The science and engineering of thermal spray coatings (original) (raw)

THERMAL SPRAY COATING: A STUDY

Surface coating is a reliable and cost effective process, used for the production of tools, materials, and machine components that requires desired surface properties like corrosion, erosion and wear resistance. The main purpose of applying the coating may be decorative, functional, or both. To improve surface properties such as adhesion, erosion, corrosion and wear resistance, functional coating may be used. Whereas, decorative coating such as artists paints are applied to make the product/material more attractive. Now days, number of coating methods are applied to achieve the desired functional or decorative properties. Thermal spray coating process is one of the effective and attractive methods to preserve/protect the new machine components from wear, hot corrosion, and erosion. This paper briefly explains various spray processes, their basic principles, advantages, and applications. In addition to this features, characteristics of coating methods and various corrosion prevention methods are briefly described.

Design of experiments in thermal spraying: A review

Surface and Coatings Technology, 2008

The designs of experiments (DOE) methodology useful for thermal spraying and associated processes of post-spray treatment are thoroughly reviewed. The designs Hadamard (Plackett-Burman), two-level full and fractional factorial and also the response surfaces methodology are briefly described. The designs enable to obtain a polynomial regression equation which expresses the influence of process parameters on the response. The methods of determining of the significant coefficients of the regression equation (factors) are discussed. Examples of the application of different designs to determine the response equations with the responses related to microstructure, mechanical, electrical and other properties of coatings deposited using different thermal spray and post-spray processes are presented and discussed.

Suspension and solution thermal spray coatings

Surface and Coatings Technology, 2009

The emerging methods of coating deposition by suspension and solution thermal spraying are described. The liquid suspensions of fine powders and liquid precursors are injected into flames and/or jets generated in the torches. The formulation and stability of suspensions as well as the methods of fine powders synthesis are briefly described. Typical solutions, being often the liquid organo-metallics are also briefly described. An important problem of injection of liquids into jets and flames is then presented. Two principal modes of injection, used at present, are outlined, i.e.: (i) atomization; and, (ii) injection of a continuous jet. Subsequently, the phenomena occurring in flames and plasma jets are discussed and the major differences to these occurring during conventional spraying are stressed up. The build up of coatings starting from the impact of fine particles on the substrate is described and typical microstructures of suspension and solution sprayed coatings are shown. Some properties of the sprayed coatings, including mechanical, electrical, chemical, and thermophysical ones are collected and presented. Finally, the emerging applications of coatings are shown and the possible future applications are discussed.

Thermal Sprayed Coatings Used Against Corrosion and Corrosive Wear

Advanced Plasma Spray Applications, 2012

Coatings have historically been developed to provide protection against corrosion and erosion that is to protect the material from chemical and physical interaction with its environment. Corrosion and wear problems are still of great relevance in a wide range of industrial applications and products as they result in the degradation and eventual failure of components and systems both in the processing and manufacturing industries and in the service life of many components. Various technologies can be used to deposit the appropriate surface protection that can resist under specific conditions. They are usually distinguished by coating thickness: deposition of thin films (below 10 to 20 µm according to authors) and deposition of thick films. The latter, mostly produced at atmospheric pressure have a thickness over 30 µm, up to several millimeters and are used when the functional performance and life of component depend on the protective layer thickness. Both coating technology can also be divided into two distinct categories: "wet" and " dry " coating methods, the crucial difference being the medium in which the deposited material is processed. The former group mainly involves electroplating, electroless plating and hot-dip galvanizing while the second includes, among others methods, vapor deposition, thermal spray techniques, brazing, or weld overlays. This chapter deals with coatings deposited by thermal spraying. It is defined by Hermanek (2001) as follows , "Thermal spraying comprises a group of coating processes in which finely divided metallic or non-metallic materials are deposited in a molten or semi-molten condition to form a coating". The processes comprise: direct current (d.c.) arcs or radio frequency (r.f.) discharges-generated plasmas, plasma transferred arcs (PTA), wire arcs, flames, high velocity oxy-fuel flames (HVOF), high velocity air-fuel flames (HVAF), detonation guns (D-gun). Another spray technology has emerged recently ; it is called cold gas-dynamic spray technology, or Cold Spray (CS). It is not really a thermal spray technology as the high energy gas flow is produced by a compressed relatively cold gas (T < 800°C) expanding in a nozzle and will not be included in this presentation. Most processes are used at atmospheric pressure in air, except r.f. plasma spraying, necessarily operated in soft vacuum. Also, d.c. plasma spraying can be carried out in inert atmosphere or vacuum and Cold Spray is generally performed at atmospheric pressure but in a controlled atmosphere chamber to collect and recycle the spray gas (nitrogen or helium) because of the huge gas flow rates used (up to 5 m 3 .min-1). In the following only processes www.intechopen.com Advanced Plasma Spray Applications 4 operated in air at atmospheric pressure will be considered, except when the coating material is very expensive, such as platinum that must be sprayed in a chamber to recover the overspray. The coating material may be in the form of powder, ceramic rod, wire or molten materials. The central part of the system is a torch converting the supplied energy (chemical energy for combustion or electrical energy for plasma-and arc-based processes), into a stream of hot gases. The coating material is heated, eventually melted, and accelerated by this hightemperature, high-velocity gas stream towards a substrate. It impacts on the substrate in the form of a stream of droplets that are generated by the melting of powders or of the tips of wires or rods in the high-energy gas stream. The droplets flatten or deform on the substrate and generate lamellae called "splats". The piling up of multiple layered splats forms the coating.

Recent Advancements in Thermal Spray Coatings

Different methods and techniques are employed to protect the materials from degradation. Within most industry segments, significant financial losses may be incurred due to accelerated wear of various components. In order to minimize the effects of mechanical wear and extend product life, thermal spray coating solutions introduced into production and is further developing them to meet even more demanding wear applications. Applying coatings using thermal spray is an established industrial method for resurfacing metal parts. The process is characterized by simultaneously melting and transporting sprayed materials, usually metal or ceramics, onto parts. In this paper some studies on Thermal sprayed wear resistant coatings have been reviewed.

Oxidation of coatings in thermal spraying

Materials Letters, 1998

Mechanisms of the coating oxidation during thermal spraying are discussed. They are associated with the in-flight oxidation of the powder particles and oxidation of the solidifying splat on the substrate surface. Influence of oxidation on the splat morphology, porosity and the splat-substrate adhesion is outlined. Theoretical results agree well with the observed tendencies of the development of oxidation and its effect on the coating formation.

Thermal Sprayed Coatings Adherence - Influencing Parameters

2008

Thermal spraying represents the process of obtaining new special multilayer structure materials, with good mechanical and chemical characteristics. These materials are often used in solving real important problems, like repairing worn parts working under severe wearing conditions or, ensuring efficient corrosion protection of parts used in sea, as platform, bridges, or obtaining high refractory surfaces. Most of the times, once obtained, these coatings need additional machining and, it is of interest to study how, and, if, any of the machining parameters do influence one of their very important characteristic, meaning, adherence to the basic substrate. The paper presents a study on the adherence of thermal sprayed coatings, obtained from some Romanian thermal sprayed materials and submitted to exterior cylindrical turning.