Effects of IR Laser Shots on the Surface Hardness and Electrical Resistivity of High-Purity Iron

Annealed specimens of 99.99% pure iron were irradiated with 500, 750, 1000, and 1250 Nd:YAG laser shots. The laser fluence and laser intensity at the laser irradiation spot on the target surface were 4.4 × 103 J/cm2 and 4.8 × 1011 W/cm2, respectively. Vickers hardness of irradiated specimens was measured at various points separated by 0.5 mm in four different mutually perpendicular directions around the laser irradiation spot. The surface hardness profile for each irradiated specimen shows an increasing trend in surface hardness till a distance of 3.5 mm from the reference point. The average surface hardness (ASH) is found to increase up to 21% and electrical resistivity increases up to 50% as the number of laser shots is increased to 1250. A linear relationship between electrical resistivity and ASH is observed. Moreover, the ASH follows the well-known Hall-Petch relation, indicating that the crystallite boundaries impede the motion of dislocations to a greater extent as the crystallite size gets smaller.