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

Tuesday, April 24, 2007 8:00 AM in Room Sunrise

Tuesday Morning

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8:00 AM A1-2-1 Formation of Novel Diffusion Barrier Coatings and their Influence on the Oxidation Behavior of Coated Heat Resistant Alloys
T. Narita (Hokkaido University)

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.

8:40 AM A1-2-4 Long-Term Testing of Aluminide Coatings on Fe-Base Alloys
B.A. Pint (Oak Ridge National Laboratory); Y. Zhang (Tennessee Technological University); I.G. Wright (Oak Ridge National Laboratory)
Aluminide coatings made by chemical vapor deposition on ferritic (Fe-9Cr-1Mo) and austenitic (Type 304L) substrates are being tested in humid air at 650°-800°C. A humid air environment was used to identify coating failure as uncoated substrates experience rapid oxidation at these temperatures. One goal of this work is to demonstrate the potential benefits and problems with alumina-forming coatings. The highest test temperature was selected to accelerate the degradation of the coating by interdiffusion with the substrate. Cycle frequency and coating thickness have been varied in order to demonstrate the effect of coating-substrate thermal expansion mismatch. Another goal is to develop a lifetime model based on the results. A critical Al content of the coating at failure is needed to complete the model.
9:00 AM A1-2-5 Investigation of High Temperature Oxidation Properties of Pack Aluminized High Entropy Alloy at 1100°C
Y.C. Chang, J.-W. Lee, J.S. Yang (Tung Nan Institute of Technology, Taiwan); J.H. Lai, W.J. Wang (Industrial Technology Research Institute, Taiwan)
Many efforts have been devoted to the High Entropy Alloy (HEA) due to their excellent properties and performance since 1995. It has been found that the HEA exhibits sufficient high temperature strength, unusual high temperature precipitation behavior and excellent high temperature oxidation resistance. In this work, a HEA with the following chemical compositions: 17.91%Fe-34.60%Cr-5.18%Al-24.64%Ni-17.66%Co (in wt%) had been pack aluminized at 900°C for 4 hrs. The (Ni,Co,Cr,Fe)Al layers around 25µm in thickness was observed on surfaces of alloy. Protective and adherent alumina scale was found on surfaces of the aluminide layers after oxidized at 1100°C for 196 hrs. A better high temperature oxidation resistance was observed due to the aluminizing process. Complex phase transformation reactions occurred during the test at 1100°C. It is also found that an aluminum depletion layer with lower hardness was formed on the outermost surface of the aluminide layer, which was induced by the outward diffusion of aluminum during high temperature oxidation. In addition, except some precipitation phases, the hardness of HEA substrates decreases after high temperature oxidation test.
9:40 AM A1-2-7 Oxidation Behavior of Multi-Component (Ti-Al-Cr-Si-V)xNy Nitride Coatings at Elevated Temperature
C.-H. Lin, J.G. Duh (National Tsing-Hua University, Taiwan)
The multi-component nitride coatings (Ti-Al-Cr-Si-V)xNy were deposited on Si and mild steel substrates by RF magnetron sputtering process. By adjusting nitrogen inlet and substrate bias in the sputtering, coatings with various Me/N(Me=Ti-Al-Cr-Si-V) ratio and orientation could be obtained. The as-deposited coatings were annealed at 600, 700, 800, 900°C in the air atmosphere for 4 hours, and the oxygen content was saturated at 64 at.% according to EPMA quantitative analysis. In the initial stage of oxidation reaction (600, 700°C), favored oxide phase V2O5 was identified by XRD and top-view observation. However, it was difficult to identify oxide formation for coatings annealed at 900°C only by X-ray diffraction patterns due to their complicated diffraction peaks. To have a detailed investigation of fully oxidized multi-component nitrides, cross-section of the coatings were polished by special ion-milling technique. After cross-section polishing, distribution of two different oxides (Ti-Al-Cr-V mixed oxides and SiOx) could be clearly distinguished and composition of each oxide phase was also analyzed by high resolution FE-EPMA.
10:00 AM A1-2-9 Towards the Understanding of Wear Mechanisms for Alumina Coatings in Metal Cutting Operations.
I. Reineck, M. Collin (Sandvik Tooling AB, Sweden); K. Back, D. Trinh, H. Högberg, L. Hultman (Linköping University, Sweden)
Alumina coatings are widely applied on today's tools used for metal machining. This material is beneficial in such a context because of its high temperature hardness and chemical inertness. CVD deposited alpha and kappa alumina coatings are currently dominating the market for cutting tools. PVD gamma alumina coatings are, however, emerging as a possible replacement in several applications where thermal or mechanical fatigue cracking determines the tool life. The materials properties relevant for metal machining are hard to evaluate with standard analysis techniques such as electron microscopy and x-ray diffraction since the machining operations typically are associated with an extreme environment with temperatures up to 1000°C at the cutting edge and pressures well above 100MPa. In this report, the phase transformation of kappa alumina has been investigated in-service through the use of point focus XRD. This has been supported with in-situ HT XRD measurements that have purely isolated the effect of temperature on the phase transformation. It is found that the phase transformation from kappa to alpha takes place considerably much faster in the real application compared to the HT XRD environment. In addition, the phase stability of gamma alumina deposited by dual magnetron sputtering will be presented for the first time. Possible mechanisms for the phase transformations will also be discussed.
10:20 AM A1-2-10 Triboactive Materials for Dry High-Speed Sliding Applications up to 800°C
W.O. Woydt (BAM, Germany)
This paper presents tribological results elaborated under unlubricated conditions at RT, 400°C, 600°C and 800°C in the range of 0,1 m/s to 7,5 m/s with two distinct different, non-commercial and novel metallurgical concepts based on Magn©li-type phases displaying wear rates, which are one to two orders of magnitude below those published for NASA PS30x-coatings: a. (Ti,Mo)(C,N) grades as monolithic materials and thermal sprayed coatings forming Magn©li-type phases by tribooxidation and b. Tin-2Cr2O2n-1, Magn©li-phases of titania stabilized by Chromium as thermal sprayed coatings resulting in oxidation resistant Magn©li-phases. The established wear rates qualify both metallurgical concepts for radial and axial air foil bearings in small jet turbines and turbochargers, but are not limited to these, as well as the values of wear rates are comparable to those typically attributed under mixed/boundary lubrication of liquid lubricated tribosystems.
10:40 AM A1-2-11 Oxidation Behavior of Chromium Nitride and Chromium Aluminum Nitride Thin Films by DSC and TG Analysis
J. Lin, J.J. Moore, B. Mishra (Colorado School of Mines)
Freestanding CrN and CrAlN (22 at% and 58.5 at% Al) thin films have been produced by pulsed closed field unbalanced magnetron sputtering. The oxidation behavior of the films has been characterized by a combination of dynamic and isothermal thermal analysis using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure and mechanical properties of the thin films at different heat-annealing states were investigated by nanoindentation and scanning electron spectroscopy (SEM). Different phase transitions observed on the DSC plot, e.g. Cr2N, AlN, Cr2O3, Al2O3, have been confirmed by further X-ray diffraction (XRD) studies. The peak temperatures of the exothermic oxidation reactions in the DSC signals at different heating rates were applied to the Kissinger model for determination of activation energy. It was found that the addition of Al into the CrN film can effectively increase the film onset oxidation temperature from 550°C to 700°C, and the temperature at which full oxidation takes place can be increased from 1000°C to 1250°C. The CrAlN film with 58.5 at% Al content retained a hardness of 25 GPa after annealing at 800°C.
Time Period TuM Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF2007 Schedule