Surface & Coatings Technology 205 (2010) S11–S14
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Nitrogen inducing effect on preparation of AlON–Al2O3 coatings on Al6061 alloy by electrolytic plasma processing Kai Wang, Sang Sik Byeon, Bon Heun Koo ⁎ School of Nano and Advanced Materials Engineering, Changwon National University, Changwon, Republic of Korea
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Article history: Received 21 September 2009 Accepted in revised form 25 January 2010 Available online 29 January 2010 Keywords: Electrolytic plasma processing Al2O3–AlON coating Aluminum alloy Nitrogen
a b s t r a c t In this work, AlON–Al2O3 coatings were prepared on Al6061 alloy by the electrolytic plasma processing (EPP) method. The experimental electrolytes include: 10 g/l NaAlO2 as alumina formative agent, 2 g/l NaOH as the electrolytic conductive agent, 0.3–0.9 g/l NH4NO3 and 0.3–0.7 g/l NaNO3 as nitrogen supply agents. The nitrogen inducing effects were studied by a combined composition and structure analysis of the coating layer carried out by X-ray diffractometer (XRD), scanning electron microscopy (SEM) for the specimens EPP treated at room temperature for 15 min under a hybrid voltage of 260 V DC plus 200 V AC (50 Hz) power supplies. In addition, microhardness values were measured to correlate the evolution of microstructure and resulting mechanical properties. The results demonstrate that NaNO3 is a good nitrogen inducing agent to produce high quality AlON–Al2O3 coatings. © 2010 Published by Elsevier B.V.
1. Introduction A spinel phase AlON is a unique material exhibiting many important properties which make it useful in many applications. AlON has been produced by several techniques such as sintering, reaction sintering, hot-pressing, hot isostatic pressing (HIP), selfpropagating high-temperature synthesis (SHS) and plasma arc synthesis [1–3]. However, all the traditional techniques require special equipments to provide high temperature and high pressure, and carefully previous powder preparation, which are complicated and high economic costs. It is strongly required a new technique, with simple serviceability and low economic costs, to replace these old techniques. The combination of plasma arc synthesis and plasma electrolytic deposition is hopeful to deal with this problem, which can produce AlON under low voltage and low temperature. Plasma electrolytic oxidation (PEO), also called macro arc oxidation (MAO), has attracted great interest and is being studied widely for its high efficiency of fabricating ceramic coatings with exceptional properties on light metals (Al, Mg, Ti etc.) [4–6]. The processing includes a combined physical and chemical reaction with high discharge temperature (3000–6000 K) [7]. The PEO produced ceramic coatings are yet limited in microhardness and anti-abrasion properties for only Al2O3 hardening phase in the ceramic coatings. In this paper, nitrogen was induced into the PEO reaction system to form AlON phase. Therefore, the new compromised PEO reaction is called as electrolytic
⁎ Corresponding author. Tel.: +82 55 24 5431. E-mail address:
[email protected] (B.H. Koo). 0257-8972/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2010.01.026
plasma processing (EPP). The effect of nitrogen inducing agents on the properties of coatings produced by EPP treatment was studied. 2. Experimental Column samples (D 20 mm × H 30 mm) of Al6061 alloy (1% Mg, 0.65% Si, 0.3% Cu, 0.2% Cr, 0.15% Mn, 0.7% Fe, 0.15% Ti and Al balance) were used as the substrates. All the samples were polished to a uniform surface roughness of 0.1 ± 0.05 μm by diamond paste and then were degreased with acetone and alcohol in ultrasonic cleaner, and then immersed in the electrolyte for EPP treatment. An EPP coating unit designed and built by the authors has been employed in the present study, which mainly consists of a power supply unit, a bath container and a cooling system. The metallic samples immersed in the electrolyte were used as the anode and stainless steel was used as the counter electrode. Throughout the entire range of experimentation, the temperature of the electrolyte was maintained constant at approximately 25 ± 2 °C using a cooling system. The electrolysis environment was an aqueous electrolyte containing 10 g/l NaAlO2, 2 g/l NaOH, and 0.3–0.9 g/l NaNO3 and 0.3–0.7 g/l NH4NO3. Samples were rinsed with distilled water after EPP procedures. The AC amplitude was slowly increased to 200 V to maintain the current density at 5 A/cm2 throughout the experimentation, afterwards the DC value was increased gradually with time till 260 V so as to maintain the reset current density even as the coating thickness gradually increased. After the hybrid voltage reached up to the anticipated value, the treating time was calculated to 15 min. In this work, when the inducing content of NH4NO3 is over 0.9 g/l and NaNO3 is over 0.7 g/l, no plasmas could occur. For this reason, no coating could form in when the content of NaNO3 is over 0.9 g/l.
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Fig. 1. a. Surface morphologies of the coatings EPP treated in NH4NO3 contained electrolytes of a) 0.3 g/l, b) 0.5 g/l, c) 0.7 g/l and d) 0.9 g/l. b. Surface morphologies of the coatings EPP treated in NaNO3 contained electrolytes of a) 0.3 g/l, b) 0.5 g/l and c) 0.7 g/l.
The different phases present in the coatings were investigated with Philips-X'Pert system X-ray diffractometer (XRD) (Cu Kα radiation) and the scans were performed with 0.02° θ step size in the 2θ range of 20–90°. The microstructure of surfaces and cross sections of samples by different nitrogen inducing agents were examined by a JSM 5610 scanning electron microscopy (SEM). All the samples detected by SEM were sputtered with a thin gold layer in order to prevent surface charging effects. The microhardness of the coating layers was measured in 10 different places by a VLPAK2000 Mitutoyo hardness test machine using 0.1 N load with a 30 s dwell time, and then the average microhardness was calculated and reported. 3. Result and discussion Fig. 1a and b illustrates the surface features of the EPP-treated Al6061 alloys. During the EPP treatments, plenty amount of water vapor was
generated by the exothermal EPP reactions and violent plasmas sparked across the whole substrates. The AlON–Al2O3 coatings present uniformly distributed pancake like cavities resulted by the plasma discharge channels in NH4NO3 containing electrolyte (Fig. 1(a)). The center hole is the plasma discharge channel. The morphologies (Fig. 1(b)) appeared as continuously strip islands with circle plasma discharge channels in NaNO3 containing electrolyte, as is quite different from genernal surface morphologies of traditional PEO coating [4]. It can be concluded from these SEM observations that, with increasing content of NH4NO3 and NaNO3, the numbers of the discharge channels decreased synchronously. Oppositely, the sizes of discharge channels increased gradually. As no more plasma discharges occured when NaNO3 over 0.7 g/l or NH4NO3 over 0.9 g/l, no coating formed when nitrogen inducing agents excessed. It may need higher voltage to simulate plasmas in the nitrogen induced electrolytes. However, as the limitation of our power supplier, only low voltage was available.
Fig. 2. a. Cross sections of the coatings EPP treated in NH4NO3 contained electrolytes of a) 0.3 g/l, b) 0.5 g/l, c) 0.7 g/l and d) 0.9 g/l. b. Cross sections of the coatings EPP treated in NaNO3 contained electrolytes of a) 0.3 g/l, b) 0.5 g/l and c) 0.7 g/l.
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Fig. 3. Microhardness gradient of the coatings EPP treated in different content of NH4NO3 and NaNO3 contained electrolytes.
The cross-sectional SEM images of coatings EPP treated in different electrolytes are shown in Fig. 2. Molten aluminum flew outwards through the discharging channels, as to oxide to form γ-Al2O3 [8]. With the participation of nitrogen from the electrolytes, then the γ-Al2O3 reacted with NO− 3 to form AlON phase [9]. At the same time, γ-Al2O3 would transform to α-Al2O3 at a sharp temperature gradient. The formation of AlON and phase transformation of Al2O3 resulted in the growth of the layers when being ejected from the channels and rapidly quenched at the interface between electrolyte and substrates. In Fig. 2a, the differences between the thickness of 0.3 g/l and 0.5 g/l are not obvious. As means the function of NH4NO3 in advancing the growth of the coatings was weak. When excess NH4NO3 was added into the electrolytes, the reactions became more furious, higher plasma initial voltage was needed. The coatings began shattering, and took shape of porous structures. On the other hand, when the content of NaNO3 was over 0.5 g/l, the growths of the coatings were suspended. From Fig. 2b, the variation of coating thickness is more obvious. It means that NaNO3 potently acts on the growth of the coatings. It is considered that proper content of induced NH4NO3 and NaNO3 is helpful to prepare thick and uniform coatings. The proper content of nitrogen inducing agents is 0.7 g/l NH4NO3 and 0.5 g/l NaNO3, respectively. Low content of nitrogen induced agents makes thin coatings. Excess nitrogen additions would result in porous coatings (NH4NO3 containing electrolytes) or none coatings (NaNO3 containing electrolytes).
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The microhardness tests indicated that EPP coatings on 6061 Al alloys exhibit extremely high hardness, as shown in Fig. 3. It is obvious that the Al2O3–AlON coating can significantly improve the strength of the coating compared to other deposition methods [7]. The microhardness gradient of the coatings treated in different electrolytes is closely coincided with the content of nitrogen inducing addition. In NaNO3 induced electrolytes, the microhardness can reach as high as 2450 HV0.1 when NaNO3 is 0.5 g/l. In NH4NO3 induced electrolytes, the highest microhardness value is around 2100 HV0.1 when NH4NO3 is 0.7 g/l. It is proved that 0.5 g/l NaNO3 is a suitable content to produce high performance coatings. The phase analysis of the oxide coatings by XRD on 6061 Al alloys produced in optimum electrolytes is presented in Fig. 4. It can detect strong diffraction peaks of the Al, because the coatings are so thin and porous that X-rays could penetrate easily. The typical phase composition of EPP coatings consists of α-Al2O3, γ-Al2O3 and AlON according to the XRD spectra of coatings EPP treated both in 0.5 g/l NaNO3 and 0.7 g/l NH4NO3 containing electrolytes. From the XRD patterns, high content of AlON and α-Al2O3 phases can be found in the coatings prepared in 0.5 g/l NaNO3 containing electrolyte, as perfectly explained by the high microhardness of the ceramic coatings in this condition. While lower content of AlON and α-Al2O3 phases in the ceramic coating prepared in 0.7 g/l NH4NO3 containing electrolyte resulted a relatively softer coating. In conclusion, the formation of hard AlON and α-Al2O3 phases significantly enhances the coating properties. 4. Conclusions EPP processes were carried out in alkaline aluminate and nitride induced electrolytes on 6061 Al alloy substrates in 15 min under a hybrid voltage (260 V DC value combined with 200 V AC amplitude). The surface morphologies of EPP Al2O3–AlON coatings present pancake structures in NH4NO3 induced electrolytes, and island morphologies in NaNO3 induced electrolytes. The coatings phases consist of AlON, γ-Al2O3 and α-Al2O3. The microhardness results show that NaNO3 is a better nitrogen inducing agent than NH4NO3. Moreover, proper content of 0.5 g/l NaNO3 and 0.7 g/l NH4NO3 is available for the preparation of hard ceramic coatings. Acknowledgement This research was supported by the MKE (The Ministry of Knowledge Economy), Korea, under the ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency) (NIPA-2009-C1090-0903-0007).
Fig. 4. XRD partterns of a) 6061 Al alloy substrate, and EPP-treated coatings in b) 0.5 g/l NaNO3 and 0.7 g/l NH4NO3.
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