Effects of CeO2 nanoparticles on microstructure and properties of laser cladded NiCoCrAlY coatings

JOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010, p. 246

Effects of CeO2 nanoparticles on microstructure and properties of laser cladded NiCoCrAlY coatings WANG Hongyu (⥟ᅣᅛ)1,2, ZUO Dunwen (Ꮊᬺ〇)1, LI Xiangfeng (咢৥䫟)1, CHEN Kangmin (䰜ᒋᬣ)3, HUANG Mingmin (咘䫁ᬣ)1 (1. College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; 2. School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China; 3. School of Materials and Engineering, Jiangsu University, Zhenjiang 212013, China) Received 4 August 2009; revised 10 November 2009

Abstract: CeO2 nanoparticles (nano-CeO2p) were added into laser cladded NiCoCrAlY coatings on Ni-based superalloy substrate to improve the microstructure and properties. Scanning electron microscope (SEM), X-ray diffractometer (XRD), micro-hardness tester, and heat treatment furnace were employed to investigate their morphologies, phases, micro-hardness and thermal shock resistance, compared with the coating without nanoparticles added. The results showed that the microstructure and properties of the coatings with the addition of nano-CeO2p were better than the coating without nanoparticles added, in which the coating with the addition of 2.0 wt.% nano-CeO2p showed the most significant improvement effects. With the addition of nano-CeO2p, the growth pattern of interface grain was changed, the microstructure of cladding layer was refined, and the solid solubility of Cr was increased. These changes of microstructure after adding nano-CeO2p improved micro-hardness and its distribution uniformity on the cross section, and the thermal shock resistance as well. Keywords: MCrAlY coating; nano-CeO2p; laser cladding; microstructure; micro hardness distribution; thermal shock resistance; rare earths

Double-layer thermal barrier coating (TBC) is one of the most practical TBCs, of which NiCoCrAlY alloy is the first choice for metal bondcoat[1]. The improvement of TBCs working temperature sets higher requirements for the microstructure and properties of NiCoCrAlY coatings[2,3]. Rare earth (RE) materials show large capacities in improving microstructure and properties of alloys due to its excellent physicochemical characteristics, such as special electronic structure, outstanding chemical activity and large atomic radius. The CeO2p has been widely used as a RE modification material[4–7]. Many studies have demonstrated that nanoparticle reinforced metal matrix composite coatings have many advantages in strength, toughness, heat resistance and other aspects compared with the general coatings[8–10]. Therefore, it is possible to obtain better microstructure and properties through introducing CeO2 nanoparticles (nanoCeO2p) into NiCoCrAlY coatings. However, few studies were reported on microstructure and properties of NiCoCrAlY coatings strengthened by nano-CeO2p. In this work, laser cladding that is one of the most representative technologies for preparing NiCoCrAlY coatings was used for the fabrication of NiCoCrAlY coatings strengthened by three contents of nano-CeO2p on Ni-based superalloy substrate. Their morphologies, phases, microhardness and thermal shock resistance were investigated, compared with the coating without nanoparticles added,

which aimed at studying effects of nano-CeO2p on microstructure and properties of NiCoCrAlY coatings and providing some reference for the optimization of strengthening component of NiCoCrAlY composition.

1 Materials and methods The substrate material used was Ni-based superalloy GH4033. Its nominal chemical composition is 19–22Cr, 0.6–1.0Al, 2.4–2.8Ti, 4Fe and 70–74Ni. The cladding material used was NiCoCrAlY powder, which possesses the chemical composition of 18.64Cr, 3.39Al, 2.6Co, 0.55C, 1Y and 73.82Ni. The average sizes of the NiCoCrAlY powders and nano-CeO2p were 75 Pm and 20 nm, respectively. According to the mixture ratio model of micro-nanometer composite powders proposed in a previous study[11], the optimal ratio of nano-CeO2p is 2.0 wt.%. However, for comparison, micro-nanometer composite powders were made with contents of 1.0 wt.%, 2.0 wt.% and 3.0 wt.% nano- CeO2p respectively using a self-developed method[12]. The cladding powders were preset onto the substrates using the squashpresetting method described in a Chinese patent[13] and the thickness of presetting powders layer was 0.4 mm. The multi-channel laser cladding was carried out using a crosscurrent CO2 laser with the optimized process parameters such as laser power of 1000 W, spot diameter of 2 mm and scanning

Foundation item: Project supported by the Doctoral Program of Higher Education of China (20060287019), the Opening Research Fund of Jiangsu Key Laboratory of Tribology of China (kjsmcx07001) and Graduate Innovation Foundation of Jiangsu Province of China (CX08B-039Z) Corresponding author: ZUO Dunwen (E-mail: [email protected]; Tel.: +86-25-84890249) DOI: 10.1016/S1002-0721(09)60089-2

WANG Hongyu et al., Effects of CeO2 nanoparticles on microstructure and properties of laser cladded NiCoCrAlY coatings

speed of 5 mm/s, as well as a nitrogen gas environment. The thermal shock test was conducted in heat treatment furnace with a temperature control precision of less than ±1 °C. The size of polished samples for thermal shock test was 10 mm×1.5 mm×4 mm. The cycling temperature of thermal shock test was from 1050 ºC to room temperature (about 15 ºC). The spalled area of oxide scale after 10 thermal shock cycles and the crack state in cross section after 100 thermal shock cycles were as the evaluation indexes of thermal shock performances. The spalled area was calculated by dividing into grids and then taking the average value of three ones selected randomly. Three samples were made for each test. Morphologies, phases and micro-hardness of the coatings were examined using a scanning electron microscope (SEM, JSM-7001F), an X-ray diffraction spectrometer (XRD, D/max2500PC) and a micro-hardness tester (HXS-1000A, 0.1 kg), respectively.

2 Results and analysis 2.1 Cross-sectional morphologies Fig. 1 shows cross-sectional morphologies of the coating without nanoparticles added. Fig. 1(a) displays the morphology of interfacial zone. The solidification microstructure is a dendrite and it is vertical, directional and epitaxial to the bonding interface. This result is consistent with that reported by Li et al.[10] Some cracks exist in the substrate near interface and they expand to the interface. Fig. 1(b) is the local amplification of area A in Fig. 1(a). The microstructure of cladding layer shown in Fig. 1(c) is an equiaxial grain, from which some cellular substructure with diameter of about 5

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ȝm is observed. Fig. 2 shows cross-sectional morphologies of the coating with addition of 2.0 wt.% nano-CeO2p. These morphologies indicate that the solidification microstructure of interfacial zone is no longer epitaxial and directional to the bonding interface after adding nano-CeO2p. There is no crack in the cross section. The dendrites have turned to equiaxed grains (Fig. 2(b)). The microstructure of cladding layer is a fine equiaxed grains (Fig. 2(c)). Compared cross-sectional morphologies of NiCoCrAlY coatings with different contents of nano-CeO2p, some dendrites still exist in the interfacial zone when the content of nano-CeO2p is 1.0 wt.% (Fig. 3(a)), while some pores appear in the substrate near the bonding interface when adding 3.0 wt.% nano-CeO2p (Fig. 3(b)). This indicates that insufficient addition of nano-CeO2p did not improve microstructure enough, while too much addition of nano-CeO2p brought some new defects. 2.2 Phases The effects of CeO2p on phases of NiCoCrAlY coatings are identical to that of other nanoparticles[14]. It is indicated by XRD patterns that the phases of coating without nanoparticles added are Ȗ-Ni, ȕ-NiAl, Ȗ’-Ni3Al and Į-Cr, while the phases of coating with the addition of 2.0 wt.% nano-CeO2p are Ȗ-Ni, ȕ-NiAl and Ȗ’-Ni3Al. The disappearance of Į-Cr directly relates to the increase of Cr solid solubility[15]. No CeO2 or new generated phase is observed in the XRD patterns of coating with the addition of 2.0 wt.% CeO2p. This phenomenon might be caused by the small addition of nano-CeO2p. Moreover, no different effects on phases of the coating have been found after adding different contents of nano-CeO2p.

Fig. 1 Interfacial zone morphology (a), the local amplification of area A in Fig. 1(a) (b), and cladding layer morphology (c) of the coating without nanoparticles added

Fig. 2 Interfacial zone morphology (a), the local amplification of area A in Fig. 2(a) (b), and cladding layer morphology (c) of the coating with the addition of 2.0 wt.% nano-CeO2p

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Fig. 3 Cross-sectional morphologies of the coating with the addition of 1.0 wt.% nano-CeO2p (a) and 3.0 wt.% nano-CeO2p (b)

2.3 Micro-hardness distribution on the cross section

2.4 Thermal shock resistance

Fig. 4 gives the curves of micro-hardness distribution on cross section of four NiCoCrAlY coatings with different contents of nano-CeO2p. We can see from Fig. 4 that microhardness of the coatings with the addition of nano-CeO2p has all been improved compared with the one without nanoparticles added. The micro-hardness is the highest when adding 2.0 wt.% nano-CeO2p, which is 14% higher than that of the one without nanoparticles added. Following are coatings with the addition of 1.0 wt.% and 3.0 wt.% nano-CeO2p. Moreover, the uniformity of micro hardness distribution on the cross-section is also all improved.

Fig. 5(a) and (b) show surface morphologies of NiCoCrAlY coatings without nanoparticles added and with the addition of 2.0 wt.% nano-CeO2p after 10 thermal shock cycle, respectively. We can see from Fig. 5(a) that the oxide scale spallation of the coating without nanoparticles added belongs to an extended spalling[16] during the thermal shock cycles. The calculations suggest that the sum of spalled and spalling area reaches 27.7%. After adding 2.0 wt.% nanoCeO2p, as shown in Fig. 5(b), the spallation is in unitspalling instead of extended spalling. The calculations indicate that the spalled area of coatings with the addition of 1.0 wt.%, 2.0 wt.% and 3.0 wt.% nano-CeO2p are 14.77%, 5.23% and 19.67%, respectively. Fig. 6(a) and (b) show cross-sectional morphologies of NiCoCrAlY coatings without nanoparticles added and with the addition of 2.0 wt.% nano-CeO2p after 100 thermal shock cycles, respectively. We can see from Fig. 6(a) that many thermal cracks appear in the coating without nanoparticles added after 100 thermal shock cycles. These cracks can be divided into two types: one is a propagating crack, another is an internal crack. Both have an adherent tendency. While after adding nano-CeO2p as shown in Fig. 6(b), no internal crack is observed and the amount and size of propagating cracks are reduced. Further comparing the coatings with different contents of nano-CeO2p added indicates that

Fig. 4 Cross-sectional micro-hardness distribution of coatings with different contents of nano-CeO2p

Fig. 5 Surface morphologies of coatings after 10 thermal shock cycles without nanoparticles added (a) and with the addition of 2.0 wt.% nano-CeO2p (b)

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Fig. 6 Cross-sectional morphologies of coatings after 100 thermal shock cycles without nanoparticles added (a) and with the addition of 2.0 wt.% nano-CeO2p (b)

the one with the addition of 2.0 wt.% nano-CeO2p performs the least.

3 Discussion The effects of nano-CeO2p on microstructure of NiCoCrAlY coating might be attributed to some factors as follows. Firstly, RE as the surfactant can reduce the surface tension and nucleation energy, improving the ratio of nucleation. Secondly, many fine nanometer granules distribute in the fusing zone during the cladding, promoting the heterogeneous nucleation of liquid metal in large area. Thirdly, RE possesses large atomic radius and large specific surface area, and they are internal adsorption elements. The adhesion of these granules on solid-liquid boundary prevents the grains from growing. Finally, the addition of nano-CeO2p can hinder the diffusion of alloying elements, which decreases the activity of Ni while improves the solubility of Cr. With the addition of nano-CeO2p, the growth pattern of interface grain was changed, the microstructure of cladding layer was refined, and the solid solubility of Cr was increased. These changes of microstructure after adding nano-CeO2p improve micro-hardness and its distribution uniformity on the cross section, and reduce stress concentration of interface zone, inhibit defects such as cracks and porosities in the substrate near interface, and the thermal shock resistance as well. Moreover, RE can also improve the antispalling capacity of coating’s oxide scale[4], which further improves the thermal shock resistance. It can be inferred from the mixture ratio model that the difference of effects on microstructure and properties due to different contents of nano-CeO2p relate to the active compounds of nanometer particles and micron particles in the micro-nanometer composite powders.

4 Conclusions (1) After adding nano-CeO2p, the growth of interface grain translated from epitaxial growth to non-epitaxial growth, the slender dendrites became short and tended to be cellular, the microstructure of cladding layer was refined,

and the solid solubility of Cr was increased. These changes of microstructure improved micro-hardness and its distribution uniformity on the cross section, and reduced stress concentration of interface zone, inhibited defects such as cracks and porosities in the substrate near interface, and the thermal shock resistance as well. (2) The improvement of microstructure and properties of NiCoCrAlY coatings with 2 wt.% nano-CeO2p added was the best. This was coincident with the optimal ratio of micro-nanometer composite powders.

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