GB2065882A - Coated shape memory effect elements - Google Patents

Abstract

An element of shape memory effect alloy has a coating applied to it. The coating may serve to protect the surface of the element from environmental attack, or, when insulated from the element, may be electrically conductive to enable the temperature of the element to be varied by passing an electrical current through the coating. Specified coating materials include metals such as nickel, or an alloy, a lacquer, or a plastics material. The element may be helical or tubular in form, or in the form of a torsion bar.

Description

SPECIFICATION Shape memory effect elements This invention relates to shape memory effect elements, methods of making those elements, and actuators which include those elements. By a shape memory effect (SME) element is meant an element made of an alloy which is capabie, on change of temperature in a transformation range, of exhibiting a martensitic transformation, the metallurgical structure changing from austenite at a temperature above the transformation range to martensite at a temperature below that range, and which exhibits an elastic modulus which varies significantly with temperature in a reversible manner which it is subject to temperature change in the transformation range. A number of SME alloys have been described in the literature; examples are nickel-titanium and copper-zincaluminium.

After an SME alloy has been formed into the shape required for the element, the alloy is brought into memory condition capable of performing the martensitic transformation. To that end, it is customary to heat treat the alloy by raising its temperature to bring it into beta state and then quenching it. If the element is then deformed at a temperature below the transformation range, and is heated through the range, it returns towards its original shape. On subsequent cooling, it changes again towards its deformed shape. The shape change is thus reversible and the cycle of shape change can be repeated many times.

The present invention resides, in one aspect, in an SME element as defined above, having a coating of another material covering at least a portion of its surface. We have found that such a coating can be beneficial to the SME element to which it is applied. For example, when the SME element is used in a temperature-responsive actuator, performing various temperaturedependent control operations such as the opening and closing of electric switches, valves and windows, reliability of the element is important, the SME element being required to execute the control operation almost indefinitely without failure, and without substantial alteration in performance.

We have found that some SME elements, and particularly those made from copper-zincaluminium, may fail prematurely, in that they are liable to suffer rapid deterioration of memory after a limited number of temperature cycles so that they can no longer perform their required actuating functions. The resulting short useful life detracts from the element's commercial viability.

In a preferred form of the present invention, premature deterioration of memory of an SME element is reduced, and its useful life is increased, by applying to the surface of the element a coating of a material which protects the surface from the environment. Normally, the entire surface is so coated but, where shape change occurs over a part only of the element, the coating may be limited to the surface of only that part. The shape memory itself, involving deformation of the element, is preferably implanted after coating.

The coating material should be capable of protecting the SME element from environmental attack, should be non-reactive with the material of the element, should be inert with respect to the environment, and should be capable of withstanding strain without spalling or delaminating. Further, the coating should offer minimum resistance to deformation of the SME element, unless it is required to bias the element.

The coating material may be a suitable metal, such as nickel, an alloy, a lacquer, or a plastics material according to the function it is to perform, other than that of protecting the SME element.

It is believed that the applied coating prevents loss of memory by protecting the surface of the element from environmental attack, which is thought to alter the composition of the surface of the element. When the element is initially deformed, maximum strain is located at or near the surface, and consequently it is the skin of the element that supplies the greater part of the memory causing change of shape on change of temperature. If the composition of that skin is altered by the atmosphere, as may occur particularly at temperatures above ambient, the part of the element that causes change of shape may no longer have the composition required for shape memory. By protecting the surface from the atmosphere, deterioration of the surface, and consequentially of the memory, of the element is prevented.

In a second preferred form of the invention, the coating is electrically conductive and is electrically isolated from the element itself. The element may then be used as an electrically controlled actuator responding to the passage of an electrical current to effect a displacement performing a control operation, such as the opening or closing of a valve or of electrical contacts.

Normally a solenoid has been used for that purpose. However, a solenoid does not perform satisfactorily in all applications. For example, when it is used to operate a valve, the valve is caused to close very rapidly, giving rise to hydraulic hammer in the pipework or atQeast the production of noise which is often undesirable from an environmental point of view and by reason of the stresses it sets up. In such applications, a gradual operation of the actuator is to be preferred.

An actuator has been proposed using an element of shape memory effect (SME) material in conjunction with an electrical heating element disposed in the vicinity of the SME element; on passage of current through the heating element, the temperature of the SME is caused to rise, resulting in a temperature-dependent displacement of the SME element. However, in theprior proposal, the heating element has been separate from the SME element with consequential bulkiness of the actuator.

By making the coating electrically conducting, the temperature of the element and hence its shape condition can be varied by the passage of a controlled current through the coating. The size of the resulting actuator is then substantially that of the SME element itself. Preferably the conductor envelops the SME element.

The invention also provides a method of making the SME element.

The SME alloy is preferably a copper-zincaluminium alloy which has been heat-treated to exhibit a martensitic transformation when the temperature is varied within the transition temperature range. The composition by weight of the alloy is advantageously Copper 70% Zinc 26% Aluminium 4% the proportions varying in minor respects from those values according to the transition temperature range required.

In one method of making an SME element illustrating the invention, a rod of an SME alloy as described above is shaped into a helix, and is heat treated to bring it into memory condition. A thin coating of insulating varnish is deposited on the helix and is cured at a temperature which is below that at which the memory state of the alloy is destroyed. Thus, the varnish may have a composition such that by chemical reaction it hardens with time at ambient temperature, or may be such that it may be cured at moderate temperature with or without irradiation with ultra violet light.

Next the cured insulating coating is covered by a layer of electrically conductive paint in a medium that can be cured as described above in relation to the insulating coating.

Finally, a second, outermost, insulating coating is applied to the cured conductive coating by the above method, to produce a compact actuator which, when current at low voltage is supplied to the conductive coating, performs a control exerting displacement by expansion of the helix.

Because of the time period needed for the conductive coating to cause heating of the helix, the helix expands slowly compared with the operation of a solenoid. The conductive coating itself has little thermal mass, and cools relatively quickly so that a rapid return of the actuator occurs once the current has been cut off.

Although it is preferred to have the SME element in helical form, it may take other forms as described in the literature. For example, the element may be a torsion bar or tube.

The coating described above may have the added benefit that the SME element is protected for the deleterious effects of exposure to the atmosphere, described above. Where electric control of the actuator constituted by the SME element is not required, a single protective coating may instead be applied to the surface of the element, or at least the surface of that part of the element that performs shape change on temperature variation. The coating is then a flexible material that is impervious to the atmosphere and may be a suitable metal or alloy, such as nickel, a lacquer, or a plastics material according to the functions it and the element itself are to perform.

In our earlier patents and patent applications we have described temperature-responsive actuators consisting essentially of an SME element, usually in the form of a helix, and a biasing spring made of steel or other material not exhibiting shape memory effect. A coating of nickel or some other metal or alloy elastically resists movement of the element and can itself act as the biasing spring.

Claims (14)

1. An SME element as defined having a coating of another material covering at least a portion of its surface.

2. An SME element according to claim 1, in which the coating material is such as to protect the coated surface from the environment.

3. An SME element according to claim 2, in which the coating material is a metal, an alloy, a lacquer or a plastics material.

4. An SME element according to claim 2 or claim 3, so designed that a part only of the element performs a shape change with change of temperature, and having the coating on the surface of only that part.

5. An SME element according to claim 1 , in which the coating is electrically conductive to provide electrical heating of the element, and is electrically insulated from the element.

6. An SME element according to claim 2, in which the electrically conductive coating is separated from the element by an electrically insulating coating and is itself covered by a further electrically insulating coating.

7. An SME element according to any one of the preceding claims, in the form of an alloy helix.

8. An SME element according to any one of the preceding claims in which element is composed of a copper-zinc-aluminium alloy.

9. An SME element according to claim 8, in which the alloy has an approximate composite by weight of 70% copper, 26% zinc and 4% aluminium.

10. A method of making an SME actuator, which includes the steps of shaping an SME alloy to form an SME element as defined, heat treating the element to bring it to a metallurgical condition capable of displaying shape memory, and applying a coating of a material other than that of the alloy to cover at least a portion of the element's surface.

1 A method according to claim 10, in which the material of the coating is such as to protect the coated surface from the environment.

12. A method according to claim 10, in which the coating is electrically conductive to enable the element to be electrically heated and is electrically insulated from the element.

13. A method according to claim 10, in which the element is given in sequence a first coating of an electrically insulating material, a second coating of an electrically conductive material, and a third coating also of electrically insulating material.

14. A coated SME element as defined, substantially as herein described.

1 5. A method of making an SME actuator, substantially as herein defined.