Oxidation Behavior of LPPS MCrAlY Coatings at High Temperature: Part I TGO Growth and β Phase Depletion

MCrAlY can be used as bond coats for thermal barrier coatings (TBCs) with good ductility and excellent resistance against high temperature oxidation and hot corrosion. The behavior of the thermally grown oxide (TGO) scale formed at the MCrAlY coatings plays a key role on the oxidation resistance. In this paper, the oxidation kinetic curves of a MCrAlY coating at 900~1000 °C were obtained by measuring the thickness of the TGO scales. The curves basically conveyed parabolic laws, indicating a diffusion-controlled mechanism of the TGO growth. The thickness of TGO was positively correlated with the consumption of β phase during the early stage of the oxidation processes. After about the half-life of the β phase consumption, the depletion of the β phase significantly accelerated, which was caused by coating-substrate interdiffusion. In addition, the microstructure of the TGO was analyzed

Dynamic Hardness of MCrAlY Abradable Seal Coating

The purpose of this investigation is to study the dynamic hardness of MCrAlY abradable coatings under different strain rates. A dynamic indentation device based on the split Hopkinson pressure bar system (SHPB) was used. The results show that the hardness of MCrAlY coating increased with the increase of the strain rate, which has a positive strain rate effect. In addition, the difference of the static hardness of MCrAlY coating prepared by HVOF and LPPS was only 4%, while the difference in dynamic hardness was 16%.

Effects of Heat Accumulation on Temperature Field during Multi-Track Laser Cladding of Preset MCrAlY Coating

To investigate influences of multi-track overlapping on melting of preset MCrAlY coating during laser cladding plasma spraying, a three-dimensional finite element model of the continuously moving temperature field during multi-track laser cladding was constructed using the ANSYS parametric design language (APDL) based on the existing temperature field model during single-track laser cladding. According to analysis results of temperature field, temperature of samples increases gradually during laser cladding due to heat accumulation effect of laser scanning, and the molten pool expands gradually. There are evident differences among different scanning pathways. Therefore, it is impossible to get high-quality cladding coating with uniform melting and small dilution ratio. Molten pools with basically same sizes in different scanning pathways can be gained by decreasing laser power or increasing scanning speed in different tracks one by one. Similarly, differences of molten pools in different scanning pathways can be relieved effectively through preheating of samples. Through a closed-loop control over the highest cladding temperature, a more even cladding coating can be gained through adaptive control of laser power and / or scanning speed.

Computational Analysis of Laser Cladding of Preset MCrAlY Coating Based on ANSYS I-Temperature Field

A finite element model of temperature field for plasma spraying preset MCrAlY coating during laser cladding was constructed using ANSYS parametric design language (APDL) in accordance to characteristics of preset laser cladding. Influencing laws of laser cladding parameters on temperature field were analyzed. Results show that laser power influences temperature field of cladding samples more than laser scanning speed. Experimental results agree well with simulation results, which prove the accuracy and reliability of the constructed calculation model of temperature field. Heating and cooling laws in the laser cladding process could be mastered through this calculation model. Research conclusions provide some references to optimization parameters in preparing high-performance laser cladding coatings.

Computational Analysis of Laser Cladding of Preset MCrAlY Coating Based on ANSYS Ii-Stress Field

A finite element model of thermal coupling stress field during laser cladding plasma spraying of preset MCrAlY coating was constructed based on the finite element model of temperature field by using the indirect thermal coupling method in ANSYS finite element software. Moreover, stress field during laser cladding was analyzed. Through the constructed model, variation laws of stress field with time during laser cladding and cooling process could be mastered. Based on the stress field, the formation mechanism of cracks in laser cladding coating and influencing factors were further analyzed and some solutions to cracks of laser cladding coating were proposed.

A comprehensive study on the microstructure evolution and oxidation resistance of conventional and nanocrystalline MCrAlY coatings

Conventional and nanocrystalline MCrAlY coatings were applied by the high-velocity oxy-fuel (HVOF) deposition process. The ball-milling method was used to prepare the nanocrystalline MCrAlY powder feedstock. The microstructure examinations of the conventional and nanocrystalline powders and coatings were performed using X-ray diffraction (XRD), high-resolution field emission scanning electron microscope (FESEM) equipped with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). Williamson–Hall analyzing method was also used for estimation of the crystalline size and lattice strain of the as-milled powders and sprayed coatings. Owing to the investigation of the oxidation behavior, the freestanding coatings were subjected to isothermal and cyclic oxidation testing at 1000 and 1100 °C under static air. The results showed that the conventional as-sprayed MCrAlY coating had a parabolic behavior in the early stage and prolonged oxidation process. On the contrary, in the case of the nanocrystalline MCrAlY coating, the long-term oxidation behavior has deviated from parabolic to sub-parabolic rate law. Moreover, the results also exemplified that the nanocrystalline MCrAlY coating had a greater oxidation resistance following the creation of a continuous and slow-growing Al2O3 scale with a fine-grained structure. The nucleation and growth mechanisms of the oxides formed on the nanocrystalline coating have also been discussed in detail.