GB1564335A - Gas turbine engine igniter - Google Patents

Description

(54) GAS TURBINE ENGINE IGNITER (71) We, GENERAL ELECTRIC COM PANY, a corporation organized and existing under the laws of the State of New York, United States of America, residing at 1 River Road, Schenectady, 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a gas turbine engine igniter and, more particularly, to the igniter electrode and its material of construction.

Combustion in various power generating apparatus such as gas turbine engines, etc., is initiated by a spark igniter or igniter plug, hereinafter referred to as an igniter. The igniter contains two electrodes between which an electrical arc is discharged in order to ignite the combustible mixture, or, in continuous or extended duty operational modes, to promote the continuing stability of the combustion process. Because the combustion process in general is one of rapid oxidation, an important characteristic of an igniter electrode is that it have adequate resistance to oxidation at combustion generated temperatures such as up to 2000"F. An igniter electrode must also possess adequate arc erosion characteristics, which involve temperatures at the arc interface of at least 4500"F.

According to the present invention there is provided a gas turbine engine with an electrode made of a material comprising 2 to 16 volume percent of at least one oxide selected from oxides of Y, Th and the rare earth elements, with the balance selected from Ru, Ir and alloys based on these metals, and incidental impurities. In its preferred form, the electrode material consists of 2 to 12 volume percent of oxide, with the balance selected from Ru and Ir, and more specifically of 4 to 8 volume percent of Y203 with the balance Ru.

The invention will now be described in more detail making reference to the accompanying drawing which is a graphical comparison of electrode weight loss with Y2O, content in an Ru electrode.

Spark igniters or igniter plugs are relatively critical components of gas turbine engines such as aircraft jet engines. They must be reliable and exhibit long life under severe environmental conditions. Their electrodes, made of high temperature alloys or metallic materials, must exhibit resistance to spark erosion and oxidation.

During service, it has been observed that an igniter can fail as a result of electrode tip erosion which results from a combination of spark erosion and oxidation. Such failure can result in foreign object impact damage to other components downstream of the igniter, for example in the turbine. It is believed that predictability with regard to spark erosion of materials in such a complex combustion environment as is found in a gas turbine engine is relatively poor. However, in an inert environment, spark erosion resistance increases as the melting temperature increases and the vapor pressure decreases. Therefore, it is desirable that the melting point the boiling point and heat of vaporization of an igniter electrode material be as high as possible. This was confirmed by examination of igniters which had been used in modern production gas turbine engines, for example using electrodes based on nickel-base alloys and on tungsten-base alloys.

Examination of the properties of metals indicate that relatively few metals exhibit the combination of properties ideal for igniter electrodes. The refractory metals such as Re, Ta, Mo and Cb exhibit high spark erosion resistance but relatively poor oxidation resistance and must be maintained at a temperature below about 1000"F (about 540"C). The oxidation-resistant alloys based on Ni, Fe and G exhibit too low melting points and high vapor pressures. It has been recognized that such noble metals as Ir, Ru, Pt, Pd and Rh exhibit good oxidation resistance. However, Pt, Pd and Rh exhibit significantly lower melting and boiling temperatures than do Ru and Ir which have intermediate erosion resistance.

In order to improve the arc erosion resistance of Ru and Ir, the present invention provides the addition of oxides selected from those of Y, Th and the rare earth elements, for example Y2O3, to Ru, Ir or alloys based thereon, in an amount which has been found to provide an igniter electrode material with the combination of imptoved spark erosion resistance, good oxidation resistance and the capability to be manufactured into an electrode.

During the evaluation of the present invention, a variety of materials and combinations of materials were considered from the standpoint of oxidation resistance as well as spark erosion resistance. The test apparatus used to determine the relative spark erosion resistance between materials was a commercial electrodischarge hole drilling machine with its electrode being a specimen of the igniter electrode material to be tested. Such specimens were rods ground to a 0.09" diameter and with the ends squared. Such specimen rods were used to drill holes through a 0.065" thick plate of a nickel-base superalloy commercially available as Rene' 41 alloy and described in U.S. Patent 2,945,758. The machine settings were held constant and the temperature was ambient. Measurements of electrode wear, including weight loss, length loss and change in tip radius, were obtained. The following Table shows data selected from the results of such testing.

TABLE Electrode Wear Electrode Length Loss Weight Loss Material (mils) (mug) Hastelloy X 74.0 86.6 Ru 10.2 20.4 Ir 5.0 23.9 Ru+2 vol.

% Y2O. 7.0 14.1 Ru+4 vol.

% Y2Q. 6.5 12.7 From these data, it is clearly recognizable that oxide additions significantly improve erosion resistance. Included in the above Table are data for Hastelloy X alloy which is a nickel-base superalloy commonly used as an electrode material in gas turbine engines and consisting nominally, by weight, of 18.5:1 Fe, 22 Yc, Cr, 9"' Mo, 0.6% W, 1.5O" Co with the balance Ni. The very significant difference between Hastelloy X alloy and specimens made of Ru and Ir are shown by these data, with Ir showing further improvement over Ru. Although the weight loss for Ir is greater than for Ru, it should be recognized that the atomic weight of Ir is almost twice that of Ru and hence its weight loss per unit of volume is significantly less. The selection of Ru in the preferred material of the present invention is based on present cost rather than on the inability of Ir to perform as well as Ru.

As shown by the addition of Y2O, to Ru, there is achieved an unusual increase in spark erosion resistance, for example the addition of about 4 volume percent Y,O, about doubles the erosion resistance of the electrode material based on Ru alone. This same improvement is to be expected from additions of oxides of Th and the rare earth metals.

In one evaluation associated with the present invention, the effect of YO, additions to Ru was evaluated in the same test described above in connection with the data of the above Table. Results of that evaluation are shown in the graphical presentation of the drawing, comparing volume percent (vol. %) addition of Y2O, to Ru wth electrode weight loss. As shown by the data of the drawing, the arc erosion resistance of the electrode increases as Y203 is increased. However, the effect of that oxide addition is different in the range of about 2 vol. % or less than it is for additions greater than that amount: a small amount of Y,O3 causes an initial drop in electrode weight loss but such drop in weight loss does not substantially increase until an addition of about 20 vol. % is made. Therefore, the present invention recognizes an unusual result from the inclusion of at least about 20,' and preferably at least about 4t,Ó by volume of such oxides as Y2Oa.

In an additional evaluation of the present invention, manufacturing procedures were studied for producing electrodes of Ru to which varying amounts of YO, had been added. Ru and Y203 powders were blended to prepare mixtures including 8, 12, 16, 20 and 24 vol. % Y,O,. Rod specimens, as described above, were prepared by pressing and sintering followed by grinding and heat treatment. Radiographic examination was performed after 30 oSO and 60 9Y, reductions of the rods. This inspection showed a tendency for the higher percentages of yttrium (20 and 24 vol. %) to crack and break into short pieces. Sound rod specimens were produced from the 8, 12 and 16 vol. 116 Y,O, rods. Thus, the present invention recognizes not only the advantage of such characteristics as spark erosion resistance through the addition of such oxides as Y2O3, but also the limitation on the oxide content of the material in order to allow practical fabrication. Therefore, the present invention is defined by the range of 2-16 vol.

% oxide addition. More specifically, it has been recognized that such additions in the range of 2-12% 2--12'1:, and preferably 8 % by volume are particularly advantageous.

Although the present invention has been described in connection with certain specific examples, it will be readily recognized by those skilled in the art that modifications to the broad concept of adding oxides of Y, Th and the rare earth elements to Ir and Ru and alloys based thereon for improvement in resistance to both spark erosion and oxidation, can be made without departing from the scope of present invention as defined by the appended

Claims (3)

claims. WHAT WE CLAIM IS:-

1. A gas turbine engine igniter with an electrode made of a metallic material consisting of 2 to 16 vol. c of at least one oxide selected from oxides of Y, Th and the rare earth elements, with the balance selected from Ru, Ir and alloys based on those metals, and incidental impurities.

2. An igniter according to Claim 1 in which the oxide is present at 2 to 12 vol. /".

3. An igniter according to Claim 1 in which the electrode consists of 4 to 8 vol. %' of Y203 with the balance Ru and incidental impurities.