For obtaining an intentional cracking behaviour of such ceramic composites various suggestions have been made, for in- stance according to WO 93/22258 to the Applicant. In practice, however, it has turned out that an interface layer of ZrO2 pro- vides a bonding which mostly is too strong. On the other hand, carbon provides a suitably weak bonding between the reinforcing material and the matrix but degrades the fibre material. Over ZrO2 also HfO2 fulfils the requirements as to a weak interface zone but in many connections a still weaker interface is desir- able.

Therefore, the main object of the present invention is to suggest a method by which an even weaker interface is obtained.

In one aspect of the invention, the reinforcing fibre material is immersed into a powder slurry containing carbon and ZrO2 so as to be coated thereby and then dried, after which the compos- ite material is subjected to green forming and densification steps as known per se, and finally a heat-treatment in air leaving a porous structure of the interface material. In an- other aspect of the invention, the reinforcing fibre material is immersed into and coated with a first powder slurry contain-

ing ZrO2 and then-dried and immersed into and. coated with a second powder slurry containing carbon and dried, after which the composite material is subjected to green forming and densi- fication steps as known per se, and finally a heat-treatment in air leaving a gap between the reinforcing material and the ma- trix. It is advantageous to use a powder slurry technique, in which thus rather great particles are used which create a sta- ble porous layer when a coating is used including C/ZrO2.

By the present invention it is thus possible to obtain a weak interface securing the necessary crack deflection and fi- bre pullout behaviour during fracture of the composite.

The coating formed on fibres of which the reinforcing ma- terial consist and which are immersed in the slurry has proved to be adherent and strong enough to survive green forming and densification processes. After densification the C is removed by heat treatment in air, leaving a gap or porosity in the ox- ide interface. The volume fraction of C can be varied to achieve the desired interface strength. The interface made by this invention is stable at high temperatures for long times because the pores have got the right size, i. a. are large enough.

According to the further aspect of the invention, there is provided, however, another way to obtain an interface weak enough, namely to add a fugitive layer. In this method, the re- inforcement is first immersed in a C powder slurry, dried and then immersed in an oxide (ZrO2) powder slurry, forming a dou- ble. or sequential coating. Again, this coating is strong and adherent enough to survive green forming and densification. Af- ter densification the C is removed by heat treatment leaving a gap between the reinforcing material and the oxide interface.

Surface roughness of the reinforcing and matrix materials is sufficient to give load transfer between reinforcing material

and matrix material. The absence of bonding between the oxide interface and the reinforcing material will ensure effective crack deflection. The degree of load transfer and frictional sliding resistance between reinforcing material and matrix con- trolling the fibre pullout behaviour can be varied by varying the thickness of the fugitive C layer.

The presence of an oxide interface will prevent a possi- ble carbothermal reduction of the surface of the reinforcing fibre material of single crystal Al203 fibres (to A14C3) which would degrade the mechanical properties by defects created. The presence of oxide interface (especially in the case of a C/ZrO2 mixture) will locally raise the partial pressure of oxygen and thus prevent the carbothermal reduction to take place.

Example1 Single crystal fibres of Al203 (from Saphikon Inc., USA) were covered with a thin layer of C/ZrO2 mixture. This was made by immersing the fibres in a slurry of C/ZrO2-powder in water.

The volume proportions of C to ZrO2 was 1/1. After drying the coated fibres were stacked to a fibre preform in a plaster mould. A Al203 powder slurry was poured thereon and a pressure gradient was applied to give good infiltration of the fibre preform.

After drying the green bodies were sintered by hot- pressing at 1400°C, 10 MPa for 70 minutes. The C was burnt out by heat treatment at 1250°C for 10 hours leaving a porous ZrO2 layer. After sintering and heat treatment the porous Zr02 was about 3 um thick. The porous ZrO2 layer provided crack deflec- tion and fibre pullout which was proved by bending tests. Fur- thermore, the porous ZrO2 layer was stable at 1400°C for 1000 hours and still provided crack deflection and fibre pullout af- ter this heat treatment which was proved by bending tests.

Example 2 Example 1 was repeated however using HfO2 instead of ZrO2.

Example 3 Single crystal fibres of Al203 (from Saphikon Inc., USA) were covered with at thin double layer of C and ZrO2. This was made by first immersing the fibres in a slurry of ZrO2-powder in water and then immersing the fibres in a slurry of C-powder in water. After drying the coated fibres were stacked to a fibre preform in a plaster mould. An Al203 powder slurry was poured thereon and a pressure gradient was applied to give good infiltration of the fibre preform.

After drying the green bodies were sintered by hot- pressing at 1400°C, at 10 MPa for 70 minutes. The C was burnt out by heat treatment at 1250°C for 10 hours leaving a gap of about 1 um between the zirconia layer and the Al203 matrix. Af- ter sintering and heat treatment the ZrO2 layer was about 3 um of thickness and not bonded to the matrix. The gap between fi- bre and ZrO2 layer provided crack deflection and fibre pullout which was proved by bending tests. (Furthermore, the ZrO2 layer was stable at 1400°C for 1000 hours and still provided crack deflection and fibre pullout after this heat treatment which was proved by bending tests.) Example4 Example 3 was repeated however using HfO2 instead of ZrO2.

Example 5 Example 3 was repeated, however with the steps in slightly reversed order, i. e. by first immersing the fibres in a slurry of C-powder in water and then in a slurry of ZrO2- powder, with similar advantageous result.