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Adhesion of YSZ suspension plasma-sprayed coating on smooth and thin substrates
Surface & Coatings Technology 205 (2010) 999–1003
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Adhesion of YSZ suspension plasma-sprayed coating on smooth and thin substrates R. Vert a,⁎, D. Chicot b, C. Dublanche-Tixier c, E. Meillot a, A. Vardelle c, G. Mariaux c a b c
CEA, DAM, Le Ripault, F-37260 MONTS, France SPCTS-UMR CNRS 6638, ENSIL, University of Limoges, Limoges, France LML-UMR CNRS, UMR 8107, F-59650 Villeneuve d'Ascq, France
a r t i c l e
i n f o
Available online 6 August 2010 Keywords: Plasma spraying Nanostructure Zirconium oxide Nano-indentation X-ray diffraction Vickers indentation cracking
a b s t r a c t The design concept of the gas-cooled fast reactor which is a 4th generation nuclear reactor, requires protective coatings able to operate at 850 °C and protect the underlying structure in case of sudden increase of the functional temperature up to 1250 °C and depressurization from 0.70 MPa to atmospheric pressure. The parts to be covered are made of 1-mm thick materials resistant to heat and erosion and exhibiting high mechanical properties at high temperatures, such as the Haynes® 230 nickel-based alloy. In this study, the use of suspension plasma spraying to manufacture zirconia coatings is explored. The spraying conditions were optimized for the elaboration of coatings on stainless steel AISI 304L substrates and then adapted for Haynes 230 substrates. A special attention was paid to coating adhesion that was investigated by using a Vickers indentation cracking method. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The gas-cooled fast reactor is one of the next generation nuclear energy systems selected by the Generation IV International Forum for further study [1]. It features a fast-neutron-spectrum, helium-cooled reactor and closed fuel cycle. The key challenges associated with this system concern, in particular, the development of materials capable of operating at temperatures of 850 °C and the design of features intended to overcome the consequences of using a high-pressure gas with poor thermal characteristics to cool down a core with a low thermal inertia during depressurization events [1]. A protective coating able to resist high temperatures, high-pressure variations and wear, could help to insulate the installation of the high temperature produced by the core. The material that has to be protected is a superalloy, like Inconel or Haynes, 1-mm thick for the sake of conception convenience. A ceramic coating 150–250-μm thick is thought to meet the specifications. The high adhesion of coating to substrate is a precondition for the application. Plasma spraying that is a widely-used deposition technique to elaborate ceramic coatings for thermal barrier use [2], is a potential candidate for the application. However, the adhesion of plasma-sprayed coatings is essentially controlled by the roughness and cleanness of the surface and, most of the plasma spray deposition procedures involve the roughening of the surface by grit-blasting prior to coating deposition in order to adapt the roughness of the surface to the size of the sprayed particles. This surface preparation
⁎ Corresponding author. E-mail address: [email protected] (R. Vert). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.090
stage brings about compressive stresses [3] that generally result in the deformation of thin substrates [4]. Other processes could be used to enhance the adhesion of coatings as the PROTAL® process [5], electric discharges machining [6,7] or plasma treatment [8,9] in order to clean the surface, modify its roughness or the wettability of the impacting liquid material that will form the coating [10]. However, the industrialisation of such processes is not completely finalized. An innovative spray process has been emerging since more than one decade with the suspension plasma spraying (SPS) process [11] that makes it possible to achieve the elaboration of coatings with fine particles (from nanoscale to a few micrometers) and their deposition on smooth surfaces (Ra b 1 μm) [12]. This process consists in mixing the fine particles with a carrier fluid and injecting the liquid, in the form of a spray or liquid jet, with a controlled velocity, into the plasma jet [13]. In this study, the use of the SPS process for manufacturing nanostructured coatings on thin Haynes and stainless steel substrates is investigated. A special attention is turned to the effect of the spray parameters on the adhesion of coatings. 2. Experimental procedure The specifications for the protective coating include stability up to 1250 °C, high toughness, coefficient of thermal expansion close to that of Haynes 230 (15.2 × 10− 6 m/m °C between 25 and 800 °C) and low thermal conductivity (b2.5 W m− 1 K− 1). Stabilized zirconia meets these specifications and has been chosen for the study. It is widelyused as thermal barrier coating material for gas turbine engine applications because of its low thermal conductivity, and its relatively high (compared to many other ceramics) thermal expansion
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coefficient (9.0 × 10− 6 m/m °C) close to that of nickel-based superalloys used for turbine blades (14.0 to 16.0 × 10− 6 m/m °C) [14]. 2.1. Materials Two zirconia powders were selected: a partially yttria stabilized zirconia (Y-PSZ) with 3 mol% Y2O3 and a fully yttria stabilized zirconia (YSZ) with 8 mol% Y2O3. The particle size of both powders ranged between 30 and 60 nm. These powders were provided by Inframat Corp. (Willington, CT, USA) and prepared via an aqueous chemical synthesis route. The powders were mixed with distilled water and ultrasonically and mechanically stirred to break up the agglomerates. The solid concentration in the suspension was 6% by weight. No dispersant was added to the suspension since a sedimentation test showed that the sedimentation time was by far higher than the spraying time. Yttria partially stabilized zirconia (Y-PSZ) with 3 mol% Y2O3 is now considered as a standard for many TBC applications and usually leads to coatings made of tetragonal zirconia (often referred to as non transformable zirconia). However, Scardi et al. [15] showed the effect of yttria content on the stress field in the coating and in particular the effect of yttria depletion from Y-PSZ matrix that led to the development of catastrophic stresses in service because of the important volume changes resulting from the phase transformation. So, a second set of experiments was carried out with yttria fully stabilized zirconia (YSZ) with 8 mol% Y2O3. Suspensions were sprayed onto Haynes 230 and AISI 304L stainless steel substrates. The latter were used to carry out the preliminary experiments for the optimization of the spray parameters. Both substrates were plates of 25 cm2, 1 mm thick and exhibited an average roughness (Ra) of about 0.5 μm. They were degreased by acetone and alcohol before deposition. The chemical composition of Haynes 230 by weight is: Ni 57%, Cr 22%, W 14%, Mo 2, Fe max 3%, and that of 304L is: Ni 8–12%, Cr 18–20%, Mn b 2%, Si b 1%, C b 0.03% and Fe balance. 2.2. Plasma spray conditions The suspension was injected in the plasma jet by using a mechanical feeder consisting of a pressurized reservoir in which the suspension is stored and forced through a precision nozzle 250 μm in internal diameter [16]. The diameter of the nozzle flow was about 250 μm [17]. The liquid stream might undergo a primary break-up caused by waves that form on the surface of the stream after it has travelled some distance from the nozzle outlet plane. In the conditions used for the study, the liquid injector was fixed on the torch nozzle in such a way that the liquid penetrated against the plasma stream prior to its natural fragmentation [18]. The plasma torch was a Sulzer-Metco F4-VB (Sulzer-Metco
AG, Wohlen, Switzerland). Table 1. shows the torch operating conditions used in this study. The substrate temperature, during the pre-heating phase before deposition and during deposition phase, was monitored by an infrared pyrometer (Modline 4, Ircon, 8–14 μm wavelength range) and a thermal sensor (K-type thermocouple, TC direct, France). The temperature of coating was controlled during the spray process by CO2 cryogenic cooling that made it possible to maintain the coating temperature at the temperature set point for temperatures equal or higher than 400 °C. However, for lower substrate pre-heating temperatures, the coating temperature increased gradually and reached 400 °C at the end of the deposition process for a 30-μm thick coating. Under these conditions, 100 passes were necessary to make a coating 30-μm thick. The methodology followed in this study can be described as the following: 1. Optimization of the plasma spraying conditions in order to get a nanostructured coating that exhibits good mechanical properties and also good cohesion and adhesion to a smooth substrate (Ra = 0.5 μm). These experiments were conducted on AISI 304L stainless steel substrate on account of the cost of Haynes 230 (about 8€ per kg for AISI 304L compared to 75€ per kg for Haynes 230) and its availability. The targeted coating thickness was about 30 μm as this nanostructured coating is thought to be used as a first layer for a thicker thermal barrier coating. The main function of this layer is to promote the adhesion of the TBC to the smooth and thin substrate. 2. Adaptation of the spraying conditions to the Haynes substrate. For this study, six sets of spraying parameters differing in liquid injector nozzle diameter, plasma torch nozzle diameter, plasmaforming gas composition and spraying distance were used to spray the Y-PSZ suspension on 304L substrates pre-heated at 200 °C that is at a temperature close to the transition temperature for the zirconia/stainless steel couple [19,20] (Table 1). The choice of these four parameters resulted from preliminary experiments and survey of literature [16,18,19,21]. The arc current was fixed at 700 A. Because of the application of coatings, the properties that were investigated at the end of the spraying stage were their architecture, phase composition and mechanical properties (hardness and Young Modulus). The latter was determined from nano-indentation tests (see Section 2.3.2). 2.3. Characterization techniques 2.3.1. Coating microstructure and architecture The microstructures of coating were observed by Scanning Electron Microscopy (SEM, Phillips XL30), either with the Secondary
Table 1 Operating parameters for manufacturing the coatings on AISI 304L substrates. Plasma torch type
F4-VB Sulzer Metco
Sample Anode internal diameter (mm) Plasma gas mixture Plasma gas flow rate (slpm) Arc current intensity (A) Plasma effective power (kW) Pre-heating temperature (°C) Injector exit diameter (μm) Injection pressure(MPa) Liquid velocity (m s−1) Gun traverse speed (m s−1) Spray distance (mm) Scanning step (mm)
a 8 Ar/He 45/45 700 20.0 ± 0.1 200 200 0.22 15.6 1.5 40 5
b 6 Ar/He 45/45
c 6 Ar/He/H2 45/45/3
d 6 Ar/He/H2 45/45/3
e 6 Ar/He/H2 45/45/3
f 6 Ar/He/H2 45/45/3
200 3.0 17.7
200 3.5 19.1
150 4.0 20.1
250 3.2 18.2
250 3.2 18.2
40
40
40
40
30
27.5 ± 0.2
R. Vert et al. / Surface & Coatings Technology 205 (2010) 999–1003
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Electron (SE) or Back-Scattered Electron (BSE) modes. The former mode enabled a higher resolution compared to the BSE mode (in the order of 100 nm compared to 50 nm) while the latter mode should enhance the contrast between the various phases present in coatings. The samples were prepared by standard metallographic methods. Phase analysis was carried out by XRD, using a Siemens D501 diffractometer with Cu-Kα radiation (λ = 1.5406 ) at an acquisition rate of 0.005°/s. 2.3.2. Mechanical properties The elastic modulus and hardness of coating were estimated by using nano-indentation test (Nano Indenter XP™, MTS Nano Instruments) with a Berkovich indenter that possesses a three-sided pyramidal tip. The properties were evaluated thanks to the Oliver and Pharr's analysis technique [22,23]. The instrument was operated in the continuous stiffness mode (CSM) allowing hardness and Young's modulus to be calculated for each data point acquired during the indentation experiment. The indentations were made under a constant nominal strain rate of 0.05 s− 1 and frequency of 45 Hz. The harmonic displacement was 2 nm. A Poisson's ratio of 0.25 was used to calculate the elastic modulus. The reported data correspond to an average value of twenty indents. They are representative of the mean values at a penetration depth of about 400 nm. 2.3.3. Adhesive properties The adhesive properties of coatings were evaluated from a Vickers indentation cracking (VIC) test [24] by using a specific technique that consisted in indenting the substrate with a Vickers indent. Analysis by indentation was performed on a cross-section of the coated material. After cutting the sample from the coating toward the substrate to avoid a possible coating delamination, the cross-section was polished with abrasive grade papers (ranging from 80 to 1200 mesh) in order to get mirror polishing by using 6-μm diamond paste. The indentation tests were performed on a macrohardness tester Wölpert using loads ranging between 10 and 200 N. According to the indentation conditions applied in the interfacial indentation test [25], the time of application of the maximum load was 2 minutes. The geometrical dimensions of the indent, the distance from the centre of the indent to the plan of the interface and the crack length were measured directly on the apparatus by means of its optical system. To characterize the adhesion of coating/substrate couples, more than 10 indentations were performed for each applied load at different distances from the interface in order to create various crack lengths. The indent generates a deformation of the substrate that can results in a crack at the interface between substrate and coating. The distance of the indentation to the interface which corresponds to the appearance of crack is thought to be representative of coating adhesion. In other words, for a given load condition, the coating, characterized by the shorter distance between indentation and interface for crack generation at interface, exhibits the higher adhesive properties.
Fig. 1. Variation of the hardness and Young modulus of the Y-PSZ coating with the spray conditions. The spray conditions a to f are defined in Table 1. Ref [26] corresponds to Y-PSZ TBC manufactured by APS.
technique [27]. Finally the selected spray conditions involved a ternary gas mixture of argon, helium and hydrogen, a plasma torch with a 6-mm nozzle, a liquid injector of 250 μm in diameter in order to limit the spraying time and possible clogging of the injector, and a spray distance of 40 mm. The latter was chosen to facilitate the spray procedure although the mechanical properties of the coatings deposited at this spray distance were slightly lower than that of coatings deposited at a standoff distance of 30 mm. A coating deposited under these conditions is shown in Fig. 2.
3.2. Y-PSZ coating on Haynes 230 substrate Y-PSZ coatings were elaborated on Haynes 230 substrates under the set of spray conditions e that was identified as the best parameter set for the sought-after coating properties. The resulting coating exhibited the same architecture, phase composition and mechanical properties but a weaker adhesion to substrate as it could be observed when using a fretsaw to cut the sample. It is now well established in thermal spraying that the coating adhesion depends to a great extent on substrate cleanness and temperature In particular, coating adhesion substantially increases if the substrate is pre-heated over a transition temperature [28] that corresponds to the desorption of absorbates and condensates and the formation of lenticular shape splats on the substrate. This operation could also bring about the formation of nanoscale roughness with a positive skewness (SK ~ 1) which induces a better contact between the splat and the substrate [29].
3. Results and discussions 3.1. Y-PSZ coating on AISI 304L substrate The variation of coating hardness and Young Modulus with the set of spraying conditions is shown in Fig. 1. The XRD analysis of coatings indicated that whatever were the spraying conditions, the Y-PSZ coatings crystallized in the tetragonal phase as expected. The parametric study showed that the “best properties” for hardness and elastic modulus (hardness of 8.7 GPa and elasticity modulus of 145 GPa) were achieved with a high-velocity plasma jet exhibiting also a high specific enthalpy and a short spraying distance (30 mm), The values of elasticity seems to be very high in survey of literature but seems to be characteristic of the nano-indentation
Fig. 2. SEM micrograph of Y-PSZ coating on AISI 304L substrate deposited under the spray set of parameters f (see Table 1).
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To determine the transition temperature for the Haynes 230zirconia couple, the sprayed particles were collected on mirrorpolished Haynes substrates pre-heated at different temperatures ranging from 25 °C to 400 °C while all the other spray conditions were kept constant (set of parameters e in Table 1). From the SEM
(scanning electron microscopy) observation of the material collected on the substrate [30], the transition temperature of the Haynes 230 substrate was estimated to be around 400 °C [30]. This estimation was confirmed by the evaluation of the adhesion of Y-PSZ coatings to Haynes substrates by using the Vickers indentation cracking (VIC) method that is well adapted to thin coatings. Fig. 3a and b shows the relationship between the distance of indentation and resulting crack length for loads ranging between 10 and 200 N. The distance of indentation when the crack appears at the interface is a “measure” of the adhesion of coating. Fig. 4 shows, in a bi-logarithmic representation, the variation of this distance with the applied load. It confirms that the adhesion of coatings was improved when the substrate was pre-heated at 400 °C. Indeed, it can be shown that the adhesive properties increase when the straight lines, characteristic of the cracking, are moved toward the right and down side of the graph. 3.3. YSZ coating on Haynes substrate YSZ coatings were elaborated on Haynes 230 substrates, preheated at 400 °C, under the set of spray conditions (e) of Table 1. No appreciable differences can be observed in the architecture, phase composition and mechanical properties of YSZ coatings in comparison to that of Y-PSZ coatings. Fig. 3c shows the relationship between the applied load, distance of indentation and resulting crack length. In the VIC test, the crack deviation into the coating is supposed to be representative of the cohesive properties of the coating [24] while the crack initiation at the interface is characteristic of the adhesive properties of coating. Crack deviation in coating is illustrated by a change in the slope of the straight lines as shown in Fig. 3c, the upper part with lower slope is representative of the crack deviation into the coating and the lower part with higher slope is representative of the crack through the interface [24]. The bi-logarithmic representation of the variation of indentation distance with the applied load, presented in Fig. 4, clearly indicates an improvement in the adhesion of YSZ coatings. 4. Conclusion • The elaboration by plasma spraying of P-YSZ nanostructured coating on AISI 304L substrate, and the optimization of the spraying parameters were explored. • The “best” spray parameters, used to elaborate coatings on AISI 304L substrate, were also used on Haynes 230 substrate, but resulted in
Fig. 3. a. Results of Vickers indentation cracking test performed on the P-YSZ/Haynes 230 coated system, pre-heated at 200 °C, using indentation loads varying between 10 and 200 N. b. Results of Vickers indentation cracking test performed on the P-YSZ/ Haynes 230 coated system, pre-heated at 400 °C, using indentation loads varying between 10 and 200 N. c. Results of Vickers indentation cracking performed on the YSZ/ Haynes 230 coated system, pre-heated at 400 °C, using indentation loads varying between 10 and 200 N.
Fig. 4. Representation of the adhesive indentation distance (distance of indentation where a crack appears at the interface), Zadh, as a function of the applied load, L, for the P-YSZ and YSZ/Haynes 230 coated systems.
R. Vert et al. / Surface & Coatings Technology 205 (2010) 999–1003
coatings exhibiting poor adhesion to Haynes substrate. Therefore, the transition temperature for the couple P-YSZ/Haynes 230 was determined and coating deposited on substrate whose pre-heating temperature was close to that transition temperature. • The adhesion properties of coatings manufactured on Haynes 230 substrates was studied by a Vickers indentation cracking (VIC) method that made it possible to compare the adhesion of the various coatings and confirmed the effect of the substrate temperature on adhesion. • Further work involves the measure of the residual stresses in the nanostructured coatings by various methods and the comparison of the adhesion values obtained by the VIC method with that obtained with other measurement methods. References [1] J. Bouchard, R. Bennett, Generation IV International Forum, Energy Focus Spring, 2009. [2] A. Feuerstein, J. Knapp, T. Taylor, A. Ashary, A. Bolcavage, N. Hitchman, J. Therm. Spray Technol. 17 (2) (2008). [3] H. Liao, P. Vaslin, Y. Yang, C. Coddet, J. Therm. Spray Technol. 6 (2) (1997) 235. [4] J. Patru (in French), Ph.D. thesis, University of Limoges, France, 2005. [5] G. Barbezat, F. Folio, C. Coddet, G. Montavon, Proceedings of the 1st International Thermal Spray Conference, Montréal, Québec, Canada, 2000. [6] A. Hasçalik, U. Caydas, Appl. Surf. Sci. 253 (22) (2007) 9007. [7] H. Ramasawmy, L. Blunt, J. Mater. Process. Technol. 148 (2) (2004) 155. [8] C.H. Yi, Y.H. Lee, G.Y. Yeom, C.H. Jeong, Y.W. Wook Ko, Surf. And Coat. Technol. 177–178 (2004) 711.
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[9] B. Kegel, H. Schmid, Surf. Coat. Technol. 112 (1–3) (1999) 63. [10] M.E. Schrader, J. Colloid Interface Sci. 100 (2) (1984) 372. [11] J.J. Karthikeyan, C.C. Berndt, S. Reddy, J.Y. Wang, A.H. King, H. Herman, J. Am. Ceram. Soc. 81 (1) (1998) 121–28. [12] G. Darut, F. Ben-Ettouil, A. Denoirjean, G. Montavon, H. Ageorges, P. Fauchais, J. Therm. Spray Technol. 19 (2010) 275. [13] K. Wittmann-Ténèze (in French), Ph.D. thesis, University of Limoges, France, 2001. [14] J. Matejicek, S. Sampath, P.C. Brand, H.J. Prask, Acta Mater. 47 (2) (1999) 607. [15] P. Scardi, M. Leoni, L. Bertamini, Surf. Coat. Technol. 76–77 (1995) 106. [16] K Wittmann-Ténèze, K. Vallé, L. Bianchi, F. Blein, P. Belleville, CEA Le Ripault, France, Fr patent n°04 52390, (2004). [17] C. Delbos (in French), Ph.D. thesis, University of Limoges, France, 2004. [18] A. Vardelle, A., C. Chazelas, C. Marchand, G. Mariaux, Pure Appl. Chem. 80 (9) (2008) 1981. [19] P. Fauchais, M. Vardelle, A. Vardelle, L. Bianchi, A.C. Leger, Plasma Chem. Plasma Process. 16 (1996) 99. [20] R. Etchart-Salas, P. Fauchais, V. Rat, J.F. Coudert, Adv. Sci. Technol. 45 (2006) 1163. [21] O. Tingaud, A. Bacciochini, G. Montavon, A. Denoirjean, P. Fauchais, Surf. Coat. Technol. 203 (15) (2009) 2157. [22] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [23] W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (2004) 3. [24] Vert, R., Decoopman, X., Gruescu, I.C., Meillot, E., Chicot, D., Vardelle, A., Mariaux, G., Submitted for publication in Thin Solid Films (2010). [25] P. Démarécaux, D. Chicot, J. Lesage, J. Mater. Sci. Lett. 15 (1996) 1377. [26] J.A. Thompson, T.W. Clyne, Acta Mater. 49 (2001) 1565. [27] M. Ahrens, S. Lampenscherf, R. Vaßen, D. Stöver, J. Therm. Spray Technol. 13 (3) (2004) 432. [28] L. Bianchi, A. Denoirjean, F. Blein, P. Fauchais, Thin Solid Films 299 (1997) 125. [29] J. Cedelle, M. Vardelle, P. Fauchais, Surf. Coat. Technol. 201 (3–4) (2006) 1378. [30] R. Vert, E. Meillot, A. Vardelle, G. Mariaux, D. Chicot, C. Dublanche-Tixier, Proceedings of ITSC 2010, Singapore, Republic of Singapore, May 3–5 2010.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Adhesion of YSZ suspension plasma-sprayed coating on smooth and thin substrates R. Vert a,⁎, D. Chicot b, C. Dublanche-Tixier c, E. Meillot a, A. Vardelle c, G. Mariaux c a b c
CEA, DAM, Le Ripault, F-37260 MONTS, France SPCTS-UMR CNRS 6638, ENSIL, University of Limoges, Limoges, France LML-UMR CNRS, UMR 8107, F-59650 Villeneuve d'Ascq, France
a r t i c l e
i n f o
Available online 6 August 2010 Keywords: Plasma spraying Nanostructure Zirconium oxide Nano-indentation X-ray diffraction Vickers indentation cracking
a b s t r a c t The design concept of the gas-cooled fast reactor which is a 4th generation nuclear reactor, requires protective coatings able to operate at 850 °C and protect the underlying structure in case of sudden increase of the functional temperature up to 1250 °C and depressurization from 0.70 MPa to atmospheric pressure. The parts to be covered are made of 1-mm thick materials resistant to heat and erosion and exhibiting high mechanical properties at high temperatures, such as the Haynes® 230 nickel-based alloy. In this study, the use of suspension plasma spraying to manufacture zirconia coatings is explored. The spraying conditions were optimized for the elaboration of coatings on stainless steel AISI 304L substrates and then adapted for Haynes 230 substrates. A special attention was paid to coating adhesion that was investigated by using a Vickers indentation cracking method. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The gas-cooled fast reactor is one of the next generation nuclear energy systems selected by the Generation IV International Forum for further study [1]. It features a fast-neutron-spectrum, helium-cooled reactor and closed fuel cycle. The key challenges associated with this system concern, in particular, the development of materials capable of operating at temperatures of 850 °C and the design of features intended to overcome the consequences of using a high-pressure gas with poor thermal characteristics to cool down a core with a low thermal inertia during depressurization events [1]. A protective coating able to resist high temperatures, high-pressure variations and wear, could help to insulate the installation of the high temperature produced by the core. The material that has to be protected is a superalloy, like Inconel or Haynes, 1-mm thick for the sake of conception convenience. A ceramic coating 150–250-μm thick is thought to meet the specifications. The high adhesion of coating to substrate is a precondition for the application. Plasma spraying that is a widely-used deposition technique to elaborate ceramic coatings for thermal barrier use [2], is a potential candidate for the application. However, the adhesion of plasma-sprayed coatings is essentially controlled by the roughness and cleanness of the surface and, most of the plasma spray deposition procedures involve the roughening of the surface by grit-blasting prior to coating deposition in order to adapt the roughness of the surface to the size of the sprayed particles. This surface preparation
⁎ Corresponding author. E-mail address: [email protected] (R. Vert). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.090
stage brings about compressive stresses [3] that generally result in the deformation of thin substrates [4]. Other processes could be used to enhance the adhesion of coatings as the PROTAL® process [5], electric discharges machining [6,7] or plasma treatment [8,9] in order to clean the surface, modify its roughness or the wettability of the impacting liquid material that will form the coating [10]. However, the industrialisation of such processes is not completely finalized. An innovative spray process has been emerging since more than one decade with the suspension plasma spraying (SPS) process [11] that makes it possible to achieve the elaboration of coatings with fine particles (from nanoscale to a few micrometers) and their deposition on smooth surfaces (Ra b 1 μm) [12]. This process consists in mixing the fine particles with a carrier fluid and injecting the liquid, in the form of a spray or liquid jet, with a controlled velocity, into the plasma jet [13]. In this study, the use of the SPS process for manufacturing nanostructured coatings on thin Haynes and stainless steel substrates is investigated. A special attention is turned to the effect of the spray parameters on the adhesion of coatings. 2. Experimental procedure The specifications for the protective coating include stability up to 1250 °C, high toughness, coefficient of thermal expansion close to that of Haynes 230 (15.2 × 10− 6 m/m °C between 25 and 800 °C) and low thermal conductivity (b2.5 W m− 1 K− 1). Stabilized zirconia meets these specifications and has been chosen for the study. It is widelyused as thermal barrier coating material for gas turbine engine applications because of its low thermal conductivity, and its relatively high (compared to many other ceramics) thermal expansion
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coefficient (9.0 × 10− 6 m/m °C) close to that of nickel-based superalloys used for turbine blades (14.0 to 16.0 × 10− 6 m/m °C) [14]. 2.1. Materials Two zirconia powders were selected: a partially yttria stabilized zirconia (Y-PSZ) with 3 mol% Y2O3 and a fully yttria stabilized zirconia (YSZ) with 8 mol% Y2O3. The particle size of both powders ranged between 30 and 60 nm. These powders were provided by Inframat Corp. (Willington, CT, USA) and prepared via an aqueous chemical synthesis route. The powders were mixed with distilled water and ultrasonically and mechanically stirred to break up the agglomerates. The solid concentration in the suspension was 6% by weight. No dispersant was added to the suspension since a sedimentation test showed that the sedimentation time was by far higher than the spraying time. Yttria partially stabilized zirconia (Y-PSZ) with 3 mol% Y2O3 is now considered as a standard for many TBC applications and usually leads to coatings made of tetragonal zirconia (often referred to as non transformable zirconia). However, Scardi et al. [15] showed the effect of yttria content on the stress field in the coating and in particular the effect of yttria depletion from Y-PSZ matrix that led to the development of catastrophic stresses in service because of the important volume changes resulting from the phase transformation. So, a second set of experiments was carried out with yttria fully stabilized zirconia (YSZ) with 8 mol% Y2O3. Suspensions were sprayed onto Haynes 230 and AISI 304L stainless steel substrates. The latter were used to carry out the preliminary experiments for the optimization of the spray parameters. Both substrates were plates of 25 cm2, 1 mm thick and exhibited an average roughness (Ra) of about 0.5 μm. They were degreased by acetone and alcohol before deposition. The chemical composition of Haynes 230 by weight is: Ni 57%, Cr 22%, W 14%, Mo 2, Fe max 3%, and that of 304L is: Ni 8–12%, Cr 18–20%, Mn b 2%, Si b 1%, C b 0.03% and Fe balance. 2.2. Plasma spray conditions The suspension was injected in the plasma jet by using a mechanical feeder consisting of a pressurized reservoir in which the suspension is stored and forced through a precision nozzle 250 μm in internal diameter [16]. The diameter of the nozzle flow was about 250 μm [17]. The liquid stream might undergo a primary break-up caused by waves that form on the surface of the stream after it has travelled some distance from the nozzle outlet plane. In the conditions used for the study, the liquid injector was fixed on the torch nozzle in such a way that the liquid penetrated against the plasma stream prior to its natural fragmentation [18]. The plasma torch was a Sulzer-Metco F4-VB (Sulzer-Metco
AG, Wohlen, Switzerland). Table 1. shows the torch operating conditions used in this study. The substrate temperature, during the pre-heating phase before deposition and during deposition phase, was monitored by an infrared pyrometer (Modline 4, Ircon, 8–14 μm wavelength range) and a thermal sensor (K-type thermocouple, TC direct, France). The temperature of coating was controlled during the spray process by CO2 cryogenic cooling that made it possible to maintain the coating temperature at the temperature set point for temperatures equal or higher than 400 °C. However, for lower substrate pre-heating temperatures, the coating temperature increased gradually and reached 400 °C at the end of the deposition process for a 30-μm thick coating. Under these conditions, 100 passes were necessary to make a coating 30-μm thick. The methodology followed in this study can be described as the following: 1. Optimization of the plasma spraying conditions in order to get a nanostructured coating that exhibits good mechanical properties and also good cohesion and adhesion to a smooth substrate (Ra = 0.5 μm). These experiments were conducted on AISI 304L stainless steel substrate on account of the cost of Haynes 230 (about 8€ per kg for AISI 304L compared to 75€ per kg for Haynes 230) and its availability. The targeted coating thickness was about 30 μm as this nanostructured coating is thought to be used as a first layer for a thicker thermal barrier coating. The main function of this layer is to promote the adhesion of the TBC to the smooth and thin substrate. 2. Adaptation of the spraying conditions to the Haynes substrate. For this study, six sets of spraying parameters differing in liquid injector nozzle diameter, plasma torch nozzle diameter, plasmaforming gas composition and spraying distance were used to spray the Y-PSZ suspension on 304L substrates pre-heated at 200 °C that is at a temperature close to the transition temperature for the zirconia/stainless steel couple [19,20] (Table 1). The choice of these four parameters resulted from preliminary experiments and survey of literature [16,18,19,21]. The arc current was fixed at 700 A. Because of the application of coatings, the properties that were investigated at the end of the spraying stage were their architecture, phase composition and mechanical properties (hardness and Young Modulus). The latter was determined from nano-indentation tests (see Section 2.3.2). 2.3. Characterization techniques 2.3.1. Coating microstructure and architecture The microstructures of coating were observed by Scanning Electron Microscopy (SEM, Phillips XL30), either with the Secondary
Table 1 Operating parameters for manufacturing the coatings on AISI 304L substrates. Plasma torch type
F4-VB Sulzer Metco
Sample Anode internal diameter (mm) Plasma gas mixture Plasma gas flow rate (slpm) Arc current intensity (A) Plasma effective power (kW) Pre-heating temperature (°C) Injector exit diameter (μm) Injection pressure(MPa) Liquid velocity (m s−1) Gun traverse speed (m s−1) Spray distance (mm) Scanning step (mm)
a 8 Ar/He 45/45 700 20.0 ± 0.1 200 200 0.22 15.6 1.5 40 5
b 6 Ar/He 45/45
c 6 Ar/He/H2 45/45/3
d 6 Ar/He/H2 45/45/3
e 6 Ar/He/H2 45/45/3
f 6 Ar/He/H2 45/45/3
200 3.0 17.7
200 3.5 19.1
150 4.0 20.1
250 3.2 18.2
250 3.2 18.2
40
40
40
40
30
27.5 ± 0.2
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Electron (SE) or Back-Scattered Electron (BSE) modes. The former mode enabled a higher resolution compared to the BSE mode (in the order of 100 nm compared to 50 nm) while the latter mode should enhance the contrast between the various phases present in coatings. The samples were prepared by standard metallographic methods. Phase analysis was carried out by XRD, using a Siemens D501 diffractometer with Cu-Kα radiation (λ = 1.5406 ) at an acquisition rate of 0.005°/s. 2.3.2. Mechanical properties The elastic modulus and hardness of coating were estimated by using nano-indentation test (Nano Indenter XP™, MTS Nano Instruments) with a Berkovich indenter that possesses a three-sided pyramidal tip. The properties were evaluated thanks to the Oliver and Pharr's analysis technique [22,23]. The instrument was operated in the continuous stiffness mode (CSM) allowing hardness and Young's modulus to be calculated for each data point acquired during the indentation experiment. The indentations were made under a constant nominal strain rate of 0.05 s− 1 and frequency of 45 Hz. The harmonic displacement was 2 nm. A Poisson's ratio of 0.25 was used to calculate the elastic modulus. The reported data correspond to an average value of twenty indents. They are representative of the mean values at a penetration depth of about 400 nm. 2.3.3. Adhesive properties The adhesive properties of coatings were evaluated from a Vickers indentation cracking (VIC) test [24] by using a specific technique that consisted in indenting the substrate with a Vickers indent. Analysis by indentation was performed on a cross-section of the coated material. After cutting the sample from the coating toward the substrate to avoid a possible coating delamination, the cross-section was polished with abrasive grade papers (ranging from 80 to 1200 mesh) in order to get mirror polishing by using 6-μm diamond paste. The indentation tests were performed on a macrohardness tester Wölpert using loads ranging between 10 and 200 N. According to the indentation conditions applied in the interfacial indentation test [25], the time of application of the maximum load was 2 minutes. The geometrical dimensions of the indent, the distance from the centre of the indent to the plan of the interface and the crack length were measured directly on the apparatus by means of its optical system. To characterize the adhesion of coating/substrate couples, more than 10 indentations were performed for each applied load at different distances from the interface in order to create various crack lengths. The indent generates a deformation of the substrate that can results in a crack at the interface between substrate and coating. The distance of the indentation to the interface which corresponds to the appearance of crack is thought to be representative of coating adhesion. In other words, for a given load condition, the coating, characterized by the shorter distance between indentation and interface for crack generation at interface, exhibits the higher adhesive properties.
Fig. 1. Variation of the hardness and Young modulus of the Y-PSZ coating with the spray conditions. The spray conditions a to f are defined in Table 1. Ref [26] corresponds to Y-PSZ TBC manufactured by APS.
technique [27]. Finally the selected spray conditions involved a ternary gas mixture of argon, helium and hydrogen, a plasma torch with a 6-mm nozzle, a liquid injector of 250 μm in diameter in order to limit the spraying time and possible clogging of the injector, and a spray distance of 40 mm. The latter was chosen to facilitate the spray procedure although the mechanical properties of the coatings deposited at this spray distance were slightly lower than that of coatings deposited at a standoff distance of 30 mm. A coating deposited under these conditions is shown in Fig. 2.
3.2. Y-PSZ coating on Haynes 230 substrate Y-PSZ coatings were elaborated on Haynes 230 substrates under the set of spray conditions e that was identified as the best parameter set for the sought-after coating properties. The resulting coating exhibited the same architecture, phase composition and mechanical properties but a weaker adhesion to substrate as it could be observed when using a fretsaw to cut the sample. It is now well established in thermal spraying that the coating adhesion depends to a great extent on substrate cleanness and temperature In particular, coating adhesion substantially increases if the substrate is pre-heated over a transition temperature [28] that corresponds to the desorption of absorbates and condensates and the formation of lenticular shape splats on the substrate. This operation could also bring about the formation of nanoscale roughness with a positive skewness (SK ~ 1) which induces a better contact between the splat and the substrate [29].
3. Results and discussions 3.1. Y-PSZ coating on AISI 304L substrate The variation of coating hardness and Young Modulus with the set of spraying conditions is shown in Fig. 1. The XRD analysis of coatings indicated that whatever were the spraying conditions, the Y-PSZ coatings crystallized in the tetragonal phase as expected. The parametric study showed that the “best properties” for hardness and elastic modulus (hardness of 8.7 GPa and elasticity modulus of 145 GPa) were achieved with a high-velocity plasma jet exhibiting also a high specific enthalpy and a short spraying distance (30 mm), The values of elasticity seems to be very high in survey of literature but seems to be characteristic of the nano-indentation
Fig. 2. SEM micrograph of Y-PSZ coating on AISI 304L substrate deposited under the spray set of parameters f (see Table 1).
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To determine the transition temperature for the Haynes 230zirconia couple, the sprayed particles were collected on mirrorpolished Haynes substrates pre-heated at different temperatures ranging from 25 °C to 400 °C while all the other spray conditions were kept constant (set of parameters e in Table 1). From the SEM
(scanning electron microscopy) observation of the material collected on the substrate [30], the transition temperature of the Haynes 230 substrate was estimated to be around 400 °C [30]. This estimation was confirmed by the evaluation of the adhesion of Y-PSZ coatings to Haynes substrates by using the Vickers indentation cracking (VIC) method that is well adapted to thin coatings. Fig. 3a and b shows the relationship between the distance of indentation and resulting crack length for loads ranging between 10 and 200 N. The distance of indentation when the crack appears at the interface is a “measure” of the adhesion of coating. Fig. 4 shows, in a bi-logarithmic representation, the variation of this distance with the applied load. It confirms that the adhesion of coatings was improved when the substrate was pre-heated at 400 °C. Indeed, it can be shown that the adhesive properties increase when the straight lines, characteristic of the cracking, are moved toward the right and down side of the graph. 3.3. YSZ coating on Haynes substrate YSZ coatings were elaborated on Haynes 230 substrates, preheated at 400 °C, under the set of spray conditions (e) of Table 1. No appreciable differences can be observed in the architecture, phase composition and mechanical properties of YSZ coatings in comparison to that of Y-PSZ coatings. Fig. 3c shows the relationship between the applied load, distance of indentation and resulting crack length. In the VIC test, the crack deviation into the coating is supposed to be representative of the cohesive properties of the coating [24] while the crack initiation at the interface is characteristic of the adhesive properties of coating. Crack deviation in coating is illustrated by a change in the slope of the straight lines as shown in Fig. 3c, the upper part with lower slope is representative of the crack deviation into the coating and the lower part with higher slope is representative of the crack through the interface [24]. The bi-logarithmic representation of the variation of indentation distance with the applied load, presented in Fig. 4, clearly indicates an improvement in the adhesion of YSZ coatings. 4. Conclusion • The elaboration by plasma spraying of P-YSZ nanostructured coating on AISI 304L substrate, and the optimization of the spraying parameters were explored. • The “best” spray parameters, used to elaborate coatings on AISI 304L substrate, were also used on Haynes 230 substrate, but resulted in
Fig. 3. a. Results of Vickers indentation cracking test performed on the P-YSZ/Haynes 230 coated system, pre-heated at 200 °C, using indentation loads varying between 10 and 200 N. b. Results of Vickers indentation cracking test performed on the P-YSZ/ Haynes 230 coated system, pre-heated at 400 °C, using indentation loads varying between 10 and 200 N. c. Results of Vickers indentation cracking performed on the YSZ/ Haynes 230 coated system, pre-heated at 400 °C, using indentation loads varying between 10 and 200 N.
Fig. 4. Representation of the adhesive indentation distance (distance of indentation where a crack appears at the interface), Zadh, as a function of the applied load, L, for the P-YSZ and YSZ/Haynes 230 coated systems.
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coatings exhibiting poor adhesion to Haynes substrate. Therefore, the transition temperature for the couple P-YSZ/Haynes 230 was determined and coating deposited on substrate whose pre-heating temperature was close to that transition temperature. • The adhesion properties of coatings manufactured on Haynes 230 substrates was studied by a Vickers indentation cracking (VIC) method that made it possible to compare the adhesion of the various coatings and confirmed the effect of the substrate temperature on adhesion. • Further work involves the measure of the residual stresses in the nanostructured coatings by various methods and the comparison of the adhesion values obtained by the VIC method with that obtained with other measurement methods. References [1] J. Bouchard, R. Bennett, Generation IV International Forum, Energy Focus Spring, 2009. [2] A. Feuerstein, J. Knapp, T. Taylor, A. Ashary, A. Bolcavage, N. Hitchman, J. Therm. Spray Technol. 17 (2) (2008). [3] H. Liao, P. Vaslin, Y. Yang, C. Coddet, J. Therm. Spray Technol. 6 (2) (1997) 235. [4] J. Patru (in French), Ph.D. thesis, University of Limoges, France, 2005. [5] G. Barbezat, F. Folio, C. Coddet, G. Montavon, Proceedings of the 1st International Thermal Spray Conference, Montréal, Québec, Canada, 2000. [6] A. Hasçalik, U. Caydas, Appl. Surf. Sci. 253 (22) (2007) 9007. [7] H. Ramasawmy, L. Blunt, J. Mater. Process. Technol. 148 (2) (2004) 155. [8] C.H. Yi, Y.H. Lee, G.Y. Yeom, C.H. Jeong, Y.W. Wook Ko, Surf. And Coat. Technol. 177–178 (2004) 711.
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