Unlike their metal counterparts, composite structures do not readily conduct away the electrical currents generated by lightning strikes. Cost reduction and expected production growth of the next middle range airplanes require automated manufacturing process of polymer components. The development of an automated technology to metallize polymer based composite for lightning strike protection is the aim of the CO3 project (EU Grant agreement: ID831979). In this study, thermal and electrical conductivities of composites were achieved by cold spray deposition of Cu or Al coatings. Critical points to be addressed were substrate erosion during cold spray, lack of polymer-metal adhesion and poor deposition efficiency. Several strategies were tested: i) a thin polymer film was cocured at the substrate surface before cold spraying, to enable implantation of metallic particles in the film, helping coating build-up and protecting the fibers of the composite. ii) Cold spraying a mix of metal and polymer powders to improve coating adhesion and prevent fiber damage. iii) Supercritical Nitrogen Deposition technology, prior to cold spray, to mechanically anchor metallic particles into the polymer. Subsequent cold spraying of purely metallic coatings was more efficient and showed better adhesion. All coatings were tested in terms of adhesion strength and electrical conductivity.

Cold spray process is a low temperature spraying method (<600°C) based on particle high kinetic energy at impact (velocity in the range 300-1200 m/s). Powder is injected in a specific designed nozzle and, there, reaches high velocity. Even if bounding mechanisms are not perfectly known, it is established that adhesion can’t exist below a definite particle velocity called critical velocity. This specific velocity depends on both powder and substrate characteristics (chemical composition, mechanical properties…). The large number of parameters makes the determination of critical velocity very complex. But it is a fundamental value to determine the correct operating conditions. This work presents an imaging technique that allows a fast measurement of critical velocity. In this paper, the measuring method is first validated by comparing the critical velocity of copper (sprayed on copper substrate) found in the literature, with the measured one. Its accuracy is then tested with other materials and, finally, some improvements of the method are proposed.

Cold spray process is an emerging technique that produces high density coatings. Particles are carried by a supersonic gas stream through a De Laval nozzle and, finally, impact on a substrate with high kinetic energy. Low gas temperatures make it possible to maintain sprayed material in solid state during the whole process. Beyond a given velocity, called “particle critical velocity”, particles can bind to the surface and create a coating. This velocity is clearly dependent on both sprayed material and substrate properties. This study deals with the investigation of the influence of the substrate hardness on nickel coating properties. Substrates with different hardness but same chemical composition were used. Samples are then coated with pure nickel or NiCrAlY under the same operating conditions. The measurements of some coating properties and process parameters (deposition efficiency (D.E.), level of porosity, micro hardness) allow the analysis of the effect of substrate hardness on sprayability. It was found that only D.E. is widely influenced by hardness of the substrate. It decreases drastically (especially for NiCrAlY) since hardness of the substrate is higher than hardness of the particle. Porosity level and coating hardness remain constant whatever substrate hardness is.

The Cold Gas Dynamic Spray Method (CGDS) requires high-kinetic energy particles to obtain dense coatings. The aim of this study is to characterize some properties of CGDS coatings and compare them to the bulk materials. Pure nickel and nickel-base alloys (NiCrAlY, CoNiCrAlY, and Hastelloy C) are sprayed on 316L steel substrates. Coating thickness is about 700 µm. The particle velocity at impact is measured using an imaging technique based on a fast-shutter CCD camera and a high-power diode laser. The original powder characterization involves particle size, density, chemical composition and hardness, and coating analysis includes micro hardness, nano hardness (performed in the first 60 µm of the coating), Young’s modulus determination and porosity level. The hardness test results make it possible to determine effects of this specific property on the sprayability with this process.