Air gas dynamic spraying of powder mixtures: Theory and application

The radial injection gas dynamic spray (RIGDS) technology of powder coatings deposition was considered for this work. A coating was created by injecting powders with variable compositions into a supersonic air jet and depositing powder on the substrate. This study describes the preliminary analysis of an air gas dynamic spray method realized by a portable RIGDS apparatus with a radial injection of powder. Attention was given to shock compaction processes during the coating structure formation and examples of powder mixtures utilization in RIGDS. It was shown that the operational parameters of supersonic powder-gas jet have a significant influence on the coating's microstructure, thus defining the high performance of the coating. Compaction and bonding of particles were analyzed. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

Numerical Studies of the Application of Shock Tube Technology for Cold Gas Dynamic Spray Process

A new method for a combustion-free spraying is studied fundamentally by modeling and simulation in comparison with first experiments. The article focuses on the numerical simulation of the gas-particle nozzle flow, which is generated by the shock reflection at the end wall section of a shock tube. To study the physical fundamentals of this process, at present only a single shot operation is considered. The particles are injected downstream of the nozzle throat into a supersonic nozzle flow. The measurements of the particle velocity made by a laser Doppler anemometry (LDA) set up show that the maximum velocity amounts to 1220 m/s for stainless steel particles of 15 μm diameter. The CFD-Code (Fluent) is first verified by a comparison with available numerical and experimental data for gas and gas-particle flow fields in a long Laval-nozzle. The good agreement implied the great potential of the new dynamic process concept for cold-gas coating applications. Then the flow fields in the short Laval nozzle designed and realized by the Shock Wave Laboratory (SWL) are investigated. The gas flow for experimentally obtained stagnation conditions is simulated. The gas-particle flow without and with the influence of the particles on the gas flow is calculated by the Surface Engineering Institute (IOT) and compared with experiments. The influence of the injection parameters on the particle velocities is investigated, as well. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Liquid Precursor Plasma Spraying: Modeling the Interactions Between the Transient Plasma Jet and the Droplets

Plasma spraying using liquid feedstock makes it possible to produce thin coatings (<100 μm) with more refined microstructures than in conventional plasma spraying. However, the low density of the feedstock droplets makes them very sensitive to the instantaneous characteristics of the fluctuating plasma jet at the location where they are injected. In this study, the interactions between the fluctuating plasma jet and droplets are explored by using numerical simulations. The computations are based on a three-dimensional and time-dependent model of the plasma jet that couples the dynamic behaviour of the arc inside the torch and the plasma jet issuing from the plasma torch. The turbulence that develops in the jet flow issuing in air is modeled by a large Eddy simulation model that computes the largest structures of the flow which carry most of the energy and momentum. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Effect of Particle Size Distribution on Isothermal Oxidation Characteristics of Plasma Sprayed CoNi- and CoCrAlY Coatings

The effect of particle size distribution on the degradation behavior of plasma sprayed CoNi- and CoCrAlY coatings during isothermal oxidation was investigated, in terms of the oxygen content, porosity, surface roughness, and oxide scale formation. The results show that the degradation of both coatings was considerably influenced by the starting particle size distribution. It also shows that in the as-sprayed vacuum plasma spray (VPS) coatings the oxygen content on the coating surface increased significantly with decreased average particle size. But after thermal exposure, the difference of the oxygen contents between the coatings with different particle size was decreased. The powder with various particle size resulted in low porosity inside the coatings during the deposition process. The surface roughness of the coatings increased with increased particle size. The small particles produced a relatively smooth surface, and the oxide growth in the coating deposited by small particle was slower than that in the large particle coating. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Rapid and Continuous Deposition of Porous Nanocrystalline SnO2 Coating with Interpenetrating Pores for Gas Sensor Applications

In this article, we proposed a rapid and continuous process for the production of nanoporous coatings for functional applications. Experiments following two statistical designs were implemented to screen and investigate the spraying parameters’ effects on coating crystallinity and porosity in order to gain a better understanding. The spraying standoff distance, solution flow rate and power were identified as having significant effects on coating porosity and crystallinity. The result yielded a peculiar microstructure comprised of interpenetrating pores and layered structures with embedded pores. A deposition mechanism was postulated to explain this microstructure. Ethanol gas sensors that are constructed from the coatings had comparable sensitivities to those reported in the literature for thick-film coatings and had a maximum sensitivity near 200 °C. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Alumina coatings by plasma spraying of monosize sapphire particles

A series of plasma sprayed coatings of controlled microstructure was obtained by spraying three monosize sapphire powders using an axial injection torch in which the plasma gas composition and nozzle diameter were the only processing parameters varied. The effects of changes in these parameters on the coating splat morphology, porosity, angular crack distribution, and hardness are reported. The uniform, dense microstructure and the high hardness of 14 GPa (a level usually only associated with chromia thermal spray coatings) of the best alumina coatings resulted from using tightly controlled processing conditions and monodispersed precursor powders. The microstructural quality of plasma sprayed coatings and, hence, the coating properties can be improved significantly by minimizing variations in processing and raw material parameters. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

Mechanical Characteristics of Al-Co-Ce Coatings Produced by the Cold Spray Process

Gas atomized feedstock particles of an Al-13Co-26Ce alloy system were sprayed using the Cold Spray deposition technique. The microstructures of the coatings produced are examined and the mechanical characteristics, in particular the bending fatigue and the bond strength, of the Al-Co-Ce coatings are reported. The results show that the Al-Co-Ce coatings improved the fatigue behavior of AA 2024-T3 specimens when compared to uncoated and Alclad specimens. During the bond strength tests, the bonding agent failed and no delamination of the coating from the substrate occurred. The microstructural features of the feedstock powder were also found in the coatings. It is suggested that the increase in the fatigue properties of the specimens can be attributed to the residual compressive stresses induced in the coatings and to the high adhesion strength of the coatings to the substrates. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Microstructural Characterization of Cold-Sprayed Nanostructured FeAl Intermetallic Compound Coating and its Ball-Milled Feedstock Powders

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using the compound feedstock powders due to their intrinsic low-temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post-heat treatment. In this study, a nanostructured Fe/Al alloy powder was prepared by ball-milling process. The cold-sprayed Fe/Al alloy coating was evolved in-situ to intermetallic compound coating through a post-heat treatment. The microstructural evolution of the Fe-40Al powder during mechanical alloying and the effect of the post-heat treatment on the microstructure of the cold-sprayed Fe(Al) coating were characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), and x-ray diffraction analysis. The results showed that the milled Fe-40Al powder exhibits lamellar microstructure. The microstructure of the as-sprayed Fe(Al) coating depends significantly on that of the as-milled powder. The heat-treatment temperature significantly influences the in-situ evolution of the intermetallic compound. The heat treatment at a temperature of 500 °C results in the complete transformation of Fe(Al) solid solution to FeAl intermetallic compound. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Fluid dynamic effects on plasma torch anode erosion

Anode erosion in plasma spray torches results in coating deterioration. The usable life of a torch anode is strongly dependent on the fluid dynamic behavior of the plasma inside the torch, which in turn depends on the geometric design of the anode and the operating parameters. To study the relative importance of these effects, cold flow investigations have been performed with a torch having a glass anode with the same geometric dimensions as a commercial plasma torch. The density differences between the arc and the cold gas were simulated by injecting heated helium from the tip of the cathode into the cold argon gas flow from the regular gas injector. Flow visualization was achieved by seeding the flow with micron-sized particles. A finite-element computational fluid dynamics code was used to simulate the cold flow structure. The results were compared with erosion patterns observed with an actual plasma torch. The results indicate that recirculation eddies inside the torch will force a preferred anode attachment, which is different for different gas injectors. The minimization of such recirculation regions by appropriate fluid dynamic design will result in more random attachment of the arc and prolonged anode life. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Plasma sprayed cast iron coatings containing solid lubricant graphite and h-BN structure

Water-atomized cast iron powder of Fe-2.17 at.%C-9.93at.%Si-3.75at.%Al were deposited onto an aluminum alloy substrate by atmospheric direct current plasma spraying to improve its tribological properties. Preannealing of the cast iron powder allows the precipitation of considerable amounts of graphite structure in the powder. However, significant reduction in graphitized carbon in cast iron coatings is inevitable after plasma spraying in air atmosphere due to the in-flight burning and dissolution into molten iron droplets. Hexagonal boron nitride (h-BN) powders, which have excellent lubricating properties like graphite, were incorporated into the cast iron powder as a solid lubricant by the sintering process (1300°C) to obtain protective coatings with a low friction coefficient. The performance of each coating was evaluated using a ring-on-disk-type wear tester under a paraffin-based oil condition in an air atmosphere. A conventional cast iron liner, which had a flaky graphite embedded in the pearlitic matrix, was also tested under similar conditions for comparison. Sections of worn surfaces and debris were characterized, and the wear behavior of plasma-sprayed coatings was discussed. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

On the Role of Spraying Process on Microstructural, Mechanical, and Thermal Response of Alumina Coatings

Thermal spraying is a widely used technology for industrial applications to provide coatings that improve the surface characteristics. According to the specificities of processes (APS, VPS, flame, electric arc), any kind of material can be sprayed. Among materials, ceramic coatings present several interesting aspects such as wear resistance, corrosion protection as well as thermal or electrical insulation; particularly alumina coatings which appear as the most commonly used. From all spraying processes, atmospheric plasma spraying (APS) is a rather well-established process but some others can also be used with a lower economical impact such as the flame technology. The aim of this study was to analyze the alumina coating properties according to the technology employed such as APS or wire flame spraying using the Rokide™ and the Master Jet^? guns. After micrographic analyses by SEM, physical and mechanical properties were measured considering the thermal conductivity and the hardness. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Substrate roughness and thickness effects on cold spray nanocrystalline Al−Mg coatings

Nanocrystalline Al−Mg coatings were produced using the cold gas dynamic-spraying technique. Unsieved Al−Mg powder of average nanocrystalline grain size in the range of 10 to 30 nm and with a particle size distribution from 10 to >100 μm was used as the feedstock powder. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy, as well as microhardness and nanoindentation measurements. Coating observations suggest that the wide particle size distribution of the feedstock powder has a detrimental effect on the coating quality but that it can be successfully mitigated by optimizing the spraying parameters. Nanohardness values close to 3.6 GPa were observed in both the feedstock powder and coatings, suggesting the absence of cold-working hardening effects during the process. The effects of the substrate surface roughness and thickness on coating quality were investigated. The deposited mass measurements performed on the coatings showed that the effect of using different grit sizes for the substrate preparation is limited to small changes in the deposition efficiency of only the first few layers of deposited material. The SEM observation showed that the substrate surface roughness has no significant effect on the macrostructures and microstructures of the coating. The ability to use the cold gas dynamic spraying process to produce coatings on thin parts without noticeable substrate damage and with the same quality as coatings produced on thicker substrates was demonstrated in this work. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and IIW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

New attachment for controlling gas flow in the HVOF process

During the decade, the high-velocity oxyfuel (HVOF) process proved to be a technological alternative to the many conventional thermal spray processes. It would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow deposition of ceramic coatings. Based on a numerical analysis, a new attachment to a standard HVOF torch was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the attachment was investigated by spraying several coating materials including metal and ceramic powders. Particle conditions and spatial distribution, as well as gas phase composition, corresponding to the new attachment and the standard HVOF gun, were compared. The attachment provides better particle spatial distribution, combined with higher particle velocity and temperature. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Microstructures and characterization of zirconia-yttria coatings formed in laser and hybrid spray process

Hybrid plasma spraying combined with yttrium-aluminum-garnet laser irradiation was studied to obtain optimum zirconia coatings for thermal barrier use. Zirconia coatings of approximately 150 μm thickness were formed on NiCrAlY bond coated steel substrates both by means of conventional plasma spraying and hybrid plasma spraying under a variety of conditions. Post-laser irradiation was also conducted on the plasma as-sprayed coating. The microstructure of each coating was studied and, for some representative coatings, thermal barrier properties were evaluated by hot erosion and hot oxidation tests. With hybrid spraying, performed under optimum conditions, it was found that a microstructure with appropriate partial densification and without connected porosity was formed and that cracks, which are generally produced in the post-laser irradiation treatment, were completely inhibited. In addition, hybrid spraying formed a smooth coating surface. These microstructural changes resulted in improved coating properties with regard to hardness, high temperature erosion resistance, and oxidation resistance. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

Hypersonic plasma particle deposition—A hybrid between plasma spraying and vapor deposition

In the hypersonic plasma particle deposition process, vapor phase reactants are injected into a plasma and rapidly quenched in a supersonic nozzle, leading to nucleation of nanosize particles. These particles impact a substrate at high velocity, forming a coating with grain sizes of 10 to 40 nm. As previously reported, coatings of a variety of materials have been obtained, including silicon, silicon carbide, titanium carbide and nitride, and composites of these, all deposited at very high rates. Recent studies have shown that slight modifications of the process can result in nanosize structures consisting of single crystal silicon nanowires covered with nanoparticles. These nanowires are believed to grow in a vapor deposition process, catalyzed by the presence of titanium in the underlying nanoparticle film. However, simultaneously nanoparticles are nucleated in the nozzle and deposited on the nanowires, leading to structures that are the result of a plasma chemical vapor deposition (CVD) process combined with a nanoparticle spray process. The combination of these two process paths opens new dimensions in the nanophase materials processing area. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Preparation of perovskite powders and coatings by radio frequency suspension plasma spraying

Perovskite-type LaMnO₃ powders and coatings have been prepared by a novel technique: reactive suspension plasma spraying (SPS) using an inductively coupled plasma of approximately 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility, and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO₂ powder in a saturated ethanol solution of LaCl₃ with a 1 to 1 molar ratio of lanthanum and manganese. A low reactor pressure was helpful in diminishing the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70 to 90%. After a posttreatment with an 80% oxygen plasma, an almost pure LaMnO₃ deposit was achieved in the center of the incident plasma jet. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

Characterization of Nanostructured WC-Co Deposited by Cold Spraying

Nanostructured WC-Co coating was deposited by cold spraying using a nanostructured WC-12Co powder. The critical velocity for the particle to deposit was measured. The coating microstructure was characterized by X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy. The coating hardness was tested using a Vickers hardness tester. The deposition behavior of single WC-Co particle was examined. WC particle size was measured for comparison of deposit properties to that of sintered bulk. The result shows that the nanostructured WC-Co coating can be successfully deposited by cold spraying using nanostructured powders. The coating exhibited a dense microstructure with full retention of the original nanostructure in the powder to the coating. The test of microhardness of the coating yielded a value of over 1820 Hv_0.3, which is comparable to that of sintered nanostructured WC-Co. The deposition behavior of WC-Co powders as superhard cermet materials in cold spraying and powder structure effects is discussed. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

A Novel Hybrid Axial-Radial Atmospheric Plasma Spraying Technique for the Fabrication of Solid Oxide Fuel Cell Anodes Containing Cu, Co, Ni, and Samaria-Doped Ceria

Composite coatings containing Cu, Co, Ni, and samaria-doped ceria (SDC) have been fabricated using a novel hybrid atmospheric plasma spraying technique, in which a multi-component aqueous suspension of CuO, Co₃O₄, and NiO was injected axially simultaneously with SDC injected radially in a dry powder form. Coatings were characterized for their microstructure, permeability, porosity, and composition over a range of plasma spray conditions. Deposition efficiency of the metal oxides and SDC was also estimated. Depending on the conditions, coatings displayed either layering or high levels of mixing between the SDC and metal phases. The deposition efficiencies of both feedstock types were strongly dependent on the nozzle diameter. Plasma-sprayed metal-supported solid oxide fuel cells utilizing anodes fabricated with this technique demonstrated power densities at 0.7 V as high as 366 and 113 mW/cm² in humidified hydrogen and methane, respectively, at 800 °C.     

Yttria-Stabilized Zirconia Thermal Barriers Sprayed Using N<Subscript>2</Subscript>-H<Subscript>2</Subscript> and Ar-H<Subscript>2</Subscript> Plasmas: Influence of Processing and Heat Treatment on Coating Properties

Thermal barrier coatings were produced using both Ar and N₂ as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N₂, but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N₂ as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus, and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness, and thermal diffusivity) when heat-treated at 1400 °C. However, the N₂-processed coatings tended to have lower values of Young’s modulus and thermal diffusivity following such treatment. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N₂ as compared to Ar as the primary plasma gas for TBC deposition. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.