Suspension plasma sprayed composite coating using amorphous ...
Applied Surface Science 255 (2009) 5935–5938
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Suspension plasma sprayed composite coating using amorphous powder feedstock Dianying Chen a,*, Eric H. Jordan b, Maurice Gell a a b
Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, 97 N Eagleville Rd U-3136, Storrs, CT 06269, USA Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 August 2008 Received in revised form 12 December 2008 Accepted 13 January 2009 Available online 23 January 2009
Al2O3–ZrO2 composite coatings were deposited by the suspension plasma spray process using molecularly mixed amorphous powders. X-ray diffraction (XRD) analysis shows that the as-sprayed coating is composed of a-Al2O3 and tetragonal ZrO2 phases with grain sizes of 26 nm and 18 nm, respectively. The as-sprayed coating has 93% density with a hardness of 9.9 GPa. Heat treatment of the as-sprayed coating reveals that the Al2O3 and ZrO2 phases are homogeneously distributed in the composite coating. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Alumina Zirconia Suspension plasma spray Amorphous powders Nanocomposites
1. Introduction The thermal spray process has been widely used to deposit nanostructured coatings for industrial applications, including aerospace, pulp and paper, machinery, petroleum and petrochemical, biomedical, etc. [1]. Nanostructured coatings can have improved mechanical properties compared to that observed in conventional coatings [2–8]. Thermal spray ceramic coatings are usually made from a powder feedstock. Individual nanoparticles cannot be thermally sprayed using production powder feeders. These nanosized particles would clog the hoses and fittings that transport the powder particles from the powder feeder to the thermal spray torch [7,9]. To overcome this, reconstitution of individual nanoparticles into spherical micrometer-sized granules is necessary [2,3,8]. Recently, a suspension plasma spray (SPS) process has been developed for the deposition of nanostructured coatings [9–15]. In SPS, nanoparticles are dispersed in a solvent such as water or ethanol to form a suspension and then the suspension is injected into the plasma torch. The nanoparticles will melt and form the nanostructured coatings upon impacting the substrate. In both conventional and suspension plasma spray, crystalline nanosized powders are often used. However, the preparation of nanocrystalline powders often requires high temperature and long heat treatments and
* Corresponding author. Tel.: +1 860 486 2371; fax: +1 860 486 5088. E-mail address: [email protected] (D. Chen). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.038
therefore, increases the powder preparation cost. For example, Chandradass et al. [16] prepared zirconia doped alumina nanocrystalline powders at 1200 8C for 2 h. O et al. [17] synthesized aalumina nanopowders at 1150 8C for 3 h. In contrast, preparation of amorphous powders instead of crystalline ones requires low temperature and short heat treatment, and thus decreases the powder preparation cost. So far, there is no report on the SPS deposition of nanostructured coatings using amorphous feedstock. Alumina–zirconia composites have gained wide applications as structural ceramics or protective coatings due to their excellent mechanical and thermal properties [18–20]. In this research, a novel SPS process using amorphous powders as feedstock for the deposition of Al2O3–ZrO2 composite coatings is presented. 2. Experimental procedures Aluminum nitrate (Al(NO3)9H2O)3, powder, >97% (Alfa Aesar) and zirconium acetate (ZrO(OOCCH3)2, liquid, >99.9% (Inframat Corporation, Farmington, CT) were used as starting materials. Aluminum nitrate was dissolved in deionized water and then mixed with zirconium acetate based on molar volumes to produce a ceramic eutectic composition of Al2O3–40 wt%ZrO2. The obtained solution was heated at 80 8C and stirred continually to get the sol transformed into dried gel. To study the phase evolution of the solution precursor, the dried gel powders were then heated to various temperatures (750–1200 8C) at a heating rate of 10 8C/min, and then held for 2 h. The amorphous powders were prepared at 750 8C for 2 h.
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The suspension was prepared by mixing and ball-milling the amorphous powders in ethanol using ZrO2 balls with a loading rate of 50 wt%. The particle size and morphology of the as-prepared and ball-milled amorphous powders are shown in Section 3.1. Suspension plasma spray is similar to solution precursor plasma spray (SPPS); the schematic diagram can be found in previous publications [21–23]. The Al2O3–ZrO2 coatings were deposited using the direct current (DC) plasma torch (Metco 9MB, Sulzer Metco, Westbury, NY, USA), which was attached to a six-axis robotic arm. Argon and hydrogen are used as the primary and the secondary plasma gases, respectively. The suspension was delivered to the atomizing nozzle by a peristaltic pump with a flow rate of 20 ml/min. Air is used as the suspension atomizing gas. The coatings were deposited on the grit-blasted (Al2O3 grit of #30 mesh size) 304 stainless steel disk substrates (25 mm diameter, 3 mm thickness). Both differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) experiments were performed simultaneously on the as-dried precursor powders using a SDT-Q6000 thermal analyser (TA Inc., New Castle, DE). For each thermal analysis run, 30 mg of powder was placed in an Al2O3 crucible. The crystalline phase composition of all samples was determined using X-ray diffraction (XRD, Cu Ka radiation; D5005, Bruker AXS, Karlsruhe, Germany). The XRD patterns were collected in a 2u range from 208 to 808 with a scanning rate of 28/min. An environmental scanning electron microscope (ESEM 2020, Philips Electron Optics, Eindhoven, The Netherlands) and a JEOL JSM-6335F field emission scanning electron microscope (FESEM, Tokyo, Japan) were used to characterize the coating microstructure. Coating porosity was measured on the polished cross-section (500 magnification) by image analysis. The Vickers hardness of the as-sprayed coatings was measured on the polished crosssection with a 1.96 N normal load and a dwell time of 15 s. The hardness value for each sample is the average of 10 measurements. 3. Results and discussion
Fig. 2. Typical TG–DTA curves of dried precursor powders at a heating rate of 10 8C/ min in air.
The sample weight decreases with increasing temperature continuously from room temperature to 500 8C, and the total weight loss is about 44 wt%. The exothermic peaks at 277 8C and 353 8C can be attributed to the pyrolysis of the precursor, because a significant weight loss occurs in this temperature range. The exothermic DTA peak at 939 8C and 1287 8C can be ascribed to the crystallization of zirconia and alumina, respectively, as confirmed by XRD. Based on the XRD and TG–DTA analyses, the preparation temperature of the amorphous powders was chosen to be 750 8C. The XRD pattern of the as-prepared powders heat treated at 750 8C for 2 h is shown in Fig. 3; a large hump is observed at 308 and no crystalline peaks appear. These results indicate that the powders are amorphous. The as-prepared amorphous powders have an average particles size of about 40 mm (Fig. 4a). After ball-milling, the average particles size was reduced to 5 mm (Fig. 4b). The ballmilled powders were then used in the suspension plasma spray process.
3.1. Amorphous powder 3.2. Coatings The XRD patterns of the Al2O3–ZrO2 composite powders heated in the lab furnace at various temperatures are displayed in Fig. 1. It can be seen that at 800 8C, the composite powders are still amorphous. The zirconia crystalline peak in the composite powders begins to appear at 900 8C. a-Al2O3 was formed at 1200 8C. Fig. 2 shows the typical TG–DTA curves for the crystallization of as-dried precursor obtained at a heating rate of 10 8C/min in air.
Fig. 1. XRD patterns of as-calcined powders (Z: zirconia; a: a-Al2O3).
In the suspension plasma spray process, 20 coating scans were carried out. The thickness of the coating is approximately 250 mm. A typical polished cross-section of the Al2O3–ZrO2 coating is shown in Fig. 5. The coating is quite dense with measured porosity only 7% based on image analysis. The average hardness of the as-sprayed coating is 9.9 GPa. There are no coarse splat boundaries or layered structures in the as-sprayed SPS coatings using amorphous powders. These microstructural features are always present in
Fig. 3. XRD of as-prepared powders at 750 8C.
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Fig. 4. SEM of as-prepared amorphous powders at 750 8C (a) before and (b) after ball milling.
Fig. 7. Surface morphology of the as-sprayed coating.
Fig. 5. SEM micrograph of a polished cross-section of the as-sprayed coating.
the conventional APS and suspension plasma spray coatings [24,25]. The results indicate that Al and Zr are uniformly distributed in the as-sprayed coatings. Figs. 6 and 7 show the microstructure of single scan deposits and the representative surface morphology of the as-sprayed Al2O3– ZrO2 coating. The coating surface microstructure is very similar to that of the single scan deposits, which are mainly composed of ultrafine splats (1–5 mm) and dense fine spheres (100 nm). The
Fig. 6. Microstructure of deposits collected on room temperature substrate during single scan experiment.
ultrafine splats are quite similar to what is observed in APS coatings; however, the diameters of the splats in the suspension plasma sprayed coating from amorphous powders are much smaller than that in a conventional air plasma spray coatings (100–150 mm) [26] and is very similar to that reported by Fauchais in the suspension plasma spray coatings (1–5 mm) [15]. These splats and spherical particles indicate that the amorphous powders experience melting and solidification during coating formation. The XRD patterns (Fig. 8) show that the as-sprayed coatings are composed of a-alumina and tetragonal zirconia phases. The grain size of Al2O3 and ZrO2 determined by the Scherrer equation is 26 nm and 18 nm, respectively. Therefore, a nano-grained composite coating was produced.
Fig. 8. XRD pattern of the as-sprayed coating.
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Fig. 9. Coating microstructure following heat treatment at 1400 8C for 2 h showing the Al2O3 (black phase) and ZrO2 (bright phase) grains.
To reveal the Al2O3 and ZrO2 phase distribution in the coating, the coating was heat treated at 1400 8C for 2 h. Fig. 9 shows the coating surface microstructure following heat treatment at 1400 8C for 2 h. It can be seen that the splats disappear and evolve to grains after heat treatment. The Al2O3 (black phase) and ZrO2 (bright phase) grains are interpenetrated and distributed homogeneously. In the conventional air plasma spray and suspension plasma spray multicomponent ceramics coatings, layer structures are always observed due to the inhomogeneous distribution of phases in the original powders [24,25]. The suspension plasma spray process using amorphous powders as feedstock developed in the present research overcome this problem. It is an ideal process for the deposition of homogeneously distributed multicomponent nanostructured ceramics coatings. The homogeneously distributed phases in the composite coatings are derived from the molecularly mixed amorphous powders. It should be pointed out that using the amorphous powders as feedstock should not be limited to the suspension plasma spray; it can also be used in the conventional thermal spray process once the amorphous powders are agglomerated to flowable powders. 4. Conclusions Al2O3–ZrO2 nanocomposite hard coatings have been deposited by the suspension plasma spray process using amorphous powders feedstock. TG–DTA was used to determine the crystallization temperature of the precursor powders. The coating is mainly composed of ultrafine splats (1–5 mm) and dense fine spheres (100 nm). XRD analysis shows that the coating is composed of aalumina and tetragonal zirconia phases with grain sizes of 26 nm and 18 nm, respectively. The as-sprayed coating has 93% density with a hardness of 9.9 GPa. Heat treatment of the as-sprayed coating shows Al2O3 and ZrO2 phases are homogeneously distributed in the coating. The suspension plasma spray process using amorphous powders as feedstock is an ideal process for the deposition of homogeneously distributed multicomponent ceramics coatings. Acknowledgements This work is supported by a sub-contract from Raytheon Corporation, from a prime contract funded by DARPA/ONR. References [1] D. Mateyka, Plasma Spraying of Metallic and Ceramic Coatings, John Wiley & Sons, New York, 1989.
[2] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Development and implementation of plasma sprayed nanostructured ceramic coatings, Surface & Coatings Technology 146 (2001) 48–54. [3] E.H. Jordan, M. Gell, Y.H. Sohn, D. Goberman, L. Shaw, S. Jiang, M. Wang, T.D. Xiao, Y. Wang, P. Strutt, Fabrication and evaluation of plasma sprayed nanostructured alumina–titania coatings with superior properties, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 301 (1) (2001) 80–89. [4] R.S. Lima, B.R. Marple, Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO2 in comparison to air plasma-sprayed conventional Al2O3– 13TiO2, Journal of Thermal Spray Technology 14 (3) (2005) 397–404. [5] R.S. Lima, B.R. Marple, Enhanced ductility in thermally sprayed titania coating synthesized using a nanostructured feedstock, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 395 (1/2) (2005) 269–280. [6] R.S. Lima, B.R. Marple, From APS to HVOF spraying of conventional and nanostructured titania feedstock powders: a study on the enhancement of the mechanical properties, Surface & Coatings Technology 200 (11) (2006) 3428–3437. [7] R.S. Lima, B.R. Marple, Thermal spray coatings engineered from nanostructured ceramic agglomerated powders for structural, thermal barrier and biomedical applications: a review, Journal of Thermal Spray Technology 16 (1) (2007) 40–63. [8] L.L. Shaw, D. Goberman, R.M. Ren, M. Gell, S. Jiang, Y. Wang, T.D. Xiao, P.R. Strutt, The dependency of microstructure and properties of nanostructured coatings on plasma spray conditions, Surface & Coatings Technology 130 (1) (2000) 1–8. [9] Z. Chen, R.W. Trice, M. Besser, X.Y. Yang, D. Sordelet, Air-plasma spraying colloidal solutions of nanosized ceramic powders, Journal of Materials Science 39 (13) (2004) 4171–4178. [10] P. Fauchais, R. Etchart-Salas, C. Delbos, M. Tognonvi, V. Rat, J.F. Coudert, T. Chartier, Suspension and solution plasma spraying of finely structured layers: potential application to SOFCs, Journal of Physics D Applied Physics 40 (8) (2007) 2394–2406. [11] I. Burlacov, J. Jirkovsky, M. Muller, R.B. Heimann, Induction plasma-sprayed photocatalytically active titania coatings and their characterisation by microRaman spectroscopy, Surface & Coatings Technology 201 (1/2) (2006) 255–264. [12] R. Tomaszek, L. Pawlowski, L. Gengembre, J. Laureyns, Z. Znamirowski, J. Zdanowski, Microstructural characterization of plasma sprayed TiO2 functional coating with gradient of crystal grain size, Surface & Coatings Technology 201 (1/2) (2006) 45–56. [13] F.L. Toma, G. Bertrand, D. Klein, C. Coddet, C. Meunier, Nanostructured photocatalytic titania coatings formed by suspension plasma spraying, Journal of Thermal Spray Technology 15 (4) (2006) 587–592. [14] J.O. Berghaus, B. Marple, C. Moreau, Suspension plasma spraying of nanostructured WC-12Co coatings, Journal of Thermal Spray Technology 15 (4) (2006) 676–681. [15] P. Fauchais, V. Rat, U. Delbos, J.F. Coudert, T. Chartier, L. Bianchi, Understanding of suspension DC plasma spraying of finely structured coatings for SOFC, IEEE Transactions on Plasma Science 33 (2) (2005) 920–930. [16] J. Chandradass, J.H. Yoon, D.S. Bae, Synthesis and characterization of zirconia doped alumina nanopowder by citrate–nitrate process, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 473 (1/2) (2008) 360–364. [17] Y.T. O, S.W. Kim, D.C. Shin, Fabrication and synthesis of alpha-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), Colloids and Surfaces A Physicochemical and Engineering Aspects 313 (2008) 415–418. [18] J. Chevalier, A.H. De Aza, G. Fantozzi, M. Schehl, R. Torrecillas, Extending the lifetime of ceramic orthopaedic implants, Advanced Materials 12 (21) (2000) 1619. [19] J. Chevalier, S. Deville, G. Fantozzi, J.F. Bartolome, C. Pecharroman, J.S. Moya, L.A. Diaz, R. Torrecillas, Nanostructured ceramic oxides with a slow crack growth resistance close to covalent materials, Nano Letters 5 (7) (2005) 1297–1301. [20] A. Afrasiabi, M. Saremi, A. Kobayashi, A comparative study on hot corrosion resistance of three types of thermal barrier coatings: YSZ, YSZ + Al2O3 and YSZ/ Al2O3, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 478 (1/2) (2008) 264–269. [21] D. Chen, E.H. Jordan, M. Gell, Effect of solution concentration on splat formation and coating microstructure using the solution precursor plasma spray process, Surface & Coatings Technology 202 (10) (2008) 2132–2138. [22] D. Chen, E.H. Jordan, M. Gell, X. Ma, Dense alumina–zirconia coatings using the solution precursor plasma spray process, Journal of the American Ceramic Society 91 (2) (2008) 359–365. [23] M. Gell, L.D. Xie, X.Q. Ma, E.H. Jordan, N.P. Padture, Highly durable thermal barrier coatings made by the solution precursor plasma spray process, Surface & Coatings Technology 177 (2004) 97–102. [24] J.O. Berghaus, J.G. Legoux, C. Moreau, F. Tarasi, T. Chraska, Mechanical and thermal transport properties of suspension thermal-sprayed alumina–zirconia composite coatings, Journal of Thermal Spray Technology 17 (1) (2008) 91–104. [25] H.J. Kim, Y.J. Kim, Amorphous phase formation of the pseudo-binary Al2O3–ZrO2 alloy during plasma spray processing, Journal of Materials Science 34 (1) (1999) 29–33. [26] S. Sampath, X.Y. Jiang, J. Matejicek, A.C. Leger, A. Vardelle, Substrate temperature effects on splat formation, microstructure development and properties of plasma sprayed coatings. Part I. Case study for partially stabilized zirconia, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 272 (1) (1999) 181–188.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Suspension plasma sprayed composite coating using amorphous powder feedstock Dianying Chen a,*, Eric H. Jordan b, Maurice Gell a a b
Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, 97 N Eagleville Rd U-3136, Storrs, CT 06269, USA Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 August 2008 Received in revised form 12 December 2008 Accepted 13 January 2009 Available online 23 January 2009
Al2O3–ZrO2 composite coatings were deposited by the suspension plasma spray process using molecularly mixed amorphous powders. X-ray diffraction (XRD) analysis shows that the as-sprayed coating is composed of a-Al2O3 and tetragonal ZrO2 phases with grain sizes of 26 nm and 18 nm, respectively. The as-sprayed coating has 93% density with a hardness of 9.9 GPa. Heat treatment of the as-sprayed coating reveals that the Al2O3 and ZrO2 phases are homogeneously distributed in the composite coating. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Alumina Zirconia Suspension plasma spray Amorphous powders Nanocomposites
1. Introduction The thermal spray process has been widely used to deposit nanostructured coatings for industrial applications, including aerospace, pulp and paper, machinery, petroleum and petrochemical, biomedical, etc. [1]. Nanostructured coatings can have improved mechanical properties compared to that observed in conventional coatings [2–8]. Thermal spray ceramic coatings are usually made from a powder feedstock. Individual nanoparticles cannot be thermally sprayed using production powder feeders. These nanosized particles would clog the hoses and fittings that transport the powder particles from the powder feeder to the thermal spray torch [7,9]. To overcome this, reconstitution of individual nanoparticles into spherical micrometer-sized granules is necessary [2,3,8]. Recently, a suspension plasma spray (SPS) process has been developed for the deposition of nanostructured coatings [9–15]. In SPS, nanoparticles are dispersed in a solvent such as water or ethanol to form a suspension and then the suspension is injected into the plasma torch. The nanoparticles will melt and form the nanostructured coatings upon impacting the substrate. In both conventional and suspension plasma spray, crystalline nanosized powders are often used. However, the preparation of nanocrystalline powders often requires high temperature and long heat treatments and
* Corresponding author. Tel.: +1 860 486 2371; fax: +1 860 486 5088. E-mail address: [email protected] (D. Chen). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.038
therefore, increases the powder preparation cost. For example, Chandradass et al. [16] prepared zirconia doped alumina nanocrystalline powders at 1200 8C for 2 h. O et al. [17] synthesized aalumina nanopowders at 1150 8C for 3 h. In contrast, preparation of amorphous powders instead of crystalline ones requires low temperature and short heat treatment, and thus decreases the powder preparation cost. So far, there is no report on the SPS deposition of nanostructured coatings using amorphous feedstock. Alumina–zirconia composites have gained wide applications as structural ceramics or protective coatings due to their excellent mechanical and thermal properties [18–20]. In this research, a novel SPS process using amorphous powders as feedstock for the deposition of Al2O3–ZrO2 composite coatings is presented. 2. Experimental procedures Aluminum nitrate (Al(NO3)9H2O)3, powder, >97% (Alfa Aesar) and zirconium acetate (ZrO(OOCCH3)2, liquid, >99.9% (Inframat Corporation, Farmington, CT) were used as starting materials. Aluminum nitrate was dissolved in deionized water and then mixed with zirconium acetate based on molar volumes to produce a ceramic eutectic composition of Al2O3–40 wt%ZrO2. The obtained solution was heated at 80 8C and stirred continually to get the sol transformed into dried gel. To study the phase evolution of the solution precursor, the dried gel powders were then heated to various temperatures (750–1200 8C) at a heating rate of 10 8C/min, and then held for 2 h. The amorphous powders were prepared at 750 8C for 2 h.
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The suspension was prepared by mixing and ball-milling the amorphous powders in ethanol using ZrO2 balls with a loading rate of 50 wt%. The particle size and morphology of the as-prepared and ball-milled amorphous powders are shown in Section 3.1. Suspension plasma spray is similar to solution precursor plasma spray (SPPS); the schematic diagram can be found in previous publications [21–23]. The Al2O3–ZrO2 coatings were deposited using the direct current (DC) plasma torch (Metco 9MB, Sulzer Metco, Westbury, NY, USA), which was attached to a six-axis robotic arm. Argon and hydrogen are used as the primary and the secondary plasma gases, respectively. The suspension was delivered to the atomizing nozzle by a peristaltic pump with a flow rate of 20 ml/min. Air is used as the suspension atomizing gas. The coatings were deposited on the grit-blasted (Al2O3 grit of #30 mesh size) 304 stainless steel disk substrates (25 mm diameter, 3 mm thickness). Both differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) experiments were performed simultaneously on the as-dried precursor powders using a SDT-Q6000 thermal analyser (TA Inc., New Castle, DE). For each thermal analysis run, 30 mg of powder was placed in an Al2O3 crucible. The crystalline phase composition of all samples was determined using X-ray diffraction (XRD, Cu Ka radiation; D5005, Bruker AXS, Karlsruhe, Germany). The XRD patterns were collected in a 2u range from 208 to 808 with a scanning rate of 28/min. An environmental scanning electron microscope (ESEM 2020, Philips Electron Optics, Eindhoven, The Netherlands) and a JEOL JSM-6335F field emission scanning electron microscope (FESEM, Tokyo, Japan) were used to characterize the coating microstructure. Coating porosity was measured on the polished cross-section (500 magnification) by image analysis. The Vickers hardness of the as-sprayed coatings was measured on the polished crosssection with a 1.96 N normal load and a dwell time of 15 s. The hardness value for each sample is the average of 10 measurements. 3. Results and discussion
Fig. 2. Typical TG–DTA curves of dried precursor powders at a heating rate of 10 8C/ min in air.
The sample weight decreases with increasing temperature continuously from room temperature to 500 8C, and the total weight loss is about 44 wt%. The exothermic peaks at 277 8C and 353 8C can be attributed to the pyrolysis of the precursor, because a significant weight loss occurs in this temperature range. The exothermic DTA peak at 939 8C and 1287 8C can be ascribed to the crystallization of zirconia and alumina, respectively, as confirmed by XRD. Based on the XRD and TG–DTA analyses, the preparation temperature of the amorphous powders was chosen to be 750 8C. The XRD pattern of the as-prepared powders heat treated at 750 8C for 2 h is shown in Fig. 3; a large hump is observed at 308 and no crystalline peaks appear. These results indicate that the powders are amorphous. The as-prepared amorphous powders have an average particles size of about 40 mm (Fig. 4a). After ball-milling, the average particles size was reduced to 5 mm (Fig. 4b). The ballmilled powders were then used in the suspension plasma spray process.
3.1. Amorphous powder 3.2. Coatings The XRD patterns of the Al2O3–ZrO2 composite powders heated in the lab furnace at various temperatures are displayed in Fig. 1. It can be seen that at 800 8C, the composite powders are still amorphous. The zirconia crystalline peak in the composite powders begins to appear at 900 8C. a-Al2O3 was formed at 1200 8C. Fig. 2 shows the typical TG–DTA curves for the crystallization of as-dried precursor obtained at a heating rate of 10 8C/min in air.
Fig. 1. XRD patterns of as-calcined powders (Z: zirconia; a: a-Al2O3).
In the suspension plasma spray process, 20 coating scans were carried out. The thickness of the coating is approximately 250 mm. A typical polished cross-section of the Al2O3–ZrO2 coating is shown in Fig. 5. The coating is quite dense with measured porosity only 7% based on image analysis. The average hardness of the as-sprayed coating is 9.9 GPa. There are no coarse splat boundaries or layered structures in the as-sprayed SPS coatings using amorphous powders. These microstructural features are always present in
Fig. 3. XRD of as-prepared powders at 750 8C.
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Fig. 4. SEM of as-prepared amorphous powders at 750 8C (a) before and (b) after ball milling.
Fig. 7. Surface morphology of the as-sprayed coating.
Fig. 5. SEM micrograph of a polished cross-section of the as-sprayed coating.
the conventional APS and suspension plasma spray coatings [24,25]. The results indicate that Al and Zr are uniformly distributed in the as-sprayed coatings. Figs. 6 and 7 show the microstructure of single scan deposits and the representative surface morphology of the as-sprayed Al2O3– ZrO2 coating. The coating surface microstructure is very similar to that of the single scan deposits, which are mainly composed of ultrafine splats (1–5 mm) and dense fine spheres (100 nm). The
Fig. 6. Microstructure of deposits collected on room temperature substrate during single scan experiment.
ultrafine splats are quite similar to what is observed in APS coatings; however, the diameters of the splats in the suspension plasma sprayed coating from amorphous powders are much smaller than that in a conventional air plasma spray coatings (100–150 mm) [26] and is very similar to that reported by Fauchais in the suspension plasma spray coatings (1–5 mm) [15]. These splats and spherical particles indicate that the amorphous powders experience melting and solidification during coating formation. The XRD patterns (Fig. 8) show that the as-sprayed coatings are composed of a-alumina and tetragonal zirconia phases. The grain size of Al2O3 and ZrO2 determined by the Scherrer equation is 26 nm and 18 nm, respectively. Therefore, a nano-grained composite coating was produced.
Fig. 8. XRD pattern of the as-sprayed coating.
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Fig. 9. Coating microstructure following heat treatment at 1400 8C for 2 h showing the Al2O3 (black phase) and ZrO2 (bright phase) grains.
To reveal the Al2O3 and ZrO2 phase distribution in the coating, the coating was heat treated at 1400 8C for 2 h. Fig. 9 shows the coating surface microstructure following heat treatment at 1400 8C for 2 h. It can be seen that the splats disappear and evolve to grains after heat treatment. The Al2O3 (black phase) and ZrO2 (bright phase) grains are interpenetrated and distributed homogeneously. In the conventional air plasma spray and suspension plasma spray multicomponent ceramics coatings, layer structures are always observed due to the inhomogeneous distribution of phases in the original powders [24,25]. The suspension plasma spray process using amorphous powders as feedstock developed in the present research overcome this problem. It is an ideal process for the deposition of homogeneously distributed multicomponent nanostructured ceramics coatings. The homogeneously distributed phases in the composite coatings are derived from the molecularly mixed amorphous powders. It should be pointed out that using the amorphous powders as feedstock should not be limited to the suspension plasma spray; it can also be used in the conventional thermal spray process once the amorphous powders are agglomerated to flowable powders. 4. Conclusions Al2O3–ZrO2 nanocomposite hard coatings have been deposited by the suspension plasma spray process using amorphous powders feedstock. TG–DTA was used to determine the crystallization temperature of the precursor powders. The coating is mainly composed of ultrafine splats (1–5 mm) and dense fine spheres (100 nm). XRD analysis shows that the coating is composed of aalumina and tetragonal zirconia phases with grain sizes of 26 nm and 18 nm, respectively. The as-sprayed coating has 93% density with a hardness of 9.9 GPa. Heat treatment of the as-sprayed coating shows Al2O3 and ZrO2 phases are homogeneously distributed in the coating. The suspension plasma spray process using amorphous powders as feedstock is an ideal process for the deposition of homogeneously distributed multicomponent ceramics coatings. Acknowledgements This work is supported by a sub-contract from Raytheon Corporation, from a prime contract funded by DARPA/ONR. References [1] D. Mateyka, Plasma Spraying of Metallic and Ceramic Coatings, John Wiley & Sons, New York, 1989.
[2] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Development and implementation of plasma sprayed nanostructured ceramic coatings, Surface & Coatings Technology 146 (2001) 48–54. [3] E.H. Jordan, M. Gell, Y.H. Sohn, D. Goberman, L. Shaw, S. Jiang, M. Wang, T.D. Xiao, Y. Wang, P. Strutt, Fabrication and evaluation of plasma sprayed nanostructured alumina–titania coatings with superior properties, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 301 (1) (2001) 80–89. [4] R.S. Lima, B.R. Marple, Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO2 in comparison to air plasma-sprayed conventional Al2O3– 13TiO2, Journal of Thermal Spray Technology 14 (3) (2005) 397–404. [5] R.S. Lima, B.R. Marple, Enhanced ductility in thermally sprayed titania coating synthesized using a nanostructured feedstock, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 395 (1/2) (2005) 269–280. [6] R.S. Lima, B.R. Marple, From APS to HVOF spraying of conventional and nanostructured titania feedstock powders: a study on the enhancement of the mechanical properties, Surface & Coatings Technology 200 (11) (2006) 3428–3437. [7] R.S. Lima, B.R. Marple, Thermal spray coatings engineered from nanostructured ceramic agglomerated powders for structural, thermal barrier and biomedical applications: a review, Journal of Thermal Spray Technology 16 (1) (2007) 40–63. [8] L.L. Shaw, D. Goberman, R.M. Ren, M. Gell, S. Jiang, Y. Wang, T.D. Xiao, P.R. Strutt, The dependency of microstructure and properties of nanostructured coatings on plasma spray conditions, Surface & Coatings Technology 130 (1) (2000) 1–8. [9] Z. Chen, R.W. Trice, M. Besser, X.Y. Yang, D. Sordelet, Air-plasma spraying colloidal solutions of nanosized ceramic powders, Journal of Materials Science 39 (13) (2004) 4171–4178. [10] P. Fauchais, R. Etchart-Salas, C. Delbos, M. Tognonvi, V. Rat, J.F. Coudert, T. Chartier, Suspension and solution plasma spraying of finely structured layers: potential application to SOFCs, Journal of Physics D Applied Physics 40 (8) (2007) 2394–2406. [11] I. Burlacov, J. Jirkovsky, M. Muller, R.B. Heimann, Induction plasma-sprayed photocatalytically active titania coatings and their characterisation by microRaman spectroscopy, Surface & Coatings Technology 201 (1/2) (2006) 255–264. [12] R. Tomaszek, L. Pawlowski, L. Gengembre, J. Laureyns, Z. Znamirowski, J. Zdanowski, Microstructural characterization of plasma sprayed TiO2 functional coating with gradient of crystal grain size, Surface & Coatings Technology 201 (1/2) (2006) 45–56. [13] F.L. Toma, G. Bertrand, D. Klein, C. Coddet, C. Meunier, Nanostructured photocatalytic titania coatings formed by suspension plasma spraying, Journal of Thermal Spray Technology 15 (4) (2006) 587–592. [14] J.O. Berghaus, B. Marple, C. Moreau, Suspension plasma spraying of nanostructured WC-12Co coatings, Journal of Thermal Spray Technology 15 (4) (2006) 676–681. [15] P. Fauchais, V. Rat, U. Delbos, J.F. Coudert, T. Chartier, L. Bianchi, Understanding of suspension DC plasma spraying of finely structured coatings for SOFC, IEEE Transactions on Plasma Science 33 (2) (2005) 920–930. [16] J. Chandradass, J.H. Yoon, D.S. Bae, Synthesis and characterization of zirconia doped alumina nanopowder by citrate–nitrate process, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 473 (1/2) (2008) 360–364. [17] Y.T. O, S.W. Kim, D.C. Shin, Fabrication and synthesis of alpha-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), Colloids and Surfaces A Physicochemical and Engineering Aspects 313 (2008) 415–418. [18] J. Chevalier, A.H. De Aza, G. Fantozzi, M. Schehl, R. Torrecillas, Extending the lifetime of ceramic orthopaedic implants, Advanced Materials 12 (21) (2000) 1619. [19] J. Chevalier, S. Deville, G. Fantozzi, J.F. Bartolome, C. Pecharroman, J.S. Moya, L.A. Diaz, R. Torrecillas, Nanostructured ceramic oxides with a slow crack growth resistance close to covalent materials, Nano Letters 5 (7) (2005) 1297–1301. [20] A. Afrasiabi, M. Saremi, A. Kobayashi, A comparative study on hot corrosion resistance of three types of thermal barrier coatings: YSZ, YSZ + Al2O3 and YSZ/ Al2O3, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 478 (1/2) (2008) 264–269. [21] D. Chen, E.H. Jordan, M. Gell, Effect of solution concentration on splat formation and coating microstructure using the solution precursor plasma spray process, Surface & Coatings Technology 202 (10) (2008) 2132–2138. [22] D. Chen, E.H. Jordan, M. Gell, X. Ma, Dense alumina–zirconia coatings using the solution precursor plasma spray process, Journal of the American Ceramic Society 91 (2) (2008) 359–365. [23] M. Gell, L.D. Xie, X.Q. Ma, E.H. Jordan, N.P. Padture, Highly durable thermal barrier coatings made by the solution precursor plasma spray process, Surface & Coatings Technology 177 (2004) 97–102. [24] J.O. Berghaus, J.G. Legoux, C. Moreau, F. Tarasi, T. Chraska, Mechanical and thermal transport properties of suspension thermal-sprayed alumina–zirconia composite coatings, Journal of Thermal Spray Technology 17 (1) (2008) 91–104. [25] H.J. Kim, Y.J. Kim, Amorphous phase formation of the pseudo-binary Al2O3–ZrO2 alloy during plasma spray processing, Journal of Materials Science 34 (1) (1999) 29–33. [26] S. Sampath, X.Y. Jiang, J. Matejicek, A.C. Leger, A. Vardelle, Substrate temperature effects on splat formation, microstructure development and properties of plasma sprayed coatings. Part I. Case study for partially stabilized zirconia, Materials Science and Engineering A Structural Materials Properties Microstructure and Processing 272 (1) (1999) 181–188.
