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The new technology in thermal plasma spray coating process
Coating using plasma spraying process is achieved when a surface is subjected to heated or molten materials. These are normally accelerated using plasma mechanisms. Defined as cloud of ionised gas with subatomic particles, plasma is the product of a gas passing through a high intensity electric field where large amount of energy is released by ultraviolet radiation. Plasma process normally produce coating ranging from a few micrometres to several millimetres in thickness using metals, ceramics and sometimes a mixture of both. In plasma spraying, the powder is normally fed into a high temperature plasma flame where they are accelerated at high velocity after being heated to a substance where it forms a coating after cooling (Bergmann and Vicenzi 8). The figure1 below shows the plasma spray process and figure 2 shows the coating thickness.
Figure 1Plasma spray process (Bergmann and Vicenzi 9).
Figure 2 coating thickness (Bergmann and Vicenzi 9)
These solids normally have different heat capacities. This is normally estimated using Einstein equation where the temperature of solid is estimated. Elements with strong bonds are termed as hard solids and normally have high temperatures while some elements with weaker bonds normally have lower temperature (Wunderlich 111). The figure below shows temperatures of elements that can be used to calculate heat capacities.
Figure 3Temperatures several of elements that can be used to calculate heat capacities (Wunderlich 115).
Different techniques, which are distinguished by the surrounding atmosphere, are normally used. . Air plasma spraying (APS) and high power plasma spraying (HPPS) rely on air. The ones on the special atmospheres are inert gas plasma spray for vacuum there is lower pressure plasma and controlled atmosphere plasma spraying as shown below.
Figure 4 Plasma surrounding atmosphere (Heimann 18)
New technologies intend to achieve Thermal Barrier Coatings (TBC). The method to achieve this is by use of plasma enhances chemical vapour deposition. Enhancement normally uses microwave frequency plasma together with metal organic precursors at lower temperature and inorganic compounds such chlorides at high temperatures. Currently the technologies in plasma spraying are based on depositing the coating through complete or partial melting of feedstock powder, which are sprinkled on the surface of a substance. The PS-PVD technology uses physical vapour deposition process. Here the electron-beam vaporised materials forms a continuous thin and compact coating after the material condenses from vapour. This was a designed coating structures may be created (Goral, Kotowski and Sieniawski 690).
Conventional plasma vacuum plasma spray normally operates at pressures above 3Kpa but new development on the technology has made changes to the technology , for instance the physical vapour deposition (PVD) working at lower pressure which ranges from 5pa to 0.1 Pa(Von Niessen and Gindrat 736). The figure below show the plasma jet at different pressures a 95 kPa b, 5kPa and c kPa.
Figure 5 Plasma jet at different pressures a 95 kPa b, 5kPa and c kPa (Von Niessen and Gindrat 737)
Thermal plasma sprays that operate under a defined and controlled atmosphere normally operate under reduced pressure. For vacuum thermal, spray the working pressure ranges from 3kPa up to 20kPa. With this pressure deposits with thickness of about2mm to 20mm is achieved. Having pressured reduced to a level lower than atmospheric pressure results in plasma plume enlargement from 50mm to 500 mm and 10mm to 40mm in length and diameter respectively. When the plume is enlarged, the result is big spray spot. This makes particle velocity and temperature to be homogenously distributed over the plume cross section and this allows for homogenous thickness coating especially in parts having complex geometries (Von Niessen and Gindrat 736). PS-PVD is based on low pressure plasma techniques. LPPS is conducted at 50-200 mbar and attains a deposition of thickness 20 μm-1 mm (Goral, Kotowski and Sieniawski 690).
Figure 6 Properties of LPPS thin film and PS-PVD processes (Goral, Kotowski and Sieniawski 691)
There are different coating made from different parameters that can be achieved by plasma spray. Either from flat type layer to a porous coating type and the a columnar structure achieved by combination of low powder feed rate, argon, secondary plasma gases and a large spray distance. The figures below show the two types of coating.
Figure 7 Flat porous coating (Von Niessen and Gindrat 738)
Figure 8 columnar structure (Von Niessen and Gindrat 738)
Bergmann, Carlos Pe´rez, and Juliane Vicenzi. Protection Against Erosive Wear Using Thermal Sprayed Cermet. 1st ed. Berlin: Springer, 2011. Print.
Goral, Marek, Slawomir Kotowski, and Jan Sieniawski. ‘The Technology Of Plasma Spray Physical Vapour Deposition’. High Temperature Materials and Processes 32.1 (2013): 33—39. Print.
Heimann, R. B. Plasma-Spray Coating. 1st ed. Weinheim: VCH, 1996. Print.
Von Niessen, Konstantin, and Malko Gindrat. ‘Plasma Spray-PVD: A New Thermal Spray Process To Deposit Out Of The Vapor Phase’. Journal of thermal spray technology 20.4 (2011): 736—743. Print.
Wunderlich, Bernhard. Thermal Analysis Of Polymeric Materials. 1st ed. New York: Springer Heidelberg, 2005. Print.
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