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Preparation of thermal barrier coatings for gas turbine blades by EB-PVD
Thin Solid Films 334 Ž1998. 98]102
Preparation of thermal barrier coatings for gas turbine blades by EB-PVD Huibin XuU , Shengkai Gong, Liang Deng Department of Materials Science and Engineering, Beijing Uni¨ ersity of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China
Abstract Thermal barrier coatings for gas turbine blades have been coated by the EB-PVD method. This paper will mainly focus on two aspects of thermal barrier coatings: Ži. the formation mechanism of the initial cracks during thermal cycle process; and Žii. the thermal barrier effect. The microcracks were only discovered inside the ceramic top coat of the thermalcycled TBCs. SEMrEDS observations indicated that some different oxides exist at the point just below the cracks. It seems that the formation of initial cracks resulted from oxidation stress as well as thermal stress. A simulated test facility has been designed to investigate the thermal barrier effect. Based on the heat transfer theory of one-dimensional steady-state heat conduction, the thermal barrier effect Ž DT . of two-layer structure TBCs was theoretically and experimentally studied as a function of the thickness of the coatings, the thermal conductivity, l, and the temperatures of the operating conditions. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: TBCs; EB-PVD; Thermal barrier effect
1. Introduction Thermal barrier coatings ŽTBCs. have been widely studied over the past 20 years because they increase the durability and efficiency of gas turbine engines by allowing an increase in turbine inlet temperature and by reducing the amount of cooling air required by the hot-section components. It has been reported that applications of such coatings throughout the combustion chamber and early stages of the turbine would save an operator around 1]2% of fuel cost, translating into savings of over $10 million per annum for some companies. TBCs used in current applications such as combustors and turbine blades commonly consist of two layers. The top coat, typically 8 wt.% Y2 O 3 stabilized zirconia ŽYSZ., is a thermally insulating ceramic material, which is mainly to protect the substrate from high temperature. The metallic bond layer, typically U
Corresponding author. Fax: q86 10 82314871; e-mail: [email protected]
MCrAlY Žwhere M indicates Ni, Co, Fe, or their combinations., is deposited next to the substrate to protect it from oxidation and corrosion at high temperature. This bond layer also provides the mechanical bonding between the substrate and the top coat w1,2x. Combustors and blades can be coated by lowpressure plasma spray for the bond coat and air plasma spray for the top coat, or both can be deposited by an electron beam physical vapor deposition ŽEB-PVD. process. Previous investigations mainly focused on plasma sprayed TBCs, the most prominent failure of which is the loss of adherence and spalling of ZrO 2 . The use of EB-PVD methods opened a new era of fabricating TBCs with high quality. It has been shown that the properties of TBCs deposited by EBPVD process would be better than that deposited by plasma spraying because of: Ž1. the possibility of achieving a columnar structure for the ceramic coating, which exhibits a higher strain tolerance than sprayed coatings with an arch structure, and thus can lead to longer component lives in cyclic oxidation environment; Ž2. the possibility of forming a smooth
0040-6090r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved. PII S0040-6090Ž98.01124-9
H. Xu et al. r Thin Solid Films 334 (1998) 98]102
99
surface coating to avoid disturbing the aerodynamic flow around turbine blades; and Ž3. the ability to control the microstructure of the deposited ceramics by adjusting coating process parameters such as substrate temperature and deposition rate, thus leading to fabricate graded TBCs w3x. In this paper, we focused on two main aspects of thermal barrier coatings prepared by EB-PVD technique. One is the mechanism in the formation of initial cracks, which relates to the lifetime of TBCs, and another is the thermal barrier effect. 2. Experiment Two-layer system thermal barrier coatings were deposited by EB-PVD method. Fig. 1 shows the EB-PVD equipment used in the present study. Four electron beam guns are installed in the two gun chambers and three water-cooled crucibles are placed in the main chamber. The substrate is mounted on the holder which can be fed from the pre-vacuum chamber to the main chamber and rotates around the horizontal axis. Bond coat material NiCoCrAlY and top coat material 8% Y2 O 3 stabilized ZrO 2 ingots, 50 mm in diameter and 200 mm in length, were used as the evaporation sources. These were heated and evaporated by two electron beam guns, respectively. The substrate was heated during the coating process by electron beam gun. The evaporation rates of NiCoCrAlY and YSZ were about 10 m mrmin and 2 m mrmin, respectively. For testing the thermal cycle life and investigating the formation mechanism of initial cracks, an IN-100 cast alloy blade was coated with a bond coat thickness of about 40 m m, with a top coat of about 110 m m. The thermal barrier effect was examined by coating a stainless steel tube Ž2 mm thick. with a top coat ranging in thickness from 100 to 500 m m. The coated samples were subjected to a furnace thermal cycling test which consisted of repeatedly heating the samples in air to 10508C and maintaining that temperature for about 20 min, followed by rapid
Fig. 1. Schematic drawing of EB-PVD equipment.
Fig. 2. Gas turbine blade coated with TBCs.
cooling by nitrogen gas until macrocracks were formed. The composition and microstructure of both as-deposited and cycled TBCs were analyzed by scanning electron microscopy ŽSEM. equipped with electron probe microanalyzers. A simulated facility was designed to test the thermal barrier effects by using the TBCs. Two thermocouples Ž1 mm in diameter. were used as sensors, one was located between the top coating and the substrate to test inner side temperature and another was bonded on the ceramic coating surface to test outer side temperature. Both of them were linked to a computer and monitored in real time. When the two temperatures became stable, the data were recorded. In addition, the effects of cooling gas flow rates has been also investigated to improve the heat exchange coefficient a 1. 3. Results and discussion 3.1. Micromorphology and composition analysis Fig. 2 shows the turbine blade coated with NiCoCrAlY bond coat and YSZ ceramic top coat by EBPVD. The total thickness is 150 m m with a bond coat of about 40 m m and a top coat of about 110 m m, respectively. The sample used for thermal cycling testing is cut from such blade TBCs perpendicular to the axis direction. The cross-sectional SEM morphology of the cycled TBCs is shown in Fig. 3. A microcrack is observed to be initiated from the interface between the top coat and bond coat and propagates through the ceramic top coat perpendicular to the interface. Along a line normal to the interface near the crack, at almost 8 m m intervals, element concentration analyses were made using electron probe microanalyzers. Fig. 4 shows the elemental con-
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H. Xu et al. r Thin Solid Films 334 (1998) 98]102
Fig. 3. SEM image of thermalcycled TBCs.
centration profiles of Ni, Co, Cr and Al measured by SEMrEDS in weight percent. Further microanalysis showed that just beneath the crack, Al element is about 12 wt.%, which is much higher than the area nearby. This indicates that just below the crack there exists different oxides which contain higher Al and Cr elements. This compositional difference is one of the reasons for the formation of microcracks. Several mechanisms of microcrack formation have been reported in previous studies. Movchan et al. reported that in two-layer metalrceramic PVD coatings, the cracks initiated from the base metal and then spreads out into the ceramic layer, finally causing ceramic spallation w3x. Tsai et al. found that the spalling of the top coat was initiated and propagated mainly by the cracks within the laser-glazed plasmasprayed top coat near the interface of the topcoatrbond-coat. The failure features are similar to those of plasma sprayed TBCs w4x. In this study, no crack was recognized in the bond coat layer. It appeared that the cracks first initiated
Fig. 4. Element distribution perpendicular to the interface after being thermalcycled from room temperature to 10508C for 80 times.
from the interface between the ceramic and oxidation layer as shown in Fig. 3. It is well known that during the thermal cycling process, the oxygen from the air can easily diffuse through the columnar structure zirconia owing to the high porosity, segmentation and ionic conductivity of the zirconia top coat, and this can cause the MCrAlY bond coat to suffer oxidation attack. Hence an oxide layer will form along the ceramicrmetallic interface. On the other hand, further microanalysis shows that the Al concentration is not uniform along the interface. At the interface, oxidation stress will be generated by the volume expansion of the oxide layer formation. According to calculation, an oxide layer with a thickness of 3 m m can produce about 12 MPa of oxidation
Fig. 5. Thermal barrier effect DT test results.
H. Xu et al. r Thin Solid Films 334 (1998) 98]102
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stress. At the point where different oxides form, stresses will be even much higher. Another factor is the thermal stress, that can reach about 9.88 MPa. This is caused by the difference between the thermal expansion coefficients of the top and bond coats when the specimens were being cooled to between 700 and 6008C. If the specimens were cooled to room temperature, a greater thermal stress can be expected. Combined with the factors discussed above, we see that cracks in the EB-PVD TBCs are first observed on the ceramicrbond-coat interface, and that these cracks propagate along a direction normal to the interface in the ceramic layer. The growth of these cracks finally results in the spallation of the TBCs. 3.2. Thermal barrier effect The thermal barrier effect is one of the important properties of TBCs. It mainly depends on the thickness of the top, TBC coat, and cooling technique of the blade Žsuch as the flow rate of the cooling-gas, the temperature of the cooling-gas and so on.. Fig. 5 shows the change of the thermal barrier effect with the top coat thickness and the flow rate of the cooling-gas. It is obvious that the thermal barrier effect, DT, increases with the thickness of the TBCs and the flow rate of the cooling-gas. The thicker the coating is, the larger DT has been observed by increasing the cooling gas flow rate. In the present study, in order to obtain a relationship between the thermal barrier effect and the parameters described above, the thermal barrier effect was analyzed theoretically on the basis of the one dimensional steady-state conduction theory for hollow cylinders. From this theory we can get an approximate equation by omitting the high temperature side heat exchange resistance. DT s
T1 y T2 d2 2 1 1 q ln q ln Ž 1 q 2 drd2 . a 1 d1 l b d1 lc ?
ln Ž 1 q 2 drd2 . lc
where, d1 is the inner diameter of the stainless tube; a 1 is the heat exchange coefficient of the cooling gas; d 2 is the outer diameter of the stainless tube; l b is the thermal conductivity of the stainless tube; d is the thickness of the ceramic coating; l c is the thermal conductivity of the ceramic coating; T1 is the hot gas average temperature; T2 , the cooling gas average temperature. Using the following known values: Ž1. a 1 10 wrm2 k, 100 wrm2 k; Ž2. ŽT1 y T2 . 870 k; Ž3. d 0.1 mm]1 mm; Ž4. l c 0.1 wrmk]3 wrmk; Ž5. l b 20 wrmk; Ž6.
Fig. 6. Thermal barrier effect of the cylinder model.
Fig. 7. Diagram of thermal barrier effect DT vs. coating thickness.
d1 s 28 mm, d 2 s 32 mm, we can draw a surface plot Žin Fig. 6. of the thermal barrier effect DT vs. thickness and thermal conductivity of the ceramic coating. Fig. 7 shows the thermal barrier effect for different thicknesses of ceramic coatings for several cooling conditions Ž l c s 0.1 wrmk.. From Fig. 6 it can be seen that DT increases with the increase of the thickness of the ceramic coating and the heat exchange coefficient of the cooling gas, which agrees with the experiment results. Another important phenomenon as shown in Fig. 6 is that there exists a sensitive zone of the thermal conductivity in which a little change can lead to substantial increase of the DT.
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4. Conclusions In this study, TBCs were prepared by EB-PVD method. 1. In the thermalcycled TBCs the cracks were observed to initiate from the interface between the ceramicroxide layer and propagate into the ceramic coat. No cracks were recognized in the bond coat layer. 2. At a point just below the cracks, abnormally higher aluminum concentration has been found. It is thought that different oxides formed and caused the initial cracks. 3. Thermal barrier effect: DT increased almost linearly with the increase of thickness of the ceramic top coat in the present thickness range, and also increased by improving the cooling gas heat exchange Žvia increasing the flow rate of the cooling gas..
4. Thermal barrier effect: DT also depends on the thermal conductivity values of the selected materials. It is important to choose the appropriate materials to satisfy the design condition.
Acknowledgements This work is sponsored by NSFC and Aviation Science Foundation of China. References w1x H.B. Xu, S.K. Gong et al., Aviat. Prod. Eng. 11 Ž1995. 23 Žin Chinese.. w2x H.B. Xu, S.K. Gong et al., Aviat. Prod. Eng. 12 Ž1995. 18 Žin Chinese.. w3x B.A. Movchan, et al., Surf. Coat. Technol. 67 Ž1994. 55. w4x H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 Ž1995. 53.
Preparation of thermal barrier coatings for gas turbine blades by EB-PVD Huibin XuU , Shengkai Gong, Liang Deng Department of Materials Science and Engineering, Beijing Uni¨ ersity of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China
Abstract Thermal barrier coatings for gas turbine blades have been coated by the EB-PVD method. This paper will mainly focus on two aspects of thermal barrier coatings: Ži. the formation mechanism of the initial cracks during thermal cycle process; and Žii. the thermal barrier effect. The microcracks were only discovered inside the ceramic top coat of the thermalcycled TBCs. SEMrEDS observations indicated that some different oxides exist at the point just below the cracks. It seems that the formation of initial cracks resulted from oxidation stress as well as thermal stress. A simulated test facility has been designed to investigate the thermal barrier effect. Based on the heat transfer theory of one-dimensional steady-state heat conduction, the thermal barrier effect Ž DT . of two-layer structure TBCs was theoretically and experimentally studied as a function of the thickness of the coatings, the thermal conductivity, l, and the temperatures of the operating conditions. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: TBCs; EB-PVD; Thermal barrier effect
1. Introduction Thermal barrier coatings ŽTBCs. have been widely studied over the past 20 years because they increase the durability and efficiency of gas turbine engines by allowing an increase in turbine inlet temperature and by reducing the amount of cooling air required by the hot-section components. It has been reported that applications of such coatings throughout the combustion chamber and early stages of the turbine would save an operator around 1]2% of fuel cost, translating into savings of over $10 million per annum for some companies. TBCs used in current applications such as combustors and turbine blades commonly consist of two layers. The top coat, typically 8 wt.% Y2 O 3 stabilized zirconia ŽYSZ., is a thermally insulating ceramic material, which is mainly to protect the substrate from high temperature. The metallic bond layer, typically U
Corresponding author. Fax: q86 10 82314871; e-mail: [email protected]
MCrAlY Žwhere M indicates Ni, Co, Fe, or their combinations., is deposited next to the substrate to protect it from oxidation and corrosion at high temperature. This bond layer also provides the mechanical bonding between the substrate and the top coat w1,2x. Combustors and blades can be coated by lowpressure plasma spray for the bond coat and air plasma spray for the top coat, or both can be deposited by an electron beam physical vapor deposition ŽEB-PVD. process. Previous investigations mainly focused on plasma sprayed TBCs, the most prominent failure of which is the loss of adherence and spalling of ZrO 2 . The use of EB-PVD methods opened a new era of fabricating TBCs with high quality. It has been shown that the properties of TBCs deposited by EBPVD process would be better than that deposited by plasma spraying because of: Ž1. the possibility of achieving a columnar structure for the ceramic coating, which exhibits a higher strain tolerance than sprayed coatings with an arch structure, and thus can lead to longer component lives in cyclic oxidation environment; Ž2. the possibility of forming a smooth
0040-6090r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved. PII S0040-6090Ž98.01124-9
H. Xu et al. r Thin Solid Films 334 (1998) 98]102
99
surface coating to avoid disturbing the aerodynamic flow around turbine blades; and Ž3. the ability to control the microstructure of the deposited ceramics by adjusting coating process parameters such as substrate temperature and deposition rate, thus leading to fabricate graded TBCs w3x. In this paper, we focused on two main aspects of thermal barrier coatings prepared by EB-PVD technique. One is the mechanism in the formation of initial cracks, which relates to the lifetime of TBCs, and another is the thermal barrier effect. 2. Experiment Two-layer system thermal barrier coatings were deposited by EB-PVD method. Fig. 1 shows the EB-PVD equipment used in the present study. Four electron beam guns are installed in the two gun chambers and three water-cooled crucibles are placed in the main chamber. The substrate is mounted on the holder which can be fed from the pre-vacuum chamber to the main chamber and rotates around the horizontal axis. Bond coat material NiCoCrAlY and top coat material 8% Y2 O 3 stabilized ZrO 2 ingots, 50 mm in diameter and 200 mm in length, were used as the evaporation sources. These were heated and evaporated by two electron beam guns, respectively. The substrate was heated during the coating process by electron beam gun. The evaporation rates of NiCoCrAlY and YSZ were about 10 m mrmin and 2 m mrmin, respectively. For testing the thermal cycle life and investigating the formation mechanism of initial cracks, an IN-100 cast alloy blade was coated with a bond coat thickness of about 40 m m, with a top coat of about 110 m m. The thermal barrier effect was examined by coating a stainless steel tube Ž2 mm thick. with a top coat ranging in thickness from 100 to 500 m m. The coated samples were subjected to a furnace thermal cycling test which consisted of repeatedly heating the samples in air to 10508C and maintaining that temperature for about 20 min, followed by rapid
Fig. 1. Schematic drawing of EB-PVD equipment.
Fig. 2. Gas turbine blade coated with TBCs.
cooling by nitrogen gas until macrocracks were formed. The composition and microstructure of both as-deposited and cycled TBCs were analyzed by scanning electron microscopy ŽSEM. equipped with electron probe microanalyzers. A simulated facility was designed to test the thermal barrier effects by using the TBCs. Two thermocouples Ž1 mm in diameter. were used as sensors, one was located between the top coating and the substrate to test inner side temperature and another was bonded on the ceramic coating surface to test outer side temperature. Both of them were linked to a computer and monitored in real time. When the two temperatures became stable, the data were recorded. In addition, the effects of cooling gas flow rates has been also investigated to improve the heat exchange coefficient a 1. 3. Results and discussion 3.1. Micromorphology and composition analysis Fig. 2 shows the turbine blade coated with NiCoCrAlY bond coat and YSZ ceramic top coat by EBPVD. The total thickness is 150 m m with a bond coat of about 40 m m and a top coat of about 110 m m, respectively. The sample used for thermal cycling testing is cut from such blade TBCs perpendicular to the axis direction. The cross-sectional SEM morphology of the cycled TBCs is shown in Fig. 3. A microcrack is observed to be initiated from the interface between the top coat and bond coat and propagates through the ceramic top coat perpendicular to the interface. Along a line normal to the interface near the crack, at almost 8 m m intervals, element concentration analyses were made using electron probe microanalyzers. Fig. 4 shows the elemental con-
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Fig. 3. SEM image of thermalcycled TBCs.
centration profiles of Ni, Co, Cr and Al measured by SEMrEDS in weight percent. Further microanalysis showed that just beneath the crack, Al element is about 12 wt.%, which is much higher than the area nearby. This indicates that just below the crack there exists different oxides which contain higher Al and Cr elements. This compositional difference is one of the reasons for the formation of microcracks. Several mechanisms of microcrack formation have been reported in previous studies. Movchan et al. reported that in two-layer metalrceramic PVD coatings, the cracks initiated from the base metal and then spreads out into the ceramic layer, finally causing ceramic spallation w3x. Tsai et al. found that the spalling of the top coat was initiated and propagated mainly by the cracks within the laser-glazed plasmasprayed top coat near the interface of the topcoatrbond-coat. The failure features are similar to those of plasma sprayed TBCs w4x. In this study, no crack was recognized in the bond coat layer. It appeared that the cracks first initiated
Fig. 4. Element distribution perpendicular to the interface after being thermalcycled from room temperature to 10508C for 80 times.
from the interface between the ceramic and oxidation layer as shown in Fig. 3. It is well known that during the thermal cycling process, the oxygen from the air can easily diffuse through the columnar structure zirconia owing to the high porosity, segmentation and ionic conductivity of the zirconia top coat, and this can cause the MCrAlY bond coat to suffer oxidation attack. Hence an oxide layer will form along the ceramicrmetallic interface. On the other hand, further microanalysis shows that the Al concentration is not uniform along the interface. At the interface, oxidation stress will be generated by the volume expansion of the oxide layer formation. According to calculation, an oxide layer with a thickness of 3 m m can produce about 12 MPa of oxidation
Fig. 5. Thermal barrier effect DT test results.
H. Xu et al. r Thin Solid Films 334 (1998) 98]102
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stress. At the point where different oxides form, stresses will be even much higher. Another factor is the thermal stress, that can reach about 9.88 MPa. This is caused by the difference between the thermal expansion coefficients of the top and bond coats when the specimens were being cooled to between 700 and 6008C. If the specimens were cooled to room temperature, a greater thermal stress can be expected. Combined with the factors discussed above, we see that cracks in the EB-PVD TBCs are first observed on the ceramicrbond-coat interface, and that these cracks propagate along a direction normal to the interface in the ceramic layer. The growth of these cracks finally results in the spallation of the TBCs. 3.2. Thermal barrier effect The thermal barrier effect is one of the important properties of TBCs. It mainly depends on the thickness of the top, TBC coat, and cooling technique of the blade Žsuch as the flow rate of the cooling-gas, the temperature of the cooling-gas and so on.. Fig. 5 shows the change of the thermal barrier effect with the top coat thickness and the flow rate of the cooling-gas. It is obvious that the thermal barrier effect, DT, increases with the thickness of the TBCs and the flow rate of the cooling-gas. The thicker the coating is, the larger DT has been observed by increasing the cooling gas flow rate. In the present study, in order to obtain a relationship between the thermal barrier effect and the parameters described above, the thermal barrier effect was analyzed theoretically on the basis of the one dimensional steady-state conduction theory for hollow cylinders. From this theory we can get an approximate equation by omitting the high temperature side heat exchange resistance. DT s
T1 y T2 d2 2 1 1 q ln q ln Ž 1 q 2 drd2 . a 1 d1 l b d1 lc ?
ln Ž 1 q 2 drd2 . lc
where, d1 is the inner diameter of the stainless tube; a 1 is the heat exchange coefficient of the cooling gas; d 2 is the outer diameter of the stainless tube; l b is the thermal conductivity of the stainless tube; d is the thickness of the ceramic coating; l c is the thermal conductivity of the ceramic coating; T1 is the hot gas average temperature; T2 , the cooling gas average temperature. Using the following known values: Ž1. a 1 10 wrm2 k, 100 wrm2 k; Ž2. ŽT1 y T2 . 870 k; Ž3. d 0.1 mm]1 mm; Ž4. l c 0.1 wrmk]3 wrmk; Ž5. l b 20 wrmk; Ž6.
Fig. 6. Thermal barrier effect of the cylinder model.
Fig. 7. Diagram of thermal barrier effect DT vs. coating thickness.
d1 s 28 mm, d 2 s 32 mm, we can draw a surface plot Žin Fig. 6. of the thermal barrier effect DT vs. thickness and thermal conductivity of the ceramic coating. Fig. 7 shows the thermal barrier effect for different thicknesses of ceramic coatings for several cooling conditions Ž l c s 0.1 wrmk.. From Fig. 6 it can be seen that DT increases with the increase of the thickness of the ceramic coating and the heat exchange coefficient of the cooling gas, which agrees with the experiment results. Another important phenomenon as shown in Fig. 6 is that there exists a sensitive zone of the thermal conductivity in which a little change can lead to substantial increase of the DT.
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4. Conclusions In this study, TBCs were prepared by EB-PVD method. 1. In the thermalcycled TBCs the cracks were observed to initiate from the interface between the ceramicroxide layer and propagate into the ceramic coat. No cracks were recognized in the bond coat layer. 2. At a point just below the cracks, abnormally higher aluminum concentration has been found. It is thought that different oxides formed and caused the initial cracks. 3. Thermal barrier effect: DT increased almost linearly with the increase of thickness of the ceramic top coat in the present thickness range, and also increased by improving the cooling gas heat exchange Žvia increasing the flow rate of the cooling gas..
4. Thermal barrier effect: DT also depends on the thermal conductivity values of the selected materials. It is important to choose the appropriate materials to satisfy the design condition.
Acknowledgements This work is sponsored by NSFC and Aviation Science Foundation of China. References w1x H.B. Xu, S.K. Gong et al., Aviat. Prod. Eng. 11 Ž1995. 23 Žin Chinese.. w2x H.B. Xu, S.K. Gong et al., Aviat. Prod. Eng. 12 Ž1995. 18 Žin Chinese.. w3x B.A. Movchan, et al., Surf. Coat. Technol. 67 Ž1994. 55. w4x H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 Ž1995. 53.