Iridium

I've read that iridium is used in the impact shielding of radioisotope thermoelectric generators (RTG) on spacecraft because of its hardness, high
melting point, and extreme corrosion resistance. With stats like that, why isn't it being used in more applications? I know it is expensive, but so are many other exotic alloys and composites. It seems that it would be perfect for numerous aggressive environment applications but is largely ignored. Any thoughts on other uses/reasons for non-use?
Thanks!
Jim Painter Materials Science and Engineering Georgia Institue of Technology
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Jim Painter wrote:

You will find a nearly420 year long low level of interest in the refractory platinum group metals for uses such as rocket nozzles and reentry heat shields.
There was some interest in Rhenium infiltrated carbon/carbon composites as a hypervelocity impact resistant material for nuclear weapons heat shields that could fly through a nuclear debris cloud.
The subject is OLD. After all, the basic properties of all of the platinum group have long been known, and there have always been curious people who think of high temperature uses of materials.
If something has been found wanting a long time ago, not paying attention to hope reborn today for the same old thing isn't "ignoring" the issue.
Research, involves sometimes finding out old stuff, and old reasons for certain actions and attitudes.
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jbuch wrote:

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Jim Painter wrote:

Iridium has found a small niche in high performance spark plug manufacture where people seem to be willing to pay for the miniscule performance gains over platinum. http://www.spark-plugs.co.uk/pages/technical/iridium_spark_plugs.htm
In the end the rarity of iridium is the killer.
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Ir is not so rare, nor expensive. Output is adjusted to demand.
See: www.platinum.matthey.com--->click PLATNUM 2005 under "Platinum Reviews" in the upper right corner.
J.J.
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