GB2095604A - Seal between two components - Google Patents

Abstract

A seal between two components, such as an aluminium alloy electrode (1) and a glass ceramic monoblock (2) of an integral cavity laser gyroscope, is made by applying a layer of metal, e.g. gold, (6, 7) to each component (1, 2) by vacuum deposition, compressing a member, e.g. an O-ring (10), of a further metal, e.g. indium, between the layers (6, 7) so as to promote diffusion, and subjecting the resulting seal to heat treatment to enhance the mechanical strength thereof. <IMAGE>

Description

SPECIFICATION Improved seal and method of production This invention relates to a method of making a seal between two components and a seal made by such a method. The invention is particularly, but not exclusively, concerned with the provision of a seal between a metal component and a component of an ultra low expansion glass ceramic material (i.e. between components having widely differing coefficients of expansion) in the construction of an integral cavity laser gyroscope.

Conventional glass/metal seals rely on wellestablished techniques that have been used in the electron tube and physics related industries for many years. The techniques available can be adapted or applied to meet most of the criteria demanded by specific applications, e.g. hermetic qualities, resistance to vibration and temperature, and minimum contamination of sealed gas environments.

The two more commonly used seals are usually called (a) matched or direct fusion seals and (b) mismatched seals. In both cases the term "matching" refers to the compatibility of the thermal coefficients of expansion of the materials to be joined.

In the case of the matched or direct fusion seal, a technique commonly employed is based on the ability of certain glasses and glass ceramics to "wet" specially prepared metal surfaces under controlled heating conditions. For the direct fusion seal, typical materials would be borosilicate type glasses and the nickel/iron/cobalt alloys such as the Nylo series, Telcoseal and Kovar. The metals are treated, prior to sealing, to form a precisely controlled oxide layer which partially diffuses into the glass when the two components are brought together and heated under slight pressure.

An alternative matched seal is the metal soldered type. This seal relies on the deposition of a metallic film (such as nickel by electroless deposition) on a glass component, and the subsequent soldering of the metal component to the deposited film. The deposited film must have excellent adhesion to the glass component and must not be chemically etched by the selected solder and/or flux.

Unlike the matched seal, the mismatched seal can be used where it is necessary to join two components whose expansion coefficients differ.

Seals of this type usually rely on the technique of interposing a number of glasses of differing expansion coefficients which effectively grade the seal between the metal and glass components. A metal soldered seal can also be used, where suitable, as a mismatched sealing technique.

It should be noted that the above techniques, with the exception of the metal soldered seal, rely on the softening and deformation of the glass using either a glass blowing torch, induction heating (where appropriate), or furnace heating.

Glass ceramic to metal seals can also be made where hermetically sealed electrical lead-through type connections are required, i.e. where it is desired to seal a metal pin within an aperture in an insulating support plate, the pin projecting from both surfaces of the latter. A typical glass ceramic to metal seal of this type uses a glass ceramic preform which surrounds and supports the lead through pin and also provides electrical insulation between the pin and the outer metal support plate. Following the careful preparation and jigging of the components, the complete unit is heated to a temperature usually sufficient to soften the glass ceramic pre-form, and then cooled under controlled conditions, to produce the desired type of seal assembly.

The present invention arose from the requirement for a method of attaching aluminium alloy electrodes to precisely machined blocks of ultra low expansion glass ceramic in the manufacture of laser gyroscopes of the integral cavity type.

A laser gyroscope utilises the properties of laser light to measure the frequency difference between two contra-rotating beams propagating within an optically closed ring cavity, usually in the form of a triangle or square. An integral cavity laser gyroscope has the optical cavity machined within a single block (usually referred to as the monoblock) of material exhibiting suitable properties.

The monoblock must provide features for:~ (i) Enabling the laser cavity to be optically closed by the attachment of suitable laser mirrors.

(ii) Completing a symmetrical gas discharge path within the cavity such that when the cavity is filled with gas or gas mix (typically helium and neon isotopes at predetermined partial pressures), an electrical gas discharge may be initiated and maintained by means of appropriately placed electrodes.

One of the many features required to ensure that the gyroscope performance is optimised is the maintenance of the loop or cavity path length as a constant over a prescribed temperature range.

This is usually achieved by (i) selecting the material of the gyroscope monoblock to have the lowest possible thermal expansion coefficient to minimise path length changes with temperature and (ii) to compensate for the changes which do occur by constructing one or more of the laser mirrors in the form of a flexible diaphragm whose axial movement may be precisely controlled effectively to tune the laser cavity.

A material which meets the requirement for the ultra-low expansion of the monoblock is a glass ceramic known by the trade name Zerodur. For the purposes of machining, grinding and polishing, this material behaves as a conventional glass, but it cannot be flame manipulated or softened without risk to its structure and alteration in its properties. Its expansion coefficient in the range 0 to 500C is typically 0 + 0.15 x l0-e/OC (third expansion grade) and 0 + 0.05 x 1 O-6/0C (first expansion grade). Suitable materials for the electrodes of an integral cavity laser gyroscope are aluminium alloys to British Standard L44 and L65.

The expansion coefficient of these alloys at 200C is typically 23 x 1 O-6/0C. The ratio of the expansion coefficients as between these alloys and Zerodur is, therefore, 153:1 and 460:1, respectively, at 200 C.

The requirements for the seals of an integral cavity laser gyroscope are as follows:~ (i) to have good hermetic sealing capability (ii) to be non-contaminating to the helium/neon gas fill (iii) to survive the required environment, with an adequate safety margin.

(iv) to be suited to fixturised assembly techniques whenever possible.

(v) to be repeatable and consistent with high yield rates.

(vi) to have no effect on the machined outer profile of the glass ceramic block.

(vii) to impart no residual strain to the glass ceramic block on completion of the seal.

Since no softening or deformation of the glass ceramic material is possible without physically damaging the monoblock, affecting the other cavity features or losing the ultra-low expansion properties of the material, none of the described conventional glass/metal sealing techniques can be applied for attaching the electrodes to the glass ceramic material.

The use of adhesives or fluxes cannot be seriously considered due to the risk of contamination of the helium/neon gas mixture by hydrocarbons and the resulting loss of performance and life of the completed gyroscope.

Therefor a specialised technique is required which maintains the properties of the glass ceramic material and yet fulfils all the aforementioned seal requirements.

In its broadest aspect the invention provides a method of making a seal between two components, the method comprising applying a layer of a first metal to one of the components, applying a layer of the first metal or a different second metal to the other of the components, positioning a further metal between the layers and applying pressure to compress the further metal between the layers and thereby effect a seal between the further metal and each of the layers.

This broad aspect covers methods which may not necessarily satisfy all the stringent requirements for seals in laser gyroscopes but which may be perfectly adequate for less demanding applications where an effective seal is required between two components which cannot or must not be softened by heating or deformed.

For the special case of seals for ring laser gyroscopes, it has been found by experiment that best results are obtained if the first metal is gold (which is conveniently applied by vacuum deposition to the aluminium alloy electrode and the glass ceramic block forming the respective components) and if the further metal is indium.

Diffusion of the two metals takes place, thus producing a firm bond and an excellent hermetic seal between the indium and gold. It is expected that other metals exhibiting the property of diffusion might be usable in the method accoridng to the present invention. In particular, it is thought that the first and/or second metal might be platinum or palladium and that the further metal might be aluminium (preferably of high purity).

The invention also provides a seal made by the inventive method, as recited in the appended claims.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:~ Figures 1 a and 1 b illustrate the preliminary production of an indium O-ring used in making a seal according to the invention, Figure 2 illustrates the parts of the seal before the seal is made, and Figure 3 illustrates the completed seal.

Referring first to Figures 2 and 3, the seal to be described is made between two components respectively in the form of an aluminium alloy electrode 1 and a glass ceramic monoblock 2 having an access hole 3 over which the electrode is to be sealed. The electrode 1 is to be sealed to the monoblock 2 as a step in the manufacture of an integral cavity laser gyroscope. The monoblock 2 is made of the aforementioned material Zerodur and is drilled parallel to a mounting surface 4 to form a passage 5. The passage 5, together with other similar passages drilled in the block, forms a continuous cavity through which the contra-rotating laser beams propagate. The elctrode 1 is shown schematically in block form, affording a flat surface for confronting the mounting surface 4 of the monoblock 2.The electrode 1 may be an anode or a cathode, and the invention may be applied to sealing the anode (or anodes) and the cathode to the glass ceramic monoblock 2 of the laser gyroscope.

At least the or each mounting surface 4 of the monoblock 2 is optically polished to transparency with a typical flatness of A/2. The sealing face of the or each electrode 1 is machined to have a surface finish of approximately 0.8 microns and to be free of surface defects. These sealing surface characteristics of the mounting surface 4 and electrode 1 are those which have been employed in and respect of which satisfactory seals have been produced but these characteristics are not essential for the production of satisfactory seals.

However, due care should be exercised in the preparation of the sealing surfaces to avoid excessive surface irregularities, scratches or other blemishes.

The monoblock 2 and the aluminium alloy electrode 1 are thoroughly cleaned using techniques to ensure the removal of all organic and inorganic contamination from both internal and external surfaces. Each component 1 or 2 is coated with gold by vacuum deposition, such that a complete annular layer 6 or 7 is formed. The layer 7 surrounds and is concentric with the access hole 3, the layer 6 being the same size as the layer 7 and directly overlying the latter when the component 1 is located in its desired position on the component 2. It is essential that the adhesion of the evaporated gold of the layers 6 and 7 to the components 1 and 2 is optimised.

Referring to Figures 1 a and 1 b, an indium O-ring is made from cleaned high purity indium wire 8 (metallic impurities typically < 1 5 parts per million) by cutting the ends of the wire 8 at an angle and joining the cut ends by a lap joint 9 (Figure 1 b. The diameter of the resulting O-ring 10 (Figure 2) is chosen to lie in the middle of each deposited annular layer 6 or 7 of gold.

The two gold-coated components 1 and 2 are brought together so as to sandwich the indium ring 10 between the two gold layers 6 and 7. A force whose axis (denoted by the arrow 1 1 ) is precisely controlled to be perpendicular to the gold coated surfaces of the components 1 and 2, is applied such that the indium O-ring is uniformly compressed between the gold layers 6 and 7 at room temperature.

A satisfactory seal requires intimate contact to be made between the pure gold and the pure indium. Since gold does not form a stable oxide, there is no problem in producing a pure gold surface, free of contaminants. Vacuum deposition provides a suitable and controllable method for meeting this requirement. Indium, however, rapidly forms an oxide layer and it is this layer which must be removed to expose the pure indium. In applying the compressive force to the indium O-ring 10, the pure indium metal is effectively squeezed out of its oxide skin and, due to its intrinsic softness, flows over the gold layers 6 and 7 to achieve pure metal to metal contact.

Once this stage has been reached there is no barrier to prevent the diffusion process taking place between the gold and indium to produce a bond at each indium/gold interface.

The typical effect of a 1 ,Q00 Kg force applied to a 19 mm diameter O-ring made from 0.5 mm diameter indium wire is to change the O-ring to a flat annular ring 12 (Figure 3) of washer shape whose width is approximately 3 mm and whose thickness is 0.075 mm.

At this stage, the resulting hermetic seal between the components 1 and 2 has a typical helium leakage rate of less than 5 x 10-11 torr litres sec-l as tested using a helium mass reflectometer. However a further step is required to optimise the mechanical strength of the seal.

Following the removal of the compressive force, the seal assembly is strength optimised by heating to 1 400C (slightly below the melting point of indium) for approximately twelve hours to promote the gold/indium diffusion process. The catalytic effect of high temperature on the gold/indium diffusion process is inversely proportional to the time required to optimise the seal. The heat treatment cycle may, therefore, be carried out at any temperature which does not jeopardise the integrity of the seal. At the conclusion of the heat treatment, visible signs of diffusion between the gold and indium may be observed by viewing the seal area through the glass ceramic block 2.

Tests on many seals using the described technique have shown that they are capable of meeting the requirements for integral cavity laser gyroscopes previously mentioned since:~ (i) their hermetic sealing capability for helium is typically better than 5 x 10-11 torr litres sec-'.

(ii) they are non-contaminating to the helium/neon gas fill in the laser gyroscope cavity since the materials used have insignificant vapour pressures at temperatures up to 1400. Typical values are ~gold 10-" torr at 6420C ~indium 10-11 tarr at 3680C ~aluminium alloy 10-11 torr at 5420C (iii) they exhibit a sufficiently high mechanical failure characteristic (disruption of the seal between electrode and monoblock by peeling forces) to give a typical safety factor of 50:1 over the required survival limit.

(iv) despite the large difference in expansion coefficients of the glass ceramic material and aluminium alloy the seal assemblies have survived temperatures of -400C to + 1 300C with no degradation of strength or leakage characteristics, as subsequently tested at room temperature.

(v) the fact that no softening is required of any of the materials involved in the seal, and since the highest temperature required is below the melting point of indium (1 57 C), the described technique has proved ideal for use with relatively simple fixturisation which allows the seals to be repeatedly made with minimum skill and time required.

(vi) yield rates of completed seals in fulfilling the hermetic and mechanical requirements have exceeded 95%.

(vii) since no significant deformation of the Zerodur takes place, the outer profile of the monoblock is maintained, allowing "machined-in" accuracies to be fully utilised.

(viii) using principles and techniques of photoelasticity, no change in strain is evident in the monoblock on completion of the seals.

In addition, the described method of sealing has been successfully used to attach stainless steel and nickel-iron alloy (known as Invar) components to low expansion glass-ceramics but would be equally suited for use with most glass type materials (such as the range of optical glasses, other glass-ceramics, and fused silica) and with most metals.

Claims (14)

1. A method of making a seal between two components, comprising applying a layer of a first metal to one of the components, applying a layer of the first metal or a different second metal to the other of the components, positioning a further metal between the layers and applying a force to compress the further metal between the layers and thereby effect a seal between the further metal and each of the layers.

2. A method according to claim 1 , wherein after said force is applied, the seal is heated and maintained at an elevated temperature below the melting point of the further metal to facilitate diffusion of the further metal and the first metal, or of the first and second metals.

3. A method according to claim 1 or 2, wherein the layers are applied to the respective components by vacuum deposition.

4. A method according to any of the preceding claims, wherein the first metal is gold, platinum or palladium and is applied as said layers to both said components.

5. A method according to any of the preceding claims, wherein the further metal is indium or aluminium.

6. A method according to any of the preceding claims, wherein the further metal is, before compression between the layers, made into the shape of an annular ring and wherein the resulting seal is of annular shape.

7. A method according to claim 6, wherein the annular ring is formed from a length of wire of the further metal, the ends of the wire being cut and joined by a lap joint to form the annular ring.

8. A method according to any of the preceding claims, wherein the components are made respectively of a metal and a glass type material.

9. A method according to claim 8, wherein the components are made respectively of an aluminium alloy and a glass ceramic material having a low coefficient of thermal expansion.

10. A method according to claim 8, wherein one of the components is made from stainless steel or a nickel-iron alloy having a low coefficient of thermal expansion, and the other component is made of a glass ceramic material also having a low coefficient of thermal expansion.

11. A seal made by a method according to any of the preceding claims.

1 2. A seal between a first component of aluminium alloy and a second component of a glass ceramic material, the seal being formed by a first layer of gold applied to the first component, a second layer of gold applied to the second component and an intermediate layer of indium which is disposed between the layers of gold, wherein at each of the two gold/indium interfaces diffusion has taken place between the indium and the gold.

1 3. A ring laser gyroscope having an aluminium alloy electrode bonded to a glass ceramic block by means of a seal according to claim 11 or 12.

14. A method of making a seal between two components, substantially as herein particularly described with reference to the accompanying drawings.

1 5. A seal between two components, substantially as herein particularly described with reference to the accompanying drawings.