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Electron beam treatments of electrophoretic ceramic coatings
Applied Surface Science 254 (2008) 1830–1836 www.elsevier.com/locate/apsusc
Electron beam treatments of electrophoretic ceramic coatings M.F. De Riccardis *, D. Carbone, E. Piscopiello, M. Vittori Antisari ENEA-C.R. Brindisi, S.S. 7 APPIA, Km. 712-72100 Brindisi, Italy Received 21 May 2007; received in revised form 24 July 2007; accepted 24 July 2007 Available online 28 July 2007
Abstract In this work a method to densify ceramic coating obtained by electrophoresis and to improve its adhesion to the substrate is proposed. It consists in irradiating the coating surface by electron beam (EB). Alumina and alumina–zirconia coatings were deposited on stainless steel substrates and treated by low power EB. SEM, XRD and TEM characterizations demonstrated that the sintering occurred. Moreover, it is shown that on alumina–zirconia coating the EB irradiation produced a composite material consisting principally of tetragonal zirconia particles immersed in an amorphous alumina matrix. The adhesion stress of EB treated coating was estimated by stud pull test and it was found to be comparable to that of plasma-sprayed coatings. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Je; 81.40. z Keywords: Alumina; Zirconia; EPD; Electron beam; Microstructure; Adhesion stress
1. Introduction Electrophoretic deposition (EPD) is an attractive coating technique because it is a simple, versatile and not expensive method to produce ceramic coatings on conductive substrates, even of complex geometries [1–4]. However, coatings obtained by EPD exhibit some drawbacks such as low density and poor adhesion to substrate. Sintering process can increase the coating density but, due to the different thermal properties between ceramic coating and metallic substrate, it could cause at the same time coating cracking and delamination. Furthermore, the metallic support should be resistant to the sintering high temperature. For ceramic densification microwave or laser treatments are also used, often combined with heating treatments at temperature lower than those required by ceramic sintering [5,6]. In microwave treatments the control of dielectric properties is a critical issue, while laser beam treatment is successfully used in surface re-melting of sintered ceramic coating [7–10], much less for sintering of green powder coating [6]. In this work an alternative method has been used to densify the electrophoretic deposit and to make it more adherent, based
on the irradiation of the green ceramic coating surface by a high power density energy source, such as an electron beam (EB). Compared to laser energy sources, in EB treatment the reflectivity by the irradiated material is lower and therefore the beam efficiency is higher. Moreover, the thickness of affected material can be varied by changing the beam power: 10 kW power is usually employed to cut or weld materials, whereas lower power allows to treat few micrometers of the material surface. In our work, EB treatment has been used to sinter the EPD coating, by confining the high temperature to the ceramic material without affecting deeply the metallic substrate. Alumina and alumina–zirconia suspensions were used to coat stainless steel substrates by EPD. Alumina is the most diffuse material in advanced ceramic processing; moreover, alumina dispersed in sintered tetragonal zirconia induces transformation toughness improving mechanical properties. After coating deposition, EB was used to irradiate the surface of coated stainless steel. Microstructural characterization was performed by X-ray diffraction and transmission electron microscopy. In order to evaluate the adhesion between coating and substrate, pull tests were performed. 2. Experimental details
* Corresponding author. Tel.: +39 831 201472; fax: +39 831 201540. E-mail address: [email protected] (M.F. De Riccardis). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.164
Submicrometric a-Al2O3 powder (Sumitomo AES 11) with a median particle size of 220 nm, and ZrO2 powder (Inframat
M.F. De Riccardis et al. / Applied Surface Science 254 (2008) 1830–1836
Advanced Materials) having monoclinic phase and median particle size of 750 nm were used to prepare a suspension with 5 wt.% of solid in ethylic alcohol, as described in a previous work [11]. Stainless steel (AISI 316L) plates with exposed area of 4 cm2 were used as substrates. Electrophoretic deposition was carried out on as-received stainless steel (SS) substrates as well as on sandblasted SS (Ra = 0.5 mm) substrates; in some cases, a bond layer of 500 nm thick titanium was deposited before EPD on as-received SS substrate by radio frequency (rf) sputtering. Electrophoretic deposition was performed by using a voltage supply; for both the suspensions, voltage and deposition time were optimized on differently treated substrates in order to obtain deposits of 3.5 mg/cm2 on average, with uniform thickness and composition. Generally it was sufficient a deposition time of about 3 min and an electric field of 20 or 40 V/cm, depending on material [11]. Immediately after deposition, alumina and alumina–zirconia coatings were dried in ethanol vapours for 24 h; in this way, alumina (A) or alumina–zirconia (AZ) coatings on SS were obtained. Heat treatments were performed on ceramic coating–metal substrate system by Lenton furnace in nitrogen flux, if not otherwise indicated. Whole pre- and post-deposition conditions of the most representative samples are reported in Table 1. To sinter the ceramic coating, electron beam (Lara 52 TECHMETA) operating with maximum acceleration voltage of 80 KV and maximum beam current of 0.8 A was used in focused beam condition. The EB parameters, i.e. beam current, accelerating voltage, beam scanning frequency and speed of the specimen table, were varied in order to optimize the sintering process. The effect of EB parameters variation is usually evaluated through the irradiated energy density (fluence). EB fluence was varied between 5.5 and 13.75 J/mm2, as reported in Table 1. SEM, TEM and XRD characterizations were performed, respectively, by XL40 LaB6 Philips scanning electron
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Fig. 1. EB track on EPD coating observed at low magnification (a); inside the EB track, some cracks and pores are visible (b).
microscope, TECNAI G2 F30 transmission electron microscope, operating at 300 kV, and Multi-Purpose Diffractometer Philips X’Pert with Cu Ka radiation. Stud pull tests and scratch tests were conducted by using Romulus III-A (Quad Group).
Table 1 Experimental conditions of EPD and EB treatments: pre-EPD treatment was sandblasting (SB) or application of a 550 nm Ti bond layer (B); post-deposition thermal treatments were carried out at the indicated conditions System
Designation
Pre-deposition treatment
Post-deposition treatment
Al2O3
A1 A2 A3 A4 A5 A6
– – SB SB B B
– 1 h at 900 8C in N2 – 1 h at 900 8C in N2 – 1 h at 550 8C in air
Al2O3 + ZrO2
AZ1 AZ2 AZ3 AZ4 AZ5 AZ6 AZ7 AZ8 AZ9 AZ10 AZ11
– – – SB SB SB SB B B B B
– 1h 1h – – 1h 1h – 1h 1h 1h
at 900 8C in N2 at 900 8C in N2
at 900 8C in N2 at 900 8C in N2 at 550 8C in air at 550 8C in air at 550 8C in N2
Electron beam fluence (J/mm2) 7.5 7.5 7.5 7.5 7.5 7.5 13.75 5.5 10 10 13.75 10 13.75 10 6.25 10 10
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3. Results and discussion In order to improve the sintering process, it is convenient that the ceramic coating is composed of two materials with different grain sizes and melting point. Here alumina mixed to zirconia was used, since in principle a composite material with metastable structures and improved properties would be created. Sandblasted, Ti bonded and as-received SS substrates were subjected to thermal treatments after electrophoretic deposition in order to further improve the adhesion between coating and
substrate. In these post-deposition heat treatments, the temperature was varied between 500 and 950 8C under inert atmosphere (N2) or in air. In order to not damage the substrate, the temperatures were intentionally kept lower than sintering temperature of the ceramic materials. An inert atmosphere was used in order to minimize the metallic substrate oxidation that was observed at temperature higher than 800 8C. In heating treatment on ceramic coated stainless steel with a Ti bond coat, a maximum temperature of 550 8C and an inert atmosphere were used to prevent the complete oxidation of titanium.
Fig. 2. SEM images (in SE signal) of typical morphology of alumina ((a) and (b)) and alumina–zirconia ((c) and (d)) coatings. The untreated coatings are reported in (a) and (c), whereas the EPD coatings after EB treatment, respectively, at 7.5 and 10 J/mm2 are visible in (b) and (d). Image (d) is equivalent to image (e) (in BSE signal); here it is possible to appreciate as the alumina and zirconia phases are mixed but still distinguishable.
M.F. De Riccardis et al. / Applied Surface Science 254 (2008) 1830–1836
A first adhesion screening among the samples was made by means of qualitative scratch test. This preliminary test on A and AZ coatings showed a better adhesion for three particular conditions: (i) SS with Ti bond layer and heat treatment at 550 8C, (ii) as-received SS with heat treatment at 900 8C, (iii) sandblasted SS with heat treatment at 900 8C. All prepared samples, with or without post-deposition heat treatment, were irradiated by electron beam. EB treatment parameters were chosen so that the steel substrate was not heavily affected, being the sintering of the ceramic coating the aim of EB treatment. The surface of EB treated samples was observed by using SEM; at low magnification, the EB track is clearly distinguishable because of the different contrast of the signals (Fig. 1a). It can be seen that the untreated regions, adjacent to treated area, do not present evidence of damage, suggesting that the heat produced by EB irradiation was confined to beam incident spot area. Some large pores and cracks inside the coating are evident (Fig. 1b), attributable to sintering shrinkage. Nevertheless, a large part of the coating remained adherent to the steel, especially when the substrate was sandblasted and Ti bond layer was applied. This finding suggests that a sort of physical bond at the interface between coating and metallic substrate occurred. At high magnification SEM images (Fig. 2), the different morphology of EB treated and untreated A and AZ coating was evidenced. The treated area of A coating (Fig. 2b) was more compact than untreated area, with close grains (Fig. 2a). This morphology was observed when the used fluence was 7.5 J/ mm2; at lower fluence values the material was not sintered, whereas at higher fluences the coating was completely damaged, sometimes up to the vaporization of the deposited material. In order to estimate the sintering degree, image analysis on several SEM images of ceramic coating surface, before and after the EB treatment, was performed; in this way the porosity was evaluated about 35% in untreated coating and about 30% in EB treated coating. Certainly, the final porosity of EB treated A coating is higher than a completely sintered ceramic, but local sintering has occurred. In Fig. 2c–e typical structure of AZ coating before and after the EB treatments is reported. Differently from A coatings, at varying fluence the AZ coating seemed sufficiently densified although, as visible in backscattered electron image for one of the samples (Fig. 2e), the alumina and zirconia separate phases were still distinguishable. All AZ coatings exhibited this feature but with different extent, depending on the fluence. The best sintering degree was carried out by EB fluence of 10 J/ mm2. As for A coating, also for AZ coating the porosity was estimated by performing image analysis on coating surface SEM images; in AZ coatings, the porosity decrease due to EB treatment is more evident than in A coatings: it is about 20% in untreated coating and about 8% in EB treated coating. All things considered, by comparing treated A and AZ coatings, it can be deduced that the densification degree of AZ coating was better than that of A coating. As expected, in AZ coating the densification was clearly enhanced by the
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Fig. 3. XRD diagram acquired on EPD mixed coating after deposition (a) and after EB treatment (b); in inset an amorphous structure is recognizable.
different grain size and melting temperature of the two materials. Due to the more successful results, hereafter the discussion will be focused exclusively on this kind of coating. XRD pattern acquired on as-deposited AZ coating showed peaks related to zirconia monoclinic phase and a-alumina (Fig. 3a). EB treatment produced zirconia phase transition from monoclinic phase to metastable tetragonal phase, as shown in Fig. 3b. It is worth to note that in the angular range 20–658, XRD pattern showed a broad peak due to an amorphous phase (inset in Fig. 3b). To perform TEM investigations, the EPD coatings treated with EB were removed from metallic substrate and thinned by ion milling impinging at the glazing angle of 48 with 5 keV Ar2+. Fig. 4 reports two scanning transmission electron microscopy (STEM) images recorded on the same region of AZ coating by using high angle annular dark field (Fig. 4a) and bright field detector (Fig. 4b). The elemental identification was done by X-ray microanalysis. TEM analysis evidenced that the EB treated coating consisted of micro-particles with globular shape and different
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Fig. 4. Representative STEM images of the alumina–zirconia coating; HAADF-STEM (a) and BF-STEM (b) micrographs give an overview of the polycrystalline structure. Three different phases are distinguishable on the basis of different contrast, high-resolution imaging and X-ray microanalysis: zirconia (dot 1), alumina (dot 2) and amorphous phase (dot 3).
size, embedded in a sufficiently homogeneous matrix; three different phases could be distinguished by crystallinity and chemical composition. These phases are well evidenced in the STEM images, showing a roughly complementary contrast: the lightest phase (dot 1) consisting of ZrO2 particles, the light-gray phase of alumina crystals (dot 2). Both phases were embedded in a matrix phase (dot 3) having an amorphous structure and containing also nano-sized crystal particles, as shown on HRTEM image (Fig. 5). X-ray microanalysis indicated that the main constituent of the matrix phase was alumina, whereas zirconia was a second phase. From the experimental results, some considerations on the phenomena occurring during the electron beam processing of AZ coating could be deduced. The peak temperature achieved
Fig. 5. High-resolution TEM images of crystalline phase confining with amorphous matrix (a) and of the amorphous phase (b) where some nano-crystal particles are contained (indicated by arrows).
during the process was higher than the alumina melting point but lower than that of zirconia, sufficient however to allow the transformation of zirconia into the tetragonal phase, almost for the most of the particles. The high thermal gradient, evaluated about 2 106 K m 1, together with the high cooling rate, due to EB irradiation, induced an incomplete melting of the alumina particles and a partial dissolution of the zirconia ones. Once solidified, the melted phase constituted the amorphous matrix of the sintered composite material; in this matrix some crystalline nanoparticles were nucleated during the cooling stage. As also reported in literature, the presence of amorphous phase together with some nano-crystals is caused by competition between cooling rate in liquid phase and highest nucleation rate among all the thermodynamically feasible crystalline phases [12,13]. In our case, high cooling rates promoted
M.F. De Riccardis et al. / Applied Surface Science 254 (2008) 1830–1836
principally amorphous solidification to the detriment of other crystalline formations. Structures consisting of amorphous alumina and crystalline zirconia are not unusual; Muller et al. [14] observed the simultaneous presence of amorphous and crystalline phases in zirconia–alumina particles prepared by laser evaporation. In that case, the achieved temperature and the cooling rate were so high to allow material evaporation and formation of a mixed amorphous–crystal structure [14]. In our experiment, we started from solid zirconia and alumina particles. The temperature achieved was no so high to allow the complete melting of alumina and zirconia particles; nevertheless, a composite nanostructured material was obtained, containing both metastable tetragonal zirconia and amorphous phase with nano-crystals. Besides the positive effect on microstructure, EB treatment also enhanced the adhesion of EPD coating to the metallic
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substrate. To verify the improvement of coating adhesion, stud pull test was conducted. For this test, a stud was glued onto the coating; then an increasing load was applied to the stud. The stress to which detachment between the stud and the sample occurred, was recorded as adhesion stress. The adhesion stress value is significant if the detachment occurs between stud and glue or if the coating delaminates; however, the measured stress values are used only to compare different cases and do not make absolute sense. We observed three different behaviours in ceramic coatings subjected to stud pull test. In the first case, the steel substrate remained almost uncovered by the coating, meaning the adhesion force between coating and substrate was weak. This behaviour was observed on EB treated coating without any treatment of the substrate (Fig. 6a). In the second case, the applied load produced a sort of coating delamination, as observed on Ti bonded substrate. The
Fig. 6. Pull test marks observed on EPD alumina–zirconia coating applied on stainless steel, treated by EB: (a) without any pre- or post-deposition treatment, (b) with application of Ti bond layer, and (c) on sandblasted steel substrate.
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adhesion force between coating and substrate was higher than the coating strength (Fig. 6b). In the third case, the detachment occurred mostly between stud and glue, meaning that the adhesion force between coating and substrate was higher than that one necessary to pull off the stud. This case was observed on EPD coating applied onto sandblasted substrate (Fig. 6c). SEM images of Fig. 6 were acquired by backscattered signal (BSE) in compositional contrast in order to better point out where the detachment occurred. The coating adhesion improvement is qualitatively demonstrated by SEM observations as well as quantitatively by the measured adhesion stress. On as-deposited coating, the stress to which the detachment occurred was 46 kPa; this stress increased up to about 3 MPa on coating EB treated without any pre- or post-deposition treatment and it increased again up to 4 MPa on coating applied on steel with Ti bond layer. The highest stress level of 7 MPa was achieved when the coating was deposited on sandblasted steel. It is worthwhile to mention that the stud pull test does not measure the real value of adhesion stress between metallic substrate and ceramic coating. Since the stress values here reported were evaluated when coating delamination or detachment occurred, it is expected that the effective adhesion stress should have been higher. In conclusion, the electron beam treatment had two positive effects: it increased the ceramic coating density and improved the adhesion between coating and substrate. In particular the adhesion stress values evaluated for coating applied on sandblasted substrate were comparable to those typical of plasma-sprayed coatings [15]. The mechanical and wear properties of this composite material as well as the coating adhesion mechanism are subject of a future study. 4. Conclusions In this work alumina and alumina–zirconia coatings deposited by EPD were densified by electron beam irradiation with the choice of optimal parameters. The microstructural characterization of treated AZ coatings was conducted by X-ray diffraction and transmission electron microscopy. The analyses evidenced that the EB irradiation on alumina–zirconia coating produced a nanostructured ceramic composite material, formed by micro-particles of a-Al2O3 and tetragonal ZrO2, embedded
in an amorphous matrix containing also nano-sized crystals. The porosity, as estimated by image analysis, is reduced by EB treatment; it is not so low as in classical sintered ceramics, specially in EB treated alumina coating. Nevertheless the presence of sometimes quite large pores can be useful in gas or liquid permeation applications. Moreover, it was demonstrated that EB treatment improved the adhesion of AZ EPD coating onto substrate; pull tests evidenced that the adhesion stress between EB treated coating and substrate was higher than that measured on untreated coatings. The measured adhesion stress values were comparable with those of typical plasma-sprayed coatings. The positive results obtained in this work are encouraging; with the described method sintered ceramic coating could be obtained on metallic support having low temperature resistance. Moreover, it was observed that EB irradiation does not damage the material outside the track therefore making possible to perform several adjacent tracks in order to obtain densified coating on large area. Further investigations are in progress. Acknowledgements The authors wish to thank Mr. S. Paiano for EB treatments, Dr. A. Rizzo for RF sputtering deposition, Dr. L. Tapfer and Mr. T. Nocco for XRD measurements. References [1] A.R. Boccaccini, C. Kaya, K.K. Chawa, Compos. Part A 32 (2001) 997. [2] O. Van der Biest, L.J. Vandeperre, Annu. Rev. Mater. Sci. 29 (1999) 327. [3] A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci. 6 (2002) 251. [4] P. Sarkar, P. Nicholson, J. Am. Ceram. Soc. 79 (1996) 1987. [5] R. Peelamedu, A. Badzian, R. Roy, J. Am. Ceram. Soc. 87 (2004) 1806. [6] J. Wang, J. Binner, B. Vaidhynathan, J. Am. Ceram. Soc. 89 (2006) 1977. [7] S.O. Chwa, A. Ohmori, Surf. Coat. Technol. 148 (2001) 88. [8] Y. Fu, A.W. Batchelor, H. Xing, Y. Gu, Wear 210 (1997) 157. [9] R. Krishnan, S. Dash, C. Babu Rao, R.V. Subba Rao, A.K. Tyagi, B. Raj, Scripta Mater. 45 (2001) 693. [10] J. Lawrence, L. Li, J. Phys. D: Appl. Phys. 32 (1999) 1075. [11] M.F. De Riccardis, D. Carbone, A. Rizzo, J. Colloid Interf. Sci. 307 (2007) 109. [12] H.J. Kim, Y.J. Kim, J. Mat. Sci. 34 (1999) 29. [13] T. Ando, Y. Shiohara, J. Am. Ceram. Soc. 74 (1991) 410. [14] E. Muller, C. Oestreich, V. Klemm, E. Brendler, H. Ferkel, W. Reihemann, Part. Part. Syst. Charact. 19 (2002) 169. [15] S. Yilmaz, M. Ipek, G.F. Celebi, C. Bindal, Vacuum 77 (2005) 315.
Electron beam treatments of electrophoretic ceramic coatings M.F. De Riccardis *, D. Carbone, E. Piscopiello, M. Vittori Antisari ENEA-C.R. Brindisi, S.S. 7 APPIA, Km. 712-72100 Brindisi, Italy Received 21 May 2007; received in revised form 24 July 2007; accepted 24 July 2007 Available online 28 July 2007
Abstract In this work a method to densify ceramic coating obtained by electrophoresis and to improve its adhesion to the substrate is proposed. It consists in irradiating the coating surface by electron beam (EB). Alumina and alumina–zirconia coatings were deposited on stainless steel substrates and treated by low power EB. SEM, XRD and TEM characterizations demonstrated that the sintering occurred. Moreover, it is shown that on alumina–zirconia coating the EB irradiation produced a composite material consisting principally of tetragonal zirconia particles immersed in an amorphous alumina matrix. The adhesion stress of EB treated coating was estimated by stud pull test and it was found to be comparable to that of plasma-sprayed coatings. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Je; 81.40. z Keywords: Alumina; Zirconia; EPD; Electron beam; Microstructure; Adhesion stress
1. Introduction Electrophoretic deposition (EPD) is an attractive coating technique because it is a simple, versatile and not expensive method to produce ceramic coatings on conductive substrates, even of complex geometries [1–4]. However, coatings obtained by EPD exhibit some drawbacks such as low density and poor adhesion to substrate. Sintering process can increase the coating density but, due to the different thermal properties between ceramic coating and metallic substrate, it could cause at the same time coating cracking and delamination. Furthermore, the metallic support should be resistant to the sintering high temperature. For ceramic densification microwave or laser treatments are also used, often combined with heating treatments at temperature lower than those required by ceramic sintering [5,6]. In microwave treatments the control of dielectric properties is a critical issue, while laser beam treatment is successfully used in surface re-melting of sintered ceramic coating [7–10], much less for sintering of green powder coating [6]. In this work an alternative method has been used to densify the electrophoretic deposit and to make it more adherent, based
on the irradiation of the green ceramic coating surface by a high power density energy source, such as an electron beam (EB). Compared to laser energy sources, in EB treatment the reflectivity by the irradiated material is lower and therefore the beam efficiency is higher. Moreover, the thickness of affected material can be varied by changing the beam power: 10 kW power is usually employed to cut or weld materials, whereas lower power allows to treat few micrometers of the material surface. In our work, EB treatment has been used to sinter the EPD coating, by confining the high temperature to the ceramic material without affecting deeply the metallic substrate. Alumina and alumina–zirconia suspensions were used to coat stainless steel substrates by EPD. Alumina is the most diffuse material in advanced ceramic processing; moreover, alumina dispersed in sintered tetragonal zirconia induces transformation toughness improving mechanical properties. After coating deposition, EB was used to irradiate the surface of coated stainless steel. Microstructural characterization was performed by X-ray diffraction and transmission electron microscopy. In order to evaluate the adhesion between coating and substrate, pull tests were performed. 2. Experimental details
* Corresponding author. Tel.: +39 831 201472; fax: +39 831 201540. E-mail address: [email protected] (M.F. De Riccardis). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.164
Submicrometric a-Al2O3 powder (Sumitomo AES 11) with a median particle size of 220 nm, and ZrO2 powder (Inframat
M.F. De Riccardis et al. / Applied Surface Science 254 (2008) 1830–1836
Advanced Materials) having monoclinic phase and median particle size of 750 nm were used to prepare a suspension with 5 wt.% of solid in ethylic alcohol, as described in a previous work [11]. Stainless steel (AISI 316L) plates with exposed area of 4 cm2 were used as substrates. Electrophoretic deposition was carried out on as-received stainless steel (SS) substrates as well as on sandblasted SS (Ra = 0.5 mm) substrates; in some cases, a bond layer of 500 nm thick titanium was deposited before EPD on as-received SS substrate by radio frequency (rf) sputtering. Electrophoretic deposition was performed by using a voltage supply; for both the suspensions, voltage and deposition time were optimized on differently treated substrates in order to obtain deposits of 3.5 mg/cm2 on average, with uniform thickness and composition. Generally it was sufficient a deposition time of about 3 min and an electric field of 20 or 40 V/cm, depending on material [11]. Immediately after deposition, alumina and alumina–zirconia coatings were dried in ethanol vapours for 24 h; in this way, alumina (A) or alumina–zirconia (AZ) coatings on SS were obtained. Heat treatments were performed on ceramic coating–metal substrate system by Lenton furnace in nitrogen flux, if not otherwise indicated. Whole pre- and post-deposition conditions of the most representative samples are reported in Table 1. To sinter the ceramic coating, electron beam (Lara 52 TECHMETA) operating with maximum acceleration voltage of 80 KV and maximum beam current of 0.8 A was used in focused beam condition. The EB parameters, i.e. beam current, accelerating voltage, beam scanning frequency and speed of the specimen table, were varied in order to optimize the sintering process. The effect of EB parameters variation is usually evaluated through the irradiated energy density (fluence). EB fluence was varied between 5.5 and 13.75 J/mm2, as reported in Table 1. SEM, TEM and XRD characterizations were performed, respectively, by XL40 LaB6 Philips scanning electron
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Fig. 1. EB track on EPD coating observed at low magnification (a); inside the EB track, some cracks and pores are visible (b).
microscope, TECNAI G2 F30 transmission electron microscope, operating at 300 kV, and Multi-Purpose Diffractometer Philips X’Pert with Cu Ka radiation. Stud pull tests and scratch tests were conducted by using Romulus III-A (Quad Group).
Table 1 Experimental conditions of EPD and EB treatments: pre-EPD treatment was sandblasting (SB) or application of a 550 nm Ti bond layer (B); post-deposition thermal treatments were carried out at the indicated conditions System
Designation
Pre-deposition treatment
Post-deposition treatment
Al2O3
A1 A2 A3 A4 A5 A6
– – SB SB B B
– 1 h at 900 8C in N2 – 1 h at 900 8C in N2 – 1 h at 550 8C in air
Al2O3 + ZrO2
AZ1 AZ2 AZ3 AZ4 AZ5 AZ6 AZ7 AZ8 AZ9 AZ10 AZ11
– – – SB SB SB SB B B B B
– 1h 1h – – 1h 1h – 1h 1h 1h
at 900 8C in N2 at 900 8C in N2
at 900 8C in N2 at 900 8C in N2 at 550 8C in air at 550 8C in air at 550 8C in N2
Electron beam fluence (J/mm2) 7.5 7.5 7.5 7.5 7.5 7.5 13.75 5.5 10 10 13.75 10 13.75 10 6.25 10 10
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3. Results and discussion In order to improve the sintering process, it is convenient that the ceramic coating is composed of two materials with different grain sizes and melting point. Here alumina mixed to zirconia was used, since in principle a composite material with metastable structures and improved properties would be created. Sandblasted, Ti bonded and as-received SS substrates were subjected to thermal treatments after electrophoretic deposition in order to further improve the adhesion between coating and
substrate. In these post-deposition heat treatments, the temperature was varied between 500 and 950 8C under inert atmosphere (N2) or in air. In order to not damage the substrate, the temperatures were intentionally kept lower than sintering temperature of the ceramic materials. An inert atmosphere was used in order to minimize the metallic substrate oxidation that was observed at temperature higher than 800 8C. In heating treatment on ceramic coated stainless steel with a Ti bond coat, a maximum temperature of 550 8C and an inert atmosphere were used to prevent the complete oxidation of titanium.
Fig. 2. SEM images (in SE signal) of typical morphology of alumina ((a) and (b)) and alumina–zirconia ((c) and (d)) coatings. The untreated coatings are reported in (a) and (c), whereas the EPD coatings after EB treatment, respectively, at 7.5 and 10 J/mm2 are visible in (b) and (d). Image (d) is equivalent to image (e) (in BSE signal); here it is possible to appreciate as the alumina and zirconia phases are mixed but still distinguishable.
M.F. De Riccardis et al. / Applied Surface Science 254 (2008) 1830–1836
A first adhesion screening among the samples was made by means of qualitative scratch test. This preliminary test on A and AZ coatings showed a better adhesion for three particular conditions: (i) SS with Ti bond layer and heat treatment at 550 8C, (ii) as-received SS with heat treatment at 900 8C, (iii) sandblasted SS with heat treatment at 900 8C. All prepared samples, with or without post-deposition heat treatment, were irradiated by electron beam. EB treatment parameters were chosen so that the steel substrate was not heavily affected, being the sintering of the ceramic coating the aim of EB treatment. The surface of EB treated samples was observed by using SEM; at low magnification, the EB track is clearly distinguishable because of the different contrast of the signals (Fig. 1a). It can be seen that the untreated regions, adjacent to treated area, do not present evidence of damage, suggesting that the heat produced by EB irradiation was confined to beam incident spot area. Some large pores and cracks inside the coating are evident (Fig. 1b), attributable to sintering shrinkage. Nevertheless, a large part of the coating remained adherent to the steel, especially when the substrate was sandblasted and Ti bond layer was applied. This finding suggests that a sort of physical bond at the interface between coating and metallic substrate occurred. At high magnification SEM images (Fig. 2), the different morphology of EB treated and untreated A and AZ coating was evidenced. The treated area of A coating (Fig. 2b) was more compact than untreated area, with close grains (Fig. 2a). This morphology was observed when the used fluence was 7.5 J/ mm2; at lower fluence values the material was not sintered, whereas at higher fluences the coating was completely damaged, sometimes up to the vaporization of the deposited material. In order to estimate the sintering degree, image analysis on several SEM images of ceramic coating surface, before and after the EB treatment, was performed; in this way the porosity was evaluated about 35% in untreated coating and about 30% in EB treated coating. Certainly, the final porosity of EB treated A coating is higher than a completely sintered ceramic, but local sintering has occurred. In Fig. 2c–e typical structure of AZ coating before and after the EB treatments is reported. Differently from A coatings, at varying fluence the AZ coating seemed sufficiently densified although, as visible in backscattered electron image for one of the samples (Fig. 2e), the alumina and zirconia separate phases were still distinguishable. All AZ coatings exhibited this feature but with different extent, depending on the fluence. The best sintering degree was carried out by EB fluence of 10 J/ mm2. As for A coating, also for AZ coating the porosity was estimated by performing image analysis on coating surface SEM images; in AZ coatings, the porosity decrease due to EB treatment is more evident than in A coatings: it is about 20% in untreated coating and about 8% in EB treated coating. All things considered, by comparing treated A and AZ coatings, it can be deduced that the densification degree of AZ coating was better than that of A coating. As expected, in AZ coating the densification was clearly enhanced by the
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Fig. 3. XRD diagram acquired on EPD mixed coating after deposition (a) and after EB treatment (b); in inset an amorphous structure is recognizable.
different grain size and melting temperature of the two materials. Due to the more successful results, hereafter the discussion will be focused exclusively on this kind of coating. XRD pattern acquired on as-deposited AZ coating showed peaks related to zirconia monoclinic phase and a-alumina (Fig. 3a). EB treatment produced zirconia phase transition from monoclinic phase to metastable tetragonal phase, as shown in Fig. 3b. It is worth to note that in the angular range 20–658, XRD pattern showed a broad peak due to an amorphous phase (inset in Fig. 3b). To perform TEM investigations, the EPD coatings treated with EB were removed from metallic substrate and thinned by ion milling impinging at the glazing angle of 48 with 5 keV Ar2+. Fig. 4 reports two scanning transmission electron microscopy (STEM) images recorded on the same region of AZ coating by using high angle annular dark field (Fig. 4a) and bright field detector (Fig. 4b). The elemental identification was done by X-ray microanalysis. TEM analysis evidenced that the EB treated coating consisted of micro-particles with globular shape and different
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Fig. 4. Representative STEM images of the alumina–zirconia coating; HAADF-STEM (a) and BF-STEM (b) micrographs give an overview of the polycrystalline structure. Three different phases are distinguishable on the basis of different contrast, high-resolution imaging and X-ray microanalysis: zirconia (dot 1), alumina (dot 2) and amorphous phase (dot 3).
size, embedded in a sufficiently homogeneous matrix; three different phases could be distinguished by crystallinity and chemical composition. These phases are well evidenced in the STEM images, showing a roughly complementary contrast: the lightest phase (dot 1) consisting of ZrO2 particles, the light-gray phase of alumina crystals (dot 2). Both phases were embedded in a matrix phase (dot 3) having an amorphous structure and containing also nano-sized crystal particles, as shown on HRTEM image (Fig. 5). X-ray microanalysis indicated that the main constituent of the matrix phase was alumina, whereas zirconia was a second phase. From the experimental results, some considerations on the phenomena occurring during the electron beam processing of AZ coating could be deduced. The peak temperature achieved
Fig. 5. High-resolution TEM images of crystalline phase confining with amorphous matrix (a) and of the amorphous phase (b) where some nano-crystal particles are contained (indicated by arrows).
during the process was higher than the alumina melting point but lower than that of zirconia, sufficient however to allow the transformation of zirconia into the tetragonal phase, almost for the most of the particles. The high thermal gradient, evaluated about 2 106 K m 1, together with the high cooling rate, due to EB irradiation, induced an incomplete melting of the alumina particles and a partial dissolution of the zirconia ones. Once solidified, the melted phase constituted the amorphous matrix of the sintered composite material; in this matrix some crystalline nanoparticles were nucleated during the cooling stage. As also reported in literature, the presence of amorphous phase together with some nano-crystals is caused by competition between cooling rate in liquid phase and highest nucleation rate among all the thermodynamically feasible crystalline phases [12,13]. In our case, high cooling rates promoted
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principally amorphous solidification to the detriment of other crystalline formations. Structures consisting of amorphous alumina and crystalline zirconia are not unusual; Muller et al. [14] observed the simultaneous presence of amorphous and crystalline phases in zirconia–alumina particles prepared by laser evaporation. In that case, the achieved temperature and the cooling rate were so high to allow material evaporation and formation of a mixed amorphous–crystal structure [14]. In our experiment, we started from solid zirconia and alumina particles. The temperature achieved was no so high to allow the complete melting of alumina and zirconia particles; nevertheless, a composite nanostructured material was obtained, containing both metastable tetragonal zirconia and amorphous phase with nano-crystals. Besides the positive effect on microstructure, EB treatment also enhanced the adhesion of EPD coating to the metallic
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substrate. To verify the improvement of coating adhesion, stud pull test was conducted. For this test, a stud was glued onto the coating; then an increasing load was applied to the stud. The stress to which detachment between the stud and the sample occurred, was recorded as adhesion stress. The adhesion stress value is significant if the detachment occurs between stud and glue or if the coating delaminates; however, the measured stress values are used only to compare different cases and do not make absolute sense. We observed three different behaviours in ceramic coatings subjected to stud pull test. In the first case, the steel substrate remained almost uncovered by the coating, meaning the adhesion force between coating and substrate was weak. This behaviour was observed on EB treated coating without any treatment of the substrate (Fig. 6a). In the second case, the applied load produced a sort of coating delamination, as observed on Ti bonded substrate. The
Fig. 6. Pull test marks observed on EPD alumina–zirconia coating applied on stainless steel, treated by EB: (a) without any pre- or post-deposition treatment, (b) with application of Ti bond layer, and (c) on sandblasted steel substrate.
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adhesion force between coating and substrate was higher than the coating strength (Fig. 6b). In the third case, the detachment occurred mostly between stud and glue, meaning that the adhesion force between coating and substrate was higher than that one necessary to pull off the stud. This case was observed on EPD coating applied onto sandblasted substrate (Fig. 6c). SEM images of Fig. 6 were acquired by backscattered signal (BSE) in compositional contrast in order to better point out where the detachment occurred. The coating adhesion improvement is qualitatively demonstrated by SEM observations as well as quantitatively by the measured adhesion stress. On as-deposited coating, the stress to which the detachment occurred was 46 kPa; this stress increased up to about 3 MPa on coating EB treated without any pre- or post-deposition treatment and it increased again up to 4 MPa on coating applied on steel with Ti bond layer. The highest stress level of 7 MPa was achieved when the coating was deposited on sandblasted steel. It is worthwhile to mention that the stud pull test does not measure the real value of adhesion stress between metallic substrate and ceramic coating. Since the stress values here reported were evaluated when coating delamination or detachment occurred, it is expected that the effective adhesion stress should have been higher. In conclusion, the electron beam treatment had two positive effects: it increased the ceramic coating density and improved the adhesion between coating and substrate. In particular the adhesion stress values evaluated for coating applied on sandblasted substrate were comparable to those typical of plasma-sprayed coatings [15]. The mechanical and wear properties of this composite material as well as the coating adhesion mechanism are subject of a future study. 4. Conclusions In this work alumina and alumina–zirconia coatings deposited by EPD were densified by electron beam irradiation with the choice of optimal parameters. The microstructural characterization of treated AZ coatings was conducted by X-ray diffraction and transmission electron microscopy. The analyses evidenced that the EB irradiation on alumina–zirconia coating produced a nanostructured ceramic composite material, formed by micro-particles of a-Al2O3 and tetragonal ZrO2, embedded
in an amorphous matrix containing also nano-sized crystals. The porosity, as estimated by image analysis, is reduced by EB treatment; it is not so low as in classical sintered ceramics, specially in EB treated alumina coating. Nevertheless the presence of sometimes quite large pores can be useful in gas or liquid permeation applications. Moreover, it was demonstrated that EB treatment improved the adhesion of AZ EPD coating onto substrate; pull tests evidenced that the adhesion stress between EB treated coating and substrate was higher than that measured on untreated coatings. The measured adhesion stress values were comparable with those of typical plasma-sprayed coatings. The positive results obtained in this work are encouraging; with the described method sintered ceramic coating could be obtained on metallic support having low temperature resistance. Moreover, it was observed that EB irradiation does not damage the material outside the track therefore making possible to perform several adjacent tracks in order to obtain densified coating on large area. Further investigations are in progress. Acknowledgements The authors wish to thank Mr. S. Paiano for EB treatments, Dr. A. Rizzo for RF sputtering deposition, Dr. L. Tapfer and Mr. T. Nocco for XRD measurements. References [1] A.R. Boccaccini, C. Kaya, K.K. Chawa, Compos. Part A 32 (2001) 997. [2] O. Van der Biest, L.J. Vandeperre, Annu. Rev. Mater. Sci. 29 (1999) 327. [3] A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci. 6 (2002) 251. [4] P. Sarkar, P. Nicholson, J. Am. Ceram. Soc. 79 (1996) 1987. [5] R. Peelamedu, A. Badzian, R. Roy, J. Am. Ceram. Soc. 87 (2004) 1806. [6] J. Wang, J. Binner, B. Vaidhynathan, J. Am. Ceram. Soc. 89 (2006) 1977. [7] S.O. Chwa, A. Ohmori, Surf. Coat. Technol. 148 (2001) 88. [8] Y. Fu, A.W. Batchelor, H. Xing, Y. Gu, Wear 210 (1997) 157. [9] R. Krishnan, S. Dash, C. Babu Rao, R.V. Subba Rao, A.K. Tyagi, B. Raj, Scripta Mater. 45 (2001) 693. [10] J. Lawrence, L. Li, J. Phys. D: Appl. Phys. 32 (1999) 1075. [11] M.F. De Riccardis, D. Carbone, A. Rizzo, J. Colloid Interf. Sci. 307 (2007) 109. [12] H.J. Kim, Y.J. Kim, J. Mat. Sci. 34 (1999) 29. [13] T. Ando, Y. Shiohara, J. Am. Ceram. Soc. 74 (1991) 410. [14] E. Muller, C. Oestreich, V. Klemm, E. Brendler, H. Ferkel, W. Reihemann, Part. Part. Syst. Charact. 19 (2002) 169. [15] S. Yilmaz, M. Ipek, G.F. Celebi, C. Bindal, Vacuum 77 (2005) 315.