Abstract
The influence of two different fluoride-based activator salts (NH4F and AlF3) was studied for diffusion aluminide coatings obtained via pack cementation on a Ni-based superalloy (René 108DS). The resistance to oxidation and hot corrosion was assessed as a function of the concentration of activator salts used during the synthesis process by means of pack cementation. Two different concentrations were selected for activator salts (respecting the equimolarity of fluoride in the synthesis) and the obtained diffusion coatings were compared in terms of morphology, thickness and composition, as well as in terms of microstructural evolution after high temperature exposure. Isothermal oxidation tests were conducted at 1050 °C in air for 100 h in a tubular furnace. The oxidation kinetics were evaluated by measuring the weight variation with exposure time. The microstructural evolution induced by the high temperature exposure was investigated by SEM microscopy, EDS analysis and X-ray diffraction. Results showed that the coatings obtained with AlF3 activator salt are thicker than those obtained using NH4F as a consequence of different growth mechanism during pack-cementation. Despite this evidence, it was found that the NH4F coatings show a better oxidation resistance, both in terms of total mass gain and of quality of the microstructure of the thermally grown oxide. On the other hand, coatings produced with high concentration of AlF3 exhibited a better resistance in hot corrosion conditions, showing negligible mass variations after 200 h of high temperature exposure to aggressive NaCl and Na2SO4 salts.
Highlights
IntroductionPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
A great scientific and technological interest is devoted to thermal barrier coatings (TBCs) [4,5,6,7,8,9] as heat resistant surface layers deposited on metallic components of gas turbine engines
The polyhedral structure is typical of diffusion aluminide coatings, and continuous network β-NiAl phase, and consists consists of of polygonal polygonal grains grains surrounded surrounded by by aa continuous network of of β-NiAl phase, as observed in the literature
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Materials used in hot sections of aero- and land-based gas turbines are designed to endure severe operating conditions and must be able to resist both hot corrosion and high temperature oxidation. Nickel-based superalloys are usually employed in high temperature sections of the engine, whereas TiAl components have aroused great interest in recent decades for the less thermally stressed areas [1,2,3]. A great scientific and technological interest is devoted to thermal barrier coatings (TBCs) [4,5,6,7,8,9] as heat resistant surface layers deposited on metallic components of gas turbine engines
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