ICMCTF2007 Session A1-2: Coatings to Resist High Temperature Oxidation and Wear

To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxide growth at very high temperatures, advanced diffusion-barrier type coatings were developed. These barrier coatings are typically duplex in structure, with an inner Ω(Re, W, Cr, Ni) as a diffusion barrier and outer Ni-aluminides as an Al reservoir. The coatings have been applied to high-temperature alloys such as the single-crystal nickel-based superalloys TMS-82+, CMSX-4, and TMS-138, and the wrought Hastelloy X. As will be presented, many aspects of the barrier coatings have been investigated, including:. (1) their formation process; ( 2) their oxidation behavior with or without an imposed external stress; (3) mutual diffusion among Re and Ni-aluminides; (4) permeability of alloying elements through the barrier layer; and (5) compatibility of barrier layer with Ni-based superalloys and Ni-aluminides. The results obtained may be summarized as follows. The inner Ω layer with a composition (at%) of (35-40)Re, (15-20)W, (15-25)Cr and (15-25)Ni was produced by electro-deposition of Ni-70Re and Ni-20W films from aqueous solutions followed by Cr-pack cementation at temperatures between 1200 and 1300°C, and the outer Ni-aluminides of β-(Ni,Cr)Al + γγ'-(Ni,Cr)₃Al was formed by electro-deposition of a Ni film, followed by Al pack cementation. After 100 h oxidation at 1150°C in air it was found that the structure and composition of both Ωlayer and alloy substrate were retained with little change. Furthermore, there was little Al in the Ω layer. It could be concluded that the Re-based alloys such as Ω! Re(W),Cr,Ni) act as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.

Interdiffusion behavior was investigated by using diffusion couples of Re with Ni aluminides such as Ni-12Al (γNi(Al)), Ni-25Al(γ'Ni₃Al), Ni-50Al(βNiAl) and Ni-60Al(ΩNi₂Al₃), as well as a Re-60Ni / β-NiAl couple, at temperatures between 1000 and 1200°C for up to 400 h. In the Re / γ-Ni(12Al) couple, Ni diffused into the Re accompanied with a little diffusion of Re into the γ-Ni(Al). In both the Re / γ'-Ni₃Al and Re / β-NiAl couples, Ni diffused into the Re but diffusion of Al in the Re and Re in the Ni-aluminides were negligible. Penetration depth and maximum solubility of Ni in the Re decreased with increasing Al content in the Ni-aluminides. In the Re / Ω-Ni₂Al₃ couple there was a formation of intermediate layers of ReAl and Re4Al11, accompanied with β-NiAl transformed from the Ω-Ni₂Al₃. Coated with Ni-aluminides such as γ’-Ni₃Al and β-NiAl, diffusion barriers of Re, and likely also the Re-based alloys, remain after long durations at high temperatures. Diffusion couple studies among TMS-82+ with or without a diffusion-barrier coating and mated with Ni, γ’-Ni₃Alβ-NiAl were carried out at 1150°C for up to 1000 h. The barrier layer consisted of Ω-Re (W, Cr, Ni, Mo) with little amounts of Al, Ti, and Ta. For couples with the Ni, the barrier layer disappeared after the 100 h diffusion, due to rapid diffusion of Ni and Cr. In couples of the γ’-Ni₃Al or β-NiAl, the barrier layer was stable, being retained up to 1000 h. Moreover, the presence of the barrier layer reduced remarkably the formation of unwanted TCP precipitates. Reaction diffusion and the feasibility of using Ω diffusion barriers will be discussed.

Creep tests were carried out on a Ni-based alloy, Hastelloy-X, with and without a diffusion-barrier coating at 970°C in air for up to 400 h and external tensile stresses of 22.5, 27.5 and 40MPa. Strain rates of the diffusion barrier coated alloy decreased rapidly during the first couple of hours, followed by a slow creep-deformation with a strain of 3.4% and strain rates of (0.7~0.2)x10^-7 /s for up to 200 h, showing little change in the coating structure and composition (at%) of the inner Re-based alloy layer, (33~37)Cr, (20~23)Re, (9~14)Fe, (11~15)Mo, and (15~21)Ni. Considering the creep behavior of the uncoated alloy, as well as the fact that there were few cracks and flaws in the Re-based alloy layer, it was concluded that the inner Re-based alloy layer in the coating is subject to creep-deformation along with the alloy substrate. After the 200 h oxidation the outer β-NiAl contains about 40at%Al, while the alloy substrate near the inner layer has less than 1at%Al. It was found that the Re based alloy acts as an effective barrier against inward Al diffusion and outward diffusion of alloying elements during creep deformation.