An AlPO4/SiC coating prepared by pulse arc ...

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Nov 19, 2013 - for oxidation protection of carbon/carbon composites. Huang Jian-Feng. *. , Hao Wei, Cao Li-Yun, Yin Li-Xiong, Ouyang Haibo, Yao Chun-Yan,.
Corrosion Science 79 (2014) 192–197

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An AlPO4/SiC coating prepared by pulse arc discharge deposition for oxidation protection of carbon/carbon composites Huang Jian-Feng ⇑, Hao Wei, Cao Li-Yun, Yin Li-Xiong, Ouyang Haibo, Yao Chun-Yan, Wu Jian-Peng, Fei Jie Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, PR China

a r t i c l e

i n f o

Article history: Received 16 June 2013 Accepted 11 November 2013 Available online 19 November 2013 Keywords: A. Ceramic matrix composites A. Ceramic B. Weight loss C. Oxidation

a b s t r a c t Pulse arc discharge deposition was developed to prepare an oxidation-protective AlPO4 coating for SiC pre-coated carbon/carbon composites (SiC–C/C). The influence of pulse duty ratio on composition, microstructure and oxidation resistance of the as-prepared coatings was investigated. Results show that thickness, compactness and oxidation protective property of the AlPO4 coating were improved with the increase of pulse duty ratio from 10% to 30%. The multi-layer coating prepared at a pulse duty ratio of 30% exhibits good oxidation protective property, which can effectively protect C/C composites from oxidation in air at 1773 K for 148 h with a weight loss of 0.82%. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Carbon/carbon (C/C) composites are a promising candidate material for the high thrust weight ratio turbine engines because of their high temperature mechanical properties [1–3]. Nevertheless, the oxidation of C/C composites above 723 K limits their applications in oxygen containing environments [4,5]. Therefore, it is important to improve the oxidation resistance of C/C composites to widen their applications in an oxidizing environment [6]. SiC ceramic coating with good physical and chemical compatibility to C/C substrate was usually considered as one of the best bonding layers between C/C composites and outer ceramic layer [7]. But a single SiC layer can not provide effective oxidation protection [8]. To solve the problem mentioned above, multi-layer coatings are considered as one of the best choices to protect C/C composites from oxidation at high temperature, such as MoSi2/SiC [9], SiC nanowire-SiC–Si/SiC–Si coating [10] and C/SiC/Mo–Si–Cr [11] multi-layer coating. Cristobalite aluminum phosphate (cristobaliteAlPO4) has been considered as a promising candidate outer coating materials due to its high melting point, good stability, lower oxygen permeability at high temperature and good erosion resistance [12]. In addition, the good match of thermal expansion coefficient between AlPO4 (5.5  106/°C) and SiC (4.3–5.4  106/°C) is another advantage for the adoption of cristobalite-AlPO4 as outer coating material [7,12].

⇑ Corresponding author. Tel./fax: +86 29 86168802. E-mail address: [email protected] (J.-F. Huang). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.11.006

However, the traditional technologies, such as chemical vapor deposition [13] and pack cementation [10,11], are usually carried out at high temperature, which will result in the generation of thermal stress in the coatings. Moreover, the coatings prepared by hydrothermal electrophoretic deposition [9,14] are loose, which may weaken the oxidation protective property. Double pulse electrodeposition [15,16] and pulse laser-assisted electron beam deposition [17] are rarely reported on the anti-oxidation coatings for C/C composites, but mainly used for preparing coatings of alloy or polymer at low temperature. Pulse arc discharge deposition has been demonstrated to be a new low temperature method with pulse technique and arc discharge sintering process combined. The dense coatings can be deposited by pulse arc discharge deposition with high efficiency and simple operation by the control of the pulse duty ratio. In the present work, an AlPO4 coating was prepared by a pulse arc discharge deposition process on the SiC bonding layer prepared by pack cementation. The phase compositions and microstructures of the asprepared AlPO4/SiC multi-layer coating were characterized. The oxidation protective, the thermal shock properties and the failure mechanism of the multi-layer coated C/C composites were investigated. 2. Experimental 2.1. Pulse arc discharge deposition system Fig. 1 shows a schematic diagram of pulse arc discharge deposition system. The anode of the autoclave was a graphite substrate (20  10  3 mm3) and the SiC–C/C substrate was fixed on the

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cathode of the autoclave. The suspension was used to deposite coating for deposition processes. Pulse power supply takes the place of DC power supply in hydrothermal electrophoretic deposition process [7,12], and discharges take place in an asymmetrical electric field [18]. The pulse duty ratio of pulse power supply in same output voltage can be regulated continuously. Pulse duty ratio is the percent of time that an entity spends in an conduction state as a fraction of the total cycle time under consideration. 2.2. Specimens and coating preparation

Fig. 1. The schematic diagram of pulse arc discharge deposition system.

Small specimens (10  10  10 mm3) used as substrates were cut from bulk 2D C/C composites with a density of 1.75 g cm3. The specimens were hand-abraded with 300-grit SiC paper, cleaned with absolute ethanol and dried at 333 K for 2 h. The SiC bonding layer was prepared by pack cementation procedure. Details for the preparation of the bonding SiC layer were reported in Ref. [9]. For the pulse arc discharge deposition process, cristobaliteAlPO4 powders with the mass of 3.4 g were dispersed in 170 ml isopropanol with an ultrasonic bath for 30 min (the ultrasonic power was kept at 100 W) with a later magnetic stirring for 24 h. Next, 0.17 g crystalline analytical reagent iodine (Made in San Pu chemical factory in Xi’an, China) as charge agent was dissolved in the above suspension with an ultrasonic bath for 30 min (the ultrasonic power was kept at 100 W) followed by constant magnetic stirring for 24 h. Protons were produced by the reaction between iodine and isopropanol shown in Eq. (A.1) [9]. Finally, the above suspension was transferred into a hydrothermal autoclave. After being sealed, the autoclave was put into a furnace and the pulse duty ratios were kept at 10%, 20%, 30% and 50%. During this process, the autoclave temperature and the deposition voltage were kept at 373 K and 390 V, respectively. And after 15 min deposition, the specimens were taken out from the autoclave and cooled naturally down to room temperature following by drying at 333 K in air for 4 h. 2.3. Characterization

Fig. 2. Surface XRD patterns of the AlPO4 coating on SiC–C/C composites prepared by pulse arc discharge deposition process at different pulse duty ratios.

The isothermal oxidation tests of the coated samples were carried out in an electrical furnace at 1773 K in static air. The isother-

Fig. 3. Surface SEM images of the AlPO4 coating on SiC–C/C composites deposited at different pulse duty ratios. (a) 10%; (b) 20%; (c) 30%; and (d) 50%.

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Fig. 4. Cross-section SEM images of the AlPO4 coating on SiC–C/C composites deposited at different pulse duty ratios: (a) 10%; (b) 20%; (c) 30%; and (d) 50%.

Fig. 6. Isothermal oxidation curves of SiC–C/C composites and the AlPO4 coating coated SiC–C/C composites at different pulse duty ratios in air at 1773 K.

Fig. 5. EDS element line scan analysis of the cross-section of AlPO4/SiC multi-layer coating according to Fig. 4(c).

mal and thermal cycling oxidation behaviors were investigated. The cumulative weight change of the group of five samples after each thermal cycle was measured by an electrical balance with a sensitivity of ±0.1 mg. The mass loss was calculated by Eqs. (A.2)–(A.4). The end mass loss is from average value of the group of five samples after oxidation at high temperature for a certain time. Among which m0 is the original mass of the coated C/C composites, kg; mt is the mass of the coated C/C composites after oxidation

Fig. 7. Isothermal oxidation curves of the AlPO4 coating coated SiC–C/C composites in air at 1773 K: (a) The relationship between weight loss and oxidation time; and (b) the relationship between weight loss rate and oxidation time.

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Fig. 8. Surface SEM images of the AlPO4 coating on SiC–C/C composites after oxidation at 1773 K in air for different hours: (a) 12 h; (b) 22 h; (c) 76 h; and (d)148 h.

at high temperature for a certain time, kg; t is the oxidation time, h and S is the surface area of the specimen, m2. DW% is percent of weight loss, DW is weight loss per unit area, kg m2 and voxidation is weight loss rate, kg m2 h1. The morphology, the crystalline structure, and the element composition of the multi-layer coatings were analyzed by a scanning electron microscope (SEM, JSM-6390A) with energy-dispersive spectroscopy (EDS) and a X-ray diffractrometer (XRD, Rigaku D/max-3C). 3. Results and discussion 3.1. XRD analysis of the coating Fig. 2 shows the surface XRD patterns of the AlPO4 outer layers prepared by pulse arc discharge deposition. It reveals that the as-prepared coating is mainly composed of AlPO4 (JCPDS No. 11– 0500) phase, which agrees with the original powder phase composition. In addition, the peak intensity of the AlPO4 increases with the increase of pulse duty ratio. And when the pulse duty ratio reaches up to 50%, the highest peak intensity of the AlPO4 is obtained. This may be attributed to the higher diffusion velocity of the charged AlPO4 particles from the cathode to anode at higher pulse duty ratio, which is helpful for the increase of the coating thickness and may lead to the improvement in crystallization of the AlPO4 coating. The peaks of SiC are detected at 10%. This may be due to the AlPO4 outer layer with insufficient thickness. From 20% to 50%, no peaks of SiC and Si are found but peaks of Al2O3 are observed, indicating that a compact AlPO4 outer layer with enough thickness is achieved and part of AlPO4 is decomposed into Al2O3 and POx (Eq. (A.8)) during the arc discharge sintering process. 3.2. Morphologies of the multi-composition coating Fig. 3 shows the surface SEM images of the AlPO4 coating on SiC–C/C composites deposited at different pulse duty ratios. Clearly, the coating surface of all the samples is composed of some small particles with microholes, but no cracks are observed on the coating surfaces. With the increase of pulse duty ratios from 10% to 50%, the surface of the coatings exhibits different morphology.

Loose coating is obtained at 10% (Fig. 3(a)), inhomogeneous and porous AlPO4 coating is prepared at 20% (Fig. 3(b)), and very compact and homogeneous AlPO4 coating is achieved at 30% (Fig. 2(c)). However, when the pulse duty ratio reaches 50% (Fig. 3(d)), the asprepared coating shows an inhomogeneous and porous morphology. This may be caused by high pulse duty ratio with high deposition voltage, which provides sufficient energy for the large deposition mass per unit time, recrystallization and growth of AlPO4 crystalline during the arc discharge sintering process. Cross-section SEM images of the AlPO4 coating deposited at different pulse duty ratios are shown in Fig. 4. A clear two-layer structure without microholes or penetrative cracks is achieved, which may be due to the AlPO4 coating prepared at low temperature with the arc discharge sintering process. With the increase of pulse duty ratio, the improvement in compactness and thickness of the AlPO4 coating is clear. The thickness of AlPO4 outer layer is about 130, 245, 290 and 500 lm at the pulse duty ratios of 10%, 20%, 30%, and 50%, respectively. This indicates that the coating thickness increases with pulse duty ratio correspondingly, the improvement in coating compact with the increase of pulse duty ratio is observed. Homogenous and crack-free AlPO4 coating is obtained when pulse duty ratio is 30%. Above 30%, microcracks in the outer coating are found at 50%, possibly as a result of the fast deposition velocity and large thickness of the AlPO4 layer leading to the generation of thermal stress. Fig. 5 displays the EDS element line scan analysis of the crosssection of the AlPO4/SiC multi-layer coating. It shows the concentration distributions of C, O, Al, Si and P elements along the coating cross direction. The element line scan analysis demonstrates that the multi-layer coating could be divided into three parts, designated as A, B and C. Part A is the C/C composites matrix, Part B is the SiC bonding layer, and part C is the AlPO4 coating that agrees well with the experimental designation, XRD and SEM analyses. 3.3. Oxidation test of the coated C/C composites The isothermal oxidation curves of the SiC and AlPO4 coating coated SiC–C/C composites at different pulse duty ratios in air at 1773 K are shown in Fig. 6. After oxidation in air for 40 h at 1773 K, the mass loss of SiC–C/C composites is about 2.61%, which

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Fig. 7 exhibits the isothermal oxidation curve of AlPO4 coating (Deposited at 30%) coated SiC–C/C composites in air at 1773 K. Clearly, the as-prepared AlPO4 coating can protect SiC–C/C composites from oxidation at 1773 K for 148 h with the weight loss of only 1.69 kg m2 and the corresponding weight loss rate is 1.14  102 kg m2 h1. The oxidation behavior of the coated sample can be divided into three processes marked as A, B, and C by the analyses of the oxidation curves and the corresponding oxidation kinetic equations are shown in Eqs. (A.5)–(A.7), Among which t (h) is the oxidation time and DW (kg m2) is weight loss per unit area. At the initial oxidation stage (0–12 h), the weight loss of the sample with time follows a parabolic law and the obvious weight loss of sample is detected. It is clear that the samples gain weight quickly to 7.21  101 kg m2 during the initial 12 h oxidation. Additionally, by SEM observation, the surface of the coating is harsh and rough with many microholes (Fig. 8(a)), which indicates that a possible cause of the weight loss of the sample is the oxidation of C/C matrix through the microholes. At this oxidation stage, part of AlPO4 is exposed to air and is decomposed into Al2O3 and POx (Eq. (A.8)) with a later transformation into AlPO4 molten phase (Eq. (A.9)) [12]. Fig. 9 shows the SEM image and EDS analyses of the surface of the coatings. Clearly, EDS analyses show that the small particles shown in Fig. 9(a) are mainly composed of Al 33.30 at.%, O 62.85 at.% and Si 3.85 at.% (spot 1 in Fig. 9(b)) on the surface of the coating after oxidation at 1773 K in air for 12 h. The composition of the phases is Al 32.35 at.%, O 63.18 at.% and Si 4.47 at.% (spot 2 in Fig. 9(c)). It is suggested that the small particles may result from Al2O3 grains embedded in the microholes and the Si element may be due to the oxidation of SiC binding layer into SiO2. In the meantime, oxygen diffuses along the microholes of the AlPO4 coating to the interface between the AlPO4 and SiC–C/C. As a result, the oxygen then reacts with SiC (Eqs. (A.11) and (A.12)) bonding layer. As oxidation time increases (Process A), the AlPO4 outer layer can be gradually transformed into the metaphosphate (Eq. (A.10)) layer [12] and simultaneously the Al2O3 may be generated (Eq. (A.10)). From 22 h to 76 h (Process B), the dense and smooth metaphosphate can be completely formed (Fig. 8(b)), which may result in the best protection for the C/C matrix due to their low oxygen permeation constant and good self-cure ability (Fig. 8(c)). The average weight loss rate in this process keeps below 1.53  102 kg m2 h1, which suggests that the oxidation rate of the coated sample is mainly controlled by the diffusion rate of oxygen along the molten layer. After 76 h oxidation (Process C) at 1773 K, the weight loss of the coated sample increases quickly with time passing, which may result from the volatilization of the

Fig. 9. SEM image and EDS spectra of the AlPO4/SiC coated samples after oxidation at 1773 K in air for 12 h: (a) surface SEM image for 12 h; (b) EDS spectra for spot 1; and (c) EDS spectra for spot 2.

suggests that the single SiC coating can not effectively protect C/C composites from oxidation for a long time. Compared with it, the AlPO4 coated SiC–C/C composites exhibit much better oxidation protective properties. The rate of mass loss decreases with the increase of thickness, homogeneity and compact of AlPO4 coating. Thus, the improvement of oxidation protective property is observed as pulse duty ratio increases to 30%. When the pulse duty ratio reaches 50%, the decrease in oxidation resistance of the coated samples is found with the generation of microcracks in outer coating. Moreover, the mass loss is only 0.82% after 148 h oxidation at 1773 K with the sample prepared at 30%.

Fig. 10. Cross-section SEM images of the AlPO4 coating on SiC–C/C composites after oxidation at 1773 K in air for 148 h.

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molten layer and lead to the decrease in coating thickness. As a result, oxygen will diffuse through the thin metaphosphate and glass layer to react with the C/C matrix as is shown in Eqs. (A.13) and (A.14). These reactions may lead to the formation of microholes and microcracks (Figs. 8(d) and 10) in the surface glass layer, which may possibly result from the escape of CO and CO2. In addition, microcracks and oxidation holes can also be generated in the coating possibly due to the frequent thermal shocks between high temperature and room temperature during the isothermal oxidation tests process. The as-generated microholes, microcracks and oxidation holes provide the channels for oxygen to attack the C/C matrix, which may lead to the decrease in oxidation resistance of the coated sample.

AlPO4 ðsÞ ! Al2 O3 ðsÞ þ POx ðgÞ

ðA:8Þ

AlPO4 ðsÞ ! AlPO4 ðmÞ

ðA:9Þ

4. Conclusions

References

In conclusion, an improved compact and homogeneous AlPO4 coating for SiC–C/C composites was successfully prepared by a novel pulse arc discharge process at low temperature. The thickness, compactness and oxidation resistance of the as-prepared coating improve with the increase of pulse duty ratios from 10% to 50%. The AlPO4 coating prepared at the pulse duty ratio of 30% can effectively protect C/C composites from oxidation in air at 1773 K for 148 h with a weight loss of only 1.69 kg m2, and a corresponding weight loss rate of 1.14  102 kg m2 h1. The failure of the multilayer coatings is attributed to the generation of microholes and microcracks. This may due to the escape of CO and CO2 and the volatilization of metaphosphate molten layer.

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Acknowledgements This work has been supported by the National Natural Science Foundation of China (51272146 and 51172134), Natural Science Foundation of Shaanxi Province Educational Department (2010JK444 and 2010JM6017), Scientific Research Innovation Team Foundation of Shaanxi Province (2013KCT-06) and the Graduate Innovation Foundation of Shaanxi University of Science and Technology. Appendix A

CH3 CHOHCH3 þ 2I2 $ ICH2 CHOHCH2 I þ 2Hþ þ 2I

DW% ¼ DW ¼

m0  mt  100% m0

ðA:2Þ

m0  mt  100% S

v oxidation ¼

ðA:1Þ

ðA:3Þ

m0  mt  100% St

DW ¼ 0:84865 þ 0:72493t  0:01597t2

ðA:4Þ ðProcess A : 0

2

< t 6 22 h; R ¼ 0:99413Þ

DW ¼ 7:35152 þ 0:09698t  5:4553  104 t2

ðA:5Þ ðProcess B

2

: 22 h < t 6 76 h; R ¼ 0:99850Þ

ðA:6Þ

DW ¼ 5:9619 þ 0:07381t ðProcess C : 76 h < t 6 148 h; R2 ¼ 0:99961Þ

ðA:7Þ

3AlPO4 ðsÞ ! AlðPO3 Þ3 ðsÞ þ Al2 O3 ðsÞ

ðA:10Þ

SiCðsÞ þ O2 ðgÞ ! SiOðgÞ þ COðgÞ

ðA:11Þ

SiCðsÞ þ 2O2 ðgÞ ! SiO2 ðsÞ þ CO2 ðgÞ

ðA:12Þ

2CðsÞ þ O2 ðgÞ ! 2COðgÞ

ðA:13Þ

CðsÞ þ O2 ðgÞ ! CO2 ðgÞ

ðA:14Þ