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Nanostructured zirconia–30 vol.% alumina composite coatings deposited by atmospheric plasma spraying
Thin Solid Films 484 (2005) 225 – 231 www.elsevier.com/locate/tsf
Nanostructured zirconia–30 vol.% alumina composite coatings deposited by atmospheric plasma spraying Bo Lianga,*, Hanlin Liaob, Chuanxian Dinga, Christian Coddetb b
a Shanghai Institute of Ceramics, Chinese Academic of Sciences, Shanghai 200050, PR China Laboratoire d’Etudes et de Recherches sur les Mae´riaux, les Proce´de´s et les Surfaces, Universite´ de Technologie de Belfort-Montbe´liard, 90010 Belfort Cedex, France
Received 30 June 2004; accepted in revised form 25 February 2005 Available online 12 May 2005
Abstract Nanostructured zirconia – 30 vol.% alumina composite coatings were deposited by atmospheric plasma spraying using nanosized ZrO2 and Al2O3 powders. The microstructure and phase composition of the coatings were characterized by electron probe X-ray microanalyser (EPMA), field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Raman spectroscopy. Microhardness (HV0.2 kg) and roughness (Ra) were also measured. The as-sprayed coating exhibited a bimodal structure: the typical lamellar structure comprised of nanosized columnar grains and the nanosized equiaxed grains. Alumina-rich splats were dark grey colored and non-uniformly embedded in light grey zirconia-rich splats. No monoclinic or cubic zirconia phase and alumina phase were detected in the as-sprayed coating. Compared to pure zirconia coating, the microhardness of the as-sprayed coating increased from 5.7 GPa to 8.4 GPa, and the roughness increased from 3.74 Am to 6.03 Am with the addition of alumina into feedstock. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructured; Zirconia – 30 vol.% alumina composite; Atmospheric plasma spraying
1. Introduction ZrO2 – Al2O3 composite coatings deposited by plasma spraying or electron beam physical vapor deposition (EBPVD) have been actively studied in order to improve properties of zirconia thermal barrier coatings. It was reported that deposits of ZrO2 and Al2O3 alternating layers manufactured using plasma spraying or EB-PVD exhibited the reduction of oxygen diffusion through the deposits. Plasma sprayed laminated composites showed an increase in thermal resistance, compared with EB-PVD and single bulk coatings [1– 3]. Mixing alumina into the zirconia layers was effective in reducing the residual stresses and the formation of cracks [4]. The plasma spraying of these premixed powders also improved mechanical and chemical properties without compromising the thermal behavior of
* Corresponding author. Tel.: +86 21 52414103; fax: +86 21 52413903. E-mail address: [email protected] (B. Liang). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.02.040
the coatings [5]. In addition, the combination of high hardness alumina with the low thermal conductivity zirconia contributed to the development of the thermal shock resistance, microhardness and wear resistance of the as-sprayed coatings [6 –9]. Nanostructured coatings are attractive because of their potential superior mechanical and physical properties [10 – 14]. The plasma spraying of nanostructured powders is one of the effective means for obtaining nanostructured coatings because of its cost effectiveness [15 – 18]. Although the microstructure and other properties of the nanostructured as-sprayed zirconia coatings or alumina coatings have been discussed in many reports, only limited researches were published on nanostructured ZrO2 – Al2O3 composite coatings. In this work, reconstituted nanosized premixed powder was used as starting powders for manufacturing nanostructured ZrO2 – 30 vol.% Al2O3 composite coatings by atmospheric plasma spraying. The powders and coatings were characterized by X-ray diffraction (XRD), Raman
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Fig. 1. (a) TEM image of the ZrO2 – 30 vol.% Al2O3 reconstituted granules. (b) Morphology of the spray-dried ZrO2 – 30 vol.% Al2O3 reconstituted granules.
spectroscopy, energy dispersive spectroscopy (EDS), field emission scanning electron microscope (FESEM), and electron probe X-ray microanalyser (EPMA). The objective of this work was to identify the phase composition and characterize the microstructure of the ZrO2 –30 vol.% Al2O3 composite coating. The average roughness and microhardness of the as-sprayed coating were also measured.
2. Experimental details 2.1. Preparation of coating Commercial nanosized powders of yttria stabilized zirconia (3 mol% Y2O3) with the mean diameter of 50 nm (Farmeiya Advanced Materials Co. Ltd., Jiujiang) and alumina powders with the mean diameter of 20 nm (High Technology Nano Co. Ltd., Nanjing) were used as the starting particles. The nanosized zirconia and alumina powders were wet-mixed and milled together for 20 h in a
ball mill, then subsequently reconstituted into granules with the mean diameter of 60 Am by spray drying process. The microstructure and morphology of the ZrO2 – 30 vol.% Al2O3 reconstituted granules are shown in Fig. 1. It can be seen that the spray-dried ZrO2 –30 vol.% Al2O3 granules are spherical or ellipsoidal, which improve powders feeding behavior during plasma spraying process. The nanosized alumina particles were distributed homogeneously within zirconia particles (Fig. 2). The coatings were deposited with the help of a Metco A2000 atmospheric plasma spraying system (Sulzer Metco AG, Switzerland).The powder was fed with Twin-System (Plasma-Technick AG, Switzerland). A mixture of argon and hydrogen was used as plasma gas. Compressed air was used as substrate cooling gas during plasma spraying. The plasma spray parameters are listed in Table 1. Stainless steel coupons with the dimension of 40 mm 20 mm 2 mm were used as substrates. Before spraying, the substrates were degreased ultrasonically in acetone and grit blasted with white corundum. 2.2. Characterization of coatings The phase composition of the as-sprayed coating was examined by XRD using nickel filtered CuKa (k = 1.54056 ˚ ) radiation on a Rigaku D/Max2550 diffractometer and a A LabRam-1B micro-Raman spectrometer (Dilor, France). The surface and cross-section morphologies of the coatings were studied using EPMA (EPMA-8705QH22, Table 1 Plasma spraying parameters for nanostructured ZrO2 – 30 vol.% Al2O3 coating Parameter Current Voltage Primary Secondary Carrier Spray Feeding (A) (V) gas (Ar, gas (H2, gas (Ar, distance distance slpm) slpm) slpm) (mm) (mm)
Fig. 2. The line scanning result of electron probe microanalysis of the reconstituted granule.
Value
620
68
40
12
4
100
6
B. Liang et al. / Thin Solid Films 484 (2005) 225 – 231
Fig. 3. XRD patterns of (a) the as-sprayed coating and (b) starting powders.
Shimadzu, Japan) with EDS detector (IneA Energy, Oxford Instruments, UK), and FESEM (JSM-6700F, JEOL, Japan). 2.3. The roughness and microhardness The surface roughness of the as-sprayed coating was determined using a TK300 HOMMEL WERKE roughness tester (Wave, Germany) with 0.5 mm/s traverse speed at 4.8 mm length. The results from five measurements were averaged to determine roughness data. The Vickers microhardness was measured by HX-1000 microhardness tester (Shanghai, China) at 0.2 kg loads for 15 s on polished crosssection area of the as-sprayed coatings, and the average microhardness resulted from 20 measurements.
3. Results and discussion 3.1. Phase composition analysis Fig. 3 shows the XRD patterns of the reconstituted powder and the as-sprayed coating. It shows that the reconstituted powders were composed by a mixture,
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Fig. 5. XRD pattern between 2h = 72- and 76- of the as-sprayed coating.
including the metastable tetragonal zirconia phase, cubic zirconia phase, monoclinic zirconia phase, gamma alumina phase and alpha alumina phase. The relative amounts of monoclinic and tetragonal zirconia phase in the reconstituted powder were calculated by the following formula [19]: m 111¯þ mð111Þ m ð% Þ ¼ m 111¯þ t ð111Þ þ mð111Þ where m(111), m(111¯) and t(111) represent the intensity of the corresponding diffraction peaks in the XRD patterns of the reconstituted powder. The percentage of monoclinic phase calculated was at least 35%. However, in the as-sprayed coating, only the broad zirconia diffraction peaks at 2h = 30.2-, 35.0-, 50.5-, 59.5and 74.2- were observed. The characteristic peaks of monoclinic zirconia phase at 2h = 28.2-, 31.5- disappeared after plasma spraying. This result is consistent with the results of Raman scattering measurements on the same sample (Fig. 4). Comparing Fig. 4a with Fig. 4b, it can be seen that only the very broad spectra lines of tetragonal phase located at 154.7 cm1, 260 cm1, 472.8 cm1 and 639.7 cm1 were observed. The typical characteristic Raman lines of monoclinic phase at 177.1 cm1, 188.1
Fig. 4. Raman spectrum of (a) the as-sprayed coating and (b) the reconstituted powders.
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cm1, 219.9 cm1 and 379.8 cm1 disappeared. Also, a new Raman line appeared at 688.6 cm1, which was not identified in the present work and needs to be further examined in the future. All the results obtained from XRD and Raman spectra showed clearly broadening phenomenon occurring in the as-sprayed coating. This indicated that the tetragonal structure was seriously distorted. This result was in agreement with many previous studies [20 –22]. Such tetragonal zirconia phase with distorted structure was defined as t¶-phase, resulting from the good melting of particles and high rate of cooling during plasma spraying process. The good melting of reconstituted particles resulted in the transformation of monoclinic to tetragonal or cubic phase, and subsequently, these high temperature stable phases were retained at room temperature under the high cooling conditions. Although it seemed that there were overlap diffraction peaks of cubic and tetragonal phase around 2h = 29.8-, 35.0- and 74.2- in XRD pattern of the as-sprayed coating, no characteristic Raman lines of cubic zirconia phase were observed in the as-sprayed coating. The further XRD study between 2h = 72- and 76- was conducted on the same sample (Fig. 5). The result showed that the (004) and (400) diffraction peaks of tetragonal zirconia phase were observed, corresponding to 2h = 73.1- and 74.2-, respectively. The characteristic cubic phase diffraction peak at 2h = 73.7- was not detected. Associated with the Raman spectra analysis, it can be concluded that there was no cubic zirconia phase in the as-sprayed coating. As the cubic phase in 3 mol% yttria partially stabilized zirconia is not stable, therefore it should not appear [23]. Alumina phase exhibited a similar phenomenon as that of the monoclinic zirconia phase. In the Raman spectra of starting powder, the Raman line occurring at 978.4 cm1 was clearly observed, which was characteristic of alumina, and another in the vicinity of 400 cm1 appeared to be masked by the strong broad lines of zirconia phase with wave number ranging from 379.8 cm1 to 473.0 cm1.
However, none of the two characteristic Raman lines of alumina was observed in the as-sprayed coating. This was consistent with result of the XRD analysis. This phenomenon could be partially explained in terms of formation of the metastable Al2O3 – ZrO2 solid solutions with tetragonal structure. The solubility of Al2O3 in ZrO2 could be up to 2 mol% at temperatures 1400 -C, and slightly increases with the increase of temperature [24 – 26]. In the present work, nanosized Al2O3 or ZrO2 particles experienced a very high temperature in plasma jet ranging from 3000 -C to 10,000 -C [27]. Such a high temperature combined with the high surface energy of nanosized composite particles resulted in the perfect melting and the formation of solid solution. It was verified by the EDS results, as shown in Fig. 6. The results of points A and B revealed that there was a certain amount of Al element in the ZrO2 splat and some Zr elements in the Al2O3 splat. The different splat colors resulted from the different Al2O3 amounts. The dark grey color indicated the high alumina in the as-sprayed coating. In addition, it was also observed that the dark grey aluminarich splats were non-uniformly embedded in light grey zirconia-rich splats. The boundaries between the zirconiarich and alumina-rich splats were clearly discernable and relatively free of voids. 3.2. Microstructural analysis Fig. 7a shows the representative splat morphology of the ZrO2 – 30 vol.% Al2O3 composite particle on the glass substrate processed at room temperature (T = 293 K). The splat morphology was a nearly regular disk-like shape with limited splashing, indicating a rather good particle flattening [28 – 32] and a strong bonding [33]. These results conformed to the SEM analyses (Fig. 7b and c). Fig. 7b revealed that the as-sprayed coating possesses a rather smooth surface. In fact, the measured average roughness (Ra) of the as-sprayed coating was about 6.03 Am. Fig. 7c shows that excellent bonding exists between the substrate
Fig. 6. EDS analyses of zirconia-rich splats (point A) and alumina-rich splats (point B).
B. Liang et al. / Thin Solid Films 484 (2005) 225 – 231
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observed in the cross-section area. One kind was smaller pores of less than 5 Am; the other was some large pores of more than 30 Am. The thickness of the as-sprayed coating was approximately 450 Am. Fig. 8a– c presents the high resolution FESEM morphologies of the surface and fracture surface of the as-sprayed coating. It shows that the grains visible on the top surface
Fig. 7. SEM morphology of (a) single splat, (b) surface of the coating and (c) cross-section area of the coating.
and the as-sprayed coating. The bonding strength was about 40 MPa, tested on American standard of testing materials C 633-79 [34]. The porosity of the as-sprayed coating was approximately 6%, measured using SEM image of the cross-section. Two kinds of pores were
Fig. 8. FESEM micrographs of (a) as-sprayed coating surface morphology, (b) fracture surface morphology of lamellar structure, and (c) fracture surface of bimodal structure.
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are less than 100 nm (Fig. 8a), and the as-sprayed coating exhibited the typical lamellar structure with the average splat thickness of about 1 Am, comprised of columnar grains with the average diameter size less than 150 nm (Fig. 8b). It was noted that there was a bimodal structure in the assprayed coating (Fig. 8c). One was the above lamellar structure, which was invariably aligned parallel to the substrate and previously deposited material, similar to the conventional coatings deposited by plasma spraying [35 – 37]; the other was the equiaxed grains with the grain size less than 100 nm. The former was the main structure in the as-sprayed coating. The equiaxed grains embedded in the as-sprayed coating resulted from the high cooling velocity and very short dwelling time of nanosized particles in plasma spraying process. Under the above conditions, the partially melted or unmelted nanosized composite powder could not grow up and their nanostructure was preserved in the as-sprayed coating. However, the study about the nanostructure forming of the as-sprayed coating is still in progress. 3.3. Microhardness The average microhardness tested in the present work was about 8.4 GPa, which was higher than the average value (å 5.7 GPa) of the nanostructured zirconia coating deposited by plasma spraying using the same nanostructured zirconia powder. Corresponding with the improvement of the microhardness, the average surface roughness value also increased from 3.74 Am to 6.03 Am. This result indicated that the microhardness of the coating could be changed by addition of alumina. This may result in the improvement of tribology properties of as-sprayed coating.
4. Conclusions The phase composition of as-sprayed ZrO2 – 30 vol.% Al2O3 composite coatings deposited by atmospheric plasma spraying using premixed nanosized Al2O3 and ZrO2 powders mainly consisted of nontransformable tetragonal phase of ZrO2; no monoclinic and cubic zirconia phases of ZrO2 were observed. Alumina phase was also not observed in XRD pattern or Raman spectra of the as-sprayed coating. The phenomenon of Al2O3 – ZrO2 solid solution was observed during the plasma spraying process. The as-sprayed composite coating exhibited typical bimodal structures: the lamellar structure comprised of nanosized columnar grains and equiaxed nanosized grains. It also showed lower porosity (å 6%), higher bonding strength and microhardness; particularly, the microhardness of the as-sprayed composite coatings was 8.4 GPa, which was about 1.5 times that of the nanostructured zirconia coating deposited using same nanosized zirconia powder.
Acknowledgement This work was supported by Chine –France PRA dans ledomaine des mat e´ riaux 2002 under grant PRA MX0203(C.Coddet and C.Ding). The authors gratefully acknowledge Mrs. Gao Jianhua, Mrs. Qian Weijun for FESEM and EDS analysis, and Mrs. Zhou Xiaming for preparing the specimens.
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Nanostructured zirconia–30 vol.% alumina composite coatings deposited by atmospheric plasma spraying Bo Lianga,*, Hanlin Liaob, Chuanxian Dinga, Christian Coddetb b
a Shanghai Institute of Ceramics, Chinese Academic of Sciences, Shanghai 200050, PR China Laboratoire d’Etudes et de Recherches sur les Mae´riaux, les Proce´de´s et les Surfaces, Universite´ de Technologie de Belfort-Montbe´liard, 90010 Belfort Cedex, France
Received 30 June 2004; accepted in revised form 25 February 2005 Available online 12 May 2005
Abstract Nanostructured zirconia – 30 vol.% alumina composite coatings were deposited by atmospheric plasma spraying using nanosized ZrO2 and Al2O3 powders. The microstructure and phase composition of the coatings were characterized by electron probe X-ray microanalyser (EPMA), field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Raman spectroscopy. Microhardness (HV0.2 kg) and roughness (Ra) were also measured. The as-sprayed coating exhibited a bimodal structure: the typical lamellar structure comprised of nanosized columnar grains and the nanosized equiaxed grains. Alumina-rich splats were dark grey colored and non-uniformly embedded in light grey zirconia-rich splats. No monoclinic or cubic zirconia phase and alumina phase were detected in the as-sprayed coating. Compared to pure zirconia coating, the microhardness of the as-sprayed coating increased from 5.7 GPa to 8.4 GPa, and the roughness increased from 3.74 Am to 6.03 Am with the addition of alumina into feedstock. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructured; Zirconia – 30 vol.% alumina composite; Atmospheric plasma spraying
1. Introduction ZrO2 – Al2O3 composite coatings deposited by plasma spraying or electron beam physical vapor deposition (EBPVD) have been actively studied in order to improve properties of zirconia thermal barrier coatings. It was reported that deposits of ZrO2 and Al2O3 alternating layers manufactured using plasma spraying or EB-PVD exhibited the reduction of oxygen diffusion through the deposits. Plasma sprayed laminated composites showed an increase in thermal resistance, compared with EB-PVD and single bulk coatings [1– 3]. Mixing alumina into the zirconia layers was effective in reducing the residual stresses and the formation of cracks [4]. The plasma spraying of these premixed powders also improved mechanical and chemical properties without compromising the thermal behavior of
* Corresponding author. Tel.: +86 21 52414103; fax: +86 21 52413903. E-mail address: [email protected] (B. Liang). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.02.040
the coatings [5]. In addition, the combination of high hardness alumina with the low thermal conductivity zirconia contributed to the development of the thermal shock resistance, microhardness and wear resistance of the as-sprayed coatings [6 –9]. Nanostructured coatings are attractive because of their potential superior mechanical and physical properties [10 – 14]. The plasma spraying of nanostructured powders is one of the effective means for obtaining nanostructured coatings because of its cost effectiveness [15 – 18]. Although the microstructure and other properties of the nanostructured as-sprayed zirconia coatings or alumina coatings have been discussed in many reports, only limited researches were published on nanostructured ZrO2 – Al2O3 composite coatings. In this work, reconstituted nanosized premixed powder was used as starting powders for manufacturing nanostructured ZrO2 – 30 vol.% Al2O3 composite coatings by atmospheric plasma spraying. The powders and coatings were characterized by X-ray diffraction (XRD), Raman
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Fig. 1. (a) TEM image of the ZrO2 – 30 vol.% Al2O3 reconstituted granules. (b) Morphology of the spray-dried ZrO2 – 30 vol.% Al2O3 reconstituted granules.
spectroscopy, energy dispersive spectroscopy (EDS), field emission scanning electron microscope (FESEM), and electron probe X-ray microanalyser (EPMA). The objective of this work was to identify the phase composition and characterize the microstructure of the ZrO2 –30 vol.% Al2O3 composite coating. The average roughness and microhardness of the as-sprayed coating were also measured.
2. Experimental details 2.1. Preparation of coating Commercial nanosized powders of yttria stabilized zirconia (3 mol% Y2O3) with the mean diameter of 50 nm (Farmeiya Advanced Materials Co. Ltd., Jiujiang) and alumina powders with the mean diameter of 20 nm (High Technology Nano Co. Ltd., Nanjing) were used as the starting particles. The nanosized zirconia and alumina powders were wet-mixed and milled together for 20 h in a
ball mill, then subsequently reconstituted into granules with the mean diameter of 60 Am by spray drying process. The microstructure and morphology of the ZrO2 – 30 vol.% Al2O3 reconstituted granules are shown in Fig. 1. It can be seen that the spray-dried ZrO2 –30 vol.% Al2O3 granules are spherical or ellipsoidal, which improve powders feeding behavior during plasma spraying process. The nanosized alumina particles were distributed homogeneously within zirconia particles (Fig. 2). The coatings were deposited with the help of a Metco A2000 atmospheric plasma spraying system (Sulzer Metco AG, Switzerland).The powder was fed with Twin-System (Plasma-Technick AG, Switzerland). A mixture of argon and hydrogen was used as plasma gas. Compressed air was used as substrate cooling gas during plasma spraying. The plasma spray parameters are listed in Table 1. Stainless steel coupons with the dimension of 40 mm 20 mm 2 mm were used as substrates. Before spraying, the substrates were degreased ultrasonically in acetone and grit blasted with white corundum. 2.2. Characterization of coatings The phase composition of the as-sprayed coating was examined by XRD using nickel filtered CuKa (k = 1.54056 ˚ ) radiation on a Rigaku D/Max2550 diffractometer and a A LabRam-1B micro-Raman spectrometer (Dilor, France). The surface and cross-section morphologies of the coatings were studied using EPMA (EPMA-8705QH22, Table 1 Plasma spraying parameters for nanostructured ZrO2 – 30 vol.% Al2O3 coating Parameter Current Voltage Primary Secondary Carrier Spray Feeding (A) (V) gas (Ar, gas (H2, gas (Ar, distance distance slpm) slpm) slpm) (mm) (mm)
Fig. 2. The line scanning result of electron probe microanalysis of the reconstituted granule.
Value
620
68
40
12
4
100
6
B. Liang et al. / Thin Solid Films 484 (2005) 225 – 231
Fig. 3. XRD patterns of (a) the as-sprayed coating and (b) starting powders.
Shimadzu, Japan) with EDS detector (IneA Energy, Oxford Instruments, UK), and FESEM (JSM-6700F, JEOL, Japan). 2.3. The roughness and microhardness The surface roughness of the as-sprayed coating was determined using a TK300 HOMMEL WERKE roughness tester (Wave, Germany) with 0.5 mm/s traverse speed at 4.8 mm length. The results from five measurements were averaged to determine roughness data. The Vickers microhardness was measured by HX-1000 microhardness tester (Shanghai, China) at 0.2 kg loads for 15 s on polished crosssection area of the as-sprayed coatings, and the average microhardness resulted from 20 measurements.
3. Results and discussion 3.1. Phase composition analysis Fig. 3 shows the XRD patterns of the reconstituted powder and the as-sprayed coating. It shows that the reconstituted powders were composed by a mixture,
227
Fig. 5. XRD pattern between 2h = 72- and 76- of the as-sprayed coating.
including the metastable tetragonal zirconia phase, cubic zirconia phase, monoclinic zirconia phase, gamma alumina phase and alpha alumina phase. The relative amounts of monoclinic and tetragonal zirconia phase in the reconstituted powder were calculated by the following formula [19]: m 111¯þ mð111Þ m ð% Þ ¼ m 111¯þ t ð111Þ þ mð111Þ where m(111), m(111¯) and t(111) represent the intensity of the corresponding diffraction peaks in the XRD patterns of the reconstituted powder. The percentage of monoclinic phase calculated was at least 35%. However, in the as-sprayed coating, only the broad zirconia diffraction peaks at 2h = 30.2-, 35.0-, 50.5-, 59.5and 74.2- were observed. The characteristic peaks of monoclinic zirconia phase at 2h = 28.2-, 31.5- disappeared after plasma spraying. This result is consistent with the results of Raman scattering measurements on the same sample (Fig. 4). Comparing Fig. 4a with Fig. 4b, it can be seen that only the very broad spectra lines of tetragonal phase located at 154.7 cm1, 260 cm1, 472.8 cm1 and 639.7 cm1 were observed. The typical characteristic Raman lines of monoclinic phase at 177.1 cm1, 188.1
Fig. 4. Raman spectrum of (a) the as-sprayed coating and (b) the reconstituted powders.
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cm1, 219.9 cm1 and 379.8 cm1 disappeared. Also, a new Raman line appeared at 688.6 cm1, which was not identified in the present work and needs to be further examined in the future. All the results obtained from XRD and Raman spectra showed clearly broadening phenomenon occurring in the as-sprayed coating. This indicated that the tetragonal structure was seriously distorted. This result was in agreement with many previous studies [20 –22]. Such tetragonal zirconia phase with distorted structure was defined as t¶-phase, resulting from the good melting of particles and high rate of cooling during plasma spraying process. The good melting of reconstituted particles resulted in the transformation of monoclinic to tetragonal or cubic phase, and subsequently, these high temperature stable phases were retained at room temperature under the high cooling conditions. Although it seemed that there were overlap diffraction peaks of cubic and tetragonal phase around 2h = 29.8-, 35.0- and 74.2- in XRD pattern of the as-sprayed coating, no characteristic Raman lines of cubic zirconia phase were observed in the as-sprayed coating. The further XRD study between 2h = 72- and 76- was conducted on the same sample (Fig. 5). The result showed that the (004) and (400) diffraction peaks of tetragonal zirconia phase were observed, corresponding to 2h = 73.1- and 74.2-, respectively. The characteristic cubic phase diffraction peak at 2h = 73.7- was not detected. Associated with the Raman spectra analysis, it can be concluded that there was no cubic zirconia phase in the as-sprayed coating. As the cubic phase in 3 mol% yttria partially stabilized zirconia is not stable, therefore it should not appear [23]. Alumina phase exhibited a similar phenomenon as that of the monoclinic zirconia phase. In the Raman spectra of starting powder, the Raman line occurring at 978.4 cm1 was clearly observed, which was characteristic of alumina, and another in the vicinity of 400 cm1 appeared to be masked by the strong broad lines of zirconia phase with wave number ranging from 379.8 cm1 to 473.0 cm1.
However, none of the two characteristic Raman lines of alumina was observed in the as-sprayed coating. This was consistent with result of the XRD analysis. This phenomenon could be partially explained in terms of formation of the metastable Al2O3 – ZrO2 solid solutions with tetragonal structure. The solubility of Al2O3 in ZrO2 could be up to 2 mol% at temperatures 1400 -C, and slightly increases with the increase of temperature [24 – 26]. In the present work, nanosized Al2O3 or ZrO2 particles experienced a very high temperature in plasma jet ranging from 3000 -C to 10,000 -C [27]. Such a high temperature combined with the high surface energy of nanosized composite particles resulted in the perfect melting and the formation of solid solution. It was verified by the EDS results, as shown in Fig. 6. The results of points A and B revealed that there was a certain amount of Al element in the ZrO2 splat and some Zr elements in the Al2O3 splat. The different splat colors resulted from the different Al2O3 amounts. The dark grey color indicated the high alumina in the as-sprayed coating. In addition, it was also observed that the dark grey aluminarich splats were non-uniformly embedded in light grey zirconia-rich splats. The boundaries between the zirconiarich and alumina-rich splats were clearly discernable and relatively free of voids. 3.2. Microstructural analysis Fig. 7a shows the representative splat morphology of the ZrO2 – 30 vol.% Al2O3 composite particle on the glass substrate processed at room temperature (T = 293 K). The splat morphology was a nearly regular disk-like shape with limited splashing, indicating a rather good particle flattening [28 – 32] and a strong bonding [33]. These results conformed to the SEM analyses (Fig. 7b and c). Fig. 7b revealed that the as-sprayed coating possesses a rather smooth surface. In fact, the measured average roughness (Ra) of the as-sprayed coating was about 6.03 Am. Fig. 7c shows that excellent bonding exists between the substrate
Fig. 6. EDS analyses of zirconia-rich splats (point A) and alumina-rich splats (point B).
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observed in the cross-section area. One kind was smaller pores of less than 5 Am; the other was some large pores of more than 30 Am. The thickness of the as-sprayed coating was approximately 450 Am. Fig. 8a– c presents the high resolution FESEM morphologies of the surface and fracture surface of the as-sprayed coating. It shows that the grains visible on the top surface
Fig. 7. SEM morphology of (a) single splat, (b) surface of the coating and (c) cross-section area of the coating.
and the as-sprayed coating. The bonding strength was about 40 MPa, tested on American standard of testing materials C 633-79 [34]. The porosity of the as-sprayed coating was approximately 6%, measured using SEM image of the cross-section. Two kinds of pores were
Fig. 8. FESEM micrographs of (a) as-sprayed coating surface morphology, (b) fracture surface morphology of lamellar structure, and (c) fracture surface of bimodal structure.
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are less than 100 nm (Fig. 8a), and the as-sprayed coating exhibited the typical lamellar structure with the average splat thickness of about 1 Am, comprised of columnar grains with the average diameter size less than 150 nm (Fig. 8b). It was noted that there was a bimodal structure in the assprayed coating (Fig. 8c). One was the above lamellar structure, which was invariably aligned parallel to the substrate and previously deposited material, similar to the conventional coatings deposited by plasma spraying [35 – 37]; the other was the equiaxed grains with the grain size less than 100 nm. The former was the main structure in the as-sprayed coating. The equiaxed grains embedded in the as-sprayed coating resulted from the high cooling velocity and very short dwelling time of nanosized particles in plasma spraying process. Under the above conditions, the partially melted or unmelted nanosized composite powder could not grow up and their nanostructure was preserved in the as-sprayed coating. However, the study about the nanostructure forming of the as-sprayed coating is still in progress. 3.3. Microhardness The average microhardness tested in the present work was about 8.4 GPa, which was higher than the average value (å 5.7 GPa) of the nanostructured zirconia coating deposited by plasma spraying using the same nanostructured zirconia powder. Corresponding with the improvement of the microhardness, the average surface roughness value also increased from 3.74 Am to 6.03 Am. This result indicated that the microhardness of the coating could be changed by addition of alumina. This may result in the improvement of tribology properties of as-sprayed coating.
4. Conclusions The phase composition of as-sprayed ZrO2 – 30 vol.% Al2O3 composite coatings deposited by atmospheric plasma spraying using premixed nanosized Al2O3 and ZrO2 powders mainly consisted of nontransformable tetragonal phase of ZrO2; no monoclinic and cubic zirconia phases of ZrO2 were observed. Alumina phase was also not observed in XRD pattern or Raman spectra of the as-sprayed coating. The phenomenon of Al2O3 – ZrO2 solid solution was observed during the plasma spraying process. The as-sprayed composite coating exhibited typical bimodal structures: the lamellar structure comprised of nanosized columnar grains and equiaxed nanosized grains. It also showed lower porosity (å 6%), higher bonding strength and microhardness; particularly, the microhardness of the as-sprayed composite coatings was 8.4 GPa, which was about 1.5 times that of the nanostructured zirconia coating deposited using same nanosized zirconia powder.
Acknowledgement This work was supported by Chine –France PRA dans ledomaine des mat e´ riaux 2002 under grant PRA MX0203(C.Coddet and C.Ding). The authors gratefully acknowledge Mrs. Gao Jianhua, Mrs. Qian Weijun for FESEM and EDS analysis, and Mrs. Zhou Xiaming for preparing the specimens.
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