Ti-6Al-4V(Ti64) alloy has excellent mechanical properties and corrosion resistances, but poor oxidation properties at high temperatures. It is necessary to improve the oxidation resistances of the alloy in order to expand its application in the aerospace and automotive fields. This study aimed to improve oxidation resistances through diffusion coatings on the surface of the alloy. Aluminide coatings on Ti64 alloys have been investigated via pack aluminium process at temperature ranges of 400-800 degrees C. Al and/or TiAl3 layers were formed at the interface between the Ti64 alloy and the Al coating layer, when the Ti64 alloys were heat-treated at 400 and 500 degrees C. TiAl3 layer was manufactured on Ti64 alloys at 600, 700 and 800 degrees C. The growth of the coated TiAl3 layer followed diffusional growth manner, and the estimated value of the nucleation barrier was the lowest among the intermediate phases of the Ti-Al system. The coating kinetics were discussed together with free energy diagram observations. The activation energy for growth of the TiAl3 layer was estimated as similar to 82 kJ/mol in the experimental temperature ranges. At the same time, isothermal oxidation tests of the bare Ti64 alloy and the aluminized Ti64 alloys were performed at 1000 degrees C. By confirming the diffusion pathway of the Ti-Al-O ternary system, the oxidation behaviours and resistances of the alloys were compared and analysed.

This paper aims to improve the oxidation resistance of MoNbTaVW (at%) refractory high-entropy alloys after flame tests at 1350 ℃ with Si/Al pack cementation coatings. After the Si/Al pack cementation coatings were applied, a coating layer of about 80 μm with a sequence of Si-rich layer (silicide)/interdiffusion layer/Al-rich layer (aluminide)/Al diffusion layer was formed. After the flame tests at 1350 °C for 20 min, the uncoated specimens formed composite oxides of Nb2O5 (s), Ta2O5 (s), and V2O5 (s) with porosity of MoO3 (g) and WO3 (g) with a weight gain. On the other hand, the Si/Al coated specimens were composed of Si-rich layer (silicide)/interdiffusion layer/Al-rich layer (aluminide)/Al diffusion layer. After the flame tests, SiO2 and Al2O3 ceramic protective layers were formed on the surface and prevented additional oxidation. The oxidation behaviors of MoNbTaVW alloys and the Si/Al pack cementation coated alloys were discussed with microstructures and phase development.

The degradation and stability behavior of AISI 4130 steels have been examined under high-temperature dynamic plasma flame environments. A Fe2Al5 layer was formed as a result of aluminizing on the steel. An Al2O3 layer was produced on top of the Fe2Al5 layer, when the coated steel was oxidized at 500 °C (pre-oxidation treatment). The uncoated steels and the Fe2Al5 coated steels (i.e., the aluminized steels) showed large weight loss and complete changes in their original shapes due to their melting during the plasma flame tests at 1480 °C. However, the steels with the Al2O3 and Fe2Al5 (pre-oxidation treatment) were very stable without showing noticeable weight loss even after exposures to the plasma flame tests mainly due to the presence of the Al2O3 layer. SEM and XPS analyses were used to provide the presence of Fe2Al5 and Al2O3 phases on the surface. The effects of synthesized surface layers of the steels on high-temperature plasma flame exposure are discussed together with the surface morphology and microstructural observations.

Cr-Si based alloys are not only excellent in corrosion resistance at high temperatures, but also have good wear resistance due to the formation of Cr3Si phase, therefore they are promising as metallic coating materials. Aluminum is often added to Cr-Si alloys to improve the oxidation resistance through which stable alumina surface film is formed. On the other hand, due to the addition of aluminum, various Al-containing phases may be formed and may negatively affect the heat resistance of the Cr-Si-Al alloys, so detailed investigation is required. In this study, two Cr-Si-Al alloys (high-Si & high-Al) were prepared in the form of cast ingots through a vacuum arc melting process and the microstructural changes after high temperature heating process were investigated. In the case of the cast high-Si alloy, a considerable amount of Cr3Si phase was formed, and its hardness was significantly higher than that of the cast high-Al alloy. Also, Al-rich phases (with the high Al/Cr ratio) were not found much compared to the high-Al alloy. Meanwhile, it was observed that the amount of the Al-rich phases reduced by the annealing heat treatment for both alloys. In the case of the high temperature heating at 1,400 degrees C, no significant microstructural change was observed in the high Si alloy, but a little more coarse and segregated AlCr phases were found in the high Al alloy compared to the cast state.

In this study, we tried to improve the oxidation resistance of Nb-12Si (wt%) alloys at 1200 °C or higher through pack cementation coatings. Nb-12Si (wt%) alloys were prepared by arc-melting under Ar atmosphere. When the alloys were coated using pack powder mixtures composed of Si, Al2O3 and NaF, two silicide layers composed of NbSi2 and Nb5Si3 phases were successfully produced on the substrate. The Si-pack coatings were performed with various heat treatment temperatures and time conditions. The microstructures and thickness changes of the coating layers were analyzed to determine the growth behaviors of the coating layer. The growth constant of 8.4 10–9 cm2/sec was obtained with a diffusion growth mode. In addition, in order to examine the resistance of the Si-pack coated alloys, isothermal static oxidation tests were performed at 1200 °C and higher temperatures. As a result, the oxidation resistance of the alloys was determined by protecting the surface of the alloys with silicide oxide layers formed by the silicide coatings. The uncoated specimens exhibited an abnormal weight increase due to the formation of Nb oxide. The coated specimen showed excellent oxidation resistance at 1200 °C for up to 12 hrs, while the previous reports on the same alloy verified oxidation resistance only up to 1100 °C. It appears that the excellent oxidation resistance is closely related to the NbSi2 coating layer thickness. The oxidation behaviors of the coating layers after the oxidation tests were discussed in terms of microstructural and phase analyses.

Refractory metal alloys, such as Mo–Si–B systems, can extend high-temperature capability more than commercial Ni-based superalloys, but Mo–Si–B alloys require surface coatings to improve their poor oxidation resistance. In this study, aluminide coating layers were created on Mo–3Si–1B (wt%) alloys by pack cementation with NH4Cl, Al and Al2O3 powder. The aluminide coating layers consisted of MoAl4, Mo3Al8, and precipitates. The growth kinetics of the coating layers were estimated by identifying diffusion behaviors. The aluminide coating layer growth constant (k0) was estimated as ~ 741.3 μm/h0.5, and the activation energy (Q) for the growth of the diffusion coating layer was evaluated as ~ 44.2 kJ/mol for the examined coating temperatures of 800, 900 and 1000 °C. The thicknesses of the coating layers calculated by an estimated kinetic equation were compared with the experimental results. The coating layer successfully protected the Mo–Si–B substrate during isothermal oxidation at 1400 °C under an air atmosphere. The growth kinetics of the coated layer and oxidation behaviors were discussed in terms of microstructural analyses.

In this study, we tried to improve the oxidation resistance of Mo–3Si–1B (wt%) alloy at intermediate (800 °C) and high temperature (1300 °C) after applying Si/Al pack cementation coatings. In addition, we examined structural changes and phase change in the intermediate and high temperature oxidizing atmospheres to observe oxidation behaviors. The diffusion pack coatings were carried out with an NH4F activator and both Si and Al powder mixtures. Mo–Si–B alloys were subjected to diffusion coatings of Si/Al powders at various temperatures in various time conditions at a temperature of 1100 °C. When the pack coatings were carried out for 48 h, Mo(Si, Al)2 and Mo3Al8 layers together with a T2 (Mo5SiB2) layer were successfully synthesized on the substrate. After oxidation tests, an Al2O3 protective oxide layer was formed by the diffusion of Al in an ambient atmosphere. The Si/Al pack cementation coatings provided excellent oxidation resistance through the formation of an Al2O3 oxide layer. The enhanced oxidation behaviors are discussed in terms of microstructural evolution at high temperatures.

Research is being conducted on Mo- and Nb-based alloys that are used in the aerospace sector, including those used for advanced gas turbines and aircraft engines. There is a limit to using Mo, which has a high density among refractory metals, and a few studies exist describing the addition of Nb to Mo–silicide alloys. There is a lack of guidance research on the basic Nb:Mo ratio of alloys, and it is necessary to study how to improve oxidation resistance. Therefore, this study aims to improve oxidation resistance by controlling the ratio of Nb and Mo in (Nbx, Moy)Si2 coating layers with Si pack cementation coatings on Nb–Mo alloys. Static oxidation tests were carried out at 1200 °C for 6 h to confirm the oxidation characteristics. As a result, a SiO2 or SiO2 + Nb2O5 ceramic protective layer was formed on the surface. After the oxidation tests, alloys with a Nb content of less than 35 at.% were found to protect the surface. The ratios of Nb and Mo in the Nb–Mo alloy and silicide coating layer were compared, and the improvement of oxidation resistance is discussed in terms of microstructural evolution.

Stainless steel is being used in various industries such as automobile and aerospace for its cheap manufacturing cost and excellent mechanical properties. However, stainless steel failed to stably protect a specimen with a Cr2O3 protective layer at temperatures above 1000 oC. Thus, improving the high temperature flame resistance of the specimen through additional surface coating was needed. In this study, multilayer coatings of YSZ and Al2O3 were performed on SUS 304 specimens using pack cementation coatings and thermal plasma spray. The multilayer coated specimen showed enhanced thermal properties due to the coated layers. The microstructures and phase stability are discussed together with flame conditions at 1350 oC.