Surface Treatment of Aluminum/Silicon Alloys

This invention relates to processes for the surface treatment of alloys to promote wear resistance.

Many metal alloy piece parts require some form of surface treatment prior to use to provide or enhance wear resistance. For example, aluminium silicon alloys are conventionally provided with a surface coating of nickel to achieve this effect. This of course is relatively costly.

Attempts have been made to treat alloys of this nature by a much cheaper anodisation process to produce a hard oxide film. However it has been found that the presence of a non-anodisable phase in the alloy surface prevents the formation of a continuous film having the necessary wear resistant properties.

The object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a process for surface treatment of a metal alloy comprising an anodisable and a non-anodisable phase, the process including exposing the alloy to a selective etchants whereby the non-anodisable phase is removed from the surface, and anodising the etched surface to provide a continuous wear resistant oxide film.

In one application the process may be applied to aluminium/silicon alloy piece parts for use in load bearing applications, e.g. vehicle brake systems. The process is not however limited to aluminium/silicon alloys, but is of general application to alloys incorporating an anodisable and a non-anodisable phase.

Typically the process is employed as a finishing stage after the alloy has been formed into a particular shape or configuration by conventional metal working processes. In some applications a dye may be incorporated in the anodic film to provide a decorative coating. The selective etching process may comprise a single etch or a plurality of etches applied sequentially.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIGS. 1 and 2 are X-ray maps obtained from an aluminium/silicon alloy surface respectively before and after selective etching,

and FIGS. 3 and 4 are scanning electron microscope (SEM) views corresponding to the X-ray maps of FIGS. 1 and 2.

We have found for example that aluminium/silicon alloys may be surface etched to remove the silicon phase by a two stage process, each stage employing a selective etchant. Typically we employ fluoride coating etchants. The first etch may employ a solution of ammonium bifluoride in nitric acid followed by a second etch in aqueous hydrofluoric acid. Typically the concentration of ammonium bifluoride in the first etch is 30 g/1 and the alloy is exposed to this etch at room temperature for a period of 20 to 60 seconds. Advantageously the exposure time is 28 to 32 seconds. A 10% solution of hydrogen fluoride in water is suitable for the second etch for which the exposure time is 5 to 20 seconds and preferably 14 to 16 seconds. In some applications this two stage etching may be repeated one or more times. The effects of this selective etching in removing the silicon phase from the alloy surface is demonstrated in FIGS. 1 to 4. In FIGS. 1 and 2 it should be noted that X-rays are emitted only from silicon atoms in the alloy surface, the lighter aluminium atoms having substantially no effect on the X-ray pattern. As can be seen from FIGS. 3 and 4, the effect of the etch is to remove silicon from the alloy surface to leave a substantially pure aluminium surface which can then be anodised.

After etching has been effected the treated surface may be protected against wear by anodisation. For this purpose we prefer to employ a saturated solution of oxalic acid in aqueous sulphuric acid at a reduced temperature. Advantageously the anodisation process is performed at or below 10.degree. C. and typically at 0.degree. C. Other anodisation processes can of course be employed and these will be well known to those skilled in the art.

To illustrate the techniques described herein an aluminium/silicon alloy was selectively etched and then anodised to provide an oxide film thickness of 20 microns. The treated surface was then wear tested by an abrasion technique. Breakdown of the oxide film was detected by electrical conductivity between the wear inducing tool and the bulk alloy. For comparison similar wear test measurements were carried out on an anodised unetched conventional alloy. The results are summarised in Table 1 below.

As can be seen from the Table, the results achieved by the technique described herein are comparable with those obtained with conventional alloys. The technique also compares very favourably with conventional processes for the treatment of aluminium silicon alloys.

Aluminium/copper alloys may be treated in a similar way. For this purpose we prefer to employ an etch comprising a mixture of sulphuric acid and nitric acid. After treatment with the etch, the alloy surface may be anodised as previously described.

The technique is not of course limited to aluminium alloys but is of general application to alloys incorporating an anodisable and a non-anodisable phase.

It will be appreciated that alloys comprising an anodisable and a non-anodisable phase may also incorporate minor quantities of other metals, e.g. manganese, iron, nickel, magnesium, zinc or mixtures thereof.