Friction transitions and connections to third bodies for a Cd coating on steel substrate

Cd coating is used in aerospace industries from last five decades due to its sacrificial protection and lubrication properties. Although Cd coating is primarily used due to its sacrificial corrosion protection when applied on steel substrate, the added benefit of modifying the tribological behavior by acting as a lubricious layer gives it a leading-edge than other coatings. Often the measurement of friction coefficient (CoF) is reported as a value generated after full sliding cycle. This measurement of average CoF generally limits the study of local variation in CoF occurring within one sliding cycle, which can be significantly different with change in spatial position due to change in third body morphology. In this study, a linearly reciprocating sliding test is used to measure the CoF at a sampling rate of 800 Hz along the track length to generate triboscopic image with steel countersphere. The instantaneous CoF obtained with triboscopy is correlated with the wear track morphology using scanning electron microscope (SEM) and optical profilometer for variation in contact conditions. Tribological test performed in dry atmosphere shows an average CoF of 0.4 till the end of the test whereas with increase in relative humidity to 60%, the average CoF changes from 0.4 to 0.8 at the end of the test due to change in contact conditions. Soft Cd coating on low carbon steel substrate is used to study these variations in third body morphology.