| The diffusion and oxidation behavior of electrodeposited composite coatings were studied using thermogravimetric analysis, electron probe microanalysis, and microstructural characterization techniques. Deposition of Al and Al-Si particles into a Ni matrix produced binary and ternary composite coatings with a uniform particle distribution. During heat treatment, coating morphological development was found to occur by the exothermic reaction Ni + Al → NiAl followed by interdiffusion to form a final γ + γ′ microstructure. During oxidation at 800°C, the γ + γ ′ coatings formed a γ-A12O3 layer below an outer NiO scale. At 1000°C, &thetas;-Al2O3 needles are formed in the early stages. At later times, an α-Al2O3 layer is found below an outer spinel NiAl2O4 scale. At both temperatures, the Al2O3 layer provides good oxidation resistance during long-term exposure. The coatings were compared to bulk Ni-Al alloys, and a critical Al content of about 6 wt. % was found, below which internal oxidation attack may occur. During exposure at high temperatures, interdiffusion occurs between the Ni-Al coating and the Ni substrate. The diffusion process was modeled using an analytical solution to Fick's second law. The Al content at the coating surface was monitored using EPMA and the results were in agreement with the predicted values from the diffusion equation. Estimates of coating lifetime were made by combining the diffusion results with knowledge of the effect of Al content on oxidation behavior. The results showed that practical use temperatures for Ni-Al coatings are 700–800°C. At 1000–1100°C, Al depletion begins after a few hundred hours and predicted coating lifetimes are in the range of a few thousand hours. |
