- Email: [email protected]
NiCrAl coatings
Surface and Coatings Technology 162 (2003) 194–201
Microstructural characteristics in plasma sprayed functionally graded ZrO2 yNiCrAl coatings Chunxu Pana,1,*, Xiaorong Xub a
Department of Physics, Wuhan University, Wuhan, Hubei 430072, PR China School of Materials Science and Engineering, Wuhan University of Technology, Yujiatou Campus, Wuhan, Hubei 430072, PR China
b
Received 5 March 2002; accepted in revised form 24 June 2002
Abstract Different functionally graded 20 wt.% MgO-ZrO2 yNiCrAl thermal barrier coatings were obtained through the plasma spraying process. The microstructures, chemical compositions and fractured surface were examined by means of electron probe microscopic analysis (EPMA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and EPMA results showed that: (1) microstructures and compositions varied gradually in the coatings; (2) the elements Cr and Al were enriched along the sprayed lamellae boundaries and leaded to the formation of oxides. TEM observation enabled finding the formation of dense dislocations and deformation twins, and also the formation of the oxides Al2 O3 and Cr2O3 between grain boundaries. The SEM observations of the fractured surface revealed that the intermediate graded layer had the compositive mechanical properties in strength and toughness, due to the microstructure improvement and relaxation of residual stress concentration. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: MgO-ZrO2 yNiCrAl; Plasma spraying; Coatings
1. Introduction Thermal barrier coatings (TBCs) are being widely used to increase performance of high temperature components in the hot gas section of gas turbine engines and also in aerospace and aircraft applications w1–3x. In recent years, a new composite such as functionally graded materials (FGM) w4–8x has been studied and used to replace regular plasma sprayed TBCs. Generally, in the duplex TBCs system, the oxidation resistant alloy, such as NiCrAlY or NiCoCrAlY, is deposited on the metallic substrate as a bond coat and the heat-resistant ceramic yttria stabilized zirconia (YSZ) as a top coat to provide adequate heat resistance w9–12x. However, in a functionally graded thermal barrier coating (FGTBC) system, an intermediate layer with a gradual compositional variation is employed in between the above topcoat and bond coat w7,8,13–17x. This intermediate *Corresponding author. Tel.: q86-27-8721-3376; fax: q86-278765-4569. E-mail address: [email protected] (C. Pan). 1 Also: [email protected].
layer generally consists of several individual sections with the compositions of ceramic and alloy in various ratios, which not only achieve a gradual compositions variation, but also gradual changes in microstructures and mechanical properties w14–16x. Obviously, compared with regular duplex TBCs, FGTBCs can effectively reduce the discontinuity in thermal expansion coefficients between the intermediate layer and metallic substrate, and minimize the residual stress in the coating w18,19x. Up to now, most studies are concentrated upon the preparation, physical and mechanical properties of FGTBCs w7,8,13,15,16x. However, microstructural characteristics in coatings are still not sufficiently examined w14,20x. Especially, transmission electron microscopy (TEM) has not been performed on the microstructures. In the present work, the functionally graded ZrO2 y NiCrAl thermal barrier coatings were prepared by the plasma spraying process. The microstructures, chemical compositions and fractured surfaces of the coatings were examined using electron microscopic techniques EPMA, SEM and TEM.
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 1 1 - 5
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Table 1 Design and manufacturing of functionally graded thermal barrier coatings Sample no.
NiCrAl bond coat Thickness (mm)
20 wt.%MgO-ZrO2 yNiCrAl intermediate layer Thickness and blend ratio
20 wt.%MgOZrO2 top coat Thickness (mm)
1 2 3 4
0.2 0.2 0.2 0.2
– 0.6 0.6 0.4 0.4
1.8 1.2 0.6 0.2
2. Experimental procedures Commercial powders of 20 wt.%MgO stabilized zirconia ZrO2 with size range from 43 to 74 mm and nickel alloy NiCrAl (Cr:17–19 wt.%; Al: 5.5–6.6 wt.%; Ni: balance; size range from 43 to 147 mm) were used as the starting spray materials. The substrate metal was austenitic stainless steel (304 grade) with a diameter of 29 mm and length of 15 mm. The total thickness of the coating was 2 mm. The bond coat was 0.2 mm. The intermediate coating layers varied in different samples, as given in Table 1. The plasma spraying process was taken in a Type Qinghua (China) QZNI machine with arc current 450 A, arc voltage 80 V and spray distance 10–12 cm. Table 2 lists the main plasma-spray parameters. The cross-sections of the functionally graded thermal barrier coatings for the metallographic examinations were mechanically polished and electrolytically etched with a solution of 10-g CrO3 in 100-ml water. Some samples were broken in liquid nitrogen for the observation of the fractured surfaces. A thin carbon film was deposited on the polished and fractured surfaces in order to achieve a better conductivity for SEM and EPMA experiments. The microstructures and chemical compo-
mm mm mm mm
50y50 33y67q0.6 mm 67y33 20y80q0.4 mm 40y60q 60y40q0.4 mm 80y20
sitions were permored using a SEM of Hitachi S-570 and an EPMA of JEOL JSA8800R. Two kinds of TEM specimens—powder and thin foil—were prepared. For the powder specimen, the coating was ground into a tiny powder, and then sonicated in ethanol for 15 min to separate the powder. A droplet was dispersed on a carbon TEM microgrid. For the thin foil specimen, the cross-section slices of 1 mm were cut from the bulk coating using a diamond wafering saw. Then the slices were thinned mechanically to approximately 40 mm. Finally, the discs, 3 mm in diameter, were thinned in an ion-beam thinning device. The specimens were observed in a TEM of Hitachi 200 kV H-800. 3. Results and discussion 3.1. Metallographic characteristics Fig. 1 illustrates the overview morphology of sample 4. The microstructure of the coating exhibits a graded distribution, in which the metal components are bright and the ceramic is gray or dark.
Table 2 Plasma-spray conditions Sample no.
Fraction of 20%MgO-ZrO2 (%)
Thickness (mm)
Nitrogen gas flow rate (m3yh)
Argon gas flow rate (m3yh)
S-1
0 100 0 50 100 0 33 66 100 0 20 40 60 80 100
0.2 1.8 0.2 0.6 1.2 0.2 0.6 0.6 0.6 0.2 0.4 0.4 0.4 0.4 0.2
0.0 2.0 0.0 1.5 2.0 0.0 1.0 1.5 2.0 0.0 0.4 0.5 1.5 1.8 2.0
2.0 0.0 2.0 0.5 0.0 2.0 1.0 0.5 0.0 2.0 1.6 1.5 0.5 0.2 0.0
S-2
S-3
S-4
Fig. 1. SEM morphology (low magnification) of sample 4.
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In the topcoat, which contained 100 vol.%ZrO2, the sprayed lamellas of ceramic component were broken into small grains (Fig. 2a, signed b). This morphology is called the ‘mosaic’ microstructure in the present paper. In Rabiei and Evans w21x, it was considered that a kind of amorphous silicate layer exists between the intact splats of ‘mosaic’ structure. However, according to the present work, there were many tiny cracks in the ceramic phase, which were related to the release of stress concentration due to rapid cooling at spraying, and formed a ‘mosaic’ microstructure. In this layer, few unmelted 20 wt.%MgO-ZrO2 raw particles (Fig. 2a, signed c) and porosity (Fig. 2a, large dark dots) were also observed. Fig. 3 shows an original raw 20 wt.%MgO-ZrO2 ceramics particle. It has a eutectic structure with the dark MgO phase and bright ZrO2 phase. This result indicated that the unmelted raw ceramics could remain after spraying. In the intermediate graded layer, the more or less NiCrAl metal lamellae (Fig. 2b–d, signed a) and ‘mosaic’ (Fig. 2b–d, signed b) ceramic co-existed according to the metalyceramic mixed ratios. Inter-lamellae cracking and porosity were also found in the layers. In a layer containing more ceramic components, few melted metal particles were squeezed between the ‘mosaic’ ceramic and the typical sprayed lamellae structures were not clearly observed. However, when the metal component became equal to or more than the ceramic, the sprayed lamellas became dominative in the layers. The boundaries were distinctively observed between metal lamellas, ceramic lamellas or metal to ceramic lamellas. These boundaries were not closely connected and seemed like a cracking, which was induced due to the
Fig. 3. SEM morphology of raw 20 wt.%MgO-ZrO2 ceramics particle.
large difference in thermal expansion coefficient between metal and ceramic, rapid cooling and oxidation during plasma spraying. Fig. 2e shows the typical lamellae microstructures in the 100% NiCrAl alloy bond coat. The large cracking found between the lamellae was the results of Cr and Al oxidation and improper adhesion during plasma spraying. In most cases, the bond coat had a close adhesion with the substrate, which provides possible applications of the coating, as shown in Fig. 2f. 3.2. Chemical compositions distributions The chemical compositions distributions in macroand micro-scales in the coatings are illustrated in Fig. 4
Fig. 2. SEM morphologies in different graded layers: (a) top 100 vol.%ZrO2 layer; (b) 67 vol.%ZrO2 q33 vol.%NiCrAl layer; (c) 50 vol.%ZrO2q50 vol.%NiCrAl layer; (d) 33 vol.%ZrO2 q67 vol.%NiCrAl layer; (e) bond 100 vol.%NiCrAl layer; and (f) interface between the coated layer and substrate. (a) NiCrAl metal lamellae; (b) Ceramic ‘mosaic’ structure; (c) unmelted 20 wt.%MgO-ZrO2 raw particles.
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Fig. 4. EPMA overviews of the chemical compositions in sample 4.
and Fig. 5, respectively. Corresponding to the above microstructures, Fig. 4 shows the same graded distributions in coatings (sample 4). Fig. 5 shows the high magnifications of local elements distributions in the 50 vol.%ZrO2q50 vol.%NiCrAl coating layer of sample 2. EPMA measurements gave the following results: (1) both Zr and Mg had the same distributions; (2) Zr and Ni were located at the completely opposite locations; (3) Cr and Al distribute along the Ni boundaries; (4) oxygen’s distribution was with strong oxidizing elements Zr, Mg, Cr and Al. Generally, in the ceramic ZrO2 phase, the oxide MgO was added to stabilize the cubic ZrO2 phase and prevent
it from transforming into other phases. Generally, most of MgO was dissolved in ZrO2 during plasma spray, which therefore resulted in the similar distribution of elements Mg and Zr. However, the bright spots of Mg in Fig. 5 showed that the unmelted raw ceramics particles and other spots were the precipitation of Mg due to local supersaturation. In the parts of NiCrAl alloy metal phase, the distributions of elements Cr and Al were not homogeneous. Fig. 5 illustrates that the elements Cr and Al were located mainly along the Ni lamellae boundaries. Clearly, Al had a stronger tendency to segregate at the boundary than Cr. The distribution of element oxygen
Fig. 5. EPMA elements distributions in 50 vol.%ZrO2q50 vol.%NiCrAl layer of sample 2.
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Fig. 6. TEM micrograph face centered cubic (f.c.c.) ZrO2 ceramic phase: (a) bright-field image; and (b) w255x axis selected area electron diffraction (SAD) pattern.
revealed that the oxides, such as Al2O3 and Cr2O3, were formed on the surface of melted particles during plasma spray. In addition, because the Al element had small solid solubility in Zr, Mg and Ni and also has strong affinity to oxygen, most of the Al element was precipitated from Ni-based alloy and formed the oxide Al2O3 along the lamellae surfaces or boundaries w14,16,21x. 3.3. TEM microstructures When the samples were performed in TEM, the selected-area electron diffraction (SAD) revealed that the predominant ceramic phase was the face-centered cubic (f.c.c.) ZrO2 (Fig. 6). And only few other ZrO2 phases were observed. Inside the ZrO2 grains, no substructures, such as dislocation, slip line and twin, were observed. MgO was found as a solution in ZrO2 and as a precipitation of eutectic structure (Fig. 7). TEM observations at NiCrAl enabled finding dense dislocations and a large number of deformation twins, as shown in Fig. 8 and Fig. 9. These dislocations and twins were caused by the stresses concentration which might come from the following reasons: (1) residual stress during plasma spray coating; (2) temperature gradient between coating layers; (3) large difference of
Fig. 7. TEM micrograph of the MgO phase.
thermal coefficient of expansion between metal and ceramics; (4) phase transformations happened in both metal and ceramic; (5) oxide formation, etc. Actually, the induced dislocations and twins relaxed these stress concentrations during coating. The advantages of the ceramicymetal graded coating process was also confirmed. It is well known that slip is a major plastic deformation in f.c.c. materials. Twinning appears only at low temperature or under fast strain conditions w22x. However, at plasma spraying the solidification rate is almost 106 8Cys w23x. The internal stress introduced at this rapid solidification process impedes the excitation of slip system in f.f.c. crystalline structure. In order to relax the deformation status, twinning has to be initiated w22x. In the present case, the deformation twinning was not able to relax the stresses in the NiCrAl alloy components. Therefore, the high density of dislocations was necessarily requisite and was generated simultaneously during solidification of spraying. Theoretically, the action of deformation twinning just only provided an additional optimal path for the dislocation generation and moving, while these phenomena were suppressed.
Fig. 8. TEM micrograph of the dislocations in the NiCrAl metal component.
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Fig. 9. TEM micrographs deformation twins in the NiCrAl metal component: (a) bright-field image; (b) dark-field image; and (c) w110xyw110xT axis selected area electron diffraction (SAD) pattern.
Fig. 10. TEM micrograph of Al2O3 existed between grains: (a) bright-field image; and (b) selected area electron diffraction (SAD) pattern.
Fig. 11. SEM morphologies of fractured surface in cross-section of sample 2: (a) intergranular cleavage fracture in the top 100 vol.%ZrO2 layer; (b) 50 vol.%ZrO2q50 vol.%NiCrAl layer (A: ductile tearing, B: intergranular cleavage fracture; and (c) ductile tearing in the bond 100 vol.%NiCrAl layer (arrow: secondary crack).
Fig. 10 shows the TEM micrograph and SAD pattern of Al2O3. The discrete rings indicated that the Al2O3 oxide was a kind of polycrystalline. Many observations found that this Al2O3 inclusion could exist between the metalymetal, metalyceramic, or ceramicyceramic phases, which was presumed to be formed at elevated
temperatures during plasma spraying w14,21x. Obviously, the metal oxides Al3O2 and Cr2O3 weakened the adhesion strength of sprayed lamellae boundaries. It was an undesired microstructure and Ar gas shroud protection was necessary for the manufacturing of functionally graded coatings. However, according to the present
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work, the oxide films still formed more or less, even when the shroud gases were used. 3.4. Fractographs Fig. 11 shows the SEM morphologies of the fractured surfaces in the cross-section of sample 2. It was clearly illustrated that the ceramic component possesses an intergranular cleavage fracture (Fig. 11a and Fig. 11b, signed b). And, however, the metal component of NiCrAl alloy exhibited primarily the ductile tearing fracture (Fig. 11b, signed a) and Fig. 11c). It was also found that the secondary cracks (arrows indicated) were formed at the lamellae boundaries. According to the above studies on the microstructures characteristics and compositions segregations, the present results were reasonable. In the area with full hard ceramic component, the intergranular brittle failure showed a so-called ‘rock candy pattern’ revealing that the fracture propagated along the ‘mosaic’ microstructural boundaries, not along the lamellae boundaries. This could be conformed from the size comparison between the ‘rock candy pattern’ (Fig. 11a) and the ‘mosaic’ grain (Fig. 2a). In addition, no large secondary crack was observed. In contrary, in the area with full of NiCrAl metal component, the metal phase exhibited a good toughness with ductile tearing fracture, and however, the secondary cracks were formed along the sprayed lamellae boundaries (Fig. 11c, arrows indicated). This result showed that the formation of oxides between the lamellas weakened the adhesion strength of the sprayed lamellae boundaries. When examining the mixed area of ceramic and metal, both brittle and ductile morphologies were co-exited (Fig. 11b). The secondary cracks were also generated along the lamellae boundaries (Fig. 11b, arrow indicated), which however, was smaller than that in the area of full metal lamellas. It hinted that the intermediate graded layer had the compositive mechanical properties in strength and toughness, due to the microstructure improvement and relaxation of residual stress concentration. 4. Conclusions 1. Corresponding to the graded microstructural variations, the main elements Zr, Mg and Ni also showed the graded distributions. However, elements Cr and Al were segregatively located along the Ni lamellae boundaries, due to their small solid solubility in Zr, Mg and Ni, and strong affinity to oxygen. 2. TEM observations revealed that the Ni alloy metal components were deformed with dense dislocations and deformation twinning which relaxed the stress concentration in the coatings, while the stabilized ZrO2 was not deformed.
3. The metal oxides Al2O3 and Cr2O3 were a kind of polycrystalline formed between metalymetal, metaly ceramic or ceramicsyceramic lamellae boundaries, which influences the adhesion strength between boundaries. 4. The fractured surface observations showed that the ceramic and metal components exhibited intergranular cleavage and ductile tearing morphologies, respectively, and the intermediate graded layer had the compositive mechanical properties in strength and toughness. Acknowledgments The authors would like to thank Professor Jinsheng Xiao for preparation of the samples and Senior Engineering Shibo Yang and Associate Professor Jin Liu for their helpful discussions and experiments. This work was supported by the Science Foundation for Outstanding Youth Scientists, Ministry of Communication, PR China under contract number 32010. References w1x Z. Mutasim, W. Brentnall, Thermal barrier coatings for industrial gas turbine applications, an industrial note, J. Therm. Spray Technol. 6 (1997) 105–108. w2x W.P. Parks, E.E. Hoffman, W.Y. Lee, I.G. Wright, Thermal barrier coatings issues in advanced land-based gas turbines, J. Therm. Spray Technol. 6 (1997) 187–192. w3x J.A. Haynes, E.D. Rigney, M.K. Ferber, W.D. Porter, Oxidation degrafation of a plasma-sprayed thermal barrier coating system, Surf. Coat. Technol. 86–87 (1996) 102–108. w4x A. Mortensen, S. Suresh, Functionally graded metals and metal–ceramic composites: Part 1 process, Inter. Mater. Rev. 40 (1995) 239–265. w5x A. Mortensen, S. Suresh, Functionally graded metals and metal–ceramic composites: Part 2 thermomechanical behavior, Inter. Mater. Rev. 42 (1997) 85–116. w6x J. Musil, J. Fiala, Metal–ceramic coatings prepared by plasma spraying, Surf. Coat. Technol. 52 (1992) 211–220. w7x W. Hu, H. Guan, X. Sun, S. Li, Graded coatings prepared by plasma spraying with Ni-coated ZrO2 powders, Surf. Coat. Technol. 105 (1998) 102–108. w8x Y.W. Gu, K.A. Khor, Y.Q. Fu, Y. Wang, Functionally graded ZrO2 –NiCrAlY coatings prepared by plasma spraying using pre-mixed, spheroidized powders, Surf. Coat. Technol. 96 (1997) 305–312. w9x R. Hamacha, B. Dionnet, A. Grimaud, F. Nardou, Residual stress evolution during the thermal cycling of plasma-sprayed zirconia coatings, Surf. Coat. Technol. 80 (1996) 295–302. w10x E.F. Rejda, D.F. Socie, T. Itoh, Deformation behavior of plasma-sprayed thick thermal barrier coatings, Surf. Coat. Technol. 113 (1999) 218–226. w11x C.H. Lee, H.K. Kim, H.S. Choi, H.S. Ahn, Phase transformation and bond coat oxidation behavior of plasma-sprayed zirconia thermal barrier coatings, Surf. Coat. Technol. 124 (2000) 1–12. w12x P. Diaz, M.J. Edirisinghe, B. Ralph, Microstructural changes and phase transformations in a plasma-sprayed zirconia–yttria–
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titania thermal barrier coating, Surf. Coat. Technol. 82 (1996) 284–290. X. Xiang, J. Zhu, Z. Yin, Z. Lai, Fabrication and microstructure of ZrO2yNiCrCoAlY graded coating by plasma spraying, Surf. Coat. Technol. 88 (1996) 66–69. Z.L. Dong, K.A. Khor, Y.W. Gu, Microstructure formation in plasma-sprayed functionally graded NiCoCrAlYyyttria-stabilized zirconia coatings, Surf. Coat. Technol. 114 (1999) 181–186. K.A. Khor, Y.W. Gu, Z.L. Dong, Mechanical behavior of plasma sprayed functionally graded YSZyNiCoCrAlY composite coatings, Surf. Coat. Technol. 139 (2001) 200–206. A.S. Demirkiran, E. Celik, M. Yargan, E. Avei, Oxidation behavior of functionally gradient coatings including different composition of cermets, Surf. Coat. Technol. 142–144 (2001) 551–556. K.A. Khor, Y.W. Gu, C.H. Quek, Z.L. Dong, Post spray treatment of plasma sprayed functionally graded ZrO2 yNiCoCrAlY coatings, in: T.S. Sudarshan, K.A. Khor, M. Jeandin
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(Eds.), Surface Modification Technology XII, ASM International, Materials Park, OH, USA, 1998, pp. 495–500. M. Dao, P. Gu, A. Maewal, R.J. Asaro, A micromechanical study of residual stress in functionally graded materials, Acta Materialia 45 (1997) 3265–3276. J.I. Lavsky, C.C. Berndt, Thermal expansion properties of metallic and cement coatings, Surf. Coat. Technol. 102 (1998) 19–24. D.L. Porter, A.H. Heuer, Microstructural development in MgO—partially stabilized zirconia (Mg-PSZ), J. Am. Ceram. Soc. 62 (1997) 298–305. A. Rabiei, A.G. Evans, Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings, Acta Materialia 48 (2000) 3963–3976. C. Pan, B. Chen, Formation of the deformation twinning in austenitic stainless steel weld metal, J. Mater. Sci. Lett. 14 (1995) 1798–1800. C. Hu, Technical Handbook for Surface Treatment, Beijing Industrial University Press, 1997, pp. 364–368, (In Chinese).
Microstructural characteristics in plasma sprayed functionally graded ZrO2 yNiCrAl coatings Chunxu Pana,1,*, Xiaorong Xub a
Department of Physics, Wuhan University, Wuhan, Hubei 430072, PR China School of Materials Science and Engineering, Wuhan University of Technology, Yujiatou Campus, Wuhan, Hubei 430072, PR China
b
Received 5 March 2002; accepted in revised form 24 June 2002
Abstract Different functionally graded 20 wt.% MgO-ZrO2 yNiCrAl thermal barrier coatings were obtained through the plasma spraying process. The microstructures, chemical compositions and fractured surface were examined by means of electron probe microscopic analysis (EPMA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and EPMA results showed that: (1) microstructures and compositions varied gradually in the coatings; (2) the elements Cr and Al were enriched along the sprayed lamellae boundaries and leaded to the formation of oxides. TEM observation enabled finding the formation of dense dislocations and deformation twins, and also the formation of the oxides Al2 O3 and Cr2O3 between grain boundaries. The SEM observations of the fractured surface revealed that the intermediate graded layer had the compositive mechanical properties in strength and toughness, due to the microstructure improvement and relaxation of residual stress concentration. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: MgO-ZrO2 yNiCrAl; Plasma spraying; Coatings
1. Introduction Thermal barrier coatings (TBCs) are being widely used to increase performance of high temperature components in the hot gas section of gas turbine engines and also in aerospace and aircraft applications w1–3x. In recent years, a new composite such as functionally graded materials (FGM) w4–8x has been studied and used to replace regular plasma sprayed TBCs. Generally, in the duplex TBCs system, the oxidation resistant alloy, such as NiCrAlY or NiCoCrAlY, is deposited on the metallic substrate as a bond coat and the heat-resistant ceramic yttria stabilized zirconia (YSZ) as a top coat to provide adequate heat resistance w9–12x. However, in a functionally graded thermal barrier coating (FGTBC) system, an intermediate layer with a gradual compositional variation is employed in between the above topcoat and bond coat w7,8,13–17x. This intermediate *Corresponding author. Tel.: q86-27-8721-3376; fax: q86-278765-4569. E-mail address: [email protected] (C. Pan). 1 Also: [email protected].
layer generally consists of several individual sections with the compositions of ceramic and alloy in various ratios, which not only achieve a gradual compositions variation, but also gradual changes in microstructures and mechanical properties w14–16x. Obviously, compared with regular duplex TBCs, FGTBCs can effectively reduce the discontinuity in thermal expansion coefficients between the intermediate layer and metallic substrate, and minimize the residual stress in the coating w18,19x. Up to now, most studies are concentrated upon the preparation, physical and mechanical properties of FGTBCs w7,8,13,15,16x. However, microstructural characteristics in coatings are still not sufficiently examined w14,20x. Especially, transmission electron microscopy (TEM) has not been performed on the microstructures. In the present work, the functionally graded ZrO2 y NiCrAl thermal barrier coatings were prepared by the plasma spraying process. The microstructures, chemical compositions and fractured surfaces of the coatings were examined using electron microscopic techniques EPMA, SEM and TEM.
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 1 1 - 5
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195
Table 1 Design and manufacturing of functionally graded thermal barrier coatings Sample no.
NiCrAl bond coat Thickness (mm)
20 wt.%MgO-ZrO2 yNiCrAl intermediate layer Thickness and blend ratio
20 wt.%MgOZrO2 top coat Thickness (mm)
1 2 3 4
0.2 0.2 0.2 0.2
– 0.6 0.6 0.4 0.4
1.8 1.2 0.6 0.2
2. Experimental procedures Commercial powders of 20 wt.%MgO stabilized zirconia ZrO2 with size range from 43 to 74 mm and nickel alloy NiCrAl (Cr:17–19 wt.%; Al: 5.5–6.6 wt.%; Ni: balance; size range from 43 to 147 mm) were used as the starting spray materials. The substrate metal was austenitic stainless steel (304 grade) with a diameter of 29 mm and length of 15 mm. The total thickness of the coating was 2 mm. The bond coat was 0.2 mm. The intermediate coating layers varied in different samples, as given in Table 1. The plasma spraying process was taken in a Type Qinghua (China) QZNI machine with arc current 450 A, arc voltage 80 V and spray distance 10–12 cm. Table 2 lists the main plasma-spray parameters. The cross-sections of the functionally graded thermal barrier coatings for the metallographic examinations were mechanically polished and electrolytically etched with a solution of 10-g CrO3 in 100-ml water. Some samples were broken in liquid nitrogen for the observation of the fractured surfaces. A thin carbon film was deposited on the polished and fractured surfaces in order to achieve a better conductivity for SEM and EPMA experiments. The microstructures and chemical compo-
mm mm mm mm
50y50 33y67q0.6 mm 67y33 20y80q0.4 mm 40y60q 60y40q0.4 mm 80y20
sitions were permored using a SEM of Hitachi S-570 and an EPMA of JEOL JSA8800R. Two kinds of TEM specimens—powder and thin foil—were prepared. For the powder specimen, the coating was ground into a tiny powder, and then sonicated in ethanol for 15 min to separate the powder. A droplet was dispersed on a carbon TEM microgrid. For the thin foil specimen, the cross-section slices of 1 mm were cut from the bulk coating using a diamond wafering saw. Then the slices were thinned mechanically to approximately 40 mm. Finally, the discs, 3 mm in diameter, were thinned in an ion-beam thinning device. The specimens were observed in a TEM of Hitachi 200 kV H-800. 3. Results and discussion 3.1. Metallographic characteristics Fig. 1 illustrates the overview morphology of sample 4. The microstructure of the coating exhibits a graded distribution, in which the metal components are bright and the ceramic is gray or dark.
Table 2 Plasma-spray conditions Sample no.
Fraction of 20%MgO-ZrO2 (%)
Thickness (mm)
Nitrogen gas flow rate (m3yh)
Argon gas flow rate (m3yh)
S-1
0 100 0 50 100 0 33 66 100 0 20 40 60 80 100
0.2 1.8 0.2 0.6 1.2 0.2 0.6 0.6 0.6 0.2 0.4 0.4 0.4 0.4 0.2
0.0 2.0 0.0 1.5 2.0 0.0 1.0 1.5 2.0 0.0 0.4 0.5 1.5 1.8 2.0
2.0 0.0 2.0 0.5 0.0 2.0 1.0 0.5 0.0 2.0 1.6 1.5 0.5 0.2 0.0
S-2
S-3
S-4
Fig. 1. SEM morphology (low magnification) of sample 4.
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In the topcoat, which contained 100 vol.%ZrO2, the sprayed lamellas of ceramic component were broken into small grains (Fig. 2a, signed b). This morphology is called the ‘mosaic’ microstructure in the present paper. In Rabiei and Evans w21x, it was considered that a kind of amorphous silicate layer exists between the intact splats of ‘mosaic’ structure. However, according to the present work, there were many tiny cracks in the ceramic phase, which were related to the release of stress concentration due to rapid cooling at spraying, and formed a ‘mosaic’ microstructure. In this layer, few unmelted 20 wt.%MgO-ZrO2 raw particles (Fig. 2a, signed c) and porosity (Fig. 2a, large dark dots) were also observed. Fig. 3 shows an original raw 20 wt.%MgO-ZrO2 ceramics particle. It has a eutectic structure with the dark MgO phase and bright ZrO2 phase. This result indicated that the unmelted raw ceramics could remain after spraying. In the intermediate graded layer, the more or less NiCrAl metal lamellae (Fig. 2b–d, signed a) and ‘mosaic’ (Fig. 2b–d, signed b) ceramic co-existed according to the metalyceramic mixed ratios. Inter-lamellae cracking and porosity were also found in the layers. In a layer containing more ceramic components, few melted metal particles were squeezed between the ‘mosaic’ ceramic and the typical sprayed lamellae structures were not clearly observed. However, when the metal component became equal to or more than the ceramic, the sprayed lamellas became dominative in the layers. The boundaries were distinctively observed between metal lamellas, ceramic lamellas or metal to ceramic lamellas. These boundaries were not closely connected and seemed like a cracking, which was induced due to the
Fig. 3. SEM morphology of raw 20 wt.%MgO-ZrO2 ceramics particle.
large difference in thermal expansion coefficient between metal and ceramic, rapid cooling and oxidation during plasma spraying. Fig. 2e shows the typical lamellae microstructures in the 100% NiCrAl alloy bond coat. The large cracking found between the lamellae was the results of Cr and Al oxidation and improper adhesion during plasma spraying. In most cases, the bond coat had a close adhesion with the substrate, which provides possible applications of the coating, as shown in Fig. 2f. 3.2. Chemical compositions distributions The chemical compositions distributions in macroand micro-scales in the coatings are illustrated in Fig. 4
Fig. 2. SEM morphologies in different graded layers: (a) top 100 vol.%ZrO2 layer; (b) 67 vol.%ZrO2 q33 vol.%NiCrAl layer; (c) 50 vol.%ZrO2q50 vol.%NiCrAl layer; (d) 33 vol.%ZrO2 q67 vol.%NiCrAl layer; (e) bond 100 vol.%NiCrAl layer; and (f) interface between the coated layer and substrate. (a) NiCrAl metal lamellae; (b) Ceramic ‘mosaic’ structure; (c) unmelted 20 wt.%MgO-ZrO2 raw particles.
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Fig. 4. EPMA overviews of the chemical compositions in sample 4.
and Fig. 5, respectively. Corresponding to the above microstructures, Fig. 4 shows the same graded distributions in coatings (sample 4). Fig. 5 shows the high magnifications of local elements distributions in the 50 vol.%ZrO2q50 vol.%NiCrAl coating layer of sample 2. EPMA measurements gave the following results: (1) both Zr and Mg had the same distributions; (2) Zr and Ni were located at the completely opposite locations; (3) Cr and Al distribute along the Ni boundaries; (4) oxygen’s distribution was with strong oxidizing elements Zr, Mg, Cr and Al. Generally, in the ceramic ZrO2 phase, the oxide MgO was added to stabilize the cubic ZrO2 phase and prevent
it from transforming into other phases. Generally, most of MgO was dissolved in ZrO2 during plasma spray, which therefore resulted in the similar distribution of elements Mg and Zr. However, the bright spots of Mg in Fig. 5 showed that the unmelted raw ceramics particles and other spots were the precipitation of Mg due to local supersaturation. In the parts of NiCrAl alloy metal phase, the distributions of elements Cr and Al were not homogeneous. Fig. 5 illustrates that the elements Cr and Al were located mainly along the Ni lamellae boundaries. Clearly, Al had a stronger tendency to segregate at the boundary than Cr. The distribution of element oxygen
Fig. 5. EPMA elements distributions in 50 vol.%ZrO2q50 vol.%NiCrAl layer of sample 2.
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Fig. 6. TEM micrograph face centered cubic (f.c.c.) ZrO2 ceramic phase: (a) bright-field image; and (b) w255x axis selected area electron diffraction (SAD) pattern.
revealed that the oxides, such as Al2O3 and Cr2O3, were formed on the surface of melted particles during plasma spray. In addition, because the Al element had small solid solubility in Zr, Mg and Ni and also has strong affinity to oxygen, most of the Al element was precipitated from Ni-based alloy and formed the oxide Al2O3 along the lamellae surfaces or boundaries w14,16,21x. 3.3. TEM microstructures When the samples were performed in TEM, the selected-area electron diffraction (SAD) revealed that the predominant ceramic phase was the face-centered cubic (f.c.c.) ZrO2 (Fig. 6). And only few other ZrO2 phases were observed. Inside the ZrO2 grains, no substructures, such as dislocation, slip line and twin, were observed. MgO was found as a solution in ZrO2 and as a precipitation of eutectic structure (Fig. 7). TEM observations at NiCrAl enabled finding dense dislocations and a large number of deformation twins, as shown in Fig. 8 and Fig. 9. These dislocations and twins were caused by the stresses concentration which might come from the following reasons: (1) residual stress during plasma spray coating; (2) temperature gradient between coating layers; (3) large difference of
Fig. 7. TEM micrograph of the MgO phase.
thermal coefficient of expansion between metal and ceramics; (4) phase transformations happened in both metal and ceramic; (5) oxide formation, etc. Actually, the induced dislocations and twins relaxed these stress concentrations during coating. The advantages of the ceramicymetal graded coating process was also confirmed. It is well known that slip is a major plastic deformation in f.c.c. materials. Twinning appears only at low temperature or under fast strain conditions w22x. However, at plasma spraying the solidification rate is almost 106 8Cys w23x. The internal stress introduced at this rapid solidification process impedes the excitation of slip system in f.f.c. crystalline structure. In order to relax the deformation status, twinning has to be initiated w22x. In the present case, the deformation twinning was not able to relax the stresses in the NiCrAl alloy components. Therefore, the high density of dislocations was necessarily requisite and was generated simultaneously during solidification of spraying. Theoretically, the action of deformation twinning just only provided an additional optimal path for the dislocation generation and moving, while these phenomena were suppressed.
Fig. 8. TEM micrograph of the dislocations in the NiCrAl metal component.
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Fig. 9. TEM micrographs deformation twins in the NiCrAl metal component: (a) bright-field image; (b) dark-field image; and (c) w110xyw110xT axis selected area electron diffraction (SAD) pattern.
Fig. 10. TEM micrograph of Al2O3 existed between grains: (a) bright-field image; and (b) selected area electron diffraction (SAD) pattern.
Fig. 11. SEM morphologies of fractured surface in cross-section of sample 2: (a) intergranular cleavage fracture in the top 100 vol.%ZrO2 layer; (b) 50 vol.%ZrO2q50 vol.%NiCrAl layer (A: ductile tearing, B: intergranular cleavage fracture; and (c) ductile tearing in the bond 100 vol.%NiCrAl layer (arrow: secondary crack).
Fig. 10 shows the TEM micrograph and SAD pattern of Al2O3. The discrete rings indicated that the Al2O3 oxide was a kind of polycrystalline. Many observations found that this Al2O3 inclusion could exist between the metalymetal, metalyceramic, or ceramicyceramic phases, which was presumed to be formed at elevated
temperatures during plasma spraying w14,21x. Obviously, the metal oxides Al3O2 and Cr2O3 weakened the adhesion strength of sprayed lamellae boundaries. It was an undesired microstructure and Ar gas shroud protection was necessary for the manufacturing of functionally graded coatings. However, according to the present
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work, the oxide films still formed more or less, even when the shroud gases were used. 3.4. Fractographs Fig. 11 shows the SEM morphologies of the fractured surfaces in the cross-section of sample 2. It was clearly illustrated that the ceramic component possesses an intergranular cleavage fracture (Fig. 11a and Fig. 11b, signed b). And, however, the metal component of NiCrAl alloy exhibited primarily the ductile tearing fracture (Fig. 11b, signed a) and Fig. 11c). It was also found that the secondary cracks (arrows indicated) were formed at the lamellae boundaries. According to the above studies on the microstructures characteristics and compositions segregations, the present results were reasonable. In the area with full hard ceramic component, the intergranular brittle failure showed a so-called ‘rock candy pattern’ revealing that the fracture propagated along the ‘mosaic’ microstructural boundaries, not along the lamellae boundaries. This could be conformed from the size comparison between the ‘rock candy pattern’ (Fig. 11a) and the ‘mosaic’ grain (Fig. 2a). In addition, no large secondary crack was observed. In contrary, in the area with full of NiCrAl metal component, the metal phase exhibited a good toughness with ductile tearing fracture, and however, the secondary cracks were formed along the sprayed lamellae boundaries (Fig. 11c, arrows indicated). This result showed that the formation of oxides between the lamellas weakened the adhesion strength of the sprayed lamellae boundaries. When examining the mixed area of ceramic and metal, both brittle and ductile morphologies were co-exited (Fig. 11b). The secondary cracks were also generated along the lamellae boundaries (Fig. 11b, arrow indicated), which however, was smaller than that in the area of full metal lamellas. It hinted that the intermediate graded layer had the compositive mechanical properties in strength and toughness, due to the microstructure improvement and relaxation of residual stress concentration. 4. Conclusions 1. Corresponding to the graded microstructural variations, the main elements Zr, Mg and Ni also showed the graded distributions. However, elements Cr and Al were segregatively located along the Ni lamellae boundaries, due to their small solid solubility in Zr, Mg and Ni, and strong affinity to oxygen. 2. TEM observations revealed that the Ni alloy metal components were deformed with dense dislocations and deformation twinning which relaxed the stress concentration in the coatings, while the stabilized ZrO2 was not deformed.
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