FIELD OF THE INVENTION The present invention relates to coating technology. Especially the present invention relates to protective coatings, and to methods for protecting substrates from the environment.

BACKGROUND OF THE INVENTION

Objects which are utilized in demanding environmental conditions often require e.g. mechanical or chemical protection, so as to prevent the environmental conditions from affecting the object. Examples of such objects include sensors, electronic components, and objects with a functional surface such as mirrors or solar cells. Protection to the object can be realized by applying a coating on the surface of the object, i.e. on the substrate. The known art dis- closes protective coatings for various purposes; hard- coatings that protect the substrate from mechanical effects, diffusion barriers for protection against chemical effects, thermal barriers (or insulators) and electrical insulators. Protective coatings to be efficient often have to be relatively thick. For example, the protective properties of a diffusion barrier, thermal barrier or an electrical insulator improve in proportion to the thickness of the coating with a given coating material. For this reason, among others, protective coatings of the prior art are fabricated with methods possessing a relatively high growth rate; physical vapour deposition (PVD), chemical vapour deposition (CVD), or liquid or aerosol based deposition methods. E.g. patent application publication US2009087585 discloses processes for fabricating a barrier layer com- prising titanium nitride and aluminum using both the PVD and the CVD methods.

A problem with the aforementioned deposition methods of the prior art, and with the protective coatings fabricated with these methods, is that the deposited coating material includes small pores and pin holes as defects. These defects are often called residual porosity. These properties of the coating may result in serious degradation of the barrier proper- ties of the coating. The pores and pin holes in e.g. a chemical barrier may enable diffusion of material through these defects from the environment onto the substrate which the coating is intended to protect. Pores and pin holes may also e.g. decrease the thresh- old of breakdown caused by an electrical field or deteriorate the mechanical properties of the protective coating.

PURPOSE OF THE INVENTION A purpose of the present invention is to reduce the aforementioned technical problems of the prior art by providing a method, use for the method and a coating for protecting a substrate from the environment .

SUMN[ARY OF THE INVENTION

The method according to the present invention is characterized by what is presented in independent claim 1. The structure according to the present invention is characterized by what is presented in independent claim 10.

The use according to the present invention is characterized by what is presented in claim 14. A method according to the present invention, for protecting a substrate from effects caused by an interaction of the substrate with the environment, by depositing on the substrate a first layer comprising a top surface oriented essentially parallel to the surface of the substrate, comprises the step of deposit- ing a second layer on the first layer by exposing the first layer to alternately repeated surface reactions of two or more different precursors, to at least partially fill pores in the first layer, the pores opening through to the top surface, with the material of the second layer, such that the second layer conforms to the shape of the surface of the pores, wherein the first layer consists predominantly of a compound of titanium, aluminum and nitrogen, and the material of the second layer is predominantly metal oxide. A protective coating on a substrate, according to the present invention, for protecting a substrate from effects caused by an interaction of the substrate with the environment, comprises a first layer having a top surface oriented essentially paral- IeI to the surface of the substrate. The first layer comprises pores opening through to the top surface. The protective coating comprises a second layer on the first layer, the material of the second layer at least partially filling the pores of the first layer, such that the second layer conforms to the shape of the surface of the pores. The first layer consists predominantly of a compound of titanium, aluminum and nitrogen, and the material of the second layer is predominantly metal oxide. According to the present invention a method of the present invention is used to protect the substrate from effects caused by a chemical interaction of the substrate with the environment.

In this context "a pore" should be understood as any hollow region within a layer, including a pore, a pin hole or the like. The pores referred to in this document should be understood as microscopically small defects on or inside the layer. These defects are part of the residual porosity of the layer, which results from the deposition method employed for the layer.

The present invention provides a simple way to fabricate coatings that act as efficient barriers against material transfer through the coating, and additionally possess good mechanical strength and durability. The method and the coating of the present invention efficiently inhibit chemical reactions from occurring between the environment and the substrate as material is not able to diffuse or otherwise drift through the coating from the environment onto the surface of the substrate under the coating. The at least partial filling of the pores in the first layer also improves the mechanical stability of the coating making it more durable and improving the mechanical protection of the substrate.

In one embodiment of the invention, depositing the second layer comprises exposing the first layer to alternately repeated essentially self- limiting surface reactions, for depositing the second layer by atomic layer deposition (ALD) . When the second layer is deposited by alternately repeated essentially self-limiting surface reactions in a process like ALD, the second layer can get very conformally deposited even inside very small pores or pin holes and the thickness of the second layer is very uniform.

In one embodiment of the invention the effects caused by the interaction of the substrate with the environment are caused by a chemical interaction. In another embodiment of the invention the effects caused by the interaction of the substrate with the environment are caused by an electrochemical interaction. In yet another embodiment of the invention the effects caused by the interaction of the substrate with the environment are the effects of corrosion. In one embodiment of the invention, the thickness of the second layer is below 20 nanometres (nm) . By depositing only a very thin second layer on the first layer it may be possible to impart the good protective properties to the coating while still preserving the functionality of the underlying second layer. This may be very useful in applications such as solar cells, or on optical coatings where a very thick second layer would significantly affect the optical properties, e.g. reflectance, of the layered structure .

The material of the second layer is predominantly metal oxide. In one embodiment of the invention the material of the second layer is predominantly alu- minum oxide. In one embodiment of the invention, depositing the second layer comprises exposing the first layer to alternately repeated surface reactions of trimethylaluminum and water, to deposit a layer of aluminum oxide. In one embodiment of the invention, the substrate consists predominantly of metal. In one embodiment of the invention, the substrate comprises a cutting tool, for example a cutting blade. In one embodiment of the present invention, the substrate comprises a cutting tool comprising metal.

In another embodiment of the invention, the chemical interaction is an interaction causing corrosion. The method and the protective coating according to the present invention is well suited for protecting metal substrates from corrosion as coatings where the second layer conforms to the shape of the surface of the pores in the first layer can efficiently reduce diffusion of water (moisture) and/or oxygen through the coating onto the substrate. Aluminum oxide and metal oxides in general are materials that possess good barrier properties against water and oxygen. These materials are also well suited for deposition by alternately repeated surface reactions, e.g. by ALD.

The first layer consists predominantly of a compound of titanium, aluminum and nitrogen. In one embodiment of the present invention the first layer consists predominantly of TiAlN. In one embodiment of the invention the first layer consists predominantly of Tii- _X A1 _X N (0<x<l) . In one embodiment of the present invention the ratio of titanium and aluminium is var- ied. In one embodiment of the present invention the ratio is 40 % aluminum and 60 % titanium. In another embodiment of the present invention the ratio is 70 % aluminum and 30 % titanium.

A compound of titanium, aluminum and nitro- gen, e.g. TiAlN, as the first layer on the substrate can efficiently serve the purpose of a hard coating mechanically protecting the substrate. Applying the second layer on this first layer can make the coating according to this embodiment of the invention also e.g. a good corrosion barrier by clogging the pores in the first layer.

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to- gether to form a further embodiment of the invention. A method, a structure or a use, to which the invention is related, may comprise at least one of the embodiments of the invention described hereinbefore.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the present invention will be described in more detail with exemplary embodiments by referring to the accompanying figures in which

Fig. 1 is a schematic illustration of a pro- tective coating of the prior art, Figs. 2 to 6 schematically illustrate the method for fabricating a protective coating according one embodiment of the present invention,

Fig. 7 schematically illustrates the protec- tive effect of the protective coating of the prior art,

Fig. 8a and Fig. 8b schematically illustrate the protective effect of the protective coating according to one embodiment of the present invention, Fig. 9 illustrates a pulsing sequence for an

ALD cycle in the fabrication of a protective coating according to one embodiment of the present invention,

Fig. 10 presents experimental data from electrochemical corrosion measurements, and Fig. 11 presents data from wear test measurements .

For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components. The structure of Fig. 1 comprises a substrate

1, and on the substrate 1 there are a first layer 3 and a second layer 5. The first layer 3 is fabricated with a conventional deposition method such as PVD or CVD, and comprises pores 7 opening through to the top surface 4 of the first layer 3. On the first layer 3 there is a second layer 5, which is an essentially planar layer lying over the pores 7 of the first layer 3; i.e. the second layer 5 is not able to penetrate into the pores 7 to clog them but only covers them on the top surface 4 of the first layer 3. The second layer 5 may also comprise pores 9 when it is fabricated with the conventional PVD or CVD methods.

The coating comprising the first layer 3 and the second layer 5 in Fig. 1 is not able to effi- ciently act as a protective coating for the substrate 1 for two reasons. Firstly the pores 7 in the first layer 3 make the first layer 3 mechanically fragile. Secondly the pores 7 in the first layer 3 and the pores 9 in the second layer 5 may act as passages for material, e.g. molecules in the gas phase, to drift (e.g. diffuse) through the coating onto the substrate 1. This exposes the substrate 1 to effects caused by a chemical or an electrochemical interaction of the substrate 1 with the environment, e.g. an interaction leading to corrosion of the substrate 1.

Atomic Layer Deposition (ALD) is a well known method for depositing uniform thin-films over substrates of various shapes, even over complex three dimensional structures, with excellent conformality . In ALD the coating is grown by alternately repeating, essentially self-limiting, surface reactions between a precursor and a surface to be coated (the substrate) . Therefore the growth mechanism in an ALD process is commonly not as sensitive as in other coating methods to e.g. the flow dynamics inside a reaction chamber which may be a source for non-uniformity, especially in coating methods relying on gas-phase reactions. In an ALD process two or more different reactants (precursors) are introduced to the reaction chamber in a sequential, alternating, manner and the precursors adsorb on surfaces, e.g. on a substrate, inside the re- action chamber. The sequential, alternating, introduction of precursors is commonly called pulsing (of precursors) .

In between each precursor pulse there is commonly a purging period during which a flow of inert gas, often called the carrier gas, purges the reaction chamber from e.g. surplus precursor and by-products resulting from the adsorption reactions of the previous precursor pulse. A film can be grown by an ALD process by repeating several times a pulsing sequence comprising the aforementioned precursor pulses and purging periods. The number of how many times this se- quence called the "ALD cycle" is repeated depends on the targeted film, or coating, thickness.

The construction of an apparatus suitable for depositing material by ALD, i.e. the construction of a conventional ALD reactor system, is obvious for a skilled person, and the construction will therefore not be discussed.

The series of figures from Fig. 2 to Fig. 6 illustrates how a protective coating according to one embodiment of the present invention can be fabricated on a substrate 1. The series of figures presents the cross section of the coating structure at different stages of the process in chronological order, starting from Fig. 2 which presents the bare substrate 1. In Fig. 3 the substrate is coated with a thick first layer 3 using e.g. PVD or CVD. The first layer comprises pores 7 opening through to the top surface 4 of the first layer 3. In Fig. 4 the deposition of the second layer 5 has begun by exposing the substrate 1 to alternately repeating surface reactions of two or more different precursors. In this embodiment of the invention ALD is used to deposit the second layer 5.

Fig. 4 schematically presents how the second layer 5 gets deposited conformally into the pores 7 in the first layer 3. This is a result of the growth mechanism of the second layer 5 which is governed by surface reactions between a vaporized (or gaseous) precursor and the surface of the structure onto which the layer is deposited. As the first layer 3 is porous there may exist passages for the precursor molecules to drift into pores 7 significantly below the top surface 4 of the first layer 3. These passages are not shown in the figures. The material of the second layer 5 may therefore be able to penetrate well into the po- rous structure of the first layer 3, contrary to conventional deposition methods, and to fill and clog pores 7 in the first layer 3. As growth of the second layer 5 is continued some pores 7 in the first layer 3 may get completely filled with the material of the second layer 5. This is illustrated schematically by Fig. 5 and Fig. 6. The material of the second layer 5 penetrating into the pores 7 of the first layer 3 can increase the durability of the first layer 3 as voids causing fragility to the first layer 3 are filled. Additionally barrier properties of the first layer 3 are sig- nificantly improved as the material of the second layer 5 clogs or closes off passages in the first layer 3 through which material, e.g. molecules in the gas phase, may diffuse or otherwise drift from the environment through the first layer 3 onto the substrate 1. Therefore the presented coating surprisingly provides excellent mechanical and chemical protection for the substrate 1 and can be simultaneously used e.g. as a corrosion barrier and a hard coating.

In protective coatings of the prior art, the second layer 5 resides on top of the pores 7 and only covers the pores 7 and supports the first layer 3 from its surface. This is illustrated in Fig. 7. The protective coating structure and the method for fabricating the protective coating according to an embodiment of the present invention can, on the contrary, provide excellent protection to the substrate 1 even if the second layer 5 is very thin as the second layer 5 penetrates into the pores 7 and reinforces the first layer 3 from within. Depending on the size of the pore 7 even a thin second layer 5 is able to fill up the pore 7 as the conformally deposited second layer 5 grows on all sides of the pore 7 (Fig. 8a) . As the growth of the second layer 5 continues, the deposits growing on all sides of the pore 7 eventually merge, and the void in the middle of the pore 7 closes up (Fig. 8b) . This blocks (or clogs) a possible passage for molecules through which they could drift (e.g. diffuse). Looking at Fig. 7, Fig. 8a and Fig. 8b it can therefore be appreciated that in order to achieve the same barrier effect, a thinner second layer 5, compared to protec- tive coatings of the prior art, on a porous first layer 3 is sufficient when the protective coating of an embodiment of the present invention is employed. Furthermore, in the case that the second layer 5 is deposited by a process which relies on alternately re- peated, possibly self limiting, surface reactions of two or more different precursors according to an embodiment of the present invention, the second layer 5 itself possesses less residual porosity (less pores 9) than a layer in the protective coatings of the prior art. This further decreases the probability of molecules diffusing through the protective coating according to an embodiment of the invention. Especially ALD processes may result in layers essentially free of pores 9 (e.g. essentially free of pin holes or resid- ual porosity) .

EXAMPLES

A protective coating according to one embodi- ment of the present invention was fabricated by first coating a 2 micrometers (μm) thick first layer 3 of titanium-aluminum-nitride (TiAlN) on an M2 steel substrate 1 by PVD. This coating process was a conventional PVD process and can be readily repeated by the skilled person.

Subsequently the M2 steel substrate 1 having the first layer 3 of TiAlN on its surface was inserted into the reaction chamber of a widely used P400 ALD tool (from Beneq Oy, Finland) . After pumping the reac- tion chamber down to a pressure of about 1 mbar, the substrate 1 was heated to a temperature of 300 °C . After this the substrate was alternately exposed to va- porized trimethylaluminum (TMA) and vaporized deion- ized water. In between each precursor exposure a purging period of one second was employed to purge the reaction chamber with nitrogen gas from e.g. surplus precursor and reaction by-products. The ALD cycle, consisting of a 0.5 second exposure to TMA, followed by a one second purging with nitrogen gas (Pl), followed by a 0.5 second exposure to deionized H ₂ O, followed by another one second purging with nitrogen gas (P2), is presented in Fig. 9. This ALD cycle was repeated 40 times to grow 4 nanometres (nm) of aluminum oxide (AI2O3) on the titanium-aluminum-nitride (TiAlN) PVD-deposited layer, through alternately repeated essentially self-limiting surface reactions. The afore- mentioned ALD process is widely used and can be readily optimized by a skilled professional for different ALD tools in light of this disclosure.

The barrier properties of the exemplary structure with an M2 steel substrate 1, a PVD-grown TiAlN first layer 3, and a thin ALD-grown AI2O3 second layer 5, were electrochemically tested by introducing the structure to a corroding water solution of NaCl (0.15M) . The corrosion of the exemplary structure according to an embodiment of the invention was compared with the corrosion of an otherwise identical structure which did not have the ALD-grown AI2O3 second layer 5 on the PVD-grown TiAlN first layer 3, and with a bare M2 steel substrate 1. The results of the electrochemical corrosion measurements from the three structures are presented in Fig. 10. The graphs of Fig. 10 present the electric potential "E" for the three test structures versus the cell current density "i" of the electrochemical cell. The electric potential of the test structure was referenced to an Ag/AgCl reference- cell.

As can be observed from the Fig. 10, the corrosion rate of the bare M2 steel substrate 1 (M2) was significantly higher than the corrosion rate of the structure having the PVD-grown TiAlN first layer 3 on the M2 steel substrate 1 (M2+TiAlN) . The graphs show that the protective coating according to an embodiment of the present invention on the substrate 1 (M2+TiAlN+ALD) resulted in clearly the lowest corrosion rate. Additionally a passivation zone, where the cell current does not significantly change as a function of the electric potential, can be observed for the embodiment of the invention. The width of this passivation zone is about 550 mV. Hence, the addition of the thin ALD-grown AI ₂ O ₃ second layer 5 significantly decreased the corrosion rate further compared to the "M2+TiAlN"-structure. TiAlN coatings can commonly be used in the cutting industry, where these coatings exhibit properties enabling the use of higher working temperatures and thus higher cutting speeds. In a TiAlN coating the corrosion resistance is due to Al-segregation appear- ing on the surface during the cutting process in high temperatures. When the TiAlN layer is coated with an AI2O3 layer according to one embodiment of the present invention, one does not need to wait for this natural Al-segregation to appear. Fig. 11 presents the results from wear-test measurements with two different coatings. One of the coatings had a TiAlN first layer alone and the other coating had the TiAlN first layer coated with an AI ₂ O ₃ second layer according to an embodiment of the present invention. The wear tests were carried out using a tribometer in a ball-on-disc configuration. The tests were performed with a constant sliding velocity (0.5 cm/s) and in conditions of dry friction and mild wear. The duration of the test was restricted to 30 minutes, which equals to a sliding distance of 900 m. The counterpart used was a pin of tungsten carbide (3.2 mm diameter) and the load employed was F _n = 3.3 N. The sensor used allowed a high precision survey (up to 1.2 mN) of the applied load and of the tangential force, leading to the friction coefficient value.

As can be observed from the data of Fig. 11 the fric- tion coefficient of the coating with the TiAlN first layer alone stabilizes initially around 0.2 and tends to rise to 0.6 after 1400 s. Further, as can be observed from the data of Fig. 11 the AI2O3 second layer on the TiAlN first layer provides the additional ad- vantage of stabilizing the friction coefficient already in the beginning of the performed wear test, compared to the TiAlN first layer alone. The additional advantage of stabilized friction coefficient was achieved without changing the general wear behav- iour of the coating structure. I.e. the AI2O3 second layer fills the pores or the cavities of the first layer and thereby improves the corrosion resistance.

Naturally many other precursors suitable for

ALD exist for various coating materials, in addition to the ones disclosed in the examples above. The precursors and the corresponding chemistries and processes suitable for synthesizing the materials by ALD are obvious for a skilled person in light of this disclosure and will not be listed in this context. Dif- ferent ALD process chemistries can be found in e.g. patent application publications # US2005277780 and US2004043149 which are added as references herein. There are many examples of materials for which there exist known ALD processes. These materials include, but are not limited to, hafnium oxide, aluminum oxide, titanium oxide, tantalum oxide, niobium oxide, zinc oxide, titanium nitride, tantalum nitride, platinum, and many more. Many combinations of the aforementioned materials may also be deposited by ALD, even in a sin- gle process. In light of this disclosure, the processes for combining the various materials, e.g. in a nanolaminate layer, can be readily prepared by the skilled professional. In light of this disclosure, the process parameters in an ALD process, such as temperature and pressure, can also be readily varied and optimized by a skilled person for a specific deposited material and to accommodate for e.g. a particular substrate material.

As an example only, many materials deposited by a thermal ALD process are commonly deposited at a substrate temperature of 150 °C - 500 °C, and in a re- action environment where the pressure is in the region of 0.1 mbar - 100 mbar. Plasma-enhanced ALD processes can also be conceived, the deposition temperatures of which are potentially much lower than in thermal ALD processes. There also exist thermal ALD processes for some deposited materials in which the essentially self-liming surface reactions, characteristic for ALD, can be achieved already at temperatures below 100 °C and even close to room temperature (25 °C) . Examples of these materials include aluminum oxide (AI2O3) and titanium oxide (TiO ₂ ) • With these processes a protective coating according to an embodiment of the present invention could be fabricated on a substrate 1 which would not be able to sustain high temperatures, e.g. some polymer substrates 1.

As is clear for a person skilled in the art, the invention is not limited to the examples described above but the embodiments can freely vary within the scope of the claims.