Tarp.docx

TECHNICAL ANSWERS FOR REAL WORLD PROBLEMS (MEE3999) ASSESSMENT-4 SUBMITTED TO- PROF. SITARAM DASH NAME - HIMANSHU KHANDELWAL REGISTRATION NO. - 16BME0897 SLOT- TF1 (MB-110)

1. Discuss thermal spray processes used in Industry to meet Surface Engineering needs. Discuss the role of turbulent momentum transfer in coating formation? Ans) In the simplest terms possible, thermal spray coating involves heating a material in powder or wire form to a molten or semi-molten state. The material is propelled using a stream of gas or compressed air to deposit it, creating a surface structure on a given substrate. The coating material may consist of a single element, but is often an alloy or composite with unique physical properties that are only achievable through the thermal spray process.

Generally speaking, thermal sprayed coatings are a highly cost effective way to add superior performance qualities to a given substrate. The variations on this technical theme are virtually limitless. Coatings can be metallic, ceramic, plastic, or any combination desired to meet a broad range of physical criteria.

Thermal sprayed coatings can be the most cost-effective means of protecting substrate surfaces from wear or corrosion. Other primary uses of thermal sprayed coatings include dimensional restoration, maintaining precise clearances, and modifying thermal and electrical properties.

The coating materials can be applied using several different processes. Thermal coating methods utilize fuel combustion, plasma spray and electric arc delivery systems. Coatings can be applied under standard atmospheric conditions or in specialized, highly controlled atmospheric environments – even under water! Coatings can be applied manually or with the automated precision of software-driven robotics. Many industries use our coatings to extend product life, increase performance and reduce production and maintenance costs. Thermal spray processes are classified through:  Feed stock material  Thermal Energy source  Kinetic Energy produce

Different types of Thermal spray processes used for surface engineering needs areI.

Flame Powder Spray

Flame Powder Spray has been serving industries for decades with economical, reliable surfaces. Relatively low initial investment cost makes this an ideal process for entry level thermal spray coating.

Theory of Operation: The Powder Flame Spray process is similar to the Wire Flame Spray process except that is has the advantage of using powdered materials as the coating feedstock. This offers a much wider range of coating material options than the Wire Flame Spray process. In addition, the use of powder allows for a greater degree of freedom for spray gun manipulation. The spray material in powdered form is fed continually into a fuel gas oxygen flame where it is typically melted by the heat of combustion. A powder feed carrier gas transports the powder particles into the combustion flame, and the mixed gases transport the material towards the prepared work piece surface. Typical choices for fuel gases are acetylene or hydrogen.

II.

Flame Wire Spray

Wire Flame Spray is the earliest thermal spray processes to be developed, with its usefulness enduring even today. A low initial investment cost makes this an ideal process for entry level thermal spray coating. Easily transported for on-site applications makes wire flame spray a favorite for infrastructure corrosion applications, such as bridgework.

Theory of Operation: The spray material in wire form is fed continually into a fuel gas-oxygen flame where it is melted by the heat of that combustion. Compressed air surrounds the flame and atomizes the molten tip of the wire. This accelerates the spray of molten particles towards the prepared work piece surface. Typical choices for fuel gases are acetylene, propane, hydrogen or MAPP. III.

Arc wire spray

Simple, Fast and Economical. The simplicity of this process, which uses electricity and compressed air, and the speed at which is applies coatings without fuel gas are the hallmarks of electric wire arc spraying.

Theory of Operation: Arc Wire Spray uses two metallic wires as the coating feedstock. The two wires are electrically charged with opposing polarity and are fed into the arc gun at matched, controlled speeds. When the wires are brought together at the nozzle of the (spray gun), the opposing charges on the wires

create enough heat to continuously melt the tips of the wires. Compressed air is used to atomize the now molten material and accelerate it onto the workpiece surface to form the coating. In arc wire spray, the weight of coating that can be deposited per unit of time is a function of the electrical power (amperage) of the system and the density and melting point of the wire. IV.

Air (atmospheric) Plasma Spray

Versatile. Only one word is necessary to describe what plasma spray apply the widest variety of coating materials, by far, of any thermal spray process for an unlimited number of applications. Plasma spray performs where other processes cannot and is the best choice for facilities where many different surfaces must be applied. A superb choice for high-quality ceramic coatings. Plasma spray produces high-performance coatings that deliver workhorse durability and reliability.

Theory of Operation: Plasma Spray is perhaps the most flexible of all of the thermal spray processes as it can develop sufficient energy to melt any material. Since it uses powder as the coating feedstock, the number of coating materials that can be used in the plasma spray process is almost unlimited. The plasma gun incorporates a cathode (electrode) and an anode (nozzle) separated by a small gap forming a chamber between the two. DC power is applied to the cathode and arcs across to the anode. At the same time, gases are passed through the chamber. The powerful arc is sufficient to strip the gases of their electrons and the state of matter known as plasma is formed. As the unstable plasma recombines back to the gaseous state thermal energy is released.

V.

High Velocity Oxy-Fuel (HVOF) Spray

Tenacious. Durable. Tough. HVOF produces premium quality hard, dense coatings exhibiting high adhesion to the substrate and excellent wear resistance for extended component longevity and profitability.

Theory of Operation: The HVOF process efficiently uses high kinetic energy and controlled thermal output to produce dense, very low porosity coatings that exhibit very high bond strengths (some exceeding 12,000 PSI or 83 MPa), low oxides and extremely fine as-sprayed finishes. The coatings have low residual internal stresses and can be sprayed to thicknesses not normally associated with dense, thermal sprayed coatings. The process uses an oxygen-fuel mixture. Depending on user requirements, propylene, propane, hydrogen, natural gas or liquid fuel may be used as the fuel. VI.

Cold Spray

Cold spray can be considered as a new “HVOF” technology were the kinetic energy is increased while the thermal energy is lowered. With Cold Spray is possible to spray virtually oxide free coatings. The coating material particles are being accelerated in a heated gas stream (600°C/1112°F), up to a particle velocity of >1000 m/s. The extreme high particle velocity in combination with the low particle temperature results in very dense and oxide free coatings. Applications are found in the automotive industry, corrosion protection and electronics industry.

Theory of Operation: The cold spray process efficiently uses high kinetic gas jet with limited thermal output to produce dense, very low porosity coatings that exhibit very high bonding and lowest oxides levels. Cold gas spraying is a coating deposition method using a supersonic gas jet to accelerate a ductile spherical

powder to velocities up to 500–1000 m/s. During impact with the substrate, particles undergo plastic deformation and adhere to the surface.  The role of turbulent momentum transfer in coating formation isIn contrast to plasma spraying, the role of momentum and mass transfer is• Fast cameras showing the transient behavior of plasma jets that act like a piston flow. This behavior is mainly controlled by the arc root fluctuations at the anode wall. They depend on the thickness of the boundary layer between the plasma column and the anode-nozzle internal diameter. Even if this piston flow is not taken into account in the k–ε models used to model plasma jets they give a good trend of the jet behavior with the torch working parameters. The agreement is especially good if the inlet velocity and temperature profiles are measured.

• Devices to measure in flight the key microscopic parameters: particle trajectory, velocity, temperature and diameter. It is worth underlining that such devices use more and more imaging techniques. Such set-ups have been simplified to determine the same parameters in the harsh environment of spray booths. They are now the basis of on-line monitoring of the spray process and works are in progress to transform them in on-line control with a feedback to the input macroscopic parameters (arc current, gas flow rates and composition, etc.). 2. Briefly describe thermal barrier coating (TBC) architecture in low bypass turbofan engines? Suggest a technique to deposit TBC overlay. Ans) Thermal barrier coatingThermal barrier coatings (TBC) are advanced materials systems usually applied to metallic surfaces, such as on gas turbine or aero-engine parts, operating at elevated temperatures, as a form of exhaust heat management. These 100 μm to 2 mm coatings serve to insulate components from large and prolonged heat loads by utilizing thermally insulating materials which can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface.[1] In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue. In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications. Due to increasing demand for higher engine operation (efficiency increases at higher temperatures), better durability/lifetime, and

thinner coatings to reduce parasitic weight for rotating/moving components, there is significant motivation to develop new and advanced TBCs. StructureAn effective TBC needs to meet certain requirements to perform well in aggressive thermomechanical environments.[2] To deal with thermal expansion stresses during heating and cooling, adequate porosity is needed, as well as appropriate matching of thermal expansion coefficients with the metal surface that the TBC is coating. Phase stability is required to prevent significant volume changes (which occur during phase changes), which would cause the coating to crack or spall. In airbreathing engines, oxidation resistance is necessary, as well as decent mechanical properties for rotating/moving parts or parts in contact. Therefore, general requirements for an effective TBC can be summarize as needing: 1) a high melting point. 2) no phase transformation between room temperature and operating temperature. 3) low thermal conductivity. 4) chemical inertness. 5) similar thermal expansion match with the metallic substrate. 6) good adherence to the substrate. 7) low sintering rate for a porous microstructure. These requirements severely limit the number of materials that can be used, with ceramic materials usually being able to satisfy the required properties.

Thermal barrier coatings typically consist of four layers: the metal substrate, metallic bond coat, thermally-grown oxide (TGO), and ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ) which is desirable for having very low conductivity while remaining stable at nominal operating temperatures typically seen in applications. This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface. However, above 1200 °C, YSZ suffers from unfavorable phase transformations, going from t'-tetragonal to tetragonal to cubic to monoclinic. Such phase transformations lead to crack formation within the top coating. Recent advancements in finding an alternative for YSZ ceramic topcoat identified many novel ceramics (rare earth zirconates) having superior performance at temperatures above 1200 °C, however with inferior fracture toughness compared to that of YSZ. In addition, such zirconates may have a high concentration of oxygen ion vacancies, which may facilitate oxygen transport and exacerbate the formation of the TGO. With a large enough TGO, spalling of the coating may occur, which is a catastrophic mode of failure for TBCs. The use of such coatings would require addition coatings that are more oxidation resistant, such as alumina or mullite. The bond-coat is an oxidation-resistant metallic layer which is deposited directly on top of the metal substrate. It is typically 75-150 μm thick and made of a NiCrAlY or NiCoCrAlY alloy, though other bond coats made of Ni and Pt aluminides also exist. The primary purpose of the bond-coat is to protect the metal substrate from oxidation and corrosion, particularly from oxygen and corrosive elements that pass through the porous ceramic top-coat. At peak operating conditions found in gas-turbine engines with temperatures in excess of 700 °C, oxidation of the bond-coat leads to the formation of a thermally-grown oxide (TGO) layer. Formation of the TGO layer is inevitable for many high-temperature applications, so thermal barrier coatings are often designed so that the TGO layer grows slowly and uniformly. Such a TGO will have a structure that has a low diffusivity for oxygen, so that further growth is controlled by diffusion of metal from the bond-coat rather than the diffusion of oxygen from the top-coat. The TBC can also be locally modified at the interface between the bondcoat and the thermally grown oxide so that it acts as a thermographic phosphor, which allows for remote temperature measurement.  Some techniques to deposit TBC overlay by giving example of material type-

YSZ YSZ is the most widely studied and used TBC because it provides excellent performance in applications such as diesel engines and gas turbines.

Additionally, it was one of the few refractory oxides that could be deposited as thick films using the then-known technology of plasma spraying.As for properties, it has low thermal conductivity, high thermal expansion coefficient, and low thermal shock resistance. However, it has a fairly low operating limit of 1200C due to phase instability, and can corrode due to its oxygen transparency.

Mullite Mullite is a compound of alumina and silica, with the formula 3Al2O3-2SiO2. It has a low density, along with good mechanical properties, high thermal stability, low thermal conductivity, and is corrosion and oxidation resistant.

However, it suffers from crystallization and volume contraction above 800C, which leads to cracking and delamination. Therefore, this material is suitable as a zirconia alternative for applications such as diesel engines, where surface temperatures are relatively low and temperature variations across the coating may be large. Alumina Only α-phase Al2O3 is stable among aluminum oxides. With a high hardness and chemical inertness, but high thermal conductivity and low thermal expansion coefficient, alumina is often used as an addition to an existing TBC coating. By incorporating alumina in YSZ TBC, oxidation and corrosion resistance can be improved, as well as hardness and bond strength without significant change in the elastic modulus or toughness.

One challenge with alumina is applying the coating through plasma spraying, which tends to create a variety of unstable phases, such as γ-alumina. When these phases eventually transform into the stable α-phase through thermal cycling, a significant volume change of ~15% (γ to α) follows, which can lead to microcrack formation in the coating.

CeO2 + YSZ CeO2 (Ceria) has a higher thermal expansion coefficient and lower thermal conductivity than YSZ. Adding ceria into a YSZ coating can significantly improve the TBC performance, especially in thermal shock resistance. This is most likely due to less bond coat stress due to better insulation and a better net thermal expansion coefficient. Some negative effects of the addition of ceria include the decrease of hardness and accelerated rate of sintering of the coating (less porous). Rare-earth zirconates La2Zr2O7, also referred to as LZ, is an example of a rare-earth zirconate that shows potential for use as a TBC. This material is phase stable up to its melting point and can largely tolerate vacancies on any of its sublattices.

Along with the ability for site-substitution with other elements, this means that thermal properties could potentially be tailored. Although it also has very low thermal conductivity compared to YSZ, it also has a low thermal expansion coefficient and low toughness. Rare earth oxides The mixture of rare earth oxides is readily available, cheap, and may have promise as effective TBCs. The coatings of rare earth oxides (ex: La2O3, Nb2O5, Pr2O3, CeO2 as main phases) have lower thermal conductivity and higher thermal expansion coefficients when compared to YSZ.

The main challenge to overcome is the polymorphic nature of most rare earth oxides at elevated temperatures, as phase instability tends to negatively impact thermal shock resistance. Metal-Glass Composites A powder mixture of metal and normal glass can be plasma-sprayed in vacuum, with a suitable composition resulting in a TBC comparable to YSZ. Additionally, metal-glass composites have

superior bond-coat adherence, higher thermal expansion coefficients, and no open porosity, which prevents oxidation of the bond-coat.

In order to improve the hot corrosion resistance of conventional YSZ TBC system, the overlay of Al2O3 coating was deposited on the TBC by EB-PVD techniques. Hot corrosion tests were carried out on the TBC with and without Al2O3 coating in molten salts mixtures (Na2SO4 + 5%V2O5) at 950o C for 10h. The microstructures of TBC and overlay before and after exposure were examined by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD).

3. How is particle propulsion achieved in Detonation Gun Sprayed coatings? Give an example of application of D-Gun sprayed coatings? ANS) Detonation Gun Spraying D-gun spray process is a thermal spray coating process, which gives an extremely good adhesive strength, low porosity and coating surface with compressive residual stresses . A precisely measured quantity of the combustion mixture consisting of oxygen and acetylene is fed through a tubular barrel closed at one end. In order to prevent the possible back firing a blanket of nitrogen gas is allowed to cover the gas inlets. Simultaneously, a predetermined quantity of the coating powder is fed into the combustion chamber.

The gas mixture inside the chamber is ignited by a simple spark plug. The combustion of the gas mixture generates high pressure shock waves (detonation wave), which then propagate through the

gas stream. Depending upon the ratio of the combustion gases, the temperature of the hot gas stream can go up to 4000 dig C and the velocity of the shock wave can reach 3500m/sec. The hot gases generated in the detonation chamber travel down the barrel at a high velocity and in the process heat the particles to a plasticizing stage (only skin melting of particle) and also accelerate the particles to a velocity of 1200m/sec.

These particles then come out of the barrel and impact the component held by the manipulator to form a coating. The high kinetic energy of the hot powder particles on impact with the substrate result in a buildup of a very dense and strong coating. The coating thickness developed on the work piece per shot depends on the ratio of combustion gases, powder particle size, carrier gas flow rate, frequency and distance between the barrel end and the substrate. An example of application of D-Gun sprayed coatingsDepending on the required coating thickness and the type of coating material the detonation spraying cycle can be repeated at the rate of 1-10 shots per second. The chamber is finally flushed with nitrogen again to remove all the remaining “hot” powder particles from the chamber as these can otherwise detonate the explosive mixture in an irregular fashion and render the whole process uncontrollable. With this, one detonation cycle is completed above procedure is repeated at a particular frequency until the required thickness of coating is deposited.

Detonation Gun process. The chamber is finally flushed with nitrogen again to remove all the remaining “hot” powder particles from the chamber as these can otherwise detonate the explosive mixture in an irregular fashion and render the whole process uncontrollable. With this, one detonation cycle is completed above procedure is repeated at a particular frequency until the required thickness of coating is deposited. 4. Comment upon microstructure of coatings formed by air plasma spray (APS). Discuss the mechanism of coating formation. ANS) Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma). Thermal spraying can provide thick coatings approx. thickness range is 20 microns to several mm, over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition.

In plasma spraying process, the material to be deposited (feedstock) — typically as a powder, sometimes as a liquid,[2] suspension [3] or wire — is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is on the order of 10,000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc. Deposit properties The deposits consist of a multitude of pancake-like 'splats' called lamellae, formed by flattening of the liquid droplets. As the feedstock powders typically have sizes from micrometers to above 100 micrometers, the lamellae have thickness in the micrometer range and lateral dimension from

several to hundreds of micrometers. Between these lamellae, there are small voids, such as pores, cracks and regions of incomplete bonding. As a result of this unique structure, the deposits can have properties significantly different from bulk materials. These are generally mechanical properties, such as lower strength and modulus, higher strain tolerance, and lower thermal and electrical conductivity. Also, due to the rapid solidification, metastable phases can be present in the deposits. During the spray process, feed powder particles are injected into a plasma flame, melted and accelerated towards a substrate where they impact to form a coating. Such processes are used to achieve surface modification of substrates. The high particle temperatures and speeds achieved result in significant droplet deformation on impact at a surface, producing thin layers or lamellae, often called “splats,” that conform and adhere to the substrate surface. Solidified droplets build up rapidly, particle by particle, as a continuous stream of droplets impact to form continuous rapidly solidified layers. Individual splats are generally thin (~1 to 20 µm), and each droplet cools at very high rates (>106 K/s for metals) to form uniform, very fine-grained, polycrystalline coatings or deposits.Sprayed deposits usually contain some level of porosity, typically between 0 and ~10%, some unmelted or partially melted particles, fully melted and deformed “splats,” metastable phases, and oxidation from entrained air.

Feedstock is usually in powdered form with a distribution of particle sizes. When these powdered materials are fed into the plume, portions of the powder distribution take preferred paths according

to their inertia. As a result, some particles may be completely unmelted and can create porosity or become trapped as “unmelts” in the coating. The microstructure includes partially melted particles and dark oxide inclusions that are characteristic of many metallic coatings sprayed in air. Such coatings exhibit characteristic lamellar microstructures, with the long axis of the impacted splats oriented parallel to the substrate surface, together with a distribution of similarly oriented oxides. Coating oxide content varies with the process—wire arc, plasma, or HVOF. Oxides may increase coating hardness and wear resistance and may provide lubricity. Conversely, excessive and continuous oxide networks can lead to cohesive failure of a coating and contribute to excessive wear debris. Oxides can also reduce corrosion resistance. Coating Characterization Depending on the application of the coating, different characteristics are important but there are some characteristics which are the same for all applications: coating thickness, porosity, structure, presence of unmelted particles and oxide inclusions, microhardness and bond strength. Coating characteristics are very dependent on spray parameters. According to some researchers [12], there are more than 50 macroscopic parameters that influence the quality of the coating and the production of coating is still based on trial and error approach. The resultant coating is a composite of the alloy with oxides formed during deposition. Presence of the FeO (wustite) and Fe3O4 (magnetite) oxides, as solid lubricants, improves the tribological properties of the coating. The formation of Fe2O3 (hematite) oxide should be avoided, because it is abrasive. The residual porosity of the plasma coating should help to reduce the coefficient of friction through a micro-cavity lubrication system, where micro-cavity serves as a lubricant reservoir allowing improved lubrication process.

Porosity may be beneficial in tribological applications through retention of lubricating oil films. Porosity also is beneficial in coatings on biomedical implants. Lamellar oxide layers can also lead to lower wear and friction due to the lubricity of some oxides. The porosity of thermal spray coatings is typically 5% by volume. The retention of some unmelted and/or resolidified particles can lead to lower deposit cohesive strengths, especially in the case of “as-sprayed” materials with no postdeposition heat treatment or fusion.

Other key features of thermal spray deposits are their generally very fine grain structures and columnar orientation (Fig. 1b). Thermal-sprayed metals, for example, have reported grain sizes of less than 1 µm prior to postdeposition heat treatment. Grain structure across an individual splat normally ranges from 10 to 50 µm, with typical grain diameters of 0.25 to 0.5 µm, owing to the high cooling rates achieved (~106 K/s). Tensile Strength The tensile strengths of as-sprayed deposits can range from 10 to 60% of those of cast or wrought materials, depending on the spray process used. Spray conditions leading to higher oxide levels and lower deposit densities result in the lowest strengths. Controlled-atmosphere spraying leads to ~60% strength, but requires postdeposition heat treatment to achieve near 100% values. Low assprayed strengths are related somewhat to limited intersplat diffusion and limited grain recrystallization during the rapid solidification characteristic of thermal spray processes. The primary factor limiting adhesion and cohesion is residual stress resulting from rapid solidification of the splats. Accumulated residual stress also limits thickness buildup. The conventional plasma spray process is commonly referred to as air or atmospheric plasma spray (APS). Plasma temperatures in the powder heating region range from about 6000 to 15,000 °C (11,000 to 27,000 °F), significantly above the melting point of any known material. To generate the plasma, an inert gas—typically argon or an argon-hydrogen mixture—is superheated by a dc arc. Powder feedstock is introduced via an inert carrier gas and is accelerated toward the workpiece by the plasma jet. Provisions for cooling or regulating the spray rate may be required to maintain substrate temperatures in the 95 to 205 °C (200 to 400 °F) range.

Commercial plasma spray guns operate in the range of 20 to 200 kW. Accordingly, spray rates greatly depend on gun design, plasma gases, powder injection schemes, and materials properties, particularly particle characteristics such as size, distribution, melting point, morphology, and apparent density. Vacuum Plasma. Vacuum plasma spraying (VPS), also commonly referred to as low-pressure plasma spraying (LPPS, a registered trademark of Sulzer Metco), uses modified plasma spray torches in a chamber at pressures in the range of 10 to 50 kPa (0.1 to 0.5 atm). At low pressures the plasma becomes larger in diameter and length, and, through the use of convergent/divergent nozzles, has a higher gas speed. The absence of oxygen and the abil-ity to

operate with higher substrate temperatures produce denser, more adherent coatings with much lower oxide contents.

5. Both cold spray and HVOF utilize supersonic acceleration of gases. How is this acceleration achieved? ANS) The process of Cold Spraying is “a kinetic spray process utilizing supersonic jets of a compressed gas to accelerate, at or near-room temperature, powder particles to ultra-high velocities (up to 1,500 m/s). The unmolten particles traveling at speeds between 500 and 1,500 m/s plastically deform and consolidate on impact with their substrate to create a coating”. The basis of the cold spray process is the gas-dynamic acceleration of particles to supersonic velocities and hence high kinetic energies. This is achieved using convergent–divergent Laval nozzles. The upstream pressure is between 2 and 2.5 MPa for typical nozzle throat internal diameters in the range of 2–3 mm. Gases used are N2, He, or their mixtures at very high flow rates (up to 5 m3 /min). For stable conditions, typically the mass flow rate of the gas must be ten times that of the entrained powder. For a powder flow rate of 6 kg/h, this means a volumetric flow rate of 336 m3 /h for helium and 52.3 m3 /h for nitrogen. Gases introduced (nitrogen or helium) are preheated up to 700–800 C to avoid their liquefaction under expansion and increase their velocity. With the highest gas flow rate this means that the heating device must be capable of heating 90 m3 /min to temperatures up to 700–800 C. As particles are injected upstream of the nozzle throat, the powder feeder has to be at a slightly higher pressure compared to the upstream chamber pressure. Especially when spraying with He, the spray is performed within an enclosure in order to recycle the gas. Particles adhere to the substrate only if their impact velocity is above a critical value, depending on the sprayed material varying between about 500 and 900 m/s.

The spray pattern covers an area of roughly 20–60 mm2 , and spray rates are about 3–6 kg/h. Feed stock particle sizes are typically between 1 and 50 μm and deposition efficiencies reach easily 70– 90%. Only ductile metals or alloys are sprayed (Zn, Ag, Cu, Al, Ti, Nb, Mo, Ni–Cr, Cu–Al, Ni alloys, MCrAlYs, and polymers), owing to the impact-fusion coating build-up. Blends of ductile materials (>50 vol. %) with brittle metals or ceramics are also used. It is also important to emphasize that the substrate is not really heated by the gas exiting the gun (up to 200 C at the maximum). Current and expected applications for cold spray coatings are electronic and electrical coatings (Cu, Fe–NdFeB),

and coatings for the aircraft (superalloy) and automotive industries for localized corrosion protection (Al, Zn), rapid tooling repair, etc In HVOF the combustion is achieved in a pressurized chamber, water-cooled or not, followed by a Laval-type nozzle . The combustion at pressures higher than atmospheric pressure (between 0.2 and 1 MPa) increases slightly the combustion temperature. For example, stoichiometric combustion of methane with oxygen at atmospheric pressure results in a temperature of 3,030 K, while at 2 MPa it is 3,733 K.

But the main advantage is the high gas velocity (up to 2,000 m/s) achieved due to the hot gas expansion through the Laval nozzle. Such velocities are supersonic as shown by shock diamonds observed downstream of the nozzle Two types of guns exist characterized by the chamber pressure. The low-pressure HVOF uses a gun at pressures between 0.24 and 0.6 MPa with heat inputs below 600–700 MJ, while the high-pressure guns are operated in the pressure range of 0.62–0.82 MPa, generally fueled with kerosene burning either with oxygen or air with heat inputs over 1 GJ. These guns are often termed “hyper velocity guns.” The “low” pressure guns are fed with hydrogen, propylene, methane, propane, heptane, and a few trademark gases together with oxygen, or also kerosene with air.

The first HVOF gun was introduced in the early eighties by Browning and Witfield. Particles are injected either axially upstream of the nozzle (a pressurized powder feeder is required), as shown in

or radially downstream of the nozzle (with a conventional powder feeder). Compared to flame spraying, HVOF guns are fed with high fuel gas flow rates of 60–120 slm, and oxygen flow rates ranging from 280 to 600 slm, which corresponds to power levels of a few hundreds of kW. With kerosene the liquid is fed between 20 and 30 slm with oxygen flows in the order of 1 m3 /min, or air up to 5 m3 /min. In flame spraying, the gas-specific mass is about one-tenth of that of the cold gas. The coupling of having highly plasticized particles impacting with high inertia allows achieving very dense coatings. The flame mixing with surrounding air can be delayed and the particles can be kept accelerating by extending the gun nozzle using a barrel . For the spraying of wires, special HVOF guns have been developed. The principle is the same as that of wire flame spraying. The flame (propane–oxygen) is positioned at the nozzle face and the molten tip of the wire is atomized both by the pressurized flame and the airflow. Under such conditions, the gas temperature reaches values of 3,100 K, and the velocities reach values of up to 1,600 m/s, and particle velocities are much higher than with wire flame spraying. The initial success of HVOF was in spraying dense and wear-resistant WC–Co coatings, almost as good as those sprayed by the D-gun, which at that time was only available as a service. The HVOF coatings (metals, alloys, and cermets) are dense, adherent with low-oxide content, compared to flame spraying, and they compare favorably with high-energy plasma-sprayed coatings. Typically with WC + Co (WC + NiCr or CrCo are also used) and dependent on the applications hardness values between 1,100 and 1,400 150 HV5N are obtained.