Initial Oxidation of β-NiAl(Pt) Coatings and Model Alloys
Doctoral thesis, 2006
The aim of this work was to investigate the oxidation properties of NiAl coatings and model materials during high temperature oxidation. One of the important issues was to elucidate the beneficial role of Pt additions. A better understanding of the involved processes provides a key for the further improvement of the coatings performance.
The aluminide coatings are used to improve the oxidation behaviour of superalloys during high temperature exposure. The high Al content in the coatings ensures the formation of a protective Al2O3 scale. However, at the initial stages of oxidation, fast growing transient Al2O3 phases form. Then the oxide scale is not fully protective. Thus, factors like: oxide adhesion, structure, transformation- and oxidation rates, are essential for the coating performance and were a subject of this PhD work. TEM and SEM were used as the experimental tools. Specimen preparation was performed using FIB milling. To obtain a measure of the oxide binding to the metal, theoretical calculations using DF theory were applied.
The investigations showed that as the initial transient Al2O3 grows, mainly by an outward diffusion of cations, cavities form at the coating-oxide interface and drastically decreases the contact between the metal and the scale. During further oxidation these cavities are filled with oxide. This is most probably due to the formation of small cracks in the oxide that ensure supply of molecular oxygen. The presence of Pt in the alloys decreases the amount of interfacial cavities and speeds up their filling. This is connected to the increased Al diffusion in the presence of Pt.
After prolonged oxidation new voids form due to the super-saturation of vacancies injected into the oxide-metal interface by faster diffusion of Ni into the bulk than that of Al into the interface. This may lead to oxide spallation, which is a life-limiting factor for the component. Presence of Pt in the alloys retards the formation of these mature voids by speeding up Al diffusion.
Although theoretical calculations showed a reduction of the work of separation when Pt is added to the alloys, experiments reveal that, due to much larger effective contact area between the metal and the oxide in the Pt- containing materials, the overall adhesion of the scale is improved.
High Temperature Oxidation