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CaTiO3 coating on TiAl by hydrothermal-electrochemical technique
Inrernwtullics 3 (1995) 125-128 Elserier Science Limited Printed tn Great Britain. 0966-9795195/$09.50 ELSEVIER
CaTiO, coating on TiAl by hydrothermal-electrochemical technique Masahiro Yoshimura, Wataru Urushihara, Masatomo Yashima & Masato Kakihana Reseurch Laboratory
of’ Engineeritzg Materials, Tokyo Institute ofTechnology. 4259, Nagatsuta, Midori, Yokohama, 226, Japan (Received 22 March 1994; accepted 3 June 1994)
Smooth and dense films of CaTiO, were prepared on the TiAl specimens at 473K in Ca(OH)* solutions by the hydrothermal and the hydrothermal-electrochemical treatments. The films produced were well-crystallized tetragonal CaTiO, and no appreciable intermediate layer such as TiO, were identified. The result of a cyclic oxidation test at 1173K in air indicated that the oxidation resistance of TiAl was improved by the CaTi03 coating. Key words: TiAl, hydrothermal-electrochemical coating, CaTiO,, cyclic oxidation.
reaction,
resisting oxidation
tion of which was Ti-37.12wt%Al (Mitsubishi Material Co.) as seen in Table 1 and 50 X 10 X 1 mm3 in size. Specimen surfaces were mechanically polished with emery papers of up to #lOOO and washed with acetone in an ultrasonic cleaner. CaTiO, films were prepared on these specimens in hydrothermal treatment or a hydrothermal-electrochemical treatment at 473K for l-2 h in mixed aqueous solutions of 0.25-l .OON-NaOH and excess Ca(OH), under a saturated water vapor pressure of 2.0 MPa. In the case of the hydrothermalelectrochemical treatment, a constant electrical current of lo-30 mA/cm2 was passed between the anode of a TiAl specimen and the cathode of Pt to promote film formation during this treatment (Fig. 1). The thin films obtained were studied by a grazing incidence X-ray diffractometer (XRD, MXP 3VA, MAC Science Co.). Radiation of CuKcv was used at an operating power of 4OkV4OmA. The grazing angle of the incident beam on the film surface was 0.5-l .O”. The samples were investigated using a scanning electron microscopy (SEMJSM-T200 and JSM-T300, JEOL Co.), an energydispersive X-ray analysis (EOXANl OOO-QX200J, LINK Co.) and by examining the weight change of specimens. The oxidation resistance of the specimens with the thin film coating and without coating was investigated by a cyclic oxidation test at 1173K in static air. One cycle of the test was chosen to be 0.5 h. The specimens were also investigated by
1 INTRODUCTION TiAl is an attractive material as a high temperature structural material because of its high specific strength at high temperatures.’ Since it has poor oxidation resistance at high temperatures,2,3 various surface treatments such as aluminizing,4 CVD,’ plasma spraying6 and preoxidation at low oxygen pressures7 have been studied to form a protective coating layer. The authors have proposed new techniques of titanate coating on Ti metal and Ti-alloy by hydrothermal or hydrothermal-electrochemical treatments.8’o These treatments can yield a thin film of such double oxides as BaTiO,, SrTiO, and CaTiO, at temperatures of 473K or below. It was found that these titanate films could also be formed on TiAl substrate and that they were effective in improving the oxidation resistance of the TiAl substrate.” In the present paper, it is shown that the CaTiO, coating on TiAl substrate by the hydrothermal-electrochemical treatment is effective in improving oxidation resistance of TiAl at high temperatures.
2 EXPERIMENTAL The TiAl specimens used in the present study were cut from a TiAl ingot, the stoichiometric composi125
126
hi.
Yoshimura, W. Urushihara, M. Yashima, M. Kakihana Table 1. Chemical composition of TL37wt%Al
Composition wt’%,
Al 36.6
0 0.045
C 0,006
N 0.0039
H 0+004
Ti balance
XRD, SEM, EDX and examining their weight changes after oxidising for several hours.
-@--
3 RESULTS AND DISCUSSION
Potediometer
The hydrothermal treatment yielded very thin films of CaTiO, (JCPDS number 22-153) on TiAl in a mixed aqueous solution of O.SON-NaOH and excess Ca(OH), at 473K for 2 h. The thin films obtained had lustrous interference colors of blue, purple, red, etc, probably depending on their thickness, and they had thicknesses of 0.1-0.2 pm, similar to BaTi03 or SrTiO, films reported previously.” On the other hand, the hydrothermal-electrochemical treatment resulted in the formation of much thicker CaTiO, films; for example, a treatment at 473K with the current density of 20 mA/cm2 for 1 h in a mixed aqueous solution of O.SON-NaOH and excess Ca(OH), yielded a film of single phase CaTiO, showing a grayish color. Figure 2 shows the XRD pattern from the specimen surface formed in the above condition indicating the well-crystallized almost single phase CaTiO,. The crystalline A1203 and TiO, phases were not detected by the XRD
Fig. 1. Schematic illustration of experimental equipment for hydrothermal-electrochemical method: (A) cathode (Pt plate); (B) thermocouple; (C) stirrer; (D) anode (TiAl plate).
CaTi
t
I
20
o unknown
I
30
I
I
40 50 28/degi (CuKa)
I
60
I
7C
Fig. 2. Grazing incidence X-ray diffraction pattern at the incidence angle of 0.5” for the surface of film by hydrothermal-electro
chemical method.
CaTiO,
coating
127
on TiAl 1
-A-o-
non-coaled
CaTi SrTiOa - v - BaTi - 0 - CaTi
-o-
(H ) (H) (H ) ( HE )
Oxidation time
/h
Fig. 4. Weight gain due to cyclic oxidation at 1173K of the TiAl specimens coated with CaTiO,, SrTiO, BaTiO, and without the coating: H = hydrothermal method; HE = hydrothermalelectrochemical method.
Fig. 3. SEM
micrographes of the specimen coated with CaTiO,: (a) the outer surface; (b) the fractured cross section.
analysis in both treatments. The optimum conditions for the formation of CaTiO, film with good oxidation resistance seems to be 0.5&075N-NaOH with the current density of 20 mA/cm2. Figures 3(a) and (b) show SEM micrographs of the outer surface and fractured cross section of the specimen coated with the CaTiO, film by the hydrothermal-electrochemical treatment, respectively. These photographs reveal that the film was composed of CaTiO, crystals with approximate 1 pm size and that it had a thickness of 4-5 pm. Figure 4 shows the result of a cyclic oxidation test at 1173K of the TiAl specimens coated with CaTiO,, BaTi03” or SrTi03” and without coating. Weight gains of the coated specimens were smaller than those of the specimen without the coating, particularly in the early stage of the test. It indicates that the titanate coating can improve the oxidation resistance of TiAl. The CaTiO, coating by the hydrothermal treatment was similar to SrTiO, coating in improving the oxidation resistance of TiA 1. Among the samples tested, the CaTiO, coat-
ing by the hydrothermalelectrochemical treatment was most effective, probably because the thickness of CaTiO, (45 pm) by the hydrothermal+lectrochemical treatment is greater than those of BaTiO, (0.1 pm), SrTiO, (0.12 pm) and CaTiO, (0.1-0.2 pm) by the hydrothermal treatment. The present study revealed that the CaTiO, coating is effective in increasing the oxidation resistance of the TiAl sample. However, the formation of TiO, grains was observed by XRD and SEM at several points where the coated layer was broken after the oxidation of a certain period of time. The other regions remained unchanged to be coated CaTiO, grains because of the good adhesion to the substrate. Thus, the breakdown of the coated film might occur at some defective points of the film and/or of the substrate. Much improved oxidation resistance is expected when denser and/or non-defective CaTiO, coating could be produced on the TiAl specimen.
REFERENCES 1. Kawabata,
T., Kanai, T. & Izumi, (1985) 1355. 2. Becker, S., Schtitze, M. & Rahmel, (1993) 93. 3. Becker, S., Schiitze, M. & Rahmel, (1992) 425. 4. Takai, A., & Ishida, A., Proceedings
O., Acta Met&l., 33 A., Oxid. Met., 39 A., Oxid. Met., 38
of the 86th National Symposium on Corrosion and Protection, ( I99 1) 44. 5. Taniguchi, S., Shibata, T., & Takeuchi, K., Metar. Trans. JIM., 32 (1991) 299.
128
A4. Yoshimura, W. Urushihara, A4. Yashima, A4. Kakihana
6. Furukawa, H., Proceeding of the 86th National Symposium on Corrosion and Protection, (199.1) 54. 7. Kobayashi, I., Yoshihara, M. & Tanaka, R., J. Jpn. Inst. Met., 53, (1989) 251. 8. Yoshimura, M., Yoo, S. E., Hayashi, M. & Ishizawa, N., Jpn. J. Appl. Phys., 28( 11) (1989) L2007.
9. Yoo, S.E., Hayashi, M., Ishizawa, N. & Yoshimura, M., J. Am. Ceram. Sot., 73(8) (1990) 256 1. 10. Kajiyoshi, K., Ishizawa, N. & Yoshimura, M., Jpn J. Appl. Phys., 3O(lb) (1990) L120. 11. Yoshimura, M., Mizunuma, S., Yashima, M. & Kakihana, M., J. Mater Sci. Lett. (submitted).
CaTiO, coating on TiAl by hydrothermal-electrochemical technique Masahiro Yoshimura, Wataru Urushihara, Masatomo Yashima & Masato Kakihana Reseurch Laboratory
of’ Engineeritzg Materials, Tokyo Institute ofTechnology. 4259, Nagatsuta, Midori, Yokohama, 226, Japan (Received 22 March 1994; accepted 3 June 1994)
Smooth and dense films of CaTiO, were prepared on the TiAl specimens at 473K in Ca(OH)* solutions by the hydrothermal and the hydrothermal-electrochemical treatments. The films produced were well-crystallized tetragonal CaTiO, and no appreciable intermediate layer such as TiO, were identified. The result of a cyclic oxidation test at 1173K in air indicated that the oxidation resistance of TiAl was improved by the CaTi03 coating. Key words: TiAl, hydrothermal-electrochemical coating, CaTiO,, cyclic oxidation.
reaction,
resisting oxidation
tion of which was Ti-37.12wt%Al (Mitsubishi Material Co.) as seen in Table 1 and 50 X 10 X 1 mm3 in size. Specimen surfaces were mechanically polished with emery papers of up to #lOOO and washed with acetone in an ultrasonic cleaner. CaTiO, films were prepared on these specimens in hydrothermal treatment or a hydrothermal-electrochemical treatment at 473K for l-2 h in mixed aqueous solutions of 0.25-l .OON-NaOH and excess Ca(OH), under a saturated water vapor pressure of 2.0 MPa. In the case of the hydrothermalelectrochemical treatment, a constant electrical current of lo-30 mA/cm2 was passed between the anode of a TiAl specimen and the cathode of Pt to promote film formation during this treatment (Fig. 1). The thin films obtained were studied by a grazing incidence X-ray diffractometer (XRD, MXP 3VA, MAC Science Co.). Radiation of CuKcv was used at an operating power of 4OkV4OmA. The grazing angle of the incident beam on the film surface was 0.5-l .O”. The samples were investigated using a scanning electron microscopy (SEMJSM-T200 and JSM-T300, JEOL Co.), an energydispersive X-ray analysis (EOXANl OOO-QX200J, LINK Co.) and by examining the weight change of specimens. The oxidation resistance of the specimens with the thin film coating and without coating was investigated by a cyclic oxidation test at 1173K in static air. One cycle of the test was chosen to be 0.5 h. The specimens were also investigated by
1 INTRODUCTION TiAl is an attractive material as a high temperature structural material because of its high specific strength at high temperatures.’ Since it has poor oxidation resistance at high temperatures,2,3 various surface treatments such as aluminizing,4 CVD,’ plasma spraying6 and preoxidation at low oxygen pressures7 have been studied to form a protective coating layer. The authors have proposed new techniques of titanate coating on Ti metal and Ti-alloy by hydrothermal or hydrothermal-electrochemical treatments.8’o These treatments can yield a thin film of such double oxides as BaTiO,, SrTiO, and CaTiO, at temperatures of 473K or below. It was found that these titanate films could also be formed on TiAl substrate and that they were effective in improving the oxidation resistance of the TiAl substrate.” In the present paper, it is shown that the CaTiO, coating on TiAl substrate by the hydrothermal-electrochemical treatment is effective in improving oxidation resistance of TiAl at high temperatures.
2 EXPERIMENTAL The TiAl specimens used in the present study were cut from a TiAl ingot, the stoichiometric composi125
126
hi.
Yoshimura, W. Urushihara, M. Yashima, M. Kakihana Table 1. Chemical composition of TL37wt%Al
Composition wt’%,
Al 36.6
0 0.045
C 0,006
N 0.0039
H 0+004
Ti balance
XRD, SEM, EDX and examining their weight changes after oxidising for several hours.
-@--
3 RESULTS AND DISCUSSION
Potediometer
The hydrothermal treatment yielded very thin films of CaTiO, (JCPDS number 22-153) on TiAl in a mixed aqueous solution of O.SON-NaOH and excess Ca(OH), at 473K for 2 h. The thin films obtained had lustrous interference colors of blue, purple, red, etc, probably depending on their thickness, and they had thicknesses of 0.1-0.2 pm, similar to BaTi03 or SrTiO, films reported previously.” On the other hand, the hydrothermal-electrochemical treatment resulted in the formation of much thicker CaTiO, films; for example, a treatment at 473K with the current density of 20 mA/cm2 for 1 h in a mixed aqueous solution of O.SON-NaOH and excess Ca(OH), yielded a film of single phase CaTiO, showing a grayish color. Figure 2 shows the XRD pattern from the specimen surface formed in the above condition indicating the well-crystallized almost single phase CaTiO,. The crystalline A1203 and TiO, phases were not detected by the XRD
Fig. 1. Schematic illustration of experimental equipment for hydrothermal-electrochemical method: (A) cathode (Pt plate); (B) thermocouple; (C) stirrer; (D) anode (TiAl plate).
CaTi
t
I
20
o unknown
I
30
I
I
40 50 28/degi (CuKa)
I
60
I
7C
Fig. 2. Grazing incidence X-ray diffraction pattern at the incidence angle of 0.5” for the surface of film by hydrothermal-electro
chemical method.
CaTiO,
coating
127
on TiAl 1
-A-o-
non-coaled
CaTi SrTiOa - v - BaTi - 0 - CaTi
-o-
(H ) (H) (H ) ( HE )
Oxidation time
/h
Fig. 4. Weight gain due to cyclic oxidation at 1173K of the TiAl specimens coated with CaTiO,, SrTiO, BaTiO, and without the coating: H = hydrothermal method; HE = hydrothermalelectrochemical method.
Fig. 3. SEM
micrographes of the specimen coated with CaTiO,: (a) the outer surface; (b) the fractured cross section.
analysis in both treatments. The optimum conditions for the formation of CaTiO, film with good oxidation resistance seems to be 0.5&075N-NaOH with the current density of 20 mA/cm2. Figures 3(a) and (b) show SEM micrographs of the outer surface and fractured cross section of the specimen coated with the CaTiO, film by the hydrothermal-electrochemical treatment, respectively. These photographs reveal that the film was composed of CaTiO, crystals with approximate 1 pm size and that it had a thickness of 4-5 pm. Figure 4 shows the result of a cyclic oxidation test at 1173K of the TiAl specimens coated with CaTiO,, BaTi03” or SrTi03” and without coating. Weight gains of the coated specimens were smaller than those of the specimen without the coating, particularly in the early stage of the test. It indicates that the titanate coating can improve the oxidation resistance of TiAl. The CaTiO, coating by the hydrothermal treatment was similar to SrTiO, coating in improving the oxidation resistance of TiA 1. Among the samples tested, the CaTiO, coat-
ing by the hydrothermalelectrochemical treatment was most effective, probably because the thickness of CaTiO, (45 pm) by the hydrothermal+lectrochemical treatment is greater than those of BaTiO, (0.1 pm), SrTiO, (0.12 pm) and CaTiO, (0.1-0.2 pm) by the hydrothermal treatment. The present study revealed that the CaTiO, coating is effective in increasing the oxidation resistance of the TiAl sample. However, the formation of TiO, grains was observed by XRD and SEM at several points where the coated layer was broken after the oxidation of a certain period of time. The other regions remained unchanged to be coated CaTiO, grains because of the good adhesion to the substrate. Thus, the breakdown of the coated film might occur at some defective points of the film and/or of the substrate. Much improved oxidation resistance is expected when denser and/or non-defective CaTiO, coating could be produced on the TiAl specimen.
REFERENCES 1. Kawabata,
T., Kanai, T. & Izumi, (1985) 1355. 2. Becker, S., Schtitze, M. & Rahmel, (1993) 93. 3. Becker, S., Schiitze, M. & Rahmel, (1992) 425. 4. Takai, A., & Ishida, A., Proceedings
O., Acta Met&l., 33 A., Oxid. Met., 39 A., Oxid. Met., 38
of the 86th National Symposium on Corrosion and Protection, ( I99 1) 44. 5. Taniguchi, S., Shibata, T., & Takeuchi, K., Metar. Trans. JIM., 32 (1991) 299.
128
A4. Yoshimura, W. Urushihara, A4. Yashima, A4. Kakihana
6. Furukawa, H., Proceeding of the 86th National Symposium on Corrosion and Protection, (199.1) 54. 7. Kobayashi, I., Yoshihara, M. & Tanaka, R., J. Jpn. Inst. Met., 53, (1989) 251. 8. Yoshimura, M., Yoo, S. E., Hayashi, M. & Ishizawa, N., Jpn. J. Appl. Phys., 28( 11) (1989) L2007.
9. Yoo, S.E., Hayashi, M., Ishizawa, N. & Yoshimura, M., J. Am. Ceram. Sot., 73(8) (1990) 256 1. 10. Kajiyoshi, K., Ishizawa, N. & Yoshimura, M., Jpn J. Appl. Phys., 3O(lb) (1990) L120. 11. Yoshimura, M., Mizunuma, S., Yashima, M. & Kakihana, M., J. Mater Sci. Lett. (submitted).