Thermal/plasma-sprayed ceramics

Thursday, September 27, 2012

Tags :  coatings

The main use for ceramic coatings for racing powertrains is on exhaust system components and various heat shields. The very low thermal conductivity of many metal oxides makes them ideal candidates where we would like a coating to reduce heat transfer. This might be desirable in order to improve the transient response of a turbocharger by keeping the temperature of the exhaust gas as high as possible, to protect other components that are sensitive to temperature or to reduce heat transfer to the main structure of the engine and the fluids within.

Trials of ceramic thermally sprayed bore coatings have been successfully undertaken in race engines; this was done by a major coating company in order to prove the process before transferring it to passenger cars. The coating was part of a trial involving both ceramic and metallic sprayed coatings in a NASCAR engine, and the trials were successful in proving both the durability of the coatings and the potential for increased engine performance due to decreased friction.

NASCAR engines don't usually use a bore coating, one of the reasons being that engines are routinely re-bored when required and the block can be used for a number of builds. However, this strategy means keeping a large inventory of pistons and piston rings. Sprayed ceramic coatings on bores might prove to be an equally economical way to run an engine, but without the need to stock varying sizes of piston, rings and so on.

With the increasing adoption of hybrid systems in motorsport comes a proliferation of electric motors designed to exploit the new regulations allowing energy recovery. It is necessary to electrically insulate the rotor shaft from the stator assembly. If the rotor is electrically connected to the stator, a circuit is formed and the bearings become damaged very quickly, essentially due to 'spark erosion' of the races. There are various reasons why a voltage is induced on the rotor shaft which are far beyond the scope of this article.

The characteristic pattern of damage resulting from the arcing across the gap between the races and the balls is referred to as 'fluting' or 'washboarding'. This quickly leads to notchy bearings and increased clearance in them. As with many other wear problems, once there is clearance and loss of precision, wear can quickly accelerate.

There are a number of ways to counter this, but one is to use a ceramic coating on either the bearing outer race or to the bore of the bearing housing. Some bearing companies have offered such coated races on steel bearings for electric motor applications, although there are other materials for rolling element bearing construction that can achieve the same effect.

The methods for applying ceramics to the outside surfaces of the outer races are generally thermal or plasma spraying. I have seen high velocity oxy-fuel (HVOF) methods used to coat bearing bores on end caps of bespoke high-performance electric motors, although this is not as easy as buying a bearing that is already coated by the manufacturer.

The one advantage of developing a relationship with a thermal/plasma-spraying company is that you can tailor the coating to your requirements. Some ceramics that aren't offered by bearing companies offer attractive combinations of properties for the design engineer.

Written by Wayne Ward

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With SI recip piston engines, ceramic coatings can be detrimental in some applications.  For example, when applied to cylinder bores, piston crowns, or combustion chamber surfaces, the reduced thermal conductivity can cause lubrication-related failures or greater susceptibility to combustion detonation.  A modern SI recip piston engine is a device that has very carefully controlled heat transfer conditions in the cylinder bore wall, piston crown/rings/skirt, and combustion chamber surfaces.  It only works as well as it does due to the proper amount of heat transfer that occurs at these interfaces.