Disk Lubricant Additives,A20H and C2: Characteristics and Chemistry in the Disk Environment

Disk lubricant additives A20H and C2 are Fomblin Z type perfluoropolyether with the hydroxyl end-group, –O–CF₂–CH₂–OH, at one end, and the cyclo-triphosphazene end-group, R₅(PN)₃–O–, at the other end. Here, R is an m-trifluoromethyl-phenoxy group for A20H and a trifluoroethoxy group for C2. These additives were examined for miscibility with benzene, spin-off rate, water contact angle, and the diffusion rate over the carbon overcoat. It is revealed that A20H adheres to the carbon overcoat spontaneously. The attractive interaction arises from the charge–transfer type interaction between the aromatic rings of the phosphazene end and the graphitic regime of the carbon overcoat. No spontaneous adherence occurs between the lubricant C2 and the carbon overcoat. A TOF-SIMS study of disks coated with A20H and C2, respectively, with and without subsequent curing by short-UV (185 nm) was performed. It is revealed: (1) if presented with a low energy electron, the phenoxy groups of A20H readily undergo the dissociative electron capture, while the trifluoroethoxy group does not, and (2) photoelectrons generated by short-UV have little kinetic energy and the electron capture occurs only if an electrophilic molecular sector is in intimate contact with the carbon. Thus, in the case of disks coated with A20H, UV-curing results in detachment of a phenoxy group in contact with the carbon, generation of a radical center at the phosphorus atom and subsequent formation of a bona fide chemical bond between the phosphor and the carbon overcoat. No reaction of consequence occurs when disks coated with C2 are irradiated with short-UV.     

Effects of Nano-TiO2 Additive in Oil-in-Water Lubricant on Contact Angle and Antiscratch Behavior

Contact angle and scratch tests have been conducted to investigate the effects of nano-TiO₂ additive in oil-in-water (O/W) lubricant. The results show that the contact angle between high-speed steel with oxide scale and 1% (oil concentration) O/W lubricant decreases first and then increases as the concentration of nano-TiO₂ particle in the O/W lubricant increases. The smallest contact angle is obtained after an addition of 4% nano-TiO₂ additive to the O/W lubricant. This is because the nano-TiO₂ can enhance the surface excess of the oil when the nano-TiO₂ particles distribute throughout the surface of the oil droplets, and after saturation they can distribute throughout the water and also improve the surface excess of the water in the O/W lubricant. The scratch and hot rolling tests show that the nano-TiO₂ particles in the O/W lubricant can also reduce friction, improve scratch resistance, and reduce rolling force. A method for measuring the adhesion force of the oxide scale is proposed and the effect of nanoparticles is discussed. It is demonstrated that the effect of self-lubrication of nanoparticles in the O/W lubricant plays a more significant role in the tribological behavior during hot rolling than wettability.     

End-grafted Sugar Chains as Aqueous Lubricant Additives: Synthesis and Macrotribological Tests of Poly(<Emphasis Type="SmallCaps">l</Emphasis>-lysine)-<Emphasis Type="Italic">graft</Emphasis>-Dextran (PLL-<Emphasis Type="Italic">g</Emphasis>-dex) Copolymers

Comb-like graft copolymers with carbohydrate side chains have been developed as aqueous lubricant additives for oxide-based tribosystems, in an attempt to mimic biological lubrication systems, whose surfaces are known to be covered with sugar-rich layers. As adopted in the previous studies of the graft copolymer poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), which showed both excellent lubricating and antifouling properties, a similar approach was chosen to graft dextran chains onto the same backbone, thus generating PLL-g-dex. PLL-g-dex copolymers readily adsorb from aqueous solution onto negatively charged oxide surfaces. Tribological characterization at the macroscopic scale, either under pure sliding conditions or a mixed sliding/rolling contact regime, shows that PLL-g-dex is very effective for the lubrication of oxide-based tribosystems. The relative lubricating capabilities of PLL-g-dex copolymers compared with PLL-g-PEG copolymers were observed to be highly dependent on the molecular structure of the copolymers (in particular, side-chain density along the backbone) and the measurement conditions (in particular, time between tribocontacts); the PLL-g-dex copolymers with a low degree of grafted side chains (≤20% grafting of available protonated primary amine groups along the backbone) showed better lubricating performance than their PLL-g-PEG counterparts at high tribocontact frequency (≥ca. 0.32 Hz).