|
|
|
|
Ceramic Materials and Bearing Compatibility
Silicon Nitride is an Optimal Rolling Element Material
| |
Oxides |
Nitrides |
Carbides |
| Al203 |
Zr203 |
Si3N4 |
SiC |
B4C |
| Rolling Contact Fatigue |
Poor |
Poor |
Excellent |
Poor |
Poor |
| Density g/cc |
3.9 |
6.1 |
3.2 |
3.1 |
2.5 |
| Flexural Strength, MPa, RT |
300-600 |
900-1200 |
800-1100 |
400-600 |
300-400 |
Fracture Toughness
MPa-m1/2 |
3-4 |
6-10 |
5-7 |
3-4 |
2-3 |
Thermal Cof. Exp.
(x10-6/°K |
7-8 |
9-11 |
2-3 |
4-5 |
5-6 |
| Hardness, HV (10kg load) |
15-18 |
11-13 |
15-17 |
25-27 |
27-29 |
| Thermal Shock Resistance, ºC |
Fair |
Fair |
Very Good |
Very Good |
Excellent |
The green text in the chart identifies properties
that are desirable in a rolling element such as
low density, high hardness, and good
fracture toughness. However, only silicon
nitride possesses superior performance in
rolling contact fatigue (RCF) which is
essential to the performance of a rolling
element in a bearing.
Properties of silicon nitride, other ceramics
and steel.
Three basic types of ceramics could be
considered as bearing ball materials.
The carbides are extremely hard and very
light materials while the oxides provide
very good toughness. However, neither of
these materials performs well in rolling
contact fatigue, a critical parameter
associated with rolling elements in bearings.
Silicon nitride possesses high hardness,
high stiffness, and low weight compared
to steel. Silicon nitride performs much better
than steel in rolling contact fatigue making
it an ideal rolling element material for bearings.
Silicon nitride is significantly harder, has
a 60% lower density, and a 50% higher
Young's Modulus than typical bearing steels.
These properties, combined with a low thermal
expansion and low coefficient of friction
against steel are essential to improved
rolling element performance in a bearing.
Rolling contact fatigue (RCF) of nitride
and steel
A critical performance measure for rolling
elements is rolling contact fatigue (RCF) life.
This is a statistical measure of rolling
element failure and load. While many materials
have desirable characteristics (light weight,
high hardness, etc), few can withstand the
contact stresses imposed internally on a
bearing. The rolling contact fatigue life
of silicon nitride is far superior to steel.
In fact, to create a Weibull plot of Si3N4
similar to the illustration, the stresses
must be increased to a level where steel
failure is instantaneous.
Si3N4 vs Steel as a Bearing Material
| Property |
Typical Steel |
CERBEC Si3N4 |
CERBEC Difference |
| Density [g/cc] |
7.6 |
3.2 |
-58% Lighter |
| Hardness [Vickers] |
700 |
1550 |
+121% Harder |
| Elastic Modulus [GPa] |
190 |
320 |
+68% Stiffer |
Thermal Expansion Coefficient (10-6K)
[RT to 800C] |
12.3 |
2.9 |
-76% |
| Max Usage Temperature [°C] |
320 |
1000 |
+680%°C |
| Surface Finish Grade 5 [micron] |
0.02 |
0.005 |
+75% Smoother |
| Material Fatigue, Life Wear Resistance |
- |
<10x |
<10x |
|
|
|
