Pure
zinc oxide (ZnO) and magnesium-doped zinc oxide (ZMO) nanoparticles
(NPs) with the composition of Zn0.88Mg0.12O
have been prepared by an autocombustion method. The as-synthesized
ZMO nanoparticles were calcined for 2 h at 800 and 1000 °C to
yield different-sized ZMO nanoparticles abbreviated as ZMO-1 and ZMO-2,
respectively. These nanoparticles have been characterized by powder
X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX),
and transmission electron microscopy (TEM) techniques. The average
size of these nanoparticles is found to be 30, 27, 39, and 44 nm,
respectively for ZnO, ZMO, ZMO-1, and ZMO-2. A stable dispersion of
ZMOs (ZMO, ZMO-1, and ZMO-2) nanoparticles in paraffin oil has been
achieved with an appropriate percentage of surfactant sodium dodecylsulfate
(SDS), abbreviated as SZMOs (SZMO, SZMO-1, and SZMO-2). The effect
of the particle size of these nanoparticles on the tribological behavior
of the paraffin oil has been investigated at an optimized additive
concentration (0.25% w/v with 0.10% SDS) using different ASTM D4172
and D5183 standards. In addition, a test has been conducted by varying
the loads for a 30 min time duration and by varying the test durations
at 392 N load. All tribological testing of SZMOs nanoparticles were
conducted on a four-ball lubricant tester. These tribological tests
revealed that the SZMOs nanoparticles act as excellent antiwear agents
and friction modifiers and also enhance the load-bearing ability.
Being the smaller particle size, SZMO nanoparticles (27 nm) exhibited
better tribological behavior than SZMO-1 (39 nm) and SZMO-2 (44 nm).
The morphology of the worn surfaces lubricated with nanoparticles
and without SZMOs at a 392 N applied load for 60 min and at higher
loads for a 30 min test duration has been examined by scanning electron
microscopy (SEM) and contact mode atomic force microscopy (AFM) analyses.
Energy-dispersive X-ray (EDX) analysis of the surface lubricated with
SZMO nanoparticles shows the presence of zinc, magnesium, iron, carbon,
and oxygen on the steel surface which confirmed the adsorption of
the additive on the interacting/rubbing surface. These elements form
tribochemical film on the interacting surfaces to prevent the metal–metal
contact thereby reducing wear and friction.