Hardness Enhancement Using Plasma Nitriding SKD11 Die Steel in an Atmospheric Pressure Plasma Jet

Metals are typically altered for varying the color appearance and improving the physical properties of surface hardness, resistance of wear and abrasion, and anticorrosion. We employed an atmospheric pressure plasma jet (APPJ) to increase the surface hardness of SKD11 die steel. The H2/N2 mixture gas was used as the nitriding gas for plasma nitriding process. Materials characteristics by Vickers hardness tester, optical microscope, and X-ray diffractometer (XRD) were employed. The optical emission spectroscopy (OES) was used to detect the optical emission of plasma species from atmospheric pressure plasma jet. The pristine SKD11 die steel represented a hardness of $558\pm 5.9\text{HV}_{0.3}$, while plasma nitriding process apparently increased to reach the value around $1095\pm 15.5$ HV0.3. The hardness enhancement was believed to be contributed from the self-growth of the nitride layer and diffusion layer with a total thickness of $45\ \mu \mathrm{m}$ above the matrix within 17 min analyzed by optical microscope. We also proposed that the accumulated heat induced by plasma bombardment under H2/N2 mixture gas in an APPJ system was obtained, in which give a substrate temperature around 500°C detected by the thermal couple offering suitable nitriding temperature. Additionally, from OES observation, the NH radicals were the key species to control the hardening mechanism of SKD11 die steel, as well as the exposure time and substrate temperatures. Based on the thermochemical case hardening process using H2/N2-APPJ system, the improved surface hardness can be used to promote high resistance to wear, scuffing, galling and seizure. Plasma nitriding process is implemented in typical applications in the manufacturing industries such as gears, valve parts, pressure-die-casting tools, forging dies, moulds, and engine parts. We also believed that plasma nitriding using H2/N2 mixture gas in an APPJ system is a modern thermochemical treatment for offering short time operation on the nitridation of steel objects.