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Commercial and Coprecipitated of Al2O3-20 wt.% ZrO2 Plasma Sprayed Coatings

Mediha İpek, Cuma Bindal, Sakin Zeytin

Sakarya University - Türkiye

Abstract The aim of this study is to investigate atmospheric plasma sprayed coating using coprecipitated Al2O3-20 wt.% ZrO2 powders and commercial Al2O3-20 wt.% ZrO2 (8 mol% Y2O3) powders. Alumina-20 wt.%zirconia powders were synthesized by the coprecipitation method from Al2(SO4)3 and Zr(SO4)2 salts without stabilizing component. First, aluminum sulfate salt was dissolved in hot distilled water and it was cooled down to room temperature, for powder mixture. Then aqueous zirconium sulfate salt was added into cooled aluminum salt solution with continuous stirring. For precipitation, the pH of solution was adjusted to 10 with addition of NH4OH. After drying at 80°C for 72 h, precipitate was calcinated at 1300°C for 1 h. For a comparison commercial alumina and Y2O3 stabilized ZrO2 powders are mixed by conventionally ball milling for 2 h. The morphology of powders and coatings were examined by means of SEM and phase analyses were performed by XRD instrument. 1. Introduction Alumina is one of the most important technical ceramics. Its mechanical, electrical, thermal and optical properties have been extensively studied because of its wide range of uses such as lasers, lamp covers, thread guides, catalytic converters and substrates for microelectronic computer chips. However, the major disadvantage which restricts use is its tendency to fracture in a brittle fashion. Moreover, failure often occurs without prior warning. This can be overcome by increasing the toughness of alumina. The fracture toughness and strength of alumina can be significantly increased by adding zirconia which shows transformation toughening effect due to tetragonal monoclinic phase transformation [1,2]. Zirconia containing alumina is known as zirconia toughened alumina (ZTA). ZTA is a high purity combination of the low cost of alumina and high strength of zirconia. The enhanced strength and toughness have made the ZTAs more widely applicable and more productive than plain ceramics and cermets in machining steels and cast irons. It was proved that the combination

of high hardness alumina (19.3 GPa in the dense form) with the low thermal conductivity zirconia (2.2–2.6W/mK in the dense form) contributed to the development of the microhardness and wear resistance of the as-sprayed coatings. In addition, their mechanical properties are known to depend strongly on their microstructure. With the development of nanoscience and nanotechnology, the interest in the preparation of ultra-structured coatings is growing, since they have improved mechanical properties and might find promising application in engineering. Noticing that although the microstructures and properties of plasma sprayed alumina or zirconia coatings with nano powders have been extensively dealt with in many reports, only limited researches have been published on Al2O3-ZrO2 nano composite coatings by plasma spraying method and on the microstructure and properties of the composite coatings as well in open literature [3-7]. In the present study, both nano and coarse powders of Al2O3-20 wt.% ZrO2 were used as the starting materials for the feedstocks to prepare ultra-structured and micro-structured Al2O3-20 wt.% ZrO2 composite coatings by atmospheric plasma spraying. The microstructures and phases were comparatively investigated. 2. Experimental details In this study, alumina with 20 wt.% zirconia powders were prepared by coprecipitation method. Composite powders were processed from Al2(SO4)3 salt and aqueous solution of Zr(SO4)2 salt. Firstly, aluminum sulfate salt was dissolved in hot distilled water and then it was cooled room temperature. Aqueous zirconium sulfate salt was added into cooled aluminum salt solution with continuous stirring. The pH of salt solution was adjusted as 10 for precipitation with addition of NH4OH. Precipitate was calcinated at 1300°C for 1 h after dried at 80°C for 72h. Commercial alumina and Y2O3 stabilized ZrO2 (YSZ) powders are mixed by conventionally ball milling for 2 h. The powders size of alumina is -31 +3.9 mm, and the powders size of YSZ is -106 +11 mm. The substrate used for the coatings was AISI 316 stainless steel plates with the dimensions of 20 mm in length, 15 mm in width

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and 10 mm in height. The surface of the substrates was grit-blasted to provide surface roughness for better adherence between the coating and metallic substrate with 35 grid Al2O3 under 0.2 MPa pressure and a flow rate of 2 kg.min-1. Alumina-20 wt.% zirconia coatings were produced using an atmospheric plasma spraying (APS) technique on substrates with NiCoCrAlY bond coating. The plasma-spraying parameters are given in Table 1. Table 1. Plasma spraying parameters Parameter Value Plasma Gun 3 MB Arc current (A) 600 Voltage (V) 65-75 Gas flow for Ar (l/min) 100 Gas flow for H2 (l/min) 15 Spray distance (mm) 100 Powder feed rate (g/min) 60

Scanning electron microscopy (SEM) was used to examine the microstructure evolution of the plasma sprayed coatings. The distribution of elements in coatings layer was determined by means of energy dispersive spectroscopy (EDS) analysis. Phases of started powders and coatings were described by X-ray diffraction (XRD) analysis technique. 3. Results and discussion Figure 1 shows micrographs of alumina- 20 wt.%ZrO2 powders calcinated at 1300°C for 1 h. The morphologies of coprecipitated powders have sharp and complex geometrical shape and particle size of powders ranged from 1 mm to 100 mm. In fact, each particle consists of agglomeration of nano size powders (Figure 1b). SEM micrographs of atmospheric plasma spray coatings produced from coprecipitated and commercial powders are given in Figure 2. In coating layers, randomly distributed small pores with different sizes and lamellar structure which is characteristic for this kinds of coatings [5-8] are observed in all coatings. Lamellar structure is clearer in coatings produced from commercial powders (Figure 2b) than coprecipitated ones (Figure 2a). In coating produced from commercial powders (Figure 3) the open gray regions are zirconium-rich areas, while dark gray regions indicate aluminum rich areas. Zirconia and alumina particles can be distinguished very easily. But alumina and zirconia areas are not clear in the coatings produced from coprecipitated powders (Figure 4). The distributions of aluminum and zirconium elements in coating layer are more homogenous than commercial ones which confirmed by EDS analysis (Figure 3 and 4). The reason of this formation is agglomeration of

particles in nano scale which produced by coprecipitation method.

(a)

(b)

Figure 1. SEM micrographs of coprecipitated alumina-zirconia powders calcinated at 1300°C for 1 h, a) low and b) high magnifications XRD analysis showed that the coprecipitated and commercial powders have alpha ( ) alumina, tetragonal and monoclinic zirconia phases (Figure 5). It is interesting to note that the coprecipitated powders have tetragonal zirconia phase which do not include any stabilizer. Also coatings have similar phases to powders have, but differently the coating include gamma ( ) alumina phase. In general, sprayed alumina coatings consist of not only the expected stable form, which is the most desirable phase because of its relatively high corrosion and chemical resistance, hardness, but also the metastable , , and phase forms of Al2O3. Plasma spraying may lead to rapid solidification phenomena in the droplets following deposition at a surface, resulting in metastable crystalline phases and amorphous structures [8-11]. The intensity of alumina phase in coating produced from coprecipitated powders is very low than coating produced from commercial powders (see Figure 5).

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

182 IMMC 2016 | 18th International Metallurgy & Materials Congress

(a)

(b)

Figure 2. SEM micrographs of coatings produced from a) coprecipitated and b) commercial powders

wt. % Mark

Al Zr Y O 1 - 64.208 11.585 24.207 2 57,870 - - 42.130

Figure 3. SEM micrographs and EDS point analysis of coatings (commercial powders)

wt. % Mark Al Zr O

1 33.426 22.101 44.473 2 37.747 17.267 44.986 3 33.087 22.359 44.554 4 34.882 20.304 44.814 5 36.385 17.598 46.017 6 38.358 15.100 46.542

Figure 4. SEM micrographs and EDS point analysis of coatings, (coprecipitated powders)

Figure 5. XRD analysis of powders and coatings a) coprecipitated and b) commercial

Substrate

Coating

Bond coat

Bond coat

Coating

Bond coat

a

b

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4. Conclusions In this study, Al2O3-20 wt.% ZrO2 coatings were produced by using coprecipitated nanocomposite powders and coarse commercial powders. The results obtained from this study are listed below: 1. Coprecipitated powders is formed from

agglomerated particles which are mixture of alumina and zirconia powders in nanoscale.

2. Coprecipitated and commercial powders have stable phases composed of -alumina, monoclinic and tetragonal zirconia.

3. Coatings have typical lamellar morphology and small porosity.

4. Each coating has -alumina, -alumina, monoclinic and tetragonal zirconia, respectively. But the amount of -alumina phase is very low in coating performed with coprecipitated powders comparing to commercial one.

5. Although, synthesized alumina-20 wt.% zirconia powders from salts reveal that tetragonal zirconia phase have been obtained without using any stabilizer for powders and coating.

5. References [1] I. Iordanova, V. Antonov, R. Mirchev, A. Schwenk, G. Nutsch, J. Electrochemistry and Plating Technology, 1 (2008) 25-38. [2] P.P. Bandyopadhyay, D. Chicot, B. Venkateshwarlu, V. Racherla, X. Decoopman, J. Lesage, Mechanics of Materials, 53 (2012) 61-71 [3] J. Chandradass, J. Hong Yoon, D. Bae, Materials Science and Engineering, A 473 (2008) 360-364. [4] M. H. Maneshiana, M. K. Banerjee, Journal of Alloys and Compounds, 493 (2010) 613-618 X. Zhao, Y. An, J. Chen, H. Zhou, B. Yin, Wear, 265 (2008) 1642-1648 [5] B. Liang, H. Liao, C. Ding, C. Coddet, Thin Solid Films, 484 (2005) 225-231 [6] B. Liang, G. Zhang, H. Liao, C. Coddet , C. Ding, Surface & Coatings Technology, 203 (2009) 3235-3242 [7] G. Shanmugavelayutham, S. Yano, A. Kobayashi, Vacuum, 80 (2006) 1336-1340 [8] G Shanmugavelayutham, A. Kobayashi, Materials Chemistry and Physics, 103 (2007) 283-289 [9] S. Jiansirisomboona, K.J.D. MacKenzieb, S.G. Robertsa, P.S. Granta, J. European Ceramic Society, 23 (2003) 961-976

[10] M. Uma Devi, Ceramics International, 30 (2004) 555-565