Porous Architecture of SPS Thick YSZ Coatings Structured at the Nanometer Scale (~50 nm)

Suspension plasma spraying (SPS) is a fairly recent technology that is able to process sub-micrometer-sized or nanometer-sized feedstock particles and permits the deposition of coatings thinner (from 20 to 100 μm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists of mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 μm. Due to the large volume fraction of the internal interfaces and reduced size of stacking defects, thick nanometer- or sub-micrometer-sized coatings exhibit better properties than conventional micrometer-sized ones (e.g., higher coefficients of thermal expansion, lower thermal diffusivity, higher hardness and toughness, better wear resistance, among other coating characteristics and functional properties). They could hence offer pertinent solutions to numerous emerging applications, particularly for energy production, energy saving, etc. Coatings structured at the nanometer scale exhibit nanometer-sized voids. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers, from connected to non-connected network. Nevertheless, the discrimination of porosity in different classes of criteria such as size, shape, orientation, specific surface area, etc., is essential to describe the coating architecture. Moreover, the primary steps of the coating manufacturing process affect significantly the coating porous architecture. These steps need to be further understood. Different types of imaging experiments were performed to understand, describe and quantify the pore level of thick finely structured ceramics coatings.     

Suspension Plasma-Sprayed Alumina Coating Structures: Operating Parameters Versus Coating Architecture

Suspension plasma spraying (SPS) is able to process sub-micrometric-sized feedstock particles and permits the deposition of layers thinner (from 5 to 50 μm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 μm, average values. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and evaporation. Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers. Nevertheless, the coupling between the parameters controlling the coating microstructure and properties are not yet fully identified. The aim of this study is to further understand the influence of parameters controlling the manufacturing mechanisms of SPS alumina coatings, particularly the spray beads influence.     

Latest Developments in Suspension and Liquid Precursor Thermal Spraying

The interest to manufacture onto large surfaces thick (i.e., 10-20 μm, average thickness) finely structured or nanostructured layers is increasingly growing since the past 10 years. This explains the interest for suspension thermal spraying (STS) and solution precursor thermal spraying (SPTS), both allowing manufacturing finely structured layers of thicknesses varying between a few micrometers up to a few hundreds of micrometers. STS aims at processing a suspension of sub-micrometer-sized or even nanometer-sized solid particles dispersed in a liquid phase. The liquid phase permits the injection of particles in the thermal flow (i.e., due to their size, a carrier gas cannot play this role). SPTS aims at processing a solution of precursors under the same conditions. Upon evaporation of the liquid phase, the precursor concentration increases until precipitation, pyrolysis, and melting of small droplets occur. Compared to conventional thermal spray routes, STS and SPTS are by far more complex because fragmentation and vaporization of the liquid control the coating build-up mechanisms. Numerous studies are still necessary to reach a better understanding of the involved phenomena and to further develop the technology, among which are injection systems, suspension and solution optimizations, spray kinematics, etc. This review presents some recent developments and our present knowledge in this field together with the available tools implemented to characterize the plasma-liquid interaction and the coating formation.     

Operating parameters for suspension and solution plasma-spray coatings

The interest to manufacture on large surfaces thick (i.e., 10 to 20 μm, average thickness) finely structured or nano-structured layers is increasingly growing since about 10 years. This explains the interest for suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS), both allowing manufacturing finely structured layers of thicknesses varying between a few micrometers up to a few hundred of micrometers. SPS aims at processing a suspension of sub-micrometric-sized or even nano-metric-sized solid particles dispersed in a solvent. The liquid solvent permits to inject particles in the thermal flow (i.e., due to their size, a carrier gas cannot play this role). SPTS aims at processing a solution of precursors under the same conditions. Upon evaporation of the liquid, the precursor concentration increases until precipitation, pyrolysis and melting of small droplets. Compared to conventional plasma spraying, SPS and SPPS are by far more complex because fragmentation and vaporization of the liquid control the coating build-up mechanisms. Numerous studies are still necessary to reach a better understanding of involved phenomena and to further develop the technology, among which injection systems, suspension and solution optimizations, spray kinematics, etc. This review presents some recent developments in this field.     

Plasma Spraying of Metal Coatings Using CO2-Based Gas Mixtures

Compared to argon, plasmas generated by molecular gases and operated at the same electric current level contain much higher enthalpy. This is because molecular gases must dissociate before ionization, which requires larger energy input. The authors have developed a new DC plasma torch, which operates with a mixture of carbon dioxide (CO₂) and methane (CH₄). The torch was employed for plasma spray coating with input power in a range of 30 to 45 kW. This paper reports on the effect of CO₂/CH₄ mixtures on the particle conditions during spraying of nickel alloy powders. Results of gas composition analysis at various distances from the nozzle exit are presented. The particle in-flight conditions, coating microstructure, and deposition efficiency are analyzed.     

From Powders to Thermally Sprayed Coatings

Since the early stages of thermal spray, it has been recognized that the powder composition, size distribution, shape, mass density, mechanical resistance, components distribution for composite particles play a key role in coating microstructure and thermo mechanical properties. The principal characteristics of particles are strongly linked to the manufacturing process. Coatings also depend on the process used to spray particles and spray parameters. Many papers have been devoted to the relationships existing between coating properties and structures at different scales and manufacturing processes. In many conventional spray conditions resulting in micrometric structures, among the different parameters, good powder flow ability, and dense particles are important features. Thermal plasma treatment, especially by RF plasma, of particles, prepared by different manufacturing processes, allows achieving such properties and it is now developed at an industrial scale. Advantages and drawbacks of this process will be discussed. Another point, which will be approached, is the self-propagating high-temperature synthesis, depending very strongly upon the starting composite particle manufacturing. However, as everybody knows, “small is beautiful” and nano- or finely structured coatings are now extensively studied with spraying of: (i) very complex alloys containing multiple elements which exhibit a glass forming capability when cooled-down, their under-cooling temperature being below the glass transition temperature; (ii) conventional micrometer-sized particles (in the 30-90 μm range) made of agglomerated nanometer-sized particles; (iii) sub-micrometer- or nanometer-sized particles via a suspension in which also, instead of particles, stable sol of nanometer-sized particles can be introduced; and (iv) spray solutions of final material precursor. These different processes using plasma, HVOF or sometimes flame and also cold-gas spray will be discussed together with the production of nanometer-sized particles via the chemical reaction method or by a special type of milling: the cryogenic milling process often referred to as “cryomilling.”     

Application of Plasma Spraying as a Precursor in the Synthesis of Oxidation-Resistant Coatings

Thermal spray methods offer a versatile and flexible approach to the manufacture of coatings as a final product. A novel application of thermal spray coating is demonstrated by incorporating a plasma-sprayed Mo layer coating as a precursor step within an integrated costing design. The effectiveness of the two-step design is illustrated for aluminoborosilica coatings on SiC/C composites and W substrates based on the plasma-sprayed Mo precursor and subsequent codeposition of Si and B by a pack cementation method. Even with incomplete precursor coverage, an aluminoborosilica coating is developed because of the high initial fluidity of the as-pack coating. An effective oxidation resistance is observed following exposure at elevated temperatures (1373-1673 K) in ambient air and during torch testing at 1773 K, providing clear evidence that the plasma spraying of Mo is a viable precursor step in the formation of the oxidation-resistant Mo-Si-B-based coating.     

Permeability and Microstructure of Suspension Plasma-Sprayed YSZ Electrolytes for SOFCs on Various Substrates

Yttria-stabilized zirconia electrolyte coatings for solid oxide fuel cells were deposited by suspension plasma spraying using a range of spray conditions and a variety of substrates, including finely structured porous stainless steel disks and cathode layers on stainless steel supports. Electrolyte permeability values and trends were found to be highly dependent on which substrate was used. The most gas-tight electrolyte coatings were those deposited directly on the porous metal disks. With this substrate, permeability was reduced by increasing the torch power and reducing the stand-off distance to produce dense coating microstructures. On the substrates with cathodes, electrolyte permeability was reduced by increasing the stand-off distance, which reduced the formation of segmentation cracks and regions of aligned and concentrated porosity. The formation mechanisms of the various permeability-related coating features are discussed and strategies for reducing permeability are presented. The dependences of electrolyte deposition efficiency and surface roughness on process conditions and substrate properties are also presented.     

Improvement of Plasma Spray Torch Stability by Controlling Pressure and Voltage Dynamic Coupling

The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.     

Characterization of Yttria-Stabilized Zirconia Coatings Deposited by Low-Pressure Plasma Spraying

The research presented here aimed to apply plasma spraying at a low pressure of 100 Pa for fabricating the columnar structure or dense coatings. These coatings with different structures were elaborated from the vapor condensation and molten droplets, respectively, using the agglomerated YSZ powders and a relatively low power commercial F4-VB torch. It was shown that the crystallite size of coating deposited from the vapor condensation at a spraying distance of 200 mm was reduced to 17.1 nm from 43.7 nm of the feedstock. Observations indicated that a thin columnar structured coating was produced out of the line of sight of projection. In the line of sight of projection, the hybrid structured coating was obtained. The relatively dense coating was fabricated using a specifically designed extended nozzle. Investigations by means of optical emission spectroscopy were performed to analyze the nature of the plasma jet with YSZ powders. The Vickers microhardness was also conducted. It was found that the relatively dense coating showed a higher value in comparison to the hybrid structure coating, up to 1273 ± 56 Hv_100g.     

Vapors and Droplets Mixture Deposition of Metallic Coatings by Very Low Pressure Plasma Spraying

In recent years, the very low pressure plasma-spraying (VLPPS) process has been intensely developed and implemented to manufacture thin, dense and finely structured ceramic coatings for various applications, such as Y₂O₃ for diffusion barriers, among other examples. This paper aims at presenting developments carried out on metallic coatings. Aluminum was chosen as a demonstrative material due to its “moderate” vaporization enthalpy (i.e., 38.23 KJ cm^?3) compared to the one of copper (i.e., 55.33 KJ cm^?3), cobalt (i.e., 75.03 KJ cm^?3), or even tantalum (i.e., 87.18 KJ cm^?3). The objective of this work is primarily to better understand the behavior of a solid precursor injected into the plasma jet leading to the formation of vapors and to better control the factors affecting the coating structure. Nearly dense aluminum coatings were successfully deposited by VLPPS at 100 Pa with an intermediate power plasma torch (i.e., Sulzer Metco F4 type gun with maximum power of 45 kW). Optical emission spectroscopy (OES) was implemented to study and analyze the vapor behavior into the plasma jet. Simplified CFD modeling allowed better understanding of some of the thermo-physical mechanisms. The effect of powder-size distribution, substrate temperature and spray distance were studied. The phase composition and microstructural features of the coatings were characterized by XRD and SEM. Moreover, Vickers microhardness measurements were implemented.     

Atmospheric Plasma Spraying Low-Temperature Cathode Materials for Solid Oxide Fuel Cells

Atmospheric plasma spraying (APS) is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (500-700 °C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. However, standard cathode materials used in commercial SOFCs exhibit high polarization resistances at low operating temperatures. Therefore, alternative cathode materials with high performance at low temperatures are essential to facilitate the use of metallic supports. Coatings of lanthanum strontium cobalt ferrite (LSCF) were fabricated on steel substrates using axial-injection APS. The thickness and microstructure of the coating layers were evaluated, and x-ray diffraction analysis was performed on the coatings to detect material decomposition and the formation of undesired phases in the plasma. These results determined the envelope of plasma spray parameters in which coatings of LSCF can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured.     

Developments in direct current plasma spraying

Thermal spray processing is used to confer specific in-service properties to components via the production of a coating between 50 μm (minimum value) to a few millimeters thick. Thermal spray represents a global market of about 4.8 Billion Euros (i.e., ∼ US$5 billion) in 2004; 30% of which is European based. 50% of this activity is devoted to plasma spray processing with about 90% dedicated to direct current (DC) plasma torches. Several developments of new torch architectures, among which three-cathode torches, have evolved recently. However, most of the recent progress has been applied to conventional DC torches. The advances were related to two prime factors: (i) the development of industrial sensors permitting to diagnose the processes during spray operation (especially in-flight particle characteristics in terms of their surface temperature and velocity) and additionally the monitoring of the substrate and coating temperatures with the objective of controlling the operating parameters via a close-loop controller; (ii) the adaptation of plasma spray systems to manufacture nano-structured coatings via the development of suspension plasma spray and solution plasma spray. As well, there has been an enhanced understanding of the mechanisms controlling the coating formation and of the effects of the arc root fluctuations; thereby permitting a more robust process. This paper develops the above points by presenting focused examples.     

Desirability-Based Multi-Criteria Optimization of HVOF Spray Experiments to Manufacture Fine Structured Wear-Resistant 75Cr3C2-25(NiCr20) Coatings

Thermal spraying of fine feedstock powders allow the deposition of cermet coatings with significantly improved characteristics and is currently of great interest in science and industry. However, due to the high surface to volume ratio and the low specific weight, fine particles are not only difficult to spray but also show a poor flowability in the feeding process. In order to process fine powders reliably and to preserve the fine structure of the feedstock material in the final coating morphology, the use of novel thermal spray equipment as well as a thorough selection and optimization of the process parameters are fundamentally required. In this study, HVOF spray experiments have been conducted to manufacture fine structured, wear-resistant cermet coatings using fine 75Cr₃C₂-25(Ni20Cr) powders (?8 + 2 μm). Statistical design of experiments (DOE) has been utilized to identify the most relevant process parameters with their linear, quadratic and interaction effects using Plackett-Burman, Fractional-Factorial and Central Composite designs to model the deposition efficiency of the process and the majorly important coating properties: roughness, hardness and porosity. The concept of desirability functions and the desirability index have been applied to combine these response variables in order to find a process parameter combination that yields either optimum results for all responses, or at least the best possible compromise. Verification experiments in the so found optimum obtained very satisfying or even excellent results. The coatings featured an average microhardness of 1004 HV 0.1, a roughness Ra = 1.9 μm and a porosity of 1.7%. In addition, a high deposition efficiency of 71% could be obtained.     

An evaluation of the electric arc spray and (HPPS) processes for the manufacturing of high power plasma spraying MCrAIY coatings

The high power plasma torch (PlazJet) can be used to spray refractory ceramics with high spray rates and deposition efficiency. It can provide dense and hard coating with high bond strengths. When manufacturing thermal barrier coatings, the PlazJet gun is well adapted to spraying the ceramic top coat but not the MCrAIY materials that are used as bond coat. Arc spraying can compete with plasma spraying for metallic coatings since cored wires can be used to spray alloys and composites. In addition, the high production rate of arc spraying enables a significant decrease in coating cost. This paper discusses the performances of the PlazJet gun, and a twin-wire are spray system, and compares the properties and cost of MCrAIY coatings made with these two processes. For arc spraying, the use of air or nitrogen as atomizing gas is also investigated.     

Modeling the Influence of Injection Modes on the Evolution of Solution Sprays in a Plasma Jet

Solution precursor plasma spraying (SPPS) is a novel technology with great potential for depositing finely structured ceramic coatings with nano- and sub-micrometric features. The solution is injected into the plasma jet either as a liquid stream or gas atomized droplets. Solution droplets or the stream interact with the plasma jet and break up into fine droplets. The solvent vaporizes very fast as the droplets travel downstream. Solid particles are finally formed, and the particle are heated up and accelerated to the substrate to generate the coating. The deposition process and the properties of coatings obtained are extremely sensitive to the process parameters, such as torch operating conditions, injection modes, injection parameters, and substrate temperatures. This article numerically investigates the effect of injection modes, a liquid stream injection and a gas-blast injection, on the size distribution of injected droplets. The particle/droplet size, temperature, and position distributions on the substrate are predicted for different injection modes.     

Influence of the HVOF Gas Composition on the Thermal Spraying of WC-Co Submicron Powders (−8 + 1 μm) to Produce Superfine Structured Cermet Coatings

Thermal spraying technology represents a novel and promising approach to protect forming tools with complex surfaces and highest shape accuracy against abrasive wear and galling. However, due to high or nonuniform layer thicknesses or inappropriate surface roughness conventional coarse-structured coatings are not suitable to achieve this aim. The application of novel submicron or nanoscaled feedstock materials in the thermal spray process can provide the deposition of cermet coatings with significantly improved characteristics and is recently of great interest in science and industry. In this collaborative study, the feeding and HVOF spraying of WC-Co submicron powders (?8 + 1 μm) have been investigated to manufacture superfine structured, wear resistant, near-net-shape coatings with improved macroscopic properties and smooth surfaces. The influences of varying HVOF gas compositions on the spray process and the coating properties have been analyzed.     

Suspension DC plasma spraying of thick finely-structured ceramic coatings: Process manufacturing mechanisms

Due to the large volume fraction of the internal interfaces and reduced size of stacking defects, thick (from 20 to 100 µm) nano- or sub-micron structured coatings exhibit better properties than conventional micron structured ones (e.g. higher coefficients of thermal expansion, lower thermal diffusivity, higher hardness and toughness, better wear resistance, among other coating characteristics and functional properties). They could hence offer pertinent solutions to numerous emerging applications, in particular for energy production, energy saving, diffusion and environmental barriers, etc.Suspension plasma spraying (SPS) permits to manufacture such finely-structured layers and consists in mechanically injecting within the plasma flow a liquid suspension of sub-micrometric-sized or nano-sized particles through an injector of diameter of the order of one hundred micrometers. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and then solvent evaporation. Particles are then processed by the plasma flow (heat and momentum transfers) prior to their impact, flattening and solidification upon the surface to be covered.Compared to plasma spraying of micrometer-sized particles (APS), SPS exhibit several major differences : i) a more pronounced sensitivity to electric are root fluctuation requiring to operate the spray gun in a relatively stable mode (take over) unless to process inhomogeneously the suspension which would results in heterogeneous coating structure; ii) a shorter spray distance (since small particles decelerate faster than bigger ones) leading to higher thermal flux transmitted from the plasma flow to the substrate (5 to 10 times higher than conventional plasma spraying); iii) an emphasized thermophoresis effect; iv) a typical cohesive structure made of the stacking of granular and flattened particles with low density of stacking defects.This paper aims at presenting recent developments carried-out on this process in terms of process optimization and coating manufacturing mechanisms.     

等离子熔-喷WC-17%Co涂层工艺参数对外观形貌的影响

等离子熔-喷技术同时具备等离子喷涂、熔焊技术的综合特点,可以实现各种陶瓷材料涂层与基体的冶金结合。Co基WC具有硬度高、耐磨损、耐腐蚀、耐高温等一系列优点。采用等离子熔-喷技术在Q235钢基体表面制备WC-17％Co涂层,通过研究等离子熔-喷工艺参数（等离子熔枪和喷枪角度、熔枪和喷枪与基材表面的距离、熔-喷速度、喷涂送粉量）对涂层外观形貌的影响,获得最佳的工艺参数,制备出外观形貌良好、无缺陷的熔-喷涂层。     

The Influence of Temperature on Frictional Behavior of Plasma-Sprayed NiAl-Cr2O3 Based Self-Adaptive Nanocomposite Coatings

Frictional behavior of nano and hybrid-structured NiAl-Cr₂O₃-Ag-CNT-WS₂ adaptive self-lubricant coatings was evaluated at a range of temperatures, from room temperature to 700 °C. For this purpose, hybrid structured (HS) and nanostructured (NS) composite powders with the same nominal compositions were prepared by spray drying and heat treatment techniques. A series of HS and NS coating samples were deposited on steel substrate by an atmospheric plasma spraying process. The tribological behavior of both coatings was studied from room temperature to 700 °C at 100° intervals using a custom designed high temperature wear test machine. Scanning electron microscopy was employed for the evaluation of the composite coatings and worn surfaces. Experimental results indicated that the hybrid coating had inferior tribological properties when compared to the nanostructured coating, showing the attractive frictional behavior on the basis of low friction and high wear resistance; the NS coating possessed a more stable friction coefficient in the temperature range of 25-700 °C against alumina counterface. Microstructural examinations revealed more uniformity in NS plasma-sprayed coatings.