Hydrocarbon Lubricants Can Control Hydrogen Embrittlement.

While it is well known that during RCF tests the formation of nascent catalytic sites on the wear track can break down hydrocarbon molecules to release atomic hydrogen, the potential of the hydrogen environment in fuel cells to hydrocrack the hydrocarbon lubricant in high pressure rolling contacts has so far been ignored. Here we investigate for the first time the ability of the hydrogen environment to generate a chemical tribofilm on the wear track most likely through lubricant hydrocracking, as compared with argon and air environments. Despite the ability of the hydrogen environment to generate a notably larger amount of atomic hydrogen, the chemical tribofilm significantly prevents the formation of atomic hydrogen and its subsequent diffusion through the lattice of steel rolling element bearings. This is of great importance in the lubrication of hydrogen technology and the prevention of Hydrogen embrittlement (HE). An investigation into the prospects of high energy micro-computed-tomography (Micro-CT) as a non-destructive technique for sub-surface damage characterisation in RCF was comparatively performed alongside traditional sectioning methods.

Formation of Surface Deposits on Steel and Titanium Aviation Fuel Tubes under Real Operating Conditions.

In this study, stainless steel and titanium (Ti) tubes obtained from a turbofan engine after the end of its lifetime were analyzed in order to compare the amount of pyrolytic coke present and its influence on the parent, base material. Various analytical techniques including microhardness and topographical evaluations, optical emission spectrometry (OES), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were applied. On steel surfaces, a thick pyrolytic coke deposition layer consisting of carbon and oxygen and also containing elements from the tube material, fuel, and fuel additives was found. The concentration of elements from the pyrolytic coke continuously decreased with distance from the surface of the deposit, while the concentrations of elements from the tube material continuously increased, with the concentrations of elements from the fuel and the fuel additives being relatively constant. With ultrasonic cleaning in distilled water, most of the deposits could be removed. Only carbon-rich patches with a thickness of more than 300 nm remained adhered to the surface and/or had diffused into the original material. On Ti surfaces, the thickness of the C-rich fuel deposit layer was significantly thinner as compared to that on the stainless steel; however, the surface was covered with an ∼3 µm-thick oxide layer, which consisted of elements from the fuel additives. It is believed that the beneficial properties of Ti covered with a thin layer of TiO2, such as low adhesion and/or surface energy, have promoted different deposition mechanisms compared to those of stainless steel and thus prevented pyrolytic coke deposition and the related material deterioration observed on stainless steel.