The discovery of the spontaneous reaction of boric oxides with moisture in the air to form lubricious HBO films has led to great interest in the tribology of boron compounds in general. Despite this, a study of the growth kinetics of HBO on a BO substrate under controlled relative humidity (RH) has not yet been reported in the literature. Here, we describe the tribological properties of HBO-BO glass systems after aging under controlled RH over different lengths of time. A series of tribological tests has been performed applying a normal load of 15 N, at both room temperature and 100 °C in YUBASE 4 oil. In addition, the cause of HBO film failure under high-pressure and high-temperature conditions has been studied to find out whether the temperature, the tribostress, or both influence the removal of the lubricious film from the contact points. The following techniques were exploited: confocal Raman spectroscopy to characterize the structure and chemical nature of the glass systems, environmental scanning electron microscopy to examine the morphology of the HBO films developed, atomic force microscopy to monitor changes in roughness as a consequence of the air exposure, focused-ion-beam scanning electron microscopy to measure the average thickness of the HBO films grown over various times on BO glass substrates and to reveal the morphology of the sample in the vertical section, tribological tests to shed light on the system's lubricating properties, and finally small-area X-ray photoelectron spectroscopy to investigate the composition of the transfer film formed on the steel ball while tribotesting.
Thermally formed, additive-derived films are often a key precursor to tribologically protective films that are formed under the influence of mechanical stress. We have examined the tribological stability of model thermal films, tested in pure base oil under severe experimental conditions. Our aim was to exclude the self-healing mechanism of the protective film, which would be expected in a fully formulated lubricant. The thermal films were grown in the presence of various oil formulations containing combinations of ZnDTP and dispersants. The films were fully characterized by means of scanning electron microscopy and atomic force microscopy (for investigating the film morphology) in combination with X-ray photoelectron spectroscopy (for determining the chemical composition and thickness of the films). The mechanical stability of the films was evaluated via a series of tribological tests carried out applying two different contact pressures, 1.04 and 0.7 GPa, in base oil—a hydro-treated,\ud
heavy paraffinic fraction known as Yubase 4—at 100°C. The tribological tests were performed using a ball-on-disc set-up and run until the failure of the protective film occurred. It was found that boron-containing thermal films displayed a much higher mechanical stability than their boron-free counterparts. It was also shown that the presence of boron species (8.4–11.1 at.% B) in the thermal films seems to be able to limit the formation of wear particles and prolong the lifetime of the protective coatings.\ud
Furthermore, it appears that very thin, boron-based thermal films can exhibit higher durability than films that are thicker but boron-free
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