Spurious Mild Wear Measurement Using White Light Interference Microscopy in the Presence of Antiwear Films

Commercial scanning white light interference microscopes are able to map surface topography to very high resolution and can thus measure very small amounts of wear on rubbed surfaces. These instruments are therefore being increasingly used to study aspects of mild wear. It is shown in this Technical Note that, when used on surfaces that have been rubbed in lubricants containing the antiwear additive zinc dialkyldithiophosphate (ZDDP), some white light interference microscopes can produce an apparent but spurious wear measurement. The transparent and relatively thick antiwear film generated by ZDDP on rubbed surfaces produces an optical path difference that is incorrectly interpreted as wear. It is thus suggested that surfaces that have been rubbed in lubricants containing ZDDP or other thick film-forming additives should be treated to remove tribofilms prior to wear measurement using scanning interference microscopy.