TECHNICAL FIELD

The invention relates to a method of manufacturing bodies with improved fracture toughness by means of hot-isostatic pressing/isostatic pressure sintering, in the form of composites in which long fibres are arranged in a matrix of ceramic or metal.

BACKGROUND ART

In many demanding applications, such as components for gas turbines, rocket engines and other types of structural members for which a high reliability is required, a brittle fracture behaviour is unacceptable. Brittle materials often lead to fracture without plastic deformation having preceded the fracture, or if the breaking stress is locally exceeded following the stress concentration which arises near a microcrack or other defect. This reduces the possibility of detecting faulty parts by inspection or estimating service lives by statistical methods and replacing these parts prior to a breakdown.

The above applies preferably to ceramics which are very strong but for which the strength is reduced and the variation in strength values between individuals becomes great as a consequence of the inability of absorbing stresses by plastic deformation as well as the occurrence of internal defects in the form of microcracks and other inhomogeneities. Thus, they result in fracture when the weakest link breaks. The same is also true of certain metallic materials, preferably in high-temperature applications, where changes in the microstructure as a result of diffusion and grain growth increase the tendency to brittle fracture behaviour.

One way of avoiding brittle fractures is to reinforce with fibres. Fibre reinforcement gives greater fracture toughness and safety against fracture because when the strength is locally exceeded, the fracture does not propagate spontane- ously through the part but the fibres take up and redis¬ tribute the stresses in the same way as when one of a large number of parallel-connected links breaks. In this way, defects can be detected and critical components be replaced during a regular overhaul.

Known methods for manufacturing ceramics reinforced with long fibres are of two main types, namely:

Preform infiltration: A three-dimensional fibre preform is manufactured. The preform is then infiltrated with a ceramic matrix. During the infiltration the ceramic matrix may be supplied:

- in gaseous phase, whereby the fibres included in the fibre preform constitute a substrate on which the ceramic matrix is built by precipitation from the gaseous phase, Chemical Vapour Infiltration (CVI) ;

- in liquid form, whereby the ceramic particles which-build up the ceramic matrix are supplied to the fibre preform suspended in a liquid polymer, in the form of a sol or suspended in a powder slurry, and the built-up ceramic matrix is bonded to the fibre preform by reaction with a gas. Irrespective of how the fibre preform is infiltrated, the ceramic matrix is built in the void between the fibres without the fibres being packed more densely. Ceramic fibre composites produced by infiltration in the manner described above typically result in a material with a porosity of from 10 to 15 per cent by volume.

ϋhiaxial pressure sintering: Fibres are mixed with ceramic powder and are formed into a green body. The green body is consolidated and sintered under uniaxial pressure. Often the ceramic matrix comprises a ceramic with a low melting point which is densified via a viscous flow. Typical matrices for

ceramic fibre composites produced by means of uniaxial pressure sintering are glass and glass ceramics in the X- aluminium silicate system where X preferably consists of any of the substances Li, Mg, Ca, Zr or Y. During the pressure sintering both the ceramic matrix and the fibres are packed together, and completely dense materials can be obtained under favourable conditions. Also green bodies formed by winding of infiltrated fibre bundles have been consolidated and sintered by means of uniaxial pressure sintering. The applicability of the method, because the pressure is applied uniaxially, is limited to simple geometries. Also limita¬ tions as regards magnitude and compacting pressure are very pronounced in the case of uniaxial pressure sintering.

In addition to the methods described above, test bars, with simple geometries, of silicon nitride containing continuous carbon fibres have been manufactured by enclosing green bodies in glass, consolidating the bodies and sintering them by means of hot-isostatic pressing.

SUMMARY OF THE INVENTION

One object of the present invention is to propose a method for the manufacture of composites reinforced with long fibres without the limitations regarding the geometries of the body which are described above.

During the manufacture of a composite body according to the invention, a fibrous material, in the form of a long fibre thread comprising a large number of fibres, a single long fibre, a mono-filament, or a fibre fabric, in the following referred to as fibre thread/fibre fabric, is infiltrated with a matrix whereafter a green body is preformed from the infiltrated fibre thread/fibre fabric. The matrix preferably comprises ceramic materials such as silicon nitride, silicon carbide, aluminium oxide, mullite, yttrium oxide or yttrium aluminium garnet (YAG) but can also comprise metallic materials. The fibre thread/fibre fabric preferably consists

of carbon, boron, silicon carbide, silicon nitride, Si-N-C- IQ I fibres or aluminium (sapphire) fibres. The fibre thread/fibre fabric may be coated, for example with boron nitride, to prevent undesired reactions between the fibres and the matrix or to control the shearing strength between the fibres and the matrix.

According to the invention, the infiltrated fibre thread/fibre fabric is formed into a green body of the desired shape against a shape-imparting body. This shape- imparting body may be a core which gives the composite body its inner contour or a shape-imparting outer casing which gives the composite body its outer contour or comprise shape-imparting parts which shape both the inner and outer contours of the composite body. After drying, the green body together with the shape-imparting body is surrounded by an encapsulation which is impenetrable to the pressure medium used during the isostatic pressure sintering. Thereafter, the green body is sintered and consolidated into an essentially dense ceramic-ceramic body by isostatic pressure sintering, hot-isostatic pressing. After the isostatic pressure sintering, the encapsulation and the shape- imparting body are removed. The shape-imparting body is preferably made of a material which retains its shape- imparting properties under the pressure and temperature conditions which prevail during the isostatic pressure sintering, such as graphite or boron nitride.

The pressure and the temperature for the isostatic pressure sintering are, of course, chosen in dependence on the material included in the matrix and in the fibre thread/fibre fabric. By preforming a green body starting from an infiltrated fibre thread/fibre fabric, surrounding the green body together with the shape-imparting body by an encapsulation before the encapsulated green body is sintered and consolidated by isostatic pressure sintering, essen¬ tially dense composite bodies can be manufactured irrespective of the geometry.

Infiltration of the fibre thread/fibre fabric preferably takes place by passing the fibre thread/fibre fabric through at least one bath containing the above-mentioned powder slurry, whereby the fibre thread/fibre fabric is infiltrated with the material suspended in the powder slurry containing at least the ceramic materials from which the ceramic matrix is to be built up.

The powder slurry is prepared by the addition of a solvent such as cyclohexane, a ketone, ethanol or any other alcohol, to a content of 30 to 80, preferably to around 60, parts by volume, to the powdered material. Of course, any suitable dispersion and suspension-promoting agents as well as other surface-active agents are added to ensure that an essen- tially fully covering and even slurry infiltrates the fibre thread/fibre fabric. In addition, if there is a need of green strength, a suitable organic binder is added, such as, for example, an acrylic binder. As described above, the fibre thread/fibre fabric may pass through more than one powder slurry with or without intermediate drying.

As encapsulation there is often used a glass br a glass- forming material which is applied in the form of a powder. After the glass powder has been applied around the preformed green body and the shape-imparting body, the green body, the shape-imparting body and the glass powder are heated, usually under vacuum, to a temperature such that the glass or the glass-forming material forms a dense coherent encapsulation around the green body. During the heating, any organic constituents present in the green body are driven off before the dense encapsulation is formed. As glass which may be used may be mentioned a glass containing 80.3 per cent by weight Siθ ₂ , 12.2 per cent_.by weight B ₂ O3, 2.8 per cent by weight AI ₂ O ₃ , 4.0 per cent -by weight Na ₂ θ, 0.4 per cent by weight K ₂ O and 0.3 per cent by weight CaO-(Pyrex®) -, a glass consisting of 20-60 per cent by weight B ₂ O ₃ , 80-40 per cent by weight Siθ ₂ and 0-6 per cent by weight AI ₂ O ₃ , further an aluminium silicate containing 58 per cent by

weight Siθ ₂ , 9 per cent by weight B ₂ O3, 20 per cent by weight AI ₂ O ₃ , 5 per cent by weight CaO and 8 per cent by weight MgO, as well as mixtures of particles of substances such as Siθ ₂ , B ₂ O ₃ , AI ₂ O ₃ and alkali and alkaline earth metal oxides which form glass upon heating. Further, there may be used a glass containing 96.7 per cent by weight Siθ ₂ , 2.9 per cent by weight B ₂ O3 and 0.4 per cent by weight AI ₂ O3- CVycor®)- and mixtures of particles which form glass upon heating.

To prevent the encapsulation material, preferably a glass, from penetrating into open pores or other surface-connected cavities in the green body, from reacting with or adhering to the surface of the ceramic body, at least the green body is first coated with a barrier layer. Normally, the exposed surfaces on both the green body and the shape-imparting body are coated with this barrier layer. The barrier layer may also function as or be supplemented by a release layer to facilitate the removal of the encapsulation after the hot- isostatic pressing.

The above-mentioned barrier layer suitably consists of materials which maintain their crystalline states in contact with the glass as well as the green body and the shape- imparting body at the pressing temperature. Examples of suitable materials are intermediary phases in the system l2θ3-Siθ ₂ of which mullite 3Al ₂ θ3 ^« 2Siθ ₂ , sillimanite Al ₂ θ3-Siθ ₂ and kyanite Al ₂ θ ₃ -Siθ ₂ (a high pressure modifi¬ cation of sillimanite) may be mentioned. The barrier layer may also contain a powdered additive consisting of one or more of substances such as aluminium oxide, zirconium oxide, titanium boride, silicon nitride, silicon carbide, titanium nitride, boron nitride, or a high-melting glass which does not, or only insignificantly, react with the material in the barrier layer, such that the barrier layer in all essentials maintain its crystalline state. As examples of useful high- melting glass may be mentioned quartz glass and a glass containing 96.7 per cent Siθ ₂ , 2.9 per cent by weight B ₂ O ₃

and 0.4 per cent by weight AI ₂ O ₃ -(Vycor ^® )-. The additive may be included in combination with intermediary phases in the system Al ₂ θ3-Siθ ₂ such as mullite 3Al ₂ θ3-2Siθ2, sillimanite Al ₂ θ3-Siθ ₂ and kyanite Al ₂ θ3-Siθ ₂ in contents of up to 60%, preferably to at most 30% of the total dry weight of all constituents in the barrier layer.

In one embodiment, the barrier layer is supplemented by an internal layer which, in addition to preventing the glass from penetrating into open pores or reacting with the castings, also acts as release layer. The release layer is arranged nearest the body to be easily removed from the body, after the hot-isostatic pressing, together with the external barrier layer and the glass encapsulation. The release layer preferably consists of boron nitride. Boron nitride has insignificant or no tendency to react with the ceramic body. Boron nitride in commercially available qualities is well suited for this purpose. As an alternative to boron nitride, it is possible, depending on the material in the composite body, to use other substances with surface structure such as graphite and molybdenum sulphide as well as boron nitride mixed with silicon nitride, silicon carbide, titanium nitride, titanium boride, etc.

The particle size of the powder in the barrier layer is suitably chosen to be between 0.1 and 200 μm, preferably to less than 150 μm and the material is applied into a layer with a thickness of between 0.1 and 3 mm, preferably to a thickness of between 0.3 and 0.6 mm.

For the supplementary release layer, which is applied inside the intermediate layer, the particle size is suitably chosen to be between 0.1 and 100 μm, preferably to less than 45 μm. The layer is suitably applied into a thickness of between 0.1 and 2 mm, preferably into a thickness of between 0.2 and 0.6 mm.

Both the intermediate layer and the supplementary release layer can be applied to the green body and the shape- imparting body by immersion into a slurry of the powdered materials which are included in the respective layers, by spraying or in some other suitable way. A slurry is suitably prepared by the addition of a solvent such as a cyclohexane, a ketone, an ethanol or another alcohol, to a content of 80 to 92 parts by volume to the powdered material. Suitable dispersion and suspension-promoting agents and other surface-active agents can be added to ensure that an essen¬ tially fully covering and even coating of the slurry remains on the castings. The application is followed by a drying when the solvent is driven off and a covering even layer of the desired thickness is obtained. Each layer can be applied in one or more steps, with or without intermediate drying, to obtain the desired thickness of the layer.

According to one embodiment of the invention, ceramic- ceramic composites can be produced by passing a fibre thread, in the form of a bundle of a large number of ceramic fibres, or alternatively, a single fibre, mono-filament, or a fibre fabric, through at least one bath containing ceramic particles suspended in a polymer melt, whereupon the fibre thread/fibre fabric thereby infiltrated with ceramic par- tides and polymer melt, in the manner described above, is preformed into a green body against a shape-imparting body, is encapsulated together with the shape-imparting body, before the green body is consolidated and sintered into an essentially dense ceramic-ceramic composite by isostatic pressure sintering.

Metal-ceramic composites can be produced while utilizing the invention, whereby a fibre thread in the form of a bundle of a large number of ceramic fibres, or alternatively, a single fibre, mono-filament, or a glass fabric is passed through at least one bath containing a metal melt whereupon the fibre thread/fibre fabric infiltrated with metal melt, in the manner described above, is preformed into a green body

against a shape-imparting body, is encapsulated together with the shape-imparting body, before the green body is consolidated and sintered into an essentially dense metal- ceramic composite by isostatic pressure sintering.

Composite bodies with an internal contour or shape are produced while utilizing the invention, whereby the fibre thread/fibre fabric is infiltrated and the infiltrated fibre thread/fibre fabric is preformed by winding it onto a shape- imparting core of a material which maintains its shape- imparting properties under the pressures and temperature conditions which prevail during the isostatic pressure sintering. After preforming, the core is encapsulated and the green body preformed on the core is consolidated and sintered in the manner described above. Preferably, a core of graphite or boron nitride is used. Cores of these materials also have the advantage that they can be easily removed from the sintered composite body by mechanical methods such as blasting if the internal structure should not allow the core to be withdrawn from the body. If necessary, graphite cores can be coated with a diffusion- preventing barrier layer to avoid carbon absorption during the pressure sintering. Preferably, the graphite core is coated with a layer of boron nitride.

Preforming of a green body by winding of infiltrated fibre thread/fibre fabric, and surrounding the green body with a dense encapsulation followed by isostatic pressure sinte¬ ring, is applicable to the manufacture of composite bodies irrespective of the geometry of the body. For example, a base frame of infiltrated fibre thread/fibre fabric can first be clamped by means of a fixture, whereafter infiltrated fibre thread/fibre fabric is wound onto the frame to form a green body of optional shape. A green body can also be built up by winding infiltrated fibre thread/fibre fabric, be removed from the core used during the winding and be shaped into a desired outer contour by applying around the green body an outer mould or a casing of

a material which maintains its shape-imparting properties under the pressure and temperature conditions which prevail during the isostatic pressure sintering, for example a casing of graphite or boron nitride. The green body and the casing mentioned are encapsulated, after which the green body is consolidated and sintered in the manner described above.

Composite bodies can also be produced while utilizing the invention whereby fibre fabrics are infiltrated with a matrix according to the above description. These fibre fabrics are then stacked in a mould or a shape-imparting casing, of a material which retains its shape-imparting properties at the pressures and temperature conditions which prevail during the isostatic pressure sintering. After preforming, the mould and the green body preformed in the mould are encapsulated in accordance with the description above, whereafter the green body is consolidated and sintered by isostatic pressure sintering in the manner described above. Preferably, a mould of graphite or boron nitride is used. Moulds of these materials also have the advantage of being easily removable from the sintered- composite body by mechanical methods such as blasting if the outer form should not allow the mould to be withdrawn from the body. If necessary, graphite moulds can be coated with a diffusion-preventing barrier layer to avoid carbon absorption during the pressure sintering. The graphite mould is preferably coated with a layer of boron nitride.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail in the following with reference to the accompanying figures and examples. Figure 1 shows infiltration and winding of fibre thread according to one embodiment of the invention, and Figure 2 shows preforming by arrangement or stacking of infiltrated fibre fabric in a mould or a shape-imparting

casing. Figure 3 shows a preformed body and a core arranged surrounded by glass in a crucible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS AND EXAMPLES

EXAMPLE 1

Carbon-silicon nitride composite

A powder slurry was prepared starting from a ceramic powder mixture comprising silicon nitride powder - PERMASCAND P95C - with addition of 6 per cent by weight yttrium oxide - HC STARCK, Grade fine - and 2 per cent by weight aluminium oxide - SUMITOMO, AKP-30 -. To this powder mixture was added water with an addition of 0.2 per cent by weight ligno- sulphate as dispersing agent. After the addition of water, the slurry was adjusted to a pH of 10 and a dry content of around 70 per cent by weight, 41.7 per cent by volume. The dispersion was carried out while grinding in a ball mill for 70 hours.

The slurry obtained was degassed for 24 hours while being magnetically stirred.

After degassing, the slurry was transferred to a tank 2 included in a device for winding fibre thread. A carbon fibre thread - Tonen F700 - was taken from a fibre spool 1 and led through the slurry tank 2 where it was infiltrated with the slurry before it was wound into a green body 4 on the graphite core 3.

The body 4 preformed by winding was dried at room tempera¬ ture.

After drying, the graphite core 3 and the body 4 preformed on the graphite core were placed in a graphite crucible 5, which is shown in Figure 3. The crucible 5 was internally coated with a release layer 6 of boron nitride. A glass

powder 7 was supplied to the crucible 5 whereafter the graphite crucible 5 with the core 3, the preformed body 4 and the glass powder 7 was placed in a high-pressure furnace (not shown) , where the preformed body 4 with the surrounding glass powder 7 was degassed for 2 hours at room temperature and under vacuum. Thereafter, while maintaining the vacuum, the furnace was heated to a temperature of about 600°C, whereby organic constituents present in the remainder of the green body .4 were driven off. Thereafter, argon was added until a pressure of 0.1 MPa was attained, after which the temperature was raised to 1000 to 1220°C. The furnace was maintained under these conditions for 1 hour, whereby the glass powder melted and formed a dense coherent casing, impenetrable to the pressure medium used, around the green body and the core. The pressure was then raised by pumping in argon gas to 100 MPa and the temperature was raised to between 1650 and 1750°C. The furnace was maintained under these conditions for 2 hours, whereby the green body was sintered and consolidated into an essentially dense ceramic- ceramic composite. After the sintering, the glass encapsu¬ lation and the shape-imparting core were removed.

The driving off of organic constituents can also be carried out under flushing gas, preferably nitrogen gas and/or hydrogen gas. The treatment up to 600°C can also be carried out in a separate furnace, which need not be a high-pressure furnace, after which the crucible with the core, the body and the glass powder is transferred to a high-pressure furnace. The gas, pressure medium, pumped in during the pressure increase may also comprise other inert gases, such as helium and nitrogen gas.

EXAMPLE 2

Aluminium oxide-aluminium oxide composite

A powder slurry was prepared by dispersing aluminium oxide - ALCOA, A 16 SG - in water to which 0.5 per cent by weight

polyacrylic acid - DISPEX A40 - had been added. The dry content of the slurry was adjusted to about 72 per cent by weight. The preparation was carried out by means of ball mill grinding for 70 hours.

The slurry was degassed for 24 hours while being magnetically stirred.

After the degassing, the slurry was transferred to a tank 2 included in the equipment for winding fibre thread. An aluminium oxide fibre - ALMAX, MITSUI - was led from the fibre spool 1, through the tank 2, where it was infiltrated with slurry before it was wound into a green body 4 on the graphite core 3.

The body 4 preformed by winding was dried at room temperature before it was encapsulated and sintered in a high-pressure furnace in acrordance with the description in Example 1. However, the high-pressure sintering was carried out at a temperature of between 1450 and 1550°C.

EXAMPLE 3

Carbon-silicon nitride composite

A polymer melt was prepared by mixing, at a temperature of about 150°C,

- 60 parts by volume of a silicon nitride-based ceramic powder with a mean grain size of 1 μm-H C Starck, grade Hl- and containing 1.1 part by volume yttrium oxide, with

- 40 parts by volume of a temporary binder comprising: 32.4 parts by volume paraffin, melting temperature 58-60°C, 6.9 parts by volume ethylene-vinyl acetate polymer with a melting index of 400 g/min (ASTM 1238 Modified) and a density of 0.951 g/cm3 - ELVAX® - from Du Pont), and 0.7 parts by volume ^" Carnauba wax.

The mixture of ceramic powder and polymer melt was supplied to a tank 2 included in the winding equipment and was main¬ tained at 150°C. Carbon fibre - Tonen, F700 - was led from the fibre spool 1, through the tank 2, where it was infiltrated with ceramic powder and polymer melt before it was wound into a green body 4 on the graphite core 3.

The wound structure 4 was placed in a vacuum furnace where the polymer was driven off under a vacuum of about 0.1 mbar while successively raising the temperature under slow heating, about 1.5°C/hour to 350°C, thereafter somewhat faster, 15°C/hour, to 600°C. After cooling and pressure equalization, the body 4 was withdrawn from the vacuum furnace, whereupon it was placed in a graphite crucible 5, was surrounded and pressure sintered in accordance with Example 1.

EXAMPLE 4

Silicon carbide- silicon nitride composite

A slurry was prepared and degassed in accordance with- Example 1 and was transferred to a tank for infiltration of fibre fabrics. Fibre fabrics 8 of silicon carbide - NICALON NL-607 from NIPPON ^" CARBON, Japan - of HS type with a size of 100x100 mm were immersed into the slurry tank and stacked in a graphite mould 9.

The green body preformed by stacking of slurry-infiltrated fibre fabrics was dried at room temperature before the green body, formed from infiltrated fibre fabrics, and the shape- imparting graphite mould 9 were encapsulated. Thereafter, the green body was sintered in a high-pressure furnace in accordance with the description in Example 1. However, the high-pressure sintering was performed at a temperature of between 1550 and 1650°C.