IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF METAL/FIBRE COMPOSITES

Abstract: In the persuit of the forming of light composite materials which may be substituted for metal without loss of strength, composites have been manufactured from silicon carbide fibres in a titanium matrix. Titanium diffuses into the given boundaries of the silicon carbide and degrades it. Yttria (16) has been sandwiched between the titanium (18) and silicon carbide, to provide a reaction skin of yttria which reduces the extent of diffusion. The present invention oxidises the silicon carbide prior to applying the yttrium and thus further improves the resistance to diffusion.

Description: The present invention relates to composite materials comprising metal sheathed ceramic fibres. In our co-pending published patent application GB2219006A there is disclosed and claimed a method of reducing the diffusion of defined metals into silicon carbide fibres when making a composite structure, which comprising covering the fibre with a contact coating of one of the metals defined therein and then covering the metal with a coating of an oxide thereof. The method claimed by the invention in GB2219006A was an improvement over the prior art, but some diffusion still occurs. It has been found however, that now, the diffusion is not of the kind where there is interchange of atoms across the interface between the differing materials, which if it were, would result in an alloy consisting of the metal and silicon carbide; rather, the metal diffuses into the grain boundaries of the silicon carbide. The present invention seeks to further improve the method of forming a composite material from a defined metal or its alloys, and ceramic fibres. According to the present invention a method of inhibiting the diffusion of a metal coating which is capable of producing a thermodynamically stable oxide, into the grain boundaries of a silicon carbide substrate comprising the steps of heating the substrate (10) in air prior to applying the metal (14) so as to oxidise the substrate surface, then applying the metal (14) to the oxidised surface of the substrate (10) in a manner which causes the metal (14) to oxidise at the interface and then applying a further metal (18) to the metal (14) and heating the whole to cause further oxidation at all of the interfaces. Preferable features of the present invention are included in claims 2 to 5. The invention will now be described by way of example and with reference to the accompanying drawings in which Figures 1 to 2 inclusive depict the inventive step and Figures 3 and 4 depict the further steps necessary to complete the composite material. Referring to Figures 1 and 2. Figure 1 depicts a silicon carbide fibre 10 of cylindrical cross section. Figure 2 depicts the same fibre 10 after it has been heated in air for eight hours at a temperature of 1000°C. The outer surface of the fibre has oxidised i.e. it has been converted to a layer of silica 12. Referring now to Figure 3. Yttrium 14 has been applied to the oxidised outer layer 12 of the fibre 10, under conditions of vacuum or alternatively whilst in an inert atmosphere of e.g. argon. The yttrium 14 in the present example has been applied by the known process of sputtering, but other methods such as vapour deposition may be utilised. In Figure 4, the sputtered yttrium 14 has reacted with the silica 12, at their interface to form a layer of yttria 16. A layer of titanium 18 is then applied to the silica, yttria and yttrium covered fibre by wrapping a titanium foil around the yttrium layer, heating the whole in vacuum to a temperature of 925°C and holding that temperature for twenty five minutes and over the same time period, applying a pressure of 15 mega pascals to the structure. A subsequent heat treatment is then effected by way of heating the whole to 950°C and maintaining that temperature for two hours. This final step effects further reaction at all of the interfaces of the layers and thus increases the ability of the silica to inhibit diffusion of yttrium into the silicon carbide fibre and the ability of the yttria to inhibit diffusion of the yttrium.

Claims: A method of inhibiting the diffusion of a metal coating which is capable of producing a thermodynamically stable oxide into the grain boundaries of a silicon carbide substrate characterised by the steps of heating the substrate (10) in air prior to applying the metal (14) so as to oxidise the substrate surface, then applying the metal (14) to the oxidised surface of the substrate (10) in a manner which causes the metal (14) to oxidise at the interface and then applying a further metal (18) to the metal (14) and heating the whole to cause further oxidation at all of the interfaces. The method of claim 1 characterised by heating the substrate in air at 1000°C. The method of claim 2 characterised by heating the substrate in air for a period of eight hours. The method as claimed in any previous claim characterised by applying Yttrium (14) to the oxidised surface of the substrate. The method as claimed in any previous claim characterised by applying Titanium (18) or an alloy thereof to the metal (14).

Inventors: CLYNE, TREVOR WILLIAM, KIESCHKE, ROBERT RICHARD
Applicants: ROLLS-ROYCE PLC
Application year: 1990
Main classification: C04B41
IPC codes: D06M11, C04B35, C04B41, C23C8