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MCrAlYHf bond coats are employed in jet and aircraft engines, stationary gas turbines, and power plants, which require strong resistance to oxidation at high temperatures. The oxidation behavior of a free-standing CoNiCrAlYHf coating was investigated in this study. Isothermal oxidation tests were performed in air at 1050 °C on the specimens with various levels of surface roughness to investigate the oxidation kinetics. X-ray diffraction, focused ion beam, scanning electron microscopy (SEM), and scanning transmission electron microscopy were used to characterize the surface oxides. The surface roughness was examined using a contact profilometer and SEM. An empirical model was proposed to elucidate the impact of surface roughness on the oxidation behavior based on the correlation between the surface roughness level and oxidation rates.

MAX-phase has excellent high-temperature lubrication performance, which has the potential to be used as the abradable seal coating in aircraft engine combustion chamber. Various concentrations of MAX-phase Ti3AlC2 (2, 5 and 10 wt %) are doped in YSZ-based abradable seal coating as the lubricating phase, and the coating produced by air plasma spraying (APS) on Ni-based alloy. The microstructure and element distribution of the seal coating are studied by field emission scanning electron microscopy (FE-SEM), electron X-ray probe microanalysis (EPMA). The mechanical properties are measured by Vickers hardness test, nanoindentation test and tribological test. The results reveal that with the content of the MAX-phase Ti3AlC2 increasing, the hardness of the abradable seal coating is increased and the resultant wear rate is reduced. The reduction of wear rate is due to the production of some tiny TiC and Al2O3 phases during deposition of Ti3AlC2 in plasma spraying. Comparing with the different MAX-phase content, the wear rate of the composite coating is different. The changes of the nanohardness and wear mechanism of the composite coating after plasma spraying will be discussed.

To improve the oxidation resistance of a Mo substrate at high temperatures, MoSi2–MoB double layers were prepared on the surface of the substrate using a two-step pack cementation method. The isothermal oxidation tests indicate that the weight loss of MoSi2–MoB double layers was only 6.3824 mg/cm2 after oxidation at 1300 °C for 15 h, whereas the untreated Mo experienced serious weight loss and the weight loss was 606.775 mg/cm2 after only 90 min. Following oxidation exposure, the oxidized coating became six complex layers due to correlation diffusion among Mo, Si and B. The double layers showed excellent high-temperature oxidation resistance due to the formation of a dense and continuous SiO2 layer with low O2 permeability. Meanwhile, in situ fabrication of a novel Mo5SiB2 layer could prevent inward diffusion of Si.

Alumina and zirconia ceramic particles exhibit high hardness and excellent wear resistance at high temperature, and hence are used as ceramic reinforcement phases in some plasma sprayed coatings. In this study, the interface evolution of a zirconia/alumina eutectic ceramic and the phase transition of zirconia in a plasma-sprayed coating were investigated. Scanning electron microscopy and transmission electron microscopy combined with focused-ion beam and energy dispersive X-ray were used to analyze the microstructure and composition of the ceramic interface. The results showed that the eutectic ceramic particles consisted of alumina (outer) and columnar zirconia (inner) before and after the plasma spraying process. The inner zirconia part showed the martensitic transformation of t-type zirconia to stripe-like m-type zirconia. After the plasma spraying, the interface between alumina and zirconia changed significantly, which formed a new oxide layer. The phase transition mechanism in the ceramic particle and oxide layer formation mechanism at the alumina/zirconia interface were investigated.

Temperature and atmosphere have a significant effect on the oxidation of MCrAlY (M = Ni, Co) bond coating. The initial growth behavior of the NiCrAlHf bond coat alloy was investigated at 1100 °C under different atmospheric conditions and using heating methods. A thick Al2O3 oxide layer and large HfO2 particles were observed, perhaps due to metastable oxide growth at low temperatures when using stepped heating. However, in air and water vapor atmospheres, the oxide scale was thinner and the HfO2 precipitates were smaller in stepped heating than in constant heating. The size and distribution of the HfO2 particles might have induced different microstructures, particularly voids within the oxide scale.

In this work, we investigated FeCrMo coatings with 35 wt% ceramic, which were produced on a cylinder surface by an internal rotating plasma spraying and oxidized in air. The ceramic particles consisted of 80 wt% alumina and 20 wt% zirconia, which can improve the mechanical properties of the coating. For microstructural characterization, scanning electron microscopy was combined with energy dispersive X-ray analysis and electron probe micro analysis, and the nanomechanical properties were measured by a nanoindentation tester. The structure of Fe-base coating matrix consists of Al 2 O 3 , ZrO 2 and a small amount of mixed Fe–Cr oxides, and the ceramic particles exhibit a uniform distribution. During oxidation, a mixed oxide layer containing Fe 2 O 3 and Fe 3 O 4 forms on the surface of the Fe-base coating, and the thickness of the oxide layer increases with increasing oxidation temperature. Due to the in-situ oxidation, the nanohardness and Young's modulus of the Fe-base coating decreased with increasing oxidation temperature. The nanohardness evolution of Fe-base coatings oxidized at different temperatures is discussed.

This study investigated the structural evolution and microhardness of Al2O3–40 wt.% TiO2/NiCoCrAl gradient coatings deposited by atmospheric plasma spraying (APS) after high-temperature treatment carried out at 800 °C in air for 600 h. The gradient region exhibited a different ceramic growth and oxidation behaviour at different zones. The nonhomogeneous needle-like γ-Al2O3 phase was formed at the ceramic region, whereas the homogeneous needle-like NiAl2O4 phase was formed at the NiCoCrAl oxidation region close to the ceramic coating. And the ingrowth Cr-rich phase existed inside the NiCoCrAl splats at the gradient region close to the bond coating. An obviously microhardness improvement of the coating can be measured by a Vickers microhardness tester. The formation of the needle-like phase has an effect on the reinforcement of coating.

The low fracture toughness of ceramic coatings has always hindered their wide application. In this study, an in-situ nanocomposite coating was prepared by the atmospheric plasma spraying of a 50 wt% Ti3AlC2-50 wt% Cu mixed powder. The in-situ nanocomposite coating was found to have an unusual microstructure with a nano-micrometre phase synergistic enhancement, which consisted of submicrometre-thick layers of Cu and nanoparticles of Cu(Al), Ti4O5, TiO2, and Al2TiO5. Thus, in the spraying process, Al was delinked out of Ti3AlC2, forming a large amount of plastic Cu(Al) with Cu. The delinked channel provided a path for Cu to diffuse into Ti3AlC2, which a spatial Cu network structure was formed in the coating. The in-situ nanocomposite coating has high fracture toughness and crack growth resistance by a three-point bending test. This paper reports a new method to prepare a high-fracture-toughness composite ceramic coating.

The growth behaviour of the modified surface of NiCoCrAl alloy by Pt and Hf was investigated at 1050 °C in air atmosphere. The undoped NiCoCrAl alloy exhibited an extremely serious oxide scale spallation. Compared to the undoped alloy, the Hf and Pt depositing on the surface of NiCoCrAl can greatly increase the alumina scale lifetime. Furthermore, altering the deposition order of Pt and Hf influences the cyclic oxidation resistance. The higher content of Hf on the alumina scale of NiCoCrAl alloys resulted in the formation of spinel (NiCr2O4). The spinel as a second phase in the oxide layer of NiCoCrAl + Hf alloys led to the internal oxidation. Internal defects were caused by the diffusion of Al during the oxidation process. Due to the phase transformation and grains growth, the internal stress of alumina scale increased, so that the spallation occurred on the surface of NiCoCrAl + Pt alloys. Pt particles agglomerated on the NiCoCrAl + Hf + Pt alloy surface to form irregular particles, and a lower TGO growth rate has been observed for the NiCoCrAl + Hf + Pt alloy.

The hot corrosion behaviour was investigated for pack-cemented Si-modified aluminide coating on the K438 Ni-base alloy when covered by NaCl, Na2SO4, and 50% NaCl +50% Na2SO4 (mass fractions) at 1050 °C in air. The primary aim of this work was to add NaCl and Na2SO4 only once to the surface of coating to study the effect on the subsequent and long-term hot corrosion behaviour of coating. After 120 h hot corrosion, covered by NaCl and 50% NaCl +50% Na2SO4 (mass fractions) for the coating, generated the maximum and minimum oxides spallation per unit area, respectively. Combined-effect of NaCl and Na2SO4 decreases oxides spallation.

A layer of platinum was fabricated through electroplating on the surface of NiCoCrAl model alloy. Diffusion characteristics and oxidation behaviour of Pt-coated NiCoCrAl compared to uncoated was investigated at 1050 °C. The platinum inter-diffusion layer within the alloy was obtained through the heat treatment. For microstructural characterization results of scanning electron microscopy were combined with X-ray diffraction data. Due to different covalent bonding calculated by first principle calculation, the platinum presented different diffusion abilities in β-NiAl and γ′-Ni3Al phases during the heat treatment. The calculated elastic constant and elastic moduli revealed that Pt-doped β-NiAl and γ′-Ni3Al possess better elastic and mechanical properties than β-NiAl and γ′-Ni3Al. The platinum addition restrained the spallation of Ni-rich oxide on Pt-coated NiCoCrAl alloy compared to uncoated, hence the adhesion between the oxide layer and NiCoCrAl alloy was improved.

The cyclic oxidation experiment of yttria-stabilized zirconia coatings deposited on NiCoCrAlYHf alloys by air plasma spraying was investigated at 1050 °C in air and in air containing water vapor. The results revealed that water vapor has a great influence on the oxidation resistance of the thermal barrier coatings (TBCs). Compared with the samples oxidized in air atmosphere, TBCs oxidized in air containing water vapor had a longer lifetime. It was also found that different atmospheres could lead to different HfO2 formation positions, which could decrease the rumpling in the oxide layer. In particular, after the coatings on Hf-doped NiCoCrAlY were first pretreated in air containing water vapor for 24 h at 1050 °C, the lifetime of the pretreated coating was doubled compared to the coating in laboratory air only. The water vapor pretreatment of the coatings could be an important method for optimizing the lifetime of TBCs.

The interfacial reactions of the plastic Cu-metal barrier layer in the Al2O3–40 wt% TiO2 (AT40) composite coating prepared by plasma spraying and its effect on the crack propagation behaviour during thermal cycling were investigated. The phenomena of Cu dendrites appearing on the upper surface of the Cu layer and the Cu whiskers growing along the crack of the ceramic coating under the Cu layer after thermal cycling (900 °C, Ar), and the formation mechanism are explained using FE-SEM, XRD, EPMA, and EBSD. The Cu dendritic and Cu whiskers changed the stress state of the interface and enhanced the anti-crack-propagation ability of the coating. In addition, nanoparticles surrounding the Cu layer using EBSD were observed in the Cu particles added to the AT40 coatings (C-AT coating) by plasma spraying. The calculations of surface energy indicated that the nanoparticles improved the wettability of the interface and promoted the interfacial coupling behaviour. A model of the relationship between stress and crack propagation in the process of heating and cooling was used to analyse the effect of the plastic metallic-barrier layer (Cu) on crack propagation. This work demonstrates that the Cu layer improved the anti-crack-propagation ability of the coating. Such C-AT coatings may find potential applications in high-temperature materials and wear-resistant material surfaces.

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine. We also demonstrate a cost-efficient, robust, and easy-handling setup for correlative analysis based solely on commercially available components, which can be used with all conventional TEM tools without the need to modify the specimen holder assembly.

A new method for improving the adhesion of thermally sprayed metal-ceramic coatings is proposed. Experimental results show that an in-situ Al2O3 layer formed between the surface of a FeCrAl-ceramic coatings during the arc spraying rapid deposition and a hypoxic heat treatment, which it pinning in the FeCrAl bondcoat, significantly improving the bonding performance between metal and ceramic. Based on these experimental results, a new theory of in-situ oxidation-enhanced interfaces is proposed to explain the bonding between metal and ceramic coatings which can be improved after hypoxic heat treatment.

In this paper, an Al2O3–40 wt%TiO2/NiCoCrAl gradient coating is deposited by atmospheric plasma spraying (APS). The microstructure and element distribution of the coating are studied by scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA). The crack propagation behaviour in the coating under applied and thermal stress is analysed through a three-point bending test and a thermal shock test, respectively. Two rapid propagation processes of the cracks can be found during the three-point bending test, which lead to two peaks in the load-displacement curves of the gradient coating. The gradual change in the composition also has an effect on the crack propagation process within the coating. Non-directional propagation paths of cracks and the formation of oxides can be observed in the gradient region under the effect of thermal stress, which lead to the spallation failure of the coating.

FeCrAl-base alloys are well known for their excellent oxidation resistance due to formation of a slowly growing alumina surface scale during high-temperature service. The actual scale growth mechanism and especially adherence are strongly affected by the presence of oxygen-active elements such as yttrium, titanium or hafnium. In the present study, the effect of titanium addition on the scale growth mechanisms of an yttrium oxide dispersion-strengthened FeCrAl base alloy was studied during oxidation at 1200 °C in Ar–O2. For microstructural characterization results of scanning electron microscopy and electron backscatter diffraction were combined with X-ray diffraction data. Scale growth mechanisms were investigated by two-stage oxidation using 18O tracer with subsequent scale analyses using secondary neutrals mass spectrometry. The scale on the alloy without intentionally added titanium grew virtually exclusively by oxygen diffusion along oxide grain boundaries and exhibited a columnar structure with the grain size increasing in growth direction. The addition of titanium resulted in formation of an outer oxide zone of equiaxed grains on top of the inner columnar part. The equiaxed grains increased in size with increasing exposure time. Comparison with the tracer studies revealed that the titanium-induced equiaxed zone was the result of outer scale growth. Mechanisms for the initiation of outward aluminium transport are discussed. Indications were found that the effect of titanium on the scale growth mechanisms already occurred for titanium additions as low as 0.02 wt%.