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Microstructure and mechanical properties of plasma sprayed alumina-based coatings
Author's Accepted Manuscript
Microstructure and mechanical properties of plasma sprayed alumina-based coatings_R1 G. Di Girolamo, A. Brentari, C. Blasi, E. Serra
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)00698-1 http://dx.doi.org/10.1016/j.ceramint.2014.04.143 CERI8511
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
26 March 2014 15 April 2014 27 April 2014
Cite this article as: G. Di Girolamo, A. Brentari, C. Blasi, E. Serra, Microstructure and mechanical properties of plasma sprayed alumina-based coatings_R1, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.04.143 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of plasma sprayed alumina-based coatings_R1 G. Di Girolamoa,*, A. Brentarib, C. Blasic, E. Serraa a
ENEA, Materials Technology Unit, Casaccia Research Center, Rome, Italy
b
ENEA, Materials Technology Unit, Faenza Research Center, Faenza, Italy
c
ENEA, Materials Technology Unit, Brindisi Research Center, Brindisi, Italy
* corresponding author: [email protected] Abstract Alumina-based coatings are employed in many industrial applications, in order to protect the surface of metal components against high temperature, wear, corrosion and erosion. In this work two different alumina-based coatings were fabricated by atmospheric plasma spraying (APS), starting from powder particles composed of pure Al2O3 and Al2O3-3 wt.% TiO2, respectively. Their phase composition was investigated by X-Ray Diffraction (XRD) and revealed that both the assprayed coatings were mainly composed of metastable - and -Al2O3 phases. The phase recrystallized to phase after heat treatment. The porous microstructure was analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Thin TiO2-rich splats were observed within the microstructure of alumina-titania coatings. The pure alumina coatings exhibited similar porosity and higher microhardness than the alumina-titania ones (12.8 against 9.9 GPa). Both the coatings herein analyzed are particularly promising for high-temperature anti-wear applications, because of their enhanced mechanical properties.
Keywords plasma spraying; coatings; alumina; microstructure
1
1 Introduction Ceramic coatings are commonly employed for thermal and environmental protection of metal components operating at severe working conditions [1]. Their application is able to improve the resistance and the durability of the underlying components, thus reducing the replacement of worn out parts and the related idle times. Among them Al2O3 coatings are good candidates for anti-wear and anti-corrosion applications, due to their high hardness, chemical inertness and high melting point, as well as to their high resistance to abrasion and erosion [2]. Alumina coatings can retain up to 90 % of their strength at 1100 °C [3]. The addition of titania (3, 13 and 40 wt. %) allows to increase their toughness and then their resistance to wear and erosion [4,5]. For example, aluminatitania coatings are used to cover the surface of turbine and jet engine parts (blades and seal seats), whose durability is strongly affected by thermal loading, hot combustion products and erosion by dust and sand. Moreover, they can be applied on hydraulic system components (butterfly valves, pump shafts, pump seats, rocket nozzles, bearings), textile manufacturing industry tools (thread guiding and distribution rollers, ridge thread brakes), integrated circuits, orthopaedic and dental implants [1,6,7,8]. Plasma spraying is a cost-effective technique to deposit thick coatings on metal substrates starting from micronsized powders [9,10]. In such process the powder particles are injected into a hightemperature plasma jet, melted and accelerated toward the substrate. When impacting on the substrate surface, they are flattened and quenched, thus producing the build-up of a coating with a layered microstructure containing typical defects such as pores with different size, splat boundaries and microcracks. The high deposition rate of the plasma spraying process involves shorter manufacturing times and lower costs in comparison with the most expensive electron beam physical vapor deposition (EB-PVD) process. The aim of this work is to manufacture very hard alumina-based coatings for anti-wear applications and to investigate the effect of TiO2 addition on their microstructure and microhardness. 2
Therefore, alumina and alumina-titania coatings were deposited by atmospheric plasma spraying and characterized about their phase composition and microstructure using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), respectively. The microhardness of the coatings was also measured by indentation tests.
2 Material and methods 2.1 Plasma Spraying Thick coatings (300-450 m) were deposited by atmospheric plasma spraying (APS) equipped with F4-MB plasma gun (Sulzer Metco, Wolhen, Switzerland) with 6 mm internal diameter nozzle. Stainless steel plates (25 x 25 x 4 mm3) were sand blasted with alumina powder particles (Metcolite F, Sulzer Metco, Westbury, NY, USA) to increase their surface roughness and the mechanical interlocking between coating and substrate. Then they were mounted on a rotating sample holder and coated. Two different commercial powder feedstocks, herein named alumina and alumina-titania, were used. The former was composed of pure Al2O3 particles (Metco 105 NS, Westbury, NY, USA), whereas the latter was an Al2O3-3TiO2 powder (Amdry 187, Sulzer Metco, Westbury, USA) with the following chemical composition: 94 Al2O3, 1 SiO2, 1 Fe2O3, 3 TiO2, 1 T.A.O. (wt.%). The process parameters used for plasma spraying are summarized in Table 1. The arc current and the carrier gas flow have been set by using an empirical method, based on previous experiments performed on mullite [11] and alumina coatings, with the purpose to guarantee good powder flowability and good degree of melting, i.e. a powder feed rate close to 30 g/min for both the powders and a thickness per torch pass close to 10 μm. Higher arc current was used to melt coarser Al2O3-3TiO2 powder particles. The thickness per torch pass, measured after deposition, was 11 and 7 μm for alumina and alumina-titania coatings, respectively. The substrates were preheated by three 3
consecutive torch passes on their surface (current = 400 A, stand-off distance = 100 mm) and then were cooled by two air jets during deposition. 2.2 Characterization
The phase composition of powder feedstocks and coatings was investigated using an X-Ray powder Diffractometer (XRD, PW 1880, Philips, Almelo, Netherlands) with CuK (=0.154186 nm) radiation source produced at 40 kV and 40 mA. The analyzed range of the diffraction angle -2 was from 20 to 80°, by step width of 0.02° and 5 s time per step.
The surface roughness of the substrate and the as-sprayed coatings was measured using a 3D optical surface profilometer (New View 5000 system, Zygo Corporation, Connecticut, USA). This whitelight interferometer system allows to study the topography of the coating surface, which was previously metallized by Al thin film deposition (thickness = 40 nm), in order to allow reflection. The measurements were performed on areas of 0.7 x 0.5 μm2. The interferogram of the coating surface was then processed and transformed to three-dimensional high-resolution surface image by frequency domain analysis. The morphology of the powder particles was studied by Field Emission Gun Scanning Electron Microscopy (SEM-FEG, Leo Gemini mod. 1530, Carl Zeiss, Oberkochen, Germany) equipped with high-resolution secondary electron detector (in-lens detector). The measured resolution of this SEM was less than 0.9 nm at 20 kV – 1mm of working distance. The microstructure of as-sprayed coatings was investigated by Scanning Electron Microscopy (SEM, Gemini NVision 40, Carl Zeiss, Oberkochen, Germany) equipped with Energy Dispersive Spectroscopy (EDS, Oxford Link ISIS 300). To the purpose, the coatings were cut by low-speed diamond saw and their cross sections were polished to 0.25 m. The related SEM pictures were then processed by image analysis software
4
(Image J, U.S. National Institutes of Health, Bethesda, USA) to calculate the porosity level and the pore distribution. Microhardness measurements were carried out at room temperature on the polished cross sections of the as-sprayed coatings, using a Vickers microindenter (VMHT MOT, Leica Microsystems, Weztlar, Germany). The indentations were performed at load of 2.94 N for a dwell time of 15 s. The spacing between the indentations was kept at least three times the diagonal to avoid further stresses produced by the interaction between consecutive indentations. The microhardness data were then analyzed by Weibull statistics. 3 Results and discussion 3.1 Phase composition and microstructure As shown in Fig. 1, the alumina powder is composed of pure -Al2O3 (corundum) phase, while the as-sprayed coating is composed of both - and -Al2O3 phases. The presence of the broad hump between 25 and 40° in the related pattern suggests the formation of an amorphous phase at lesser extent. The phase is associated to unmelted sprayed powder particles or to the molten particles cooled at lower rate, while the nucleation of the metastable phase from the liquid occurred due to the high quenching rate of the molten splats at the substrate surface [12].The metastable phase exhibits broad peaks, suggesting fine crystallite size. It tends to recrystallize to -Al2O3 after isothermal treatment at 1100 °C for 2 h. The phases have been identified on the basis of the Joint Committee on Powder Diffraction Standards (JCPDS) available at International Centre for Diffraction Data (No. 10-0425 for -Al2O3 and No. 43-1484 for -Al2O3). As displayed in Fig. 2 the alumina-titania powder is composed of -Al2O3 phase, while the assprayed coating is composed of both - and -Al2O3 phases. The presence of an amorphous phase can be also noticed, as previously observed for pure Al2O3 coating. In alumina-titania coating a higher - to -Al2O3 ratio can be observed in comparison with pure alumina one. The metastable
5
phase tends to recrystallize to -Al2O3 after isothermal treatment at 1100 °C for 2 h. Some peaks for rutile-TiO2 can be also observed after heat treatment (card No. 88-1175). As displayed in Fig. 3, the alumina powder is composed of fused and crushed particles with blocky and angular morphology, and with size ranging from 5 to 55 μm, while the alumina-titania powder is composed of coarser particles with blocky and angular morphology, whose size was in the range 25 to 135 m. The measured substrate roughness was found to be 6.95 ± 1.13 m. In turn, the surface roughness Ra was found to be 6.19 ± 1.04 m and 8.03 ± 0.58 m for alumina and alumina-titania coatings, respectively. The melting and flattening of coarser alumina-titania particles with broader size distribution determined better intersplat bonding as well as higher surface roughness of the corresponding coatings in comparison with pure alumina ones. As shown in Fig. 4a, the cross section of the as-sprayed alumina coating exhibits a typical porous microstructure, characterized by pores with different size. Figure 4b shows some pores, interlamellar microcracks (denoted by white arrows) and intralamellar microcracks (denoted by black arrows) embedded in coating microstructure. Partially melted particles are also observable in some areas. By analyzing the cross sectional SEM microstructure in Fig. 5, it can be seen that the aluminatitania coating exhibits a lamellar and relatively dense microstructure with embedded splat boundaries and pores with different size. The results of pore distribution analysis are reported in Fig. 6. The splat boundaries, highlighted by white arrows in Fig. 5b, derived from solidification of the deposited splats during coating build-up. In turn the globular pores were originated by gas entrapped and filling defects during impact and solidification of the molten splats at the surface. The release of thermal stresses during quenching also promoted the formation of vertical microcracks ranging from 0.01 to 0.5 μm, in turn denoted by black arrow in Fig. 5b. 6
Globular porosity includes large circular and elongated pores (d > 3 μm) and more fine pores (d < 3 μm). The latter derived from gas entrapped under the liquid droplets and are homogeneously distributed, while the coarse pores are associated with filling defects caused by unmelted or semimolten particles and are not equally distributed. Some of them were determined by pull-out of unmelted particles during polishing step. EDS analysis was performed to determine the atomic composition of the different phases found within the microstructure. Some examples of EDS spectra are reported in Fig. 7. According to the EDS analysis dissolved TiO2 rich splat-like particles can be detected within the matrix. The thickness of these lamellae was in the range from sub-micron to a few micrometers. These areas are characterized by Ti segregation, lower content of Al and higher content of Ti in comparison with the matrix. This segregation has been also noticed in previous works [13,14]. The spot areas indicated in Fig. 5b highlight areas with different composition. Spot 1 highlights a splat-like particle composed of Ti and O and embedded in the Al2O3 matrix. The spot 2 shows an area composed of O, Al, Si and Fe, while the area denoted by spot 3 characterizes the Al2O3 matrix with Fe traces. The presence of Si and Fe is related to the chemical composition of the starting powder feedstock. Figure 8 shows the EDS map of the cross section of alumina-titania coating, showing the presence of titania-rich particles embedded in the microstructure. Traces of Ti, Si and Fe can be also detected. As reported in Table 2, the porosity value of the alumina coatings was found to be 7.1 %, while the porosity of the alumina-titania coatings was equal to 6.7 %. These porosity values are not quite different than those obtained by other investigators using the same powder feedstock, the same spraying distance and coating thickness (6.7-8.7 %) [15]. Moreover, it should be also noted that higher arc current was employed to melt coarser alumina-titania powder particles, in order to fabricate coatings with porosity similar to that of pure alumina ones. 7
3.2 Microhardness The microhardness data of both the coatings were broadly distributed due to the presence of microstructural defects such as pores, splat boundaries and microcracks, so that the Weibull statistics were employed. Figure 9 shows the Weibull plots for the microhardness data. The Weibull modulus (m), reflecting the scatter in the data distribution, was 12.6 and 9.2 for alumina and alumina-titania coatings, respectively. As reported in Table 2, the average microhardness of plasma sprayed alumina coating was 12.8 GPa (S.D. = 1.2), higher than the values reported in previous works [15] which were in the range 9.310.6 GPa, and were obtained using the same spraying distance and coating thickness. In turn, the average microhardness of plasma sprayed alumina-titania coating was 9.9 GPa (S.D. = 1.3). Values in the range 5.8-7.6 GPa have been obtained by other investigators by studying similar coatings deposited at lower spraying distance [16]. To this purpose, it is worth noting that higher spraying distance increases the residence time of the sprayed particles in the plasma jet, allowing better melting and thus involving higher coating microhardness. It has been also reported that with increasing the percentage of TiO2 the hardness decreased [17]. For example, Sanchez et al. reported values of 7.6 GPa for plasma sprayed Al2O3-13TiO2 coatings tested at 1.96 N [18]. In a recent work nanostructured alumina and alumina-titania coatings have been manufactured by APS and High Velocity Oxygen Fuel (HVOF), respectively, and have been characterized about their mechanical properties [19]. The microhardness of pure Al2O3 coatings was of about 10.8-11 GPa, whereas the hardness of Al2O3-13TiO2 coatings, produced starting from fused and crushed powder particles and tested at 2.94 N, was in the range 10.5-10.7 GPa.
8
Therefore, in the present work very hard alumina-based coatings were produced, even if a decrease in microhardness was noticed (~ 23%) when 3 wt.% TiO2 was present in the starting powder feedstock. Because of their microstructural and mechanical properties the plasma-sprayed alumina-based coatings herein studied are also promising as oxygen barrier layers in TBC systems, with the purpose to extend their lifetime, by reducing the oxygen diffusion and the oxidation at the top coat/bond coat interface (much TBC materials are transparent to oxygen and prone to hightemperature phase changes) as well as by reducing the attack promoted by erosive and corrosive media [20,21,22]. 4 Conclusions Very hard Al2O3 and Al2O3-3TiO2 coatings were deposited by atmospheric plasma spraying. Higher plasma power was employed to melt coarser alumina-titania particles with broad size distribution. Larger particles tended to increase the surface roughness of the as-produced coatings. Both the coatings were mainly composed of - and -Al2O3 phases. The presence of an amorphous phase was also noticed, resulting from the high quenching rate of the molten droplets at the substrate surface. The heat treatment of the as-sprayed coatings at 1100 °C produced the formation of a crystalline structure, mainly composed of -Al2O3 phase. Both the coatings exhibited a porous microstructure with embedded pores, splat boundaries and microcracks. The fine pores and microcracks were homogeneously distributed, while the coarse pores, some of them derived from pull-out effects, were not homogeneouly distributed within the microstructure. Alumina-titania coating also contained splat-like particles enriched in Ti, Fe and Si with respect to pure alumina one. The porosity level was found to be 7.1 % for the alumina coating and 6.7 % for the aluminatitania one. The microhardness of the alumina-titania coating was lower than that of pure alumina one (9.9 against 12.8 GPa).
9
In conclusion, very hard alumina-based coatings for high-temperature anti-wear applications were manufactured by plasma spraying. The presence of TiO2 and other impurities in the starting powder feedstock resulted in lower microhardness.
10
References
[1] J.R. Davis, Handbook of Thermal Spray Technology, ASM International: Materials Park, OH, 2004. [2] D.I. Pantelis, P. Psyllaki, N. Alexopoulos, Tribological behaviour of plasma-sprayed Al2O3 coatings under severe wear conditions, Wear 237 (2000) 197-204. [3] E. Celik, I. Ozdemir, E. Avci, Y. Tsunekawa, Corrosion behavior of plasma sprayed coatings, Surf. Coat. Technol. 193 (2005) 297-302. [4] Y. Liu, T.E. Fischer, A. Dent, Comparison of HVOF and plasma-sprayed alumina-titania coatings—microstructure, mechanical properties and abrasion behavior , Surf. Coat. Technol. 167 (2003) 68-76. [5] M. Wang, L.L. Shaw, Effects of the powder manufacturing method on microstructure and wear performance of plasma sprayed alumina–titania coatings, Surf. Coat. Technol. 202 (2007) 34-44. [6] P.P. Bandyopadhyay, D. Chicot, B. Venkateshwarlu, V. Racherla, X. Decoopman, J. Lesage, Mechanical properties of conventional and nanostructured plasma sprayed alumina coatings, Mech. Mater. 53 (2012) 61-71. [7] L. Pawlowski, The relationship between strcuture and dielectric properties in plasma sprayed alumina coatings, Surf. Coat. Technol. 35 (1988) 285-298. [8] M. Khalid, M. Mujahid, A.N. Khan, R.S. Rawat, I. Salam, K. Mahmood, Plasma sprayed alumina coatings on Ti6Al4V alloy for orthopaedic implants: microstructure and phase analysis, Key Eng. Mater. 510-511 (2012) 547-553. [9] O. Knotek, Thermal spraying and detonation gun process, in: R.F. Bunshah (Ed.), Handbook of Hard Coating, Noyes Publications, New Yersey, 2001, pp. 77-107. [10] R.B. Heimann, Applications of plasma-sprayed ceramic coatings, Key Eng. Mater. 122-124 (1996) 399-442.
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[11] G. Di Girolamo, C. Blasi, L. Pilloni, M. Schioppa, Microstructural and thermal properties of plasma sprayed mullite coatings, Ceram. Int. 36 (2010) 1389-1395. [12] R. McPherson, On the Formation of Thermally Sprayed Alumina Coatings, J. Mater. Sci. 15 (1980) 3141–3149. [13] E. Sanchez, E. Bannier, V. Cantavella, M.D. Salvador, E. Klyatskina, J. Morgiel, J. Grzonka, A.R. Boccaccini, Deposition of Al2O3–TiO2 Nanostructured Powders by Atmospheric Plasma Spraying, J. Therm. Spray Technol. 17 (2008) 329–337. [14] X. Lin, Y. Zeng, S. W. Lee, C. Ding, Characterization of Alumina –3 wt% Titania Coating Prepared by Plasma Spraying of Nanostructured Powders, J. Eur. Ceram. Soc. 24 (2004) 627–634. [15] O. Sarikaya, Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process, Surf. Coat. Technol. 190 (2005) 388-393. [16] A.R.M. Sahab, N.H. Saad, S. Kasolang, J. Saedon, Impact of plasma spray variables parameters on mechanical and wear behaviour of plasma sprayed Al2O3 3% wt TiO2 coating in abrasion and erosion application, Proc. Eng. 41 (2012) 1689-1695. [17] R.A. Zatorski, H. Herman, High performance ceramic films and coatings, Elsevier, Amsterdam, 1991, pp. 591-601. [18] E. Sanchez, E. Bannier, M. Vincent, A. Moreno, M.D. Salvador, V. Bonache, E. Klyatskina, A.R. Boccaccini, Characterization of nanostructured ceramic and cermet coatings deposited by plasma spraying, Int. J. Appl. Ceram. Technol. 8 (2011) 1136-1146. [19] V. Matikainen, K. Niemi, H. Koivuluoto, P, Vuoristo, Abrasion, erosion and cavitation erosion wear properties of thermally sprayed alumina based coatings, Coatings 4 (2014) 18-36. [20] J. Mueller, D. Neuschuetz, Efficiency of -Al2O3 as diffusion barrier between bond coat and
bulk material of gas turbine blades, Vacuum 71 (2003) 247-251 [21] M. Saremi, A. Afrasiabi , A. Kobayashi, Microstructural Analysis of YSZ and YSZ/Al2O3
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Plasma Sprayed Thermal Barrier Coatings after High Temperature Oxidation, Surf. Coat. Technol. 202 (2008) 3233–3238. [22] M. Nejati, M.R. Rahimipour, I. Mobasherpour, Evaluation of hot corrosion behavior of CSZ, CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings, Ceram. Int. 40 (2014) 4579-4590.
13
Alumina
Alumina-titania
Current [A]
500
600
Substrate
2083
2083
4
4
40
38
10
9
100
100
5
3
27
28
5
6
2
1.8
90
90
tangential speed [mm/s] Gun velocity [mm/s] Primary gas Ar flow rate [slpm*] Secondary gas H2 flow rate [slpm*] Stand-off distance [mm] Carrier gas flow rate [slpm] Powder feed rate [g/min] Distance torchinjector [mm] Injector diameter [mm] Injector angle [°]
Table 1 – Plasma spraying parameters used in this work. * slpm – standard litres per minute.
14
Coating
Average porosity [%]
Microhardness [GPa]
Al2O3
7.1 ± 1.1
12.8 ± 1.2
Al2O3-3TiO2
6.7 ± 0.8
9.9 ± 1.3
Table 2 – Porosity and microhardness values for alumina and alumina-titania coatings.
Figure Captions Figure 1 – XRD patterns of a) powder, b) as-sprayed and c) heat treated (1100 °C) alumina coating. Figure 2 – XRD patterns of a) powder, b) as-sprayed and c) heat treated (1100 °C) alumina-titania coating. Figure 3 – SEM morphology of alumina and alumina-titania powder particles, respectively. Figure 4 – Cross sectional SEM microstructure of plasma sprayed alumina coating. Fig. 4b shows pores, interlamellar microcracks (white arrows) and intralamellar microcrcaks (black arrows). Figure 5 – Cross sectional SEM microstructure of alumina-titania coating at different magnification. Fig.5b denotes pores, splat boundaries, microcracks and areas with different elemental composition. Figure 6 - Image analysis procedure for separating coating microstructural defects: (a) original greyscale image, (b) binary image showing total porosity, (c) large pores (d > 3 m), (d) fine porosity, (e) globular fine pores (d < 3 m) and (f) traces of interlamellar and intralamellar cracks. Figure 7 - Spot EDS spectra taken from different areas in coating microstructure. Figure 8 – EDS map of alumina-titania coating cross section. Figure 9 – Weibull plots for microhardness data referred to plasma sprayed alumina and aluminatitania coatings.
16
Figure 1
Figure 2
Figure 3a
Figure 3b
Figure 4a
Figure 4b
Figure 5a
Figure 5b
Figure 6
Figure 7
Figure 8
Figure 9
Microstructure and mechanical properties of plasma sprayed alumina-based coatings_R1 G. Di Girolamo, A. Brentari, C. Blasi, E. Serra
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)00698-1 http://dx.doi.org/10.1016/j.ceramint.2014.04.143 CERI8511
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
26 March 2014 15 April 2014 27 April 2014
Cite this article as: G. Di Girolamo, A. Brentari, C. Blasi, E. Serra, Microstructure and mechanical properties of plasma sprayed alumina-based coatings_R1, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.04.143 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of plasma sprayed alumina-based coatings_R1 G. Di Girolamoa,*, A. Brentarib, C. Blasic, E. Serraa a
ENEA, Materials Technology Unit, Casaccia Research Center, Rome, Italy
b
ENEA, Materials Technology Unit, Faenza Research Center, Faenza, Italy
c
ENEA, Materials Technology Unit, Brindisi Research Center, Brindisi, Italy
* corresponding author: [email protected] Abstract Alumina-based coatings are employed in many industrial applications, in order to protect the surface of metal components against high temperature, wear, corrosion and erosion. In this work two different alumina-based coatings were fabricated by atmospheric plasma spraying (APS), starting from powder particles composed of pure Al2O3 and Al2O3-3 wt.% TiO2, respectively. Their phase composition was investigated by X-Ray Diffraction (XRD) and revealed that both the assprayed coatings were mainly composed of metastable - and -Al2O3 phases. The phase recrystallized to phase after heat treatment. The porous microstructure was analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Thin TiO2-rich splats were observed within the microstructure of alumina-titania coatings. The pure alumina coatings exhibited similar porosity and higher microhardness than the alumina-titania ones (12.8 against 9.9 GPa). Both the coatings herein analyzed are particularly promising for high-temperature anti-wear applications, because of their enhanced mechanical properties.
Keywords plasma spraying; coatings; alumina; microstructure
1
1 Introduction Ceramic coatings are commonly employed for thermal and environmental protection of metal components operating at severe working conditions [1]. Their application is able to improve the resistance and the durability of the underlying components, thus reducing the replacement of worn out parts and the related idle times. Among them Al2O3 coatings are good candidates for anti-wear and anti-corrosion applications, due to their high hardness, chemical inertness and high melting point, as well as to their high resistance to abrasion and erosion [2]. Alumina coatings can retain up to 90 % of their strength at 1100 °C [3]. The addition of titania (3, 13 and 40 wt. %) allows to increase their toughness and then their resistance to wear and erosion [4,5]. For example, aluminatitania coatings are used to cover the surface of turbine and jet engine parts (blades and seal seats), whose durability is strongly affected by thermal loading, hot combustion products and erosion by dust and sand. Moreover, they can be applied on hydraulic system components (butterfly valves, pump shafts, pump seats, rocket nozzles, bearings), textile manufacturing industry tools (thread guiding and distribution rollers, ridge thread brakes), integrated circuits, orthopaedic and dental implants [1,6,7,8]. Plasma spraying is a cost-effective technique to deposit thick coatings on metal substrates starting from micronsized powders [9,10]. In such process the powder particles are injected into a hightemperature plasma jet, melted and accelerated toward the substrate. When impacting on the substrate surface, they are flattened and quenched, thus producing the build-up of a coating with a layered microstructure containing typical defects such as pores with different size, splat boundaries and microcracks. The high deposition rate of the plasma spraying process involves shorter manufacturing times and lower costs in comparison with the most expensive electron beam physical vapor deposition (EB-PVD) process. The aim of this work is to manufacture very hard alumina-based coatings for anti-wear applications and to investigate the effect of TiO2 addition on their microstructure and microhardness. 2
Therefore, alumina and alumina-titania coatings were deposited by atmospheric plasma spraying and characterized about their phase composition and microstructure using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), respectively. The microhardness of the coatings was also measured by indentation tests.
2 Material and methods 2.1 Plasma Spraying Thick coatings (300-450 m) were deposited by atmospheric plasma spraying (APS) equipped with F4-MB plasma gun (Sulzer Metco, Wolhen, Switzerland) with 6 mm internal diameter nozzle. Stainless steel plates (25 x 25 x 4 mm3) were sand blasted with alumina powder particles (Metcolite F, Sulzer Metco, Westbury, NY, USA) to increase their surface roughness and the mechanical interlocking between coating and substrate. Then they were mounted on a rotating sample holder and coated. Two different commercial powder feedstocks, herein named alumina and alumina-titania, were used. The former was composed of pure Al2O3 particles (Metco 105 NS, Westbury, NY, USA), whereas the latter was an Al2O3-3TiO2 powder (Amdry 187, Sulzer Metco, Westbury, USA) with the following chemical composition: 94 Al2O3, 1 SiO2, 1 Fe2O3, 3 TiO2, 1 T.A.O. (wt.%). The process parameters used for plasma spraying are summarized in Table 1. The arc current and the carrier gas flow have been set by using an empirical method, based on previous experiments performed on mullite [11] and alumina coatings, with the purpose to guarantee good powder flowability and good degree of melting, i.e. a powder feed rate close to 30 g/min for both the powders and a thickness per torch pass close to 10 μm. Higher arc current was used to melt coarser Al2O3-3TiO2 powder particles. The thickness per torch pass, measured after deposition, was 11 and 7 μm for alumina and alumina-titania coatings, respectively. The substrates were preheated by three 3
consecutive torch passes on their surface (current = 400 A, stand-off distance = 100 mm) and then were cooled by two air jets during deposition. 2.2 Characterization
The phase composition of powder feedstocks and coatings was investigated using an X-Ray powder Diffractometer (XRD, PW 1880, Philips, Almelo, Netherlands) with CuK (=0.154186 nm) radiation source produced at 40 kV and 40 mA. The analyzed range of the diffraction angle -2 was from 20 to 80°, by step width of 0.02° and 5 s time per step.
The surface roughness of the substrate and the as-sprayed coatings was measured using a 3D optical surface profilometer (New View 5000 system, Zygo Corporation, Connecticut, USA). This whitelight interferometer system allows to study the topography of the coating surface, which was previously metallized by Al thin film deposition (thickness = 40 nm), in order to allow reflection. The measurements were performed on areas of 0.7 x 0.5 μm2. The interferogram of the coating surface was then processed and transformed to three-dimensional high-resolution surface image by frequency domain analysis. The morphology of the powder particles was studied by Field Emission Gun Scanning Electron Microscopy (SEM-FEG, Leo Gemini mod. 1530, Carl Zeiss, Oberkochen, Germany) equipped with high-resolution secondary electron detector (in-lens detector). The measured resolution of this SEM was less than 0.9 nm at 20 kV – 1mm of working distance. The microstructure of as-sprayed coatings was investigated by Scanning Electron Microscopy (SEM, Gemini NVision 40, Carl Zeiss, Oberkochen, Germany) equipped with Energy Dispersive Spectroscopy (EDS, Oxford Link ISIS 300). To the purpose, the coatings were cut by low-speed diamond saw and their cross sections were polished to 0.25 m. The related SEM pictures were then processed by image analysis software
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(Image J, U.S. National Institutes of Health, Bethesda, USA) to calculate the porosity level and the pore distribution. Microhardness measurements were carried out at room temperature on the polished cross sections of the as-sprayed coatings, using a Vickers microindenter (VMHT MOT, Leica Microsystems, Weztlar, Germany). The indentations were performed at load of 2.94 N for a dwell time of 15 s. The spacing between the indentations was kept at least three times the diagonal to avoid further stresses produced by the interaction between consecutive indentations. The microhardness data were then analyzed by Weibull statistics. 3 Results and discussion 3.1 Phase composition and microstructure As shown in Fig. 1, the alumina powder is composed of pure -Al2O3 (corundum) phase, while the as-sprayed coating is composed of both - and -Al2O3 phases. The presence of the broad hump between 25 and 40° in the related pattern suggests the formation of an amorphous phase at lesser extent. The phase is associated to unmelted sprayed powder particles or to the molten particles cooled at lower rate, while the nucleation of the metastable phase from the liquid occurred due to the high quenching rate of the molten splats at the substrate surface [12].The metastable phase exhibits broad peaks, suggesting fine crystallite size. It tends to recrystallize to -Al2O3 after isothermal treatment at 1100 °C for 2 h. The phases have been identified on the basis of the Joint Committee on Powder Diffraction Standards (JCPDS) available at International Centre for Diffraction Data (No. 10-0425 for -Al2O3 and No. 43-1484 for -Al2O3). As displayed in Fig. 2 the alumina-titania powder is composed of -Al2O3 phase, while the assprayed coating is composed of both - and -Al2O3 phases. The presence of an amorphous phase can be also noticed, as previously observed for pure Al2O3 coating. In alumina-titania coating a higher - to -Al2O3 ratio can be observed in comparison with pure alumina one. The metastable
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phase tends to recrystallize to -Al2O3 after isothermal treatment at 1100 °C for 2 h. Some peaks for rutile-TiO2 can be also observed after heat treatment (card No. 88-1175). As displayed in Fig. 3, the alumina powder is composed of fused and crushed particles with blocky and angular morphology, and with size ranging from 5 to 55 μm, while the alumina-titania powder is composed of coarser particles with blocky and angular morphology, whose size was in the range 25 to 135 m. The measured substrate roughness was found to be 6.95 ± 1.13 m. In turn, the surface roughness Ra was found to be 6.19 ± 1.04 m and 8.03 ± 0.58 m for alumina and alumina-titania coatings, respectively. The melting and flattening of coarser alumina-titania particles with broader size distribution determined better intersplat bonding as well as higher surface roughness of the corresponding coatings in comparison with pure alumina ones. As shown in Fig. 4a, the cross section of the as-sprayed alumina coating exhibits a typical porous microstructure, characterized by pores with different size. Figure 4b shows some pores, interlamellar microcracks (denoted by white arrows) and intralamellar microcracks (denoted by black arrows) embedded in coating microstructure. Partially melted particles are also observable in some areas. By analyzing the cross sectional SEM microstructure in Fig. 5, it can be seen that the aluminatitania coating exhibits a lamellar and relatively dense microstructure with embedded splat boundaries and pores with different size. The results of pore distribution analysis are reported in Fig. 6. The splat boundaries, highlighted by white arrows in Fig. 5b, derived from solidification of the deposited splats during coating build-up. In turn the globular pores were originated by gas entrapped and filling defects during impact and solidification of the molten splats at the surface. The release of thermal stresses during quenching also promoted the formation of vertical microcracks ranging from 0.01 to 0.5 μm, in turn denoted by black arrow in Fig. 5b. 6
Globular porosity includes large circular and elongated pores (d > 3 μm) and more fine pores (d < 3 μm). The latter derived from gas entrapped under the liquid droplets and are homogeneously distributed, while the coarse pores are associated with filling defects caused by unmelted or semimolten particles and are not equally distributed. Some of them were determined by pull-out of unmelted particles during polishing step. EDS analysis was performed to determine the atomic composition of the different phases found within the microstructure. Some examples of EDS spectra are reported in Fig. 7. According to the EDS analysis dissolved TiO2 rich splat-like particles can be detected within the matrix. The thickness of these lamellae was in the range from sub-micron to a few micrometers. These areas are characterized by Ti segregation, lower content of Al and higher content of Ti in comparison with the matrix. This segregation has been also noticed in previous works [13,14]. The spot areas indicated in Fig. 5b highlight areas with different composition. Spot 1 highlights a splat-like particle composed of Ti and O and embedded in the Al2O3 matrix. The spot 2 shows an area composed of O, Al, Si and Fe, while the area denoted by spot 3 characterizes the Al2O3 matrix with Fe traces. The presence of Si and Fe is related to the chemical composition of the starting powder feedstock. Figure 8 shows the EDS map of the cross section of alumina-titania coating, showing the presence of titania-rich particles embedded in the microstructure. Traces of Ti, Si and Fe can be also detected. As reported in Table 2, the porosity value of the alumina coatings was found to be 7.1 %, while the porosity of the alumina-titania coatings was equal to 6.7 %. These porosity values are not quite different than those obtained by other investigators using the same powder feedstock, the same spraying distance and coating thickness (6.7-8.7 %) [15]. Moreover, it should be also noted that higher arc current was employed to melt coarser alumina-titania powder particles, in order to fabricate coatings with porosity similar to that of pure alumina ones. 7
3.2 Microhardness The microhardness data of both the coatings were broadly distributed due to the presence of microstructural defects such as pores, splat boundaries and microcracks, so that the Weibull statistics were employed. Figure 9 shows the Weibull plots for the microhardness data. The Weibull modulus (m), reflecting the scatter in the data distribution, was 12.6 and 9.2 for alumina and alumina-titania coatings, respectively. As reported in Table 2, the average microhardness of plasma sprayed alumina coating was 12.8 GPa (S.D. = 1.2), higher than the values reported in previous works [15] which were in the range 9.310.6 GPa, and were obtained using the same spraying distance and coating thickness. In turn, the average microhardness of plasma sprayed alumina-titania coating was 9.9 GPa (S.D. = 1.3). Values in the range 5.8-7.6 GPa have been obtained by other investigators by studying similar coatings deposited at lower spraying distance [16]. To this purpose, it is worth noting that higher spraying distance increases the residence time of the sprayed particles in the plasma jet, allowing better melting and thus involving higher coating microhardness. It has been also reported that with increasing the percentage of TiO2 the hardness decreased [17]. For example, Sanchez et al. reported values of 7.6 GPa for plasma sprayed Al2O3-13TiO2 coatings tested at 1.96 N [18]. In a recent work nanostructured alumina and alumina-titania coatings have been manufactured by APS and High Velocity Oxygen Fuel (HVOF), respectively, and have been characterized about their mechanical properties [19]. The microhardness of pure Al2O3 coatings was of about 10.8-11 GPa, whereas the hardness of Al2O3-13TiO2 coatings, produced starting from fused and crushed powder particles and tested at 2.94 N, was in the range 10.5-10.7 GPa.
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Therefore, in the present work very hard alumina-based coatings were produced, even if a decrease in microhardness was noticed (~ 23%) when 3 wt.% TiO2 was present in the starting powder feedstock. Because of their microstructural and mechanical properties the plasma-sprayed alumina-based coatings herein studied are also promising as oxygen barrier layers in TBC systems, with the purpose to extend their lifetime, by reducing the oxygen diffusion and the oxidation at the top coat/bond coat interface (much TBC materials are transparent to oxygen and prone to hightemperature phase changes) as well as by reducing the attack promoted by erosive and corrosive media [20,21,22]. 4 Conclusions Very hard Al2O3 and Al2O3-3TiO2 coatings were deposited by atmospheric plasma spraying. Higher plasma power was employed to melt coarser alumina-titania particles with broad size distribution. Larger particles tended to increase the surface roughness of the as-produced coatings. Both the coatings were mainly composed of - and -Al2O3 phases. The presence of an amorphous phase was also noticed, resulting from the high quenching rate of the molten droplets at the substrate surface. The heat treatment of the as-sprayed coatings at 1100 °C produced the formation of a crystalline structure, mainly composed of -Al2O3 phase. Both the coatings exhibited a porous microstructure with embedded pores, splat boundaries and microcracks. The fine pores and microcracks were homogeneously distributed, while the coarse pores, some of them derived from pull-out effects, were not homogeneouly distributed within the microstructure. Alumina-titania coating also contained splat-like particles enriched in Ti, Fe and Si with respect to pure alumina one. The porosity level was found to be 7.1 % for the alumina coating and 6.7 % for the aluminatitania one. The microhardness of the alumina-titania coating was lower than that of pure alumina one (9.9 against 12.8 GPa).
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In conclusion, very hard alumina-based coatings for high-temperature anti-wear applications were manufactured by plasma spraying. The presence of TiO2 and other impurities in the starting powder feedstock resulted in lower microhardness.
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References
[1] J.R. Davis, Handbook of Thermal Spray Technology, ASM International: Materials Park, OH, 2004. [2] D.I. Pantelis, P. Psyllaki, N. Alexopoulos, Tribological behaviour of plasma-sprayed Al2O3 coatings under severe wear conditions, Wear 237 (2000) 197-204. [3] E. Celik, I. Ozdemir, E. Avci, Y. Tsunekawa, Corrosion behavior of plasma sprayed coatings, Surf. Coat. Technol. 193 (2005) 297-302. [4] Y. Liu, T.E. Fischer, A. Dent, Comparison of HVOF and plasma-sprayed alumina-titania coatings—microstructure, mechanical properties and abrasion behavior , Surf. Coat. Technol. 167 (2003) 68-76. [5] M. Wang, L.L. Shaw, Effects of the powder manufacturing method on microstructure and wear performance of plasma sprayed alumina–titania coatings, Surf. Coat. Technol. 202 (2007) 34-44. [6] P.P. Bandyopadhyay, D. Chicot, B. Venkateshwarlu, V. Racherla, X. Decoopman, J. Lesage, Mechanical properties of conventional and nanostructured plasma sprayed alumina coatings, Mech. Mater. 53 (2012) 61-71. [7] L. Pawlowski, The relationship between strcuture and dielectric properties in plasma sprayed alumina coatings, Surf. Coat. Technol. 35 (1988) 285-298. [8] M. Khalid, M. Mujahid, A.N. Khan, R.S. Rawat, I. Salam, K. Mahmood, Plasma sprayed alumina coatings on Ti6Al4V alloy for orthopaedic implants: microstructure and phase analysis, Key Eng. Mater. 510-511 (2012) 547-553. [9] O. Knotek, Thermal spraying and detonation gun process, in: R.F. Bunshah (Ed.), Handbook of Hard Coating, Noyes Publications, New Yersey, 2001, pp. 77-107. [10] R.B. Heimann, Applications of plasma-sprayed ceramic coatings, Key Eng. Mater. 122-124 (1996) 399-442.
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[11] G. Di Girolamo, C. Blasi, L. Pilloni, M. Schioppa, Microstructural and thermal properties of plasma sprayed mullite coatings, Ceram. Int. 36 (2010) 1389-1395. [12] R. McPherson, On the Formation of Thermally Sprayed Alumina Coatings, J. Mater. Sci. 15 (1980) 3141–3149. [13] E. Sanchez, E. Bannier, V. Cantavella, M.D. Salvador, E. Klyatskina, J. Morgiel, J. Grzonka, A.R. Boccaccini, Deposition of Al2O3–TiO2 Nanostructured Powders by Atmospheric Plasma Spraying, J. Therm. Spray Technol. 17 (2008) 329–337. [14] X. Lin, Y. Zeng, S. W. Lee, C. Ding, Characterization of Alumina –3 wt% Titania Coating Prepared by Plasma Spraying of Nanostructured Powders, J. Eur. Ceram. Soc. 24 (2004) 627–634. [15] O. Sarikaya, Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process, Surf. Coat. Technol. 190 (2005) 388-393. [16] A.R.M. Sahab, N.H. Saad, S. Kasolang, J. Saedon, Impact of plasma spray variables parameters on mechanical and wear behaviour of plasma sprayed Al2O3 3% wt TiO2 coating in abrasion and erosion application, Proc. Eng. 41 (2012) 1689-1695. [17] R.A. Zatorski, H. Herman, High performance ceramic films and coatings, Elsevier, Amsterdam, 1991, pp. 591-601. [18] E. Sanchez, E. Bannier, M. Vincent, A. Moreno, M.D. Salvador, V. Bonache, E. Klyatskina, A.R. Boccaccini, Characterization of nanostructured ceramic and cermet coatings deposited by plasma spraying, Int. J. Appl. Ceram. Technol. 8 (2011) 1136-1146. [19] V. Matikainen, K. Niemi, H. Koivuluoto, P, Vuoristo, Abrasion, erosion and cavitation erosion wear properties of thermally sprayed alumina based coatings, Coatings 4 (2014) 18-36. [20] J. Mueller, D. Neuschuetz, Efficiency of -Al2O3 as diffusion barrier between bond coat and
bulk material of gas turbine blades, Vacuum 71 (2003) 247-251 [21] M. Saremi, A. Afrasiabi , A. Kobayashi, Microstructural Analysis of YSZ and YSZ/Al2O3
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Plasma Sprayed Thermal Barrier Coatings after High Temperature Oxidation, Surf. Coat. Technol. 202 (2008) 3233–3238. [22] M. Nejati, M.R. Rahimipour, I. Mobasherpour, Evaluation of hot corrosion behavior of CSZ, CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings, Ceram. Int. 40 (2014) 4579-4590.
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Alumina
Alumina-titania
Current [A]
500
600
Substrate
2083
2083
4
4
40
38
10
9
100
100
5
3
27
28
5
6
2
1.8
90
90
tangential speed [mm/s] Gun velocity [mm/s] Primary gas Ar flow rate [slpm*] Secondary gas H2 flow rate [slpm*] Stand-off distance [mm] Carrier gas flow rate [slpm] Powder feed rate [g/min] Distance torchinjector [mm] Injector diameter [mm] Injector angle [°]
Table 1 – Plasma spraying parameters used in this work. * slpm – standard litres per minute.
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Coating
Average porosity [%]
Microhardness [GPa]
Al2O3
7.1 ± 1.1
12.8 ± 1.2
Al2O3-3TiO2
6.7 ± 0.8
9.9 ± 1.3
Table 2 – Porosity and microhardness values for alumina and alumina-titania coatings.
Figure Captions Figure 1 – XRD patterns of a) powder, b) as-sprayed and c) heat treated (1100 °C) alumina coating. Figure 2 – XRD patterns of a) powder, b) as-sprayed and c) heat treated (1100 °C) alumina-titania coating. Figure 3 – SEM morphology of alumina and alumina-titania powder particles, respectively. Figure 4 – Cross sectional SEM microstructure of plasma sprayed alumina coating. Fig. 4b shows pores, interlamellar microcracks (white arrows) and intralamellar microcrcaks (black arrows). Figure 5 – Cross sectional SEM microstructure of alumina-titania coating at different magnification. Fig.5b denotes pores, splat boundaries, microcracks and areas with different elemental composition. Figure 6 - Image analysis procedure for separating coating microstructural defects: (a) original greyscale image, (b) binary image showing total porosity, (c) large pores (d > 3 m), (d) fine porosity, (e) globular fine pores (d < 3 m) and (f) traces of interlamellar and intralamellar cracks. Figure 7 - Spot EDS spectra taken from different areas in coating microstructure. Figure 8 – EDS map of alumina-titania coating cross section. Figure 9 – Weibull plots for microhardness data referred to plasma sprayed alumina and aluminatitania coatings.
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Figure 1
Figure 2
Figure 3a
Figure 3b
Figure 4a
Figure 4b
Figure 5a
Figure 5b
Figure 6
Figure 7
Figure 8
Figure 9