Transition from elastohydrodynamic to mixed lubrication in highly loaded squeeze contacts

We analyze the highly loaded strongly non-stationary squeeze process of an oil film sandwiched between an elastic spherical ball and a rigid rough substrate. We show that the coupling between the elastic properties of the contacting solids, the oil rheology, the surface roughness and the applied load determines a wide range of lubrication conditions from fully elastohydrodynamic to mixed and even boundary lubrication. In particular we find that increasing (decreasing) the surface roughness (the applied normal load) speeds up the squeeze process, anticipates and shrinks the time interval during which the transition to mixed lubrication conditions occurs. On the contrary, the initial separation between the approaching bodies only marginally affects the transition time. We also observe that, in mixed lubrication conditions, the highest asperity–asperity contact pressure occurs in the annular region where the separation between solids takes its minimum value. One then concludes that surface damage and wear should nucleate in the outer region of the contact.