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.     

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.     

Characterization of microstructure of Nano-TiO2 coating deposited by vacuum cold spraying

Control of the microstructure of TiO₂ coatings through preparation methods significantly influences the coating performance. In this study, a vacuum cold-spray process, as a new coating technology, is used to deposit nanocrystalline TiO₂ coatings on conducting glass and stainless steel substrates. TiO₂ deposits were formed using two types of nanocrystalline TiO₂ powders with mean particle diameters of 200 and 25 nm. Coating microstructures were characterized by scanning electron microscopy and x-ray diffraction analysis. Results demonstrate that a thick nanocrystalline TiO₂ coating can be deposited by the vacuum cold-spray process. The coating was found to consist of particles stacked as agglomerates that build up to several hundred nanometers. The coating also presents a mesoporous microstructure that could be effective in such applications as photocatalytic degradation and dye-sensitized solar cells. 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.     

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.     

Flame-Sprayed Glaze Coatings: Effects of Operating Parameters and Feedstock Characteristics Onto Coating Structures

Many substrates do not sustain the conventional glazing process (i.e., vitreous glazing) due to the relatively high temperature required by this treatment (i.e., up to 1400 °C in some cases) to fuse glazes after their application on the surface to be covered. Flame spraying could appear as a solution to circumvent this limitation and to avoid thermal decomposition of substrates. This contribution describes some structural attributes of glaze coatings manufactured by flame spraying. It also discusses the influence of the feedstock powder morphology and some of its physical properties on coating characteristics. 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.     

Production of titanium deposits by cold-gas dynamic spray: Numerical modeling and experimental characterization

Over the past five years, interest in cold-gas dynamics spraying (CGDS) has increased substantially. Considerable effort has been devoted to process development and optimization for such metals as copper and aluminium. This paper describes work undertaken to expand the understanding of the deposition of titanium by cold-spray methods. CGDS deposits have been produced from commercially pure titanium using room-temperature helium gas. The effect of different powder paticle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on powder deposition have been investigated. Microhardness testing of deposits was conducted, and their microstructures have been examined by scanning electron microscopy. Samples for pull-off bond-strength tests have been prepared from a number of the more promising sets of spray parameters and adhesive strengths determined. A one-dimensional numerical model of particle acceleration, employing isentropic gas flow behavior in the nozzle, has also been used to estimate particle exit velocities. This model explicitly addresses the dependence of the drag coefficient on gas compressibility and demonstrates its significance in terms of predicted particel velocities. By linking this model with the measured particle size distributions, estimates of particle velocity distributions at the nozzle exit plane have been computed. These allow an approximate value of the critical velocity for deposition of titanium to be made. Experimental observations on the microstructure and properties of the deposits are discussed in light of powder particle size and velocity distributions and the underlying physical and mechanical properties of the powders and substrates. The original version of this paper was published in the CD ROM Thermal Spray Commects: 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.     

Cold-Spray processing of a nanocrystalline Al−Cu−Mg−Fe−Ni alloy with Sc

This work describes recent progress in cold-spray processing of conventional and nanocrystalline 2618 (Al−Cu−Mg−Fe−Ni) aluminum alloy containing scandium (Sc). As-atomized and cryomilled 2618+Sc aluminum powder were sprayed onto aluminum substrates. The mechanical behavior of the powders and the coatings were studied using micro-and nanoindentation techniques, and the microstructure was analyzed using scanning and transmission electron microscopy (SEM and TEM). The influence of powder microstructure, morphology, and behavior during deposition on the coating properties was analyzed. This work shows that Al−Cu−Mg−Fe−Ni−Sc coatings with a nanocrystalline grain structure can be successfully produced by the cold-spray process. Inspection of the scientific literature suggests that this is the first time a hardness value of 181 HV has been reported for this specific alloy. 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.     

Role of the laser surface preparation on the adhesion of Ni−5%Al coatings deposited using the PROTAL process

The thermal spraying assisted by laser (PROTAL) process combines a laser surface preparation followed by a thermal spray process. This type of surface preparation avoids some drawbacks of the degreasing and sand blasting processes. Previous studies showed that the adhesion of the deposits obtained with the help of the PROTAL process is similar to that achieved by traditional surface preparation. To obtain better insight into the effects of the laser treatment, a Ni−5%Al coating was plasma sprayed using the PROTAL process under different surface conditions. The morphology of the impinging splats and adhesion of the deposits were examined. Removal of contaminants, adsorbates, and oxides at substrate surface is confirmed. The role of the laser irradiation on the coating adhesion is 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 IIW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

Effect of Substrate Temperature on the Formation Mechanism of Cold-Sprayed Aluminum, Zinc and Tin Coatings

When describing the cold-spray process, one of the most widely used concepts is the critical velocity. Current models predicting critical velocities take the temperature of the sprayed particles explicitly into account, but not the surface temperature (substrate or already deposited layers) on which the particle impacts. This surface temperature is expected to play an important role, since the deformation process leading to particle bonding and coating formation takes place both on the particle and the substrate side. The aim of this work is to investigate the effect of the substrate temperature on the coating formation process. Experiments were performed using aluminum, zinc, and tin powders as coating materials. These materials have a rather large difference in critical velocities that gives the possibility to cover a broad range of deposition velocity to critical velocity ratio using commercial low-pressure cold-spray system. The sample surface was heated and the temperature was varied from room temperature to a high fraction of the melting point of the coating material for all three materials. The change in temperature of the substrate during the deposition process was measured by means of a high speed IR camera. The coating formation was investigated as a function of (1) the measured surface temperature of the substrate during deposition, (2) the gun transverse speed, and (3) the particle velocity. Both single particle impact samples and thick coatings were produced and characterized. Both the particle-substrate and interparticle bonding were evaluated by scanning electron microscopy (SEM) and confocal microscopy. 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.     

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.     

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.     

Development of Detonation Flame Sprayed Cu-Base Coatings Containing Large Ceramic Particles

Metal-matrix composites (MMCs) containing large ceramic particles as superabrasives are typically used for grinding stone, minerals, and concrete. Sintering and brazing are the key manufacturing technologies for grinding tool production. However, restricted geometry flexibility and the absence of repair possibilities for damaged tool surfaces, as well as difficulties of controlling material interfaces, are the main weaknesses of these production processes. Thermal spraying offers the possibility to avoid these restrictions. The research for this paper investigated a fabrication method based on the use of detonation flame spraying technology to bond large superabrasive particles (150-600 μm, needed for grinding minerals and stones) in a metallic matrix. Layer morphology and bonding quality are evaluated with respect to superabrasive material, geometry, spraying, and powder-injection parameters. The influence of process temperature and the possibilities of thermal treatment of MMC layers are analyzed. 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.     

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.     

Thermal Fatigue Behavior of Thick and Porous Thermal Barrier Coatings Systems

High-temperature thermal fatigue causes the failure of thermal barrier coating (TBC) systems. This paper addresses the development of thick TBCs, focusing on the microstructure and the porosity of the yttria partially stabilized zirconia (YPSZ) coating, regarding its resistance to thermal fatigue. Thick TBCs, with different porosity levels, were produced by means of a CoNiCrAlY bond coat and YPSZ top coat, both had been sprayed by air plasma spray. The thermal fatigue resistance of new TBC systems and the evolution of the coatings before and after thermal cycling was then evaluated. The limit of thermal fatigue resistance increases depending on the amount of porosity in the top coat. Raman analysis shows that the compressive in-plane stress increases in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend which is contrary to the porosity level of top coat. 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.     

Plasma Electrolytic Oxidation of Arc-Sprayed Aluminum Coatings

Different posttreatment methods, such as heat treatment, mechanical processing, sealing, etc., are known to be capable to improve microstructure and exploitation properties of thermal spray coatings. In this work, a plasma electrolytic oxidation of aluminum coatings obtained by arc spraying on aluminum and carbon steel substrates is carried out. Microstructure and properties of oxidized layers formed on sprayed coating as well as on bulk material are investigated. Oxidation is performed in electrolyte containing KOH and liquid glass under different process parameters. It is shown that thick uniform oxidized layers can be formed on arc-sprayed aluminum coatings as well as on solid material. Distribution of alloying elements and phase composition of obtained layers are investigated. A significant improvement of wear resistance of treated layers in two types of abrasive wear conditions is observed. 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.     

Nozzle developments for thermal spray at very low pressure

Very low pressure plasma spraying has been the object of numerous studies for the past years. However, numerical simulations and experiments revealed some difficulties such as the presence of successive expansions and constrictions of the plasma jet that have an influence on the deposition efficiency and on the coating structure. Optimization of the plasma gun is thus required and the use of bell-contoured De Laval nozzle extensions seems particularly promising. In this paper, new developments concerning the use of an in-house bell-contoured nozzle extension are presented, and both numerical calculations and experiments were performed. 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.     

A systematic approach to material eligibility for the cold-spray process

This article represents an effort to systematize an understanding of the cold-spray process and the suitability of materials for such a process. The evaluation is based on a brief analysis of the powder particle impact and literature research concerning shock-compression phenomena in matter and related physical effects, such as impact heating and dynamic yielding. The finite-element modeling (FEM) allows the estimation of the maximum impact pressures, the deformation rates, and the deformation kinetics during impact. The calculations can be verified experimentally and are supported by the published data. From a brief analysis of the equations of state applied to shock compression, key material parameters are derived and investigated. A parameterization of physical properties and correlation with the crystal types endeavors to provide a qualitative ranking of material suitability. 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, Eds., ASM International, 2003.     

Investigation about the Chrome Steel Wire Arc Spray Process and the Resulting Coating Properties

Nowadays, wire-arc spraying of chromium steel has gained an important market share for corrosion and wear protection applications. However, detailed studies are the basis for further process optimization. In order to optimize the process parameters and to evaluate the effects of the spray parameters DoE-based experiments had been carried out with high-speed camera shoots. In this article, the effects of spray current, voltage, and atomizing gas pressure on the particle jet properties, mean particle velocity and mean particle temperature and plume width on X46Cr13 wire are presented using an online process monitoring device. Moreover, the properties of the coatings concerning the morphology, composition and phase formation were subject of the investigations using SEM, EDX, and XRD-analysis. These deep investigations allow a defined verification of the influence of process parameters on spray plume and coating properties and are the basis for further process optimization. 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 Powder Injection Location on Ceramic Coatings Properties When Using Plasma Spray

The effect of powder injecting location of the plasma spraying on spraying properties was studied. Three different powder-injecting methods were applied in the experiment. In the first method, the particles were axially injected into the plasma flow from the cathode tip. In the second method, the particles were radially injected into the plasma flow just downstream of the anode arc root inside the anode nozzle. In the third method, the particles were radially injected into the plasma jet at the nozzle exit. The alumina particles with a mean diameter of 20 μm were used to deposit coatings. Spraying properties, such as the deposition efficiency, the melting rate of the powder particles, and the coating quality were investigated. The results show that the spraying with axial particle injecting can heat and melt the powder particles more effectively, produce coatings with better quality, and have higher deposition efficiency. 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.