Thermal spray deposition

The principle of this deposition technique is the spraying on a substrate, generally metallie, of droplets of the material to be deposited, molten or in a plastic state using a high temperature source. The thickness of the depositions obtained can vary from a hundred micrometers to a few millimeters. The applications of thick depositions produced by thermal spraying are mainly thermomechanical (thermal 

WC-Co), but also electric insulation (AI2O3). Ceramic oxides are sprayed in flee air, whereas carbides and nitrides require a controlled atmosphere. 

The sprayed powders, generally obtained by fusion-milling, have sizes ranging between 22 and 45 pm. They can also be prepared by agglomeration-sintering, coating, atomization or mechanofusion. 

The substrates are sanded before spraying in order to ensure a satisfactory mechanical anchorage. 

The deposition obtained by plasma spraying consists of a microcracked stacking of splats with a porosity varying from 1 to 20%. The quality of the deposition is controlled by a large number of experimental parameters, which makes the process difficult to master. However, it is clear that the trajectory and thermal history of the particles in the plasma jet, as well as the sttrface temperature during deposition, are the critical parameters that must be controlled. 

Continuous or step-wise gradients in composition through the thickness of the layer can be achieved by use of multiple nozzles whereby the 

The plasma spray technique offers a straightforward and cost-effective means to spray deposits of metals and ceramics that are tens to hundreds of micrometers in thickness onto a variety of substrates in applications involving thermal-barrier or insulator coatings. Typical plasma-spray deposits are porous, with only 85-90% of theoretical density. 

For applications requiring higher density coatings with a strong adhesion to the substrate, low-pressure plasma spray is employed where spraying is done in an inert-gas container operating at a reduced pressure. Vacuum plasma spray is another thermal spray process which is used to improve purity of the deposited material and to reduce porosity and defect content, albeit at a higher cost than air plasma spray. 

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Thermal spray deposition from plasmas or flames Aircraft engine parts... 

S. Chandra, P. Fauchais Formation of solid splats during thermal spray deposition, J. Thermal Spray Technol., 18, 148-180 (2009). 

Thermal spray deposition is a three-stage industrial process where a material is heated, accelerated and impacted onto a surface so that the material splats and cools. The sprayed materials are deposited in thin layers, usually less than 13 xm per pass of the thermal spray device. The use of the line-of-sight additive process enables either an entire part or selected areas to be coated. Moreover the coating thickness may be varied from area to area. 

A. Edrisy, T. Perry, Y. T. Cheng and A. T. Alpas, Wear of Thermal Spray Deposited Low Carbon Steel Coatings on Aluminum Alloys , Wear 8872 (2001) 1023-1033. 

The selection of a particular deposition process depends on the material to be deposited and its availabiUty rate of deposition limitations imposed by the substrate, eg, maximum deposition temperature adhesion of deposit to substrate throwing power apparatus required cost and ecological considerations. Criteria for CVD, electro deposition, and thermal spraying are given in Table 2 (13). 

Characteristic Evaporation Ion plating Sputtering Chemical vapor deposition Electro- dep 0 sitio n Thermal spraying... 

Protecting a surface from corrosion by coating can be accomplished by a number of well-established processes which include paints, metal plating (with zinc or cadmium), diffusion, thermal spraying, and, more recently, vapor deposition processes. Of these physical vapor deposition (PVD) is used extensively in corrosion protection. Typical applications are ... 

Chemical vapor deposition competes directly with other coating processes which, in many cases, are more suitable for the application under consideration. These competing processes comprise the physical vapor deposition (PVD) processes of evaporation, sputtering, and ion plating, as well as the molten-material process of thermal spray and the liquid-phase process of solgel. A short description of each process follows. For greater detail, the listed references should be consulted. 

Thermal spray is a well-established, relatively low-cost, industrial processes which is used widely for the deposition of metals and compounds, including the refractory carbides and nitrides. Examples are coatings of tungsten carbide with a cobalt binder which are of major industrial importance. PI... 

The use of spray deposition increases the range of solvents which can be used in moving-belt LC-MS and the range of solutes that can be studied by this technique. Since less heat is required to remove the solvent, it is less likely that the solute will be inadvertently removed from the belt or undergo thermal degradation. It is not, however, unknown for particularly volatile and labile analytes to be lost when using spray deposition. 

Thermal spray, laser deposition, physical vapor deposition, and magnetron sputtering are physical processes that are used for fabrication of electrolyte thin films. Sputtering is a reliable technique for film deposition and is being used in industry... 

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