Design, processing and characterization of flexible hybrid coatings: A comparative evaluation M. Barletta a,⇑ , S. Pezzola a , M. Puopolo b , V. Tagliaferri a , S. Vesco a a Dipartimento di Ingegneria dell’Impresa, Università degli Studi di Roma Tor Vergata, Via del Politecnico 1, 00133 Roma, Italy b Dipartimento di Ingegneria Meccanica ed Aerospaziale, Università degli Studi di Roma La Sapienza, Via Eudossiana 18, 00184 Roma, Italy article info Article history: Received 26 April 2013 Accepted 12 September 2013 Available online 20 September 2013 Keywords: Coating process Phenyl methyl silicone Scratch Wear Chemical endurance abstract Visual appearance as well as mechanical and chemical resistance of a phenyl methyl silicone resin deposited on cylindrical aluminum substrates by dipping and spraying is investigated. The morphological features of the coatings were analyzed by contact gauge profiler and scanning electron microscopy. Scratch and wear endurance was tested by progressive load scratch and dry sliding linear reciprocating tribological tests. The chemical resistance was tested by contact tests during which the coatings were dipped in acid, solvent-based and surfactant-rich solutions. The machinability of the coated substrates was tested by three-point bending and tensile static tests. Performance of the phenyl methyl silicone resin was also evaluated by comparison with two organic coating systems based on epoxy and polyurethane resins, widely used in several industrial domains as for their outstanding properties of chemical insulation and ductility. The experimental findings highlight significant performances of silicone resins as for protective and decorative purposes. Yet, the character- istics of flexibility showed by pure organic materials remain unparalleled, thus making epoxy and/or polyurethane good choice as far as the machinability of the coated substrate is concerned. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Aluminums are strategic candidates in automotive and aircraft industrial domains to replace heavier and more expensive materials like high performance Fe-based alloys. Aluminum alloys must meet specific requirements of resistance to oxidation and corrosion by dif- ferent chemicals with which it may come into contact during ser- vice. Up till now, protection on aluminum alloys involves mostly chromate conversion coatings through a process, which implies the usage of hexavalent chromium [1]. The carcinogenicity and tox- icity for humans and environments of such a class of material has been often proved in scientific and technical literature [2]. In this re- spect, strong efforts have been providing to find safer and healthier alternatives. For engineering applications, emerging alternatives in- clude the design of innovative materials based on organic polymers [3–7] or on the most promising hybrid organic–inorganic materials [8]. Hybrid materials are constituted of a main inorganic silicon backbone endowed with organic lateral groups. The modulation of the organic fraction can confer to the inorganic backbone specific properties, which are a good compromise between rigidity and stiff- ness of the inorganic chains and ductility and flexibility of the organ- ic matters [9]. Accordingly, the material properties can be tailored varying the inorganic/organic ratio and improved adhesion on underneath metals can be reached by adding to the Si-based back- bone customized organic functional group like –OH. Such hydroxyl groups can react with the ones on passivated aluminum substrate by a sol–gel addition reaction with formation of covalent –Si–O– Al– bonds, which ensure improved adhesion between coating and underlying substrate [8,10]. Indeed, adherence to underlying sub- strates and intrinsic ductility are fundamental requirements for the design of tough, scratch and wear resistant as well as chemi- cal-proof materials [10]. Mechanical properties of organic inorganic hybrid coatings were thoroughly investigated together with their chemical and corrosion endurance [8–13]. Hardness, adhesion, flexibility and wear resistance were studied as function of organic/inorganic and/or of organic lateral chains/cross linkers ratio. Han et al. [11] showed the brittleness of an hybrid TEOS-polyaniline coating was strongly amplified with the increase in the inorganic/organic ratio. This result was associated to changes in the coating morphol- ogy which turned from smooth and even to micro-cracked and, ultimately, porous for larger concentrations in TEOS [11]. In 2009, Han et al supported the aforementioned findings by investi- gating the scratch resistance of a similar coating class and deducing a strict dependence on coating brittleness and amount of TEOS in the mixture [12]. They also found coatings with more ductile response to scratch tests can be achieved by increasing 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.033 ⇑ Corresponding author. Tel.: + 39 (0)672597591; fax: +39 (0)62021351. E-mail address: barletta@ing.uniroma2.it (M. Barletta). Materials and Design 54 (2014) 924–933 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes