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Oxidation control in plasma spraying NiCrCoAlY coating
Applied Surface Science 258 (2012) 5094–5099
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Oxidation control in plasma spraying NiCrCoAlY coating Qi Wei ∗ , Zhiyong Yin, Hui Li The College of Materials Science and Engineering, Beijing University of Technology, 100 PingLeYuan, Beijing 100124, PR China
a r t i c l e
i n f o
Article history: Received 16 June 2011 Received in revised form 22 December 2011 Accepted 24 January 2012 Available online 2 February 2012 Key words: Plasma spray Convective oxidation Diffusible oxidation Oxidation resistance
a b s t r a c t Atmospheric plasma spraying is usually accompanied by oxidation reactions, which result in the formation of oxides in the coatings. The presence of oxides in metallic coatings is usually undesirable because they cause the coating properties to deteriorate. This study highlights how the high temperature oxidation resistance of plasma sprayed NiCrCoAlY coating is influenced by both the oxidation behavior of NiCrCoAlY particles and by the shrouding gas during the spray process. It is shown that two different oxidation mechanisms are involved in the in-flight oxidation. One is diffusion oxidation, and the other is convective oxidation. The convective oxidation of NiCrCoAlY particles is the dominating oxidation mechanism when the plasma jet is at a distance of 55 mm from the torch nozzle exit; while diffusion oxidation was found to be the dominant mechanism when the spray distance is greater than 55 mm. Oxidation mainly occurs during in-flight and after impact on the substrate. In-flight oxidation is the dominant mechanism for NiCrCoAlY particles in plasma spray. Adding inert-gas shrouding is an effective method for decreasing the oxide content of the NiCrCoAlY coating, which significantly increases the coating’s oxidation resistance. © 2012 Elsevier B.V. All rights reserved.
1. 1.Introduction The plasma spray process is carried out by feeding powders into a plasma torch where the material is melted and propelled as molten or semi-molten particles toward the substrate. The coating is built up on the substrate surface by successive impacts of molten metal droplets, originating from the powder particles fed into the plasma torch [1]. Generally, the plasma spray process is conducted in an ambient air environment. Turbulent flow is caused by the jet flow and the surrounding air with shearing force will inevitably lead to the entrainment of ambient air into the plasma jet involved in the oxidation of the sprayed metallic particles. Many studies have indicated that the oxidation of thermally sprayed materials can significantly influence the phase composition, microstructure, properties and therefore the performance of sprayed coatings [2]. Metal oxides are usually brittle, their thermal expansion coefficients are different from those of the base metal. The presence of the oxide in the coating disrupts the chemical uniformity. For example, oxide layer at interfaces being brittle can potentially reduce the strength and ductility of the deposit, which will lead to decrease in the corrosion resistance of the coating [3,4]. Therefore, experiments have been carried out to study the oxidation of NiCrCoAlY particles
∗ Corresponding author. Tel.: +86 10 67396168; fax: +86 10 67396168. E-mail address: [email protected] (Q. Wei). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.134
during the spray process, which are expected to have guiding significance to optimize the performance of the coating. For this reason, experiments have been carried out to study the oxidation of NiCrCoAlY particles during the spray process. The results of this testing are expected to be significant in optimizing the performance of the coating. Thermal barrier coatings (TBCs) are widely used in gas turbines to reduce thermal effect and increase turbine efficiency. A usual TBC includes a MCrAlY bond coat (M Ni and Co) as oxidation resistant layer and a ceramic top coat as thermal barrier layer. Application of TBC at high temperature causes the transfer of oxygen through the top coat toward the bond coat, so an oxidized scale can be formed on the bond coat which is termed the thermally grown oxide (TGO). Although this layer protects the substrate against further oxidation, the excessive growth of TGO during thermal cycling can lead to the failure of the ceramic top layer [5–7]. Low-pressure plasma spray (LPPS) or vacuum plasma spray (VPS) can obtain homogeneous and compact microstructure, and the present coatings were free of oxide inclusions; but the operational cost of the system goes up tremendously (the capital cost of a LPPS system is around ten times than that of APS system). The size of the workpieces is also limited by the vacuum chamber [8]. Adding a coaxial shrouding gas can effectively reduce the entrainment of ambient air into the plasma jet and the oxidation of metallic particles can be mitigated. This experiment takes the NiCrCoAlY particle as the object, and studies the particle’s oxidation mechanism in the
Q. Wei et al. / Applied Surface Science 258 (2012) 5094–5099
spray process in conjunction with the impact of the shielded gas on the high temperature oxidation resistance of the coating.
Table 1 Element content of points 1, 2 of Fig. 3 (wt%) determined by EDS. Element
2. Experimental The oxidation of the coating test specimen is cut from the Haynes 230 nickel-based super alloy, the test sample is 15 mm × 10 mm × 4 mm, the nominal composition of the alloy (wt%): Cr 22%, W 14%, Mo 2%, other Ni. Grit blasting was used prior to plasma spraying, and then the sample was cleaned ultrasonically in an acetone bath. The spraying material used for the experimentation was NiCrCoAlY powder (China Institute of Mining and Metallurgy) having particle size in the range 44–104 m, the main composition of the powder (wt%): C < 0.7%, O 0.95%, Y2 O3 1.5–2.0%, Co 3–5%, Al 4–6%, Cr 14–16%, other Ni. Metco 9M is employed to spray the powder and coating, the powder feeder is Metco 5MP. The parameters for the spray process are as follows: voltage and amperage were set to 75 V and 500 A, the main gas flow rate (Ar) was 45 L min−1 , the auxiliary gas flow rate (H2 ) was 10 L min−1 , powder feed rate was 35 g min−1 , spraying distances were 55, 75, and 120 mm. Spraying was conducted at the different spraying distances, and the sprayed particles were into a cylindrical particle collector which is thenfilled with argon gas of the Ø 130 mm × 650 mm. A schematic of the collection setup is shown in Fig. 1a. The particles entering the collector were quenched by argon jets flowing through three inlets, two fixed at the collector entrance and one introduced at the bottom, the argon gas flow rate is 60 L min−1 . The self-made additional shield spray nozzle was used to spray the powder and the coating (Fig. 1b). The flow rate of the argon used as protective gas is 65 L min−1 . The cross-section of the structure was observed, and the porosity of the coating was measured by image analysis software MATLAB. The microstructure and the composition were observed by FEI Quanta 200 SEM–EDS. The phase analysis of the coating with different oxidation time was conducted by BRUKER D8 Advance X-ray diffraction (XRD). LECO TC-436 oxygen–nitrogen analyzer was used to measure the oxygen content before and after deposition. The coating sample was oxidized at 1080 ◦ C for 100 h under the furnace oven, and all the surfaces of the samples were coated. Weighing the samples in the oxidation process intermittently, the oxide weight gain of the samples was measured by electro-optical balance, whose accuracy is 0.01 mg.
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O Al Cr Co Ni
Point 1 5 8 80 1 5
Point 2 2 13 17 1 66
3.1.2. Oxidation behavior of particle during in-flight Fig. 4a shows the surface morphology of particles at the spraying distance of 55 mm, and ripples can be found at the surface of the particle. In fluid flow conditions where kinematic viscosity ratio of surrounding gases to liquid particle (i = g /p ) is greater than 55 and relative Reynolds number (Re) is higher than 20, the particle surface is likely to attain velocity superior to the average particle velocity generating eddy current within the particle. These conditions are very probable for a liquid particle in the plasma core and convective movements within the liquid particle can therefore be expected. These movements lead to the mass transfer of the oxide
3. Results and discussion 3.1. The oxidation behavior of particles in plasma spray process 3.1.1. The microstructure of particles Fig. 2 shows the surface and the section morphology of the asreceived NiCrCoAlY powder. It can be seen that the shape of the powder is irregular (Fig. 2a), and a small amount of residual oxide exists which is dissolved oxygen in the metal on the preparation stage. The oxide can be found neither on the surface nor on the interior of the particles (Fig. 2b). The morphology of the sprayed powder is shown in Fig. 3. The powder appears spherical after spraying. The morphology on the typical cross-section of particles after spraying is shown in Fig. 3b. It can be seen that a layer of cap area is distributed over the surface of the particle, and a small amount of dark gray nodular exists in the interior of the particle. The cap area on the surface and nodular material are separately analyzed by EDS point analysis (see Fig. 3b points 1, 2), Obvious oxide can be seen on the surface and interior of the powder after spraying, which is the result of the reaction between the particles and the air involved in jet during the spray process (Table 1).
Fig. 1. (a) Schematic of in-flight particles collection setup (b) schematics of plasma torch equipped with shroud.
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Fig. 2. Micrographs of as-sprayed powder:(a) surface of particles and (b) cross section of particles.
Fig. 3. Micrographs of collected particles: (a) surface of particles collected at 55 mm and (b) cross section of particles collected at 120 mm.
Fig. 4. Micrographs of particles collected at different standoff distance: (a) at 55 mm and (b) at 75 mm.
and the absorbed oxygen from the particle’s surface to its interior. The particle surface is continuously renewed and fresh metal is always present that enhances the particle’s chemical reactivity [9]. For plasma spraying, convective oxidation can be the dominating oxidization process, if the plasma to liquid particle kinematic viscosity ratio g /p is 50–2000 and relative Re attains values superior to 20. The discovery from Fig. 2a makes clear the existence of this convection. Fig. 4a shows the morphology on the cross-section of particles at the spraying distance of 55 mm. Nodular structure of oxide appears in the interior of particles, but no significant oxide is found on the surface, indicating that convective oxidation is the primary oxidation mechanism of the particles in this case. Because the surface tension between the metal oxides and molten metals
is different, the oxides which enter into the interior of particles become nearly spherical independently. Fig. 4b shows the morphology on the cross-section of particles at the spraying distance of 75 mm. When spraying distance increases to 75 mm, a thin oxide layer also appears at the periphery of the particles in addition to an internal oxide. As the distance from particles to nozzle increased, the relative Reynolds number of particles gets smaller, the plasma viscosity also decreases rapidly, and the convection oxidation of particles will decrease gradually. The traditional diffusion oxidation will become the main source of particle oxidation until the convection oxidation stops. The oxides formed on the surface are no longer involved in the interior of the particles; however, oxide accumulates and solidifies as a thin oxide layer on particle’s surface.
Q. Wei et al. / Applied Surface Science 258 (2012) 5094–5099 Table 2 Oxygen content (wt%) of collected particles and coatings determined by oxygen–nitrogen analyzer. Spraying into collector (C) or onto substrate (S)
Shrouding gas
O (wt%)
C–Argon S S
– – Argon
2.17 2.31 1.35
Fig. 3b shows the morphology on the cross-section of particles at the spray distance of 120 mm. The oxide on the surface of particles is obviously thicker compared with the particles sprayed at a distance of 75 mm, indicating that the particles oxidize further in the interior, while the accumulation of oxide on the surface continues. The oxidizing species (such as O2 , O, O+ , OH, etc.) and reducing species (such as H2 , H, H+ , NH, N+ , etc.) exists in the plasma jet, and the plasma has strong oxidation activity away from the latter half of the spray nozzle [10]. Some studies [11] point out that the quantity of oxidizing species has reached 100 times higher than that of the reducing species 100 mm beyond the exit of the spray nozzle, the intensity of oxidizability of plasma jet is especially high at this time. The oxide present on the surface accumulates when the particle was placed in the high intensity of oxidizability of the jet flow at the distance of 75–120 mm. Analysis of the surface oxide in the section of the particles showed that the average thickness of the surface oxide is 1–2 m. Fig. 3b also shows that more oxide is formed on one side. Because of the relative velocity existing between the particles and the plasma jet, the molten metal which has low viscosity will be pushed to the back-surface of the particles, while the higherviscosity oxide is pushed to the front-surface of the particles. In general, oxide accumulates on one side of the particles which have high speed, high temperature and low viscosity. Fig. 5 shows the microstructure of coating at the spraying distance of 120 mm, the coating is a typical laminated structure, and some defects such as porosity and oxide exist in the coating. Coating lamellae are separated alternately by the black oxide strip which can be seen clearly from the local magnification, and a few dots of a black substance are distributed in the interior of flat layer. The result of EDS on two spots 1, 2 from Fig. 5 shows that they are both the oxide inclusions. The oxide in the interior of particles remains in the coating which forms the oxide inclusion in the interior of laminar on splat. 3.1.3. Oxidation after impact on substrate Table 2 shows the content of oxygen of the particles and the coating at the spraying distance of 120 mm. The oxygen content increases from 0.95 wt% to 2.17 wt% at the spraying distance of 120 mm, and the oxygen content in the coating is 2.31 wt%,
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Table 3 Porosities of the SAPS and APS coatings. Coatings Porosity (%)
SAPS bond coat 3.2
APS bond coat 4.3
therefore, the degree of oxidation of NiCrCoAlY particles during inflight is higher than that which is formed on substrate after impact. Particle oxidation can be classified as in flight oxidation, oxidation at deposition and after deposition: (1) oxidation during flight. Inflight oxidation depends strongly upon the amount of entrained air, particle temperature and nature and turbulence of plasma jet. (2) Oxidation at deposition. Oxidation occurs in this period when the particles and the substrate contact, spread, and flat forms the coating but are not covered by subsequent particle deposition. The oxidation of the particles can almost be neglected during this time, because the splats are covered by the identical position of other particles within 10–20 s after the deposition of splat [9]. (3) Oxidation on the substrate after deposition. Oxidation occurs after the coating starts to spread. The oxidation during this period of time is mainly due to high temperature, high oxygen content of the plasma jet constantly sweeping in the deposition of coating during the spray process. In the spray process, particle oxidation can be divided into oxidation during in-flight process and oxidation after impact on substrate, which depends on the spray technology and the nature of the spray powder. The results of this experiment show that the oxidation of NiCrCoAlY particles mainly occurs in the in-flight process phase of the plasma spraying process.
3.2. Effect of the gas shroud on coating properties 3.2.1. The microstructure of the coating Fig. 6 shows the morphology on the coating surface of the atmospheric plasma spraying (APS) and the shrouding atmospheric plasma spraying (SAPS). The surface of the APS coating is rougher than the SAPS coating. Adding gas shrouding can reduce the disturbance of the surrounding air, and the velocity and temperature of the particle can be increased [12]. The melting condition of particles are therefore improved, the unmolten and the semi-molten particles on the surface of SAPS coating are decreased, and the surface roughness of SAPS coating is smaller than that one sprayed by APS. Adding gas shrouding can reduce the oxidation (Table 2) and oxide inclusion on the coating of the spray process, and the spreading of splat can also be improved. Therefore, the porosity contents in the coating are decreased (Table 3); thus, the quality of the coating will be improved.
Fig. 5. Micrographs of coatings at 120 mm (a), (b).
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Fig. 6. Surface morphology of the as-sprayed coatings: (a), (c) APS coatings and (b), (d) SAPS coatings.
3.2.2. The high temperature oxidation resistance of coating The oxidizing kinetics curve for the SAPS and the APS of NiCrCoAlY coating is shown in Fig. 7, and the coating sample oxidized at 1080 ◦ C for 100 h under the furnace oven, and weighing the samples in the oxidation process intermittently. The result reveals that the two oxidation kinetics curves are relatively consistent with the parabolic law, the oxide contents increase quickly during the initial period, and the oxidizing kinetics curve of APS coating is higher
than that of the SAPS coating. Along with the oxidation time, the oxidation rate of the coatings becomes lower, and the oxidizing kinetics curve becomes more gradual. The kinetics curve shows that the oxidation resistance of SAPS coating is better than that of ASP coating. The XRD pattern of the oxidized coating of SAPS and APS NiCrCoAlY particles over a 24 h period is shown in Fig. 8. It can be seen
Fig. 7. Comparison of oxidation kinetics at 1080 ◦ C for the SAPS and APS bond coats.
Fig. 8. XRD patterns of SAPS and APS coatings after 1080 ◦ C for 24 h.
Q. Wei et al. / Applied Surface Science 258 (2012) 5094–5099
that ␣-Al2 O3 , Cr2 O3 and NiCr2 O4 oxide is the main phase on the oxidized surface of SAPS coating, while for APS coating Cr2 O3 and NiCr2 O4 oxide is the main phase after 1080 ◦ C. In this experiment, the aluminum contents of NiCrCoAlY alloy powder itself are 4–6%, and a part of the aluminum is lost during the oxidation process between particles and air of the APS. The number of unmolten or semi-molten particles on the surface of APS coating is much greater than that during SAPS, and the surface roughness in APS coating is also much larger. The coatings were oxidized seriously during the initial period of time at 1080 ◦ C in air. As the time increases, these particles are separated completely from the coating by the mixed oxidation products. Because of the aluminum which diffuses from the interior coating to the attached particles are obstructed by the early-formed oxide (such as Cr2 O3 and NiCr2 O4 ), the depletion of Al from the interior of particles appears more easily. The ␣-Al2 O3 coating is inhibited by the absence of aluminum supply on the surface of these insolated particles, but some other mixed oxides are generated (such as NiCr2 O4 ), and the oxidation weight gain rate of APS coating is increased. However, the whole melting situation of the particles in SAPS coating is better, and the porosity is decreased, so the oxygen diffusion from outside to the interior of the coating is mitigated. Adding gas shrouding is an effective method for reducing the entrainment of ambient air into the plasma jet and the retained air content in the porosity of coatings. The decrease in oxide contents in the high temperature oxidation process during SAPS coatings is compared to APS coatings. Consequently, the high temperature oxidation resistance of the coatings prepared by SAPS is expected to be better than that one sprayed by APS. 4. Conclusion (a) For NiCrCoAlY particles, convective oxidation is the dominating oxidation mechanism at the center of the plasma jet at a distance of 55 mm from the torch nozzle exit; diffusion oxidation is the dominating mechanism when the spray distance is longer than 55 mm, and the degree of particle oxidation increases with the distance beyond the nozzle. (b) In-flight oxidation and after impact oxidation on substrate are two different mechanisms during the spray process. The dominant oxidation mechanism of APS sprayed NiCrCoAlY coating is in-flight oxidation.
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(c) Adding gas shrouding is an effective method for reducing the oxide inclusion in the coating and improving the particles’ melting, the density and the porosity content of the coatings can also be improved. It is shown that by adding gas shrouding, the oxidation resistance of the coatings during SAPS is better than that during APS. Acknowledgments This research was supported in part by the Science and Technology Development Project KM200610005026 and JP009012200803 funded by Beijing Municipal Education Commission, PR China, and this project was also partially supported by the National Natural Science Foundation of China (50805002). References [1] K. Volenik, J. Letnner, F. Hanousek, J. Dubsky, B. Kolman, Oxides in plasmasprayed chromium steel, J. Therm. Spray Technol. 6 (3) (1997) 327–334. [2] S. Deshpande, S. Sampath, H. Zhang, Mechanisms of oxidation and its role in microstructural evolution of metallic thermal spray coatings—case study for Ni–Al, Surf. Coat. Technol. 200 (2006) 5395–5406. [3] J. Alcala, F. Gaudette, S. Suresh, S. Sampath, Instrumented spherical microindentation of plasma-sprayed coatings, Mater. Sci. Eng. A316 (2001) 1–10. [4] R.A. Neiser, M.F. Smith, R.C. Dykhuizen, Oxidation in wire HVOF-sprayed steel, J. Therm. Spray Technol. 7 (4) (1998) 537–545. [5] U. Schulz, M. Menzebach, C. Leyens, Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems, Surf. Coat. Technol. 146 (2001) 117–123. [6] A.M. Limarga, S. Widjaja, T.H.L.K. Yip, Modeling of the effect of Al2O3 interlayer on residual stress due to oxide scale in thermal barrier coatings, Surf. Coat. Technol. 153 (2002) 16–24. [7] E.A.G. Shillington, D.R. Clarke, Spalling failure of a thermal barrier coating associated with aluminum depletion in the bond-coat, Acta Mater. 47 (1999) 1297–1305. [8] B.K. Pant, V. Arya, B.S. Mann, Development of low-oxide MCrAlY coatings for gas turbine applications, J. Therm. Spray Technol. 16 (2007) 275–280. [9] A.A. Syed, A. Denoirjean, P. Fauchais, J.C. Labbe, On the oxidation of stainless steel particles in the plasma jet, Surf. Coat. Technol. 200 (2006) 4368–4382. [10] K. Cheng, X. Chen, H.-X. Wang, W. Pan, Modeling study of shrouding gas effects on a laminar argon plasma jet impinging upon a flat substrate in air surroundings, Thin Solid Films 506 (2006) 724–728. [11] G. Espe, A. Denoirjean, P. Fauchais, J.C. Labbe, J. Dubsky, O. Schneeweiss, K. Volenik, In-flight oxidation of iron particles sprayed using gas and water stabilized plasma torch, Surf. Coat. Technol. 195 (2005) 17–28. [12] M.P. Planche, H. Liao, C. Coddet, Oxidation control in atmospheric plasma spraying coating, Surf. Coat. Technol. 202 (2007) 69–76.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Oxidation control in plasma spraying NiCrCoAlY coating Qi Wei ∗ , Zhiyong Yin, Hui Li The College of Materials Science and Engineering, Beijing University of Technology, 100 PingLeYuan, Beijing 100124, PR China
a r t i c l e
i n f o
Article history: Received 16 June 2011 Received in revised form 22 December 2011 Accepted 24 January 2012 Available online 2 February 2012 Key words: Plasma spray Convective oxidation Diffusible oxidation Oxidation resistance
a b s t r a c t Atmospheric plasma spraying is usually accompanied by oxidation reactions, which result in the formation of oxides in the coatings. The presence of oxides in metallic coatings is usually undesirable because they cause the coating properties to deteriorate. This study highlights how the high temperature oxidation resistance of plasma sprayed NiCrCoAlY coating is influenced by both the oxidation behavior of NiCrCoAlY particles and by the shrouding gas during the spray process. It is shown that two different oxidation mechanisms are involved in the in-flight oxidation. One is diffusion oxidation, and the other is convective oxidation. The convective oxidation of NiCrCoAlY particles is the dominating oxidation mechanism when the plasma jet is at a distance of 55 mm from the torch nozzle exit; while diffusion oxidation was found to be the dominant mechanism when the spray distance is greater than 55 mm. Oxidation mainly occurs during in-flight and after impact on the substrate. In-flight oxidation is the dominant mechanism for NiCrCoAlY particles in plasma spray. Adding inert-gas shrouding is an effective method for decreasing the oxide content of the NiCrCoAlY coating, which significantly increases the coating’s oxidation resistance. © 2012 Elsevier B.V. All rights reserved.
1. 1.Introduction The plasma spray process is carried out by feeding powders into a plasma torch where the material is melted and propelled as molten or semi-molten particles toward the substrate. The coating is built up on the substrate surface by successive impacts of molten metal droplets, originating from the powder particles fed into the plasma torch [1]. Generally, the plasma spray process is conducted in an ambient air environment. Turbulent flow is caused by the jet flow and the surrounding air with shearing force will inevitably lead to the entrainment of ambient air into the plasma jet involved in the oxidation of the sprayed metallic particles. Many studies have indicated that the oxidation of thermally sprayed materials can significantly influence the phase composition, microstructure, properties and therefore the performance of sprayed coatings [2]. Metal oxides are usually brittle, their thermal expansion coefficients are different from those of the base metal. The presence of the oxide in the coating disrupts the chemical uniformity. For example, oxide layer at interfaces being brittle can potentially reduce the strength and ductility of the deposit, which will lead to decrease in the corrosion resistance of the coating [3,4]. Therefore, experiments have been carried out to study the oxidation of NiCrCoAlY particles
∗ Corresponding author. Tel.: +86 10 67396168; fax: +86 10 67396168. E-mail address: [email protected] (Q. Wei). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.134
during the spray process, which are expected to have guiding significance to optimize the performance of the coating. For this reason, experiments have been carried out to study the oxidation of NiCrCoAlY particles during the spray process. The results of this testing are expected to be significant in optimizing the performance of the coating. Thermal barrier coatings (TBCs) are widely used in gas turbines to reduce thermal effect and increase turbine efficiency. A usual TBC includes a MCrAlY bond coat (M Ni and Co) as oxidation resistant layer and a ceramic top coat as thermal barrier layer. Application of TBC at high temperature causes the transfer of oxygen through the top coat toward the bond coat, so an oxidized scale can be formed on the bond coat which is termed the thermally grown oxide (TGO). Although this layer protects the substrate against further oxidation, the excessive growth of TGO during thermal cycling can lead to the failure of the ceramic top layer [5–7]. Low-pressure plasma spray (LPPS) or vacuum plasma spray (VPS) can obtain homogeneous and compact microstructure, and the present coatings were free of oxide inclusions; but the operational cost of the system goes up tremendously (the capital cost of a LPPS system is around ten times than that of APS system). The size of the workpieces is also limited by the vacuum chamber [8]. Adding a coaxial shrouding gas can effectively reduce the entrainment of ambient air into the plasma jet and the oxidation of metallic particles can be mitigated. This experiment takes the NiCrCoAlY particle as the object, and studies the particle’s oxidation mechanism in the
Q. Wei et al. / Applied Surface Science 258 (2012) 5094–5099
spray process in conjunction with the impact of the shielded gas on the high temperature oxidation resistance of the coating.
Table 1 Element content of points 1, 2 of Fig. 3 (wt%) determined by EDS. Element
2. Experimental The oxidation of the coating test specimen is cut from the Haynes 230 nickel-based super alloy, the test sample is 15 mm × 10 mm × 4 mm, the nominal composition of the alloy (wt%): Cr 22%, W 14%, Mo 2%, other Ni. Grit blasting was used prior to plasma spraying, and then the sample was cleaned ultrasonically in an acetone bath. The spraying material used for the experimentation was NiCrCoAlY powder (China Institute of Mining and Metallurgy) having particle size in the range 44–104 m, the main composition of the powder (wt%): C < 0.7%, O 0.95%, Y2 O3 1.5–2.0%, Co 3–5%, Al 4–6%, Cr 14–16%, other Ni. Metco 9M is employed to spray the powder and coating, the powder feeder is Metco 5MP. The parameters for the spray process are as follows: voltage and amperage were set to 75 V and 500 A, the main gas flow rate (Ar) was 45 L min−1 , the auxiliary gas flow rate (H2 ) was 10 L min−1 , powder feed rate was 35 g min−1 , spraying distances were 55, 75, and 120 mm. Spraying was conducted at the different spraying distances, and the sprayed particles were into a cylindrical particle collector which is thenfilled with argon gas of the Ø 130 mm × 650 mm. A schematic of the collection setup is shown in Fig. 1a. The particles entering the collector were quenched by argon jets flowing through three inlets, two fixed at the collector entrance and one introduced at the bottom, the argon gas flow rate is 60 L min−1 . The self-made additional shield spray nozzle was used to spray the powder and the coating (Fig. 1b). The flow rate of the argon used as protective gas is 65 L min−1 . The cross-section of the structure was observed, and the porosity of the coating was measured by image analysis software MATLAB. The microstructure and the composition were observed by FEI Quanta 200 SEM–EDS. The phase analysis of the coating with different oxidation time was conducted by BRUKER D8 Advance X-ray diffraction (XRD). LECO TC-436 oxygen–nitrogen analyzer was used to measure the oxygen content before and after deposition. The coating sample was oxidized at 1080 ◦ C for 100 h under the furnace oven, and all the surfaces of the samples were coated. Weighing the samples in the oxidation process intermittently, the oxide weight gain of the samples was measured by electro-optical balance, whose accuracy is 0.01 mg.
5095
O Al Cr Co Ni
Point 1 5 8 80 1 5
Point 2 2 13 17 1 66
3.1.2. Oxidation behavior of particle during in-flight Fig. 4a shows the surface morphology of particles at the spraying distance of 55 mm, and ripples can be found at the surface of the particle. In fluid flow conditions where kinematic viscosity ratio of surrounding gases to liquid particle (i = g /p ) is greater than 55 and relative Reynolds number (Re) is higher than 20, the particle surface is likely to attain velocity superior to the average particle velocity generating eddy current within the particle. These conditions are very probable for a liquid particle in the plasma core and convective movements within the liquid particle can therefore be expected. These movements lead to the mass transfer of the oxide
3. Results and discussion 3.1. The oxidation behavior of particles in plasma spray process 3.1.1. The microstructure of particles Fig. 2 shows the surface and the section morphology of the asreceived NiCrCoAlY powder. It can be seen that the shape of the powder is irregular (Fig. 2a), and a small amount of residual oxide exists which is dissolved oxygen in the metal on the preparation stage. The oxide can be found neither on the surface nor on the interior of the particles (Fig. 2b). The morphology of the sprayed powder is shown in Fig. 3. The powder appears spherical after spraying. The morphology on the typical cross-section of particles after spraying is shown in Fig. 3b. It can be seen that a layer of cap area is distributed over the surface of the particle, and a small amount of dark gray nodular exists in the interior of the particle. The cap area on the surface and nodular material are separately analyzed by EDS point analysis (see Fig. 3b points 1, 2), Obvious oxide can be seen on the surface and interior of the powder after spraying, which is the result of the reaction between the particles and the air involved in jet during the spray process (Table 1).
Fig. 1. (a) Schematic of in-flight particles collection setup (b) schematics of plasma torch equipped with shroud.
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Fig. 2. Micrographs of as-sprayed powder:(a) surface of particles and (b) cross section of particles.
Fig. 3. Micrographs of collected particles: (a) surface of particles collected at 55 mm and (b) cross section of particles collected at 120 mm.
Fig. 4. Micrographs of particles collected at different standoff distance: (a) at 55 mm and (b) at 75 mm.
and the absorbed oxygen from the particle’s surface to its interior. The particle surface is continuously renewed and fresh metal is always present that enhances the particle’s chemical reactivity [9]. For plasma spraying, convective oxidation can be the dominating oxidization process, if the plasma to liquid particle kinematic viscosity ratio g /p is 50–2000 and relative Re attains values superior to 20. The discovery from Fig. 2a makes clear the existence of this convection. Fig. 4a shows the morphology on the cross-section of particles at the spraying distance of 55 mm. Nodular structure of oxide appears in the interior of particles, but no significant oxide is found on the surface, indicating that convective oxidation is the primary oxidation mechanism of the particles in this case. Because the surface tension between the metal oxides and molten metals
is different, the oxides which enter into the interior of particles become nearly spherical independently. Fig. 4b shows the morphology on the cross-section of particles at the spraying distance of 75 mm. When spraying distance increases to 75 mm, a thin oxide layer also appears at the periphery of the particles in addition to an internal oxide. As the distance from particles to nozzle increased, the relative Reynolds number of particles gets smaller, the plasma viscosity also decreases rapidly, and the convection oxidation of particles will decrease gradually. The traditional diffusion oxidation will become the main source of particle oxidation until the convection oxidation stops. The oxides formed on the surface are no longer involved in the interior of the particles; however, oxide accumulates and solidifies as a thin oxide layer on particle’s surface.
Q. Wei et al. / Applied Surface Science 258 (2012) 5094–5099 Table 2 Oxygen content (wt%) of collected particles and coatings determined by oxygen–nitrogen analyzer. Spraying into collector (C) or onto substrate (S)
Shrouding gas
O (wt%)
C–Argon S S
– – Argon
2.17 2.31 1.35
Fig. 3b shows the morphology on the cross-section of particles at the spray distance of 120 mm. The oxide on the surface of particles is obviously thicker compared with the particles sprayed at a distance of 75 mm, indicating that the particles oxidize further in the interior, while the accumulation of oxide on the surface continues. The oxidizing species (such as O2 , O, O+ , OH, etc.) and reducing species (such as H2 , H, H+ , NH, N+ , etc.) exists in the plasma jet, and the plasma has strong oxidation activity away from the latter half of the spray nozzle [10]. Some studies [11] point out that the quantity of oxidizing species has reached 100 times higher than that of the reducing species 100 mm beyond the exit of the spray nozzle, the intensity of oxidizability of plasma jet is especially high at this time. The oxide present on the surface accumulates when the particle was placed in the high intensity of oxidizability of the jet flow at the distance of 75–120 mm. Analysis of the surface oxide in the section of the particles showed that the average thickness of the surface oxide is 1–2 m. Fig. 3b also shows that more oxide is formed on one side. Because of the relative velocity existing between the particles and the plasma jet, the molten metal which has low viscosity will be pushed to the back-surface of the particles, while the higherviscosity oxide is pushed to the front-surface of the particles. In general, oxide accumulates on one side of the particles which have high speed, high temperature and low viscosity. Fig. 5 shows the microstructure of coating at the spraying distance of 120 mm, the coating is a typical laminated structure, and some defects such as porosity and oxide exist in the coating. Coating lamellae are separated alternately by the black oxide strip which can be seen clearly from the local magnification, and a few dots of a black substance are distributed in the interior of flat layer. The result of EDS on two spots 1, 2 from Fig. 5 shows that they are both the oxide inclusions. The oxide in the interior of particles remains in the coating which forms the oxide inclusion in the interior of laminar on splat. 3.1.3. Oxidation after impact on substrate Table 2 shows the content of oxygen of the particles and the coating at the spraying distance of 120 mm. The oxygen content increases from 0.95 wt% to 2.17 wt% at the spraying distance of 120 mm, and the oxygen content in the coating is 2.31 wt%,
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Table 3 Porosities of the SAPS and APS coatings. Coatings Porosity (%)
SAPS bond coat 3.2
APS bond coat 4.3
therefore, the degree of oxidation of NiCrCoAlY particles during inflight is higher than that which is formed on substrate after impact. Particle oxidation can be classified as in flight oxidation, oxidation at deposition and after deposition: (1) oxidation during flight. Inflight oxidation depends strongly upon the amount of entrained air, particle temperature and nature and turbulence of plasma jet. (2) Oxidation at deposition. Oxidation occurs in this period when the particles and the substrate contact, spread, and flat forms the coating but are not covered by subsequent particle deposition. The oxidation of the particles can almost be neglected during this time, because the splats are covered by the identical position of other particles within 10–20 s after the deposition of splat [9]. (3) Oxidation on the substrate after deposition. Oxidation occurs after the coating starts to spread. The oxidation during this period of time is mainly due to high temperature, high oxygen content of the plasma jet constantly sweeping in the deposition of coating during the spray process. In the spray process, particle oxidation can be divided into oxidation during in-flight process and oxidation after impact on substrate, which depends on the spray technology and the nature of the spray powder. The results of this experiment show that the oxidation of NiCrCoAlY particles mainly occurs in the in-flight process phase of the plasma spraying process.
3.2. Effect of the gas shroud on coating properties 3.2.1. The microstructure of the coating Fig. 6 shows the morphology on the coating surface of the atmospheric plasma spraying (APS) and the shrouding atmospheric plasma spraying (SAPS). The surface of the APS coating is rougher than the SAPS coating. Adding gas shrouding can reduce the disturbance of the surrounding air, and the velocity and temperature of the particle can be increased [12]. The melting condition of particles are therefore improved, the unmolten and the semi-molten particles on the surface of SAPS coating are decreased, and the surface roughness of SAPS coating is smaller than that one sprayed by APS. Adding gas shrouding can reduce the oxidation (Table 2) and oxide inclusion on the coating of the spray process, and the spreading of splat can also be improved. Therefore, the porosity contents in the coating are decreased (Table 3); thus, the quality of the coating will be improved.
Fig. 5. Micrographs of coatings at 120 mm (a), (b).
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Fig. 6. Surface morphology of the as-sprayed coatings: (a), (c) APS coatings and (b), (d) SAPS coatings.
3.2.2. The high temperature oxidation resistance of coating The oxidizing kinetics curve for the SAPS and the APS of NiCrCoAlY coating is shown in Fig. 7, and the coating sample oxidized at 1080 ◦ C for 100 h under the furnace oven, and weighing the samples in the oxidation process intermittently. The result reveals that the two oxidation kinetics curves are relatively consistent with the parabolic law, the oxide contents increase quickly during the initial period, and the oxidizing kinetics curve of APS coating is higher
than that of the SAPS coating. Along with the oxidation time, the oxidation rate of the coatings becomes lower, and the oxidizing kinetics curve becomes more gradual. The kinetics curve shows that the oxidation resistance of SAPS coating is better than that of ASP coating. The XRD pattern of the oxidized coating of SAPS and APS NiCrCoAlY particles over a 24 h period is shown in Fig. 8. It can be seen
Fig. 7. Comparison of oxidation kinetics at 1080 ◦ C for the SAPS and APS bond coats.
Fig. 8. XRD patterns of SAPS and APS coatings after 1080 ◦ C for 24 h.
Q. Wei et al. / Applied Surface Science 258 (2012) 5094–5099
that ␣-Al2 O3 , Cr2 O3 and NiCr2 O4 oxide is the main phase on the oxidized surface of SAPS coating, while for APS coating Cr2 O3 and NiCr2 O4 oxide is the main phase after 1080 ◦ C. In this experiment, the aluminum contents of NiCrCoAlY alloy powder itself are 4–6%, and a part of the aluminum is lost during the oxidation process between particles and air of the APS. The number of unmolten or semi-molten particles on the surface of APS coating is much greater than that during SAPS, and the surface roughness in APS coating is also much larger. The coatings were oxidized seriously during the initial period of time at 1080 ◦ C in air. As the time increases, these particles are separated completely from the coating by the mixed oxidation products. Because of the aluminum which diffuses from the interior coating to the attached particles are obstructed by the early-formed oxide (such as Cr2 O3 and NiCr2 O4 ), the depletion of Al from the interior of particles appears more easily. The ␣-Al2 O3 coating is inhibited by the absence of aluminum supply on the surface of these insolated particles, but some other mixed oxides are generated (such as NiCr2 O4 ), and the oxidation weight gain rate of APS coating is increased. However, the whole melting situation of the particles in SAPS coating is better, and the porosity is decreased, so the oxygen diffusion from outside to the interior of the coating is mitigated. Adding gas shrouding is an effective method for reducing the entrainment of ambient air into the plasma jet and the retained air content in the porosity of coatings. The decrease in oxide contents in the high temperature oxidation process during SAPS coatings is compared to APS coatings. Consequently, the high temperature oxidation resistance of the coatings prepared by SAPS is expected to be better than that one sprayed by APS. 4. Conclusion (a) For NiCrCoAlY particles, convective oxidation is the dominating oxidation mechanism at the center of the plasma jet at a distance of 55 mm from the torch nozzle exit; diffusion oxidation is the dominating mechanism when the spray distance is longer than 55 mm, and the degree of particle oxidation increases with the distance beyond the nozzle. (b) In-flight oxidation and after impact oxidation on substrate are two different mechanisms during the spray process. The dominant oxidation mechanism of APS sprayed NiCrCoAlY coating is in-flight oxidation.
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(c) Adding gas shrouding is an effective method for reducing the oxide inclusion in the coating and improving the particles’ melting, the density and the porosity content of the coatings can also be improved. It is shown that by adding gas shrouding, the oxidation resistance of the coatings during SAPS is better than that during APS. Acknowledgments This research was supported in part by the Science and Technology Development Project KM200610005026 and JP009012200803 funded by Beijing Municipal Education Commission, PR China, and this project was also partially supported by the National Natural Science Foundation of China (50805002). References [1] K. Volenik, J. Letnner, F. Hanousek, J. Dubsky, B. Kolman, Oxides in plasmasprayed chromium steel, J. Therm. Spray Technol. 6 (3) (1997) 327–334. [2] S. Deshpande, S. Sampath, H. Zhang, Mechanisms of oxidation and its role in microstructural evolution of metallic thermal spray coatings—case study for Ni–Al, Surf. Coat. Technol. 200 (2006) 5395–5406. [3] J. Alcala, F. Gaudette, S. Suresh, S. Sampath, Instrumented spherical microindentation of plasma-sprayed coatings, Mater. Sci. Eng. A316 (2001) 1–10. [4] R.A. Neiser, M.F. Smith, R.C. Dykhuizen, Oxidation in wire HVOF-sprayed steel, J. Therm. Spray Technol. 7 (4) (1998) 537–545. [5] U. Schulz, M. Menzebach, C. Leyens, Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems, Surf. Coat. Technol. 146 (2001) 117–123. [6] A.M. Limarga, S. Widjaja, T.H.L.K. Yip, Modeling of the effect of Al2O3 interlayer on residual stress due to oxide scale in thermal barrier coatings, Surf. Coat. Technol. 153 (2002) 16–24. [7] E.A.G. Shillington, D.R. Clarke, Spalling failure of a thermal barrier coating associated with aluminum depletion in the bond-coat, Acta Mater. 47 (1999) 1297–1305. [8] B.K. Pant, V. Arya, B.S. Mann, Development of low-oxide MCrAlY coatings for gas turbine applications, J. Therm. Spray Technol. 16 (2007) 275–280. [9] A.A. Syed, A. Denoirjean, P. Fauchais, J.C. Labbe, On the oxidation of stainless steel particles in the plasma jet, Surf. Coat. Technol. 200 (2006) 4368–4382. [10] K. Cheng, X. Chen, H.-X. Wang, W. Pan, Modeling study of shrouding gas effects on a laminar argon plasma jet impinging upon a flat substrate in air surroundings, Thin Solid Films 506 (2006) 724–728. [11] G. Espe, A. Denoirjean, P. Fauchais, J.C. Labbe, J. Dubsky, O. Schneeweiss, K. Volenik, In-flight oxidation of iron particles sprayed using gas and water stabilized plasma torch, Surf. Coat. Technol. 195 (2005) 17–28. [12] M.P. Planche, H. Liao, C. Coddet, Oxidation control in atmospheric plasma spraying coating, Surf. Coat. Technol. 202 (2007) 69–76.