ASTM F2094/F2094-14,Standard Specification for Silicon Nitride Bearing Balls.ASTM F2094/F2094-14 pdf free download.
4. Classification
4.I Silicon nitride materials for bearing and specialty ball applications are specified according to the following material classes (see Appendix XI for typical current applications)：
4.1.1 Class I-Highest grade of material in terms of prop- erties and microstructure. Suitable for use in the most demanding applications. This group adds high reliability and durability for extreme performance requirements.
4.1.2 Class I-General class of material for most bearin and specialty ball applications. This group addresses the concerns of ball defects as is relative to fatigue life, levels of torque, and noise.
4.1.3 Class III–Lower grade of material for low duty applications only. This group of applications primarily takes advantage of silicon nitride material properties. For exampl Light weight, chemical inertness, lubricant life extension due to dissimilarity with race materials, and so forth.
6. Material
6.1 Unless otherwise specified, physical and mechanical property requirements will apply to all material classes.
6.2 To be classified as Class I. silicon nitride balls shall be produced from either silicon nitride powder having the compositional limits listed in Table I or from silicon metal powder which after nitridation complies with the compositional limits listed in Table 1.