GB2129619A - Reed contact blade, reed switch and process for the production thereof - Google Patents

Abstract

In a reed switch comprising an electric coil (10), an elongated sealed glass capsule (12) and two ferromagnetic reed blades (14), each blade (4) has a ferromagnetic substrate with a contact end (16) and a coating comprising a layer of tungsten alloy extending over at least the contact end (16) of the substrate. Tungsten alloys have superior anti-sticking properties but tend to have a higher contact resistance than conventional coatings, so the tungsten alloy may be coated with a metal having a lower contact resistance, such as rhodium, or ruthenium. To enhance the adhesion and reduce the electrical resistance between the substrate and alloy and between the alloy and outer coating a layer of metals such as gold, nickel, cobalt, platinum or silver may be provided. <IMAGE>

Description

SPECIFICATION Reed blade, reed switch and process for the production thereof Field of the Invention This invention relates to reed switches and more particularly to an improved coating system for reed blades of reed switches.

Background of the Invention Reed switches or reed relays generally consist of a generally cylindrical electric coil, an elongated sealed glass capsule concentrically mounted within the coil and a pair of elongated, flexible reed blades having inner contact ends sealed within the glass capsule. The first reed blade extends through one end of the capsule so that the inner contact end of the reed blade is positioned within the space of the capsule. The second reed blade extends through the opposite end of the glass capsule such that the inner contact end of the second reed blade is spacedapart from and overlaps the inner contact end of the first reed blade. In other words, the inner contact ends of the two reed blades are positioned adjacent each other within the glass capsule in a spaced-apart overlapping relationship.The outer ends of the reed blades extend through opposite ends of the capsule wall and are connected outside the capsule to an electrical circuit.

The reed blades are generally made of a ferromagnetic alloy, typically a nickel-iron alloy.

When the coil is activated, i.e., when the current is passed through the coil, a magnetic field is generated which causes the ferromagnetic reed blades to be magnetically attracted to each other, thereby establishing electrical contact between the inner contact ends of the reed blades closing an electrical circuit and allowing current to pass between the reed blades for activating the electrical circuit. When the coil is deactivated, contact between the reed blades is broken and the electrical circuit is opened.

Reed switches are used in many electrical devices and have found extensive use in computers, communication equipment and automatic testing equipment. Such an application may require many millions of switching operations. If, after extended use, the contact resistance of the reed blades, i.e., the electrical resistance between the reed blades when they are in contact becomes too high, insufficient current will pass between the reed blades during contact to activate the electrical circuit. In most applications, while this type of failure is undesirable, it presents no hazard. For example, in computer applications this type of failure does not pesent a hazard to the computer circuitry or the data stored in the computer.

On the other hand, one failure caused by "sticking" of the reed blades, i.e., the reed blades not breaking contact when the externally applied magnetic field is removed may be extremely hazardous. In computer applications, this type of failure may result in loss of valuable data or damage to computer circuitry and therefore must be avoided at all costs.

To reduce "sticking" failures, various materials have been used to coat the reed blade. For example, coatings of hard gold alloys and ruthenium are known. The occlusion of carbon in a gold alloy coating is also known.

The number of switching operations that a reed switch can undergo before sticking failures occur depends on several factors includng the load under which the reed switch is operated. For example, reed switches can presently undergo about 20 million switching operations under low resistive loads, e.g., 10 volts direct current (d.c.) and 4 milliamperes. There is a continuing demand for reed switches having longer lifetimes, i.e., capable of undergoing a greater number of switching operations. There is a competing demand, however, for reed switches that eliminate, or at least minimize, the possibility of sticking failures or maintain a dynamic contact resistance below one ohm.

Summary of the Invention According to the invention, there is provided a reed blade comprising an elongated ferromagnetic substrate and a coating comprising a layer of tungsten alloy covering at least the contact end of the substrate. The tungsten alloy is preferably selected from a group consisting of tungsten-cobalt alloys, tungsten-nickel alloys, and tungsten-iron alloys.

In a preferred embodiment of the invention, the layer of tungsten alloy serves as an undercoat to a second layer comprising a metal having a contact resistance lower than the tungsten alloy.

Preferred metals of the second layer include rhodium and ruthenium.

In a particularly -preferred embodiment of the invention, the coating comprises a bottom layer of tungsten alloy, an intermediate layer of rhodium and a top layer of ruthenium.

There is further provided a process for making a reed blade that comprises an elongated ferromagnetic substrate and a coating comprising a layer of tungsten alloy. The process comprises first electroplating a thin strike layer comprising a metal capable of forming cohesive bonds with both the substrate and the tungsten alloy onto at least the contact end of a ferromagnetic reed blade substrate. A layer of tungsten alloy is then electroplated onto the strike layer.

In a preferred embodiment of the invention, a second thin strike layer is electroplated over the tungsten alloy layer. A layer of rhodium or ruthenium is then electroplated over the second strike layer.

In a particularly preferred embodiment of the invention, a layer of rhodium is electroplated over the second strike layer. A layer of ruthenium is then electroplated directly over the rhodium layer.

Brief Description of the Drawings These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing which is a cross-sectional view of a reed relay.

Detailed Description This invention is particularly applicable to conventional reed switches such as that illustrated in Fig. 1. Such a reed switch comprises an electric coil 10, an elongated sealed glass capsule 1 2 and two ferromagnetic reed blades 14. The capsule 1 2 is concentrically mounted in a generally cylindrical coil 10. The reed blades 14 extend through the wall of the glass capsule at opposite ends of the glass capsule so that each reed blade has an inner contact end positioned within the interior of the glass capsule. The inner or contact ends 16 of the two reed blades overlap each other and are spaced-apart slightly. The outer ends 1 8 of the two reed blades extend away from opposite the glass capsule and are connectable to an electric circuit.When current is passed through the coil, a magnetic field is generated which causes the ferromagnetic reed blades to be magnetically attracted to each other thereby establishing electrical contact between the inner contact ends of the reed blades to contact each other for closing an electrical circuit and allowing current for activating such an external circuit to pass between the reed blades.

As used herein, "contact end of the reed blades" refers to the portion of the reed blade that makes contact with the other reed blade when the external magnetic field is applied.

In accordance with the invention, each reed blade 14 comprises an elongated ferromagnetic substrate, preferably made of a nickel-iron alloy, and a coating comprising a layer of tungsten alloy over at least the contact end of the substrate.

As used herein, "contact end of the substrate" refers to the substrate portion of the contact end of the reed blade.

Preferred tungsten alloys include tungsten-cobalt alloys, tungsten-nickel alloys, and tungsteniron alloys. Such tungsten alloys are extremely hard, have exceptional wear-resistance properties and have anti-sticking properties superior to conventional reed blade coatings.

Reed blade coatings are generally limited to about 200 microinches in thickness. Greater thicknesses of the coating cause the magnetic substrates of the reed blades, when assembled in a reed switch, to be spaced too far apart for contact to occur when the external magnetic field is applied.

Many of the tungsten alloys exhibit at least some degree of magnetism. The degree of magnetism exhibited by such a tungsten alloy depends on the metal alloyed with tungsten and the amount of tungsten in the alloy. For example, tungsten alloys tend to exhibit magnetic properties when tungsten in the alloy is less than about 20% to 30% by weight. The thickness of the layer of tungsten alloy can therefore exceed about 200 microinches if such a magnetic tungsten alloy is used. Thicknesses up to about 400 microinches have been used successfully.

The thickness of the layer of tungsten alloy is generally not critical. The preferred thickness of the tungsten-alloy coating generally depends on the number of switching operations required in the particular application in which the reed switch having such reed blades is used. Generally, the greater the number of switching operations required by the application, the greater the preferred thickness is. For example, a thickness of from about 1 90 to about 210 microinches is presently preferred for high load applications, e.g., a 60 volt inductive load, which requires about 5 million switching operations.

It is desirable, although optional, to provide a thin strike layer comprising a metal capable of forming cohesive bonds with both the substrate and the tungsten alloy and having a thickness of from about 5 to about 10 microinches, between the substrate and the tungsten alloy layer.

Strike layers comprising gold, nickel or cobalt are preferred; however, strike layers comprising other metals, e.g.,-platinum and silver, are also suitable. The use of such a strike layer depends on the method of application of the tungsten alloy over the substrate. Generally, the strike layer enhances the adhesion and reduces the electrical resistance between the tungsten alloy layer and the substrate.

It has been found that tungsten alloys, while having superior anti-sticking properties as compared with metals used in conventional coatings, e.g., hard gold alloys and ruthenium, tend to have a higher contact resistance than such metals. For many applications, this does not present a problem. However, for some load applications, e.g., resistive loads of 10 volts d.c.

and 4 milliamperes, there may be insufficient current passing between the reed blades of the relay switch during contact to activate the external circuit. For such applications, it is preferred to coat the tungsten alloy layer with at least one layer comprising a metal having a contact resistance lower than the tungsten alloy.

In a preferred embodiment of the invention, a layer of rhodium is applied over the tungsten alloy layer. Rhodium has a lower contact resistance than tungsten alloys. It has been found that reed blades having a layer of rhodium over a layer of tungsten alloy can undergo in excess of 100 million operations without sticking.

The thicknesses of both the rhodium and the tungsten alloy layers are not critical. A rhodium layer thickness in a range of at least about 60 microinches is preferred for applications requiring about 100 million switching operations. Such a thickness enables about 100 million switching operations to occur before the rhodium layer wears through to the underlying tungsten alloy layer. It is presently preferred that the underlying tungsten alloy layer has a thickness in the range of about 20 to about 50 microinches. It has been found that such a thickness of the tungsten alloy layer tends to maximize the number of switching operations that occur before the rhodium layer wears through to the tungsten alloy layer.

It is desirable, although again optional, to have a thin strike layer comprising a metal capable of forming cohesive bonds with both the tungsten alloy and rhodium and having a thickness of from about 5 to about 10 microinches, between the rhodium layer and the tungsten alloy layer to enhance the adhesion and to reduce the electrical resistance between those layers. Strike layers comprising gold, nickel and cobalt are preferred.

In another preferred embodiment of the invention, a layer of ruthenium is applied over the tungsten alloy layer. Ruthenium is presently most preferred as the exterior or top layer of the coating. This is because ruthenium has the most preferred combination of properties including excellent anti-sticking properties and stability, and contact resistance sufficiently low for low load applications. By conventional application techniques, ruthenium tends to be highly stressed when applied and therefore can be applied only as a very thin layer, generally no more than about 20 to about 50 microinches thick. Such a thickness, i.e., about 20 to about 50 microinches, is preferred as it maximizes the benefits of the ruthenium.A ruthenium layer having a iesser thickness is less preferred because such a layer wears through to the underlying tungsten alloy layer more rapidly than a thicker layer and thus, the benefits of the ruthenium layer are more rapidly lost.

It has been found that in combination with a ruthenium layer an underlying tungsten alloy layer having a thickness of from about 20 to about 50 microinches tends to maximize the number of switching operations that occur before the ruthenium layer wears through to the tungsten alloy layer. Accordingly, an underlying tungsten alloy layer having a thickness of from about 20 to about 50 microinches is presently preferred.

It is desirable, although optional, to have a strike layer comprising a metal capable of forming cohesive bonds with both the tungsten alloy and ruthenium having a thickness of from about 5 to about 10 microinches, between the tungsten alloy layer and the ruthenium layer to enhance adhesion and to reduce electrical resistance between the layers. Strike layers comprising gold, nickel or cobalt are preferred.

It has been found that a reed switch comprising reed blades coated with a layer of tungsten alloy having a thickness of about 20 to about 50 microinches and a top layer of ruthenium having a thickness of from about 20 to about 50 microinches can undergo up to about 20 million switching operations before the ruthenium layer wears down to the tungsten alloy layer at resistive loads of 10 volts d.c. and 4 milliamperes.

A particularly preferred embodiment of the invention comprises reed blades in which at least the contact ends of the reed blades have a coating comprising an underlying layer of tungsten alloy, an intermediate layer of rhodium and a top layer of ruthenium.

Ruthenium is preferred as the top layer because ruthenium has the most preferred combination of anti-sticking properties, stability, wear-resistance, and contact resistance.

It is presently preferred that the top layer of ruthenium has a thickness of from about 20 to about 50 microinches.

A layer of rhodium is particularly preferred as an intermediate layer because it has sufficiently low contact resistance for use in low load applications and also very good anti-sticking properties and because it tends to increase the life of the ruthenium layer.

While not being-bound by theory, it is believed that the rhodium layer acts as a contributory low contact resistance "back-up" having different crystalline structure than the ruthenium layer (face-centered cubic structure for rhodium and'hexagonal closed packed structure for ruthenium). It is believed that these structural differences increase the life, i.e., the wear-resistance, of the ruthenium layer and prevents or at least reduces cracking of the ruthenium layer.

Rhodium continues to provide good switching characteristics in the event the ruthenium layer is worn through to the rhodium layer.

The thickness of the rhodium layer is not critical, however, it is presently preferred that the rhodium layer have a thickness of at least about 60 microinches. For applications requiring about 100 million operations, thicknesses of from about 80 to about 90 microinches provide ample excess capability to assure that at least 100 million operations can be achieved and is thus presently preferred for such applications. It has been found that when the rhodium layer has a thickness in the range of about 80 to about 90 microinches, the relay switch can undergo from about 200 to about 300 million operations before the contact resistance begins to increase as a result of the higher contact resistance of the tungsten alloy. In other words, it takes from 200 to 300 million operations before the ruthenium and the rhodium layers wear through to the underlying tungsten alloy layer.

A thickness greater than about 90 microinches can be used but provides no added benefits for such applications. If the application requires more than 100 million operations, a thicker rhodium layer would be preferred.

Again, the thickness of the underlying tungsten alloy layer is also not critical. It has been found that reed blades having a top layer of ruthenium which has a thickness of from about 20 to about 50 microinches and an intermediate layer of rhodium which has a thickness of at least about 60 microinches, the highest number of switching operations tend to occur when the tungsten alloy layer has a thickness of from about 20 to about 50 microinches.

In the preferred embodiments, the underlying tungsten alloy layer provides a unique advantage in that it assures that, if the ruthenium and/or rhodium layers wear through, no sticking will occur. The tungsten alloy, although having a higher contact resistance than ruthenium or rhodium, is very hard and has exceptional anti-sticking properties. If the contact resistance of the tungsten alloy is too high for the particular application, the relay switch can be replaced. Thus, switching failures due to the high contact resistance provide a hon-hazardous signal that the switch requires replacement. If the contact resistance of the tungsten alloy is sufficiently low for the particular application, the switch can be operated for additional time after the ruthenium and/or rhodium layers wear through.

Another unique advantage is presented by such a tungsten alloy layer, whether it is used as the top layer or as an underlying layer, is that tungsten alloys tend to harden in response to heat. The electrical arc that is generated during a switching operation heats the surface of the tungsten alloy resulting in an increase in the hardness of the surface, which improves the wearresistance of the tungsten alloy.

A preferred process for manufacturing such reed blades comprises the electrolytic deposition, i.e., electroplating, of such layers onto ferromagnetic reed blade substrates. Electroplating is the preferred method of application because it provides a smoother coating than other conventional application methods, e.g., sputtering and flame spraying. In such a process, reed blade substrates are first cleaned and activated. The substrate can also be polished, e.g., by conventional electropolishing techniques prior to activation.

The activated substrates are first immersed in a strike solution for depositing a thin metallic layer capable of forming cohesive bonds with the substrate and with a subsequently deposited tungsten alloy layer. Commercially available strike solutions, e.g., gold strike solutions, nickel strike solutions and cobalt strike solutions, are suitable. A strike layer is electrolytically deposited onto at least the contact end of the substrates to a thickness of from about 5 to about 10 microinches.

The substrates are then rinsed in water and immersed in a tungsten alloy electroplating bath.

A tungsten alloy layer is deposited over the strike layer to a desired thickness. Conventional tungsten alloy electroplating solutions, e.g., see Electrodeposition of Alloys, Vol. 2, pages 347-412 by A. Bramer, Academie Press-(1963), are suitable.

It has been found that tungsten alloy deposits having a greater amount of tungsten tend to be harder and exhibit better wear-resistance and anti-sticking properties than tungsten alloys having a lesser amount of tungsten. It is.therefore preferred that the concentration of tungsten in the tungsten alloy electroplating solution be maintained at the upper end of the suggested concentration range for the particular solution that is used. For example, if a formulation for a tungsten alloy electroplating solution suggests the use of 5 to 10 g/l sodium tungstate dihydrate, it is preferred that about 10 g/l sodium tungstate dihydrate be used. This tends to maximize the percentage of tungsten in the deposit for that particular solution formulation.

Following electrolytic deposition of the tungsten alloy layer, the reed blades are preferably heat treated. The heat treatment enhances the bonding of the tungsten alloy layer to the substrate. Generally, during the heat treatment, at least a portion of the strike layer diffuses into both the substrate and the tungsten alloy layer, thereby forming excellent bonds to both. A heat treatment of about 1 hour at about 500"C in hydrogen gas is suitable. Hydrogen gas is preferred because it is a reducing agent and therefore not only prevents the formation of an oxide film on the surface of the tungsten alloy, but reduces any oxide film already present on the surface.

For applications requiring reed blades having lower contact resistance than that of the tungsten alloy layer, the tungsten alloy plated reed blades, with or without a heat treatment, are rinsed in water and again immersed in a strike solution for depositing a thin metallic layer capable of cohesive bonding to both the tungsten alloy layer and a subsequently deposited layer comprising a metal having a lower contact resistance than the tungsten alloy. Suitable strike solutions are the same as those previously described. A second strike layer about 5 to about 10 micro inches thick is deposited over the tungsten alloy layer.

The reed blades are again rinsed and then immersed in a conventional rhodium electroplating solution. Such solutions are commercially available. A layer of rhodium is then deposited to a desired thickness, generally at least about 60 microinches.

Alternatively, the reed blades having been plated with the second strike layer are immersed in a conventional ruthenium electroplating solution. Such solutions are also commercially available.

A layer of ruthenium is then deposited to a desired thickness, generally from about 20 to about 50 microinches.

In a particularly preferred embodiment of the invention, rhodium is deposited over the second strike layer as described above. The reed blades are then rinsed and plated with ruthenium, also as described above, directly over the rhodium.

In such an electroplating process it is desirable to maintain a minimum amount of impurities in all electroplating solutions. To reduce the possibility of contamination of solutions and staining of the deposit, the reed blades can be dried following rinsing prior to the deposition of the rhodium layer and prior to the deposition of the ruthenium layer.

Following electrolytic deposition of the final layer, the reed blades are preferably heat treated for a time and at a temperature sufficient to form an electrically conductive oxide coating on the ruthenium layer. This oxide coating enhances the wear-resistance and/or anti-sticking properties of the ruthenium layer. A heat treatment of about 30 minutes of about 450"C in air or other oxidizing atmosphere is suitable.

In addition to the formation of an oxide coating on the ruthenium, such a heat treatment causes migration of at least a portion of the gold or gold alloy layer, positioned between the tungsten alloy layer and the rhodium or ruthenium layer, into the tungsten alloy layer. This reduces any sticking problem caused by the gold layer when the ruthenium layer and the rhodium layer are worn through to that layer.

EXAMPLE 1 The contact ends of reed blade substrates comprising a nickel-iron alloy were first cleaned and polished by immersion in an electropolish solution marketed by Electro-Glo Co. under the trade name Electro-Glo "300" for about 1 minute. The solution was agitated and maintained at a temperature from about 50-55"C. The current density was maintained at about 1 50 amperes per square foot.

Following electropolishing, the reed blade substrates were rinsed in hot deionized water (50-70"C) and then activated by immersion in an electroactivation solution containing about 10% by volume sulfuric acid for about 2 minutes. The electroactivation solution was maintained at room temperature. Current density was maintained at about 5 amperes per square foot.

The reed blade substrates were again rinsed in a hot deionized water rinse and then immersed in a gold strike solution, namely, EAS low pH pure Gold Strike marketed by Engelhard Inc. The gold strike solution was maintained at 40"C. Gold was deposited over the contact end to a thickness of 5-10 microinches at a current density of about 5 amperes per square foot.

The gold plated reed blades were then rinsed in hot deionized water and then dried at 60 to 70"C. The reed blades were then immersed in a tungsten-cobalt alloy electroplating solution having the following formulation: citric acid 65 g/l cobalt chloride 6-hydrate 10 g/l ammonium sulfate 50 9/1 sodium tungstate dihydrate 10 g/l ammonium hydroxide to pH 8.5 to 9.0 The solution was maintained at about 70"C, without agitation.

A tungsten-cobalt alloy was deposited over the gold layer at a current density of about 25 amperes per square foot to a thickness of from about 1 90 to about 210 microinches.

The tungsten-cobalt alloy plated reed blades were then rinsed in hot deionized water and dried in air at 60 to 70"C. The reed blades were then cleaned by immersion in neutralizing rinses followed by sequential rinsing in hot and cold high purity deionized water with ultrasonic energy. The reed blades were then heat treated for about 1 hour at about 500"C in pure hydrogen.

The reed blades were then assembled into reed switches and tested under a 60 volt inductive load. Such reed switches underwent 8 million switching operations without a sticking failure.

EXAMPLE 2 Reed blade substrates were electropolished, electro-activated and plated with a gold strike as described in Example 1. A tungsten-cobalt alloy layer was deposited over the gold strike layer as described in Example 1 to a thickness of about 40 microinches.

The tungsten-cobalt alloy plated reed blades were then rinsed in hot deionized water and immersed again in the gold strike solution. A second layer of gold having a thickness of about 5 to 10 microinches was deposited over the tungsten-cobalt layer.

The reed blades were then rinsed in hot deionized water and dried in air at 60 to 70"C. The dry reed blades were then immersed in a rhodium electroplating solution, namely Rhodex Solution, marketed by Sel Rex Division of Occidental Chemical Corp. The rhodium solution was maintained at about 40 to 50"C and agitated. Rhodium was deposited at a current density of about 10 amperes per square foot to a thickness of about 60 microinches.

The reed blades were rinsed in hot deionized water and air dried at about 60 to 70"C. The reed blades were then cleaned by immersion in neutralizing rinses followed by sequential rinsing in hot and cold high purity deionized water with ultrasonic energy. The reed blades were then dried for about 30 minutes at about 450"C in air.

The reed blades were assembled into reed switches and tested under various loads. The reed switches underwent over 10 million switching operations without a sticking failure under resistive loads of about 10 volts d.c. and 1 milliampere, 10 volts d.c. and 10 milliamperes, 10 volts d.c. and 100 milliamperes, 20 volts d.c. and 1 milliampere, 20 volts d.c. and 10 milliamperes and 20 volts d.c. and 100 milliamperes. Some reed switches exhibited a contact resistance greater than 0.5 ohm after 10 million switching operations when tested under a resistive load of about 20 volts d.c. and 100 milliamperes.

EXAMPLE 3 Reed blade substrates were electropolished, electro-activated and plated with a gold strike as described in Example 1. A tungsten-cobalt alloy layer was deposited over the gold strike layer as described in Example 1 to a thickness of about 40 microinches.

The tungsten-cobalt alloy plated reed blades were then rinsed in hot deionized water and immersed again in the gold strike solution. A second layer of gold having a thickness of about 5 to 10 microinches was deposited over the tungsten-cobalt layer.

The reed blades were then rinsed in hot deionized water, dried in air at about 60 to 70"C and then immersed in a ruthenium electroplating solution. The ruthenium electroplating solution was Ru 8X plating bath from Engelhard Inc. and was maintained at about 70"C with agitation.

Ruthenium was deposited at a current density of about 5 amperes per square foot to a thickness of about 30 microinches.

The reed blades were rinsed in hot deionized water and air dried at about 60 to 70"C. The reed blades were then cleaned by immersion in neutralizing rinses followed by sequential rinsing in hot and cold high purity deionized water with ultrasonic energy. The reed blades were then dried for about 30 minutes at about 450"C in air.

The reed blades were assembled into reed switches and tested under various loads The reed switches underwent over 5 million switching operations without a sticking failure under resistive loads of about 10 volts d.c. and 1 milliampere, 10 volts d.c. and 10 milliamperes, 10 volts d.c.

and 100 milliamperes, 20 volts d.c. and 1 milliampere, 20 volts d.c. and 10 milliamperes and 20 volts d.c. and 100 milliamperes. Some reed switches exhibited a contact resistance greater than 0.5 ohm after one million switching operations when tested under a resistive load of about 20 volts d.c. and 100 milliamperes.

EXAMPLE 4 Reed blade substrates were electropolished, electro-activated and plated with a gold strike as described in Example 1. A tungsten-cobalt alloy layer was deposited over the gold strike layer as described in Example 1 to a thickness of about 20-25 microinches.

The tungsten-cobalt alloy plated reed blades were then rinsed in hot deionized water and immersed again in the gold strike solution. A second layer of gold having a thickness of about 5 to 10 microinches was deposited over the tungsten-cobalt layer.

The reed blades were then rinsed in hot deionized water and dried in air at 60 to 70"C. The dry reed blades were then immersed in a rhodium electroplating solution, namely Rhodex Solution, marketed by Sel Rex Division of Occidental Chemical Corp. The rhodium solution was maintained at about 40 to 50"C and agitated. Rhodium was deposited at a current density of about 10 amperes per square foot to a thickness of about 80 to 90 microinches.

The rhodium plated reed blades were then rinsed in hot deionized water, dried in air at about 60 to 70"C and then immersed in a ruthenium electroplating solution. The ruthenium electroplating solution was Ru 8X plating bath from Engelhard Inc. and was maintained at about 70"C with agitation. Ruthenium was deposited at a current density of about 5 amperes per square foot to a thickness of about 30 to 40 microinches.

The reed blades were rinsed in hot deionized water and air dried at about 60 to 70"C. The reed blades were then cleaned by immersion in neutralizing rinses followed by sequential rinsing in hot and cold high purity deionized water with ultrasonic energy. The reed blades were then dried for about 30 minutes at about 450"C in air.

The reed blades were assembled into reed switches and tested. The reed switches underwent over 200 million switching operations without a sticking failure under a resistive load of about 10 volts d.c. and about 4 milliamperes. Such reed switches also underwent over 200 million switching operations without a sticking failure under a resistive load of about 5 volts d.c. and 100 milliamperes or a dynamic contact resistance failure at or over one ohm.

EXAMPLE 5 The procedure of Example 2 was followed with the exception that a tungsten-nickel alloy electroplating solution having the formulation: citric acid 100 g/l nickel sulfate hexahydrate 90 g/l ammonium sulfate 50 g/l sodium tungstate dihydrate 63 g/l ammonium hydroxide to pH 8.5 to 9.0 was used in place of the tungsten-cobalt electroplating solution. Plating conditions were the same as those previously described.

The reed blades were assembled into reed switches, some of which were tested under a resistive load of about 10 volts d.c. and about 4 milliamperes and others under a resistive load of about 5 volts d.c. and 100 milliamperes. Such reed switches underwent over 200 million switching operations without a sticking failure under both loads.

The preceding description has been presented with reference to the presently preferred embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate thfl alterations and changes in the described compositions and processes can be practiced without meaningfully departing from the principles, spirit and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise compositions and processes described, but rather should be read consistent with and as support for the following claims which are to have their fullest fair scope.

Claims (29)

1. A reed blade comprising a ferromagnetic substrate having a contact end and a coating comprising a layer of tungsten alloy over at least the contact end of the substrate.

2. A reed blade as claimed in claim 1 wherein the tungsten alloy is selected from the group consisting of tungsten-cobalt alloys, tungsten-nickel alloys, and tungsten-iron alloys.

3. A reed blade as claimed in claim 1 wherein the layer of tungsten alloy has a thickness of up to about 400 microinches.

4. A reed blade as claimed in claim 1 wherein the coating further comprises a second layer over the layer of tungsten alloy, said second layer comprising a metal having a lower contact resistance than the tungsten alloy.

5. A reed blade as claimed in claim 4 wherein the second layer comprises a metal selected from the group consisting of rhodium and ruthenium.

6. A reed blade as claimed in claim 4 wherein the layer of tungsten alloy has a thickness of from about 20 to about 50 microinches.

7. A reed blade as claimed in claim 5 wherein the second layer comprises ruthenium.

8. A reed blade as claimed in claim 7 wherein the second layer has a thickness of from about 20 to about 50 microinches.

9. A reed blade as claimed in claim 5 wherein the second layer comprises rhodium.

10. A reed blade as claimed in claim 9 wherein the second layer has a thickness of at least about 60 microinches.

11. A reed blade as claimed in claim 9 wherein the coating further comprises a third layer over the second layer, said third layer comprising ruthenium.

1 2. A reed blade as claimed in claim 11 wherein the third layer has a thickness of from about 20 to about 50 microinches.

1 3. A reed blade as claimed in claim 1 wherein the coating further comprises a strike layer between the substrate and the layer of tungsten alloy, said strike layer comprising a metal capable of forming cohesive bonds with both the substrate and the layer of tungsten alloy.

14. A reed blade as claimed in claim 1 3 wherein the strike layer comprises a metal selected from the group consisting of gold, nickel and cobalt.

1 5. A reed blade as claimed in claim 1 3 wherein the strike layer has a thickness of from about 5 to about 10 microinches.

16. A reed blade as claimed in claim 4 wherein the coating further comprises a strike layer between the layer of tungsten alloy and the second layer, said strike layer comprising a metal capable of forming cohesive bonds with both the layer of tungsten alloy and the metal of the second layer.

1 7. A reed blade as claimed in claim 1 6 wherein the strike layer comprises a metal selected from the group consisting of gold, nickel and cobalt.

1 8. A reed blade as claimed in claim 1 6 wherein the strike layer has a thickness of from about 5 to about 10 microinches.

1 9. A reed switch comprising: an electric coil; an electrically insulated capsule disposed within the coil; and a pair of ferromagnetic reed blades as defined in any of claims 1-18, said reed blades extending through the capsule so that the contact ends of the reed blades are disposed within the capsule in a spaced-apart relationship to each other and the ends of the reed blades opposite the contact ends extend outwardly from the capsule.

20. A process for the production of a reed blade comprising: providing a ferromagnetic reed blade substrate having a contact end; electrolytically depositing a metal capable of forming cohesive bonds with the substrate and tungsten alloys onto at least the contact end of the substrate to thereby form a first strike layer having a thickness of from about 5 to about 10 microinches; and electrolytically depositing a tungsten alloy onto the first strike layer to thereby form a tungsten alloy layer.

21. A process as claimed in claim 20 wherein the tunsten alloy is deposited to a thickness of up to about 400 microinches.

22. A process as claimed in claim 20 further comprising heating the reed blade in hydrogen gas for a select time at a temperature sufficient to reduce metal oxides present on the surface of the tungsten alloy layer.

23. A process as claimed in claim 22 wherein the reed blades are heated for about 1 hour at about 500"C in hydrogen gas.

24. A process as claimed in claim 20 further comprising: electrolytically depositing a metal capable of forming cohesive bonds with the tungsten alloy and with rhodium and ruthenium onto the tungsten alloy layer to thereby form a second strike layer having a thickness of from about 5 to about 10 microinches; and electrolytically depositing a metal comprising rhodium and ruthenium onto the second strike layer to thereby form a second layer.

25. A process as claimed in claim 24 wherein the second layer comprises rhodium and the process further comprises: electrolytically depositing a metal comprising ruthenium onto the second layer to thereby form a third layer.

26. A process as claimed in claim 25 wherein the tungsten alloy layer is deposited to a thickness of from about 20 about 50 microinches, the second layer is deposited to a thickness of at least about 60 microinches, and the third layer is deposited to a thickness of from about 20 to about 50 microinches.

27. A process as claimed in claims 23, 24 or 25 further comprising heating the reed blade for a select time at a temperature sufficient to form a conductive oxide film on the surface of at least the contact end of the reed blade.

28. A process as claimed in claim 27 wherein the reed blade is heated for about 30 minutes at a temperature of about 450"C.

29. A reed blade, a reed switch or a process for the production of a reed blade, substantially as hereinbefore described with reference to the accompanying drawings.