Figure 1a shows the microstructure of the sintered NCF composite. EDS analysis revealed that the gray area is the NiAl phase, the black region is Cr and Mo phases, while the white area is the fluoride phase. Fluoride was observed to distribute uniformly at the grain boundary. In terms of microstructure, examination of the sample by high-magnification SEM revealed that the sample was etched with HF+H2O2+H2O solution (1:4:5, in volume). Figure 1b shows that the black area at the grain boundary is in the bonding phase, which was found to consist of the Cr-rich and Mo-rich alloy phases by EDS analysis.
Fig. 1 Scanning electronic microscopy (SEM) image showing the microstructure of the sintered NiAl–Cr–Mo–CaF2 (NCF) composite
The XRD results in Fig. 2 indicate that small amounts of intermetallic compounds containing CrAl and MoAl are present; these can be attributed to the solid-state reaction at high temperatures during the fabrication process. There are no reactants between the NiAl (or Cr, Mo) and CaF2 detected after the hot-sintering process, indicating that the
NiAl–Cr–Mo–CaF2composite can be employed in a high-temperature environment requiring thermal stability. Moreover, after wearing at 600℃, it can be observed from
Fig. 2 that NiO, CaCrO4, and CaMoO4appear in the form of weak peaks on the worn surface due to oxidation of NiAl and CaF2, respectively. Above 800℃, the CaCrO4 and CaMoO4 peaks get stronger but those of CaF2 disappear from the XRD patterns.
Fig. 2 X-ray photoelectron spectroscopy (XRD) patterns of the sintered sample (a) and worn surfaces of the NCF composite after tests at 600 ℃ (b), 800 ℃ (c), and 1000℃ (d)
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