On the damping behaviour of single and multi-layer coatings

Mechanical components with high stiffness, resistance and vibration damping specifications are tipically required in most aerospace and industrial mechanical applications. Composite materials such as multi layer materials can be properly designed and optimized to this aim. Most coating layer deposition technologies can be effectively used to increase the component dissipative behaviour, as it is also shown by some experimental results reported in this work. This result can be obtained by applying coating layers with high internal hysteresis or by maximizing frictional actions between the different layer interfaces. Different deposition techniques, such as the reactive plasma vapor deposition and the anodizing process, are considered in this work. Single-layer and multi-layer coatings are applied on metallic substrates. A theoretical model for multi-layered beams is proposed and critically discussed. Conventional beam theories theories, like the Timoshenko-Bernoulli beam theory, can not be employed to accurately describe the complex behavior of multi-layer beams and even higher order theories show some shortcomings when dealing with nonlinear contributions such as interface slipping and friction. A third order zig-zag layer-wise model approach is considered in this work, where the number of kinematic variables is not dependent on the number of layers considered in the model. The contribution of the frictional actions at the layer interfaces and of the viscoelastic behavior of the coating layer materials is considered. The increase of the damping behaviour of coated specimens can be obtained by properly designing the interface frictional actions at the modeling stage, by adopting a consistent coating deposition technology and then by experimentally validating it. Different material solutions are tested for both the substrate and the coating technology, i.e. aluminum alloy, structural steel and stainless steel for the substrate and metal, metal oxide and metal nitride for the coating layers. Dynamical measurement data in a wide range of excitation frequency are obtained from slender beam specimens by means of a dynamic mechanical analyzer, in a flexural forced excitation experimental set-up, with clamped sliding boundary conditions. Homogeneous, uncoated specimens are also tested and experimental measurements are used to compare the effectiveness of the different coating solutions. The complex material modulus is estimated from force and displacement experimental data taking into account of the contribution of the inertial actions. The material constitutive relationship is modeled by means of a high order generalized Kelvin model, and this results in non-trivial constitutive material equations. A robust identification procedure resulting from previous work of the authors of this paper is used to identify the optimal material model and its parameters, and to eliminate the non-physical model components. The ratio of imaginary and real part of the estimated complex modulus is considered as a measure estimate of the material dissipative behaviour. Results related to different technologies are presented and compared. Some engineering test cases are considered and critically discussed in this work.