In this section you can learn about Material Properties:

• Intrinsic Material Properties Are Derived From Atomic Bonding
• Bearing Grade Silicon Nitride Requirements
• CERBEC Silicon Nitrides For Bearings

Intrinsic Material Properties Are Derived From Atomic Bonding

Metals	Polymers	Ceramics
Metalic Bonding	Covalent Bonding	Ionic Bonding
		Covalent Bonding
		Oxygen
		Cation in octahedral site
		Cation in tetrahedal site

METALS Metals predominantly are bonded by non-directional electron sharing between neighboring atoms which is known as metallic or electronic bonding. At any moment in time, metal atoms care only that they have an overall electrical neutrality with their surrounding neighborhood. They do not need to “own” the electrons surrounding them. Non-directional electron sharing between atoms allows for tightly packed structures (which results in high density) with numerous slip planes (which allows for ductility). Since electrons are not tightly bound to specific neighboring atoms, they are free to move through the metallic lattice (which results in electrical and thermal conductivity).

POLYMERS Polymers are made up of individual molecules that are attached in long chains at specific, discrete points to satisfy the molecules’ total electrical neutrality. The atoms within each molecule are bound by strong directional bonds to their neighbors known as covalent bonding. Each atom secures its electrical neutrality by maintaining a very fixed set of neighboring atoms with which it shares electrons directionally. Within each molecule, atoms do not have a high freedom to move with respect to their neighbors since electrical neutrality must be guaranteed by its neighbors. Typically, the molecules in a polymer bond to each other at discrete attachment points to form very, very long chains. The combination of very tightly bound individual molecules and long chain arrangements of multiple molecules results in an interesting combination of properties. Corrosion and electrical resistance come from the individual molecules not allowing their electrons to move. Elasticity and lubricity come from the long chain bonding between molecules. The chains can stretch and move around within the material.

CERAMICS Ceramics predominantly are bonded by very directional bonds between neighboring atoms in expansive lattice structures (as compared to the chains of polymers). Because every atom in a ceramic directionally shares its electrons within its lattice neighborhood through ionic and/or covalent type bonding, ceramics tend not to be tightly packed like metals. Due to strong directional bonding and non-close packed lattices, ceramics tend to have high stiffness, low electrical conductivity, low density, high hardness, low thermal expansion, high melting or dissociation temperatures, electrical resistance, high strength and corrosion resistance. In essence, ceramics tend to be very “non-reactive” or “inert” because their atoms are essentially electrically neutral through strong directional bonds within a very fixed lattice neighborhood – they have no need to “react” with the outside world looking for electrons to satisfy neutrality.

Si₃N₄ vs Steel as a Bearing Material