4. Results and discussion
4.1. Phase and microstructure of the coating
Experiments were carried out initially at 1100 °C using steel 45# as substrate. The specimen obtained after coating at this temperature for 4 h had a quite smooth surface with a bright and shining metallic appearance, and the specimen weight gain was 11.4 mg/cm2, indicating that a surface layer was produced. No entrapped pack powder particles were present in the as-coated surface.
A simple comparison of XRD pattern measured directly from the as-coated surface with that from Cr2N powder confirmed that the phase of the outmost surface layer was Cr2N (Fig. 2). But, it can be noted that its reflection peaks were shifted slightly to the right relative to those of Cr2N powder, indicating that the chemical composition of the Cr2N coating layer differed slightly from that of the pure Cr2N. The EDS results to be presented later will show that the Cr2N coating layer contained a small amount of Fe, and a small amount of carbon may also be present in it. It is also evident that the Cr2N coating layer resulted from a highly oriented growth with the (002) reflection showing the highest intensity. For a non-textured Cr2N coating layer, the highest peak intensity is expected to be from the (111) reflection as shown by the XRD pattern measured for Cr2N powder (Fig. 2), but, it is one of the weakest among all the reflection peaks in the XRD pattern measured from the Cr2N coating layer. This oriented growth feature for the Cr2N coating layer produced in the pack process differs from that of the Cr2N coating formed in magnetron sputtering process in which it grew predominantly to have a (111) orientation [3]. Whether this difference in texture is mainly due to the difference in the steel grades used as substrates by different investigators is not clear at present and requires further investigation.