Microindentation fatigue tests on submicron carbon films

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Microindentation fatigue tests have been conducted on submicron coatings using a microindenter. The cyclic indentation was implemented by servo controlling the motion of the indenter so that the loadcell output followed a 0.1 Hz sinusoidal loading pattern. A conical diamond indenter with a nominal 1 μm tip radius was used. A model system composed of two 0.11 μm sputtered carbon films was chosen for investigating the indentation fatigue effects on brittle coatings. Both carbon films were deposited on silicon substrates but with substantially different interfacial adhesion strengths. For the carbon film with poor adhesion tested at the condition of a 4 mN maximum load, an annular coating buckling pattern concentric with the center of the indent appeared during the first loading stage whereas coating spallation occurred at loading cycle numbers ranging from 50 to 140. In contrast to this, under the same test condition, the carbon coating with good adhesion did not show any noticeable failure up to 500 cycles where the experiments were truncated. When the maximum load was increased, fine cracks initiated in the carbon coating and along the circular indent boundaries but the coating still adhered well to the substrate. These fine cracks continued to propagate during subsequent indentations, and eventually spallation of the carbon coating occurred. Because the microindenter has the advantage of continuously monitoring both the applied load and penetration depth during an entire test, the evolution of the fatigue failure of a coating system can be clearly visualized from the corresponding load vs. depth curve. A typical brittle failure event is manifested by several repeatable loading-unloading cycles with a gradual depth increase after the first indentation, then a couple of consecutive abrupt changes in the applied load as well as in the depth followed by stabilized indentation cycles.