There is a difference between cermet coatings and thermal barrier coatings.
Cermets are ceramic particles bound in metallic binders, usually applied by
thermal spray processes at thicknesses of around .010". Examples are

tungsten

carbide in cobalt, chrome carbide in ni-chrome, titanium carbide in steel.
These are used for exceptional wear resistance, and that is why they were
used on the Mazda aluminum housings. The influence on heat transfer (and
efficiency) would be pretty well nil.

Thermal barrier coatings are much thicker ceramics such as zirconia, and are
(were - they have fallen from fashion by and large) also applied via thermal
spray. Usually there is a bond coating of NiCrAlY to attempt to reduce the
thermal stresses caused by the difference in expansion coefficient between
the parent metal and the top coating. There is also a steep temperature
gradient, causing high internal stresses within the coating itself. Attempts
are often made to apply the coating with a graded composition to combat the
internal stresses. Diesel engine research showed that the coatings had to be
rather thick (on the order of .125") to effect any improvement in heat
rejection. Performance gains were not as dramatic as predicted. After many
millions of dollars, attempts to create an adiabatic engine have largely

been

abandoned due to the extreme difficulties encountered and the rather meager
benefits gained. Military applications are still interested because
elimination of the liquid cooling systems gets rid of a major combat
vulnerability (as well as a maintenance issue).

In rotary engines, Curtiss Wright found that a thin TBC was able to

raise the

rotor surface temperature enough to reduce hydrocarbon emissions but no

other

real benefits were noted. Other researchers (Moller, for one -- Jeff know
anything about this?) have had mixed results. Basically, the challenge is to
keep the stuff on the rotor because if it comes off the engine will fail. So
I guess the risks have been shown to outweigh the benefits.

Jon Lauter
Rotary Power Marine Corporation

I don't recall too many specifics from the Moller tests.  I think Paul has
the SAE paper that summarizes the results.  One thing I recall is an
indication that more of the fuel was being burned due to reduced flame
quenching.  In my diesel studies I was shocked to find plenty of data that
suggests that TBC's are a waste of time.  The best theory I heard (that was
based on some testing) was that the hot surface temps allowed the flame to
progress closer to the surface without queching in the "thermal boundary
layer [of cool air]" thus leading to higher heat transfer.  Bummer.

Jeff Spitzer

 
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