Improvement of moisture barrier properties of CFRP for high-precision engineering applications through silicon-like hybrid films prepared by Plasma Enhanced Chemical Vapour Deposition

The use of carbon-fibre-reinforced polymers (CFRP) laminates in structural applications where high strength-to-weight and high stiffness-to-weight ratios are required is wellestablished. However, because of structural distortions induced by moisture absorption effects, polymer matrix composites are not yet definitely considered suitable candidate for ultra-high precision engineering applications. Typical examples are ultra-high precision machine structures (e.g. mobile parts of micro-milling machines, grinding machines, coordinates measuring machines, ...) for which high values and stability of mechanical properties (stiffness, damping) and at the same time extremely good long-term geometrical stability are required. The introduction of composite materials in this new industrial market will surely bring a great advances in term of dynamic performances and will represent a new challenge for design engineers and huge opportunities for the composite manufacturers. However, moisture absorption of CFRP materials presents an important problem. Absorbed moisture causes unpredictable and unacceptable structural distortions over time (even a geometrical drift of a few microns is not tolerated!) and this is still an open issue. In order to solve this problem barrier coatings, including metallic sheet coatings, elastomer polymerbased coatings, vinylester coatings and other specialty gel coats are used. An alternative can be represented by amorphous silicon-like (SiOx) thin films produced by Plasma Enhanced Chemical Vapour Deposition (PECVD). This method uses a low-pressure glow discharge to create active species, such as radicals and ions, from an original monomer (hexamethyldisiloxane in this specific case) so that polymer-like films are deposited through reactions between these active species present in plasma phase and the surface of the sample. In PECVD process the monomer gains energy from the plasma through inelastic collisions, especially with electrons, so that the plasma is about at room temperature, and is activated and fragmented into small molecules (or atoms) which are recombined to form larger molecules in plasma phase and at the substrate surface and, sometimes, oxygen incorporation due to residual air in the reaction chamber can occur. So the chemical structure of plasma polymers is never predicted from the structure of the monomer because fragmentation and rearrangements of the monomers occur in plasma phase. How the starting molecules are fragmented into activated small molecular fragments in plasma phase depends from plasma parameters such as monomer and gas flow rates, radio frequency (RF) power, pressure in the reaction chamber, deposition time and geometry of the system, even if the same starting molecules are used for the plasma polymerization. Therefore PECVD technology, compared with other traditional deposition techniques, presents the possibility of gradually changing chemical composition and physical properties of the deposited films simply by the modification of the process operative conditions. If the plasma parameters are changed during the deposition process, hybrid films which join different chemical-physical properties can be realized from the same starting monomer (during the same process). In the present work a SiOx barrier film was deposited on CFRP substrate (unidirectional UHM carbon fibres in epoxy matrix) by PECVD from a mixture of hexamethyldisiloxane (HMDSO), O2 and Ar in conditions of high monomer (HMDSO) fragmentation [1]. To prevent the absorption of water during machining operations in engineering applications, a more superficial thin layer was grown from the same monomer in conditions of low monomer fragmentation so that the film acquires also hydrophobic properties [2]. The result is an hybrid film which joins two properties, resistance to the permeation of water vapour and hydrophobicity, without presenting the adhesion problems that could occur in multilayer films where each layer has a totally different chemical composition. A further advantage of this film is represented by its thin thickness (submicron domain) which does not affect the final roughness of the surface (this is very important for applications where high quality smooth surfaces are required) and then no supplementary surface finishing process operations will be necessary. Moreover PECVD process does not requires high temperatures and is considered a sustainable environmental friendly process as it is performed in a closed vacuum apparatus with no emissions of VOCs (Volatile Organic Compounds). The film chemical composition and its morphology were analyzed by Fourier Transform Infrared spectrometry (FT-IR) and Atomic Force Microscopy (AFM) respectively. AFM images show the film is very dense because of the small features grown during the process, as a consequence of the high monomer fragmentation in plasma phase. The high density prevents the permeation of gases and vapours through the film and so through the CFRP. The film hydrophobicity was investigated by water contact angle measurements. The WVTR (Water Vapour Transmission Rate) coefficient of the film was measured through permeance tests [3]: a value of WVTR = 0.5 g/m/day (ASTM E398-83 standard) was obtained, and that represents a very good results in term of barrier properties against moisture absorption, considering also that conventional polymer barrier films show a WVTR value of about 2÷10 g/m/day. Micro-mechanical tests (using a micro-tribometer developed at Cambridge University [4]) were carried out on CFRP samples to characterize the wear-scratch behaviour of the film. The number of cycles to wear under a given normal force was measured and compared to conventional coating ones. Finally the adhesion of the film to the composite substrate was investigated by Scanning Electron Microscopy (SEM). The scratch tests show that the specimen fails in a brittle way with micro-cracks emanating from the damaged region. SEM images show also a good adhesion of the film to the composite substrate because the failure of the film and of the substrate (such as fibre failure) takes place simultaneously. The film here proposed for CFRP materials used in ultra-high precision mechanical applications can be extended to other classes of FRP, including natural fibres composites, to prevent the degradation of their Young’s module due to the moisture absorption effects.