Thermal Spray Fundamentals

Materials have been developed for exceptional functional performance in specific applications, such as a number of specialty steels or super alloys; however, the increasing demands for combined functional requirements such as high temperatures and corrosive atmospheres in addition to abrasive wear, and the difficulty in machining some of the specialty alloys to the final form, as well as the costs of having an entire part made of such a material has led to the ever increasing demand for coatings. First the difference between thin and thick films is discussed to introduce the thermal spray concept with a short description of the different processes used. A brief history of these processes introduces the different thermal spray applications and shows that these processes are complementary and not competitive.

In order to be competitive in the market, it is important to be able to produce surfaces that wear only a little, are more resistant to tarnishing and corrosion, and retain their electrical, optical, or thermal properties over a long period. It is also interesting to have technologies to simplify product ranges or maintenance requirements. Surface treatments and coatings have a prominent role to play in this respect. This chapter is an overview of the different surface treatments; among them are set thermal spray processes, whose applications are briefly described. Then the different processes are summarily discussed together with the different ways to supply powders, wires, rods, cords, and also liquids (suspensions or solutions). Then the interactions high-energy gas particles or liquids are briefly described, before the coating formation is presented.

Except for Cold Spray, thermal spray processes are based either on combustion or thermal plasmas. This chapter recalls the basic phenomena involved allowing understanding how the high temperature jets are generated and what are their properties. First the bases of combustion are presented with flames, detonation, and explosions: their stability limits, their temperatures and velocities. Then bases of thermal plasmas are discussed with a short presentation of how are calculated their compositions, specific masses, enthalpies, viscosities, and electrical and thermal conductivities with finally the results for the main gases used in thermal spraying. At last the basic concepts in modeling are presented: conservation equations (continuity, momentum, and energy), electromagnetic field equations, and at last laminar and turbulent flows. Bases presented in this chapter are then used in the different chapters related to the various spray processes.

In the thermal processing of powders for spraying of protective coatings or free-standing bodies, the proper control of particle or droplet (suspension or solution) trajectories and their temperature and momentum histories in the gas flow represents one of the critical aspects on which the overall success of the operation strongly depends. In fact, a slight deviation from near optimal conditions can easily lead to poor results due to either the lack of melting of particles, or insufficient impact velocities, or the modification of their composition due to particle evaporation or unwanted chemical reactions. The first parts of the chapter deal with a single particle trajectory and heating, including heat propagation, with mass transfers and chemical reactions for liquid or gaseous phases. Then ensemble of particles is considered with the injection problems and possible loading effects. The last section is devoted to the interaction between a high-energy gas and a liquid, which fragmentation and then vaporization cools the hot gases. Moreover liquid fragmentation and vaporization is drastically modified by arc root fluctuations for plasmas produced by direct current torches

The growth of combustion processes along the years was driven both by scientific and technical developments providing disruptive innovations and by the market requirements )(e.g., the development of HVOF spraying by Browning in 1983 was pushed forward by the need to produce WC-Co cermet coatings with superior properties). The different processes are presented with for each one the principle, the type of materials used (powder, liquid wire cord or rod), then materials sprayed, sprayed particle temperatures, velocities and oxidation, types of coatings obtained and finally the process modeling. Successively are presented flame spraying, High Velocity Oxy-Fuel (HVOF), and High Velocity Air-Fuel spraying and modified HVOF processes and finally Detonation gun (D-gun).

Cold spray is a kinetic spray process, utilizing supersonic jets of compressed gas to accelerate near-room temperature powder particles to ultrahigh velocities. The solid ductile particles, travelling at velocities between 300 and 1500 m/s, plastically deform on impact with the substrate and consolidate to create a coating. First are presented the different types of cold sprays: conventional ones, using nitrogen or helium, with particle velocities in the 500–900 m/s range, the kinetic spray process working in air, the pulsed gas dynamic spraying process, using a shock generator, the low pressure cold spray (p < 1 MPa), and finally the vacuum cold spray. Conventional cold spray, representing most processes, is developed: models, deposition of the first layer, critical and erosion velocities, coating formation with deposition parameters, and the influence of substrates and deposited materials. The sprayed materials and applications are then discussed. At last is presented summarily low pressure cold spray process.

In plasma spraying, an electric arc generates plasma within a plasma torch. The arc is struck between a cathode (usually a rod or button-type design) and a cylindrical anode nozzle, and the plasma gas is injected at the base of the cathode, heated by the arc, and exits the nozzle as a high temperature, high velocity jet (see Fig. 7.1). Figure 2.​1 presents the details of coating generation by plasma spraying.

Radio Frequency (r.f.) Induction plasma spraying, or as more commonly known as Vacuum Induction Plasma Spraying (VIPS), has attracted increasing attention over the past three decades. While it is not a technology that is posed to replace any of the other thermal spray processes, it is commercially used in niche applications such as to perform cladding in the fiber optics industry, or X-ray target manufacturing, where the high purity and density of the deposit obtained are of critical importance. VIPS is characterized by the electrodeless nature of the discharge, which allows for high purity and greater flexibility with regard to the chemistry of the plasma gas, and the ease of axial injection of the powder into the center of the plasma jet. In this context, one of the fastest growing applications of induction plasma spraying is in the area of powder treatment for powder densification, purification, and/or spheroidization. The r.f. induction plasma torch is first presented with the basic concepts, its design and resulting temperature, fluid flow, and concentration fields. Then the discharge and the plasma–particle interactions modeling are discussed and typical results presented. At last vacuum induction plasma spraying (VIPS) with the torch operating conditions, the reactive, the suspension, and the supersonic spraying are described.

Wire arc spraying is the oldest thermal spray process, having been patented in the USA in 1915 [1]. However, only since the 1960s, there has been progress in expanding the applications based on improvements of the understanding of the essentials of the process using systematic studies with high time resolution [2]. This expansion has accelerated over the past twenty years with several significant improvements in the equipment and processes [3, 4].

The development of the plasma transferred arc coating process was directed towards reducing the cost of corrosion and wear resistant parts. Regular steel parts with an appropriate PTA coating can exhibit superior corrosion and wear behavior even compared to specialty alloys. The process is significantly different from the other coating processes as the substrate is part of the electrical circuit that delivers the power for the coating process. The substrate in most of the cases serves as the anode of the arc transferred from the torch, and only sometimes as the cathode. Thus it must consist of an electrically conducting material. First equipment and operating parameters are described with the coating materials used and the corresponding applications. Then the process characterization is presented with the temperature distributions in the arc and arc voltages, heat flux to substrate and process modeling. The different process modifications and adaptations are described, especially with the influence of the pilot arc, the nitriding of coating, the modulation of deposition parameters, the PTA combination with tape casting and the PTA deposition with a negative work piece polarity. Examples of applications are at last presented, especially against wear and abrasive wear, against wear and corrosion, refurbishing worn parts and finally free standing shape fabrication

The structure and properties of coatings obtained by thermal or cold gas spraying depend strongly upon different parameters among which the quality of the powder, wire, rod or cord used plays a key role. Powder quality can affect coating performance through different effects. First are presented the different powder manufacturing techniques: atomization, fusing and crushing, milling and sintering, ball-, attrition-, cryo-milling, mechanical alloying and milling, spray-drying, spheroidization, cladding, sol-gel and solutions, self-propagating high-temperature synthesis, cermets. The influence of powder morphologies on coating properties is discussed through examples. Then are presented powder classification methods and characterization: sampling, XRD, elements distribution, composition and purity, particle shape, size distribution, flow ability and surface area. The different types of powder feeders and the hazards related to particles are finally presented. Wires, cored wires, rods and cords are also presented and the chapter finishes with problems linked to polymer powders according to their physical, chemical and mechanical properties widely different from those of metals, alloys ceramics and cermets

Thermal spraying begins with proper surface preparation, which is absolutely essential. Steps must be undertaken correctly in order for the coating to perform the design expectation because coating adhesion quality is directly related to the cleanliness, the roughness and sometimes the proper machining for optimal coating performance. The coating material and the nature of the substrate are the major factors in determining what kind of surface preparation is necessary to achieve a resistant bonding. It is also important to keep in mind that the coating must never end abruptly at the part extremity. The different surface preparations comprise: machining, cleaning by various means, and masking preventing deposit formation on areas where it is not wanted. The surface roughening, characterized by different measurements, can be achieved by three main means: grit blasting, high pressure water roughening and laser treatment, which are described in details in this chapter. This description comprised the equipment, the main parameters and their action on surface roughness, with, for grit blasting the influence of the grit used (material and size).

Thermal spray processes are relatively mature technologies widely used in industry. They mostly involve the introduction of either, particles (in the tens of micrometers size range) into the high-energy gas stream where they are, except for cold spray, accelerated and heated over or below their meting point, or wires, cored wires, rods, cords, which have their tip melted and atomized. The thermal and kinetic energy content of the ductile particles or droplets impinging on the substrate can widely vary with the process used. Moreover, for metals or alloys or composites sprayed in air, high process temperatures tend to increase the in-flight particle oxidation, increasing the oxide content embedded into the coating. At last the coating is formed by ductile particles or droplets flattening to form splats, which layering forms the coating. Thus the coating formation depends also strongly on substrate surface composition, microstructure, roughness and pollution. This chapter starts with the physical and chemical description of substrates with the drastic influence of the oxide layer and the mean to get rid of adsorbates and condensates. Then the impact of a single ductile particle (metal, alloy, cermet, ceramic, polymer) or a droplet is considered first on a smooth surface and then on a rough one. The way parameters characterizing flattening (Reynolds, Weber, Sommerfeld numbers), must be calculated to fit with experiments is discussed, as well as the impact direction. Coating formation is discussed from splats layering with the formation of beads and passes and the importance and means, such as robots and cooling devices, to control the coating temperature during its formation. As pointed out in previous chapters the influence of powder or wire…manufacturing process on coating properties is discussed. The different residual stresses formed during spraying are presented with their influence on coating adhesion-cohesion. Chapters ends-up with coatings finishing and the different post-treatments.

According to the definition of the US National Nanotechnology Initiative, USNNI, “nanometer-sized” materials, or more commonly referred to as “nanosized” or “nanostructured” materials, are materials with a particle diameter or an internal structure with at least one dimension smaller than 100 nm. From the pioneering works of McPherson in 1973, who was among the first to identify nano-size features in thermally sprayed alumina coatings, to recent and most advanced work aimed at manufacturing nanostructured coatings from nanosize feedstock particles, the thermal spray community has been actively involved in this area for more than 30 years. Nano-sized materials are at the frontier of materials science due to their remarkable, and in some cases novel, properties [1, 2]. This results in particular from the surface-to-bulk ratio, and interface density between features, that are much higher in nanostructured materials compared to those of coarser particle (e.g., micro-sized) materials. The physical mechanisms involved leading to those remarkable properties lies, at such dimensions, between atom quantum effects (e.g., phonon scattering, surface plasmon, etc.) and bulk behaviors (e.g., cohesion, etc.).

Coatings, as most industrial products, must be tested at the Research and Development stage, in production environment, but it should be kept in mind that coating properties depend strongly on both the spray conditions and powder used and both must be regularly tested. Tests at the research and development level use techniques more or less sophisticated such as metallography and image analysis. In production tests deal with the control of quality (adhesion–cohesion, mechanical properties, thermal properties, wear resistance, corrosion resistance…) and are more targeted towards the service conditions, without neglecting some simple tests from simple visual observation of the coated part to some specific characteristics required by the coating or component specifications. The aim of this chapter is not to describe in detail all the characterization and testing methods that could be used for thermal-spray coatings but to give the reader information about the most used techniques and which information can be drawn from them. It starts with the specificity of coating characterization methods and presents the nondestructive methods. They are successively described: the metallography and image analysis, materials characterization, void content and network architecture, adhesion–cohesion, mechanical properties, and testing of wear resistance and corrosion.

Whatever maybe the spray process, the user expects that coating thickness and weight tolerances are respected, the reproducibility of coating microstructure and reliability of service properties are at least good and, if possible, excellent. Good quality control of coatings, before (powders… substrate preparation), during, and after the spray process presents many benefits: reduced rework, predictable performance and life linked to coatings, and high reproducibility with narrow variability. This chapter defines first what are coatings repeatability, reliability, and reproducibility and place the process control relatively to the other different errors observed in a production unit. A brief history of the influence of the spray process monitoring on coating quality is presented, with the spray process parameters that should and could be controlled. The high-energy jet characterizations (temperatures, velocities, turbulences, electrodes erosion), developed in laboratories, are presented. Sensors able to work in the harsh environment of spray both and developed since the nineties are then described. They include enthalpy probes, sensors for hot and cold in-flight particles, to measure their trajectories distribution, their steady or transient temperatures, velocities, and diameters either as ensemble (large measurement volume) or local measurements. Measurements of the coating under formation (hot gases flux, temperature, stress development, thickness) are then considered. The use of robots, artificial neural networks (ANN), and fuzzy logic (FL) to monitor and further control coating generation is then discussed. Finally, this chapter presents a few measurements that are used in laboratories and are very important for a better understanding of coatings generation such as particle vaporization, particle flattening and splat formation, and high-energy jet–liquid interaction.

This section is devoted to important items that are not directly linked to the different coatings formation and applications, but without which coatings production could not exist. Thermal spray induces potential risks due to gases and powders used, vapors and dusts produced, noise, radiation, high temperatures of the different processes, electrical equipment, use of robots,… Such risks must be prevented not only by individual protections but also mainly by using especially designed enclosures (spray booth), with adapted exhaust and ventilation systems. The controlled atmosphere spraying equipments are also described. Moreover, in most cases coatings are not usable as-sprayed because the dimensions cannot be as precise as requested by specifications; they might be too porous, not enough impervious to gas liquids,…. Thus, they have to be machined, grinded, polished,…, to achieve the right dimensions, densified for a better wear resistance, sealed to improve their corrosion resistance. The fusing processes of self-fluxing alloys and the heat-treating or annealing of coated parts are thus described. At last examples of different posttreatments illustrate processes presented.

At its early stages of development, thermal spray technology was mostly used for the repair, rebuilding, retrofitting, and for surface protection against corrosion, erosion and wear. The wider acceptance of the technology for industrial-scale production has started in the late eighties and early nineties, with applications limited to high added-value components in the aeronautic and nuclear industry. Over the two past decades, a wide range of industrial-scale surface modification processes became available. The choice of a specific coating and/or thermal spray process, for a given service condition, depends, however, on the expectation of the user and the cost that could be tolerated for the application. This chapter presents the advantages and limitations of the different spray processes. Then the different coating applications are described, with coatings resistant to wear, corrosion and oxidation, providing thermal protection, clearance control, good bonding, electrical and electronic properties, free standing spray-formed parts, medical applications, replacement of hard chromium… potential applications. These applications are then presented according to the industrial users: aerospace, land-based turbines, automotive, electrical and electronic industries, corrosion applications for land-based and marine applications, medical engineering, ceramic and glass manufacturing, printing, pulp and paper, metal processing, petroleum and chemical industries, electrical utilities, textile and plastic, polymers, reclamation… The development of thermal sprayed coatings in the different countries is then discussed, the last part of the chapter being about the economic analysis of the different spray processes.

These are presently accepted for applications ranging from tribological and wear resistant applications including lubricity and low-friction surfaces, to resistance to corrosion and/or oxidation, thermal protection, freestanding components, electrical and optical components, electromagnetic shielding, electrical insulation, abradable seals, biomedical applications, superconducting oxides, components with coefficient of thermal expansion tailored to service conditions, magnetic coatings, solid oxide fuel cells, replacement of hard chromium, as well as ornamental applications.. This affected, in turn, the selection of the material to be applied for the coating, and the spray process to be used. The coating design process is often complicated, by the fact that in practice components are not always devoted to a single requirement such as wear or corrosion or electrical insulation or thermal insulation. In most cases, coatings must resist to different combined needs: for example, wear is often linked to corrosion.