Micropitting is a concern in gears and sometimes bearings. Small pits are formed on the the surface of the material, leading to loss of geomerty. Micropitting frequently leads to pitting/spalling and eventually part failure. The material ejected by micropitting can also cause abrasive wear. Surface treatments, materials and chemical additives can be used to reduce the occurrence of micropitting. We can evaluate this performance.
We use a 3 ring on roller set-up to accelerate micropitting: the PCS Instruments MPR. This instrument was designed to subject the small central roller to many contact cycles, accelerating any fatigue type failure modes. The 3 counter rings are relatively hard and rough. Measuring ~ 0.5 µm Ra. This high roughness and hardness will accelerate the micropitting on the relatively soft and smooth central roller. For each revolution of the central roller it will be subjected to 3 contact cycles. Accelerating the test further.
Images are taken of the central roller periodically throughout a test. Some example images of the surface are shown below.
The image below show two reference oils, one which passes the FZG micropitting test (FVA 54/7) and one which passes. Thus this method can be used to quickly screen potential chemistries in terms of their micropitting performance.
The same method has been used to investigate the effect of a base oil, a ZnDDP oil and a ashless antiwear oil. The results of these are shown below:
Microptting and mild wear are competing mechanisms. Thus oils with high wear on the steel surfaces, may reduce the ring roughness resulting in the appearance of a “good” micropitting performance. We monitor the ring roughness in-situ to log and understand the progression of wear on the rings. This is to ensure a valid accelerated micropitting test.