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This article reports the structure and hardness responses of TiN/CrN superlattice coatings to elevated-temperature annealing. Polycrystalline TiN/CrN superlattices with bilayer periods of 5.6–39 nm were deposited on a Ni-base alloy substrate by reactive unbalanced magnetron sputtering. The superlattices were subsequently annealed in vacuum at elevated temperatures for 2–100 h, followed by characterization using small-angle x-ray reflection, high-angle x-ray diffraction, and hardness testing. The superlattices can sustain their hardness up to 650 °C for 4 h or 575 °C for 100 h. A strong correlation exists between the hardness and the x-ray reflection or x-ray diffraction characteristics. A marked reduction in the high-angle satellite/Bragg peak ratio or in the small-angle reflection intensity corresponds to a rapid decrease in the hardness. This phenomenon is related to a physical transition during which the loss of hardness is caused by the structural instability resulting from accelerated interdiffusion between TiN and CrN layers, which leads to reduced compositional modulation amplitude and diffuse layer interfaces. © 2003 American Vacuum Society.