Role of X-Ray Diffraction Analysis in Surface Engineering: Investigation of Microstructure of Nitrided Iron and Steels

X-ray diffraction analysis can be very usefully applied in the field of surface engineering, since a depth of a few micrometres only is usually probed. This paper provides an overview of current X-ray diffraction methods for the analysis of surface layers. The treatment is illustrated by examples taken from recent work on nitrided iron and steels. Distinct ways to characterize the effective depth and layer thickness probed are presented. Composition-depth profiles can be measured accurately by tracing the lattice parameter (taking into account the possible presence of stresses) as a function of depth below the surface by successive sublayer removals. A correction for the penetration effect is required and methods for that are presented and demonstrated. The basis of the determination of (macro) stress (the sin²ψ method) is indicated. Attention is paid to the determination of the strain free lattice spacing and (diffraction) elastic constants, to the effect of a concentration-depth profile, and to the comparison of theoretically predicted and experimentally determined strain values. A stress (profile) in a surface layer often originates from the thermal history (cooling after treatment) and the presence of a concentration profile, as is illustrated by examples. Further, the analysis of X-ray diffraction line broadening is discussed. It is shown that nowadays an easily applicable method exists for line profile analysis (the single line Voigt method), allowing the determination of crystallite size and microstrain parameters. As demonstrated by the examples, knowledge of in particular microstrain in conjunction with macrostress is of importance in understanding and predicting material behaviour.