Effect of WB on oxidation behavior and microstructure evolution of ZrB2-SiC coating

Accepted Manuscript Title: Effect of WB on oxidation behavior and microstructure evolution of ZrB2 -SiC coating Authors: Chong Li, Yaran Niu, Tao Liu, Liping Huang, Xin Zhong, Xuebin Zheng, Chuanxian Ding PII: DOI: Reference:

S0010-938X(18)31043-6 https://doi.org/10.1016/j.corsci.2019.04.034 CS 8000

To appear in: Received date: Revised date: Accepted date:

8 June 2018 23 April 2019 25 April 2019

Please cite this article as: Li C, Niu Y, Liu T, Huang L, Zhong X, Zheng X, Ding C, Effect of WB on oxidation behavior and microstructure evolution of ZrB2 -SiC coating, Corrosion Science (2019), https://doi.org/10.1016/j.corsci.2019.04.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of WB on oxidation behavior and microstructure evolution of ZrB2-SiC coating Chong Lia, b, Yaran Niua, *, Tao Liua, b, Liping Huanga, Xin Zhonga, Xuebin Zhenga, *, Chuanxian Dinga Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China

SC R

b

IP T

a

U

Contact emails: [email protected], [email protected]

ZrB2-SiC-WB ternary composite coatings prepared by vacuum plasma spray were

A



N

Highlights

Appropriate amount of WB greatly improved the oxidation resistance of the

ED



M

firstly reported and their oxidation behaviors at 1500oC were evaluated.

coatings, as reducing the weight gain and increasing the thickness of liquid layer. WB is more stable and a better boron source compared with ZrB2 for promoting

PT



CC E

self-healing ability. 

WO3, oxidation product of WB, plays positive function through increasing liquid

A

viscosity, limiting oxygen diffusion, stabilizing B2O3 and forming WO3-ZrO2 eutectic.

Abstract In this work, different contents (5, 10, 15 mol.%) of WB were introduced into

ZrB2-SiC coatings which were fabricated by vacuum plasma spray technique. The oxidation resistant property of the ZrB2-SiC-WB ternary composite coatings was evaluated in static air at 1500 oC. The results showed that appropriate amount of WB addition significantly improved the oxidation resistance of ZrB2-SiC coating, as

IP T

proved by the reduced weight gain and increased thickness of the liquid layer. The influence of WB and its oxidation product WO3 were analyzed based on the

SC R

microstructure changes and thermodynamic calculations in detail.

Key words: ZrB2-SiC composite coating; WB; vacuum plasma spray; oxidation

N

U

behavior; microstructure

A

Ⅰ. Introduction

M

With the key requirements for hypersonic vehicles to withstand high temperatures in oxidant atmosphere, it is necessary to develop outstanding thermal protection

ED

systems for extreme environment applications. Ultra-high temperature ceramics

PT

(UHTCs) have attracted great attention due to their excellent adaptability for critical conditions [1, 2]. ZrB2 is one of the most important candidate materials in UHTCs

CC E

family due to its high melting point (3050 oC), high thermal conductivity (~ 100 W/(m⋅K)) and relatively low density (6.09 g/cm3) [3, 4].

A

The oxidation behavior of ZrB2 has been studied since 1960s [5, 6]. It has been

found that ZrB2 is easy to be oxidized above 800 oC, forming B2O3 and ZrO2. The formed ZrO2 is loose and porous, which is resulted from the volume expansion and phase transformation. B2O3 exhibits relatively low melting point (450 oC) and can form a continuous liquid layer on the sample surface, which would effectively inhibit

oxygen diffusion. However, when the temperature increased above 1200 oC, B2O3 would be consumed completely owing to its severe evaporation and can no longer protect the matrix [7, 8]. It has been reported that SiC introduction could improve the oxidation resistance of ZrB2 at high temperatures above 1200 oC by generating a

IP T

borosilicate liquid layer [9-11]. ZrB2-SiC system has been extensively studied in the past and the optimum amount of SiC is confirmed to be 15 - 20 vol.% [12].

SC R

Nevertheless, it may not be the best choice to add SiC exclusively because of the following drawbacks: i. in the deep interior region of the bulk, where the oxygen

U

partial pressure is low, SiC forms gaseous products (e.g. SiO and CO) through active

N

oxidation and thus a SiC-depleted layer is formed [13]; ii. the integrity of SiO2 liquid

A

layer would be damaged when the temperature is above 1600 oC because of the

M

transition from passive oxidation to active oxidation of SiC [14]. Metal silicides, such

ED

as MoSi2 and TaSi2, have also been introduced to ZrB2 to meliorate the oxidation resistance [15, 16]. However, the melting points of metal silicides are relatively low,

PT

which limited their applications at high temperatures.

CC E

In order to enhance the oxidation resistance of ZrB2, tungsten containing additives (e.g. WC, WB, WSi2 and W) were introduced. Zhang et al. [17-19] studied the oxidation resistance of ZrB2-WC ceramics and found that WC addition effectively

A

reduced the weight gain and oxide thickness of samples at 1500-1600 oC. They confirmed that the presence of WO3 in the oxide scale resulted in liquid-phase sintering of ZrO2 and made the microstructure denser in comparison to the non-containing WC samples. Recently, Zou et al. [22] evaluated the ablation

resistance of ZrB2-SiC-WC ceramics through oxyacetylene flame at 2400 oC and found that the addition of WC could avoid the formation of the SiC-depleted layer. Table 1 is the summary of literatures that reported the effect of tungsten containing additives on the oxidation resistance of UHTCs. WB is characterized with high

IP T

melting point (2670 oC) and could provide B2O3 during oxidation. Therefore, WB can better improve the self-healing ability of UHTCs compared to WC and W, yet few

SC R

studies have discussed about the function of WB in the oxidation resistant property of ZrB2-SiC composites. What’s more, few reporters focused on coating materials. In

U

order to ameliorate the intrinsic brittleness of ceramics, UHTCs could be used as

N

coating materials to protect thermal structure components such as C/C and C/SiC

A

composites [23-26]. Especially, vacuum plasma spray (VPS) technology is suitable

M

for the preparation of UHTCs coatings for the following reasons: i. the temperature of

ED

the central plasma flame is above 10000 oC, which is effective to melt materials with high melting points; ii. the process is done in an inert gas atmosphere, which could

PT

avoid the introduction of oxygen impurity to some content; iii. the high efficiency and

CC E

automation of VPS could ensure high coating quality [16, 27-29]. To the best of our knowledge, few scholars have studied the oxidation resistant behaviors of

A

ZrB2-SiC-WB ternary composite coatings. In this work, ZrB2-SiC coatings with different contents of WB were fabricated by

VPS method. The oxidation resistance of the composite coatings was evaluated in ambient air at 1500 oC for different time. The phase composition and microstructure changes of the coatings were characterized in detail. The function and influence

mechanism of WB addition on the composition and microstructure changes of the oxide scale were focused on. It is expected that this work would shed some light on the role of WB in optimizing the oxidation resistant properties of UHTC composites.

IP T

Ⅱ.Experimental procedure The ZrB2-20 vol.%SiC powders containing different contents of WB (0 mol%, 5

SC R

mol%, 10 mol% and 15 mol%, noted as ZS, ZSW5, ZSW10 and ZSW15, respectively) were sprayed on graphite substrates via a vacuum plasma spray system (A-2000,

U

Sulzer Metco AG, Switzerland) equipped with an F4-VB torch. Then the graphite

N

substrates were removed by mechanical method to get free-standing coating samples

A

for this research. The thickness of these obtained coatings was about 1.0 mm and the

M

coatings were cut to an average size of 5.0 mm×5.0 mm×1.0 mm for later

ED

experiments.

The coatings were placed in a tube-type furnace and were heated up to 1500 oC at a

PT

rate of 10 oC/min. The samples were incubated for 1 h, 3 h and 6 h respectively, and

CC E

then cooled to room temperature naturally. The mass change of samples was measured by an electronic balance with an accuracy of 0.1 mg. The mass gain rate Wm% was

A

calculated as:

Wm % =

m2 − m1 × 100% m1

where m1 and m2 represent the measured weight before and after the oxidation, respectively. The phase compositions of the powders, as-sprayed and oxidized coatings were

identified by X-ray diffraction (XRD, RAX-10, Rigaku, Japan) with Cu Kα (λ = 1.5406 Å) radiation. The microstructure and chemical composition of the as-sprayed and oxidized coatings were characterized by scanning electron microscopy (SEM, Magellan 400, FEI, UK) equipped with X-ray energy-dispersive spectroscopy (EDS,

IP T

INCA SERIES, Oxford Instrument, UK). The porosity of the as-sprayed coatings were evaluated by three cross-sectional images with a magnification of 1000× using

SC R

an image analysis software (Leica Qwin, Germany).

U

Ⅲ Results and discussion

N

3.1 As-sprayed coatings

A

The XRD patterns of the as-received powders and as-sprayed coatings are

M

presented in Fig.1. The peaks corresponding to WB were observed for the ZSW5,

ED

ZSW10 and ZSW15 powders. The major peaks of hexagonal ZrB2 were indexed in the ZS powders (PDF card number: 34-0423). Besides, it is worth mentioning that the

PT

ZrB2 peak of the WB-containing composite powders shifted to a higher 2θ angle (Fig.

CC E

1a). The main reason is that the W substitution for Zr in ZrB2 reduced the average unit cell size, as the covalent radius of W (1.4 Å) is smaller than that of Zr (1.6 Å) [17,19]. These results indicated that a solid solution, (Zr, W)B2, was formed in the

A

ZrB2-SiC-WB composite powders. Fig.1b shows the XRD patterns of the as-sprayed coatings. The peaks corresponding to ZrB2 and ZrO phases were observed, while SiC and WB phases were not detected. Although they were not observed in the XRD patterns, the following microstructure and element mapping results have confirmed

the existence of WB additive in the composite coatings. The surface and fracture morphologies of the as-sprayed coatings were analyzed by SEM as shown in Fig. 2 and Fig. 3, respectively. All coatings were built up by melted and semi-melted particles. The porosity of the composite coating was calculated using

IP T

the cross-section SEM images, which were about 15-20%. A cross-sectional microstructure image and element maps of the ZSW10 coating are revealed in Fig. 4.

SC R

It can be seen that the elements of Zr, B, Si and W were uniformly distributed.

Combined the XRD and EDS results, it is inferred that ZrB2, SiC, WB and (Zr ,W)B2

U

existed in the ternary composite coatings.

N

3.2 Oxidized coatings

A

3.2.1 Macroscopic changes after oxidation tests

M

The macroscopic morphologies of the samples after oxidation at 1500 oC for 1 h, 3

ED

h and 6 h are shown in Fig. 5. The surface of samples changed from dark color to white color and the white region enlarged with the oxidation time increasing. The

PT

surface color of the ZS and ZSW15 coatings became white after 1 h oxidation, and the

CC E

ZSW5 coating was completely changed to white color after 3 h oxidation. The ZSW10 coating kept the original dark color after 6 h oxidation. The mass gain rates of the composite coatings after oxidation for different time at

A

1500 oC are plotted as a histogram in Fig. 6. The mass gain rate of the ZS coating was about 20.9% after 1 h oxidation, which was far greater than other samples under the same oxidation conditions. And much smaller weight gain changes were observed in the WB-containing coatings. This suggested that WB was effective in improving the

oxidation resistance of the ZrB2-SiC coating. It is worth noting that the mass gain rate of the ZSW10 coating was about 18.9% after 6 h oxidation, showing the lowest weight gain among all samples. 3.2.2 Oxidation at 1500 oC for 1 h

IP T

The fractured SEM images of the oxide scales of the coatings after 1 h oxidation are showed in Fig. 7. For the ZS and ZSW15 coatings, no liquid layer was formed on

SC R

the surface, while some liquid was observed inside the ZS coating (Fig. 7a). A liquid

layer appeared on the surface of the ZSW5 and ZSW10 coatings. It can be seen that a

U

thicker liquid layer could form in a short time for the ZSW10 coating (about 15 μm

N

for the first one hour). The composition of the liquid layer was paid attention to as

A

well. The liquid layer in the ZSW10 coating was mainly composed of SiO2, according

M

to the EDS analysis (Fig. 7e). The scale under the liquid layer contained Zr, W, O, Si

ED

and B elements. It is worth noting that W and B mainly existed in the lower part owing to the evaporation of WO3 and B2O3.

PT

3.2.3 Oxidation at 1500 oC for 3 h

CC E

The phase compositions of the coating surface after 3 h oxidation were characterized by XRD, as shown in Fig. 8. It is discovered that the main composition was ZrO2. Within the resolution limit of XRD, no other crystalline phases were

A

detected. The fracture morphologies of the ZS and ZSW10 coatings after 3 h oxidation are present in Fig. 9. A thin liquid layer and some needle-like particles were observed in the ZS coating (Fig. 9a). The needle-like particles (point A) was composed of ZrO2 while the liquid layer area (point B) was mainly composed of SiO2,

as confirmed by the EDS results. The SiO2-rich layer was formed after 3 h oxidation in the ZS coating, indicating that the liquid content gradually increased with the oxidation time prolonging. A liquid layer with a thickness about 50 μm was formed in the ZSW10 coating, which was the thickest among all coatings (Fig. 9b).

IP T

3.2.4 Oxidation at 1500 oC for 6 h The fracture morphologies of the four kinds of coatings after 6 h oxidation are

SC R

shown in Fig. 10. The liquid layer disappeared and some holes were observed inside the ZS coating. ZrO2 grains in columnar shape were observed in the holes from the

U

magnified picture (Fig. 10a). A discontinuous liquid layer was formed on the surface

N

of the ZSW5 coating (Fig. 10b). A continuous glass layer covered the surface of the

A

ZSW10 coating, the thickness of which was about 40 μm (Fig. 10c). In contrast, the

M

microstructure of the ZSW15 coating was very loose (Fig. 10d). It is inferred that the

ED

ZSW15 coating was destroyed by bubbles due to the accumulation of gases. In addition, it is observed that some internal grains in darker color were wrapped by

PT

grains in whiter color in the ZSW5 coating (Fig. 10e). The EDS analysis indicated

CC E

that the region B, C and D were composed of ZrO2, WO3 and borosilicate glass, respectively. This result suggested that ZrO2 was wrapped by WO3. Silvestroni et al. [15, 30] also found that (Zr, W)B2 solid solution was formed in ZrB2-WSi2 composite

A

ceramics and a core-shell structure, of which the core was ZrB2 and the shell was (Zr, W)B2, was detected by TEM. Therefore, the unique structure of WO3-wrapped ZrO2 was formed after oxidation for the ternary composite coatings. 3.3 Discussion

3.3.1 Thermodynamic analysis on function of WB and WO3 The oxidation process of ZrB2-SiC-WB composite coatings, which could take place in ambient air under high temperature (1500 oC), followed the reactions below [7, 13,

ZrB2(s)+5/2O2(g) = ZrO2(s)+B2O3(l)

(1)

SiC(s)+O2(g) = SiO2(l)+C(s)

IP T

21, 22]:

(2)

(3)

SC R

SiC(s)+3/2O2(g) = SiO2(l)+CO(g) WB(s)+3/4O2(g) = W(s)+1/2B2O3(l)

(4)

(5)

U

WB(s)+9/4O2(g) = WO3(s)+1/2B2O3(l)

N

The oxidation processes and the oxidation products have great influence on the

A

oxidation behavior of the composite coatings. We try to elaborate the effects of WB

M

and its oxidation product (WO3) based on thermodynamic analysis in the following

(i) Function of WB

ED

part.

PT

Figure 11 describes the oxygen partial pressure change as a function of temperature

CC E

for the oxidation of ZrB2 and WB. The blue line indicates that ZrB2 is oxidized into ZrO2 and B2O3 (Reaction 1) and the red line indicates the oxidation of WB into WO3 and B2O3 (Reaction 5). It is demonstrated that the required oxygen partial pressure

A

(pO2) for ZrB2 oxidation is lower than that of WB at 1000-2000 oC temperature range, namely, the oxidation of WB requires higher oxygen content than ZrB2 under the same condition. Therefore, WB is more stable than ZrB2 in an environment of low oxygen partial pressure.

The volatility diagram of ZrB2-SiC-WB system at 1800 K was calculated based on NIST-JANAF thermochemical tables [31], as depicted in Fig. 12. It can be seen that the equilibrium oxygen partial pressures (pO2) for SiC oxidized to SiO2 (Reaction 2) and for ZrB2 oxidized to ZrO2 (Reaction 1) are 10-10.48 Pa and 10-10.38 Pa, respectively.

IP T

When pO2 is below 10-8.59 Pa, WB is stable. When pO2 is between 10-8.59-10-4.67 Pa, W and B2O3 are produced (Reaction 4). When pO2 is above 10-4.67, WB will be oxidized

SC R

into WO3 and B2O3 (Reaction 5). It can be seen that WB is the most stable substance at low oxygen partial pressure compared with ZrB2 and SiC.

U

(ii) Function of oxidation product WO3

N

The formation of the liquid layer on the coatings’ surface is vital for their oxidation

A

resistance. The thickness of the liquid layer is one critical factor affecting the

M

oxidation resistance of ZrB2 based composite coatings. The thickness of the liquid

ED

layer is related to the formation (generation) rate and consumption (vaporization) rate of borosilicate glass. If the formation rate is faster than the consumption rate, it means

PT

that a thicker liquid layer could form on the surface, which can more effectively

CC E

hinder the transmission of oxygen to the coating interior. If the consumption rate is faster than the formation rate, it means that the liquid layer becomes thinner or broken, which could not effectively protect the matrix anymore. Compared with ZrB2-SiC

A

coating, the addition of WB resulted in a thick liquid layer (Fig.7, 9, 10) and a reduction of weight gain (Fig.6), for the reason that appropriate amount of WO3, the oxidation product of WB, could significantly reduce the volatilization rate of borosilicate glass.

Cations are commonly regarded as network modifier in glass network. Cation field strength usually has direct impact on the arrangement of molecules for the liquid glass. The cation field strength is defined as Z/r2, where Z is the atomic number and r is the ionic radius (Å). The cation field strengths of groups IVB, VB and VIB elements are

IP T

labeled in Fig. 14. It can be seen that V5+, Mo6+ and W6+ had relatively high field strengths of 23.62, 17.23 and 16.67 Å-2, respectively. While the cation field strengths

SC R

of Zr4+ was the lowest among all elements (7.72 Å-2). The high cation field strength of W6+ would attract non-bridging oxygen around them, resulting in the formation of

U

SiO2 phase separation and then the increasing of the liquid viscosity. Based on the

N

Stoke-Einstein relation, the high viscosity glass could also help decrease the oxygen

A

diffusion rate [32].

M

It has been reported that metal oxide additions could modify the structure of B2O3,

ED

and then the stability of B2O3 [33-35]. Certain amount of WO3 is beneficial for decreasing the evaporation rate of B2O3 and then improving the stability of the liquid

PT

layer. However, the existence of more WO3 would decrease the stability and enhance

CC E

the evaporation of B2O3. Fig. 13 reveals the partial pressures of some oxidation products under different temperatures, including WO3, B2O3, SiO2 and ZrO2. It can be seen that the vapor pressures of B2O3 and WO3 at 1500 oC are very high, reaching 238

A

Pa and 2133 Pa, respectively. The accumulation and volatilization of large amount of gases would generate holes or bubbles to destroy the integrity of the oxide layer. With the extending of the oxidation time, the liquid would be exhausted and the ZrO2 scale would come into direct contact with oxygen. According to ZrO2-WO3

phase diagram, WO3 could form a eutectic with 75 mol% ZrO2 above 1231 oC [36]. It is inferred that WO3 and ZrO2 could produce some liquid as well to fill holes and cracks, therefore a relatively dense scale could be formed to further impede oxygen diffusion (Fig. 7d).

IP T

3.3.2 Oxidation mechanism analysis of coatings For ZS coating: Some liquid was formed after 1 h oxidation (Fig. 7a). Then a thin

SC R

continuous liquid layer formed on the coating surface after 3 h oxidation (Fig. 9a),

while the liquid phase disappeared and some holes were generated after 6 h oxidation

U

(Fig. 10a). It can be concluded that the disappearance of liquid is the main reason for

N

the poor oxidation resistance.

A

For ZSW5 and ZSW10 coatings: A continuous liquid layer was formed on the

M

surface of both coatings after 1 h oxidation (Fig. 7b and Fig. 7c). It is worth noting

ED

that the ZSW10 coating generated a thicker liquid layer even after 6 h oxidation (Fig. 10c), exhibiting the best oxidation resistance among the four kinds of coatings. It

PT

indicates that proper amount of WB addition decreased the vaporization rate of the

CC E

liquid and improved the stability of the liquid layer. At the same time, the solid oxide scale was denser compared with that of the ZS coating (Fig. 10a), due to the higher stability of WB compared with ZrB2 in the low oxygen partial pressure environment.

A

Both the thick liquid layer and relative dense solid oxide layer contributed to the enhanced oxidation resistance of the ZSW5 and ZSW10 coating. For ZSW15 coating: No liquid phase was observed on the surface of the ZSW15 coating after 1 h oxidation (Fig. 7d) and the oxide layer become more loose with

many big holes after 6 h oxidation (Fig. 10d). It is inferred that the excessive WO3 formation will reduce the liquid stability and promote the liquid evaporation. What’s more, the accumulation of large amount of gases resulted in loose solid oxide layer. That is, the excessive amount of WB addition would lead to the degradation of the

IP T

oxidation resistance. IV. Conclusions

SC R

ZrB2-SiC-WB ternary composite coatings were fabricated by vacuum plasma spray and their oxidation resistance and microstructure changes at 1500 oC were studied.

U

Based on the experimental results and theoretical analyses, the following conclusions

Appropriate amount of WB addition could effectively improve the oxidation

A

(i)

N

can be drawn:

M

resistance of ZrB2-SiC coating. While excessive of WB addition would reduce

ED

the oxidation resistance of ZrB2-SiC coating. The ZSW10 coating exhibited the lowest weight gain and thickest liquid layer among all samples. WB is a better boron source compared with ZrB2 for promoting self-healing

PT

(ii)

CC E

ability. Based on thermodynamic analysis, WB is the most stable substance compared with ZrB2 and SiC in the low oxygen partial pressure environment.

A

(iii)

The formed WO3 oxidation product plays great function on the oxidation behavior of the ZrB2-SiC coating: the appropriate dissolution of WO3 could increase the viscosity of the liquid and then decrease the oxygen diffusion; WO3 with optimum content could stabilize borosilicate glass and reduce its evaporation; the eutectic of WO3-ZrO2 phases contributed to forming a denser

ZrO2 layer to impede oxygen diffusion.

Acknowledgement This work was supported by the National Natural Science Foundation (for Young

IP T

Scholar) of China under Grant 51102267, Engineering case study in extreme conditions using system mechanics approach (XDB22010202) and Youth Innovation

SC R

Promotion Association CAS (2014223).

U

Data Availability

N

The research data supporting this publication are directly available within this

M

A

publication.

ED

References

PT

[1] W.G. Fahrenholtz, G.E. Hilmas, Ultra-high temperature ceramics: materials for extreme environments, Scripta Mater. 129 (2017) 94-99.

CC E

[2] F. Monteverde, L. Scatteia, Resistance to thermal shock and to oxidation of metal diborides-SiC ceramics for aerospace application, J. Am. Ceram. Soc. 90 (2007)

A

1130-1138.

[3] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (2007) 1347-1364. [4] A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, D.T. Ellerby, High-strength zirconium diboride-based ceramics, J. Am. Ceram. Soc. 87 (2004) 1170-1172.

[5] A.K. Kuriakose, J.L. Margrave, The oxidation kinetics of zirconium diboride and zirconium carbide at high temperatures, J. Electrochem. Soc. 111 (1964) 827-831. [6] B. Mattuck, High-temperature oxidation. III.zriconium and hafunium diborides, J. Electrochem. Soc. 113 (1966) 908-914.

of ZrB2, HfB2 and TiB2, Acta Mater. 55 (2007) 5999-6010.

IP T

[7] T.A. Parthasarathy, R.A. Rapp, M. Opeka, R. J. Kerans, A model for the oxidation

SC R

[8] F. Monteverde, A. Bellosi, Oxidation of ZrB2-based ceramics in dry air, J. Electrochem. Soc. 150 (2003) B552-B559.

U

[9] J. Han, P. Hu, X. Zhang, S. Meng, W. Han, Oxidation-resistant ZrB2-SiC

N

composites at 2200 oC, Compos. Sci. Technol. 68 (2008) 799-806.

A

[10] S.S. Hwang, A.L. Vasiliev, N.P. Padture, Improved processing, and

M

oxidation-resistance of ZrB2 ultra-high temperature ceramics containing SiC

ED

nanodispersoids, Mat. Sci. Eng. A-Struct. 464 (2007) 216-224. [11] S.N. Karlsdottir, J.W. Halloran, Oxidation of ZrB2-SiC: influence of SiC content

PT

on solid and liquid oxide phase formation, J. Am. Ceram. Soc. 92 (2009) 481-486.

CC E

[12] E. Eakins, D.D. Jayaseelan, W.E. Lee, Toward oxidation-resistant ZrB2-SiC ultra high temperature ceramics, Metall. Mater. Trans. A. 42A (2011) 878-887. [13] W.G. Fahrenholtz, Thermodynamic analysis of ZrB2-SiC oxidation: formation of

A

a SiC-depleted region, J. Am. Ceram. Soc. 90 (2007) 143-148. [14] J. Wang, L. Zhang, Q. Zeng, G.L. Vignoles, A. Guette, Theoretical investigation for the active-to-passive transition in the oxidation of silicon carbide, J. Am. Ceram. Soc. 91 (2008) 1665-1673.

[15] L. Silvestroni, G. Meriggi, D. Sciti, Oxidation behavior of ZrB2 composites doped with various transition metal silicides, Corros. Sci. 83 (2014) 281-291. [16] Z. Wang, Y. Niu, C. Hu, H. Li, Y. Zeng, X. Zheng, M. Ren, J. Sun, High temperature oxidation resistance of metal silicide incorporated ZrB2 composite

IP T

coatings prepared by vacuum plasma spray, Ceram. Int. 41 (2015) 14868-14875. [17] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Improved oxidation resistance of

SC R

zirconium diboride by tungsten carbide additions, J. Am. Ceram. Soc. 91 (2008) 3530-3535.

U

[18] S.C. Zhang, W.G. Fahrenholtz, G.E. Hilmas, Oxidation of ZrB2 and ZrB2-SiC

N

ceramics with tungsten additions, ECS Transactions 16 (2009) 137-145.

A

[19] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Oxidation of zirconium diboride

M

with tungsten carbide additions, J. Am. Ceram. Soc. 94 (2011) 1198-1205.

ED

[20] M.K. Dehdashti, W.G. Fahrenholtz, G.E. Hilmas, Effects of temperature and the

221-228.

PT

incorporation of W on the oxidation of ZrB2 ceramics, Corros. Sci. 80 (2014)

CC E

[21] L. Silvestroni, D. Sciti, F. Monteverde, K. Stricker, H. Kleebe, Microstructure evolution of a W-doped ZrB2 ceramic upon high-temperature oxidation, J. Am. Ceram. Soc. 100 (2017) 1760-1772.

A

[22] J. Zou, V. Rubio, J. Binner, Thermoablative resistance of ZrB2-SiC-WC ceramics at 2400 oC, Acta Mater. 133 (2017) 293-302. [23] C. Bartuli, T. Valente, M. Tului, Plasma spray deposition and high temperature characterization of ZrB2-SiC protective coatings, Surf. Coat. Tech. 155 (2002)

260-273. [24] C.A.A. Cairo, M.L.A. Graca, C.R.M. Silva, J.C. Bressiani, Functionally gradient ceramic coating for carbon-carbon antioxidation protection, J. Eur. Ceram. Soc. 21 (2001) 325-329.

IP T

[25] E.L. Corral, L.S. Walker, Improved ablation resistance of C-C composites using zirconium diboride and boron carbide, J. Eur. Ceram. Soc. 30 (2010) 2357-2364.

SC R

[26] S. Tang, J. Deng, S. Wang, W. Liu, Comparison of thermal and ablation

behaviors of C/SiC composites and C/ZrB2-SiC composites, Corros. Sci. 51 (2009)

U

54-61.

N

[27] Y. Niu, L. Huang, C. Zhai, Y. Zeng, X. Zheng, C. Ding, Microstructure and

A

thermal stability of TaSi2 coating fabricated by vacuum plasma spray, Surf. Coat. Tech.

M

279 (2015) 1-8.

ED

[28] H. Pu, Y. Niu, C. Hu, G. Wang, H. Li, Y. Zeng, X. Zheng, Ablation of vacuum plasma sprayed TaC-based composite coatings, Ceram. Int. 41 (2015) 11387-11395.

PT

[29] C. Zhai, Y. Niu, L. Huang, H. Pan, H. Li, X. Zheng, J. Sun, Microstructure

CC E

characteristics and oxidation behavior of vacuum plasma sprayed tungsten disilicide coating, Ceram. Int. 42 (2016) 18798-18805. [30] L. Silvestroni, D. Sciti, TEM analysis, mechanical characterization and oxidation

A

resistance of a highly refractory ZrB2 composite, J. Alloy. Compd. 602 (2014) 346-355. [31] M.W. Chase Jr., NIST-JANAF Thermochemical Tables, fourth ed., American Institute of Physics, Woodbury, New York, 1998.

[32] D.D. Jayaseelan, E. Zapata-Solvas, R.J. Chater, W.E. Lee, Structural and compositional analyses of oxidised layers of ZrB2-based UHTCs, J. Eur. Ceram. Soc. 35 (2015) 4059-4071. [33] R. Akagi, N. Ohtori, N. Umesaki, Raman spectra of K2O-B2O3 glasses and melts,

IP T

J. Non-Cryst. Solids 293 (2001) 471-476. [34] D. Maniu, T. Iliescu, I. Ardelean, R. Ciceo-Lucacel, M. Bolboaca, W. Kiefer,

SC R

Raman study of B2O3-SrO-CuO glasses, Vib. Spectrosc. 29 (2002) 241-244.

[35] D. Maniu, T. Iliescu, I. Ardelean, S. Cinta-Pinzaru, N. Tarcea, W. Kiefer, Raman

U

study on B2O3-CaO glasses, J. Mol. Struct. 651 (2003) 485-488.

N

[36] L.L.Y. Chang, M.G. Scroger, B. Phillips, Condensed phase relations in the

A

systems ZrO2‐WO2‐WO3 and HfO2‐WO2‐WO3, J. Am. Ceram. Soc. 50 (1967)

A

CC E

PT

ED

M

211-215.

U

SC R

IP T

Figure Captions

A

CC E

PT

ED

M

A

N

Figure 1: XRD patterns of as-received powders (a) and as-sprayed coatings (b).

Figure 2: Surface morphologies of ZS, ZSW5, ZSW10 and ZSW15 coatings.

IP T SC R U N A

A

CC E

PT

ED

M

Figure 3: Fracture morphologies of ZS, ZSW5, ZSW10 and ZSW15 coatings.

Figure 4: Cross-sectional morphology and element maps of ZSW10 coating.

IP T SC R

N

U

Figure 5: Macroscopic photos of ZS, ZSW5, ZSW10 and ZSW15 coatings after

A

CC E

PT

ED

M

A

oxidation at 1500 oC for 1, 3, 6 h.

Figure 6: Specific weight changes as a function of oxidation time for ZS, ZSW5, ZSW10 and ZSW15 coatings.

IP T SC R U N A M ED PT

Figure 7: Fracture morphologies and element maps of the coatings after oxidation for

CC E

1 h: (a) ZS coating, (b) ZSW5 coating, (c) ZSW10 coating, (d) ZSW15 coating and (e)

A

the element maps of ZSW10 coating.

IP T SC R

Figure 8: X-ray diffraction patterns of ZS, ZSW5, ZSW10 and ZSW15 coatings after

A

CC E

PT

ED

M

A

N

U

oxidation at 1500 oC for 3 h.

Figure 9: Fracture morphologies of ZS (a) and ZSW10 (b) coatings after 3 h

CC E

PT

ED

M

A

N

U

SC R

IP T

oxidation.

A

Figure 10: Fracture morphologies of coatings after 6 h oxidation: (a) ZS coating, (b) ZSW5 coating, (c) ZSW10 coating, (d) ZSW15 coating and (e) typical microstructure of ZSW5 coating.

IP T SC R

Figure 11: Thermodynamic stability diagram of ZrB2 and WB under different oxygen

A

CC E

PT

ED

M

A

N

U

pressures.

Figure 12: Volatility diagram of ZrB2-SiC-WB system at 1800 K.

IP T SC R U N A M ED

A

CC E

PT

Figure 13: Vapor pressure diagram of WO3, B2O3, SiO2 and ZrO2.

Figure 14: Cation field strength of groups IVB, VB and VIB elements.

IP T

Table 1 Summary of literatures on tungsten-containing ZrB2 based ceramics. Material

Fbrication method

Test conditon

ZrB2-WC-B4C

bulk ceramic

pressureless sintering

oxidation, 1500, 1600 oC for 1, 2, 3h

[17]

ZrB2-WC-B4C ZrB2-WC-SiC-B4C

bulk ceramic

pressureless sintering

oxidation, 1500, 1600 oC for 1, 2, 3h

[18]

ZrB2-WC-B4C

bulk ceramic

pressureless sintering

oxidation, 1500, 1600 oC for 1, 2, 3h

[19]

ZrB2-MeSi2 (Me=Zr, Mo, Ta, W)

bulk ceramic

hot pressing

oxidation, 1200, 1300, 1500, 1650oC for 15min

[15]

ZrB2-W-B4C

bulk ceramic

hot pressing

heated from 800 to 1600 oC

[20]

ZrB2-WSi2

bulk ceramic

hot pressing

oxidation, 1650 oC for 15 min

[21]

ZrB2-SiC-WC

bulk ceramic

spark plasma sintering

ablation, 2400 oC for 300 s

[22]

U

N

A

M

ED PT CC E A

SC R

Composition

Ref.