High-temperature Corrosion of AlCrTiSiN Film in Ar-1%SO 2 Gas
AlCrSiN film with a composition of 29.1Al-17.1Cr-2.1Si-51.7N in at. % was deposited on a steel substrate by cathodic arc ion plating at a thickness of 1.8 μm. It consisted of nanocrystalline hcp-AlN and fcc-CrN, where a small amount of Si was dissolved. Corrosion tests were carried out at 800 ◦C for 5–200 h in Ar-1%SO2 gas. The major corrosion reaction was oxidation owing to the high oxygen affinity of Al and Cr in the film. The formed oxide scale consisted primarily of (Al,Cr)2O3, within which Fe, Si, and S were dissolved. Even after corrosion for 200 h, the thickness of the scale was about 0.7–1.2 μm, indicating that the film had good corrosion resistance in the SO2-containing atmosphere.
Characteristics of (Cr1-x, Alx)N Films Prepared by Pulsed Laser Deposition
Chromium aluminum nitride ((Cr1-x, Alx)N) films have been successfully prepared by pulsed laser deposition (PLD). Experiments were carried out by changing the surface area ratio of the target (Al/(Cr+Al)) from 0 to 100%. From the results of energy-dispersive X-ray spectroscopy (EDS) and Rutherford backscattering spectroscopy (RBS), we have found that the metal content ratio of the film agreed well with the surface area ratio of the target. The hardness of the (Cr1-x, Alx)N films showed the maximum value (HV∼3000) at x=0.75. Phase transition from the B1 (NaCl) structure to the B4 (würtzite) structure occurred above x=0.75, which was in reasonable agreement with the theoretical prediction. The result of the oxidation test revealed that the CrN film was oxidized at approximately 600°C, whereas in the (Cr0.5, Al0.5)N film, the existence of oxide was not detected in the annealing temperature range of RT to 900°C.
Application of Band Parameters to Materials Design
Availability of the two band parameters to materials design is reviewed. In order to express the chemical character between two different atoms in solid state, the concept of two band parameters for any bond between two different atoms is proposed on the basis of the bond orbital model and the empirical relation between observed band gap and pseudopotential radii of Zunger. The two band parameters are called as the hybrid function (H) and gap reduction parameter (S), which substantially correspond to the energy difference between the bonding and anti-bonding levels and the band widening effect due to the formation of crystalline state. These parameters roughly correspond to ionic and covalent band gaps which are constructed by one gap model. By comparing with the definition of ionicity based on the ionic and covalent band gaps, it is found that the ionicity for any A–B bond can be estimated from the two band parameters. Extensive availability of the band parameters to materials design is suggested by exemplifying several successful results such as the construction of crystal structure maps for compounds, determination of site preference of third alloying elements in intermetallics and prediction of phase changes in pseudobinary nitride films in the energetic processes.