Characterization of pulsed laser deposited PbO/MoS₂ by transmission electron microscopy

Films of PbO/MoS₂, grown by pulsed laser deposition, exhibit a significant improvement in tribological performance compared to MoS₂ films grown by the same process. The microstructure and crystallography of PbO/MoS₂ composite films were investigated using transmission electron microscopy (TEM) to identify the features responsible for this tribological improvement. Self-supporting samples were prepared from pulsed laser deposited, PbO/MoS₂ thin films grown on single crystal sodium chloride substrates. Films deposited at room temperature exhibited a two-phase microstructure with one of the phases being amorphous. X-ray microanalysis results showed that the crystalline phase had significantly higher concentration ratios of Mo/Pb, Mo/S, and Pb/S than did the amorphous phase. Films grown at 300 °C were polycrystalline, with a grain size of about 20 nm, and had a NaCl type structure which was isomorphous to PbS. The grains had rectangular shape, and exhibited preferred orientation with the sodium chloride substrate. The concentration of S for these films was approximately 80% of the S concentration for films grown at room temperature. Both the high temperature and room temperature films had S concentrations which were higher than expected from the MoS₂ in the target; this was attributed to gettering of the S in the vacuum chamber by Pb. The electron diffraction results, together with previously published results, suggest that the crystal structure of the phases in these films is not responsible for the improvement in tribological properties. However, the microstructural components formed during film growth do determine the wear-induced chemical reaction pathways.