Search

Plasma spray physical vapor deposition (PS-PVD) is a technology that combines the advantages of traditional atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). As the feedstock of the PS-PVD, nano-agglomerated powder is critical on determining the microstructure of the obtained coating. In this study, a method to characterize the cohesion of nano-agglomerated powders was investigated. The nano-agglomerated powders fractured into smaller particles under ultrasonic waves. Their particle size distributions were measured to quantitatively compare their cohesiveness. The change rate in the percentage of powders with particle size less than 5 μm was selected as the value for the cohesion comparison. A high change rate corresponded to a faster fracture and lower powder cohesion. Furthermore, the fracture behavior and heat and mass transfer process of nano-agglomerated powders in the plasma torch were studied by combining 3-D simulation and observation of the microstructures of PS-PVD coatings sprayed with different powders. To obtain a quasi-columnar coating, the nano-agglomerated powder required high cohesion. Finally, a suitable powder was selected and quasi-columnar structure coatings were obtained by optimizing the PS-PVD parameters. [ABSTRACT FROM AUTHOR]

Currently, the self-shadowing effect of impinging particles is recognized as a vital factor to form columnar-like coating in the manufacture of thermal barrier coatings. Most of these quasi-columnar coatings are usually prepared under a very low-pressure condition. This paper investigates a novel quasi-columnar yttria stabilized zirconia (YSZ) coating using an atmospheric plasma spray-physical vapor deposition method. The microstructures of the coating present a quasi-columnar structure that is distributed along the cross-section of the coating within certain intervals with a large number of cluster-like structures on the top surface of the coating. A lower particle velocity that contributes to the generation of a mass of vapor YSZ materials is studied via experimental and numerical analyses and these results are compared with other current plasma spray methods. The mechanism of the self-shadowing effect from impinging particles that leads to the formation of a quasi-columnar feature at the boundary layer of the substrate is demonstrated by a three-dimensional numerical simulation and experimental observation. Furthermore, the hybrid growth model of the vapor and droplet co-deposited coating is clarified in this paper. [ABSTRACT FROM AUTHOR]

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 structure of yttria stabilized zirconia coatings was developed in this study by a newly laminar plasma spraying technology in atmospheric environment. The unique microstructures of coatings showed the multi-island protrusions feature at the top surface, quasi-columnar structures along the cross-section distributed as a certain interval and hybrid droplet/vapor deposited structures at the fracture surface. The distributions of particle velocity and surface temperature along the axial direction of long laminar plasma jet were investigated and compared with other plasma spraying methods. The effects of different microstructures to thermal conductivities comparing with other current plasma spray methods were also demonstrated. [ABSTRACT FROM AUTHOR]

A novel expansion and super long plasma plume at low vacuum conditions (200 Pa–10000 Pa) has been developed to deposit the ceramic thermal barrier coating (TBC) for aircraft engines. The maximum length of the Ar–He plasma plume is 1900 mm when the plasma torch operates at a chamber pressure of 200 Pa and an input power of 64 kW (I = 2000 A). Meanwhile, the radial expansion of the plasma plume is 90 mm in an axisymmetric plane at the same condition. The maximum substrate temperature ranges from 1100 °C to 1250 °C when the plasma plume impinges perpendicularly on the graphite substrate at the distance from 800 mm to 1500 mm. It is also found that the Mach number of the plasma flow is 3.4. The numerical simulation predicts the maximum velocity and temperature of the plasma plume are 6474.8 m/s and 12823.7 K, respectively. • A novel expansion and super long plasma plume at low vacuum are studied. • The maximum length of the Ar–He plasma plume is 1900 mm. • The length of the plasma plume increases with the decreasing of chamber pressure. • Mach number of the plasma flow at 200 Pa is 3.4. [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]

La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is a promising cathode material for solid oxide fuel cells (SOFCs). Hence, structured LSCF coatings were deposited in this study by atmospheric laminar plasma spraying (ALPS) at the spraying distances from 150 mm to 250 mm. Particles with high temperature were obtained at low plasma arc power of only 15 kW. The microstructural characterization of the coatings showed cluster-like morphologies on the top surfaces and columnar features with large-scale vertical cracks at the cross section. An evolution mechanism of the coatings was proposed based on particle size. The movement of particles during laminar plasma jet depended on particle size, which was studied experimentally and by simulation to explain the selective deposition. The coatings showed lower polarization resistance when compared to LSCF coatings deposited by atmospheric plasma spraying (APS) due to large vertical crack densities. The high output performance of the SOFCs with the LSCF cathode also proved this phenomenon. The results suggest that ALPS provides extensive options for SOFCs manufacturing. • LSCF coatings were deposited at the long spraying distances, although at low plasma arc power of only 15 kW. • The selective deposition depended on particle size was proposed to explain various structures of coatings. • Structured coatings with large-scale vertical cracks revealed lower polarization resistances compared to APS coatings. [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]

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]