Fig. 19a). The thickness of this oxide scale

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for ZSS3 coating was about 70 m. According to reaction (3) and (7), the B2O3 begins to evaporate above 1100 C and such behavior becomes dominant way when temperature is higher than 1400 C due to the high vapor pressure of B2O3. So the amount of remaining oxidation product glassy SiO2 is not enough to fill whole oxide layer forming a dense layer. And due to the existence of voids in the outer layer, oxygen is easy to infiltrate into coating resulting a thick oxide layer. With the increase of the ablation temperature, microstructure evolution of oxide scale is

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significant after ablation at 1700 C, the oxide scale grew to a thickness nearly 100 m (as shown in Fig. 19c). One reason is melted original SiO2 begins to participate in protect work in this temperature, which is main source for the glassy phase together with oxidation of SiC. Another reason is SiO(g) re-oxidation near the surface. Noting that active oxidation of SiC was occurred in this temperature which caused a significant increase in the oxidation rate of SiC leading to preferential oxidation of SiC in the coating. During ablation test, the gaseous by-products outward diffuse, while oxidants O2 and O diffuse in the opposite direction. When SiO(g) approach the surface with enough high oxygen partial pressure, SiO(g) prefers to get re-oxidized to supply the glassy layer[45]. Fig. 19d shows high magnification of Fig. 19c, similar porous regions still appeared but they trend to aggregation together and grown up, which is different with the results of previous works. For ZrB2-SiC coating, a continuous SiC consumption layer would form beneath the outmost layer after ablation test. But in our work SiO2 additive could slow down the SiC consumption by destroying interconnected SiC network and reducing its chances of contact with oxygen. That is also the reason for the formation of discontinuous porous SiC consumption layer.

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Fig. 19. The cross-sectional morphologies of ZSS3 coatings after ablation for 600s: (a)(b) 1400 C; (c)(d) 1700 C.

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Theoretically, the anti-ablation resistance of plasma-spraying ZrB2-based coatings depends significantly on the oxidation rate of these species, SiC contents and distribution, the amounts of non-negligible voids caused by plasma spray technology. A schematic ablation model of ZSS coatings at different temperatures is shown in Fig. 20. This paper tries to use a model to interpret the effect of temperature on the ablation resistance of ZrB2-SiC-SiO2 coatings. Logically, both ZrB2 and SiC should be oxidized and could protect C/C composites for long time at high temperature, but previous studies show that ZrB2-SiC coating is susceptible to failure at short time at medial high temperature. In fact, SiC formed a network in three dimensions and the dominant chemical process for SiC is active oxidation under low oxygen partial pressure. The oxidation of SiC is more rapid than the oxidation of ZrB2 at this situation. In present case, introduction of SiO2 as the third phase could obviously improve the densification of coatings and reduce the defects served as the oxygen diffusion paths, so the as-sprayed ZSS coating shows compact structure(fig.18a). And material SiO2 could alleviate the CTE mismatch between the coating and

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substrate. As the surface temperature of the sample is up to 1400 C, oxygen diffused to the surface of coating and reacted with the ZrB2–SiC–SiO2 coating. The oxidation of SiC on the surface is mainly determined by Reaction (5). ZrO2 formed during ablation by Reaction (3), which can provide a framework for glassy phase. In this temperature, it is noted that the B2O3 volatilization is much larger than its formation and raw material SiO2 not occurred melting. So the oxide layer including ZrO2 granules, SiO2 granules and glassy SiO2 formed on the surface. As the ablation went on, not enough product of glassy SiO2 completely filled the oxide layer with discontinuous compact regions and porous regions, which may be the channels for the O2 to diffuse into the interior of the coatings. And during the ablation test, ZrSiO4 was formed by reaction (8) and (9), which is benefic to lengthen the lifetime of ZSS coating. When surface temperature of the coating approached to 1700 C, larger than the melting temperature of SiO2, the ablation reactions on the coatings may intensify at this temperature. The thickness of the oxide layer increased and melted SiO2 and ZrO2 granules formed a compact structure in the outermost layer. And the oxide layer could limit the inward transportation of oxygen and resulted in the lower partial pressure of oxygen in the inner coating. Therefore, according to reaction (6), the active oxidation of SiC could take place in the inner coating. In fact, previous studies show that this reaction is considered as main reason for porous SiC-depleted region. In present study, SiO2 raw in the coating could play a significant role to destroy the SiC interconnectivity in three 15

Journal Pre-proof dimensions, which could slow the active reaction of SiC. The degree of SiC network decreases with increasing SiO2 content. Compared to formation of fully-connected porous layer in previous studies, therefore, discontinuous rounded porous regions formed in the subsurface. Under the driving of oxygen, the regions that in contact with porous regions directly exposed to oxygen and thus the oxidation simultaneously occurred resulting in the dense regions surround the open pore channels. It is noted, during the ablation test, the samples were also subjected to the large aerodynamic loads especial at higher temperature. Therefore, as the temperature approached 1800

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C, the ZSS coating was suffered from the severe aerodynamic loads and heating. So the ZrO2 phase on the surface coarsened with SiO2 as the bonding phase showing complanate morphology, shown in fig. 20. Meanwhile, the more oxygen diffused inward between the outer layer and the SA inner coating, and resulted in more formation of gaseous by-products and interconnectivity tendency among porous regions. Due to lack of SiO2 and SiC, the coefficient of thermal expansion (CTE) of the porous subscale of crystalline zirconia will much larger than C/C composites easily causing a penetrating crack in this layer.

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Fig.20. Schematic ablation model of ZSS coatings at different temperatures.

4. Conclusion

ZrB2-SiC-SiO2 coatings were fabricated by plasma spray technique. The effect of SiO2 content on ZSS coatings’ densification and ablation behaviors at middle and high temperatures was investigated. The as-sprayed ZSS coatings displayed dense and homogeneous microstructures after coating deposition. Besides, since the SiO2 volatilization problem increased with the increase of the content during coating deposition, the density of as-sprayed coating showed a trend of increasing first and then decreasing with the content of SiO2 and reached the optimal value when the SiO2 content is 30%. The microstructural evolution of the ZSS composite coatings were significantly affected by the void content, temperature composition and aerodynamic loads, especially at temperatures above 1800 C. Though the ZSS1 coating got a minimum weight loss of 3.4×10-4 g/s after ablation for 600 s, the ZSS3 coating showed an optimal cross-sectional microstructure among the 4 kinds of coatings because of less voids than others. Interestingly, excessive SiO2 content may form a dense oxide layer on the outermost layer of the coating, thereby hindering the outward diffusion of the gaseous byproducts inside the coating resulting in an increase of local pressure in the coating, which would form voids or even cracks on the surface 16

Journal Pre-proof of the coating. Ablation mechanism revealed that ZrO2 skeletons would played an important role to configurational stability and the glassy component is beneficial to seal the coating. Compared with the ZSS3 coating ablated at 1400 C, the active oxidation of SiC was evident at 1700 C, which led to the quick consumption of the SiC inner coating at high temperature, and as a result, the porous regions appeared beneath the outmost glassy layer after ablation for 600 s. And it is worth noting that the original SiO2 exhibits good resistance to active reaction of SiC during ablation at 1700 C, which contributes to slow down the ablation rate and prolong the lifetime of composite coating.

Acknowledgements

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This work was supported by the National Natural Science Foundation of China (51302013) and ’Excellent Young Scholars Research Fund’ of the Beijing Institute of Technology.

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Table 1. Details of the spraying parameters for ZrB2-SiC-SiO2 coatings. Parameters

Spraying current, A

950

Primary gas Ar, SCFH

90

Carrier gas Ar, SCFH

10

Second gas He, SCFH

50

Powder feed rate, RPM

2.0

Spraying distance, mm

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Content

SiC （vol.%）

SiO2 （vol.%）

ZSS1

67.5

22.5

ZSS2

60

ZSS3

52.5

ZSS4

45

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ZrB2 （vol.%）

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Table 2. 4 kinds of composition ratios of ZrB2-SiC-SiO2 powders.

10 20

17.5

30

15

40

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Table 3. Void contents of different ZSS coatings. ZSS1

ZSS2

ZSS3

ZSS4

16.5

15.82

15.42

15.73

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Void content (%)

Table 4. The mass ablation rates of different ZSS coatings after ablation tests. Sample

Temperature

Time

The mass ablation rates

(C)

(s)

(10-4g/s)

ZSS1

1800

600

3.4

ZSS2

1800

600

6.67

ZSS3

1800

600

8.3

ZSS4

1800

600

6.67

ZSS3

1700

600

1.67

ZSS3

1400

600

0

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Highlights ● The as-sprayed ZSS coatings displayed dense and homogeneous microstructures after adding SiO2. ● The ZrB2-SiC-10Vol.%SiO2 coating got a minimum weight loss of 3.4×10-4 g/s after ablation at 1800ºC for 600s. ● SiO2 additive in ZrB2-SiC coating could slow down the active oxidation of SiC during ablation test by destroying interconnected SiC network in coating.

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