Elsevier

Corrosion Science

Volume 192, November 2021, 109814
Corrosion Science

Effect of WB addition on long ablation behavior of ZrB2-MoSi2 coating

https://doi.org/10.1016/j.corsci.2021.109814Get rights and content

Highlights

  • The competitive oxidation of MoSi2 and WB inhibits the formation of Mo-containing oxides.

  • The addition of WB is beneficial to the decrease of surface temperature, and then stabilize the existence of intermediate MoB.

  • The ablation mechanism of the ZMW coating was also analyzed based on thermodynamics predictions.

Abstract

Improving the long time ablation resistance of thermal protective coating is the key index to promote the development of hypervelocity aerocrafts. In this work, the ablation behaviors of ZrB2-MoSi2 and ZrB2-MoSi2-WB composite coatings fabricated by vacuum plasma spray (VPS) were evaluated by plasma flame. The microstructure evolution induced by prolonged ablation was focused on and discussed in detail. The results shows that the addition of WB significantly improved the long-term ablation resistant performance of the ZrB2-MoSi2 coating, as characterized by integrated macrostructure, compact microstructure and decreased thickness of oxide layer. The function of WB was explained in detail based on microstructure observation and thermodynamic calculations, which would give some inspirations for the design of UHTCs.

Introduction

Advanced materials are required to withstand the extreme conditions provoked by the development of hypersonic vehicles [1], [2]. High temperature thermal structural materials, mainly referring to ultra-high temperature ceramics (UHTCs), have attracted great attention for their promising potential applications in the oxygen containing environments above 1600 °C [3], [4], [5], [6]. Generally, ZrB2 is used as a suitable framework structure for its oxidation product (ZrO2) possessing a high melting point and specific strength [7], [8], which can effectively resist air flow erosion. Besides, the B2O3 liquid phase formed on the outer surface of ZrB2 at low temperatures acts as oxygen diffusion barrier, which can initially reduce the oxygen partial pressure inside [9], [10], [11], [12].

In recent years, SiC has been selected as silicon sources to be incorporated into the UHTC systems. However, under ultrahigh temperature oxygen-containing environments, the passive oxidation of SiC gradually turns to active oxidation as the internal oxygen partial pressure decreases, and the generated SiO gas is easy to cause the coating to fall off [13], [14], [15]. Therefore, metal silicides have been chosen as the second phase to enhance the ablation resistant performance of UHTC materials [16], [17], [18]. Silvestroni et al. [16] investigated the effect of various metal silicides (ZrSi2, MoSi2, TaSi2 and WSi2) additives on the static anti-oxidant performance of ZrB2, respectively. They found that MoSi2 was the most beneficial doping for improving the oxidation resistance, even up to 1800 °C. They speculated that the formation of stable MoB played some function besides the protective SiO2 scale [16], [17]. However, some researchers found that during the ablation process, the accumulation and rapid volatilization of gaseous phase accelerated the oxidation of ZrB2-MoSi2 system, and the formation of the porous exterior oxide layer was one of the main reasons for the failure [19], [20]. In the previous work in our laboratory [21], the ablation behavior of ZrB2-SiC coating doped with tungsten-containing additives (W, WB and WSi2) was evaluated and compared. It was found that the ZrB2-SiC-WB coating exhibited excellent ablation resistant performance under plasma flame. It was revealed that the addition of WB could enhance the densification of the oxide layer due to the effect of liquid phase sintering. Zhang et al. [22] found that WB component was beneficial to improve the oxidation resistance of ZrB2 bulk ceramics. Under a static oxidation environment at 1600 °C, the weight gain rate of modified composite system was only 8 mg/cm2, which was 63.6% lower than that of pure ZrB2, and the oxide layer thickness was greatly weakened as well. Based on the references and our previous work, we designed a new ZrB2-MoSi2-WB coating system, which is expected to achieve better long-term ablation resistance.

In this work, two kinds of coatings, including ZrB2-15 mol.%MoSi2 and ZrB2-15 mol.%MoSi2-5 mol.%WB coatings were fabricated by vacuum plasma spray (VPS) technique and their ablation resistance were examined by plasma flame. The detailed surface temperature, evolution of phase composition, microstructure and ablation behaviors of the ZrB2-MoSi2-WB coatings were characterized and compared with the ZrB2-MoSi2 coating. Based on microstructure analysis and thermodynamic calculations, the effects of WB on the ablation behavior of composite coating were elaborated. This work would give some inspirations for the design of UHTCs.

Section snippets

Powder and coating preparations

The following available commercial powders were used as virgin feedstocks: ZrB2, MoSi2 and WB (Qinhuangdao Yinuo Advanced Material Co., Ltd., China). The mixed powders ZrB2-15 mol.%MoSi2 (labeled as ZM) and ZrB2-15 mol.%MoSi2-5 mol.%WB (labeled as ZMW) were prepared in turn for spray drying.

Graphite wafers (Φ 40 mm × 5 mm) (Beijing Jinglong Carbon Materials Co., Ltd., China) coated with a SiC bonding layer were selected as substrates. Subsequently, the prepared powders were used to fabricated

Phase compositions and morphologies of the as-sprayed coatings

The surface and cross-sectional morphologies and related EDS results of the as-sprayed ZM and ZMW coatings are shown in Fig. 1. It can be seen that the surface of the coatings was mainly composed of flat molten droplets and a small amount of insufficient molten particles, showing a typical plasma sprayed microstructure (Fig. 1a, c). The cross-sectional picture further confirmed that the two coatings had relatively dense microstructure with a few pores (Fig. 1b, d). According to the element

Discussion

The following reactions may occur in the ablation processes of the ZM and ZMW coatings at oxygen-containing high temperature environments [24], [25], [26]:ZrB2 (s)+5/2 O2 (g)→ZrO2 (s)+B2O3 (l)MoSi2 (s)+O2 (g)→Mo5Si3 (s)+SiO2 (l)MoSi2 (s)+7/2 O2 (g)→2SiO2 (l) + MoO3 (g)2MoSi2 (s)+B2O3 (l)+5/2 O2 (g)→4SiO2 (l) + 2MoB (s)MoB (s)+9/4 O2 (g)→MoO3 (g)+1/2B2O3 (l, g)WB (s)+3/2 O2 (g)→W (s)+1/2B2O3 (l)WB (s)+9/4 O2 (g)→WO3 (g)+1/2B2O3 (l, g)2 W (s)+3 O2 (g)→2WO3 (g)Where (s), (g) and (l) represent

Conclusions

ZrB2-MoSi2 and ZrB2-MoSi2-WB composite coatings were fabricated by vacuum plasma spray and their ablation behaviors were investigated. Based on microstructure observation and thermodynamic calculations, the function of WB was explained in detail. Several conclusions can be drawn as following:

  • (i)

    The addition of WB can greatly improve the anti-oxidation / ablation performance of ZrB2-MoSi2 coating, which kept the coating surface temperature at about 1700 °C and integrated macro-morphology after

CRediT authorship contribution statement

Xueting Xu: Data acquisition, Writing – original draft & editing, Xiaohui Pan: Conceptualization, Resources, Data curation, Shansong Huang: Experimental support, Dandan Qin: Experimental support, Yaran Niu: Project administration, Supervision, Writing – review & editing, Hong Li: Project administration, Writing – review & editing, Supervision, Xuebin Zheng: Project administration, Writing – review & editing, Supervision, Jinliang Sun: Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key R&D Program of China (2018YFB0704400) and Shanghai Technical Platform for testing on inorganic materials (19DZ2290700).

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