A design of experiment study of plasma-sprayed alumina-titania coatings

This article presents an experimental study of the plasma spraying of alumina- titania powder. This powder system is being used to fabricate heater tubes that emulate nuclear fuel tubes for use in thermal-hydraulic testing. Coating experiments were conducted using a Taguchi fractional- factorial design parametric study. Operating parameters were varied around the typical spray parameters in a systematic design of experiments to display the range of plasma processing conditions and their effect on the resultant coating. The coatings were characterized by hardness and electrical tests, image analysis, and optical metallography. Coating qualities are discussed with respect to dielectric strength, hardness, porosity, surface roughness, deposition efficiency, and microstructure. The attributes of the coatings are correlated with the changes in operating parameters.     

Effect of Spray Parameters on the Corrosion Behavior of HVOF Sprayed WC-Co-Cr Coatings

WC-10Co-4Cr cermet coatings were deposited on the substrate of AISI 1045 steel by using high-velocity oxygen-fuel (HVOF) thermal spraying process. The Taguchi method including the signal-to-noise (S/N) ratio and the analysis of variance (ANOVA) was employed to optimize the porosity and, in turn, the corrosion resistance of the coatings. The spray parameters evaluated in this study were spray distance, oxygen flow, and kerosene flow. The results indicated that the important sequence of spray parameters on the porosity of the coatings was spray distance > oxygen flow > kerosene flow, and the spray distance was the only significant factor. The optimum spraying condition was 300 mm for the spray distance, 1900 scfh for the oxygen flow, and 6.0 gph for the kerosene flow. The results showed the significant influence of the microstructure on the corrosion resistance of the coatings. Potentiodynamic polarization and electrochemical impendence spectroscopy (EIS) results showed that the WC-10Co-4Cr cermet coating obtained by the optimum spraying condition with the lowest porosity exhibits the best corrosion resistance and seems to be an alternative to hard chromium coating.     

Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings

Seven different Al₂O₃-based suspensions were prepared by dispersing two nano-sized Al₂O₃ powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water + isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters.The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well-flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~ 1-2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to > 50%), as no particle or agglomerate larger than ~ 2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to a few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surface chemistry), poor deposition efficiencies and porous layers are obtained.Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~ 70%) is obtained by suitable mixtures of micron- and nano-sized powders, on account of synergistic effects.     

Optimization of Plasma Spray Process Using Statistical Methods

The microstructure features of coatings produced by a plasma spray process are affected significantly by the process parameters such as powder size, spray gun nozzle size, total plasma gas flow, ratio of H₂ + N₂ over total gas flow, and so on. This article presents a study of the effects of these parameters on the microstructure (porosity, formation of crack, unmelted particle and oxide phase) of NiCrAlY coatings deposited by the Mettech Axial III™ System. A Taguchi array is used to design the spraying process parameters. The results of the microstructure evaluation are used to generate regression equations for the prediction of coating microstructure based on process parameters. The results predicted from the regression equations are in good agreement with the experimental results according to a confidence level of 0.95. Among the parameters examined, the powder size and the ratio of H₂ + N₂ over total gas flow rate are the most significant parameters affecting the occurrence of crack, porosity, unmelted particle and oxide. Within the range of the designed process parameters, lower powder size and higher ratio of H₂ + N₂ over total gas flow rate lead to less cracks, pores, unmelted particles but more oxides. Nozzle size has marginal influence on oxides which increase with nozzle size. Gas flow rate has no direct influence on any coating feature evaluated with the range of variation.     

Processing-microstructure relationships for plasma-sprayed yttrium aluminum garnet

Plasma-sprayed yttrium aluminum garnet (YAG) has been considered as a component in a thermal barrier coating system to reduce oxidation of the bond coat by impeding oxygen diffusion through the coating. For this application, a fully crystalline, dense YAG coating would be advantageous to maximize lifetime and minimize oxygen diffusion. The effects of nine processing variables on the porosity and percent crystallinity in plasma-sprayed YAG were determined. Two powder types were investigated to compare a commercial, off-the-shelf, fused-and-crushed powder to a specially-processed, spherical plasma-spray powder. The resultant models suggest that plasma torch power and spray distance had the largest effect on the responses. It was determined that the processing parameters that lead to a coating with low porosity produce a coating with low crystallinity and vice versa. A possible route to producing a dense, crystalline coating was explored where a dense, amorphous coating was subsequently heat treated to produce crystalline material that retained the microstructure of the as-sprayed YAG in the bulk of the coating. However, macrocracking in the system due to thermal mismatch and crystallization stresses would need to be addressed for a viable multilayer TBC.     

Formation of molybdenum boride cermet coating by the detonation spray process

The effects of the powder particle size and the acetylene/oxygen gas flow ratio during the detonation spray process on the amount of molybdenum phase, porosity, and hardness of the coatings using MoB powder were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), etc. The results show that the presence of metallic molybdenum in the coating results from decomposition of MoB powder during thermal spray. The compositions of the coatings are metallic Mo, MoB, and Mo₂B, which are different from the phases of the original powder. The amount of molybdenum phase increases monotonously with the oxygen/acetylene ratio, but the increasing rate for the fine powder is faster than that for the coarse powder. The porosity and hardness of the coating are related to the amount of molybdenum phase. The phase constitution of the coating is discussed.     

Cold Spray Al-5% Mg Coatings for the Corrosion Protection of Magnesium Alloys

Poor corrosion resistance is a significant limitation of magnesium alloys as structural materials. To address this problem, the objective of this study was to apply to a magnesium alloy a corrosion-resistant barrier coating that has galvanic compatibility with magnesium and a hardness value no less than that of magnesium. Aluminum coatings were applied to ZE41A-T5 Mg by the cold spray process. A custom-made high-purity Al-5 wt.% Mg powder was produced by spray metal forming for the coating evaluation. In addition, coatings of commercially pure Al (99.5 wt.%), high-purity Al (99.95 wt.%), AA5356, and AA4047 were used for comparison. Coating evaluation included mechanical testing (hardness and adhesion strength) and corrosion testing (salt spray, galvanic coupling, and crevice corrosion). The Al-5% Mg powder resulted in the best overall performance, including a high hardness, 125 H_v100, and an adhesion strength, over 60 MPa, when treated for over 1000 h in a salt spray chamber and with a low galvanic current.     

超音速火焰喷涂合成TiC-Ni涂层滑动磨损性能研究

采用二次正交回归试验方法得到了喷涂工艺参数与反应超音速喷涂合成涂层滑动磨损性能的定量关系，研究了涂层的磨损失效机制，并分析了氧气流量、燃气流量和喷涂距离对超音速火焰喷涂合成TiC-Ni涂层滑动磨损性能的影响。结果表明：涂层的磨损机制以粘结相的优先磨损以及硬质相的剥落引起的磨粒磨损为主。氧气流量、燃气流量和喷涂距离对涂层滑动磨损性能有较大影响。适中的氧气流量、燃气流量有利于获得耐磨性较好的涂层，喷涂距离较小时，涂层的磨损失重量变化不明显．喷涂距离较大时，失重量较高。     

Novel application of nanocrystalline nickel electrodeposit: Making good diamond tools easily, efficiently and economically

Electrodeposition has been identified as a feasible and economical technique for nanomaterials application. This article details an improved approach to producing better diamond tools at lower cost and with higher productivity. Pulse-electroformed nanocrystalline nickel was used as the new matrix. The pulse parameters were determined after examination of the microstructure, grain size, hardness and tensile strength of the deposits obtained at different average current densities (J_m) with constant pulse-on time and pulse-off time. It is shown that, with J_m ranging from 1 Adm^− 2 to 14 Adm^− 2, the grain size decreases sharply from 180 nm to about 10 nm while the hardness and tensile strength significantly increase at first and then reach their peaks respectively, although the strength fails to stay long. Current density J_m that produced the highest hardness and strength of deposit (with grain size of 20 nm) was chosen for new diamond tools that exhibited 20.2% longer service life than their usual Ni-Co counterparts. Therefore, nanocrystalline electrodeposits are expected to be an upgrading substitute for conventional polycrystalline matrix.     

Elimination of porosity in directly fabricated titanium via cold gas dynamic spraying

Cold gas dynamic spray (cold spray) is a deposition technology in which particles achieve supersonic velocities. Commercial purity titanium (CP Ti) is directly fabricated to exploit the potential for cold spray to be utilised as a new fabrication technology. Deposition parameters for elimination of porosity were identified. Results show that using helium as the deposition gas has a significant influence on the elimination of pores. The volume fraction of porosity decreased with a decrease in the distance between the nozzle and the substrate. Furthermore, a decrease in average particle size led to a reduction in porosity of the deposited material. The results show that the directly fabricated CP Ti exhibits a higher hardness compared with wrought CP Ti. Optimisation of cold spray parameters led to a significant reduction of pores in directly fabricated parts.     

HVOF Combustion spraying of inconel powder

A major trend in the thermal spray industry has been to increase the gas jet velocity to obtain better coating attributes. One emerging technology now used in industry is the high-velocity oxygen fuel process (HVOF). High-velocity spray guns combine oxygen and a fuel gas to generate heat and extremely high particle velocities. In this study, Inconel 718 powder was deposited on steel substrates. The primary coating function was electrical resistivity for a heater application. Experiments were conducted using a Taguchi L8 statistical fractional/factorial design parametric study. The Taguchi experiment evaluated the effect of six HVOF processing variables on the measured responses. The parameters were oxygen flow, fuel flow, air envelope gas flow, powder feed rate, spray distance, and nozzle configuration. The coatings were characterized by hardness tests, surface profilometry, optical metallography, and image analysis. This article investigates coating hardness, porosity, surface roughness, deposition efficiency, and microstructure with respect to the influence of the processing parameters. Analytical studies were conducted to investigate gas, particle, and coating dynamics for two of the HVOF thermal spray experiments.     

Atmospheric plasma spraying of yttria-stabilized zirconia coatings with specific porosity

The interdependence between plasma spray process parameters and porosity of YSZ coating microstructures was investigated with simultaneous consideration of the deposition efficiency. Based on a factorial experimental plan, the argon plasma gas flow, the current, the interaction of argon flow and current, and the spray distance for the Triplex II plasma gun were found to yield the main contributions to porosity as well as to deposition efficiency.Each of these three process parameters has a significant individual effect on the in-flight particle velocities and temperatures. The contribution to the effects on porosity arises almost exclusively from the particle temperature. Regarding the deposition efficiency, the larger contribution originates from the particle velocity.To achieve a targeted high porosity at reasonable deposition efficiency a simple linear regression model was applied yielding an argon flow of 50 slpm and a current of 470 A at a spray distance of 200 mm as the optimum parameter set. The average particle temperature estimated for this optimum is just above the melting temperature. At this setting, a porosity of 17.7% and a deposition efficiency of 32.5% may be expected.At a greater spray distance and lower power density (lower current and/or higher argon plasma gas flow) the deposition efficiency was observed to drop considerably. The cooling of the particles here becomes critical, i.e. the particles are only partly molten. This was verified by an analysis of the density distributions of measured in-flight particle temperatures.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

Effect of processing parameters on the microstructures and corrosion behaviour of high‐velocity oxy‐fuel (HVOF) sprayed Fe‐based amorphous metallic coatings

A FeCrMoMnWBCSi amorphous metallic coating was prepared by using high‐velocity oxy‐fuel spray. The influence of processing parameters on microstructure, porosity level, amorphous phase fraction and corrosion behaviour of the coatings was characterised by scanning electron microscopy, X‐ray diffraction, differential scanning calorimeter and electrochemical methods. The results indicated that the microstructures of the coatings were sensitive to the spray parameters considerably. Porosity and unmelted particle proportion decreased with the oxygen/fuel (O/F) ratio and increased with the powder feed rate. The trend of oxides content was opposite to the porosity and unmelted particle proportion. The coatings obtained with higher O/F ratio and lower powder feed rate exhibited higher hardness. The low coating hardness was mainly due to the high porosity especially when the porosity was higher than 1.21%. The spraying parameters strongly affected the amorphous phase fraction. There was a critical passive current density for balancing the porosity and the amorphous phase fraction. Corrosion resistance is dominant by the amorphous phase fraction when the porosity is less than 1.21%, while by porosity when it is higher than that. Open‐circuit potential, potentiodynamic polarisation and electrochemical impedance spectroscopy results showed that the coatings obtained with the O/F ratio of 4.2 and the powder feed rate of 40 g/min exhibiting the best corrosion resistance in 1 wt% sodium chloride solution.     

超音速等离子喷涂工艺参数对 Cr2O3涂层硬度的影响

目的研究影响超音速等离子喷涂Cr2O3涂层硬度的主要因素,制备出高硬度涂层。方法首先采用单变量法研究超音速等离子生成气体压力(空气压力)和喷涂距离对涂层显微结构的影响,然后采用正交试验研究喷涂电流、空气压力、喷涂距离对Cr2O3涂层硬度的影响。结果工艺参数对Cr2O3涂层硬度影响的主次顺序为:空气压力>喷涂电流>喷涂距离。结论获得高硬度涂层的最佳工艺参数组合为:空气压力0.4 MPa,喷涂电流270 A,喷涂距离200 mm。在该工艺条件下获得的涂层致密、均匀,孔隙率小。     

Optimization of Atmospheric Plasma Spray Process Parameters using a Design of Experiment for Alloy 625 coatings

Alloy 625 is a Ni-based superalloy which is often a good solution to surface engineering problems involving high temperature corrosion, wear, and thermal degradation. Coatings of alloy 625 can be efficiently deposited by thermal spray methods such as Air Plasma Spraying. As in all thermal spray processes, the final properties of the coatings are determined by the spraying parameters. In the present study, a D-optimal experimental design was used to characterize the effects of the APS process parameters on in-flight particle temperature and velocity, and on the oxide content and porosity in the coatings. These results were used to create an empirical model to predict the optimum deposition conditions. A second set of coatings was then deposited to test the model predictions. The optimum spraying conditions produced a coating with less than 4% oxide and less than 2.5% porosity. The process parameters which exhibited the most important effects directly on the oxide content in the coating were particle size, spray distance, and Ar flow rate. The parameters with the largest effects directly on porosity were spray distance, particle size, and current. The particle size, current, and Ar flow rate have an influence on particle velocity and temperature but spray distance did not have a significant effect on either of those characteristics. Thus, knowledge of the in-flight particle characteristics alone was not sufficient to control the final microstructure. The oxidation index and the melting index incorporate all the parameters that were found to be significant in the statistical analyses and correlate well with the measured oxide content and porosity in the coatings.

Nanocrystalline NiAl Coating Prepared by HVOF Thermal Spraying

Nanocrystalline NiAl intermetallic powder was prepared by mechanical alloying (MA) of Ni₅₀Al₅₀ powder mixture and then deposited on low carbon steel substrates by high velocity oxy fuel (HVOF) thermal spray technique using two sets of spraying parameters. X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM), differential scanning calorimetry (DSC), and hardness test were used to characterize the prepared powders and coatings. The MA of Ni₅₀Al₅₀ powder mixture led to the formation of NiAl intermetallic compound. The resulting powder particles were three dimensional in nature with irregular morphology and a crystallite size of ~10 nm. This powder was thermally sprayed by HVOF technique to produce coating. The deposited coating had a nanocrystalline structure with low oxide and porosity contents. The hardness of coatings was in the range of 5.40-6.08 GPa, which is higher than that obtained for NiAl coating deposited using conventional powders.     

Improved Protection Properties by Using Nanostructured Ceramic Powders for HVOF Coatings

The potential of the high-velocity oxy-fuel (HVOF) thermal spray process for reduced porosity in coatings compared to those produced by other ambient thermal spray processes is well known. The ability to produce high-density ceramic coatings offers potential in high-performance applications in the field of wear, corrosion resistance, and dielectric coatings. However, due to operational limit of the HVOF process to effectively melt the ceramic particles, the process—structure relationship must be well optimized. It has been also demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. One strategy to improve melting of ceramic particles in relative low-flame temperatures of HVOF process is to modify particle crystal structure and composition. In this paper the effect of the powder manufacturing method and the composition on deposition efficiency of spray process as well as on the mechanical properties of the HVOF sprayed are studied. Effect of fuel gas, hydrogen vs. propane, was also demonstrated. Studied materials were alumina-, chromia-, and titania-based agglomerated powders. Coating properties such as microstructure, hardness, abrasive wear resistance, and relative fracture toughness were compared to the coating manufactured by using conventional fused and crushed powders. It can be concluded that powder size distribution and microstructure should be optimized to fulfill process requirements very carefully to produce coatings with high deposition efficiency, dense structure, improved fracture toughness, and adhesion.     

高速电弧喷涂不锈钢涂层的旋转回归试验研究

对比研究了３Ｃｒ１３不锈钢高速电弧喷涂涂层和普通电弧喷涂层的孔隙率、氧化物含量、涂层结合强度和滑动磨损性能,并采用多因素正交旋转回归设计和单因素对比试验的方法了高速电弧涂工艺参数对３Ｃｒ１３不钢涂层显微硬度的影响,用微机回归得到了喷涂工艺参数与涂层硬度之间的定量关系式。结果表明,和普通电弧喷涂涂层相比,高速电弧喷涂涂层组织致密、孔隙度低、涂层氧化物含量增多,涂层－基体界面结合强度大于４０ＭＰａ,比     

Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al₂O₃ coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al₂O₃ coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al₂O₃ coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 μm and the substrate temperature of 500 °C. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness.