Superior thermal barrier coatings using solution precursor plasma spray

A novel process, solution precursor plasma spray (SPPS), is presented for depositing thermal barrier coatings (TBCs), in which aqueous chemical precursors are injected into a standard direct current plasma spray system. The resulting coatings microstructure has three unique features: (1) ultra fine splats (1 μm), (2) nanometer and micron-sized interconnected porosity, and (3) closely spaced, through-thickness cracks. Coatings over 3 mm thick can be readily deposited using the SPPS process. Coating durability is excellent, with SPPS coatings showing, in furnace cycling tests, 2.5 times the spallation life of air plasma coatings (APS) and 1.5 times the life of electron beam physical vapor deposited (EB-PVD) coatings. The conductivity of SPPS coatings is lower than EB-PVD coatings and higher than the best APS coatings. Manufacturing cost is expected to be similar to APS coatings and much lower than EB-PVD coatings. The SPPS deposition process includes droplet break-up and material arriving at the deposition surface in various physical states ranging from aqueous solution, gel phase, to fully-molten ceramic. The relation between the arrival state of the material and the microstructure is described.     

Effects of substrate rotation speed during deposition on the thermal cycle life of thermal barrier coatings fabricated by electron beam physical vapor deposition

The effects of substrate rotation speed during deposition of an Y₂O₃ stabilized ZrO₂ (YSZ) layer fabricated by electron beam physical vapor deposition (EB-PVD) on the microstructure, elastic modulus and lifetime were investigated. The microstructure and elastic modulus of EB-PVD YSZ coatings were highly influenced by the rotation speed of the substrates. The elastic modulus of the coatings was found to decrease as the rotation speed was increased, which led to a longer thermal cycle life.     

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m^?1 K^?1 for EB-PVD YSZ coatings to about 0.7 W m^?1 K^?1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni₃Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.     

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti-Al-Cr-Y-N and Ti-Al-Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti-45Al-8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD).Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti-Al-Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti-Al-Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti-Al-Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.     

Comparison of the photocatalytic behavior of TiO2 coatings elaborated by different thermal spraying processes

This paper proposes a comparative study on the microstructure and photocatalytic performances of titanium dioxide coatings elaborated by various thermal spraying methods (plasma spraying in atmospheric conditions, suspension plasma spraying, and high-velocity oxyfuel spraying). Agglomerated spray dried anatase TiO₂ powder was used as feedstock material for spraying. Morphology and microstructural characteristics of the coatings were studied mainly by scanning electron microscopy and x-ray diffraction. The photocatalytic behavior of the TiO₂-base surfaces was evaluated from the conversion rate of gaseous nitrogen oxides (NOx). It was found that the crystalline structure depended strongly on the technique of thermal spraying deposition. Moreover, a high amount of anatase was suitable for the photocatalytic degradation of the pollutants. Suspension plasma spraying has allowed retention of the original anatase phase and for very reactive TiO₂ surfaces to be obtained for the removal of nitrogen oxides. 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.     

Thermal barrier coating experience in gas turbine engines at Pratt & Whitney

Pratt & Whitney has accumulated more than three decades of experience with thermal barrier coatings (TBCs). These coatings were originally developed to reduce surface temperatures of combustors of JT8D gas turbine engines to increase the thermal fatigue life of the components. Continual improvements in de-sign, processing, and properties of TBCs have extended their applications to other turbine components, such as vanes, vane platforms, and blades, with attendant increases in performance and component du-rability. Plasma-spray-based generation I (Gen I) combustor TBCs with 7 wt % yttria partially stabilized zirconia deposited by air plasma spray (APS) on an APS NiCoCrAlY bond coat continues to perform ex-tremely well in all product line engines. Durability of this TBC has been further improved in Gen II TBCs for vanes by incorporating low-pressure chamber plasma-sprayed NiCoCrAl Y as a bond coat. The modi-fication has improved TBC durability by a factor of 2.5 and altered the failure mode from a “black fail-ure” within the bond coat to a “white failure” within the ceramic. Further improvements have been accomplished by instituting a more strain-tolerant ceramic top layer with electron beam/physical vapor deposition (EB-PVD) processing. This Gen III TBC has demonstrated exceptional performance on rotating airfoils in high-thrust-rated engines, improving blade durability by three times through elimination of blade creep, fracture, and rumpling of metallic coatings used for oxi-dation protection of the airfoil surfaces. A TBC durability model for plasma-sprayed as well as EB-PVD systems is proposed that involves the accumulation of compressive stresses during cyclic thermal expo-sure. The model attempts to correlate failure of the various TBCs with elements of their structure and its degradation with thermocyclic exposure.     

Deposition and Characterization of HVOF Thermal Sprayed Functionally Graded Coatings Deposited onto a Lightweight Material

There is a significant interest in lightweight materials (like aluminum, magnesium, titanium, and so on) containing a wear resistance coating, in such industries as the automotive industry, to replace heavy components with lighter parts in order to decrease vehicle weight and increase fuel efficiency. Functionally graded coatings, in which the composition, microstructure, and/or properties vary gradually from the bond coat to the top coat, may be applied to lightweight materials, not only to decrease weight, but also to enhance components mechanical properties by ensuring gradual microstructural (changes) together with lower residual stress. In the current work, aluminum/tool-steel functionally graded coatings were deposited onto lightweight aluminum substrates. The graded coatings were then characterized in terms of residual stress and hardness. Results show that residual stress increased with an increase in deposition thickness and a decrease in number of layers. However, the hardness also increased with an increase in deposition thickness and decrease in number of layers. Therefore, an engineer must compromise between the hardness and stress values while designing a functionally graded coating-substrate system.     

Forming nanostructured hypereutectic aluminum via high-velocity oxyfuel spray deposition

Al-Si alloys are widely used in aerospace and automobile industries due to their high strength-to-weight ratio and superior wear and corrosion resistance. Increasing the percentage of silicon in these alloys improves their wear resistance and strength considerably, but impairs their ductility. This problem can be overcome by refining the microstructure of hypereutectic alloys by varying the processing conditions. High velocity oxyfuel (HVOF) spray deposition is an efficient process to deposit nanostructured Al-Si coatings and near-net-shape spray-formed structures. This article deals with the microstructural evolution in an Al-21wt.% Si alloy processed through HVOF spraying. T. Laha is pursuing his doctoral degree in the same department. For more information, contact A. Agarwal, Florida International University, 10555 West Flagler Street, CEAS 3464, Department of Mechanical and Materials Engineering, Miami, Florida 33174; agarwala@fiu.edu.     

基板温度对EB-PVD ZrO2涂层微观组织及残余应力的影响

采用电子束物理气相沉积(EB-PVD)技术在不锈钢基板上沉积ZrO2涂层.研究基板温度对涂层微观组织和残余应力的影响.结果表明:沉积的涂层均为t-ZrO2结构,涂层表面平整致密;随着基板温度的升高,涂层表面颗粒逐渐长大,表面粗糙度增大;涂层残余应力也随着基板温度的升高而增加,当基板温度为1000 ℃时,涂层残余应力的增量最大,ZrO2涂层的残余应力主要是由于涂层与基板热膨胀系数差别而产生的热应力引起.     

Columnar-Structured Thermal Barrier Coatings (TBCs) by Thin Film Low-Pressure Plasma Spraying (LPPS-TF)

The very low-pressure plasma Spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with coverage of a large area by plasma spraying. At typical pressures of 100-200 Pa, the characteristics of the plasma jet change compared to conventional low-pressure plasma-spraying processes (LPPS) operating at 5-20 kPa. The combination of plasma spraying at low pressures with enhanced electrical input power has led to the development of the LPPS-TF process (TF = thin film). At appropriate parameters, it is possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This technique offers new possibilities for the manufacturing of thermal barrier coatings (TBCs). Besides the material composition, the microstructure is an important key to reduce thermal conductivity and to increase strain tolerance. In this regard, columnar microstructures deposited from the vapor phase show considerable advantages. Therefore, physical vapor deposition by electron beam evaporation (EB-PVD) is applied to achieve such columnar-structured TBCs. However, the deposition rate is low, and the line-of-sight nature of the process involves specific restrictions. In this article, the deposition of TBCs by the LPPS-TF process is shown. How the evaporation of the feedstock powder could be improved and to what extent the deposition rates could be increased were investigated.     

An investigation of tribological properties of CN and TiCN coatings

Today the tool industry on a worldwide basis uses hard, wear-resistant, and low-friction coatings produced by different processes such as electrochemical or electroless methods, spray technologies, thermochemical, chemical-vapor deposition (CVD), and physical vapor deposition (PVD). In the current work, two different coatings, nitrocarburized (CN) and titanium carbonitride (TiCN) on M2-grade tool steel, were prepared by commercial diffusion and PVD techniques, respectively. Properties such as thickness, roughness, and hardness were characterized using a variety of techniques, including glow-discharge optical emission spectrometry (GD-OES) and scanning electron microscopy (SEM). A crossed-cylinders wear-testing machine was used to investigate the performances of both coatings under lubrication. The effect of coatings on the performance of lubricants under a range of wear-test conditions was also examined. Degradation of lubricants during tribological testing was explored by Fourier transform infrared (FTIR) spectroscopy. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appears on pp. 560–66 of the Proceedings.     

Characterization and properties of MTCVD Ti(C,N) coated cemented carbide substrates with Fe/Ni binder

Two developments, including the deposition of hard chemical vapour deposition (CVD) coatings on cemented carbides and the use of innovative binders instead of Co for cemented carbides, have attracted worldwide interest. In this paper, ISO grade P30 cemented carbides with Fe/Ni and Co binders are prepared as the substrates, and adherent Ti(C,N) coatings are deposited on them by moderate temperature chemical vapour deposition (MTCVD) technique. The microstructure and properties of both the substrates and coated cemented carbides are studied.     

The hydrogen embrittlement of titanium-based alloys

Titanium-based alloys provide an excellent combination of a high strength/weight ratio and good corrosion behavior, which makes these alloys among the most important advanced materials for a variety of aerospace, marine, industrial, and commercial applications. Although titanium is considered to be reasonably resistant to chemical attack, severe problems can arise when titanium-based alloys come in contact with hydrogen-containing environments, where they can pick up large amounts of hydrogen, especially at elevated temperatures. The severity and the extent of the hydrogen interaction with titanium-based alloys are directly related to the microstructure and composition of the titanium alloys. This paper addresses the hydrogen embrittlement of titanium-based alloys. The hydrogen-titanium interaction is reviewed, including the solubility of hydrogen in α and β phases of titanium and hydride formation. Also, the paper summarizes the detrimental effects of hydrogen in different titanium alloys. For more information, contact Dan Eliezer, Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel; +972-7-646-1467; fax +972-7-646-1475; e-mail deliezer@bgumail.bgu.ac.il.     

Application of USAXS analysis and non-interacting approximation to determine the influence of process parameters and ageing on the thermal conductivity of electron-beam physical vapor deposited thermal barrier coatings

Electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) display a lower thermal conductivity compared with the deposited bulk material. This effect is achieved due to the presence of pores within these films. The spatial and geometrical characteristics of the porosity influence directly the magnitude of the achieved reduction of the thermal conductivity. In this work, three EB-PVD coating containing different microstructures were manufactured by varying the manufacturing process parameters during the deposition process. Their corresponding thermal conductivities were measured via the laser flash analysis method (LFA) in both the as-coated state and after ageing (1100 °C/100 h). Analysis of the pore formation during processing was carried out by ultrasmall-angle X-ray scattering (USAXS). This technique is supported with a computer based modeling developed by researchers at Advanced Photon Source (APS) in ANL, USA, and in National Institute of Standards and Technology (NIST), USA. The model enables the characterization of the size, shape, volume and orientation of each of the pore populations in EB-PVD TBCs. The effect of these spatial and geometrical characteristics of the porosity on the thermal conductivity of the EB-PVD coatings were studied via a non-interacting approximation based on Maxwell's model. Results of LFA measurements and the applied approximation indicate an interrelation between the microstructure and the thermal properties of the analyzed EB-PVD coatings. Microstructures containing a higher volume fraction of fine anisotropic intra-columnar pores, and larger voids between feather-arms oriented at lower angles toward the substrate plane correspond to lower thermal conductivity values. Inter-columnar gaps do no significantly contribute to lowering the thermal conductivity due to their orientation parallel to the heat flux and their lower volume fraction compared with the volume occupied by the primary columns. On heat treatment, the deepest section of the gaps between feather-arms break-up into arrays of nano-sized low aspect ratio voids. The anisotropic, elongated intra-columnar pores evolve toward low aspect ratio shapes that are less effective in reducing the thermal conductivity.     

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.     

Large area electron beam physical vapor deposition (EB-PVD) and plasma activated electron beam (EB) evaporation — Status and prospects

Electron beam physical vapor deposition (EB-PVD) is an industrially well established large area PVD technology. Highly productive air-to-air coaters for metal strip, roll-to-roll coaters for plastic web or thin metal foils as well as inline carrier coaters for substrates or lots of substrates have been introduced into industrial production. Axial type high power EB-guns having a nominal power of several 100 kW at 60 kV are available as technological key components in order to generate high vapor densities.Great efforts have been made to combine high rate EB-PVD with plasma generation processes in order to activate the generated vapor. The industrial break through depends on the potential of long term process stability and of large area extension of the process combination as well as on the availability of robust and cost-saving solutions. Two versions of process combination which are suitable to meet these requirements will be introduced: Spotless arc activated deposition (SAD) and a combination of electron beam evaporation with magnetron sputtering (EB-Mag process).The paper presents some examples of coating results concerning these process combinations. Coatings of titanium and titanium based compounds have been deposited by SAD. Coatings of tin and copper have been deposited by the EB-Mag process.     

Nanostructured photocatalytic titania coatings formed by suspension plasma spraying

This paper describes formation of titanium dioxide coatings designed for photocatalytic applications, obtained by suspension plasma spraying (SPS), an alternative of the atmospheric plasma spraying (APS) technique in which the material feedstock is a suspension of the material to be sprayed. Two different TiO₂ powders were dispersed in distilled water and ethanol and injected in Ar-H₂ or Ar-H₂-He plasma under atmospheric conditions. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were performed to study the microstructure of the titania coatings. Photocatalytic efficiency of the elaborated samples was evaluated from the conversion ratio of different air pollutants: nitrogen oxides (NOx) and sulfur dioxide (SO₂). The morphology and crystalline structure of the deposits depended mainly on the nature of the solvent (water or alcohol) used in the preparation of the slurries. Dense coatings were obtained starting from aqueous suspensions and porous deposits were elaborated by plasma spraying of a PC105 alcoholic suspension. A significant phase transformation from anatase to rutile occurred when ethanol was used as a solvent. Different photocatalytic performances were observed as a function of the nature of the liquid material feed-stock, the spraying parameters, and the nature of the pollutant. 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.     

Alumina coatings obtained by thermal spraying and plasma anodising — A comparison

Thermally sprayed alumina coatings are widely used in a range of industrial applications to improve wear and erosion resistance, corrosion protection and thermal insulation of metallic surfaces. These properties are required for many components to be used for production processes in the paper and printing industry.Another appropriate method to produce ceramic coatings is the plasma electrolytic oxidation (PEO). However PEO can only be applied on self-passivating metals like aluminium, titanium, magnesium and their alloys. The present paper concerns a combination of cost-efficient arc spraying and flame spraying of Al coatings (Al99.5, AlCu4Mg1) on steel substrates and post-treatment by plasma-electrolytic oxidation (PEO). The microstructure and phase composition of generated oxide coatings are examined and discussed. The created Al₂O₃ layers show outstanding hardness up to 1600 HV0.1, good bonding strength and excellent abrasion resistance compared to atmospheric plasma-sprayed Al₂O₃-coatings. The results show the superior performance of PEO-coatings and demonstrate their applicability for technical components in extreme operating conditions.     

The evaluation of powder processing on microstructure and mechanical properties of hydroxyapatite (HA)/yttria stabilized zirconia (YSZ) composite coatings

In clinical applications, the mechanical failure of HA-coated titanium alloy implants suffered at the interface of the HA coating and titanium alloy substrate will be a potential weakness in prosthesis. Yttria stabilized zirconia (YSZ) reinforced HA coatings have been proven to enhance the mechanical properties of the HA coating significantly and reduce the formation of calcium oxide (CaO). In this paper, HA/YSZ (30 wt.% YSZ) composite coatings were sprayed by the plasma technique. The effects of the powder processing–mechanical ball milling method and spheroidization method on the microstructure and mechanical properties of the HA/YSZ composite coatings were evaluated. The experimental results showed that the spheroidized powders melted better than the ball milled powders during plasma spraying and formed higher mechanical property coatings (1.6326±0.08 MPa m^−0.5 of fracture toughness, 58.59±2.91 GPa of elastic modulus and 43.42±2.53 MPa of tensile bond strength). HA/YSZ solid solution formed during deposition on the substrate, which played a very important role in the mechanical properties of the HA/YSZ composite coatings. Tensile bond strength tests showed that the fracture mode was cohesive and that failure occurred at the interface of HA and unmelted YSZ particles. The molten state of YSZ had a great influence on the properties of the HA/YSZ composite coatings.