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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 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
Thermal barrier coatings (TBCs) are thick films applied to metal components to provide thermal protection against hot combustion gases.1 They are most often used in gas turbines, used to power and move ships and airplanes, or to generate electricity, such as land-based gas turbines (generators).2,3 TBCs are typically applied to high-temperature components such as combustor liners, blades, and vanes by spraying a heated or molten powder or vaporizing it.2,4 They also have applications in brake systems, internal combustion engines (i.e. combustion chamber, pistons, and valves), rocket engines, and nuclear reactors.2,5 Before TBCs were introduced, 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 to last tens of thousands of service hours.2 In the early 1990s, the focus shifted to reducing the thermal conductivity of ceramic material to impede thermal transport.2
Developing a coating to meet conflicting goals requires tailoring its chemical composition and microstructure.2,5,6 TBCs must withstand thermal, corrosive, and mechanical wear in harsh environments7 including erosion from particulate debris or liquid droplets like fuel or water1,2,4 Prolonged exposure to high temperatures can cause TBCs to fail due to thermal fatigue and thermally grown oxide (TGO) layer growth.5,6,8 To combat the strain caused by repeated heating and cooling and TGO layer growth, TBCs must have thermo-cyclic fatigue (TCF) resistance5,6,8 and vertical cracks can be intentionally introduced in the ceramic layer to reduce thermal stress mismatch.5 The residual stress in the TBC system caused by the difference in the thermal expansion (TE) of the airfoil and TBC system layers3 can be reduced by controlling the porosity architecture and thickness of the TGO layer.2,3,9 TBCs deposited on high-pressure turbine blades typically have stresses of 2.4 – 2.8 GPa as-prepared and 2.3 – 3.1 GPa after service.5 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%8 (15%10) and fully penetrating pores help with bond retention.3 Plasma spraying (PS) can create voids and incomplete overlap of splats11, resulting in porosity that can lower thermal conductivity.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
TBCs have the remarkable ability to lower component temperatures by as much as 190 °C8 (150 °C9), enabling engines to operate at higher temperatures and generate increased output while ensuring cooler parts.2,6 This advancement not only enhances engine performance, efficiency, and fuel economy but also boosts part durability and lifespan.2,6 Additionally, TBCs provide crucial protection to the substrate against the adverse effects of oxidation and high combustion temperatures.2,11
TBCs feature a three-layer structure, including a metallic bond coat, a thermally grown oxide (TGO) layer, and a ceramic topcoat, as shown in Figure 1.11 To optimize the performance of these coatings, various improvements can be implemented. These include reducing the thermal conductivity, controlling porosity by introducing vertical cracks, and increasing the thickness of the ceramic topcoat.5 For example, decreasing the density of the coatings can result in a 15% reduction in thermal conductivity.2 Additionally, TBCs deposited at high chamber pressure (1.5 X 10-3 Torr) in an oxygen and inert gas environment have demonstrated reduced thermal conductivity and increased TCF (thermomechanical fatigue) lifetime.2 The typical thickness of TBC layers ranges from 0.1 to 2 mm (300 – 500 μm12), but TBCs produced by air-plasma spraying (APS) may fail when the thickness exceeds 1.5 mm. The Thornton Structure Zone model, illustrated in Figure 2, illustrates how changes in temperature and pressure can result in significant variations in microstructure.13
The substrate is protected from corrosion by a metallic layer, which provides resistance to oxidation in high-temperature environments where air is present.8 The ceramic layer, due to its porous structure, offers little or no corrosion resistance. 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
When the bond coat oxidizes, a TGO layer forms.17 This occurs either before or during the ceramic layer deposition.17 Initially, the TGO layer consists of dense alumina (α and γ phase Al2O3 from α-Al2O3 powder7,19).11 The TGO layer is 1 nm thick, grows to 3-9 nm, and will fail at 10 nm.5
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 > 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 adding Y2O3 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 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
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 > 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
The methods used to apply TBCs are physical vapor deposition (PVD) and plasma (thermal) spraying (PS).2 Still under development are 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 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 and 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 it 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
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 ability 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.
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
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
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
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 and the source. If the unreacted gas is not incorporated into the film, it provides kinetic energy and enhances chemical reactions to produce a denser coating.28
In reactive sputtering, the reactive gas replaces the inert gas such as O, carbides, or nitrides.28 A magnet is usually used to localize the plasma around the target area, which is intended to emit material, rather than somewhere else on the cathode, and prolongs the residence time – the mean time it remains in the process chamber before being pumped away.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 used to prevent charge buildup.30
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 (anodic arc PVD is uncommon – the source material is the anode instead of the cathode28).30 This leads to a cathode spot, and this highly energetic emitting area of a few μm in size produces a high-velocity jet (102 m/s) of vaporized cathode material when exposed to a high current density of 106 – 1012 A/cm2.28,33 The vapor (> 85% ionized, 10-100 eV)33 is directed 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).28 They should be filtered out so that a high-quality thin film can be achieved.28
CVD was used in incandescent lamps in 1880.13,25 It involves the decomposition or reduction of a chemical vapor precursor, at high temperatures, 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 hybrid-physical CVD process (HPCVD) illustrates the overlap that can occur. CVD is performed at much higher temperatures than PVD (> 800 °C vs 200 – 500 °C), and at higher temperatures than its reactive gases.34 The higher substrate temperature helps desorb the reacting gas and results in more diffusion bonding, which gives the metallic layer a better adhesion.28 Chemical reactions in CVD lead to corrosive volatile byproducts and unused reactive gases that must be removed by gas flow from the chamber.28
The source material is subject to a certain pressure and temperature by applying a vacuum and 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
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 that it is 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 number of parameters: temperature, pressure, and components that can be varied independently without changing the number of phases.25 The phase rule applies to all deposition processes, as long as the system is at equilibrium.25
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 from evaporating 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
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 a surface and/or gaseous reaction.25
The degree of vacuum should be > 10-4 Torr28, so that the mean free path (MFP) – 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
Controlling the angle-of-incidence of deposition leads to unique morphologies.28 This angled, off-normal, deposition results in column growth in the direction of the flux.28 Very oblique deposition angles (GLAD) exacerbate the column growth, as the valleys do not get any flux, resulting in a more porous structure.28 Rotating the substrate during GLAD yields corkscrew column growth.28 This “Zig-Zag” or “Herringbone” structure has an increased path length for thermal transport and a 40% reduction in thermal conductivity.2
Deposition is when a vapor becomes a solid without first becoming a liquid. Deposition rates in PVD vary greatly, 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
Atoms that land on a substrate and do not react immediately will have some mobility.28 These mobile atoms on the substrate (crystal plane) are known as adatoms.28 Adatom is a contraction of adsorbed atoms. Many arriving atoms are in a thermodynamically unfavorable state, and depending on the energy of the atom, the interaction between the atom and the surface (chemical bonding), and the temperature of the surface determine the amount of mobility (diffusion).28 It is unlikely that adparticles diffuse into stable lattice positions if the substrate temperature is low and there is low ion-bombardment (figure 6).28 Ballistic deposition is when the atom is immobilized where it lands, aptly named, as the projectile motion of the atom is the only effect on deposition.29
Condensation describes the process of cooling the vapor atom to a liquid (or compressing it to its saturation limit). Not every atom condenses, some atoms are re-evaporated or reflected, especially if the energy of the vaporized atoms or the substrate temperature is too high.28 The sticking coefficient is the ratio of condensing atoms to impinging atoms.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.28 The adatom loses energy through diffusion, bonding with other atoms, and collisions with deposited and incoming atoms.28 Most of the energy is given up in the form of the heat of vaporization or the heat of sublimation.28
In plasma spraying, rapid solidification occurs when liquid particles (some vapor) strike the substrate surface, flatten, and cool (106 K/sec) to form splats (fine, non-columnar, equiaxed grains).28 The splats develop a lamellar structure parallel to the interface through directional solidification.11 In PVD, columnar growth describes solidification, 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 is condensation from both molten feedstocks, which form splats, and vapor transported in a supersonic plasma.35 PS-PVD has superior coating properties than PS or PVD alone.35
Sintering is a heat treatment that causes particles to diffuse together. It fuses the ceramic layer without reaching its melting point and liquefying. The sintering resistance during service is important for the ceramic topcoat. During the spray process, a desirable structure is created that gives strain tolerance and low conductivity due to the high volume of pores.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 Furthermore, 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 is higher at 1.8 – 2.0 W/mK2 (1.5 – 1.9 W/mK17) than APS at 0.6 – 1.2 W/mK2 (0.8-1.0 W/mK17).2,17 The trade-off for low 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 The sintering of the microcracks at the splat interfaces is the main cause of increased thermal conductivity.19
Improved thermal surface protection is important for improving engine technology, which continually strives to run engines hotter.4 Advances in processing, which led to single crystal blades and directional solidification, yielded a temperature increase of 120 °C for turbine blades.4 Cooling the hot structure (blades, disc, and vanes) is the most effective method to achieve longer part lifetimes, but a cooler blade leads to lower engine efficiency.4 The use of cooling provides a temperature reduction of 370 °C4 (air cooling- 200-300 °C36) on the blade surface while using only 1-3%36 of the main airflow.4,36 Air or liquid cooling schemes 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 10 million gallons of fuel annually in 250 aircraft.8 TBCs allow both the durability and high efficiency of the blades.4 The advantage of liquid cooling compared to air cooling is its higher specific heat capacity, but leaks can occur.36 Convection cooling is a common method that routes compressor air inside the hollow airfoil and out of the many pinholes.36
Historically, the emphasis was on preventing creep.4 Today, the main concern is to prevent oxidation and hot corrosion (sulfidation) of the metallic bond coat and thermal-mechanical fatigue (TMF).4 Elements that provide corrosion resistance are Al, Ta, Zn, Ni, and Cr (and Ca, which is being phased out as the dust is carcinogenic37).28 As engine operating temperatures increased, the high Cr content (20%4, 25-35%8), which prevented hot corrosion, was phased out with a higher Al content (4-5%4, 6%8) to ensure better oxidation resistance.4 Resistance to chemical reactions with gas impurities such as S and V, or deposits such as calcium-magnesium aluminosilicate (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 with constitutive oxides other than yttria (figure 7) have been put into service or tested with an underlying P-YSZ ceramic layer.2 This is partly due to the chemical incompatibility of the new ceramic material with the TGO, especially for pyrochlores (cerium metals).2 Furthermore, the new ceramic altered the formation of the TGO layer, resulting in reduced lifetimes.2 Improvements in TBC systems are possible by using multi-layer (micro-layering) or phase mixture ceramics.2,4,28
One method of layering with a thickness of 1 μm is to switch on and off a bias voltage.2,4,28 This produces layers of different densities and yields an impressive 37-45%2 reduction in thermal conductivity.2,4 Functionally graded materials (FGMs) offer increased part durability by preventing spallation.2,4,28 One method of blending TBC layers is the vaporization of metal and metal oxides simultaneously.2,4,28 This creates a gradient structure between the bond coat and the ceramic layer, which helps minimize TE mismatch.2,4,28 This is already achieved by vaporizing alloys from a single source, which produces a graded composition since the evaporant is selectively vaporized.2,4,28 FGM may require the use of alternative, multiple, and/or both coarse and fine powdered rare-earth metal dopants.2,4,28 In addition, 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 due to its nanopores and nanograins.2,4 Finally, when composites are used in the next generation of turbine blades, the substrate will have different diffusion rates and surface chemistry.4
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