Thermal Barrier Coatings

Presentation

Table of Contents

  1. Thermal Barrier Coatings
    1.1. History
    1.2. Overview
    1.3. Function
    1.4. Structure
    1.4.1. Metallic Bond Coat
    1.4.2. Thermally Grown Oxide
    1.4.3. Ceramic Topcoat
    1.5. Applications
    1.6. Processes
  2. Physical Vapor Deposition
    2.1. History
    2.2. Overview
    2.3. Processes
    2.3.1. Thermal
    2.3.2. Electron Beam
    2.3.3. Sputtering
    2.3.4. Pulsed Laser Deposition
    2.4. Physical Vapor Deposition vs Chemical Vapor Deposition
  3. Physics
    3.1. Evaporation
    3.1.1. Vapor Pressure
    3.1.2. Gibb’s Phase Rule
    3.1.3. Multiple Atoms
    3.2. Transport
    3.2.1. Mean Free Path
    3.2.2. Glancing Angle Deposition
    3.3. Deposition
    3.3.1. Diffusion
    3.3.2. Condensation
    3.3.3. Solidification
    3.3.4. Sintering
  4. Conclusion
  5. References
  6. Acronyms
  7. Figures

1. Thermal Barrier Coatings

1.1. History

Thermal barrier coatings (TBCs) are thick films applied to metal components to provide thermal protection against hot combustion gases.1 TBCs are most often used in gas turbines, used to power and move ships and airplanes, or to generate electricity, such as a land-based gas turbines (generators).2,3 The coating is sprayed as a heated or molten powder or vaporized onto such high-temperature components as the combustor liner, blades, and vanes.2,4 They have also found use in brake systems, internal combustion engines (i.e. combustion chamber, pistons, and valves), rocket engines, and nuclear reactors.2,5 Before the introduction of TBCs, extensive research was conducted into structural materials with greater temperature performance using textured microstructure, single-crystal blades, improved alloy design, and airflow cooling through internal channels.1 Convection cooling offers the most cooling to the component, even more than TBCs.4 Early research in TBC coatings focused on increased adhesion between the ceramic topcoat and metallic bond coat, since the ceramic was prone to spalling.2 Ensuring chemical and mechanical compatibility between layers helped demonstrate the reliability of the coatings.2 Today, coatings have improved their lifetime and last tens of thousands of service hours.2 In the early 1990s, the focus shifted from improving reliability by preventing spalling to reducing thermal conductivity of ceramic material to impede thermal transport.2

1.2. Overview

Developing a coating to meet the many, often conflicting, goals is achieved by tailoring the chemical composition and coating microstructure.2,5,6 TBCs must be resistant to thermal, corrosive, and mechanical wear encountered in the harsh environments in which they are used.7 TBCs should have erosion resistance, that is, resistance against mechanical wear caused by impinging particulate debris or liquid droplets, such as fuel or water.1,2,4 The failure mechanism proposed for a TBC exposed to high temperatures for extended duration is thermal fatigue and and thermally grown oxide (TGO) layer growth.5,6,8 TBCs should have thermo-cyclic fatigue (TCF) resistance, and thus be tolerant of the strain caused by repeated heating and cooling, as well as the strain caused by TGO layer growth.5,6,8 To lessen the thermal stress mismatch, vertical cracks can be intentionally introduced in the ceramic layer.5 The difference in thermal expansion (TE) of the airfoil and TBC system layers leads to strain, causing residual stress in the TBC system.3 TBCs deposited on high-pressure turbine blades have stresses of 2.4 – 2.8 GPa as-prepared and 2.3 – 3.1 GPa after service, which can vary based on the morphology of the TGO.5 To reduce the elastic strain energy, it is important to control the porosity architecture (pore volume fraction, geometry, morphology, and distribution) and thickness of the TGO layer.2,3,9 Since the growth of the TGO layer applies significant strain to the TBC system, controlling the TGO layer thickness ultimately controls the lifetime of the coating.5,6 TBCs with good durability are achieved at a porosity level of 10% (15%10).8 Entrapped air creates these voids and a lack of complete overlap of splats (flattened molten particles that occur during the impact on the substrate) in plasma spraying (PS).11 Pores that fully penetrate the ceramic coating help the ceramic and metallic layers remain bonded.3 Porosity is also important in lowering the thermal conductivity of the ceramic.2,3,9 The use of a porosity-forming polymer, which is burned out during heat treatment, leads to the highest porosity rate at 23-27%.6

1.3. Function

TBCs reduce component temperatures by 190 °C8 (150 °C9), allowing the engine to run hotter and produce more work, while keeping parts cooler.2,6 This improves engine performance, efficiency, and fuel economy (and reduces pollution), as well as part durability and thus lifetime.2,6 TBCs protect the substrate from high temperatures, as well as harsh oxidation and combustion environments.2,11

1.4 Structure

These well-engineered structures consist of a three-layer case: a metallic bond coat, a TGO, and a ceramic topcoat.11 To enhance the performance of coatings, improvements can be made in lowering the intrinsic thermal conductivity, controlling porosity (introducing vertical cracks), and increasing the thickness of the ceramic topcoat.5 A low density offers a 15% reduction in thermal conductivity.2 A substantial reduction in thermal conductivity and improved TCF lifetime have been found for TBCs deposited at high chamber pressure (1.5 X 10-3 Torr) with an oxygen and inert gas environment.2 TBC layers are 0.1 – 2 mm thick1 (300 – 500 m12). However, the failure of TBCs produced by using air-plasma spraying (APS) occurs when the thickness is >1.5 mm.5 The different coating layers and dimensions are shown in figure 1. The Thornton Structure Zone model shows how varying temperature and pressure results in considerably different microstructures (figure 2).13

1.4.1. Metallic Bond Coat

The metallic layer provides corrosion resistance to the substrate, since the ceramic layer provides little or no corrosion resistance due to its porous structure.8 Oxidation is a specific type of corrosion that occurs in applications at high temperatures where air is present. The metallic bond coat has strong adhesion to the ceramic layer due to its roughness (Ra = 5 – 10 m).6 The two types of metallic bond coats are diffusion and overlay.5,14 Diffusion coatings lead to better adhesion due to increased diffusion, are less expensive, and tend to be thinner.4 Overlay coatings bond well and offer greater composition flexibility even with minimal elemental interdiffusion.4 Both diffusion and overlay coatings are corrosion resistant and lead to a spall-resistant, slow-growing aluminide scale (intermetallic, i.e. NiAl, Ni3Al, CoAl) to protect against oxidation.4,15 NiCrSi is another composition option, but protects only up to 850 °C.15 The metallic bond coat layer is 130 m thick8 (200 m6, 190±25 nm11, 120 nm16). The bond coat is either a low sulfur platinum aluminide diffusion type or MCrAlY overlay type (where the M represents Ni and/or Co5,11,15,17,18, Ni protects against high-temperature oxidation and Co against hot corrosion15).5,17 The bond coat composition is Ni-22Cr-10Al-1.0Y5,11 and is applied to a substrate of composition Ni-20Cr-20Co-5.9Mo-0.5Al-2.1Ti-0.4Mn-0.3Si-0.06C in wt%.5 The particle size is 56 – 106 nm.5

1.4.2. Thermally Grown Oxide

When the bond coat oxidizes, a TGO layer is formed.17 This occurs either before or during the ceramic layer deposition.17 Initially, the TGO layer consists of dense alumina (α & γ phase Al2O3 from α-Al2O3 powder7,19).11 The TGO layer is 1 nm thick, grows to 3-9 nm, and if grown to 10 nm, it will fail.5

1.4.3. Ceramic Topcoat

The topcoat reduces the surface temperature of the bond coat by 100 – 200 °C3 (110 °C4, 140 °C20, 100 – 120 °C21, 170 °C7). In one experiment, the temperatures on the surface of a 600±50 nm and 300±50 nm ceramic topcoat (both applied to a 300±50 nm bond coat and a 5 mm thick substrate) were 1700 – 1900 °C and 280 °C respectively.5 The temperature drop across the ceramic layer depends on the coating thickness and its thermal resistance.3 The insulating layer should have low thermal conductivity and high-temperature capability (sintering resistance), as well as chemical and mechanical compatibility with the bond coat.2 The standard refractory used is 6 – 9 wt% yttria-stabilized zirconia (YSZ).21 Maximum ionic conductivity is obtained at a yttria (Y2O3) concentration of 8 wt% (1000 °C).17 The substitution of some zirconium (Z4+) ions with slightly larger yttrium (Y3+) ions stabilizes the crystal structure of zirconia (ZrO2) and prevents high-temperature phase transformations.6,17 The phase transformation of the ceramic causes volume change and thus increased lattice strain.6 This occurs when the YSZ exceeds 1200 °C (1170 °C7) in operation and changes from a monoclinic to tetragonal crystal structure.6 This is a slow process and may require temperatures more than 1350 °C for over 100 hours before the monoclinic zirconia appears.10 Upon cooling, the crystal lattice turns back into monoclinic, resulting in a damaging volume expansion of 4%7,17 (4-5%2).6

The introduction of oxygen vacancies that occur when Y2O3 is added to ZrO2, is the other mechanism that slows down the rate of decomposition to the tetragonal phase: the more stabilizer added, the more oxygen vacancies are created.2 An oxygen vacancy is created when two Z4+ ions (Rion = 0.82 Å), which reside in the zirconia structure, are replaced by a Y3+ ion (Rion = 0.96 Å) and four O2- ions with only three O2 ions.1 TBCs containing rare-earth metals other than Y3+ are being developed with even lower thermal conductivity.2 One dopant alternative is dysprosium-stabilized zirconia (DySZ), in which substituting the Y3+ ions with larger Dy3+ ions (adding Dy2O3) achieves lower conductivity through enhanced phonon scattering (lattice vibration).6 Similar spraying capability and thermal cycling behavior were observed compared to conventional YSZ powders.6 The ceramic, topcoat layer is 300 – 500 nm thick (APS- 600±50 nm5, 130 – 380 nm8, 230±25 nm11, 150 nm22, 100 nm – 1 mm10).12 However, ceramic layers thicker than 300 nm are more likely to spall.5 The particle size is 11 – 150 nm.5,11

1.5. Applications

In general, PS is used to coat nozzle vanes and EBPVD for turbine airfoils.21 EBPVD is preferable for the blades because it applies a smooth surface, which leads to better aerodynamic properties, with less interference due to cooling holes.4 Gas turbine engines are a $42B industry worldwide; 65% are used in jet engines and 35% in land-based generators.10 Land-based generators run off natural gas or liquid fuels, producing 25% of all electricity in the US and 20% of all global electricity (2010).10 In recent decades, alloy and process development has led to blades without a TBC, which can handle average temperatures of 1050 °C with occasional excursions (hot spots) of 1200 °C.23 However, the temperatures in the hot section (combustor and turbine) can reach 1650 – 2200 °C if there is advanced cooling, and 1550 °C if there is no cooling.4 Modern turbines experience temperatures of 1370 °C, and a jet engine turbine such as the Snecma M88 of 1590 °C.13 In a 747 and DC10, gases reach temperatures above 2,000 °C.9 The material commonly used for turbine blades is the superalloy NiCoCrAlY, which has a melting point of 1330 °C.23 The melting temperature of a TGO (i.e. Al2O3) is 2053 °C.24 The blades are therefore exposed to gas temperatures higher than their melting point.18 The use of cooling schemes along with TBCs has allowed components to withstand gas temperatures above the melting point of the superalloy (>250 °C above the melting point20).4,20

1.6. Processes

The methods used to apply TBCs are physical vapor deposition (PVD) and plasma (thermal) spraying (PS).2 Under development is chemical vapor deposition (CVD) and solution precursor plasma spraying (SPPS) (2004).2 PVD coatings form by condensation, while CVD coatings form by decomposition.25 Thermal spray coatings are prepared by spraying melted or heated material on the substrate where it solidifies.4 Heating is performed with a plasma, an arc, or chemical means.4 Techniques include APS, high-velocity oxy-fuel (HVOF), low-pressure plasma spray (LPPS), high-frequency pulse detonation, and pack cementation.5,6,17,26 APS uses a plasma jet (electrical method), in which a mixture of inert gases (i.e. Ar & H) is passed over an electrode that forms the plasma.4 The powder particles are then fed into the plasma and attain a speed of 350 – 560 m/s.5 HVOF uses a combustion flame (chemical method) consisting of a liquid (i.e. kerosene, etc.) or gaseous (i.e. acetylene, propane, propylene, hydrogen, methane, natural gas, etc.) fuel and oxygen to spray powder at 700 m/s.5 The powder fed into the combustion flame is melted on the way to the substrate.4 The simpler HVOF process produces coatings with better performance, as they are denser, stronger, and have improved bond coating oxidation resistance.6 However, the coatings in APS are cleaner, as fuel combustion in HVOF produces impurities.6 The LPPS method is the most cost-prohibitive, but gives a dense coat without oxide formation.26 The macroscopic properties of a coating depend on which deposition process is used (and its parameters) and the properties of the particles, such as size, size distribution18, and morphology.11

2. Physical Vapor Deposition

2.1. History

Physical vapor deposition (PVD) was popularized in the 1966 book Vapor Deposition, and in 1838 Michael Faraday created the first glow discharge in a vacuum tube.27 The vacuum tube, which controls the electric current in a vacuum, consisted of two brass electrodes and a 2 Torr vacuum.27 Enough electrons were made to make enough ions (and electric neutrals) to create enough electrons (ejection of secondary electrons) to sustain the discharge.28 As of 1996, the worldwide market for functional coatings was $83B, with vacuum deposition accounting for $200 million.27 The distinction between thin and thick film coatings is made based on the processing techniques used, not on the thickness of the film.29 Thin films are made by building up individual molecules (i.e. PVD), while thick films are made by depositing particles (i.e. PS).29 Electrochemical deposition has found widespread commercial application.25 However, this method has a limited capability to deposit ceramics.4 The process deposits material (anode) on a conductive surface (cathode) by immersing them in an electrolyte solution with metal salts and ions that permit the flow of electricity.25 A power supply oxidizes the anode material, causing it to dissolve in the liquid medium, then the electrolyte deposits the ions on the cathode.25 PVD compared to electroplating (i.e. chrome plating) results in more uniform coatings that typically weigh less.4 This is important in reducing centrifugal stresses on turbine blades during operation.2,3 Plus, the use of a chromic acid oxidation process is toxic and carcinogenic.

2.2. Overview

The PVD process takes place in a vacuum chamber, and either requires a heat source to evaporate/sublimate the target material, or energetic ions to bombard it.28 The volatile vapor is transported to the substrate in a vacuum or low-pressure plasma environment.28 The vacuum removes air from the system, which can act as a contaminant and lower the density of the coating.28 It also reduces collisions between air and source vapor, making the deposition less diffuse.28 Air introduces point defects by adding extra oxygen atoms, which disrupts the atomic, and ionic, arrangement in the crystal structure.28 Typical PVD coating growth is 1-10 nm/sec.28 A substrate surface can be modified before and/or after deposition of the TBC system.28 Plasma nitriding, or ionitriding, thermally diffuses nitrogen into the substrate by heating the substrate to 500-550 °C and placing it in a gaseous nitride atmosphere (other environments are carbide, boride, etc.) for 48 hours.13,28 Shot peening can modify a sputter-deposited overlay coat, making it denser and adding residual stresses.28

2.3. Processes

2.3.1. Thermal

Thermal evaporation is the most common technique of vaporizing material at temperatures <1500 °C.28 The source material is heated in a vacuum (>10-4 Torr28), to a temperature at which its vapor pressure is 10-2 Torr13,25,28 (0.1-0.3 eV30) giving an effective deposition rate.28,30 PVD is a distilling process, so it yields coatings of high purity.30,31 The vaporized source material, assuming no collisions during transport, travels to the substrate in a line-of-sight deposition, and the flux is assumed from a point source or cosine emitter (flux distribution from a Knudsen cell, used in MBE, is given by the Knudsen effusion model by Roth and Hirth28).3,28,30 Heating is achieved by conduction and radiation; convection in a vacuum environment is negligible.28 One method of conduction heating is to resistively evaporate material either directly, using a heated coil (filament) or, more commonly, heating a dimpled boat containing the material (charge) (figure 4).28,31 The filament (charge rod) is a refractory metal of either W, Ta, Mo, C, or BN/TiB2 and melts the source rod.28 The source rod should be in good thermal contact with the filament for efficient heating and to allow wetting or flow of the molten source over the filament.28 An advanced form of resistive evaporation is Molecular Beam Epitaxy (MBE) or Vapor Phase Epitaxy (VPE), which uses a high or ultra-high vacuum (>10^-9 Torr) and Knudsen cells for evaporation.28 The effusion cells allow careful control of temperature and thus evaporation.28 The much higher vacuum and the absence of carrier gas lead to the highest achievable purity of the grown films.28 In the MBE configuration, neutral thermal energy beams (molecular or atomic) impinge on the heated substrate.30 MBE would not be effective, since the TBC ceramic layer should have a low density, which occurs at high chamber pressure and low substrate temperature.2

2.3.2. Electron Beam

As opposed to radiative heating with an electric filament, the electron beam PVD (EBPVD) process is commonly used when vaporizing material at temperatures >1500 °C, and is also useful for evaporating large quantities of material.28 This method uses a high-energy e-beam to deposit refractory materials.28 Electrons are generated from a thermionic emission source and accelerated through a high voltage (70 kv3, 60 kv17, 10-20 kV9,28).28 The accelerating electrons are focused and deflected with electromagnetic (EM) fields (figure 5).28 E-beam guns usually operate at 50-100 kW, with some operating at 150 kW (similar to plasma guns operating at 50-120 kW4).28 The methods used to apply the topcoat are either APS or EBPVD (or ion-enhanced EBPVD- IE EBPVD).5,26 EBPVD deposits coatings of uniform thickness (in line-of-sight) on all but inner surfaces of complex geometry due to high chamber pressure, which scatters the vapor, creating a ribbon-like structure normal to the plane.9

The downside of this columnar structure and segmentation cracks is higher thermal conductivity and greater permeability to oxygen and molten salts (i.e. Na and K sulfates).4,8 The electrochemical reaction, known as hot corrosion, between the substrate and the salts accelerates spallation.4 When the salt builds up and the temperature exceeds the melting point of the salt, it wicks through cracks and pores in the TBC and causes cracks after freezing.4 However, this structure is more strain tolerant and thus compliant in the direction parallel to the interface, resulting in good thermal fatigue resistance.8,9 EBPVD coatings last 8-10 times longer before spalling compared to APS coatings.32 Therefore, EBPVD coatings are preferred in aerospace applications where extreme thermal cycling occurs.32 IE EBPVD has the advantage of increased coating adhesion due to ion bombardment, which provides in-situ cleaning and surface modification, leading to a denser microstructure from smoothing pits, scratches, and evaporated droplets on the surface.28 Additionally, it can be used to heat the substrate, which enhances diffusion and chemical reactions.28 In EBPVD, the vacuum is at least 7.5 X 10-5 Torr to allow the passage of electrons and ionized gaseous atoms.33 Electrons are generated either from a thermionic source (i.e. electron gun), field electron emission (i.e. electrostatic field), or an anodic arc.33 When using a gaseous arc, a vacuum of only 10-2 Torr is used so that the gas is not trapped in the coating.28 A typical arc is initiated when a high current density (104-106 A/cm2)28 (> 1010 A/cm2)33 and a low voltage (>25 V28, 10-30 V33) is passed through a gas or vapor of the electrode material.28,33

2.3.3. Sputtering

Sputtering can apply various materials, such as ceramics, metals, and alloys, as well as organic and inorganic compounds.4 Sputtering is not currently used to apply TBCs due to the slow deposition rate, even though it creates a columnar structure with elongated grains normal to the interface, which is the ideal microstructure for TBCs.4 The sputtering process physically erodes solid material from the source by ion-bombardment (plasma: Ar-H or Ar-He, a few percent of enthalpy-enhancing gas such as H).4 Surface coverage on complex substrates is achieved due to the higher gas pressure (10-1-10-3 Torr)4 which scatters the source material.3,4,29 These ions (few eV to 10’s eV)30 are created when electrons are accelerated and bump into gas molecules, which decreases or increases the number of valence electrons in the gas atoms, which creates positively or negatively charged ions respectively.29 These ions arrive at the substrate at a normal angle-of-incidence (collimation) and can modify the surface by creating defects and reactive sites.28

Thermal and sputter deposition can be applied in the same system; sputtering the minor constituent for example.28 The target is bonded to a water-cooled Cu backing plate to prevent the target material from melting or outgassing due to ion-bombardment.29 Additionally, since the radiative heating is less compared with thermal PVD, the substrate can be placed closer to the source. The ions could be replaced by neutral fragments of gas molecules or atoms (i.e. free radicals), which have high chemical reactivity, but it is easier to accelerate ions.29 The plasma is used to clean the substrate before deposition (“back-sputtering”), and to modify the film during deposition (“re-sputtering”).29 High-speed ions can implant or otherwise damage the substrate.31 The setup can include a high-voltage DC power supply connected between the sputtering target (cathode) and the workpiece (anode).30 By biasing the substrate, the ions can be accelerated at the substrate, as well as the source. If the unreacted gas is not incorporated into the film, it will enhance chemical reactions and provide kinetic energy, creating a denser coating.28

In reactive sputtering, the inert gas is replaced with reactive gas such as O, carbides, or nitrides.28 A magnet is usually used to localize the plasma around the area of the target that is to emit material, rather than somewhere else on the cathode, and it also prolongs residence time.30 Sputtering with a magnet (cathode) is known as magnetron sputtering.30 When coating a non-conductive substrate (i.e. polymer), radio-frequency (RF) sputtering is employed to prevent charge build-up.30

2.3.4. Pulsed Laser Deposition

Cathodic arc PVD (Arc – PVD), like EBPVD, uses a high current, low voltage arc to strike the surface of the source material.28 The positively charged cations (i.e. Ar+) are accelerated towards the cathode source (uncommon is anodic arc PVD where the source material is the anode instead of the cathode28).30 This gives rise to a cathode spot; a highly energetic emitting area.28 The cathode spot, size of a few μm, yields a high velocity 102 m/s jet of vaporized cathode material when subjected to a high current density of 106 – 1012 A/cm2.33 The vapor (>85% ionized, 10-100 eV)33 is guided to an anode which is either the vacuum chamber wall or the surface to be coated.33 An EM field is used to direct and rapidly move the arc over the entire surface of the cathode target.28 The plasma beam from the cathodic arc source contains some large clusters of atoms, or molecules (macro-particles), which should be filtered out so that a high-quality thin film can be achieved.28

2.4. PVD vs CVD

CVD was employed as early as 1880 in incandescent lamps.13,25 It involves high-temperature decomposition or reduction of a chemical vapor precursor and differs from PVD during the deposition of the vapor on the substrate.28,30 CVD uses gases, evaporating liquids, and chemically gasified solids as the source, and deposition involves a chemical reaction, not merely a phase change.29 The term vacuum or evaporation (vapor) deposition was coined to integrate the two methods.25 The existence of the hybrid-physical CVD process, HPCVD, illustrates the overlap that can take place. CVD is carried out at much higher temperatures than PVD (>800 °C vs 200 – 500 °C), and higher temperatures than its reactive gases.34 The increased temperature at the substrate helps desorb the reacting gas and results in more diffusion bonding, giving the metallic layer better adhesion.28 The chemical reactions in CVD result in corrosive volatile byproducts and unused reactive gas that must be removed by gas flow from the chamber.28

3. Physics

3.1. Evaporation

3.1.1. Vapor Pressure

The source material is subjected to a certain pressure and temperature by using a vacuum and applying heat. By using a high vacuum and lowering the pressure on the source to below its triple point, then adding heat, the source sublimes instead of evaporates.29,31

3.1.2. Gibb’s Phase Rule

Any system under equilibrium conditions conforms to the phase rule; the equality is F = C – P + 2, where F is the number of degrees of freedom (dof), C is the number of components, and P is the number of phases in thermodynamic equilibrium.25 Phases are similar to the four states of matter; gas, liquid, solid, and plasma, except it’s possible to have several separable, immiscible phases of the same state of matter.25,28 Components are the amount of chemically different species.25 Dof is the amount of parameters: temperature, pressure, and components which may be varied independently without causing a change in the number of phases.25 The phase rule applies to all deposition processes, as long as the system is at equilibrium.25

3.1.3. Multiple Atoms

When depositing multiple atoms, the term partial pressure is used to describe the vapor pressure of each of the gaseous species.28 Partial pressure describes the pressure of each constituent as if it was the only one in the system.28 Adding together the partial pressures for each vapor yields the total pressure of the system; and the vapor pressure of the source determines the lowest pressure the vacuum can achieve.28 If multiple atoms with different vapor pressures are evaporated from a single source, described by Raoult’s Law, this will result in compositional fluctuations across the TBC thickness.2,28 This can be avoided by separating the components and using different energy levels for evaporation.2 Fortunately, the vapor pressure of most rare earth oxides is close enough to Zr, that no major composition problems should result by evaporating from a single source.2 However, oxides that tend to sublime, such as ceria, magnesia, and silica, require additional processing efforts.2 To avoid sublimation, an element that normally sublimes can be alloyed with another element to give a liquid melt.28 Compounds often vaporize, or sublime, as anywhere from atoms to clusters of molecules to dissociated, or partially dissociated molecules.28

3.2. Transport

Transport consists of mainly three steps: transport reactants to the substrate, chemical reaction at or on the substrate, and transport of products away from the substrate.25 The first and third steps are interdependent, and the middle step involves surface and/or gaseous reaction.25

3.2.1. Mean Free Path

The degree of vacuum should be >10-4 Torr28 so that the mean free path (MFP), i.e. the distance an atom travels before colliding with another atom, is greater than the distance between the source and the target (10-16 inches4).28

3.2.2. Glancing Angle Deposition

Controlling the angle-of-incidence of deposition results in unique morphologies.28 This angled, off-normal, deposition results in columnar growth in the direction of the flux.28 Very oblique deposition angles (GLAD) exacerbate the columnar growth, since the valleys do not get any flux, resulting in a more porous structure.28 Rotating the substrate during GLAD results in corkscrew columnar growth.28 This “Zig-Zag”, or “Herringbone”, the structure has an increased path length for thermal transport and offers a 40% reduction in thermal conductivity.2

3.3. Deposition

Deposition is when a vapor becomes a solid without first becoming a liquid. Deposition rates vary greatly in PVD, ranging from <1 MLS (<3 μ/s) to >104 MLS (>30 μm/s).28 The further the substrate is moved from the source, the uniformer the coating, but the deposition rate is decreased as 1/r2.28

3.3.1. Diffusion

Atoms that land on a substrate and do not react immediately will have some mobility.28 These mobile atoms, that lie on the substrate (crystal plane), are known as adatoms.28 Adatom is a contraction of adsorbed atoms (another term is adparticles). Many arriving atoms are in a thermodynamically unfavorable state, and depending on the energy of the atom, the atom-surface interactions (chemical bonding), and the temperature of the surface will dictate the amount of mobility (diffusion) that takes place.28 It is unlikely that adatoms diffuse into stable lattice positions if the substrate temperature is low and there is reduced ion-bombardment (figure 6).28 Ballistic deposition is when the atom is immobilized where it lands, aptly named, since the projectile motion of the atom is the only effect on the deposition.29

3.3.2. Condensation

Condensation describes the process of vapor atom cooling (or being compressed to its saturation limit) to a liquid. Not every atom will condense, some atoms are re-evaporated or reflected, especially if the energy of the vaporized atoms or substrate temperature is too high.28 The term used for the ratio of the condensing atoms to impinging atoms is called the sticking coefficient.28 After finding preferential nucleation sites, the adatom will condense on the surface, assuming the temperature of the part is less than the adatoms melting temperature, by losing energy through diffusion, bonding to other atoms, and collisions with deposited and incoming atoms.28 Energy mostly in the form of heat of vaporization (enthalpy change on condensation) or heat of sublimation is given up.28

3.3.3. Solidification

In plasma spraying, rapid solidification occurs when liquid particles (some vapor) strike the substrate surface, flatten and cool (106 K/sec) to form splat (fine, non-columnar, equiaxed grains).28 The splats develop a lamellar structure parallel to the interface by directional solidification.11 In PVD, solidification is described by columnar growth, where elongated grains are oriented normal to the substrate.4,28 A new process known as plasma spray-PVD (PS-PVD) is a combination of LPPS and PVD.35 There are both molten feedstocks that form splats and vapor transported in a supersonic plasma plume to condense.35 PS-PVD gives superior coating properties than PS or PVD achieve on their own.35

3.3.4. Sintering

Sintering is the high temperature (heat) treatment that causes particles to diffuse together, fusing the ceramic layer, without the high melting point of the ceramic being reached and the ceramic liquefying. Sintering resistance during service is important for the ceramic topcoat. A desirable structure is created during the spray process that gives strain tolerance and low conductivity, due to the high volume of pore space.6 Improvements in lowering the thermal conductivity of the ceramic layer result in a hotter ceramic surface and thus a greater likelihood of sintering.2 In addition, a thicker coating increases the chances of sintering, because the heat conduction distance is increased, which decreases the rate of heat transfer.2 Thermal conductivity of P-YSZ coatings produced by EBPVD are higher at 1.8 – 2.0 W/mK2 (1.5 – 1.9 W/mK17) compared to APS at 0.6 – 1.2 W/mK2 (0.8-1.0 W/mK17).2,17 The tradeoff for low values of thermal conductivity, in APS, will be an increase in sintering due to the higher temperatures generated on the surface, and thus an increase in thermal conductivity.2 Sintering of the microcracks at the splat interfaces is the main cause of increased thermal conductivity.19

4. Conclusion

Improved thermal surface protection is important for improving engine technology, which continually strives to run engines hotter.4 Advances in processing resulting in single crystal blades and directional solidification provided a 120 °C temperature improvement for turbine blades.4 Cooling of the hot structure (blades, disc, and vanes) is the most effective technique at achieving longer part lifetimes, but a cooler blade results in lower engine efficiency.4 Using cooling provides a 370 °C4 (air cooling- 200-300 °C36) temperature reduction on the blade surface while only using 1-3%36 of the main airflow.4,36 Cooling schemes, using either air or liquid, are essential to protect the blade from degradation.36 However, if the cooling air could instead be used for propulsion, it is estimated that this would save 10M gallons of fuel annually in 250 aircraft.8 TBCs, on the other hand, allow both blade durability and high efficiency.4 The advantage of using liquid is its higher specific heat capacity compared with air, however, leaks can occur.36 A common method, using convection cooling, routes compressor air inside the hollow airfoil and out the many pinholes in its surface.36 Historically, the focus was on preventing creep.4 Now the main concern is preventing oxidation and hot corrosion (sulfidation) of the metallic bond coat and thermal-mechanical fatigue (TMF).4 Elements that provided corrosion resistance are Al, Ta, Zn, Ni, and Cr (and Ca, which is being phased out since the dust is carcinogenic37).28 As engine operating temperatures increased, the high Cr contents (20%4, 25-35%8) that prevented hot corrosion were phased out with higher Al content (4-5%4, 6%8) to provide better oxidation resistance.4 Resistance to chemical reactions with gas impurities, such as S and V, or deposits like Calcium-Magnesium-Alumino-Silicate (CMAS), is important.2 Recent studies have shown success with other ceramic types that use earth dopant oxides other than yttria.2,6 Many TBCs containing constituent oxides other than yttria (figure 7) have been introduced into service or testing with an underlying P-YSZ ceramic layer.2 This is partly due to the chemical incompatibility of the new ceramic material with the TGO, as found especially for pyrochlores (cerium metals), and partly the new ceramic altered the formation of the TGO layer, resulting in reduced lifetimes.2 Improvements in TBC systems are possible through the use of multi-layer (i.e. micro-layering), or phase mixture ceramics.2,4,28 One method of layering, at a thickness of 1 μm, is to switch on and off a bias voltage.2,4,28 This produces layers of different density and yields an impressive 37-45%2 reduction in thermal conductivity.2,4 Functionally graded materials (FGM) offer increased part durability by preventing spallation.2,4,28 One method of blending TBC layers is by vaporizing metal and metal oxides at the same time.2,4,28 This creates a gradient structure between the bond coat and the ceramic layer that helps to minimize TE mismatch.2,4,28 This is already performed by vaporizing alloys from a single source, yielding a graded composition as the evaporant is selectively vaporized.2,4,28 FGM may demand the use of alternative, multiple, and/or both coarse and fine powdered rare-earth metal dopants.2,4,28 Additionally, improved coating performance is achieved by using ultrafine, or nanopowders, (i.e. gas evaporation28) due to the unique microstructure that results.2,4,28 This structure has greater sintering resistance owing to its nanopores and nanograins.2,4 Additionally, using both coarse and fine powders.2,4 Lastly, if composites are used in the next generation of turbine blades the substrate will possess very different diffusion rates and surface chemistry.4

5. References

1. D. Clarke, S. Phillpot. 2005. “Thermal Barrier Coating Materials.” Mat’ls Today. Elsevier. 22-9

2. U. Schulz, B. Saruhan, K. Fritscher, C. Leyens. 2004.”Review on Advanced EB-PVD Ceramic Topcoats for TBC Applications.” Int. Journal of Applied Ceramic Technology. 1 [4] 302-1

3. T. Rodgers, H. Zhao, and H. Wadley. 2013. “Thermal Barrier Coating Deposition by Rarefied Gas Jet Assisted Processes: Simulations of Deposition on a Stationary Airfoil.” Journal Vacuum Science Technology A. American Vacuum Society. 31 [6] 1-14.

4. “Coatings for High-Temperature Structural Materials: Trends and Opportunities.” 1996. Committee on Coatings for High-Temperature Structural Materials, Commission on Engineering and Technical Systems, National Research Council. National Academy Press. 102 p.

5. Z. Lu, S. Myoung, H. Kim, et. al. 2013.”Microstructure Evolution and Interface Stability of Thermal Barrier Coatings with Vertical Type Cracks in Cyclic Thermal Exposure.” Journal of Thermal Spray Technology. ASM International. 22 [5] 671-9

6. Curry, N. Markocsan, L. Östergren, et. al. 2013. “Evaluation of the Lifetime and Thermal Conductivity of Dysprosia-Stabilized Thermal Barrier Coating Systems.” Journal of Thermal Spray Technology. ASM International. 22 [6] 864-72

7. A. Karaoglanli, K. Ogawa, A. Türk, et. al. 2013. “Thermal Schock and Cycling Behavior of Thermal Barrier Coatings (TBCs) Used in Gas Turbines.” InTech Open

8. R. Miller, W. Brindley, M. Bailey. 1989. “Thermal Barrier Coatings for Gas Turbine and Diesel Engines.” NASA Technical Memorandum

9. S. Terry, J. Litty, C. Levi. 1999. “Evolution of Porosity and Texture in Thermal Barrier Coatings Grown by EB-PVD.” The Minerals, Metals and Materials. Elevated Temperature Coatings: Science and Technology III Society. 13-26

10. L. Wang, X. Zhong, Y. Zhao, et. al. 2014. “Effect of Interface on the Thermal Conductivity of Thermal Barrier Coatings: A Numerical Simulation Study.” International Journal of Heat and Mass Transfer. 79 954-967

11. R. Ghasemi, R. Shoja-Razavi, R. Mozafarinia, et. al. 2013. “Comparison of Microstructure and Mechanical Properties of Plasma-sprayed Nanostructured and Conventional Yttria Stabilized
Zirconia Thermal Barrier Coatings.” Ceramics International. Elsevier. 39 8805-13

12. N. Fleck, A. Cocks, S. Lampenscherf. 2014. “Thermal Shock Resistance of Air Plasma Sprayed Thermal Barrier Coatings.” Journal of the European Ceramic Society. Elsevier. 34 2687-94

13. D. Mattox. 2003. The Foundations of Vacuum Coating Technology. Springer, 164 p

14. Z. Lu, U. Paik, Y. Jung, et. al. 2013. “Thermal Fatigue Behavior of Air-Plasma Sprayed Thermal Barrier Coating with Bond Coat Species in Cyclic Thermal Exposure.” Mat’ls. [6] 3387-403

15. H. Bernstein. 1999. “High-Temperature Coatings for Industrial Gas Turbine Users.” Proceedings of the 28th Turbomachinery Symposium. Texas A&M University. 179-88.

16. “Progress in Thermal Barrier Coatings.” 2009. The American Ceramics Society. 583 p.

17. D. Hass, A. Slifka, H. Wadley. 2001. “Low Thermal Conductivity Vapor Deposited Zirconia Microstructures.” Acta Materialia. [49] 973-83.

18. M. Dorfman, C. Dambra, S. Metco. 2001. “Thermal Barrier Coatings: Improving Thermal Protection.” Sulzer Technical Review. [4] 10-13.

19. R. Dutton, R. Wheeler, K. Ravichandran, et. al. 2000. “Effect of Heat Treatment on the Thermal Conductivity of Plasma-Sprayed Thermal Barrier Coatings.” Journal of Thermal Spray Technology. 9 (2) 204-9

20. B. Gleeson. 2006. “Thermal Barrier Coatings for Aeroengine Applications.” Journal of Propulsion and Power. 22 [2]

21. A. Lepeshkin. 2012. “Investigations of Thermal Barrier Coatings for Turbine Parts.” Intech Open

22. H. Purwaningsih, L. Noerochim, R. Fajarain, et. al. 2010. “Phase Transformation on Interface between NiCoCrAlY Bond Coat and Substrate and Study of Thermal Barrier Coating as High-Temperature Material.” The Journal for Technology and Science. 21 (4).

23. T. Pollock, S. Tin. 2006. “Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure, and Properties.” Journal of Propulsion and Power. 22 [2]

24. M. Bäker. 2014. “Influence of Material Models on the Stress State in Thermal Barrier Coating Simulations.” Surface & Coatings Technology. Elsevier. 240 301-10

25. C. Powell, J. Oxley, J. Blocher Jr. 1966. Vapor Deposition. John Wiley & Sons, Inc. New York, London, and Sydney. 725 p.

26. “Thermal Barrier Coating System.” 1977

27. M. Jarratt. “PVD Coatings: Welcome to the Future.”

28. Mattox, D.M. 2010. Handbook of Physical Vapor Deposition (PVD) Processing, 2nd Ed. Elsevier. Oxford. 746 p.

29. D. Smith. 1995. Thin-Film Deposition. McGraw-Hill. 616 p.

30. “M12: Thin Films.” 2006

31. A. Doolittle. “Physical Vapor Deposition: Evaporation and Sputtering.” Georgia Tech

32. D. Ribeiro. “Thermal barrier coatings (TBCs) – State of the Art.” The University of the Minho

33. Harsha, K.S.S. 2006. “Principles of Physical Vapor Deposition of Thin Films.” Elsevier. Great Britain. p.400

34. J. Davis. “ASM Specialty Handbook: Tool Materials.” The Materials Information Society. 505 p.

35. K. Niesson, M. Gindrat. 2011. “Plasma Spray-PVD: A New Thermal Spray Process to Deposit Out of the Vapor Phase.” Journal of Thermal Spray Technology. 20 (4) 736-43

36. Yahya. 2010. “Turbines Compressors and Fans.” Mc-Graw Hill. 944 p.

37. “Technology Transfer Network – Air Toxics Web Site.” 2013. EPA.

38. N. Tamilselvam, Y. Marcian. “Improving the Thermal Sustainability of Modern Gas Turbine Blade using Newly Proposed Coolant Passage Shapes.” International Journal of Systems, Algorithms, and Applications

6. Acronyms

adatom = adsorbed atom

adparticle = adsorbed particle

APS = air-plasma spraying

Arc – PVD = cathodic arc physical vapor deposition

C = number of components

CMAS = Calcium-Magnesium-Alumino-Silicate

dof = degrees of freedom

DySZ = dysprosium-stabilized zirconia

EM = electromagnetic

F = number of dof

FGM = functionally graded materials

GLAD = glancing angle deposition

HPCVD = hybrid-physical chemical vapor deposition

HVOF = high velocity oxy-fuel

IE EBPVD = ion enhanced electron beam physical vapor deposition

ionitriding = ion-nitriding

LPCVD = low-pressure chemical vapor deposition

LPPS = low-pressure plasma spray

MBE = molecular beam epitaxy

MFP = mean free path

MLS = monolayer per second

MOCVD = metalorganic chemical vapor deposition

P = number of phases

PACVD = plasma-assisted chemical vapor deposition

PECVD = plasma-enhanced chemical vapor deposition

PS = plasma spray

PS-PVD = plasma spray-physical vapor deposition

P-T = pressure and temperature

PVD = physical vapor deposition

P-YSZ = partially yttria-stabilized zirconia

Rion = ionic radius

Ra = roughness

RF = radio-frequency

TBC = thermal barrier coating

TCF = thermo-cyclic fatigue

TE = thermal expansion

TGO = thermally grown oxide

TMF = thermal-mechanical fatigue

tribo = tribological

VPE = vapor phase epitaxy

wt = weight

Y3+ = yttrium

Y2O3 = yttria

YSZ = yttria-stabilized zirconia

Z4+ = zirconium

ZrO2 = zirconia

7. Figures

Figure 1: A Cross Section of TBC System Showing the Thicknesses (mm) of the Substrate, Bond Boat, TGO, and Top Coat. S. Akwaboa, P. Mensah, E. Beyazouglu, et. al. 2012. “Thermal Modeling and Analysis of a Thermal Barrier Coating Structure Using Non-Fourier Heat Conduction.” Journal of Heat Transfer. 134 (11) 111301
Figure 2. Coating Structure For a Certain Argon Pressure and Substrate Temperature. John A. Thornton “Structure-Zone Models Of Thin Films”, Proc. SPIE 0821, Modeling of Optical Thin Films, 95. 1988
Figure 3. TBC Structure using APS versus EBPVD Techniques: (2 Representations)
1. (Top) Source
2. (Bottom). A. Karaoglanli, K. Ogawa, A. Türk, et. al. 2013. “Thermal Schock and Cycling Behavior of Thermal Barrier Coatings (TBCs) Used in Gas Turbines.” InTech Open
Figure 4: Resistively Heated Thermal Vaporization Source Configurations. Mattox, D.M. 2010. Handbook of Physical Vapor Deposition (PVD) Processing, 2nd Ed. Elsevier. Oxford. 746 p.
Figure 5: Focused E-Beam Vaporization.
Left: Linear (Pierce) Beam.
Right: Bent Beam Source. Mattox, D.M. 2010. Handbook of Physical Vapor Deposition (PVD) Processing, 2nd Ed. Elsevier. Oxford. 746 p.
Figure 6: Adatom and Substrate Surface “Surfaces, Growth and Strain Relaxation.” 2010. Warwick. Department of Physics.
Figure 7: Vapor Pressures of Various Oxides as a Function of Inverse Temperatures. U. Schulz, B. Saruhan, K. Fritscher, C. Leyens. 2004.”Review on Advanced EB-PVD Ceramic Topcoats for TBC Applications.” Int. Journal of Applied Ceramic Technology. 1 [4] 302-1