The Study of Tribological Phenomena using Linear and Nonlinear Ultrasound

Tribology is the science of friction, lubrication, and wear. A single
surface or a contact created by multiple parts can be studied in tribology.
The contact is either dry or lubricated. In addition, lubricants can
be studied for their viscosities, degradation or film thickness when used.
When two solid surfaces come into contact, the asperity peaks touch, and
the real contact area is substantially less than the nominal contact area.
Applying pressure to the surfaces causes a small approach of the mean
lines of their roughness. The interfacial stiffness (per unit area) is then
defined as the rate of change in contact pressure with the approach of the
mean lines of the roughness of the contacting surfaces. If the real contact
area is low, then a low nominal contact pressure is all that is required
to deflect the asperities. Increasing the nominal contact pressure brings
more asperities into contact, making the interface stiffer; hence, the interfacial
stiffness is a nonlinear variable. Interfacial stiffness is important in
tribology because it affects machine element deflection, wear and friction
since stiffer interfaces tend to have less deflection. Machining accuracy,
for example, depends on the stiffness of the joints in the machine tool
assembly.
Lubricants have a significant impact on energy consumption and the
level of environmental pollution. As lubricants circulate in an engine or
machinery, they become degraded. Various sources of lubricant degradation
include oxidation, water, soot, wear debris and other contaminations.
The two approaches for lubricant drain times are hours/rotation/mileage
operation of the engine or machinery and degradation monitoring.
In the current study, high-power ultrasonic waves have been used in
two ways. First, to explain the nonlinear stiffness of dry contacts and,
second, the nonlinear behaviour of lubricants as they degrade. The nonlinearity
due to longitudinal ultrasonic waves was investigated both analytically
and numerically. The reason for the peak of the amplitude of nonlinearity was investigated as it was not sufficiently studied in the previous
research. The effect of frequency and the amplitude of excitation
and the mechanical properties of the contacting surfaces on the amplitude
of fundamental frequency and second-order harmonic was studied. The
effect of consecutive loading/unloading cycles and surface roughness on
nonlinear interfacial stiffness was experimentally investigated.