Electrochemical Impedance Characterization of Ultra-Thin Carbon Overcoats

Abstract

Ultra-thin carbon overcoats deposited on Al2O3-TiC (AlTiC) substrates were characterized using electrochemical impedance spectroscopy. By depositing diamond-like carbon (DLC) directly on AlTiC, the polarization resistance increased to 10^7 ohm-cm2 from ~10^5 ohm-cm2 for bare AlTiC. Two adhesion layer materials were studied: Si and Si3N4. The polarization resistance of DLC/Si3N4 films was 100 times higher than that of DLC/Si. The impedance, wear and magnetic test results indicated that Si3N4 is a more suitable seed material. Comparing films with the same DLC thickness, the Si3N4 layer thickness showed no influence on the resistance. For the same Si3N4 thickness, a thicker DLC layer yielded slightly higher pore resistance. This study reveals that the thinner carbon overcoats (DLC/Si3N4 thickness = 8/12 or 12/8 Aå) provided equal, if not better, corrosion protection than the thicker films. The advantage of a thinner carbon overcoat is that it facilitates an improvement in sensitivity of the magnetic sensor.