Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance

doi:10.1016/j.cap.2009.09.003

Current Applied Physics

Volume 10, Issue 2, March 2010, Pages 698-702

https://doi.org/10.1016/j.cap.2009.09.003 Get rights and content

Abstract

Ceramic coatings were prepared in Na₂SiO₃–Na₂CO₃–NaOH system by pulsed bi-polar plasma electrolytic oxidation on Ti–6Al–4V alloy. The phase composition, structure and the elemental distribution of the coatings were studied by XRD, SEM and energy dispersive spectroscopy, respectively. The thermal shock resistance of the coated samples at 850 °C was evaluated by the thermal shock tests. The high temperature oxidation resistance of the coating samples at 500 °C was investigated. The results showed that the coating was mainly 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 particles on the surface of the coatings. Increasing concentration of Na₂SiO₃, the sintered particles on the surface turned large, and the Si content increased while the Ti content decreased gradually. When the concentration of Na₂SiO₃ was 15 g/L, the thermal shock resistance of the coatings was better than that of the coatings that prepared under other Na₂SiO₃ concentrations. The coating samples prepared under the optimized technique process based on the thermal shock tests improved the high temperature oxidation resistance at 500 °C greatly, whether considering the isothermal oxidation or the cyclic oxidation.

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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Citation Excerpt :
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Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance

Abstract

Introduction

Section snippets

Preparation of the ceramic coatings by plasma electrolytic oxidation

The thickness of the coatings

Surface SEM of the coatings and EDS analyses

Conclusions

Acknowledgement

Mater. Sci. Eng.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Mater. Lett.

Appl. Surf. Sci.

J. Mater. Process. Technol.