Mechanical properties of solution-precursor plasma-sprayed thermal ...
Surface & Coatings Technology 202 (2008) 4976–4979
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Mechanical properties of solution-precursor plasma-sprayed thermal barrier coatings Amol D. Jadhav 1, Nitin P. Padture ⁎ Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA
A R T I C L E
I N F O
Article history: Received 10 March 2008 Accepted in revised form 28 April 2008 Available online 2 May 2008 Keywords: Thermal barrier coatings Toughness Mechanical testing Finite element analysis
A B S T R A C T The microstructure of thermal barrier coatings (TBCs) of 7 wt.% Y2O3 stabilized ZrO2 (7YSZ) deposited using the solution-precursor plasma spray (SPPS) method has: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale “splat” boundaries. An unusual feature of such SPPS TBCs is that they are well-adherent in ultra-thick forms (~ 4 mm thickness), where most other types of ultra-thick ceramic coatings fail spontaneously. Here a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the “barb” shear test, which is found to be 66 J m− 2. Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low, as a result of the strain-tolerant microstructure of the SPPS TBCs. The corresponding strain energy release rates that drive mode II cracks in the three different thickness SPPS TBCs have been found to be less than the mode II toughness. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs) made of low thermal conductivity ceramics (e.g. ZrO2 stabilized with 7 wt.% Y2O3 or 7YSZ) are routinely used to provide thermal insulation and protection to metallic gasturbine engine components from the hot gas stream (see e.g. reviews [1–3]). While thin TBCs (150 to 300 μm thickness) provide adequate temperature reductions (up to 150 °C) at the metal/ceramic interface in moving components, there is a growing need for thicker TBCs (N1 mm thickness) that can provide even higher temperature reductions in static components (combustors, shrouds) for gas-turbine engines, and in certain diesel-engine components. The air plasma spray (APS) process is perhaps best suited, and most economical, method for depositing such thick TBCs [4]. However, thick APS TBCs (N1.5 mm thickness) either fail prematurely in-service or spontaneously during deposition, requiring complex graded metal/ceramic interfaces [4,5] or very high porosities [6] or segmented cracks in dense coatings [6,7]. Graded coatings can be expensive to fabricate, while highly porous coatings do not have adequate hardness and resistance to foreignobject damage (FOD). Segmented-crack coatings are generally dense and, thus, they can have higher thermal conductivities. In this context, we recently reported the feasibility of depositing well-adherent, ultra-thick 7YSZ TBCs (3–4 mm thickness) on metallic substrates using the solution-precursor plasma spray (SPPS) process
⁎ Corresponding author. Tel.: +1 614 247 8114; fax: +1 614 292 1537. E-mail address: [email protected] (N.P. Padture). 1 Present address: Intel Corp., Chandler, AZ 85226, USA. 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.091
[8,9]. This potentially low-cost deposition process [10–12] uses liquidprecursor solutions that are injected directly into the plasma jet, instead of the ceramic powder feedstock used in the conventional APS process. The deposition mechanisms in SPPS are fundamentally different from those involved in conventional APS [13–20], which results in SPPS TBCs having some unique and desirable microstructural characteristics: (i) controlled porosity, (ii) through-thickness vertical cracks, and (iii) lack of large-scale “splat” boundaries that are omnipresent in APS TBCs. The porosity and the through-thickness cracks impart strain tolerance, while the lack of large-scale “splat” boundaries are thought to effectively toughen the ceramic top-coat [8]. These attributes make SSPS TBCs highly durable, in thin [12,21] and in thick [8] forms, relative to their APS TBC counterparts. In this work a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs do not fail spontaneously. 2. Experimental procedure Since thick TBCs fail primarily in mode II by edge delamination, at or near the metal/ceramic interface [2], the mode II toughness for SPPS TBCs (7YSZ) was measured using the “barb” shear test (Fig. 1) [22]. In order to minimize the mode II driving force from macroscopic residual thermal stresses in the TBCs, “barb” tests were carried out on thin TBCs (0.2 mm). The SPPS TBCs were deposited on several identical bondcoated stainless steel substrates 3 mm thick (hSUB) and 6 mm wide (wSUB) using a procedure described elsewhere [8,12]. The final TBCs on each of the specimens were machined to be 3 mm long (lTBC), 6 mm wide (wTBC), and 0.2 mm thick (hTBC). These specimens were then tested using a procedure described elsewhere [22] in a universal
A.D. Jadhav, N.P. Padture / Surface & Coatings Technology 202 (2008) 4976–4979
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Fig. 2. Typical load–displacement trace of “barb” shear test of a 7YSZ SPPS TBC of thickness 0.2 mm.
SPPS TBCs (measured using indentations within the ceramic TBCs) was previously found to be KIC ~ 1.7 MPa m0.5, or GC ~ 59 J m assuming ETBC ~ 49 GPa [8]. Now consider the steady-state strain energy release rate, GA, for spontaneous mode II failure of a porous SPPS TBC with no additional strain tolerance, i.e. no vertical cracks. This is given by [26]:
Fig. 1. Schematic illustration of the “barb” shear test (after [22]).
mechanical testing machine (Model 1322, Instron Corp., Dayton, OH), where the peak loads, PMAX, for each of the specimens were recorded. The mode II toughness values, GC, were calculated using the following relation [22,23]: GC c
2 PMAX 4
1 ESUB hSUB w2SUB
þ
1 ETBC hTBC w2TBC
! ;
ð1Þ
where, ESUB and ETBC are the Young's moduli of the substrate (=200 GPa for steel) and the ceramic TBC (=49 GPa for SPPS TBC [8]), respectively. The fracture surfaces of the failed specimens were observed in a scanning electron microscope (Sirion, FEI Company, Hillsboro, OR). Polished cross-sections of the as-deposited thin SPPS TBCs from this study (0.2 mm thick), and some thicker ones from a previous study [8] were observed in the SEM and optical microscope. The resulting micrographs serve as input to a microstructure-based object-oriented finite element (OOF) method [24] that was used to estimate the macroscopic residual stresses in the as-deposited SPPS TBCs of thicknesses 0.2, 1.5, and 4.0 mm. 3. Results and discussion Fig. 2 shows a load–displacement trace from the “barb” test of a SPPS TBC specimen. This trace is typical of what has been observed in TBC “barb” tests by others [22]. The monotonic increase in the load to a peak load of PMAX over a crosshead displacement of ~ 300 μm, followed by abrupt failure indicates that the “barb” test is valid. (Note that finite loads carried over much longer displacements can be the result of the knife edges used in this test ploughing into the substrate, making the test invalid.) Fig. 3 is a SEM micrograph of the fracture surface (substrate side) of a failed specimen. The fracture surface shows no evidence of wear tracks, indicating a clean mode II failure at or near (within the TBC) the metal/ceramic interface. Fig. 4 shows the average GC value (2 specimens) measured for the thin (0.2 mm thickness) SPPS TBCs, which is 66 J m− 2. This is comparable to the mode II toughness of electron-beam physical vapor deposited (EB-PVD) TBCs (~ 70 J m− 2) [22], but it is somewhat lower than that of APS TBCs (~ 100 J m− 2) [25], all measured using the same “barb” test on ~ 0.2 mm thick TBCs. Note that the mode I toughness of
GA ¼
r2TBC hTBC ; 2ETBC
ð2Þ
where σTBC is the residual stress in the TBC resulting from the coefficient of thermal expansion (CTE) mismatch between the ceramic TBC and the substrate, and assuming that this is the only stress that drives mode II fracture in this system. This stress is estimated at σTBC ~ 154 MPa using [26]: rTBC e ETBC ðaTBC aSUB ÞDT;
ð3Þ
where αSUB = 14.5 × 10− 6 °C (superalloy) and αTBC = 10 × 10− 6 °C are the CTEs of the TBC and the substrate, respectively [27], and ΔT is ~ 700 °C (deposition temperature of ~ 725 °C [13]). The TBC thickness hTBC dependence of GA according to Eq. (2) is plotted in Fig. 4 as a dashed line. Assuming that the intrinsic toughness of the TBC, GC, is independent of the TBC thickness, GA = GC at hTBC = 0.27 mm. This implies that TBCs thicker than ~ 0.27 mm should fail spontaneously upon cooling to room temperature after deposition. However, as mentioned earlier, and as seen in Fig. 5, as-deposited SPPS TBCs as thick as 4 mm are found to be well-adherent [8]. This indicates clearly that the residual stresses have been grossly overestimated using Eq. (3). It appears that the microstructure of the SPPS TBCs, vertical cracks in particular, are responsible for relaxing most of the residual stresses and thereby reducing the strain energy release rate, GA, driving mode II failure.
Fig. 3. SEM micrograph showing fracture surface of a failed 7YSZ SPPS TBC specimen (substrate side) in the “barb” shear test. The arrow indicates direction of shear.
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A.D. Jadhav, N.P. Padture / Surface & Coatings Technology 202 (2008) 4976–4979 Table 1 Thermal and mechanical properties data for constituent phases used in OOF analysis [8,33]
Fig. 4. Plot showing: (i) mode II toughness, GC, of the 7YSZ SPPS TBCs, (ii) strain energy release rate (GA) from Eq. (2), and (iii) strain energy release rate, GASPPS, for SPPS TBCs of 3 different thicknesses.
To estimate the macroscopic residual stresses in the as-deposited SPPS TBCs of thicknesses 0.2, 1.5 and 4.0 mm a microstructure-based object-oriented finite element (OOF) method [24] was used. This is because, while spectroscopic and diffraction experimental methods have been used to measure microscopic residuals stresses, they are not well-suited to measure macroscopic residual stresses in ultrathick TBCs (e.g. TBCs of 4.0 mm thickness). Also, most analytical and numerical methods cannot be used to estimate accurately macroscopic residual stresses while capturing the microstructural subtleties. OOF combines data in the form of 2-dimensional microstructures and fundamental material data of the constitutive phases (elastic constants and CTE) to evaluate the effective material behavior. OOF is a public domain software [28] and it has been used to simulate the thermo-mechanical behavior of composites and TBCs [9,29–32].
Material
Young's modulus E (GPa)
Poisson's ratio ν
CTE α (°C− 1)
Dense 7YSZ SPPS 7YSZ (without vertical cracks) Superalloy Substrate
200 49
0.25 0.25
10 × 10− 6 10 × 10− 6
213
0.30
14.5 × 10− 6
Although the OOF method used here considers only 2-dimensional microstructures, it has been shown to capture the nature of the residual stresses in coatings, such as TBCs [29], quite accurately. Fig. 5A, B, and C shows examples of cross-sectional SEM micrographs of SPPS TBCs of thicknesses 0.2, 1.5, and 4.0 mm. Several such micrographs for each of the TBCs were converted to a binary images containing ceramic phase, porosity, and cracks, which were used as input to OOF. Then, the images were attached (perfect bonding) to a 5.4 mm thick superalloy substrate (elastic continuum) to complete the as-sprayed TBC system for OOF simulation. This was followed by assigning intrinsic material properties given in Table 1. In the case of the thin TBC (0.2 mm) the solid phase was assigned the properties of dense 7YSZ, while the pores and the cracks are considered to be voids. In the case of the thick TBCs (1.5 and 4.0 mm) the scale of the pores is significantly smaller (microns) than the overall thickness of the TBC (millimeters). Thus, individual pores could not be resolved at the mm scale. In those cases, the solid phase is assigned the properties of the porous SPPS 7YSZ without the vertical cracks (Table 1), while the vertical cracks are considered to be voids. The binary images were then meshed using an adaptive meshing procedure, which allows the subdivision of the elements and movement of nodes to conform to the microstructure. The mesh is adjusted by minimizing a fictitious mesh energy parameter. This energy minimization is achieved by an “annealing” procedure at the conclusion of the meshing process. After meshing the TBC micrographs effectively in OOF, pertinent boundary conditions were applied, as explained by Hsueh et al. [29]. Finally, the TBC system was equilibrated at its assumed high-
Fig. 5. Cross-sectional SEM micrographs 7YSZ SPPS TBCs of different thicknesses: (A) 0.2 mm, (B) 1.5 mm, and (C) 4.0 mm [8]. Note the proliferation of vertical cracks in these TBCs.
A.D. Jadhav, N.P. Padture / Surface & Coatings Technology 202 (2008) 4976–4979 Table 2 Average in-plane residual stress (σTBC) estimated from OOF analysis and corresponding mode II strain energy release rates (GASPPS) for 7YSZ SPPS TBCs 7YSZ SPPS TBC thickness hTBC (mm)
Average in-plane residual stress σTBC (MPa)
Strain energy release rate GASPPS (J m− 2)
0.2 1.5 4.0
− 77.4 −27.5 −13.4
12.2 11.6 7.3
temperature stress-free temperature (deposition temperature of 725 °C) and cooled to 25 °C (ΔT = 700 °C). Note that in the SPPS process the substrates are always preheated. Also, unlike APS, where rapid solidification of large “splats” results in buildup of residual stresses during deposition, such large “splat” are absent in SPPS. Thus, this simplifying assumption of stress-free state at deposition temperature is better justified in the case of SPPS TBCs considered here. The average in-plane compressive residual stresses, σTBC, calculated from OOF analysis for the SPPS TBCs of various thicknesses are reported in Table 2. These are significantly lower than those estimated using Eq. (3), and are found to decrease with increasing TBC thickness. These low residual stresses are the direct consequence of the microstructure of the SPPS TBCs, in particular the vertical cracks. The corresponding strain energy release rates, GSPPS (calculated using Eq. (2)), are also reported in Table 2 and Fig. 4. The GSPPS values are significantly lower than the intrinsic toughness of the SPPS TBC, GC. This provides a quantitative explanation for why ultra-thick SPPS TBCs do not fail spontaneously. This analysis is based on the assumption that the mode II toughness of SPPS TBCs, GC, is independent of the TBC thickness, and that the residual stress, σTBC, is the only stress driving the mode II failure. However, during in-service high-temperature thermal cycling of these TBCs, interfaces will degrade, decreasing GC. At the same time, other stresses, such as those generated during the build up of the thermally grown oxide (TGO) and the sintering of the TBC, could assist in driving the mode II failure, leading to eventual failure of the ultra-thick SPPS TBCs [8]. 4. Summary The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the “barb” shear test, which is found to be 66 J m− 2. Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low as result of the strain-tolerant microstructure of these TBCs which have: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale “splat” boundaries. The corresponding strain energy release rates in the three thickness cases have been found to be less than the intrinsic mode II toughness. This provides a quantitative explanation as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. Acknowledgements The authors thank: Dr. X. Ma and Mr. J. Roth for depositing the coatings; Profs. E.H. Jordan and M. Gell for fruitful discussions; and Dr.
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E.R. Fuller, Jr. for help with the OOF program. This publication was prepared with the support of the US Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory (Grant No. 0301-SR107). References [1] R.L. Jones, in: K.H. Stern (Ed.), Metallurgical and Ceramic Coatings, Chapman & Hall, London, 1996, p. 194. [2] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505. [3] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [4] M.B. Beardsley, J. Therm. Spray Technol. 6 (1997) 181. [5] S. Suresh, A. Mortensen, Fundamentals of Functionally Graded Materials, IOM Communications Ltd., London, UK, 1998. [6] T.A. Taylor, D.L. Appleby, A.E. Weatherill, J. Griffiths, Surf. Coat. Technol. 43–44 (1990) 470. [7] H.B. Guo, R. Vassen, D. Stoever, Surf. Coat. Technol. 186 (2004) 353. [8] A. Jadhav, N.P. Padture, F. Wu, E.H. Jordan, M. Gell, Mater. Sci. Engr. A 405 (2005) 313. [9] A.D. Jadhav, N.P. Padture, E.H. Jordan, M. Gell, P. Miranzo, E.R. Fuller, Acta Mater. 54 (2006) 3343. [10] J. Karthikeyan, C.C. Berndt, S. Reddy, J.Y. Wang, A.H. King, H. Herman, J. Am. Ceram. Soc. 81 (1998) 121. [11] P.R. Strutt, B.H. Kear, R. Boland, Method of Manufacture of Nanostructured Feeds, U. S. Patent No. 6,025,034 (2000). [12] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [13] T. Bhatia, A. Ozturk, L. Xie, E.H. Jordan, B.M. Cetegen, M. Gell, X. Ma, N.P. Padture, J. Mater. Res. 17 (2002) 2363. [14] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, Mater. Sci. Eng. A362 (2003) 204. [15] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, Surf. Coat. Technol. 177–178 (2004) 103. [16] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, J. Mater. Sci. 39 (2004) 1639. [17] L. Xie, X. Ma, A. Ozturk, E.H. Jordan, N.P. Padture, B.M. Cetegen, T.D. Xiao, M. Gell, Surf. Coat. Technol. 183 (2004) 51. [18] E.H. Jordan, L. Xie, X. Ma, M. Gell, N.P. Padture, B.M. Cetegen, A. Ozturk, J. Roth, T.D. Xiao, P.E.C. Bryant, J. Therm. Spray Technol. 13 (2004) 57. [19] L. Xie, E.H. Jordan, N.P. Padture, M. Gell, Mater. Sci. Eng., A 381 (2004) 189. [20] M. Gell, E.H. Jordan, M. Teicholz, B.M. Cetegen, N.P. Padture, L. Xie, D. Chen, X. Ma, J. Roth, J. Therm. Spray Technol. 17 (2008) 124. [21] M. Gell, L. Xie, X. Ma, E.H. Jordan, N.P. Padture, Surf. Coat. Technol. 177–178 (2004) 97. [22] S.Q. Guo, D.R. Mumm, A.M. Karlsson, Y. Kagawa, Scripta Mater. 53 (2005) 1043. [23] J.W. Hutchinson, Z. Suo, Adv. Appl. Mech. 29 (1991) 63. [24] S.A. Langer, J.E.R. Fuller, W.C. Carter, Comput. Sci. Eng. 3 (2001) 15. [25] K. Taguchi, M. Kawamura, T. Suemitsu, Y. Kagawa, 31st International Conference & Exposition on Advanced Ceramics and Composites, Daytona Beach, FL, January, 2007, Paper No. ICACC-S2-033-2007, 2007. [26] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation and Surface Evolution, Cambridge University Press, Cambridge, UK, 2003. [27] K.W. Schlichting, N.P. Padture, E.H. Jordan, M. Gell, Mater. Sci. Eng. A342 (2003) 120. [28] http://www.ctcms.nist.gov/oof/ (accessed on March 10, 2008). [29] C.H. Hsueh, J.A. Haynes, M.J. Lance, P.F. Becher, M.K. Ferber, J.E.R. Fuller, S.A. Langer, W.C. Carter, W.R. Cannon, J. Am. Ceram. Soc. 82 (1999) 1073. [30] A. Saigal, J.E.R. Fuller, Comput. Mater. Sci. 21 (2001) 149. [31] N. Chawla, B.V. Patel, M. Koopman, K.K. Chawla, R. Saha, B.R. Patterson, E.R. Fuller, S.A. Langer, Mater. Charac. 49 (2003) 395. [32] Z. Wang, A. Kulkarni, S.A. Deshpande, T. Nakamura, H. Herman, Acta Mater. 51 (2003) 5319. [33] C.H. Hsueh, E.R. Fuller, Mater. Sci. Eng. A283 (2000) 46.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Mechanical properties of solution-precursor plasma-sprayed thermal barrier coatings Amol D. Jadhav 1, Nitin P. Padture ⁎ Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA
A R T I C L E
I N F O
Article history: Received 10 March 2008 Accepted in revised form 28 April 2008 Available online 2 May 2008 Keywords: Thermal barrier coatings Toughness Mechanical testing Finite element analysis
A B S T R A C T The microstructure of thermal barrier coatings (TBCs) of 7 wt.% Y2O3 stabilized ZrO2 (7YSZ) deposited using the solution-precursor plasma spray (SPPS) method has: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale “splat” boundaries. An unusual feature of such SPPS TBCs is that they are well-adherent in ultra-thick forms (~ 4 mm thickness), where most other types of ultra-thick ceramic coatings fail spontaneously. Here a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the “barb” shear test, which is found to be 66 J m− 2. Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low, as a result of the strain-tolerant microstructure of the SPPS TBCs. The corresponding strain energy release rates that drive mode II cracks in the three different thickness SPPS TBCs have been found to be less than the mode II toughness. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs) made of low thermal conductivity ceramics (e.g. ZrO2 stabilized with 7 wt.% Y2O3 or 7YSZ) are routinely used to provide thermal insulation and protection to metallic gasturbine engine components from the hot gas stream (see e.g. reviews [1–3]). While thin TBCs (150 to 300 μm thickness) provide adequate temperature reductions (up to 150 °C) at the metal/ceramic interface in moving components, there is a growing need for thicker TBCs (N1 mm thickness) that can provide even higher temperature reductions in static components (combustors, shrouds) for gas-turbine engines, and in certain diesel-engine components. The air plasma spray (APS) process is perhaps best suited, and most economical, method for depositing such thick TBCs [4]. However, thick APS TBCs (N1.5 mm thickness) either fail prematurely in-service or spontaneously during deposition, requiring complex graded metal/ceramic interfaces [4,5] or very high porosities [6] or segmented cracks in dense coatings [6,7]. Graded coatings can be expensive to fabricate, while highly porous coatings do not have adequate hardness and resistance to foreignobject damage (FOD). Segmented-crack coatings are generally dense and, thus, they can have higher thermal conductivities. In this context, we recently reported the feasibility of depositing well-adherent, ultra-thick 7YSZ TBCs (3–4 mm thickness) on metallic substrates using the solution-precursor plasma spray (SPPS) process
⁎ Corresponding author. Tel.: +1 614 247 8114; fax: +1 614 292 1537. E-mail address: [email protected] (N.P. Padture). 1 Present address: Intel Corp., Chandler, AZ 85226, USA. 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.091
[8,9]. This potentially low-cost deposition process [10–12] uses liquidprecursor solutions that are injected directly into the plasma jet, instead of the ceramic powder feedstock used in the conventional APS process. The deposition mechanisms in SPPS are fundamentally different from those involved in conventional APS [13–20], which results in SPPS TBCs having some unique and desirable microstructural characteristics: (i) controlled porosity, (ii) through-thickness vertical cracks, and (iii) lack of large-scale “splat” boundaries that are omnipresent in APS TBCs. The porosity and the through-thickness cracks impart strain tolerance, while the lack of large-scale “splat” boundaries are thought to effectively toughen the ceramic top-coat [8]. These attributes make SSPS TBCs highly durable, in thin [12,21] and in thick [8] forms, relative to their APS TBC counterparts. In this work a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs do not fail spontaneously. 2. Experimental procedure Since thick TBCs fail primarily in mode II by edge delamination, at or near the metal/ceramic interface [2], the mode II toughness for SPPS TBCs (7YSZ) was measured using the “barb” shear test (Fig. 1) [22]. In order to minimize the mode II driving force from macroscopic residual thermal stresses in the TBCs, “barb” tests were carried out on thin TBCs (0.2 mm). The SPPS TBCs were deposited on several identical bondcoated stainless steel substrates 3 mm thick (hSUB) and 6 mm wide (wSUB) using a procedure described elsewhere [8,12]. The final TBCs on each of the specimens were machined to be 3 mm long (lTBC), 6 mm wide (wTBC), and 0.2 mm thick (hTBC). These specimens were then tested using a procedure described elsewhere [22] in a universal
A.D. Jadhav, N.P. Padture / Surface & Coatings Technology 202 (2008) 4976–4979
4977
Fig. 2. Typical load–displacement trace of “barb” shear test of a 7YSZ SPPS TBC of thickness 0.2 mm.
SPPS TBCs (measured using indentations within the ceramic TBCs) was previously found to be KIC ~ 1.7 MPa m0.5, or GC ~ 59 J m assuming ETBC ~ 49 GPa [8]. Now consider the steady-state strain energy release rate, GA, for spontaneous mode II failure of a porous SPPS TBC with no additional strain tolerance, i.e. no vertical cracks. This is given by [26]:
Fig. 1. Schematic illustration of the “barb” shear test (after [22]).
mechanical testing machine (Model 1322, Instron Corp., Dayton, OH), where the peak loads, PMAX, for each of the specimens were recorded. The mode II toughness values, GC, were calculated using the following relation [22,23]: GC c
2 PMAX 4
1 ESUB hSUB w2SUB
þ
1 ETBC hTBC w2TBC
! ;
ð1Þ
where, ESUB and ETBC are the Young's moduli of the substrate (=200 GPa for steel) and the ceramic TBC (=49 GPa for SPPS TBC [8]), respectively. The fracture surfaces of the failed specimens were observed in a scanning electron microscope (Sirion, FEI Company, Hillsboro, OR). Polished cross-sections of the as-deposited thin SPPS TBCs from this study (0.2 mm thick), and some thicker ones from a previous study [8] were observed in the SEM and optical microscope. The resulting micrographs serve as input to a microstructure-based object-oriented finite element (OOF) method [24] that was used to estimate the macroscopic residual stresses in the as-deposited SPPS TBCs of thicknesses 0.2, 1.5, and 4.0 mm. 3. Results and discussion Fig. 2 shows a load–displacement trace from the “barb” test of a SPPS TBC specimen. This trace is typical of what has been observed in TBC “barb” tests by others [22]. The monotonic increase in the load to a peak load of PMAX over a crosshead displacement of ~ 300 μm, followed by abrupt failure indicates that the “barb” test is valid. (Note that finite loads carried over much longer displacements can be the result of the knife edges used in this test ploughing into the substrate, making the test invalid.) Fig. 3 is a SEM micrograph of the fracture surface (substrate side) of a failed specimen. The fracture surface shows no evidence of wear tracks, indicating a clean mode II failure at or near (within the TBC) the metal/ceramic interface. Fig. 4 shows the average GC value (2 specimens) measured for the thin (0.2 mm thickness) SPPS TBCs, which is 66 J m− 2. This is comparable to the mode II toughness of electron-beam physical vapor deposited (EB-PVD) TBCs (~ 70 J m− 2) [22], but it is somewhat lower than that of APS TBCs (~ 100 J m− 2) [25], all measured using the same “barb” test on ~ 0.2 mm thick TBCs. Note that the mode I toughness of
GA ¼
r2TBC hTBC ; 2ETBC
ð2Þ
where σTBC is the residual stress in the TBC resulting from the coefficient of thermal expansion (CTE) mismatch between the ceramic TBC and the substrate, and assuming that this is the only stress that drives mode II fracture in this system. This stress is estimated at σTBC ~ 154 MPa using [26]: rTBC e ETBC ðaTBC aSUB ÞDT;
ð3Þ
where αSUB = 14.5 × 10− 6 °C (superalloy) and αTBC = 10 × 10− 6 °C are the CTEs of the TBC and the substrate, respectively [27], and ΔT is ~ 700 °C (deposition temperature of ~ 725 °C [13]). The TBC thickness hTBC dependence of GA according to Eq. (2) is plotted in Fig. 4 as a dashed line. Assuming that the intrinsic toughness of the TBC, GC, is independent of the TBC thickness, GA = GC at hTBC = 0.27 mm. This implies that TBCs thicker than ~ 0.27 mm should fail spontaneously upon cooling to room temperature after deposition. However, as mentioned earlier, and as seen in Fig. 5, as-deposited SPPS TBCs as thick as 4 mm are found to be well-adherent [8]. This indicates clearly that the residual stresses have been grossly overestimated using Eq. (3). It appears that the microstructure of the SPPS TBCs, vertical cracks in particular, are responsible for relaxing most of the residual stresses and thereby reducing the strain energy release rate, GA, driving mode II failure.
Fig. 3. SEM micrograph showing fracture surface of a failed 7YSZ SPPS TBC specimen (substrate side) in the “barb” shear test. The arrow indicates direction of shear.
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A.D. Jadhav, N.P. Padture / Surface & Coatings Technology 202 (2008) 4976–4979 Table 1 Thermal and mechanical properties data for constituent phases used in OOF analysis [8,33]
Fig. 4. Plot showing: (i) mode II toughness, GC, of the 7YSZ SPPS TBCs, (ii) strain energy release rate (GA) from Eq. (2), and (iii) strain energy release rate, GASPPS, for SPPS TBCs of 3 different thicknesses.
To estimate the macroscopic residual stresses in the as-deposited SPPS TBCs of thicknesses 0.2, 1.5 and 4.0 mm a microstructure-based object-oriented finite element (OOF) method [24] was used. This is because, while spectroscopic and diffraction experimental methods have been used to measure microscopic residuals stresses, they are not well-suited to measure macroscopic residual stresses in ultrathick TBCs (e.g. TBCs of 4.0 mm thickness). Also, most analytical and numerical methods cannot be used to estimate accurately macroscopic residual stresses while capturing the microstructural subtleties. OOF combines data in the form of 2-dimensional microstructures and fundamental material data of the constitutive phases (elastic constants and CTE) to evaluate the effective material behavior. OOF is a public domain software [28] and it has been used to simulate the thermo-mechanical behavior of composites and TBCs [9,29–32].
Material
Young's modulus E (GPa)
Poisson's ratio ν
CTE α (°C− 1)
Dense 7YSZ SPPS 7YSZ (without vertical cracks) Superalloy Substrate
200 49
0.25 0.25
10 × 10− 6 10 × 10− 6
213
0.30
14.5 × 10− 6
Although the OOF method used here considers only 2-dimensional microstructures, it has been shown to capture the nature of the residual stresses in coatings, such as TBCs [29], quite accurately. Fig. 5A, B, and C shows examples of cross-sectional SEM micrographs of SPPS TBCs of thicknesses 0.2, 1.5, and 4.0 mm. Several such micrographs for each of the TBCs were converted to a binary images containing ceramic phase, porosity, and cracks, which were used as input to OOF. Then, the images were attached (perfect bonding) to a 5.4 mm thick superalloy substrate (elastic continuum) to complete the as-sprayed TBC system for OOF simulation. This was followed by assigning intrinsic material properties given in Table 1. In the case of the thin TBC (0.2 mm) the solid phase was assigned the properties of dense 7YSZ, while the pores and the cracks are considered to be voids. In the case of the thick TBCs (1.5 and 4.0 mm) the scale of the pores is significantly smaller (microns) than the overall thickness of the TBC (millimeters). Thus, individual pores could not be resolved at the mm scale. In those cases, the solid phase is assigned the properties of the porous SPPS 7YSZ without the vertical cracks (Table 1), while the vertical cracks are considered to be voids. The binary images were then meshed using an adaptive meshing procedure, which allows the subdivision of the elements and movement of nodes to conform to the microstructure. The mesh is adjusted by minimizing a fictitious mesh energy parameter. This energy minimization is achieved by an “annealing” procedure at the conclusion of the meshing process. After meshing the TBC micrographs effectively in OOF, pertinent boundary conditions were applied, as explained by Hsueh et al. [29]. Finally, the TBC system was equilibrated at its assumed high-
Fig. 5. Cross-sectional SEM micrographs 7YSZ SPPS TBCs of different thicknesses: (A) 0.2 mm, (B) 1.5 mm, and (C) 4.0 mm [8]. Note the proliferation of vertical cracks in these TBCs.
A.D. Jadhav, N.P. Padture / Surface & Coatings Technology 202 (2008) 4976–4979 Table 2 Average in-plane residual stress (σTBC) estimated from OOF analysis and corresponding mode II strain energy release rates (GASPPS) for 7YSZ SPPS TBCs 7YSZ SPPS TBC thickness hTBC (mm)
Average in-plane residual stress σTBC (MPa)
Strain energy release rate GASPPS (J m− 2)
0.2 1.5 4.0
− 77.4 −27.5 −13.4
12.2 11.6 7.3
temperature stress-free temperature (deposition temperature of 725 °C) and cooled to 25 °C (ΔT = 700 °C). Note that in the SPPS process the substrates are always preheated. Also, unlike APS, where rapid solidification of large “splats” results in buildup of residual stresses during deposition, such large “splat” are absent in SPPS. Thus, this simplifying assumption of stress-free state at deposition temperature is better justified in the case of SPPS TBCs considered here. The average in-plane compressive residual stresses, σTBC, calculated from OOF analysis for the SPPS TBCs of various thicknesses are reported in Table 2. These are significantly lower than those estimated using Eq. (3), and are found to decrease with increasing TBC thickness. These low residual stresses are the direct consequence of the microstructure of the SPPS TBCs, in particular the vertical cracks. The corresponding strain energy release rates, GSPPS (calculated using Eq. (2)), are also reported in Table 2 and Fig. 4. The GSPPS values are significantly lower than the intrinsic toughness of the SPPS TBC, GC. This provides a quantitative explanation for why ultra-thick SPPS TBCs do not fail spontaneously. This analysis is based on the assumption that the mode II toughness of SPPS TBCs, GC, is independent of the TBC thickness, and that the residual stress, σTBC, is the only stress driving the mode II failure. However, during in-service high-temperature thermal cycling of these TBCs, interfaces will degrade, decreasing GC. At the same time, other stresses, such as those generated during the build up of the thermally grown oxide (TGO) and the sintering of the TBC, could assist in driving the mode II failure, leading to eventual failure of the ultra-thick SPPS TBCs [8]. 4. Summary The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the “barb” shear test, which is found to be 66 J m− 2. Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low as result of the strain-tolerant microstructure of these TBCs which have: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale “splat” boundaries. The corresponding strain energy release rates in the three thickness cases have been found to be less than the intrinsic mode II toughness. This provides a quantitative explanation as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. Acknowledgements The authors thank: Dr. X. Ma and Mr. J. Roth for depositing the coatings; Profs. E.H. Jordan and M. Gell for fruitful discussions; and Dr.
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E.R. Fuller, Jr. for help with the OOF program. This publication was prepared with the support of the US Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory (Grant No. 0301-SR107). References [1] R.L. Jones, in: K.H. Stern (Ed.), Metallurgical and Ceramic Coatings, Chapman & Hall, London, 1996, p. 194. [2] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505. [3] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [4] M.B. Beardsley, J. Therm. Spray Technol. 6 (1997) 181. [5] S. Suresh, A. Mortensen, Fundamentals of Functionally Graded Materials, IOM Communications Ltd., London, UK, 1998. [6] T.A. Taylor, D.L. Appleby, A.E. Weatherill, J. Griffiths, Surf. Coat. Technol. 43–44 (1990) 470. [7] H.B. Guo, R. Vassen, D. Stoever, Surf. Coat. Technol. 186 (2004) 353. [8] A. Jadhav, N.P. Padture, F. Wu, E.H. Jordan, M. Gell, Mater. Sci. Engr. A 405 (2005) 313. [9] A.D. Jadhav, N.P. Padture, E.H. Jordan, M. Gell, P. Miranzo, E.R. Fuller, Acta Mater. 54 (2006) 3343. [10] J. Karthikeyan, C.C. Berndt, S. Reddy, J.Y. Wang, A.H. King, H. Herman, J. Am. Ceram. Soc. 81 (1998) 121. [11] P.R. Strutt, B.H. Kear, R. Boland, Method of Manufacture of Nanostructured Feeds, U. S. Patent No. 6,025,034 (2000). [12] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [13] T. Bhatia, A. Ozturk, L. Xie, E.H. Jordan, B.M. Cetegen, M. Gell, X. Ma, N.P. Padture, J. Mater. Res. 17 (2002) 2363. [14] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, Mater. Sci. Eng. A362 (2003) 204. [15] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, Surf. Coat. Technol. 177–178 (2004) 103. [16] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, J. Mater. Sci. 39 (2004) 1639. [17] L. Xie, X. Ma, A. Ozturk, E.H. Jordan, N.P. Padture, B.M. Cetegen, T.D. Xiao, M. Gell, Surf. Coat. Technol. 183 (2004) 51. [18] E.H. Jordan, L. Xie, X. Ma, M. Gell, N.P. Padture, B.M. Cetegen, A. Ozturk, J. Roth, T.D. Xiao, P.E.C. Bryant, J. Therm. Spray Technol. 13 (2004) 57. [19] L. Xie, E.H. Jordan, N.P. Padture, M. Gell, Mater. Sci. Eng., A 381 (2004) 189. [20] M. Gell, E.H. Jordan, M. Teicholz, B.M. Cetegen, N.P. Padture, L. Xie, D. Chen, X. Ma, J. Roth, J. Therm. Spray Technol. 17 (2008) 124. [21] M. Gell, L. Xie, X. Ma, E.H. Jordan, N.P. Padture, Surf. Coat. Technol. 177–178 (2004) 97. [22] S.Q. Guo, D.R. Mumm, A.M. Karlsson, Y. Kagawa, Scripta Mater. 53 (2005) 1043. [23] J.W. Hutchinson, Z. Suo, Adv. Appl. Mech. 29 (1991) 63. [24] S.A. Langer, J.E.R. Fuller, W.C. Carter, Comput. Sci. Eng. 3 (2001) 15. [25] K. Taguchi, M. Kawamura, T. Suemitsu, Y. Kagawa, 31st International Conference & Exposition on Advanced Ceramics and Composites, Daytona Beach, FL, January, 2007, Paper No. ICACC-S2-033-2007, 2007. [26] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation and Surface Evolution, Cambridge University Press, Cambridge, UK, 2003. [27] K.W. Schlichting, N.P. Padture, E.H. Jordan, M. Gell, Mater. Sci. Eng. A342 (2003) 120. [28] http://www.ctcms.nist.gov/oof/ (accessed on March 10, 2008). [29] C.H. Hsueh, J.A. Haynes, M.J. Lance, P.F. Becher, M.K. Ferber, J.E.R. Fuller, S.A. Langer, W.C. Carter, W.R. Cannon, J. Am. Ceram. Soc. 82 (1999) 1073. [30] A. Saigal, J.E.R. Fuller, Comput. Mater. Sci. 21 (2001) 149. [31] N. Chawla, B.V. Patel, M. Koopman, K.K. Chawla, R. Saha, B.R. Patterson, E.R. Fuller, S.A. Langer, Mater. Charac. 49 (2003) 395. [32] Z. Wang, A. Kulkarni, S.A. Deshpande, T. Nakamura, H. Herman, Acta Mater. 51 (2003) 5319. [33] C.H. Hsueh, E.R. Fuller, Mater. Sci. Eng. A283 (2000) 46.
