WO2008078346A1

WO2008078346A1 - Hybrid coating that is water-repellent and humidity-proof for reinforced composite polymer materials deposited through pecvd

Info

Publication number: WO2008078346A1
Application number: PCT/IT2007/000604
Authority: WO
Inventors: Anna Cremona; Espedito Vassallo; Franco Taddei; Norman Fleck; Angelo Merlo
Original assignee: Ce.S.I. Centro Studi Industriali Di Taddei Ing. Franco & C. S.A.S.
Priority date: 2006-12-22
Filing date: 2007-09-03
Publication date: 2008-07-03
Also published as: ITMI20062482A1; EP2094798A1

Abstract

A hybrid coating is described, for reinforced composite polymer materials to be used for structural mechanical applications in which a high geometric stability is required. The composite materials has lightweight and stiffness properties, but are subjected to geometric distortions due to the humidity absorption being present in the atmosphere or following the accidental contact with fluids in the working environment in which the composite itself is used. Purpose of this coating is conferring to a composite material properties that improve its dimensional stability and that therefore prevent alteration of the dimensions of structures made of the affected material. Such coating is obtained by means of a Plasma Enhanced Chemical Vapour Deposition (PECVD) process. Preferred embodiments of such coating are realised by using, as precursors, organosilane monomers (for example hexamethyldisiloxane or HDMSO) combined with oxygen (O2), and possibly argon (Ar).The coating is composed of two layers. The most internal layer is humidity-proof, and therefore operates as protecting barrier for the composite material on which it is applied. The layer more on the surface has water-repellence characteristics and is composed of the same chemical elements present in the more internal layer but with different percentages and with a different stiochiometry. Among the composite materials on which it is possible to lay this coating, there are, for example, materials reinforced with aramide (ex. Kelvar), carbon (CFRP) or glass (GFRP) fibres. Among the structural applications of said composite materials in which a high geometric stability is required, there is, for example, the field of operating machines.

Description

HYBRID COATING THAT IS WATER-REPELLENT AND HUMIDITY-PROOF FOR REINFORCED COMPOSITE POLYMER MATERIALS DEPOSITED THROUGH PECVD

The present invention refers to the application of the Plasma Enhanced Chemical Vapour Deposition (PECVD) process for coating reinforced composite polymer materials to be used for structural mechanical applications in which a high geometrical stability is required. A possible field of application of the present invention is the sector of operating machines (for example, machine tools, measuring machines, machines for high accuracy mechanical working, robots, etc. ) .

In such sector (and, in particular, in those applications in which measuring or mechanical working accuracies are required on the order of micrometers) , , the structural elements of the operating machine must have a high dimensional stability since possible distortions of a geometric nature are transformed into errors that, if not p_reve^ted or corrected, impair functionality and accuracy of the mecϊ^an^-^cs' ^anc* therefore in the end the quality of the worked pi^{ece or} ^^e performed measure. The use of reinforced composite polymer materials (for example with aramide, carbon (CFRP) or glass (GFRP) fibres) for making moving components of an operating machine allows, with respect to the use of traditional materials, reducing the inertia of such components without impairing their mechanical characteristics.

Composite materials are defined as two-phase hybrid materials, in which one phase is composed of a polymer matrix (that can be a thermoplastic or thermosetting resin) , while the second phase is composed of (short or long) fibres, among which the most common ones are carbon, glass and aramide (ex. Kevlar) fibres.

Matrix and fibres are in a suitable stoichiometric ratio. Fibres can further assume different orientations inside the matrix itself, according to functional requirements that have to be satisfied.

Among the main advantages offered by the use of the above composite materials, there are: high stiffness-weight ratio (main reason that provides for their use) , good structural dampening and excellent resistance to corrosion and fatigue. If suitably designed, the composite materials can further confer to components of operating machines a high dimensional stability from the point of view of thermal distortions . However, a typical characteristic of composites based on a polymer matrix that cannot be found in metals, is the trend to absorb humidity from the outside environment. At macroscopic level, humidity absorption implies a dimensional variation whose value is given by a swelling coefficient β (defined below). In the majority of composites, the humidity absorption can be attributed to the matrix.

If there are no adequate surface protection measures, due to the effect of the humidity absorption present in the environment, the composite materials can therefore be unstable from the dimensional point of view, giving rise to distortions. These geometric distortions can be generated, for example, also by the accidental contact of the surface of the composite material with fluids commonly used during the machine operation (such as, for example, cutting fluids used as lubricants-coolants in the tool-piece contact area when working or as means for removing chips from the cutting area) . As previously stated, when the composite material is used in making components of an operating machine, these distortions generate errors that, if not corrected, impair the quality of worked piece or performed measure.

Water diffusion inside the polymer resin can generate a series of other undesired effects, such as: variation of structural properties, generation of local stresses that can give rise to micro-cracks, hydrolysis, and in general irreversible ageing phenomena. Moreover, in aerospace applications (such as, for example, in case of orbital satellites), water absorbed on the ground by the composite material is completely released in the space, determining shrinkage phenomena of structural parts/components that have very severe dimensional stability tolerances.

As better described below, the hybrid coating to which the present invention refers, prevents above mentioned geometric distortions and undesired effects from occurring (thereby preventing measuring and working errors that would happen in case of use of the composite material for making structural elements of an operating machine) . In fact, the polymer composite material reinforced with a pair of protecting layers that compose a water-repellent and humidity-proof coating, is coated through PECVD. The modification of the composite surface does not modify in any way the structural properties of the "bulk", namely the composite itself that composes the substrate, and increases its dimensional stability.

In addition to the field of operating machines, there are other structural applications for which the requirement of a high dimensional stability can be satisfied through the use of composite materials coated with the hybrid film of the present invention. Among these applications, there are, as an example, manufacturing precision guides and bearings with hydrostatic or pneumatic-static support, manufacturing of aeronautic structural elements or structural elements for motor vehicles, optical telescopes, radio-telescopes, orbital telescopes, satellites and orbital stations.

Plasma Enhanced Chemical Vapour Deposition (PECVD)

The Plasma Enhanced Chemical Vapour Deposition treatment is a surface modification process for a material consisting in generating a thin film on the material surface exposed to plasma. Plasma is an ionised gas in which electrons and ions move independently and are not mutually linked like in the remaining states of the matter. As such, plasma is considered the fourth state of the matter.

The PECVD method uses a gas discharge to create active chemical species (radicals or ions) starting from a precursor monomer under vapour phase. These chemical species react with the material surface to be treated giving rise to a thin deposit (or film) having specific chemical-physical properties. Fragmentation of the precursor monomer (breakage of chemical links and possible ionisation) therefore does not occur through a thermal activation, but occurs by means of energy supplied to particles being present in the plasma. This makes it possible to perform the process at ambient temperature (for this reason, it is also called "cold plasma'') . The PECVD technology provides for the use of a vacuum chamber within which the substrate to be subjected to surface treatment is placed. Such chamber is evacuated by means of a pumping system that, in addition to taking care of reaching vacuum, must keep the pressure during the process constant. Typical working pressures are included in a range of 0.1 ÷ 50 Pa. Plasma is generated through an electric field whose frequency is on the order of MHz.

After having inserted the sample in the reactor and having reached a vacuum, the substrate can be subjected to a short surface cleaning treatment through inert gas plasma (typically argon) to remove possible contaminants and impurities present on the surface of the material to be treated. The substrate can also be subjected to a pre- treatment, usually with argon, oxygen or hydrogen plasma, whose purpose is improving the film adherence to the substrate. After having ended this starting step, the vaporised precursors are inserted in the vacuum chamber. Turning on the radio-frequency discharge generates fragmentation, excitation and ionisation of precursors. The active species generated in the plasma phase are chemically linked to the substrate surface, thereby generating the coating layer. Different types of monomers and gas mixtures can be used, according to film characteristics that have to be obtained. Deposits can be realised with chemical-physical properties that are uniform or variable along the thickness, in order to satisfy a plurality of requests depending on the application in which the material must be inserted. In order to do this, it is enough to operate during the process on deposition parameters. The effect of the plasma-solid surface interaction in fact does not depend only on the choice of precursors (and therefore on their chemical composition) and on the substrate composition, but also on parameters such as flows of used monomer and gases, chamber pressure, deposition time, power of the electric field with which plasma is generated and reactor geometry. By changing these parameters, it is possible to grow on the substrate, starting from the same precursors, a layer of a certain thickness having certain characteristics and an upper layer over this layer, having different characteristics. A single hybrid film is thereby obtained, that does not show adhesion problems between the two layers, that could instead happen if the coating were composed of two films with very different chemical composition, obtained with the same deposition technique or with different techniques. Moreover, the coated material can afterwards be possibly subjected to a final, non-PECVD treatment, that confers it further functional properties .

The PECVD technology has peculiar advantages with respect to traditional techniques (especially galvanic ones) , first of all the chance of depositing films with desired chemical-physical characteristics operating, as already stated, simply on the above mentioned process parameters. Among the other advantages, there are the low environmental impact and the limited consumption of energy and process gases and vapours. Moreover, it is important to underline that the deposition process does not need high temperatures, but occurs at ambient temperature since the necessary energy for chemical reactions is supplied by particles present in the plasma.

Through PECVD, it is possible to confer specific surface properties to a material, without modifying substrate properties and without excessively increasing costs. Among the functional properties that can be obtained through PECVD, there are mechanical . properties such as hardness and low friction coefficient; opto-electronic properties such as low dielectric constant; chemical properties such as permeability or impermeability to gas and vapours (barrier property) , hydrophobicity or hydrophily, biocompatibility, resistance to corrosion and humidity, paintability.

With reference to the present invention, the PECVD process has been used for obtaining materials that combine structural properties typical of composite materials

(lightweight and high stiffness) with hydro-repellence and humidity-proof properties, required by the particular type of application.

State of the art

In the past, several patent applications have been filed related to Plasma Enhanced Chemical Vapour Deposition (herein below designated with its acronym PECVD) in order to obtain water-repellent coatings and/or coatings with barrier properties against humidity absorption.

Herein below, the most important projects studied in this context are briefly summarised and classified by application sector. Electronics and microelectronics Patent US5260236 (November 1993)

This patent deals with a process for obtaining, through PECVD, a layer of passivation for wafers of semiconductor integrated circuits. Such layer has a constant thickness and is characterised by a high resistance to humidity and abrasion. Patent US6083313 (July 2000)

This patent deals with a process for making a surface hardening coating, to be applied onto substrates made of plastic material, such as plane displays. Such coating is composed of carbon, hydrogen, silicon and oxygen and is applied to the substrate through PECVD. Through such process, a coating is obtained that is transparent, resistant to abrasion, water-repellent, and scarcely permeable to humidity, oxygen, helium and other vapours. Patent WO00989 (November 2000)

This patent deals with a process for obtaining materials characterised by a reduced dielectric constant to be used in making semiconductor devices containing integrated circuits. The process provides for the deposition of two layers of oxides on interconnecting structures of the two integrated circuits, structures that generally assume the appearance of metal lines that mutually connect the integrated circuits according to a particular scheme.

The first layer of oxides is obtained through High- Density Plasma Deposition (HPD) . Such layer is doped through fluorine in order to reduce the dielectric constant of the laid layer.

The second layer of oxides is laid through PECVD. Such layer is doped through phosphor in order to keep mobile ions (for example, sodium and potassium ions) whose migration towards the interconnecting structures could impair the electronic device operation. This second layer is an optimum barrier against humidity. This is fundamental since if humidity reaches the above first layer and reacts with fluorine, hydrofluoric acid is formed, that corrodes the interconnecting structures. The process finally provides for a chemical mechanical polishing operation to level the upper surface of the above second layer of oxides. Patent US6820481 (November 2004)

This patent deals with a mass flow sensor characterised by a membrane that has a high mechanical stability due to the deposition, on the upper sensor layer, of a coating composed of silicon oxide and obtained through PECVD. Such coating increases the membrane thickness and operates as barrier against humidity in the atmosphere. Patent PR2864110 (June 2005)

This patent deals with a coating characterised by a carpet of fibres with nanometre sizes and hydrocarbon nature. Fibres directly grow on the substrate to be coated due to the deposition in some points of the substrate of a catalyst, and afterwards adding the hydrocarbon precursor from which nanofibres originate. The nanofibre growth therefore occurs only in substrate points in which the catalyst has been laid. Nanofibres and free substrate surface included between them are afterwards coated with a hydrophobic and/or lipophobic polymer whose deposition is performed through PECVD. Textile sector Patent MI2002A000360 (February 2002)

This patent deals with the surface treatment through plasma aimed to confer liquid-repellent properties to textile fibres and animal skins without modifying the physical- mechanical properties of fabric or skin. Cultural property sector Patent MI2004A000068 (January 2004)

This patent deals with the restoration through plasma of paper materials stored in archives and libraries. Deterioration of books and archive materials is due to the cellulose decay that can be caused by many factors, such as acid hydrolysis, oxidising agents, light, environment pollution, presence of micro-organisms. The treatment through PECVD is aimed to remove micro-organisms, to reduce surface stains intensity, to confer surface properties of barriers against gases and vapours and water-repellence .

Among the other fields in which the PECVD process is used, optics, packaging, biomedicine, mechanics can be mentioned.

In the field of optics, the treatment through plasma is aimed to confer water-repellence properties to eyeglass lenses and optical devices.

In packaging, very dense thin films, obtained through plasma from organosilane monomers, can confer to packages a very reduced permeability to gases, such as oxygen, steam and aromas. If the film is applied inside plastic containers (for example bottles for food use) , the "internal vitrification", namely the deposition of glass films, allows storing fluids without gas and vapour leaks, and prevents the possible release of materials contained in the envelopes.

In the biomedical field, the PECVD is used for sterilising and depositing anti-bacterial and anti-fouling coatings on biomedical devices. The surfaces of materials having affinity for biologic components (proteins, cells, bacteria, algae, etc.) placed in non-sterile environments are subjected to deterioration due to the formation of bio-films with varying composition that "foul" the material and promote the adhesion and growth of bacteria, thereby modifying the performance of the medical device. The deposition of non- fouling films of the PEO (Polyethylene Oxide) type on polymer materials for biomedical applications is currently being studied.

In the mechanical field, through plasma it is possible to realise (especially through Physical Vapour Deposition, PVD) coatings with wear-preventing properties and very hard, or surfaces that are not paintable can be made so. Through PECVD it is possible to make car windshield water-repellent.

Finally, it must be remembered that there are several applications of plasma treatments with reinforced composite polymer materials.

Reinforcing fibres of a composite material can for example be subjected to a plasma treatment in order to modify the chemical and physical surface properties of said fibres. In particular, the fibre surface can be treated in order to increase its wettability and favour its adhesion to the polymer matrix within which the fibres are dispersed.

Plasma treatments can also be applied to reinforced composite polymer materials in order to increase their surface roughness. Such treatment can be used in the chemical industry as method for preparing samples to be analysed under an electronic microscope to locate arrangement and orientation of particles inside the polymer matrix.

It is finally possible to use plasma treatment for increasing the adhesion between the surface of a composite material and a structural adhesive with epoxy nature.

In spite of the fact that there are already applications to plasma treatments with reinforced composite polymer materials, the coating of the present invention is an innovation since none of the studied application has as its aim the conferment of water-repellence and humidity barrier properties to said composite materials.

Object of the present invention is providing a hybrid coating for reinforced composite polymer materials (for example with aramide, carbon (CFRP) or glass (GFRP) fibres) to be used as structural elements in mechanical applications in which a high geometric stability is required, such as, for example, in the field of operating machines. Such coating operates as protecting layer and is obtained by means of a Plasma Enhanced Chemical Vapour Deposition (PECVD) process. By adequately choosing the gas mixture to be used and the process parameters, the coating becomes water-repellent and steam-proof. This increases the dimensional stability of the composite material and consequently of the structural elements realised with this material.

Object of the present invention is providing a coating for reinforced composite polymer materials (for example with aramide, carbon (CFRP) or glass (GFRP) fibres) that can be used as structural elements in mechanical applications where a high geometric stability is required (such as, for example, in the field of operating machines) , so that said coating has specific desired properties.

A further object of the present invention is providing a coating for reinforced composite polymer materials that can be obtained through a Plasma Enhanced Chemical Vapour Deposition (PECVD) process.

Another object of the present invention is providing a coating for reinforced composite polymer materials that is water-repellent and humidity-proof, thereby being a protecting layer for coated composite materials.

The above and other objects and advantages of the invention, as will result from the following description, are obtained with a hybrid coating for composite materials as claimed in claim 1. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims .

The present invention will be better described by some preferred embodiments thereof, provided as a non-limiting example, with reference to the enclosed drawings, in which:

- Figure 1 shows a schematic view of a preferred embodiment of the hybrid coating, that is water-repellent and with humidity-barrier properties, laid on a preferred embodiment of a panel made of reinforced polymer composite material, according to the present invention;

- Figure 2 shows a schematic view of a drop of water laid on the hybrid coating schematically shown in Fig. 1. The Figure points out what is meant by "Water Contact Angle" or WCA;

- Figure 3 shows a transmittance spectrum that is the result on an analysis performed on a layer of polyurethane paint laid on a preferred embodiment of a panel made of reinforced polymer composite material, after a three-month exposure to an environment with its relative humidity increased from 40 % to 80 % in a few days, and remained at 80 % for the whole affected period^' (three months) ;

- Figure 4 shows a transmittance spectrum that is the result on an analysis performed on the hybrid coating schematically shown in Fig. 1, after a three-month exposure to an environment with its relative humidity increased from 40 % to 80 % in a few days, and remained at 80 % for the whole affected period (three months);

Figure 5 shows a graph that describes how humidity concentration changes in a preferred embodiment of a panel made of reinforced polymer composite material, depending on the relative humidity of the environment to which said panel is exposed;

Figure β shows a graph that describes how humidity concentration changes depending on time in a preferred embodiment of a panel made of reinforced polymer composite material exposed to an environment whose relative humidity is equal to 80%; and

- Figure 7 shows a schematic perspective view of a preferred embodiment of a machine tool.

The present invention refers to a hybrid coating (1) for reinforced polymer composite materials to be used in mechanical applications where a high dimensional stability is required. Such coating (1) , obtained through PECVD, is composed of a humidity-proof thin layer (4) and a still thinner layer (3) with water-repellence characteristics.

As a non-limiting example, the substrate to which said coating (1) is applied can be a panel (2) made of a polymer composite material reinforced with aramide, carbon (CFRP) or glass (GFRP) fibres. With reference to said substrate (2), as a non-limiting example, a process is described below through which it is possible to lay a preferred embodiment of the coating (1) of the present invention onto said substrate (2) .

A preferred embodiment of said coating (2) is obtained by using as gas mixture for the plasma treatment, for example, hexamethyldisiloxane (or HDMSO: [ (CH₃) ₃Si] ₂0) as monomer, oxygen (O₂) and argon (Ar) . Said coating (1) is therefore a silicon-like coating, namely based on silicon, and also containing carbon, oxygen and hydrogen. Starting from the gas mixture, on the composite material (2) , two layers (3) and (4) are laid, composed of the same chemical elements being present, but in different percentages and with a different stoichiometry. The deeper layer (4) directly adheres to the composite material (2) skin and has steam- barrier properties. The layer (3) more on the surface is laid on the humidity-proof layer (4) and shows water-repellence characteristics .

Deposition in the present case occurs by using as process chamber a reactor with capacitive coupling in an asymmetric configuration (namely with electrodes with different sizes) in which biasing/ionisation of gaseous products (and therefore plasma generation) occurs through an electric field at a frequency, for example, of 13.56 MHz. The material to be coated is placed on the electrode with bigger sizes, that is electrically grounded.

The first layer (4) of which the hybrid coating (1) is composed, is laid at a pressure, for example, ranging between 1 Pa and 10 Pa and using a gas mixture composed of HMDSO as monomer and O₂. In order to obtain a steam-impermeable film, it is necessary to perform the deposition process in a high fragmentation state (and therefore with high power, for example greater than 200 W) and at a high dilution of HMDSO in O₂ (for example: O₂ flow/HMDSO flow > 10) . By operating under these conditions, a thin layer (4) is laid with a thickness that changes, according to the deposition time, from a few tens to a few hundreds of nanometres, having a very high density and whose composition is characterised by a low content of carbon and silanol groups (SiOH) (elements that lower the humidity barrier properties), so that its stoichiometry is next to the silica (SiO₂) one. With the above mentioned process parameters as an example, the optimum speed with which the above film (1) is obtained is 20 ÷ 30 nm/minute .

Following a variation of the process parameters, on said barrier film (4) a second, very thin layer (3) (for example, with a thickness of a few tents of nanometres) is laid, with a different chemical composition and with water-repellence characteristics. Such layer (3) is laid at a pressure, for example, included between 1 Pa and 10 Pa and using a gas mixture composed of HMDSO as monomer, O₂ and Ar. Differently from what happens for the deposition of the layer (4) with barrier property, the water-repellent layer (3) is obtained by operating in a low fragmentation state (and therefore with low power, for example less than 100 W) and with a lower dilution of HMDSO in O₂ (for example: O₂ flow/HMDSO flow < 10). Possibly, O₂ can be completely removed from the deposition process. The thereby obtained layer (3) is less dense with respect to the layer (4) below, but due to its composition (presence of apolar methyl groups (-CH₃) ) has water-repellence properties. Moreover, if such layer (3) is extremely thin, its possible humidity absorption is negligible. With the above mentioned process parameters as an example, the optimum speed with which the above film (1) is obtained is 30 ÷ 50 nm/minute.

Mechanical and tribologic tests performed on the coating

(1) have pointed out an excellent adherence between such coating and the substrate (2) . Moreover, the hybrid coating

(1) obtained through the above described deposition process is characterised by optimum steam barrier properties and water-repellence. Should the application require a higher hardness and resistance to abrasion, it is possible to deposit (through direct insertion, pouring or injection moulding), on the layer (4) with barrier property, as an alternative to the water-repellent later (3), a layer of resins/fillers loaded with metal or ceramic powders, having a thickness of some tens of micrometer.

As proof of the excellent water-repellence and humidity barrier properties of the hybrid coating (1) of the present invention, some experimental results are included below, of tests performed on samples of composite materials CFRP on which said coating (1) has been laid.

The first experimental test consisted in measuring the contact angle with a water drop (5) laid on the surface of a CFRP sample coated through PECVD with previously mentioned process parameters as an example.

As shown in Figure 2, the term contact angle of a water drop (designated in the Figure as WCA, "Water Contact Angle") means the angle included between the contact surface (in such case, the coating (1) of the CFRP sample) and a surface that is tangent to the drop and passing by a point (6) in which the contact between drop (5) and contact surface begins. The higher the value assumed by such angle, the lower the surface energy (and therefore the adhesion force of the water drop (5) to the coating (1) of the CFRP sample) and the higher its water-repellence. Such angle is therefore a quantitative expression of coating (1) water-repellence.

Measures have been repeated many times and the mean value of performed measures has been compared with the mean value of the contact angle of a water drop laid on the surface of an identical CFRP sample coated with a layer of protecting paint (ex. polyurethane paint). Polyurethane paints are traditional coatings used for conferring properties of water-repellence and barrier against humidity absorption. Said paints, being based on resins, however tend to absorb humidity and then diffuse it to the composite layers below.

Experimental tests have demonstrated that, while in case of coating with protecting paints, said contact angle assumes a mean value of 80°, in case of coating (1) with PECVD, said contact angle is greater than 90° and can reach values even equal to 120°.

The above stated results demonstrate how the coating (1) of the present invention is more efficient than protecting paints in terms of water-repellence.

The second experimental test consisted in qualitatively evaluating the barrier properties of the protecting film, through a survey of the presence of -OH groups in a CFRP sample initially not containing -OH groups, after a three- month exposure to an environment with a relative humidity grown from 40 % to 80 % in a few days, and remained at 80 % for the whole affected period (three months) . The CFRP sample has been coated with PECVD adopting the previously mentioned process parameters as an example. The presence of -OH groups testifies the absorption of water molecules by any material. The lower the concentration of -OH groups, the lower the absorption of water molecules and the better the humidity barrier properties of the coating laid on the material. In order to establish the presence of -OH groups in the CFRP sample, a spectroscopic analysis with infrared radiations has been employed. The presence of -OH groups determines the disappearance of a negative resonance peak in the transmittance spectrum of the analysis, in a range of wave numbers between 3200 cm^"1 and 3600 cm^"1. It must be remembered that the term wave number means the wavelength reverse. Such number is the number of oscillations that a wave performs in a unit space .

Similarly to what has been made for the first experimental test, also for the second experimental test the survey result has been compared with the result of a similar survey performed on an identical CFRP sample coated with a layer of protecting paint (ex. polyurethane paint).

As shown in Figure 3, after three months of exposure to a "humid" environment, the sample spectrum coated with polyurethane paint has the negative peak (7) that testifies the presence of -OH groups. Such peak instead is absent in the spectrum (shown in Figure 4) that is the result of the analysis performed on the sample coated with PECVD. Again, the above stated results demonstrate how the coating (1) of the present invention is extremely more efficient than polyurethane paints in terms of humidity- barrier .

Another not negligible advantage is given by the lower environmental impact of a PECVD process with respect to the use of traditional coatings such as the above stated paints.

As previously stated, the coating (1) of the present invention has been conceived to be applied to reinforced composite polymer materials to be used as structural elements in mechanical applications that require a high dimensional stability, and a possible field of application of the present invention is the field of operating machines.

As a non-limiting example, among the operating machines that can be made of composite materials with water-repellent and water-permeable coating according to what is described in the present invention, there are machine tools, measuring machines, machines for high accuracy mechanical working, robots and machinery in general.

In particular, among the machine tools, the following are listed as an example:

• milling machines for working dies and sculptured surfaces;

• working centres for working prism-shaped work-pieces; • laser cutting machines;

• grinding machines;

• wood-working and plastic-working machines;

• water-jet machines;

• punching machines.

As proof of geometric distortions caused by a humidity absorption by a structural element of a machine tool, herein below the result is included of an analytical survey performed on a CFRP panel not subjected to any hydrophobic and humidity barrier treatment.

The length variation AL [m] of a panel size produced by efforts of a mechanical and thermal nature and by humidity absorption, can be approximately expressed by the following relationship: ^«-

where: ε is the distortion due to mechanical efforts;

Lo [m] is the initial length of the affected characteristic size; β [%^~1] is the swelling coefficient due to humidity absorption;

AM / Mo [in % units] is the percentage variation of panel humidity concentration due to the humidity absorption effect, with respect to initial panel humidity concentration; a [K^"1] is the thermal expansion coefficient;

AT [K] is the temperature variation with respect to the initial state.

The swelling coefficient β due to humidity absorption is defined as:

namely the relative length variation with respect to the initial length, with respect to the percentage variation of a panel mass unit due to humidity absorption with respect to the initial state.

The thermal expansion coefficient a is the relative length variation with respect to the initial length, due to the temperature variation with respect to the initial state.

Geometric distortions AS produced only by humidity absorption by a structural element can therefore be expressed through the following relationship:

In general, since composite materials are orthotropic, the beta coefficient has different values, according to considered sizes. In particular, if fibres are unidirectional, β_L is defined as the value assumed by the swelling coefficient along the fibre direction and β_τ the value assumed by the swelling coefficient in a direction perpendicular to the fibres.

From experimental tests, it results that, for a CFRP panel, β_L ~ 0 while β_τ assumes values included between 3-1CT³ and β-10^"3 for every Δ% of absorbed humidity.

Geometric distortions ΔS produced by humidity absorption by a structural element are therefore expressed through the following relationship:

By exposing a CFRP panel for three months to an environment with a relative humidity increased from 40 % to 80 % in a few days, and remained at 80 % for the whole affected period (three months) , the percentage variation of humidity concentration in the panel made of composite (with respect to a initial humidity concentration) passes (due to a diffusive effect) from a value of about 0.8% to about 1.4% (equilibrium value) as can be deduced by the experimental graph in Figure 5.

Humidity concentration in the panel matrix grows with time according to the Fick law. The relationship between humidity concentration in the panel matrix under the above stated conditions and time is shown by the experimental graph in Figure 6. In particular, from such graph it is possible to deduce that the humidity concentration in the panel matrix goes from about 0.8 % to the equilibrium value (about 1.4 %) within 50 days (1200 hours) .

Assuming that Lo is 300 mm, from computations,- it is obtained that the humidity absorption determines an expansion included between 0.54 mm and 1.08 mm. If the panel being analysed is used for making a vertical sleeve (8) of a machine tool (9) and the size around which the analysis is focused were axis X or axis Y of said sleeve (8), such distortion would be considered extremely high and would generate an intolerable working error in high accuracy working. By protecting the panel with a coating similar to the one of the present invention, said distortion would not occur.

Claims

1. Hybrid coating (1) for reinforced composite polymer materials (2) used as structural elements in applications where a high geometric stability is required, said hybrid coating having water-repellence and humidity-impermeability properties and being obtained by means of a Plasma Enhanced Chemical Vapour Deposition (PECVD) process.

2. Hybrid coating (1) according to claim 1, characterised in that said composite polymer materials (2) are reinforced with carbon fibres (CFRP) .

3. Hybrid coating (1) according to claim 1, characterised in that said composite polymer materials (2) are reinforced with glass fibres (GFRP) .

4. Hybrid coating (1) according to claim 1, characterised in that said composite polymer materials (2) are reinforced with aramide fibres.

5. Hybrid coating (1) according to claim 1, characterised in that it is silicon-like, namely it is based on silicon.

6. Hybrid coating (1) according to claim 1, characterised in that it is composed of at least two layers (3) and (4) .

7. Hybrid coating (1) according to claim 6, characterised in that said two layers (3) and (4) are laid on said reinforced composite polymer material (2) inside a reactor suitable for the PECVD process.

8. Hybrid coating (1) according to claim β, characterised in that at least one (4) of said layers (3, 4) is humidity- proof.

9. Hybrid coating (1) according to claim 8, characterised in that said humidity-proof layer (4) directly adheres to the composite material (2) .

10. Hybrid coating (1) according to claim 8, characterised in that said humidity-proof layer (4) is laid by using a gas mixture for the plasma treatment composed of hexamethyldisiloxane (HDMSO) as monomer, oxygen and possibly argon.

11. Hybrid coating (1) according to claim 8, characterised in that said humidity-proof layer (4) is laid at a pressure included between 0.1 and 50 Pa.

12. Hybrid coating (1) according to claim 8, characterised in that said humidity-proof layer (4) is laid at a greater power than 200 W so that the process occurs under high fragmentation conditions.

13. Hybrid coating (1) according to claim 10, characterised in that said humidity-proof layer (4) is laid under conditions of a high dilution of the monomer in oxygen (O₂ flow/HMDSO flow > 10) .

14. Hybrid coating (1) according to claim 8, characterised in that said humidity-proof (4) layer has a chemical composition characterised by a low content of carbon and silanol groups (SiOH) .

15. Hybrid coating (1) according to claim 6, characterised in that at least one (3) of said layers (3, 4) is water- repellent .

16. Hybrid coating (1) according to claim 15, characterised in that said water-repellent layer (3) perfectly adheres to the humidity-proof layer (4) below.

17. Hybrid coating (1) according to claim 15, characterised in that said water-repellent layer (3) is laid starting from hexamethyldisiloxane (HDMSO) as monomer, oxygen and possibly argon.

18. Hybrid coating (1) according to claim 15, characterised in that said water-repellent layer (3) is laid at a pressure included between 0.1 and 50 Pa.

19. Hybrid coating (1) according to claim 15, characterised in that said water-repellent layer (3) is laid at a lower power than 100 W so that the process occurs under low fragmentation conditions.

20. Hybrid coating (1) according to claim 15, characterised in that said water-repellent layer (3) is laid under conditions of low dilution of monomer (HMDSO) in oxygen (O₂ flow/HMDSO flow < 10) .

21. Hybrid coating (1) according to claim 15, characterised in that said water-repellent layer (3) has a chemical composition characterised by the presence of apolar metal groups (-CH₃) .

22. Hybrid coating (1) according to claim 1, characterised in that a contact angle of a water drop (5) laid onto said coating (1) is at least 90°.

23. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements of operating machines .

24. Hybrid coating (1) according to claim 23, characterised in that said operating machine is a machine tool.

25. Hybrid coating (1) according to claim 23, characterised in that said operating machine is a measuring machine.

26. Hybrid coating (1) according to claim 23, characterised in that said operating machine is a machine for high accuracy mechanical working.

27. Hybrid coating (1) according to claim 23, characterised in that said operating machine is a robot.

28. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a milling machine for working dies and sculptured surfaces .

29. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a working centre for working prism-shaped work-pieces.

30. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a laser cutting machine.

31. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a grinding machine.

32. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a wood-working or plastic- working machine.

33. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a water-jet machine.

34. Hybrid coating (1) according to claim 24, characterised in that said machine tool is a punching machine.

35. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements of precision guides and bearings with hydrostatic support.

36. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements of precision guides and bearings with pneumatic-static support.

37. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as aeronautic structural elements.

38. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements for motor vehicles.

39. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements for optical telescopes .

40. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements for radio- telescopes .

41. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements for orbital telescopes .

42. Hybrid coating (1) according to claim 1, characterised in that said coating (1) is laid on reinforced composite polymer materials used as structural elements for satellites and orbital stations.