GB2082205A

GB2082205A - Dispersion-hardened platinum- group metal articles

Info

Publication number: GB2082205A
Application number: GB8123778A
Authority: GB
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1980-08-14
Filing date: 1981-08-04
Publication date: 1982-03-03
Also published as: GB2082205B; FR2488536A1; JPS5754202A; DE3030751A1

Abstract

Semi-finished articles of dispersion hardened platinum-group- metal alloys are made by co- precipitation of the precious metal together with one or more base metal oxides to give a fine powder mixture, sintering the mixture without prior compression in a two-step process at 700-1100 DEG C and 1300-1600 DEG C respectively, turning the agglomerate through 180 DEG between the steps, optionally subjecting the base metal component to oxidation, and processing the agglomerate to the semi-finished article e.g. by forging. The sintering steps are preferably carried out under argon but may also be carried out in vacuo, or under air or H2. In an example, platinum containing 0.16% zirconium oxide is heated under argon for 8 hours at a temperature rising to 800 DEG C, then for 7 hours at a temperature rising to 1600 DEG C to give an agglomerate of 60% density, then forged at 1200 DEG C to 90% density before being cold rolled to sheet with intermediate anneals.

Description

SPECIFICATION Process for the production of semi-finished articles of dispersion-hardened platinum This invention relates to a process for the production of semi-finished articles of platinum and/or other platinum metals dispersion-hardened with base metal oxides by chemically coprecipitating the components, forming a sintered agglomerate from the powder mixture, optionally oxidising the base metal component and further processing to form the semi-finished article.

Noble metals and noble metal alloys, particularly platinum metals and platinum metal alloys, are widely used in applications requiring high resistance to corrosion or to oxidation at elevated temperatures. However, one disadvantage of materials such as these lies in their relatively poor strength at elevated temperatures.

Accordingly, there has been no shortage of attempts to harden and increase the density of noble metals and their alloys by the addition of substances which are inert in an oxidising atmosphere, for example high-melting oxides.

For example, it is possible to mix fine-grained platinum powder with fine-grained powder of high-melting oxides by mechanical means and to subject the resulting mixture to powdermetallurgical processing. However, the reproducibility of the properties of a material obtained in this way is far from satisfactory and the improvement for example in the long-term strength of a material such as this at high temperatures by comparison with additive-free materials is minimal because it is very difficult in this process to achieve homogeneous distribution of the oxide particles in the platinum.

It has also been proposed to considerably increase the long-term strength of platinum metals at high temperatures by subjecting them to dispersion hardening after the addition of a metal, of which the oxide has a very high heat of formation, by internal oxidation at temperatures in the range from 900 to 14000 C. In addition to greater tensile strength and hardness at room temperature, it is possible by this process to obtain greater long-term strength at elevated temperatures.

However, the diameter of the oxide particles thus produced by internal oxidation is of the order of a few ym so that the dispersion-hardening effect obtainable by them is relatively poor. In addition, on account of the low rate of diffusion of oxygen in the metallic platinum, it takes considerable time to obtain a fully oxidised material.

It is also known that the hardening particles may be introduced by simultaneous precipitation from salt solutions, by the concentration of solutions containing both components through evaporation or by spraying solutions such as these into flames. Further processes include the surface oxidation of alloy powders by a combination of electrodeposition and electrophoresis.

All these processes are attended by the disadvantage that they are relatively complicated and expensive to carry out and still do not achieve any significant improvement in the properties.

Although homogeneous distribution of the components in the precipitated powder is obtained in the chemical co-precipitation of platinum and base metals from solutions, conventional further processing by calcining the powder at 8000C and then subjecting it to compression-moulding, sintering and forming into semi-finished articles gives products which tend to form bubbles on annealing and hence are only of limited use.

Accordingly, an object of the present invention is to provide a process for the production of semifinished articles of platinum and/or other platinum metals dispersion-hardened with base metals by chemically co-precipitating the components, forming a sintered agglomerate from the powder mixture, optionally oxidising the base metal component and further processing into a semifinished article, in which the semi-finished article formed does not show any tendency to form bubbles on annealing.

The present invention provides a process for the production of semi-finished articles of platinum and/or other platinum metals dispersionhardened with base metal oxides by chemical coprecipitation of the components, which comprises forming a sintered agglomerate from the powder mixture, optionally oxidising the base metal component and further processing to form the semi-finished article, the sintered agglomerate being formed by sintering the loose powder in a first step at 700 to 11 000C after which the sintered agglomerate is turned through 1 800 and sintered to completion in a second step at 1 300 to 1 6000 C. Preferably, a temperature of 800"C is applied in the first step and a temperature of 1 6000C in the second step.Sintering may be carried out in air, in hydrogen or in vacuo.

However, it has proved to be particularly effective to carry out sintering in an argon atmosphere, particularly in the second step.

Sintering the powder in loose form means that the platinum powder is not precompacted by mechanical or isostatic pressing before sintering, as has hitherto been the case in the production of corresponding sintered agglomerates. The powder obtained by chemical precipitation is introduced into a crucible and only gently shaken in.

The starting material used is preferably a platinum powder containing from 0.05 to 1% by weight of zirconium oxide. However, it is also possible to use other platinum metal powders, such as palladium, iridium, rhodium, ruthenium or osmium or mixtures thereof, and other base metal oxides, such as for example titanium, thorium, beryllium, aluminium or tantalum either individually or in admixture. The oxide-forming metals are generally added in quantities of from 0.05 to 5% by weight, expressed as oxide.

Further processing of the sintered agglomerates into semi-finished articles is preferably carried out by a forging treatment at 1200 to 14000C in a hydrogen atmosphere until the agglomerate has reached a density of more than 80% of the theoretical density. This is advantageously followed by another cold-forming treatment with intermediate annealing treatments in air at 10000 C. In this connection, it has been found to be effective repeatedly to change the direction of the forming treatment, particularly where it is carried out by rolling.

After annealing in air at 1 4000C, the semifinished articles thus produced to not show any signs of bubble formation, even after 100 hours.

Their long-term strength values (100 hours in air at 1 4000C) amount to between 6 and 8.5 N/mm2 and their hardness values HV3 (after 1000 hours in air at 14000 C) to between 560 and 660 N/mm2.

The process according to the invention is illustrated by the following Example: 1 kg of powder of platinum containing approximately 0.16% of zirconium oxide, which had been produced in known manner by simultaneous precipitation from hexachloroplatinic acid and zirconium nitrate, was introduced into a graphite crucible and shaken in for 1 to 2 minutes. In the first sintering step carried out in a furnace under an argon pressure of approximateiy 1025 mbars, the temperature was increased to 8000C over a period of about 6 hours and left at that level for about 2 hours. The sintered agglomerate was then turned through 1800 and further sintered on a ceramic support in a second step. To that end, the temperature was increased to 1 6000C over a period of 4 hours and left at that level for 3 hours.

An agglomerate having a sinter density of the order of 60% is thus obtained, being further processed by forging under hydrogen at 1 2000C until it has reached a density of 90% of the theoretical density. The production of sheet is carried out by cold rolling with intermediate annealing treatments (20 minutes, 1 0000C, air).

The sheets thus produced did not show any signs of bubble formation, even after annealing for 1000 hours (14000C, air) and had a long-term strength of 7.5 N/mm2 (100 h, 1 4000C, air).

Claims

1. A process for the production of semi-finished articles of platinum and/or other platinum metals.

dispersion-hardened with base metal oxides by chemical co-precipitation of the components, which comprises forming a sintered agglomerate from the powder mixture, optionally oxidising the base metal component and further processing to form the semi-finished article, the sintered agglomerate being formed by sintering the loose powder in a first step at 700 to 11 000C after which the sintered agglomerate is turned through 1 800 and sintered to completion in a second step at 1300 to 16000C.

2. A process for the production of semi-finished articles as claimed in Claim 1, wherein sintering is carried out at 8000C in the first step and at 1 6000C in the second step.

3. A process for the production of semi-finished articles as claimed in Claim 1 or 2, wherein sintering is carried out in an argon atmosphere.

4. A process for the production of semi-finished articles as claimed in any of Claims 1 to 3, wherein platinum containing from 0.05 to 1% by weight of zirconium oxide is used as the powder mixture.

5. A process for the production of semi-finished articles as claimed in any of Claims 1 to 4, wherein the sintered agglomerates are subjected to a forging treatment in a hydrogen atmosphere at 1200 to 1 4000C up to a density of greater than 80% of the theoretical density.

6. A process for the production of semi-finished articles as claimed in Claim 5, wherein the forged sintered agglomerates are subjected to cold forming with intermediate annealing treatments.

7. A process tor the production of semi-finished articles of platinum and/or other platinum metals substantially as described with particular reference to the Example.

8. Semi-finished articles of platinum and/or other platinum metals when prepared by a process as claimed in any of Claims 1 to 7.