The present invention relates to the deposition of tin alloys, especially tin-cobalt alloys, tin-nickel alloys and tin-nickel-cobalt alloys and to electrolyte media for use in the deposition of such alloys. The invention relates especially to the use of such alloys as bearing overlays.
The “End of Life” vehicle regulations in Europe for 2003 aim to increase the “recyclability” of vehicles by removing toxic materials such as hexavalent chromium and lead. One common use of lead in automotive applications is the use of lead alloys in bearing overlays. Overlays are generally soft alloys deposited onto harder bearing alloys to produce a surface having compatibility and conformability with a co-operating shaft and also to provide a means of embedding debris particles to prevent damage to the shaft. More than 300,000,000 bearing shells are currently plated every year. The most commonly applied bearing overlay material is a lead-tin-copper alloy containing at least 90% lead. In higher performance engines, lead-indium (where the indium is plated on top of the bearing and diffused into the underlying lead) is commonly applied. Clearly, in order to comply with the 2003 regulations, a replacement for lead must be found.
A suitable replacement alloy must be soft enough to allow the bearing to “bed in” correctly and the melting point of the alloy must be higher than 250° C. because engine operating temperatures can approach this level. Tin-based alloys are an obvious choice as they have good lubrication properties and are soft enough. The low toxicity of tin is also an advantage. Tin cannot be used alone because the melting point of tin is too low. The easiest alloy of tin to produce would be a tin-copper alloy. A tin-copper alloy containing approximately 5% copper would have the required melting point. However, tests have shown that tin-copper alloys do not have the necessary fatigue strength. Tin zinc alloys can readily be produced but these alloys fail corrosion testing due to the appearance of white corrosion products from the sacrificial corrosion of the zinc in the alloy.
Other tin based alloys include tin-nickel, tin-cobalt or ternary alloys comprising all three of these metals. The use of these alloys for bearing overlays has already been suggested. In particular, tin cobalt has been found to be advantageous. U.S. Pat. No. 4,795,682 discloses the use of tin cobalt alloys containing preferably between 2-8% cobalt. These alloys are claimed to have superior fatigue resistance as compared to standard lead-tin-copper bearing overlays (90-100 Mpa as compared to 60-70 Mpa when tested on a “Sapphire” testing machine). However, as far as the applicant is aware, these alloys have not been exploited commercially due to the difficulty of producing a tin cobalt alloy of the required composition and thickness. According to U.S. Pat. No. 4,795,682, the tin cobalt alloy overlays were produced by the technique known as “Brush Plating” where the coating is applied manually by brushing the bearing with an anode coated with an absorbent material soaked in an electrolyte containing tin and cobalt salts and a gluconate complexant. This technique is not easily applicable to mass production techniques and so the use of tin-cobalt alloy for bearing overlays has not been possible in commercial practice in spite of its performance advantages.
The production of tin-cobalt alloys is also described in other prior art documents. One commercial use of tin-cobalt alloys is in the production of thin overlays for nickel plated components as a replacement for chromium. However, it is not possible to produce thick coatings from these electrolytes as the content of tin is only 2-4 g/l. These electrolytes also produce coatings of a composition approximating to the intermetallic alloy composition (20-25% cobalt), whereas the optimum desired composition is about 2-8% cobalt for bearing alloys since the intermetallic composition is too hard.
Several compositions have been proposed in the patent literature which claim to be suitable for producing thicker deposits of tin-cobalt alloys.
U.S. Pat. Nos. 3,951,760 and 4,021,316 suggest an alkaline bath based on pyrophosphate and utilising an organo sulphur compound as a brightening agent and peptones as grain refining agents. Baths based on pyrophosphate have the disadvantage that the stannous tin ions are not stable in alkaline media and quickly oxidise to stannic tin rendering the bath useless. Also, insoluble anodes have to be used in these baths as tin does not dissolve effectively in these pyrophosphate baths. These baths would therefore be unsuitable for the high volume production of plated bearings.
Several patents have been granted for baths based on stannous chloride and cobalt chloride also containing fluoride in order to complex the tin ions and facilitate co-deposition of cobalt (U.S. Pat. Nos. 3,966,564 and 4,029,556). These electrolytes are very corrosive and toxic and because they contain large amounts of ammonium ions, they are difficult to effluent treat. Additionally, these electrolytes produce an intermetallic alloy over a wide range of current densities and so are unsuitable for producing alloys of the required composition.
U.S. Pat. No. 4,168,223 describes a citrate-based bath from which it is claimed tin-cobalt alloys could be deposited However, attempts by the present applicant to reproduce the examples cited in U.S. Pat. No. 4,168,223 resulted in only deposits of pure tin with no co-deposition of cobalt (when examined by Energy Dispersive X-ray Analysis).
A more recent patent (U.S. Pat. No. 4,828,657) discloses baths based on stannic tin in either alkaline or acidic media. Maintenance of tin concentration in these baths is very difficult as it is not possible to directly dissolve tin anodes in stannic baths because build-up of stannite ions in the bath leads to spongy deposition. Also, if acid baths are formulated based on stannic tin, the tin tends to eventually precipitate as alpha or metastannic acid.