EP0298151A2 - Composite material with a great toughness - Google Patents

Abstract

(A) In a composite material comprising reinforcing fibres uniformly distributed in a metal or cermet matrix, the novelty is that the fibres are ceramic fibres which are sepd. or in cloth form and which are covered with a thin protective coating of material selected from carbon, boron and transition metal carbides, nitrides, borides, carbonitrides and oxides. (B) Prodn. of the composite material comprises coating the fibres with a protective coating, mixing the fibres with matrix powder, homogenising the mxit. in a liq. medium, shaping and sintering.

Description

The present invention relates to a high tenacity composite material comprising a binder matrix and a fibrous reinforcing phase, as well as to a process for the manufacture of such a composite material.

_r sont comprises entre 200 et 700 MPa.Recent industrial developments imply the use of ceramic materials for high temperature applications (turbines, cutting tools, motors, etc.). For these materials, the stress intensity factor and the toughness are essential criteria. Traditional ceramic materials have a relatively low toughness, with the possible exception of alumina-zirconia with partially stabilized zirconia. The values of K _IC are of the order of 3 to 5 MPa m½ for monolithic ceramics, such as Al₂O₃, SiC, Si₃N₄, for the compounds Al₂O₃ + TiC and Sialons, while the values of

_r are between 200 and 700 MPa.

The use of solid ceramics in the construction of components for use at high temperatures is therefore limited by their brittleness, and to compensate for this, refractory layers (TiC - Al₂O₃, etc.) are deposited on non-ceramic materials having higher toughness; these layers can be up to 50µ thick.

It is therefore desirable to develop materials having a higher tenacity, but having the characteristics of resistance to high temperatures and wear resistance of ceramics: these are ceramic materials reinforced with fibers, for example graphite fibers, which have a stress intensity factor and a very high tenacity at low temperatures; on the other hand, at high temperatures, their performance is poor, because on the one hand there is an oxidation of the carbon fibers and on the other hand a reaction between the carbon of the fibers and the ceramic matrix. Another example of reinforcing fibers are the fibers of inorganic materials, such as silicon carbide. These fibers have a polycrystalline structure which recrystallizes at high temperature and makes the use of such materials delicate at these temperatures, because they become brittle by reducing the reinforcing characteristics of the fibers. These materials are therefore ill-suited to shaping by hot isostatic pressing in particular.

On the other hand, we have hitherto encountered the difficulty or even the impossibility of manufacturing composite materials with a metal binder reinforced by particles or ceramic fibers. We know that the difficulty of manufacturing such materials was linked to the near impossibility of bonding the ceramic phases with a metallic binder. The reason for this poor adhesion is to be sought in the high chemical stability, the poor wettability and the immiscibility of the ceramic phases in metals or metal alloys. To improve these wettability properties, it has been proposed to add a platinum group metal to the binding phase.

The notion of composite materials implicitly associates the idea of reinforcing a material constituting the matrix of low mechanical strength by very strong and very rigid fibers. This effect is all the more important that the Young's modulus of the fibers is large compared to that of the matrix and that the volume fraction of the reinforcement is high; but at constant volume, the reinforcing effect is greater for fibers with very high elastic modulus. The strain at break of the matrix must be greater than that of the fiber and the fiber-matrix adhesion must be good so that there is no reduction in the mechanical properties by default of matrix-fiber adhesion. Several solutions have been proposed, including hot pressing or hot isostatic repressing. However, all of these solutions prove to be insufficient in certain cases, in particular when the impact resistance must be high.

The object of this invention therefore consists in remedying the abovementioned drawbacks and in providing a composite material which, in addition to resistance to high temperatures, has a high tenacity and is very chemically stable.

The composite material which is the subject of the invention and which aims to achieve the above object has the characteristics defined in claim 1.

Another object of the present invention consists of a process for the production of this composite material, which is as defined in claim 8.

Thus, the fact that the ceramic reinforcing fibers, embedded in the binder matrix, are previously "encapsulated" in a thin refractory sheath, makes it possible to obtain excellent wettability of the fibers in the matrix, without destruction of the refractory layer during sintering. The composite material thus obtained therefore has a high tenacity, and is practically free from cracks, the fiber itself contributing to the tensile strength, while the refractory sheath and the binding phase lead to obtaining good compression strength.

The matrix or binding phase can be constituted by a metal, such as Fe, Ni or Co, by an alloy of these or another metallic alloy, optionally with a transition metal carbide. In addition, this binding phase may contain additional elements or compounds intended to improve the wettability of this metallic binding phase. These additional elements or compounds are, for example, a metal from the Pt group, such as Ru, Os, Ir, Rh, Pd and Pt, Ru being preferred, or Re, or alternatively transition metal carbides such as carbides of Ti, W, V and Mo. These additional elements or compounds should in principle not be present in an amount greater than about 20% by weight of the binding phase; preferably they represent from 5 to 20% of this binding phase. The choice of the binding phase must be made taking into account that it must wet the protective coating of the fibers and not dissolve it.

The fibrous phase generally represents from 5 to 40% volume of the composite material, preferably from 15 to 25% vol. Above 40%, it is difficult to remove all the sintering porosities, even by hot isostatic pressing, while below 5% the reinforcing effect is too weak. The ceramic fibers constituting it have a diameter of between approximately 0.5 to 5 microns. The length of these fibers can vary, from a few microns for short fibers or "whiskers" to a few mm for long fibers, more particularly from 0.01 to 10 mm.

By way of example, it is possible to use as fibers in the present invention, those consisting of simple elements such as W, B and C, or else carbides, nitrides, borides, carbonitrides, oxides, etc. transition elements, such as Al₂O₃, SiO₂, BeO, ZrO₂, B₄C, SiC, WC, Si₃N₄, BN, AlN, etc.

As mentioned previously, one of the important characteristics in the present invention consists in rendering the matrix or binding phase non-reactive with the fibers, by first coating the latter with a protective deposit, the thickness of which is generally of the order of 1 / 10 of the diameter of the fibers coated by said deposit. This deposit therefore has the role of preventing a chemical reaction at the interface between the matrix and the fibers during sintering in the liquid phase or use at high temperature.

The thin protective coating consists for example of C, of B, of a carbide, nitride, boride, carbonitride and oxide of transition metals, such as B₄C, TiC, SiC, W₂C, WC, HfC, BN, TiN, Si₃N₄ , HfN, TiB₂ Al₂ O₃ and TiAlON.

The coating of the fibers can be carried out using any known technique in the gas, liquid or solid phase. Preferably, in the present invention, the deposition by gas (chemical or physical) is used. By way of example, the following layers can be obtained by CVD, using as the gaseous reagent that indicated in parentheses:
C (CH₄C₃H₈); B (BCl₃-H₂); B₄C (BCl₃-CH₄-H₂); BN (BF₃-NH₃ / BCl₃-NH₃); TiC (TiCl₄-CH₄-H₂); TiN (TiCl₄-N₂-H₂); Si₃N₄ SiCl₄-NH₃); Al₂O₃ (AlCl₃-CO₂-H₂); SiC (CH₃SiCl₃-N₂) and TiB₂ (TiCl₄-BCl₄-H₂)

Similarly, the following layers can be obtained by PVD, using the combinations indicated in parentheses as a cathode:
TiC (TiC-Ni); TiN (TiC-Ti-N₂); W₂C (W₂C-W); WC (WC-W); HfN (HfN-Hf-N₂), HfC (HfC-Hf) and TiAlON (Al₂O₃-TiN).

The choice of coating material obviously depends on the two binding and fibrous phases respectively present, with the aim of protecting the fibers by preventing their reaction with the binding phase during sintering or use at high temperature. For this, the coating at the interface of the two phases must be put in compression, that is to say that the expansion coefficient of the protective coating must be less than or equal to that of the matrix and the fibers, at temperature ambient. In addition, the binder phase can be provided so that it reacts with the protective coating, however without dissolving the latter, for example by dissolution-precipitation reaction; in this case, the binding phase must for example be saturated with the element or the compound constituting the protective coating.

• a) phase liante : Ni et/ou Mo
  phase fibreuse : fibres de Al₂O₃ recouvertes de TiC
• b) phase liante : Ni + TiC
  phase fibreuse : fibres de Al₂O₃ recouvertes de Cr

By way of example, the following composite materials can be mentioned:

• a) binding phase: Ni and / or Mo
  fibrous phase: Al₂O₃ fibers covered with TiC
• b) binding phase: Ni + TiC
  fibrous phase: Al₂O₃ fibers covered with Cr

The manufacturing of composite material according to the invention firstly comprises mixing the metallic binder phase, in powder form, with the fibrous phase formed from fibers previously covered with the protective layer, this mixture generally being carried out in a liquid medium. Then, the shaping of the material obtained is carried out by any technique of powder metallurgy, for example by pressing, by extrusion, by injection, by casting, etc. It may be important, during the shaping step, to orient the fibers, in particular to obtain articles intended to be subjected to particularly high stresses at high temperature. In this case, the shaping techniques to be used are those by injection, by extrusion or by formation of flexible film. Then a sintering is carried out, by conventional treatment (pre-sintering-sintering) followed or not by hot repressing, or else directly by hot isostatic pressing.

More particularly, a slip is first prepared by mixing the metallic binder phase, in powder form, with an organic product in the liquid state, for example paraffin or a mixture of paraffins, polyethylene glycol, oil. castor oil, etc., or a mixture thereof, for example paraffin and polyethylene glycol. This slip is ground for 48 hours in a ball mill, then degassed to remove gas bubbles and adjust its viscosity.

The fibers can be added at two stages: if they are short fibers, they can be mixed during the grinding of the slip, in order to obtain a homogeneous distribution and eliminate the risk of agglomeration of said fibers; if it is about long fibers, they can be incorporated after degassing of the slip, or, in the case of a sheet of fibers, be impregnated with the slip to make shaped articles.

Once the composite material has been shaped as mentioned above, possibly with a particular orientation of the fibers,: 1 is subjected to sintering, at a temperature of between 300 and 700 ° C., which leads in particular to the decomposition and the volatilization of the liquid organic support used to prepare the slip. Sintering is carried out under vacuum or in the presence of an inert gas, the type of gas depending in particular on the nature of the fibers and of the matrix.

The composite material according to the invention has advantageous characteristics due to the combination of the high elastic modulus, the high hardness and the high chemical stability of the fibers used, and by the high elastic limit and the ductility of the binder phase. Its remarkable mechanical properties, in particular its toughness, its resistance to wear, to creep, to rupture, to shock, to oxidation at high temperature, etc., make it a new material suitable for a large number of applications, such as cutting tools, wearing parts, structural elements such as motor, turbine, etc. For the production of complex parts, such as turbine blades and cutting inserts, which are exposed in use to high stresses at high temperatures, it is preferable that the whiskeys are oriented in the matrix. In general, it can be seen that the composite materials according to the invention have an increase in the stress intensity factor up to 2.5, and for tensile strength an increase by a factor of 10.

The composite material according to the invention and the manufacture thereof are now illustrated in more detail with reference to the following examples:

Example 1

* fibers not covered
** covered fibers

Alumina fibers 1µ in diameter and 5mm long, whether or not covered by chemical vapor deposition with a 0.1µ titanium carbide layer mixed with titanium carbide, nickel and other elements :
sample 1 20% Al2O3 * + 80% (40% TiC + 40% Ni + 10% Mo₂C + 8% Co + 2% Ru)
sample 2 20% Al2O3 ** + 80% (40% TiC + 40% Ni + 10% Mo₂C + 8% Co + 2% Ru)

These samples are ground for 24 hours in Inconel mills with alumina beads in petroleum ether in the presence of 2% paraffin. After distillation, the mixture is pressed into 150 x 40 mm bars from which test pieces are cut (dimensions after sintering and rectification 35 x 7 x 3.5 mm.). These test pieces are then sintered in a "SINTER HIP" type oven, where a pressure of 50 bars is applied after sintering. Comparative measurements of K _lC on the two types of samples give the following results (toughness according to standards ASTM E813-81)

sample 1 K _lC = 7.5 MN / m MPa√m)
sample 2 K _lC = 12.5 MN / m (MPa√m)

Example 2

* fibers not covered
** covered fibers

WC fibers 0.8 µ in diameter and 8 mm long whether or not covered with a chemical vapor deposition of a 0.08 µ thick layer of titanium carbide are mixed with WC, TiC , Ni and Mo₂C:
sample 1 20% WC * + 80% (42.5% WC + 42.5% TiC + 5% Mo₂C + 12% Ni)
sample 2 20% WC ** + 80% (42.5% WC + 42.5% TiC + 5% Mo₂C + 12% Ni)

R = 1700 N/mm2 (MPa)
échantillon 2

R = 2400 N/mm2 (MPa)The mixtures undergo the same treatment as in Example 1 and identical bars are machined. The measurement of the breaking strength at 3 points on these samples gives the following results: (transverse failure according to ASTM B406-76 standards)
sample 1

R = 1700 N / mm2 (MPa)
sample 2

R = 2400 N / mm2 (MPa)

Example 3

A 2µ diameter fiber fabric high modulus carbon, whether or not covered with a 0.2 μ tungsten carbide layer, is impregnated with a slip composed of an organic medium and a suspension of powder pre-ground for 48 h. :
sample 1 89% WC + 11% (30% Co + 30% Ni + 20% CV + 10% Mo + 6% WC + 4% Ru) + fabric of uncoated fibers
sample 2 same as sample 1, but with a fabric of fibers covered with tungsten carbide.

_R, qui donnent les résultats suivants :
échantillon 1 K_IC = 8,7 MN/m (MPa√m)

R = 2500 N/mm (MPa)
échantillon 2 K_IC = 17,9 MN/m (MPa√m)

R = 3500 N/mm (MPa)After evaporation of the organic products, sintering and machining of the test pieces, as for sample 1, the K factor _IC and the breaking strength are measured.

_R , which give the following results:
sample 1 K _IC = 8.7 MN / m (MPa√m)

R = 2500 N / mm (MPa)
sample 2 K _IC = 17.9 MN / m (MPa√m)

R = 3500 N / mm (MPa)

It is clear from the above examples that the mechanical characteristics are clearly improved when the fibers are protected by an appropriate coating before their incorporation into the matrix, here by a factor of about 1.4 to 1.7.

As additional examples, mention should also be made of (a) possible combinations of fibers and protective coatings, and (b) possible combinations of fibers and matrix or binder.
(a) fibers: W, WC, Si₃N₄, Al₂O₃, BeO₂, ZrO₂, B₄C, SiC, BN, B and C.
with coating in: TiC, TiN, WC, HfN and HfC
(b) matrix fiber (coating) Co WC (TiC), C (WC), B (WC) Co-Ni-Fe WC (TiC), Al₂O₃ (TiC), SiC (TiC), BN (TiC), WC (TiN), Al₂O₃ (TiN), BN (TiN), SiC (TiN), C (TiN), C (WC), B (WC), B (TiN) Ni-TiC WC (TiC), Al₂O₃ (TiC), SiC (TiC), BN (TiC), WC (TiN), Al₂O₃ (TiN), BN (TiN), SiC (TiN), C (TiAlON), C (TiN), C (WC), B (WC), B (TiN) "Super-alloy" (*) WC (TiC), Al₂O₃ (TiC), SiC (TiC), BN (TiC), WC (TiN), Al₂O₃ (TiN), BN (TiN), SiC (TiN), Al₂O₃ (Y₂O₃), ZrO₂ (Y₂O₃), SiC (Y₂O₃), C (TiN), C (WC), B (WC), B (TiN) (*) Co (35%), Ni (35), Cr (15), Mo (5), W (6), Ru (4)

Claims (12)

1. Composite material comprising a binder phase, constituted by a metallic or ceramic-metallic matrix, and a fibrous reinforcing phase distributed uniformly in the matrix, characterized in that this fibrous phase is constituted by ceramic fibers, separated or in the form of sheet, and covered with a thin protective coating of a material chosen from the group comprising carbon, boron and carbides, nitrides, borides, carbonitrides and oxides of the transition metals. 2. Composite material according to claim 1, characterized in that the ceramic fibers have a diameter of 0.5 to 5 microns and a length of 0.01 to 10 mm. 3. Composite material according to claim 1 or claim 2, characterized in that the fibrous phase represents from 5 to 40% vol. of the composite material, preferably from 15 to 25% vol. 4. Composite material according to one of claims 1 to 3, characterized in that the ceramic fibers are chosen from the group comprising carbon, boron, tungsten, carbides, nitrides, borides, carbonitrides and metal oxides of transition. 5. Composite material according to one of claims 1 to 4, characterized in that the thickness of the protective coating corresponds to approximately 1/10 of the diameter of the coated fibers. 6. Composite material according to one of claims 1 to 5, characterized in that the matrix consists of iron, nickel, cobalt, an alloy of these, optionally with a transition metal carbide. 7. Composite material according to claim 1 or claim 6, characterized in that the binding phase contains from 5 to 20% by weight of additional elements, for example chosen from metals of the platinum group, rhenium and carbides of transition metals. 8. Method for the manufacture of a composite material according to one of claims 1 to 7, characterized in that the fibers of the fibrous phase are coated with a protective coating, that the binder phase is mixed in the form of powder and the fibrous phase, that the mixture is homogenized in a liquid medium, that the material is shaped and that the sintering is carried out. 9. Method according to claim 8, characterized in that the material is pressed isostatically during sintering. 10. Method according to claim 8, characterized in that one proceeds to a hot repressing after sintering. 11. Method according to one of claims 8 to 10, characterized in that the shaping is carried out under conditions such that the fibers are oriented uniaxially in the matrix. 12. Method according to one of claims 8 to 10, characterized in that the fibers in the form of a sheet, previously coated with the protective coating, are impregnated with a slip containing the other constituents of the composite material, then the material is placed in shaped and sintered, if necessary repressed.