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Abstract A super long and stable laminar plasma jet was used in the atmospheric plasma spray process in this work. Microstructures evolution and properties of alumina coatings that were obtained using a long spraying distance ranging from 200 mm to 350 mm in an atmospheric environment were studied in this work. These processes were carried out by using a total gas flow rate of 14 slpm with 70% nitrogen and 30% argon in volume and output power of 25.4 kW (current of 160 A). A long particle residence time was obtained in this study, which compared with other current atmospheric plasma spray methods. The microstructures of coatings showed the multi-island protrusions on the top surfaces and overlapped lamellar splats at the fracture surfaces. The microstructural evolution was significantly affected by the heating and motion behaviors of alumina particles when these were flowing in the long laminar plasma jet at different spraying distances. Highlights • A long and stable laminar plasma jet in air is used in this work. • A super long spraying distance of 350 mm is used in this article. • Long particle residence time when flowing in the laminar plasma plume • Multi-island protrusions at the top surface of coatings [ABSTRACT FROM AUTHOR]

A novel direct current non-transferred arc plasma torch that can generate long, silent and stable laminar plasma jet in air is applied to thermal spraying process in this article. Such long laminar plasma jet, making a low level of noise (< 80 dB) in air, features a jet length over 350 mm, and presents regular variation in length with the increase of output power and gas flow rate. Microstructures and properties of YSZ coatings obtained under spraying distances ranging from 130 mm to 300 mm are discussed in the following. The results show different intervals of vertical cracks across the transverse sections of the coatings and orderly distributed island-protrusions on the top surface of the coatings. A mass of vapor deposited structures can be found in the samples, that means the refractory 8YSZ powders can be melted sufficiently when flowing in long laminar plasma jet, although the stable output power of the laminar plasma torch is only 25–26 kW during the spraying process. This may provide extensive options for different plasma spraying applications and improve the controllability of plasma spraying technology. [ABSTRACT FROM AUTHOR]

A stable laminar plasma jet with a jet length of above 600 mm was adopted to deposit molybdenum coatings to investigate the behavior of high-melting point metal, molybdenum, and particles during laminar plasma spraying process. The temperature, velocity, and species distribution of the plasma jet were calculated by numerical simulation. In the experiment, the temperature and velocity distribution of the in-flight particles were measured, and the microstructure and properties of the coatings were analyzed. Combining the results of the simulation and experimental results, the deposition behavior and oxidation mechanism of the coatings prepared at the spray distance from 200 mm to 400 mm were discussed. The results indicate that both the laminar plasma jet and injected particles exhibit high temperature and low velocity, and low temperature and velocity gradients. The microstructure of the coatings shows a high similarity and hardly changes when the spray distance exceeds 250 mm. When the spray distance is 200 mm, post-deposition oxidation occurs due to the heating effect of the long plasma jet on the substrate. In contrast, when the spray distance is longer, the particles undergo severe in-flight oxidation, and the oxide formed is uniformly distributed in the coating improving the hardness from 500 HV 0.3 to 700 HV 0.3. • A stable laminar plasma jet with length over 600 mm was adopted. • A super long spray distance of 400 mm was realized in atmospheric environment. • The microstructure and properties of coatings show insensitivity on spray distance. • Particles mainly endure inflight oxidation enhancing the hardness. [ABSTRACT FROM AUTHOR]

In this paper, by means of numerical simulation and experimental analysis, the performance of the engulfment based annular coaxial powder feeding method and the particle behaviors were studied in the laminar plasma spraying process. First, numerical simulation methods are used to analyze the flow state of the laminar plasma jet and its interaction with the powder particles. Then, the laminar plasma spray was applied to deposit coatings based on the annular coaxial powder feeding method, and the performance of this method was evaluated by observing the coatings. The numerical simulation results show that the powder particles can be fed into the laminar plasma jet without affecting the flow state of the plasma jet, and can be sufficiently heated. The experimental results are consistent with the numerical simulation results. The surface morphologies of the coatings indicates that vapor phase deposition and liquid phase deposition occur simultaneously during the spraying process, and the laminar plasma spray - physical vapor deposition process has the potential to be applied in atmospheric environments. • Engulfment based annular coaxial powder feeding method is applied in laminar plasma spray. • The engulfment of the plasma jet was evaluated and the best feeding rate was confirmed. • The particle melting and evaporating behavior is numerically studied. • Vapor phase and cluster deposition structure can be observed in atmospheric environment. [ABSTRACT FROM AUTHOR]

A promising new bond coat using CuAlNiCrFe high-entropy alloy was proposed in this paper. The difference in pre-oxidation conditions between the traditional thermal-sprayed MCrAlY bond coat and CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding was investigated. The result confirmed that the CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding can complete the pre-oxidation faster and form a continuous α-Al 2 O 3 thermally grown oxide (TGO), which shortens the initial oxidation stage, thereby avoiding the formation of other oxides and spinel structures. In the subsequent isothermal oxidation process, the block-like structure of the high-speed laser cladding layer and the sluggish diffusion effect of the high-entropy alloy work together to ensure the slow and continuous supply of aluminum element to TGO layer and obtain a low growth rate. Also, the diffusion between the bonding layer and the substrate is controlled at a low level, and the new CuAlNiCrFe bond coat exhibits excellent oxidation and diffusion resistance. With the consumption of aluminum, the phase structure of the high-entropy alloy bond coat changed from BCC to FCC, but it still maintained a stable simple solid solution structure. • A high entropy alloys bond coat was prepared by ultra-high speed laser cladding. • The laser Cladded high-entropy alloys ensure the sluggish diffusion effect. • The CuAlNiCrFe bond coat shows great oxidation resistance in 1100 °C. • The phase structure of CuAlNiCrFe change to FCC single-phase solid solution. • A new thermal barrier coatings system with high entropy alloys was proposed. [ABSTRACT FROM AUTHOR]

The behavior of low-velocity NiCrBSi particles with a high surface temperature of 1800 °C during the long laminar plasma spraying process was studied. During the experiment, the maximum length of the laminar plasma jet exceeded 600 mm, and coatings were obtained using a laminar plasma spray at a wide spray distance ranging from 250 to 500 mm. Interestingly, the coatings obtained at a short spray distance of 250 mm showed special alternate porous and compact lamellar structure with island-like bulges that were evenly distributed on the coating surface. This type of structure was possibly formed by a low-velocity splashes deposition process intensified by the furry-like structure, which can be attributed to vapor phase deposition. The coatings obtained at an extremely long spray distance of 500 mm showed massive cracks. Moreover, the medium-distance sprayed coatings of 350–450 mm showed similar lamellar-like microstructure and mechanical performance because of the similarity in the particle states in the low-energy-gradient laminar plasma jet. Thus, a laminar plasma spray shows great reproducibility and potential for improving the spraying efficiency on an uneven plane. Unlabelled Image • Stable laminar plasma jet longer than 600 mm was adopted. • NiCrBSi coatings show similar properties regardless of spray distance within 500 mm. • Island-like bulges were found in atmospheric laminar plasma sprayed metal coatings. • A splashes gathering model was proposed and deposition mechanism was established. [ABSTRACT FROM AUTHOR]