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OXIDATION
Acta Astronautica Vol. 35, No. I, pp. 3541, 1995 Copyright c 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0095-5765/95 $9.50 + 0.00
PROTECTION FOR 3D CARBON/CARBON COMPOSITESt
SHU-EN Hsu, HUAN-DERWV, TSUNG-MINGWV, SHIAN-TSONGCHOU, KUNG-LONG WANG and CHUNG-I CHEN Materials R&D Center, Chung Shan Institute of Science and Technology, Lung-Tan, Taiwan 32526, Republic of China (Received
I2 May 1993; revised 22 June 1993; received for publication
26 May 1994)
Abstract-The main objective of this study is to produce an effective protection coating for 3D carbon/carbon composites (C/C) to prevent oxygen attacks at elevated temperatures. Silicon and zirconium base alloys were used to stack a multilayer coating in an array of elemental gradients. The oxidation rate was detected by an in situ measurement at various temperatures as well as by long-term isothermal oxidation at elevated temperatures up to 1650°C. A maximum oxidation rate was found at a medial temperature of around 800°C. The rate converged to zero at 1150°C and weight gain occurred at higher temperatures. The multilayer coating proved useful in encountering the severe oxidation circumstances for hundreds of hours without deterioration of the C/C substrate.
metallic compound in the present study minimizes the oxidation rate and simultaneously prolongs the effectiveness of the protection coating. The oxide product, silica, is formed from both Sic and silicide after oxidation. The silica then exhibits a passive effect on the oxidation resistance and the C/C substrate survives for a longer time period. In this work, silicon, zirconium and the subsequent formation of Si-Zr intermetallics were first utilized as reagents of the thermal functional gradient. The concept of the functional gradient array of the coating[8] was also applied in this work to overcome the problem of thermal mismatches between the C/C matrix and the coating layer. A gradual change in the composition of the coating layer minimized the difference of the coefficient of thermal expansion (CTE) between each layer, and so lowered the induced stress to bring about microcracking in the coating layer.
1. INTRODUCPION The development of a high performance carbon/carbon composite (C/C) has attracted a lot of interest due to its excellent performance of strength and reliability at elevated temperatures. The study of the oxidation protection of the C/C is one of the main branches extending its applications to severe circumstances. Multilayer coating has been mentioned to compromise the inconsistency of the mechanical and thermodynamic properties between the C/C and its protection coating[ 11. Among the candidate coating materials, SIC exhibits excellent properties when encountering oxygen attacks at temperatures as high as 1800°C and has been widely studied[24]. The microcracks induced by the thermal stress in the SIC coating seem inevitable when encountering a large temperature variation[S]. A microcrack sealant is thus developed to cover the exposure site of the coated C/C, where the protection coating cracks when cooling down from the high processing temperature to room temperature. Silica and boron oxide are often used as reagents for microcrack sealing in the coated C/C due to their simplicity in processing[1,6,7]. The viscous property of the sealant at a medial temperature exhibits a good healing effect on the microcracks but evaporates quickly at higher temperatures. Increasing the thickness of the sealant can only prolong the time of evacuation to some extent because spallation occurs when the layer becomes too thick. Combining Sic and silicide inter-
2. EXPERIMENTAL PROCEDURE The 3D C/C was manufactured by performing a Throne1 high modulus carbon fiber (Union Carbide) to a 3D skeleton, followed by cycles of pitch impregnation pyrolysis by hot isostatic press (HIP) with a final density of 1.82 g/cm). The sample was finally graphitized at 2500°C to enhance its crystalline perfection as described elsewhere[9]. Specimens were machined to 10 mm in diameter and 10 mm in length for thermal gravimetry analysis (TGA) measurement. A reference specimen as large as 35 mm in diameter and 70mm in length was also prepared to compare with the TGA specimens. The protection coating was prepared by traditional reactions coating layer by layer. The raw powders of silicon (Ecka, Germany)
tPaper IAF-92-328 presented at the 43rd Astronautical Congress, Washington, September 1992.
D.C., U.S.A.,
28 August-S
35
36
Hsu el al.
&W-EN Table I. The properties of metal powders used for protection coatings
Purity (%) Particle size (pm) Apparent density (g/cm’) True density (g/cm’) Surface area (m’/g)
Si
Zr
99.1 5 0.44 2.20 2.32
99.5 44 2.65 6.35 0.23
0
$
.;
and zirconium (Elecmat, U.S.A.) were wet-sprayed on the surface of the C/C in various compositions in each layer. The physical properties of the raw powders are listed in Table 1. The primary coating of Sic was conducted at 1500°C for 2 h in an argon atmosphere. A similar procedure to produce Si-Zr intermetallic hybrid coating (Si/ZrSi, in the second layer and ZrSi, in the outermost layer) was conducted at 1500°C for 2 h in a vacuum. The overall process produced a SiC-(Si/ZrSi,)-ZrSi, multilayer coating to decrease the gradient concentration for silicon and to increase the gradient for zirconium elements. The oxidation resistance of the coated C/C was measured by both in situ monitoring at various temperatures and long-term isothermal oxidation. The isothermal oxidation behavior of the coated sample at an elevated temperature was carried out in a tube furnace. The temperature was raised to the testing point and the sample was loaded directly to the hot zone. The weight change was measured after it was taken out of the furnace to cool down to room temperature. A thermal cycle test was performed between room temperature and the elevated temperature to explore the tolerance of the combined coating in protecting the C/C matrix from oxidation,
,
0
,
IO 20
/
30 40
1650°C
;::z,
SO 60 Time
,
70
,
(h)
Fig. 2. Accumulative weight change of the coated C/C after oxidation at various temperatures.
and the thermal shock resistance of the coating was evaluated at the same time. The in situ oxidation behavior of the coated sample between 600 and 1300°C was measured with CAHN 2000 TGA in a fixed and controlled atmosphere. The operation temperature of the TGA was limited by an accessory furnace, which was equipped with SiC heaters. An analysis of the weight loss with respect to the temperature was calculated to determine whether the mechanism of oxidation obeyed Arrhenius’ law. Morphological observation of the coating layer was conducted by a JEOL 35CF SEM at 25 kV, with an X-ray energy dispersive spectrometer (EDS). Crystallography information was obtained by using a Philips PW 1729 diffractometer.
3D-C/C + Sic coating
I
2
3
,
80 90 100 II0 120
3D-C/C wlthout coating
0
,
s
4 Time
6
7
(h)
Fig. 1. Weight change as a function of heating time due to oxidation of 3D-C/C (a) without coating; (b) reaction coating with silica; and (c) multilayer coating with Si and Zr.
C/C composites
37
l/* /._._
o
._._.-._.
-.
-1 7
0 e
2h
1650
2h
2h
2h
2h
!z ;;i B z G
Fig. 3. The outer appearance of the SiC/ZrSi,coated C/C after oxidation for a long period of time at elevated temperatures.
3. RESULTS
3. I. Oxidation
RT -
1
Time Fig. 4. Weight variation of the multilayer-coated C/C composite following thermal cycles of severe oxidation in fast loading from RT to 1650°C.
AND DISCUSSIONS
behavior
Figure 1 shows the results of the oxidation measurements of the 3D-C/C composites in terms of the rate of weight change as a function of testing time/temperature at various coating processes. Comparing the weight change shown in Fig. 1, it is realized that the multilayer gradient coating is a very effective process for oxidation protection of the 3D-C/C, especially at elevated temperatures. A longterm isothermal oxidation test for the multilayer coated C/C was performed at 1250,145O and 165O”C, respectively, at various time intervals. The sample was loaded from room temperature to the hot zone in seconds. A detailed weight variation after longterm oxidation is plotted in Fig. 2. The amount of
weight gain in the beginning had a higher rate but became slower and slower. It is believed that the fast rate came from oxidation of the protection coating of the C/C, and the resulting lower rate of weight gain could be explained by the oxide layer barricade of SiOz, ZrO, and ZrSiO,. The performance of the oxidation of the multilayer coated C/C was further examined with a larger specimen. A 35 mm diameter x 70 mm long block also performed excellent oxidation resistance to exhibit a 1.4% weight gain after four cycles of 1450°C oxidation, with 30 h for each cycle. The outer appearance of the coated C/C after a total of 120 h oxidation at 1450°C is shown in Fig. 3. The oxide scale on the
4 2
----
500
700
900
0 A
As
After oxidized for 15 h
at 1250°C
0
After oxidized for 60 h
at 1250°C
1100
Temperature
prepared
1300
1500
1700
(“C)
Fig. 5. Weight change of the multilayer-coated C/C composite after oxidation at the temperature range of 60@-I 100°C.
&U-EN Hsu et a/
38
outer surface was visible and strongly adhered to its base coating. In order to simulate a more realistic application, a thermal cycle test was conducted for the multilayer coated C/C. The sample was plunged into the furnace directly from room temperature. The variation of the temperature from room temperature to 1650°C was completed in seconds. Figure 4 shows the weight change of a five cycle test, each lasting 2 h at 1650°C and then dropping to room temperature. The overall weight change is a combined result of the oxidation of the coating layer and the oxidation of the C/C substrate, with the former dominating and resulting in weight gain. From this cycle test, the thermal shock resistance of the coating layer was simultaneously qualified. As depicted in Fig. l(c), there is an apparent weight loss when the coated C/C is oxidized at temperatures between 600 and 1100°C. Weight loss became pronounced at 800°C and then converged to a lower rate as the temperature further increased, and eventually approached zero at a temperature beyond 1100°C. A small weight gain was found at higher temperatures. This phenomenon controverted our knowledge about the means of oxygen transportation and the chemical reaction of carbon
(4
with oxygen. The tendency of weight change with respect to temperature variation did not change even though the sample was pre-oxidized at elevated temperatures for a long period. The rate of weight change was minimum with the fastest oxidation at 800°C and then converged to zero at 1150°C for the sample pre-oxidized at 1250°C for 15 h. The sample with a longer duration of pre-oxidation of 60 h at 1250°C further degrades the protective coating layer. It has a slightly higher oxidation rate than that of the original or the slightly oxidized sample. However, the oxidation rate converges to zero at 1250°C instead of at 1150°C and the tendency of weight change remains unchanged, as depicted in Fig. 5. In summary of the above phenomenon, it is concluded that the overall weight change is balanced by weight loss due to the oxidation of carbon with oxygen and weight gain due to the oxidation of silicon and zirconium by forming oxide scales of SiO, , ZrO, as well as ZrSiO,. The protective coating exhibits some permeability for oxygen at 60@-1000°C due to the presence of microcracks. It is suspected that oxygen permeated through the microcracks in the Sic layer at medial and lower temperatures and that the microcracks would be
(b)
(cl
(0
(ii)
(iii)
__~_ ____~__ Fig. 6. Microstructure and EDS analysis of multilayer coating after oxidation in air (a) as received; (b) 1650°C x I5 h; (c) 1650°C x 30 h; (i) SEM microstructure; (ii) Si mapping and (iii) Zr mapping.
.~__
39
C/C composites
healed by the temperatures.
Si/ZrSi,
intermetallic
at
higher
3.2. Description of the coating layer The coated sample was examined by SEM/EDS to reveal the phases and the composition of the coatings. EDS examination, as shown in Fig. 6(a), depicts that Si and Zr are distributed in a step gradient in opposite directions to each other. The Si concentration in the coating layer decreases when it is far from the C/C matrix, and the Zr element increases in a reverse direction. This is consistent with our design to produce a coatrng with a gradual change in concentration of Si and Zr. The crystalline phases of the coating were determined by the XRD method and are demonstrated in Fig. 7(a). The phases of the coating layer consisted of Sic, ZrSi, and the residual unre-
9500
acted Si and Zr. The appearance of the graphite crystal structure in the pattern came from the C/C substrate. The adherence of the coating layer in the C/C matrix is well established as can be observed from the micrograph of the polished cross section in Fig. 6(a). The thickness of the SIC coating in the innermost layer was about 10-20 pm. The following Si/ZrSi, hybrid layer was around 30pm and the outermost ZrSir was about 50 pm. The thinness of the SIC layer is due to the difficulty of mass transport of Si and C to overcome the barricade of the produced Sic layer. A further study on the variation of the coating layer was performed. A thickness of 6Opm of the outer layer of the protection coating, presumably SiO,, ZrO, or ZrSiO,, was found after oxidation at 1650°C for 15 h, as shown in Fig. 6(b). The oxide
(4
7600
A:C
D : Si E : Si F : Sic
B : Zr C : ZrSiz 5700 5 3800
1900
0
4600 (b) a : SiO
b : C, $0, c :
zro
d : ZrS:O,
Ok--
25
40
c:zr02 f : SiO,. ZrSiOJ g : Sic h : ZrSiO,
55
i : ZrSiZ j : ZrSiO, k : Sic
70
85
20 Fig. 7. (a) XRD spectrum of the multilayer coating and (b) XRD spectrum of the multilayer coating after oxidation at 1650°C for 15 h.
SITU-EN Hsu et al.
40
scale was easily divided into Si-rich layers near the C/C substrate and a Zr-rich zone in the outer portion, as observed from the elemental analysis by EDS. The XRD analysis revealed that there are oxides, such as silica, zirconia and zirconium silicate as shown in Fig. 7(b), and residual coating materials of Sic and ZrSi, were also identified. The chemical reaction between the coating and oxygen is suggested as the following:
200
E
180
x
160t\
3.3. Kinetics consideration The kinetics of oxidation, of the 3D-C/C without a coating seemed to obey Arrhenius’ law, as shown in Fig. l(a). For the Sic-coated C/C composites, as shown in Fig. l(b), the rate of weight loss, i.e. the rate of oxidation, was still high at elevated temperatures. For the multilayer-coated sample, the situation of weight change was entirely different. As shown in Fig. l(c), the oxidation rate converged to zero when it encountered an elevated temperature. Apparently, the kinetics of oxidation is a heterogeneous reaction. The mechanism includes the adsorption of oxygen on the oxide scale, the transportation/diffusion of oxygen through oxide, the oxidation of Zr-Si intermetallics, the opening and healing of microcracks and finally the reaction of oxygen with BC and the C/C. Therefore, the weight change is a combined result of the heterogeneous reaction and the conventional rate law for oxidation no longer applies. The oxidation rate of the coated sample was traced at 1250, 1450 and 1650°C for a long period of time without interruption. The result is illustrated in Fig. 8 with the time scale of the square root. The rate of weight gain, due to the growth of the thickness of the oxide scale, decayed very rapidly
\
t\
Si + Or+ SiO, ZrSi, + 3O,+ZrOr + 2Si0, ZrSi, + 30,+ZrSiO, + SiOr The Zr-rich layer on the oxide scale is reasonably suggested to be zirconia and zirconium silicate, and the Si-rich layer as silica. Furthermore, it is found that the unoxidized coating layer changes its morphology after heat treatment at higher temperature, as compared in Fig. 6(a) and (b). A laminate texture was developed after eutectic solidification during thermal history[lO]. The elemental distribution in each part was easily identified by EDS analysis as displayed in Fig. 6(b-ii) A severe oxidation of the coated sample, at 1650°C for 30 h, was performed to observe the variation of the coating layer. The cross section of the coating after oxidation at 1650°C for 30 h is shown in Fig. 6(c). There was less variation in the morphology except the increase of thickness in the oxide layer, however a more serious one was observed in the microcrack. The Sic coating remained easily distinguished from the micrograph and no observable oxidation was found.
t
. L l
'0.
0
1650°C
0
1450°C
0
1250°C
I3 IO@ x IOHmm
l
%h.._
0
149
I6
_. 25
36
l-
49
-. 64
4 Si'160
(Time)“z (h)
Fig. 8. In situ weight variation of the coated temperatures.
C/C at various
and ceased at 1650°C for 25 h and at 1450°C for 49 h. The weight gain lasts longer than 240 h with a rate of 1.5 x lo-‘mg/min at 1250°C. The rate of weight gain was consistent with the growth of the thickness of the oxide scale. The rate, however, did not obey the square rule (the rate proportional to the square root of time) except in the very beginning. The whole process implied that weight change in an isothermal process is not controlled by the single mechanism of the oxygen diffusion. Concerning the performance of the oxidation resistance of the multilayer-coated C/C, there are some unusual phenomena in our study at temperatures below 1100°C. An interesting issue for discussion is that maximum weight loss occurs at the medial temperature and finally converges to zero at an elevated temperature and, thus, the only reason for this anomalous phenomena is that the healing/opening of the microcracks of the SiC as well as oxides can only occur at specific temperatures. The formation of microcracks in the various layers of the coating is due to the difference of CTE between two adjunct layers. The factors concerning the overall oxidation rate are: (1) the transport of oxygen and the chemical reaction of oxygen with the C/C, both accelerated by elevated temperature and (2) the site area of oxygen transport and the occurrence of oxidation. Most probably the microcrack of the Sic exhibits the largest oxygen flux at 800°C permitting oxygen to transport and react with the C/C substrate. As the temperature increases, the microcracks are healed by Si-ZrSi, or thermal stress, and the infiltrated oxygen reacts with Si-Zr intermetallics to form an oxide scale. As a result, the combined oxidation rate ceases. The mechanism of oxidation resistance of the coated C/C is suggested and illustrated in Fig. 9.
41
C/C composites
-
Sic Primary coating of Sic
c/c nTlTn ZrSiZ SilZrSi, Sir
Secondary coating of intermetallic compound
ZrSi, Si/ZrSi2 Sic
Microcrack formed after cooling down to RT
ZrSi,
1
Si/ZrSi, Sic
The largest opening of microcrack at 800 “C
SilZrSi, Sic ‘
Microcrack healed above 900 ‘C
Fig. 9. Illustration of the variation of the multilayer coating before and after oxidation.
4. CONCLUSION
Multilayer coating of SiC-(Si/ZrSi,)-ZrSi, is an effective process for oxidation protection of the 3D-C/C. The following conclusions can be drawn from this work. (1) The multilayer coating of Si/ZrSi,-ZrSi, can heal microcracks in the SIC coating. This leaves a small amount of C/C oxidation during 600-l lOO”C, but oxidation ceases at 1150°C after the microcrack is healed by the formation of the oxide scale and by thermal stress. (2) The application of ZrSi, is useful in elevating the oxidation resistance of the C/C at high temperatures. It exhibits a passive effect by crack sealing and acts as an oxygen barrier after oxidation. (3) The gradual change in composition of the coating layers performs an excellent oxidation resistance at 12%1650°C. The coating system is extremely useful, in long-term purposes, for as long as 240 h at 1250°C without deteriorating the function of the C/C. A longer duration is expected because the oxide of a protective coating (silica, zirconia and zirconium silicate) bears elevated temperatures. REFERENCES 1.
J. R. Strife and J. E. Sheehan, Ceramics coatings for carbon/carbon composite. Am. Ceram. Sot. BUN. 67, 369-374 (1988).
2. D. C. Roger, D. M. Shuford and J. I. Muller, Formation mechanism of a silicon carbide coating for a reinforced carbon/carbon composite. 7fh National SAMPE Technical Co& New Mexico, pp. 319-336 (1975). 3. R. D. Veltri, Composite silicon carbide/silicon nitride coatings for carbon/carbon composite. U.S. patent 4472476 (1984). 4. J. W. Patten, R. W. Moss and B. A. Forcht, Composite protective coating for carbon/carbon substrates. U.S. patent 4500602 (1985). 5. T. M. Wu, Study on the coating process and antioxidation model analysis of the carbon/carbon composites. Ph.D. thesis, National Taiwan University, Taiwan (1991). 6. D. W. McKee, Oxidation behavior and protection of carbon/carbon composite. Carbon 25, 551-557 (1987). 7. D. M. Shuford, Enhancement coating and process for carbonaceous substrates. U.S. patent 4471023 (1984). 8. S. T. Chou, H. D. Wu, H. Y. Chou, K. L. Wang, C. I. Chen and S. E. Hsu, Sic-Si-ZrSi, coating of 3D carbon/carbon composite for improved oxidation resistance. Presented at the First European East-Wesr Symposium on Materials and Processed, IO-18 June (1990). 9. S. E. Hsu and C. I. Chen, Superalloys, Supercomposites and Superceramics (Edited by J. K. Tien and T. Caulfield), pp. 721-744. Academic Press, San Diego (1989). 10. A. J. Whitehead and T. F. Page, Novel siliconized mixed-phase ceramics. Ceram. Engng Sci. Proc. 10 (9-10) 1108-I 120 (1989). 11. F. F. Lange, Healing of surface cracks in Sic by oxidation. J. Am. Ceram. Sot. 53, 290 (1970).