Effects of shallow plasma nitriding on the surface topography of gray cast iron specimens

Abstract A concept of plasma treatment for enhancing pre-textured surfaces of components made in pearlitic gray cast iron is proposed. It consists of a plasma nitriding at low temperatures and short times, to increase the hardness of the surface employing a shallow depth nitride formation in the surface of the iron. As a result, the roughness pattern of a previously textured surface is preserved after the treatment. A series of plasma nitriding experiments was run on specimens produced from gray cast iron. Flat specimens, obtained from blanks extracted from the cylinder wall of an engine cylinder block, were textured in laboratory to emulate a typical roughness distribution of an ordinary cylinder bore plateau honing process. e‑iron nitride (Fe3N) formation was identified via X-ray diffraction analysis. Also, the nitrogen concentration profile was quantitatively evaluated using wavelength-dispersive (X-ray) spectroscopy microanalysis, substantiating the nitride formation at shallow depth. Microhardness measurements indicated that e‑iron nitride and solid solution hardening at shallow depth increased the hardness of the textured surface. 3-D interferometry roughness measurements were performed before and after the plasma treatment to assess the topographic stability of the textured specimens. The results demonstrated that the roughness texture pattern was satisfactorily preserved after the treatment, which was supported by direct observation of the surfaces via scanning electron microscopy.