Effect of Pulsed Laser Deposited Ceramic Coatings on Microhardness and Corrosion Behavior of Titanium, Ti6al4v and Inconel

Abstract

Coatings are used to modify and increase the functionality of a bulk surface or substrate without modifying the bulk properties of the material. The present work aims at obtaining uniform adhesive coatings of alumina and silicon carbide on different substrates viz., titanium, Ti6Al4V and inconel by pulsed laser deposition technique using Q-switched Nd: YAG laser at low temperature. Processing parameters such as laser fluence, substrate target distance, substrate temperature and target density during deposition were standardized to get adhesive films. Coated films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, spectrophotometer, optical microscope, nanoindentation, surface roughness measurements using 3D optical profilometer, adhesion test. Microhardness and corrosion studies were carried on substrates and after coating. Composite microhardness of ceramic coated substrates was measured using Knoop indenter and its film hardness was separated from composite hardness using a mathematical model based on modified area-law of mixture. Then by including indentation size effect the film hardness was compared with values obtained using nanoindentation method. Composite hardness as well as film hardness of the ceramic coating was found to be higher compared to the substrates. Corrosion behavior of substrates after ceramic coating was studied using 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The Nyquist and the Bode plots obtained from the electrochemical impedance spectroscopy data are fitted by appropriate equivalent circuits. The pore resistance, the charge transfer resistance, the coating capacitance and the double layer capacitance of the coatings were obtained from the equivalent circuit. Alumina coated substrates showed more corrosion resistance than silicon carbide coated substrates. After the corrosion testing, the surface topography of the uncoated and the coated system were examined under scanning electron microscopy. Experimental results confirmed the possibility of using Nd: YAG laser for ceramic film deposition which improves the microhardness and corrosion resistance of the substrate considerably