Isothermal and cyclic oxidation behaviors of chromium samples with and without nanometric CeO2 coating were studied at 900 ℃ in air. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM) were used to examine the morphology and microstructure of the oxide film. It was found that ceria coating greatly improved the oxidation resistance of Cr both in isothermal and cyclic oxidizing experiments. Acoustic emission (AE) technique was used in situ to monitor the cracking and spalling of oxide film, and AE signals were analyzed in time-domain and number-domain according to related oxide fracture model. Laser Raman spectrometer was also used to study the stress of oxide film formed on Cr with and without ceria. The improvement in oxidation resistance of chromium is believed mainly due to that ceria greatly reduced the growth speed and grain size of Cr2O3. This fine-grained Cr2O3 oxide film might have better high temperature plasticity and could relieve parts of the compressive stress by means of creeping and maintained ridge character and relatively lower level of internal stress. Meanwhile, ceria application reduced the size and number of interfacial defects, remarkably enhanced the adhesive property of Cr2O3 oxide scale formed on Cr substrate.
Isothermal and cyctic oxidation behaviours of pure and yttrium-implanted nickelwere studied at 1000℃ in air.Scanning electronic microscopy (SEM) and transmission electronicmicroscopy (TEM) were used to examine the micro-morphology and structure of oxide scalesformed on the nickel substrate.It was found that Y-implantation significantly improved the anti-oxidation ability of nickel in both isothermal and cyclic oxidizing experiments.Laser Ramanmicroscopy was also used to study the stress status of oxide scales formed on nickel with andwithout yttrium.The main reason for the improvement in anti-oxidation of nickel was that Y-implantation greatly reduced the growing speed and grain size of NiO.This fine-grained NiO oxidefilm might have better high temperature plasticity and could relieve parts of compressive stressby means of creeping,and maintained a ridge character and a relatively low internal stress level.Hence yttrium ion-implantation remarkably enhanced the adhesion of protective NiO oxide scaleformed on the nickel substrate.
The isothermal oxidizing kinetics of Co-40Cr alloy and its yttrium ion-implanted samples arc studied at 1000℃in air by thermal-gravity analysis(TGA).Scanning electronic microscopy (SEM)is used to examine the Cr_2O_3 oxide film's morphology after oxidation.Acoustic emission(AE) method is used in situ for monitoring the cracking and spalling of oxide films formed on both samples during oxidizing and subsequent air-cooling stages.Theoretical model is proposed relating to the film fracture process and is used for analyzing the acoustic emission spectrum both on time domain and on AE-cvcnt number domain.It is found that yttrium implantation remarkably reduces the isothermal oxidizing rate of Co-40Cr and improves the anti-cracking and anti-spalling properties of Cr_2O_3 oxide film.The reasons for the improvement arc mainly that the implanted yttrium reduced the grain size of Cr_2O_3 oxide,increased the high temperature plasticity of oxide film,and remarkably reduced the number and size of Cr_2O_3/Co-40Cr interfacial defects.
Isothermal and cyclic oxidation behaviors of pure and yttrium-implanted nickel were studied at 1000 ℃ in the air. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the micro-morphology and structure of oxide scales formed on nickel substrate. It was found that Y-implantation greatly improved the anti-oxidation ability of nickel both in isothermal and cyclic oxidizing experiments. Laser Raman Microscopy was also used to study the stress status of oxide scales formed on nickel with and without yttrium. The main reason for the improvement in anti-oxidation of nickel was that Y-implantation greatly reduced the growing speed and grain size of NiO. This fine-grained NiO oxide film might have better high temperature plasticity and could relieve parts of compressive stress by means of creeping, and maintained ridge character and relatively lower internal stress level, hence remarkably enhanced the adhesion of protective NiO oxide scale formed on nickel substrate. The actual existing form of yttrium in oxide can be Y2O3 and NiY2O4 spinel nanometric particles, and even be Y3+ ions segregated at NiO grain boundaries.
Isothermal and cyclic oxidizing behavior of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000 °C in the air by thermal-gravimetric analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the morphology and structure of oxide film after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on the formation of Cr2O3 film. Laser Raman spectrum was used to examine the stress changes within the oxide film. It was found that lanthanum implantation remarkably reduced isothermal oxidizing rate of Co-40Cr and improved anti-cracking and anti-spalling properties of Cr2O3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduced the grain size and internal stress of Cr2O3 oxide and increased high temperature plasticity of the oxide film. Lanthanum mainly existed on the outer surface of Cr2O3 oxide film in the form of fine La2O3 and LaCrO3 spinel particles.
