Accepted Manuscript

Ablation Behavior and Mechanism of SiC/Zr-Si-C Multilayer Coating for PIP-

C/SiC Composites Under Oxyacetylene Torch Flame

Xiang Yang, Li Wei, Wang Song, Chen Zhao-hui, Zhang Yan-zhi

PII: S1359-8368(13)00608-2

DOI: http://dx.doi.org/10.1016/j.compositesb.2013.10.033

Reference: JCOMB 2710

To appear in: Composites: Part B

Received Date: 31 October 2012

Revised Date: 9 October 2013

Accepted Date: 25 October 2013

Please cite this article as: Yang, X., Wei, L., Song, W., Zhao-hui, C., Yan-zhi, Z., Ablation Behavior and Mechanism

of SiC/Zr-Si-C Multilayer Coating for PIP-C/SiC Composites Under Oxyacetylene Torch Flame, Composites: Part

B (2013), doi: http://dx.doi.org/10.1016/j.compositesb.2013.10.033

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1

Ablation Behavior and Mechanism of SiC/Zr-Si-C Multilayer Coating for

PIP-C/SiC Composites Under Oxyacetylene Torch Flame

Xiang Yang, Li Wei∗, Wang Song, Chen Zhao-hui, Zhang Yan-zhi

Science and Technology on Advanced Ceramic Fibers and Composites Laboratory,

National University of Defense Technology, Changsha 410073, China

Abstract: To improve the ablation resistance of PIP-C/SiC composites, SiC/Zr-Si-C

multilayer coating was prepared by chemical vapor deposition (CVD) using

methyltrichlorosilane (MTS) and hydrogen as the precursors and molten salt reaction

using KCl–NaCl, sponge Zr and K2ZrF6, then the ablation capability of the coated

composites was tested under oxyacetylene torch flame. The linear and mass ablation

rates were much lower than those of uncoated samples. The linear and mass ablation

rates of the three coating coated samples reached 0.0452mm/s and 0.031g/s, decreased

by 27.3% and 27.1%, respectively. Moreover, the linear and mass ablation rates of the

five coating coated samples reached 0.0255mm/s and 0.0274g/s, decreased by 59.0%

and 35.5%. The gases released during ablation could take away a lot of heat, which

was also helpful to the protection of the composites.

Key words: A. Ceramic-matrix composites (CMCs) ; B. Corrosion;

B. High-temperature properties; E. Chemical vapor deposition (CVD); Molten Salt;

1 Introduction

As one of the most popular methods to fabricate C/SiC composites, PIP route has ∗ Corresponding author: Tel: +86 731 84576441; Fax: +86 731 84573165

E-mail address: liwei3205@163.com (Dr. Li Wei).

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been actively developed [1~5]. However, before the practical application of C/SiC in

these cases, investigation on the evolution of morphology and microstructure during

ablation is required.

Oxyacetylene torch flame ablation method is the simplest and easiest way to

conduct with the lowest cost [6]. In our earlier research, the evolution of the

morphology and microstructure of PIP-C/SiC composites during ablation were

reported [7], which meant that PIP-C/SiC composites showed a worse ablative

property under oxyacetylene torch flame. Thus, the protection coating of PIP-C/SiC

composites needed further research.

Ultra high temperature ceramics (UHTCs), such as refractory carbides/borides

ceramics, were considered as candidates for protection coating on the surface of

PIP-C/SiC composites [8]. Among this material family, the compound of Zirconium

had the lowest theoretical density but a high melting point [9]. By adding fine SiC, the

oxidation resistance property and high temperature strength of the Zirconium

compound based ceramics were further enhanced [10, 11]. So multilayer coatings

containing both Si and Zr were especially attractive and should be extensively

explored as oxidation-resistant coatings.

In this paper, we report a new multilayer coating, dense SiC layers alternated

with porous Zr-Si-C layers, for PIP-C/SiC composites. The dense SiC layers are

prepared by CVD and the porous Zr-Si-C layers are made by molten salt reaction. The

Zr-Si-C layers in the new multilayer coating are porous, which are obviously different

from the traditional dense and crack-free coatings made by pack cementation or single

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CVD. To our knowledge, there have been no reports on using CVD combined with

molten salt reaction to prepare coatings on the surface of PIP-C/SiC composites. The

ablation property is evaluated under an oxyacetylene torch flame.

2 Experimental

2.1 Composites fabrication

Three-dimensional braided carbon fibers (T-300, ex-PAN carbon fiber, Toray)

were used as the reinforcement [4]. The fiber volume fraction was ∼45%.

Polycarbosilane (PCS) powers with molecular weight ~1742 and soften point ~448K

were synthesized in our laboratory. Xylene was used as solvent for PCS. C/SiC

composites denoted as raw samples were prepared using 9~12 cycles of infiltration of

PCS-Xylene solution (mass ratio 1:1) and subsequently pyrolysised at 1473K under

N2 (purity: 99.99%) atmosphere [4].

The substrates (30mm×30mm×4 mm) were cut from C/SiC composites with a

density of 2.01 g/cm3 and an average open porosity of 11.4% [4].

2.2 Coating fabrication

As for the CVD SiC process, MTS with a molar ratio of 10 between H2 and MTS

was carried by bubbling hydrogen in gas phase. Argon was used as the dilute gas to

slow down the chemical reaction rate during deposition. The deposition temperature

was controlled at 1373K for 5 h under a reduced pressure of 3kPa [8].

As for Zr-Si-C coating process, a mixture of equimolar KCl-NaCl, sponge Zr and

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20wt.% K2ZrF6 was put in the alumina crucible and heated to 1373K under an Ar

atmosphere. The samples were immersed in the bath of molten salt for 3h.

2.3 Ablation experiment

The ablation properties of the samples were tested with an oxyacetylene torch,

and the oxyacetylene flame was parallel to the axial orientation of samples. The

pressure and flux of O2 were 0.4 MPa and 1512L/h, and those of C2H2 were 0.095

MPa and 1116 L/h, respectively. The inner diameter of the nozzle was 8 mm. The

distance between the nozzle tip and the samples was 10 mm. The samples were fixed

in a water-cooled copper concave fixture and exposed to the flame with an estimated

temperature of 2373 K for 30s. The linear and mass ablation rates of the samples

could be obtained according to the formulas below:

t

ΔdR l =

(1)

t

ΔmR m =

(2)

Rl is the linear ablation rate; Δd is the change of the samples’s thickness at center

region before and after ablation; Rm is the mass ablation rate; Δm is the samples’s

mass change before and after ablation; t is the ablation time.

The microstructure and morphology of the samples were analyzed by a field

emission scanning electron microscopy (SEM, JSM-5600LV) combined with energy

dispersive spectroscopy (EDS).

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3 Results and discussion

3.1 Composition and Microstructure of SiC/Zr-Si-C Multilayer Coating

Fig. 1 shows the cross-section images of the two different coatings. Two types

were prepared: the first is SiC/Zr-Si-C/SiC, the second is

SiC/Zr-Si-C/SiC/Zr-Si-C/SiC. The characteristic of CVD SiC coating is dominant in

the coatings and no visible microcracks exist. The first coating is about 70 µm thick

and display a sandwich-like structure, dense CVD SiC layer alternates with one

Zr-Si-C layer. Obvious interfaces are found between the SiC bonding layer and the

substrate, and between SiC layer and the Zr-Si-C layer. The average diameter of SiC

coating is 30~40μm and that of Zr-Si-C coating is only 10μm. The second coating is

about 100 µm thick and display a sandwich-like structure too. Three dense CVD SiC

layers alternate with two Zr-Si-C layers.

The results are still confirmed by EDS patterns of the coatings (Fig. 2). The

results show that the SiC/Zr-Si-C/SiC coating can be fabricated by CVD and molten

salt reaction. It is believed that dense coating can be gained at the temperature.

3.2 Ablation behavior of coated PIP-C/SiC composites

Tab.1 shows the ablation property of PIP-C/SiC composites, which indicates that

uncoated C/SiC samples exhibit severe ablation under oxyacetylene torch (Tab.1).

Without protection, composites can be consumed rapidly, and the linear and mass

ablation rates reach 0.0622mm/s and 0.0425g/s. Compare with the uncoated C/SiC

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samples, the coated C/SiC samples exhibits better anti-ablative property, the coating

prevents the matrix from ablation efficiently. The linear and mass ablation rates of the

three coating coated samples reach 0.0452mm/s and 0.031g/s and decrease by 27.3%

and 27.1%, respectively, and the linear and mass ablation rates of the five coating

coated samples reach 0.0255mm/s and 0.0274g/s and decrease by 59.0% and 35.5%.

The compounds of Zr and SiC are high melted point materials, the oxidation of the

coating, the melting of the coating oxides, and the evaporation of SiO2 take away

large amount of heat, which is also beneficial to the protection for C/SiC composites.

The surface morphologies of PIP-C/SiC composites with the three coating after

ablating for 30s is shown in Fig.3. A great change in morphology is found on the

surface of the composites. At the beginning of ablation, the coating becomes loosen

and porous because of oxidization, with the ablation going on, the coating is

consumed out by the shearing action of the oxyacetylene flame, then PIP-C/SiC

composites is oxidized. SiC matrix is oxidized and its oxides sublimate mostly,

leaving naked needle-shaped carbon fibers (Fig.3(c)). Few of the oxides remain on the

surface of samples (Fig.3 (b)).

The surface morphologies of PIP-C/SiC composites with the five coating after

ablation for 30s are shown in Fig.4. The coating becomes loosen and porous too. SiC

matrix is oxidized, but its oxides do not sublimate completely, leaving some matrix in

the composites after ablation (Fig.4 (b)). More oxides of the coating remain on the

surface (Fig.4(c)).

To reveal the ablation behavior further, EDS patterns of different coatings are

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listed in Fig. 5. There are no obvious Zr phase generated in the three coating region.

EDS results show that there are C, O and Si, which indicated that the phases are

composed of carbides and oxides of Si, and the white and gray phases could be

distinguished as SiC and SiO2, respectively. So it can be inferred that the coating is

almost consumed out.

In the five coating region, the oxides of the coating still remain on the surface,

EDS results show that there are Zr, C, O and Si, which indicate that the phase is

composed of carbides and oxides of Si and Zr. The residual ZrO2 combines with SiC

grains because of the adhesive effect of glassy phase SiO2.

SEM images of the cross-section of PIP-C/SiC composites with SiC/Zr-Si-C

multilayer coating after ablation for 30s are shown in Fig.6. A great change in

morphology occurs on the surface of the composites. From the morphologies of the

center (Fig.6 (b)) and transition (Fig.6 (a)) region, the ablation behavior can be

inferred. The central region is ablated by the core of oxyacetylene flame and its

ablation is more severe. The coating is oxidized and washed away by the shearing

action of the oxyacetylene flame, leaving naked needle-shaped carbon fibers. In the

transition region, the mechanical denudation becomes weakened; the chemical erosion

becomes the leading mechanism. The coating cannot be washed out during ablation.

So the coating and C/SiC remains in this region.

3.3 Ablation mechanism of coated PIP-C/SiC composites

Ablation is an erosive phenomenon with a removal of material by a combination

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of thermo-mechanical, thermo-chemical, and thermo-physical factors from high

temperature, pressure, and velocity of combustion flame. There are mainly two kinds

of ablation mechanism: chemical erosion and mechanical denudation. The chemical

erosion refers to the reactions between the material and the combustion gases (O2,

H2O, etc.). The mechanical denudation means the peeling of the coating caused by the

flame with high-temperature, high velocity and pressure [12].

The ablation mechanism can be inferred from the SEM images of the composites

after ablation. The samples with the three coating are more severely ablated. The

coating is oxidized in priority and becomes loosen and porous, which provides more

channels for oxygen diffusing into composites. With the ablation going on, the oxides

are washed away by the shearing action of the oxyacetylene flame. While, after 30s’

ablation, the coating is consumed out, few of the oxides remain on the surface. As a

result, C/SiC composite is oxidized. SiC matrix is oxidized and its oxides sublimate

completely, leaving naked needle-shaped carbon fibers in the central region of the

sample surface (Fig.3).

With the five coating, the mechanical denudation becomes weakened. The matrix

cannot be washed out during ablation. So there are more fiber and SiC matrix in this

region (Fig.5).

During ablation, the samples suffer the highest temperature (>2273K) and

pressure, the main ablation behaviors are the chemical erosion related to oxidation

and erosion. There are severe reactions between reactive gases and PIP-C/SiC

composites with multilayer coating. In the present case, the main expected reactions

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during the oxidation process are as follows [13, 14]:

SiC(l)SiC(s) → (3)

CO(g)SiO(g)(g)OSiC(l) 2 +→+ (4)

CO(g)(l)SiO(g)OSiC(l) 22 +→+ (5)

CO(g)SiO(g)(l)SiOSiC(l) 2 +→+ (6)

O(g)S2(s)ZrO4(g)O5Si(s)2Zr 222 i+→+ (7)

CO(g)(s)ZrO(g)OZrC(s) 22 +→+ (8)

CO(g)(g)OC(s) 2 +→+ (9)

During ablation, as the ablation temperature reaches 2273K, the reactions about

SiC based coating ((3)-(6)) happen firstly, and then the reactions (7) and (8) happen.

After the coating being consumed out, SiC matrix ((3)-(6)) is oxidized, and the

reaction (9) follows. It can be found that: reaction (3) is a melting process and can

consume lots of heat during ablation; there are large amounts of gas generated, which

can also take heat away from the surface of the coating. The temperature of the flame

region can be lowered according to the reactions, which is the reason of well

anti-ablative property of multilayer coating.

Additionally, reactions (5), (7) and (8) are all weight-gain process, which is the

reason why the coated samples have low mass ablation rate.

Concerning the surface morphology of uncoated PIP-C/SiC composites, as

shows by Li Wei [7], evident discrepancy can be found in comparison with coated

PIP-C/SiC composites. The matrix is almost consumed out and fibers are oxidized

after 30s’ ablation, which is more serious. It indicates that the uncoated PIP-C/SiC

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composites show worse anti-ablative property under oxyacetylene torch. After 30s’

ablation, the composites are ablated severely and the sample shape is destroyed

completely. Thus, the multilayer coating exhibits good ablation resistance.

According to the ablation behavior of PIP-C/SiC composites, the ablation

process may be divided into three steps. Fig.7 shows the sketch map of the ablation

process of PIP-C/SiC composites. Firstly, oxidation of SiC coating begins at a relative

lower temperature to form a porous structure, then the oxides of SiC is blew away by

the flame, leaving the Zr-Si-C coating. In this step, the ablation rate is controlled by

the oxidation rate of SiC. Secondly, the oxides of Zr-Si-C coating melt while the

temperature raises over 2473K. In the center of the samples, high speed gas flow led

to ZrO2-SiO2 melt and spread on the ablated surface. In other regions, some of the

melt ZrO2-SiO2 fill some pores. The melt ZrO2-SiO2 formation prevented the further

oxidation of carbon fibers and SiC matrix, showing a better anti-ablative behavior.

Finally, with the ablation time going on, the coating is consumed out in the center of

the samples, and C/SiC composites show worse ablative property under oxyacetylene

torch.

4. Conclusions

By studying the ablation behaviors of PIP-C/SiC composites with SiC/Zr-Si-C

multilayer coating in the oxyacetylene flame conditions, the following conclusions

could be made:

(1) The as-prepared coating protected the PIP-C/SiC samples efficiently. The

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linear and mass ablation rates were much lower than that of uncoated samples. The

linear and mass ablation rates of the three coating coated samples reached

0.0452mm/s and 0.031g/s and decreased by 27.3% and 27.1%, respectively, and the

linear and mass ablation rates of the five coating coated samples reached 0.0255mm/s

and 0.0274g/s and decreased by 59.0% and 35.5%.

(2) ZrO2-SiO2 produced from Zr-Si-C oxidation could improve the ablation

resistance of PIP-C/SiC composites. The gases released during ablation could take

away a lot of heat, which was also helpful to the protection for the composites.

Acknowledgements

The authors are grateful to National Natural Science Foundation of China

(90916002) for financial support. In addition the authors are grateful to Aid Program

for Science and Technology Innovative Research Team in Higher Educational

Institutions of Hunan Province.

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Tables Caption

Tab.1The ablation property of PIP-C/SiC composites

Figures Caption

Fig.1 Cross-section morphology of as-prepared SiC/Zr-Si-C multilayer coating (a: the

three coating; (b) the five coating)

Fig.2 EDS of SiC/Zr-Si-C multilayer coating

Fig.3 Ablation morphology of the three coating coated samples

Fig.4 Ablation morphology of the five coating coated samples

Fig.5 EDS of the coated samples after ablation.

Fig.6 Cross-section morphology of the coated samples after ablation.

Fig.7 Schematical of the coated samples during ablation

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16

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Table.1 The ablation property of PIP-C/SiC composites

Samples Linear ablation rate

(mm/s)

Mass ablation rate

(g/s)

PIP-C/SiC composites [8]

0.0622 0.0425

SiC/Zr-Si-C/SiC coating

0.0452 0.031

SiC/Zr-Si-C/SiC/Zr-Si-C/SiC

coating

0.0255 0.0274