Aluminization of High Purity Nickel by Powder Liquid Coating

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1 Chiang Mai J. Sci. 2009; 36(3) 331 Chiang Mai J. Sci. 2009; 36(3) : Contributed Paper Aluminization of High Purity Nickel by Powder Liquid Coating Patama Visuttipitukul [a], Nantiya Limvanutpong [b], Niti Yongvanich * [c,d], Prasonk Srichroenchai [d] and Panyawat Wangyao ** [a] [a] Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand. [b] Graduate School of Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand. [c] Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom, Thailand. [d] Center of Excellent for Petroleum, Petrochemical and Advanced Materials, Chulalongkorn University, Bangkok, Thailand. *Author for correspondence; * niti@su.ac.th ** panyawat@hotmail.com Received: 28 May 2009 Accepted: 1 July 2009 ABSTRACT This research aims to study feasibility of applying powder liquid coating process to form nickel aluminide layer which is superior for oxidation resistance on nickel. In powder liquid coating process, mixture slurries of Al and Al 2 powders were pasted onto samples and heat treated. The microstructures of coated samples were analyzed by SEM and optical microscope. GIXD, EDS and EPMA were employed to investigate phase formations and element distributions in the coating layer. Thickness of the coating layer was also measured. The coating layer can be either single-phase (Ni 2 ) or multi-phases (NiAl, (Al) and Ni 2 ) coating layer depending on the coating condition. Thickness and homogeneity of the coating layer are used as the criteria to determine quality of the coating layer. Increasing of Al content in mixture slurries produces thicker coating layer with less degree of homogeneity. To increase homogeneity, longer aluminizing time is required. The thickness uniformity depends on Al powder distribution in the mixture slurries which can be controlled by adding appropriate amount of Al 2 powder into the mixture slurries. In this experiment, the optimum condition to achieve uniformly thick single-phase (Ni 2 ) coating layer is to use slurry with Al:Al 2 ratio of 7:3 and 4 hours aluminizing time at 1000 o C. Keywords: aluminizing, Ni 2, oxidation resistance, powder liquid coating. 1. INTRODUCTION Nickel, a main component for high temperature resistance materials such as nickel base superalloys, has been industrially employed for various machine parts. However, in order to utilize nickel base superalloys at high temperature, failure due to oxidation must be carefully considered [1, 2]. Oxidation resistance of nickel and its

2 332 Chiang Mai J. Sci. 2009; 36(3) alloys can be improved by coating of a protective layer composed of nickel aluminide compounds such as NiAl and Ni 2 [3-5]. The superior oxidation resistance of nickel aluminide can be explained by the formation of a dense continuous alumina layer which can form after exposure to a high temperature under oxidizing atmosphere. This continuous alumina layer, acting as a protective layer, is a good barrier to oxygen diffusion; as a result, the oxidation process is suppressed. In this research, powder liquid coating is employed as a new technique for aluminizing. The process involves mixture of Al and Al 2 powders pasted onto the surface and heat treated. Such process was recently developed to synthesize an aluminide layer on iron and steel [6]. In order to extend the application of this process to nickel and its alloys, the feasibility study of nickel aluminide layer formation by powder liquid coating method must be carried out by using pure nickel. The present research is aimed to understand the effect of different processing parameters: powder ratios and holding time. In addition, the optimal conditions to obtain the films with the best possible microstructures will be determined. 2. EXPERIMENTAL PROCEDURE Nickel with a purity of 99.9% was selected as a substrate. The nickel was cut into rectangular pieces (10mm 10mm 2mm). All samples were ground by SiC paper and alumina polishing powder. The polished samples were rinsed and cleaned in acetone by a sonicator for 5 minutes. Coating slurries were prepared from a mixture of Al and Al 2 powders with various compositions. An alumina powder (average size 160 mm) was mixed with an aluminum powder (average size 3 μm). The Al 2 powder was added intended to minimize agglomeration of the Al powder. The mass ratios of Al:Al 2 were 3:7, 5:5, 7:3 and 10:0. Ethylene glycol was added to liquefy the mixture, converting it into coating slurries. The slurries were pasted onto the samples with a density of 0.1 g/cm 2. The pasted samples were then heated to remove ethylene glycol under argon atmosphere at 200 o C for 1.5 hour. The temperature was then increased to 1000 o C aluminizing temperature for 2.25 hours and 4 hours. The coating conditions along with the sample numbers are summarized in Table 1. Both surface and cross-sectional microstructures were examined by an optical microscope. The optimal slurry composition for coating was determined by the thickness and uniformity of the coating layer. Energy Dispersive Spectroscopy (EDS) and Electron Table 1 Coating conditions. Sample No. Al:Al 2 powder ratio Time (hour) Temperature ( o C) Atmosphere 1 3: Argon 2 5:5 3 7:3 4 10:0 5 3: :5 7 7:3 8 10:0

3 Chiang Mai J. Sci. 2009; 36(3) 333 Probe Micro Analysis (EPMA) were performed to observe the type and distribution of elements in the coating layer. Phase formations were characterized by an X-rays Diffractrometer (XRD) and Glacing Incident X-rays Diffractometer (GIXD) at the incident angle of 5 degree. 3. RESULTS AND DISCUSSION Figure 1 shows the surface and crosssectional microstructures of samples No.1 and No.2, which were aluminized by using slurries with Al:Al 2 of 3:7 and 5:5, respectively, at 1000 o C for 2.25 hours. The surface microstructure of both samples appears rough, which is caused by formation of nodular islands (grey phase). The GIXD profiles analyzed at the top surface of these samples in Figure 2 show only Ni 2 and nickel peaks; therefore, the grey phase in the nodular island is Ni 2, which forms according to reaction between liquid aluminum and nickel. The remaining aluminum and Al 2 powders agglomerate into large particles which can be easily removed from the sample surface. The surface of the nickel samples coated by slurries with Al:Al 2 ratios of 7:3 and 10:0 (sample No.3 and No.4, respectively) are shown in Figure 3. The surface appears rough; however, there are no observable nodular islands. The cross-sectional microstructure shows a continuous coating layer which can be divided into two sub-layers. The first sublayer is grey in color layer and is located adjacent to the pure nickel matrix. The second sub-layer is right beneath the surface and is composed of dark grey and white color Figure 1. Surface morphology of sample (a) No.1, (b) No.2, and cross-sectional microstructure of sample, (c) No 1, and (d) No.2.

4 334 Chiang Mai J. Sci. 2009; 36(3) phases. The depth profile of these two sub layers were investigated by GIXD and are shown Figure 4. It can be clearly seen that at the depth of 210 μm (the first sub-layer), there exists only Ni 2. The second sub-layer (less than 210 μm from surface) mainly consists of Ni and (Al). The EPMA micrographs in Figure 5 shows that the dark grey phase, as previously observed by optical microscope, contains mostly aluminum with a small amount of nickel. The nickel is believed to be dissolved in the aluminum matrix. The phase that is white in color has a higher content of nickel, indicating a Ni phase. Figure 2. GIXD profiles at the surface of aluminized samples (No.1-No.4) with various Al:Al 2 ratios. Figure 3. Surface morphology of sample (a) No.3, (b) No.4, and crosssectional microstructure of sample, (c) No 3, and (d) No.4.

5 Chiang Mai J. Sci. 2009; 36(3) 335 Figure 4. GIXD profiles analysis at various depth from the surface. Figure 5. Distributions of elements on the second sub-layer analyzed by EPMA (a) secondary image (b) nickel and (c) aluminum. Low aluminum content slurries (samples No.1 and No.2) result in discontinuous layers. A continuous coating layer, which is appropriate to be used as a protective layer, can be achieved by using high aluminum content slurries: Al:Al 2 ratios of 7:3 and 10:0 (sample No. 3 and No.4). The results show that the aluminum contents in the slurries play an important role on both the microstructure and phase formation within the coating layer. Since wettability of aluminum in nickel is poor due to the instantaneous formation of nickel aluminide layer, it is necessary to have a sufficient content of aluminum in the slurries to yield a complete coverage, forming a continuous layer. Note that an effective coating layer for oxidation resistance requires homogeneity as well as thickness uniformity. Homogeneity means even distribution of all phases throughout the coating layer, while, thickness uniformity means minimum deviation of coating layer thickness from the average thickness value. The effect of longer aluminizing time (4 hours) on the microstructure can be seen in Figure 6. The coating layer tends to be more

6 336 Chiang Mai J. Sci. 2009; 36(3) homogeneous as compared to those in Figure 1 and 3. The accumulative thicknesses of the coating layers, including both the first and the second sub layers, are approximately similar regardless the aluminizing time. However, the thicknesses of each sub layers which indicate the homogeneity of the coating layer microstructure strongly depend on aluminizing time. The thickness of the first sub-layer increases with increasing aluminizing time and vice versa for the second sub-layer. With appropriate Al:Al 2 ratio content in slurry and holding time, the homogenous coating layer containing only the first sub layer, Ni 2, can be formed. Although the slurry with higher Al:Al 2 ratio requires a longer time to achieve the same level of homogeneity, the resulting layer thickness is higher. From Table 2, adding the alumina powder reduces the thickness of coating layer. The standard deviation of the thickness, as shown in Table 2, decreases with increasing Al 2 content, indicating a higher degree of uniformity of the layer thickness. These results are in agreement with those previously reported in which Al 2 particles disperse molten Al on the substrate and trap some excess aluminum liquid [6]. Figure 6. Cross-sectional microstructure of aluminized samples with different Al:Al 2 ratios (a) No.5 (3:7), (b) No.6 (5:5), (c) No.7 (7:3) and (d) No.8 (10:0). Table 2. Thickness of the coated layer. Holding time (hours) Al:Al 2 ratio 10:0 7:3 10:0 7:3 Thickness (μm) S.D. of thickness (μm)

7 Chiang Mai J. Sci. 2009; 36(3) 337 The formation mechanism of the coating layer might be explained as follows. At the aluminizing temperature, a aluminum powder melts into a liquid phase into which nickel is dissolved. The nickel content in the liquid is highest at the interface between the liquid and the nickel matrix. Beyond the solubility limit, the excess nickel in the liquid reacts with aluminum, resulting in the formation of a Ni 2 layer at the interface. A high content of nickel in aluminum at the interface as well as a concentration gradient of nickel, can be detected by elemental line scans as shown in Figure 7. This result suggests that there is an outward diffusion of nickel, resulting in the growth of the Ni 2 layer. For longer aluminizing processing (4 hours), the diffusion time is sufficient for the growth of the Ni 2 layer by consumption of all aluminum. This explanation is manifested by the fact that there is no liquid left on the top surface; the first sub-layer is a homogeneous Ni 2 layer. For shorter aluminizing time (2.25 hours), the aluminizing time is insufficient for nickel to diffuse and to consume all aluminum. Therefore, some Al-rich liquids are left on the top surface. These liquids solidify upon cooling and form the second sub-layer as previously described. The phasic type and structure detected in the second sub-layer can be explained by using a Ni-Al phase diagram (Figure 8). Ni 2 precipitates in the liquid between 1000 o C (aluminizing temperature) and 854 o C. Below 854 o C, primary Ni is formed. The eutectic reaction takes place at o C resulting in formations of both (Al) and Ni as a eutectic structure. The precipitated Ni 2, primary Ni and eutectic structure of (Al) and Ni, as described previously, are shown in Figure 9. Consequently, for shorter aluminizing time, the coating layer is inhomogeneous and can be divided into two sub-layers containing (Al), Ni and Ni 2 phases. Figure 7. Distribution of elements in the coated layer.

8 338 Chiang Mai J. Sci. 2009; 36(3) Figure 8. Ni-Al binary phase diagram [8]. Figure 9. SEM Micrograph of the coated layer.

9 Chiang Mai J. Sci. 2009; 36(3) CONCLUSION Aluminizing of nickel was successfully performed by powder liquid coating process using slurry mixtures of Al 2 and Al powders. The degree of homogeneity depends on the ratio of Al:Al 2 and aluminizing time. Ni 2 forms in a homogeneous coating layer, whereas (Al), Ni and Ni 2 occur in an inhomogeneous layer. Although the higher ratio of Al:Al 2 tends to produce more continuous coating layer, phase homogeneity and thickness uniformity are inferior to those with lower ratios. Phase homogeneity has been shown to be improved by allowing longer aluminizing time without obvious changes in the coating layer thickness. The optimum coating condition in this experiment to achieve homogeneous Ni 2 layer is to utilize a slurry with a Al:Al 2 ratio of 7:3 and an aluminizing time of 4 hours at 1000 o C. ACKNOWLEDGEMENT The authors would like express their gratitude to the Thailand Research Fund for financial support under the contract number MRG The authors also thank Ms.Wanaporn Khanitnuntarak from Thai Technical Center, Thai Parkerizing Co.Ltd. for her supports on EPMA analysis. REFERENCES [1] Decker R.F. and Sims C.T., The metallurgy of nickel-base alloys in the Superalloys, edited by Sims C.T. and Hagel W.C., John Wieley & son Inc. (Publisher), 1972: [2] Davis J.R., ASM Specialty Handbook - Nickel, Cobalt, and Their Alloys. 1 st ed. US, ASM International, 2000: [3] Chester T. Sims, and William C. Hagel. The Superalloy. Canada, John Wiley & Sons, 1972: [4] Goward G.W., Progress in coating for gas turbine airfoils, Surface and Coating Tech. 1998; 73-79: [5] Lee S.Y., Lee J.S., Kim K.B., Kim G.S., Lee B.Y., Moon H.S., Eun H.B., Lee J.H. and Lee S.Y., Effects of aluminizing on the oxidation and hot corrosion behaviors of two phase nickel aluminides, Intermetallics 2003; 11: [6] Murakami K., Nishida N., Osamura K. and Tomota Y., Aluminizing of high purity iron by powder liquid coating, Acta meteria 2004; 52: [7] Ip S.W., Sridhar R., Toguri J.M., Stephenson T.F. and Warner A.E.M., Wettability of Nickel coated graphite by aluminum, Materials Science and Engineering 1998; A244: [8] JOM/9712/Kattner-9712.fig.2d.gif