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First things first, I came across this interesting article after which I stumbled upon this mysterious composite of an aluminium oxide and silicon carbide. Specifically stating this on wikipedia:

"In 1982 an exceptionally strong composite of aluminium oxide and silicon carbide whiskers was discovered. Development of this laboratory-produced composite to a commercial product took only three years. In 1985, the first commercial cutting tools made from this alumina and silicon carbide whisker-reinforced composite were introduced into the market."

leaving me with this

So, Does it happens for you to know what this mystery aluminium oxide is? And what other aluminium oxides could be used for this strong composite?

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    $\begingroup$ It's ordinary aluminium (III) oxide, $\mathrm{Al}_2\mathrm{O}_3$, a.k.a. alumina. The word "composite" refers to a composite material, in this particular case a ceramic. (And the word "strong" here means "hard". I don't fully understand why hardness would be such an important quality for bones.) (Besides the ordinary aluminium (III) oxide, there exist aluminium (I) and aluminium (II) oxides; they are both unstable and have no practical use.) $\endgroup$
    – AlexP
    Sep 6 at 17:33
  • $\begingroup$ Hello Explunky. Please forgive me for pointing this out, but you're not asking a worldbuilding question. You're asking a straight real-world question that would have been more appropriate on Chemistry. Normally I'd VTC as not about worldbuilding, but you're new. Please remember, we're here to help you build a world and real-world questions are expected to be asked in the context of designing a fictional world. If you're just hunting for real-world answers, please use the other Stacks. $\endgroup$ Sep 6 at 21:59
  • $\begingroup$ Hard is not the same as strong. The materials you speak of are hard and brittle (not strong). $\endgroup$
    – DKNguyen
    Sep 7 at 5:21
  • $\begingroup$ corundum. See also Ceramic matrix composite for a starter and go from there. Don't know what your question has to do with biology. $\endgroup$ Nov 17 at 5:49
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Ceramics are incredibly strong and hard but are not very tough (i.e. they initiate cracks easily), and are brittle with graceless failure, as the cracks propagate easily from the crack tip right through the ceramic.

So a ceramic is only as tough as its weakest point that experiences any sudden force; it's all about preventing a crack beginning in the first place, because once it begins, it's going to rip through the ceramic item.

Composite ceramics are much stronger because they have two phases of ceramic, generally one which is a matrix (or surround), with another one as a fibre or filler particles.

If the fibres/fillers adhere to the matrix strongly but not too strongly, then cracks that strike them don't propagate further. The tip "bounces" off the filler and the item is saved.

If adhesion is weak, then the fibres pull out and act more like holes than reinforcements. If adhesion is too strong, then the fibres cease to act like fibres and cracks propagate right through them just like through an unreinforced ceramic.

All this is to say that when you are talking about reinforced ceramics, the magic lies usually not in the bulk chemical composition (although minor additives can be really important), but in the morphology, i.e. the distribution and adhesion of the filler, along with the all-important defect reduction during production.

Re: aluminium oxide (Al2O3), the most common form is alpha-alumina, which is also the strongest and form. What would have made that ceramic exceptional would have been the successful insertion of defect-free SiC fibres into defect-free Al2O3 with just the right adhesion strength and in some way that resulted in useful shaped items (you don't get to machine them afterwards).

In the 1980s, it was pretty hard to make long ceramic fibres, all they had was short fibres. I'm guessing the fibres in question would have been unusually long and defect free.

FWIW, porcelain (real porcelain, not white stonewear) is similar to an engineering ceramic; it's got long mullite (engineering ceramic) needles in glass, which is why it's so much tougher and able to be made finer than average stonewear. You can compare them using a microscope.

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