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ZrO2 intermediate on the combination ability of coatings
Surface and Coatings Technology 140 Ž2001. 231᎐237
Effects of plasma-sprayed NiCrAlrZrO2 intermediate on the combination ability of coatings Xiaodong Wu, Duan WengU , Zhen Chen, Luhua Xu Department of Materials Science and Engineering, Tsinghua Uni¨ ersity, Beijing 100084, PR China Received 5 May 2000; accepted in revised form 9 March 2001
Abstract A NiCrAlrZrO2 composite coating was deposited on the surface of metal carrier FeCrAl alloy by a plasma-spray technique. After static-state oxidation at 800⬚C, the transitions in structure and composition of the coating was analyzed by XRD, SEM and EDX. The results showed that the surface phases of the as-sprayed coating were mainly composed of Ni and ZrO 2 . When the oxidation time was extended from 8 to 50 h, NiO crystallites were formed and these grew coarse on the coating surface, and alloy elements were diffused between the NiCrAlrZrO2 coating and the FeCrAl substrate. With the pretreatment, an intermediate coating was prepared with a coarse and porous structure, high cohesive strength and high heat resistance. These developed properties could provide high geometric surface area for a catalytic ␥-Al 2 O 3 washcoat, and enhance the adhesive strength between ceramic washcoat and metal substrate so as to extend the lifetime of the washcoat. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma spraying; Intermediate coating; Microstructure; Combination ability; Thermal stability
1. Introduction Substrate materials have been developed in recent years for exhaust gas emission converters in motor vehicles. It can be advantageous to use a metal foil instead of a cordierite ceramic honeycomb as the substrate for the catalytic washcoat. FeCrAl is a heat- and corrosion-resistant supperalloy, whose low thermal capacity and fast heat-up performance is beneficial to the quick light-off function of the exhaust gas. With high resistance to mechanical vibration and thermal shock, the metal substrate is especially suitable for the driving conditions and exhaust characteristics of motorcycles. However, the surface of the metal substrate is too smooth to load the catalytic washcoat, and its linear U
Corresponding author. Tel.: q86-10-6278-2806; fax: q86-106278-2806. E-mail address: [email protected] ŽD. Weng..
thermal expansion coefficient is different from that of the ceramic coating so as to cause thermal mismatch. At high temperatures, some active components would migrate into the substrate by way of thermal diffusion, thus decreasing catalytic activity. Therefore, oxygen could simultaneously filtrate through the washcoat up to the substrate, causing the surface oxidation of metal substrate and the peeling of ␥-alumna washcoat w1x. Thus, it is difficult to improve the combination ability between the washcoat and the substrate since the FeCrAl alloy was applied to the motorcycle exhaust converter. Several pre-treatments have been mentioned in order to improve the combination ability. For example, a hydrothermal treatment had been developed by Ferrandon et al., which made a lot of alumna whiskers on the surface of the metallic support resulting in good adhesion of the washcoat w2x. Another precoating technology was also developed to yield a converter capable of withstanding hot vibration w3x.
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 6 6 - 5
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
232
Table 1 Chemical composition of 0Cr20Al7Y, wt.% Component
C
P, S
Mn
Si
Ni
Re
Cr
Al
Fe
Concentration
F 0.04
F 0.02
0.50᎐0.65
F 0.55
F 0.55
0.2᎐0.50
19.0᎐21.0
6.5᎐8.5
etc.
This paper reported a pre-treatment technique on the surface of a FeCrAl metal substrate for motorcycle exhaust catalysis by plasma spraying. The microstructures of the coatings and the interlayer diffusion processes of different elements were analyzed after various oxidation times, and their impacts on the combination quality between the substrate and the washcoat were investigated.
2. Experimental 2.1. Materials 0Cr20Al7Y-ferrite stainless steel was drawn and twisted into wire meshes as the substrate material. Its chemical composition was listed in Table 1. NiCrAl powders were composed of Ni Ž17᎐19%., Al Ž5᎐6.5%. and Ni Ž74.5᎐78%. with a granularity of 43᎐147 m. The granularity of ZrO 2 Ž7% Y2 O 3 stabilized. powder was 43᎐74 m. These two raw materials were made up of spraying powders in the weight proportion of 75:25.
3. Results 3.1. Phase composition analysis with X-ray diffraction Fig. 1 shows the X-ray diffraction profiles of NiCrAlrZrO2 intermediate coating before and after 8-h oxidation. Cubic-structured ZrO 2 and Ni were easily detected in such an as-sprayed coating. There were also a few NiO crystallites with hexagonal cubic structure on the coating surface. After thermal treatment at 800⬚C for 8 h, the results from XRD analysis showed an increase of the peak value of NiO. It indicated that Ni had oxidized to form a great deal of NiO crytallites on the coating surface. After 50-h static oxidation, the peak shape of NiO was more acuminated compared to that with 8-h oxidation treatment. It could be anticipated that the relative content of NiO increased in the coating but the phase composition varied little, mainly composed of Ni, NiO, ZrO 2 , etc.
2.2. Preparation and analytical methods
3.2. Morphology analysis with scanning electronic microscope
A plasma spraying technique was adapted to prepare NiCrAlrZrO2 intermediate coating. The spraying technical parameters were listed in Table 2. The thickness of sprayed coating was approximately 45 m. After spraying, the specimens were heated in the furnace at 800⬚C for 8- and 50-h static oxidation tests, respectively. The phase compositions of the coating surfaces were measured by Drmax-RB X-ray diffraction before and after the oxidation. The surface and sectional micrographes were observed by S-450 scanning electrical microscope. The linear distributions of elements across the coating section were analyzed by EDX.
Fig. 2 shows SEM surface images of NiCrAlrZrO2 intermediate coatings after various oxidation times at 800⬚C. The microstructure of the coating surface consisted of crystal and amorphous areas at the early stage of oxidation, as revealed in Fig. 2a. The crystal areas were mainly composed of regular NiO crystallites with hexagonal cubic structure, while the amorphous areas were composed of agglomerates of sintered ZrO 2 particles. After exposure to high temperature for as long as 50 h, the surface micro-morphology of the coating was greatly varied, as displayed in Fig. 2b. The crystal area increased with a laminated structure and the
Table 2 Technical parameters of plasma spraying Voltage ŽV.
Current ŽA.
Arc gas Ar flow rate Žlrmin.
Aux gas H2 flow rate Žlrmin.
Powder gas N2 flow rate Žlrmin.
Specimen linear speed Žmrmin.
Powder transfer speed Žgrmin.
Spray distance Žmm.
75᎐80
500
35
4.0
12
40
50
; 100
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
233
Fig. 1. XRD profiles of NiCrAlrZrO2 intermediate coating before and after 8-h oxidation.
amorphous area decreased accordingly. At the same time, NiO crystallites grew coarser on the coating surface. It was confirmed by the result of XRD analysis that the NiO content on the coating surface increased with oxidation time. Also, the NiO scale was relatively loose-structured and could not provide effective protection for the coating.
Fig. 3a presents the SEM surface view of as-sprayed coating. It could be found in the picture that the coating surface was very coarse and porous with very few droplets. This structure provided a high geometric surface area to load the ␥-alumna washcoat and helped to enhance the adhesive strength between the washcoat and metal substrate.
Fig. 2. Micromorphologies of NiCrAlrZrO2 intermediate coating after Ža. 8- and Žb. 50-h oxidation.
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X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
Fig. 3. Micromorphologies of as-sprayed NiCrAlrZrO2 intermediate coating: Ža. surface and Žb. sectional view.
Fig. 4. EDX analysis of NiCrAlrZrO2 intermediate coating with different oxidation times: Ža. sectional view after 8-h oxidation; Žb. linear elemental ditribution after 8-h oxidation; Žc. sectional view after 50-h oxidation; and Žd. linear elemental ditribution after 50-h oxidation.
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
Fig. 3b demonstrates the SEM sectional view of as-sprayed coating. The laminated structure was formed when the molten raw material particles were sprayed on the substrate surface. The coating was constructed with wavy layers of sprayed materials, oxide impurities, random micro-pores and a few irregular additive particles. In the left part of Fig. 3b, the EDX results shows that the grayish areas are mostly NiCrAl alloy, while the fuscous areas were considered to be complex oxides of Al 2 O 3 and Cr2 O 3 . Furthermore, the dark areas were gas cavities and the white regions were infusible ZrO 2 particles strongly adhered to alloy powders. The metallurgical phase of FeCrAl alloy was ␣-ferrite, as shown in the right part of Fig. 3b. The bright thread-shaped zone was the interface between the coating and the substrate, which showed an alloyed combination with a compacted structure. The obvious diffusion phenomenon was not observed from the coating into the substrate, because the interacting process between molten spraying particles and substrate materials lasted for only a few tenths of a second.
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than the original value of 60 wt.% in the substrate. All the data indicated that the Ni element began to diffuse from the coating into the substrate under oxidation at 800⬚C for such a long time, while the high density of grain boundary in the coating provided short circuits to the diffusivity of Al. Thus, there was much more Ni and Al at the interface, which reacted and formed the inter-metallic compounds of Ni 3 Al or NiAl. The oxygen content on the coating surface was greatly increased after a long time of high temperature treatment, as shown in Fig. 4d. It suggested that a quite thick scale had been formed with the main composition of NiO crystallites. At the same time, the results demonstrated that Ni content was not quickly dropped but decreased in a stepwise manner after entering the substrate. Iron remained in abundance at the interface. The thickness of the diffusion layer was approximately 12 m. It showed a high degree of thermal effects on the specimen.
4. Discussion 3.3. Elements distribution analysis with energy dispersi¨ e X-ray
4.1. Effects of the coating processing
Fig. 4 presents the sectional images of NiCrAlrZrO2 intermediate coatings after the oxidation tests at 800⬚C with the linear element distributions of O, Al, Zr, Cr, Fe and Ni through the coating section measured by EDX. Fig. 4a,b shows the coating status after 8-h oxidation. At the early stage of thermal treatment, the oxygen content on the coating surface was low. The coating was mainly composed of Ni, Cr and ZrO 2 . The low Al content in the coating was related to the low Al content in spraying materials. Furthermore, Al would be easily blown off during the spraying process because of its small density, and oxidized as discontinuous ␣-Al 2 O 3 film due to the active characteristics. The Ni content was rapidly decreased to zero from the coating into the substrate. Similarly, the Fe content decreased dramatically after infiltrating into the coating. The thickness of the diffusion layer was approximately 4 m. It indicated that the inter-diffusion effect of the alloy elements had started between the NiCrAlrZrO2 intermediate coating and the FeCrAl substrate. The interface zone in Fig. 4c was relatively darker than that in Fig. 4a, which implied the formation of a new phase in the coating after 50-h oxidation. Chemical composition point-analysis was carried out in this area. There was as much as 56-wt.% Ni in the substrate of the interface zone, which had only existed in the coating before the oxidation test. The Al content also reached 23 wt.%, much higher than the initial value of 6.5᎐8.5 wt.% in the substrate. On the other hand, the Fe content was approximately 16 wt.%, much lower
From the above analysis, it revealed an optimized performance of NiCrAlrZrO2 intermediate coating for loading ␥-Al 2 O 3 washcoat on the metal substrate. The structure and performance of the coating was seriously affected by coating parameters in the plasma spraying process. In the plasma spraying experiments, the porosity of the coating would generally increase along with the increase in particle size of raw powders. The granularity of the mixed powders was chosen from 43 to 147 m. In this case, the porosity of the coating was 5᎐8%. It was unavailable for the oxidative gas to penetrate through, thus preventing the oxidation of the substrate surface and peeling of the coating. Furthermore, the coarse and porous structure of the coating created a number of anchors for ␥-Al 2 O 3 washcoat. The metal wire-mesh support was not transcalent because of its small size and complicated structure. If the spray distance were too short, the specimen would oxidize owing to quick heat-up. The performance of the coating would be deteriorated as the spraying powder remained for only a short time in the plasma flame and their melting status was not good. On the other hand, the longer the spray distance, the lower the deposit efficiency of the powder. A satisfactory distance was found to be approximately 80᎐100 mm in the spray experiment. Similarly, if the linear speed of the specimen movement was too slow as well as the transverse speed of the spray gun, the specimen surface would be superheated resulting in an increase of residual stress in the
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
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Table 3 Some optimized parameters of plasma spraying affected on the coating structure and performance
Table 4 Linear thermal expansion coefficients of FeCrAl substrate, NiCrAlrZrO2 intermediate coating and ␥-Al 2 O 3 washcoat
Granularity of mixed powder Žm.
Spray distance Žmm.
Specimen linear speed Žmrmin.
Spray gun transfer speed Žmrmin.
Materials
Linear thermal expansion coefficients Ž=10y6 r⬚C.
Temperature Ž⬚C.
43᎐147
80᎐100
40
; 300
NiCrAl ZrO2 NiCrAlrZrO2 ␥-Al2 O3 FeCrAl
13.5᎐20.0 7᎐8 7.5᎐15.4 5.1᎐7.3 14᎐16
100᎐800 200᎐2000 500᎐1000 500᎐1000 20᎐1000
coating. This caused significant damage to the combination ability if the residual stress was not released. However, the deposit efficiency of the powders also decreased if the specimen and spray gun moved too quickly. Thus, it was fit for the substrate with a linear speed of 40 mrmin and the spray gun with a transverse speed of ; 300 mrmin. See Table 3 for the best recommended spraying parameters in the coating process.
that to spray an intermediate coating could improve the co-ordination of thermal expansion between the washcoat and the metal substrate. Thus, it could help reduce thermal stress at the interface between the washcoat and the intermediate coating, so as to enhance the thermal-shock resistance of the catalyst.
4.2. Effects of the intermediate coating
5. Conclusion
It was clear in Figs. 2 and 3 that the infusible ZrO 2 particles formed the second phase of the NiCrAl matrix. Along with the oxides of Al 2 O 3 , Cr2 O 3 , they acted as a strengthening mechanism providing many pinning sources for the laminated layers themselves w4x, as well as for the coating with the washcoat. The ceramic᎐ceramic chemical bond energy between ZrO 2 in the coating and ␥-Al 2 O 3 was stronger than the metal᎐ceramic adsorptive bond between FeCrAl and ␥-Al 2 O 3 . This resulted in strong adhesion of the catalytic washcoat on the surface of the sprayed coating. At the same time, there are less micro-cracks within the ZrO 2 layers by adding the metallic components into the composite coating w5x. The ZrO 2 additive lowered the coating porosity of, inhibited the outward diffusion of the cations of Ni 2q, Al 3q, Cr 3q, and reduced the inward diffusion coefficient of oxygen. Furthermore, the combination ability was effectively improved between the intermediate coating and the metal substrate due to the NiCrAl alloy. The compact structure of alloy layers also decreased the infiltration of oxidative gas, avoiding the further oxidation of the substrate and the peeling of the coating. Thus, the weight proportion of 75:25 for NiCrAl and ZrO 2 is preferable. As shown in Table 4, the integrated thermal expansion coefficient ŽCTE. of the composite coating was from 7.5 to 15.4 = 10y6 r⬚C at a temperature of 500᎐800⬚C. This value was between the CTE of 14᎐16 = 10y6 r⬚C for FeCrAl substrate, and the CTE of 5.1᎐7.3= 10y6 r⬚C for ␥-Al 2 O 3 washcoat. It indicated
1. A NiCrAlrZrO2 composite coating was deposited on the surface of a metal carrier FeCrAl alloy by plasma-spray technique. The surface of as-sprayed coating was mainly composed of cubic-structured ZrO 2 and Ni, along with a few hexagonal-cubicstructured NiO crystallites. With the oxidation time extended from 8 to 50 h at 800⬚C, the phase composition of the coating surface was mainly unchanged; just the relative content of NiO was increased and the crystallites became much coarser. 2. The coating was constructed with wavy layers of sprayed materials, oxide impurities, random micropores and a few irregular additive particles. The bright thread-shaped zone was the interface between the coating and the substrate. The obvious diffusion phenomenon from the coating into the substrate was not observed due to the brief interacting process between molten coating particles and substrate materials. 3. The alloy elements were inter-diffused between NiCrAlrZrO2 coating and FeCrAl substrate when the specimen was treated at high temperature for a long time. The contents of Ni and Al were obviously increased at the interface and they reacted as the intermetallic compounds of Ni 3 Al and NiAl. The second phase and inborn oxides brought the pinning effect, as well as enhancing the adhesive strength of the sprayed coating with the washcoat and substrate. At the same time, the compact alloy layers lowered the coating porosity and enhanced the oxidation resistance.
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
4. With the pre-treatment, an intermediate coating was prepared with coarse and porous structure, high cohesive strength and high heat resistance. These properties could provide a large geometric surface area for Al 2 O 3 washcoat, and enhance the adhesive strength between the ceramic washcoat and the metal substrate so as to extend the lifetime of the washcoat.
Acknowledgements The authors would like to acknowledge the Ministry of Science and Technology, PR China for the financial support of the project no. 96-910-03-04A, as well as the State Environment Protection Agency for guidance. We also thank the Center of Testing and Analyzing of
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Materials, Tsinghua University for the help in the tests of XRD, SEM and EDX. References w1x J.G. McCarty, M. Gusman, D.M. Lowe, D.L. Hilbenbrand, K.N. Lau, Stability of supported metal and supported metal oxide combustion catalysts, Catal. Today 47 Ž1999. 5᎐17. w2x M. Ferrandon, M. Berg, E. Bjornbom, Thermal stability of ¨ metal-supported catalysts for reduction of cold-start emissions in a wood-fired domestic boiler, Catal. Today 53 Ž1999. 647᎐659. w3x J.R. Adomaitis, M.P. Galligan, J.E. Kubsh, W.A. Whittenberger, Metal converter technology using precoated metal foil, SAE 962080 Ž1996. 2099᎐2108. w4x G. Chen, H. Lou, The effect of nanocrystallization on the oxidation resistance of Ni᎐5Cr᎐5Al alloy, Scripta Mater. 41 Ž8. Ž1999. 883᎐887. w5x C.T. Chia, K.A. Khor, Y.W. Gu, Dynamic mechanical properties of ZrO 2 rNiCoCrAlY composite coatings, Thin Solid Films 358 Ž2000. 139᎐145.
Effects of plasma-sprayed NiCrAlrZrO2 intermediate on the combination ability of coatings Xiaodong Wu, Duan WengU , Zhen Chen, Luhua Xu Department of Materials Science and Engineering, Tsinghua Uni¨ ersity, Beijing 100084, PR China Received 5 May 2000; accepted in revised form 9 March 2001
Abstract A NiCrAlrZrO2 composite coating was deposited on the surface of metal carrier FeCrAl alloy by a plasma-spray technique. After static-state oxidation at 800⬚C, the transitions in structure and composition of the coating was analyzed by XRD, SEM and EDX. The results showed that the surface phases of the as-sprayed coating were mainly composed of Ni and ZrO 2 . When the oxidation time was extended from 8 to 50 h, NiO crystallites were formed and these grew coarse on the coating surface, and alloy elements were diffused between the NiCrAlrZrO2 coating and the FeCrAl substrate. With the pretreatment, an intermediate coating was prepared with a coarse and porous structure, high cohesive strength and high heat resistance. These developed properties could provide high geometric surface area for a catalytic ␥-Al 2 O 3 washcoat, and enhance the adhesive strength between ceramic washcoat and metal substrate so as to extend the lifetime of the washcoat. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma spraying; Intermediate coating; Microstructure; Combination ability; Thermal stability
1. Introduction Substrate materials have been developed in recent years for exhaust gas emission converters in motor vehicles. It can be advantageous to use a metal foil instead of a cordierite ceramic honeycomb as the substrate for the catalytic washcoat. FeCrAl is a heat- and corrosion-resistant supperalloy, whose low thermal capacity and fast heat-up performance is beneficial to the quick light-off function of the exhaust gas. With high resistance to mechanical vibration and thermal shock, the metal substrate is especially suitable for the driving conditions and exhaust characteristics of motorcycles. However, the surface of the metal substrate is too smooth to load the catalytic washcoat, and its linear U
Corresponding author. Tel.: q86-10-6278-2806; fax: q86-106278-2806. E-mail address: [email protected] ŽD. Weng..
thermal expansion coefficient is different from that of the ceramic coating so as to cause thermal mismatch. At high temperatures, some active components would migrate into the substrate by way of thermal diffusion, thus decreasing catalytic activity. Therefore, oxygen could simultaneously filtrate through the washcoat up to the substrate, causing the surface oxidation of metal substrate and the peeling of ␥-alumna washcoat w1x. Thus, it is difficult to improve the combination ability between the washcoat and the substrate since the FeCrAl alloy was applied to the motorcycle exhaust converter. Several pre-treatments have been mentioned in order to improve the combination ability. For example, a hydrothermal treatment had been developed by Ferrandon et al., which made a lot of alumna whiskers on the surface of the metallic support resulting in good adhesion of the washcoat w2x. Another precoating technology was also developed to yield a converter capable of withstanding hot vibration w3x.
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 6 6 - 5
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
232
Table 1 Chemical composition of 0Cr20Al7Y, wt.% Component
C
P, S
Mn
Si
Ni
Re
Cr
Al
Fe
Concentration
F 0.04
F 0.02
0.50᎐0.65
F 0.55
F 0.55
0.2᎐0.50
19.0᎐21.0
6.5᎐8.5
etc.
This paper reported a pre-treatment technique on the surface of a FeCrAl metal substrate for motorcycle exhaust catalysis by plasma spraying. The microstructures of the coatings and the interlayer diffusion processes of different elements were analyzed after various oxidation times, and their impacts on the combination quality between the substrate and the washcoat were investigated.
2. Experimental 2.1. Materials 0Cr20Al7Y-ferrite stainless steel was drawn and twisted into wire meshes as the substrate material. Its chemical composition was listed in Table 1. NiCrAl powders were composed of Ni Ž17᎐19%., Al Ž5᎐6.5%. and Ni Ž74.5᎐78%. with a granularity of 43᎐147 m. The granularity of ZrO 2 Ž7% Y2 O 3 stabilized. powder was 43᎐74 m. These two raw materials were made up of spraying powders in the weight proportion of 75:25.
3. Results 3.1. Phase composition analysis with X-ray diffraction Fig. 1 shows the X-ray diffraction profiles of NiCrAlrZrO2 intermediate coating before and after 8-h oxidation. Cubic-structured ZrO 2 and Ni were easily detected in such an as-sprayed coating. There were also a few NiO crystallites with hexagonal cubic structure on the coating surface. After thermal treatment at 800⬚C for 8 h, the results from XRD analysis showed an increase of the peak value of NiO. It indicated that Ni had oxidized to form a great deal of NiO crytallites on the coating surface. After 50-h static oxidation, the peak shape of NiO was more acuminated compared to that with 8-h oxidation treatment. It could be anticipated that the relative content of NiO increased in the coating but the phase composition varied little, mainly composed of Ni, NiO, ZrO 2 , etc.
2.2. Preparation and analytical methods
3.2. Morphology analysis with scanning electronic microscope
A plasma spraying technique was adapted to prepare NiCrAlrZrO2 intermediate coating. The spraying technical parameters were listed in Table 2. The thickness of sprayed coating was approximately 45 m. After spraying, the specimens were heated in the furnace at 800⬚C for 8- and 50-h static oxidation tests, respectively. The phase compositions of the coating surfaces were measured by Drmax-RB X-ray diffraction before and after the oxidation. The surface and sectional micrographes were observed by S-450 scanning electrical microscope. The linear distributions of elements across the coating section were analyzed by EDX.
Fig. 2 shows SEM surface images of NiCrAlrZrO2 intermediate coatings after various oxidation times at 800⬚C. The microstructure of the coating surface consisted of crystal and amorphous areas at the early stage of oxidation, as revealed in Fig. 2a. The crystal areas were mainly composed of regular NiO crystallites with hexagonal cubic structure, while the amorphous areas were composed of agglomerates of sintered ZrO 2 particles. After exposure to high temperature for as long as 50 h, the surface micro-morphology of the coating was greatly varied, as displayed in Fig. 2b. The crystal area increased with a laminated structure and the
Table 2 Technical parameters of plasma spraying Voltage ŽV.
Current ŽA.
Arc gas Ar flow rate Žlrmin.
Aux gas H2 flow rate Žlrmin.
Powder gas N2 flow rate Žlrmin.
Specimen linear speed Žmrmin.
Powder transfer speed Žgrmin.
Spray distance Žmm.
75᎐80
500
35
4.0
12
40
50
; 100
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
233
Fig. 1. XRD profiles of NiCrAlrZrO2 intermediate coating before and after 8-h oxidation.
amorphous area decreased accordingly. At the same time, NiO crystallites grew coarser on the coating surface. It was confirmed by the result of XRD analysis that the NiO content on the coating surface increased with oxidation time. Also, the NiO scale was relatively loose-structured and could not provide effective protection for the coating.
Fig. 3a presents the SEM surface view of as-sprayed coating. It could be found in the picture that the coating surface was very coarse and porous with very few droplets. This structure provided a high geometric surface area to load the ␥-alumna washcoat and helped to enhance the adhesive strength between the washcoat and metal substrate.
Fig. 2. Micromorphologies of NiCrAlrZrO2 intermediate coating after Ža. 8- and Žb. 50-h oxidation.
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X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
Fig. 3. Micromorphologies of as-sprayed NiCrAlrZrO2 intermediate coating: Ža. surface and Žb. sectional view.
Fig. 4. EDX analysis of NiCrAlrZrO2 intermediate coating with different oxidation times: Ža. sectional view after 8-h oxidation; Žb. linear elemental ditribution after 8-h oxidation; Žc. sectional view after 50-h oxidation; and Žd. linear elemental ditribution after 50-h oxidation.
X. Wu et al. r Surface and Coatings Technology 140 (2001) 231᎐237
Fig. 3b demonstrates the SEM sectional view of as-sprayed coating. The laminated structure was formed when the molten raw material particles were sprayed on the substrate surface. The coating was constructed with wavy layers of sprayed materials, oxide impurities, random micro-pores and a few irregular additive particles. In the left part of Fig. 3b, the EDX results shows that the grayish areas are mostly NiCrAl alloy, while the fuscous areas were considered to be complex oxides of Al 2 O 3 and Cr2 O 3 . Furthermore, the dark areas were gas cavities and the white regions were infusible ZrO 2 particles strongly adhered to alloy powders. The metallurgical phase of FeCrAl alloy was ␣-ferrite, as shown in the right part of Fig. 3b. The bright thread-shaped zone was the interface between the coating and the substrate, which showed an alloyed combination with a compacted structure. The obvious diffusion phenomenon was not observed from the coating into the substrate, because the interacting process between molten spraying particles and substrate materials lasted for only a few tenths of a second.
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than the original value of 60 wt.% in the substrate. All the data indicated that the Ni element began to diffuse from the coating into the substrate under oxidation at 800⬚C for such a long time, while the high density of grain boundary in the coating provided short circuits to the diffusivity of Al. Thus, there was much more Ni and Al at the interface, which reacted and formed the inter-metallic compounds of Ni 3 Al or NiAl. The oxygen content on the coating surface was greatly increased after a long time of high temperature treatment, as shown in Fig. 4d. It suggested that a quite thick scale had been formed with the main composition of NiO crystallites. At the same time, the results demonstrated that Ni content was not quickly dropped but decreased in a stepwise manner after entering the substrate. Iron remained in abundance at the interface. The thickness of the diffusion layer was approximately 12 m. It showed a high degree of thermal effects on the specimen.
4. Discussion 3.3. Elements distribution analysis with energy dispersi¨ e X-ray
4.1. Effects of the coating processing
Fig. 4 presents the sectional images of NiCrAlrZrO2 intermediate coatings after the oxidation tests at 800⬚C with the linear element distributions of O, Al, Zr, Cr, Fe and Ni through the coating section measured by EDX. Fig. 4a,b shows the coating status after 8-h oxidation. At the early stage of thermal treatment, the oxygen content on the coating surface was low. The coating was mainly composed of Ni, Cr and ZrO 2 . The low Al content in the coating was related to the low Al content in spraying materials. Furthermore, Al would be easily blown off during the spraying process because of its small density, and oxidized as discontinuous ␣-Al 2 O 3 film due to the active characteristics. The Ni content was rapidly decreased to zero from the coating into the substrate. Similarly, the Fe content decreased dramatically after infiltrating into the coating. The thickness of the diffusion layer was approximately 4 m. It indicated that the inter-diffusion effect of the alloy elements had started between the NiCrAlrZrO2 intermediate coating and the FeCrAl substrate. The interface zone in Fig. 4c was relatively darker than that in Fig. 4a, which implied the formation of a new phase in the coating after 50-h oxidation. Chemical composition point-analysis was carried out in this area. There was as much as 56-wt.% Ni in the substrate of the interface zone, which had only existed in the coating before the oxidation test. The Al content also reached 23 wt.%, much higher than the initial value of 6.5᎐8.5 wt.% in the substrate. On the other hand, the Fe content was approximately 16 wt.%, much lower
From the above analysis, it revealed an optimized performance of NiCrAlrZrO2 intermediate coating for loading ␥-Al 2 O 3 washcoat on the metal substrate. The structure and performance of the coating was seriously affected by coating parameters in the plasma spraying process. In the plasma spraying experiments, the porosity of the coating would generally increase along with the increase in particle size of raw powders. The granularity of the mixed powders was chosen from 43 to 147 m. In this case, the porosity of the coating was 5᎐8%. It was unavailable for the oxidative gas to penetrate through, thus preventing the oxidation of the substrate surface and peeling of the coating. Furthermore, the coarse and porous structure of the coating created a number of anchors for ␥-Al 2 O 3 washcoat. The metal wire-mesh support was not transcalent because of its small size and complicated structure. If the spray distance were too short, the specimen would oxidize owing to quick heat-up. The performance of the coating would be deteriorated as the spraying powder remained for only a short time in the plasma flame and their melting status was not good. On the other hand, the longer the spray distance, the lower the deposit efficiency of the powder. A satisfactory distance was found to be approximately 80᎐100 mm in the spray experiment. Similarly, if the linear speed of the specimen movement was too slow as well as the transverse speed of the spray gun, the specimen surface would be superheated resulting in an increase of residual stress in the
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Table 3 Some optimized parameters of plasma spraying affected on the coating structure and performance
Table 4 Linear thermal expansion coefficients of FeCrAl substrate, NiCrAlrZrO2 intermediate coating and ␥-Al 2 O 3 washcoat
Granularity of mixed powder Žm.
Spray distance Žmm.
Specimen linear speed Žmrmin.
Spray gun transfer speed Žmrmin.
Materials
Linear thermal expansion coefficients Ž=10y6 r⬚C.
Temperature Ž⬚C.
43᎐147
80᎐100
40
; 300
NiCrAl ZrO2 NiCrAlrZrO2 ␥-Al2 O3 FeCrAl
13.5᎐20.0 7᎐8 7.5᎐15.4 5.1᎐7.3 14᎐16
100᎐800 200᎐2000 500᎐1000 500᎐1000 20᎐1000
coating. This caused significant damage to the combination ability if the residual stress was not released. However, the deposit efficiency of the powders also decreased if the specimen and spray gun moved too quickly. Thus, it was fit for the substrate with a linear speed of 40 mrmin and the spray gun with a transverse speed of ; 300 mrmin. See Table 3 for the best recommended spraying parameters in the coating process.
that to spray an intermediate coating could improve the co-ordination of thermal expansion between the washcoat and the metal substrate. Thus, it could help reduce thermal stress at the interface between the washcoat and the intermediate coating, so as to enhance the thermal-shock resistance of the catalyst.
4.2. Effects of the intermediate coating
5. Conclusion
It was clear in Figs. 2 and 3 that the infusible ZrO 2 particles formed the second phase of the NiCrAl matrix. Along with the oxides of Al 2 O 3 , Cr2 O 3 , they acted as a strengthening mechanism providing many pinning sources for the laminated layers themselves w4x, as well as for the coating with the washcoat. The ceramic᎐ceramic chemical bond energy between ZrO 2 in the coating and ␥-Al 2 O 3 was stronger than the metal᎐ceramic adsorptive bond between FeCrAl and ␥-Al 2 O 3 . This resulted in strong adhesion of the catalytic washcoat on the surface of the sprayed coating. At the same time, there are less micro-cracks within the ZrO 2 layers by adding the metallic components into the composite coating w5x. The ZrO 2 additive lowered the coating porosity of, inhibited the outward diffusion of the cations of Ni 2q, Al 3q, Cr 3q, and reduced the inward diffusion coefficient of oxygen. Furthermore, the combination ability was effectively improved between the intermediate coating and the metal substrate due to the NiCrAl alloy. The compact structure of alloy layers also decreased the infiltration of oxidative gas, avoiding the further oxidation of the substrate and the peeling of the coating. Thus, the weight proportion of 75:25 for NiCrAl and ZrO 2 is preferable. As shown in Table 4, the integrated thermal expansion coefficient ŽCTE. of the composite coating was from 7.5 to 15.4 = 10y6 r⬚C at a temperature of 500᎐800⬚C. This value was between the CTE of 14᎐16 = 10y6 r⬚C for FeCrAl substrate, and the CTE of 5.1᎐7.3= 10y6 r⬚C for ␥-Al 2 O 3 washcoat. It indicated
1. A NiCrAlrZrO2 composite coating was deposited on the surface of a metal carrier FeCrAl alloy by plasma-spray technique. The surface of as-sprayed coating was mainly composed of cubic-structured ZrO 2 and Ni, along with a few hexagonal-cubicstructured NiO crystallites. With the oxidation time extended from 8 to 50 h at 800⬚C, the phase composition of the coating surface was mainly unchanged; just the relative content of NiO was increased and the crystallites became much coarser. 2. The coating was constructed with wavy layers of sprayed materials, oxide impurities, random micropores and a few irregular additive particles. The bright thread-shaped zone was the interface between the coating and the substrate. The obvious diffusion phenomenon from the coating into the substrate was not observed due to the brief interacting process between molten coating particles and substrate materials. 3. The alloy elements were inter-diffused between NiCrAlrZrO2 coating and FeCrAl substrate when the specimen was treated at high temperature for a long time. The contents of Ni and Al were obviously increased at the interface and they reacted as the intermetallic compounds of Ni 3 Al and NiAl. The second phase and inborn oxides brought the pinning effect, as well as enhancing the adhesive strength of the sprayed coating with the washcoat and substrate. At the same time, the compact alloy layers lowered the coating porosity and enhanced the oxidation resistance.
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4. With the pre-treatment, an intermediate coating was prepared with coarse and porous structure, high cohesive strength and high heat resistance. These properties could provide a large geometric surface area for Al 2 O 3 washcoat, and enhance the adhesive strength between the ceramic washcoat and the metal substrate so as to extend the lifetime of the washcoat.
Acknowledgements The authors would like to acknowledge the Ministry of Science and Technology, PR China for the financial support of the project no. 96-910-03-04A, as well as the State Environment Protection Agency for guidance. We also thank the Center of Testing and Analyzing of
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Materials, Tsinghua University for the help in the tests of XRD, SEM and EDX. References w1x J.G. McCarty, M. Gusman, D.M. Lowe, D.L. Hilbenbrand, K.N. Lau, Stability of supported metal and supported metal oxide combustion catalysts, Catal. Today 47 Ž1999. 5᎐17. w2x M. Ferrandon, M. Berg, E. Bjornbom, Thermal stability of ¨ metal-supported catalysts for reduction of cold-start emissions in a wood-fired domestic boiler, Catal. Today 53 Ž1999. 647᎐659. w3x J.R. Adomaitis, M.P. Galligan, J.E. Kubsh, W.A. Whittenberger, Metal converter technology using precoated metal foil, SAE 962080 Ž1996. 2099᎐2108. w4x G. Chen, H. Lou, The effect of nanocrystallization on the oxidation resistance of Ni᎐5Cr᎐5Al alloy, Scripta Mater. 41 Ž8. Ž1999. 883᎐887. w5x C.T. Chia, K.A. Khor, Y.W. Gu, Dynamic mechanical properties of ZrO 2 rNiCoCrAlY composite coatings, Thin Solid Films 358 Ž2000. 139᎐145.