Characterization of Dynamic Elastic Modulus and Damping Property of CNx Coating Material by Experimental Modal Analysis and Finite Element Approach

Abstract: Free-layer hard or soft coating material can be used for enhancing the inherent damping capacity (energy dissipation ability) of a structure under cyclic bending deformation. This may help to attenuate the vibration amplitude at the resonance frequency. In this study, dynamic mechanical and damping properties of a carbon based (CNx) coating material have been investigated. For determining the material properties of this coating, two samples (600 μm and 800 μm thick carbon nitride (CNx) film layers) were produced and deposited onto two internal turning tools by using the plasma enhanced chemical vapor deposition (PECVD) process. The deposition process was conducted at the room temperature with the magnetron sputtering of a copper and a subsequent graphite target plate in a highly ionized plasma and reactive environment of Ar, N2 and C2H2 gases. Eigen frequencies and system loss factors of the uncoated and coated tools were extracted, for the first two fundamental bending modes (mode X and mode Y), from the ‘drive point’ measurements of free hanging impact tests at the free-free boundary condition. Modulus of elasticity and loss factor of the coating material has been deduced through the comparison between the eigen frequencies and resonance amplitudes of the identical bending modes extracted from the experimental and analytical frequency response functions. The results obtained from the experimental modal analyses and the iterative finite element analyses show that, compared to the substrate, the flexural stiffness and the damping capacity of the coated tools have increased notably. The resonant frequencies of the coated samples have been shifted to the higher frequency levels, and the frequency response acceleration amplitudes have been attenuated dramatically. Elastic modulus and loss factor range of the coating material have been found to be in the range of 32.5 GPa to 49.1 GPa and 0.004 to 0.0245 respectively. Comparison between the analytical frequency response functions of the CNx coating material and 3M-112 viscoelastic material coated samples (for 800 μm film thickness) has anticipated that the coating material has higher loss modulus (energy dissipation ability) as opposed to the viscoelastic material. Scanning electron microscope images of the cross-section of a coated sample have revealed that the frictional energy losses between the interfaces of the carbon-nitride columnar micro-structures dominate the inherent damping mechanism of the coating material. Voids and porosities, present between the columnar clusters, further increase the energy dissipation ability of the coating material by enhancing the interface slippage mechanism during the cyclic bending deformation.