Available online at www.amse.org.cn Acta Metall. Sin.(Engl. Lett.)Vol.23 No.1 pp1-7 February 2010 Application of low-emissivity Pt layer on Ni alloy to high temperature Zhibin HUANG, Wancheng ZHOU and Xiufeng TANG State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi an 710072, China Manuscript received 26 June 2009; in revised form 13 October 2009 Platinum films were sputter-deposited on two groups of nickel alloy substrates, in which the first group was the samples with rough surface, and the other group with polished surface. The platinum thin-films were applied to serve as the low-emissivity layers to reflect thermal radiation. Then, the platinum-coated samples were heated in air at 600 C for 200 h to explore the effect of high-temperature environment on the emissivity of coated platinum film. After annealing, the average IR emissivity (at the wavelength of 3 14 µm) of the platinum film was only about 0.1 for polished sample and 0.45 for rough sample. The diffusion between platinum and the nickelalloy elements at 600 C had been also discussed. KEY WORDS Low-emissivity layer; Platinum film; Infrared emissivity 1 Introduction In recent years, the low emissivity materials have attracted more and more attention [1 7], such as conductive polymeric materials, strongly magnetic materials and semiconductive materials, but these materials either have low working temperature or have poor thermal stability. The low-emissivity materials at high temperature application are limited. Pt has many exceptional properties such as high melting point, low IR emissivity, excellent electrical conductivity, great chemical stability and good thermal stability, especially, it has good resistivity against oxidation, and therefore it has the potential to be used as low-emissivity layer at high temperature. Actually, the infrared emissivity of a clean metal surface is very low and stable, but it becomes high when the surface is oxidized [8,9]. The infrared emissivity of the metals is strongly affected by the growth of metal oxide film on the surface [10]. Therefore, the resistivity against oxidation is a key factor to estimate low-emissivity materials at high temperature. The low-emissivity materials have been exploited in the several technical fields in recent years. An important application area of the low-emissivity materials is as the aerospace Corresponding author. Engineer, PhD; Tel.:+86 29 88494574; Fax: +86 29 88494574. E-mail address: huangzhibin83@163.com (Zhibin HUANG)
2 materials used at high temperature environments. The aerospace materials generally require thermal insulation and low-emissivity layer to reduce the amount of absorbed thermal radiation [11]. Coating low-emissivity film is a good way to reduce the IR radiation of the aerospace while increasing weight a little. The low-emissivity films and coatings are widely used for infrared emissivity control [12 18]. The aerospace materials used in different position have different roughness, and the roughness influencing the IR emissivity. It is necessary to explore the long-term maintenance of low-emissivity effectiveness both on rough surface and polished surface. There is a serious problem for the use of low-emissivity layer on nickel alloy. The metal atoms in the nickel alloy will diffuse into the low-emissivity layer at high temperatures [19]. If the diffusion time is long enough, the metal atoms will diffuse to the surface and then be oxidized. This situation is a catastrophic for the application of the low-emissivity layer because of the very high infrared emissivity value of the metal oxide. Therefore, the study of the diffusion is also important. In this paper, the IR emissivity control effectiveness of the Pt film on nickel alloy at high temperature was studied. The effects of the annealing and the substrate roughness on IR emissivity were explored. Meanwhile, the diffusion between Pt and alloy atoms was also studied. 2 Experimental The Ni-based alloy K424 is a high temperature alloy and its nominal composition (wt pct) is Cr 8.5 10.5, Co 12.0 15.0, Al 5.0 5.7, Ti 4.2 4.7, W 1.0 1.8, Mo 2.7 3.4, C 0.14 0.20 and Ni balanced. The samples with a size of about 15 mm 15 mm 1 mm were separated into two groups which have different surface roughness. The first group was the samples with rough surface (polished with No.200 grits SiC water proof abrasive paper) and the second group was polished surface (polished with SiC water proof abrasive paper from No.200 grits to No.2000 grits and finally with φ1 diamond polishing paste). Before deposition, each group of the substrates was ultrasonically rinsed in acetone, distilled water and ethanol, successively. Pt films were deposited on the two groups by using a SBC-12-type DC sputtering system [KYKY] [20,21]. Prior to deposition, the chamber was evacuated to a pressure of 2 Pa using a rotary pump for 1 min. The sputtering started at an Ar gas pressure of 4 Pa. The sputtering power was 1000 V 10 ma. The substrate was also a grounded anode. The distance between the target and the substrate was 5 cm. The deposition time of the Pt layer was 50 min and the deposition rate was about 10 nm/min. Thickness of the layers was confirmed by multiple beam interferometry, and the error rate is less than 5%. After deposition, the two groups of platinum-coated test panels were subjected to a constant high temperature exposure in air at 600 C for 200 h respectively to simulate the exposure in use environment. The crystalline phase was examined by X-ray powder diffraction analysis using a CuK α radiation (Philips X Pert diffractometer). The microstructure was observed by a scanning electron microscope (SEM) (model JSM-6360, Japan). IR emissivity values of the samples were determined using an infrared emissometer (SR5000 USA). The parameters of the infrared emissivity were measured.
3 3 Results and Discussion After annealing at 600 C for 200 h, a distinct color change visible in the platinum film was revealed. The color of all the samples changed from black to silvery white. Fig.1 shows the small angle X-ray diffraction (XRD) spectra of the Pt films before and after annealing. As can be seen, the XRD spectra show Pt (111), Pt (200), Pt (220) Pt (311) and Pt (222) diffraction peaks. The half width of the Pt peaks of the as-deposited Pt films is broad. But it becomes much narrower after annealing, meaning that the annealing changes the structure of the Pt layer. Additionally, no preferred orientation is observed in the XRD spectra before and after annealing. Fig.1 Small angle X-ray diffraction patterns of the different samples: rough sample before (a) and after (c) annealing, polished sample before (b) and after (d) annealing. The grain sizes of the Pt films before annealing are 102 nm and 52 nm for rough substrate and polished substrate, respectively, and after annealing are 280 nm and 252 nm for rough substrate and polished substrate, respectively, which are calculated by the Scherrer formula. As we can see, the grain size of the as-deposited Pt film is small but it greatly increases through annealing. The grain size of the rough sample is bigger than that of polished sample. The grain size of the as-deposited Pt film is only 50 nm for polished sample and 90 nm for rough samples respectively, which is corresponding to the low sputtering power of the deposition. Then the grain size of the Pt film increases to 252 nm and 280 nm after annealing at 600 C for 200 h. It is obvious that the annealing has a great influence on the transformation of film structure, which will greatly affect the infrared emissivity of the Pt film. The color changes can be explained by this increase of the Pt grain size. Most of the metal materials composed of the small grains are black in appearance. But when the grain size increases, the color will be gradually close to the bulk metal. Fig.2a shows the SEM micrographs of the Pt films on rough samples before annealing. It can be seen that there are many agglomerated platinum particles distributed on the uneven surface and the platinum film is discontinuous. The evolution of this morphology is likely attributed to the surface energy reduction by atom agglomeration during the deposition. After annealing, as can be seen from Fig.2b, the platinum particles are connected each other, and the film is much smoother than that before annealing. Doubtlessly, the smoothing of Pt film will decrease the surface area and hence lower the IR emissivity of the samples. Therefore, from the angle of surface roughness, the heat-treatment process is helpful to decrease the IR emissivity of the platinum film on rough substrate. Fig.3 shows energy dispersive X-ray spectrum (EDXS) of the platinum-coated sample after annealing within a square region approximately 50 µm 50 µm in size. As can be seen, a significant amount of Pt, Ni, Cr, Co and O is detected. The element content of Pt films after annealing is also listed in Table 1. It can be seen that the contents of O, Ni, Cr and Co are 16.13 at. pct, 15.61 at. pct, 2.19 at. pct and 2.51 at. pct, respectively, meaning
4 Fig.2 SEM micrographs of Pt films on rough samples before (a) and after (b) annealing. Table 1 EDXS results of the Pt films deposited on different substrate after annealing Samples with different substrates Content of element (atomic fraction%) Pt Ni Co Cr O Rough substrate 63.55 15.61 2.51 2.19 16.13 Polished substrate 91.38 8.62 that there is a metal oxide forming on the surface. These oxides should be mostly nickel oxide and a little chromium oxide and cobalt oxide. Because the platinum film deposited on rough sample is uneven and non-uniform in thickness (Fig.1a), the alloy atoms can easily diffuse into the platinum film forming the metal oxide in the position where the Pt film is relatively thin on the surface. According to the EDS results, it could be concluded that the alloy elements diffused through the platinum over-layer, forming the oxide on the surface. As we know, the metal the oxide ionic crystals have strong absorption effect in the infrared band because of the molecular vibration [8]. The generation of the metal oxide, such as nickel oxide, chromium oxide and cobalt oxide, will increase the IR emissivity of the samples. The IR emissivity results are shown in Fig.4. It can be seen that the IR emissivity of the as-deposited film is about 0.6 and it then decreases to 0.45 after annealing. Interestingly, the annealing has the advantage to reduce the IR emissivity, which is different from that of other metals [8]. This can be explained as foll- Fig.3 Energy dispersive X-ray spectrum of the platinum-coated rough sample after annealing within a square regions approximately 50 µm 50 µm in size. Fig.4 Infrared emissivity of Pt films on rough sample as a function of wavelength.
5 ows: The O content in the platinum film is only about 16 at. pct. The effect of the generation of the metal oxides on the IR emissivity is limited. Considering the effect of the surface roughness, it seems that the surface roughness effect is greater than the effect of the metal oxides. Our previous research [22] showed that the average IR emissivity value of the Ni-based alloy K424 without any low-emissivity films is about 0.7 after exposure at 600 C for 100 h. Therefore, it seems to be said that the low-emissivity effectiveness of the platinum film on rough sample is not very obvious. Fig.5 shows the SEM micrographs of the Pt films on polished substrates before and after annealing. As can be seen, the films are continuous over a large area and are characterized by uniformly distributed feature before and after annealing. The grain size of the asdeposited film is extremely small (20 70 nm) while it greatly increases during annealing (Fig.5b). In addition, the grain growth of the platinum film on polished surface is finer than that on rough samples (Fig.1a). The surface morphology of the as-deposited Pt films is found to be strongly dependent on the roughness of substrate. As discussed above, the surface with higher roughness will have bigger radiation absorption area leading to higher IR emissivity. It is obviously that the roughness of the as-deposited Pt films on the polished substrates is lower that of rough substrate. Therefore, from the angle of surface roughness, the polished substrate has an advantage to maintain the low-emissivity effectiveness of the platinum over-layer. Fig.5 SEM micrographs of the Pt films on polished samples before (a) and after (b) annealing. Fig.6 shows EDXS result of the Pt-coated polished sample after annealing. It can be seen that the over-layer consists mostly of Pt. The Ni atom inside the Pt layer is only 8.62 at. pct and it probably originates from the nickel alloy. The EDS analysis of the surface is noticeably free of any O, implying that there are no significant oxides on the surface. The Pt film grown on the polished Ni alloy exhibits a good resistance to oxidation, which is strongly required for the low-emissivity films used at high temperature. Fig.7 shows the IR emissivity spectrum of platinum-coated polished sample as a function of wavelength. As we can see, the IR emissivity at the wavelength of 3 14 µm is greatly decreased after annealing, especially at the low wavelength band. In the case of the rough samples, the decrease of the IR emissivity is attributed to the surface smooth. However, the surface roughness of the polished samples is changed little before and after annealing. The IR emissivity change of the polished samples can not be attributed to the surface roughness change.
6 Fig.6 Energy dispersive X-ray spectrum of the platinum-coated polished sample after annealing within a square regions approximately 50 µm 50 µm in size. Fig.7 Infrared emissivity of Pt film on polished sample as a function of wavelength. The interactions between electromagnetic wave and the metal materials have a great relationship with the electrical properties of the metal materials. The IR emissivity of the metal film can be evaluated by the famous Hagen-Rubens relation [23], ρ ε(λ) = 36.50 λ where ρ is the d.c. electrical resistivity in Ω cm, and λ is the wavelength of the infrared radiation in µm. The Hagen-Rubens relation does predict the emissivity of metal films in the infrared, particularly in the region λ>4 µm. It is clear that the IR emissivity decrease with the increase of the electrical conductivity. As described above, the grain size of the Pt film on polished sample increases from 52 nm before annealing to 252 nm after annealing. The grain size of annealed Pt film is 5 times bigger than that of the as-deposited one. The intuitive image of the grains is also displayed in Fig.5a. Doubtlessly, the as-deposited Pt film composed of extremely fine grain is impossible to get high electrical conductivity. However, after annealing, the grain size of the Pt film greatly increases, which is helpful to improve its electrical conductivity because the grain boundary and the particle boundary are greatly reduced. It is clear that the electrical conductivity of the Pt film is very low before annealing but greatly increases after annealing. The main reason for the IR emissivity reduction of the platinum film should be the increase of the electrical conductivity. In summary, the average IR emissivity at the wavelength of 3 14 µm is less than 0.1 after annealing even for 200 h. It can be concluded that Pt film deposited on polished Ni alloy is very effective to control the IR emissivity even at high temperature. 4 Conclusion The surface morphology of the Pt films is strongly dependent on the roughness of substrate. Both of the substrate roughness and the annealing have great effect on the IR emissivity of the Pt films. Pt films deposited on polished substrate have better lowemissivity effectiveness than that on rough substrate. The IR emissivity of the Pt films decrease after annealing, which is different from the situation of the other metals.
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