| As a special additive, dialkyldithiophosphates (ZDDPs) have been used as an antiwear and antioxidant in engine oil formulations for more than 70 years. ZDDPs can break down thermally and form so-called thermal films on metal surfaces. ZDDPs can also form so-called AW films on the surfaces in a tribological environment. These films have been studied for decades, but the mechanism of their formation remains a puzzle.;This thesis focuses on nanoscale studies on the films formed by ZDDPs and the novel method development of low-wear-rate measurements. The first part (Chapters 3 to 6) studies the formation mechanism of thermal films and antiwear (AW) films formed from ZDDP containing oil under various conditions and with different substrates. The second part (Chapter 7) is related to a novel method of low-wear-rate measurement, including method development and its applications.;The ZDDP thermal films (Chapter 3) and AW films (Chapters 4 and 5) formed on AISI 52100 steel surfaces under various conditions have been investigated by X-ray absorption near edge structure (XANES) spectroscopy and atomic force microscopy (AFM). Nanoindentation was used to probe the mechanical properties of these thermal and AW films. The thickness of the thermal films was measured by both XANES P K-edge and FIB/SEM methods and consistent data were obtained for thicker films (over 40 nm). Friction coefficients, mu, were monitored for most wear tests. It was found that mu is highly correlated to the wear scar width of the pins. The oil residue after thermal reactions or rubbing experiments was analyzed by 31-P nuclear magnetic resonance (31P NMR); the secondary alkyl ZDDP shows better antiwear effectiveness and lower thermal stability compared with the primary alkyl ZDDP.;The ZDDP AW films formed on AISI 1095 steel surfaces with various Vickers hardness values have also been investigated (Chapter 6). It was found that wear performance is highly correlated to the hardness value of the substrate, and the AW films formed on the various substrates show different indentation moduli although the chemical composition of these films are similar.;In the second part of the thesis (Chapter 7), a new method of measuring low-wear-rate relevant to engine wear has been established and the applications are described.;Gold implantation, Rutherford backscattering spectrometry (RBS) and neutron activation analysis (NAA) are the main techniques used in the method. The method has a depth-sensitivity range which depends on the energy of Au implantation. By controlling the Au implantation, the method can achieve a sensitivity in the order a few nm/hr. By applying this method, the effect of roughness orientation (referred to the sliding direction) effect on wear performance has been reported quantitatively for the first time.;Keywords. ZDDP, AW films, hardness, Low-wear-rate, polyphosphates, roughness orientation, XANES, AFM, FIB/SEM, 31P NMR, RBS, NAA, implantation... |
