Atmospheric Plasma Spray Process Research Articles

Hydroxyapatite coatings deposited by liquid precursor plasma spraying: controlled dense and porous microstructures and osteoblastic cell responses

Hydroxyapatite coatings were deposited on Ti-6Al-4V substrates by a novel plasma spraying process, the liquid precursor plasma spraying (LPPS) process. X-ray diffraction results showed that the coatings obtained by the LPPS process were mainly composed of hydroxyapatite. The LPPS process also showed excellent control on the coating microstructure, and both nearly fully dense and highly porous hydroxyapatite coatings were obtained by simply adjusting the solid content of the hydroxyapatite liquid precursor. Scanning electron microscope observations indicated that the porous hydroxyapatite coatings had pore size in the range of 10–200 µm and an average porosity of 48.26 ± 0.10%. The osteoblastic cell responses to the dense and porous hydroxyapatite coatings were evaluated with human osteoblastic cell MG-63, in respect of the cell morphology, proliferation and differentiation, with the hydroxyapatite coatings deposited by the atmospheric plasma spraying (APS) process as control. The cell experiment results indicated that the heat-treated LPPS coatings with a porous structure showed the best cell proliferation and differentiation among all the hydroxyapatite coatings. Our results suggest that the LPPS process is a promising plasma spraying technique for fabricating hydroxyapatite coatings with a controllable microstructure, which has great potential in bone repair and replacement applications.

Reactive Atmospheric Plasma Spraying of AlN Coatings: Influence of Aluminum Feedstock Particle Size

Feedstock powder characteristics (size distribution, morphology, shape, specific mass, and injection rate) are considered to be one of the key factors in controlling plasma-sprayed coatings microstructure and properties. The influence of feedstock powder characteristics to control the reaction and coatings microstructure in reactive plasma spraying process (RPS) is still unclear. This study, investigated the influence of feedstock particle size in RPS of aluminum nitride (AlN) coatings, through plasma nitriding of aluminum (Al) feedstock powders. It was possible to fabricate AlN-based coatings through plasma nitriding of all kinds of Al powders in atmospheric plasma spray (APS) process. The nitriding ratio was improved with decreasing the particle size of feedstock powder, due to improving the nitriding reaction during flight. However, decreasing the particle size of feedstock powder suppressed the coatings thickness. Due to the loss of the powder during the injection, the excessive vaporization of fine Al particles and the completing nitriding reaction of some fine Al particles during flight. The feedstock particle size directly affects on the nitriding, melting, flowability, and the vaporization behaviors of Al powders during spraying. It concluded that using smaller particle size powders is useful for improving the nitriding ratio and not suitable for fabrication thick AlN coatings in reactive plasma spray process. To fabricate thick AlN coatings through RPS, enhancing the nitriding reaction of Al powders with large particle size during spraying is required.

Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO 2 detection

This work studied the possibility of using a sensor based on plasma-sprayed zinc oxide (ZnO) sensitive layer for NO 2 detection. The atmospheric plasma spray process was employed to deposit ZnO gas sensing layer and the obtained coating structure was characterized by scanning electron microscopy and X-ray diffraction analysis. The influences of gas concentration, working temperature, water vapor in testing air on NO 2 sensing performance of the ZnO sensors were studied. ZnO sensors showed a good sensor response and selectivity to NO 2 at an optimal working temperature.

Microstructures and tribological properties of plasma sprayed WC–Co–Cu–BaF 2/CaF 2 self-lubricating wear resistant coatings

A promising WC–Co–Cu–BaF 2/CaF 2 self-lubricating wear resistant coating was deposited via atmospheric plasma spraying (APS) process by using homemade feedstock powders composed of WC–Co, Cu and BaF 2/CaF 2 eutectic. The as-prepared cermet coatings had better frictional behavior comparing with the WC–Co coating. Moreover, the often-occurred decarburization of WC in APS process was noticeably improved due to the binding of copper and BaF 2/CaF 2 phase, which not only offered effective solid lubrication, but also acted as bind phases to mend the microstructure and protected WC from decomposition. The optimized specimen contained 10 wt.% Cu and 10 wt.% BaF 2/CaF 2 in a WC–Co matrix, which had excellent frictional and wear performance. The wear mechanism of the self-lubricating wear resistant coating was discussed with the microstructures, compositions and mechanical properties of the composite materials in detail.

Cubic Aluminum Nitride Coating Through Atmospheric Reactive Plasma Nitriding

Aluminum nitride is a promising material for structural and functional applications. Cubic AlN (c-AlN) is expected to have higher thermal conductivity due to their high symmetry; however, its fabrication is difficult. In this study, c-AlN was synthesized by atmospheric plasma spray process through the reaction between Al feedstock powder and nitrogen plasma. Al powders were supplied to the plasma stream by Ar carrier gas and reacted with surrounding N2 plasma, then deposit onto substrate. The obtained coatings were c-AlN/Al mixture at 150 mm of spray distance, and the nitride content was improved by increasing the spray distance. The coatings almost consist of c-AlN at 300 mm of spray distance. The coatings thickness decreased from 100 to 10 μm with increasing spray distance from 150 to 300 mm. Using carrier gas, N2 enable to fabricate thick c-AlN coating with hardness 1020 Hv.

Thermal and Electrical Properties of Copper Coatings Produced by Cold Spraying

A cold spray process has been developed for dense and clean and tightly bonded coatings in contract with other established thermal spray processes. The cold sprayed copper is particularly well suited for metallic coatings, such as electric conductor and heat-sink. However, the thermal and electrical properties of copper coatings formed by the cold spray process have not always been clarified. Four kinds of free-standing copper specimens, which were formed by the cold spray process and an atmospheric plasma spray process, were machined from the thick coatings and were used for measuring the thermal conductivity, specific heat and electrical resistivity etc. The experimental results suggested that the porosity and oxide content had important effects on the thermal and electrical properties. Namely, high thermal conductivity and the low electrical resistivity of coatings could be obtained from the cold spray process, because of the low porosity and the low oxide content. Also, it was confirmed that the thermal conductivity of cold sprayed copper could be improved at high temperature by the sintering effect.

대기 플라즈마 용사공정을 이용한 Cu계 벌크 비정질 금속 코팅의 미세조직 분석과 나노 압입시험을 이용한 상 분석

In this study, Cu-based bulk metallic glass (BMG) coatings were deposited by atmospheric plasma spraying (APS) process with different process conditions (with- and without hydrogen gas). As adding the hydrogen gas, thermal energy in the plasma flame increased and induced difference in the melting state of the Cu-based BMG particles. The microstructure and mechanical properties of the coatings were analyzed using a scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) and nano-indentation tester in the light of phase analysis. It was elucidated by the nano-indentation tests that un-melted region was a mainly amorphous phase which showed discrete plasticity observed as the flow serrations on the load.displacement (P - h) curves, and the curves of solidified region showed lower flow serrations as amorphous phase mingled with crystalline phase. Oxides produced during the spraying process had the highest hardness value among the phases and were well mixed with other phases resulted from the increase in melting degree.

Effect of Spraying Parameters on Deposition Efficiency and Wear Behavior of Plasma Sprayed Alumina-Titania Composite Coatings

The effects of parameters, in the process of plasma-sprayed ceramic coating, upon the deposition efficiency of alumina-13 wt.% titania composite coatings are reported. The coatings were prepared by the atmospheric plasma spray process. The plasma torch input power, flow rates of primary, secondary and carrier gas, powder feed rate and spraying distance were considered as variables. The results show that the variations in all the selected spraying parameters strongly affect the deposition efficiency. The micro-hardness, as well as erosive and sliding wear rates of the coating are also affected by these parameters. Especially the input power strongly affects the phase and microstructure of the coatings.

Oxidation and crystallization mechanisms in plasma-sprayed Cu-based bulk metallic glass coatings

Cu-based bulk metallic glass (BMG) coatings were built-up by the atmospheric plasma spraying (APS) process at different hydrogen flow rates. As the hydrogen flow rate increased, thermal energy in the plasma jet increased the melting state of the Cu-based BMG particles. Although it was expected that the difference in melting states would quantitatively affect the amorphous fractions in the coatings, both the amorphous fraction and the oxide content of the layers were independent of hydrogen flow rate. However, different melting states resulting from different hydrogen gas flow rates, and the subsequent oxidation which inevitably occurred during spraying, generated different crystallization processes in the coating. In this paper, crystallization mechanisms in plasma-sprayed BMG were studied using their microstructures. Specific analyses of the particles deposited at high travel speed and quenched in cold water also served for further understanding of the melting state and change in composition of the particles undergo during deposition.

Manufacturing Optimization for Bondcoat/Thermal Barrier Coating Systems

A reliable lifetime prediction rule for bondcoat/thermal barrier coating (BC/TBC) coated parts in gas turbine operation is necessary to determine remnant service life. The specimens investigated were coated with MCrAlY plus yttria partially stabilized zirconia applied by vacuum plasma spraying and atmospheric plasma spraying processes, respectively. The performances of these laboratory specimens were statistically assessed, combining long term oxidation testing with thermal cycling, thus superimposing thermomechanical loading on the laboratory specimens to more accurately represent engine conditions. A design of experiment (DOE) approach was used for manufacturing optimization of the BC/TBC system. The life of the coating system is influenced by several manufacturing parameters such as BC thickness, BC roughness, TBC thickness, TBC porosity, and TBC stiffness. Specimens with a suitable variation in these parameters were produced to ensure a balanced test matrix of fractional factorial DOE. Based on results derived from laboratory testing the specifically tailored parts, first and second order effects of manufacturing parameters on lifetime were quantified. The findings revealed that the second order effects (the interaction of manufacturing parameters) were more important on the lifetime of the BC/TBC system than the corresponding first order effect (single parameter). For instance, the variation in BC thickness or BC roughness led to a scatter of lifetimes of 10% and 60%, respectively, whereas their interaction resulted in a scatter of lifetime of 150% for the same range of coating parameters. Further examples of such pairings are also demonstrated. Finally, a lifetime prediction for three quality classes (high, medium, and low qualities) has been demonstrated. The difference in achievable lifetime highlights the importance of manufacturing parameters in determining the life of the BC/TBC system.

In-flight and upon impact particle characteristics modelling in plasma spray process

In Atmospheric plasma spray process, the in-flight particle characteristics such as their particle size, velocity and surface temperature influence significantly their flight duration and consequently their melting degree. The knowledge of the correlations between process parameters and in-flight particle characteristics is very important for optimizing the coating qualities. Artificial neural networks was trained and optimized to establish the relationships linking in-flight particle average diameter and process parameters to in-flight particle average velocity and surface temperature. Then, the established ANN relationships permitted to determine the in-flight particle average velocity and surface temperature versus their diameter for given process parameters. These predicted average velocity and surface temperature data were then used to determine the time for complete melting of the particle and its dwell-time before impact by an analytical model for given operating conditions.

APS Deposition of MnCo2O4 on Commercial Alloys K41X used as Solid Oxide Fuel Cell Interconnect: The Importance of Post Heat-treatment for Densification of the Protective Layer

The continuous contamination of the cathode with chromium released from metallic interconnects speeds up the degradation of the performance of the cell. For this reason Solid Oxide Fuel Cell (SOFC) interconnects are coated with a dense protective layer. This layer plays the role of a diffusion barrier for the chromium evaporation at operating conditions. The deposition of a dense, gas tight and stable MnCo2O4 spinel layer on ferritic alloy using the atmospheric plasma spraying (APS) process is studied. The first experimental results show that the plasma spray process combined with an appropriate post thermal-treatment is suitable to form a dense spinel protective layer.

Effect of in-flight particle characteristics on the coating properties of atmospheric plasma-sprayed 8mol% Y2O3–ZrO2 electrolyte coating studying by artificial neural networks

Abstract In-flight particle characteristics (surface temperature and velocity upon impact) are among the most important parameters which influence the coating microstructures and properties in atmospheric plasma spraying (APS) process. The purpose of this paper is to study hydrogen fraction used as secondary plasma forming gas on the in-flight particle surface temperature and by extension on the coating microstructures of atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia electrolyte coatings implementing in particular artificial neural networks (ANN). Then, the predicted in-flight particle characteristics were on the one hand compared to experimental values and on the other hand correlated to some of the coating structural attributes (porosity and gas specific permeability). The predicted results were in good accordance with the experimental data. Results showed that the H 2 flow rate had obvious influence on particle temperature and had almost no significant effect on particle velocity. Increasing the particle temperatures induced dense coating microstructure and improved the gas-tightness performance.

Nanomechanical properties and microstructure of ZrO 2/Al 2O 3 plasma sprayed coatings

Nanocomposite coatings were prepared by atmospheric plasma spraying process using the Yttria-stabilized zirconia (YSZ) power mixed 5 wt.% nano-Al 2O 3. The mechanical properties of the nanocomposite coatings were studied by nanoindentation and the microstructures were studied by SEM and TEM. Compared to the microcomposite coatings prepared with YSZ powers without nano-Al 2O 3, the nanocomposite coatings can better tolerate applied loads and has higher hardness and elastic modulus values, better elastic recovery, and excellent microstructure. Associated with the investigation of the microstructure, the nano-Al 2O 3 is shown to play a major role in the grain refinement, densification, and microcrack formation, and be beneficial for better mechanical properties of the coatings.

Artificial neural networks implementation in plasma spray process: Prediction of power parameters and in-flight particle characteristics vs. desired coating structural attributes

Artificial neural networks (ANN) were implemented to predict atmospheric plasma spraying (APS) process parameters to manufacture a coating with the desired structural characteristics. The specific case of predicting power parameters to manufacture grey alumina (Al 2O 3–TiO 2, 13% by wt.) coatings was considered. Deposition yield and porosity were the coating structural characteristics. After having defined, trained and tested ANN, power parameters (arc current intensity, total plasma gas flow, hydrogen content) and resulting in-flight particle characteristics (average temperature and velocity) were computed considering several scenarios. The first one deals at the same time with the two structural characteristics as constraints. The others one deals with one structural characteristic as constraint while the other is fixed at a constant value.

Plasma Spray Coatings of Ni-Al-SiC Composite

Ni-Al-SiC powder mixture containing 12 wt.% SiC was prepared by conventional ball milling. Morphological and microstructural investigations showed that powder particles after 15 h of milling time had the optimum characteristics with respect to their size and microstructure. X-ray diffraction patterns of powder particles included only the elemental Ni, Al, and SiC peaks without any traces of oxides or intermetallic phases. The powder mixture was then deposited onto a steel substrate by atmospheric plasma spray (APS) process under different conditions. The results showed that under APS conditions used here, the coatings were composed of various intermetallics including Ni-Al and Ni2Al3. The mean hardness of coating was found to be about 567 HV. It was also found that by increasing current density of APS, the coating/substrate adhesive strength was increased.

Plasma Spray of Free-Standing Components for Bone Tissue Engineering

This research work deals with the development of free-standing hydroxyapatite (HA) components produced using the Atmospheric Plasma Spraying Process. The spray parameters were based on the optimal values found in previous work for the HA powder used. The deposition time used to produce the free-standing coupons was varied between 70 and 150 seconds. The influence of spray time on the deposit thickness and the resulting crystallinity of the coupons were investigated. The surface of the samples was characterised by means of SEM and surface roughness was measured using a laser profiler. The crystallinity of the samples was analysed using XRD. The phase content of the coupons was investigated using XRD and Raman Spectroscopy. The crystallinity and thickness of the coupons was found to increase with increasing spray time. A maximum crystallinity of 89% and maximum average thickness of 2.45 mm were obtained.

Development of high quality thermal spraying process by shielding control

The shielding controlled plasma spraying process was investigated to improve corrosion resistance of metal surfaces. In this process, a shielding nozzle that covered only spraying area was attached in front of the tip of a commercial plasma spray gun nozzle, and the environment surrounding the plasma jet was controlled by nitrogen flow. When the oxygen concentration in the shielding nozzle was maintained at 0·5%, the metal oxide contents in CoNiCrAlY coating and the porosity of the coating reduced to 0·2 and 0·3% at optimal spray particle size respectively. The corrosion potential in an acid solution including chloride ions was almost constant for 1000 h for CoNiCrAlY coating deposited by shielding controlled plasma spraying. On the other hand, those obtained by atmospheric plasma spraying process decreased in the corrosion potential largely after 10 h. Thus, it can be concluded that the shielding controlled plasma spraying process improves the corrosion resistance of the metal.

Evaluation of Microstructure for Copper Coatings Produced by Cold Spraying

A cold spraying process has been developed for dense and clean and tightly bonded coatings in contract with other established thermal spraying processes. The cold sprayed copper is particularly well suited for metallic coatings, such as electric conductor and heat-sink. However, the microstructure of copper coatings formed by the cold spraying have not always been clarified everything. Two kinds of free-standing copper specimens, which were formed by the cold spray process and an atmospheric plasma spray process, were machined from the thick coatings and were used for measuring the density, microstructure, oxygen distribution and pore size distribution. As a result, it was confirmed that the cold sprayed copper showed remarkable high density and low oxygen content in comparison with the atmospheric plasma sprayed copper. Also, these experimental results showed that the pores below the size of a few μm in diameter could not be observed in case of the cold sprayed copper though the pore size distribution of the atmospheric plasma sprayed copper was extensive.

Development and microstructure optimization of atmospheric plasma-sprayed NiO/YSZ anode coatings for SOFCs

In this paper, preparation and characterization of porous anode layers with uniform phase distribution are discussed for solid oxide fuel cell (SOFC) application. The Ni/8YSZ cermet coatings were fabricated by atmospheric plasma spray (APS) process using oxidized nickel coated graphite (Ni-graphite) and 8 mol% yittria — stabilized zirconia (8YSZ) blend as feedstock. To control the microstructure of the coating, the nickel coated graphite with low density was used as a starting feedstock instead of conventional pure nickel (Ni) powder. To balance the conductivity, uniform porosity, and structural stability of the coatings, the effects of process parameters such as hydrogen gas flow rate, stand off distance and pore formation precursor (graphite) addition on the microstructures of the resulting coatings are investigated. The results show that the anode coatings with high conductivity, structural stability and porosity could be deposited with moderate hydrogen gas flow rate and short stand off distance.