US3657784A

US3657784A - Cladding of metals

Info

Publication number: US3657784A
Application number: US16943A
Authority: US
Inventors: Gordon Leslie Selman; Alan S Darling
Original assignee: Johnson Matthey PLC
Current assignee: KLOSK BELLA 4063 OAKRIDGE DEERFIELD BEACH FL 33441; KLOSK MICHAEL J 15 MILL RD EXTENSION WOODCLIFF LANE NJ 07675; Johnson Matthey PLC
Priority date: 1970-03-05
Filing date: 1970-03-05
Publication date: 1972-04-25
Anticipated expiration: 1989-04-25

Abstract

This invention relates to articles for use at high operating temperatures (1,100*-1,500* C.) and comprising a core made from a refractory metal or alloy and clad with a sheath of a platinum group metal or alloy based on at least one platinum group metal. Such articles in the form of stirrers, crucibles, spinning dies and the like have particular application in the glass industry.

Description

United States Patent Selman et al.

[ 51 Apr. 25, 1972 CLADDING OF METALS inventors: Gordon Leslie Selman, High Wycombe;

Alan S. Darling, Northwood, both of En- [56] References Cited UNITED STATES PATENTS 2,681,876 6/1954 De Santis et al. ..29/l98 X 2,497,090 2/1950 Miller et al. ..250/27.5 2,947,1 14 8/1960 Hill ..49/53 2,536,673 1/1951 Widell ..250/27.5

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney-Hofgren, Wegner, Allen, Stellman & McCord [57] ABSTRACT This invention relates to articles for use at high operating temperatures (1,100-1,500 C.) and comprising a core made from a refractory metal or alloy and clad with a sheath of a platinum group metal or alloy based on at least one platinum group metal. Such articles in the form of stirrers, crucibles, spinning dies and the like have particular application in the glass industry.

17 Claims, No Drawings CLADDING F METALS This is a continuationin-part of application Ser. No. 631,592 filed Apr. 18, 1967, now abandoned.

This invention relates to the cladding of refractory metals with platinum group metals or alloys based on at least one metal of the platinum group. Such alloys will be referred to herein as platinum base alloys.

The invention is particularly but not exclusively applicable to the cladding with the aforesaid metals or alloys of articles which are used in the glass industry, such as, s'tirrers, crucibles, spinning dies, tube fabricating dies and the like.

Of the platinum group metals we have found platinum, rhodium, palladium and iridium particularly suitable cladding metals. 0f the alloys consisting predominantly of one or more platinum group metals, satisfactory results have been obtained from the following cladding alloys; rhodium/iridium; platinum/iridium; palladium/iridium, and platinum/rhodium/iridium.

Such articles are required, in use, to withstand the action of molten glass at temperatures at least as high as 1,100" C. and sometimes as high as 1,500 C. Further, they are generally required not to introduce measurable quantities of impurities into molten glass with which they come into contact.

Platinum metal and certain rhodium/platinum alloys, for example, resist attack by molten glass and also have good hot strengths. It would thus be possible to make articles of the kind mentioned previously from platinum or rhodium/platinum alloys. The cost would, however, be prohibitive. A stirrer is typically in the form of an inverted T in which the vertical shaft is about 6 feet long and 1% inch diameter and the horizontal shaft is 4 feet long and 1% inch diameter. It has accordingly become the practice to fabricate articles such as stirrers from a core of a metal such as molybdenum which is very much cheaper than platinum and which has good hot strength but which would be completely and rapidly oxidized when exposed to air at the temperature of molten glass, and to enclose or c1ad" this core in a thin, continuous, closely fitting sheath of platinum metal or platinum base alloy. It is found in practice that such an article has the advantages not only of being cheaper than a corresponding article made wholly of platinum but also of being stronger at temperatures in the region of 1,400 C. since molybdenum, at this temperature, is stronger than platinum.

The platinum or rhodium/platinum sheath is usually applied by fitting suitably shaped pieces of platinum of platinum/rhodium alloy sheet round the article to be clad and then welding these pieces together so as to form a closely fitting cladding or covering for the core. The cladding is generally provided with one or more small exhaust tubes which communicate with the interfacial space between the cladding and the core but is otherwise gas-tight.

The final stage in the cladding of the core involves a reduction in the pressure of the fluid (e.g. gas, vapor) in the interfacial space via the exhaust tube(s) down to a low value following which the tube(s) are pinch sealed. The pressure in the interfacial space is reduced in this way so as to minimize the oxidation of the molybdenum core in service and to improve the fit of the cladding. The value to which the pressure of the fluid in the interfacial space is reduced will depend inter alia on the geometry of the core and may be within the range of a few microns to 2 mm. of H,.

Molydenum articles clad in this way were, however, found to be prone to early failure in service. In general the failure takes the form of a fracture or split in the cladding which permits molten glass'to reach the underlying molybdenum core. It was originally thought that such failures were due to the difl usion of molybdenum from the core into the platinum cladding at the region of contact between the cladding and the core. Attempts were, therefore, madeat least to reduce this effect by interposing a barrier layer of some refractory material such 'as alumina between the core and the cladding so as to prevent direct contact therebetween. Some improved results were obl,250 C. when the alumina was flame sprayed on to the core but, although this procedure was found to prolong the operating life of a stirrer, the improvement was not marked.

We have now discovered that, although the failure of the platinum cladding is due to diflusion of the molybdenum into the platinum, the molybdenum is transferred to the platinum cladding in the vapour phase as one or more oxides of molybdenum. It is believed that the trioxide is predominant. At the temperatures at which a stirrer is used in the glass industry, the molybdenum oxides formed by the interaction of the core material and any residual oxygen in the interfacial space are tained at the lower operating temperatures in the region of volatilized and, when they contact the inner surface of the platinum cladding, they are reduced to the metal, which then alloys with the platinum. The oxygen liberated in this way is then available to oxidize more of the molybdenum core so that the action is continuous and we have found that the inner regions of the cladding begin to breakdown, thus leading to ultimate failure.

We have also found that the operating life of a core clad with platinum or with a platinum base alloy can be greatly improved if the core is made from a metal selected from the group consisting of niobium, tantalum, niobium-tantalum alloys, niobium-chromium alloys, tantalum-chromium alloys and niobium-tantalum-chromium alloys.

The oxides of niobium and tantalum are much less volatile than the oxides of molybdenum at the operating temperatures of l,l00 1,400" C. or l,500 C. The platinum (or platinum base alloy) cladding is affected by cores of these metals much less, and, consequently, the operating life of the article is considerably increased.

For example, an article consisting of a molybdenum core loosely sheathed with 0.020 inch thick platinum and in which the interfacial volume was not fully evacuated and thus contained some residual oxygen, was found to be protected for about 400 hours at an operative temperature of 1,400 C. in air. A similar sheath on a core of niobium under the same conditions provided protection for 1,000 to 2,000 hours, thus showing the advantages provided by the non-volatility of niobium oxide.

However, even with niobium and tantalum core materials there must still be certain points of contact between the sheath and the underlying core. At these points the core metal tends to diffuse through the sheath and, upon reaching the exterior, oxidation again occurs leading to sudden failure of the sheath. In the case of articles in contact with molten glass this is unacceptable as it leads to discolouration of the glass.

Very much longer life times than can be obtained with niobium and tantalum core materials therefore necessitate th use of alternative materials.

Other metals which might be considered, such as titanium, zirconium and vanadium do not form volatile oxides but their melting points are too low to allow of their effective use. They are weak mechanically at the average operating temperatures of 1,350 1,400 C. and moreover they have the undesirable tendency to alloy with the platinum or platinum based sheath to form diffused alloy layers, intermediate phases, and in some instances, phases with melting points lower than the temperature at which the component is intended to operate.

As previously mentioned, an arrangement using a molybdenum core has a shorter operating life than is desirable, and attempts to prolong the operating life by reducing the pressure in the interfacial spaces between the core and the sheath have not up to the present time met with success.

However, our investigations indicate that a further reduction of the pressure in the interfacial space may reduce to an even greater degree the migration of the molybdenum (in the form of oxide) to the cladding material.

We have found that the application to the molybdenum or tungsten core of a getter in the form of a refractory coating greatly aids evacuation of oxygen from the interfacial volume and increases to aconsiderable extent the operating life of a molybdenum/tungsten core platinum/platinum alloy sheath arrangement.

Suitable getter materials are metals such as zirconium, tantalum, niobium, titanium, vanadium or hafnium. These metals may also be applied in smaller quantities as dilute alloys of molybdenum or platinum.

As direct contact between the core or layer of getter material has been found to be undesirable, the invention also includes the use of a barrier layer between the core and getter layers and the outer sheath of the article.

The barrier layer may comprise:

a. refractory oxides (i.e. oxides which are themselves refractory; not necessarily the oxides of refractory metals;)

b. refractory carbides;

c. refractory nitrides (for example boron nitride and silicon nitride);

d. Refractory sulphides; and

e. other refractory compounds which are compatible at operating temperature with the two materials with which they come into contact.

The above items (a) (e) include the compounds of the rare earth metals.

According to a further feature of the present invention, therefore, an article intended to be used at high operating temperature, above l,000 C. comprises:

a. a core comprising a refractory metal selected from the group consisting of molybdenum, tungsten and an alloy based on at least one of said metals;

b. a getter" applied to the core in the form of a refractory coating;

c. a barrier layer applied to the getter comprising a refractory compound selected from the group consisting of refractory carbides, silicides, borides, sulphides, nitrides and oxides, whereby volatilization of the oxides of said core material is prevented from forming with an outer sheath an alloy of lower melting point than that of the sheath, and

d. an outer sheath enclosing the core, "getter" and barrier layers and comprising a material selected from the group consisting of the platinum group metals and an alloy based on at least one platinum group metal.

In a preferred embodiment of the invention, the pressure in the interfacial space between core and sheath is within the range of a few microns to 2 mm. of mercury.

The low oxygen pressure required to ensure the continued operation of the molybdenum cored component may be easily obtained by the use of a getter as detailed above. However, the use of these getters produce such a low partial pressure of oxygen that the more usual refractory oxides, such as alumina, zirconia and thoria, for example, tend to decompose. The metal so released tends to alloy with the platinum of the sheath and this again produces early failure of the component.

We have found that a barrier layer to which this does not apply is composed of magnesia. A preferred embodiment of the invention, therefore, is the use of magnesia as the barrier layer.

As an example of the invention a molybdenum or tungsten core may be flame-sprayed with zirconium metal. This results in a coating comprising a mixture of zirconium metal, zirconium oxide and zirconium nitride. In order to prevent contact of a platinum sheath with zirconium metal, zirconia is then flame-sprayed onto the previously flame-sprayed layer of zirconium. The quantity of zirconium first sprayed is chosen so that the amount of zirconium available is only slightly greater than that required to take-up the oxygen present in the interfacial space. Preferably, the zirconium oxide is porous to allow a rapid movement of gas.

In carrying out an experiment to determine the effect of zirconium as a getter" material in platinum clad molybdenum, a core of molybdenum was sprayed with zirconium metal to prove a layer 0.003 inch thick. This layer was then oversprayed with zirconia to provide an outer layer 0.010 inch thick. The composite core was then sealed into a platinum sheath 0.020 inch thick without evacuation of the interfacial volume. The core had a life of over 2,000 hours at a temperature of l,400 C.

If desired, zirconium or other getter may be incorporated within the molybdenum or tungsten core so as to form an alloy of for example 0.5 1 .0 wt. percent zirconium/molybdenum.

Further, it will be appreciated that a composite core comprising a central core of molybdenum having a first layer of titanium, zirconium or vanadium as getter and finally an outer barrier layer formed from a rare earth compound oxide with refractory properties, particularly magnesia, may be used.

The getter may be plasmaor flame-sprayed in a layer uniformly over the molybdenum or tungsten surface of the core.

Alternatively it may be concentrated at a point in a fairly cool area of the core (e.g. 700 1,200 C.) so that it can absorb oxygen very effectively without coming into direct contact with barrier layer materials at high temperature because such contact would lead to decomposition.

For example, in a molten glass stirrer the getter may be concentrated around the upper part of the stem, above the surface of the molten glass.

When tlameor plasma-sprayed magnesia is used as the barrier layer it may be applied in combination with a small quantity of silica to assist adhesion to the core.

What is claimed is:

1. An article intended to be used at high operating temperatures above l,000 C. comprising a molybdenum core, a zirconium metal coating on said core as a getter for oxygen, a barrier layer of the group consisting essentially of zirconia and magnesia on said zirconium metal getter, and a sheath enclosing the so coated core and consisting essentially of a material selected from the group consisting of a platinum group metal and an alloy having at least one platinum group metal.

2. An article according to claim 1 wherein said core comprises an alloy of said molybdenum with 0.5 1.0 wt. percent of zirconium.

3. An article according to claim 1 wherein the barrier layer comprises magnesia.

4. An article intended to be used at high operating temperatures above l,000.C. comprising a core of a metal selected from the group consisting of niobium, tantalum, niobium-tantalum alloy, niobium-chromium alloy, tantalum-chromium alloy and niobium-tantalum-chromium alloy, a getter for oxygen on the core consisting essentially of a refractory coating, a sheath of a material selected from the group consisting of a platinum group metal and an alloy having at least one platinum group metal, and a barrier layer interposed between said core and said sheath comprising a refractory compound compatible with said core and said sheath at the operating temperature, whereby volatilization of oxides of said core material is reduced and said core material is prevented from forming with said sheath an alloy having a melting point below said operating temperature.

5. An article according to claim 4 further having an interfacial space between the core and the sheath wherein the said space has a partial pressure of not more than 2 mm. of mercury.

6. An article intended to be used at high operating temperatures above l,000 C. comprising: (a) a core consisting essentially of a refractory metal selected from the group consisting of molybdenum, tungsten and a refractory alloy based on at least one of said metals; (b) a getter for oxygen on the core consisting essentially of a refractory coating; (c) a barrier layer on the getter consisting essentially of a refractory compound selected from the group consisting of refractory carbides, silicides, borides, sulphides, nitrides and oxides, and (d) an outer sheath enclosing the core, getter and barrier layers and consisting essentially of a material selected from the group consisting of the platinum group metals and a platinum group metal alloy, volatilization of the oxides of said core material thereby being prevented from forming with said outer sheath an alloy of lower melting point than that of the sheath.

7. An article according to claim 6 having an interfacial space between the core and the sheath wherein the said space has a partial pressure of not more than 2 mm. of mercury.

8. An article according to claim 7 wherein the getter is a material selected from the group consisting of zirconium, tantalum, niobium, titanium, vanadium and hafnium.

9. An article according to claim 8 wherein the getter material is present in relatively small quantities as dilute alloys of molybdenum or platinum.

10. An article according to claim 6 wherein the getter is in the form of a plasma or flame sprayed coating.

11. An article according to claim 6 wherein the getter is concentrated in an area which, in use, is not subject to temperatures greater than 1,200 C. so that the getter absorbs oxygen without contact with the barrier layer, thereby avoiding decomposition of the sheath material.

12. An article according to claim 6 wherein the core material is molybdenum alloyed with minor quantities of at least one of the materials of the group consisting of titanium and zirconium.

13. An article according to claim 6 wherein the core material is alloyed with at least one of the metals selected from the group consisting of titanium, zirconium, niobium, tantalum and hafnium.

14. An article according to claim 6 wherein the getter material comprises zirconium metal and the barrier layer is zirconia.

15. An article according to claim 6 wherein the getter material is zirconium and the barrier layer is magnesia.

16. An article according to claim 15 wherein the barrier layer of magnesia includes a minor amount of silica.

17. An article intended to be used at high operating temperatures above 1,000 C. having a core of the class consisting of molybdenum, tungsten, an alloy of molybdenum, and an alloy of tungsten, and a flame-sprayed coating on said core of zirconium, a further flame-sprayed coating thereon selected from the group consisting of zirconia and magnesia, and a sheath enclosing said coated core and comprising a material selected from the group consisting of a platinum group metal and an alloy of at least one platinum group metal.

i l l l

Claims

2. An article according to claim 1 wherein said core comprises an alloy of said molybdenum with 0.5 - 1.0 wt. percent of zirconium.
3. An article according to claim 1 wherein the barrier layer comprises magnesia.
4. An article intended to be used at high operating temperatures above 1,000* C. comprising a core of a metal selected from the group consisting of niobium, tantalum, niobium-tantalum alloy, niobium-chromium alloy, tantalum-chromium alloy and niobium-tantalum-chromium alloy, a getter for oxygen on the core consisting essentially of a refractory coating, a sheath of a material selected from the groUp consisting of a platinum group metal and an alloy having at least one platinum group metal, and a barrier layer interposed between said core and said sheath comprising a refractory compound compatible with said core and said sheath at the operating temperature, whereby volatilization of oxides of said core material is reduced and said core material is prevented from forming with said sheath an alloy having a melting point below said operating temperature.
5. An article according to claim 4 further having an interfacial space between the core and the sheath wherein the said space has a partial pressure of not more than 2 mm. of mercury.
6. An article intended to be used at high operating temperatures above 1,000* C. comprising: (a) a core consisting essentially of a refractory metal selected from the group consisting of molybdenum, tungsten and a refractory alloy based on at least one of said metals; (b) a getter for oxygen on the core consisting essentially of a refractory coating; (c) a barrier layer on the getter consisting essentially of a refractory compound selected from the group consisting of refractory carbides, silicides, borides, sulphides, nitrides and oxides, and (d) an outer sheath enclosing the core, getter and barrier layers and consisting essentially of a material selected from the group consisting of the platinum group metals and a platinum group metal alloy, volatilization of the oxides of said core material thereby being prevented from forming with said outer sheath an alloy of lower melting point than that of the sheath.
7. An article according to claim 6 having an interfacial space between the core and the sheath wherein the said space has a partial pressure of not more than 2 mm. of mercury.
8. An article according to claim 7 wherein the getter is a material selected from the group consisting of zirconium, tantalum, niobium, titanium, vanadium and hafnium.
9. An article according to claim 8 wherein the getter material is present in relatively small quantities as dilute alloys of molybdenum or platinum.
10. An article according to claim 6 wherein the getter is in the form of a plasma or flame sprayed coating.
11. An article according to claim 6 wherein the getter is concentrated in an area which, in use, is not subject to temperatures greater than 1,200* C. so that the getter absorbs oxygen without contact with the barrier layer, thereby avoiding decomposition of the sheath material.
12. An article according to claim 6 wherein the core material is molybdenum alloyed with minor quantities of at least one of the materials of the group consisting of titanium and zirconium.
13. An article according to claim 6 wherein the core material is alloyed with at least one of the metals selected from the group consisting of titanium, zirconium, niobium, tantalum and hafnium.
14. An article according to claim 6 wherein the getter material comprises zirconium metal and the barrier layer is zirconia.
15. An article according to claim 6 wherein the getter material is zirconium and the barrier layer is magnesia.
16. An article according to claim 15 wherein the barrier layer of magnesia includes a minor amount of silica.
17. An article intended to be used at high operating temperatures above 1,000* C. having a core of the class consisting of molybdenum, tungsten, an alloy of molybdenum, and an alloy of tungsten, and a flame-sprayed coating on said core of zirconium, a further flame-sprayed coating thereon selected from the group consisting of zirconia and magnesia, and a sheath enclosing said coated core and comprising a material selected from the group consisting of a platinum group metal and an alloy of at least one platinum group metal.