IL93644A - Method for enhancing adhesion of a metal coating on a substrate and coated substrates produced produced by such method - Google Patents

Description

IT no*©:, o'lrn'o o'aiKn METHOD FOR ENHANCING ADHESION OF A METAL COATING ON A,SUBSTRATE, AND COATED SUBSTRATES PRODUCED BY SUCH METHOD METHOD FOR ENHANCING ADHESION OF A METAL COATING ON A SUBSTRATE, AND COATED SUBSTRATES PRODUCED BY SUCH METHOD The present invention relates to a method for enhancing the adhesion of a metal coating on a metal substrate, and also to a coated metal substrate produced by the novel method.

In recent years, new methods and technologies have been developed for applying coatings to various parts, e.g., tools, to withstand harsh environments, such as those involving high temperatures, or. high rates of corrosion or wear. Among the recently-developed technologies for this purpose are laser techniques used either for hardening the surface of the part or tool, or for applying a hard cladding or alloy coating to the surface. Ion beam techniques have also been developed for either producing or enhancing the properties of such coatings.

One of the problems in applying such coatings, particularly metal coatings, to substates is the difficulty in producing a good mechanical and chemical bond between the coating material and the substrate. A poor bond increases the tendency of the coating material to peel or crack, and thereby substantially reduces the useful life of the part or, tool .

An object of the present invention is to provide a novel method of enhancing the adhesion of a metal coating on a metal substrate. Another object of the invention is to provide coated substrates produced by the novel method.

According to the present invention, there is provided a method of enhancing the adhesion of a metal coating on a metal substrate, wherein the coating and substrate are of materials which form a eutectiferous alloy, comprising: heating the coated substrate to a temperature slightly above the eutectic temperature of the eutectiferous alloy but below the melting temperature of either of the materials, to thereby melt the interface between the coating and substrate, but not to melt either the substrate or coating; and cooling the coated substrate to room temperature.

A eutectiferous alloy is an alloy of metals which, while completely soluble in one another in a liquid state, separate into distinct crystals on cooling. The "eutectic" so formed has a lower melting point than the melting point of either of the two alloys of which it is composed. The composition of the eutectic is determined by the rate of cooling to room temperature .

Eutectiferous alloys are to be distinguished from "solid-solution" alloys, in which the two metals retain in the solid state the mutual miscibility which exists when they are molten.

Thus, the method of the present invention involves a process which is opposite to that of eutectic solidification from a melt. That is, eutectic solidification is a cooling process, in which the melt is cooled first to solidify individual metals of the alloy and then to solidify the eutectic of the two alloys, which eutectic solidifies at the lowest temperature. The present invention, however, involves a heating process, in which the coated substrate is heated to a temperature slightly above the eutectic temperature of the eutectiferous alloy of the substrate and coating materials and then cooled. The heating causes melting of the two metals at the interface, and since the heating is to a temperature slightly above the eutectic temperature and below the melting temperature of the two materials, the melting is restricted to the interface. In the subsequent cooling step, the cooling rate determines the eutectic composition at the interface. Preferably, a slow cooling rate is used.

The heating step can be effected by thermal heating in a vacuum, or in an inert or reactive gas environment. It can also be effected by an acoustical source. The heating step may last from seconds to minutes, which provides an additional advantage over other known thermal techniques which may take up to several hours.

If the materials of the substrate and coating form eutectiferous alloys having high eutectic temperatures, an intermediate material may be applied between the substrate and coating in order to reduce the heating temperature. This can be done by selecting a material for the intermediate layer which forms a first eutectiferous alloy with the substrate having a first eutectic temperature, and a second eutectiferous alloy with the coating having a second eutectic temperature, and heating the coated substrate to a temperature which is slightly higher than either of the above two eutectic temperatures.

The method may also be used where the coating is of a material which does not form a eutectiferous alloy with the material of the substrate. In such a case, an intermediate layer could be applied selected of a material which forms a eutectiferous alloy with the substrate, and which also forms a eutectiferous alloy with the material of the coating, and heating the coated substrate, including the intermediate layer, to a temperature which is slightly higher than the higher of the above two eutectic temperatures.

The intermediate layer may be of sufficient thickness so that it remains as an intermediate layer following the heating and cooling steps; alternatively, it may be applied so thin that substantially only the eutectiferous alloy formed by the intermediate layer remains in the coated substrate after the heating and cooling steps have been completed.

The heating step may be performed in an atmosphere containing a gas which reacts with the outer surface of the coating to form a protective layer, for example a protective oxide, nitride or carbide layer.

Further features of the invention will be apparent from the description below.

The invention is herein described with reference to a number of examples set forth below and also with reference to the attached drawings, in which : Fig. 1 is an equilibrium diagram of an alloy system of two metals forming a eutectiferous alloy, showing the composition in percentages as the base line with temperature as the ordinate; Fig. 2 illustrates a conventional coating-substrate system; Fig. 3 illustrates the coating-substrate system of Fig. 2 after the coated substrate has been subjected to the thermal treatment of the present invention; Fig. 4 illustrates a coated-substrate system including an intermediate layer befgore subjected to the thermal treatment of the present invention; Fig. 5 illustrates the system of Fig. 4, but after the coated substrate, including the intermediate layer, has been subjected to the thermal treatment of the present invention; Fig. 6 illustrates a coated substrate system similar to Fig. 4 but with the intermediate layer applied as a very thin layer; Fig. 7 illustrates the system of Fig. 6 after the coated substrate has been thermally treated in accordance with the present invention; Fig. 8 illustrates a coated substrate including an intermediate layer, similar to the system of Fig. 4; Fig. 9 illustrates the system of Fig. 8 after it has been subjected to a thermal treatment in accordance with the present invention, which thermal treatment is performed in an atmosphere containing a gas which reacts with the outer surface of the coating to form a protective layer thereover; Fig. 10 illustrates the coated substrate according to Fig. 8, but omitting the intermediate layer; and Fig. 11 illustrates the coated substrate of Fig. 10 after it has been heated, in accordance with the present invention, in an atmosphere containing a gas to prdouce both the intermediate eutectiferous alloy and the outer protective layer.

Fig. 1 is an equilibrium diagram of an alloy system of two metals A, B which form an eutectiferous alloy. In this diagram, the vertical axis is the temperature, and the horizontal axis is the composition of the alloy. It will be seen from the diagram of Fig. 1 that the melting point of alloy B (TmB) is higher than the melting point of alloy A (TmA) . Thus, starting with the molten alloy, the first change which occurs is the start of solidification of alloy B (at temperature mB), which is followed by the start of solidification of alloy A (at temperature mA) . Point E is the eutectic temperature of the alloy which, as can be seen from Fig. 1, is lower than the melting points of both of alloys A, B. Eutectic point Te determines the composition at the beginning of the solidification of the alloy, and the composition following solidification is determined by the cooling rate to room temperature .

The present invention involves heating two (or more) materials (e.g., the coating and substrate) which form a eutectiferous alloy, to a temperature slightly above the eutectic temperature of the alloy, but below the melting temperature of either of the materials, and then cooling the two materials to room temperature. The heating thereby melts the interface between the two materials (coating and substrate), but does not melt either of these materials. It has been found that such a method greatly enhances the adhesion of the one material to the other.

The heating may be established over large areas, and may be performed in a vacuum environment, inert environment, or a reactive gas environment.

Fig. 2 illustrates the construction wherein material A is a metal coating applied over a substrate of metal B. When such a coated substrate is heated to (preferably slightly above), the eutectic temperature Te of the eutectiferous alloy formed by these two materials, the interface between the coating A and substrate B is melted to form a eutectic mixture (M) . If the cooling rate is low, then layer M has a eutectic composition A B1 ; other cooling rates will cause a family of other eutectic segregations.

It has been found that this process, particularly the formation of the eutectic intermediate layer M, greatly enhances the adhesion of the metal coating A to the metal substrate B.

Following are a number of examples of applications of the invention: Example 1 This example involves iron (Fe) as a substrate, and titanium (Ti) as a coating. The melting point of Fe is 1,538°C, and the melting point of Ti is 1,670°C. The Ti-Fe phase diagram has a eutectic point of 1,317°C. Thus, by heating the coated substrate to slightly above 1,317°C, preferably about 1,320°C, and cooling slowly, the interface between the coating and substrate will be melted to produce a eutectiferous alloy which enhances the adhesion of the coating to the substrate. The heating step is preferably performed no higher than 1,320°C, so as not to cause any melting of either the Ti or Fe.

Example 2 Another example of the invention is one wherein the coating is nickel, and the substrate is titanium. Nickel has a melting temperature of 1,455°C, and titanium has a melting temperature of 1,670°C. The eutectic temperature of the alloy formed by these two metals is 1,310°C. Accordingly, this coated substrate may be heated to a temperature slightly higher than 1,310°C, e.g., about 1,315°C, to melt the interface between the coating and the substrate, but without melting either the substrate or coating, and then cooled slowly.

Certain pairs of materials have high eutectic temperatures, like the examples set forth above.

Others have none at all, like Cu-Ni, and Al-Pb. Where the materials have high eutectic temperatures, the heating temperature of the process may be reduced by including an intermediate layer selected of a material forming a eutectiferous alloy with the material of the substrate as well as with the material of the coating, which eutectiferous alloys have relatively low eutectic temperatures .

Fig. 4 illustrates a construction including an intermediate layer C applied between the substrate B and the coating A before the heat treatment of the present invention; and Fig. 5 illustrates the construction following the heat treatment according to the present invention. Thus, the intermediate layer C forms a eutectic mixture with the substrate B having a composition B^C^_^, depending on the cooling rate, and also forms a eutectic mixture M¾ with the coating A of a composition A C1 , also depending on the cooling rate .

Following are several examples of the invention utilizing an intermediate layer C: Example 3 This example is similar to Example 2 above wherein titanium is the substrate and nickel is the coating. Nickel and titanium form a eutectiferous alloy, but one having a high eutectic temperature, being 1 ,310°C.

Aluminum also forms a eutectiferous alloy with both nickel and titanium. The eutectic temperature of the titanium aluminum alloy is 665°C, and the eutectic temperature of the nickel and aluminum alloy is 639°C. Accordingly, by including the aluminum intermediate layer (layer C, Fig. 5), the coated substrate may be heated to the eutectic temperature of the nickel-aluminum alloy, namey 665°C or slightly above, which thereby melts both interfaces between the aluminum and nickel and the aluminum and titanium, and then cooling the coated substrate.

It will thus be seen that the use of the intermediate aluminum layer between the nickel and titanium enables the heating step to be performed at a substantially lower temperature (670°C as compared to 1,315°C), than would be possible without including the aluminum layer.

Example 4 Iron having a melting point of 1,538°C and manganese having a melting point of 1,246°C form a eutectiferous alloy having a eutectic temperature of 1,232°C. However germanium, having a melting point of 936°C, forms a eutectiferous alloy with manganese having a eutectic temperature of 697°C, and a eutectiferous alloy with iron havinga eutectic temperature of 838°C. Accordingly, by using germanium as the intermediate layer, the coated substrate may be heated to a temperature of 840°C, slightly higher than the eutectic temperature of 838°C, in order to melt the interface without melting the coating or the substrate, thereby substantially reducing the heating temperature from 1,232°C to 840°C (a decrease of approximately 32%) .

An intermediate layer may also be used where the coating material and the substrate material do not form eutectiferous alloys. Following is an example of this application of the invention: Example 5 Iron having a melting point of 1,538°C does not form a eutectiferous alloy with nickel having a melting point of 1,455°C. However, they have a concave type of phase diagram where the minimum melting temperature of both is 1,425°C.

In such a case, silicon may be used as an intermediate layer for promoting the adhesion of the nickel to the iron. Silicon forms a eutectiferous alloy with iron having a eutectic temperature of 1,203°C, and forms a eutectiferous alloy with nickel having a eutectic temperature of 1,143°C (23% atomic Si), and 966°C (55% atomic Si). Therefore, heating the system to 1,205°C, and cooling, will result in alloying the interface at a 15% lower temperature.

Example 7 Copper having a melting point of 1,084°C and nickel having a melting point of 1,455°C do not form a eutectiferous alloy. However, aluminum, having a melting point of 660°C, forms a eutectiferous alloy with copper having a eutectic temperature of 548°C, and a eutectiferous alloy with nickel having a eutectic temperature of 639 °C. Thus, by using aluminum as an intermediate layer, the heat treatment can be performed at a temperature slightly above 640°C, which is about 31% of the melting point of copper.

Example 8 Chromium having a melting point of 1,863°C does not form a eutectiferous alloy with tungsten having a melting point of 3,422°C. However, silicon forms a eutectiferous alloy with chromium having a eutectic temperature of 1,391°C, and also with tungsten having a eutectic temperature of 1,392°C. Thus, by using silicon as an intermediate layer, the temperature of heating the coated substrate to 1,395°C in order to alloy the interfaces may be lowered by 25% below the melting point of the chromium.

In all the foregoing methods, the intermediate layer may be applied for a significant thickness so as to form not only the two alloys (Μβ, M , Fig. 5) at the interfaces of the two metals B, A, but also to remain as a separate intermediate layer (M, Fig. 5). However, the intermediate layer C may also be applied sufficiently thin, as shown in Fig. 6, so that it is substantially completely converted to the alloy AX' BY' C £°H° i-n<3 the heating and cooling steps, as shown in Fig. 7.

As indicated earlier, the process may be performed in a vacuum atmosphere, an inert atmosphere, or a reactive gas atmosphere. The latter is particularly useful when it is desired to prvide an outer protective layer to protect against particularly harsh conditions. Thus, the heating and cooling steps may be performed in a gas atmosphere which reacts with the outer layer of the coated substrate to form a new surface composition. Thus, the diffusion of atoms from the reative gas to the surface may form a new composition, such as a nitride, oxide, carbide, or any combination, such as a carbo-nitride, oxy-nitride, etc.

Fig. 8 illustrates the construction using an intermediate layer C before the heat treatment, and Fig. 9 illustrates the construction following the heat treatment. As shown in Fig. 9, the process produces the new layer M, which is the alloy Αχ, Βγ, C described above. In addition, the reactive gas forms an outer protective layer N having the composition Ay , Dz , including the nitride, oxide, carbide, or combination described earlier formed by diffusion of atoms from the reactive gas.

The above process can also be performed without including an intermediate layer. Fig. 1 0 illustrates such a construction before the heating process, and Fig. 1 1 illustrates the construction following the heating process. It will be seen that the same results are produced as described above with respect to Figs. 2 and 3 , respectively, except that the reactive gas produces the outer protective layer (N) as in Fig. 9 .

Following are examples of the above variations : Example 9 A substrate of chromium having a melting point of 1 , 863 ° C , coated with a coating of zirconium having a melting point of 1 , 855 °C was heated to a temperature of 1 , 335°C (sightly above the eutectic temperature of 1 , 332 °C ) in an atmosphere of oxygen, whereby the adhesion of the zirconium coating to the substrate was enhanced, and also a coating of zirconium oxide was formed on the outer face of the coating.

Example 1 0 The same coated substrate as in the preceding example was heated in an atomosphere of nitrogen, whereby an external coating of zirconium nitride was formed .

FURTHER EXAMPLES Chromium-coated substrates when heated in oxygen will form outer surfaces of chromium oxide, and when heated in methane will form outer coatings of chromium carbide. Titanium-coated substrates heated in oxygen will form outer coatings of titanium oxide, and when heated in methane will. form outer coatings of titanium carbide.

The invention can also be performed by coating an intermediate layer on the substrate and performing the coating process at eutectic conditions; i.e., heating the substrate and the intermediate layer to the eutectic temperature during the coating process .

Further, the invention can also be applied with respect to ceramics. For example, the invention could be applied with respect to a system including A^O^ as a substrate and AlN as a coating, these materials having melting temperatures of 2,050°C and 2,600°C, respectively, and a eutectic point at 1,700°C, which is 17 percent lower than the melting temperature of the A^O^ in such a system, the coated substrate could be heated to a temperature of approximately 1,703°C, slightly higher than the eutectic temperature.

Another ceramic system is MgO and SiC^, having melting temperatures of 2 800°C and 1,700°C, respectively, and a eutectic point at 1,453°C, which is 9 percent lower than the melting temperature of the SiC^ in this case, the coated substrate could be heated to a temperature of about 1,546°C, slightly higher than the eutectic point.

It will be appreciated, therefore, that the examples described above are merely illustrative examples of practising the invention, and that the invention could be practised with many other materials.

Claims (22)

WHAT IS CLAIMED IS;

1. A method of enhancing the adhesion of a metal coating on a metal substrate, wherein the coating and substrate are of materials which form a eutectiferous alloy, comprising: heating the coated substrate to a temperature slightly above the eutectic temperature of said eutectiferous alloy but below the melting temperature of either of said materials, to thereby melt the interface between the coating and substrate, but not to melt either the substrate or coating; and cooling the coated substrate to room temperature .

2. The method according to Claim 1 , wherein said coating is titanium, said substrate is iron, and the temperature to which the coated substrate is heated is about 1 , 330°C.

3. The method according to Claim 1 , wherein said coating is nickel, said substrate is titanium, and the temperature at which the coated substrate is heated is about 1 ,310°C.

4. The method according to Claim 1, including a second coating over said first-mentioned coating, said first-mentioned coating being a material which forms a eutectiferous alloy having a first eutectic temperature with the material of the substrate; said second coating being of a material which forms a eutectiferous alloy having a second eutectic temperature with the material of the first-mentioned coating; said heating temperature being slightly higher than the higher one of said first and second eutectic temperatures.

5. The method according to Claim 1 , including a second coating over said first-mentioned coating, said first-mentioned coating being of a material which does not form a eutectiferous alloy with the material of the substrate; said second coating being of a material which forms a eutectiferous alloy with the material of the first-mentioned coating; said heating temperature being slightly higher than the eutectic temperature of said eutectiferous alloy.

6. The method according to Claim 1, wherein the coating and substrate materials form a eutectiferous alloy of high eutectic temperature, said method including the further step of applying an intermediate layer between the coating and substrate which intermeidate layer is selected of a material which forms a eutectiferous alloy with each of the coating and the substrate materials, the latter eutectiferous alloys having lower eutectic temperatures than the eutectic temperature of the coating and substrate materials; the coated substrate with the intermediate layer therebetween being heated to a slightly higher temperature than the higher eutectic temperature of the material of the intermediate layer with the coating and substrate materials.

7. The method according to Claim 6, wherein the substrate is nickel, the coating is titanium, the intermediate layer is aluminum, and the temperature of heating is about 665°C.

8. The method according to Claim 6, wherein the substrate is iron, the coating is manganese, the intermediate layer is germanium, and the heating temperature is about 840°C.

9. The method according to Claim 6, wherein said substrate is iron, said coating is nickel, said intermediate layer is silicon, and said heating temperature is about 1,205°C.

10. The method according to Claim 1 , wherein the coating and substrate materials do not form eutectiferous alloys; said method including the further step of applying an intermediate layer between the coating and substrate which intermediate layer is selected of a material which forms a eutectiferous alloy with each of the coating and the substrate materials; the coated substrate with the intermediate layer therebetween being heated to a slightly higher temperature than the higher eutectic temperature of the material of the intermediate layer with the coating and substrate materials.

11. The method according to Claim 10, wherein said substrate is copper, said coating is nickel, said intermediate layer is aluminum, and said heating temperature is about 548°C.

12. The method according to Claim 10, wherein said substrate is chromium, said coating is tungsten, said intermediate layer is silicon, and said heating temperature is about 1,395°C.

13. The method according to any of Claims 6-12, wherein said intermediate layer is sufficiently thin so as to leave substantially only the eutectiferous alloy between the substrate and coating after the coated substrate has cooled.

14. The method according to any one of Claims 1-13, wherein said cooling of the coated substrate to room temperature is performed at a low rate.

15. The method according to any one of Claims 1-14, wherein said heating step is performed in an atmosphere containing a gas which reacts with the outer surface of said coating to form a protective layer thereover.

16. The method according to Claim 14, wherein said gas is oxygen, forming an outer protective oxide layer .

17. The method according to Claim 14, wherein said gas is methane, forming a protective nitride layer.

18. The method according to any one of Claims 1-14, wherein the heating step is performed in an inert atmosphere .

19. The method according to any one of Claims 1-14, wherein said heating step is performed in a vacuum.

20. A coated substrate made according to the method of any one of Claims 1-19.

21. A method of enhancing the adhesion of a coating on a substrate substantially as described with reference to any of the disclosed examples.

22. A coated substrate substantially as described with reference to any of the disclosed examples . Tel-Aviv, 61 230