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Thermal diffusivity and erosion resistance of ZrO2–8 wt.% Y2O3 coatings prepared by a laser hybrid spraying technique
Thin Solid Films 415 (2002) 160–166
Thermal diffusivity and erosion resistance of ZrO2 –8 wt.% Y2O3 coatings prepared by a laser hybrid spraying technique Sang Ok Chwa*, Akira Ohmori Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, 567-0047 Osaka, Japan Received 6 February 2002; received in revised form 26 April 2002; accepted 5 June 2002
Abstract The results of thermal diffusivity measurement revealed that post-treated thermal barrier coatings (TBCs), which contain a remelted dense layer with numerous vertical cracks, are not effective in reducing the heat transfer. However, laser–hybrid sprayed TBCs with segmented feather-like microcracks decreased the thermal diffusivity by 60% of the as-sprayed value at the specific laser–hybrid condition (Ps120 W mmy2 ). The post-treated TBCs with dense layers endured at the early stage of erosion but collapsed gradually due to consecutively impacting particles. On the other hand, weight losses of laser–hybrid sprayed coatings (Ps80 and 120 W mmy2) were decreased compared with the usual sprayed coatings and the erosion rates were also constant even if blasted by much more erodent than used for post-treated samples (erodent weight, 210 g). From the results of this work, it is suggested that the laser–hybrid spraying process can be a good choice for production of durable TBCs used in hot sandy environments. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: TBCs; Laser treatment; Microstructure; Thermal diffusivity; Erosion
1. Introduction Thermal barrier coatings (TBCs) have been widely used for the protection of hot-section components used in gas turbine engines for aircrafts and power generators w1–4x. A typical TBC system consists of an oxidationresistant metallic layer on the superalloy and the topcoat of a partially stabilized zirconia (PSZ) layer, which provides thermal insulation. Among the various top-coat materials of PSZ, yttria partially stabilized zirconia (YPSZ) is the currently preferred top-coat material for gas turbine engine applications because it has a low thermal conductivity and relatively good match of thermal expansion coefficient with the superalloy when compared to other ceramics and good erosion resistance w5x. Generally, the YPSZ top-coat is deposited by either electron beam physical vapor deposition (EB-PVD) or air plasma spraying (APS). The EB-PVD process gives *Corresponding author. Tel.: q81-6-6879-8683; fax: q81-6-68798683. E-mail address: [email protected] (S.O. Chwa).
columnar microstructure with defects such as intercolumnar pores and grain boundaries running normal to the plane of coating w6–9x. These aligned columnar microstructures are effective at improving the strain tolerance but are not favorable for reducing heat transfer through the thickness of the TBC coating. In contrast, coatings prepared by the APS method are characterized as layers with randomly distributed inter-splat pores, which resulted from impingement of molten droplets on to a substrate. These defects, while effective in reducing the heat flux, leave the top-coat relatively less straintolerant and are a source of weakness that ultimately result in the TBC delamination andyor spallation. To enhance the lifetime of TBC coatings prepared by APS under thermal shock, laser irradiations have been tried, and it was shown that the vertical cracks in coatings induced by laser treatment improved the thermal shock resistance w10–13x. On the other hand, such a microstructural modification by laser treatment can alter the path of heat transfer and a wear property of coatings. However, as far as we know, no reports can
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 5 3 3 - 3
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chamber equipped with an exhaust system. Nd:YAG laser (NEC, Kanagawa, Japan) working in continuous wave mode at a wave-length of 1.06 mm was used. The laser beam coming from the oscillator was delivered by optical fiber to the laser nozzle w15x. The samples for the laser treatment were fixed on the X–Y table, moved by a six-axes articulated robot. For the uniform laser treatment of sprayed coatings, a kaleidoscope composed of two pairs of gold mirrors (3.6=51 mm2) was installed in a conventional YAG laser, and the laser system was combined with the plasma spraying equipment w13,16x. The schematic diagram of the laser– hybrid plasma spraying process is depicted in Fig. 1. The post-treatment and hybrid spraying were carried out under the conditions given in Table 1. Laser power density, P (W mmy2) and energy density E (J mmy2), were calculated from the following equations:
Fig. 1. The schematic illustration of laser–hybrid plasma spraying process.
be found about the thermal diffusivity of laser-treated coatings, especially the laser–hybrid sprayed TBCs. Here, not only the investigation of thermal diffusivity and wear property of laser-treated coatings, but also the control of those properties in TBC coatings obtained by employing the laser–hybrid spraying process were tried w14x. 2. Materials and experimental procedures 2.1. Materials Thermal barrier coatings consisting of 8 wt.% YPSZ top-coat and CoNiCrAlY bond-coat were prepared on Ni-superalloy (Inconel 738 LC) specimens with a diameter of 10 mm and a thickness of 5 mm for thermal diffusivity measurements and JIS SUS 304 plates for erosion tests using commercial powders (Showa Denko, Nagano, Japan). CoNiCrAlY bond coating of approximately 100 mm thickness was deposited at a pressure of 13.30 kPa argon gas atmosphere, and then a ZrO2 coating with a thickness of 330 mm was produced in air by using the general plasma spraying conditions for TBCs. 2.2. Laser combined plasma spraying system Post-treatments of pre-deposited coatings were carried out using the laser combined spraying system. Both the plasma gun and the laser gun were installed inside a
PsWyA
(1)
EsPyLV
(2)
where, W is the measured laser power, A is the area of a rectangular beam spot emerged on the specimen, L is a length of beam, and V is the transverse speed of the X–Y table. The transverse speed of the sample for laser– hybrid treatments was fixed at the same conditions as used for the non-hybrid spraying process. 2.3. Vacuum impregnation The mechanical stresses that arise during the mounting, cutting, grinding, and polishing operations of usual metallographic procedures can result in material pullout and alter the structural features of segmented microcracks w17x. For the careful observation of the microstructure of as-sprayed and laser-treated coatings, Spurr resin with a very low viscosity of 6=10y2 PaØsy1 was used as an embedding material for vacuum impregnation w14,18x. The specimen mounted in a mold was placed in a vacuum furnace, and then, resin mix was poured over the specimen. After pumping to reach the pressure less than 10y2 Pa, the vacuum was broken and vented to atmospheric pressure to accelerate the infiltration of resin, and this procedure was repeated three times, and then, the sample was cured at 345 K for 48 h. Table 1 Nd:YAG laser parameters used for post-treatment and hybrid process Process
Post-treatment
Hybrid treatment
Power (kW) Beam shape and size (mm2) Scan speed (mm sy1) Shield gas flow rate (Ar, m3 sy1)
1.0–4.0 Rectangular beam (7=7–12=12) 8–100 1.67=10y4
2.0–4.0 Rectangular beam (5=5) 250 1.67=10y4
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powders with an average grain size of 330 mm, are accelerated by precisely controlled compressed air to get a particle velocity of approximately 60 m sy1. The wear of the nozzle during the test was strictly monitored. The impact angle was fixed at 608, and the experiment was carried out at room temperature. Mass loss from a sample is determined by weighing before and after each exposure with a precise valance. Details of our erosion apparatus could be found elsewhere w19x. 3. Results and discussions 3.1. Microstructure and thermal diffusivity of posttreated coatings Fig. 3 shows the SEM cross-sections of as-sprayed and laser-treated coatings as a function of power and Fig. 2. A scheme of the ACT–JP (Arata coating tester with jet particles) erosion test.
2.4. Characterization of coatings The phase composition of the sprayed and laser treated coatings was investigated by X-ray diffractome˚ with a ter using Cu Ka1 radiation (ls1.5405 A) graphite crystal monocromator (JDX3530, JEOL, Tokyo, Japan). The surface morphology and cross-section of coatings were observed by the scanning electron microscopy (SEM) (JSM-6400, JEOL, Tokyo, Japan). The micro-hardness of the coating was measured using a Vickers testing machine at a load of 2000 g for 15 s. 2.5. Thermal diffusivity measurement Thermal diffusivity was measured on a specimen consisting of three layers of materials including the Nisuperalloys (Inconel 738 LC) using the laser flash method (TC-7000, ULVAC-RIKO, Tokyo, Japan). After finishing the post-laser treatment or hybrid spraying, specimens composed of three layers (YPSZyCoNiCrAlYyInconel 738 LC) were cut by electrical discharge machining to obtain a total thickness of 1 mm, and then the specimens were carefully polished to reach a thickness of 0.95 ("0.02) mm with a diamond paste. To enhance the absorption and emission of the laser beam, thin layers of carbon were deposited in a vacuum evaporator. The metal substrate side was always positioned towards the laser flash at each measurement. 2.6. Erosion test Wear property of post-treated and laser–hybrid sprayed coatings was evaluated by means of the erosion tester, ACT–JP (Arata coating tester with jet particles) as shown in Fig. 2. The jet particles, angular Al2O3
Fig. 3. SEM cross-sections of as-sprayed and laser-treated coatings as a function of power and energy density of the laser beam: (a) assprayed coating; (b,c) post-treated coatings at the power and energy density of 20.4 W mmy2, Es17 J mmy2 and 30.6 W mmy2, Es21 J mmy2, respectively.
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diffusivity after laser treatment was mainly due to the structural changes of coatings. Laser-treatment of spayed zirconia TBCs have been extensively carried out to improve their poor spallation resistance by the intentional introduction of vertical cracks formed from the cooling down procedure of remelted coatings. However, from the result of the thermal diffusivity measurement of post-treated sprayed coatings, it was found that densification and full remelting of sprayed layers are not effective at reducing the heat flux through the TBCs. Accordingly, it can be suggested that the post-treatment, generally tried for improving the life-time of TBCs in hot section components of gas turbine, have to be carefully monitored to optimize the remelting depth and crack density of sprayed coatings for escaping the significantly increased heat transfer problems. Fig. 4. Thermal diffusivity of post-treated TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of laser power density.
energy density of the laser beam. In the laser-treated coating (Ps20.4 W mmy2, Es17 J mmy2), pores of the surface region were slightly decreased and a relatively good contact between the splats was observed compared with the as-sprayed coating (see Fig. 3a,b). After the laser treatment at increased power and energy density (Ps30.6 W mmy2, Es21 J mmy2), the surface region of the coating layer was fully melted and the vertical cracks were formed (Fig. 3c). Fig. 4 shows the thermal diffusivity of TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of post-treatment laser power density. When the normalized value divided by the thermal diffusivity of the as-sprayed coating was compared, approximately 20% increase of thermal diffusivity was observed in the laser-treated coating (Ps20.4 W mmy2, Es17 J mmy2) which showed a relatively good contact between the splats in the surface region compared with the as-sprayed coating. Generally, the sprayed coatings have very thin interlamellar pores or cracks with a thickness of a few tenths of micrometers. Therefore, it is considered that the increase of contact between those lamellars after laser treatment lowers the thermal resistance between the splats and, consequently, results in the higher thermal diffusivity of the coating. Laser-treated coatings (Ps 30.6 W mmy2, Es21 J mmy2), which have a thoroughly remelted layer and sharp vertical cracks, showed approximately 85% increase of thermal diffusivity compared with the value for as-sprayed coatings. From the XRD analysis, no considerable phase changes were observed and a non-transformable t9 phase was still predominant even after the remelting of coating layers by the laser w13x. So, the drastic increase of thermal
3.2. Microstructure and thermal diffusivity of laser– hybrid sprayed coatings It was previously explained that post-laser treatments of coatings resulted in mainly vertical cracks, which extended to the interface of the bond-coat and top-coat. In the case of coatings prepared by hybrid spraying processes, the feature of cracks in the coating was different from the post-treated coatings as shown in Fig. 5. The feature of cracks in the coatings prepared by hybrid spraying processes was characterized as a featherlike structure that consisted of a vertical crack with a significant number of branching cracks that were not observed in the post-treated coatings. From our previous study, it was found that TBCs with such microstructure also have a good thermal shock resistance w14x. Bengtsson et al. w20x suggested that the better the contact between the splat and underlying material is achieved, large tensile stress is developed. This causes the initiation and propagation of vertical cracks thus acting as stress relievers. From the results of microhardness measurements carried out in non-cracked regions and porosity analysis (see Fig. 6), it can be conferred that considerable improvement of bonding leads to strengthening between the sprayed lamellars attained during the laser hybrid spraying process, and the cracks become longer and wider as shown in Fig. 5 to release such an increased stress. The formation of branching cracks during the hybrid process is not clearly understood, but the following is considered to be one of the reasons. In hybrid spraying processes, the area of molten streaming sprayed particles toward the substrate are approximately twice of the simultaneously irradiating laser beam spot. So, this causes the local difference of the substrate and particle temperature between the laser-treated and non-treated region. In consequence, the complex structure which has vertical and branching cracks is considered due to the
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Fig. 5. SEM cross-sections of laser–hybrid sprayed coatings as a function of laser power density: (a–d) is hybrid sprayed coating at the power density of a laser beam of 80, 100, 120 and 140 W mmy2, respectively.
mechanical bonding disparity between the good contact layer and relatively insufficient connected layer on the underlying material during the hybrid deposition process w14x. Fig. 7 shows the thermal diffusivity of laser–hybrid TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of laser power density. Even in the improvement of contacts between splats and a decrease
of dispersed pores, there was no considerable increase of thermal diffusivity in the laser–hybrid plasma TBCs. Moreover, it was possible to decrease the thermal diffusivity by 60% of the as-sprayed value at a specific laser–hybrid condition (Ps120 W mmy2). As shown in our previous report, the branching crack density and length showed an optimum value at the hybrid–laser power density of ;120 W mmy2 w14x. Therefore, it is
Fig. 6. Micro-hardness and porosity of laser–hybrid sprayed coatings as a function of laser power density.
Fig. 7. Thermal diffusivity of laser–hybrid TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of laser power density.
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Therefore, the coating treated at more increased laser power and energy density (Ps40.8 W mmy2, Es29 J mmy2) severely eroded away from the early stage of the test and showed an unsteady erosion rate. Fig. 9 shows a weight loss and erosion rate of laser– hybrid sprayed coatings at various laser power densities. Weight losses of laser–hybrid sprayed coatings (Ps80 and 120 W mmy2) were decreased compared with the usual sprayed coatings and the erosion rates were also constant even if blasted by much more erodent than used for post-treated samples (erodent weight, 210 g). However, the hybrid sprayed coating, prepared at the excessively high laser power density (Ps160 W mmy2), is not so tolerable against particulate erosion. Therefore, it is concluded that the wear property of TBCs can be significantly improved by applying a suitable laser–hybrid process, which forms uniformly distributed fine feather-like micro-cracks. Fig. 8. Weight loss and erosion rate of post-treated coatings at various power and energy densities of the laser beam.
considered that such an intentionally introduced branching crack effectively retarded the heat transfer even though the interlamellar gap between splats and dispersed pores, which act as a heat impeder, decreased from the laser–hybrid treatment. However, thermal diffusivity of TBCs prepared at higher power density (Ps 140 W mmy2) was slightly increased because the higher laser power results in the formation of very dense layers and columnar structures, as shown in Fig. 5d, and the reduction of heat transfer from the segmented structures was compensated. 3.3. Erosion property of post-treated and laser–hybrid sprayed coatings Fig. 8 shows a weight loss and erosion rate of posttreated coatings at various power and energy densities of laser beam. It can be noticed that weight loss of assprayed coatings and laser-treated coating (Ps20.4 W mmy2, Es17 J mmy2) is directly proportional to the applied erodent weight and the erosion rate is almost constant after 20 g of abrasive. This is due to the structure of the overall coating layers though the depth direction is not considerably changed, as shown in Fig. 3a,b. On the other hand, the laser-treated coating (Ps 30.6 W mmy2, Es21 J mmy2) shows the low weight loss up to 40 g of abrasive, and then more higher erosion wear compared with the as-sprayed coatings. It can be explained by the fact that a remelted dense coating with sharp vertical cracks endured at the early stage of erosion but collapsed gradually due to consecutively impacting particles. It was confirmed that laser-treated coatings at an excessive power and energy density have numerous vertical cracks of large width and defects such as entrapped gas bubbles from previous researches w14,21x.
4. Conclusions The results of thermal diffusivity measurement revealed that post-treated coatings (Ps30.6 W mmy2, Es21 J mmy2), which have a thoroughly remelted layer and sharp vertical cracks, showed approximately 85% increase of thermal diffusivity compared with the value for as-sprayed coatings. Accordingly, it can be concluded that the post-treatment process, generally tried for improving the life-time of TBCs in hot section components of gas turbine, have to be carefully monitored to optimize the remelting depth and crack density of sprayed coatings for escaping the significantly increased heat transfer problems. However, laser–hybrid sprayed TBCs with segmented feather-like microcracks
Fig. 9. Weight loss and erosion rate of laser–hybrid sprayed coatings at various power densities of the laser beam.
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decreased the thermal diffusivity by 60% of the assprayed value at the specific laser–hybrid conditions (Ps120 W mmy2). The post-treated TBCs with dense layers endured at the early stage of erosion but collapsed gradually due to consecutively impacting particles. Weight losses of laser–hybrid sprayed coatings (Ps80 and 120 W mmy2) were decreased compared with the usual sprayed coatings and the erosion rates were also constant even if blasted by much more erodent than used for posttreated samples (erodent weight, 210 g). However, the hybrid sprayed coating, prepared at the excessively high laser power density (Ps160 W mmy2), is not so tolerable against particulate erosion. From the results of this work, it is suggested that the laser–hybrid spraying process can be a good choice for production of durable TBCs used in hot sandy environments where not just the thermal shock resistance but also improved antierosion properties are required. References w1x W. Beele, G. Marijnissen, A. van Lieshout, Surf. Coat. Technol. 120y121 (1999) 61. w2x D. Stover, ¨ C. Funke, J. Mater. Process Technol. 92y93 (1999) 195. w3x G.W. Meetham, J. Vac. Sci. Technol. A3 (6) (1985) 2509.
w4x P. Sahoo, T. Carr, R. Martin, F. Dinh, J. Thermal Spray Technol. 7 (4) (1998) 481. w5x J.R. Brandon, R. Taylor, Surf. Coat. Technol. 46 (1991) 75. w6x U. Schulz, M. Schmucker, ¨ Mater. Sci. Eng. A276 (2000) 1. w7x E. Reinhold, P. Botzler, C. Deus, Surf. Coat. Technol. 120y121 (1999) 77. w8x E. Lugscheider, K. Bobzin, S. Barwulf, ¨ A. Etzkorn, Surf. Coat. Technol. 138 (2001) 9. w9x D.D. Hass, A.J. Slifka, H.N.G. Wadley, Acta Mater. 49 (2001) 973. w10x H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 (1995) 53. w11x I. Zaplatynsky, Thin Solid Films 95 (1982) 275. w12x D.L. Ruckle, Thin Solid Films 73 (1980) 455. w13x S.O. Chwa, A. Ohmori, H. Yang, Z. Zhou, in: C.C. Berndt, K.A. Khor, E.F. Lugscheider (Eds.), Proceedings of International Thermal Spray Conference, Singapore, 28–30 May 2001, p. 575. w14x S.O. Chwa, A. Ohmori, Surf. Coat. Technol. 153 (2002) 302. w15x Z. Zhou, N. Eguchi, H. Shirasawa, A. Ohmori, J. Thermal Spray Technol. 8 (1999) 405. w16x S.O. Chwa, A. Ohmori, Surf. Coat. Technol. 148 (2001) 88. w17x N. Llorca-lsern, M. Puig, M. Espanol, ˜ J. Thermal Spray Technol. 8 (1999) 73. w18x A.R. Spurr, J. Ultrastruct. Res. 26 (1969) 31. w19x Y. Arata, A. Ohmori, C.J. Li, Trans. JWRI 15 (2) (1987) 339, (published by Joining and Welding Research Institute of Osaka University). w20x P. Bengtsson, T. Johannesson, J. Wigren, J. Thermal Spray Technol. 4 (3) (1995) 245. w21x Y. Fu, A.W. Batcherlor, H. Xing, Y. Gu, Wear 210 (1997) 157.
Thermal diffusivity and erosion resistance of ZrO2 –8 wt.% Y2O3 coatings prepared by a laser hybrid spraying technique Sang Ok Chwa*, Akira Ohmori Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, 567-0047 Osaka, Japan Received 6 February 2002; received in revised form 26 April 2002; accepted 5 June 2002
Abstract The results of thermal diffusivity measurement revealed that post-treated thermal barrier coatings (TBCs), which contain a remelted dense layer with numerous vertical cracks, are not effective in reducing the heat transfer. However, laser–hybrid sprayed TBCs with segmented feather-like microcracks decreased the thermal diffusivity by 60% of the as-sprayed value at the specific laser–hybrid condition (Ps120 W mmy2 ). The post-treated TBCs with dense layers endured at the early stage of erosion but collapsed gradually due to consecutively impacting particles. On the other hand, weight losses of laser–hybrid sprayed coatings (Ps80 and 120 W mmy2) were decreased compared with the usual sprayed coatings and the erosion rates were also constant even if blasted by much more erodent than used for post-treated samples (erodent weight, 210 g). From the results of this work, it is suggested that the laser–hybrid spraying process can be a good choice for production of durable TBCs used in hot sandy environments. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: TBCs; Laser treatment; Microstructure; Thermal diffusivity; Erosion
1. Introduction Thermal barrier coatings (TBCs) have been widely used for the protection of hot-section components used in gas turbine engines for aircrafts and power generators w1–4x. A typical TBC system consists of an oxidationresistant metallic layer on the superalloy and the topcoat of a partially stabilized zirconia (PSZ) layer, which provides thermal insulation. Among the various top-coat materials of PSZ, yttria partially stabilized zirconia (YPSZ) is the currently preferred top-coat material for gas turbine engine applications because it has a low thermal conductivity and relatively good match of thermal expansion coefficient with the superalloy when compared to other ceramics and good erosion resistance w5x. Generally, the YPSZ top-coat is deposited by either electron beam physical vapor deposition (EB-PVD) or air plasma spraying (APS). The EB-PVD process gives *Corresponding author. Tel.: q81-6-6879-8683; fax: q81-6-68798683. E-mail address: [email protected] (S.O. Chwa).
columnar microstructure with defects such as intercolumnar pores and grain boundaries running normal to the plane of coating w6–9x. These aligned columnar microstructures are effective at improving the strain tolerance but are not favorable for reducing heat transfer through the thickness of the TBC coating. In contrast, coatings prepared by the APS method are characterized as layers with randomly distributed inter-splat pores, which resulted from impingement of molten droplets on to a substrate. These defects, while effective in reducing the heat flux, leave the top-coat relatively less straintolerant and are a source of weakness that ultimately result in the TBC delamination andyor spallation. To enhance the lifetime of TBC coatings prepared by APS under thermal shock, laser irradiations have been tried, and it was shown that the vertical cracks in coatings induced by laser treatment improved the thermal shock resistance w10–13x. On the other hand, such a microstructural modification by laser treatment can alter the path of heat transfer and a wear property of coatings. However, as far as we know, no reports can
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 5 3 3 - 3
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chamber equipped with an exhaust system. Nd:YAG laser (NEC, Kanagawa, Japan) working in continuous wave mode at a wave-length of 1.06 mm was used. The laser beam coming from the oscillator was delivered by optical fiber to the laser nozzle w15x. The samples for the laser treatment were fixed on the X–Y table, moved by a six-axes articulated robot. For the uniform laser treatment of sprayed coatings, a kaleidoscope composed of two pairs of gold mirrors (3.6=51 mm2) was installed in a conventional YAG laser, and the laser system was combined with the plasma spraying equipment w13,16x. The schematic diagram of the laser– hybrid plasma spraying process is depicted in Fig. 1. The post-treatment and hybrid spraying were carried out under the conditions given in Table 1. Laser power density, P (W mmy2) and energy density E (J mmy2), were calculated from the following equations:
Fig. 1. The schematic illustration of laser–hybrid plasma spraying process.
be found about the thermal diffusivity of laser-treated coatings, especially the laser–hybrid sprayed TBCs. Here, not only the investigation of thermal diffusivity and wear property of laser-treated coatings, but also the control of those properties in TBC coatings obtained by employing the laser–hybrid spraying process were tried w14x. 2. Materials and experimental procedures 2.1. Materials Thermal barrier coatings consisting of 8 wt.% YPSZ top-coat and CoNiCrAlY bond-coat were prepared on Ni-superalloy (Inconel 738 LC) specimens with a diameter of 10 mm and a thickness of 5 mm for thermal diffusivity measurements and JIS SUS 304 plates for erosion tests using commercial powders (Showa Denko, Nagano, Japan). CoNiCrAlY bond coating of approximately 100 mm thickness was deposited at a pressure of 13.30 kPa argon gas atmosphere, and then a ZrO2 coating with a thickness of 330 mm was produced in air by using the general plasma spraying conditions for TBCs. 2.2. Laser combined plasma spraying system Post-treatments of pre-deposited coatings were carried out using the laser combined spraying system. Both the plasma gun and the laser gun were installed inside a
PsWyA
(1)
EsPyLV
(2)
where, W is the measured laser power, A is the area of a rectangular beam spot emerged on the specimen, L is a length of beam, and V is the transverse speed of the X–Y table. The transverse speed of the sample for laser– hybrid treatments was fixed at the same conditions as used for the non-hybrid spraying process. 2.3. Vacuum impregnation The mechanical stresses that arise during the mounting, cutting, grinding, and polishing operations of usual metallographic procedures can result in material pullout and alter the structural features of segmented microcracks w17x. For the careful observation of the microstructure of as-sprayed and laser-treated coatings, Spurr resin with a very low viscosity of 6=10y2 PaØsy1 was used as an embedding material for vacuum impregnation w14,18x. The specimen mounted in a mold was placed in a vacuum furnace, and then, resin mix was poured over the specimen. After pumping to reach the pressure less than 10y2 Pa, the vacuum was broken and vented to atmospheric pressure to accelerate the infiltration of resin, and this procedure was repeated three times, and then, the sample was cured at 345 K for 48 h. Table 1 Nd:YAG laser parameters used for post-treatment and hybrid process Process
Post-treatment
Hybrid treatment
Power (kW) Beam shape and size (mm2) Scan speed (mm sy1) Shield gas flow rate (Ar, m3 sy1)
1.0–4.0 Rectangular beam (7=7–12=12) 8–100 1.67=10y4
2.0–4.0 Rectangular beam (5=5) 250 1.67=10y4
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S.O. Chwa, A. Ohmori / Thin Solid Films 415 (2002) 160–166
powders with an average grain size of 330 mm, are accelerated by precisely controlled compressed air to get a particle velocity of approximately 60 m sy1. The wear of the nozzle during the test was strictly monitored. The impact angle was fixed at 608, and the experiment was carried out at room temperature. Mass loss from a sample is determined by weighing before and after each exposure with a precise valance. Details of our erosion apparatus could be found elsewhere w19x. 3. Results and discussions 3.1. Microstructure and thermal diffusivity of posttreated coatings Fig. 3 shows the SEM cross-sections of as-sprayed and laser-treated coatings as a function of power and Fig. 2. A scheme of the ACT–JP (Arata coating tester with jet particles) erosion test.
2.4. Characterization of coatings The phase composition of the sprayed and laser treated coatings was investigated by X-ray diffractome˚ with a ter using Cu Ka1 radiation (ls1.5405 A) graphite crystal monocromator (JDX3530, JEOL, Tokyo, Japan). The surface morphology and cross-section of coatings were observed by the scanning electron microscopy (SEM) (JSM-6400, JEOL, Tokyo, Japan). The micro-hardness of the coating was measured using a Vickers testing machine at a load of 2000 g for 15 s. 2.5. Thermal diffusivity measurement Thermal diffusivity was measured on a specimen consisting of three layers of materials including the Nisuperalloys (Inconel 738 LC) using the laser flash method (TC-7000, ULVAC-RIKO, Tokyo, Japan). After finishing the post-laser treatment or hybrid spraying, specimens composed of three layers (YPSZyCoNiCrAlYyInconel 738 LC) were cut by electrical discharge machining to obtain a total thickness of 1 mm, and then the specimens were carefully polished to reach a thickness of 0.95 ("0.02) mm with a diamond paste. To enhance the absorption and emission of the laser beam, thin layers of carbon were deposited in a vacuum evaporator. The metal substrate side was always positioned towards the laser flash at each measurement. 2.6. Erosion test Wear property of post-treated and laser–hybrid sprayed coatings was evaluated by means of the erosion tester, ACT–JP (Arata coating tester with jet particles) as shown in Fig. 2. The jet particles, angular Al2O3
Fig. 3. SEM cross-sections of as-sprayed and laser-treated coatings as a function of power and energy density of the laser beam: (a) assprayed coating; (b,c) post-treated coatings at the power and energy density of 20.4 W mmy2, Es17 J mmy2 and 30.6 W mmy2, Es21 J mmy2, respectively.
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diffusivity after laser treatment was mainly due to the structural changes of coatings. Laser-treatment of spayed zirconia TBCs have been extensively carried out to improve their poor spallation resistance by the intentional introduction of vertical cracks formed from the cooling down procedure of remelted coatings. However, from the result of the thermal diffusivity measurement of post-treated sprayed coatings, it was found that densification and full remelting of sprayed layers are not effective at reducing the heat flux through the TBCs. Accordingly, it can be suggested that the post-treatment, generally tried for improving the life-time of TBCs in hot section components of gas turbine, have to be carefully monitored to optimize the remelting depth and crack density of sprayed coatings for escaping the significantly increased heat transfer problems. Fig. 4. Thermal diffusivity of post-treated TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of laser power density.
energy density of the laser beam. In the laser-treated coating (Ps20.4 W mmy2, Es17 J mmy2), pores of the surface region were slightly decreased and a relatively good contact between the splats was observed compared with the as-sprayed coating (see Fig. 3a,b). After the laser treatment at increased power and energy density (Ps30.6 W mmy2, Es21 J mmy2), the surface region of the coating layer was fully melted and the vertical cracks were formed (Fig. 3c). Fig. 4 shows the thermal diffusivity of TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of post-treatment laser power density. When the normalized value divided by the thermal diffusivity of the as-sprayed coating was compared, approximately 20% increase of thermal diffusivity was observed in the laser-treated coating (Ps20.4 W mmy2, Es17 J mmy2) which showed a relatively good contact between the splats in the surface region compared with the as-sprayed coating. Generally, the sprayed coatings have very thin interlamellar pores or cracks with a thickness of a few tenths of micrometers. Therefore, it is considered that the increase of contact between those lamellars after laser treatment lowers the thermal resistance between the splats and, consequently, results in the higher thermal diffusivity of the coating. Laser-treated coatings (Ps 30.6 W mmy2, Es21 J mmy2), which have a thoroughly remelted layer and sharp vertical cracks, showed approximately 85% increase of thermal diffusivity compared with the value for as-sprayed coatings. From the XRD analysis, no considerable phase changes were observed and a non-transformable t9 phase was still predominant even after the remelting of coating layers by the laser w13x. So, the drastic increase of thermal
3.2. Microstructure and thermal diffusivity of laser– hybrid sprayed coatings It was previously explained that post-laser treatments of coatings resulted in mainly vertical cracks, which extended to the interface of the bond-coat and top-coat. In the case of coatings prepared by hybrid spraying processes, the feature of cracks in the coating was different from the post-treated coatings as shown in Fig. 5. The feature of cracks in the coatings prepared by hybrid spraying processes was characterized as a featherlike structure that consisted of a vertical crack with a significant number of branching cracks that were not observed in the post-treated coatings. From our previous study, it was found that TBCs with such microstructure also have a good thermal shock resistance w14x. Bengtsson et al. w20x suggested that the better the contact between the splat and underlying material is achieved, large tensile stress is developed. This causes the initiation and propagation of vertical cracks thus acting as stress relievers. From the results of microhardness measurements carried out in non-cracked regions and porosity analysis (see Fig. 6), it can be conferred that considerable improvement of bonding leads to strengthening between the sprayed lamellars attained during the laser hybrid spraying process, and the cracks become longer and wider as shown in Fig. 5 to release such an increased stress. The formation of branching cracks during the hybrid process is not clearly understood, but the following is considered to be one of the reasons. In hybrid spraying processes, the area of molten streaming sprayed particles toward the substrate are approximately twice of the simultaneously irradiating laser beam spot. So, this causes the local difference of the substrate and particle temperature between the laser-treated and non-treated region. In consequence, the complex structure which has vertical and branching cracks is considered due to the
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Fig. 5. SEM cross-sections of laser–hybrid sprayed coatings as a function of laser power density: (a–d) is hybrid sprayed coating at the power density of a laser beam of 80, 100, 120 and 140 W mmy2, respectively.
mechanical bonding disparity between the good contact layer and relatively insufficient connected layer on the underlying material during the hybrid deposition process w14x. Fig. 7 shows the thermal diffusivity of laser–hybrid TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of laser power density. Even in the improvement of contacts between splats and a decrease
of dispersed pores, there was no considerable increase of thermal diffusivity in the laser–hybrid plasma TBCs. Moreover, it was possible to decrease the thermal diffusivity by 60% of the as-sprayed value at a specific laser–hybrid condition (Ps120 W mmy2). As shown in our previous report, the branching crack density and length showed an optimum value at the hybrid–laser power density of ;120 W mmy2 w14x. Therefore, it is
Fig. 6. Micro-hardness and porosity of laser–hybrid sprayed coatings as a function of laser power density.
Fig. 7. Thermal diffusivity of laser–hybrid TBCs composed of YPSZyCoNiCrAlYyInconel 738 LC as a function of laser power density.
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Therefore, the coating treated at more increased laser power and energy density (Ps40.8 W mmy2, Es29 J mmy2) severely eroded away from the early stage of the test and showed an unsteady erosion rate. Fig. 9 shows a weight loss and erosion rate of laser– hybrid sprayed coatings at various laser power densities. Weight losses of laser–hybrid sprayed coatings (Ps80 and 120 W mmy2) were decreased compared with the usual sprayed coatings and the erosion rates were also constant even if blasted by much more erodent than used for post-treated samples (erodent weight, 210 g). However, the hybrid sprayed coating, prepared at the excessively high laser power density (Ps160 W mmy2), is not so tolerable against particulate erosion. Therefore, it is concluded that the wear property of TBCs can be significantly improved by applying a suitable laser–hybrid process, which forms uniformly distributed fine feather-like micro-cracks. Fig. 8. Weight loss and erosion rate of post-treated coatings at various power and energy densities of the laser beam.
considered that such an intentionally introduced branching crack effectively retarded the heat transfer even though the interlamellar gap between splats and dispersed pores, which act as a heat impeder, decreased from the laser–hybrid treatment. However, thermal diffusivity of TBCs prepared at higher power density (Ps 140 W mmy2) was slightly increased because the higher laser power results in the formation of very dense layers and columnar structures, as shown in Fig. 5d, and the reduction of heat transfer from the segmented structures was compensated. 3.3. Erosion property of post-treated and laser–hybrid sprayed coatings Fig. 8 shows a weight loss and erosion rate of posttreated coatings at various power and energy densities of laser beam. It can be noticed that weight loss of assprayed coatings and laser-treated coating (Ps20.4 W mmy2, Es17 J mmy2) is directly proportional to the applied erodent weight and the erosion rate is almost constant after 20 g of abrasive. This is due to the structure of the overall coating layers though the depth direction is not considerably changed, as shown in Fig. 3a,b. On the other hand, the laser-treated coating (Ps 30.6 W mmy2, Es21 J mmy2) shows the low weight loss up to 40 g of abrasive, and then more higher erosion wear compared with the as-sprayed coatings. It can be explained by the fact that a remelted dense coating with sharp vertical cracks endured at the early stage of erosion but collapsed gradually due to consecutively impacting particles. It was confirmed that laser-treated coatings at an excessive power and energy density have numerous vertical cracks of large width and defects such as entrapped gas bubbles from previous researches w14,21x.
4. Conclusions The results of thermal diffusivity measurement revealed that post-treated coatings (Ps30.6 W mmy2, Es21 J mmy2), which have a thoroughly remelted layer and sharp vertical cracks, showed approximately 85% increase of thermal diffusivity compared with the value for as-sprayed coatings. Accordingly, it can be concluded that the post-treatment process, generally tried for improving the life-time of TBCs in hot section components of gas turbine, have to be carefully monitored to optimize the remelting depth and crack density of sprayed coatings for escaping the significantly increased heat transfer problems. However, laser–hybrid sprayed TBCs with segmented feather-like microcracks
Fig. 9. Weight loss and erosion rate of laser–hybrid sprayed coatings at various power densities of the laser beam.
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decreased the thermal diffusivity by 60% of the assprayed value at the specific laser–hybrid conditions (Ps120 W mmy2). The post-treated TBCs with dense layers endured at the early stage of erosion but collapsed gradually due to consecutively impacting particles. Weight losses of laser–hybrid sprayed coatings (Ps80 and 120 W mmy2) were decreased compared with the usual sprayed coatings and the erosion rates were also constant even if blasted by much more erodent than used for posttreated samples (erodent weight, 210 g). However, the hybrid sprayed coating, prepared at the excessively high laser power density (Ps160 W mmy2), is not so tolerable against particulate erosion. From the results of this work, it is suggested that the laser–hybrid spraying process can be a good choice for production of durable TBCs used in hot sandy environments where not just the thermal shock resistance but also improved antierosion properties are required. References w1x W. Beele, G. Marijnissen, A. van Lieshout, Surf. Coat. Technol. 120y121 (1999) 61. w2x D. Stover, ¨ C. Funke, J. Mater. Process Technol. 92y93 (1999) 195. w3x G.W. Meetham, J. Vac. Sci. Technol. A3 (6) (1985) 2509.
w4x P. Sahoo, T. Carr, R. Martin, F. Dinh, J. Thermal Spray Technol. 7 (4) (1998) 481. w5x J.R. Brandon, R. Taylor, Surf. Coat. Technol. 46 (1991) 75. w6x U. Schulz, M. Schmucker, ¨ Mater. Sci. Eng. A276 (2000) 1. w7x E. Reinhold, P. Botzler, C. Deus, Surf. Coat. Technol. 120y121 (1999) 77. w8x E. Lugscheider, K. Bobzin, S. Barwulf, ¨ A. Etzkorn, Surf. Coat. Technol. 138 (2001) 9. w9x D.D. Hass, A.J. Slifka, H.N.G. Wadley, Acta Mater. 49 (2001) 973. w10x H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 (1995) 53. w11x I. Zaplatynsky, Thin Solid Films 95 (1982) 275. w12x D.L. Ruckle, Thin Solid Films 73 (1980) 455. w13x S.O. Chwa, A. Ohmori, H. Yang, Z. Zhou, in: C.C. Berndt, K.A. Khor, E.F. Lugscheider (Eds.), Proceedings of International Thermal Spray Conference, Singapore, 28–30 May 2001, p. 575. w14x S.O. Chwa, A. Ohmori, Surf. Coat. Technol. 153 (2002) 302. w15x Z. Zhou, N. Eguchi, H. Shirasawa, A. Ohmori, J. Thermal Spray Technol. 8 (1999) 405. w16x S.O. Chwa, A. Ohmori, Surf. Coat. Technol. 148 (2001) 88. w17x N. Llorca-lsern, M. Puig, M. Espanol, ˜ J. Thermal Spray Technol. 8 (1999) 73. w18x A.R. Spurr, J. Ultrastruct. Res. 26 (1969) 31. w19x Y. Arata, A. Ohmori, C.J. Li, Trans. JWRI 15 (2) (1987) 339, (published by Joining and Welding Research Institute of Osaka University). w20x P. Bengtsson, T. Johannesson, J. Wigren, J. Thermal Spray Technol. 4 (3) (1995) 245. w21x Y. Fu, A.W. Batcherlor, H. Xing, Y. Gu, Wear 210 (1997) 157.