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.     

Investigation of Al-Al2O3 Cold Spray Coating Formation and Properties

Coating build-up mechanisms and properties of cold-sprayed aluminum-alumina cermets were investigated using two spherical aluminum powders having average diameters of 36 and 81 μm. Those powders were blended with alumina at several concentrations. Coatings were produced using a commercial low-pressure cold spray system. Powders and coatings were characterized by electronic microscopy and microhardness measurements. In-flight particle velocities were monitored for all powders. The deposition efficiency was measured for all experimental conditions. Coating performance and properties were investigated by performing bond strength test, abrasion test, and corrosion tests, namely, salt spray and alternated immersion in saltwater tests. These coating properties were correlated to the alumina fraction either in the starting powder or in the 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.     

Particle loading effect in cold spray

Cold gas dynamic spray is a line-of-sight, high-rate material deposition process that uses a supersonic flow to accelerate small particles (micron-sized) above a material-dependent critical velocity. When the particles impact the substrate, they plastically deform and bond to form a coating. The objective of this research is to investigate the influence of the particle mass flow rate on the properties of coatings sprayed using the cold spray process. Varying the mass flow rate at which the feedstock particles are fed into the carrier gas stream can change the thickness of the coating. It was shown that poor coating quality (peeling) was not a result of flow saturation but, instead, the result of excessive particle bombardment per unit area on the substrate. By increasing the travel speed of the substrate, this can be overcome and well-bonded dense coatings can be achieved. It has also been shown that by heating the carrier gas flow poor coating quality is avoided. The original version of this paper was published in the CD ROM Thermal Spray Comects: Explore Its Surfacing Potential, Interational 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.     

Effect of deposition conditions on the properties and annealing behavior of cold-sprayed copper

The deposition of copper by cold gas dynamic spraying has attracted much interest in recent years due to the capability to deposit low-porosity oxide-free coatings. However, it is generally found that as-deposited copper has a signicantly greater hardness, and potentially lower ductility, than bulk material. In this article, copper was deposited by cold spraying using helium as the driving gas at both 298 and 523 K. Evidence is presented indicating that the material sprayed at the lower temperature exhibits a lower dislocation density throughout the grain structure than the material sprayed at the higher temperature. The low stacking fault energy of copper restricts recovery during annealing, and thus microstructural changes during annealing only proceed once recrystallization begins. The material sprayed at low temperature (with the low dislocation density) exhibited recrystallization at annealing temperatures as low as 373 K with a corresponding reduction in hardness. However, the copper sprayed with helium at 523 K was resistant to annealing at temperatures up to 473 K where the dislocations in the structure prevented recrystallization. However, at higher temperatures, recrystallization did proceed (with corresponding reductions in hardness). The fracture behavior of the copper that was cold sprayed with helium at 523 K, both in the as-sprayed condition and following annealing, was measured and explained in terms of the annealing mechanisms proposed. 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.     

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.     

Cold Spray Forming of Inconel 718

Inconel 718 was cold spray formed to a 6-mm thickness on an 8-cm diameter aluminum alloy tube using Sulzer Amdry 1718 powder and the Plasma Giken PCS-1000 cold spray system. The effects of spray particle velocity and post-spray heat treatment were studied. Post-spray annealing was performed from 950 to 1250 °C for 1-2 h. The resulting microstructures as well as the corresponding mechanical properties were characterized. As-sprayed coatings exhibited very low ductility. The tensile strength and ductility of the heat-treated coatings were improved to varying levels depending on the heat-treatment and spray conditions. For coatings sprayed at higher particle velocity and heat treated at 1250 °C for 1 h, an elongation of 24% was obtained. SEM micrographs showed a higher fraction of interparticle metallurgical bonds due to some sintering effect. Corresponding fracture surfaces also revealed a higher fraction of dimple features, typically associated with ductile fracture, in the annealed coatings. The results demonstrate that cold spray forming of Inconel 718 is feasible, and with appropriate heat treatment, metallurgical bonding can be increased. The ductility of the spray-formed samples was comparable to that of the bulk material.     

Microstructure and properties of tungsten carbide coatings sprayed with various high-velocity oxygen fuel spray systems

This article reports on a series of experiments with various high-velocity oxygen fuel spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000, Top Gun-K) using different WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and x-ray diffraction. Carbon and oxygen content of the coatings were determined to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear, and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on type of powder, spray system, and spray parameters. Within a relatively wide range, the extent of phase transformation has only little effect on coating properties. Therefore, coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen. 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.     

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.     

Effect of Process Parameters and Heat Treatments on Properties of Cold Sprayed Copper Coatings

Cold gas dynamic spraying or cold spray is specifically suitable to obtain high-conductivity copper coatings for a variety of applications. Copper coatings at different coating parameters were deposited and subjected to various post treatments. The effect of process parameters and the treatment conditions on coating properties such as electrical conductivity, porosity, microhardness etc., was studied. The as-coated specimens exhibited low conductivities and conductivity was found to improve with heat treatment. Treatments were carried out in vacuum at different temperatures and for different durations and conductivities close to bulk annealed copper were achieved. Good correlation was observed between the conductivity, porosity and hardness of the as-coated and heat-treated specimens. Similar correlations were observed between conductivity-porosity and hardness-porosity of the coatings and the relative influence of cold work and porosity on coating properties was determined.     

The influence of process parameters and heat treatment on the properties of cold sprayed silver coatings

Cold spray coating technology is a promising low temperature variant of thermal spray technology which can deposit pure, dense and thick coatings at a rapid rate. Unlike the other thermal spray coating techniques, cold spray is especially suitable for depositing coatings with high electrical and thermal conductivity as the integrity of the feedstock is maintained during the coating process. In the present study, the effect of process parameters and heat treatment on the properties of silver coatings has been investigated. An attempt has been made to correlate the powder particle velocity with the properties and microstructure of the coating. The effect of heat treatment temperature and atmosphere on the properties of the coatings, especially electrical conductivity, has been studied in detail in conjunction with a thorough analysis of the evolution of microstructure of the coatings.     

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.     

Preparation of Aluminum Coatings Containing Homogenous Nanocrystalline Microstructures Using the Cold Spray Process

Nanostructured materials are of widespread interest because of the unique properties they offer. Well-proven techniques, such as ball milling, exist for preparing powders with nanocrystalline microstructures. Nevertheless, consolidation of nanocrystalline powders is challenging and presents an obstacle to the use of nanocrystalline metals. This work demonstrates that nanocrystalline aluminum powders can be consolidated using the cold spray process. Furthermore, transmission electron microscopy (TEM) analysis of the nanocrystalline cold spray coatings reveals that the cold spray process can cause significant grain refinement. Inert gas atomized 6061 and 5083 aluminum powders were ball milled in liquid nitrogen resulting in micron-sized powder containing 250-400 nm grains. Cold spray coatings prepared using these feed stock materials exhibited homogenous microstructures with grain sizes of 30-50 nm. TEM images of the as-received powders, ball-milled powders, and cold spray coatings are shown.     

Oxidation/carbonization/nitridation and in-service mechanical property degradation of CoCrAlY coatings in land-based gas turbine blades

This article describes variations in the microstructure/composition and mechanical properties in plasma sprayed CoCrAlY coatings and a modified René 80 substrate of gas turbine blades operated for 21,000 h under liquefied natural gas fuels. Substantial oxidation/carbonization occurred in the near surface region of concave coatings, but not in the convex coatings. Aluminum and nickel/titanium-rich nitrides formed in near interface coatings and substrates of concave side of blades, respectively. Small punch (SP) specimens were prepared from the different blade location to examine the variation of the mechanical properties in the coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave side easily initiated up to 950 °C. The convex coatings exhibited higher ductility than the concave coatings and substrate and showed a rapid increase in the ductility above 800 °C. Thus it is apparent that the oxidation/carbonization and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coatings is discussed in light of the operating temperature distribution and compared to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxidation. 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.     

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.     

Heat Treatment of Cold-Sprayed C355 Al for Repair: Microstructure and Mechanical Properties

Cold gas dynamic spraying of commercially pure aluminum is widely used for dimensional repair in the aerospace sector as it is capable of producing oxide-free deposits of hundreds of micrometer thickness with strong bonding to the substrate, based on adhesive pull-off tests, and often with enhanced hardness compared to the powder prior to spraying. There is significant interest in extending this application to structural, load-bearing repairs. Particularly, in the case of high-strength aluminum alloys, cold spray deposits can exhibit high levels of porosity and microcracks, leading to mechanical properties that are inadequate for most load-bearing applications. Here, heat treatment was investigated as a potential means of improving the properties of cold-sprayed coatings from Al alloy C355. Coatings produced with process conditions of 500 °C and 60 bar were heat-treated at 175, 200, 225, 250 °C for 4 h in air, and the evolution of the microstructure and microhardness was analyzed. Heat treatment at 225 and 250 °C revealed a decreased porosity (~ 0.14% and 0.02%, respectively) with the former yielding slightly reduced hardness (105 versus 130 HV_0.05 as-sprayed). Compressive residual stress levels were approximately halved at all depths into the coating after heat treatment, and tensile testing showed an improvement in ductility.     

Kinetic sprayed rare earth iron alloy composite coatings

One unique advantage of the kinetic spray process is its ability to mix constituents that would normally react with each other to form coatings. This attribute was used to produce composite coatings with different rare earth iron alloys (REFe₂) and several ductile matrices. Composite coatings of Terfenol-D [(Tb_0.3Dy_0.7)Fe_1.9] and SmFe₂ were combined with ductile matrices of aluminum, copper, iron, and molybdenum. Evidence of an induced magnetic coercivity was measured for the REFe₂-Mo and Fe composite coatings. Coatings were produced on flat substrates and shafts. Coating morphology as well as the physical, magnetostrictive, and magnetic properties of these coatings are discussed. 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.     

High-resolution microstructural investigations of interfaces between light metal alloy substrates and cold gas-sprayed coatings

Interfaces between light metal alloys, aluminum AA7022, and magnesium AZ91, and optimized cold gassprayed zinc-based coatings are characterized. The analyses include scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). Investigations by SEM show a seam with intensive mixing of the substrate and coating material, which is indicated by different values of gray due to element contrast. In energy-dispersive spectroscopy analyses, increased zinc concentrations compared with the substrate material are detected in <1 μm thick vortexes inside the seam. The TEM investigations prove that these areas consist of a homogeneous solid solution and submicron-sized or nanosized intermetallic phases with different concentrations of aluminum, zinc, and magnesium. Because diffusion processes cannot result in the observed microstructure. local melting followed by precipitation of intermetallic phases is concluded as the consequence of the intensive mechanical interaction at the substrate-coating interface during particle impact during the cold gas spraying of zinc on magnesium or aluminum 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.     

喷射成形Al-Zn-Mg-Cu合金的组织与性能研究

对喷射沉积制备的高zn（11．3wt％）Al-Zn-Mg-Cu合金进行挤压和热处理,并测试了力学性能。通过透射电镜和扫描电镜对拉伸试样的微观组织进行了研究,提出了合金的强化机制。结果表明,该喷射沉积Al-Zn-Mg-cu合金的抗拉强度、屈服强度和伸长率分别为849MPa、796MPa和3．3％。合金的强化主要来源于纳米晶强化、固溶强化以及沉淀强化。断口分析显示,合金的断裂方式主要为沿晶断裂。     

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.