The improved thermal radiation property of SiC doped microarc oxidation ceramic coating formed on niobium metal for metal thermal protective system
Graphical abstract
Introduction
Pure niobium (Nb) and Nb-based alloys play an important role in modern materials technology. And their abilities to function in harsh environments at high temperatures, along with other properties and characteristics unique to each metal, make them essential in applications such as aerospace [1], [2], electronics [3], medicine [4] and defense [5]. However, when serving in some elevated temperature circumstances such as thermal controlling of aerospace components in metal thermal protection systems (MTPS), the heat transfer characteristic in the way of infrared thermal radiation of niobium metals or their protective coatings, plays a crucial role on calculating the total heat transfer and transient temperature field. Since thermal radiation dominates heat transfer in a high temperature environment, a high emissivity coating is desirable to be deposited to reduce the heat flux to the metallic substrate by radiation. The high emissive surface is also required in the area such as the thermal controlling of aircraft engines, land-based gas turbines and alkali-metal heat pipes [6].
Microarc oxidation (MAO) is commonly used to fabricate ceramic coatings on Al, Ti, Mg and Zr alloys for improving corrosion- and wear- resistance properties. Most recently, some attempts have been carried out on Nb metals to improve the bio-corrosion [7], [8] and photocatalysis [9] properties. The content of elements (Ca, P and Si elements) incorporated into the oxide coating increases with voltages and concentrations in the anodizing bath. And the modification increased the corrosion resistance in Ringer's physiological solution. Stojadinovic [9] reported that the spark discharging originates from the electrolyte at oxide-electrolyte interface. Amorphous phase was firstly formed on pure Nb surface before the breakdown voltage, and Nb2O5 hexagonal phase was formed after breakdown.
Recently, MAO also exhibits the potential application in elevated temperature circumstance due to their excellent high temperature stability [10], [11], [12]. Meanwhile, most polar bonding oxides have the strong efficiency of photon emission thus exhibiting a high emissivity value [13]. Therefore, the MAO technology provides a strong possibility to design high emissivity coating, because it can grow ceramic coating with controlled polar bonding oxides by the proper selection of metal substrate types, electrolyte systems and reasonable doping. In our earlier research, MAO method was firstly applied to fabricate TiO2 based ceramic coatings on Ti alloy and Ti-Al alloy surface [13], [14], [15] for improving high temperature performances.
Tailoring the component of the MAO coatings through changing the ion types or doping available particles in the electrolytes has been studied in the past [16], [17], [18]. Most recently, some research attempts have been performed to improve the infrared emissivity value of MAO coatings through changing the composition of electrolytes, electrochemical parameters and doping with a certain amount of additives with high emissivity properties. Al Bosta et al. improved the infrared emissivity value of aluminum alloy through extending of MAO time and raising the temperature of electrolyte, by which the emissivity value raised 0.1 in the wavelength 5–10 μm and > 0.1 in the wavelength 4–16 μm, respectively [19], [20], [21]. Tang Hui et al. raised the emissivity value through doping with FeSO4, Co(CH3COO)2 and nano-Fe2O3 on Ti6Al4V alloy in the wavelength 3–8 μm [22], [23], [24]. Wang Yuanhong et al. added Cr2O5 and NH4VO3 into the electrolytes to improve the infrared emissivity value in the wavelength 3–8 μm [25].
The thermal radiation property of microarc oxidation (MAO) coated and uncoated niobium metal is not available in the elevated temperature, while it is a key requirement for dissipating heat evaluation of aerospace applications. How to improve the high emissivity of ceramic coatings by electrolyte and its doping still remains challenge. As SiC has a lot of outstanding physical and thermal properties, it has always been used in MTPS coatings with excellent infrared emissivity property [26], [27], and also been doped into the MAO coating successfully to improve the corrosion resistance [28], [29]. However, the MAO coating doped with SiC particle has not been reported on the infrared emissivity properties before. Therefore, it is essential to study the effect of SiC particle doped into the MAO coatings on its infrared emissivity properties.
In the present paper, a high emissivity ceramic coating was prepared by MAO in alkaline silicate electrolyte containing micro sized SiC particle, with the purpose to explore the effect of the additive of micro sized SiC particle on the microstructure and the thermal emissivity properties of MAO coatings on pure niobium. Furthermore, the preliminary exploration for the mechanism of thermal emissivity properties of MAO coating has been studied.
Section snippets
Sample preparation
Pure Nb sheets with a diameter 30 mm and thickness 0.7 mm were polished with waterproof abrasive paper up to 800 grit, followed by degreasing with acetone in an ultrasonic bath and rinsing by distilled water.
The MAO coatings were fabricated in the following conditions. The samples were used as anodes, while a stainless steel plate was used as cathode. A 65 kW microarc oxidation device provides voltage waveforms as described in Ref. [25]. The optimum applied voltage, frequency, duty cycle and
Characterization of coating surface
The surface morphologies for the coatings with different doping concentrations of micro sized SiC are shown in Fig. 2a) to f) respectively. The coatings with and without SiC showed a typical porous structures. However, the coating surface becomes compact due to the sealing effect of SiC incorporation into coating. The coatings surface porosity reduces from 7.5% to 1.8% with the concentration of SiC increasing from 0 g L− 1 to 12 g L− 1. The average surface roughness (Ra) of the coatings increases
Conclusion
A high emissivity ceramic coating was prepared by microarc oxidation on pure niobium. The structure, phase composition, chemical state and emissivity properties of the coatings were investigated and the following conclusions can be drawn:
- (1)
The undoped coating formed on pure niobium mainly consists of Nb2O5 phase. A small amount of micro sized SiC was incorporated into the coating during microarc discharging process with the addition of SiC particles in the electrolyte.
- (2)
The incorporation of SiC
Acknowledgements
The partial supports from the NSFC grant nos. 51371071, 51321061, 51571077 and 61575029, National Basic Science Research Program (2012CB933900), the Fundamental Research Funds for the Central Universities (HIT. BRETIII.201202) and the program for New Century Excellent Talents in University of China (NCET-08-0166) are gratefully acknowledged.
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