Originally Posted by PAR
...Aluminum oxide isn't the best "hardening" filler material you can employ. It's typical use is for as a thermal break and temperature control, not as abrasion resistant filler. Harder materials (aluminum is very soft) are the usual choices. Iron fillings make an excellent choice, though they can rust if exposed. Copper is relatively soft, but much better than aluminum. Bronze and monel dust is good, but costly, as are Kevlar fibers. Sand and stone are cost effective alternatives. Sand is typically sandstone and quartz, both relatively soft compared to decomposed granite and other, more noble stone types.
Originally Posted by Red Dwarf
Another option is mica. I have used it in epoxy, it makes it very hard and abrasion resistant. Some aerospace resins are mica filled. It will destroy a drill bit. I have no experience with it in salt water.
The Mohs scale of hardness is often used for minerals, with Talc at 1 and Diamond at 10.
Mica is relatively soft at 2.8 (roughly the same as copper, aluminum and gold)
Granite is primarily comprised of quartz (7), feldspar (6), and mica (2.8)
Most sandstone is primarily quartz and feldspar
Most inland sand is quartz, with marine sands often calcium carbonate (coral, shells, etc) (3)
Aluminum oxide is just below diamond at 9, and is commonly used for grinding wheels
The foregoing is not a recommendation for an abrasion-resistant hull material, just a clarification on hardness. http://www.rockrollers.com/features/hardness.html http://www.reade.com/Particle_Briefi...sive_grit.html http://en.wikipedia.org/wiki/Granite http://en.wikipedia.org/wiki/Sandstone http://en.wikipedia.org/wiki/Sand