EP1917370B1 - Pt-ir-based wire hardened by oxide dispersion and other alloys provided with an improved surface for spark plug electrodes - Google Patents

Abstract

A wire, strip or reshaped part is produced from an alloy based on platinum, palladium or a mixture of platinum and palladium and hardened by oxide dispersion. The wire, strip or reshaped part cross-section exhibits a peripheral zone in which at least one relatively easily volatilized oxide generator is depleted by at least 25%. In addition, a process is provided for production of such a wire, strip or reshaped part, in which a porous skin is produced thermally on the wire, strip or reshaped part, and the porous skin is compacted by conversion into a soft or impermeable skin.

Description

The present invention relates to platinum-group metal platelets or wires for use as electrodes in spark plugs and to methods of making same.

For many years, the platinum group metals and their alloys have been used as electrodes for spark plugs in internal combustion engines. Frequently, refractory base metal alloys (e.g., W) and intercalated oxides of rare earth elements are used to minimize spark erosion wear in service.

Materials that are particularly suitable for this application are Pt-based alloys with additions of Ir, Ru, W, Mo and / or Re. These alloying elements share the common feature that they oxidize much more easily than platinum and form volatile oxides upon oxidation.

Oxide-dispersion-hardened Pt-Ir and other Pt alloys are known which can be prepared by the internal oxidation of non-precious metal components ( DE 197 14 365 . DE 197 58 724 C2 and DE 100 46 456 ). However, it has been found in studies that these materials have significant disadvantages when they should be used as electrode materials in spark plugs.

Pt alloys containing both volatile oxide generators ≥ 5 wt.% And incorporated oxides at> 0.1 wt.% Tend to crack during processing into thin tapes or wires and shaped electrode tips, typically <1 in diameter 1 mm. In addition, the multiple anneals under the oxidizing atmosphere required in strip or wire and molding operations result in undesirable alloy element losses.

EP 447 820 discloses a composite wire having a core of a Pt-Ir alloy and a sheath of platinum.

It is the object of the present invention to eliminate cracking during processing into thin ribbons or wires and shaped electrode tips.

The solution of the problem is achieved by the features of the independent claims.

Preferred embodiments are described in the dependent claims.

Decisive is a significant leaning of at least one volatile component in the wire or belt surface. For this purpose, a depletion of one quarter is sufficient to achieve the effect according to the invention. The depletion is preferably greater than a quarter, in particular more than half and preferably more than 90%. The depletion refers to the initial mass or number of moles of the depleted component.

According to the invention, a tape or wire is provided whose composition is based on platinum or palladium. For this purpose, the mass fraction of Pt and Pd in the sum of at least 50 wt .-%. In this case, the strip or wire is made of an oxide-dispersion-hardened alloy doped with non-precious metal additives. The alloy may additionally contain minor group elements such as iron, cobalt, nickel, rhenium, tungsten, tantalum, hafnium, lanthanum, molybdenum, niobium, zirconium, yttrium, titanium, scandium and gold, as well as lanthanides as subordinate alloy constituents. Significantly, the tape or wire has a peripheral zone in which the more volatile components of the alloy are emaciated and their volatility is no longer critical under oxidic conditions, so that the volatile oxide formers in the belt or wire are protected from further oxidation. The emaciated edge zone is relatively soft and allows a crack-free further processing of the tape or wire. It is also important that the coat provides protection against further thinning of the volatile constituents under oxidic conditions or can be converted into such a protection.

Thus, an embodiment of the invention is based on first producing a jacket with a porous zone having a thickness of 20 to 300 microns.

The porous zone can be converted into a dense soft outer layer with a thickness of 1 to 50 μm, in particular to a thickness of 5 to 20 μm. The band or the wire can in this case have a diameter of 0.05 to 5 mm, in particular 0.1 to 2 mm.

The layer thickness is 0.1 to 5% of the diameter of the strip or wire. In this case, the layer thickness of the dense zone is preferably 0.5 to 5% of the diameter of the band or the wire, in particular 1 to 2%.

The more volatile constituents are preferably no longer present in the skin surface or have a concentration gradient in the skin, so that a concentration gradient from the skin inside to the skin outside a decrease in the more volatile component of at least 25%, in particular 50%, preferably in the order of magnitude is present.

The decrease is relative and refers to the internal concentration, especially in terms of mass or number of moles. The decrease is relative to the internal concentration, i. at a 25% decrease, the external concentration is 75% of the internal concentration; with a decrease of 50% still 50% and with a decrease by one order of magnitude still a fraction of an order reduced fraction. The concentration data can be based on mass or mol.

Specifically, it has been found that the presence of a thin layer of substantially pure platinum (Pt content> 90%, preferably> 95%) on the surface of the belt or wire or outer surface of the molded parts significantly reduces the tendency to crack during processing can be. The word "lateral surface" is here used in the sense of a lateral surface of a cylinder synonymous with the surface of the cylinder-like wire or the band. Typical layer thicknesses are 0.1-3% of the thickness of the strip or the diameter of the wire. In addition, the layer of largely pure Pt acts as a diffusion barrier, which largely prevents the further loss of the alloying elements by oxidation and evaporation of the oxide. In this layer thickness, the portion of the tape or wire or the molding can be used directly as an electrode, without the Pt layer affecting the function of the electrode. The emaciated rim significantly improves the corrosion resistance of the belt or wire.

A method for producing a strip or a wire of an oxide dispersion-hardened platinum-group-metal-based alloy according to the present invention is to thermally form a porous outer layer by thermally treating this strip or wire on that strip or a wire of a given alloy, and forming the porous outer layer by forming to densify to an impermeable layer.

The Pt layer can be conveniently produced in sifu . By aging a Pt alloy at a high temperature under an oxidizing atmosphere, the alloying element diffuses toward the surface where it oxidizes and vaporizes in the form of a volatile oxide. This results in a soft, porous layer of largely pure Pt at the surface. In the further transformation to thinner dimensions, the porous layer is compacted to form an impermeable layer, which acts as a diffusion barrier. The deformability of the oxide dispersion-hardened Pt alloy is significantly improved by this layer.

Proven ribbons or wires based on platinum or palladium alloys contain (in% by weight elements which form volatile oxides):

Ir 0,3-50% prefers 10-30% Ru 0,3-30% prefers 3-20% re 0.3-20% prefers 3-10% W 0.3-10% prefers 1-6% Not a word 0.3-10% prefers 1-6% → In total, at least 3% and a maximum of 35%.

Proven doping ranges:

Zr 0.05-3% prefers 0.1-1% Ce 0.05-3% prefers 0.1-1% Y 0.005-0.3% prefers 0.01-0.1% sc 0.005-0.3% prefers 0.01-0.1%

Optional alloying elements:

rh 0-20% Au 0-20% Ni 0-30% Co 0-25% Fe 0-10%

Proven temperature ranges:

Internal oxidation of the doping elements: 900 to 1400 ° C, preferably 900 to 1200 ° C.
Oxidation treatment to produce the surface zone: 1450 to 1750 ° C, preferably 1450 to 1650 ° C.

embodiments 1st example

A dispersion strengthened platinum material was prepared according to DE 100 46 456 and DE 197 14 365 provided. For this purpose, an alloy of 3.5 kg Pt and 1.5 kg Ir (corresponding to 5 kg of the alloy Ptlr30) was melted under vacuum in a zirconium oxide crucible. After melting and degassing, the melt was doped by means of 36 g of a master alloy consisting of Pt with 28% Zr and 2.8% Sc and poured in a mold into a billet with the approximate dimensions 40 mm × 40 mm × 150 mm. Analysis of the billet revealed a composition of PtIr30 with 1850 ppm Zr and 175 ppm Sc. The ingot was planed to eliminate casting defects and forged at 1000 ° C into a 15 mm x 15 mm cross-section bar. Subsequently, the bar was rolled at 1000 ° C to a square wire (4 mm x 4 mm). This was outsourced for 10 days at 1000 ° C under air atmosphere. By hot gas extraction analysis (LECO method), the oxygen content was determined to be 735 ppm. With complete oxidation of the Zr doping to ZrO ₂ and the Sc doping to Sc ₂ O ₃ , the oxygen content would be 742 ppm. The wire was split and the individual wire sections treated differently.

The first wire section was aged for 8 hours at 1600 ° C under air atmosphere. The cross-section metallographic examination showed a porous zone about 120 μm thick on the surface. The investigation of this zone by means of energy dispersive analysis in the scanning electron microscope revealed an Ir content which decreased from 19% to 3% from the inside to the outside. This wire section was further rolled as a square profile at 700 ° C without problems to a cross section of 2.4 mm x 2.4 mm. After a further annealing treatment of 10 minutes at 1000 ° C. under an air atmosphere, a sample was taken from the wire, which was examined by metallographic cross-section. The investigation shows a dense outer layer with uniformly fine-grained microstructure and an average layer thickness of 42 μm. A comparison of the material hardness by means of micro hardness testing according to Vickers with a load of 25 g resulted in a hardness of 295 for the inner region of the wire cross-section or for the middle of the outer layer a hardness of 155. The cross section was examined by means of energy dispersive analysis in the scanning electron microscope. The iridium content decreased from 30% in the interior of the sample to 7% below the outside surface. The remaining annealed wire was further processed on a conventional wire drawing machine at 25 ° C. It could be pulled without difficulty to a diameter of 0.6 mm.
Another cross-section survey showed a dense, soft outer layer about 8 microns thick. The wire could already be bent over a radius of 1 mm by 180 ° in the hard-drawn state, without cracks having occurred.

From this wire, spark plug electrode tips for use in cars were made.

Comparative Example 1

The second section was further rolled without further heat treatment as a square profile at 700 ° C. Even after slight deformation pronounced transverse cracks occurred. The further processing was stopped at a cross section of about 3.5 mm x 3.5 mm.

Comparative Example 2

The third section was stored for 8 h at 1600 ° C under argon atmosphere and further rolled as a square profile at 700 ° C. First transverse cracks only occurred at a cross section of about 2.8 mm x 2.8 mm.

2nd example

Analogously to Example 1, an alloy of PtIr20 with dopings of 3200 ppm Zr and 350 ppm Y was prepared. For the cross section 4 mm x 4 mm, the material was aged for 15 days at 1000 ° C in air atmosphere. The processing was carried out according to the o.a. first wire section described sequence.

3rd example

An alloy of PtIr30 doped with 5000 ppm Ce was also prepared analogously to Example 1 and processed into a wire with a diameter of 0.7 mm.

4th example

An alloy of PtRu10 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and processed into a wire with a diameter of 0.6 mm.

5th example

An alloy of PtRe10 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and made into a wire with a diameter of 0.6 mm.

6th example

An alloy of PtW5 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and made into a wire with a diameter of 0.6 mm.

7th example

An alloy of PtMo5 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and processed into a wire with a diameter of 0.6 mm.

8th example

An alloy of PtIr18W1 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and made into a wire with a diameter of 0.6 mm.

9th example

An alloy of PtIr10Ru5 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and made into a wire with a diameter of 0.6 mm.

10th example

An alloy of PtRh10Ru5 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and processed into a wire with a diameter of 0.6 mm.

11. Example

An alloy of PtAu3Ir5 doped with 1800 ppm Zr and 200 ppm Sc was also prepared analogously to Example 1 and processed into a wire with a diameter of 0.6 mm.

12. Example

The wires according to Examples 2 to 11 passed the tests carried out in Example 1 in an analogous manner.

13th example

A dispersion strengthened platinum material was prepared according to DE 100 46 456 and DE 197 14 365 provided. For this purpose, an alloy of 4.0 kg Pt and 1.0 kg Ir (corresponding to 5 kg of the alloy PtIr20) was melted under vacuum in a zirconium oxide crucible. After melting and degassing, the melt was doped by means of 36 g of a master alloy consisting of Pt with 28% Zr and 2.8% Sc and poured into a mold into a billet with the approximate dimensions 40 mm × 40 mm × 150 mm. Analysis of the billet revealed a composition of PtIr20 with 1850 ppm Zr and 175 ppm Sc. The billet was planed to eliminate casting defects and forged at 1000 ° C to a bar with a cross-section of 20 mm x 10 mm. Subsequently, the bar was rolled at 1000 ° C to a thickness of 4 mm. The tape was aged for 12 days at 1000 ° C under air atmosphere. By hot gas extraction analysis (LECO method), the oxygen content was determined to be 725 ppm. With complete oxidation of the Zr doping to ZrO ₂ as well as the Sc doping to Sc ₂ O ₃ , the oxygen content would be 742 ppm. The tape was divided into three parts and the individual tape sections treated differently.

The first band section was aged for 8 h at 1600 ° C under air atmosphere. The cross-section metallographic examination showed a porous zone about 120 μm thick on the surface. Examination of this zone by means of energy dispersive analysis in a scanning electron microscope revealed an Ir content which decreased from 14% to 2% from the inside out. This strip section was further rolled at 700 ° C. without problems to a thickness of 1.5 mm. After another annealing for 10 min at 1000 ° C under air atmosphere, a sample was taken from the tape, which was examined metallographically in transverse section. The investigation shows a dense outer layer with uniformly fine-grained microstructure and an average layer thickness of 30 μm. A comparison of the material hardness by means of micro hardness testing according to Vickers with a load of 25 g gave a hardness of 225 for the inner area of the strip cross section and a hardness of 145 for the middle of the outer layer. The cross section was examined by means of energy dispersive analysis in the scanning electron microscope. The iridium content decreased from 20% in the interior of the sample to 5% below the outside surface. The remaining annealed strip was further rolled at 25 ° C. It could be rolled without difficulty to a thickness of 0.4 mm. Another cross-section survey showed a dense, soft outer layer about 7 μm thick. Already in the hard-drawn state, the strip could be bent through 180 ° over a radius of 1 mm without causing any cracks.

From this strip, 1.2 mm diameter disks were punched, which were used as spark plug electrodes for use in gas engines.

Comparative Example 3

The second section was further rolled without further heat treatment at 700 ° C. Even after slight deformation, pronounced cracks occurred. Further processing was stopped at a thickness of 2.8 mm.

14th example

An alloy of PtW5 doped with 3200 ppm Zr and 350 ppm Sc was also prepared analogously to Example 1 and made into a strip 0.3 mm thick. From this band discs of diameter 1.5 mm were punched, which were used as spark plug electrodes in car engines. The tapes of Example 14 passed the tests made in Example 13 in an analogous manner.

Comparative Example 4

The third section was stored for 8 hours at 1600 ° C. under an argon atmosphere and further rolled at 700 ° C. First cracks first appeared at a thickness of about 2.2 m.

Claims (8)

1. Strip or wire of alloy hardened by oxide dispersion and based on platinum or palladium or a mixture of platinum and palladium, the proportion by mass of platinum and palladium amounting in total to at least 50% by weight and which contains one or several oxide generators which forms or form more volatile oxides, selected from

   0.3 - 50% by weight of iridium,

   0.3 - 30 % by weight of ruthenium,

   0.3 - 20% by weight of rhenium,

   0.3 -10% by weight of tungsten and

   0.3 -10% by weight of molybdenum,

   at least 3% and maximum 35% in total,

   characterised in that the cross-section of the strip or the wire exhibits a peripheral zone which amounts to 0.1 to 5% of the thickness of the strip or the wire in which at least one of these oxide generators forming relatively highly volatile oxides is thinned by at least one quarter.
2. Alloyed strip or alloyed wire according to claim 1 characterised in that the strip or the wire exhibits a sheathing of platinum or palladium or a platinum-rhodium alloy or a platinum-gold alloy.
3. Alloyed strip or alloyed wire according to one of claims 1 or 2 characterised in that a porous peripheral zone exhibits a thickness of 20 to 300 µm.
4. Alloyed strip or alloyed wire according to one of claims 1 or 2 characterised in that the strip or the wire exhibits a thickness of 0.05 to 5mm and a dense soft peripheral zone with a thickness of 1 to 50 µm.
5. Alloyed strip or alloyed wire according to one of claims 2 to 4 characterised in that alloy components of the platinum in the peripheral zone exhibit a decrease in concentration from inside towards the outside with a decrease to half or less.
6. Process for the production of a strip or a wire of an alloy hardened by oxide dispersion and based on platinum or palladium or a platinum-palladium mixture according to one of claims 1 to 5 characterised in that a porous skin is produced thermally on a strip or a wire of the alloy hardened by oxide dispersion and the porous skin is compacted by conversion into a soft or impermeable skin.
7. Use of a strip or a wire according to one of claims 1 to 5 as spark plug electrode.
8. Use of a strip or wire produced according to claim 6 as spark plug electrode.