Synthesis and heat treatment of sprayed high-temperature NiAl–Ni3Al coatings by in-flight combustion synthesis (CAFSY)

Abstract

Combustion-Assisted Flame SpraYing (CAFSY) of intermetallic coatings is a new, cost-efficient and on-site-applicable thermal spraying process for applying Ni–Al intermetallic overlays or bond-coats on metallic substrates for protection at high temperature. This is the synthesis of desired intermetallic phases inflight and in-situ on the substrates during oxy-acetylene thermal spraying, using only low cost base-metal powders. By adjusting the spraying conditions (initial composition, spraying distance, substrate temperature, and flame temperature), excellent Ni–Al-based coatings have been produced on various substrates, including mild steel, stainless steel, and aluminum alloys. In some cases, however, the intermetallic phases formed on the substrates during CAFSY have been found to be metastable or the nickel and aluminum powders have not reacted sufficiently. In such cases, post-spraying heat treatments of the coatings allow the solid-state combustion reactions to proceed to completion in the coating. In many cases, it was found that increasing the temperature and the duration of the heat treatment increased the amount of intermetallic compounds (NiAl, Ni3Al, NiAl3, and Ni2Al3) in the coating up to as much as 90 vol %. In particular, all remaining aluminum reacts completely by forming Ni–Al intermetallics in the coatings. In all cases, porosity of the coatings remains below 3% while adhesion strength increases and reaches 57 MPa. The CAFSY method is a special manifestation of combustion synthesis, along the lines of the well-known SHS method. Without the need for expensive pre-alloyed intermetallic powders, optimization of spraying conditions allows very fast, in-flight reactions between component base metal powders to produce the required coating alloys when they reach the surface of the substrate. The actual mixture of the intermetallic phases of the coatings and their properties can be optimized for any industrial use by controlling spraying conditions and composition of powder mixtures.