If I have a complete silicon nitride ball bearing unit, is it safe to hold without any protection? If it cracks, or disintegrates is it also safe?
I have done a minimal amount of research to try and help you understand this material. The credibility of this research is very debatable, as it basically involves me typing in silicon nitride into YouTube. That being said, understand that silicon nitride is first of all extremely strong, inert, according to that link, and being considered as a contemporary 'ideal' material for use in the human body. Inert just means it will not react which is generally safe and certainly not caustic to life.
Know however, that if this material is present in powder form I would highly recommend a breathing mask at the very least as it is usually inadvisable to inhale solid particles. In the scenario that it shatters it could potentially be sharp. However, if it is merely a bearing we are talking about, then, you probably have very little to worry about other than contaminating your materials if you are going to use the part in high performance devices, and you therefore would want to wear clean gloves so as not to get dust on the bearing.
I advise you to search you-tube and other sources for people handling the material, but more than likely it would be less beneficial to the bearing or your project to handle the material without gloves than it would be to your health.
I have a few silicon nitride balls in my drawer. They look pretty cool, that's why I keep them and touch them all of the time.
It is a very hard, glasslike material. It won't be easy to crack. There is nothing coming off the surface, ball bearings have extremely smooth surfaces (nm roughness), if anything would rub off, they would not be usable as ball bearings. Besides that, I don't think it is toxic. It is an extremely inert material, much more inert than steel, does not dissolve in much. It is even used in prosthetic parts that go into the body. I assume it needs a lot of certification for that.
If you break it (which won't be easy), you'll have some sharp pieces that might cut you, that's all.
The only risk would be from a ball bearing that was ground to a very fine dust.