Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts by N. N. Gosvami, J. A. Bares, F. Mangolini, A. R. Konicek, D. G. Yablon, and R. W. Carpick Science Volume 348(6230):102-106 April 3, 2015 Published by AAAS

Fig. 1 Morphology and volumetric growth of tribofilm. Morphology and volumetric growth of tribofilm. Tribofilm volume (mean ± SD) versus sliding cycles, with linear and power-law fits to the initial and subsequent growth regimes, respectively. Inset shows a zoom-in of the initial growth period. Around the perimeter, clockwise from upper left: periodically acquired 2 μm × 2 μm AFM images of an iron oxide surface using a DLC-coated silicon AFM tip immersed in ZDDP-containing base stock, acquired at a nonperturbative load of 20.0 ± 0.1 nN. Below each image is the number of previously acquired 1 μm × 1 μm scans (“sliding cycles”) at a load of 340 ± 2 nN (4.2 ± 0.5 GPa). The images demonstrate progressive tribofilm growth where the higher load was applied. N. N. Gosvami et al. Science 2015;348:102-106 Published by AAAS

Fig. 2 Tribofilm volumetric growth rate dependence on contact pressure. Tribofilm volumetric growth rate dependence on contact pressure. Tribofilm growth rate is exponential at low contact pressures (data are means ± SD). Further growth is inhibited above ~5 GPa as the tip wears away newly deposited material. The 2 μm × 2 μm topographic contact-mode AFM images shown were acquired at a nonperturbative load after generating tribofilms in the central 1.0 μm × 0.5 μm regions at various contact pressures. N. N. Gosvami et al. Science 2015;348:102-106 Published by AAAS

Fig. 3 Tribofilm volumetric growth rate dependence on temperature. Tribofilm volumetric growth rate dependence on temperature. Growth rate (mean ± SD) versus temperature data fitted with an exponential function (Eq. 1). The 2 μm × 2 μm topographic contact-mode AFM images shown were acquired at a nonperturbative load after generating tribofilms in the central 1.0 μm × 0.5 μm regions at 80°, 100°, 120°, and 140°C at an initial contact pressure of ~4.4 GPa. N. N. Gosvami et al. Science 2015;348:102-106 Published by AAAS

Fig. 4 Ex situ chemical characterization. Ex situ chemical characterization. (A) EDS point spectra (estimated sampling depth of ~1 μm) acquired for regions (a) inside and (b) outside the tribofilm (i.e., for the portion of the substrate covered with the thermal film). (c) Secondary electron image of the 10 μm × 5.0 μm tribofilm. Corresponding elemental maps are shown for (d) Fe, (e) Zn, (f) P, and (g) S. (B) (h) Optical and (i) secondary electron image of a 10 μm × 5.0 μm tribofilm obtained by scanning AES. (j) AES spectra for the tribofilm and the substrate (estimated sampling depth ~3 nm). N. N. Gosvami et al. Science 2015;348:102-106 Published by AAAS