Crystal habit and crystal structure of fine chromium particles: An electron microscope and electron diffraction study of fine metallic particles prepared by evaporation in argon at low pressures (III)

Evidence for a Cr metastable phase as a tracer in DLI-MOCVD chromium hard coatings usable in high temperature environment

“…It was never reported in Cr coatings grown by MOCVD and DLI-MOCVD using chlorinated inhibitors [14,[17][18][19][22][23][24] even for deposition temperature as low as 573 K. When using sulfurcontaining inhibitors it was not mentioned by MOCVD [21] nor by DLI-MOCVD [24] (Table 2). It has been observed in this work by DLI-MOCVD using C 6 H 5 SH as inhibitor of carbon incorporation only for deposition temperature ≤723 K. This is in good agreement with the fact that beyond this critical temperature a structural phase transformation into bcc-Cr occurs, as reported by [25][26][27] and confirmed by in situ XRD analysis in this study (Fig. 4).…”

“…The arrows on XRD patterns of Fig. 4 show the complete transformation of the metastable ␦-Cr phase into the stable bcc-Cr one beyond 723 K, in good agreement with literature data [25][26][27]. Traces of chromium oxide appear above 823 K, mainly due to the residual oxygen partial pressure in the XRD analysis chamber.…”

“…In this two-phase multilayer coating, the average grain size of bcc-Cr is approximately 300 nm while it is only a few nanometers for ␦-Cr. In a previous work, It was reported that the particles of this metastable ␦-Cr phase did not exhibit a definite crystal shape and their average size was lower than 20 nm [26].…”

“…The processes leading to the growth of the metastable phase are rare. Evaporation and condensation of chromium in argon at low pressures [25][26][27] led to the formation of crystalline metastable ␦-Cr. The same authors reported the structural transformation of the metastable phase to the stable one above 723 K.…”

“…The first one corresponds to the stable bcc cubic structure with Im-3m (229) space group and 2.88 Å cell parameter (PDF 00-006-0694), noted bcc-Cr (or ␣-Cr). The second one is a primitive cubic metastable phase with Pm-3n (223) space group and 4.59 Å cell parameter (PDF 00-019-0323) [25,26], noted ␦-Cr.…”