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Coatings of metastable ceramics deposited by solution-precursor plasma spray: II. Ternary ZrO2–Y2O3–Al2O3 system
Acta Materialia 54 (2006) 4921–4928 www.actamat-journals.com
Coatings of metastable ceramics deposited by solution-precursor plasma spray: II. Ternary ZrO2–Y2O3–Al2O3 system Alexander L. Vasiliev, Nitin P. Padture
*
Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210, USA Received 22 February 2006; received in revised form 19 June 2006; accepted 19 June 2006 Available online 14 September 2006
Abstract The solution-precursor plasma spray process has been used to deposit 75% dense, metastable coatings in the following ternary systems (mol%): (i) 10Al2O3–86.4ZrO2–3.6Y2O3 (10AlZrY) and (ii) 20Al2O3–76.4ZrO2–3.6Y2O3 (20AlZrY). The microstructures of these as-sprayed coatings consist of a rich variety of features and phases, as determined using X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The nanostructures (10–40 nm) in both coatings are primarily ZrO2 in tetragonal (t-ZrO2) form, with some cubic (c-ZrO2) phase present. Crystalline Al2O3 phases are absent in the nanostructured regions of these coatings, and the chemical compositions of these regions are close to the respective nominal compositions of the coatings. These results show clearly that Al3+, in addition to Y3+, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2 and c-ZrO2. The sub-micrometer regions in these coatings, in addition to t-ZrO2, contain small amounts of crystalline Al2O3 phases. Both coatings contain small amounts of spherical grains of orthorhombic ZrO2 (o-ZrO2). A heat-treatment of 1400 °C for 30 h is insufficient to effect complete t ! m transformation in ZrO2. Heat treatment at 1500 °C (30 h) results in the precipitation of a-Al2O3 and a new tetragonal form of Y2O3 in both coatings. This is accompanied by Al and Y depletion in and coarsening of the surrounding ZrO2 grains, and complete t ! m transformation. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasma spraying; Coatings; Ceramics; Analytical electron microscopy; Microstructure
1. Introduction In part I [1] of this two-part series, we presented the processing feasibility and results from detailed characterization of metastable ceramic coatings in the binary system ZrO2–Al2O3 deposited using the solution-precursor plasma spray (SPPS) method. Those coatings are 75% dense, and their microstructures consist of a rich variety of features and phases, as determined using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDXS), and electron energy loss spectroscopy (EELS). The nanostructures (10–40 nm) in those coatings are primarily tetragonal ZrO2 (t-ZrO2), with a chemical composition (atomic basis) of Zr/Al = 83/17. *
Corresponding author. E-mail address: [email protected] (N.P. Padture).
The absence of Al2O3 phases in those coatings shows clearly that Al3+, which is otherwise insoluble in ZrO2 at room temperature under equilibrium conditions, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2. The sub-micrometer regions (50–100 nm) in those coatings are primarily monoclinic ZrO2 (m-ZrO2), with a higher relative concentration of Zr (Zr/Al = 90/10). Those coatings also contain a small number of large spherical grains (0.3–1 lm) of orthorhombic ZrO2 (o-ZrO2) phase. Intergranular pockets filled with Al-rich amorphous phase are also found to exist in those coatings. After prolonged heat treatment of the ZrO2–10 mol% Al2O3 coatings, a-Al2O3 is found to precipitate out. The sources of the Al appear to be: (i) Al-containing neighboring ZrO2 grains, which results in the t ! m transformation in ZrO2 as a result of Al depletion and coarsening, and (ii) Al-rich amorphous pockets.
1359-6454/$30.00 Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.06.026
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In the present paper, we extend the study of SPPS coatings to the ternary system ZrO2–Y2O3–Al2O3. The objective here is to study co-alloying of ZrO2 SPPS coatings with Al2O3, which has been shown to stabilize ZrO2 [1], and Y2O3, which is a known ZrO2 stabilizer [2]. In an effort to elucidate the effects of Al2O3 concentration and heattreatment temperature on the microstructures and their evolution, SPPS coatings with two different concentrations of Al2O3 (10 or 20 mol%; constant Y2O3 concentration of 3.6 mol%), subjected to two different heat treatments (1400 and 1500 °C) were studied. These compositions were chosen because ZrO2-based thermal barrier coatings (TBCs) [3,4] that are co-alloyed with Y2O3 and Al2O3 may provide better thermal and environmental protection to hot-section components in gas-turbine engines compared to TBCs alloyed with Y2O3 alone. The study of the properties of the resulting coatings is beyond the scope of the both parts I and II. 2. Experimental Using the methods described in part I [1], SPPS coatings of the following compositions (mol%) were deposited by Inframat Corp. (Farmington, CT) and supplied to us: (i) 10Al2O3–86.4ZrO2–3.6Y2O3 (10AlZrY) and (ii) 20Al2O3– 76.4ZrO2–3.6Y2O3 (20AlZrY). Some free-standing SPPS coatings, of both compositions, were heat treated in a box furnace (Thermolyne, Dubuque, IA) at 1400 or 1500 °C for 30 h in air. The as-sprayed and heat-treated coatings were characterized in detail using XRD, TEM, and EDXS. The free-standing coatings (as-sprayed and heat-treated) were characterized using powder XRD (XDS 2000, Scintag, Cupertino, CA) with Cu Ka radiation, and the diffractograms for all the specimens were collected under identical conditions. TEM specimens were prepared using routine methods involving successive steps of grinding, polishing, dimpling, and ion-beam thinning (PIPS and DuoMill, Gatan Corp., Pleasanton, CA). In some cases, to rule out any artifacts introduced by mechanical and ion polishing on ZrO2 phase transformation, TEM specimens were prepared by transferring loosely attached coating material onto TEM grids (Quantifoil, Jena, Germany). The TEM specimens were observed using a conventional TEM instrument (CM200, Philips, Eindhoven, The Netherlands) with 200 kV accelerating voltage. This TEM instrument was equipped with an atmospheric-thin-window EDXS system (Phoenix, EDAX, Mahwah, NJ), and an imaging filter (GIF 2000, Gatan Pleasanton, CA). Highresolution (HREM) and EELS studies were performed using another TEM instrument (Tecnai F20 TEM/STEM, FEI, Hillsboro, OR), operated at 200 kV accelerating voltage. That TEM was equipped with an X-twin objective lens with coefficient of spherical aberration Cs = 1.3 mm (pointto-point resolution of 0.23 nm), and an imaging filter (GIF 2000, Gatan Pleasanton, CA) with EELS.
3. Results The XRD patterns of 10AlZrY and 20AlZrY SPPS coatings are shown in Figs. 1 and 2, respectively. The assprayed 10AlZrY coating (Fig. 1a) is composed primarily of t-ZrO2. Although there is no evidence of any Al2O3 phases, the presence of cubic ZrO2 (c-ZrO2) cannot be ruled out because the splitting of 0 0 2/2 0 0 and 1 1 3/3 1 1 reflections, characteristic of t-ZrO2, is not clear. Upon heat treatment (1400 °C for 30 h), the resulting coating (Fig. 1b) is found to contain t-ZrO2 predominantly, and small amounts of m-ZrO2 and a-Al2O3. Heat treatment at a higher temperature (1500 °C for 30 h) (Fig. 1c) leads to near-complete t ! m transformation in ZrO2, and further precipitation of a-Al2O3. There are several weak reflections at 2h = 26.5°, 37°, 42°, and 52° in Fig. 1c, which cannot be assigned to any known phases containing Zr, Al, Y, and/or O. Those reflections can now be attributed to a new tetragonal Y2O3 phase, as confirmed by TEM studies and discussed later. The XRD pattern of the as-sprayed 20AlZrY coating (Fig. 2a) shows that the dominant phase in that coating is also t-ZrO2, with the presence of some c-ZrO2. A set of weak reflections in Fig. 2a indicates the presence of small amounts of a-Al2O3. The XRD pattern (Fig. 2b) of the heat-treated (1400 °C for 30 h) 20AlZrY coating shows that partial t ! m transformation in ZrO2 has occurred.
Fig. 1. XRD patterns of 10AlZrY SPPS coatings: (a) as-sprayed; (b) heat treated (1400 °C, 30 h), and (c) heat treated (1500 °C, 30 h). The joint commission on powder diffraction (JCPDS) powder diffraction file (PDF) numbers 50-1089, 65-1022, and 99-0036 were used in identifying the phases present.
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Fig. 2. XRD patterns of 20AlZrY SPPS coatings: (a) as-sprayed; (b) heat treated (1400 °C, 30 h) and (c) heat treated (1500 °C, 30 h). JCPDS PDF numbers 50-1089, 81-1667, 72-1669, 78-1807, and 83-2080 were used in identifying the phases present.
The intensity of the a-Al2O3 reflections appears to increase (Fig. 2b), indicating the precipitation of a-Al2O3. Heat treatment at a higher temperature (1500 °C for 30 h) results in near complete t ! m transformation in ZrO2 (Fig. 2c) and further precipitation of a-Al2O3. Weak reflections at 2h = 26.5°, 37°, 42°, and 52° are also observed in Fig. 2c, indicating the presence of the new tetragonal Y2O3 phase. Fig. 3a–c, show bright-field TEM images, and corresponding selected area electron diffraction patterns (SAEDPs), of fine-nanostructure, coarse-nanostructure, and sub-micrometer regions, respectively, in the as-sprayed 10AlZrY coating. These regions are quite similar to those observed in the as-sprayed binary SPPS coatings in part I [1]. The fine-nanostructure region has grains 4–10 nm in size, and is primarily composed of t-ZrO2. Again, the presence of c-ZrO2 phase cannot be ruled out because the SAED ring pattern is diffuse due to the small grain size, and ring splitting, which is characteristic of t-ZrO2, is not clear. The chemical composition (atomic basis) of this region is Zr/Al/Y = 72/20/8, which is close to the nominal coating composition of 76/18/6. The coarse-nanostructure region (Fig. 3b) has grains of size 10–70 nm, and is composed of t-ZrO2, as confirmed by the SAEDP. The chemical composition of that region is Zr/Al/Y = 78/14/8. In Fig. 3c, the sub-micrometer region (100–250 nm grain size) is the most typical of what is observed in the as-sprayed 10AlZrY coating, and it contains t-ZrO2 with an overall chemical composition of Zr/Al/Y = 87/5/8. The SAEDPs of an individual grain using two different zone axes ([1 1 0] and [1 0 0]) confirm t-ZrO2 phase. Fig. 3d shows a HREM image of a bright region within an individual grain
Fig. 3. Bright-field TEM images and corresponding indexed SAEDPs of as-sprayed 10AlZrY SPPS coatings: (a) fine-nanostructure region; (b) coarse-nanostructure region and (c) sub-micrometer region. The SAEDPs in (c) are from an individual grain, using [1 1 0] and [1 0 0] zone axes; the transmitted spots are marked. (d) HREM image of a bright region indicated by arrows in (c). The EDXS spectra in (d) correspond to the Al-rich and the Al-poor regions. In (d) the (1 0 1) and ð1 0 1Þ planes are marked.
in Fig. 3c (arrows). That region does not appear to be a precipitate in the HREM image; instead it appears to be an Al-rich region within a t-ZrO2 grain according to the corresponding EDXS spectra in Fig. 3d. Fig. 4 shows a bright-field TEM image, and corresponding SAEDP, of spherical particles. In order to preclude any specimen-preparation artifacts, this TEM specimen was not mechanically polished or ion-beam thinned. Using the tilting procedure described in part I [1], the spherical particles are identified to be of o-ZrO2 phase. The spherical
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Fig. 4. Bright-field TEM image, and a corresponding SAEDP, of o-ZrO2 spherical particles in the as-sprayed 10AlZrY SPPS coatings. TEM specimen was prepared by transferring loosely attached coating material onto the TEM grid, without mechanical polishing or ion-beam thinning.
Fig. 5. EELS Al map of amorphous region at triple junction in as-sprayed 10AlZrY SPPS coating.
particles are rare, and they are typically 200 nm to 2 lm in size, with an average Al-depleted chemical composition of Zr/Al/Y = 90/1/9. Amorphous Al-rich pockets observed in the as-sprayed 10AlZrY coating (Fig. 5) are quite similar to that observed in the as-sprayed binary SPPS coatings reported in part I
Fig. 6. Bright-field TEM image of as-sprayed 10AlZrY SPPS coating showing a c-Al2O3 grain, as confirmed by SAEDP using [1 0 0] zone axis (inset), surrounded by t-ZrO2 grains.
[1]. However, unlike the as-sprayed binary coatings, the as-sprayed 10AlZrY coating is found to contain crystalline Al2O3 phases. Fig. 6 shows an example of an Al2O3 grain, which is confirmed to be the c-Al2O3 phase, with a composition Zr/Al/Y = 8/90/2. There is evidence for the presence of other transient Al2O3 phases (c 0 -Al2O3 and d-Al2O3; not shown here), but no evidence for the presence of a-Al2O3 is found (Table 1). The nanostructure, the sub-micrometer structure, and the spherical particle features observed in the as-sprayed 10AlZrY coating are also observed in the as-sprayed 20AlZrY coating (not shown here). Similar to the assprayed binary SPPS coatings in part I [1], both the 10AlZrY and the 20AlZrY as-sprayed coatings are made up of the nanostructures predominantly, while the submicrometer structures are found occasionally, and the spherical grains are found to be rare. Consistent with the higher concentration of Al in the starting precursor (nominal composition Zr/Al/Y = 80/12/8), the fine and the coarse ZrO2 nanostructures are found to be richer in Al (Table 2). However, the only key difference is the presence of a-Al2O3 grains in the as-sprayed 20AlZrY coating, as highlighted in Fig. 7.
Table 1 Microstructural characteristics of 10AlZrY SPPS coatings observed using TEM Coating
Feature
As-sprayed 10AlZrY
1. Nanostructures 1. a. Fine 1. 2. 2. 2.
b. Coarse Sub-micrometer structures a. ZrO2 b. Al2O3
2. c. Pockets 3. Spherical grains Heat treated 10AlZrY (1500 °C, 30 h)
Matrix Precipitates
Phases present
Grain size (nm)
Zr/Al/Y (at.)
Phase
Space group
t-ZrO2+ c-ZrO2 t-ZrO2
P42/nmc+ Fm3m P42/nmc
4–10
72/20/8
10–70
78/14/8
t-ZrO2 c-Al2O3 c 0 -Al2O3 d-Al2O3 Amorphous o-ZrO2
P42/nmc I41/amd P4m2 P212121 – Pbcm or Pbc21
100–250 50–150 50–150 50–150 – 200–2000
87/5/8 8/90/2 2/96/2 2/90/2 – 90/1/9
m-ZrO2 a-Al2O3 t-Y2O3
P21/c R3c 14mm
500–3000 200–2000 200–500
95/0/5 7/92/1 10/2/88
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Table 2 Microstructural characteristics of 20AlZrY SPPS coatings observed using TEM Coating
Feature
As-sprayed 20AlZrY
1. Nanostructures 1. a. Fine 1. 2. 2. 2.
b. Coarse Sub-micrometer structures a. ZrO2 b. Al2O3
2. c. Pockets 3. Spherical grains Heat treated 20AlZrY (1500 °C, 30 h)
Matrix Precipitates
Phases present
Grain size (nm)
Zr/Al/Y (at.)
Phase
Space group
t-ZrO2+ c-ZrO2 t-ZrO2
P42/nmc+ Fm3m P42/nmc
4–7
53/41/6
20–70
63/31/6
t-ZrO2 d-Al2O3 a-Al2O3 Amorphous o-ZrO2
P42/nmc P212121 R3c – Pbcm or Pbc21
100–600 – 50–150 – 200–2000
85/6/9 – 3/89/7 – 90/1/9
m-ZrO2 a-Al2O3 t-Y2O3
P21/c R3c 14 mm
500–3000 200–2000 200–500
95/1/4 4/96/0 6/2/92
Fig. 8a and b show bright-field TEM images of the heattreated (1500 °C, 30 h) 10AlZrY and 20AlZrY coatings, respectively. Both micrographs appear very similar to each other, and to the heat-treated binary SPPS coating (Fig. 11 in Ref. [1]), showing a-Al2O3 precipitates surrounded by cracked, Al-depleted m-ZrO2 grains. Significant coarsening is observed, with m-ZrO2 grains of about 2–3 lm in size. Y2O3 grains (0.2–05 lm in size) are found only in the heat-treated coatings (both 10AlZrY and 20AlZrY). These grains contained small amounts of Zr and Al in solid solution (Tables 1 and 2), and a high density of defects (dislocations). Fig. 9a shows one such example of a Y2O3 grain in the heat-treated (1500 °C, 30 h) 10AlZrY coating. Electron diffraction studies show that these Y2O3 grains are
not the typical cubic phase of Y2O3 (space group 1a3d). The absence of threefold symmetry, together with the SAEDPs being inconsistent with those expected of cubic Y2O3, indicates a new phase of Y2O3 that has not been reported before. Convergent beam electron diffraction (CBED) and tilting experiments were performed to determine the structure of these Y2O3 grains. The CBED patterns in Fig. 9b and c with [0 0 1] and [1 3 3] zone axes, respectively, confirm 4 mm symmetry and point group [5]. A series of SAEDPs was recorded using various reflection conditions (tilting), and are presented in Fig. 10 using zone axes [0 0 1], ½1 0 2, ½1 1 0, ½0 1 1, and ½1 3 3. The observed reflections are typical of 14mm or 14/mmm space groups. Thus, the CBED and SAED patterns together confirm 14mm space group for this new tetragonal Y2O3 phase, with lattice parameters a = b = 0.702 nm and c = 0.597 nm. The results from the TEM studies are summarized in Tables 1 and 2. 4. Discussion
Fig. 7. Bright-field TEM images of as-sprayed 20AlZrY SPPS coating showing: (a) larger a-Al2O3 precipitates (marked) and smaller a-Al2O3 precipitates (arrows) within t-ZrO2 grains and (b) a-Al2O3 grain (marked) surrounded by t-ZrO2 grains.
In the binary (ZrO2–10 mol% Al2O3) as-sprayed SPPS coatings in part I [1] we saw that Al3+ in solid solution with ZrO2 partially stabilizes t-ZrO2. However, m-ZrO2 is present in those coatings, while c-ZrO2 is absent [1]. In this context, the ternary as-sprayed 10AlZrY SPPS coatings studied here can be considered as 3.6 mol% Y2O3-stablized ZrO2 with additional 10 mol% Al2O3 stabilizer. The additional stabilizer appears to be responsible for the absence of m-ZrO2 and the presence of some c-ZrO2 along with t-ZrO2 in those coatings (Table 1). The same situation is observed in the as-sprayed 20AlZrY coatings (Table 2). It is well known that by virtue of its large size, Y3+ (ionic ˚ for octahedral coordination) is a more stable radius 0.9 A substitutional solute in ZrO2 compared with Al3+ (ionic ˚ for octahedral coordination) [6]. Thus, in radius 0.54 A the 10AlZrY coating it is most likely that all of the Y3+ dissolves in ZrO2 preferentially over Al3+. It appears that the
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Fig. 8. Bright-field TEM images of heat-treated (1500 °C, 30 h) SPPS coatings of (a) 10AlZrY and (b) 20AlZrY compositions. a-Al2O3 precipitates are marked in (a) and (b), surrounded by m-ZrO2 grains, with intergranular cracks visible in (b).
Al3+ that did not dissolve in ZrO2 manifests as transient Al2O3 phases (c, c 0 , d) in the as-sprayed 10AlZrY coating (Table 1). This argument is reinforced by both the absence of any Al2O3 phases in the as-sprayed binary coatings in part I [1] and the higher concentration of Al2O3 phases in the as-sprayed 20AlZrY coating with a higher Al2O3 content (Table 2). This is also consistent with the Al depletion observed in the sub-micrometer t-ZrO2 in both 10AlZrY and 20AlZrY as-sprayed coatings (Tables 1 and 2). The absence of the a-Al2O3 phase in the as-sprayed 10AlZrY coating but its presence in the as-sprayed 20AlZrY coating could be related to the higher Al2O3 content in the latter coating.
Fig. 9. (a) Bright-field TEM image of heat-treated (1500 °C, 30 h) 10SPPS coating showing a-Al2O3, m-ZrO2, and Y2O3 grains. Kossel-type CBED patterns using (b) [0 0 1] zone axis confirming 4 mm symmetry and (c) [1 3 3] zone axis confirming 1 symmetry.
The arguments presented regarding the spherical o-ZrO2 grains in the binary SPPS coatings in part I [1] also apply in the case of the as-sprayed ternary SPPS coatings. Briefly, chemically derived ZrO2 nanopowders have been found to be orthorhombic (Pmnb space group) at ambient pressures, which has been attributed to aqueous reactions in the mesoporous Zr-containing precursor during heating [7]. Since mesoporous precursor is an intermediate phase that forms during the SPPS process [8–11], it is likely that on rare occasions the reaction conditions in the plasma are conducive for the formation of orthorhombic phase ZrO2
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Fig. 10. Series of SAEDPs from the Y2O3 grain in Fig. 9a using zone axes: (a) [0 0 1]; (b) ½1 0 2; (c) ½1 1 0; (d) ½0 1 1 and (e) ½1 3 3.
particles in the SPPS coatings, as seen in Fig. 4. The XRD patterns (Figs. 1a and 2a) do not show the presence of oZrO2, which is most likely due to the rare occurrence of these orthorhombic particles in the SPPS coatings. The formation of crystalline a-Al2O3 precipitates, and the concomitant Al depletion and destabilization of t-ZrO2 into m-ZrO2, upon heat treatment (1400 °C, 30 h) of the ternary SPPS coatings (Figs. 1b and 2b) appear to be similar to that observed in the heat-treated (1400 °C, 30 h) binary SPPS coatings in part I [1]. However, only partial t ! m ZrO2 transformation occurs in both the 10AlZrY and 20AlZrY coatings heat treated at 1400 °C (30 h) (Figs. 1b and 2b), whereas complete t ! m ZrO2 transformation is observed in the binary SPPS coatings heat treated at the same temperature (1400 °C, 30 h) in part I [1]. A plausible explanation for this is as follows. The combined effect of greater stability of Y3+ substitutional solute compared with Al3+ in ZrO2 and the higher concentration of Al compared to Y in the ternary coatings results in the preferential outward diffusion of Al from ZrO2. This results in the formation of a-Al2O3 precipitates and the partial destabilization of t-ZrO2 into m-ZrO2. The Y3+ solute still in solid solution in ZrO2 is responsible for the stability of the remaining t-ZrO2. An increase in the heat-treatment temperature (1500 °C, 30 h) results in significant coarsening of ZrO2 grains, near-complete Al depletion in ZrO2, and some Y depletion in ZrO2. These factors are responsible for the near-complete t ! m ZrO2 transformation in the ternary coatings and the precipitation of a-Al2O3 (Figs. 1c and 2c; Tables 1 and 2). The microcracking associated with the a-Al2O3 observed in both heat-treated 10AlZrY (not shown here) and 20AlZrY (Fig. 8b) is most likely due to both the thermal expansion mismatch and the t ! m transformation strain; such microcracking has been observed in Al2O3 particulate-reinforced ZrO2-based ceramics [12].
The precipitation of t-Y2O3 grains in both the 10AlZrY and 20AlZrY coatings upon heat treatment (1500 °C, 30 h) appears to the result of outward diffusion of Y from the ZrO2 grains. The formation of the new tetragonal phase Y2O3, instead of the typical cubic phase, appears to be related to the small amounts of Zr and Al solute present in tetragonal Y2O3. 5. Summary The SPPS process can be used to deposit 75% dense, metastable coatings in the ternary systems 10AlZrY and 20AlZrY. The microstructures of these as-sprayed coatings consist of a rich variety of features and phases, as determined using XRD, TEM, and EDXS. The nanostructures (10–40 nm) in both coatings are primarily t-ZrO2, with some c-ZrO2. Crystalline Al2O3 phases are absent in the nanostructured regions of these coatings, and the chemical compositions of these regions are close to the respective nominal compositions of the coatings. These results show clearly that Al3+, in addition to the Y3+, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2 and cZrO2. The sub-micrometer t-ZrO2 regions in these coatings contain crystalline Al2O3 phases, the concentration of which is higher in the coating with the higher Al content (20AlZrY). Both coatings contain small amounts of spherical grains of o-ZrO2. A heat treatment of 1400 °C for 30 h is insufficient to effect complete t ! m transformation in ZrO2. Heat treatment at 1500 °C (30 h) results in the precipitation of a-Al2O3 and a new t-Y2O3 phase in both coatings. This is accompanied by coarsening of, and Al and Y depletion in, the surrounding ZrO2 grains, and complete t ! m transformation. In closing, the SPPS process offers the prospect of depositing directly coatings or free-standing structures of
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metastable ceramics in the ternary ZrO2–Y2O3–Al2O3 system in one step, and the possibility of exploiting the unusual structures and properties these ceramics may offer. Acknowledgements The authors thank Dr. X. Ma, Mr. J. Roth, Mr. X. Wang, and Dr. F. Wu for experimental assistance, Profs. M. Aindow, M. Gell, and E.H. Jordan for helpful discussions, and the Office of Naval Research (Grant No. N000014-02-1-0171) and Naval Air Systems Command (Grant No. N00421-05-1-0001) for financial support. References [1] Vasiliev AL, Padture NP, Ma X. Acta Mater 2006;54:4913.
[2] Heuer AH, Hobbs LW. Science and technology of zirconia: advances in ceramics, 3. Columbus (OH): American Ceramic Society; 1981. [3] Padture NP, Schlichting KW, Bhatia T, Ozturk A, Cetegen B, Jordan EH, et al. Acta Mater 2001;49:2251. [4] Padture NP, Gell M, Jordan EH. Science 2002;296:280. [5] Williams DB, Carter CB. Transmission electron microscopy. New York (NY): Plenum Press; 1996. [6] Balmer ML, Lange FF, Levi CG. J Am Ceram Soc 1994;77: 2069. [7] Ram S, Mondal A. Phys Stat Sol A 2004;201:696. [8] Bhatia T, Ozturk A, Xie L, Jordan EH, Cetegen BM, Gell M, et al. J Mater Res 2002;17:2363. [9] Xie L, Ma X, Jordan EH, Padture NP, Xiao TD, Gell M. Mater Sci Eng 2003;A362:204. [10] Jordan EH, Xie L, Ma X, Gell M, Padture NP, Cetegen BM, et al. J Therm Spray Technol 2004;13:57. [11] Xie L, Ma X, Jordan EH, Padture NP, Xiao TD, Gell M. J Mater Sci 2004;39:1639. [12] Green DJ, Hannink RHJ, Swain MV. Transformation toughening of ceramics. Boca Raton (FL): CRC Press; 1989.
Coatings of metastable ceramics deposited by solution-precursor plasma spray: II. Ternary ZrO2–Y2O3–Al2O3 system Alexander L. Vasiliev, Nitin P. Padture
*
Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210, USA Received 22 February 2006; received in revised form 19 June 2006; accepted 19 June 2006 Available online 14 September 2006
Abstract The solution-precursor plasma spray process has been used to deposit 75% dense, metastable coatings in the following ternary systems (mol%): (i) 10Al2O3–86.4ZrO2–3.6Y2O3 (10AlZrY) and (ii) 20Al2O3–76.4ZrO2–3.6Y2O3 (20AlZrY). The microstructures of these as-sprayed coatings consist of a rich variety of features and phases, as determined using X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The nanostructures (10–40 nm) in both coatings are primarily ZrO2 in tetragonal (t-ZrO2) form, with some cubic (c-ZrO2) phase present. Crystalline Al2O3 phases are absent in the nanostructured regions of these coatings, and the chemical compositions of these regions are close to the respective nominal compositions of the coatings. These results show clearly that Al3+, in addition to Y3+, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2 and c-ZrO2. The sub-micrometer regions in these coatings, in addition to t-ZrO2, contain small amounts of crystalline Al2O3 phases. Both coatings contain small amounts of spherical grains of orthorhombic ZrO2 (o-ZrO2). A heat-treatment of 1400 °C for 30 h is insufficient to effect complete t ! m transformation in ZrO2. Heat treatment at 1500 °C (30 h) results in the precipitation of a-Al2O3 and a new tetragonal form of Y2O3 in both coatings. This is accompanied by Al and Y depletion in and coarsening of the surrounding ZrO2 grains, and complete t ! m transformation. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Plasma spraying; Coatings; Ceramics; Analytical electron microscopy; Microstructure
1. Introduction In part I [1] of this two-part series, we presented the processing feasibility and results from detailed characterization of metastable ceramic coatings in the binary system ZrO2–Al2O3 deposited using the solution-precursor plasma spray (SPPS) method. Those coatings are 75% dense, and their microstructures consist of a rich variety of features and phases, as determined using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDXS), and electron energy loss spectroscopy (EELS). The nanostructures (10–40 nm) in those coatings are primarily tetragonal ZrO2 (t-ZrO2), with a chemical composition (atomic basis) of Zr/Al = 83/17. *
Corresponding author. E-mail address: [email protected] (N.P. Padture).
The absence of Al2O3 phases in those coatings shows clearly that Al3+, which is otherwise insoluble in ZrO2 at room temperature under equilibrium conditions, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2. The sub-micrometer regions (50–100 nm) in those coatings are primarily monoclinic ZrO2 (m-ZrO2), with a higher relative concentration of Zr (Zr/Al = 90/10). Those coatings also contain a small number of large spherical grains (0.3–1 lm) of orthorhombic ZrO2 (o-ZrO2) phase. Intergranular pockets filled with Al-rich amorphous phase are also found to exist in those coatings. After prolonged heat treatment of the ZrO2–10 mol% Al2O3 coatings, a-Al2O3 is found to precipitate out. The sources of the Al appear to be: (i) Al-containing neighboring ZrO2 grains, which results in the t ! m transformation in ZrO2 as a result of Al depletion and coarsening, and (ii) Al-rich amorphous pockets.
1359-6454/$30.00 Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.06.026
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In the present paper, we extend the study of SPPS coatings to the ternary system ZrO2–Y2O3–Al2O3. The objective here is to study co-alloying of ZrO2 SPPS coatings with Al2O3, which has been shown to stabilize ZrO2 [1], and Y2O3, which is a known ZrO2 stabilizer [2]. In an effort to elucidate the effects of Al2O3 concentration and heattreatment temperature on the microstructures and their evolution, SPPS coatings with two different concentrations of Al2O3 (10 or 20 mol%; constant Y2O3 concentration of 3.6 mol%), subjected to two different heat treatments (1400 and 1500 °C) were studied. These compositions were chosen because ZrO2-based thermal barrier coatings (TBCs) [3,4] that are co-alloyed with Y2O3 and Al2O3 may provide better thermal and environmental protection to hot-section components in gas-turbine engines compared to TBCs alloyed with Y2O3 alone. The study of the properties of the resulting coatings is beyond the scope of the both parts I and II. 2. Experimental Using the methods described in part I [1], SPPS coatings of the following compositions (mol%) were deposited by Inframat Corp. (Farmington, CT) and supplied to us: (i) 10Al2O3–86.4ZrO2–3.6Y2O3 (10AlZrY) and (ii) 20Al2O3– 76.4ZrO2–3.6Y2O3 (20AlZrY). Some free-standing SPPS coatings, of both compositions, were heat treated in a box furnace (Thermolyne, Dubuque, IA) at 1400 or 1500 °C for 30 h in air. The as-sprayed and heat-treated coatings were characterized in detail using XRD, TEM, and EDXS. The free-standing coatings (as-sprayed and heat-treated) were characterized using powder XRD (XDS 2000, Scintag, Cupertino, CA) with Cu Ka radiation, and the diffractograms for all the specimens were collected under identical conditions. TEM specimens were prepared using routine methods involving successive steps of grinding, polishing, dimpling, and ion-beam thinning (PIPS and DuoMill, Gatan Corp., Pleasanton, CA). In some cases, to rule out any artifacts introduced by mechanical and ion polishing on ZrO2 phase transformation, TEM specimens were prepared by transferring loosely attached coating material onto TEM grids (Quantifoil, Jena, Germany). The TEM specimens were observed using a conventional TEM instrument (CM200, Philips, Eindhoven, The Netherlands) with 200 kV accelerating voltage. This TEM instrument was equipped with an atmospheric-thin-window EDXS system (Phoenix, EDAX, Mahwah, NJ), and an imaging filter (GIF 2000, Gatan Pleasanton, CA). Highresolution (HREM) and EELS studies were performed using another TEM instrument (Tecnai F20 TEM/STEM, FEI, Hillsboro, OR), operated at 200 kV accelerating voltage. That TEM was equipped with an X-twin objective lens with coefficient of spherical aberration Cs = 1.3 mm (pointto-point resolution of 0.23 nm), and an imaging filter (GIF 2000, Gatan Pleasanton, CA) with EELS.
3. Results The XRD patterns of 10AlZrY and 20AlZrY SPPS coatings are shown in Figs. 1 and 2, respectively. The assprayed 10AlZrY coating (Fig. 1a) is composed primarily of t-ZrO2. Although there is no evidence of any Al2O3 phases, the presence of cubic ZrO2 (c-ZrO2) cannot be ruled out because the splitting of 0 0 2/2 0 0 and 1 1 3/3 1 1 reflections, characteristic of t-ZrO2, is not clear. Upon heat treatment (1400 °C for 30 h), the resulting coating (Fig. 1b) is found to contain t-ZrO2 predominantly, and small amounts of m-ZrO2 and a-Al2O3. Heat treatment at a higher temperature (1500 °C for 30 h) (Fig. 1c) leads to near-complete t ! m transformation in ZrO2, and further precipitation of a-Al2O3. There are several weak reflections at 2h = 26.5°, 37°, 42°, and 52° in Fig. 1c, which cannot be assigned to any known phases containing Zr, Al, Y, and/or O. Those reflections can now be attributed to a new tetragonal Y2O3 phase, as confirmed by TEM studies and discussed later. The XRD pattern of the as-sprayed 20AlZrY coating (Fig. 2a) shows that the dominant phase in that coating is also t-ZrO2, with the presence of some c-ZrO2. A set of weak reflections in Fig. 2a indicates the presence of small amounts of a-Al2O3. The XRD pattern (Fig. 2b) of the heat-treated (1400 °C for 30 h) 20AlZrY coating shows that partial t ! m transformation in ZrO2 has occurred.
Fig. 1. XRD patterns of 10AlZrY SPPS coatings: (a) as-sprayed; (b) heat treated (1400 °C, 30 h), and (c) heat treated (1500 °C, 30 h). The joint commission on powder diffraction (JCPDS) powder diffraction file (PDF) numbers 50-1089, 65-1022, and 99-0036 were used in identifying the phases present.
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Fig. 2. XRD patterns of 20AlZrY SPPS coatings: (a) as-sprayed; (b) heat treated (1400 °C, 30 h) and (c) heat treated (1500 °C, 30 h). JCPDS PDF numbers 50-1089, 81-1667, 72-1669, 78-1807, and 83-2080 were used in identifying the phases present.
The intensity of the a-Al2O3 reflections appears to increase (Fig. 2b), indicating the precipitation of a-Al2O3. Heat treatment at a higher temperature (1500 °C for 30 h) results in near complete t ! m transformation in ZrO2 (Fig. 2c) and further precipitation of a-Al2O3. Weak reflections at 2h = 26.5°, 37°, 42°, and 52° are also observed in Fig. 2c, indicating the presence of the new tetragonal Y2O3 phase. Fig. 3a–c, show bright-field TEM images, and corresponding selected area electron diffraction patterns (SAEDPs), of fine-nanostructure, coarse-nanostructure, and sub-micrometer regions, respectively, in the as-sprayed 10AlZrY coating. These regions are quite similar to those observed in the as-sprayed binary SPPS coatings in part I [1]. The fine-nanostructure region has grains 4–10 nm in size, and is primarily composed of t-ZrO2. Again, the presence of c-ZrO2 phase cannot be ruled out because the SAED ring pattern is diffuse due to the small grain size, and ring splitting, which is characteristic of t-ZrO2, is not clear. The chemical composition (atomic basis) of this region is Zr/Al/Y = 72/20/8, which is close to the nominal coating composition of 76/18/6. The coarse-nanostructure region (Fig. 3b) has grains of size 10–70 nm, and is composed of t-ZrO2, as confirmed by the SAEDP. The chemical composition of that region is Zr/Al/Y = 78/14/8. In Fig. 3c, the sub-micrometer region (100–250 nm grain size) is the most typical of what is observed in the as-sprayed 10AlZrY coating, and it contains t-ZrO2 with an overall chemical composition of Zr/Al/Y = 87/5/8. The SAEDPs of an individual grain using two different zone axes ([1 1 0] and [1 0 0]) confirm t-ZrO2 phase. Fig. 3d shows a HREM image of a bright region within an individual grain
Fig. 3. Bright-field TEM images and corresponding indexed SAEDPs of as-sprayed 10AlZrY SPPS coatings: (a) fine-nanostructure region; (b) coarse-nanostructure region and (c) sub-micrometer region. The SAEDPs in (c) are from an individual grain, using [1 1 0] and [1 0 0] zone axes; the transmitted spots are marked. (d) HREM image of a bright region indicated by arrows in (c). The EDXS spectra in (d) correspond to the Al-rich and the Al-poor regions. In (d) the (1 0 1) and ð1 0 1Þ planes are marked.
in Fig. 3c (arrows). That region does not appear to be a precipitate in the HREM image; instead it appears to be an Al-rich region within a t-ZrO2 grain according to the corresponding EDXS spectra in Fig. 3d. Fig. 4 shows a bright-field TEM image, and corresponding SAEDP, of spherical particles. In order to preclude any specimen-preparation artifacts, this TEM specimen was not mechanically polished or ion-beam thinned. Using the tilting procedure described in part I [1], the spherical particles are identified to be of o-ZrO2 phase. The spherical
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Fig. 4. Bright-field TEM image, and a corresponding SAEDP, of o-ZrO2 spherical particles in the as-sprayed 10AlZrY SPPS coatings. TEM specimen was prepared by transferring loosely attached coating material onto the TEM grid, without mechanical polishing or ion-beam thinning.
Fig. 5. EELS Al map of amorphous region at triple junction in as-sprayed 10AlZrY SPPS coating.
particles are rare, and they are typically 200 nm to 2 lm in size, with an average Al-depleted chemical composition of Zr/Al/Y = 90/1/9. Amorphous Al-rich pockets observed in the as-sprayed 10AlZrY coating (Fig. 5) are quite similar to that observed in the as-sprayed binary SPPS coatings reported in part I
Fig. 6. Bright-field TEM image of as-sprayed 10AlZrY SPPS coating showing a c-Al2O3 grain, as confirmed by SAEDP using [1 0 0] zone axis (inset), surrounded by t-ZrO2 grains.
[1]. However, unlike the as-sprayed binary coatings, the as-sprayed 10AlZrY coating is found to contain crystalline Al2O3 phases. Fig. 6 shows an example of an Al2O3 grain, which is confirmed to be the c-Al2O3 phase, with a composition Zr/Al/Y = 8/90/2. There is evidence for the presence of other transient Al2O3 phases (c 0 -Al2O3 and d-Al2O3; not shown here), but no evidence for the presence of a-Al2O3 is found (Table 1). The nanostructure, the sub-micrometer structure, and the spherical particle features observed in the as-sprayed 10AlZrY coating are also observed in the as-sprayed 20AlZrY coating (not shown here). Similar to the assprayed binary SPPS coatings in part I [1], both the 10AlZrY and the 20AlZrY as-sprayed coatings are made up of the nanostructures predominantly, while the submicrometer structures are found occasionally, and the spherical grains are found to be rare. Consistent with the higher concentration of Al in the starting precursor (nominal composition Zr/Al/Y = 80/12/8), the fine and the coarse ZrO2 nanostructures are found to be richer in Al (Table 2). However, the only key difference is the presence of a-Al2O3 grains in the as-sprayed 20AlZrY coating, as highlighted in Fig. 7.
Table 1 Microstructural characteristics of 10AlZrY SPPS coatings observed using TEM Coating
Feature
As-sprayed 10AlZrY
1. Nanostructures 1. a. Fine 1. 2. 2. 2.
b. Coarse Sub-micrometer structures a. ZrO2 b. Al2O3
2. c. Pockets 3. Spherical grains Heat treated 10AlZrY (1500 °C, 30 h)
Matrix Precipitates
Phases present
Grain size (nm)
Zr/Al/Y (at.)
Phase
Space group
t-ZrO2+ c-ZrO2 t-ZrO2
P42/nmc+ Fm3m P42/nmc
4–10
72/20/8
10–70
78/14/8
t-ZrO2 c-Al2O3 c 0 -Al2O3 d-Al2O3 Amorphous o-ZrO2
P42/nmc I41/amd P4m2 P212121 – Pbcm or Pbc21
100–250 50–150 50–150 50–150 – 200–2000
87/5/8 8/90/2 2/96/2 2/90/2 – 90/1/9
m-ZrO2 a-Al2O3 t-Y2O3
P21/c R3c 14mm
500–3000 200–2000 200–500
95/0/5 7/92/1 10/2/88
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Table 2 Microstructural characteristics of 20AlZrY SPPS coatings observed using TEM Coating
Feature
As-sprayed 20AlZrY
1. Nanostructures 1. a. Fine 1. 2. 2. 2.
b. Coarse Sub-micrometer structures a. ZrO2 b. Al2O3
2. c. Pockets 3. Spherical grains Heat treated 20AlZrY (1500 °C, 30 h)
Matrix Precipitates
Phases present
Grain size (nm)
Zr/Al/Y (at.)
Phase
Space group
t-ZrO2+ c-ZrO2 t-ZrO2
P42/nmc+ Fm3m P42/nmc
4–7
53/41/6
20–70
63/31/6
t-ZrO2 d-Al2O3 a-Al2O3 Amorphous o-ZrO2
P42/nmc P212121 R3c – Pbcm or Pbc21
100–600 – 50–150 – 200–2000
85/6/9 – 3/89/7 – 90/1/9
m-ZrO2 a-Al2O3 t-Y2O3
P21/c R3c 14 mm
500–3000 200–2000 200–500
95/1/4 4/96/0 6/2/92
Fig. 8a and b show bright-field TEM images of the heattreated (1500 °C, 30 h) 10AlZrY and 20AlZrY coatings, respectively. Both micrographs appear very similar to each other, and to the heat-treated binary SPPS coating (Fig. 11 in Ref. [1]), showing a-Al2O3 precipitates surrounded by cracked, Al-depleted m-ZrO2 grains. Significant coarsening is observed, with m-ZrO2 grains of about 2–3 lm in size. Y2O3 grains (0.2–05 lm in size) are found only in the heat-treated coatings (both 10AlZrY and 20AlZrY). These grains contained small amounts of Zr and Al in solid solution (Tables 1 and 2), and a high density of defects (dislocations). Fig. 9a shows one such example of a Y2O3 grain in the heat-treated (1500 °C, 30 h) 10AlZrY coating. Electron diffraction studies show that these Y2O3 grains are
not the typical cubic phase of Y2O3 (space group 1a3d). The absence of threefold symmetry, together with the SAEDPs being inconsistent with those expected of cubic Y2O3, indicates a new phase of Y2O3 that has not been reported before. Convergent beam electron diffraction (CBED) and tilting experiments were performed to determine the structure of these Y2O3 grains. The CBED patterns in Fig. 9b and c with [0 0 1] and [1 3 3] zone axes, respectively, confirm 4 mm symmetry and point group [5]. A series of SAEDPs was recorded using various reflection conditions (tilting), and are presented in Fig. 10 using zone axes [0 0 1], ½1 0 2, ½1 1 0, ½0 1 1, and ½1 3 3. The observed reflections are typical of 14mm or 14/mmm space groups. Thus, the CBED and SAED patterns together confirm 14mm space group for this new tetragonal Y2O3 phase, with lattice parameters a = b = 0.702 nm and c = 0.597 nm. The results from the TEM studies are summarized in Tables 1 and 2. 4. Discussion
Fig. 7. Bright-field TEM images of as-sprayed 20AlZrY SPPS coating showing: (a) larger a-Al2O3 precipitates (marked) and smaller a-Al2O3 precipitates (arrows) within t-ZrO2 grains and (b) a-Al2O3 grain (marked) surrounded by t-ZrO2 grains.
In the binary (ZrO2–10 mol% Al2O3) as-sprayed SPPS coatings in part I [1] we saw that Al3+ in solid solution with ZrO2 partially stabilizes t-ZrO2. However, m-ZrO2 is present in those coatings, while c-ZrO2 is absent [1]. In this context, the ternary as-sprayed 10AlZrY SPPS coatings studied here can be considered as 3.6 mol% Y2O3-stablized ZrO2 with additional 10 mol% Al2O3 stabilizer. The additional stabilizer appears to be responsible for the absence of m-ZrO2 and the presence of some c-ZrO2 along with t-ZrO2 in those coatings (Table 1). The same situation is observed in the as-sprayed 20AlZrY coatings (Table 2). It is well known that by virtue of its large size, Y3+ (ionic ˚ for octahedral coordination) is a more stable radius 0.9 A substitutional solute in ZrO2 compared with Al3+ (ionic ˚ for octahedral coordination) [6]. Thus, in radius 0.54 A the 10AlZrY coating it is most likely that all of the Y3+ dissolves in ZrO2 preferentially over Al3+. It appears that the
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Fig. 8. Bright-field TEM images of heat-treated (1500 °C, 30 h) SPPS coatings of (a) 10AlZrY and (b) 20AlZrY compositions. a-Al2O3 precipitates are marked in (a) and (b), surrounded by m-ZrO2 grains, with intergranular cracks visible in (b).
Al3+ that did not dissolve in ZrO2 manifests as transient Al2O3 phases (c, c 0 , d) in the as-sprayed 10AlZrY coating (Table 1). This argument is reinforced by both the absence of any Al2O3 phases in the as-sprayed binary coatings in part I [1] and the higher concentration of Al2O3 phases in the as-sprayed 20AlZrY coating with a higher Al2O3 content (Table 2). This is also consistent with the Al depletion observed in the sub-micrometer t-ZrO2 in both 10AlZrY and 20AlZrY as-sprayed coatings (Tables 1 and 2). The absence of the a-Al2O3 phase in the as-sprayed 10AlZrY coating but its presence in the as-sprayed 20AlZrY coating could be related to the higher Al2O3 content in the latter coating.
Fig. 9. (a) Bright-field TEM image of heat-treated (1500 °C, 30 h) 10SPPS coating showing a-Al2O3, m-ZrO2, and Y2O3 grains. Kossel-type CBED patterns using (b) [0 0 1] zone axis confirming 4 mm symmetry and (c) [1 3 3] zone axis confirming 1 symmetry.
The arguments presented regarding the spherical o-ZrO2 grains in the binary SPPS coatings in part I [1] also apply in the case of the as-sprayed ternary SPPS coatings. Briefly, chemically derived ZrO2 nanopowders have been found to be orthorhombic (Pmnb space group) at ambient pressures, which has been attributed to aqueous reactions in the mesoporous Zr-containing precursor during heating [7]. Since mesoporous precursor is an intermediate phase that forms during the SPPS process [8–11], it is likely that on rare occasions the reaction conditions in the plasma are conducive for the formation of orthorhombic phase ZrO2
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Fig. 10. Series of SAEDPs from the Y2O3 grain in Fig. 9a using zone axes: (a) [0 0 1]; (b) ½1 0 2; (c) ½1 1 0; (d) ½0 1 1 and (e) ½1 3 3.
particles in the SPPS coatings, as seen in Fig. 4. The XRD patterns (Figs. 1a and 2a) do not show the presence of oZrO2, which is most likely due to the rare occurrence of these orthorhombic particles in the SPPS coatings. The formation of crystalline a-Al2O3 precipitates, and the concomitant Al depletion and destabilization of t-ZrO2 into m-ZrO2, upon heat treatment (1400 °C, 30 h) of the ternary SPPS coatings (Figs. 1b and 2b) appear to be similar to that observed in the heat-treated (1400 °C, 30 h) binary SPPS coatings in part I [1]. However, only partial t ! m ZrO2 transformation occurs in both the 10AlZrY and 20AlZrY coatings heat treated at 1400 °C (30 h) (Figs. 1b and 2b), whereas complete t ! m ZrO2 transformation is observed in the binary SPPS coatings heat treated at the same temperature (1400 °C, 30 h) in part I [1]. A plausible explanation for this is as follows. The combined effect of greater stability of Y3+ substitutional solute compared with Al3+ in ZrO2 and the higher concentration of Al compared to Y in the ternary coatings results in the preferential outward diffusion of Al from ZrO2. This results in the formation of a-Al2O3 precipitates and the partial destabilization of t-ZrO2 into m-ZrO2. The Y3+ solute still in solid solution in ZrO2 is responsible for the stability of the remaining t-ZrO2. An increase in the heat-treatment temperature (1500 °C, 30 h) results in significant coarsening of ZrO2 grains, near-complete Al depletion in ZrO2, and some Y depletion in ZrO2. These factors are responsible for the near-complete t ! m ZrO2 transformation in the ternary coatings and the precipitation of a-Al2O3 (Figs. 1c and 2c; Tables 1 and 2). The microcracking associated with the a-Al2O3 observed in both heat-treated 10AlZrY (not shown here) and 20AlZrY (Fig. 8b) is most likely due to both the thermal expansion mismatch and the t ! m transformation strain; such microcracking has been observed in Al2O3 particulate-reinforced ZrO2-based ceramics [12].
The precipitation of t-Y2O3 grains in both the 10AlZrY and 20AlZrY coatings upon heat treatment (1500 °C, 30 h) appears to the result of outward diffusion of Y from the ZrO2 grains. The formation of the new tetragonal phase Y2O3, instead of the typical cubic phase, appears to be related to the small amounts of Zr and Al solute present in tetragonal Y2O3. 5. Summary The SPPS process can be used to deposit 75% dense, metastable coatings in the ternary systems 10AlZrY and 20AlZrY. The microstructures of these as-sprayed coatings consist of a rich variety of features and phases, as determined using XRD, TEM, and EDXS. The nanostructures (10–40 nm) in both coatings are primarily t-ZrO2, with some c-ZrO2. Crystalline Al2O3 phases are absent in the nanostructured regions of these coatings, and the chemical compositions of these regions are close to the respective nominal compositions of the coatings. These results show clearly that Al3+, in addition to the Y3+, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2 and cZrO2. The sub-micrometer t-ZrO2 regions in these coatings contain crystalline Al2O3 phases, the concentration of which is higher in the coating with the higher Al content (20AlZrY). Both coatings contain small amounts of spherical grains of o-ZrO2. A heat treatment of 1400 °C for 30 h is insufficient to effect complete t ! m transformation in ZrO2. Heat treatment at 1500 °C (30 h) results in the precipitation of a-Al2O3 and a new t-Y2O3 phase in both coatings. This is accompanied by coarsening of, and Al and Y depletion in, the surrounding ZrO2 grains, and complete t ! m transformation. In closing, the SPPS process offers the prospect of depositing directly coatings or free-standing structures of
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metastable ceramics in the ternary ZrO2–Y2O3–Al2O3 system in one step, and the possibility of exploiting the unusual structures and properties these ceramics may offer. Acknowledgements The authors thank Dr. X. Ma, Mr. J. Roth, Mr. X. Wang, and Dr. F. Wu for experimental assistance, Profs. M. Aindow, M. Gell, and E.H. Jordan for helpful discussions, and the Office of Naval Research (Grant No. N000014-02-1-0171) and Naval Air Systems Command (Grant No. N00421-05-1-0001) for financial support. References [1] Vasiliev AL, Padture NP, Ma X. Acta Mater 2006;54:4913.
[2] Heuer AH, Hobbs LW. Science and technology of zirconia: advances in ceramics, 3. Columbus (OH): American Ceramic Society; 1981. [3] Padture NP, Schlichting KW, Bhatia T, Ozturk A, Cetegen B, Jordan EH, et al. Acta Mater 2001;49:2251. [4] Padture NP, Gell M, Jordan EH. Science 2002;296:280. [5] Williams DB, Carter CB. Transmission electron microscopy. New York (NY): Plenum Press; 1996. [6] Balmer ML, Lange FF, Levi CG. J Am Ceram Soc 1994;77: 2069. [7] Ram S, Mondal A. Phys Stat Sol A 2004;201:696. [8] Bhatia T, Ozturk A, Xie L, Jordan EH, Cetegen BM, Gell M, et al. J Mater Res 2002;17:2363. [9] Xie L, Ma X, Jordan EH, Padture NP, Xiao TD, Gell M. Mater Sci Eng 2003;A362:204. [10] Jordan EH, Xie L, Ma X, Gell M, Padture NP, Cetegen BM, et al. J Therm Spray Technol 2004;13:57. [11] Xie L, Ma X, Jordan EH, Padture NP, Xiao TD, Gell M. J Mater Sci 2004;39:1639. [12] Green DJ, Hannink RHJ, Swain MV. Transformation toughening of ceramics. Boca Raton (FL): CRC Press; 1989.