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Optimization of Microfabricated 2D Planar Spiral Microcoils for the Micro NDT of Grinding Burn
(2020)
The approach of using current transients to model the nucleation rate as reported in the seminal work of Scharifker and Mostany is limited to electrodeposition system without bath hydrodynamics (BHD). Therefore, in this work in situ electroanalytical approach is proposed to unveil the influence of BHD on the nucleation kinetics of electrochemically deposited Nickel-Cobalt alloy system (eNiCo). Using the Hydrodynamic linear sweep voltammetry (HLSV) technique, the limiting current density as a function of BHD is computed, wherein it increased (for eNiCo alloys) from 186 mA/cm² to 222.6 mA/cm² on increasing BHD from 0 to 42 cm/s, respectively. Consecutively, the diffusion layer thickness is found to decrease from 19 µm to ca. 15.8 µm on increasing BHD from 0 to 42 cm/s, respectively. Additionally, from Nyquist plots recorded using the Galvanostatic Electrochemical Impedance Spectroscopy (GEIS), the charge transfer coefficient (Rct), exchange current density (io) and double layer capacitance (Cdl) as a function of BHD is computed. It is found that Rct decreased and io, Cdl increased as function of BHD. Thereby, indicating the enhancement in the charge transfer on the cathode surface and reduction in the thickness of the diffusion layer. Hence, with the use of BHD, it is possible to control the growth kinetics, therefore enabling the deposition of tailor-made materials possessing specific required properties.
Microfabricated 2D inductive eddy-current transducers operating in a reflection differential transmitter-receiver mode are presented for the micro nondestructive detection of micro grinding burn. 2D spiral circular microcoils are employed as excitation coils, while an innovatively conceptualized “interconnected split-D” type differential microcoil is used as a sensing coil. Finite element modelling using COMSOL revealed the efficacy of proposed concept in non-destructive testing of small grinding burn having a width of 100 µm. The induced sensing coil voltage changed as a function of presence of grinding burn, with successful recording of the signal for the investigated lift-off range of 250 µm - 1000 µm for 100 kHz to 1 MHz driving frequencies of excitation coil. Experimental validation showed a 94% increase in the induced voltage of the sensing coil in presence of grinding burn on increasing the driving frequency of excitation coil from 100 kHz to 1 MHz. Thereby, revealing the superficial nature of the grinding burn defect, and showing the efficacy of the proposed concept for the non-destructive testing of grinding burn.