Investigation of mechanical and tribological properties of Sleipner cold work tool steel borided using different laser parameters

This study comprehensively investigated the effects of different processing parameters on the tribological properties of Sleipner cold work tool steel subjected to boron coating using laser welding technology. The research evaluated the performance of boron coatings using two different laser power levels (1500 W and 1800 W) and two different oscillation frequencies (20 Hz and 40 Hz). Wear tests were conducted under dry ambient conditions at two different temperature levels: room temperature (25 °C) and high temperature (450 °C). XRD analyses revealed that FeB and Fe₂B boride phases were successfully formed under all laser coating conditions. Optical microscopy examinations showed that more homogeneous coating structures were obtained at 1500 W laser power. In microhardness measurements, the highest value was obtained as 988.9 HV under conditions of 1500 W power and 20 Hz oscillation frequency, representing a 192% increase compared to untreated Sleipner steel (338.7 HV). Heat treatment application provided a 107% increase with 700.5 HV. Tribological tests were conducted under 10 N and 20 N loads using ball-on-disc and pin-on-disc configurations. Significant improvements in friction coefficient were observed for boron coatings under room temperature conditions. While the untreated (UT) specimen exhibited the highest friction coefficient values (~0.70 μ at 10 N, ~0.62 μ at 20 N), boron-coated specimens showed friction coefficients reduced to the 0.30-0.48 μ range. The lowest specific wear under room conditions was measured as 1.95 x 10-6 mm3/N·m for the 1500-20 specimen under 20 N load. However, under high temperature (450 °C) conditions, the performance of boron-coated specimens significantly decreased, with the most resistant structure observed in the heat-treated (HT) specimen at 1.62 x 10-6 mm3/N·m under 20 N. Among boron-coated specimens, the best performance was achieved in the 1800-40 specimen at 9.50 x 10-6 mm3/N·m under 20 N. SEM and EDS analyses revealed that the WC-Co ball maintained its structural integrity under room conditions, but experienced structural degradation at high temperature (450 °C) due to cobalt phase oxidation and tungsten transfer. In wear mechanisms, adhesive wear and tribosurface formation were dominant under room conditions, while intensive oxidation was observed at high temperature. In conclusion, laser-assisted boron coating can be successfully applied as an effective surface hardening method at room temperature, but alternative approaches should be considered for high-temperature applications.