Spark plug with igniting portion chip composition

A spark plug includes a central electrode, an insulator exterior to the central electrode, a metallic shell exterior to the insulator and having the central electrode protrude from one end of the metallic shell, a ground electrode having one end coupled to the metallic shell and having another end facing the central electrode, and an igniting portion secured to either the central electrode or to the ground electrode or to both for forming a spark discharge gap, wherein the igniting portion comprises a chip including a metal-oxide composite material including at least 10 wt % of Ir, an alloy having at least one element selected from a group consisting of Rh, Mo, Nb and Pt in a total amount in a range of 0.5 to 88.4 wt %, and a rare earth oxide in an amount ranging from 1.6 to 15 wt %.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a spark plug with igniting portion chip 
composition for use in internal combustion engines. 
2. Description of the Related Art 
Conventional spark plugs used in internal combustion engines, such as 
automotive engines, have an igniting portion formed of a platinum (Pt) 
alloy chip welded to a tip end of an electrode in order to improve its 
resistance to spark consumption. However, in view of the high cost of 
platinum, it has been proposed to use less expensive iridium (Ir) as a 
chip material. 
A problem with the use of Ir as a material for the igniting portion of the 
spark plug is that Ir easily oxidizes and evaporates in a high temperature 
range of 900 to 1,000.degree. C. Therefore, if it is directly used in the 
igniting portion of the electrode, it is consumed by oxidation and 
evaporation to a greater extent than by sparking. In order to retard the 
oxidation and evaporation of Ir, it has been proposed that a material 
having a rare earth oxide, such as Y.sub.2 O.sub.3, dispersed in Ir, 
should be used as added to the chip composition (See Unexamined Japanese 
Patent Publication (kokai) No. Hei. 7-37677). However, as recent engine 
models are designed to increase output power, the range of temperatures 
over which the spark plug is used tends to shift towards the higher end, 
and even the spark plug that uses a chip made of the proposed material may 
not have a satisfactory durability. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a spark plug that is 
sufficiently resistant to consumption by oxidation and evaporation of an 
Ir component at elevated temperatures to thereby assure high durability. 
A spark plug comprises, a central electrode, an insulator provided exterior 
to the central electrode; a metallic shell exterior to the insulator such 
and having the central electrode protrude from one end of the metallic 
shell, a ground electrode having one end coupled to the metallic shell and 
having another end facing the central electrode, and an igniting portion 
secured to either the central electrode or to the ground electrode or to 
both for forming a spark discharge gap, wherein the igniting portion 
comprises a chip including a metal-oxide composite material including at 
least 10 wt % of Ir, an alloy having at least one element selected from a 
group consisting of Rh, Mo, Nb and Pt in a total amount in a range of 0.5 
to 89.9 wt % and a rare earth oxide in an amount ranging from 0.1 to 15 wt 
%. 
The spark plug according to the present invention uses an Ir containing 
chip material but is sufficiently resistant to consumption by oxidation 
and evaporation of the Ir component at elevated temperatures to assure 
high endurance.

DETAILED DESCRIPTION OF THE INVENTION 
Detailed description of the present invention will be described as follows. 
A spark plug according to the present invention has a central electrode, an 
insulator provided exterior to the central electrode, a metallic shell 
provided exterior to the insulator in such a way that the central 
electrode protrudes from one end, a ground electrode coupled at one end to 
the metallic shell and which has the other end said central electrode, and 
an igniting portion that is secured to either the central electrode or the 
ground electrode or both for forming a spark discharge gap. The igniting 
portion is composed of a chip made of a metal-oxide composite material 
that contains at least 10 wt % of Ir, which contains one or more of Rh, 
Mo, Nb and Pt in a total amount in the range of 0.5 to 89.9 wt %, 
preferably in the range of 0.5 to 20 wt %, more preferably 5 to 10 wt %, 
and which also contains a rare earth oxide in an amount in the range of 
0.1 to 15 wt %. 
If a chip made of the material set forth above is used to compose the 
igniting portion which forms a spark discharge gap, the consumption due to 
oxidation and evaporation of the Ir component at elevated temperatures is 
effectively retarded, resulting in a highly durable spark plug. 
Among the composite material of which the chip is to be made, a group of 
elements Rh, Mo, Nb and Pt (hereunder referred to as "alloy components") 
and the rare earth oxide are each effective in retarding the oxidation and 
evaporation of the Ir component. If the Ir content is least than 10 Wt %, 
the melting point of the chip will drop to a level, where the desired 
durability can no longer be assured. Therefore, the sum of the alloy 
Components and the rare earth oxide should not exceed 90 wt %. The Ir 
amount is desirably at least 50 wt %. 
If the total content of the alloy components is least than 0.5 wt %, the 
desired effectiveness of the addition of those alloy components in 
preventing the oxidation and evaporation of Ir can no longer be obtained. 
Therefore, the sum of the alloy components is preferably adjusted to be at 
least 0.5 wt %. 
On the other hand, if the content of the rare earth oxide is less than 0.1 
wt %, the desired effectiveness of the addition of the rare earth oxide in 
preventing the oxidation and evaporation of Ir can no longer be obtained. 
If the amount of the rare earth oxide exceeds 15 wt %, the resistance of 
the chip to thermal impact decreases to such a level that defects (such as 
cracking) may occur at some locations such as where the chip is secured to 
an electrode by welding. While Y.sub.2 O.sub.3 is preferably used as the 
rare earth oxide, other compounds including La.sub.2 O.sub.3 and ThO.sub.2 
may also be employed. 
The alloy components are preferably contained in amounts not exceeding 
their solubility limits with respect to Ir. If either of the alloy 
components exceeds its solubility limit with respect to Ir, a brittle 
intermetallic compound forms between the two elements, and may 
occasionally impair the durability of the igniting portion against 
sparking or its resistance to thermal impact. For example, when using Mo 
or Nb as the alloy component, the solubility limit of Mo with respect to 
Ir is about 12 wt % at room temperature, whereas the solubility limit of 
Nb with respect to Ir is about 6 wt %, so if either Nb or Mo is to be 
added individually, their content may be set at a level smaller than the 
stated values. It should be noted, however, that if the amount with which 
the intermetallic compound is to be formed is below a certain level and 
expected to cause only small effects on the durability and other 
characteristics of the igniting portion, the content of Mo or Nb may 
safely exceed their solubility limits to a slight degree. Thus, the 
content of Mo, if it is to be added alone, is preferably 20 wt % or less, 
more preferably 12 wt % or less. Similarly, the content of Nb, if it is to 
be contained alone, is preferably 10 wt % or less, more preferably 6 wt % 
or less. Needless to say, both Mo and Nb may be added in the composite 
material, and in that case, the amount of Mo and Nb are desirably set not 
to exceed their solubility limits with respect to Ir in a ternary system 
of Ir-Mo-Nb. 
Next, in the case of adding Rh as the alloy component, it the Rh amount 
exceeds 80 wt %, the melting point of the composite material will drop to 
such an extent that durability of the central electrode (or its igniting 
portion) reduced its. The Rh amount is desirably adjusted to lie within 
the range of 20 to 60 wt %, more desirably within the range of 30 to 40 wt 
%. 
When adding Pt as the alloy component, the Pt amount is in the range of 0.5 
to 40 wt %, preferably in the range of 2 to 30 wt %. 
The composite material for the chip may be a sintered material, Thus has 
the advantage of forming a uniform dispersion of the rare earth oxide to 
achieve a further improvement in the durability of the igniting portion. 
Several embodiments of the invention will now be described with reference 
to the accompanying drawings. 
FIG. 1 shows an embodiment of the invention, in which a spark plug 100 has 
a tubular metallic shell 1, an insulator 2 fitted into the metallic shell 
1 in such a way that the tip end 21 protrudes from the metallic shell 1, a 
central electrode 3 provided within the insulator 2 in such a way that the 
igniting portion 31 formed at the tip end protrudes form the insulator 2, 
and a ground electrode 4 coupled to one end to the metallic shell 1 by 
welding and that has the other end bent laterally such that its lateral 
side faces the tip end of the central electrode 3. The ground electrode 4 
has an igniting portion 32 facing the igniting portion 31 of the central 
electrode 3; the clearance between the igniting portions 31 and 32 forms a 
spark discharge gap g. 
The insulator 2 is a sinter of a ceramic material with alumina or aluminum 
nitride as a main component, and has an axial bore 6 through which the 
central electrode 3 is fitted. The metallic shell 1 is a cylindrical shape 
made of a metal, such as a low-carbon steel that provides a housing for 
the spark plug 100. The circumference of the shell 1 has a threaded 
portion 7 formed to assist in the mounting of the spark plug 100 on an 
engine block (not shown). 
The main body 3a of the central electrode 3 and the main body 4a of the 
ground electrode 4 are both typically made of a Ni alloy, The igniting 
portion 31 of the central electrode 3 and the opposed igniting portion 32 
of the ground electrode 4 are both composed of a chip made of a 
metal-oxide composite material that contains at least 10 wt % of Ir, which 
contains one or more of Rh, Mo, Nb and Pt in a total amount ranging form 
0.5 to 89.9 wt % and which also contains a rare earth oxide such as 
Y.sub.2 O.sub.3 in an amount ranging from 0.1 to 15 wt %. These chips may 
be formed from a sintered composite material obtained by providing an 
alloy powder consisting of Ir and the above-mentioned alloy components or 
a mixture of the powders of elemental metals in specified proportions, 
mixing such alloy powder or metal powders with a rare earth oxide powder 
to form a dispersion, shaping the dispersion into a compact and sintering 
the compact. 
Alternatively, the chips may be formed of a molten material obtained by 
mixing the necessary alloy components and a rare earth oxide powder to 
give the stated formula and melting the mixture. 
As shown in FIG. 2, the main body 3a of the central electrode 3 tapers at 
the tip end and its tip and face is formed flat. A disk-shaped chip having 
an alloy formula for the igniting portion 31 is placed on the flat tip end 
face and laser welding, electron beam welding, resistance welding or other 
suitable welding technique is applied to the periphery of the joined 
surfaces to form a weld line W, whereby the chip is securely fixed to the 
tip end face of the central electrode 3 to form the igniting portion 31. 
In order to form the opposed igniting portion 32, a similar chip is placed 
on the ground electrode 4 in registry with the position of the igniting 
portion 31 and a weld line W is similarly formed on the periphery of the 
joined surfaces, whereby the chip is securely fitted to the ground 
electrode 4 to form the igniting portion 32. If desired, either one of the 
two opposed igniting portions 31 and 32 may be omitted. In this case, the 
spark discharge gap is formed between the igniting portion 31 (or the 
opposed igniting portion 32) and the ground electrode 4 (or the central 
electrode 3). 
The function of the spark plug 100 will be described as follows. The spark 
plug 100 is fitted on an engine block using the threaded portion 7 and 
used to ignite an air-fuel mixture supplied into the combustion chamber. 
The igniting portion 31 and the opposed igniting portion. 32 define the 
spark discharge gap g. Since both igniting portions 31, 32 are composed of 
chips made of the aforementioned alloy, their consumption due to the 
oxidation and evaporation of It is sufficiently retarded to ensure that 
the spark discharge gap g will not increase for a prolonged period, 
thereby extending the life of the spark plug 10C. 
EXAMPLES 
Two Ir based alloy powders were tested, one containing 1 wt % of Mo and the 
other containing 5 wt % of Rh. Each powder was mixed with a Y.sub.2 
O.sub.3 powder. The mixtures were shaped into a predetermined shape and 
sintered to fabricate chips made of metal-oxide composite materials, one 
consisting of 1 wt % Mo, 1.7 wt % Y.sub.2 O.sub.3 and the balance Ir, and 
the other consisting of 5 wt % Rh, 1.7 wt % Y.sub.2 O.sub.3 and the 
balance Ir, A comparative chip was fabricated from a sintered composite 
material consisting of 1.7 wt % Y.sub.2 O.sub.3 and the balance Ir. Each 
chip was used to form the igniting portion 31 of the spark plug 100 shown 
in FIG. 1 and the opposed igniting portion 32 (to provide a spark 
discharge gap g of 1.1 mn), and the individual plugs were subjected to a 
performance test under the following condition: a six-cylinder gasoline 
engine (piston displacement=2,000 cc) was fitted with the plug under test 
and operated at full throttle for 200 hours at a rotational speed of 6,000 
rpm (with the temperature of the central electrode rising to about 
850.degree. C.); during engine operation, the volume of chip consumption 
was measured at specified time intervals, The results are shown in FIG. 3. 
Obviously, the chip of the comparative plug experienced a marked 
consumption in terms of volume but this was not the case with the plugs 
within the scope of the invention.