Phosphating
There are a wide range of applications of surface treatments for race engines and transmissions, and an even wider range of surface treatments and coatings competing for our business. One surface treatment process that continues to find use in both engines and transmissions, having been popular for several decades, is phosphating. Some of the basic applications of this family of surface treatment processes were covered in a previous article on phosphating.
There is however some debate as to the effectiveness of phosphating as an aid to running in; the use of phosphated camshafts and cam followers remains common, so we should investigate this further. Fortunately there are some well-researched technical papers to help guide us. Those by Devlin et al (1) and Chen et al (2) give us some insight into the effect of phosphate coatings on friction and wear. Farrara and Ritter (3) acknowledge the deleterious effect that the hydrogen evolved during phosphating has on the fatigue strength of high-strength steels, so where we are using high-strength steels and are considering phosphating, we need to be wary of the potential disadvantages and how we might mitigate them.
The debate about the effectiveness of phosphating is more to do with the mechanism by which it is achieved rather than whether it has proven effective. In some people’s opinion, the phosphate conversion coating acts as a fine abrasive, effectively lapping the cam and follower surfaces. However, phosphate coatings are also known to have a porous structure that promotes the retention of oil at the component surface. If the component is well-oiled before installation, it is likely that there will be more lubricant present during the initial start-up, and it’s been postulated that it is this lubricant availability at start-up which aids running-in. If there is an extended period between the engine being built and it being started, however, there is a danger that the cams and followers will have corroded, and oxidised surfaces can be abrasive. Phosphating, especially when oiled, is known to inhibit corrosion.
One area where we need to exercise caution is where fatigue properties are concerned, as there is more than one effect to consider. Phosphating can reduce friction, and in an environment where the cyclic stress is affected by friction (for example, conditions of dry sliding or boundary/mixed lubrication), this can reduce the stress amplitude, which should increase fatigue life.
As noted in the previous article, hydrogen is evolved at the component surface during the phosphating process. Hydrogen diffusion into the surface can result in embrittlement and a marked reduction in fatigue life, particularly in the case of high-strength steels. This is the same mechanism that makes electroplating of high-strength steels something to be careful of. There are ‘baking’ processes that mitigate the loss of ductility and strength due to hydrogen embrittlement, but there is no guarantee that hydrogen embrittlement can be completely eliminated.
A further mechanism by which phosphating has been shown to decrease fatigue life is where the acid reagents can attack the metal surface, corroding it and leaving ‘etch pits’ that act as local stress concentrations.
To summarise, one has to choose phosphating for the applications for which it has proven to be of benefit, and to consider the possible fatigue effects on certain materials – especially high-strength steels.
References:
1. Devlin, M.T., et al, “Effect of Phosphate Coatings on Fatigue and Wear”, paper no 722, presented at the National Lubricating Grease Institute’s 74th annual meeting, 2007
2. Chen, Y., “Improvement of Contact Fatigue Strength of Gears by Tooth Surface Modification Processing”, Proc. 12th World Congress of the International Federation for the Promotion of Mechanism and Machine Science, Besançon (France), June18-21, 2007
3. Farrara, R.A., and Ritter, J.C., “The Effect of Phosphate Coatings on Fatigue Crack Initiation in Quenched and Tempered Low Alloy Steel”, US Army Armament Research Technical Report MRL-TR-90-14, 1990
Written by Wayne Ward