Single-molecule force spectroscopy is a powerful tool to experimentally probe the effect of mechanical force on individual bonds. With increasing mechanical stability of the target bond, however, this becomes exceedingly challenging, as mechanical activation of the mechanophore competes with rupture of the surface anchor. Here we investigate and systematically modify a functionalization protocol which is based on diamond-like carbon (DLC) surfaces. We perform dynamic force spectroscopy on heterobifunctional polyethylene glycol molecules which are covalently anchored to the functionalized DLC surfaces and analyze the changes in rupture forces, distances at which single rupture events occur, as well as the properties of unspecific adhesion caused by modifications in the functionalization protocol. Average rupture forces of 1.7 nN are achieved, and 5.7 % of the rupture events yield rupture forces above 3 nN. Both values are significantly higher than for the widely used silane-based surface anchors.