CHARACTERISTICS OF APS AND VPS PLASMA SPRAY PROCESSES

Plasma is an electrically conductive gas containing ions, electrons and neutral molecules. This state of matter is created by an electrical discharge and can be maintained at steady state by introducing alternating or direct current. The paper describes the structure of plasma, the thermodynamic characteristics and a method of creating plasma that allow the application of plasma as a source of energy for plasma spray processes. In general, all existing materials in a form of powder may be deposited by plasma coating on the surfaces of various materials. At high temperatures, powder material particles are introduced into a conductive plasma gas, melting and accelerating to the substrate to form coatings. The wide use of plasma spray coatings in all industrial areas is of particular importance, because different combinations of surface layers can significantly increase the resistance of machine parts to: wear, abrasion, erosion, cavitation, corrosion and fatigue resistance at low and elevated temperatures with increased resource and reliability of the parts in service.


Introduction
Plasma is an electrically conductive gas consisting of ions, electrons and neutral molecules.The state and structure of plasma have been studied for yearsby physicists and chemists, while engineers have used plasma in applications ranging from neon lights to thermonuclear fusion.Thermal plasmas are increasingly used for the dissociation of raw materials such as carbonates, oxides, sulfides, and various polymetallic ores (Dembovsky, 1985).Remelting alloys by plasma with high melting temperatures and plasma treatment using pure argon or helium gas have proven to be useful in metal industry.As Dembovsky pointed out, plasma is possible to be used for blending a vast range of materials at pressures between 10 2 and 10 7 Pa (Dembovsky, 1985).Spraying plasma is gaseous plasma and can be considered as equilibrium or thermal plasma.The development of the plasma spray process is a result of the attempt to increase the temperature level above that of an oxy-acetylene flame.The main reason for switching from conventional methods of deposition to plasma jet deposition is to increase the temperature level and to control the jet atmosphere.The plasma jet allows the selection of inert or non-reactive gases for the medium so that the chemical reaction of oxidation can be controlled during powder deposition.Temperatures which can be obtained with commercial plasma equipment are above melting and evaporation temperatures of developed materials.Thermal plasma or plasma spraying is a technological process that takes place at atmospheric pressure (APS) or at low pressure (VPS or LPPS).Plasma spray processes have found wide applications in all industrial areas, as described in the published works of authors (Mrdak, et al., 2013, pp.559-567), (Mrdak, 2013, pp.68-88), (Mrdak, 2013, pp.26-47), (Vencl, et al., 2009, pp.398-405), (Vencl, et al., 2011(Vencl, et al., , pp.1281(Vencl, et al., -1288)).Different combinations of surface layers significantly increase the resistance of the working parts to: wear, abrasion, erosion, cavitation, corrosion and fatigue resistance at low and elevated temperatures.An important place among the twenty-first century technologies that will be intensively developed is occupied by plasma spray processes owing to the development of nano materials that form the basis for the development of many technologies.Plasma spraying is one of the surface treatment technologies and, with other technological processes, it forms an area known as surface engineering.Today, plasma spray is used in mass production as well as in the laboratory studies of new nano materials for future technologies (Herman, 1988, pp.13-21).
The aim of this study was to describe the structure of plasma, its thermodynamic properties and the way it is created, which enables it to be used as an energy source for plasma spray processes (APS and VPS), in order to protect the surface of the base material from wear, abrasion, erosion, cavitation, corrosion and fatigue resistance at low and elevated temperatures with increased resource and reliability of the parts in service.
The structure of plasma and its thermodynamic characteristics In order to understand the plasma spraying process, first of all it is necessary to know the structure of plasma.Scientists used the name 'plasma' to describe vapor materials raised to a higher energy level.Heated gases follow the classical laws of physics and thermodynamics.However, plasma does not follow the classical laws of physics and, therefore, it is considered the fourth physical state of matter.In order to explain plasma, we must clarify the state changes occurring in atoms and molecules.Figure 1 illustrates a neutral atom He.This atom has a nucleus with two electric charges and is neutral when it is in an excited state.Electrons circle around the nucleus.Each electron has the electric charge -1.If enough energy is introduced into the atom, it will arouse the atom and at least one electron will be out of its orbit as shown in Figure 1.The amount of energy required to remove an electron from its orbit is called ionization energy.Excited atoms result in two particles.One of the particles is an ionized atom, and the other particle is a free electron.The ionized atom has the electric charge +1, and the free electron has the electric charge -1.A molecule which is a fusion of two or more neutral atoms can be considered as one particle.Figure 2 shows the different types of particles.To the left, two atoms bonded together in a molecule are shown.These molecules are gases such as N 2 and H 2 , formed of two atoms and therefore referred to as diatomic gases.To the right, there is an atom representing gases such as Ar or He that are called monatomic gases.It also represents one atom of a dissociated molecule of nitrogen or hydrogen.The labels for the ion and the electron are shown on the right-hand side of the figure.Figure 3 shows four molecules which represent a diatomic gas.If enough energy is introduced, molecules break down into atoms as indicated by the arrows.Ignoring the arrows, Figure 4 shows the separated atoms which may present a monatomic gas such as Ar or He or a dissociated diatomic gas N 2 or H 2 , as previously discussed.By further introducing the energy into atoms, at least one electron from each atom is moves out of its orbit, as shown by arrows.If all the electrons leave their atoms, complete ionization is achieved -the state of stars with a temperature close to 100 million °C.In the plasma spray process, partial ionization is obtained and the operating temperatures up to 30,000 °C.It should be noted that plasma is electroneutral as a whole, since it has the same number of positive and negative electric charges.The advantage of plasma over ordinary gases is that it has a higher temperature and better heat transfer.Figure 5 shows the dependence of the enthalpy on the plasma temperature for different gas types at the atmospheric pressure (Gajić, et al., 1996, pp.448-451).For N 2, the gas curve has a very gentle slope.In this part of the diagram, the temperature is changing rapidly with the amount of heat input.When it reaches the level of dissociation, the line moves vertically showing great energy absorption with a slight change in temperature.This is the dissociation area where the N 2 molecule breaks down into atoms.With the further heating of a now atomic gas, the curve flattens out, but soon begins to climb when the ionization zone is reached.It is important to notice that diatomic gases, as a rule, have more similar curve slopes than monatomic gases.Therefore, from the standpoint of enthalpy changes, diatomic gases are superior to monatomic gases.Plasma has high electrical conductivity which is connected to very high temperatures.The electrical conductivity of plasma is to a large extent conditioned by the degree of ionization, i.e. by electron mobility, which is about 100 times higher than that of ions.Ionization can be partial or multistage.The energy of gas ionization is from 10 to 50eV, and that of dissociation is from 4 to 10eV.
Figure 5 -Dependence of the enthalpy on the temperature of plasma gases (Gajić, et al., 1996, pp.448-451) Slika 5 -Zavisnost entalpije od tempetrature plazma gasova (Gajić, et al., 1996, pp.448For monatomic gases, there is no dissociation or recombination, which, for diatomic gases, take place at lower temperatures.Because of this, for the same power supply, the Ar and He plasma jets are shorter than the N 2 and H 2 plasma jets.Thermal conductivities of plasma jets are complex functions and they have different values at different temperatures.Table 1 shows the comparative values of the specific heat and the thermal conductivity of individual gases.The shown values of the specific heat and the thermal conductivity of gases are proven at a temperature of 20 C° and a pressure of 1013 mbar (Teaching plasma spraying, Introducing plasma spray techniques, Plasma - Technik 5610,Wohlen,Switzerland).The easiest method of forming plasma is to use gases Ar, He, H 2 , N 2 or their mixtures.Ar has the highest priority because it is easily ionized.The enthalpies of Ar and He are much lower than those of diatomic gases, as shown in Figure 5. On the other hand, the temperatures of inert gases are much higher than those of N 2 and H 2 at the same enthalpy.Plasma is different from the flame of combustion gases due to the fact that the chemical reaction of combustion gases occurs at a distance of 100 to 200 mm and has a lower temperature than plasma.Plasma is a bright ionized gas in which there is no combustion and has a high temperature and a rapid temperature drop after leaving the nozzle.

The plasma spray processes
Depending on the surrounding environment, there were developed the atmospheric plasma spray -APS and the vacuum plasma spray -VPS processes that still uses the name of the LPPS (Low preassure plasma deposition).The process selection is largely determined by the properties of deposited coatings as well as by their behavior in service as described in the works of the authors (Mrdak, 2013, pp.7-25), (Mrdak, 2013, pp.26-47), (Mrdak, 2014, pp.7-22).
The APS process is the simplest to perform and has a limited range of use due to the incorporation of substantial amounts of air into the plasma jet.The air cools and slows down the plasma, while a special problem is the oxidation of particles, which results in an increased content of oxidation products in the deposited layer (Mrdak, 2010, pp.5-16), (Mrdak, 2012, pp.182-201), (Vencl, et al., 2010, pp.591-604).Plasma spraying is done using a plasma gun from which emerge focused plasma particles of inert gas at a rate of 240 to 600 m/s and a temperature of the jet from 4,500 to 20,000 o C. The plasma jet leaving the nozzle is not homogeneous due to the large differences in the temperature and velocity of plasma particles (Vardelle, et al., 1983, p.88).The plasma core zone has a relatively constant temperature of 12500-12000 o C and extends to only 10-12 mm from the nozzle.The second zone is a transitional area where plasma temperature rapidly decreases to 3000 o C, reaching a length of about 100 mm from the nozzle.The third zone is an area where there is intense plasma mixing with the surrounding air, which leads to a drop in temperature and in velocity of plasma particles.Temperature and velocity of plasma particles, as well as the length of the plasma arc depend on the type of primary and secondary gas velocity and pressure of a gas mixture, power arc discharge and operating pressure (Smith, et al., 1988, p.25).The quality of the plasma jet is highly variable during the deposition itself, because it is not a stationary system.The length of the plasma jet changes in cycles with a frequency of 300 Hz.This is explained by the instability of the arc between the cathode and the anode (Kieschke, et al., 1991, p.25).The quality of the deposited layers depends on: the characteristics of the used powder; reaction between the plasma and the powder; the reaction between the plasma and the impact of the environment as well as on the substrate.Powders used for deposition are characterized by their chemical composition, density, melting temperature, grain size, grain size distribution, particle shape, behavior when moving through plasma, purity, etc.The process parameters must be set in such a way that the optimal effect of melting the particular powder in plasma is achieved and molten powder particles are transported with optimal speed to the substrate surface.The most important parameters that must be controlled are the plasma power, the gas flow and the rate of powder addition into plasma, which is explained in the works of the authors (Mrdak, 2014, pp.7-22), (Mrdak, 2014, pp.7-26), (Mrdak, 2014, pp.7-25).An efficient addition of the powder into the plasma jet has always been a problem solved by the means of: different constructions of the powder dispenser, variation in the diameter of powder feed channels, varying the type and pressure of the gas transporting the powder and varying the position of the powder feed channels.The powder feed channels can be placed in front of the nozzle to the position above or below at a certain angle and within the plasma gun nozzle, all of which gives a large number of possible combinations.Injected powder particles in the plasma jet partially or completely melt depending on the location of injection powder, amount of time spent in plasma, and the size and range of particles.The adjustment and choice of deposition parameters is determined by analyzing the shape of particles and the degree of fusion after passing through plasma, using the methods of metallographic analyses of the microstructure of the deposited layers, microhardness, and other analyses.The process of coat depositing consists of surface preparation prior to deposition processes and deposition processes on prepared surfaces.The preparation of the substrate surface is one of the most important factors that influence the quality of the bond between the metallic substrate and the deposited coating.Surface preparation is carried out in order to clean the oxide layer from the surface of the metallic substrate, making it reactive with the coating in order to increase the surface area between the metal substrate and the coating.Figure 7 shows the Plasmadyne company equipment, used for the plasma deposition of coatings at atmospheric pressure.It consists of: the space for supplying electricity, water and gas; noise protection cabin type TB -KA; device for manipulation, robot STAR -REIS -V with a rotary table PD10; control board 3600; sources of power supply 2 x 40 KW type PS61S; plasma gun type SG -100; powder feeder model 1251; high-frequency arc starter for the closed system of water cooling of the plasma gun with a pressure control in the installation (Mrdak, 2010, pp.5-16).In the APS process, inert gas Ar is introduced through the gas injector opening between the cathode and the anode.To initiate plasma, between the electrodes there is direct current which generates an electrical impulse causing the break-through in liquid gas.After establishing an electric arc -arc plasma, secondary gas (He, H 2 or N 2 ), which ionizes, is introduced through the gas injector opening.Due to the nature of the geometry of the electrodes as well as technical gases ( in the plasma state now) hot and partially conductive gas passes through the circular aperture of the anode.In the thus formed plasma jet, powder is introducedusing a carrier gas.Powder particles accelerate and melt rapidly, being deposited on the substrate.The energy of the plasma jet can be adjusted for each type of powder, taking into account: melting temperature, thermal conductivity, and the shape, size and distribution of particles.In addition to the powder characteristics, there are the conditions of powder injection into the plasma jet.They are the powder injection location and the angle relative to the jet direction and the powder injection rate.The flow of the carrier gas is adjusted for each type of powder based on the average particle size of the powder, its density and the density of the plasma jet, so that the powder particles are injected into the plasma jet axis with the highest temperature.The flow of the secondary gas, amperage, voltage and the distanceof the plasma jet can be adjusted for each type of powder, so that the particles which are deposited have the optimal kinetic energy and the optimal melting degree.The development of the VPS technology has led to significant improvements in the quality of coatings compared to coatings produced at atmospheric pressure.VPS is a relatively new technology in which molten powder particles from 10 to 100 µm in size accelerate to the substrate, where they become flat and solid.VPS coatings generally show a higher density than coatings deposited by the atmospheric plasma spray process, where coatings have lamellae composed of columnar grains.Particles solidify with a very fast cooling rate from ~ 10 4 to 10 8 ° C/sec.The operating vacuum is in the range from 30 to 200 mbar, which enables a deposition of coatings with a thickness of 20 to 2 mm.The pressure lower than atmospheric conditions increases a length of the plasma jet from 50 to 600 mm and a diameter from 10 to 40 mm.The velocities and temperature values of particles in the plasma jet are more uniform, which allows the production of homogeneous coatings of uniform thickness on parts with complex geometries (Gindrat, et al., 2002, pp.459-464).In the process of deposition at atmospheric pressure, the plasma jet temperature rapidly decreases with distance.At low pressure, the temperature of the plasma jet gradually decreases as a function of pressure.For the same distance from the nozzle,a drop in temperature is smaller as the pressure is lower.Figure 8 shows the effect of pressure on the temperature of the plasma jet (Nikoll, 1984).The VPS process is performed at a low pressure of Ar in very clear conditions and with the use of the transferred arc for cleaning and preheating the substrate.Ar, He, H 2 , N 2 , and gas mixtures of high purity can be used as plasma gases.Figure 9 shows the VPS system of the Plasma Technik AG company, which possesses an A -2000 console and an F4 plasma gun.The VPS system is designed to protect the aircraft parts exposed to a combination of excessive oxidation and hot corrosion (Mrdak, 2013, pp.26-47).The powder deposition is usually performed with a mixture of Ar-H 2 plasma gases at low pressure in the vacuum.In the vacuum chamber there are: a rotary table, planetary systems with 48 tools, a six-axis robot and an artificial hand.The manipulation system is designed to simultaneously rotate the tool and the operating parts around their axes.(Mrdak, 2013, pp.26-47) Slika 9 -Vakuum plazma sprej sistem (Mrdak, 2013, pp.26-47) Рис. 9 -Система вакуумного плазменного нанесения (Мрдак, 2013, стр.26-47)This complex movement allows uniform cleaning by the transferred arc and uniform powder depositing over the whole surface of the substrate.All procedures, being the parts of the VPS surface treatment process, must be carried out as quickly as possible.In order to obtain fast chamber vacuuming, a high pumping capacity is required.To achieve a chamber pressure of 1000 -0.1 bar, a time of 5 minutes is necessary.Taking workpieces in and out should be done as quickly as possible after the chamber ventilation.All inside surfaces of the vacuum chamber become protected by plasma gases during the process of VPS protection.Only neutral gas atoms bind to the chamber surfaces due to Vander -Waals forces, which means that there is a possibility to save time on chamber vacuuming after the introduction and removal of workpieces.The choice of plasma gas, the flow rate and the current arc determine the content of the plasma jet energy.The characteristics of the plasma jet inside and outside the gun are influenced by the pressure in the chamber.As the pressure decreases, the plasma jet becomes longer in proportion to the increase in gas velocity.The pressure in the chamber must be maintained constant during deposition.Deposition at a low pressure of 30 to 70 mbar allows the application of the transferred arc and higher velocities of plasma particles.The transferred arc enables the cleaning of the substrate surface and its preheating prior to powder deposition, as well as additional heating during deposition.Figure 10 shows possible lengths of the Ar / He plasma jet, depending on the pressure in the vacuum chamber and the applied transferred arc (Kieschke, et al., 1991, pp.25-38).Figure 11 shows the effect of the transferred arc on the possible length of the Ar / H 2 plasma jet when the deposited powder is CoNiCrAlY (Mrdak, 2013, pp.26-47), (Nikoll, 1984).Different levels of pressure for the transferred arc can be used, depending on the characteristics of the base material and the powder.To form a plasma jet with optimum characteristics, the gas flow rate may vary between 1.5 and 20 m 3 depending on the type of powder being deposited.The deposition of coatings on substrates is performed as follows.Working parts are mounted in the supporting tools that are on the planetary system which rotates around its axis.After the mounting of the working parts, the vacuum chamber is closed.The entire system is automated and programmed on the robot's microprocessor unit.All process parameters are entered into the program.The process of chamber vacuuming, the flow of plasma gases, the substrate cleaning, the powder flow, deposition, the substrate cooling and the vacuum chamber ventilation are fully synchronized by the program.In the VPS chamber, an artificial hand on the other side of the opening of the chamber and which cannot be seen in the photograph, receives the tools with a working part from the planetary system and sets it on the rotary table.After the mounting of the tools with the working part, the chamber is vacuumed where a pressure of 10 -3 mbar is achieved in 5 min.When the vacuuming process ends, Ar is introduced into the chamber through the anode to a pressure of 25 mbar.At this pressure, cleaning of work parts surfaces is carried out by the transferred arc.The distance between the plasma gun from the surface of working parts is usually 270 mm.The plasma gun is set to the + pole, and the working part to the -pole.This bond is called direct polarity and it allows the directed ions of He as a secondary gas to clean the surface of the work part at high rate and energy by making the surface reactive.After the cleaning, powder is deposited on the surface of the work part.The secondary plasma gas H 2 is added to the primary gas Ar .The pressure in the chamber grows to the level of the operating pressure of 70-120 mbar, depending on the type of powder that is deposited.The constant pressure during the deposition is provided by the vacuum pump.When the operating pressure is reached, powder is introduced into the plasma gun.The deposition rate is constant and does not change during deposition.A coating layer of 0.1 mm is deposited for 1 min approximately.When the deposition process is completed, the working part is cooled in the chamber to a temperature of 300 °C with argon flowing from the anode of the F4 plasma gun.The cooled working part with the tool is accepted by the artificial hand and returned to its original position.The planetary system is pivoted by one step to enable the artificial arm to receive another tool with a working part.The powder deposition cycle was repeated until powder is not deposited on all working parts.

Conclusion
The paper describes the structure of plasma, the method of creating plasma, thermodynamic properties such as enthalpy, temperature, specific heat and thermal conductivity of gases and plasma spray processes at atmospheric pressure and in vacuum at low pressure of inert gas.Plasma spray processes have been developed for the deposition of molten or semi-molten powder particles on the surface of the underlying material on which the deposited particles form a coating.
The advantage of the plasma spray process is the possibility of applying a large number of different materials in the form of powder of various grain and morphology, which are suitable for the formation of coatings in order to protect the surface of the base metal from wear, abrasion, erosion, cavitation, and corrosion resistance to fatigue at low and high temperatures.
Plasma spray processes also allow the modifications of powders, such as the spheroidization of powders with sharp edges, thickening of porous particles, and more recently the formation of nanopowders.The advantage of the process is that molten powder particles with high melting points do not transfer a large amount of heat into the base material and do not violate the basic structure of the material.In the deposition process, it is essential to control the heat transfer from the coating to the substrate, which is a function of the composition of the plasma gas, power supply, and the residence time of particles in the plasma jet.

Table 1 -
Values of the specific heat and the thermal conductivity of individual gases