Possibility of spraying of copper coatings on polyamide 6 with low pressure cold spray method Aleksandra Malachowska a,b, ⁎, Marcin Winnicki b , Lukasz Konat b , Tomasz Piwowarczyk b , Lech Pawlowski a , Andrzej Ambroziak b , Mateusz Stachowicz b a Science des Procédés Céramiques et Traitements de Surface UMR 7315, University of Limoges, CNRS 12, rue Atlantis, 87068 Limoges, France b Wroclaw University of Technology, ul. Lukasiewicza 5, 50-371 Wroclaw, Poland abstract article info Article history: Received 31 July 2016 Received in revised form 30 January 2017 Accepted 1 February 2017 Available online xxxx This paper discusses metallization of polymers using a low-pressure cold spray (Dymet 413). Three commercial copper powders: i) spherical and ii) dendritic were deposited on thermoplastic polymer - polyamide 6 (PA6). It was difficult to successfully apply a copper coating directly on the polymer substrate, therefore interlayers were applied. Additionally, the copper powder was pre-treated in hydrogen atmosphere to remove the oxide layer and reduce its critical velocity. Finally, the adhesion strength, electrical conductivity, oxygen content and microstruc- ture of resulting coatings were determined. Coatings were characterized by one order of magnitude of lower con- ductivity than the bulk material and bond strength of 3.6 MPa. The powder shape turned out to have a decisive effect on the possibility on coatings formation. © 2017 Published by Elsevier B.V. Keywords: Cold spray Polymer metallization Electrical conductivity 1. Introduction Deposition of metal coatings onto polymers has been widely studied in order to improve thermal and electrical properties as well as to re- duce surface degradation. Some well-established processes were used include physical vapor deposition (PVD) [1–4] or electroless deposition [5], but the thickness of the obtained layer was limited and deposition rates were low. Therefore, thermal spraying processes [6,7] are some- times applied, which allow for the deposition of a thick coating on var- ious substrate geometries, and the recoating of damaged elements. The main challenge is temperature-sensitivity of polymers. Recently, cold spray process was tested a potential solution. In this process, material is deposited in a solid state and therefore the temperature impact is lower in comparison to traditional thermal spraying methods. The pow- der particles are accelerated in stream of heated and pressurized gas and projected towards the substrate. Critical velocity is a key concept in the cold spray method [7]. It is defined as velocity that an individual particle of powder must attain in order to be deposited after impact with the substrate [8]. This definition is valid for ductile materials as brittle materials will cause erosion for any velocity at temperatures below their melting temperature [9]. Critical velocity depends mostly on the sprayed material's mechanical properties but varies also with particle size, particle morphology, particle impact temperature or pow- der oxidation [10–12]. The critical value may be calculated with the equation proposed by Assadi et al. [11] and then developed by Schmidt et al. [10]. It takes the following form: v cr ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi F 1 ∙4∙σ TS ∙ 1− T i −T R T m −T R   ρ þ F 2 ∙C pp ∙ T m −T i ð Þ v u u u t ð1Þ where: ρ – density, σ TS – tensile strength, T m – melting point, T i – impact temperature, T R – reference temperature (293 K), c p – specific heat of particle, F 1 – mechanical calibration (for cold spray 1.2), F 2 – thermal calibration (for cold spray 0.3). The model takes into account the specific heat, tensile strength, me- chanical and thermal calibration, but particles size is not included [10]: with decrease in particle size critical velocity increase. The possible rea- son for higher critical velocity of small particles may be the higher con- tent of oxides or adsorbents hindering the bonding. Usually, powder contains a mixture of particles of varying diameters. In such cases, crit- ical velocity is calculated for larger particles due to fact that smaller par- ticles achieve a higher velocity [10]. Lupoi and O'Neill [13] pointed out that polymers might be coated when the particle impact energy calculated from the critical velocity of a given material and particle mass is sufficiently low. According to this formula, tin and lead will be easy to deposit, for aluminium and ti- tanium deposition and erosion process will be coexistent, and for cop- per, the erosion process will be prevalent [13]. This assumption was confirmed for tin, which was deposited on various substrates PC/ABS, polypropylene, polystyrene and polyamide-6 [13]. The aluminium was deposited on PEEK [14] and Lexan [15], however in case of Lexan, the Surface & Coatings Technology xxx (2017) xxx–xxx ⁎ Corresponding author at: Wroclaw University of Technology, ul. Lukasiewicza 5, 50- 371 Wroclaw, Poland. E-mail address: aleksandra.malachowska@pwr.wroc.pl (A. Malachowska). SCT-22097; No of Pages 8 http://dx.doi.org/10.1016/j.surfcoat.2017.02.001 0257-8972/© 2017 Published by Elsevier B.V. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Please cite this article as: A. Malachowska, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.02.001