Surface modification of constructional steels by irradiation with high intensity pulsed nitrogen plasma beams

This paper presents the results of the modification of two types of constructional steels induced by irradiation with multiple pulses of nitrogen plasma generated in two facilities: IBIS (at Swierk, Poland) and PROSVET (at Kharkov, Ukraine). The duration of pulses is in the microsecond range and the energy density attains a value of up to 20 J/cm2. The treatment leads to melting and solidification of a surface layer of a thickness of few micrometres, with simultaneous introduction of a significant concentration of nitrogen. Irradiation results in marked improvement of the wear properties. Several analytical techniques such as conversion electron Mössbauer spectroscopy (CEMS), conversion X-ray Mössbauer spectroscopy (CXMS), nuclear reaction analysis (NRA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to reveal the nature of this improvement. Possible mechanisms of modification are discussed. It is concluded that the improvement of the wear properties should be associated with the formation of austenitic fcc γN phase in which an Fe atom has an interstitial N atom as the nearest neighbour.