Low-friction, high-endurance, ion-beam-deposited PbMoS coatings

Abstract

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 the structure, composition and chemistry of the coatings. Worn surfaces were characterized by optical microscopy and micro-Raman spectroscopy. The average endurance (at 1.4 GPa stress) of ion-beam-deposited (IBD) PbMoS coatings (thickness, 160–830 nm) containing 4–26 at.% Pb was 160000 revolutions, more than twice that of MoS 2 coatings obtained by ion-beam-assisted deposition. In addition, the IBD PbMoS coatings had friction coefficients between 0.005 and 0.02, similar to the MoS 2 coatings obtained by ion-beam-assisted deposition. Friction coefficients were monitored as a function of the contact stress and found to obey the hertzian contact model; measured interfacial shear strengths ( S 0 ≈ 12 MPa) were similar to those observed for MoS 2 coatings. Although XRD and micro-Raman spectroscopy indicated that the IBD PbMoS coatings were initially amorphous, micro-Raman spectroscopy showed that crystalline MoS 2 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 the formation of easily sheared, MoS 2-containing sliding surfaces.