In a material for a ceramic ball according to an embodiment, a sphericity is 2% or less, and an arithmetic mean roughness Ra is 0.2 m or more and 2 m or less. In the material for the ceramic ball, a total height of profile Rt is preferably 4 m or more and 20 m or less. In the material for the ceramic ball, when a surface roughness Ra in a circumferential direction of a belt-like portion mark is defined as Ra1, and a surface roughness Ra of a circumference in a direction perpendicular to the belt-like portion mark is defined as Ra2, Ra1/Ra2 is preferably 0.2 or more and 2 or less.

Provided are a zirconia medium having improved wear resistance and durability, a zirconia bearing ball, and methods for producing the same. A zirconia medium which fulfills requirements: (a) comprising a ZrO.sub.2Y.sub.2O.sub.3 zirconia sintered material, wherein the Y.sub.2O.sub.3/ZrO.sub.2 molar ratio is from 2.5/97.5 to 3.2/96.8; (b) the amount of the contained Al.sub.2O.sub.3 being from 0.1 to 30.0% by mass; (c) containing tetragonal crystal system zirconia in an amount of 90% by volume or more; (d) having a relative density of 95% or more; (e) having an average crystal grain diameter from 0.25 to 0.50 m; (f) the minimum value Pmin of crushing load satisfying the relationship: Pmin(N)>600D.sup.2.0 (wherein D is an average of medium diameter); (g) the Weibull coefficient of crushing load being 10 or more; and (h) the coefficient of variation of medium diameter being less than 6%.

A bearing having a bearing surface with a groove, the bearing positioned adjacent to a shaft, the groove facing the shaft, and a negative thermal expansion (NTE) material positioned in the groove, the NTE material at least partially filling the groove.

Bearings with grooves and methods of producing the same are disclosed. Examples disclosed herein include a bearing having a bearing surface with a groove, the bearing positioned adjacent to a shaft, the groove facing the shaft, and a negative thermal expansion (NTE) material positioned in the groove, the NTE material at least partially filling the groove.

A ceramic ball material according to an embodiment including: a spherical portion; and a band-shaped portion formed over a circumference of a surface of the spherical portion. The ceramic ball material has a ratio Rtb/Rts of 1.0 or more, where Rtb denotes a maximum cross-sectional height of a roughness profile on an outer peripheral surface of the band-shaped portion; and Rts denotes a maximum cross-sectional height of roughness on an outer peripheral surface of the spherical portion.