GB2299813A

GB2299813A - Spark plug electrode

Info

Publication number: GB2299813A
Application number: GB9507363A
Authority: GB
Inventors: Nigel Philip Harold Fawcett; Richard Hardy Knowles; Mark Andrew Rowland; Richard William Quinn
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1995-04-08
Filing date: 1995-04-08
Publication date: 1996-10-16
Also published as: GB9507363D0

Abstract

A spark plug electrode has welded to its tip a pad of an oxide dispersion strengthened alloy comprising 80 to 97 wt% platinum, 3 to 20 wt% of a metal selected from iridium, palladium, rhodium, or gold, and 0.02 to 1.5 wt% of one or more rare earth oxides. The pad confers improved erosion resistance on the electrode, and the welded pad material resists separation or cracking caused by thermal cycling.

Description

SPARK PLUG ELECTRODE The present invention relates to a spark plug electrode and'a method of manufacturing the electrode. The spark plug electrode is for use particularly, but not exclusively, in a spark plug for a motor vehicle engine.

In order to achieve greater spark plug life and/or to allow the area of the electrode to be reduced to produce more efficient ignition, it is known to weld a pad of erosion resistant material to the electrode tip. One known material is platinum, but other materials which are more resistant to erosion than Inconel metal may also be used. Inconel metal is a material that is conventionally used for spark plug electrodes and the name Inconel is a Registered Trade Mark.

A problem with using a welded on pad is that over a period of time cracks tend to occur within the pad in the region of the weld, and the pad may partially or completely separate from the rest of the electrode.

A further problem which is encountered with erosion resistant welded pads is that pads having good fatigue resistance tend to have high tensile strength and be very hard, making them difficult to weld or to work at room temperature. Attempting to weld such materials onto a centre electrode may result in damage to the electrode, and welding may be difficult.

It has been proposed to incorporate an intermediate stress relieving layer between the electrode and the pad in order to reduce the presumed thermal expansion mismatch between the two and hence to try to reduce the tendency of the pad to crack. See for example US patent number 4 540 910.

However the use of such an intermediate layer introduces an extra manufacturing step, and extra cost in the manufacturing process.

Oxide dispersion strengthening (ODS) is a known technique in which fine particles (usually less than 5 Zm) of a rare earth oxide are dispersed in a metal to increase high temperature tensile strength and improve corrosion resistance and other properties of the metal. See for example Metal Powder Report Vol. 37, No 4, April 1982.

The rare earth particles are believed to induce a stable grain structure in the metal, particularly after working and annealing of the metal.

ODS platinum has an improved high temperature tensile strength compared to conventional platinum. However we have found that a pad of ODS platinum welded to a conventional spark plug electrode cracks when subjected to severe engine testing.

We have studied the appearance of the weld. surfaces after separation and found that they show characteristics of fatigue. We believe that the cause of separation is a high cycle thermal fatigue mechanism exacerbated by thermal expansion mismatch. We have found a material which resists cracking by virtue of its increased fatigue strength, and which can readily be welded to a centre electrode.

According to the present invention there is provided a spark plug electrode which has welded to its tip a pad of an oxide dispersion strengthened alloy comprising 80 to 97 wt% platinum, 3 to 20 wt% of a metal selected from iridium, palladium, rhodium, or gold, and 0.02 to 1.5 wt% of one or more rare earth oxides.

The invention provides a spark plug electrode which resists erosion, and in which the pad adheres well to the electrode tip without the use of an intermediate layer between the pad and the electrode.

It is particularly preferred that the platinum is alloyed with iridium. For convenience hereinafter the invention will be described with reference to an ODS alloy of platinum and iridium.

The use of an ODS alloy of platinum and iridium for welding to a spark plug electrode provides an electrode which does not crack when subjected to the same severe engine testing that produced cracks in ODS platinum.

Preferred oxide particles are yttria and zirconia, and yttria is particularly preferred. For convenience hereinafter the invention will be described with reference to the use of yttria.

Surprisingly, we have found that ODS platinum/iridium in the preferred proportions may readily be welded and worked at room temperature, facilitating manufacture of a spark plug in accordance with the invention.

Iridium is cheaper than platinum, so increasing the ratio of iridium to platinum in the alloy reduces materials costs. However iridium is also harder than platinum, and as more iridium is incorpórated in the alloy it becomes harder to weld and work at room temperature. The optimum ratio of platinum to iridium will therefore be determined by cost considerations, and the strength of the spark plug metal.

We have found that a weight ratio of nine parts platinum to one part iridium works well, being resistant to cracking and readily welded and worked at room temperature. This material will be referred to as ODS PtlOIr. A preferred concentration of yttria is 0.08 to 1.2 wt%, and a particularly preferred concentration is 0.1 wt%, with respect to the platinum/iridium alloy.

The size range of the yttria particles may be suitably be that employed for known ODS alloys.

The invention will now be further described, by way of example, with reference to the following drawing in which Figure 1 is a graph of tensile strength with temperature for a range of platinum based materials.

The materials tested were: platinum ("pure Pt"); Oxide Dispersion Strengthened platinum (ODS Pt"); a platinum/iridium alloy containing 90 wtt platinum ("PtlOIr"); and ODS PtlOIr. Both of the ODS alloys contained about 0.1 wt% yttria. The materials were supplied by Engelhard.

It will be seen that the ODS PtlOIr material has a substantially greater retained tensile strength in the temperature region 600 to 900 cm, where most spark plugs operate.

The ODS PtlOIr material also has a high tensile strength measured at room temperature. However we have surprisingly found that despite this high room temperature tensile strength the material may be worked and welded to a spark plug tip using conventional high volume production techniques.

Pads of the ODS PtlOIr and the ODS Pt materials were each welded onto a separate spark plug tip, and each spark plug was then subjected to severe engine test conditions invdlving high speed and high load to cause the required high temperature transients. After testing, the weld line between the pad and the tip was examined. The ODS Pt pad showed cracking parallel to the weld line. The ODS PtlOIr pad, however, showed no evidence of cracking.

The invention therefore provides a spark plug electrode which has improved resistance to thermal cycling.

The invention also provides a method of manufacturing the spark plug electrode, comprising welding to its tip a pad of an oxide dispersion strengthened alloy comprising 80 to 97 wt% platinum, 3 to 20 wt% of a metal selected from iridium, palladium, rhodium, or gold, and 0.02 to 1.5 wtt of one or more rare earth oxides.

Although the invention has been illustrated with reference to the use of an ODS platinum/iridium alloy, it will be understood that the invention is not limited to this embodiment. Other ODS alloys of platinum may also be used, provided that they have sufficient retained tensile strength at the operating temperatures of the spark plug, and can be worked and welded in a high volume production welding technique.

Claims

1. A spark plug electrode which has welded to its tip a pad'of an oxide dispersion strengthened alloy comprising 80 to 97 wt% platinum, 3 to 20 wt% of a metal selected from iridium, palladium, rhodium, or gold, and 0.02 to 1.5 wt% of one or more rare earth oxides.

2. A spark plug electrode as claimed in claim 1, wherein the rare earth oxide is yttria.

3. A spark plug electrode as claimed in Claim 1 or Claim 2, wherein the platinum is alloyed with iridium.

4. A spark plug electrode as claimed in any one of the preceding claims, wherein the weight ratio of platinum to the other metal is about 9 to 1.

5. A spark plug electrode as claimed in any one of the preceding claims, wherein the rare earth oxide is present at a level of 0.08 to 1.2 wt% with respect to the metals.

6. A spark plug electrode as claimed in any one of the preceding claims, wherein the rare earth oxide is present at a level of 0.1 wt%

7. A method of manufacturing a spark plug electrode as claimed in any one of the preceding claims, comprising welding to the tip of the electrode a pad of an oxide dispersion strengthened alloy comprising 80 to 97 wt% platinum, 3 to 20 wt% of a metal selected from iridium, palladium, rhodium, or gold, and 0.02 to 1.5 wt% of one or more rare earth oxides.

8. A spark plug which includes an electrode as claimed in any one of Claims 1 to 6.

9. A spark plug electrode substantially as hereinbefore described.