The paper does not mention TaC but ZrC instead. Never mind, the reasoning
should apply to TaC (and WC) too. I have the impression that what the
developers are insisting on is the hardness of the materials at ultra high
temps., which should be critical in highly abrasive episodes. Carbon
composites are surely softer, except diamond (which can be burnt.) J.J.
Most pure carbons in air will show oxidation attach becomming
significant at temperatures of 600 to 800 C.
It is largely reasonable to consider most carbon forms as synthetic
charcoal brickettes. Some may be dense enough to be harder to ignite.
There was the Windscale (or Winscale) graphite reactor in Great Britain
that underwent combustion and large radioactive loss about 1970. This is
one example of the massive combustion of hot graphite structures.
The oxidation of graphite and carbon forms has been the reason for the
inability of the material to be used structurally in air for long
duration applications when the temperatures are elevated.
Graphite and carbon/carbon composite rocket nozzles are subject to
erosion and oxidation as are graphite and carbon/carbon composite
reenntry vehicle nosetips and heat shields.
The Space Shuttle Carbon/Carbon nosetips and leading edges are protected
by "Globar(TM)" like coatings of SiC. The Globar SiC heating elements
can withstand about 1700 C (3000 F) in air. So, the development of this
particular material wasn't a fundamental breakthrough of chemistry, but
a combination of known principles.
Think of charcoal as a first order chemical representation of
Until then, forget all about the very high melting point of charcoal.
Okay, I can buy the hardness angle. But I'm still curious about
That's exactly the impression I have. I worked on the development of a
new method of making carbon-carbon composites as an undergrad lab
monkey. I look at ultra-expensive C-C composites and see fancy coal.
Anyway, I know the C-C heat shields of the shuttle are protected with
SiC, but note that the ZrB2 and HfB2 ceramics mentioned in the article
was also oxidation protected by SiC.
So...why do ZrB2 and HfB2 have higher operating temperatures than
graphite? Wouldn't they all be limited by SiC's operating temperature?
You need to think about the oxidation products and what they mean or do.
The oxidation product of graphite (carbon) is a gas (CO or Co2) which
The ozidation of HfB2 involves a solid hafnium oxide with refractory
properties and the ZrB2 also has a solid ozide reaction product.
ACtually, the products of oxidation of this Sandia set of complex
borides and carabides is also a complex set of oxides and perhaps other
stuff. So, it is difficult to conclude a lot about the detailed mecanism
of alleged higher temperature capabilities.
I like the "Expensive Coal" analogy of carbon-carbon composites.
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