Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance
Introduction
Ti–6Al–4V alloy has been applied in many fields, due to their high strength, non-magnetism, corrosion resistance and other characters. However, its safety working temperature is generally less than 400–500 °C; therefore, Ti–6Al–4V alloy is usually considered as a sort of heat-unstable material, which greatly limited their further application. Presently, many kinds of surface modification techniques such as pre-oxidation and coating technologies were developed to improve the high temperature oxidation resistance of Ti alloys [1], [2], [3].
At present, plasma electrolytic oxidation (PEO) technique can also be used for the surface treatment of Ti alloys. Using this technique, a dense ceramic coating can be grown in situ on the surface of Ti alloy [4], [5], [6]. Now, some researches have focused on the structure of the coatings on Ti alloy prepared by PEO and their properties such as corrosion resistance, anti-abrasion and so on [7], [8], [9]. However, there are only a few instances of coatings on Ti alloys with the high temperature oxidation resistance. Hao et al. and Tang et al. investigated the effects of PEO technique on the high temperature oxidation resistance of TiAl alloy under 1000 and 850 °C, respectively, and found the high temperature oxidation resistance of TiAl alloy was improved in a different degree [10], [11]. Zhou et al. and our research group preliminarily studied the high temperature oxidation resistance of PEO coatings on Ti–6Al–4V alloy at 700–1000 °C [12], [13]. In this paper, we prepared compound ceramic coatings in Na2SiO3–Na2CO3–NaOH system by pulsed bi-polar plasma electrolytic oxidation on Ti–6Al–4V alloy. Meanwhile, the structure, thermal shock resistance and high temperature oxidation resistance at 500 °C of such coatings were investigated.
Section snippets
Preparation of the ceramic coatings by plasma electrolytic oxidation
Plate samples of Ti–6Al–4V alloy with a reaction dimension of 20 mm × 10 mm × 6 mm were used as working electrode and the electrolyser made of stainless steel served as the counter electrode. The electrolyte used in the experiments was Na2SiO3 solution with different concentrations, Na2CO3 (4 g/L) and NaOH (0.5 g/L). A home-made high power pulsed bi-polar electrical source with power of 10 kW was used for plasma electrolytic oxidation under the current densities of 1.2 A/dm2 for anode pulse and 0.4 A/dm2
The thickness of the coatings
The prepared coatings are all smooth and gray. The color of the coatings turned deeply with the increase of the concentration of Na2SiO3. Fig. 1 is the mean thickness of the coatings prepared under different concentration of Na2SiO3. It can be noted that the thickness of the coating increased with the increase of the concentration of Na2SiO3. But, at low concentrations, the thickness of the coating changed not apparently, while that of at high concentrations increasing greatly.
Surface SEM of the coatings and EDS analyses
Fig. 2 is the
Conclusions
Ceramic coatings on Ti alloy were prepared in silicate solution by plasma electrolytic oxidation. The structure, thermal shock resistance and high temperature oxidation resistance of the coatings were investigated and the following conclusions can be drawn:
- (1)
The ceramic coatings were composed of rutile and anatase TiO2. Increasing the concentration of Na2SiO3, TiO2 content decreased gradually while the thickness of the coating increased. There were a large amount of micro pores and sintered
Acknowledgement
This work was financially supported by Special Foundation for New Teachers of Doctor course in Chinese Education Ministry (Grant No. 200802131065).
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