Short-term oxidation and hot corrosion resistance of a gradient CrN/Cr1−xAlxN coating
✍ Scribed by M. Zhu; M. S. Li; J. J. Xu; Y. C. Zhou
- Publisher
- John Wiley and Sons
- Year
- 2010
- Tongue
- German
- Weight
- 990 KB
- Volume
- 61
- Category
- Article
- ISSN
- 0947-5117
No coin nor oath required. For personal study only.
✦ Synopsis
A CrN/Cr 1Àx Al x N coating comprised of an inner layer of CrN and an outer layer of Cr 1Àx Al x N with a gradient distribution of Al was deposited on two different alloys by a reactive sputtering method. Oxidation and hot-corrosion tests of the gradient CrN/Cr 1Àx Al x N coating were performed at different temperatures. The phase compositions and morphologies of the as-deposited coating and the corrosion products were investigated by using XRD and SEM/EDS. The results showed that the gradient CrN/Cr 1Àx Al x N coating exhibited good oxidation resistance at temperatures above 1000 8C owing to the formation of an a-Al 2 O 3 -rich oxide scale. The coating possessed good hot-corrosion resistance in molten sulfate because the inner CrN layer could supply enough Cr to form a relatively protective Cr 2 O 3 after the Al 2 O 3 -enriched scale failed due to its dissolution in the molten sulfate.
1 Introduction
High temperature corrosion resistant coatings are widely used on superalloys used for turbine blades. These coatings can be classified into three kinds: diffusion coatings [1-3], modified diffusion coatings [4-7], and MCrAlY coatings [8][9][10][11][12]. It is well known that the protective properties of these coatings depend on the formation of either an adherent oxide layer of Al 2 O 3 or Cr 2 O 3 . Generally, Al 2 O 3 -scale forming coatings possess excellent oxidation resistance above 1000 8C, while Cr 2 O 3 -scale forming coatings can offer good oxidation and hot-corrosion resistance below 1000 8C. Because of the difference between these two protective oxides, MCrAlY coatings may have good oxidation or hot-corrosion resistance depending on their Al and Cr contents. The service environment of high temperature structural materials is becoming more variational, thus the protective coatings must have the ability to supply protection over a wide range of operating conditions. For this purpose, a smart over-layer coating system has been proposed by Nicholls et al. [13]. This so-called smart coating is a functionally gradient system, which consists of an MCrAlY base, enriched in aluminum in the outer layer and chromium in the intermediate interlayer. The aluminum-rich outer layer provides resistance to high-temperature oxidation and type I ($900 8C) hot-corrosion, while the intermediate chromium-rich interlayer permits the rapid formation of chromia once the alumina formed on the top surface loses its protection under type II ($700 8C) hot-corrosion damage. A chemically gradient MCrAlY coating has also been demonstrated in another study [14] to provide good oxidation resistance at high temperature together with good hot-corrosion resistance at intermediate temperature.
In our previous work, Cr-Al-N coatings were found to provide good protection to a Ti 3 Al-base alloy at 800-900 8C [15] and a K38G superalloy at 1000-1100 8C [16,17]. With increasing content of aluminum in the Cr-Al-N coating, its oxidation resistance was improved. For a Cr 0.50 Al 0.50 N coating, the main oxide was Cr 2 O 3 during oxidation below 1000 8C, but became a-Al 2 O 3 above 1000 8C [17]. Studies on the hot-corrosion behavior of this kind of Cr 1Àx Al x N coating are very seldom. Li et al. [18] investigated Na 2 SO 4 induced hot-corrosion of a three-layer CrAlN coating at 900, 950, and 1000 8C, with the results indicating that the coating possessed good hot-corrosion resistance owing to a high content of Cr in the coating. In order to enable the Cr-Al-N coating system to provide good protection for high temperature alloys under different aggressive conditions, a gradient CrN/ Cr 1Àx Al x N coating enriched in Al in the outer Cr 1Àx Al x N layer and Cr in the inner CrN layer has been designed according to the concept of the smart over-layer coatings mentioned above. In the present work, the gradient CrN/Cr 1Àx Al x N coating was deposited on a Ti 3 Al-base alloy and K38G superalloy by a reactive magnetron sputtering method. Oxidation and hot-corrosion resistances of this gradient coating were evaluated.
2 Experimental procedures 2.1 Coating preparation
Two different alloys, a Ti 3 Al alloy (Ti-24Al-14Nb-3V-0.5Mo-0.3Si, at%) and K38G superalloy (nominal main composition (wt%):