Bearing coatings

We could successfully argue that many plain bearings are actually little more than coatings, and that the shell is simply a carrier for the very thin layer of material which is doing the hard work for us. That would be ungenerous though, as plain bearings rely heavily on their backing to provide strength, adhesion and thermal conductivity. The shells also have a very important role to play in controlling clearances.

However, some applications are sufficiently lightly loaded to allow components to be directly coated. Con rods present two such applications. On crank-guided rods – those whose axial position is controlled by thrust faces on the crankshaft – the big-end thrust faces are commonly coated. Traditionally this was done by thermal spraying using metallic molybdenum as the coating material. It is still used, but has been supplanted to a large extent by hard, thin engineering coatings such as chromium nitride.

At the opposite end of the rod, where piston-guided rods are used (rods whose axial position is controlled by thrust faces provided on the piston) the small-end bushes often incorporate flanges that provide the thrust faces. Some people use hard engineering coatings on these bearing faces. Of course, the machined faces of the small end can also be coated.

In oil pumps, some people have found success running shafts directly in coated bores in the pump housings. The housings are often made from cast aluminium, and the machined bores are coated with a thin layer of a relatively soft resin-bonded polymer coating. Such coatings can be very durable if applied and cured properly; this is an important aspect to consider, as the temptation with aluminium housings is to cure the coatings at a temperature that is too low to fully harden the coating.

Using polymer coatings in pumps is possible owing to a number of factors that we don’t find elsewhere in the engine. There is a good supply of lubricant, temperatures are fairly steady and the sliding speed is modest owing to the rotational speed of the pumps and the small diameter of the pump shafts. It is not unusual to find that the pump shaft surface speed is a factor of ten lower than the main bearing surface speed.

Care needs to be taken though when deciding on the position of any oil-feed grooves crossing such bearings. If a soft coating is used on a relatively soft housing then any mistake in the angular position of these groove can lead to high pressures, coating wear and excessive clearances. Unless mistakes have been made in the design or manufacture of the components involved, the main period when wear will take place is during start-up, where the components are essentially running dry until sufficient oil is available.

For more highly loaded components, one option is to apply metallic bearing coatings directly to components. This has been tried in the past with con rods, and it is technically feasible with cylinder blocks and so on, but it is very difficult in practice. It would also require an improvement in the precision with which we are able to machine both the coated components and the components that run in them.

Written by Wayne Ward