This invention relates to brake pistons, and more particularly to pistons for disc brakes, and to a new method of making such components.

Disc brakes have been used in vehicles for many years and their performance has been steadily improved over that time. As the performance of the brakes increases, however, the energy which is converted by friction into heat is increased. The means for dissipating heat has changed little during development, and consequently heat soaking through a brake pad into a brake piston and thence into brake fluid has become more problematical, and has led at times to the extreme difficulties which result if the brake fluid boils.

Many proposals have been made for insulating the brake piston from the brake pad and hence reduce the rate at

which heat can soak into the brake piston, but these involve extra layers of material of reduced heat conductivity between the brake piston and the heat source (the working surface of the brake pad).

An object of this invention is to provide a radically different solution to this problem. The invention also provides a novel method of making a silicon nitride brake piston.

According to this invention, a method for the manufacture of a brake piston comprises the following steps

(i) injection moulding said piston from a composition comprising a sinteral)le or potentially sinterable silicon _. nitride powder and an organic vehicle suitable for injection moulding,

Cii) preparing the moulded piston for sintering and

(iii) sintering the piston at a temperature of approximately 1700 to 1850°C to final density.

In the manufacturing method the composition used in step (i) includes an organic vehicle to facilitate injection moulding of the composition, and the proportion of such vehicle used in the composition is kept low whilst being sufficient for its purpose. Preferably the organic vehicle

component of the composition is not more than 25% by weight. The organic vehicle component is preferably a thermoplastic polymer, and may include other ingredients such as plasticers, wax, surfactant as required.

For the purposes of step (iii) of the method, sintering aids such as chromia, yttria, alumina may be included in the composition used in step (i).

The sinterable powder may be a silicon nitride powder. By the term "potentially sinterable powder" in the context of this specification we mean powder of a material which can be converted into a sinterable material after the injection moulding step. For present purposes, potentially sinterable powder includes powdered silicon, which can be converted into silicon nitride powder by nitriding.

Step (ii) of the method includes removing any sprue from the piston formed by step (i), removing the organic vehicle from the piston by pyrolysis, and where a potentially sinterable powder is used, converting potentially sinterable powder into a sinterable form.

Removal of the organic vehicle from the moulded piston is preferably carried out by subjecting the piston to a progressive increase in temperature up to 300°C - 500°C over a period of 4 to 5 days. This treatment

degrades and drives off the organic system, which is chosen for this purpose as is known in the art.

Where silicon powder has been used as a potentially sinterable material, it is converted to a silicon nitride form (suitable for sintering) after the polymer has been removed from the moulded piston. Conversion of such silicon powder to silicon nitride is carried out by nitriding the moulded piston, which consists essentially of heating the piston up to about 1500°C in an atmosphere of nitrogen.

Step (iii) of the method may be carried out with or without application of a pressure higher than atmospheric.

After sintering, the brake piston may be finished by simply grinding those outside surfaces of the piston which are to contact other components.

^* According to a further aspect of the present invention, a brake piston having crown and skirt portions comprises a silicon nitride engineering ceramics material which has a thermal conductivity lower than steel made by the method aforesaid.

Silicon nitride is selected not only because of its thermal conductivity which is approximately 50% lower than steel, but also because of its exceptional resistance to

thermal shock. By making the entire piston of this material, the disa vantages of composite structures are avoided. Also, the difficulties of manufacturing composite pistons are avoided.

The finished brake piston may be used in an otherwise conventional brake caliper. The wholly silicon nitride ceramic piston has other advantages over steel such as superior corrosion resistance to water and salt, higher abrasion resistance, and lower density, in addition to its resistance to thermal shock.

The invention will now be described by way of example only with reference to the accompanying drawings of which

Figure 1 is a perspective view of a piston in accordance with the invention and

Figure 2 is a cross-sectional view of the piston in a brake caliper.

As shown in Figures 1 and 2 a brake piston of sintered silicon nitride (density 3.25g/cm ) has a dished crown portion 1 and a skirt portion 2, the latter being provided with a groove 3 for a seal 4 (not shown in Figure 1) to prevent brake fluid 5 seeping past the piston.

The brake piston when in use in a brake caliper represented fragmentarily by 5 in Figure 2, contacts a back plate 6 carrying a pad 7 of friction material which is forced during braking against a brake disc 8. The crown portion 1 of the brake piston is so shaped that only a peripheral ring 1A of the crown portion of the piston comes into contact with the back plate.

Those surfaces of the brake piston which need be finished by grinding during manufacture are .the peripheral ring 1A of the outside surface of the crown portion and the outer surface 2A of the skirt portion 2 (including all surfaces of the groove 3).