Method of Improving the Wear Resistance of Metals

The present invention relates to the improvement of the wear resistance of metals, and especially the wear resistance of chromium-containing steels and nickel alloys.

It is known that the high-temperature oxidation behaviour of iron and nickel alloys can be improved by the implantation of ions of yttrium and other rare earth elements. The improvement is believed to arise from the formation of perovskite-type compounds incorporating the implanted ions and oxygen which is inevitably present in or on the surfaces of the alloys. These perovskite-type compounds form preferentially in the grain boundaries, which normally provide pathways for the diffusion both of oxygen inwards into the alloys and metal cations outwards, and block these pathways, thus reducing the long-term rate of oxidation after an initial phase in which the perovskite-type compounds are formed.

The present invention arises from the appreciation that as these perovskite-type compounds are very strongly bound to the alloys in which they are formed, and reduce the tendency of the oxides at high temperatures to spall from the surfaces of the alloys, they could well prevent the disruption of bearing surfaces of artefacts at the operating temperatures normally associated with mechanisms.

According to the present invention there is provided a process for improving the wear resistance of metals, comprising the operation of implanting into a metal article the wear resistance of which is to be improved ions of a material which is capable of forming within the article oxide compound having a perovskite- or spinel-type structure, and terminating the implantation of the ions when a dose of at least 10.sup.15 ions per square centimeter has been implanted.

If the metal article is made of a ferritic alloy such as stainless steel or tool steel, or a nickel based alloy such as that known by the code number N108, then a particularly advantageous ion is yttrium.sup.+. Other ions which are particularly useful are Sc.sup.+, Yb.sup.+, Ce.sup.+, La.sup.+ and Dy.sup.+. The ions can be implanted alone, together, or with other ions species such as CO.sup.+ or N.sup.+. Also, the article can be subjected to an oxidising process after implantation to facilitate the formation of the oxide compounds.

Examples of the invention will now be described.

A region of each of a series of discs made of a medium carbon steel such as that known by the code designation En8 was implanted with one of the following ion species Sc.sup.+, Y.sup.+, Yb.sup.+, Ce.sup.+, Nd.sup.+, La.sup.+, Dy.sup.+ or Ho.sup.+. The implantation was carried out in a vacuum of 2.times.10.sup.-7 torr using an ion beam current of a few micro amperes and a beam energy of 200 keV, and the implantation was continued until an ion dose of 5.times.10.sup.. ions per sq. cm. had been implanted.

The discs were then tested in a standard pin and disc wear testing machine which it is not thought necessary to describe. The volumetric wear rates (K.sub.v), as defined by the relation K.sub.v =volume of material removed/sliding distance.times.apparent area, of the implanted and unimplanted regions of the discs are listed in the table below:

It can be seen that all the ions have a beneficial effect.

A series of similar experiments using cerium and yttrium ions only show that the improvement in the wear rate is dose dependent with a large increase at a dose rate of about 10.sup.. ions per square cm. For cerium, the optimum dose is about 3.10.sup..noteq. ions per square cm; for yttrium, the optimum dose rate is about 5.10.sup.. ions per square cm.

Each of four tool steel discs was then implanted under similar conditions to those of the previous example with one of the following ion species Y.sup.+, Ho.sup.+, Dy.sup.+, Yb.sup.+. The tool steel used had a high chromium content (12%). Each disc was implanted with two doses of the same ion, but different concentrations, one on each of the plane surfaces. The discs were then tested in the same way as before but at a higher load as the material is harder than the mild steel. The results are summarised in the table below:

The improvements measured for the tool steel are comparable to those measured for the mild steel but as the load was four times greater than the load used when testing the mild steel, the improvement is, in fact, correspondingly greater.

A single disc made of En8 mild steel was implanted under the same conditions as before with equal doses of cerium and yttrium up to a total dose of 4.times.10.sup.15 ions per square cm. The disc was then tested in the same way as before. It was found that the wear rate decreased from K.sub.v =4.1.times.10.sup.7 to K.sub.v =2.4.times.10.sup.8 for two hours, and then remained constant at K.sub.v =6.0.times.10.sup.8 for many hours. These figures correspond to an initial improvement by a factor of 17.1 followed by a final improvement by a factor of 6.7.

A single disc of En8 mild steel was implanted under the same conditions as before with a dose of 3.times.10.sup.16 ions per square cm. of aluminium. It was then wear tested both before and after being heated in an oxidising atmosphere at a temperature of 425.degree. C. for a period of 20 minutes. The final wear rate was so low as to be undetectable by the apparatus used.

A previously tested disc (E/2 face and containing 2.times.10.sup.15 Ho ions per sq. cm. was oxidised at 600.degree. C. for 20 minutes and retested. Upon subsequent testing it was found that the oxidation of the unimplanted region of the disc effected an improvement over the original wear rate, and that the wear rate of the implanted region of the disc decreased even further. The results are summarised in the table below.

The improvement in the wear rate of the unimplanted region of the disc is due to the increase in hardness of the surface of the disc as a result of the oxidation process. Bearing in mind the general increase in hardness of the surface of the disc, the improvement factor is some four times better than that achieved by implantation only.

It is apparent that oxidation in addition to implantation is very beneficial.

A ball valve seat for use in an internal combustion engine fuel injection pump was implanted with a dose of 3.times.10.sup.16 Y.sup.+ ions/cm.sup.2 at an energy of 200 keV. The complete valve was assembled into a pump and run for a period of 1000 hrs. The rate of wear of the ball valve seat compared with that of an identical but unimplanted ball valve seat was reduced by two orders of magnitude. The experiment was repeated with two other ball valves, with similar results. A practical result is that it is possible to make the ball valve seats out of a lower grade of steel than hitherto. For example they can be made out of the tool steel known as Balfour `00` instead of a high speed tool steel.