Tribochemistry of monodispersed ZDOL with hydrogenated carbon overcoats

Tribochemical studies of the lubricant molecular weight effect on the tribology of the head/disk interface were conducted using hydrogenated $(CHx)$ carbon disks coated with ZDOL lubricant. The studies involved drag tests with uncoated and carbon-coated $Al2O3–TiC$ sliders and thermal desorption experiments in an ultrahigh vacuum tribochamber. The studies showed that the lubricant interaction with the carbon overcoat varies as a function of lubricant molecular weight. The friction coefficient increases as the molecular weight increases. The higher friction is due to the higher viscosity. The friction and catalytic decomposition mechanisms of ZDOL are described. In general, the perfluoropolyethers polymers are decomposed by chain scission involving the breakage of the backbone bonds to yield free-radical segments. Chain scission can occur by three mechanisms: (1) random degradation, (2) depolymerization, and (3) weak-link degradation. Our studies further support previous observations that catalytic reactions occurred at the endgroups. The lower number of endgroups for ZDOL with higher molecular weight reduces the possibility of the occurrence of catalytic reactions. Moreover, the ZDOL desorbed peak temperatures shifted to higher temperatures with increasing molecular weight in thermal desorption tests. The spreading diffusion coefficient of ZDOL decreases with increasing molecular weight. As the mobility of the lubricant chain decreases, the desorption energy needed to break the lubricants increases, resulting in higher desorption peak temperatures.