The nitrided and coated surfaces were imaged

Fig. 2 shows microstructural measurements taken from a plasma nitrided M2 coupon. In Fig. 2(a), the surface topography (measured using AFM) is shown. The RMS roughness of this scanned surface is < 5 nm. A cross-sectioned sample showing the diffusion zone post nitriding is shown in Fig. 2(b). The acicular microstructure extends to a depth of ~ 30 μm with no observed PHA-680632 layer or grain boundary precipitation, typical of effectively nitrided HSS and consistent with a surface hardness of ~ 11 GPa (measured by nanoindentation). Fig. 2(c) shows the N1s XPS peak. The peak at ~ 397.4 eV can be attributed to FexN precipitate. The peak at ~ 400.5 eV can be attributed to CN [28]. The surface scan shows the presence of a CN nitride. However, after etching, the only visible peak associated with a nitride is that from FexN precipitate. Carbon present at the surface but absent at deeper etch levels is consistent with carbon diffusing to the stress free regions [12] and an effectively plasma nitrided M2 HSS substrate. X-ray diffractograms from heat treated coupons before and after nitriding are shown in Fig. 2(d). Peaks are indexed to the M6C and MC primary carbides and to body centred cubic αFe. Peaks attributable to a compound layer [29] were not observed, indicating that the process parameters resulted in ‘bright nitriding’. Whilst there is debate over whether a compound layer is detrimental within duplex coatings [7], [12], [13] and [30], the compound layer can decompose to produce a ‘black’ layer which reduces the bond strength between the coating and nitrided subsurface [8]. The peak associated with αFe shifts to a lower 2θ after nitriding, indicating an increased ‘d’ spacing which is consistent with a previous report [12] showing that nitriding increases the residual compressive stress in the diffusion zone. The αFe peak also broadens after nitriding. Similar broadening and peak shift have been observed previously and attributed to increases in uniform and non-uniform strain (precipitation of CrxN) [31] and [32].