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"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 p

"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.

hwillis 9y ago
> 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 https://en.wikipedia.org/wiki/Ninja_rocks
- lightedman 9y ago
  "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.
  - hwillis 9y ago
    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.
    - lightedman 9y ago
      "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.

hwillis 9y ago

> 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 https://en.wikipedia.org/wiki/Ninja_rocks

lightedman 9y ago
"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.
- hwillis 9y ago
  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.
  - lightedman 9y ago
    "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.