_2003_Mullite-Al2O3-SiC oxidation protective coating for carboncarbon composites by zahawe

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									                                                      Carbon 41 (2003) 2825–2829




                Mullite-Al 2 O 3 -SiC oxidation protective coating for
                              carbon / carbon composites
   Huang Jian-Feng a , Zeng Xie-Rong b , Li He-Jun a , *, Xiong Xin-Bo a , Huang Min a
       a
           C /C Composites Technology Research Center, Northwestern Polytechnical University, Xi’ an, Shannxi 710072, PR China
                           b
                             Department of Materials Science, Shenzhen University, Shenzhen 518060, PR China
                                            Received 6 June 2003; accepted 18 August 2003


Abstract

   In order to exploit the unique high temperature mechanical properties of carbon / carbon (C / C) composites, a new type of
oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD
analysis showed that the internal coating obtained from the first step was a gradient SiC layer that acts as a buffer layer, and
the multi-layer coating formed in the second step was an Al 2 O 3 -mullite layer. It was found that the as-received coating
characterized by excellent thermal shock resistance on the surface of C / C composites during exposure to an oxidizing
atmosphere at 1873 K, could effectively protect the C / C composites from oxidation for 45 h. The failure of the coating is
due to the formation of bubble holes on the coating surface.
 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon / carbon composites; B. Coating; Oxidation



1. Introduction                                                          Si 3 N 4 ceramics from oxidation by employing pulsed-laser
                                                                         deposition (PLD), chemical vapor deposition (CVD) and
   As an important high temperature structural material,                 plasma spraying technique [4–8].
carbon / carbon (C / C) composites offer many advantages                    Pack cementation is a good method to deposit coatings
over traditional materials, such as low density, high                    on all surfaces of the sample, such as to deposit a SiC
strength-to-weight ratio and retention of mechanical prop-               coating on the surface of a carbon material [9], and it is
erties at high temperature. Therefore, they can be used in               widely used in producing oxidation protective coatings on
aircraft and aerospace industries. These applications re-                metal surfaces [10]. But up to now, we could find no report
quire the C / C composites to operate in an oxidizing                    about using the pack cementation technique to produce a
atmosphere. Unfortunately, C / C composites are prone to                 mullite-containing coating, especially on C / C surfaces.
oxidize at ambient environment with the temperature                         In the present work, a new kind of multi-coating
above 723 K and the oxidation rate increases quickly with                including mullite, Al 2 O 3 and SiC on C / C composites was
the temperature increase [1,2].                                          produced by a two-step pack cementation method. The
   Oxidation resistant coatings are the logical choice for               structure and oxidation protection properties of the coating
protecting C / C composites at high temperatures. Mullite                were primarily investigated.
ceramic (3Al 2 O 3 ?2SiO 2 ) coatings are promising candidate
materials for high temperature applications. Mullite coated
SiC exhibits excellent oxidizing resistance in an oxidizing              2. Experimental
atmosphere by forming a slowly growing SiO 2 scale at the
mullite–SiC interface [3]. In recent papers, mullite coat-                  Small specimens (14310310 mm 3 ) used as substrates
ings were developed to protect carbon materials, SiC and                 were cut from bulk 2D-C / C composites with a density of
                                                                         1.8 g / cm 3 . Before carrying out the pack cementation
   *Corresponding author. Tel.: 186-298-495-004; fax: 186-               procedure, the specimens were hand-polished using 340
298-495-240.                                                             grit SiC paper, then cleaned with distilled water and dried
   E-mail address: lihejun2@yahoo.com (L. He-Jun).                       at 353 K for 2 h. Pack compositions for the pack

0008-6223 / 03 / $ – see front matter  2003 Elsevier Ltd. All rights reserved.
doi:10.1016 / S0008-6223(03)00397-X
2826                                       H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829

cementation process were as follow: 40–45 wt.% Si (300                formation of a b-SiC layer. Fig. 1b shows the XRD pattern
mesh), 38–42 wt.% SiC (325 mesh), 3–6 wt.% graphite                   of the multi-coating surface on C / C achieved by the
(300 mesh) and 5–9 wt.% Al 2 O 3 (325 mesh) for the first              two-step pack cementation. It is concluded that new phases
step, and 22–29 wt.% SiO 2 (400 mesh), 50–60 wt.%                     containing corundum (aluminum oxide) and mullite
Al 2 O 3 (325 mesh), 3–9 wt.% Si (300 mesh) and 4–8 wt.%              (3Al 2 O 3 ?2SiO 2 ) were generated in the second process.
graphite (325 mesh) for the second process. All the above                Fig. 2 shows the microstructure and the spot EDS
powders were reagent grade. The pack mixtures were                    analyses of the multi-coating surface on C / C composites.
weighed and then mixed by tumbling in a ball mill for up              There are two kinds of crystalline particles (characterized
to 4 h. The C / C composites and the first pack mixtures               as grey and white) found on the surface. By EDS analysis,
were placed in a graphite crucible with a graphite lid. The           the grey and white phases could be discriminated as
crucible was placed into an electric furnace, which was               corundum and mullite, respectively. The corundum par-
then heated in an argon atmosphere to 1875 K and held at              ticles are bigger than the mullite ones and most of the
that temperature for 2 h to form SiC coating. On comple-
tion of the coating deposition, the pack was allowed to
furnace cool, and the samples were removed from the pack
and ultrasonically cleaned to remove any loosely embed-
ded pack material. Then the SiC-coated samples were
immersed in the second pack mixture. The second pack
cementation process was conducted at 2073 K for 2 h to
achieve the multi-layer coatings, and the other operations
in the second process are as same as for the first one.
   The as-coated specimens were heated at 1873 K in air in
an electrical furnace to investigate the isothermal and
thermal cycling oxidation behavior. The morphology and
crystalline structure of the multi-coatings were analyzed
using scanning electron microscopy (SEM), X-ray diffrac-
tion (XRD) and energy dispersive spectroscopy (EDS).


3. Results and discussion

3.1. Structure of the multi-coating

  Fig. 1a shows the XRD pattern of the coating surface
obtained from the first pack cementation step. It shows the




Fig. 1. XRD patterns of the coatings: (a) the SiC coating on C / C
composites obtained in the first pack cementation process; (b) the
multi-coating on C / C composites prepared by two-step pack           Fig. 2. SEM micrograph and the spot EDS analyses of the surface
cementation.                                                          of the as-prepared multi-coating on C / C composites.
                                        H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829                                     2827

space among the corundum particles is filled with small             layer may consist of Al 2 O 3 and a little SiC. From the
mullite crystallites.                                              transition layer to the coating surface, the concentrations of
   It is clear from the cross-section (Fig. 3) of the multi-       Si, C, Al and O are almost constant, which means that the
coating that the as-received coating with a thickness of           pores of porous SiC may be filled with mullite and Al 2 O 3
around 200 mm is perfectly dense, and no obvious                   crystallites.
interface is found between the SiC bonding layer and the
mullite-Al 2 O 3 outer layer. By element line scanning
analyses, the concentrations of C, O, Si and Al are also           3.2. Oxidation test
shown in Fig. 3. Near the interface between coating and
C / C, there is a transition layer with little Si and C but            The results of the isothermal oxidation test in air at 1873
enriched with Al and O, which indicates that the transition        K are shown in Fig. 4. After oxidation in air for 45 h, the
                                                                   weight loss of the coated C / C composites is only 1.86%,
                                                                   and the corresponding weight loss rate is 1.51310 24
                                                                   g / cm 2 ?h. The protective temperature is higher than that of
                                                                   the mullite coating for SiC fiber, SiC and Si 3 N 4 ceramics
                                                                   (reported to 1573 K) produced by CVD and plasma
                                                                   spraying methods [2,5–7]. The effective protecting time of
                                                                   the as-prepared coating (1873 K for around 45 h) for C / C
                                                                   composites is longer than that of a mullite / Si–SiC multi-
                                                                   coating (1873 K for about 32 h) reported by Fritze et al.
                                                                   [4]. In addition, in comparison with other reported methods
                                                                   [3–8], the two-step pack cementation technique is easier to
                                                                   operate, and all the surfaces of the C / C sample could be
                                                                   coated in one process, while the PLD, CVD and plasma
                                                                   spraying techniques need to coat each surface of the
                                                                   sample separately. Therefore, the repeated deposition
                                                                   processing is avoided using the pack cementation method.
                                                                       According to the oxidation curve shown in Fig. 4, the
                                                                   oxidation behavior of the coated C / C composites could be
                                                                   divided into three processes marked as A, B and C. Under
                                                                   10 h (process A), the sample gains weight due to the
                                                                   formation of SiO 2 glass [11]. From 10 to 22 h (process B),
                                                                   the weight loss rate of the sample is almost constant.
                                                                   Above 22 h (process C), the weight loss rate increases
                                                                   linearly with time. Additionally, the sample has endured
                                                                   thermal cycling between 1873 K and room temperature for
                                                                   nine times, and no cracks and destruction were found, from
                                                                   which it can be inferred that the coating has excellent
                                                                   thermal shock resistance.




                                                                   Fig. 4. The isothermal oxidation curves of the C / C composites
Fig. 3. SEM cross-section image and the element line scanning      with a multi-coating in air at 1873 K (minus weight loss means
results of the as-prepared coating on C / C composite.             weight gain).
2828                                      H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829

3.3. Analyses of the failure of the multi-coating

   After 45 h oxidation at 1873 K in air, the coating surface
was observed with the scanning electron microscope, and
it was found that a smooth glass layer was formed on the
sample’s surface (Fig. 5), which may change from mullite
and SiC. Fig. 6 shows the XRD pattern of the coating after
oxidation. This shows that the peak intensity of SiC was
weakened and mullite and Al 2 O 3 phases disappeared,
while a weak SiO 2 phase appeared, which indicates that
mullite, Al 2 O 3 and some of the SiC have been transformed
into a glass phase during the oxidation at 1873 K. The
amorphous nature of silicate glass between 18 and 238 of
2u in Fig. 6 also verifies this deduction. From the
isothermal oxidation results, it is proposed that the glass
layer could effectively seal the pores of the coating layer at       Fig. 6. XRD pattern of the as-prepared coating surface after
the initial oxidation stage, which leads to a low weight loss        oxidation at 1873 K for 45 h.
rate for 22 h. After 22 h oxidation in air at 1873 K, the
thickness of the glass may decrease gradually by gasifica-

                                                                     tion. Subsequently, the residual glass layer cannot fully
                                                                     prevent oxygen from penetrating the coating to attack the
                                                                     C / C composites. Then gases generated during the reaction
                                                                     between the C / C and oxygen would get out through the
                                                                     coating, which produces holes and bubbles on the coating
                                                                     surface. From Fig. 5a, we can find some pinholes and some
                                                                     big gas bubbles from the surface. After the gas bubbles
                                                                     break up, they will leave big holes with diameter of almost
                                                                     30 mm on the surface (Fig. 5b). The formation of this kind
                                                                     of hole offers entrance channels for air and results in the
                                                                     linear increase of weight loss rate. Additionally, some
                                                                     microcracks are also found near the bubble hole in Fig. 5b.
                                                                     We consider that these microcracks may be generated
                                                                     during quick cooling from 1873 K to the room temperature
                                                                     during the isothermal oxidation test, and they could self-
                                                                     seal when the coating is heated to 1873 K again by the
                                                                     observation of the white traces of the sealed cracks in Fig.
                                                                     5b. Therefore, the microcracks are not the main cause of
                                                                     the efficiency loss of the coating at 1873 K, which is rather
                                                                     due to the formation of holes in the coating, though the
                                                                     convincing reason for the formation of these holes needs
                                                                     further research.



                                                                     4. Conclusions

                                                                        Based on this work, it can be concluded that the Al 2 O 3 -
                                                                     mullite-SiC multi-coatings with a thickness of around 200
                                                                     mm could be produced by a two-step pack cementation
                                                                     process. The multi-coating could effectively protect the
                                                                     C / C composites from oxidation at 1873 K for 45 h, and
                                                                     the weight loss rate of the coated C / C composites is only
                                                                     1.51310 24 g / cm 2 ?h. The failure of the coating is thought
Fig. 5. Morphologies of the as-prepared coating on C / C compos-     to occur due to the formation of bubble holes on the
ites after oxidation at 1873 K for 45 h.                             coating surface.
                                          H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829                                         2829

Acknowledgements                                                      [5] Varadarajan S, Pattanaik AK, Sarin VK. Mullite interfacial
                                                                          coatings for SiC fibers. Surf Coat Technol 2001;139:153–60.
   This work was supported by the National Natural                    [6] Lee KN, Miller RA. Development and environmental
                                                                          durability of mullite and mullite / YSZ dual layer coatings for
Science Foundation of China (Distinguished Young Scho-
                                                                          SiC and Si 3 N 4 ceramics. Surf Coat Technol 1996;86–
lar Fund) under grant No. 50225210
                                                                          87:142–8.
                                                                      [7] Basu SN, Hou P, Sarin VK. Formation of mullite coatings on
                                                                          silicon-based ceramics by chemical vapor deposition. Int J
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