Better ceramics thanks to hair crystals
If two forms of Sialon ceramic are fired in an unusual mixing ratio, a hybrid is created that combines the good properties of both raw materials. It could be produced at low cost and replace today's ceramics in ball bearings and engines. Ceramic - stronger and lighter than steel, harder than any other substance except diamonds - has long been a favorite material of engineers for making everything from ball bearings to turbine rotors. But anyone who has ever smashed a plate knows that ceramics has one major disadvantage - it's fragile. Now scientists have proven that they can make one of the hardest known ceramic materials stronger by adding whiskers to it.
I-Wei Chen and Anatoly Rosenflanz, materials scientists at the University of Pennsylvania in Philadelphia, studied a type of pottery called sialon. It is so named because it contains silicon, aluminium, oxygen and nitrogen. Sialone come in two crystal forms: an extremely hard alpha form, characterized by small, round granules, and a less hard but more robust beta form. The industry prefers to use the Beta form, which contains predominantly long, needle-shaped granules or whiskers that prevent cracks from propagating.
Now Chen and Rosenflanz have discovered how to create a sialon that combines the best properties of both forms. The secret lies in the firing, during which grains are fused into larger crystals. "To get elongated crystals, you have to give them room to grow," says Chen. Chen and Rosenflanz started with a powder that contained mostly beta grains and few alpha grains (the opposite of what other materials scientists have done). By doing so, they ensured that Alpha crystals could grow a good chunk before encountering other Alpha s that would have h alted their growth. As a result, the alpha crystal seeds "grew very quickly and ate up the betas," says Chen. Because of the hair crystals, the resulting hybrid ceramic was just as robust as beta sialon and possessed the hardness of the alpha form.
They had some very clever ideas, says Brian Sheldon, a ceramics processing specialist at Brown University. According to Chen, the new ceramic material should be able to replace beta silicon nitride - currently the most advanced material - at low cost and effort: The consumer will not notice the difference - it looks and weighs the same just as much. However, the increased hardness means that bearings made from the new ceramic material last longer and allow motors to run at higher speeds.