Surface preparation for characterization of nitride compound layers using hardness indentation and the Palmqvist method

The Palmqvist method is usually used as a measure to evaluate the toughness of ceramics and thin coatings, as these cannot be determined with classic test methods such as the tensile test or the Charpy impact test due to their brittleness or the small proportion of the coating in the specimen volume. Nitride compound layers, formed during nitriding and nitrocarburizing, are also brittle and only have a thickness of a few micrometers, so determining their toughness is also challenging. In this work, compound layers with different phase compositions and thicknesses were investigated on EN31CrMoV9 and EN42CrMo4 steels using the Palmqvist method. Barrel finishing and polishing were used for specimen preparation, for which a significant effect on the formation of Palmqvist cracks was found. Barrel finishing reduced the length of the Palmqvist cracks with increasing barrel finishing time until a constant level was reached. The surface condition caused by the porosity near the surface of the compound layer was identified as another influencing factor, thus the pore seam was removed for the further investigations. Various specimen variants were prepared and their phase compositions and compound layer thicknesses evaluated in relation to the crack lengths of the Palmqvist cracks. A correlation was found between the length of the Palmqvist cracks and the extent of the compound layer thickness.