Transparent, amorphous and hard aluminum oxide films were deposited on glass at low temperatures (less than or equal to 773 K) by r.f. plasma-enhanced chemical vapor deposition with AlCl3, H-2 and CO2 as the reactants. The adhesion of films, a predominant factor in determining the performance and reliability of a coating-substrate system, evaluated by the critical load in scratch testing strongly depends on the deposition conditions (substrate temperature and CO2 flow rate) and compressive residual stress in the films measured by radii of curvature determinations. These films have shown a tensile cracking failure mode in scratch testing and-as opposed to the prediction based on energy balance and the critical shear stress criteria, which indicates that the critical load will vary inversely with internal stress, the compressive residual stresses increase the adherence strength by increasing the resistance to the tensile cracking failure. Hence, it is important to characterize the failure modes associated with the scratch adhesion test to account for the dependence of critical load on internal stresses in the films.