Experimental Evaluation and Modeling of the Damping Properties of Multi-Layer Coated Composites

Stefano Amadori, Giuseppe Catania, Angelo Casagrande
2018 Coatings  
In this work, the dissipative properties of different coating solutions are compared and a beam mechanical model, taking into account of dissipative actions at the interface between different layers is proposed. The aim is to find optimal coatings to be employed in the production of composites with high damping properties. The investigated coating layers are obtained from different materials and production processes, and are applied on different metallic substrates. The composite specimens, in
more » ... he form of slender beams, are tested by means of forced excitation dynamic measurements. Force and displacement experimental data, in a wide range of excitation frequencies, are used to estimate the system damping properties. Homogeneous, uncoated specimens are also tested for comparison. A specific identification procedure is used to identify the specimens stress-strain relationship in the frequency domain. The ratio of the imaginary part and the modulus of the specimen estimated complex frequency response function is considered as a measurement of the damping behaviour. A modified third order multi layered beam model, based on the zig-zag beam theory, is proposed. The model takes into account the contribution to the damping behaviour of the frictional actions and slipping at the interface between layers. Frictional actions are modelled by means of a complex, elasto-hysteretic contribution. Coatings 2018, 8, 53 2 of 20 aerospace, automotive and automation industrial mechanical applications, and some composite solutions are suitable to design components with these properties. In modern high-speed applications, unwanted vibrations may result due to high inertial forces. A vibrational response associated to a small displacement and deformation field but to a wide frequency range may cause an excessive noise level, a decrease in the system efficiency and a shortening of the system service life. By increasing the component dissipative properties, high vibration levels of the contact-free, external surface of thin walled mechanical components such as a mechanical pump, motor or gearing casing may be effectively damped. Single or multiple layers of a coating material can be deposited or grown in order to produce a finished composite component with specifically designed characteristics including vibrational damping capability. Many different techniques are known [13, 14] and in this work specimens obtained by means of the screen printing technology, mainly residual stress free, are compared. The coating material structure [15] [16] [17] , the interface structure [18], the temperature dependence [19] are all factors that must be taken into account when studying the influence of coatings on the coated component damping behaviour. The coated component dissipative properties can be significantly tailored by means of the application of coating layers showing high internal hysteresis or with high frictional actions at the interface between the different layers [15, 20, 21] . Experimental research done by these authors [22, 23] and other researchers [13, 24] outlined that the application of some coating surface solutions on thin-walled components can increase the vibration damping behaviour, and that this result is mainly due to dissipative actions originating at the interface between the substrate and the coating layer. It is known from literature that dynamic mechanical measurements are an effective experimental tool to study the damping behaviour of coated components and that both forced and free vibrations tests were employed to estimate the dissipative properties of a wide range of coating materials and component geometries by means of comparing the coated and uncoated component dynamical response [25] [26] [27] . In this work, the damping behaviour of different coating solutions applied on two different metal substrates, i.e., harmonic steel and Al alloy, are compared. Coated and uncoated specimen dynamic mechanical measurement test results are processed by means of a robust parameter identification and model condensation technique to investigate the effectiveness of the different solutions. A multi layered beam model, taking into account the frictional actions localized at the interface between the layers, is proposed to help virtually test and find optimal coating solutions, to be applied to a mechanical system casing and to thin-walled components to be used in the high speed automation industry, in order to damp vibration and noise generated in working conditions. Dissipative actions are modeled by relaxing the kinematical displacement continuity at the layer interface and by introducing complex elasto-hysteretic dynamical interface coupling. The effects on the dissipative properties of the distributed constraints modeling boundary conditions are also taken into account. The model is based on zig-zag multi-layer beam theories [28] [29] [30] , and on layer wise beam theories [31] . High order layer wise beam theories are obtained by modifying the classic Bernoulli-Euler and Timoshenko beam theories in order to deal with composite beams with numerous layers in which the mechanical and geometrical characteristics significantly vary from layer to layer. The advantage of zig-zag theories with respect to other layer wise theories is that the number of state space variables required by the model is low and does not depend on the number of layers. Since large residual stress free, amorphous based structure coatings, mainly deposited by screen printing technologies, are considered in this work, no account is given here with respect to experimental measuring and modeling of residual stresses generated at the interface between two different layers. Damping-Oriented Coating Solutions In this work three innovative, different coating solutions, applied to a slender beam, uniform rectangular cross section test specimen, are compared. Both homogeneous and composite specimens Coatings 2018, 8, 53 3 of 20 are taken into account. Two different types of metallic substrates are considered, harmonic steel (C67) and Al alloy (Al1000). Three ceramic coating solutions are proposed, i.e., an alkali activated geopolymer (GP), an alkali activated alumina powder mixture (APM), an alkali activated zirconia powder mixture (ZPM). Ceramic materials, in comparison to metals and polymers, may present superior mechanical, chemical and thermal resistance properties [32]. GP Solution The geopolymer solution is made by mixing of a commercial metakaolin powder (base) with an aqueous basis binder (activator) prepared from a potassium-hydroxide solution in H 2 O with pyrogenic silica solution. The chemical composition of the resulting solution is reported in Table 1 . Potassium was preferred to sodium in the alkaline activator since a better degree of polycondensation can be achieved and because of its ability to provide geopolymeric structures, associated with high mechanical strength [33] . Geopolymers are inorganic polymers formed by linear chains or tridimensional arrays of SiO 4 and AlO 4 tetrahedra [34] . The geopolymer is produced by mechanical mixing (planetary centrifugal mixer "Thiky Mixer" ARE 500 by THINKY, Tokyo, Japan) of a reactive powder base (Metakaolin Argical M 1200S, IMERYS, Cornwall, UK) with an aqueous basis activator (H 2 O solution with potassium-hydroxide of 85% purity and an addition of a 99.8% purity pyrogenic silica solution). The prepared geopolymer has a 2.83 Si/Al ratio composition [34] and is applied to the upper and lower surfaces of the substrate beam by means of screen printing.
doi:10.3390/coatings8020053 fatcat:64xmr7cwjze7thyqffid2smzcq