Investigation of Pitting Mechanism in the FZG Pitting Test

Extended gear fatigue pitting life is an essential performance requirement for today's gear oils in automotive driveline applications. One of the important industrial standard tests used to evaluate fully formulated oil's ability to extend gear pitting fatigue life is the FZG pitting test. To understand the fatigue pitting behavior in these gears we have conducted surface analyses on the FZG gears to determine fatigue modes. We have found that micro-pitting is the major fatigue mode and pitting/spalling is mostly initiated by micro-pitting in the FZG test. To help further understand how pitting and micro-pitting relate to gear oil properties and gear surface morphology, we have also carried out a statistical analysis correlating fatigue pitting life with four major physical parameters: boundary friction coefficient, oil film thickness, oil corrosiveness, and surface roughness of the gear tooth. It is found that a model developed based on these four physical parameters is applicable to predicting the fatigue pitting life in the FZG test for both automatic transmission fluids and gear oils.