C-C vs. HfC and TaC

http://www.spacedaily.com/news/materials-03zj.html
Stupid question: why do HfC and TaC have higher operating temperatures
than graphite?
Do HfC and TaC have better oxidation resistance than protected carbon-carbon composites?
Mike Miller
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snipped-for-privacy@hotmail.com (Mike Miller) wrote:

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.
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jacques jedwab wrote:

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 carbon/carbon composites.
Until then, forget all about the very high melting point of charcoal.
Jim
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Heh. See, I said it was a stupid question. :)

Okay, I can buy the hardness angle. But I'm still curious about oxidation:

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?
Mike Miller
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Mike Miller wrote:

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 simply disappears.
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.
Jim
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