On the White Etching Crack Behavior of Lubricants and Additives

Abstract

The white etching crack (WEC) behavior of five model lubricants and one fully formulated gear oil has been investigated using the FE8 test rig. The varying concentrations of branched zinc dialkydithiophosphate (ZDDP) and overbased calcium sulfonate (OBCaSu) have been studied. The results reveal that a high concentration of branched ZDDP and OBCaSu is critical to the WEC failure, while a low concentration is not. This is attributed to tribofilm structure, composition, and mechanical properties. Tribofilms formed from less-critical lubricants show up to 110% higher hardness and 80% higher elastic modulus than the WEC-critical lubricants. Rapid tribofilm growth and higher amounts of certain tribofilm species like FeO, CaO, and ZnO appear to have a weak tribofilm structure that potentially could promote hydrogen permeation. The FeO in tribofilm is considered to act as a catalytic site for hydrogen dissociation, leading potentially to hydrogen diffusion. Changing from a branched to a linear ZDDP postpones the WEC significantly but does not prevent it. Replacing a bad reference WEC-critical oil with a good reference oil could postpone the WEC failure but cannot be prevented. The fully formulated gear oil that formed thin tribofilm and less-critical tribofilm species did not show WEC. Special micropitting rig (MPR) tests made using specimens with non-metallic inclusions showed an earlier WEC failure than those with standard specimens without such non-metallic inclusions, suggesting that non-metallic inclusions could promote WEC even in the absence of WEC-critical additives and water.