Phosphating

Phosphate coatings have been covered in a couple of previous surface treatment articles. In the first of these articles, we looked mainly at engine applications, including piston rings, camshafts and so on. In the second article, we looked in slightly more depth at some of the advantages and disadvantages of phosphating. The design engineer or metallurgist who recommends this type of coating needs to be aware of the potential problems with hydrogen embrittlement of high-strength steels and, if relevant, to specify countermeasures.

In this article, we will look at how such surface treatments might be used to advantage for transmission applications, specifically gears. Phosphated surfaces offer a number of potential advantages for gears, despite the surface being much softer than the carburised surface which is most often specified.

There are several reasons for this, some of which are a result of the softness of the treated surface. Phosphate surface treatments convert a very small depth of the surface, rather than simply depositing a material onto it, as with a coating. There is some confusion here though: there are sprayed coatings that are phosphate based, and it is a common mistake to call phosphate conversion treatments coatings. Although not a hardening treatment, phosphate conversion coatings are similar in principle – the resulting surface is different, but with an integral part of the component having been formed by a chemical reaction.

However we have finished the surface of a component, whether this is by machining or grinding, there are high-points on each surface, known as asperties. It is these asperities that first cause any hydrodynamic oil film to fail, as these are the points on the surface that will first come into contact with the mating part. As a chemical conversion process, phosphating reacts with an iron-bearing surface, and the rate of conversion is proportional to surface area. In the same way that finely atomised fuels burn more quickly, so asperities – with their high ratio of surface area to volume – are converted more fully to a soft phosphate layer. While the process may convert only microns of a surface, that is enough in many cases to change a hard asperity into a soft and weak high point, which will simply and harmlessly be deformed or detached in service.

Phosphate surface treatments are also well known for their ability to hold oil within the structure of the converted surface. This, and the softer nature of the surface itself, helps the running-in behaviour of gears and is effective in minimising scuffing of gears during early life. Bergseth (1) cites research on gears that shows that although the soft phosphate surface is quickly worn away during running, it is very effective in preventing scuffing. Sjöberg (2) carried out tests showing that under extreme contact pressures of 7 GPa (1015 ksi) phosphate treatments on standard specimens had a lower coefficient of friction and resisted scuffing, even though the same specimens without phosphate coating had scuffed at less than 1.9 GPa contact pressure.

Phosphate surface treatments are also useful, especially when they have been oiled, for inhibiting corrosion. Where transmission gears stand for periods of time, corrosion pitting may start, and phosphating can help prevent early-life corrosion. This can be especially important where transmissions are built and packed in a relatively warm environment and are then exposed to cold, as in an aircraft’s cargo hold, causing condensation.

It is not all good news though. Devlin et al (3) present research that shows that phosphate coatings may detrimentally affect the fatigue life of gears, although their study was conducted on low-hardness gears. 

References

1. Bergseth, E., “Influence of surface topography and lubricant design in gear contacts“, Kungliga Tekniska Högskolan academic thesis, ISBN 978-91-7415-427-6 

2. Sjöberg, S., “On the running-in of gears”, Kungliga Tekniska Högskolan academic thesis, ISBN 978-91-7415-656-0 

3. Devlin, M.T., Turner, T.L., Thompson, K., Kolakowski, K., Garelick, K., Guevremont, J.M., and Jao, T., “Effect of Phosphate Coatings on Fatigue and Wear”, paper presented at the Annual Meeting of the NLGI, 2007 

Written by Wayne Ward