Technical field

The present invention relates to coatings comprising a solid matrix material containing fluid filled capsules obtained by thermal spray methods and a method of applying such coatings on different substrates.

Background/prior art

Coatings are applied to different substrates for protecting the surface or to provide it with required properties. Liquid-solid coatings are coatings that comprise liquid agents (i.e.: lubricants, anti-corrosive agents, etc.) embedded in a solid matrix which is responsible for the mechanical properties of the coating. Self- healing coatings are those coatings that can repair themselves (i.e.: if a crack is produced in the coating, the coating itself repairs the crack). Self-lubricant coatings are those that include lubricants inside. Multifunctional coatings are coatings that have different properties depending on the environment or the mechanical forces acting. An example can be a multilayer coating which has different capsules filled with different liquids, giving different response depending on what is needed: i.e. multilayer coating with liquid lubricants in the top layer and self-healing agents in the inner layer.

Self-healing materials were initially designed for recovering the initial properties of the material releasing the healing agent whenever a failure or damage occurred in the coating. Healing the system as a whole (coating/material integrity and functionality) might be important in many applications for extending the life span of the components. It is a challenge at the same time restoring the material properties while keeping the operational properties of the system.

The first demonstration of an autonomic (without manual intervention) self- healing material was performed by the University of Illinois (USA) in 2001 ; cf. S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, and S. Viswanathan, Autonomic healing of polymer composites. NATURE, 2001 , 409, p 794-797. This material consisted of an epoxy matrix composite reinforced with 10% of microcapsules filled with healing agent and 2.5% catalyst. The healing agent was released when a crack was formed in the material and hit the capsule. The released healing agent came in contact with the catalyst restoring the initial properties of the coating, thus healing the damage.

Since then (just one decade ago) many different approaches have been used for developing a wide range of self-healing materials although most of the developed materials are polymer-based. The review papers, "Self healing materials: a review. R.P.Wool. Soft Matter, vol 4, (2008), 400-418" and "Self- healing: a new paradigm in materials design. M.R.Kessler. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering vol. 221 no 4 (2007), 479-495" mostly include modified polymer/based materials with liquid filled capsules.

However, the specific examples described are filled elastomers, hard elastic propylene, fiber filled composites (for both polymer and concrete matrices), epoxy, low density polyethylene (LDPE), etc., but there are no specific examples of neither metallic nor ceramic self-healing materials.

Most of the self-healing coatings found in the literature are developed for use as paints. The main drawback of paints is their limited mechanical properties, thus avoiding their use for high demanding mechanical components such as machine elements

Most recently, Fraunhofer institute developed in 2009 together with the University of Duisburg-Essen a metallic coating obtained by electrodeposition with homogenously distributed capsules that may be able to repair itself after sustaining damage; cf. "Self-healing surfaces. M.Metzner. Research News.

Fraunhoher Press. No 8 (2009) and "A Metal coating that repairs itself. P.Patel. Technology review (August 2009)". They claim that the technique used for obtaining the coating (electroplating) opens up new applications in different markets such as construction, car manufacturing, etc. Metals or alloys including copper, zinc and nickel are obtained, but theoretically it would be possible to obtain coatings from any metal that can be electroplated. However, at present the self-healing property of the coatings has not been yet demonstrated.

Thermal spraying is a process in which molten, semi-molten or solid particles are ejected towards a surface (substrate) and deposited layer by layer. Coatings can be generated if the particles can plastically deform after impacting the substrate, which may only happen at sufficiently high speed and or tern peratu red. Thermal spraying is widely used to produce coatings that are wear resistant, corrosion resistant, abradables, etc. and it is possible to spray metallic, ceramic and polymeric materials or a combination of them. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame).

Thermal spraying can provide thick coatings (approx. thickness range is 20 micrometers to several mm, depending on the process and feedstock), over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition. Coating materials available for thermal spraying include pure metals, alloys, ceramics, plastics and

composites. They are fed in powder or wire form, heated to a molten or

semimolten state and accelerated towards substrates in the form of micrometer- size particles. Combustion, detonation or electrical arc discharge is usually used as the source of energy for thermal spraying. Resulting coatings are made by the accumulation of numerous sprayed particles. The surface may not heat up significantly, allowing the coating of flammable substances.

Thermal spraying is a technology that can be divided in different techniques depending on how the jets or flames are generated. A classical classification is the following:

• Electric discharge (which includes Atmospheric Plasma Spray - APS,

Vacuum Plasma spray and Arc Spray).

• Detonation (D-Gun).

• Combustion (Flame Spray, High Velocity Oxy-Fuel - HVOF).

-Atmospheric Plasma Spray (APS): An electric arc is generated between an anode and a cathode. The gas passing through them is ionized and creates a high temperature flame (temperatures in the flame can be above 10,000 °C) melting or softening the material, which is fed radially in form of powder inside the flame. The speeds of the flame are up to 800 m/s which makes almost impossible for the material inside the flame to reach the maximum temperature of 10,000 °C.

-Vacuum Plasma Spray: In this technology the APS gun is used in a vacuum chamber, thus the oxidation of the sprayed materials is avoided/reduced due to the lack of oxygen in the chamber. -Arc spraying: In this technique an electric arc is produced between two wires, which are consumed forming a liquid molten droplet, which is accelerated towards the substrate. The wires can be made of the same material or different materials, thus different alloys can be built as coatings.

-Detonation Gun: In this spraying technique a mixture of fuel, oxygen and powder is fed into the gun barrel. The gases are ignited by a spark and explode, accelerating the powder towards the substrate. After each explosion the barrel is cleaned with an inert gas before the next batch of gases and powder is fed into the barrel.

-Flame spray: The flame is generated by the combustion of a fuel mixed with oxygen. The temperatures and velocities of the flame depend on the fuel used and the mixing ratios. In this technique the material (in form of wire, rod or powder) is fed into the flame (axially or radially). The flame heats up the material and the particles are accelerated inside the flame and propelled towards the substrate. The particles are plastically deformed when impacting the substrate, building up the coating layer by layer.

-High Velocity Oxy-Fuel (HVOF): HVOF is similar to the Detonation process, except that the gases and the powder are fed continuously, thus forming a continuous flame.

In addition cold spray and High Velocity Air Fuel can be mentioned.

Use of solid lubricants (PTFE, h-BN, graphite, MoS ₂ , etc.) embedded in the matrix has been widely used for obtaining self-lubricated coatings obtained by thermal spray technologies. The use of solid lubricants has been the only way to produce thermal sprayed self-lubricated coatings. Production of thermal spray coatings containing liquid lubricants or other liquid agents has not yet been achieved due to the complexity of keeping a liquid in a solid matrix during the spraying process. The main challenge for producing coatings containing fluid filled capsules by thermal spray is to avoid any damage of the capsules during the production process since the temperature of the flame can damage the capsule material, and thus, burn the fluid.

Capsules containing different fluid agents can be produced in different ways, among them: • Preparation of nanoemulsions, nanoparticles and nanocapsules by a miniemulsion process.

• Preparation of polyacrylate- and polystyrene-based capsules by a

miniemulsion process.

• FunzioNano™ : Patented technology for producing multifunctional organic inorganic hybrid nanoparticles.

• Preparation of hybrid and inorganic micro- and nanocapsules by

emulsification processes (membrane and stirring).

Cermets such as WC and Cr ₃ C2-based, Mo, Ni-based alloys, etc are commonly used in demanding conditions for protecting surfaces against wear or for tailoring the coefficient of friction of moving parts. Their use includes critical components such as airplanes landing gears, pistons and cylinders for engines, valves and bearings among others. Lubricants are required and used in these applications to minimize wear and friction between the moving parts. However, lubricant leakages or lubricant supply failures during operation can lead to lubricant starving conditions, eventually resulting in failure and downtimes and both energy and economical losses. Failures and downtimes in industrial applications require maintenance periods that can vary in length depending on the component. Maintenance operations of mechanical components are known to be risky and pose health and security issues for companies.

One object of the present invention is to obtain thermally sprayed self- lubricating coatings which aim at reducing the maintenance operations and downtimes by avoiding unlubricated conditions in rotating/sliding parts by releasing the lubricating agent contained in the coatings when starving situations might occur.

Further it is an object with the present invention to obtain thermally sprayed coatings containing fluid filled capsules and a method for preparing the coatings without damaging the capsules. Another object with the invention is to produce a new generation of thermally sprayed coatings containing nano- and/or

microcapsules filled with a selected fluid which can tailor the properties of the coating (i.e.: anti-abrasion components, magnetic components, etc.). Short summary of the invention

In a first aspect the invention provides a method for preparing a coating comprising a matrix material and capsules filled with a fluid onto a substrate by using a thermal spray method. The method comprises applying the matrix material on the substrate by using a thermal spray technique generating a flame or a jet. A liquid solution containing fluid filled capsules is simultaneously injected by a separate device into the flame for obtaining an homogeneously distributed capsules in the matrix. The separate device injecting the capsules inside the flame should allow full control on the exact injection position for avoiding the burning of the capsules and or the fluid. Consequently, the matrix material and the capsules are uniformly spread on the substrate, and thus, obtaining the coating.

A second aspect of the invention relates to a coated substrate wherein the substrate is coated with a coating material comprising a matrix material and capsules filled with a fluid prepared by feeding and applying the matrix material on a substrate, a liquid solution containing the capsules filled with a fluid agent is simultaneously injected into the flame.

Figures

Figure 1 shows a schematically drawing of a typical capsule used in the present invention.

Figure 2 shows a flame spraying set up for producing coatings containing capsules filled with a fluid agent.

Figure 3 shows an arch spraying set up for producing coatings containing capsules filled with a fluid agent.

Detailed description of the invention

In theory all combinations of thermal spray materials and capsules are possible to use for producing coatings with homogenously distributed capsules inside. This also includes multifunctional coatings including capsules with different fluids.

In the following coatings containing liquid filled capsules will be discussed, but the method can also be applied for coatings containing gas-filled capsules. Some embodiments of the invention will now be described more in detail. The wall thickness and the size of the capsules can be modified and tailor made depending on the requirements needed in the coating or in the production process (thicker walls can be used to minimize breakages during the spraying and different wall materials might be used to achieve specific properties in the coating).

A schematic cross-sectional view of a typical capsule used in the present invention is shown in Figure 1 . Thickness of the walls and the amount and type of liquids is only indicative since they can be tailored. The design of the capsules depends on the functionalized properties that are required on the coatings. The capsules can be made with different materials including polymers, fluor-based materials, silicon oxide, etc. and can be filled with different liquids depending on the requirements.

Typical materials used in thermal spraying include metals (i.e.: Ni-based alloys, aluminum, stainless steel, etc), polymers (polyurethane, polyester, etc.) and ceramics (zirconium oxide, aluminum oxide, chromium oxide, etc.) or cermets (WC-Co, WC-CoCr, Cr ₃ C ₂ -NiCr, etc).

Not only polymeric matrix coating can be produced by present invention, also metallic matrix coatings are obtainable by using the same method.

A homogeneous dispersion of liquid filled capsules in the coating is obtained because thermal spray techniques are possible to use.

Liquid agents can be selected from lubricants, anti-corrosive agents, anti- abrasives, etc. The liquid agents used in the capsules can also change the properties of the systems (electrical, thermal, acoustical, magnetic) by selecting proper liquid.

Figure 2 shows a flame spraying process with two independent powder and or slurry containing capsules feeders for producing the coatings containing fluid filled capsules.

The flame spray gun is fed with polymer, for example nylon, in a

conventional way. The polymer may be in powder form and is fed from a reservoir which is pressurized into the flame. The powder is fed axially into the combustion chamber where the fuel and oxygen are introduced. The main challenge is to not destroy the capsules during the spraying process. The temperature of the flame is very high (in the range of 2500-3000 °C) and the capsules, even passing at very high velocity, cannot withstand this temperature. Besides, the flame speed (in the range of 100 m/s) would promote their breakage due to the high-speed impact onto the substrate. In order to avoid the melting and breakage of the capsules the surface temperature of the capsules is preferably below 200 °C. This may be achieved by injecting radially (or axially) into the flame a liquid solution (slurry) containing the capsules. The liquid solution can be based on water, alcohols or any other suitable solvent.

The slurry injection is performed with a nozzle which has to be placed very close to the substrate, thus avoiding the thermal input from the flame. Besides, the injector can be tilted for minimizing the thermal effect of the flame.

A typical spray distance for the thermal spray torch lies between 100 mm and 300 mm. The thermal spray parameters can be selected based on the characteristics of the feedstock material used as a matrix for producing the coatings. The co-spraying of the capsules does not have an influence on the spraying parameters of the matrix. This enlarges the flexibility for producing coatings with different matrices and materials. By the present method it is possible to maintain the shape and size of the capsules and a good distribution of the capsules is obtained. This will allow controlling the mechanical and tribological properties of the coating as well as other inherent properties of the coating (corrosion resistance, thermal, acoustic, etc).

The injection of the capsules may be varied by selecting different nozzles in the spraying system.

Figure 3 shows an arc spraying process with independent capsules feeder for producing the coatings containing fluid-filled capsules. The process/set-up is similar to the flame spraying set-up.

In the following an experiment related to an embodiment wherein polymeric matrix coatings obtained by flame spray will be described more in detail. A person skilled in the art would be able to perform the necessary adjustments depending on the coating composition and the substrate to be coated.

Example 1 : Polymeric matrix coatings

Preparation of liquid filled capsules

In this example polyurea microcapsules filled with liquid lubricant were synthesized by miniemulsion polymerization process. Oil-in-water miniemulsions were prepared by emulsifying an oil-phase (20% v/v) in an aqueous phase by means of an ultrasonifier (Labsonic 2000) in small-scale experiments and a homogenizer (15MR-8TA, APV GAULIN Inc.) in large scale experiments. The oil- phase consisted of isophorone diisocyanate and lubricant in various ratios.

Table 1 : Polyurea microcapsule synthesized using various amounts and types of lubricants.

Mereta 46 (Statoil); ² silicone oil AP 100 (FLUKA); ³ 200® Fluid l OOOcs (Dow Corning)

The aqueous continuous phase consisted of a 10 g/l polyvinylalcohol (Celvol 523) stabilizer solution. Guanidine carbonate (NCO/NH ₂ molar ratio of 1 ) was added to this premade miniemulsion and interfacial polymerization performed at 70°C during 16 hours. The size and size distribution of the prepared

microcapsules was determined by means of dynamic light scattering (DLS) using a LS230 Coulter Counter (Coulter Instruments).

Thermal spray process

The substrate material used was ST-52 carbon steel, which was cleaned and degreased in an ultrasounds bath. The cleaned substrate was then grit blasted using alumina grade 24 just before the spraying process and cleaned with compressed air. The roughness (Ra) of the substrates was about 3 μηη. A commercial Nylon-1 1 powder (ET-1 1 E+C Evertuff, polyamid-1 1 hereafter called Nylon) and the aqueous suspension containing the capsules were sprayed using a flame spray system with a Eutectic Terodyn 3500 gun (Eutectic Castolin). In order to confirm the viability of thermal spray process a coating consisting only of liquid filled capsules (without any matrix) was also obtained using optimized parameters for the Nylon.

The suspension containing the capsules was fed into the flame using a feeder located close to the nozzle of the gun allowing the control of the total amount of capsules, injection angle and position. The slurry containing the capsules was sprayed independently into the flame with a specially designed feeder in such a way that the capsules were co-injected in the flame with the nylon powder. The aim with this co-injection system was to avoid the degradation of the capsule shell material during the spraying process and assuring that the aqueous solution was completely evaporated during the thermal spray process.

Lubricating properties of the coatings

The lubricating properties of the coatings were studied using a conventional reciprocating tribometer (Resmat, Canada). The tests consisted of sliding a 4.76 mm in diameter AISI 316 steel ball rubbing against the coatings and measuring the Coefficient of Friction (CoF). The test parameters are shown in table 2.

Table 2: Parameters of the tribological test

In order to compare the effect of the capsules containing the lubricant filled capsules, pure nylon coatings and capsule-containing coatings were tested.

The samples were studied using a Field Emission Scanning Electron Microscope (SEM) Zeiss Ultra 55 (Cambridge, UK). The coatings were carbon sputtered for having a better electrical conductivity and for improving the SEM images. The conductive carbon layer avoids any degradation of the capsule shell due to the voltage used in the electron beam of the SEM.

The spraying parameters of the samples of this example correspond to the typical parameters found for nylon. The gases used for spraying the nylon were propylene and compressed air. The spraying distance was kept constant between 150 and 250 mm and air was used as carrier gas.

The nylon powder was fed into the flame spraying gun through the feeding system of the equipment (axial injection) while the capsules were injected into the flame using an independent powder feeder. The capsules were injected in a water- based solution with the aim at protecting the capsules from the high temperatures of the flame. The amount of the capsules injected inside the flame was controlled using a carrier gas (air).

The nozzle used for spraying the slurry containing the capsules played a very important role in the process and it also affected the coating properties. After several trials with different nozzles it was concluded that a cone shape nozzle gives the best results. A porous coating was obtained when injecting the capsules with a stream shape cone and a non-porous coating was obtained with the cone shaped nozzle. It was also possible to observe that the capsules were not homogenously distributed when using the stream shape cone.

Coating produced only with capsules

This coating was produced to confirm the spraying performance of the liquid filled capsules. In this particular case the flame spraying gun was not fed with nylon, and only the capsules in the water-based solution were injected in the flame. All the liquid was evaporated during the spraying process and no evidence of water was found in the coatings.

The surface was studied by SEM, and it was observed that few capsules were deformed plastically and some coalesced during the thermal spray process.

However, the microstructure of the coating reveals good cohesion and adhesion between the capsules.

A very simple scratch test done manually showed the release of liquid lubricant.

This test confirms that the lubricant is still available inside the capsules and it is easy to release when the capsules are damaged at very low loads.

Nylon coating containing liquid-filled polyurea capsules

The produced coatings consisted of a Nylon-1 1 matrix containing the lubricant filled capsules. The feedstock material consisted of a Nylon-1 1 powder with particle size >10Όμηη.

The capsules and the nylon powder were co-sprayed. The nylon and the slurry containing the capsules were injected in the flame independently to avoid a high thermal input into the capsules. The nylon powder was fed axially in the

combustion chamber of the gun, whereas the capsules were fed externally and closer to the substrate material with an independently controlled injecting system. The goal of the co-spraying process was to obtain a homogeneous distribution of the capsules inside the nylon matrix. The feeding rate of the capsules must be adjusted to avoid any excess of liquid containing the capsules during the spraying process which could enhance the pores formation in the coating.

A homogeneous dispersion of the capsules on the surface and throughout the coating was obtained. The microscopic analysis of the cross section of the coatings confirmed the presence of the liquid-filled capsules inside the nylon coating. The capsules inside the coating survived the thermal spray process. It was observed that the shape and size of the capsules was similar to the starting material and were well distributed in the matrix.

Friction tests

Friction tests of pure nylon coatings and capsule-containing coatings were carried out to characterize the lubricating effect of the capsules. The coatings were tested for 30 minutes. Table 3 gives the average value of CoF of the tested coatings. Coatings composed only of Nylon gave a CoF of 0.47 while the nylon coatings containing capsules showed a CoF down to values between 0.12 and 0.19 for PAO-containing capsules. The CoF varied depending on the amount of lubricant in the capsules. Three different capsules containing different amounts (30, 50 and 70%) of PAO lubricant were used for producing the coatings.

Table 3: Average coefficient of friction (CoF) for a layer consisted solely of capsules, a pure nylon coating and capsule-containing coatings.

Coatings Normal load Average CoF Length of the

[N] test (min)

Nylon coating 5 0.47 30

10 0.47 30

Capsule-containing 5 0.19 30

coating 10 0.17 30

(30% PAO capsules)

Capsule-containing 5 0.14 30

coating 10 0.14 30

(50% PAO capsules)

Capsule-containing 5 0.12 30

coating

(70% PAO capsules) These tribological tests show the lubricating effect of the liquid containing capsules compared to a Nylon coating without liquid filled capsules. Therefore, the lubricant inside the capsules reduced the friction without affecting the mechanical properties of the coatings. The CoF was reduced due to the release of the lubricant into the contact. Besides, the capsules showed good adhesion with the coating matrix, therefore it was not possible to detach the capsules from the coatings without damaging them. The reciprocating movement of the steel ball on top of the capsules wears out the external wall of the capsules, thus the lubricant was released to the system.

Example 2: Metallic matrix coating

An arc spraying gun was used to produce zinc based coatings. In this technique a spraying distance between 50mm and 300mm was used. The schema of the process is shown in figure 2. The same concept as used in the flame spray system was tried and proven in the arc spray system. The capsules were introduced in the gas stream using the same capsule feeder used in the flame spraying process. Since arc spray is a more energetic technique than flame spray (in terms of speed and temperature of the flame), this proves that the concept is possible in a wide range of materials and thermal spray techniques.