In some cases different parts of an article may be subjected to different conditions in use, and it is known to form an article of two or more layers of different materials chosen to withstand respective operating conditions. However difficulties can arise due to differences between the physical properties of the materials forming the layers, which can lead to separation of the layers after a period of use.

Whilst it is possible to form a mechanical connection between adjacent layers, this is not always convenient, and an object of the invention is to provide a layered article which is less subject to the above mentioned difficulties but avoids the need of a mechanical connection.

Accordingly the invention provides in one aspect an article comprising two or more layers of different material compositions there being a controlled graded change of composition between layers or within at least one of the layers.

Thus it is possible to provide a controlled graded continuous or discontinuous change of thermal and ^' mechanical properties from one layer to the other. By this means a more satisfactory bond between layers can be achieved. The thickness of the graded change .may be from about one millimetre to several centimetres, or even greater. Por composite materials with three or more layers, the material or materials of one or more of the layers can be a controlled mixture or a controlled graded mixture providing controlled graded discontinuous or continuous changes of thermal and mechanical properties within the composite layer.

For composite materials with two, three or more layers, one or more of the layers can have a greater mechanical strength than the other layer or TU EΛTΓ

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layers. One or more of the layers can have a low thermal conductivity. One or more of the layers can have a low, medium or high value of mechanical elongation at rupture. For composite materials with two layers, the region of the controlled graded layer contiguous to the second layer may have a thermal expansion coefficient and/or a value of elongation at rupture which are intermediate between the value of the thermal expansion coefficient and/or the value of elongation at rupture for the second layer and the value of the thermal expansion coefficient and/or the value of elongation at rupture in the non¬ contiguous regions of the controlled graded layer. For composite materials with three or more layers, at least one intermediate layer can have a thermal expansion coefficient and/or a mechanical elongation at rupture which are intermediate between the values of the corresponding coefficients and elongations for the contiguous layers.

The properties of low thermal conductivity, high mechanical strength and non-negligible elongation may apply individually or jointly to a given layer, to contiguous layers or to non-contiguous layers, and a layer with one, two or all three of these properties can be an outer layer, an intermediate layer, or the innermost layer. In some cases, however the different layers may be provided by different zones of the article having a graded change of composition from one zone to the next.

The invention provides in a further aspect a method of manufacturing an article, comprising the steps of starting with at least two layers of different material compositions, there being a controlled graded change of composition between layers or within at least one of the layers, and subsequently bonding the layers together.

One or more of the layers may be of a material or materials produced by casting, forging, stamping, rolling or extrusion methods. One or more of the layers may be

produced as a single material or as a controlled mixture of materials bonded to the contiguous layer or layers. One or more of the layers may be bonded to another layer or layers by sintering, casting or by friction welding. One or more of the layers may be produced by existing methods used for coatings, which include roll-bonding, diffusion-bonding, spraying (including fusion spraying), sintering, deposition (including explosive deposition, electro-deposition, electro-static deposition, electroless deposition), brushing and dipping, without or with surface treatment of the layers, or the contiguous layer or layers.

The layers can have the same thickness or different thicknesses, depending on design and operating requirements. The invention has particular application to pistons for use in internal combustion engines.

Thus according to another aspect of the invention in a piston formed of at least two different materials, the skirt has a first composition and the crown has _a second composition, and between the skirt a d the crown the piston has an intermediate zone in which the relative proportions of the materials lies intermediate those in the skirt• and the crown.

By this means the thermal conductivity, mechanical strength and other properties can be graded between the skirt and the crown.

The piston may comprise three or more distinct layers, each having different proportions of said materials, the proportions being substantially uniform throughout each layer and the relative proportions of the materials being graded from one layer to the next. However there may be a continuous grading of the materials throughout part or the whole of the piston. The proportion of one of the materials in one or other of the outer layers may be zero.

Preferably aluminium or an aluminium alloy, for example silicon-aluminium, is a major constituent throughout the piston, the crown having a significant proportion of

alumina, the intermediate layer or layers having a smaller proportion of alumina, and the skirt having a still smaller proportion of alumina, which may be zero. Thus the proportion of alumina to aluminium or aluminium alloy may vary from the skirt to the crown with the range 0 to 0% by weight.

Two embodiments of the invention will now be described with reference to the accompanying drawings, wherein : Figure 1 is a vertical section of a piston having three uninterrupted layers and showing two half-sections at right angles to each other; and

Figure 2 is a similar view to Figure 1 of a pisto comprising uninterrupted layers. In Figure 1 a piston primarily for an internal combustion engine is made as a composite-material design with three uninterrupted layers 1, 2, 3 _* The outermost layer 1 (i.e. at the crown end of the piston) is a layer with a high percentage of alumina mixed with aluminium or silicon-aluminium, this layer providing most of the. mechanical strength and having a lower thermal conductivit than the- other two layers. The layer 1 extends from the crown , to the peripheral edge 5 of the piston ^' ^ and down along the side of the piston to the region 6 below the bottom compression ring.

The intermediate layer 2 extends down below the region 6 of the layer 1 and consists of a medium percentag of alumina with aluminium or silicon-aluminium, the layer having intermediate mechanical and thermal propertie between those of the outer layer 1 and the inner surface layer 3 which extends downwardly beyond the region of the intermediate layer 2 below the area of the bottom compression ring groove and further downwardly along the side of the piston to a gudgeon pin boss 7 and to an area in the bottom region of the piston skirt.

The layer 3 is made of aluminium or silicon-alumi or a mixture of a low percentage of alumina with aluminium or silicon-aluminium, and has a lower high-temperature

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mechanical strength and a higher thermal conductivity than the layer-2. As an example, layer 3 may contain from 0 to 0% by weight alumina, layer 1 up to 30% alumina, (and greater than in layer 3)» and layer 2 an intermediate amount of alumina.

The layers 1, 2, and 3 ma be made by compacting and sintering. Alternatively layers 2 and 3, or layer 3, may be cast or sprayed on layer 1 or on layers 1 and 2 respectively. In Figure 2 a piston is made as a composite-material design with three uninterrupted layers. In this example the layer 9 in the crown 4 provides low thermal conductivity and comprises a mixture of aluminium or silicon-aluminium with a significant proportion of alumina. A lower layer 10 comprises a mixture of aluminium or silicon-aluminium with a smaller proportion of alumina, and can be either a homogeneous mixture or a controlled continuously graded layer having a higher proportion of alumina near the layer 9 and a lower proportion of alumina away from the layer 9. An insert 11 containing the compression ring grooves provides low thermal conductivity and/or mechanical strength and may be of the same material as the uppermost layer 9» or of other suitable materials.- Layer 10 may be sprayed, sintered or cast. In this example the lower layer 12 of the piston, comprising the piston bosses and the skirt, may be of aluminium or silicon-aluminium but may also include a small proportion of alumina. This layer 12 may be made, for example as a separate casting. It can then be bonded, e.g. by friction welding, to the upper section of the piston. In some cases the layer 12 may comprise a continuously graded layer with a higher proportion of alumina near the layer 10 and a lower proportion of alumina (which may be zero) at the skirt end of the piston.

In an alternative arrangement the layer 10 may extend to the skirt end of the piston and be in the form of a single graded layer with a higher proportion of

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alumina nearer the layer 9 and a lower proportion of alumina (which may be zero) at the skirt end of the piston.

Also in some cases the piston may be continuously graded throughout, the proportion of alumina-increasing from the skirt towards the piston.

Although pistons made from aluminium or silicon- aluminium have been described it will be appreciated that other materials could alternatively be used. Moreover although the invention is especially applicable to pistons, clearly other articles may be made within the scope of the invention, such as, inter alia, cylinders with or without ribs or flanges, casings, turbi blades and combustors.

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