Hard damping coatings: an experimental procedure for extraction of damping characteristics and modulus of elasticity

Hard coatings can be used as damping treatments, when applied on the surface of a component (both internal and external surfaces). This can help to reduce the resonant vibration levels a component may undergo during operation. This paper discusses the experimental procedure developed for testing such materials at room and elevated temperatures. The procedure involves controlled tests for excitation level (frequency and amplitude) and temperature. The elevated temperature capability is achieved by a custom designed halogen-lamp furnace, which is attached to the test rig. The damping (internal friction) and the modulus of elasticity of the coating are extracted from response decays, where the amplitude-dependent behaviour is accounted for. The results shown here were obtained from a Nimonic C-263 beam specimen, coated with Rokide® A (aluminium oxide). The data is presented as a function of the maximum (peak) strain that the coating undergoes at the interface with the substrate. The paper also discusses the underlying damping mechanism: friction between the coatings lamellae (“splats”), which is supported by evidence from scanning electron microscope images of the cross-section of a sample.