Publications

Oxidation of Ni-Al-base electrodeposited composite coatings. I: Oxidation kinetics and morphology at 800°C

Susan, D.F.; Marder, A.R.

The oxidation of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis in air at 800°C for up to 100 hr. Long-term oxidation behavior was investigated with furnace exposures up to 2000 hr. The coatings were applied to nickel substrates by the composite electrodeposition technique and vacuum heat treated for 3-hr at 825°C prior to oxidation testing. The heat-treated coatings contained a two-phase γ(Ni) + γ′(Ni3Al) microstructure and the overall coating composition was approximately 7 wt.% Al. Also examined were uncoated nickel substrates and bulk Ni-Al alloys containing 6.2, 9.0, and 14 wt.% Al. For all samples, mass-gain kinetics were obtained from thermogravimetric (TG) experiments and furnace exposures and the composition and morphology of the oxidation products were examined using optical microscopy, scanning-electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). An outer NiO layer and an inner γ-Al2O3 layer formed on the composite-coating surface. The addition of a small amount of Si (about 1-2 at.%) was found to have little effect on Ni-Al composite-coating oxidation behavior. The Ni-Al coatings behave similarly to bulk γ + γ′(Ni3Al) or single-phase γ′(Ni3Al). In addition, at lower temperatures, such as 800°C, the coatings benefit from a small grain size that enhances Al diffusion to the surface to form the protective alumina layer. Based on oxidation kinetics and morphology, a critical Al content of about 6 wt.% was found, below which internal oxidation and higher oxidation mass gains were observed.