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Investigation of nanostructured and conventional alumina–titania coatings prepared by air plasma spray process
Materials Science and Engineering A 527 (2010) 663–668
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Investigation of nanostructured and conventional alumina–titania coatings prepared by air plasma spray process A. Ibrahim a,∗ , Z. Abdel Hamid b , A. Abdel Aal b a b
Farmingdale State College, Farmingdale, New York 11735, USA Central Metallurgical R & D Institute, CMRDI, Helwan, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 12 May 2009 Received in revised form 17 August 2009 Accepted 25 August 2009
Keywords: Nanostructured coatings Alumina–13 wt.% titania coating Abrasive wear tests Fatigue-testing Mechanical properties
a b s t r a c t Nanostructured and conventional alumina–13 wt.% titania powders were thermally sprayed using air plasma spray (APS) process. Scanning electron microscopy (SEM) was used to examine the morphology of the agglomerated powders and the cross-section of the alumina–titania coatings. The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). The fatigue and mechanical properties of the coatings were investigated. SEM analysis was also carried out on the worn surfaces of conventional and nanostructured coatings following the wear test. The experimental data indicated that the nanostructured coated samples exhibited higher hardness, wear resistance and fatigue strength compared to the conventional coated samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The air plasma spray (APS) process has been used for many years to form TiO2 and other ceramic oxide coatings. This APS process is preferred due to the high-temperature of the plasma jet, which is necessary for thermal spray high melting point materials in an effective way. During the thermal spraying of ceramic oxides, it is necessary to totally or partially melt the powder particles to make coatings. Al2 O3 coatings have been extensively used in many applications due to their thermal, chemical and mechanical stability. The Al2 O3 phase is characterised by the highest chemical resistance among all oxides, good heat and electric insulations, high hardness and wear resistance, etc. The Al2 O3 phase is a very complex material. The Al2 O3 phase exists in several polymorphs such as ␥, ␣, and , . . ., etc. from which the most important are: ␣-Al2 O3 and ␥-Al2 O3 . APS has been used to produce metastable oxide-ceramic coatings, starting with commercially available Al2 O3 –13 wt.%TiO2 (AT-13) powder feed. The feed material undergoes rapid melting in the high-temperature zone of the plasma jet. A metastable Al2 O3 –TiO2 phase is formed when the molten droplets are quenched very rapidly on the target surface [1,2]. The metastable
∗ Corresponding author at: Farmingdale State College, Route 110, Lupton Hall, Room 182, Farmingdale, NY 11735, USA. Tel.: +1 631 420 2309; fax: +1 631 420 2194. E-mail address: [email protected] (A. Ibrahim). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.08.054
phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting of ␣-Al2 O3 and -Al2 O3 –TiO2 . Both types of ceramic materials have potential as hard, wear-resistant coatings [1]. Plasma-sprayed nanostructured coatings shows improvement of resistance to wear, erosion, corrosion and mechanical properties as compared to their conventional counterparts [1–11]. For example, nanostructured AT-13 ceramic coatings show much higher wear resistance than conventional AT-13 coatings [3–7,10–12]. Plasma-sprayed AT-13 coating is one of the most important coatings for many industrial applications [4–6]. They provide a dense and hard surface coating which are resistant to abrasion, corrosion, cavitations, oxidation and erosion and are therefore regularly used for wear resistance, electrical insulation, thermal barrier applications etc. A number of papers reported that the Al2 O3 –TiO2 coating containing 13 wt.% of TiO2 showed the most excellent wear resistance among the AT-13 ones [1,4–7]. The as-sprayed AT-13 predominantly consisting of metastable nanocrystalline ␥-Al2 O3 phase and some untransformed ␣-Al2 O3 . Since the properties, surface and crystal structure of nanoparticles are size-dependent, these fine nanostructures have potential to exhibit unusual properties. This research paper presents the results of investigation of the mechanical properties of plasma-sprayed nanostructured and conventional AT-13 coatings. In this study, the microstructure of the coatings plasma-sprayed from nanostructured and conventional powders were examined by a number of characterization techniques.
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Fig. 1. Morphologies of the Al2 O3 –13 wt.% TiO2 powders: (a) conventional; (b) nanostructured.
2. Experimental procedure 2.1. Feedstock powders Nanostructured (2613S) and conventional (ALO 187) AT-13 feedstock powders employed in this study obtained from Inframat Corp. (Farmington, CT, USA) & Praxair, IN, USA. The morphologies of both AT-13 powders are shown in Fig. 1. The nanostructured was agglomerated spherical nanoparticles with high flowability and an average diameter of 30 m. The conventional powder exhibited an angular and irregular morphology with size between 10 and 45 m. Coatings were deposited using a Sulzer-Metco 9MB plasma torch under atmospheric conditions. Low-carbon steel cylindrical coupons were used as substrates. The typical spraying parameters for both conventional and nanostructured coatings are summarized in Table 1. 2.2. Characterization of coatings The phase composition of the powder, as-sprayed samples, and as-coated was determined by X-ray diffraction using a Philips X-ray diffractometer (Philips APD 3520). The microstructure of the powders, as-sprayed coatings and plan view of worn surfaces coatings was examined by scanning electron microscope (SEM, JEOL-JSM5410). The microhardness measurements were conducted on the cross-section of the as-sprayed coatings using Mitutoyo Vickers microhardness tester (HM-123).
Rotating-beam fatigue-testing was conducted on AISI lowcarbon steel. The fatigue-testing machine is an RBF-200, rotatingbeam fatigue machine (Fatigue Dynamics Inc., Walled Lake, MI). The test specimen used in the fatigue-testing was a 12.7 mm (1/2-in.) hourglass bar prepared according to ASTM E466 [13]. The nominal coating thickness was 100 m (0.004 in.). The fatigue experiments were conducted at room temperature under a rotating-beam and stress ratio of R = −1 configuration at a load frequency of 50 Hz. The surface of the specimen was prepared for coating by grit blasting with # 24 alumina, and no grinding was performed so as to not alter the surface roughness of the coatings. All fracture surfaces were subsequently examined using an optical microscope and scanning electron microscopy (SEM). 2.4. Abrasion test Abrasion test was carried out using HT-8360 Taber Abrasion Tester machine on sheet specimens (30 mm length × 15 mm width). The specimen was mounted on rotary disk fixture and a regulated load was applied on the movable arm. The weight loss due to abrasion of samples was measured with sensitivity ±0.1 mg. The abrasion test was carried out under the following conditions, rotating speed of 70 rpm, applied load of 500 g and sliding distance of 1000 m. 3. Results and discussions 3.1. Phase composition of the coatings
2.3. Fatigue-testing Several tests have been carried out during this investigation: hardness, abrasion test, and fatigue-testing. The fatigue-testing proved to be a valuable indicator of the mechanical strength of the coatings. Table 1 Summary of the plasma spraying parameters. Parameters
Conventional coating
Nanocoating
Torch metco Current Voltage Argon flow rate argon Pressure hydrogen Pressure powder carrier Flow Spray distance
9MB 470 A 75 V 80 SCFH 100 PSI 50 PSI 15–18 SCFH 4.5 in.
9MB 500 A 70 V 80 SCFH 100 PSI 50 PSI 15–8 SCFH 4 in.
The phase structure of AT-13 powders and coatings was investigated by X-ray diffractometer.The XRD patterns of the conventional and nanostructured powders are shown in Fig. 2a and b. Both powders are prominently ␣-Al2 O3 . Rutile phase of TiO2 was found in the nanostructured powder. The XRD patterns of the coatings (Fig. 2c and d) showed that most of ␣-alumina in the nanostructured powder converted into ␥-Al2 O3 after plasma spraying process, which was similar to that in the conventionally commercial powder. It is well established that ␥-Al2 O3 tends to be nucleated from the melt in preference to ␣-Al2 O3 due to the higher cooling rate [1,2–8,11]. 3.2. Microstructure of coatings The cross-sectional morphologies of the as-sprayed coatings are shown in Fig. 3. From the cross-sectional microstructures, it can be seen that both coatings consist of the lamella built up from the molten droplets impinging on the substrate. In case of the
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Fig. 2. X-ray diffraction patterns of the powders Al2 O3 –13 wt.% TiO2 and as-sprayed coatings: (a) conventional powder; (b) nanostructured powder; (c) conventional coating; (d) nanostructured coating.
nanostructured coating, the interface between the steel substrate and the coating appears stronger than that of the conventional coating. The conventional coating (Fig. 3(a)) has a layered microstructure, typical of plasma-sprayed coatings, which is the result of full melting (FM) of the ceramic feedstock powder and its solidification as “splats” on the substrate. The FM regions in the conventional coating consist of nanocrystalline ␥-Al2 O3 [1,3–8]. In all the coat-
ings, some pores are observed, and splat boundaries are not clearly visible. An SEM micrograph of the nanostructured coating is shown in Fig. 3(b). This coating shows a bimodal microstructure composed of the two regions where nanopowders were fully (FM) and partially melted (PM). A relatively large amount (∼15%) of partially melted regions (PM) is observed in the nanostructured coating (Fig. 3(b)). The partially melted region was formed when TiO2 was selectively
Fig. 3. Cross-sectional morphologies of the as-sprayed Al2 O3 –13 wt.% TiO2 coatings: (a) conventional; (b) nanostructured.
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A. Ibrahim et al. / Materials Science and Engineering A 527 (2010) 663–668
Fig. 4. (a) Cross-sectional morphology of Al2 O3 –13 wt.% TiO2 nanostructured coatings showing the partially melted regions (PM): (b) high mag of the (PM) region; (c) high mag of (b) showing the ␣-Al2 O3 fine equiaxed grains (0.5–1 m).
melted because the temperature of nanopowders was not high enough during spray coating [1,2]. Fig. 4 shows high-magnification SEM micrographs of the PM region in the nanostructured coating microstructure. The PM feature appears to consist of grains surrounded by a matrix phase, something similar to microstructures of liquid-phase-sintered materials. The PM microstructural features consists of ␣-Al2 O3 fine equiaxed grains surrounded by a TiO2 -rich amorphous phase. It is interesting to notice that many cracks were arrested at the PM regions.
out for each coating. The mean microhardness of conventional coating is about 840 Hv, which is lower than the mean microhardness of 965 Hv achieved by the nanostructured coating. The observed hardness difference is believed to result partially from different coating microstructures and phase compositions, even though the two coatings have the same nominal chemical composition. This result is in a good agreement with previous research [14]. In addition, the distribution of TiO2 in these coatings may have an effect on the coating’s hardness. It is interesting to notice that the nanostructured coating contains many micro or nanopores and thus, these regions should have a relatively low microhardness.
3.3. Hardness of the coatings 3.4. Results of fatigue tests Vickers microhardness measurements were performed under a 300 gf load for 15 s on the cross-sections of the coatings are shown in Fig. 5. A total of 10 microhardness measurements were carried
It is widely recognized that thermal spray coatings can significantly influence the fatigue strength of coated components [15–18].
A. Ibrahim et al. / Materials Science and Engineering A 527 (2010) 663–668
Fig. 5. Vickers hardness of the conventional and nanostructured coatings.
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Fig. 7. Cumulative wear mass loss as a function of sliding distance for conventional and nanostructured coatings.
number i is given by:
Pf =
Fig. 6. Fatigue life distributions of coated and uncoated specimens.
In this investigation, fatigue life data were generated from the fatigue tests and analyzed to determine the relationship between stress level, S, number of cycles to failure, N and probability of failure, Pf . The failure probability, Pf , corresponding to the order
i N+1
(1)
Fig. 6 shows the probability of failure (Pf ) as a function of the number of cycles to failure (N) for the coated and uncoated specimens. The results indicate that the nanostructured alumina–titania coating exhibited higher fatigue lives compared to the conventional alumina–titania coating. The increase in the fatigue strength of nanostructured coatings can be related to the crack propagation resistance of these coatings. Several investigators have shown that the crack propagation resistance of nanostructured coatings is superior to that of the conventional coatings [10,11,19,20]. It is important to notice that the PM regions in the coating act as a crack arrest as seen in Fig. 4. Another important factor that can play a major role in increasing the fatigue resistance is the interfacial toughness. Bansal et al. [19] have measured the interfacial toughness of the conventional and the nanostructured AT-13 plasmas prayed coatings on steel substrates; the values were 22 and 45 J m−2 , respectively.
Fig. 8. SEM images of worn surface in the (a) conventional, and (b) nanostructured coatings.
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3.5. Abrasion tests The abrasive wear resistances of the nanostructured and conventional coatings are compared in Fig. 7. The wear rate of the nanostructured coating was almost 55% lower than that of the conventional coating. This result is in a good agreement with other research [20]. There is a relatively good agreement between hardness and the abrasion resistance, harder coatings being, generally, more abrasion resistant than softer coatings. However, the relative difference in the abrasion volume loss is a lot larger than the differences in the hardness measurement. Abrasion resistance is a combination of basic factors such as elasticity, hardness, strength (both cohesive, tensile and shear strength), and toughness. In addition, abrasion resistance is intimately related to scratching and slip. Thus, coatings that enhance these properties will improve abrasion resistance. It is believed that the presence of partially melted regions inside the nanostructured coatings play a major rule in the excellent wear resistance of the nanostructured AT-13 coatings. Fig. 8 shows SEM images of the conventional and nanostructured coating following the wear test. The wear track on the conventional coating exhibited much more significant pitting and delamination. These highly decomposed regions result in large-scale material loss, and thus, high rates of wear. 4. Conclusion • The microstructure of the conventional AT-13 coating consists primarily of fully molten regions. The microstructure of nanostructured AT-13 is bimodal in nature and it consists of regions of fully molten mixed with partially molten microstructural features. • The higher fatigue strength of nanostructured coatings compared to conventional coating can be related to a higher crack propagation resistance and the interfacial toughness of these coatings.
• The presence of the partially melted regions in the nanostructured coating act as a crack arrest and play a major rule in strengthening the abrasion resistance of the coatings. • The nanostructured coatings exhibited a higher hardness and wear resistance than the comparable conventional counterpart. • The microstructure played a significant rule in improving the mechanical properties of nanostructured AT-13. References [1] S.H. Kear, R.K. Sadangi, M. Jain, R. Yao, Z. Kalman, G. Skandan, W. Mayo, J. Therm. Spray Technol. 9 (2000) 399. [2] R. Mcpherson, J. Mater. Sci. 15 (1980) 3141. [3] J. He, M. Ice, E.J. Lavernia, Metall. Mater. Trans. 31A (2000) 555–564. [4] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Surf. Coat. Technol. 146/147 (2001) 48. [5] E.H. Jordan, M. Gell, Y.H. Sohn, et al., Mater. Sci. Eng. A 301 (2001) 80. [6] D. Goberman, Y.H. Sohn, L. Shaw, E. Jordan, M. Gell, Acta Mater. 50 (2002) 1141–1152. [7] Y. Wang, S. Jiang, M. Wang, S. Wang, T.D. Xiao, P.R. Strutt, Wear 237 (2000) 176. [8] J.R. Davis, Handbook of Thermal Spray Technology, ASM International, 2004. [9] L. Xinhua, Z. Yi, Z. Xiaming, Mater. Sci. Eng. A357 (2003). [10] R.S. Lima, B.R. Marple, J. Therm. Spray Technol. 16 (1) (2007) 40–63. [11] R.S. Lima, C. Moreau, B.R. Marple, J. Therm. Spray Technol. 16 (5–6) (2007) 866–872. [12] A. Ibrahim, H. Salem, C.C. Berndt, International Thermal Spray Conference, Maastricht, Netherlands, 2008. [13] ASTM E466-07 Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. [14] L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jing, Y. Wang, T.D. Xiao, P.R. Strutt, Surf. Coat. Technol. 130 (2000) 1. [15] A. Ibrahim, C. Berndt, J. Mater. Sci. 33 (1998) 3095. [16] A. Ibrahim, C. Berndt, International Thermal Spray Conference, Osaka, Japan, 2004. [17] A. Ibrahim, C. Berndt, International Thermal Spray Conference, Orlando, FL, May, 2003. [18] A.A. Tipton, C.C. Berndt, S. Sampath (Eds.), Advances in ThermalSpray Science and Technology, ASM, Materials Park, OH, 1995, p. 463. [19] P. Bansal, N.P. Padture, A. Vasiliev, Acta Mater. 51 (2003). [20] W. Meidong, L. Shaw, Surf. Coat. Technol. 202 (1) (2007) 15.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Investigation of nanostructured and conventional alumina–titania coatings prepared by air plasma spray process A. Ibrahim a,∗ , Z. Abdel Hamid b , A. Abdel Aal b a b
Farmingdale State College, Farmingdale, New York 11735, USA Central Metallurgical R & D Institute, CMRDI, Helwan, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 12 May 2009 Received in revised form 17 August 2009 Accepted 25 August 2009
Keywords: Nanostructured coatings Alumina–13 wt.% titania coating Abrasive wear tests Fatigue-testing Mechanical properties
a b s t r a c t Nanostructured and conventional alumina–13 wt.% titania powders were thermally sprayed using air plasma spray (APS) process. Scanning electron microscopy (SEM) was used to examine the morphology of the agglomerated powders and the cross-section of the alumina–titania coatings. The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). The fatigue and mechanical properties of the coatings were investigated. SEM analysis was also carried out on the worn surfaces of conventional and nanostructured coatings following the wear test. The experimental data indicated that the nanostructured coated samples exhibited higher hardness, wear resistance and fatigue strength compared to the conventional coated samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The air plasma spray (APS) process has been used for many years to form TiO2 and other ceramic oxide coatings. This APS process is preferred due to the high-temperature of the plasma jet, which is necessary for thermal spray high melting point materials in an effective way. During the thermal spraying of ceramic oxides, it is necessary to totally or partially melt the powder particles to make coatings. Al2 O3 coatings have been extensively used in many applications due to their thermal, chemical and mechanical stability. The Al2 O3 phase is characterised by the highest chemical resistance among all oxides, good heat and electric insulations, high hardness and wear resistance, etc. The Al2 O3 phase is a very complex material. The Al2 O3 phase exists in several polymorphs such as ␥, ␣, and , . . ., etc. from which the most important are: ␣-Al2 O3 and ␥-Al2 O3 . APS has been used to produce metastable oxide-ceramic coatings, starting with commercially available Al2 O3 –13 wt.%TiO2 (AT-13) powder feed. The feed material undergoes rapid melting in the high-temperature zone of the plasma jet. A metastable Al2 O3 –TiO2 phase is formed when the molten droplets are quenched very rapidly on the target surface [1,2]. The metastable
∗ Corresponding author at: Farmingdale State College, Route 110, Lupton Hall, Room 182, Farmingdale, NY 11735, USA. Tel.: +1 631 420 2309; fax: +1 631 420 2194. E-mail address: [email protected] (A. Ibrahim). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.08.054
phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting of ␣-Al2 O3 and -Al2 O3 –TiO2 . Both types of ceramic materials have potential as hard, wear-resistant coatings [1]. Plasma-sprayed nanostructured coatings shows improvement of resistance to wear, erosion, corrosion and mechanical properties as compared to their conventional counterparts [1–11]. For example, nanostructured AT-13 ceramic coatings show much higher wear resistance than conventional AT-13 coatings [3–7,10–12]. Plasma-sprayed AT-13 coating is one of the most important coatings for many industrial applications [4–6]. They provide a dense and hard surface coating which are resistant to abrasion, corrosion, cavitations, oxidation and erosion and are therefore regularly used for wear resistance, electrical insulation, thermal barrier applications etc. A number of papers reported that the Al2 O3 –TiO2 coating containing 13 wt.% of TiO2 showed the most excellent wear resistance among the AT-13 ones [1,4–7]. The as-sprayed AT-13 predominantly consisting of metastable nanocrystalline ␥-Al2 O3 phase and some untransformed ␣-Al2 O3 . Since the properties, surface and crystal structure of nanoparticles are size-dependent, these fine nanostructures have potential to exhibit unusual properties. This research paper presents the results of investigation of the mechanical properties of plasma-sprayed nanostructured and conventional AT-13 coatings. In this study, the microstructure of the coatings plasma-sprayed from nanostructured and conventional powders were examined by a number of characterization techniques.
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Fig. 1. Morphologies of the Al2 O3 –13 wt.% TiO2 powders: (a) conventional; (b) nanostructured.
2. Experimental procedure 2.1. Feedstock powders Nanostructured (2613S) and conventional (ALO 187) AT-13 feedstock powders employed in this study obtained from Inframat Corp. (Farmington, CT, USA) & Praxair, IN, USA. The morphologies of both AT-13 powders are shown in Fig. 1. The nanostructured was agglomerated spherical nanoparticles with high flowability and an average diameter of 30 m. The conventional powder exhibited an angular and irregular morphology with size between 10 and 45 m. Coatings were deposited using a Sulzer-Metco 9MB plasma torch under atmospheric conditions. Low-carbon steel cylindrical coupons were used as substrates. The typical spraying parameters for both conventional and nanostructured coatings are summarized in Table 1. 2.2. Characterization of coatings The phase composition of the powder, as-sprayed samples, and as-coated was determined by X-ray diffraction using a Philips X-ray diffractometer (Philips APD 3520). The microstructure of the powders, as-sprayed coatings and plan view of worn surfaces coatings was examined by scanning electron microscope (SEM, JEOL-JSM5410). The microhardness measurements were conducted on the cross-section of the as-sprayed coatings using Mitutoyo Vickers microhardness tester (HM-123).
Rotating-beam fatigue-testing was conducted on AISI lowcarbon steel. The fatigue-testing machine is an RBF-200, rotatingbeam fatigue machine (Fatigue Dynamics Inc., Walled Lake, MI). The test specimen used in the fatigue-testing was a 12.7 mm (1/2-in.) hourglass bar prepared according to ASTM E466 [13]. The nominal coating thickness was 100 m (0.004 in.). The fatigue experiments were conducted at room temperature under a rotating-beam and stress ratio of R = −1 configuration at a load frequency of 50 Hz. The surface of the specimen was prepared for coating by grit blasting with # 24 alumina, and no grinding was performed so as to not alter the surface roughness of the coatings. All fracture surfaces were subsequently examined using an optical microscope and scanning electron microscopy (SEM). 2.4. Abrasion test Abrasion test was carried out using HT-8360 Taber Abrasion Tester machine on sheet specimens (30 mm length × 15 mm width). The specimen was mounted on rotary disk fixture and a regulated load was applied on the movable arm. The weight loss due to abrasion of samples was measured with sensitivity ±0.1 mg. The abrasion test was carried out under the following conditions, rotating speed of 70 rpm, applied load of 500 g and sliding distance of 1000 m. 3. Results and discussions 3.1. Phase composition of the coatings
2.3. Fatigue-testing Several tests have been carried out during this investigation: hardness, abrasion test, and fatigue-testing. The fatigue-testing proved to be a valuable indicator of the mechanical strength of the coatings. Table 1 Summary of the plasma spraying parameters. Parameters
Conventional coating
Nanocoating
Torch metco Current Voltage Argon flow rate argon Pressure hydrogen Pressure powder carrier Flow Spray distance
9MB 470 A 75 V 80 SCFH 100 PSI 50 PSI 15–18 SCFH 4.5 in.
9MB 500 A 70 V 80 SCFH 100 PSI 50 PSI 15–8 SCFH 4 in.
The phase structure of AT-13 powders and coatings was investigated by X-ray diffractometer.The XRD patterns of the conventional and nanostructured powders are shown in Fig. 2a and b. Both powders are prominently ␣-Al2 O3 . Rutile phase of TiO2 was found in the nanostructured powder. The XRD patterns of the coatings (Fig. 2c and d) showed that most of ␣-alumina in the nanostructured powder converted into ␥-Al2 O3 after plasma spraying process, which was similar to that in the conventionally commercial powder. It is well established that ␥-Al2 O3 tends to be nucleated from the melt in preference to ␣-Al2 O3 due to the higher cooling rate [1,2–8,11]. 3.2. Microstructure of coatings The cross-sectional morphologies of the as-sprayed coatings are shown in Fig. 3. From the cross-sectional microstructures, it can be seen that both coatings consist of the lamella built up from the molten droplets impinging on the substrate. In case of the
A. Ibrahim et al. / Materials Science and Engineering A 527 (2010) 663–668
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Fig. 2. X-ray diffraction patterns of the powders Al2 O3 –13 wt.% TiO2 and as-sprayed coatings: (a) conventional powder; (b) nanostructured powder; (c) conventional coating; (d) nanostructured coating.
nanostructured coating, the interface between the steel substrate and the coating appears stronger than that of the conventional coating. The conventional coating (Fig. 3(a)) has a layered microstructure, typical of plasma-sprayed coatings, which is the result of full melting (FM) of the ceramic feedstock powder and its solidification as “splats” on the substrate. The FM regions in the conventional coating consist of nanocrystalline ␥-Al2 O3 [1,3–8]. In all the coat-
ings, some pores are observed, and splat boundaries are not clearly visible. An SEM micrograph of the nanostructured coating is shown in Fig. 3(b). This coating shows a bimodal microstructure composed of the two regions where nanopowders were fully (FM) and partially melted (PM). A relatively large amount (∼15%) of partially melted regions (PM) is observed in the nanostructured coating (Fig. 3(b)). The partially melted region was formed when TiO2 was selectively
Fig. 3. Cross-sectional morphologies of the as-sprayed Al2 O3 –13 wt.% TiO2 coatings: (a) conventional; (b) nanostructured.
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Fig. 4. (a) Cross-sectional morphology of Al2 O3 –13 wt.% TiO2 nanostructured coatings showing the partially melted regions (PM): (b) high mag of the (PM) region; (c) high mag of (b) showing the ␣-Al2 O3 fine equiaxed grains (0.5–1 m).
melted because the temperature of nanopowders was not high enough during spray coating [1,2]. Fig. 4 shows high-magnification SEM micrographs of the PM region in the nanostructured coating microstructure. The PM feature appears to consist of grains surrounded by a matrix phase, something similar to microstructures of liquid-phase-sintered materials. The PM microstructural features consists of ␣-Al2 O3 fine equiaxed grains surrounded by a TiO2 -rich amorphous phase. It is interesting to notice that many cracks were arrested at the PM regions.
out for each coating. The mean microhardness of conventional coating is about 840 Hv, which is lower than the mean microhardness of 965 Hv achieved by the nanostructured coating. The observed hardness difference is believed to result partially from different coating microstructures and phase compositions, even though the two coatings have the same nominal chemical composition. This result is in a good agreement with previous research [14]. In addition, the distribution of TiO2 in these coatings may have an effect on the coating’s hardness. It is interesting to notice that the nanostructured coating contains many micro or nanopores and thus, these regions should have a relatively low microhardness.
3.3. Hardness of the coatings 3.4. Results of fatigue tests Vickers microhardness measurements were performed under a 300 gf load for 15 s on the cross-sections of the coatings are shown in Fig. 5. A total of 10 microhardness measurements were carried
It is widely recognized that thermal spray coatings can significantly influence the fatigue strength of coated components [15–18].
A. Ibrahim et al. / Materials Science and Engineering A 527 (2010) 663–668
Fig. 5. Vickers hardness of the conventional and nanostructured coatings.
667
Fig. 7. Cumulative wear mass loss as a function of sliding distance for conventional and nanostructured coatings.
number i is given by:
Pf =
Fig. 6. Fatigue life distributions of coated and uncoated specimens.
In this investigation, fatigue life data were generated from the fatigue tests and analyzed to determine the relationship between stress level, S, number of cycles to failure, N and probability of failure, Pf . The failure probability, Pf , corresponding to the order
i N+1
(1)
Fig. 6 shows the probability of failure (Pf ) as a function of the number of cycles to failure (N) for the coated and uncoated specimens. The results indicate that the nanostructured alumina–titania coating exhibited higher fatigue lives compared to the conventional alumina–titania coating. The increase in the fatigue strength of nanostructured coatings can be related to the crack propagation resistance of these coatings. Several investigators have shown that the crack propagation resistance of nanostructured coatings is superior to that of the conventional coatings [10,11,19,20]. It is important to notice that the PM regions in the coating act as a crack arrest as seen in Fig. 4. Another important factor that can play a major role in increasing the fatigue resistance is the interfacial toughness. Bansal et al. [19] have measured the interfacial toughness of the conventional and the nanostructured AT-13 plasmas prayed coatings on steel substrates; the values were 22 and 45 J m−2 , respectively.
Fig. 8. SEM images of worn surface in the (a) conventional, and (b) nanostructured coatings.
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3.5. Abrasion tests The abrasive wear resistances of the nanostructured and conventional coatings are compared in Fig. 7. The wear rate of the nanostructured coating was almost 55% lower than that of the conventional coating. This result is in a good agreement with other research [20]. There is a relatively good agreement between hardness and the abrasion resistance, harder coatings being, generally, more abrasion resistant than softer coatings. However, the relative difference in the abrasion volume loss is a lot larger than the differences in the hardness measurement. Abrasion resistance is a combination of basic factors such as elasticity, hardness, strength (both cohesive, tensile and shear strength), and toughness. In addition, abrasion resistance is intimately related to scratching and slip. Thus, coatings that enhance these properties will improve abrasion resistance. It is believed that the presence of partially melted regions inside the nanostructured coatings play a major rule in the excellent wear resistance of the nanostructured AT-13 coatings. Fig. 8 shows SEM images of the conventional and nanostructured coating following the wear test. The wear track on the conventional coating exhibited much more significant pitting and delamination. These highly decomposed regions result in large-scale material loss, and thus, high rates of wear. 4. Conclusion • The microstructure of the conventional AT-13 coating consists primarily of fully molten regions. The microstructure of nanostructured AT-13 is bimodal in nature and it consists of regions of fully molten mixed with partially molten microstructural features. • The higher fatigue strength of nanostructured coatings compared to conventional coating can be related to a higher crack propagation resistance and the interfacial toughness of these coatings.
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