Low Friction, High Endurance Ion-Beam Deposited Pb-Mo-S Coatings

Thin solid lubricating coatings of Pb-Mo-S were deposited on steel substrates via ion-beam deposition. Coating endurance and friction coefficients under dry air sliding conditions were monitored with ball-on-disk tests; additional tribological testing was performed using a ball-on-flat reciprocating test rig to investigate intermediate sliding distances (100-32000 cycles). Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), scanning Auger microscopy, and micro-Raman spectroscopy were used to examine structure, composition, and chemistry of the coatings. Worn surfaces were characterized with optical microscopy and micro-Raman spectroscopy. Average endurance (at 1.4 GPa stress) of ionbeam deposited (IBD) Pb-Mo-S coatings 160-830 nm thick containing from 4-26 at.% Pb was 160000 revolutions, more than 2 times that of IBAD (ion-beam-assisted deposition) MoS2 coatings. Additionally, the IBD Pb-Mo-S coatings had friction coefficients between 0.005 and 0.02, similar to the IBAD MoS2 coatings. Friction coefficients were monitored as a function of contact stress and found to obey the Hertzian contact model; measured interfacial shear strengths (So~12 MPa) were similar to those observed for MoS2 coatings. Although XRD and micro-Raman spectroscopy indicated that the IBD Pb-Mo-S coatings were initially amorphous, micro-Raman spectroscopy showed that crystalline MoS2 was produced both in the wear tracks on coatings and in the transfer films on balls after as few as 100 sliding cycles. The wear resistance and low friction properties of IBD Pb-Mo-S coatings are attributed to the combination of dense, adherent coatings and formation of easily-sheared, MoS2-containing sliding surfaces.