Gas detonation spray forming of Fe–Al coatings in the presence of interlayer Cezary Senderowski ⁎ , Zbigniew Bojar Department of Advanced Materials and Technologies, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland Received 10 January 2007; accepted in revised form 20 December 2007 Available online 28 December 2007 Abstract The present paper reports that gas detonation sprayed (GDS) NiAl and NiCr intermediate layers underneath of the intermetallic Fe–Al type coatings on plain carbon steel substrate form bilayer coating system interacting with external environment and/or metal elements. The interface layers are responsible for hardness, bond strength, thermal stability and adhesive strength of the whole GDS structure. The physical–chemical properties of the intermediate layers, combined with unique, very dense and pore free intermetallic Fe–Al coating obtained from self-decomposing powders resulted in new, beneficial features of the whole GDS structure which became more complex, enabled independent control of its functional properties and considerably reduced negative gradients of stress and temperature influencing the substrate and increasing adhesion strength. The achievement of homogenous and refined structure (comprising of small (b 1 μm) and equiaxed sized grains) creates a thermal barrier based on high-melting point intermetallic phases containing Al 2 O 3 ceramics which is responsible for properties of the GDS bilayer coatings. The application properties were investigated and the specific multilayer structure of the GDS coating was analyzed such as the phase composition, the degree of order, grain morphology, the quality of substrate/interlayer/external coating bonds, and first of all the influence of hardness of the NiAl or NiCr intermediate layers on the hardness and thermal stability of the FeAl coating after gas detonation spraying and additional heating at 750 °C and 950 °C for 10 h. © 2008 Elsevier B.V. All rights reserved. Keywords: Intermetallic (based on FeAl); Protective coatings; Intermediate layer; Gas detonation spraying 1. Introduction Gas detonation sprayed (GDS) intermetallic coatings com- posed of the phases in the Fe–Al system are characterized by a unique sandwich microstructure of layers which are responsible for high degree of hardness, thermal stability, high corrosion resistance, good adhesion and very good tribological properties of the GDS modified surface layer of engineering components [1–12]. It has already been shown [13–15] that the Fe–Al intermetallic compounds obtained by self-decomposition are extremely useful for GDS spraying on construction steel. The most beneficial effect as far as the layer quality is concerned (i.e. the best properties), is obtained from 40–63 μm size powders in the case of the FeAl compound matrices with initial FeAl40 (at.%) composition, practically without any alloying agents, deposited with optimum gun parameters on the carbon steel substrate pre- processed in the abrasive blasting process. The detonation gun which was used [16] was designed and constructed in the Surface Advanced and Technology Company in Warsaw. It is a valve-less gun working in cycles of (3–10) Hz. Its design allows for continual (without any valves) feeding the detonation chamber with working gases (propane–butane, acetylene, hydrogen) which depending on their volume controlled by a special system influence the detonation speed which can a couple of times exceed the sound speed. By the appropriate choice of the working gases and changes in the proportion of the gases (by changing their flow rate) as well as changing the frequency and spraying distance one can influence the formation of the metallic stream of the detonation products which impact the powder fed from the powder container and define its temperature, and velocity during coating formation. When the technological parameters of the GDS were considered [17] the research proved that even insignificant changes in one of the parameters of spraying (the volume of fuel Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2008) 3538 – 3548 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel./fax: +48 22 683 9445. E-mail address: csenderowski@wat.edu.pl (C. Senderowski). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.12.029