Low temperature alpha alumina coatings – ALALPHA

The three-year ALALPHA project is an industrial research project combining the skills of an academic partner, the Bordeaux Institute of Condensed Matter Chemistry, with those of an industrial partner, SAFRAN. This project focuses on the development of alternative processes capable of obtaining the alpha phase of alumina (?? - ????2??3) at low temperatures. Alumina is an engineered ceramic suitable for high performance applications in the defense and aerospace industries. The development of ??-alumina coatings at low temperatures still remains a challenge. However, this would allow to answer one of the major challenges of the aerospace industry (military as well as civil) which is the lightening of structures and the reduction of costs, by the use of lighter alloys (even cheaper) in substitution of superalloys, for example. The primary benefit would be significant fuel savings with positive consequences on the environmental impact. With this in mind, SAFRAN, the world's leading aircraft engine manufacturer, is continually seeking to lighten its LEAP turbojet engine (an evolution of the CFM56, the engine powering the most aircraft in the world). Indeed, in the form of a dense layer deposited on a metal alloy, alumina ?? is an effective protective barrier to the diffusion of oxygen particularly in low pressure turbines of turbojet engines.
This project is concerned with obtaining a thick, adherent and dense coating of nanostructured ??-alumina on various planar and complex shaped (ovoid type) metallic substrates, from a versatile and disruptive process based on the hydrothermal dehydration phenomenon at temperatures below 500 °C. The mechanical and temperature properties of this thick coating will be compared to those of a model thin coating, elaborated by the high power reactive magnetron sputtering process (R-HiPIMS).
The main objectives of the ALALPHA project are to: 1) Design a new generation of cold-wall reactor to obtain nanostructured coatings composed of 100% alumina ??, homogeneous in thickness for a process temperature below 500 °C, 2) Develop a model nanostructured coating of alumina ?? by R-HiPIMS in the same temperature range, 3) Establish the relationships between process, microstructure and properties, and 4) Characterize the properties in terms of adhesion to different metal substrates as well as those of roughness, hardness and oxygen diffusion resistance. Whatever the process, the challenges will be to: 1) Obtain a nanostructured and single-phase ??-alumina coating at low temperature, 2) Control its microstructure by understanding the different phenomena involved in relation to the process parameters, and 3) Coat metal substrates of different compositions while keeping similar adhesion and corrosion resistance properties.

The project is organized in 6 strongly interacting tasks and will be carried out in 3 main steps. First, to design a reactor allowing to control both the temperature and the hydrodynamics (fluid flows) in the vicinity of the substrate to be coated. Second, optimize the process parameters to control the microstructure and thickness of the single-phase ??-alumina coating. Third, characterize the different properties.

The ALALPHA Project is part of the thematic axis 8 Materials with the themes "new processes" and "surface treatments and coatings, functionalization and smart coatings". It meets the priorities: a) Materials and thermal protection for extremely severe conditions and b) Corrosion control, surface engineering.

The ALALPHA project is in the TRL range of 1 to 3.