GB2385062A

GB2385062A - Method of Applying Hard Coatings

Info

Publication number: GB2385062A
Application number: GB0302236A
Authority: GB
Inventors: Schicai Yang; Dennis Gerald Teer
Original assignee: Teer Coatings Ltd
Current assignee: Teer Coatings Ltd
Priority date: 2002-02-07
Filing date: 2003-01-31
Publication date: 2003-08-13
Also published as: GB0302236D0; GB0202855D0

Abstract

A method of coating a substrate 12, comprising electrically biasing the substrate before coating so that the ion current drawn by the substrate is sufficient for ion bombardment of the surface of the substrate, then depositing a metal nitride layer 16 followed by at least one metal alloy 18. The metal nitride may be any of titanium, chromium, molybdenum or niobium nitride. Preferably, a layer of metal 15 may be applied to the substrate before the deposition of the metal nitride. The deposition may occur from a first metal target and at least a second metal alloy target. Preferably, additional targets may be provided for the deposition of the same or further materials. The method may use closed field unbalanced magnetron sputter ion plating apparatus. Also claimed is a method of depositing a layer of metal alloy nitride, carbide, oxide, carbonitride or oxynitride using magnetron sputtering.

Description

A Method for Depositing very Hard and Smooth Metal Alloy Nitride or Multi Layer Nitride Coatings with Excellent Adhesion The invention to which this application relates is the generation of hard and smooth metal alloy nitride coatings and/or multi layer nitride coatings and a method of performing the same which provides improvements in the hardness and adhesion characteristics of the coating.

For many years metal nitride coatings have been deposited by PVD methods and in particular have been successfully applied to cutting tools to form improved cutting faces thereon. A number of known coating compositions are within the wide range of transition metal nitrides, carbides, oxides, carbo nitrides or oxynitrides and can include, more specifically, Titanium compositions such as TiN, TiC, TiCN, TiON, the Zirconium composition ZrN and Chromium compositions such as CrC, CrN. Of these, the most commonly used coating is Ti N.

These coatings can typically be deposited using electron beam guns, arc sources or magnetron sputter sources. In order to obtain good adhesion and good structures for the coating, the methods used are generally ion plating, wherein the substrate is bombarded by a flux of energetic ions before and during deposition of the coating materials. The ion bombardment prior to deposition serves to clean the substrates in an effort to ensure good adhesion between the substrate and the deposited material. The bombardment of the ions during deposition attempts to ensure a preferred dense structure of the coating but can also cause some undesirable effects such as high internal stresses within the coating. All of these advantages and disadvantages which can occur during the application of the coating are well known and well documented.

It is also known that in order to try and obtain even higher hardness and better wear resistance for the coating, metal alloy nitride coatings can be applied. An example of this type of coating is TiAlN which is harder than TiN and importantly retains its hardness and wear resistance at the higher temperatures experienced in high speed machining. There are many metal alloy nitrides, carbides, oxides, carbo nitrides or oxynitride coatings which give excellent hardness and wear resistance over a range of temperatures and these are hereinafter collectively referred to as"the coating". The particular coating TiAN is used to illustrate certain features of the invention and it should be appreciated that the scope of the invention as herein described extends to each of the other coatings indicated above and similar multi layer nitride coatings. As a result, the scope of the patent should be interpreted in this way.

The most commonly used PVD method for the deposition of the coatings is arc ion plating using, for example, TiAl alloy targets provided on the arc cathodes. The arc method is particularly suitable because of the efficiency of the ionisation produced by the arc source and the high ion current density achieved at the substrates. Good adhesion is readily achieved as are dense wear resistant structures. A problem experienced with the metal alloy deposition flux is that the same produces higher internal stresses in the coatings than when pure metal targets are used. The higher internal stresses can cause failure of the coating by delamination. This delamination may not be due to a true adhesion failure at the interface but can be due to a cohesion failure near to the interface although the appearance and result is similar to a failure of the adhesive bond between the coating and the substrate.

Magnetron sputtering can also be used to deposit the coatings but is less widely used, mainly because the ionisation efficiency is less than that for the arc and it has been claimed that the adhesion is less good than for the arc. However, despite the specific problems with the application of the coatings, magnetron sputtering can have other advantages. It can provide a more flexible technique than arc ion plating and can also produce smoother coatings. In order to make use of these advantages a combined material deposition technique has been developed as detailed in Patent application documents EP90 909 697.6 and EP 91 106 331.1. The technique described in these applications utilises an arc source to produce the ion bombardment prior to deposition to achieve good adhesion and the coating is then deposited by sputter ion plating. This is claimed to give the best of both worlds, i. e a coating with the good adhesion achieved by using an arc technique and the advantages which can be achieved by using magnetron sputter deposition.

While, in theory, the advantages of both systems may be expected to be achieved, in practice, it is found that the use of the arc for ion bombardment to improve the adhesion of the subsequently applied coating causes the deposition of droplets and therefore a less smooth coating. In addition, the method is relatively complex.

The aim of the present invention is to provide a method for applying the coatings described herein which achieves improved adhesion and hardness characteristics without encountering or causing the problems as described previously.

In a first aspect of the invention there is provided a method for the application of a coating of the type as herein described, said method comprising the steps of applying material onto at least

one substrate to form the coating thereon, characterised in that prior to the application of the material the ion current drawn by the electrically biased substrates is enhanced to perform ion bombardment of the surface of the substrate prior to the application of the material thereto and the coating is applied such that pure metal nitride is included in the coating followed by the selected metal alloy material.

Typically the pure metal nitride deposited can be selected from those metals known to give excellent adhesion characteristics for the coating e. g. Ti, Cr, Mo, Nb etc.

In one embodiment, a layer of pure metal is first applied to the substrate surface prior to the metal nitride coating.

In one embodiment, by use of the closed field concept of magnetron sputtering and material deposition an example of which is described in the applicant's patent GB 2258343, the ion bombardment can be enhanced to be 100 times greater than that which can normally be achieved with conventional magnetron sputter ion plating and can be higher than that produced by the arc ion plating method without the need to use the arc and so the problems associated with the arc method are avoided.

Typically the adhesion produced by this method, as assessed by all the recognised adhesion test methods, is at least as good as that produced by the arc deposition method or the arc ion plating before sputtering method. In addition, no droplets are produced and the coatings are smooth and so the method of the invention has all the advantages given by the flexibility of the magnetron technique. Furthermore, as the magnetron based apparatus is relatively simple to operate in comparison to arc coating apparatus, the method of the invention also has the advantage of simplicity.

In a preferred embodiment the method incorporates the controlled deposition of material from a first metal target and at least a second metal alloy target. Typically the first target is controlled to be operable following ion bombardment of the substrate surface, in conjunction with selective control of the introduction of nitrogen into the coating environment, to apply a pure metal nitride layer. Once the metal nitride layer has reached a required depth, the deposition from the first target continues and deposition of metal alloy from the second target commences and the power level gradually increased until the required coating composition is achieved.

Typically, if required, third, fourth and so on targets can be provided to be selectively and controllably operable.

In a further aspect of the invention there is provided a method of depositing a hard metal alloy nitride, carbide, oxide, carbo nitride or oxynitride coating using magnetron sputtering wherein the coating includes a base layer on the surface of the substrate consisting of pure metal followed by pure metal nitride.

In one embodiment, the coating has a contact layer with the substrate surface which comprises only metal, followed by metal nitride and, in turn, a metal alloy.

Typically the method utilises magnetron sputter ion plating apparatus or closed field unbalanced magnetron sputter ion plating apparatus in order to apply the material onto the substrate surface.

Preferably the nitrogen introduced into the coating chamber to provide the coating of the metal nitride and subsequent alloy nitride coating is controlled in conjunction with the control of

the material sputtered from the magnetrons so as to achieve a coating with a graded composition deposited on the substrate.

Typically the nitrogen flow rate is adjusted during all stages to give a stoichiometric coating.

It should also be emphasised that the method of application of the coatings as herein described includes the deposition of multi layer nitride coatings and so the features and advantages as herein described are equally applicable to multi layer nitride coatings as well as metal alloy nitride coatings.

A specific embodiment of the invention is now described with reference to the accompanying drawings in which: Figure 1 illustrates a coating apparatus in one embodiment which can be used in accordance with the method of the invention; and Figure 2 illustrates a cross sectional elevation of a substrate and coating applied in accordance with the invention.

Conventional methods for the deposition of complex alloy nitride coatings such as TiAIN rely on the deposition of material from alloy TiAL targets, typically in a magnetron sputtering apparatus. Although these techniques will give reasonably good adhesion, the use of the alloy targets results in higher stresses in the coating which are particularly damaging at or near to the interface between the applied coating and the surface of the substrate. This presents a discontinuity in mechanical properties, and failure at, or close to, the interface can and does result.

The method of the invention combines the advantages of the high ion currents produced by the magnetron closed field system

which achieves improved adhesion characteristics between the substrate and applied coating, with the flexibility offered by a magnetron sputter ion plating system. A typical magnetron sputtering apparatus which can be used to achieve the coating in accordance with the invention is shown in Figure 1.

In this example, four magnetrons, 2,4, 6,8 are provided in a vacuum chamber 10 with two magnetrons 2,4, having Ti targets and the other two magnetrons 6,8 having Al targets. The process starts by using an ion cleaning method of a closed field unbalanced magnetron sputter ion plating (CFUBMSIP) system in which the pressure of the gas (Argon) in the chamber 10 is set to about 3xl0-3torr and the magnetrons 2,4 are energised at low power. A bias voltage of around 500V is applied to the substrates 12 on a holder 14. Because of the ionisation enhancement from the system these conditions result in efficient cleaning by means of the bombardment from argon and titanium ions. The Al targets can, if desired, be energised at low power at this stage but it is not essential.

After sufficient ion cleaning has been achieved by the ion bombardment, usually after about 20 minutes of operation, the power to the Ti targets in the magnetrons 2,4 is increased while simultaneously the bias voltage is reduced to about 100V. This causes the sputtering of Ti from the targets of the magnetrons 2,4 and so a layer of Ti of between, in this example, 0.1 and 0.27 microns thickness is deposited. During this stage, the level of nitrogen that will be introduced in the next stage of the process can be calculated from a measure of the Ti deposition rate using an optical emission spectrometer. The nitrogen flow is then adjusted to this selected level and the bias voltage is simultaneously reduced to about 55V. Under these conditions a dense hard TiN coating is deposited and, as this is a pure metal (not an alloy) nitride the internal stress in the applied coating at

or near the interface with the substrate is relatively low. After a layer of TiN has been deposited, typically about 0.57 microns thick, the power applied to the Al targets is gradually increased until the required Ti/Al ratio has been achieved. Thus a coating with a graded composition is deposited.

Typically the nitrogen flow rate is adjusted during all stages to give a stoichiometric coating. Thus any internal stresses which would be caused by the deposition of an alloy nitride are introduced gradually and, importantly, are relatively remote from the substrate/coating interface which is the critical region.

During the deposition it is preferred that the deposition of pure metal occurs first after ion bombardment, followed by a pure metal nitride layer for the subsequent successful deposition of the alloy nitride coating or multi-layer nitride coating.

Typically the substrates can be rotated between the magnetrons so that a multi layer coating consisting of alternating TiN and AIN layers is deposited. The thickness of the individual layers is defined by the power applied to the magnetrons and the speed of rotation. Thus, the important feature here is that an early part of the deposition of material includes always a pure metal nitride so that any internal stresses close to the interface with the substrate are low. Figure 2 illustrates a cross sectional view of a coated substrate in accordance with the invention wherein the initial metal layer 15 on the substrate surface 12 can be seen to be followed by a metal nitride layer 16 and graduating to an alloy coating 18 or a multiplayer nitride coating. The apparatus and technique as described in the applicants patent GB2258343 is shown to ensure good adhesion and the use of a pure metal nitride coating near to the coating/substrate interface ensures low internal stresses.

The method is applicable to the wide range of complex alloy nitride coatings that are known to give hard coatings suitable for extreme tribological conditions, such as metal cutting. These include any combination of transition metals, aluminium, silicon, carbon, boron, and yttrium. The method also applies to carbides, carbo nitrides, oxides and oxynitrides of the above metal alloys. It is well known that additions such as Y and/or Si can improve the quality of these coatings and the method is applicable to such coatings. Again the important aspect is to start the coatings with a pure metal nitride and then grade in the other elements. The materials to be graded could be from individual targets, e. g. Al and Y targets or could be an alloy target e. g. AIY. The important aspect is that the initial coating is a low stress pure metal nitride. Even when the final coating is a carbide or oxide or carbo nitride or oxynitride it is preferable to start with a pure metal nitride coating and then grade in the other gases as well as the other metals.

Further it is well known that excellent properties are obtained from ternary metal alloy nitrides, carbides, oxides, carbo nitrides or oxynitrides. These will cover any combination of 3 metals selected from those listed above and also combinations of these metals with Si and or Y. Such coatings can be readily deposited from the 4 magnetron CFUBMSIP system illustrated in figure 1.

Although described with reference to a four magnetron system it is also possible for the method to be utilised in other multi magnetron systems such as a six magnetron system which is useful for the deposition of the more complex alloy nitride systems. The six magnetron system allows the deposition of combinations of up 6 metal nitrides, carbides, oxides, carbonitrides or oxynitrides selected from those listed above, and importantly allows a wider range of alloy composition by weight when using pure metal targets. For instance it is

possible to have 3 targets of metal A and 2 of metal B and 1 of metal C or 4 of A, 1 of B and 1 of C and so on so as to allow the metal compositions of the alloys to be altered by the control of the sputtering of the respective lengths.

The use of the six (or higher) magnetron apparatus to perform the method of the invention allows the deposition of a wider range of compositions than from a 4 magnetron system but again the governing principle is that the first part of the coating should be a pure metal nitride from 1,2 or more magnetrons.

The invention therefore provides the use of pure metal targets and the deposition of material therefrom during the early stages of deposition therefore giving low stress coatings particularly at the interface with the substrate surface.

Claims

Claims 1. A method for the application of a coating including metal nitride into a substrate, said method comprising the steps of electrically biasing the substrate and characterised in that prior to the application of the material onto the substrate, the ion current drawn by the electrically biased substrate is enhanced to perform ion bombardment of the surface of the substrate for a period of time whereupon the coating is then applied such that the deposition of a pure metal nitride is performed followed by at least one selected metal alloy material.
2. A method according to claim 1 characterised in that the pure metal nitride includes a metal selected to provide improved adhesion characteristics.
3. A method according to claim 2 wherein the metal selected is any of titanium, chromium, molybdenum, niobium.
4. A method according to claim 1 characterised in that a layer of pure metal is first applied to the substrate prior to the metal nitride material.
5. A method according to claim 1 characterised in that the method incorporates the controlled deposition of material from a first metal target and at least a second metal alloy target.
6. A method according to claim 5 characterised in that the first target is controlled to be operable following the ion bombardment of the substrate surface, in conjunction with selective control of nitrogen in the coating environment, to apply a pure metal nitride layer.

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7. A method according to the claim 6 characterise in that once the metal nitride layer has reached a predetermined depth, deposition from the first target continues and deposition of metal alloy from the second target commences.
8. A method according to claim 7 characterised in that as the metal alloy deposition commences and continues, the power supply to the apparatus is increased progressively.
9. A claim according to claim 5 characterised in that additional targets can be provided to be selectively operable for the deposition of the same or further materials.
10. A method according to claim 5 characterised in that following ion bombardment of the substrate surface, the first target is operated without nitrogen present to apply a metal layer onto the substrate surface, followed by operation of the first target with a controlled supply of nitrogen into the coating environment to form a metal nitride layer.
11. A method of depositing a hard metal alloy nitride, carbide, oxide, carbonitride or oxynitride coating using magnetron sputtering characterised in that the coating includes a layer applied to the surface of the substrate consisting of pure metal, followed by a layer of pure metal nitride.
12. A method according to claim 11 characterised in that the layers of pure metal and pure metal nitride are applied to the surface of the substrate prior to any other materials so forming the base layers of coating applied to the substrate surface.
13. A method according to claim 12 characterised in that the coating comprises a contact layer with a substrate surface which

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comprises metal, a layer of metal nitride and, in turn, a layer of metal alloy.
14. A method according to claim 13 characterised in that the method utilises magnetron sputter ion plating apparatus.
15. A method according to any of the preceding claims characterised in that the method utilises closed field unbalanced magnetron sputter ion plating apparatus.
16. A method according to any of the preceding claims characterised in that the nitrogen is introduced to provide the coating of the metal nitride and subsequent alloy nitride coating is controlled in conjunction with the control of the material sputtered from selected magnetrons so as to achieve a coating with a graded composition deposited on the substrate.
17. A method according to the preceding claims characterised in that the nitrogen flow rate into the coating environment is adjusted during all stages of material deposition to give a stoichiometric coating.
18. A method according to any of the preceding claims characterised in that the coating comprises the base layer of metal, a layer of metal nitride and a series of layers of metal nitride and/or metal alloy nitride coatings, formed by rotating the substrates between the two or more targets of different materials.

18. A method according to any of the preceding claims characterised in that the coating comprises the base layer of metal, a layer of metal nitride and a series of layers of metal nitride and/or metal alloy nitride coatings.

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Amendments to the claims have been filed as follows Claims

1. A method for the application of a coating including metal nitride onto a substrate, said method comprising the steps of electrically biasing the substrate and characterised in that prior to the application of the material onto the substrate, the ion current drawn by the electrically biased substrate is enhanced to perform ion bombardment of the surface of the substrate for a period of time whereupon the coating is applied to the substrate by the deposition of a pure metal nitride followed by at least one selected alloy nitride material.

2. A method according to claim 1 characterised in that the pure metal nitride includes a metal selected to provide improved adhesion characteristics.

3. A method according to claim 2 wherein the metal selected is any of titanium, chromium, molybdenum, niobium.

4. A method according to claim 1 characterised in that a layer of pure metal is first applied to the substrate followed by metal nitride material.

5. A method according to claim 1 characterised in that the method incorporates the controlled deposition of material from a first metal target and at least a second metal target to form a multilayer coating.

6. A method according to claim 5 characterised in that the first target is controlled to be operable following the ion bombardment of the substrate surface, in conjunction with selective control of nitrogen in the coating environment, to apply a pure metal nitride layer.

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7. A method according to the claim 6 characterised in that once the metal nitride layer has reached a predetermined depth, deposition from the first target continues and deposition of the metal from the second target commences to product a metal alloy nitride. coating.

8. A method according to claim 7 characterised in that as the metal alloy nitride deposition commences and continues, the power supply to the apparatus is increased progressively.

9. A claim according to claim 5 characterised in that additional targets can be provided to be selectively operable for the deposition of the same or further materials.

10. A method according to claim 5 characterised in that following ion bombardment of the substrate surface, the first target is operated without nitrogen present to apply a metal layer onto the substrate surface, followed by operation of the first target with a controlled supply of nitrogen into the coating environment to form a metal nitride layer.

11. A method of depositing a hard metal alloy nitride, carbide, oxide, carbonitride or oxynitride coating using magnetron sputtering characterised in that the coating includes a layer applied to the surface of the substrate consisting of pure metal, followed by a layer of pure metal nitride.

12. A method according to claim 11 characterised in that the layers of pure metal and pure metal nitride are applied to the surface of the substrate prior to any other materials so forming the base layers of coating applied to the substrate surface.

13. A method according to claim 12 characterised in that the coating comprises a contact layer with a substrate surface which

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comprises metal, a layer of metal nitride and, in turn, a layer or multilayers.

14. A method according to claim 13 characterised in that the 9 method utilises magnetron sputter ion plating apparatus.

15. A method according to any of the preceding claims characterised in that the method utilises closed field unbalanced magnetron sputter ion plating apparatus.

16. A method according to any of the preceding claims characterised in that the nitrogen is introduced to provide the coating of the metal nitride and subsequent alloy nitride coating is controlled in conjunction with the control of the material sputtered from selected magnetrons so as to achieve a coating with a graded composition deposited on the substrate.

17. A method according to the preceding claims characterised in that the nitrogen flow rate into the coating environment is adjusted during all stages of material deposition to give a stoichiometric coating.