P F a b c d a A R R A A K A P S A I 1 o s s H c m f c h e o f t c a s m h 0 Journal of Materials Processing Technology 237 (2016) 351–360 Contents lists available at ScienceDirect Journal of Materials Processing Technology jo ur nal ho me page: www.elsev ier .com/ locate / jmatprotec lasma torch for supersonic plasma spray at atmospheric pressure .R. Caliari a,b,∗, F.S. Mirandab, D.A.P. Reisa, G.P. Filhob, L.I. Charakhovskic, A. Essiptchoukd Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo – UNIFESP, São José dos Campos 12231-280, SP, Brazil Laboratório de Plasmas e Processos, Instituto Tecnológico de Aeronáutica – ITA, São José dos Campos 12228-900, SP, Brazil Luikov Heat and Mass Transfer Institute, HMTI – National Academy of Sciences of Belarus, Minsk 220072, Belarus Instituto de Ciência e Tecnologia, Univ. Estadual Paulista – UNESP, São José dos Campos 12247-004, SP, Brazil r t i c l e i n f o rticle history: eceived 28 October 2015 eceived in revised form 17 March 2016 ccepted 18 June 2016 vailable online 23 June 2016 a b s t r a c t This work presents a plasma torch able to operate at supersonic regime with axial injection of feedstock. In contrast to commonly used linear scheme, the principal axis of the plasma torch is perpendicular to feedstock injection direction, which is aligned with coming out plasma jet. The plasma torch has slightly ascending current voltage characteristics and fixed arc length. Electrical, thermal and kinetic characteristics outlined from comparison with conventional linear plasma spray torches are intermediate eywords: tmospheric plasma spray lasma torch upersonic plasma spray xial injection between APS, HVOF and VPS. The plasma torch developed in this work has an elevated arc voltage (370 V) and low arc current (100 A), which contribute to increase the electrode life and decrease the arc voltage relative fluctuation (10%). According to in-flight particle monitoring the CoNiCrAlY particles were sprayed at 500 m/s and temperature of 2400 ◦C, whereas the 7%YSZ at 491–683 m/s and 2535–2636 ◦C. © 2016 Elsevier B.V. All rights reserved. n-flight particle characteristics . Introduction Thermal spray embodies a family of processes where a layer f material (metallic, ceramic and/or polymer) is applied on a ubstrate in order to get protection against oxidation, corro- ion, abrasion, thermal loads, high temperature fatigue and creep. ermanek (2001) defined the thermal spraying as “a group of oating processes in which finely divided metallic or non-metallic aterials are deposited in a molten or semi-molten condition to orm a coating”. Thus the coatings are produced when the parti- les are heated and deformed at impact with the substrate, which appens if they are not only softened but have sufficient kinetic nergy. The particles may be in the form of a powder, solution r suspension, and from here on it shall be called generically as eedstock. The thermal spraying encloses several processes that use the hermal energy generated chemically (by combustion) or electri- ally (mainly by electric arc discharge) to soften and/or melt and ccelerate the feedstock at high velocities from few tens to thou- and meters per second. Fauchais and Montavon, 2007 highlighted that among the ther- al spraying processes, the atmospheric plasma spray (APS) is the ∗ Corresponding author. E-mail address: felipercaliari@yahoo.com.br (F.R. Caliari). ttp://dx.doi.org/10.1016/j.jmatprotec.2016.06.027 924-0136/© 2016 Elsevier B.V. All rights reserved. technique most commonly used due to its versatility and cost- efficiency. The high temperature of the plasma jet is suitable, but is not limited, to materials with high melting point like ceramics and refractory materials. The process usually is accomplished in the open-air environment, but to improve the coating quality a con- trolled atmosphere chamber (LPPS – low pressure plasma spray or VPS – vacuum plasma spray) may be used. As described by Pawlowski (2008) thermal energy converted from electrical determines the temperature of the flame (or jet), however, besides the efficiency of fusion of the particles, the adhe- sion of droplets to the substrate and porosity depends also of the plasma velocity. The conventional plasma spray (APS) has higher thermal loads and relatively low particles velocities when compared to High Velocity Oxygen Fuel (HVOF), Detonation Gun (D-Gun) and Vacuum Plasma Spray process (VPS). Most conventional torches for plasma spraying were devel- oped in decade 60. With the aim to improve the plasma spraying technique, a considerable number of plasma torches types, using different physical principles, have been developed in the past (see for example Zhukov and Zasypkin, 2007). Most of them used linear-circuit plasma generators where the electrodes (cathode and anode) are arranged axially. The cathode, normally, is the inner electrode and output nozzle is the anode. A discharge chamber of typical plasma torch consists of one (usually tungsten rod) surrounded by a concentric hollow anode, which acts as output nozzle, as shown by Zhukov and Zasypkin dx.doi.org/10.1016/j.jmatprotec.2016.06.027 http://www.sciencedirect.com/science/journal/09240136 http://www.elsevier.com/locate/jmatprotec http://crossmark.crossref.org/dialog/?doi=10.1016/j.jmatprotec.2016.06.027&domain=pdf mailto:felipercaliari@yahoo.com.br dx.doi.org/10.1016/j.jmatprotec.2016.06.027 3 Processing Technology 237 (2016) 351–360 ( fl ( a t w o a h e a c d a t t fl s o a 2 b a a a t i t s t t w t m d t T a p t s a b g l t w r c 2 2 g 8 B i i Table 1 Chemical composition and particle size of powders. Powder Composition (wt.%) Particle size distribution (�m) Co Ni Cr Al Y ZrO2 Y2O3 52 F.R. Caliari et al. / Journal of Materials 2007). An electric arc ignited between the electrodes and the gas ow blow-out a high enthalpy plasma jet. The feedstock is injected radially or axially) into the plasma jet where is melted and acceler- ted in the substrate direction. The advantage of this type of plasma orch is its simplicity, small number of parts and extensive set of ell-known operating parameters. The arc is practically immobile n the cathode surface, while it has axial and radial motions on the node surface, which result in oscillation of the arc voltage and, ence, in the enthalpy of the plasma jet, as described by Nogues t al. (2008). Fauchais et al. (2013a,b) pointed out that longitudinally blown rc possess best energetic characteristics, but it is commonly asso- iated with radial injection of feedstock, which stimulates elevated ispersion and spatial segregation by grain size distribution. Chyou nd Pfender (1989) reinforced that the plasma torches provide high emperatures but, to be an effective heat source, its interaction with he feedstock depends of various parameters. The estimation of in- ight particle properties can be a difficult task because one must et correctly the heat transfer boundary conditions, which depends n particle morphology and size, Biot number, injection method, mong other parameters, as observed by Fauchais and Montavon, 007. Depending on the injection method used, the relationship etween the particle and plasma jet momentum will be affected, long with the trajectory and the particle residence time. Besides, cceleration of particles at low-density plasma jet is not as effective s at high pressure and high density. Vardelle et al. (1994) developed a plasma torch with axial injec- ion considering the path of the particle through the cathode, which n principle allows a close contact with the electric arc, but reduces he lifetime of the cathode due to erosion. Mohanty et al. (2010) tudied a linear plasma torch with the particle injection through he cathode, and the results showed that the introduction of par- icles did not affected overall arc behaviour, although an elevated ear of the cathode was observed. Vardelle et al. (2015) pointed out hat the advantages provided by the axial injection method are the aximization of particles injected, uniform heating and elevated eposition efficiency. According to Fauchais et al. (2011) the Met- ech Axial III plasma torch is composed of three cathodes/anodes. he plasma jets, generated in separated channels, join together in single outlet, where the feedstock is injected. This configuration rovides high residence time. Papyrin et al. (2006) and Van Steenkiste et al. (1999) affirmed hat supersonic or hypersonic plasma jets permit to obtain dense pray coatings, similar to kinetic spray. One can hypothesize that pplication of axial injection and acceleration of particles com- ined with supersonic plasma jet would make it possible to fill the ap between dense but low melting coating materials obtained by ow-temperature kinetic spray and, refractory coatings obtained by raditional plasma spray with substantially higher porosity. In this ork, a plasma torch working with axial injection and supersonic egime, along with the in-flight particles analysis of metallic and eramic powders plasma sprayed is presented. . Materials and methods .1. Feedstock materials In this work the metallic powder Amperit 415.001, CoNiCrAlY, as atomized, from HC Stark, and the ceramic powder Amperit 25.000, 7%YSZ, fused and crushed, from HC Stark, were studied. efore each test, the powders were dried for 1 h at 100 ◦C. Chem- cal composition and the grain size distribution of powders are llustrated on Table 1. CoNiCrAlY Bal 32 21 8 0.5 – – 22–45 7%YSZ – – – – – Bal. 7 5–22 2.2. In-flight experimental analysis The kinetic and thermal characteristics of HVPS process is herein ascertained by means of in-flight particle velocity and tempera- ture. The in-flight particle state was measured using a DPV-2000, 2009 Premium (Tecnar, St. Bruno, QC, Canada, 2009), which uses an infrared pyrometer system to collect particle individual data on the plasma stream. For all in-flight particle measurements performed with DPV-2000, the auto-center procedure was performed in order to set the position within the plasma plume with maximum data acquisition. For each test at least 3000 particles data were collected, to guarantee a reasonable level of statistical significance. 2.3. Plasma spray experimental set-up This work presents a two-chamber design of plasma torch for high velocity plasma spraying, briefly described by Caliari et al. (2015). Development of this plasma torches is motivated by the request to improve the arc stability and to increase the range of powders to be used and quality of sprayed coating surface. Ther- mal and kinetic parameters from the plasma torch have been described by means of in-flight particle properties. The plasma torch is designed to work at sub- and supersonic regimes. The principal difference from other constructions is that the discharge chamber axis is perpendicular to powder injection that is aligned with coming out plasma jet. The schematic diagram of plasma torch for High Velocity Plasma Spray (HVPS) proposed in this work is shown in Fig. 1. The mixing chamber is placed between two tubu- lar copper electrodes. The swirled gas enters the electrodes (where a reversed vortex flow is formed) and the mixing chamber (with direct vortex), stabilizing the electric arc at the principal axis of the plasma torch. The nozzle is arranged in mixing chamber per- pendicularly and eccentrically from the principal axis according to the direction of vortex rotation. Therefore, the arc plasma enters the nozzle with no swirling. Feedstock is injected through nip- ple coaxially to the nozzle by the flow of additional transporting gas. Elevated temperature of the mixing chamber walls, as well as optimization of particle injection, is necessary for preventing the material adhesion on the walls and possible clogging of the noz- zle. An additional magnetic field can be used for the stabilization of the anode axial position. In presented work, in order to fix the arc length and, thus, decrease the arc voltage and enthalpy fluctuations, a plain anode with a protective argon atmosphere was used. This plasma torch is designed for up to 50 kW of power input and uses nitrogen as plasma forming gas. The power supply was arranged on the basis of six welding transformers with magnetic shunts controlling impedance and external characteristics. The transformers have been connected in series–parallel groups and, together with the standard three-phase rectifier, the DC power supply with an off-load voltage of 675 V and descending V(I) char- acteristics turned out. The plasma torch is ignited by a high frequency/voltage, which promotes simultaneous breakdown of the gaps formed between mixing chamber and electrodes (cathode and anode). Started main arc is blowing out and stabilized on torch axis by vortex. A preliminary modelling of internal cold flow of HVPS is pre- sented on Fig. 2. The internal cold flow inside of plasma torch chamber was preliminary simulated with SOLIDWORKS Flow Sim- F.R. Caliari et al. / Journal of Materials Processing Technology 237 (2016) 351–360 353 Fig. 1. Schematic diagram of plasma torch F a u a t c m s ig. 2. Velocity flow pattern in (a) transversal section of mixing chamber (b) and xial section of electrode. lation package that used Computational Fluid Dynamics (CFD) nalysis. Fluid motion is modelled by using the Navier-Stokes equa- ions, which describe, in non-stationary formulation, the laws of onservation of mass, momentum and energy of the medium. A 3D odel of inner cavity of plasma torch was studied at initial pres- ure 1.01325 MPa and temperature 20 ◦C. The inner walls of the for high velocity plasma spraying. chamber were defined as adiabatic with zero roughness. The flow type laminar and turbulent was assumed. For this, the transport equation of turbulent kinetic energy and its dissipation within k-� model of turbulence was taken into account. The turbulence energy of 1 J/kg and turbulence dissipation of 1 W/kg was assumed in simu- lation. Boundary condition of Nitrogen gas flow rate 3 g/s and outlet environment pressure of 1.01325 MPa were placed. From the gas velocity pattern, a good vortex is formed in the inner tubular elec- trode channel that must stabilize well the arc on the centre of the axis. In the mixing chamber, a vortex is off-centred and may induce augment in the arc oscillations. In order to achieve a predetermined Mach number at the exit section of a supersonic nozzle, a cross-sectional area must be suit- ably chosen in accordance with gas flow rate and, moreover, it is necessary to have an adequate supply pressure in the discharge chamber of plasma torch. On the other hand, the exhaust veloc- ity of a gas in a supersonic nozzle depends only on the stagnation temperature T0. A change of the total pressure p0 in the discharge chamber does not affect the velocity, since a local pressure p is changed proportionally and their ratio p/p0 remains unchanged, as well as the ratio of temperatures (T/T0), p p0 = ( T T0 ) � �−1 Preliminary calculations showed that at moderate, for a nitro- gen plasma gas, temperature 1720–2720 ◦C and plenum pressure 0.5 MPa, a velocity of nitrogen plasma achieves 1200–1600 m/s. Axial injection of particles at subsonic part of jet at the direction of gas flow, the same manner as used in kinetic spray, must allow a more effective acceleration. Expected parameters of HVPS pro- 354 F.R. Caliari et al. / Journal of Materials Processing Technology 237 (2016) 351–360 Fig. 3. Gas temperature versus particle velocity of thermal spray processes. APS- Atmospheric Plasma Spray, LPPS-Low Pressure Plasma Spray, VPS-Vacuum Plasma Spray. HVOF-High Velocity Oxygen Fuel, HVPS-High Velocity Plasma Spray [This p F s c s a t c t d P w p b t P w e � o r � H t p 3 W t s c e s s t t f p la sm a to rc h d u ri n g th e in -fl ig h t p ar ti cl e an al ys is an d co m p ar is on of p ar am et er s w it h co m m er ci al p la sm a to rc h es . ch /P ro ce ss M at er ia l W or ki n g ga sl @ fl ow ra te , G N 2 U l@ [V ] Il @ [A ] p to rc h l@ [M Pa ] � h l@ [M J/ kg ] Po w d er fl ow ra te , G p ow d er [g /m in ] d [m m ] C oN iC rA lY 31 4 sl p m (N 2 , 6 .0 g/ s) 38 0 10 0 0, 30 6. 3 5 50 –2 00 7% Y SZ 23 6 sl p m (N 2 , 4 .5 g/ s) 33 0 11 4 0, 28 8. 4 10 0 15 0 7% Y SZ 23 6 sl p m (N 2 , 4 .5 g/ s) 34 0 11 0 0, 26 8. 3 10 0 10 0 7% Y SZ 28 8 sl p m (N 2 , 5 .5 g/ s) 37 5 10 8 0, 3 7. 4 10 0 10 0 7% Y SZ 28 8 sl p m (N 2 , 5 .5 g/ s) 38 0 10 0 0, 3 6. 9 36 10 0 7% Y SZ 28 8 sl p m (N 2 , 5 .5 g/ s) 38 0 98 0, 3 6. 7 10 0 15 0 7% Y SZ 23 6 sl p m (N 2 , 4 .5 g/ s) 34 0 11 0 0, 24 8. 3 36 10 0 7% Y SZ 28 8 sl p m (N 2 , 5 .5 g/ s) 36 0 11 0 0, 29 7. 2 36 15 0 7% Y SZ 23 6 sl p m (N 2 , 4 .5 g/ s) 34 0 11 3 0, 24 8. 5 36 15 0 M au er et al ., 20 09 ) (S u lz er M et co )/ A PS 7% Y SZ 11 –1 25 � m 54 sl p m A r + H e 11 4 50 0 – – n ce be tw ee n n oz zl e ex it an d se n so r h ea d of D PV -2 00 0, d u ri n g in -fl ig h t p ar ti cl e m ea su re m en t. icture was published in the book: The cold spray materials deposition process undamentals and applications, M. F. Smith, Comparing cold spray with thermal pray coating technologies, pp. 43–61, Copyright Elsevier, 2007]. ess in comparison with traditional thermal spraying processes are hown in Fig. 3. During the plasma spray experiments a 4MP dual powder feeder dapted to operate at high pressures was used. Table 2 describes he set of parameters of HVPS process, as well as comparison to onventional APS parameters. The average plasma jet enthalpy (�h) has been calculated from he plasma torch parameters. The total input power of electric arc ischarge, Ptotal, W, is calculated as follow: total = UI, (2) here, U is the arc voltage, V, and I is the arc current, A. The dissi- ated energy (PAB) in plasma torch cooling system was estimated y measuring the water-cooling mass flow rate, GH2O, kg/s, and the emperature gradient �T, K, of the cooling water AB = GH2OcpH2O �T, (3) here cpH2O is the specific heat capacity of water. Thus the thermal fficiency, �, of the plasma torch is: = Ptotal − PAB Ptotal . (4) The bulk enthalpy variation of the plasma jet, �h, kJ/kg, is btained by the ratio between the net power and total mass flow ate of gas (nitrogen) injected into the plasma torch, i.e. h = �Ptotal GN2 + Gspray , (5) ere, GN2 and Gspray are the mass flow rate of the working gas and he carrier gas, respectively, kg/s. In this calculation the sprayed owder influence on �h was neglected. . Results and discussion A supersonic plasma jet produced during HVPS is shown in Fig. 4. hen the discharge chamber pressure exceeds the designed one, he exit pressure of plasma jet is higher than the external pres- ure and this flow is called under expanded. This overpressure is onsumed due to the increase of the plasma jet velocity. Accel- ration of the supersonic flow requires an increase in the jet cross ection, thus the plasma jet forms in the space an expanding “super- onic nozzle”. In certain moment, with the wider section of the jet, he pressure established is lower than the atmospheric one. After hat, the jet begins to contract, because the pressure should attain Ta b le 2 Pa ra m et er s o Pl as m a to r H V PS H V PS ru n 1 H V PS ru n 2 H V PS ru n 3 H V PS ru n 4 H V PS ru n 5 H V PS ru n 6 H V PS ru n 7 H V PS ru n 8 Tr ip le x II ( d th e d is ta F.R. Caliari et al. / Journal of Materials Processing Technology 237 (2016) 351–360 355 Fig. 4. Supersonic plasma jet. 80 85 90 95 100 105 110 320 360 400 440 480 6,9 g/s 6,3 g/s V ol ta ge , V 5,8 g/s t B w a s g s o i c s s c s n o v F s c t a r d 5,6 6,0 6,4 6,8 340 360 380 400 420 440 V ol ta ge , V Gas flow rate, g/s Fig. 6. Arc voltage vs gas flow rate. 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.30 31 32 33 34 35 36 37 38 39 40 Experi mental da ta Calcula ted data E le ct ric fi el d, V /c m Pressure, MPa 5,8 g/s 6,3 g/s Fig. 7. Electric field strength vs pressure in discharge chamber. Curren t, A Fig. 5. Volt-Current characteristics of HVPS. he atmospheric one, and the jet velocity, respectively, is reduced. reaking supersonic flow naturally leads to the appearance of shock aves. If the excess pressure is sufficiently large, the jet velocity ttains a critical value again, and after that supersonic one, i.e. a econd supersonic expanded part appears. Due to losses in the first roup of shock waves, the second overexpansion of the jet and the econd group of shocks is weaker than the first. A similar picture is bserved when the pressure in the plasma torch discharge chamber s lower than the designed one. In this case of the exhaust flow is ompressed and pressure is increased. The compression may be so trong that the plasma jet pressure exceeds the atmospheric pres- ure. As a result, in both cases, appears a diamond-like structure, learly observed on Fig. 4. For an ideal gas, this process of expan- ion and contraction would continue on forever creating an infinite umber of Mach disks. The current–voltage plot is the main electrical characteristic f a plasma torch, which describes the inter-relation between arc oltage and arc current under other fixed working parameters. In ig. 5 the current–voltage characteristics of the plasma torch for upersonic spraying are shown. Voltage is almost independent on urrent and a slight increase (within the measurement accuracy) rend can be observed. U-shaped characteristic is typical when the rc length is fixed, as proposed by Zhukov and Zasypkin (2007) . A eversed vortex, in addition to the fixed length of arc, stimulated a istributed blowing of cold gas into the arc, which has gas-dynamic effect on the strength of the electrical field, as shown experimen- tally by Essiptchouk et al. (2009). A developed turbulent flow may increase 2–3 times the electric field strength, as reported by Zhukov and Zasypkin (2007). An outstanding feature of this plasma torch is the elevated volt- age and low current, which is exactly the opposite of common plasma torches used for plasma spray (see Table 2). A low cur- rent ensures low erosion rate, low contamination of plasma and increase the lifetime of the electrodes. For the supersonic regime, the arc current was limited between 80A and 110A. The characteristic curves were obtained for three flow rates of nitrogen. The voltage dependence on the gas flow rate is shown in Fig. 6. Increase of flow rate in 20% promotes an increase of 15% of arc voltage due to intensification of heat exchange while the total pressure in the discharge chamber grows insignificantly. An effective electric field E = U/I, where U is total arc volt- age and l = constant is the inter-electrode distance, presented in Fig. 7, shows relatively high values that are typical for turbulent flow. It is observed that the electric field grows with increasing of the discharge chamber pressure. The experimental data of electric field (closed points) is compared with data from empirical depen- 356 F.R. Caliari et al. / Journal of Materials Proces 85 90 95 10 0 105 110 4,2 4,4 4,6 4,8 5,0 5,2 5,4 E nt ha lp y, M J/ kg 6,3 g/s 5,8 g/s d Z E w p o e t T s F p p m h b 2 H S C l r v c o m p o a t d w ( t w m v p ( o Current, A Fig. 8. Bulk enthalpy of plasma jet vs arc current. ence (open points) obtained via Eq. (6) as proposed by Zhukov and asypkin (2007), d = 115 ( I d )−0.23(GN2 d )0.47 (ptorchd)0.2 (6) here d is the diameter of discharge chamber, m. All this justify a practically linear increase of bulk enthalpy of lasma jet with arc current (Fig. 8). The plasma torch herein devel- ped does not only produce a more stable jet but makes it also asier to adjust the enthalpy of the plasma gas by fine correction of he arc current. The bulk enthalpy �h was estimated, as detailed on item 2.3. hermal losses in the plasma torch were obtained from the mea- urement of the cooling water flow rate and its temperature. or temperature measurement the chromel–alumel thermocou- les (type K) were used. For the torch developed in this work the ower ranged between 38 and 40 kW, the efficiency of transfor- ation electrical energy in thermal one was 75–80%, which is igher than the values of the linear plasma torches, as calculated y Shanmugavelayutham and Selvarajan (2003) and Dorier et al., 001. Preliminary studies of kinetic and thermal characteristics of VPS were carried out with application of an in-flight Particle ensor DPV-2000. In-flight measurements for the metallic powder oNiCrAlY (Fig. 9) represent the behaviour of the particles along the ength of the supersonic plasma jet. The profile of particle velocity anged between 425 and 500 m/s (Fig. 9a), whereas the particle elocity starts to decrease at 150 mm from the nozzle. The parti- le temperature remained among the range of 2270–2440 ◦C. As bserved by Fauchais (2004) the plasma spray process depends ainly on material precursor properties and transport (size, com- osition, specific mass, thermal conductivity, latent heat, method f fabrication, method of injection, etc) as well as plasma jet char- cteristics (working gas type and flow rate, torch design). From he interaction of previous parameters the respective particle resi- ence time and heat transfer efficiency between particle-plasma ill determine the range of particle velocity and temperature Fauchais et al., 2013a,b). Within the works conducted to measure he in-flight CoNiCrAlY properties, the study of Richier et al. (2008) ho deposited the CO-127 (CoNiCrAlY 5–45 �m, from Praxair) by eans of Cold Gas Dynamic Spraying – CGDS, reached a particle elocity of 560 m/s. Sampath et al. (2009) used the commercial lasma torch 7MB (Sulzer Metco), to spray the powder CO-211 CoNiCrAlY, 5–90 �m, from Praxair) at average particle velocitiy f 90 m/s. The data calculated by Yang et al. (2002) considered the sing Technology 237 (2016) 351–360 properties of powder CO-210-1 (CoNiCrAlY, 10–45 �m, from Prax- air) with average grain size of 30 �m, using High Velocity Oxygen Fuel, could provide average velocity of 410 m/s at a stand-off dis- tance of 200 mm. Comparison of velocities obtained on literature for different spray processes allows observe that the HVPS pro- cess, used in this work (see Fig. 9), can operate within the range of particle velocities of CGDS and the same range of vp and Tp of HVOF process, which along with Vacuum Plasma Spray (VPS), are the state-of-art thermal spray technologies capable to produce high particle velocities and therefore high quality coatings. A set of experiments were also performed in order to evalu- ate the influence of GN2, Gpowder and stand-off distance on the particle velocity and temperature of 7%YSZ. According to Fig. 10, the particle velocity ranged between 491 and 683 m/s and parti- cle temperatures 2535–2636 ◦C. Thermal spray processes operating at high particle velocities may find limitation to deposit high melting point materials, due to the inherent low residence time. This trade-off becomes especially critical when processing ceramic materials. Conventional APS process, with radial injection, eventu- ally encounters such limitations. The commonly adopted solution is to maximize the residence time by means, for example, of operation under quasi-laminar plasma flow, as described by Solonenko and Smirnov (2014) or increase of plasma enthalpy (Tarasi et al., 2008). The HVPS process shows, via in-flight particle velocity and temper- ature of 7%YSZ (Fig. 10), that it is plausible to operate under high velocities/temperature with high melting point materials. The fur- ther analysis of 7%YSZ in-flight particle data shows that the particle temperature (Fig. 10b) remained almost the same as commonly achieved from conventional APS process. Salimijazi et al. (2007) used a SG-100 (Praxair) plasma torch to plasma spray the Amperit 825 at 307 m/s and 2836 ◦C. Chen et al. (2008) developed a high efficient plasma torch, working with a mixture of carbon dioxide and hydrocarbons and used it to spray the Amperit 825 at 205 m/s and 2768 ◦C. When using the HVPS process developed in this work, the Amperit 825 reaches 491–683 m/s and 2535–2636 ◦C (Fig. 10). Despite the different set-up on the experiments of 7%YSZ (runs 1 through 8, see Table 2) the particle temperature was maintained almost constant, even with variation of the stand-off distance (between 100 and 150 mm). This behavior can be atributed to the high efficiency heat transfer provided the plasma torch design, supersonic plasma jet regime and inherent efficient heat trans- fer environment provided by the gas nitrogen. Another important observation is that during 7%YSZ in-flight analysis experiments, a narrow range of electric power input has been covered, which may have affected the results of particle temperature. Whilst, the 7%YSZ particle velocity showed a broader range. This may be sus- tained by the wide range of gradient pressure obtained during the experiments, the stand-off distance analysed and the low particle size. The statistical analysis of particle velocity and temperature distribution obtained from single particle measurement from DPV-2000, provides sustainable discussion on particle state, as demonstrated by Streibl et al. (2006). In-flight data of CoNiCrAlY were analysed at different standoff distances by means of frequency distribution and a mono or multimodal fitting approximation has been adjusted, considering a Gauss distribution function. Statis- tical analysis of CoNiCrAlY data showed that vp distribution is monomodal until 100 mm (Fig. 11a), after that a bimodal distri- bution is observed (Fig. 11b). The average value decrease from 522 to 494 m/s between 50 and 100 mm. At 150 mm 15% of population distribution assumes a vp of 387 m/s, and the remaining 519 m/s, whereas at 200 mm 37% of population distribution has an average vp of 393 m/s and the rest 447 m/s. The Tp assumes a bimodal Gauss distribution, regardless the standoff distance analysed from the nozzle (Fig. 11c). For a standoff distance of 50 mm the Tp frequency distribution is divided in a half, with average values of 2153 ◦C and F.R. Caliari et al. / Journal of Materials Processing Technology 237 (2016) 351–360 357 0 50 10 0 15 0 20 0 0 200 400 600 800 Caliari, 2015 Richier, 200 8 Sampa th, 200 9 Yang, 2001 (simulation for 30 um) Pa rti cl e ve lo ci ty [m /s ] Distance [mm] (a) 50 10 0 15 0 20 0 160 0 180 0 2000 2200 2400 2600 280 0 300 0 320 0 Caliari, 2015 Sampath, 200 9 Yang, 2001 (simulation for 30 um) P ar tic le te m pe ra tu re [° C ] Distan ce [mm ] (b) Fig. 9. Measurements of (a) velocity and (b) temperature of CoNiCrAlY particles on along the supersonic plasma jet. mpera 2 a r m t v p i F s d c a i v e a 3 d p a Fig. 10. Measurements of (a) velocity and (b) te 427 ◦C. Between 100 and 150 mm the 38% of the particles present n increase in average value, from 2128 ◦C to 2370 ◦C, whereas the emaining particles maintain around 2500 ◦C. The average values aintain at a distance of 200 mm further from the nozzle, never- heless an increase to 52% of the population with the lowest average alue of particle temperature (2370 ◦C) is observed. The analysis of particle properties distribution is critical when rocessing high melting point materials, such as ceramics, because t may compromise coating quality, as well as deposition efficiency. or instance, Mauer et al. (2009) studied the correlation of plasma pray parameters with the corresponding coating porosity and eposition efficiency, during the deposition of YSZ. The results indi- ated that high deposition efficiency and low porosity is achieved t increasing values of particle temperature, which presented an ncrease of the dispersion of particle temperature distribution. Con- entionally, high melting particles injected radially to plasma jet xhibit a multimodal and highly dispersive distribution of temper- ture, as described by Sampath et al. (2009). The histograms of run represented on Fig. 12 allows to demonstrate that both vp and Tp istributions assume a monomodal-Gaussian profile on the HVPS rocess. Other important parameter of plasma spraying is fluctuation of rc voltage together with the power spectrum of arc voltage. Con- ture of YSZ particles for different experiments. sidering small arc length variations, it is expected that HVPS has low current and voltage variation and, respectively, low enthalpy and plasma jet temperature fluctuations. A typical picture of arc voltage fluctuation is shown in Fig. 13a, and the power spectrum of voltage (Fig. 13b). The data was obtained by Digital Storage Oscilloscope Tektronix TDS2024C (200 MHz, 4-Ch, 2 GS/s). Besides chaotic low amplitude pulsations (low-scale shunting) it is clearly observed a large-scale oscillation of arc voltage. For the typical discharge of the average arc voltage of 370 V a standard deviation of voltage fluc- tuation of ±10% was obtained. According to Fauchais (2004), for conventional plasma torches the voltage fluctuations vary between ±15% and ±35%. The power spectrum on frequency was analyzed by means of Fast Fourier Transform (FFT), which was gathered at a sampling rate of 25 kS/s. The spectrum of voltage on the fre- quency domain was adjusted using a low pass filtering. According to Fig. 13b the spectrum does not indicated a characteristic frequency peak, except the initial peak related to the power supply. One interesting phenomenon was observed during supersonic plasma spraying. Some particles directed out of spraying axis, were forced to merge back into the plasma jet, as shown in Fig. 14. That behaviour can be explained by the Magnus effect, which determines the force acting on a rotating body in flow. The particles start to rotate when crossing a high velocity gradient in the supersonic jet 358 F.R. Caliari et al. / Journal of Materials Processing Technology 237 (2016) 351–360 100 20 0 300 40 0 50 0 60 0 70 0 800 90 0 0 500 1000 1500 2000 2500 3000 3500 Pa rti cl e C ou nt Particle velocity (m/s) (a) 100 20 0 30 0 40 0 500 60 0 70 0 80 0 0 200 400 600 800 1000 1200 Pa rti cl e C ou nt Particle vel ocity (m/s) (b) 1000 1500 200 0 250 0 300 0 350 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Pa rti cl e C ou nt Particle t empe rature (°C) (c) Fig. 11. Histograms of particle velocity and temperature distributions for CoNiCrAlY, (a) particle velocity at 100 mm and (b) at 150 mm, and (d) particle temperature at 100 mm. 200 400 600 800 10 00 0 200 400 600 800 1000 1200 1400 1600 1800 2000 P ar tic le C ou nt (a) 1500 2000 2500 3000 3500 4000 0 200 400 600 800 1000 1200 1400 1600 1800 2000 (b) P ar tic le C ou nt mpera b d i Particle velocity (m/s) Fig. 12. Histograms of particle velocity and te oundary layer (shock wave). This rotation creates a pressure gra- ient on particle surface directed to the jet axis. The acting force s proportional to the upstream velocity and the vortex strength Particle temperature (°C) ture distributions for 7%YSZ particle of run 3. established by particle rotation. The force direction can be obtained from the vector product �v × �ω, between the plasma flow velocity �v and the particle angular velocity �ω. F.R. Caliari et al. / Journal of Materials Processing Technology 237 (2016) 351–360 359 0 1 2 3 4 5 6 7 8 9 300 350 400 450 500 V ol ta ge (V ) time (ms) (a) 0 2 4 6 8 10 12 0 10 20 30 40 50 Am pl itu de (a .u .) (b) Fig. 13. (a) Arc voltage fluctuatio 4 d c a o t t ( t t t a 4 t p 7 i A # ( temperature and thermal conductivity of plasma jet in a DC plasma spray Fig. 14. Sprayed particle flow. . Conclusions A plasma torch design with discharge chamber axis perpen- icular to feedstock injection direction has been discussed. The urrent voltage characteristics has slightly ascending trend, with fixed arc length. Electrical, thermal and kinetic characteristics utlined from comparison with conventional linear plasma spray orches are intermediate between APS, HVOF and VPS. The plasma orch developed in this work showed an elevated arc voltage 370 V) and low arc current (100 A), which contribute to increase he electrode life and decrease the arc voltage relative fluctua- ion (10%). The results of in-flight analysis using DPV-2000 showed hat CoNiCrAlY particles were sprayed at velocities of 500 m/s nd temperature of 2400 ◦C. The ceramic 7%YSZ were sprayed at 91–683 m/s and 2535–2636 ◦C. Histogram of particle velocity and emperature shows a monomodal and bimodal distributions. The resented plasma torch works with overall thermal efficiency of 5–80% and provides an elevated heat transfer efficiency consider- ng the plasma-particle interaction. cknowledgments The authors acknowledge the financial support grant 2012/24851-7 provided by São Paulo Research Foundation FAPESP), CNPq and CAPES of Brazil. Frequency (KHz) n and (b) power spectrum. References Caliari, F.R., Miranda, F.S., Reis, D.A.P., Filho, G.P., Charakhovski, L.I., Essiptchouk, A., 2015. New kind of plasma torch for supersonic coatings at atmospheric pressure. In: Proceedings of the 22nd International Symposium on Plasma Chemistry, July 510, Antwerp, Belgium, 6 p. Chen, L., Pershin, L., Mostaghimi, J., 2008. A new highly efficient high-power DC plasma torch. IEEE Trans. Plasma Sci. 36 (4), 1068–1069. Chyou, Y.P., Pfender, E., 1989. Behaviour of particulates in thermal plasma flows. Plasma Chem. Plasma Process. 9, 45–71. DPV-2000 reference manual., 2009. Tecnar Rev. 5, 44. 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pressure 1 Introduction 2 Materials and methods 2.1 Feedstock materials 2.2 In-flight experimental analysis 2.3 Plasma spray experimental set-up 3 Results and discussion 4 Conclusions Acknowledgments References