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Comment by lightedman | Hacker News Reader

"It's virtually indestructible and very few materials such as diamond can scratch it."

Not really. It's a ceramic and as such will shatter if knocked against another ceramic, hardness notwithstanding. Also, for some reason, my skin eats away at it.

See my tungsten carbide ring vs my fiance's ring (mine is on the left) - both made before the requirement that the rings come with break points built-in - https://i.imgur.com/fSIOUJf.jpg

Note the differences - his ring looks brand new. Both rings bought at the same time, same maker, same style.

Mine's practically obliterated.

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Note that your ring is not really representative of real tungsten carbide. WC is a sintered material so the amount of binder (cobalt) and the size and makeup of the WC powder is extremely important. Even among tungsten inserts there are types designed for continuous turning and types for interrupted turning, which have a much higher toughness.

Toughness is what determines energy before breaking and note that while many ceramics are not at all tough, high end technical ceramics are tougher than most metals[1]. High quality WC is indeed tougher than steels. It's also twice as strong as most steel, but high-end steels surpass it in strength.

[1]: http://www.cam.ac.uk/sites/www.cam.ac.uk/files/news/research...

"Toughness is what determines energy before breaking"

With ceramics and glass, things get a little tricky. You can take a relatively weak and softer piece of pottery ceramic and obliterate the hardened bullet-resistant windows on police cars (the laminate inbetween being a different matter, but the glass itself will shatter/crack.) That happens via ultrasonic frequencies created on impact.

> With ceramics and glass, things get a little tricky. You can take a relatively weak and softer piece of pottery ceramic and obliterate the hardened bullet-resistant windows on police cars [...] That happens via ultrasonic frequencies created on impact.

All of that is wrong. You can't use pottery shards or even hard pebbles to break glass; you need very hard technical ceramics[1]. Spark plug ceramic is alumina which is extremely hard.

It has nothing to do with ultrasonic frequencies, it's caused by crack propagation. Tempered glass is under internal stress due to its quench rate and chemical treatment, so as soon as the outer shell is broken the entire thing shatters. Alumina is so hard that a corner can chip off a bit of glass, releasing that pressure.

Ceramics can't be tempered like glass, and like untempered glass they only shatter if an impact by a much harder object is above the fracture propagation energy. That means that most of the time the impact energy is about the same as the actual toughness of the material. For high toughness ceramics there are no materials hard enough to cause chipping, so they break exclusively under normal toughness.

[1]: https://en.wikipedia.org/wiki/Ninja_rocks

"You can't use pottery shards or even hard pebbles to break glass; you need very hard technical ceramics[1]. Spark plug ceramic is alumina which is extremely hard."

Plenty of pottery contains aluminum oxide, which is very hard.

That's not the same thing- coal contains microscopic diamonds but you'll have a hard time scratching sapphire with it. Incidentally, alumina is much closer to sapphire than any pottery. Alumina is the sintered powder form and you need really good quality, high density stuff to be able to break a window. It's also a near-requirement that it has very sharp points.

Alumina will effortlessly scratch gorilla glass, any metal, tungsten carbide and even a lot of sapphire glass. No kind of pottery will do any of those things.

"coal contains microscopic diamonds but you'll have a hard time scratching sapphire with it."

Coal dust will abrade and destroy almost everything and it's because of the graphite, not the diamond. I have a coal mine. It's killed two phones and a drone already.

"Alumina will effortlessly scratch gorilla glass, any metal, tungsten carbide and even a lot of sapphire glass. No kind of pottery will do any of those things."

This is entirely untrue, it depends on the composition of the clay used in making the ceramic, which will be entirely dependent upon the location the clay or clay mix is obtained from. I also happen to do lapidary and pottery work, as a profession (next to LED lighting applications.) You can get clay from Mt. Baldy, make pottery, fire it up, it will scratch the hell out of almost everything because of the sand grain-sized rubies contained within.

It might not be your skin, but your skin care products. Some contain aluminium oxide or magnesium oxide. Both of these can oxide tungsten.

My skin care products are the exact same as what my fiance uses.

Be careful with rings made out of hard materials.

if you ever get into trouble it'll be your finger that comes off, not it.

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