| Contrary to the widely accepted idea that diamond-like carbon (DLC) is inert, this thesis provides evidence that DLC readily reacts with its surrounding environments. Reactions occurring at the surface of DLC strongly affect its surface composition and structure, and consequently, its friction and wear behavior. However, experimental results presented in this thesis show that reactivity of DLC is neither limited to, nor exclusively dictated by the surface. The bulk can both influence the reactivity of DLC, and be altered by it.;The ultra-low friction behavior of DLC consists of three main components: the run-in behavior, transfer film formation, and the ultra-low steady friction. Our results prove that the cause of the run in behavior in dry environments is the removal of oxide layers of DLC. These oxide layers do not only have higher oxygen content, but are also richer in sp2 graphitic rings compared to the bulk of DLC. This structure reduces the wear resistance of the oxide layers, causing their removal and the high friction at the beginning of sliding in inert environments. In the presence of multi adsorbed water layers that can act as an electrolyte, this sp2 rich oxide protects DLC from wear when slid against more anodic surfaces. Although the oxide layer is just few nanometers thick and more susceptible to wear, they can induce galvanic corrosion on much harder counter surfaces protecting DLC during sliding in high humidity environments.;Our results also indicate that the transfer film, another component often thought to be critical for the ultra-low friction of DLC, is a byproduct of internal stresses of DLC film, rather than requirement to achieve ultra-low friction. Mitigation of internal stresses of DLC improved its wear resistance and suppressed transfer film formation from DLC to the counter-surface. Our results also suggest that these internal stresses play a critical role in the susceptibility of the bulk to react with vapor molecules. Modifying DLC surface using vapor-phase reactions is often accompanied by reaction of the subsurface layers which negatively impacts the integrity of DLC network and its mechanical properties.;Oxidation is detrimental to the ultra-low steady state friction, the third component of the friction behavior of DLC in inert environment. A chemical modification method using vaporphase reaction with silanes to increase the hydrophobicity of DLC is developed and tested. Silane-treated DLC exhibits low friction in humid environments containing up to 30% water partial pressure. However, the silane treatment compromised the bulk properties of DLC reducing its wear resistance at water partial pressures of 40% and higher. |
