As plasma spray coated cylinder surfaces eliminate the need for cast iron liners or hypereutectic alloys, thermal spray coatings are becoming the main cylinder liner technology since they offer significant weight reduction. Moreover, it has been found that mild steel coatings also lead to low friction and wear and that the spray coating forms a nanocrystalline layer. In this study we use on-line wear measurement to understand the origin of the low friction and wear.
A pin-on-disk tribometer coupled to a radionuclide wear measurement (RNT) system was used to investigate the friction and wear behavior of of plasma transferred wire arc spray (PTWA) coatings sliding against chrome diamond (GDC) composites coatings under lubricated conditions. After the friction experiments, X-ray photo electron spectroscopy (XPS) and Focused ion beam analysis (FIB) was used to characterize the worn surfaces.
By introducing a time-dependent Stribeck plot we analyzed running-in under constant and transient sliding conditions and observed a strong reduction of friction in the boundary regime. Wear rates of the PWTA disks as well as of the chromium plated pins are ultra-low with less than 0.1 µg/h. XPS revealed carbon diffusion at room temperature in wear tracks of disks run-in to a coefficient of friction (CoF) of 0.01, whereas this carbon diffusion could not be detected in the wear track of a disk without running-in, i.e. a final CoF of 0.12.
As this is the most significant difference found between differently run-in systems, the described carbon diffusion is thought to be relevant for the observed friction behaviour.
Running-in behaviour can only be discussed in terms of friction, as, even with RNT, no significant wear could be measured. The comparison of running-in under transient and constant conditions showed, that transient conditions yield lower friction values in the mixed lubrication regime.