AISI 52100 Bearing Ball Microstructure as a Source of Variability in Standard Four-Ball Testing

Abstract

AISI 52100 steel balls are extensively used in standard four-ball tribological tests for evaluating lubricant performance because of their high hardness and excellent resistance to wear and contact fatigue. However, variations in the morphology and distribution of spheroidized carbides within this material can influence not only the intrinsic wear resistance of the balls but also the measured tribological response of lubricants. This study investigates the role of spherical carbides distribution on friction and wear outcomes obtained during four-ball testing. Two commercially available AISI 52100 balls from different suppliers were characterized through metallographic analysis and nanoindentation, followed by tribological testing using a non-additized polyalphaolefin base oil (PAO6) and a fully formulated automatic transmission fluid (ATF). Ball Sample A exhibited finer and more uniformly distributed spherical carbides, resulting in approximately 99% higher carbide density and 25–35% greater hardness than Ball Sample B. Under PAO6 lubrication, Sample A produced 30% lower coefficient of friction and a 22% smaller wear scar diameter. Although the ATF significantly reduced wear for both ball samples through tribofilm formation, Sample A still exhibited approximately 31% lower wear scar diameters. The findings demonstrate that spherical carbide characteristics strongly affect the tribological response measured during four-ball testing by modifying contact stability, load support, and wear mechanisms. Consequently, differences in carbide distribution among nominally identical AISI 52100 balls can introduce significant variability in lubricant performance assessments, even following the standard method, highlighting the importance of considering steel-ball carbides content and distribution in standardized tribological evaluations.