Design
Materials
Process
Surface Modification of Ni-Alloys for TBC Applications
R.W. Jackson, J.P. Leonard, F.S. Pettit, G.H. Meier April 2010
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh PA 15261
Nickel based alloys are the materials of choice for high temperature applications in turbines and hold promise for solid oxide fuel cells. The extensive work by Meier and Pettit at the University of Pittsburgh has concerned the oxidation/corrosion behavior of these materials and thin film barrier coatings under a wide range of conditions, as well as the fundamentals of mass transport and thermomechanical effects in surface oxidation.
Figure 1. Schematic of line-scan surface processing using a pulsed excimer laser. Beam is focused onto sample surface using a cylindrical lens. Sample is translated at constant rate as pulses are delivered. Using this technique a large area can be treated under identical conditions free from stitching effects.
Our involvement with these projects has concerned surface modification of Ni-based alloy materials using the excimer laser, as well as the controlled deposition of dopants onto the surface using PLD. Research in this area has included:
- Beam-induced surface melting and smoothing. The excimer laser, configured as a line source, is scanned across the surface of a metal sample, effecting repeated melting and solidification of the top 1 µm of material. Surface tension typically acts to smooth the surface while molten. We have identified some optimum conditions concerning number of pulses, fluence, and background gas that can result in smoothing. More recently, the use of a pulsed ion beam melting system (at Sandia National Laboratories) has shown promise to produce deeper melts with lower tensile stress and fracture behavior.
- Improvement of oxidation behavior using surface dopants. With PLD we have deposited a number of oxides, including CeO2, SrO, and others, onto metal surfaces. In the case of doping with CeO2, there was a significant decrease in the NiO growth rate. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. The PLD system is ideal for quickly generating new prototype metal-oxide samples owing to its stoichoimetric transfer capabilities from target to substrate, and does not require heating or electrical conductivity of the target.
Publications relating to this project
- R.W. Jackson, J.P. Leonard, L. Niewolak, W.J. Quadakkers, F.S. Pettit, G.H. Meier, “The Effect of Copper and Ceria Additions on the High Temperature Oxidation of Nickel”, Materials Science Forum 595-598, 1057 (2008).
- R.W. Jackson, J.P. Leonard, F.S. Pettit and G.H. Meier, "The effect of minor elements on the growth and electrical properties of NiO on Ni", Solid State Ionics 179, 2111 (2008).