A solid silicon nitride ceramic insert
would be ideal. Hard, very smooth  and no expansion to speak of. Don't know
what the thermal conductivity would be. The cost would be astronomical
I am sure. Same stuff super high speed ball bearings and apex seals
are made of.

Paul, I am intrigued, You want NO expansion? I have several sources 
for Silicon nitride. My father was a engineer specializing
in ceramics. He had patents for ceramic metalizing parts. 
Silicon nitride would cost less than titainium. The expansion would be
a nearly nil. Silicon Nitride is very tough. Diamond grinding 
is a must. The material is pressed and fired to near final shape.
Might need to be thicker than .090.  Bill Jepson

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