Surface structure, composition and temperature-dependent surface free energy of hard thin films for plastic micro-molding

For state-of-the-art hard thin films deposited by various vapor deposition techniques to alter mold filling and facilitate demolding of plastic parts, relevant properties are often not explicitly reported, leaving toolmakers and plastic processors reliant on third-party expertise. The purpose of this study was to characterize the geometrical and physico-chemical properties of hard thin films, which directly influence plastic-mold interfacial phenomena, and to compare these properties with those of steel X40Cr14. Phase structure was analyzed by XRD and Raman spectroscopy, the surface morphology, film thickness, and texture were assessed by scanning electron microscopy and interferometry. These properties are primarily affected by the deposition technique and processing conditions. The elemental composition was determined by energy-dispersive X-ray spectroscopy. Compared to the coating bulk, the near-surface region exhibited material- and deposition-dependent oxygen contents, indicating surface oxidation. Surface free energy under typical micro-molding temperatures (60 – 120 °C) was determined from wetting experiments. Hard thin films exhibited more stable wettability than steel, attributed to their higher surface homogeneity. Among the transition metal nitrides, CrN showed the lowest total and polar surface energy, while TiN exhibited the highest. Amorphous hydrogenated carbon showed considerably higher values. Surface free energy decreased significantly with increasing temperature, with binary high-impulse magnetron sputtered nitride films showing the strongest inverse correlation due to smooth surfaces and chemical composition. The findings highlight the wide range of thin film properties affecting interfacial phenomena, guiding material and deposition technique selection for plastic micro-molding and informing future research in hard thin films, tool making, and micro-molding.