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.
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. Cold spray is a kinetic spray process, utilizing supersonic jets of compressed gas to accelerate near-room temperature powder particles to ultrahigh velocities. 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.
Thermal Spray Fundamentals from Powder to Part — Heberlein, Joachim | Humanitas
Figure 2. Radio Frequency r. 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.
Thermal Spray Fundamentals. From powder to part
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. The r. 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 . However, only since the s, 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 . 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.
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