Gas nitriding of chromium in NH3-H-2 atmosphere

Nitriding behaviour of pure chromium in an NH3-H-2 atmosphere is discussed. Chromium samples were nitrided for various times (1, 4, 24, 48, 72, 120 and 142 h) at 800 degreesC. The nitridation is accompanied by an increase in surface porosity, which leads to an enlarged surface brittleness. Vickers microhardness measurements along the direction of nitrogen diffusion show an overall increase in hardness as a result of the nitriding process resulting in a surface hardness as high as about 1200-1400 Vickers. X-ray diffraction studies reveal the formation of Cr2N, CrN and Cr(N) phases upon nitriding. Scanning electron microscopy and energy dispersive Xray analysis (EDAX) show an increasing fraction of the CrN phase formed at the surface with increasing nitriding times. EDAX line profile analysis of the specimen nitrided for 120 It shows a three-layered structure. Though the surface layer and the second layer adjacent to it, both consist of a mixture of CrN and Cr2N phases, the fraction of CrN is much higher in the surface layer. Nitridation of Cr2N is suggested as the possible mechanism for CrN formation at the surface with increasing nitriding times, for the nitriding parameters chosen in the present study. (C) 2002 Elsevier Science B.V. All rights reserved.status: publishe