Stress-dependent adhesion and sliding-induced nanoscale wear of diamond-like carbon studied using in situ TEM nanoindentation

We present the first in situ transmission electron microscope (TEM) study of extended sliding of diamond-like carbon (DLC) on diamond. We show how the tribological properties of amorphous carbon (a-C) reach their limit by exploring the onset of wear for the harsh condition of sliding against diamond in vacuum. A particularly hard, hydrogen-free member of the DLC family, a-C is of wide technological interest, but the mechanisms by which damage occurs during tribological contact are not well understood. Real-time TEM imaging of the nanoscale contact area provides insight into wear mechanisms and damage. We show that at high normal stresses with full sliding contributing to additional shear stress, the initially low adhesion of the a-C-diamond interface rapidly increases at a rate greater than is seen in similar silicon-diamond contact studies. We propose that high contact stresses wear away a relatively low surface energy sp²-rich layer on the outer a-C surface, exposing sp³-rich sub-layers which can form covalent bonds that require more tensile force to be broken, leading to higher adhesion and an acceleration of wear.