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Nanocomposite powder with three-dimensional network structure for preparing alumina–titania nanocomposite coating with advanced performance
Journal of Alloys and Compounds 622 (2015) 929–934
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Nanocomposite powder with three-dimensional network structure for preparing alumina–titania nanocomposite coating with advanced performance Yong Yang a,b,c,⇑, You Wang b,⇑, Wei Tian b, Dian-ran Yan a, Jian-xin Zhang a, Liang Wang c a b c
Key Lab. for New Type of Functional Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, China Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China
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Article history: Received 19 September 2014 Received in revised form 30 October 2014 Accepted 3 November 2014 Available online 11 November 2014 Keywords: Coating materials Composite materials Nanostructured materials Microstructure Nanocomposites Plasma spraying
a b s t r a c t Nanocomposite powder with three-dimensional network structure was prepared by adding nano-dopants (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing. Alumina–titania nanocomposite coating was fabricated through plasma spraying the nanocomposite powder. The formation mechanism of the nanocomposite powder was investigated, and the strengthening and toughening mechanisms of the nanocomposite coating was revealed. The internal microstructure of the powder particles experienced liquid phase sintering during plasma jet processing. The nanocomposite powder with three-dimensional network structure had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies. Microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were improved compared with that of Metco 130 coating. The strengthening mechanism was mainly microstructure refinement strengthening. The toughening mechanisms included crack deflection and crack trapping. The threedimensional network structure may work as reinforcing and toughening units in the nanocomposite coating. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Alumina ceramic matrix composite coatings are increasingly employed in applications where elevated hardness, good tribological properties, excellent corrosion resistance and high thermal resistance are required [1–4]. Al2O3–TiO2 coating is one of the most widely used alumina ceramic matrix composite coating for their excellent wear, corrosion, and erosion resistance [5,6]. With the development of nanotechnology and nanomaterials, enhanced mechanical properties in ceramic coatings with nanostructured microstructures have been reported, drawing extensive attention to the preparation, characterization and application of nanostructured ceramic coatings [7–14]. A number of techniques have been attempted to produce nanostructured ceramic coatings, such as physical vapor deposition, chemical vapor deposition, ion implantation, magnetron sputtering, electrodeposition, laser cladding, thermal spraying and cold spraying [15–18]. Among the possible processes, atmospheric ⇑ Corresponding authors at: No. 29 Guangrong Road, Hongqiao District, Tianjin 300132, China. Tel.: +86 22 60204810; fax: +86 22 26564810. E-mail address: [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.jallcom.2014.11.021 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
plasma spraying (APS) has received much attention in recent years [15,18]. Plasma spraying is a simple processing technique which can be applied on different substrates, i.e. metal or ceramic. Coatings with thickness from tens of micrometers to hundreds of micrometers even to one millimeter can be prepared with high deposition rate. Especially, it is easy to form a composite coating [19]. It provides coatings with better quality and good adherence to the substrate. Using nanoscale powder as feedstock to generate nanostructured coating by plasma spraying presents new challenges as well as new opportunities [20,21]. However, the small size of the nanoscale particles impedes the direct projection in conventional plasma spraying equipment. Individual nanoscale particles cannot be successfully deposited because of their low mass and the resultant inability to be carried in a moving gas stream and deposited on a substrate. And fine powder tends to stick to the sidewall of feed lines and spray nozzle tip, causing reduction of powder passage area. Fine powder with strong surface interaction also tends to agglomerate, leading to limited flowability. Therefore, when powder is too fine, it is difficult to maintain a constant powder flow rate during thermal spraying.
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To circumvent the obstacle, reconstitution of individual nanoscale particles into spherical micrometer-sized granules is necessary. These granules (agglomerates) with average sizes similar to those of the conventional sprayable feedstock (approximately 10–80 lm) can be deposited using standard plasma spraying equipment [21]. At present, the most widely applied granulation reconstitution technology for preparing nanostructured powders for thermal spraying is spray drying [20,22], which has the advantages of low cost, wide applicability, easy control of composition and particle size. The results [20–23] showed that the nanostructured feedstock prepared by spray drying was spherical particles with smooth surface and good fluidity. But the internal structure of the particles was loose and contained many pores, which reduced the melting degree of particles during plasma spraying process and resulted in the increased porosity of the as-prepared coating. Many investigation results [5,7–13,23] showed that alumina–titania nanocomposite coating prepared by plasma spraying had a bimodal microstructure (fully-melted region and partially melted region), and most of pores were present in the partially melted region. Pores and cracks in the coating were the most important defects that caused the failure of the coating. Summing up the above analysis, the key problem to be solved for preparing nanocomposite coatings by plasma spraying nanostructured powders is to fabricating nanostructured sprayable feedstocks with high quality, high performance and/or unique structure. It is well-known that the microstructure of material plays a decisive role in its properties. Through the above analysis, the authors think that the microstructure of the nanostructured feedstock will have an important impact on the microstructure and properties of the coating. However, to our knowledge, there are few reports on the effect of microstructure of the nanostructured feedstock on the microstructure and properties of the plasma sprayed coating. Aiming at tailoring the microstructure of nanocomposite feedstock to fabricate nanostructured feedstock with high quality and unique structure, in the present investigation, by adding nano additives (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing, nanocomposite powder with three-dimensional network structure was prepared. Subsequently, advanced performance alumina–titania nanocomposite coating was successfully fabricated through plasma spraying nanocomposite powder with three-dimensional network structure. The formation mechanism of the nanocomposite powder with threedimensional network structure was investigated, and its strengthening and toughening mechanism on the coating was revealed.
nanostructured composite powders which are microsized particles with nanosized grains. After spray drying, NN powder and NP powder were heat treated for eliminating the PVA and densifying the microstructure of the powders. At last, NP powder was plasma jet processed. During plasma jet processing, the powder was injected into the plasma jet and air quenched in a cold chamber, and then was collected. The GDP-2 type 50 kW plasma spraying system (Jiu Jiang Spraying Device Company, China) was employed for plasma jet processing powders. The carbon steel (0.14–0.22 wt.%C) coupons were used as substrate samples. These samples were grit blasted prior to coating deposition. A bond coating of NiCrAl with thickness about 50–100 lm was deposited onto the substrates. The three kinds of composite powders were then plasma sprayed onto the bond coatings for about 300 lm in thickness, respectively. The phase constitution of the powders and coatings was characterized by X-ray diffraction (XRD, D/max-cB, Japan) with Cu Ka radiation. The microstructure of the powders and coatings was characterized by scanning electron microscope (SEM, HITACHI-S4800) and transmission electron microscopy (Philips CM-12, TEM). In order to observe the cross-sectional structure of the composite powders, the asprepared composite powders were dispersed homogeneously in epoxy resin to form a bulk sample. Then, the cross-section of the epoxy resin bulk sample containing composite powders were ground and polished for cross-sectional SEM analysis of the composite powders. Coatings’ microhardness was determined under a normal load of 300 g on the cross section of the coatings. The bonding strength of the coatings was tested according to ASTM C633-79 standard. The crack propagation characteristics were determined by indenting the polished coating cross section and top surface with a Vickers indenter under a 3 kg load for 10 s. The length of cracks that emerged from the indent corners was measured. The crack growth resistance of coatings was characterized by the reciprocal of average crack length [25].
3. Results and discussion 3.1. Microstructure of the composite powders and its formation mechanism The phase constitutions of the composite powders were shown in Fig. 1. ME powder consisted of a-Al2O3 and anatase, and NN powder consisted of a-Al2O3 and rutile. NP powder consisted of a-Al2O3, rutile, t-ZrO2, cubic CeO2 and d-Al2O3, in addition, there were some solid solution of oxide additives (ZrO2 and CeO2) containing TiO2. The SEM micrographs of the composite powders were shown in Fig. 2. ME powder were microsized irregular particles with size range from 20 to 60 lm in diameter (Fig. 2a). This powder was manufactured through a so-called cladding process, in which fused-and-crushed Al2O3 powder (Fig. 2c) was clad with submicron TiO2 (Fig. 2b). After cladding process, the surface of the blocky Al2O3 powder was covered with fine TiO2 particles (Fig. 2b). NN powder possessed spherical geometry with size range from 10 to 50 lm in diameter, and there were pores in it (Fig. 2d). Each spherical particle consisted of a great number of nanosized alumina and titania grains (about 50–100 nm) (Fig. 2e), which was therefore
2. Material and methods In the present research, three kinds of alumina–titania composite powders were used to prepare alumina–titania composite coatings. It was labeled as ME powder, NN powder and NP powder. The chemical composition of ME and NN powders was 87 wt.% Al2O3 and 13 wt.% TiO2. The mass ratio of Al2O3 and TiO2 in NP powder was 87:13, and a small amount of nanosized ZrO2 and CeO2 were used as dopants. The preparation methods for the three kinds of composite powders were different. ME powder was commercial Metco 130 powder (Sulzer Metco Co., Ltd., USA). This powder was manufactured through a so-called cladding process, in which fusedand-crushed Al2O3 powder (20–60 lm diameter) was clad with submicron TiO2 [24]. After cladding process, the surface of the blocky Al2O3 powder was covered with fine TiO2 particles. NN powder was nanocomposite powder which was fabricated by spray drying and heat treating [21]. NP powder was also nanocomposite powder which was obtained by spray drying, heat treating and plasma jet processing [7,9]. For the preparing process of nanocomposite powder, as-received powders were Al2O3 (d and c phases, 99.9% grade, Degussa Co., Ltd., Germany) with grain size of 20–45 nm and TiO2 (anatase, 99.9% grade, Nanjing High Technology of Nano Material Co., Ltd., China) with grain size of 20–50 nm. These powders were blended uniformly to produce a powder mixture with composition of 87 wt.%Al2O3 and 13 wt.%TiO2 by wet ball-milling. In addition, ZrO2 with grain size of 20–50 nm and CeO2 with grain size of 20–40 nm were added during mixing for a modified powder mixture. The mixed powder slurries were then spray dried to form
Fig. 1. XRD patterns of the composite powders: (a) ME, (b) NN, and (c) NP.
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Fig. 2. SEM micrographs of the composite powders: (a) microstructured powder (ME), (b) high magnification of the surface of ME powder, (c) cross-section of ME powder, (d) cross-section of nanostructured NN powder, (e) high magnification of the cross-section of NN powder, (f) cross-section of plasma jet processed nanostructured NP powder, and (g) high magnification of the cross-section of NP powder.
called nanocomposite powder with particulate structure. Fig. 2f showed the typical microstructure of NP powder, and Fig. 2g was the high magnification cross-sectional SEM micrograph of the internal microstructure of NP powder. NP powder had better sphericity and much denser microstructure comparing with NN powder. There were a lot of equiaxed grains (with size of 50–500 nm) in the NP powder. And the equiaxed grains were surrounded by intergranular thin film (with thickness less than 100 nm), which extended inside the powder particle to form a network structure. So, the microstructure of NP powder was called three-dimensional network structure. Fig. 3 showed the TEM micrograph, and corresponding selected area diffraction (SAD) patterns and energy dispersive spectrometer (EDS) pattern of the three-dimensional network microstructure in NP powders. There were equiaxed grains with different grain size from about tens of nanometers to hundreds of nanometers in the three-dimensional network microstructure (Fig. 3a). Many small
equiaxed grains about tens of nanometers located inside or among the bigger equiaxed grains, forming intragranular and intergranular nanostructures. The SAD pattern (Fig. 3b) indicated that the equiaxed grains were mainly a-Al2O3. There was intergranular substance around the equiaxed a-Al2O3 grains. The SAD pattern (Fig. 3c) for the intergranular substance indicated that it was amorphous structure. The EDS pattern (Fig. 3d) showed that the amorphous intergranular substance was rich in Ti, Zr and Ce. Combined with the XRD, SEM and TEM results, it can be inferred that NP powder was nanocomposite powder with three-dimensional network structure. The equiaxed grains in NP powder were aAl2O3, and the intergranular network thin film was amorphous rich in Ti, Zr and Ce. That means NP powder had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies. Comparing with NN powder, the microstructure of NP powder changed a lot. The spray-dried and heat-treated NN powder had
Fig. 3. TEM micrograph, and corresponding SAD patterns and EDS pattern of the three-dimensional network microstructure in NP powder: (a) TEM bright-field micrograph of the three-dimensional network microstructure, (b) SAD pattern of ‘‘I’’ region in (a), (c) SAD pattern of ‘‘II’’ region in (a), and (d) EDS pattern of ‘‘II’’ region in (a).
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3.2. Phase constitution and microstructure of the composite coatings
Fig. 4. XRD patterns of the three coatings: (a) Metco 130 coating, (b) NN coating prepared from nanocomposite powder with particulate structure, and (c) NP coating prepared from nanocomposite powder with three-dimensional network structure.
porous and particulate microstructure, while NP powder had dense and three-dimensional network microstructure. The formation of the three-dimensional network structure was attributed to the addition of ZrO2 and CeO2 nano-dopants and the employment of plasma jet processing. The melting point of Al2O3, TiO2, ZrO2 and CeO2 is 2050 ± 10 °C, 1840 ± 10 °C, 2600 ± 100 °C and 2725 ± 20 °C, respectively [26]. TiO2, CeO2 and ZrO2 could form TiO2–CeO2–ZrO2 system liquid phase under a lower temperature (<1350 °C) below the melting point temperature of TiO2 [27]. During plasma jet processing of the composite powder, although the temperature of the plasma jet was high enough, the flight speed of the powder particles was very fast (about 150–300 m/s), and the residence time of the powder particles in the high temperature plasma jet was very short (10 3–10 4 s) [21]. Additionally, the oxide ceramics have low thermal conductivity [26]. Therefore, only the surface of the powder particles melted. The powder particles underwent rapid melting and solidifying process. The internal microstructure of most powder particles experienced relative low temperature, and did not melt completely. Only TiO2–CeO2–ZrO2 system liquid phase with low melting point was formed in the powder particles. The unmelted Al2O3 grains were surrounded by the liquid phase, which led to a similar effect as the liquid phase sintering of oxide ceramics [27]. Here, the internal microstructure of the NP powder particles just experienced very rapidly liquid phase sintering process. Threedimensional network microstructure was, therefore, formed in NP powder after plasma jet processing, which is similar to the microstructure of the liquid phase sintered oxides.
The XRD patterns of the three coatings (Fig. 4) revealed a similar phase constitution for the three coatings. There were two types of alumina in the three coatings: c-Al2O3 (most) and a-Al2O3 (minor). The melted alumina transformed to c-Al2O3 during plasma spraying deposition process due to the high cooling rate [11]. There were amorphous phase present in the three coatings indicated by the broad hump in the 2h range of 25° to 40°. The diffraction peaks of titania were not well identified in the three coatings, which may be attributed to the dissolving of titania into c-Al2O3 [9,10] and the formation of amorphous structure of titania. And the addition of nanosized ZrO2 and CeO2 in NP powder promoted the formation of amorphous in the NP coating. In addition, there were some solid solution of oxide additives (ZrO2 and CeO2) containing TiO2. Fig. 5 showed the cross-sectional SEM micrographs of the three coatings. There was typical lamellar structure in Metco 130 coating (Fig. 5a). The lamellar structure was derived from the fused and spread feedstock powders [5]. Comparing with Metco 130 coating, there was not typical lamellar structure in the coatings prepared from nanocomposite powders, which was mainly attributed to the homogeneous distribution of chemical composition in the nanocomposite feedstock powders [10,28]. There were two types of microstructures formed in the two coatings prepared from nanocomposite powders (Fig. 5b and c). ‘‘FM’’ regions with fusedand-solidified structure in NN and NP coatings were formed by the fully melted feedstock powders during plasma spraying. ‘‘PM’’ regions with particulate structure in NN coating was formed by the partially melted or unmelted NN nanocomposite powder with particulate structure, and ‘‘PM’’ regions with three-dimensional network structure in NP coating were formed by the partially melted or unmelted NP nanocomposite powder with three-dimensional network structure. The three-dimensional network structure in the NP coating was derived from the plasma jet processed feedstock powder. Fig. 6 showed the TEM micrographs of the three coatings. The grain size of Metco 130 coating was in the range of sub-micrometer to micrometer (Fig. 6a). The SAD patterns (not shown here) indicated that the sub-micrometer grains and micrometer grains were c-Al2O3. For NN coating, the grain size was in the range of a hundred nanometers to sub-micrometer (Fig. 6b), and the SAD patterns indicated that these grains were c-Al2O3. Fig. 6c showed the TEM micrograph of the fully-melted microstructure in NP coating, which was mainly composed of nano grains embedded in amorphous substrate. There were lots of equiaxed grains smaller than 100 nm in the fully-melted microstructure. The SAD pattern inserted indicated c-Al2O3 nano grains were distributed in amorphous substrate. The microstructure of the partially melted or unmelted region in NP coating retained the three-dimensional network structure in the NP powder, which was composed of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies.
Fig. 5. Cross-sectional SEM micrographs of the three coatings: (a) Metco 130 coating, (b) NN coating prepared from nanocomposite powder with particulate structure, and (c) NP coating prepared from nanocomposite powder with three-dimensional network structure.
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Fig. 6. TEM micrographs of the three coatings: (a) Metco 130 coating, (b) NN coating prepared from nanocomposite powder with particulate structure, and (c) NP coating prepared from nanocomposite powder with three-dimensional network structure.
Table 1 Microhardness, crack growth resistance and bonding strength of the three coatings. Coatings
Metco 130 coating
NN coating
NP coating
Microhardness (HV0.3) Crack growth resistance (1/lm) 10 3 Bonding strength (MPa)
849 ± 87 3.31 ± 0.64
908 ± 63 3.58 ± 0.49
939 ± 57 4.44 ± 0.51
25.0 ± 5.8
27.6 ± 3.9
31.0 ± 2.1
3.3. Properties of the coatings, and toughening and strengthening mechanism of the nanocomposite coatings The microhardness, crack growth resistance and bonding strength of the three coatings were shown in Table 1. The microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were significantly improved compared with that of Metco 130 coating, especially the nanocomposite coating prepared from the nanocomposite powder with three-dimensional network structure. The indentation cracks propagated exclusively along lamella boundaries in Metco 130 coating (Fig. 7a). By comparison, there were many different crack paths observed in the nanocomposite
coatings. It was found that some cracks were deflected at the interface between the fully-melted region and three-dimensional network structure region (Fig. 7b and e, indicated by arrows). Some cracks passed through the three-dimensional network structure region. And the cracks length in the three-dimensional network structure regions was smaller than that in the fully-melted microstructure (Fig. 7b and c). It seemed that these cracks were trapped in the three-dimensional network structure regions (Fig. 7c). Fig. 7d showed when the crack propagated through the threedimensional network structure region, it went along the intergranular film and around the a-Al2O3 grains. The crack was deflected by the a-Al2O3 grains. The crack propagation path was therefore extended, and the toughness of the coating was improved. It is well established that toughening mechanisms such as crack deflection and crack trapping can improve the crack growth resistance of bulk brittle materials markedly [26,29,30]. Therefore, it could be concluded that the improvement of the crack growth resistance was attributed to the three-dimensional network structure present in the nanocomposite coating. Cracks emerging from the edges of the indentation were deflected and trapped by the three-dimensional network structure. When the cracks propagated through the three-dimensional network structure region, the a-Al2O3
Fig. 7. SEM micrographs of cracks propagation in the coatings: (a) Metco 130 coating, (b)–(e) NP coating prepared from nanocomposite powder with three-dimensional network structure. (b) Crack was deflected at the interface between the fully-melted region and three-dimensional network structure region and propagated long distance in the fully-melted region, (c) crack was trapped in the three-dimensional network structure region, and (d) crack propagated through the three-dimensional network structure region, and it was deflected by the a-Al2O3 grains and went along the intergranular film.
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grains became obstacles to the cracks propagation. Cracks were forced to deflect around the a-Al2O3 grains, thus improving the toughness of the nanocomposite coating comparing with the conventional one. In addition, it has been demonstrated that fracture process in nanocrystalline materials are crucially influenced by intergranular and interphase boundaries [31]. During plastic deformation, grain boundaries (GBs) in nanocrystalline materials serve as sources of partial lattice dislocations and cracks often nucleate at and grow along interfaces. The intergranular network thin film with amorphous structure, however, could improve the strength and toughness of the three-dimensional network structure by intercrystalline strengthening and toughening [29–32]. Therefore, the three-dimensional network structure may work as reinforcing and toughening units in the ceramic coating, improving the strength and toughness of the coating.
The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51102074, 51272065 and 51372065), the Natural Science Foundation of Hebei Province (E2015202070), the Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences (KLICM-2013); and Outstanding Youth Fund for Science and Technology Research of Universities in Hebei Province, China (Y2012003).
4. Conclusions
References
(1) Nanocomposite powder with three-dimensional network structure was prepared by adding nano additives (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing. The internal microstructure of the powder particles experienced very rapidly liquid phase sintering during the plasma jet processing. The as-prepared nanocomposite powder with three-dimensional network structure had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies. (2) Alumina–titania nanocomposite coatings were successfully fabricated through plasma spraying. The grain size of the conventional Metco 130 coating was in the range of sub-micrometer to micrometer. The grain size of the coating prepared by the nanocomposite powder with particulate structure was in the range of a hundred nanometers to sub-micrometer. There were two types of microstructures formed in the nanocomposite coating prepared by the nanocomposite powder with three-dimensional network structure, the fully-melted region and the partially melted region. The fully-melted region was mainly composed of cAl2O3 nano grains embedded in amorphous substrate. The partially melted region retained the three-dimensional network structure in the nanocomposite powder. (3) The microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were significantly improved compared with that of the conventional Metco 130 coating. The strengthening mechanisms included microstructure refinement strengthening and grain boundary strengthening. The toughening mechanisms included crack deflection and crack trapping. Cracks were deflected and trapped by the three-dimensional network structure in the nanocomposite coating. When the cracks propagated through the three-dimensional network structure region, the a-Al2O3 grains became obstacles to the cracks propagation. Cracks were forced to deflect around the a-Al2O3 grains, thus improving the toughness of the nanocomposite coating. The intergranular network thin film with amorphous structure could improve the strength and toughness
of the three-dimensional network structure by intercrystalline strengthening and toughening. Therefore, the threedimensional network structure may work as reinforcing and toughening units in the nanocomposite coating, improving its strength and toughness.
Acknowledgments
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Nanocomposite powder with three-dimensional network structure for preparing alumina–titania nanocomposite coating with advanced performance Yong Yang a,b,c,⇑, You Wang b,⇑, Wei Tian b, Dian-ran Yan a, Jian-xin Zhang a, Liang Wang c a b c
Key Lab. for New Type of Functional Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, China Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China
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Article history: Received 19 September 2014 Received in revised form 30 October 2014 Accepted 3 November 2014 Available online 11 November 2014 Keywords: Coating materials Composite materials Nanostructured materials Microstructure Nanocomposites Plasma spraying
a b s t r a c t Nanocomposite powder with three-dimensional network structure was prepared by adding nano-dopants (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing. Alumina–titania nanocomposite coating was fabricated through plasma spraying the nanocomposite powder. The formation mechanism of the nanocomposite powder was investigated, and the strengthening and toughening mechanisms of the nanocomposite coating was revealed. The internal microstructure of the powder particles experienced liquid phase sintering during plasma jet processing. The nanocomposite powder with three-dimensional network structure had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies. Microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were improved compared with that of Metco 130 coating. The strengthening mechanism was mainly microstructure refinement strengthening. The toughening mechanisms included crack deflection and crack trapping. The threedimensional network structure may work as reinforcing and toughening units in the nanocomposite coating. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Alumina ceramic matrix composite coatings are increasingly employed in applications where elevated hardness, good tribological properties, excellent corrosion resistance and high thermal resistance are required [1–4]. Al2O3–TiO2 coating is one of the most widely used alumina ceramic matrix composite coating for their excellent wear, corrosion, and erosion resistance [5,6]. With the development of nanotechnology and nanomaterials, enhanced mechanical properties in ceramic coatings with nanostructured microstructures have been reported, drawing extensive attention to the preparation, characterization and application of nanostructured ceramic coatings [7–14]. A number of techniques have been attempted to produce nanostructured ceramic coatings, such as physical vapor deposition, chemical vapor deposition, ion implantation, magnetron sputtering, electrodeposition, laser cladding, thermal spraying and cold spraying [15–18]. Among the possible processes, atmospheric ⇑ Corresponding authors at: No. 29 Guangrong Road, Hongqiao District, Tianjin 300132, China. Tel.: +86 22 60204810; fax: +86 22 26564810. E-mail address: [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.jallcom.2014.11.021 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
plasma spraying (APS) has received much attention in recent years [15,18]. Plasma spraying is a simple processing technique which can be applied on different substrates, i.e. metal or ceramic. Coatings with thickness from tens of micrometers to hundreds of micrometers even to one millimeter can be prepared with high deposition rate. Especially, it is easy to form a composite coating [19]. It provides coatings with better quality and good adherence to the substrate. Using nanoscale powder as feedstock to generate nanostructured coating by plasma spraying presents new challenges as well as new opportunities [20,21]. However, the small size of the nanoscale particles impedes the direct projection in conventional plasma spraying equipment. Individual nanoscale particles cannot be successfully deposited because of their low mass and the resultant inability to be carried in a moving gas stream and deposited on a substrate. And fine powder tends to stick to the sidewall of feed lines and spray nozzle tip, causing reduction of powder passage area. Fine powder with strong surface interaction also tends to agglomerate, leading to limited flowability. Therefore, when powder is too fine, it is difficult to maintain a constant powder flow rate during thermal spraying.
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To circumvent the obstacle, reconstitution of individual nanoscale particles into spherical micrometer-sized granules is necessary. These granules (agglomerates) with average sizes similar to those of the conventional sprayable feedstock (approximately 10–80 lm) can be deposited using standard plasma spraying equipment [21]. At present, the most widely applied granulation reconstitution technology for preparing nanostructured powders for thermal spraying is spray drying [20,22], which has the advantages of low cost, wide applicability, easy control of composition and particle size. The results [20–23] showed that the nanostructured feedstock prepared by spray drying was spherical particles with smooth surface and good fluidity. But the internal structure of the particles was loose and contained many pores, which reduced the melting degree of particles during plasma spraying process and resulted in the increased porosity of the as-prepared coating. Many investigation results [5,7–13,23] showed that alumina–titania nanocomposite coating prepared by plasma spraying had a bimodal microstructure (fully-melted region and partially melted region), and most of pores were present in the partially melted region. Pores and cracks in the coating were the most important defects that caused the failure of the coating. Summing up the above analysis, the key problem to be solved for preparing nanocomposite coatings by plasma spraying nanostructured powders is to fabricating nanostructured sprayable feedstocks with high quality, high performance and/or unique structure. It is well-known that the microstructure of material plays a decisive role in its properties. Through the above analysis, the authors think that the microstructure of the nanostructured feedstock will have an important impact on the microstructure and properties of the coating. However, to our knowledge, there are few reports on the effect of microstructure of the nanostructured feedstock on the microstructure and properties of the plasma sprayed coating. Aiming at tailoring the microstructure of nanocomposite feedstock to fabricate nanostructured feedstock with high quality and unique structure, in the present investigation, by adding nano additives (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing, nanocomposite powder with three-dimensional network structure was prepared. Subsequently, advanced performance alumina–titania nanocomposite coating was successfully fabricated through plasma spraying nanocomposite powder with three-dimensional network structure. The formation mechanism of the nanocomposite powder with threedimensional network structure was investigated, and its strengthening and toughening mechanism on the coating was revealed.
nanostructured composite powders which are microsized particles with nanosized grains. After spray drying, NN powder and NP powder were heat treated for eliminating the PVA and densifying the microstructure of the powders. At last, NP powder was plasma jet processed. During plasma jet processing, the powder was injected into the plasma jet and air quenched in a cold chamber, and then was collected. The GDP-2 type 50 kW plasma spraying system (Jiu Jiang Spraying Device Company, China) was employed for plasma jet processing powders. The carbon steel (0.14–0.22 wt.%C) coupons were used as substrate samples. These samples were grit blasted prior to coating deposition. A bond coating of NiCrAl with thickness about 50–100 lm was deposited onto the substrates. The three kinds of composite powders were then plasma sprayed onto the bond coatings for about 300 lm in thickness, respectively. The phase constitution of the powders and coatings was characterized by X-ray diffraction (XRD, D/max-cB, Japan) with Cu Ka radiation. The microstructure of the powders and coatings was characterized by scanning electron microscope (SEM, HITACHI-S4800) and transmission electron microscopy (Philips CM-12, TEM). In order to observe the cross-sectional structure of the composite powders, the asprepared composite powders were dispersed homogeneously in epoxy resin to form a bulk sample. Then, the cross-section of the epoxy resin bulk sample containing composite powders were ground and polished for cross-sectional SEM analysis of the composite powders. Coatings’ microhardness was determined under a normal load of 300 g on the cross section of the coatings. The bonding strength of the coatings was tested according to ASTM C633-79 standard. The crack propagation characteristics were determined by indenting the polished coating cross section and top surface with a Vickers indenter under a 3 kg load for 10 s. The length of cracks that emerged from the indent corners was measured. The crack growth resistance of coatings was characterized by the reciprocal of average crack length [25].
3. Results and discussion 3.1. Microstructure of the composite powders and its formation mechanism The phase constitutions of the composite powders were shown in Fig. 1. ME powder consisted of a-Al2O3 and anatase, and NN powder consisted of a-Al2O3 and rutile. NP powder consisted of a-Al2O3, rutile, t-ZrO2, cubic CeO2 and d-Al2O3, in addition, there were some solid solution of oxide additives (ZrO2 and CeO2) containing TiO2. The SEM micrographs of the composite powders were shown in Fig. 2. ME powder were microsized irregular particles with size range from 20 to 60 lm in diameter (Fig. 2a). This powder was manufactured through a so-called cladding process, in which fused-and-crushed Al2O3 powder (Fig. 2c) was clad with submicron TiO2 (Fig. 2b). After cladding process, the surface of the blocky Al2O3 powder was covered with fine TiO2 particles (Fig. 2b). NN powder possessed spherical geometry with size range from 10 to 50 lm in diameter, and there were pores in it (Fig. 2d). Each spherical particle consisted of a great number of nanosized alumina and titania grains (about 50–100 nm) (Fig. 2e), which was therefore
2. Material and methods In the present research, three kinds of alumina–titania composite powders were used to prepare alumina–titania composite coatings. It was labeled as ME powder, NN powder and NP powder. The chemical composition of ME and NN powders was 87 wt.% Al2O3 and 13 wt.% TiO2. The mass ratio of Al2O3 and TiO2 in NP powder was 87:13, and a small amount of nanosized ZrO2 and CeO2 were used as dopants. The preparation methods for the three kinds of composite powders were different. ME powder was commercial Metco 130 powder (Sulzer Metco Co., Ltd., USA). This powder was manufactured through a so-called cladding process, in which fusedand-crushed Al2O3 powder (20–60 lm diameter) was clad with submicron TiO2 [24]. After cladding process, the surface of the blocky Al2O3 powder was covered with fine TiO2 particles. NN powder was nanocomposite powder which was fabricated by spray drying and heat treating [21]. NP powder was also nanocomposite powder which was obtained by spray drying, heat treating and plasma jet processing [7,9]. For the preparing process of nanocomposite powder, as-received powders were Al2O3 (d and c phases, 99.9% grade, Degussa Co., Ltd., Germany) with grain size of 20–45 nm and TiO2 (anatase, 99.9% grade, Nanjing High Technology of Nano Material Co., Ltd., China) with grain size of 20–50 nm. These powders were blended uniformly to produce a powder mixture with composition of 87 wt.%Al2O3 and 13 wt.%TiO2 by wet ball-milling. In addition, ZrO2 with grain size of 20–50 nm and CeO2 with grain size of 20–40 nm were added during mixing for a modified powder mixture. The mixed powder slurries were then spray dried to form
Fig. 1. XRD patterns of the composite powders: (a) ME, (b) NN, and (c) NP.
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Fig. 2. SEM micrographs of the composite powders: (a) microstructured powder (ME), (b) high magnification of the surface of ME powder, (c) cross-section of ME powder, (d) cross-section of nanostructured NN powder, (e) high magnification of the cross-section of NN powder, (f) cross-section of plasma jet processed nanostructured NP powder, and (g) high magnification of the cross-section of NP powder.
called nanocomposite powder with particulate structure. Fig. 2f showed the typical microstructure of NP powder, and Fig. 2g was the high magnification cross-sectional SEM micrograph of the internal microstructure of NP powder. NP powder had better sphericity and much denser microstructure comparing with NN powder. There were a lot of equiaxed grains (with size of 50–500 nm) in the NP powder. And the equiaxed grains were surrounded by intergranular thin film (with thickness less than 100 nm), which extended inside the powder particle to form a network structure. So, the microstructure of NP powder was called three-dimensional network structure. Fig. 3 showed the TEM micrograph, and corresponding selected area diffraction (SAD) patterns and energy dispersive spectrometer (EDS) pattern of the three-dimensional network microstructure in NP powders. There were equiaxed grains with different grain size from about tens of nanometers to hundreds of nanometers in the three-dimensional network microstructure (Fig. 3a). Many small
equiaxed grains about tens of nanometers located inside or among the bigger equiaxed grains, forming intragranular and intergranular nanostructures. The SAD pattern (Fig. 3b) indicated that the equiaxed grains were mainly a-Al2O3. There was intergranular substance around the equiaxed a-Al2O3 grains. The SAD pattern (Fig. 3c) for the intergranular substance indicated that it was amorphous structure. The EDS pattern (Fig. 3d) showed that the amorphous intergranular substance was rich in Ti, Zr and Ce. Combined with the XRD, SEM and TEM results, it can be inferred that NP powder was nanocomposite powder with three-dimensional network structure. The equiaxed grains in NP powder were aAl2O3, and the intergranular network thin film was amorphous rich in Ti, Zr and Ce. That means NP powder had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies. Comparing with NN powder, the microstructure of NP powder changed a lot. The spray-dried and heat-treated NN powder had
Fig. 3. TEM micrograph, and corresponding SAD patterns and EDS pattern of the three-dimensional network microstructure in NP powder: (a) TEM bright-field micrograph of the three-dimensional network microstructure, (b) SAD pattern of ‘‘I’’ region in (a), (c) SAD pattern of ‘‘II’’ region in (a), and (d) EDS pattern of ‘‘II’’ region in (a).
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3.2. Phase constitution and microstructure of the composite coatings
Fig. 4. XRD patterns of the three coatings: (a) Metco 130 coating, (b) NN coating prepared from nanocomposite powder with particulate structure, and (c) NP coating prepared from nanocomposite powder with three-dimensional network structure.
porous and particulate microstructure, while NP powder had dense and three-dimensional network microstructure. The formation of the three-dimensional network structure was attributed to the addition of ZrO2 and CeO2 nano-dopants and the employment of plasma jet processing. The melting point of Al2O3, TiO2, ZrO2 and CeO2 is 2050 ± 10 °C, 1840 ± 10 °C, 2600 ± 100 °C and 2725 ± 20 °C, respectively [26]. TiO2, CeO2 and ZrO2 could form TiO2–CeO2–ZrO2 system liquid phase under a lower temperature (<1350 °C) below the melting point temperature of TiO2 [27]. During plasma jet processing of the composite powder, although the temperature of the plasma jet was high enough, the flight speed of the powder particles was very fast (about 150–300 m/s), and the residence time of the powder particles in the high temperature plasma jet was very short (10 3–10 4 s) [21]. Additionally, the oxide ceramics have low thermal conductivity [26]. Therefore, only the surface of the powder particles melted. The powder particles underwent rapid melting and solidifying process. The internal microstructure of most powder particles experienced relative low temperature, and did not melt completely. Only TiO2–CeO2–ZrO2 system liquid phase with low melting point was formed in the powder particles. The unmelted Al2O3 grains were surrounded by the liquid phase, which led to a similar effect as the liquid phase sintering of oxide ceramics [27]. Here, the internal microstructure of the NP powder particles just experienced very rapidly liquid phase sintering process. Threedimensional network microstructure was, therefore, formed in NP powder after plasma jet processing, which is similar to the microstructure of the liquid phase sintered oxides.
The XRD patterns of the three coatings (Fig. 4) revealed a similar phase constitution for the three coatings. There were two types of alumina in the three coatings: c-Al2O3 (most) and a-Al2O3 (minor). The melted alumina transformed to c-Al2O3 during plasma spraying deposition process due to the high cooling rate [11]. There were amorphous phase present in the three coatings indicated by the broad hump in the 2h range of 25° to 40°. The diffraction peaks of titania were not well identified in the three coatings, which may be attributed to the dissolving of titania into c-Al2O3 [9,10] and the formation of amorphous structure of titania. And the addition of nanosized ZrO2 and CeO2 in NP powder promoted the formation of amorphous in the NP coating. In addition, there were some solid solution of oxide additives (ZrO2 and CeO2) containing TiO2. Fig. 5 showed the cross-sectional SEM micrographs of the three coatings. There was typical lamellar structure in Metco 130 coating (Fig. 5a). The lamellar structure was derived from the fused and spread feedstock powders [5]. Comparing with Metco 130 coating, there was not typical lamellar structure in the coatings prepared from nanocomposite powders, which was mainly attributed to the homogeneous distribution of chemical composition in the nanocomposite feedstock powders [10,28]. There were two types of microstructures formed in the two coatings prepared from nanocomposite powders (Fig. 5b and c). ‘‘FM’’ regions with fusedand-solidified structure in NN and NP coatings were formed by the fully melted feedstock powders during plasma spraying. ‘‘PM’’ regions with particulate structure in NN coating was formed by the partially melted or unmelted NN nanocomposite powder with particulate structure, and ‘‘PM’’ regions with three-dimensional network structure in NP coating were formed by the partially melted or unmelted NP nanocomposite powder with three-dimensional network structure. The three-dimensional network structure in the NP coating was derived from the plasma jet processed feedstock powder. Fig. 6 showed the TEM micrographs of the three coatings. The grain size of Metco 130 coating was in the range of sub-micrometer to micrometer (Fig. 6a). The SAD patterns (not shown here) indicated that the sub-micrometer grains and micrometer grains were c-Al2O3. For NN coating, the grain size was in the range of a hundred nanometers to sub-micrometer (Fig. 6b), and the SAD patterns indicated that these grains were c-Al2O3. Fig. 6c showed the TEM micrograph of the fully-melted microstructure in NP coating, which was mainly composed of nano grains embedded in amorphous substrate. There were lots of equiaxed grains smaller than 100 nm in the fully-melted microstructure. The SAD pattern inserted indicated c-Al2O3 nano grains were distributed in amorphous substrate. The microstructure of the partially melted or unmelted region in NP coating retained the three-dimensional network structure in the NP powder, which was composed of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies.
Fig. 5. Cross-sectional SEM micrographs of the three coatings: (a) Metco 130 coating, (b) NN coating prepared from nanocomposite powder with particulate structure, and (c) NP coating prepared from nanocomposite powder with three-dimensional network structure.
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Fig. 6. TEM micrographs of the three coatings: (a) Metco 130 coating, (b) NN coating prepared from nanocomposite powder with particulate structure, and (c) NP coating prepared from nanocomposite powder with three-dimensional network structure.
Table 1 Microhardness, crack growth resistance and bonding strength of the three coatings. Coatings
Metco 130 coating
NN coating
NP coating
Microhardness (HV0.3) Crack growth resistance (1/lm) 10 3 Bonding strength (MPa)
849 ± 87 3.31 ± 0.64
908 ± 63 3.58 ± 0.49
939 ± 57 4.44 ± 0.51
25.0 ± 5.8
27.6 ± 3.9
31.0 ± 2.1
3.3. Properties of the coatings, and toughening and strengthening mechanism of the nanocomposite coatings The microhardness, crack growth resistance and bonding strength of the three coatings were shown in Table 1. The microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were significantly improved compared with that of Metco 130 coating, especially the nanocomposite coating prepared from the nanocomposite powder with three-dimensional network structure. The indentation cracks propagated exclusively along lamella boundaries in Metco 130 coating (Fig. 7a). By comparison, there were many different crack paths observed in the nanocomposite
coatings. It was found that some cracks were deflected at the interface between the fully-melted region and three-dimensional network structure region (Fig. 7b and e, indicated by arrows). Some cracks passed through the three-dimensional network structure region. And the cracks length in the three-dimensional network structure regions was smaller than that in the fully-melted microstructure (Fig. 7b and c). It seemed that these cracks were trapped in the three-dimensional network structure regions (Fig. 7c). Fig. 7d showed when the crack propagated through the threedimensional network structure region, it went along the intergranular film and around the a-Al2O3 grains. The crack was deflected by the a-Al2O3 grains. The crack propagation path was therefore extended, and the toughness of the coating was improved. It is well established that toughening mechanisms such as crack deflection and crack trapping can improve the crack growth resistance of bulk brittle materials markedly [26,29,30]. Therefore, it could be concluded that the improvement of the crack growth resistance was attributed to the three-dimensional network structure present in the nanocomposite coating. Cracks emerging from the edges of the indentation were deflected and trapped by the three-dimensional network structure. When the cracks propagated through the three-dimensional network structure region, the a-Al2O3
Fig. 7. SEM micrographs of cracks propagation in the coatings: (a) Metco 130 coating, (b)–(e) NP coating prepared from nanocomposite powder with three-dimensional network structure. (b) Crack was deflected at the interface between the fully-melted region and three-dimensional network structure region and propagated long distance in the fully-melted region, (c) crack was trapped in the three-dimensional network structure region, and (d) crack propagated through the three-dimensional network structure region, and it was deflected by the a-Al2O3 grains and went along the intergranular film.
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grains became obstacles to the cracks propagation. Cracks were forced to deflect around the a-Al2O3 grains, thus improving the toughness of the nanocomposite coating comparing with the conventional one. In addition, it has been demonstrated that fracture process in nanocrystalline materials are crucially influenced by intergranular and interphase boundaries [31]. During plastic deformation, grain boundaries (GBs) in nanocrystalline materials serve as sources of partial lattice dislocations and cracks often nucleate at and grow along interfaces. The intergranular network thin film with amorphous structure, however, could improve the strength and toughness of the three-dimensional network structure by intercrystalline strengthening and toughening [29–32]. Therefore, the three-dimensional network structure may work as reinforcing and toughening units in the ceramic coating, improving the strength and toughness of the coating.
The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51102074, 51272065 and 51372065), the Natural Science Foundation of Hebei Province (E2015202070), the Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences (KLICM-2013); and Outstanding Youth Fund for Science and Technology Research of Universities in Hebei Province, China (Y2012003).
4. Conclusions
References
(1) Nanocomposite powder with three-dimensional network structure was prepared by adding nano additives (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing. The internal microstructure of the powder particles experienced very rapidly liquid phase sintering during the plasma jet processing. The as-prepared nanocomposite powder with three-dimensional network structure had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the a-Al2O3 colonies. (2) Alumina–titania nanocomposite coatings were successfully fabricated through plasma spraying. The grain size of the conventional Metco 130 coating was in the range of sub-micrometer to micrometer. The grain size of the coating prepared by the nanocomposite powder with particulate structure was in the range of a hundred nanometers to sub-micrometer. There were two types of microstructures formed in the nanocomposite coating prepared by the nanocomposite powder with three-dimensional network structure, the fully-melted region and the partially melted region. The fully-melted region was mainly composed of cAl2O3 nano grains embedded in amorphous substrate. The partially melted region retained the three-dimensional network structure in the nanocomposite powder. (3) The microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were significantly improved compared with that of the conventional Metco 130 coating. The strengthening mechanisms included microstructure refinement strengthening and grain boundary strengthening. The toughening mechanisms included crack deflection and crack trapping. Cracks were deflected and trapped by the three-dimensional network structure in the nanocomposite coating. When the cracks propagated through the three-dimensional network structure region, the a-Al2O3 grains became obstacles to the cracks propagation. Cracks were forced to deflect around the a-Al2O3 grains, thus improving the toughness of the nanocomposite coating. The intergranular network thin film with amorphous structure could improve the strength and toughness
of the three-dimensional network structure by intercrystalline strengthening and toughening. Therefore, the threedimensional network structure may work as reinforcing and toughening units in the nanocomposite coating, improving its strength and toughness.
Acknowledgments
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