STRUCTURE-ANALYSIS OF PLASMA-NITRIDED PURE IRON

Pure iron, plasma nitrided in low pressure triode ion plating equipmen t, has been investigated by optical reflection microscopy, scanning tr ansmission electron microscopy and glancing-angle X-ray diffraction, u sing conventional and synchrotron radiation. The total thickness of th e compound layer does not obey the square-root time dependence, as a r esult of the continuous Ar + N ion bombardment during the nitriding pr ocess. The grain size is also reduced in the compound layer. In additi on to the variation in thickness of the nitride layers with increasing process time, a change in the nitrogen concentration at the top of th e compound zone also is observed. Increasing the nitriding time and/or the N-2 partial pressure in the nitriding apparatus results in a larg er nitrogen concentration in the top layer of the epsilon phase. Also, a sin(2) psi method analysis has been carried out to measure the resi dual macroscopic stresses in the different nitride layers of the compo und zone.