Journal of Alloys and Compounds 781 (2019) 26e36
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and ablation properties of SiC/ZrB2eSiC/ZrB2/SiC
multilayer coating on graphite
Peng Wang a, *, Sujuan Li a, Chuncheng Wei a, Wenbo Han c, Guangwu Wen a, Xin Geng a,
Shuang Li b, Haibin Sun a, Xiaowei Li a
a
b
c
School of Materials Science and Engineering, Shandong University of Technology, Zibo, 255049, PR China
School of Resources and Environmental Engineering, Shandong University of Technology, Zibo, 255049, PR China
Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin, 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 September 2018
Received in revised form
26 November 2018
Accepted 4 December 2018
Available online 5 December 2018
To improve ablation resistance of graphite, SiC/ZrB2-SiC/ZrB2/SiC coating was prepared by pack
cementation and CVD. SiC/ZrB2eSiC/ZrB2/SiC coating showed excellent ablation resistance, its mass and
linear ablation rate were only 0.27 mg/s and 0.57 mm/s after ablation under oxyacetylene flame for 298 s.
CVD ZrB2/SiC layer significantly improved the ablation resistance of SiC/ZrB2eSiC/ZrB2/SiC coating.
During ablation, the surface temperature gradually increased until it stabilized around 2220 C. The
surface temperature response in different stages was affected by layered microstructure evolution and
emissivity of different layers.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
SiC/ZrB2eSiC/ZrB2/SiC coating
Chemical vapor deposition
Ablation resistance
Microstructure
1. Introduction
Graphite is an appealing material for high temperature application due to its high strength, high modulus, excellent thermal
shock resistance and light weight [1]. It has been widely used in
various engineering applications, such as heaters, electrical contacts, high-temperature heat exchangers, rocket nozzles and leading edges of aircraft wings, and so on [2]. However, the poor
oxidation and ablation resistance have greatly restricted its application in oxidizing atmosphere. Improving oxidation and ablation
protective ability is crucial to realize its potential in hightemperature environment.
A great deal of work has been conducted on developing effective
oxidation and ablation protective systems for graphite, carbon and
C/C composites. Preparation multilayer coating is a suitable method
to prevent graphite from oxidation and ablation [3e5]. Due to the
good chemical and physical compatibility, SiC is commonly used as
a protective layer. But, single SiC coating can not withstand high
temperature above 1600 C [6]. Preparing an ultra-high temperature ceramic (UHTC) coating onto SiC layer is expected to further
* Corresponding author.
E-mail addresses: wangpeng1@126.com, wangpeng1@sdut.edu.cn (P. Wang).
https://doi.org/10.1016/j.jallcom.2018.12.045
0925-8388/© 2018 Elsevier B.V. All rights reserved.
promote oxidation and ablation resistance, because UHTCs have
excellent high temperature performance [3,7,8]. ZrB2eSiC is
currently considered as the baseline of UHTCs [9] due to its excellent oxidation and ablation resistance in oxidizing atmosphere at
high temperature. ZrB2eSiC based coatings are promising candidates for use in extreme environments and have attracted great
interest. Currently, many methods are used to apply ZrB2eSiC
based coatings on the surface of carbon based materials: plasma
spray [10], pack cementation [11], vapor silicon infiltration (VSI)
[12], in-situ reaction method [13,14], slurry method [15], and
combination of these methods [16] But, the ZrB2eSiC based coatings usually contain many defects or pores because of the preparing
process, which seriously affect the oxidation and ablation protective ability. To further improve microstructure, vapor silicon infiltration (VSI) and CVD SiC methods are applied. In vapor silicon
infiltration (VSI) process, pores and defects in the samples could be
filled by low vapor pressure of silicon [12]. In CVD process, SiC
penetrates in the gaps between powders and a dense SiC layer
forms on the top [15].
In this work, ZrB2 and SiC layers were introduced by CVD to
prepare SiC/ZrB2-SiC/ZrB2/SiC multilayer coating to further
improve ablation resistance. The microstructural evolution of the
multilayer coatings was characterized. The ablation behavior and
mechanism of SiC/ZrB2-SiC/ZrB2/SiC coating was also discussed.
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
2. Experimental procedure
Small graphite specimens (1.765 g/cm3, purity >99%) were
chosen as substrates. The samples were hand abraded using 600
grit SiC paper, then cleaned and dried. The SiC/ZrB2-SiC (ZS50-3f)
coating was prepared on graphite substrates by the process of pack
cementation. The preparation details have been reported elsewhere [17]. The coating was used as transition layer for SiC/ZrB2SiC/ZrB2/SiC multilayer coating.
SiC/ZrB2-SiC/ZrB2/SiC coating was prepared by two step CVD
deposition on SiC/ZrB2-SiC coated graphite samples by pack
cementation. Firstly, the ZrB2 layer was deposited at 1200 C for 2 h.
240 L/h Ar (>99.99%), 5 L/h BCl3 (>99.999%) and 240 L/h 95 vol%
Are5 vol% H2 (>99.99%) gas mixture was delivered to the reactor.
51.05 g ZrCl4 (99.99%) powder was uniformly fed into the reaction
tube in 2 h by a powder feeder and carried to the central reaction
zone by the flow of BCl3, Ar and H2 mixture. Secondly, the outmost
SiC layer was deposited by CVD under normal pressure using
methyl trichlorosilane (MTS, >99%) as precursor. The deposition
temperature was 1100 C, and holding time was 2 h. During deposition, 240 L/h 95 vol% Are5 vol% H2 gas mixture was used as both
the carrier gas and diluent gas. The AreH2 gas mixture transferred
MTS through a bubbler to the reactor. 147.99 g MTS was carried into
the reactor in 2 h. The detailed deposition parameters are given in
Table 1.
The ablation resistance of the multilayer coated graphite samples were tested with an oxyacetylene torch, and the oxyacetylene
flame was parallel to the axial orientation of sample. The flux of O2
and C2H2 were 1800 L/h and 1900 L/h, respectively. The inner
diameter of the nozzle was 3 mm. The distance between the nozzle
tip and the samples was 30 mm. The flame temperature was estimated by an infrared thermometer. The linear and mass ablation
rates of the samples could be obtained according to the formulas
below:
Rl ¼ Dd=t
(1)
Rm ¼ Dm=t
(2)
where Rl is the linear ablation rate; Dd is the thickness change of
the sample in central region before and after ablation; Rm is the
mass ablation rate; Dm is the mass change of the sample before and
after ablation; and t is the ablation time.
27
The microstructure and phase composition of SiC/ZrB2-SiC/ZrB2/
SiC coating before and after ablation were also investigated by
scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and Xeray diffraction (XRD). The effect of emissivity
on surface temperature curve was also discussed.
3. Results and discussion
3.1. Microstructure of SiC/ZrB2eSiC/ZrB2/SiC coating
SiC/ZrB2eSiC/ZrB2/SiC coating was prepared by two-step
depositing of ZrB2 and SiC coating on SiC/ZrB2eSiC coated
graphite sample. SiC/ZrB2eSiC coated sample was prepared by twostep pack cementation, which has been reported previously [17].
The inner SiC layer was fabricated using Si, graphite and Al2O3 as
raw materials at 1800 C. Al2O3 powders could increase the rate of
diffusing reaction and avoid solidification of the raw materials.
Then the outer ZrB2eSiC layer was prepared on the SiC coated
graphite samples using ZrB2, Si and graphite as precursor powders
at 2000 C. The SiC/ZrB2eSiC layer was bonded tightly to graphite
matrix and its loose structure was favorable to relax thermal
mismatch and improving thermal shock resistance. Moreover, the
SiC/ZrB2eSiC layer had good oxidation protective ability [17]. Good
oxygen consumption ability of inner layer was essential for
enhancing oxidation and ablation resistance of a multilayer coating.
Hence, SiC/ZrB2eSiC layer was used as transition layer for multilayer coatings.
To prepare SiC/ZrB2eSiC/ZrB2/SiC coating, ZrB2 layer was firstly
prepared on SiC/ZrB2eSiC coated graphite. The surface images of
SiC/ZrB2eSiC/ZrB2 are shown in Fig. 1(a) and (b). The ZrB2 coating
was composed by ZrB2 particles, which were evenly distributed on
SiC/ZrB2eSiC coated graphite sample. The ZrB2 layer structure was
loose with a lot of gaps. As shown in Fig. 1(c), ZrB2 particles filled in
the pits and defects on the surface of SiC/ZrB2eSiC coating. The
thickness of ZrB2 layer on pits was larger than that on bulge. The
ZrB2 layer, with a thickness of about 45 mm, protected the X-ray
from penetrating it, thus strong ZrB2 (PDF#34-0423) phase peaks
were detected by XRD, while only a very low SiC (PDF#29-1129)
diffraction peak was observed, as shown in Fig. 2.
Then SiC layer was deposited on the SiC/ZrB2eSiC/ZrB2 coated
sample. Fig. 3 displays surface images and EDS analysis of the SiC/
ZrB2eSiC/ZrB2/SiC coating. The outermost layer was the SiC coating
with homogenous and dense structure deposited by CVD. The EDS
Table 1
Experiment parameters for depositing ZrB2/SiC layers on SiC/ZrB2eSiC coated sample.
Step 1
Step 2
Deposition temperature ( C)
Deposition time (h)
ZrCl4 consumption in 2 h (g)
MTS consumption in 2 h (g)
Gas flow rate (L/h)
95%Ar-5%H2
Ar
BCl3
1200
1100
2
2
51.05
0
0
147.99
240
240
240
240
5
0
Fig. 1. Surface and cross-section SEM images of SiC/ZrB2eSiC/ZrB2 coating: (a), (b), Surface SEM images; (c) cross-section SEM image.
28
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
ZrB2eSiC coating prepared by pack cementation was uneven and
rough, but the ZrB2 layer filled effectively in the pits and defects on
the surface of SiC/ZrB2eSiC coating. The interlock structure was
beneficial for improving bond strength between SiC/ZrB2eSiC layer
and ZrB2 layer. The chemical vapor deposited SiC outermost layer,
with thickness of about 50 mm, also effectively filled the gaps in
ZrB2 layer, and improved the compactness of the multilayer
coating. The SiC/ZrB2eSiC/ZrB2/SiC coating had integrated structure, though there were still some pores especially in the interface
between SiC/ZrB2eSiC and ZrB2 layer.
3.2. Ablation properties of SiC/ZrB2eSiC/ZrB2/SiC coating
Fig. 2. XRD pattern of SiC/ZrB2eSiC/ZrB2 coating.
analysis, as shown in Fig. 3(c), also confirmed the component elements, silicon and carbon. No cracks were observed on surface of
the SiC coating, though SiC and ZrB2 have different thermal
expansion coefficients. The loose structure of CVD ZrB2 layer
relieved thermal mismatch between SiC and ZrB2 layer.
The crossesection morphology of SiC/ZrB2eSiC/ZrB2/SiC multilayer coating is shown in Fig. 4(a). Fig. 4(b), (c) and (d) show EDS
analyses at spot A, B and C. The results indicated the outermost
layer was composed of C and Si. Combined with the previous XRD
analysis, the ZrB2 layer was mainly composed of B and Zr, though B
content was low in the EDS spectrum because it is light element.
The C and Si detected were attributed to SiC in adjacent layers. The
SiC/ZrB2eSiC coating prepared by pack cementation was mainly
constituted by SiC with a small amount of ZrB2, which was
consistent with the report before [17]. The results of the EDS
analysis indicated the composition of different layers, which was
also confirmed by XRD patterns (Figs. 2 and 5). ZrB2 (PDF#34-0423)
and SiC (PDF#29-1129) peaks were observed from the XRD patterns
of SiC/ZrB2eSiC/ZrB2/SiC coating in Fig. 5, which were caused by the
CVD ZrB2 and SiC layer. Compared with the peaks of SiC/ZrB2eSiC
coating prepared by pack cementation reported by us [17], the
strongest peak of the outmost SiC layer became wider, which
indicated that the SiC crystals in the outermost CVD SiC coating
were much smaller than that in SiC/ZrB2eSiC coating according to
Scherrer's formula.
As shown in Fig. 4(a), there was a distinct interface between SiC/
ZrB2eSiC and chemical vapor deposited ZrB2 layer as well as between ZrB2 layer and the outermost SiC layer. The surface of SiC/
3.2.1. Ablation resistance of SiC/ZrB2eSiC/ZrB2/SiC coating
Fig. 6 shows the surface temperature curve of SiC/ZrB2eSiC/
ZrB2/SiC coating during the oxyacetylene torch test. The flux of O2
and C2H2 were 1800 L/h and 1900 L/h, respectively. During ablation,
the surface temperature of SiC/ZrB2eSiC/ZrB2/SiC coating gradually
increased until it stabilized around 2220 C, with a maximum
temperature of 2259 C. Its temperature curve could be divided into
three temperature-increasing stages, as shown in Fig. 6. The layered
structure and different compounds with varied thermal-physical
properties, especially emissivity probably resulted in the stepwise
temperature rising, which would be discussed in 3.2.3 below.
After ablation for 298 s, the linear ablation and mass ablation
rate of the SiC/ZrB2eSiC/ZrB2/SiC coated sample were 0.57 mm/s
and 0.27 mg/s, respectively. For comparing and analyzing the influence of CVD ZrB2 layer on ablation resistance, SiC/ZrB2eSiC/SiC
and SiC/ZrB2eSiC/ZrB2 coatings were also prepared and tested. The
SiC/ZrB2eSiC/SiC coating was prepared by chemical vapor depositing SiC on SiC/ZrB2eSiC coated sample under the same process
parameters, but 177 g MTS was consumed in 2 h. The SiC/ZrB2eSiC/
SiC coated sample was also ablated under 1800 L/h O2 and 1900 L/h
C2H2. After ablation for 181 s, a much shorter time, the linear
ablation and mass ablation rate of the SiC/ZrB2eSiC/SiC coated
sample were 3.54 mm/s and 10.91 mg/s, respectively. The SiC/
ZrB2eSiC/ZrB2 coating was prepared by chemical vapor depositing
ZrB2 on SiC/ZrB2eSiC coated sample, but 52.21 g ZrCl4 was
consumed in 2 h. After ablation under the same condition for 180 s,
the linear ablation and mass ablation rate of SiC/ZrB2eSiC/ZrB2
coated sample were 5.0 mm/s and 6.02 mg/s, respectively. The CVD
ZrB2 layer with loose structure cannot effectively improve the
ablation resistance.
To compare the SiC/ZrB2eSiC/ZrB2/SiC coating with other coatings, the ablation properties of some SiC and ZrB2 based coatings
tested under similar ablation conditions were listed in Table 2.
Usually, coatings were tested in less than 100 s, but SiC/ZrB2eSiC/
ZrB2/SiC coating was ablated for longer time and still show excellent ablation resistance. Obviously, the ablation resistance of SiC/
ZrB2eSiC/ZrB2/SiC coating was better than that of SiC/ZrB2eSiC/SiC,
Fig. 3. Surface SEM images and EDS analysis of SiC/ZrB2eSiC/ZrB2/SiC coating: (a), (b) Surface SEM images; (c) EDS analysis of (b).
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
29
Fig. 4. Crossesection morphology of SiC/ZrB2eSiC/ZrB2/SiC multilayer coating and corresponding EDS analyses: (a) Crossesection morphology; (b) EDS at spot A; (c) EDS at spot B;
(d) EDS at spot C.
Fig. 5. Surface XRD patterns of SiC/ZrB2eSiC/ZrB2/SiC coating on graphite.
SiC/ZrB2eSiC/ZrB2 and SiC/ZrB2eSiC coatings, though it was ablated
for much longer time. The introduction of CVD ZrB2/SiC layers
substantially improved the performance of SiC/ZrB2eSiC/ZrB2/SiC
coating.
3.2.2. Microstructure of SiC/ZrB2eSiC/ZrB2/SiC coating after
ablation
Fig. 7 shows the macroscopic images of SiC/ZrB2eSiC/ZrB2/SiC
coated specimen before and after ablation. The SiC/ZrB2eSiC/ZrB2/
SiC coating showed excellent ablation resistance. After ablation for
298 s, only the central region of the specimen was damaged obviously, while the remaining area remained its integrity. In central
Fig. 6. Temperature curves vs. time during oxyacetylene torch testing of SiC/ZrB2eSiC/
ZrB2/SiC coated sample.
ablation region, white product layer with bumps was formed during ablation, as shown in Fig. 7(b). The white product in bulge region fell off when the sample was moved, as shown in Fig. 7(c),
which left pits on the surface. In contrast, the surrounding region
was not obviously ablated.
Fig. 8 gives the XRD pattern of SiC/ZrB2eSiC/ZrB2/SiC coated
sample after ablation. The pattern confirmed the presence of ZrB2,
SiC and ZrO2 phases. The presence of ZrB2 and SiC indicated a large
amount of ZrB2 and SiC phase was not oxidized after ablation,
which also mean that the coating had good ablation protective
30
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
Table 2
Properties comparison between the current and other reported coatings.
Samples
SiC/ZrB2eSiC/ZrB2/
SiCa
SiC/ZrB2eSiC/SiCa
SiC/ZrB2eSiC/ZrB2a
SiC [18]
SiC/ZrB2eSiC [19]
SiC/ZrB2-SiC [20]
SiC/ZrB2eSiCeSi [21]
SiC/SiCeZrC [22]
ZrCeSiC [23]
ZrB2-SiC/SiC [24]
ZrB2eCrSi2eSi/SiC
[25]
a
Ablation
time (s)
Linear ablation
rate (mm/s)
Flow rate (L/h)
O2
C2H2
298
0.57
1800
1900
181
180
50
60
200
30
60
20
30
90
3.54
5.0
12.6
4.27
4.647
0.21
3.3
10.5
23.6
1.64
1800
1800
1512
1512
1300
1440
7056
1512
1512
1440
1900
1900
1116
1116
1900
1440
2506
1116
1116
1440
The coatings prepared by us.
Fig. 7. Macroscopic images of SiC/ZrB2eSiC/ZrB2/SiC coated on graphite before and
after ablation: (a) before; (b), (c) after.
Fig. 8. XRD pattern of SiC/ZrB2eSiC/ZrB2/SiC coated sample after ablation.
ability. Except for the central ablation region, the remaining area
was well protected, unoxidized ZrB2 and SiC led to the XRD peaks
after ablation. Peaks of ZrO2 were mainly caused by the white
oxidation product of ZrB2 in central ablation region.
According to the surface morphology and extent of ablation, the
surface of SiC/ZrB2eSiC/ZrB2/SiC coating after ablation was divided
into five different ablation regions, as shown in Fig. 9. The surface
was divided into region II, III, IV and V (Fig. 9(a)), while a severe
ablation region I was confirmed by magnifying the image of region
II (Fig. 9(b)). The main characteristics in different regions were
summarized in Table 3. The SEM images and EDS analyses were
displayed in Figs. 10e14.
Fig. 10 shows the SEM images of ablation region I and
Fig. 9. Macroscopic and SEM image of SiC/ZrB2eSiC/ZrB2/SiC coated sample after
ablation: (a) macroscopic image; (b) SEM image of the center ablation region.
corresponding EDS analyses. In region I, the content of carbon was
high. The coating was completely ablated through in region I, so the
exposed graphite matrix resulted in high EDS peak of carbon. Some
white and gray particles scattered on the surface, as shown in the
magnified image of region I (Fig. 10(c)). By EDS analysis, the white
and gray phases could be discriminated as ZrO2 and SiC or its oxide,
respectively. The EDS analysis at spot C confirmed that the graphite
was exposed after ablation.
In ablation region II, the surface was covered by glass phase with
many pores (Fig. 11(a) and (c)). The glass phase was mainly
composed of Si and O, as indicated by Fig. 11(b) and (d), which was
deduced to be SiO2 glass formed by oxidation of SiC/ZrB2eSiC layers
adjacent to graphite matrix. Under high temperature and oxidizing
atmosphere, gas phases, such as CO2, CO, SiO2 and SiO, could be
produced because of the oxidation of SiC/ZrB2eSiC layer. The gasses
came out from bottom and inside the SiO2 glass led to pores on
surface of region II. Some white particles scattered on the glass
layer might be ZrO2. The high content of Si and C detected by EDS in
Fig. 11(e) and (f) was probably caused by the surrounding SiO2 glass
and unreacted SiC underneath the glass layer.
Fig. 12 displays SEM images and corresponding EDS of ablation
region III. Fig. 12(b) indicated high content of Zr and O in region III,
therefore the exposed region III should be ZrO2 layer formed by
oxidation of CVD ZrB2 layer. A certain content of C shown in
Fig. 12(b) might be caused by SiC or its oxide underneath the ZrO2
layer. Some pores were observed in region III from the magnified
image in Fig. 12(c) and (d). These pores might also be caused by the
escaping gasses produced by oxidation of the layers under CVD ZrB2
layer. During preparation of the multilayer coating, gaps in the
loose CVD ZrB2 layer were filled by CVD SiC. The SiC also oxidized
and its oxide, SiO2 volatilized during ablation, which could also
leave pores in region III. In Fig. 12(d), some branch-type white
oxidation product might be resulted from the strong gas flow.
Fig. 13 shows the images of ablation region IV and EDS analyses.
Some white island areas surrounded by dense glass phase were
seen from Fig. 13(a) and (b). The white areas had similar surface
morphology to region III, and the contents of Zr and O were high in
white areas (Fig. 13(c)). So, the white areas in region IV were also
the ZrO2 layer formed by oxidation of the CVD ZrB2 layer. The
surrounding area was composed by SiO2 glass phase confirmed by
Fig. 13(d), (e) and (f). The EDS at spot B, C and area D also showed
high content of C, which might be attributed to unreacted SiC underneath the glass film. Through the magnified images of region IV
(Fig. 13(g), (h) and (i)), it can be seen that the white area was loose
and mainly constituted by ZrO2 particles which was produced by
oxidation of ZrB2 particles. The microstructure shown in Fig. 13(i)
was similar to that in Fig. 1(b). The other area was covered by dense
SiO2 glass layer without pores. The EDS analysis (Fig. 13(j)) displayed a certain content of Si and C except for Zr and O in the white
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
31
Table 3
The main characteristics in different regions after ablation.
Region
I
Characteristics ablated
through
II
III
IV
V
covered by glass film with
pores
covered by ZrO2
layer
covered by dense SiO2 glass film with exposed ZrO2
regions
covered by dense SiO2 glass
film
Fig. 10. SEM images of ablation region I and corresponding EDS analyses: (a) SEM image at center ablation region; (b) EDS analyses of the region in the red box; (c) enlarged SEM
image of ablation region I; (d) EDS analysis of spot A; (e) EDS analysis of spot B; (d) EDS analysis of spot C. (For interpretation of the references to colour in this figure legend, the
reader is referred to the Web version of this article.)
area, indicating that some SiO2 glass existed in the island areas
which were also probably surrounded by neighboring SiO2 and SiC.
At the junction between the island and glass film, some white ZrO2
particles were embedded in the SiO2 glass film, as shown in
Fig. 13(h). The ZrO2 could improve the thermal stability of SiO2
glass, which was beneficial to the oxidation and ablation protective
ability.
The ablation region V was covered by compact glass film, as
shown in Fig. 14(a), (b) and (c). The surface EDS analysis (Fig. 14(d))
confirmed the film was SiO2 glass phase. The dense SiO2 glass film
protected the coating under it, therefore the region V was still
dense and integrated after ablation.
3.2.3. Ablation mechanism of SiC/ZrB2eSiC/ZrB2/SiC coating
During ablation, the outermost CVD SiC layer was first ablated.
Under high temperature and oxidizing atmosphere, the following
reactions possibly proceeded [26e28]:
SiC(s) þ O2(g) ¼ SiO2(l) þ CO2(g)
(3)
2SiC(s) þ 3O2(g) ¼ 2SiO2(l) þ 2CO(g)
(4)
SiC(s) þ O2(g) ¼ SiO2(l) þ CO(g)
(5)
2SiO2(l) þ SiC(s) ¼ 3SiO(g) þ CO(g)
(6)
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P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
Fig. 11. SEM images of ablation region II and corresponding EDS analyses: (a) SEM image at ablation region II; (b) EDS analysis of the region in the red box of (a); (c) SEM image of
ablation region II; (d) EDS analysis of spot A; (e) EDS analysis of spot B; (d) EDS analysis of spot C. (For interpretation of the references to colour in this figure legend, the reader is
referred to the Web version of this article.)
SiO2(l) ¼ SiO2(g)
(7)
In central ablation region, reactions (3)e(7) could happen at the
same time due to the high temperature. Reaction (3) was the main
reaction in regions away from the central ablation region, where
the temperature was much lower. SiO2 glass film formed by reaction (3) has low oxygen permeability and good antioxidant ability.
So, dense SiO2 glass provided a good oxidation resistant layer for
ablation region V.
In ablation region IV, closer to central ablation region, the
temperature and shear force caused by high speed gas flow were
higher than ablation region V. High temperature and different
thermal expansion coefficients led to high thermal stress between
SiC layer and ZrB2 layer. Higher shear force and thermal stress
resulted in partial spalling of SiC coating. After the SiC coating broke
off, ZrB2 was exposed to oxyacetylene flame and oxidized. Hence,
the special morphology was formed in region IV, where ZrO2 island
areas were surrounded by SiO2 glass film on SiC layer.
In ablation region III, due to more severe ablation condition than
region IV and V, the outer layers were severely oxidized and the
oxidation products were blown away by high speed gas flow. The
volatilization of liquid and gas products formed by reactions
(3)e(7), such as SiO2, CO2, etc., accelerated the ablation of CVD SiC
layer until it was completely ablated off. Then CVD ZrB2 layer was
exposed to high temperature torch and the following reactions took
place [27]:
2ZrB2(s) þ 5O2(g) ¼ 2ZrO2(s) þ 2B2O3(l)
(8)
B2O3(l) ¼ B2O3(g)
(9)
Due to low melting point (450 C) and high vapor pressure, B2O3
volatilized quickly under high temperature. The volatilization of
B2O3 led to the loss of good protective ability of B2O3 film on CVD
ZrB2 layer. So, the ZrB2 layer was oxidized quickly and ZrO2 layer
was formed in region III, which was confirmed by EDS analysis
(Fig. 12(b)).
SiO2 glass film was formed in ablation region II. As ablation
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
33
Fig. 12. SEM images of ablation region III and corresponding EDS analysis: (a) SEM image at ablation region III; (b) EDS analysis of the region in the red box of (a); (c), (d) enlarged
SEM images of ablation region III. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
continued, the inner SiC/ZrB2eSiC coating started to oxidize. During ablation test, the ZrO2 layer, formed from oxidation of CVD ZrB2,
did not fall off, though it was damaged and bulged (Fig. 7). Protected by the ZrO2 layer, the shear force caused by high speed gas
flow to SiC/ZrB2eSiC layer decreased, therefore particles and SiO2
glass were not easily blown away. Instead, SiO2 glass improved
oxidation resistance, especially combination with ZrO2 particles
which could enhance the thermal stability of SiO2 glass film. The
combination of SiO2 and ZrO2 formed by oxidation of ZrB2 was
conducive to improving the oxidation and ablation resistance of
SiC/ZrB2eSiC coating. According to reactions (1)e(5) and (7), gases
were produced during ablation of SiC/ZrB2eSiC coating. The
escaped gases from inside left pores and defects in ablation region
II. The scattering ZrO2 particles in region II probably came from the
ZrO2 layer formed by oxidation of CVD ZrB2 layer. ZrO2 particles
further improved the oxidation and ablation protective ability of
SiC/ZrB2eSiC layer.
Except ZrO2 particles and damaged coating, graphite matrix was
exposed in small areas in ablation region I. In central ablation region, the surface temperature reached to about 2220 C at the last
stage during ablation. Under the high temperature of about
2220 C, SiC/ZrB2eSiC layer was quickly oxidized, the products
volatilized and were blown away rapidly by high speed gas flow. So,
SiC/ZrB2eSiC coating lost its protective ability and graphite matrix
was exposed.
Compared with SiC/ZrB2eSiC/SiC, SiC/ZrB2eSiC/ZrB2/SiC coating
showed excellent ablation resistance. Under the same ablation
condition, only a very small area of SiC/ZrB2eSiC/ZrB2/SiC coated
sample was exposed after ablation for longer time. In contrast, SiC/
ZrB2eSiC/SiC coating was severely damaged though it was ablated
for a shorter time (181 s), and its linear ablation rate was about 6
times that of SiC/ZrB2eSiC/ZrB2/SiC coated sample while the mass
ablation rate was about 40 times as much. Compared with SiC/
ZrB2eSiC/SiC coating, introducing ZrB2 layer obviously improved
the ablation resistance of SiC/ZrB2eSiC/ZrB2/SiC coating. After
ablation, the ZrO2 layer formed in central ablation region showed
excellent ablation resistance. Moreover, SiO2 and ZrO2 formed
during ablation jointly improved ablation and oxidation resistance,
because ZrO2 improved the thermal stability of SiO2 under high
temperature. As reported before [14], ZrB2eSiC coating prepared by
pack cementation was not as dense as CVD SiC coating and the
content of ZrB2 in the coating was low, which was limited by the
preparation process and the method of introducing ZrB2 particles.
So, the ablation resistance of SiC/ZrB2eSiC/SiC coating was mainly
dependent on SiC phase. In contrast, CVD ZrB2 layer provided high
content of ZrB2. Moreover, CVD ZrB2 layer was tightly bonded with
CVD SiC layer, and the two components and their oxidation products jointly contributed to excellent ablation resistance. Besides,
the good ablation resistance of ZrO2 layer formed by oxidation of
ZrB2 layer also improved the ablation resistance. Hence, after
introducing CVD ZrB2 layer, SiC/ZrB2eSiC/ZrB2/SiC coating displayed much better ablation resistance than SiC/ZrB2eSiC/SiC
coating, because of the synergistic effect of CVD ZrB2/SiC layer and
the good ablation resistance of ZrO2 layer formed by oxidation of
ZrB2 layer.
During ablation under high temperature, surface temperature
was mainly affected by materials composition and their emissivity.
For SiC/ZrB2eSiC/ZrB2/SiC coating, it was gradually damaged, so the
temperature curve reflected the temperature of different layer.
Based on the analyses made above, CVD SiC layer was firstly
oxidized and damaged in the central ablation region. Then the CVD
ZrB2 layer was oxidized to form ZrO2 layer. Though the layers and
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P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
Fig. 13. SEM images and EDS analyses of ablation region IV: (a), (b) SEM images; (c), (d), (e), (f) EDS analyses of spot A, B, C and area D in (b); (g), (h), (i) enlarged SEM images; (j)
surface EDS analysis of (i).
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
35
Fig. 14. SEM images of ablation region V and corresponding EDS analysis: (a), (b), (c) SEM images of ablation region V; (d) EDS analysis of (c).
matrix underneath was also damaged as the ablation test proceeded, ZrO2 layer formed did not fall apart till the end of the
ablation test. Hence, the surface temperature variation was mainly
determined by the CVD ZrB2 and SiC layer.
Meng et al. [28] measured the emissivity of ZrB2eSiC based
ceramics after oxidation and calculated their total normal emissivity which was affected by the oxide composition. It was
confirmed that SiO2 glass had higher emissivity than ZrB2 and ZrO2
[28e30]. Pidan et al. [31] measured spectral emissivity of CVD SiC in
the range of 1400 Ce1600 C. According to their reports, it could
be deduced that the emissivity of SiO2 glass should be roughly
equal to that of CVD SiC layer, which was higher than the emissivity
of ZrB2 and ZrO2 layer. Moreover, the emissivity of ZrB2 was higher
than ZrO2.
As shown in Fig. 6, the temperature curve of SiC/ZrB2eSiC/ZrB2/
SiC coating could be divided into three stages. In the first stage, the
temperature grew slowly, then it accelerated to about 2200 C in
stage 2, and finally reached a steady state with the temperature
around 2220 C in stage 3. Based on the microstructure evolution
mentioned above and the relative emissivity values of different
compositions, it can be deduced that CVD SiC and SiO2 glass
determined the temperature in stage 1, while ZrO2 layer determined that in stage 3. In stage 1, CVD SiC was ablated and oxidized
to form SiO2 glass. Because both CVD SiC and SiO2 glass had relatively high emissivity, they radiated more energy which led to low
surface temperature. After the damage of CVD SiC layer, ZrB2 layer
with lower emissivity started to be exposed and oxidized to ZrO2.
Due to the evolution in central ablation region, the total emissivity
decreased, so the surface temperature rose rapidly in stage 2. At
last, CVD ZrB2 was totally transferred to ZrO2 in central region, so
ZrO2 layer determined the temperature in stage 3. And the lowest
emissivity of ZrO2 led to the highest temperature in stage 3.
4. Conclusions
SiC/ZrB2-SiC/ZrB2/SiC multilayer coating was prepared by twostep chemical vapor deposition of ZrB2/SiC layers on SiC/ZrB2-SiC
coated graphite. After ablation for 298 s, only the central ablation
region of the SiC/ZrB2eSiC/ZrB2/SiC coated sample was damaged,
which showed excellent ablation resistance. The mass and linear
ablation rate of SiC/ZrB2eSiC/ZrB2/SiC coated graphite sample were
only 0.27 mg/s and 0.57 mm/s respectively, far lower than that of
SiC/ZrB2eSiC/SiC coating. CVD ZrB2/SiC layers significantly
improved the ablation resistance of SiC/ZrB2eSiC/ZrB2/SiC coating,
because of the synergistic effect of CVD ZrB2/SiC and the good
ablation resistance of ZrO2 layer formed by oxidation of ZrB2 layer.
The microstructure evolution and emissivity of different compositions led to different surface temperature-increasing stages.
Acknowledgments
This work has been supported by the National Natural Science
Foundation of China (No. 51802176, 51872174, 51802178 and
51702189), the Natural Science Foundation of Shandong Province,
China (No. ZR2017QEM002, ZR2018MEM018 and ZR2017BEM033),
Project of Shandong Province Higher Educational Science and
Technology Program (No. J17KA020), SDUT&Zibo City Integration
Development Project (No. 2018ZBXC439).
References
[1] Qingshan Zhu, Xueliang Qiu, Changwen Ma, Oxidation resistant SiC coating for
graphite materials, Carbon 37 (1999) 1475e1484.
[2] Peng Wang, Shanbao Zhou, Xinghong Zhang, Kaixuan Gui, Yongxia Li,
Jiadong An, Wenbo Han, Thermal cycling and oxidation resistance of B
modified ZrB2eSiC coatings on SiC coated graphite, Surf. Coating. Technol. 280
(2015) 330e337.
36
P. Wang et al. / Journal of Alloys and Compounds 781 (2019) 26e36
[3] Cui Hu, Yaran Niu, Sansong Huang, Hong Li, Musu Ren, Yi Zeng, Xuebin Zheng,
Jinliang Sun, In-situ fabrication of ZrB2eSiC/SiC gradient coating on C/C
composites, J. Alloys Compd. 646 (2015) 916e923.
[4] C. He, X.L. Wu, G. Liu, W.X. Zhang, Elastic and transport properties of nanolayered crystalline Cu/amorphous Cu-Zr multilayers, Mater. Des. 106 (2016)
133e138.
[5] C. He, M. Cheng, M. Zhang, W.X. Zhang, Interfacial stability and electronic
properties of Ag/M (M¼Ni, Cu, W, and Pd) and Cu/Cr interfaces, J. Phys. Chem.
C 122 (2018) 17928e17935.
[6] Yaocan Zhu, S. Ohtani, Y. Sato, N. Iwamoto, The improvement in oxidation
resistance of CVDeSiC coated C/C composites by silicon infiltration pretreatment, Carbon 36 (1998) 929e935.
[7] Sufang Tang, Jingyi Deng, Shijun Wang, Wenchuan Liu, Ke Yang, Ablation
behaviors of ultra-high temperature ceramic composites, Mater. Sci. Eng. A
465 (2007) 1e7.
[8] Xinghong Zhang, Ping Hu, Jiecai Han, Songhe Meng, Ablation behavior of
ZrB2eSiC ultra high temperature ceramics under simulated atmospheric reentry conditions, Compos. Sci. Technol. 68 (2008) 1718e1726.
[9] Jiecai Han, Ping Hu, Xinghong Zhang, Songhe Meng, Wenbo Han, Oxidationresistant ZrB2eSiC composites at 2200 oC, Compos. Sci. Technol. 68 (2008)
799e806.
[10] Reza Aliasgarian, Malek Naderi, Seyyed Ehsan Mirsalehi, Ablation mechanism
of ZrB2-SiC coating for SiC-coated graphite under an oxyacetylene flame, Surf.
Coatings Technol. 350 (2018) 511e518.
[11] Xu Zou, Qiangang Fu, Lei Liu, Hejun Li, Yongjie Wang, Xiyuan Yao, Zibo He,
ZrB2eSiC coating to protect carbon/carbon composites against ablation, Surf.
Coatings Technol. 226 (2013) 17e21.
[12] Haijun Zhou, Le Gao, Zhen Wang, Shaoming Dong, ZrB2eSiC oxidation protective coating on C/C composites prepared by vapor silicon infiltration process, J. Am. Ceram. Soc. 93 (2010) 915e919.
[13] Xuanru Ren, Hejun Li, Yanhui Chu, Qiangang Fu, Kezhi Li, Preparation of
oxidation protective ZrB2eSiC coating by in-situ reaction method on
SiCecoated carbon/carbon composites, Surf. Coatings Technol. 247 (2014)
61e67.
[14] Alireza Abdollahi, Zia Valefi, Naser Ehsani, Shahla Torabi, High temperature
anti-oxidation behavior of in situ and ex situ nanostructured C/SiC/ZrB2-SiC
gradient coatings: thermodynamical evolution, microstructural characterization, and residual stress analysis, Intern. J. Appl. Ceram. Technol. 15 (2018)
1319e1333.
[15] Yang Xiang, Wei Li, Song Wang, Zhaohui Chen, Oxidation behavior of oxidation protective coatings for PIPeC/SiC composites at 1500 C, Ceram. Int. 38
(2012) 9e13.
[16] Houbu Li, Litong Zhang, Laifei Cheng, Yiguang Wang, Ablation resistance of
different coating structures for C/ZrB2eSiC composites under oxyacetylene
torch flame, Int. J. Appl. Ceram. Technol. 6 (2009) 145e150.
[17] Peng Wang, Changling Zhou, Xinghong Zhang, Guangdong Zhao,
Baosheng Xu, Yehong Cheng, Peng Zhou, Wenbo Han, Oxidation protective
ZrB2eSiC coatings with ferrocene addition on SiC coated graphite, Ceram. Int.
42 (2016) 2654e2661.
[18] Xin Yang, Qizhong Huang, Zhean Su, Xin Chang, Liyuan Chai, Chunxuan Liu,
Liang Xue, Dong Huang, Resistance to oxidation and ablation of SiC coating on
graphite prepared by chemical vapor reaction, Corrosion Sci. 75 (2013)
16e27.
[19] Yulei Zhang, Zhixiong Hu, Hejun Li, Jincui Ren, Ablation resistance of ZrB2eSiC
coating prepared by supersonic atmosphere plasma spraying for SiC-coated
carbon/carbon composites, Ceram. Int. 40 (2014) 14749e14755.
[20] Peng Wang, Shanbao Zhou, Ping Hu, Guiqing Chen, Xinghong Zhang,
Wenbo Han, Ablation resistance of ZrB2-SiC/SiC coating prepared by pack
cementation for graphite, J. Alloys Compd. 682 (2016) 203e207.
[21] Feng Tao, Li He-Jun, Shi Xiao-Hong, Yang Xi, Shao-Long Wang, Oxidation and
ablation resistance of ZrB2eSiCeSi/B-modified SiC coating for carbon/carbon
composites, Corrosion Sci. 67 (2013) 292e297.
[22] Dongjia Yao, Hejun Li, Heng Wu, Qiangang Fu, Xinfa Qiang, Ablation resistance
of ZrC/SiC gradient coating for SiC-coated carbon/carbon composites prepared
by supersonic plasma spraying, J. Eur. Ceram. Soc. 36 (2016) 3739e3746.
[23] Zhaoqian Li, Hejun Li, Wei Li, Jie Wang, Shouyang Zhang, Juan Guo, Preparation and ablation properties of ZrCeSiC coating for carbon/carbon composites by solid phase infiltration, Appl. Surf. Sci. 258 (2011) 565e571.
[24] Xiang Yang, Li Wei, Wang Song, Bi-feng Zhang, Chen Zhao-hui, ZrB2/SiC as a
protective coating for C/SiC composites: effect of high temperature oxidation
on mechanical properties and anti-ablation property, Composites Part B 45
(2013) 1391e1396.
[25] Tao Feng, Hejun Li, Manhong Hu, Hongjiao Lin, Lu Li, Oxidation and ablation
resistance of the ZrB2-CrSi2-Si/SiC coating for C/C composites at high temperature, J. Alloys Compd. 662 (2016) 302e307.
[26] Yongjie Wang, Hejun Li, Qiangang Fu, Heng Wu, Dongjia Yao, Bingbo Wei,
Ablative property of HfCebased multilayer coating for C/C composites under
oxyeacetylene torch, Appl. Surf. Sci. 257 (2011) 4760e4763.
[27] Xinghong Zhang, Ping Hu, Jiecai Han, Songhe Meng, Ablation behavior of
ZrB2eSiC ultra-high temperature ceramics under simulated atmospheric Reentry conditions, Compos. Sci. Technol. 68 (2008) 1718e1726.
[28] Songhe Meng, Hongbo Chen, Jianghua Hu, Zongwei Wang, Radiative properties characterization of ZrB2eSiCebased ultrahigh temperature ceramic at
high temperature, Mater. Des. 32 (2011) 377e381.
[29] Luigi Scatteia, Raffaele Borrelli, Giovanni Cosentino, Eric Beche, JeanLouis Sans, Marianne Balat-Pichelin, Catalytic and radiative behaviors of
ZrB2eSiC ultrahigh temperature ceramic composites, J. Spacecraft Rockets 43
(2006) 1004e1012.
[30] Luigi Scatteia, Davide Alfano, Frederic Monteverde, Jean-Louis Sans,
Marianne Balat-Pichelin, Effect of the machining method on the catalycity and
emissivity of ZrB2eHfB2ebased ceramics, J. Am. Ceram. Soc. 91 (2008)
1461e1468.
[31] Sergej Pidan, Monika Auweter-Kurtz, Georg Herdrich, Markus Fertig,
Recombination coefficients and spectral emissivity of silicon carbideebased
thermal protection materials, J. Thermophys. Heat Transf. 19 (2005) 566e571.