The influence of preparative parameters on the adhesion of alumina washcoats deposited on metallic supports

Applied Surface Science 253 (2007) 9099–9104 www.elsevier.com/locate/apsusc

The influence of preparative parameters on the adhesion of alumina washcoats deposited on metallic supports Jingsheng Jia a,b, Jin Zhou a, Jianguo Zhang a, Zhongshan Yuan a, Shudong Wang a,* a

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China b Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China Received 2 April 2007; received in revised form 15 May 2007; accepted 15 May 2007 Available online 21 May 2007

Abstract Well-adhered alumina washcoats on FeCrAl metallic supports were prepared using boehmite sols and alumina slurries. The microstructure and the surface performance of the washcoat/support were investigated by SEM, XRD, and ultrasonic vibration. The effects of the main preparative parameters on the coating adherence were studied. The optimal coating conditions are presented as follows: pre-oxidation of the metallic supports was performed at 900 8C for 10 h, the sol layer loadings were 2.0–6.6 wt.%, and the slurry layer loadings were less than 25.3 wt.%. The sol layer drying was performed at 30 8C for 1 h and that for the slurry layer the drying was performed at 120 8C for 2 h, and the coating calcining was performed at 900 8C for 2 h. The SEM photographs of coated samples show that alumina washcoats were well deposited on the metallic supports. # 2007 Elsevier B.V. All rights reserved. Keywords: Adhesion; Alumina coating; Metallic support

1. Introduction In recent years metallic monoliths are becoming increasingly popular due to their high thermal conductivity, lower heat capacities, greater thermal and mechanical shock resistance, the possibility of thin walls allowing high cell density and low pressure drop [1]. Metallic monoliths have been commonly proposed in many forms, such as honeycombs [2–4], meshes [5,6], foams [7,8] and compact reactors [9]. However, the metallic supports could not be applied in practice because of the low specific surface area. It is necessary to deposit a porous material (e.g., Al2O3) with high surface area over the metallic substrate. Since thermal expansion coefficient of the metallic support is different from the ceramic oxide washcoat, the major problem is how to achieve better adhesion to the surface of the metallic supports. A variety of methods for obtaining washcoats on the metallic supports have been developed, including dip coating [2–8,10], chemical vapor deposition (CVD) [11,12], plasma spraying [13] and electrophoretic deposition (EPD) [6,14]. Among these * Corresponding author. Tel.: +86 411 84662365; fax: +86 411 84662365. E-mail address: [email protected] (S. Wang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.034

methods, dip coating from the liquid phase using a sol or a slurry is an economical and simple one to be used in practice. Boehmite sol is generally used to deposit alumina coatings on metallic supports [7], but the cracking and subsequent ‘‘peeling’’ of the washcoats often appear when the coating thickness exceeds 10 mm [15]. Combining of a sol–gel and a slurry can also be used for coating alumina on metallic supports to get high loadings. Valentini et al. [10] have described the deposition of g-Al2O3 on a-Al2O3, aluminum and FeCrAlloy supports. A correlation between the apparent viscosity of the suspension and the washcoat thickness was well given. A similar method was performed by Zhao et al. and the support pre-oxidation, primer calcination and coating calcination conditions were studied [16]. However, only limited information is available so far in published scientific literature concerning the coating adhesion on the metallic supports and the description of the coating method is often not detailed. In the present work, well-adhered alumina washcoats on FeCrAl metallic supports were prepared using boehmite sols and alumina slurries. The influence of the main preparative factors on the coating adhesion between the alumina coatings and the metallic supports were studied, including the preoxidation temperature, the sol layer loading, the slurry layer

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loading, the drying temperature of the sol layer, the drying temperature of the slurry layer and the coating calcination temperature. The goal was to optimize the coating process for depositing alumina washcoats on metallic supports.

calcined at 800, 900 and 1000 8C, respectively, for 10 h to investigate the effect of the pre-oxidation temperature.

2. Experimental

A boehmite sol was used to improve the adherence between the slurry layer and the metallic support. The boehmite sol was prepared by dispersing a commercial pseudo-boehmite powder (Shandong Aluminum CO., LTD. P-DF-03, PR China, particle size 200 mesh, BET surface area 250 m2/g) in an aqueous HNO3 solution. The molar ratio of Al3+ to H+ was 1:0.07 and PEG2000 was added to the sol in order to reduce the generating of cracks during heating and calcining. The weight ratio of PEG2000 to AlOOH was 1:20. After vigorous stirring at 80 8C for 24 h, a dispersion of boehmite sol was obtained. In order to study the effect of loading and thickness for this primer, the preoxidized metallic supports were dipped in boehmite sols with concentrations of AlOOH equal to 0.86, 1.54 and 2.16 mol/L, respectively. After 1 min, these samples were withdrawn at a velocity of 3 cm/min to ensure uniformity of the coating. Then the samples were dried at 30, 70 or 120 8C for 1 h to investigate the effect of the drying temperature of the sol layer.

2.1. Preparation of the samples A three-step procedure was used for preparing alumina coatings on metallic supports in this study: (1) support pretreatment, (2) boehmite sol coating, and (3) alumina slurry coating. Samples 1–12 were designed to investigate the effects of preparative parameters on the coating adhesion. An overview of the coating conditions for samples 1–12 and the weight losses in the ultrasonic adhesion test for 20–80 min are summarized in Table 1. In order to optimize the coating process the following parameters were studied: (1) different temperatures for preoxidation (800, 900, 1000 8C; samples 1–3); (2) different sol layer loadings (3.2, 0.4 and 6.8 wt.%; samples 2, 4 and 5); (3) different slurry layer loadings (10.0, 20.0 and 30.0 wt.%; samples 2, 6 and 7); (4) different drying temperatures of the sol layer (70, 30 and 120 8C; samples 2, 8 and 9); (5) different drying temperatures of the slurry layer (120, 70 and 30 8C; samples 9–11) and (6) different calcination temperatures for the coating (500, 900 8C; samples 2 and 12). In each of the sample groups (1–6), the coating conditions were all the same. 2.2. Pre-treatment of the metallic supports FeCrAlloy foils (Beijing Ander Tech CO.) with a thickness of 50 mm were selected for the present work. The composition was 20.2 wt.% Cr, 5.75 wt.% Al, 0.04 wt.% Hf, and the balance was iron. Foil samples of 40 mm  25 mm in dimension were used for coating. The FeCrAl substrates were too smooth for coating without any pre-treated processes to obtain superficial oxides on their surfaces. Therefore, these alloy foils were first cleaned ultrasonically in acetone for 30 min and subsequently in distilled water for 10 min to remove impurities, and then

2.3. Sol coating

2.4. Slurry coating The alumina slurry was prepared by milling the mixture of Al2O3H2O and g-Al2O3 in a HNO3 aqueous solution with a planetary mixer (BM-BP, Nanjing University Instrument Plant) at room temperature for 18 h. The solid content of the slurry was 21.6 wt.% and the pH of the slurry was adjusted to 4.0 by the addition of HNO3. Agrafiotis et al. [17] have reported that reduction of the powder size of the washcoat down to less than 2 mm was necessary for achieving satisfactory adhesion and endurance of the washcoat under severe operating conditions. According to the processes stated above, particle size analysis of the slurry was conducted with a laser particle size analyzer (LS-100 Q, Beckman-Coulter Company, USA), which indicated a characteristic diameter d90 of 1.7 mm (i.e., 90% of the powder batches were smaller than this size) with main particle diameters of 0.3–2.4 mm. The samples pre-coated with the

Table 1 Coating conditions for samples 1–12 and the weight losses of coated samples in the ultrasonic adhesion test Sample

1 2 3 4 5 6 7 8 9 10 11 12

Support pre-oxidation temperature (8C)

Drying temperature (8C)

Loading (wt.%)

Sol

Slurry

Sol

Slurry

800 900 1000 900 900 900 900 900 900 900 900 900

70 70 70 70 70 70 70 30 120 120 120 70

120 120 120 120 120 120 120 120 120 70 30 120

3.2 3.2 3.2 0.4 6.8 3.2 3.2 3.2 3.2 3.2 3.2 3.2

10.0 10.0 10.0 10.0 10.0 20.0 30.0 10.0 10.0 10.0 10.0 10.0

Calcination temperature (8C) 500 500 500 500 500 500 500 500 500 500 500 900

Weight loss (%) 20 min

40 min

60 min

80 min

23.4 9.6 16.2 22.2 20.3 10.3 19.9 9.0 17.7 21.0 24.5 8.4

37.9 16.8 28.1 35.9 34.1 17.6 31.0 13.3 28.4 35.0 39.6 11.3

55.0 20.7 31.1 53.2 52.3 22.4 50.3 15.2 31.5 52.5 56.2 13.7

62.8 26.0 35.7 60.7 58.0 27.6 55.6 21.1 40.5 58.1 63.0 18.3

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boehmite primer were coated with the slurry, then dried at 30, 70 or 120 8C for 2 h, calcined at 500 or 900 8C for 2 h and cooled in the furnace. 2.5. Characterization of the samples SEM (Scanning electron microscopy XL-30, Holland) was used for the study of the morphologies of the samples. XRD (Xray diffraction) analysis was performed on a Rugaku D/max˚ ; tube 2500 instrument with Cu Ka radiation (l = 1.5418 A voltage, 40 kV; tube current, 300 mA) from 20 to 858 with a scan step size of 0.028. The adhesion of the coating was measured by ultrasonic adhesion test. The samples showed almost no weight losses under low-intensity ultrasonic vibration treatments and the data were not adequate for studying. Therefore, the adhesion of the washcoat was assessed through laundering in a high-intensity ultrasonic vibration cleaner (CQX25-24, Shanghai Branson Ultrasonic Corporation), with a power of 1000 W and a frequency of 25 kHz. The coated sample was laundered in an aqueous medium under ultrasonic vibration for 20, 40, 60 and 80 min, respectively and then the sample was dried and weighed. The weight loss after the ultrasonic vibration treatment is defined as follows:    W1  W2  DW ¼

W1

 100%

(1)

where W1 is the washcoat weight before ultrasonic vibration treatment; W2 is the washcoat weight after ultrasonic vibration treatment. 3. Results and discussion 3.1. Effect of the temperature for support pre-oxidation Fig. 1 shows the SEM micrographs of the metallic surfaces after pre-oxidation at 800, 900 and 1000 8C for 10 h. As can be seen from Fig. 1a, no alumina whiskers appeared on the surface after the foil was calcined at 800 8C for 10 h. On increasing the calcination temperature to 900 8C, a large number of alumina whiskers were formed on the surface, as shown in Fig. 1b. Further increasing the calcination temperature to 1000 8C (Fig. 1c), the alumina whiskers agglomerated. The weight losses for samples 1–3 by means of ultrasonic adhesion test are listed in Table 1. It was found that sample 2 had better coating adhesion than samples 1 and 3, with a weight loss of only 26.0% in the ultrasonic test for 80 min. It indicates that abundant alumina whiskers formed on the metallic surface at 900 8C were beneficial for the anchoring of alumina coatings on the metallic supports. 3.2. Effect of the sol layer loading Samples 2, 4 and 5 were coated with 1.54, 0.82 and 2.16 mol/L sols, respectively, as primer layers for investigating

Fig. 1. SEM photographs of the metallic surfaces pre-oxidized at different temperatures for 10 h: (a) 800 8C; (b) 900 8C; (c) 1000 8C.

the effect of sol layer loading. The corresponding sol layer loadings were 3.2, 0.4 and 6.8 wt.%, respectively. After the samples were ultrasonically tested for 80 min, the weight losses for samples 2, 4 and 5 were 26.0, 60.7 and 58.0%, respectively, as shown in Table 1. It indicates that sample 2 with a sol layer loading of 3.2 wt.% had the lowest weight loss. The surface morphologies of the sol coatings with loadings of 3.2 and 6.8 wt.%, respectively, and calcined at 500 8C for 2 h are shown in Fig. 2. Cracks can be seen on the surfaces of both samples (Fig. 2), which were generated during drying and calcination due to thermal stresses and differences in

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A sol layer with a thickness of 3–10 mm [2] was more suitable as the primer layer. If the sol coatings exceed 10 mm in thickness, the coating layers are liable to crack and separate. On the other hand, if the thickness of the sol coating is less than 3 mm, the bonding between the metallic support and the slurry coating becomes rather weak, thus causing a separation. Corresponding to the thickness of a coating, estimation of the loading, w, with respect to the metallic foil, can be calculated according to the following equation: w¼

zSr W

(2)

where z, S, r, W are the coating thickness, the specific geometric surface area, the density of the coating and the weight of the foil, respectively. The values of the parameters and the results are given in Table 2. It shows that good coatings can be obtained when the sol loading is between 2.0 and 6.6 wt.%. The sample 2 had a proper sol loading of 3.2 wt.%, thus could improve the adherence between the slurry layer and the metallic support apparently. 3.3. Effect of the slurry layer loading

Fig. 2. SEM photographs of calcined sol primers with different loadings (a) 3.2 wt.%; (b) 6.8 wt.%.

thermal expansion coefficients between the metallic supports and the alumina washcoats. It suggests that no crack-free sol primers could be made on the metallic supports. But as an intermediate layer, the wide cracks of the sol layer will be harmful to the slurry layer. Only small cracks were observed and a uniform sol primer was prepared on the support when the sol loading was 3.2 wt.% (Fig. 2a); whereas stronger and wider cracks, which will lead to the separation of the slurry layer, were seen on the surface of the coating with sol loading of 6.8 wt.% (Fig. 2b). With an increase in the sol loading, the cracks on the surfaces of the coatings became stronger. The results agree well with the findings of Agrafiotis and Tsetsekoub. It is proposed that the increase in coating thickness produced a higher shrinkage gradient, which in turn induced higher stresses during drying, thus initiating crack formation [15]. Thin sol primer with a loading of 0.4 wt.% was insufficient as a primer layer and showed high weight loss in the ultrasonic test.

Multiple depositions of the slurry onto the same sample was attempted in order to study the effect of slurry loading. For each deposition, the same procedure used for the single layer sample was repeated exactly. Samples 2, 6 and 7 were deposited with slurry loadings of 10, 20 and 30 wt.%, respectively. The weight losses of samples 2, 6 and 7 in the ultrasonic test are shown in Table 1. The data suggest that samples 2 and 6 had low weight losses of 26.0 and 27.6%, respectively, whereas sample 7 had a high weight loss of 55.6% in the ultrasonic test. The surface morphologies of the slurry layers of samples 2, 6 and 7 are shown in Fig. 3. Cracks can be seen on the surfaces of all three samples and no crack-free slurry layers could be made on the metallic supports. But the cracks can be tolerated in a catalytic carrier as long as they do not lead to spalling from the support. On the other hand, extensive cracks could be detrimental to the coating integrity. As a result, the coatings would have low crush strengths and mechanical resistances, leading to peeling off under rigorous conditions. Only tiny cracks and small blocks were observed on the surface of the coating with slurry loading of 10.0 wt.% (Fig. 3a), and the cracks and blocks augmented when the slurry loading was 20.0 wt.% (Fig. 3b). As the slurry loading was 30.0 wt.%, the integrity of the coatings could not be maintained equally well and large cracks began to appear (Fig. 3c). In fact, very small visible cracks could be seen with the naked eye on the coating surface of sample 7 with slurry loading of 30.0 wt.%. Chiu et al. reported that films would spontaneously crack during drying

Table 2 Calculation of optimal loadings for the sol and slurry layers S (cm2)

2.5 cm  4.0 cm  2

r (g/cm3)

W (g)

Sol

Slurry

1.17 [7]

0.904 [10]

0.3573

Sol loading (wt.%)

Slurry loading (wt.%)

10 mm

3 mm

50 mm

6.6

2.0

25.3

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Fig. 4. Weight loss–time curves of coated samples with different drying temperatures of the sol (Tsol) and slurry (Tslurry) layers during ultrasonic test.

good coatings could be achieved when the sol layer was dried at a low temperature of 30 8C, and the slurry layer was dried at a high temperature of 120 8C. The samples 2 and 8, of which the sol layers were dried at low temperatures of 70 8C and 30 8C, respectively, had lower weight losses than the sample 9 dried at 120 8C. Scherer [19] has proposed that by reducing the drying rate of the gels, fewer cracks would result. According to this suggestion, low temperature was preferred for sol layer drying. As for the slurry layer, drying at high temperature of 120 8C was suitable (from samples 9 to 11). This is consistent with the literature [10], in which the slurry layer was ‘flash’ dried at temperatures above 100 8C. This can be explained as follows: Because the slurry layer was thick, the surface of the slurry layer was dried first and lots of water still remained in the inner coating when it was dried at low temperatures. The residual water in the inner coating produced a high shrinkage gradient that induced high stresses during further drying and calcining at high temperatures, which in turn initiated crack formation. 3.5. Effect of calcination temperature for the coating

Fig. 3. SEM photographs of coated samples with different slurry layer loadings (a) 10.0 wt.%; (b) 20.0 wt.%; (c) 30.0 wt.%.

above the critical cracking thickness [18]. If the thickness of the slurry layer was higher than a critical value of 50 mm [2], it would be liable to crack. According to the results given in Table 2, slurry layers with loadings less than 25.3 wt.% are the suitable ones. 3.4. Effects of the drying temperatures of sol and slurry layers The weight loss-time curves for samples 2 and 8–11 obtained from ultrasonic test are shown in Fig. 4. It shows that

The weight losses of samples 2 and 12 with different calcination temperatures are shown in Table 1. The weight loss of sample 12 was 18.3% in the 80 min ultrasonic test, better than that of sample 2 (26.0%). It indicates that the coating calcined at 900 8C had lower weight loss than that at 500 8C. The XRD patterns of the metallic support pre-oxidized at 900 8C for 10 h, the coated samples 2 and 12 calcined at 500 8C and 900 8C, respectively, are shown in Fig. 5. After the preoxidization at 900 8C for 10 h, besides the characteristic peaks of FeCr (2u is 44.42, 64.64, and 81.76, JCPDS 34-0396), aAl2O3 (2u is 25.58, 35.08, 37.68, 43.30, 52.48, 57.5, 66.61 and 68.01, JCPDS 88-0826) is found on the FeCrAl surface (Fig. 5a). It is indicated that after the heat treatment, the segregation of a-Al2O3 on the FeCrAl surface is present. On the other hand, Alumina tends to transform from g-Al2O3 to aAl2O3 at above 800 8C [20]. The alumina coating calcined at 500 8C was mainly composed of g-Al2O3 (2u is 37.32, 46.12,

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4. Conclusions Several aspects of alumina washcoats on metallic (FeCrAlloy) supports were studied in the present work. The optimal preparative parameters were made certain by investigating the factors on coating adhesion. Optimal parameters were found to be as follows: pre-oxidation of the metallic supports was performed at 900 8C for 10 h, the sol layer loadings were 2.0– 6.6 wt.%, and the slurry layer loadings were less than 25.3 wt.%. The sol layer drying was performed at 30 8C for 1 h and that for the slurry layer the drying was performed at 120 8C for 2 h, and the coating calcining was performed at 900 8C for 2 h. The SEM photos of coated samples show that well-adhered alumina washcoats were prepared on the metallic supports. Fig. 5. XRD patterns of (a) the metallic support pre-oxidized at 900 8C for 10 h, the coated samples were calcined at (b) 500 8C and (c) 900 8C. (D) FeCr, (*) aAl2O3, (#) u-Al2O3, (o) g-Al2O3.

Acknowledgments The present work has partly been supported by Joint SinoGerman Project GZ 213 (101/13), National Natural Science Foundation of China (20590365 and 20476103) and National Key Foundational Research Project (973) of China (2004CB719506). The authors express their gratitude to the Sino-German Research Promotion Center and the State Science and Technology Commission of China for the supports.

References

Fig. 6. Typical SEM photograph showing the support/coating interface of a coated sample.

and 66.92 JCPDS 80-0956), as shown in Fig. 5b. By increasing the calcination temperature to 900 8C, the characteristic peaks of a-Al2O3 became more obvious (Fig. 5c). It means that more g-alumina had transformed into a-Al2O3 at 900 8C. Owing to the similarity in expansion coefficients of the alumina phases, the metallic surface and the alumina coating could combine better after the alumina coating was calcined at 900 8C. In addition, the higher the calcination temperature, the higher the ability of the samples to absorb energy for carrying out the diffusion and recrystallization processes [16]. Thus, the oxidized a-Al2O3 layer, the sol layer and the slurry layer can combine together better. The typical SEM photo about the support/coating interface of coated samples is shown in Fig. 6. It is obvious that no gaps were observed between the metal substrate and the alumina washcoat, which show the good adhesion of the alumina washcoat.

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