GB2219006A - Coated fibre for use in a metal matrix - Google Patents

Description

1 p IMPROVEMEWS IN OR RELATING TO COATED FIBRES FOR USE IN A METAL MATRIX

AND IN A COMPOSITE STRUCTURE The present invention relates to ceramic fibres which are to be set in a metal matrix, from which resulting composite, a strong, lightweight structure is achieved.

The present invention also embraces a metal matrix composite which includes ceramic fibres of the kind mentioned hereinbefore.

The aerospace industry is forever searching for materials which are lighter, yet at least as strong as presently known materials. One step in this direction merely by way of example, was the adoption of Titanium and Titanium based alloys. Lightness and strength was achieved, but at considerable cost and difficulty in working because of reaction of the material surface in ambient environmental conditions.

The next step was directed at reducing the volume of lightweight metal per volume of the finished structure without losing, and hopefully gaining, strength and to this end, ceramic fibres were substituted for the volume of metal which had been obviated. one example of such a composite is one in which silicon carbide fibres were embedded in Titanium. The fibres rapidly degraded however, as a result of the Titanium diffusing across the interface.

One attempt to cure the diffusion problem was to provide the silicon carbide fibres with a carbon rich outer layer. Diffusion of the Titanium into the fibres was slowed.

One aspect of the present invention seeks to provide ceramic fibres which include an improved coating which has an improved resistance to diffusion of the matrix material when the fibres are embedded therein.

According to one aspect of the present invention, a ceramic fibre includes a double coating, the inner one of which is a metal which produces a thermodynamically stables oxide and the outer coating is said stable oxide of said metal.

The invention further provides a metal matrix composite structure comprising a Titanium or Titanium alloy matrix containing ceramic fibres which have a first, inner coating of a metal which produces a thermodynamically stable oxide and a second, outer coating which is said stable oxide of said metal.

According to a further aspect of the present invention, a method of providing a ceramic fibre with an anti-diffusion coating comprises the step of enveloping the fibre with a metal from a group comprising yttrium, Hafnium and Zirconium and then covering the metal coating with an oxide of said metal.

The invention will be described, by way of example and with reference to the accompanying drawings in which Fig 1 is a diagrammatic crosssectional view through a composite structure which includes coated ceramic fibres in accordance-with the present invention.

Fig 2 is a graph of the extent of interface diffusion relative to barrier thickness.

Referring to Fig 1. Elongate silicon carbide fibres 10 are arranged in a matrix of Titanium 12. The whole provides a composite structure in the form of a sheet.

Each of the fibres 10 has a support core 14, which may be constructed from tungsten or carbon. Each fibre 10 is also edveloped in a double coating, the inner one 16 of which is a metal which is capable of producing an oxide which is stable under adverse conditions, with particular reference to high temperature environments. The outer coating 18 is the stable oxide of that metal.

The oxide coating 18 serves to inhibit the diffusion of the Titanium across the interface and eventually, into the ceramic fibre. Such diffusion if it occurs, degrades the ceramic and consequently, severely shortens the service life of the composite structure of which the ceramic forms a part. It follows that the oxide 18 must be of a kind, the ionic make up of which relative to :r 3 - 11 c that of the Titanium is such as to achieve the inhibition. Thus the self diffusivity of -the cations must be low, and the ionic radii must be as near to par with those of the matrix material as is possible, since a charge difference between cations and a difference in the magnitude of ionic radii will encourage the process of diffusion.

The following table lists by way of example, some metals which have the appropriate affinity for oxygen and the oxides of which have the desired ionic characteristics mentioned hereinbefore.

SUMMARY OF THERMODYNAMIC & DIFFUSION DATA

X 71 y - Zr _ Hf -Ele=nt 1 - In oxidm, of 1 X 1 cation charge; iorde radius ' defects AGloooK DX (100OK) --- (Pm) - W mol-1) __ (M2 5-1) 1 1 m01.1 +4 tZWbl,-vc--.-7 MI:CJ +3 89 0 dorective -1080 1.3 x 10-21 +4 79 0 defective -840 1.8 x 10-26, +4 1 78 0 defocdve 1 -892 D.

(M2 5-1) 1 Referring now to Fig 2. A graph was produced from the results of calculations made, as to the extent of reaction which would be achieved between titanium and yttria over a period of one thousand hours, at temperatures which were varied from 10000K to 12000K. The diffusion values are plotted as a function of barrier thickness, and it is seen t hat there is a considerable increase in reaction with increase in temperature over the constant time period. This will have some bearing on the mode of manufacture as will be described hereinafter.

The graph also shows however, that provided the barrier coating is at least 0.7 um thick, then at least up to 1100 0, the reaction layer is maintained at about 0.02 um. The yttria layer corresponds to the yttria coating 18 which envelopes the yttrium coating 16 in Fig 1. A double advantage derives from the use of the two - 4 coatings. one advantage is that the inner coating 16 of yttrium is more ductile than the outer coating 18 of yttria and thus serves to absorb some shock loads in service. The outer coating 18 of yttria may crumble. If crumbling occurs however, a fresh coating of yttrium will be exposed and will quickly oxidise, thus providing what is in effect, a self healing coating.

The yttrium is plasma sputter coated onto the fibres. The yttria however, may be applied to the yttrium by the same method or alternatively, by conventional oxide generating heat treatment of the Yttrium coated fibre or simply by covering the Yttrium coated fibres with the matrix material and allowing the Yttrium to absorb oxygen therefrom. Plasma sputtering is preferred, because it enables some magnitude of control over the stresses which evolve ie compressive stresses are generated in the sputtered coating which counter tensile hoop stresses which may evolve on cooling, should the thermal expansion coefficient of the coating differ from that of the fibre.

The composite structure consisting of Titanium matrix material with embedded silicon carbide fibres, coated as described hereinbefore with one or other of the materials and its oxide disclosed hereinbefore, may be constructed in any one of a number of ways. For example,. sheets of Titanium may be overlaid wiht elongate, coated fibres 10 between each pair of faying faces. The assembly would then be enclosed in an inert atmosphere and subjected to a pressure and temperature over an appropriate period of time, to achieve diffusion bonding of the Titanium sheets. A drawback however, is that the method needs exposure of the assembly to high temperature for a time measured in hours. This could promote excessive reaction between the Titanium and the coating, as is inferred from Fig 2.

Another method could consist of injecting molten Titanium into a mould which contained an array of the coated fibres 10, again in an inert atmosphere. High i 1 temperature is again utilised, but the cooling period reduces the time of exposure thereto relative to that needed when diffusion bonding is adopted.

A further and preferred method, is to lay a sheet of Titanium on a copper heat sink (not shown), arrange coated fibres 10 on the upper surface of the sheet and then place the assembly in an insert atmosphere and vacuum plasma spraying Titanium onto the assembly, so as to cover the fibres 10 to the desired extent.

The preference for the latter method arises because vacuum plasma spraying of Titanium powder onto the fibres, whilst exposing them to high temperature, does so for a time measured in milliseconds. This is not sufficient to effect harmful reaction.

The table given hereinafter describes plasma splutter parameters utilised, in the successful covering of Yttrium and Yttria coated silicon carbide fibres with a Titanium matrix material.

TITANIUM VPS CONDITIONS Chamber Pressure (Ar): 175 mbar Gun Current: 750 A Plasma Gas Flow Rates Powder Particle Range He H 2 Ar: 11 Litres min- I 45-63 um Gun-Substrate Distance: 330 mm Powder Feed Rate 20 g min- 1 1 min- 1 8 Litres min -1 The arrangement of ceramic fibres 10 illustrated in Fig 1, should not be regarded as limitative. The ceramic fibres 10 may be arranged in a manner which will be decided by the desired strength characteristics of the finished composite structure. For example, the ceramic fibres 10 may be arranged in a plurality of layers, wherein each layer may be at an angle to each adjacent layer or layers.

Claims (14)

Claims:

1. A ceramic f ibre having a double coating, the inner one of which is a metal which is capable of producing a thermodynamically stable oxide and the outer one of which is said stable oxide of said metal.

2. A ceramic f ibre as claimed in claim 1 wherein the metal is one from a group comprising Yttrium, Zirconium and hafnium.

3. A ceramic fibre as claimed in claim 1 or claim 2 wherein the ceramic is silicon carbide.

4. A method of applying an anti diffusion coating to a ceramic fibre comprising the step of plasma sputtering a metal selected from a group comprising Yttrium, Zirconium and Hafnium onto the ceramic fibre and then covering the metal with the oxide thereof.

5. A method of applying an anti diffusion coating to a ceramic fibre as claimed in claim 4 wherein the said oxide is applied over the metal by plasma sputtering said oxide in powder form.

6. A method-of applying an anti diffusion coating to a ceramic fibre as claimed in claim 4 wherein the oxide is generated from the metal coating by heat treatment of the metal coated fibre.

7. A method of applying an anti diffusion coating to a ceramic fibre as claimed in claim 4 including the step of embedding the metal covered ceramic fibre in a Titanium matrix so as to enable the metal outer surface to absorb oxygen from the Titanium.

8. A composite structure comprising a Titanium or Titanium alloy matrix enclosing a plurality of elongate ceramic fibres, each of which has an inner coating of metal which is capable of producing a thermodynamically stable oxide, which metal is coated in said oxide.

9. A composite structure as claimed in claim 8 wherein the ceramic fibres are silicon carbide fibres.

10. A composite structure as claimed in claim 8 or claim 9 wherein the metal is one selected from a group comprising Yttrium, Zirconium and Hafnium.

0

11. A ceramic fibre provided with an inner and outer coating substantially as described in this specification and with reference to the drawings.

12. A method of coating a ceramic fibre substantially as described in this specification and with reference to the drawings.

13. A method of forming a composite structure substantially as described in this specification and with reference to the drawings.

14. A composite structure substantially as described in this specification and with reference to the drawings.

Published 1989 at The Patent Office, State House, 66171 High Holborn. London WCIR 4TP. Further copies may be obtained from The Patent Office. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex tLchniques ltd, St Mary Cray, Kent, Con. 1/87