WO1986005774A1

WO1986005774A1 - Fibre reinforced ceramics

Info

Publication number: WO1986005774A1
Application number: PCT/GB1986/000189
Authority: WO
Inventors: Andrew Robert Baker; Julia Rosalind Mary Hood
Original assignee: Aeplc
Priority date: 1985-04-02
Filing date: 1986-04-02
Publication date: 1986-10-09
Also published as: GB2175893A; EP0217902A1

Abstract

A method for manufacturing a fibre reinforced ceramic material which comprises forming a preform (11) of ceramic fibres, placing the preform in a porous mould (12, 13) and applying a ceramic slip (14) to the preform in the mould. The slip material (14) is then caused to impregnate the preform (11) by applying a pressure differential thus forming an impregnated preform (15). This is then removed from the mould (12, 13), the slip liquid is driven off and the preform is heat treated. The ceramic fibres and the slip material may include alumina, kaowool, silicon carbide, silicon nitride and/or zirconia.

Description

FIBRE REINFORCED CERAMICS

The present invention relates to fibre reinforced ceramic materials, their uses and methods for their 5 manufacture.

Fibre reinforced ceramic materials are known in themselves and it is an object of the present invention to provide an improved method of manufacturing such materials. It is a further object of the invention 0 to provide such a material having superior strength and heat insulating properties at possibly lower specific weights.

According to the invention a method for manufacturing a fibre reinforced ceramic material comprises forming 5 a pre-form of ceramic fibres, tape or mat, applying a ceramic slip to the pre-form, causing the slip material to impregnate the pre-form by applying a pressure differentia and driving off the slip liquid; the term "ceramic slip" being used in this context to refer to a slurry 20. of ceramic material finely dispersed in a liquid carrier medium, the ceramic material of the slip incorporating silicon nitride or silicon powder which is subsequently converted to silicon nitride.

The ceramic fibres are preferably of alumina, 25. (such as that sold under the Trade Mark SAFFIL), kaowool, silicon carbide, silicon nitride or zirconia. They may of course comprise a combination of some of these materials. The slip material may also comprise one or more of these materials and may incorporate yttrium 30. oxide (Y 0-.), magnesium oxide (MgO), chromium oxide

(Cr.,0-), tungsten trioxide ( 0-), cerium dioxide (Ce0₂), silica (SiO-,) and/or magnesium carbonate (MgCO, ) (which would convert to MgO). A preferred slip composition might be 10^% Y₂°3' ^{3% M<}3^C03' ^{2% W0}3' ¹-^{5% si0}2' ^the balance being silicon nitride or zirconia, or alternatively the W0₃ might be replaced by Cr-O-.. In a similar alternative composition, the silicon nitride is replaced by silicon powder which is converted to silicon nitride at a later stage by a method known as the reaction bondinq process.

Preferably, the preform is located within a mould, for example of plaster for the slip impregnation step. The pressure differential applied may be in the form of a high pressure applied to the slip to force it into the preform, or alternatively a pressure below ambient may be applied to the preform to draw the slip into the interstices. Naturally, a combination of these alternatives may be used.

Preferably, the slip liquid is water. Fibre reinforced ceramic materials made in accordance with the invention tend to be very strong, light in weight and highly resistant to suddent impacts, particularly when silicon nitride and/or zirconia are used.

It is a further object of the invention to attach a fibre reinforced ceramic material to a metal backing. Preferably, therefore, the thickness of the preform and the amount of slip applied are arranged so that the preform is impregnated over a part only of its thickness, leaving a portion unfilled, and a molten metal is then applied under pressure to the unfilled portion to impregnate it and allowed to solidify (a so-called "squeeze-casting" process).

The present invention extends to such a ceramic/ metal fibre reinforced composite material. The molten 774

metal used can be varied to suit the intended use: a typical piston material (such as the alloy sold under the Trade Mark Lo-Ex) might be used to produce a piston with an insulating crown of fibre reinforced ceramic material which is tenaciously bonded to the piston alloy through the fibres. Similarly by using suitable nickel or cobalt alloys, gas turbine engine components such as blades may be produced having highly chemical and heat resistant ceramic surfaces strongly bonded to the blade alloy through ceramic fibres. The invention may be carried into practice in various ways and two embodiments will now be described by way of example with reference to the accompanying drawings, in Which:- Figures 1 to 3 are schematic illustrations of three successive stages according to a first embodiment of the present invention for manufacturing a ceramic fibre reinforced material;

Figures 4 to 8 are five similar successive stages according to a second embodiment for manufacturing a piston with a ceramic fibre reinforced crown;

Figures 9 and 10 are two similar stages according to a third embodiment for producing a piston with a ceramic fibre reinforced crown with a combustion bowl, and

Figure 11 is a cutaway isometric sketch of a part of a further embodiment.

Referring firstly to Figures 1 to 3, a preform 11 made up of a bundle of silicon nitride fibres having a diameter of between 1 and 5 micrometers and a length 100 to 1000 times their diameter is located in a mould

12 having a plaster base 13. The fibres may be held in position using a binding agent. A slip 14, comprising an aqueous slurry of 10% Y₂°3' ^{3% M}9 ⁰_ > ^{2% w0}3 * ¹-^5% 5 SiO„, the balance being silicon powder, is then placed in the mould 12 above the preform 11. The slip material has a particle size in the range 2-1Oum.

Pressure is applied to the inside of the mould 12 so that the slip material is forced into the interstices

10 of the preform 11 and the water is absorbed by the mould base 13. The ceramic preform body 15 so formed is then removed from the mould 12 and allowed to dry in air for 24 hours, after which it is heated to 380°C in air for 12 hours which converts the MgCO, to MgO.

15 The preform 15 is then heated to 1100°C in argon for

5 hours, after which the temperature is raised to 1400°C under an atmosphere of nitrogen at 1 bar pressure in order to convert the silicon to silicon nitride (Si-.N. ). Final full densification of the silicon nitride so

20 formed may be achieved by further heating in a nitrogen atmosphere at 1750°C. Any binding agent present in the preform will be removed during one of the heating steps.

Referring now to Figures 4 to 8, a method is illustrate

25 for manufacturing a piston with a crown of a fibre reinforced ceramic material. A similar procedure to that described with reference to Figures 1 to 3 is followed initially though the preform 21 is somewhat thicker and the amount of slip 14 used is somewhat

30. less. Thus, after the pressurising step, the preform

21 is impregnated with the slip material to a part only of its thickness as shown at 22 in Figure 6, so that the upper part of the preform remains porous. The impregnated preform is then subjected to heat treatment as in the previous embodiment and placed in a piston mould 23 and, after any machining that may be necessary, a molten piston alloy 24 is introduced as shown in Figure 7. The piston 25 is formed by a squeeze castin step in which a high pressure of perhaps 200 to 300 tons is applied to the alloy while it is allowed to solidify. This causes the piston alloy 24 to impregnate the remaining porous portion of the preform 21 as shown at 27 in Figure 8. The finished piston 25 having a crown 26 of fibre reinforced ceramic material is then removed from the mould 23.

The fibre reinforced material may have the form shown, or may have a re-entrant form to give mechanical retention in the piston body.

In the embodiment shown in Figures 9 and 10 a preform 31 is located in a mould 32 and a ceramic slip material 33 is located in an upper cavity 34 above the preform 31. The space 35 beneath the preform 31 is then subjected to a pressure reduction. The slip material impregnates the preform 31 to the desired degree and the pressure reduction is then removed.

The part-impregnated preform is removed and partially sintered. It is then machined to shape to form a bowl 36 as shown in Figure 10 and fully sintered. The finished preform is then cast into the piston by the squeeze- casting method of the previous embodiment. Some support of the bowl 36 may be required during the squeeze casting step. In another embodiment shown in Figure 11, it may be required to have fibre ends protruding both axially and radially from the finished preform 41 in order to increase the keying effect when squeeze casting the finished preform into a piston. In such a case a preform of interlocking fibres extending both axially and radially is used together with an outer cylindrical tube 44 having comb-like axial teeth 45 which surrounds the preform inside the mould 42. The radially extending fibres 46 are bent down and trapped between the teeth 45, the base 43 and the mould wall 42. When the preform is released from the mould, the ends of the radially extending fibres remain protruding from the preform.

Claims

CLAIMS.

1. A method for manufacturing a fibre reinforced ceramic material which comprises: forming a pre-form of ceramic fibres, tape or mat; applying a ceramic slip to the preform; causing the slip material to impregnate

5. the pre-form by applying a pressure differential, and driving off the slip liquid; the term "ceramic slip" being used in this context to refer to a slurry of ceramic material finely dispersed in a liquid carrier medium, the ceramic material of the slip incorporating silicon

10. nitride or silicon powder which is subsequently converted to silicon nitride.

2. A method as claimed in Claim 1 in which the ceramic fibres are of alumina, kaowool, silicon carbide, silicon

15. nitride and/or zirconia.

3. A method as claimed in Claim 1 or Claim 2 in which the slip material additionally incorporates alumina, kaowool, silicon carbide and/or zirconia.

20.

4. A method as claimed in Claim 3 in which the slip material further comprises yttrium oxide, magnesium oxide chromium oxide, tungsten trioxide, cerium dioxide, silicon dioxide and/or magnesium carbonate.

25,

5. A method as claimed in Claim 4 in which the slip composition is 10% yttrium oxide, 3% magnesium carbonate, 2% tungsten trioxide, and 1.5% silicon dioxide, the balance being silicon nitride or zirconia.

30.

6. A method as claimed in Claim 4 in which the slip composition is 10% yttrium oside, 3% magnesium carbonate, 2% tungsten trioxide, and 1.5% silicon dioxide, the balance being silicon powder which is subsequently converted to silicon nitride by a method known as the reaction bonding process.

7. A method as claimed in any preceding claim 5. in which the pre-form is located within a plaster mould for the slip impregnation step.

8. A method as claimed in any preceding claim in which the pressure differential applied is in the

10. form of a high pressure applied to the slip to force it into the preform.

15,

9. A method as claimed in any preceding claim in which the preform is heat treated in an inert atmosphere after the slip liquid has been driven off.

20,

10. A method as claimed in any preceding claim 25. in which the thickness of the preform and the amount of slip applied are arranged so that the preform is impregnated over a part only of its thickness, leaving a portion unfilled, and a molten metal is then applied under pressure to the unfilled portion to impregnate 30. it and the molten metal is then allowed to solidify.