This invention relates to a spark plug for an internal combustion engine used in an automobile or the like.
A spark plug is used in a petrol internal combustion engine for an automobile. There have heretofore been proposed various spark plugs having a noble metal tip layer (made, for example, of a platinum alloy) provided on a discharge portion of at least one of a central electrode and an earth electrode so as to achieve a long service life of the spark plug. At present, with the use of such platinum alloy tip layer, the service life of the spark plug has been prolonged to such an extent as to enable a 100,000 km running of an automobile. Recently, however, the number of parts attached to the engine has been increased because of a high-performance design of the engine, and therefore much time is required for exchanging the plug in the market. In order to prolong an exchange interval, there has now been a demand for the type of spark plug having a longer service life. This requirement can be met by increasing the amount of the platinum alloy, and there are two methods of increasing the platinum alloy amount. One is to increase the thickness of the platinum alloy tip layer, and the other is to increase the diameter of the platinum alloy tip layer.
With the method of increasing the thickness of the platinum alloy tip layer, however, when the service life expires (that is, the platinum alloy tip layer is completely consumed), the spark gap becomes greater, which leads to a possibility that a discharge may not be produced between the central electrode and the earth electrode. For this reason, this method is not practical. Therefore, to achieve a longer service life, it is necessary to increase the diameter of the platinum alloy tip layer. However, when this is done, a thermal stress at the surface of bonding between the platinum alloy tip layer and the substrate increases because of a difference in linear expansion coefficient between the platinum alloy tip layer and the substrate. As a result, the oxidation of the bonding surface between the platinum alloy tip layer and the substrate proceeds, and finally the platinum alloy tip layer becomes disengaged from the substrate. Therefore, at present, in order to achieve a long service life enabling the running of more than 100,000 km, the thermal stress must be decreased.
In order to decrease such thermal stress, it is effective to provide a stress-relieving layer between the platinum alloy tip layer and the substrate. More specifically, an alloy layer is formed at the bonding surface by a heat treatment, as disclosed in U.S. Pat. No. 4,581,558. Alternatively, as disclosed in U.S. Pat. No. 4,540,910, a stress-relieving layer of a material having a linear expansion coefficient between those of the platinum alloy tip layer and the substrate is formed between the platinum alloy tip layer and the substrate.
However, for example, if the plug is intended to achieve a service life enabling the running of 20,000 km which is twice that now attainable, it is necessary to double the area of the platinum alloy tip layer at the discharge surface, in which case the area of the bonding of the platinum alloy tip layer is doubled, and its diameter is about 1.4 times greater. Therefore, even if only the stress-relieving layer as disclosed in the above U. S. patents is provided, the platinum alloy tip layer may become disengaged, and therefore the thermal stress need to be decreased to a greater extent.