Trans. Nonferrous Met. Soc. China 23(2013) 2951-2956

Phase evolution of plasma sprayed Al₂O₃-13%TiO₂ coatings derived from nanocrystalline powders

Xue-cheng LU^1,2, Dian-ran YAN¹, Yong YANG¹, Yan-chun DONG¹, Ji-ning HE¹, Jian-xin ZHANG¹

1. College of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China;

2. Military Logistics Department, Military Transportation University, Tianjin 300161, China

Received 24 November 2012; accepted 10 June 2013

Abstract: Commercial nanosized alumina and titania particles were selected as raw materials to prepare the blended slurry with composition of Al₂O₃-13%TiO₂ (mass fraction), which were reconstituted into micrometer-sized granules by spray drying, subsequently sintering at different temperatures to form nanostructured feedstock for thermal spraying, and then Al₂O₃-13%TiO₂ nanocoatings were deposited by plasma spraying. The evolution of morphology, microstructure, and phase transformation of the agglomerated powder and as-sprayed coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that Al₂O₃ retains the same α phase as the raw material during sintering, while TiO₂ changes from anatase to rutile. During plasma spraying, some α-Al₂O₃ phases solidify to form metastable γ-Al₂O₃, and the volume fraction of α-Al₂O₃ decreases as CPSP increases. However, peaks of the TiO₂ phase are not observed from the as-sprayed coatings except for the coatings sprayed at the lower CPSP. As the CPSP increases, nanostructured TiO₂ is dissolved easily in γ-Al₂O₃ or χ-Al₂O₃·TiO₂ phase. After heat treatment, γ-Al₂O₃ in the coatings transforms to α-Al₂O₃, and rutile is precipitated.

Key words: Al₂O₃-13%TiO₂; nanocrystalline powder; nanocoatings; phase evolution

1 Introduction

In the field of thermal spray, the atmospheric plasma spraying (APS), as a well-established technology, has been widely used to produce various coatings, such as wear, corrosion resistant, hydroxyapatite, and thermal barrier coatings [1]. Plasma spraying ceramic coating technique on metal surface is a promising means to combine advantages of the metal substrate and the ceramic coating, which can simultaneously meet the mechanical properties and environmental service demands, and it has been rapidly developed [2]. However, the application of conventional plasma sprayed ceramic coatings is limited due to their brittleness and poor machining property. The unique properties of nanostructured materials have been reported for both bulk materials and coatings [3]. Plasma sprayed nanostructured ceramic coatings, by taking advantage of properties associated with nanostructures, can improve the performance and durability of conventional plasma sprayed coatings that already have a wide variety of applications in the aerospace, biomedical, automobile and chemical industries [4-7]. For example, nanostructured Al₂O₃-TiO₂ ceramic coatings show much higher wear resistance than the conventional Al₂O₃-TiO₂ coatings [6-8].

The plasma prayed Al₂O₃-TiO₂ nano coatings have been studied on processing, microstructure, interfacial mechanical properties and wear behavior [2-4,6-12]. However, there are few systematic works on the phase evolution of agglomerated Al₂O₃-13%TiO₂ nano- crystalline powder and as-sprayed coatings. In the present study, nanostructured Al₂O₃-13%TiO₂ coatings were prepared by atmosphere plasma spraying nanocrystalline powders, and the evolution of the morphology, microstructure, and phase transformation of the agglomerated powder and as-sprayed coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

2 Experimental

Commercial nanosized particles of α-Al₂O₃ with the mean diameter of 50 nm and anatase TiO₂ with the mean diameter of 10 nm were used as the starting powders, and the performance parameters of them are shown in Table 1. It is well known that individual nanoparticles cannot be plasma sprayed because of their low mass and inability to be carried in a moving gas stream and deposition on a substrate [13]. So the most important thing using nanoparticles as raw material to generate the nanostructured coatings is to reconstitute individual nanoparticles into sprayable agglomerates, which are large enough for plasma spray deposition. The process of reconstitution consisted of spray drying the slurry that contains nanoparticles in the mass ratio of Al₂O₃- 13%TiO₂, and sintering at high temperature (700-1200 °C).

Table 1 Performance parameters of nanopowders

Plasma spraying was performed on a LP-50B plasma-spraying machine made in China. The critical plasma spray parameter (CPSP) is determined by the plasma output power in the numerator and argon gas flow rate in the denominator. Other processing variables, such as carrier gas flow rate, spray distance, flow rate ratio of argon to H₂, powder feed rate, gun speed, were held constant in this study. Under these controlled processing conditions, CPSP can be directly related to the temperature of plasma and/or particles.

Mild steel substrate samples (Fe-0.45C-0.3Si- 0.75Mn-0.03P-0.035S, mass fraction, %) with dimensions of 9 mm × 8 mm × 10 mm were grit-blasted and then coated by plasma spraying Ni/Al powders to form a bonding layer about 100 μm in thickness. The nanostructured Al₂O₃-13%TiO₂ coating was deposited by plasma spray using the agglomerated powders and the coating thickness was about 300 μm.

The phase compositions of the agglomerated powder and as-sprayed coatings were examined using PHILIPS X-PertMPD X-ray diffraction (XRD) with Cu K_α radiation. The microstructures were observed on a Philips XL30 scanning electron microscope (SEM).

3 Results and discussion

3.1 Microstructure of nanostructural agglomerated powders

The morphology and line scanning results of the agglomerated Al₂O₃-13%TiO₂ nanocrystalline powders before sintering are shown in Fig. 1. It can be seen from Fig. 1(a) that the agglomerated powders are well spherical and their size ranges from 20 mm to 70 mm, which improves powders feeding behavior during plasma spraying process. Figure 1(b) shows the surface morphology of an agglomerated powder that consists of many small nano-particles combined by the organic binder of PVA. Figure 1(c) shows that the nanosized TiO₂ particles are distributed homogeneously within Al₂O₃ particles.

Fig. 1 SEM image (a), high magnification image of surface morphology (b) and line scanning result (c) of agglomerated Al₂O₃-13%TiO₂ powders

SEM images of agglomerated Al₂O₃-13%TiO₂ powder sintered at different temperature are shown in Fig. 2. It demonstrates that the nano-particles grow into 100-300 nm in size, and the nano-particles connect together, indicating that sintering of the nano-particles occurs after the organic binder of PVA is burnt. As the sintering temperature increases, the nano-particles grow from nano-size to sub-micron or micron size, and more obvious necking exists between nano-particles due to sintering effect, which improves the strength of connection between nanoparticles, necessary for plasma spraying.

Fig. 2 SEM images of agglomerated Al₂O₃-13%TiO₂ nanocrystalline powder sintered at temperature of 700 °C (a), 900 °C (b) and 1100 °C (c)

3.2 Phase evolution of nanostructural agglomerated powders during sintering

Figure 3 presents the XRD patterns of agglomerated Al₂O₃-13%TiO₂ nanocrystalline powders before and after sintering. It can be seen that Al₂O₃ retains the same α phase as the raw material after sintering, while TiO₂ changes from anatase to rutile due to an irreversible phase transition occurring at 610 °C [6]. And the reaction does not occur between alumina and titania to generate a new phase.

Fig. 3 XRD patterns for agglomerated Al₂O₃-13%TiO₂ nanocrystalline powders before and after sintering

3.3 Phase evolution during plasma spraying

To further investigate the phase evolution of agglomerated Al₂O₃-13%TiO₂ nanocrystalline powders during plasma spraying, various spraying parameters (CPSP) were taken. Figure 4 shows the XRD patterns of the nanostructured Al₂O₃-13%TiO₂ coatings sprayed at different CPSP. It can be seen that during plasma processing, the stable α-Al₂O₃ phase rapidly solidifies to form metastable γ-Al₂O₃, since γ-Al₂O₃ forms more easily from the melt than α-Al₂O₃ at a high cooling rate because of the low interfacial energy between crystal and liquid [14]. Moreover, peaks of α-Al₂O₃ and γ-Al₂O₃ are found in XRD patterns from all the as-sprayed coatings. Melting and rapid solidification is the only processing route available for the formation of γ-Al₂O₃ [10], so the peak of α-Al₂O₃ is certainly due to the presence of unmelted or partially melted alumina. However, peaks of the TiO₂ phase are not observed from the as-sprayed coatings except for the coatings sprayed at lower CPSP (i.e. 312) and only slight TiO₂ peaks exist in the crystal planes (110), (101), (211). As mentioned in Ref. [7], the solubility of TiO₂ in theα-Al₂O₃ is negligible, Ti ions are likely to be in the γ-Al₂O₃ lattice as either an interstitial or substitutional defect. The missing of Ti-containing phase peaks can be attributed to the fact that as the CPSP increases (along with particle/torch temperature), nanostructured TiO₂ is easier to dissolve in γ-Al₂O₃ or χ-Al₂O₃·TiO₂ phase presented in the nanocoatings, which is different from the investigation result of the conventional coatings reported [4,9].

Fig. 4 XRD patterns of nanostructured Al₂O₃-13%TiO₂ coatings sprayed with different CPSP

It is also seen from Fig. 4 that the phase component of the coatings is sensitive to the CPSP of the plasma spray gun during production. As the CPSP increases, the intensity of α-Al₂O₃ (113) peak, which is the main peak of α-Al₂O₃, decreases, while the intensity of γ-Al₂O₃ (400) peak increases. According to the results of XRD diffraction, the relative contents of α-Al₂O₃ and γ-Al₂O₃ in nanostructured Al₂O₃-13%TiO₂ coating can be quantitatively calculated by direct K value method [7], as shown in Fig. 5. It is clear that as CPSP increases, the volume fraction of α-Al₂O₃ decreases, conversely, the volume fraction of γ-Al₂O₃ increases. Based on the above analysis, the phase transformation of the coatings can be controlled by adjusting critical plasma spray parameter (CPSP), meanwhile the microstructure can be changed, ultimately the purpose to improve the properties of the coatings will be attained.

3.4 Microstructure transformation of as-sprayed coatings

Because the heat input applied to nanopowders increases with increasing CPSP, which is accompanied with higher probability for spray nanopowders to be melted inside the plasma flame, the morphology and microstructure transformation of the coatings can reveal the melting state during plasma spraying at different CPSP, which accords with the phase evolution rule.

Fig. 5 Relative contents of α-Al₂O₃ and γ-Al₂O₃ in nanostructured Al₂O₃-13%TiO₂ coating as function of CPSP

Figure 6 presents SEM images of the cross-section of the nanostructured Al₂O₃-13%TiO₂ coatings sprayed at different CPSP. In all the coatings, some pores are observed because of the technology characteristics of plasma spraying. And the nanostructured coatings all exhibit a particular bimodal microstructure feature, consisting of unmelted or partially melted regions (bright-gray colored, marked with PM in Fig. 6) and fully melted regions (dark-gray colored, marked with FM in Fig. 6). During the process of plasma spraying, a large amount of nanoparticles are fully melted because of high temperature of flames (more than 10⁴ K). These melting droplets struck the surface of the substrates or the as-sprayed coatings to form micron-sized lamellar structure under the action of high speed flames. Simultaneously, high speed flames are too fast to melt a part of nanoparticles completely, hence forming the partially melted regions consisting of nanoparticles. Therefore, the as-sprayed nanostructured ceramic composite coatings are composed of lamellar structure and partially unmelted nanoparticles, which can be clearly and detailedly seen from Fig. 7. It is believed that the unique bimodal microstructure contributes to the significantly enhanced mechanical properties of the nanostructured coatings.

Fig. 6 Cross sectional SEM images of nanostructured Al₂O₃-13%TiO₂ coatings sprayed at different CPSP

Fig. 7 High magnification SEM images of top surface (a) and fracture surface (b) of nanostructured Al₂O₃-13%TiO₂ coating sprayed at CPSP=437

It is obvious that increasing the CPSP makes more powders fully molten; hence reducing the proportion of the unmelted or partially melted regions in the nanostructured coatings, reducing the proportion and size of the pore, and the coating becomes denser and more uniform at the same time. It can be concluded that the plasma spray parameter (CPSP) plays an essential role in adjusting and controlling the microstructure of the coatings.

3.5 Phase evolution of as-sprayed coatings with post heat treatment

It was reported that γ-Al₂O₃ in as-sprayed Al₂O₃- 13%TiO₂ coatings is a metastable phase, and it can easily transform to α-Al₂O₃ in a certain environment or heat treatment process [13]. It can be seen from Fig. 8 that, when the Al₂O₃-13%TiO₂ nano coatings were heat treated at different temperatures, the diffraction peak of α-Al₂O₃ stable phase were significantly enhanced, while the diffraction peaks of γ-Al₂O₃ metastable phase were relatively weakened, and the diffraction peaks of rutile phase appeared, indicating that certain amount of γ-Al₂O₃ transformed to α-Al₂O₃ and TiO₂ phase precipitated during heat treatment. Because of stresses associated with the volume change, the phase transformation will deteriorate thermal shock resistance of the coatings. The higher the heating temperature is, the larger the diffraction peak intensity ratio of α-Al₂O₃ (113) to γ-Al₂O₃ (400) () is, and the more the γ-Al₂O₃ transforms to α-Al₂O₃. Then the greater stress will be induced, so that the coating will show a worse performance of thermal shock [15].

Fig. 8 XRD patterns of Al₂O₃-13%TiO₂ coating (a) and heat treated coatings at temperatures of 900 °C (b) and 1000 °C (c)

4 Conclusions

1) Agglomerated Al₂O₃-13%TiO₂ nanocrystalline powders were prepared by spray drying, and Al₂O₃ retained the same α phase as the raw material during sintering, while TiO₂ changed from anatase to rutile during sintering. The reaction did not occur between alumina and titania to generate a new phase.

2) When the nanostructured Al₂O₃-13%TiO₂ feedstocks were plasma sprayed on the steel, some α-Al₂O₃ solidified to form metastable γ-Al₂O₃, and the volume fraction of α-Al₂O₃ decreased as CPSP increased. However, the peaks of TiO₂ phase were not observed from the as-sprayed coatings except for the coatings sprayed at the lower CPSP. As the CPSP increased, nanostructured TiO₂ was be dissolved easily in γ-Al₂O₃ or χ-Al₂O₃·TiO₂ phase presented in the nanocoatings. The microstructure can be changed because the phase transformation of the coating can be controlled by adjusting critical plasma spray parameter (CPSP).

3) After heat treatment, γ-Al₂O₃ in the coatings transformed to α-Al₂O₃, and rutile will be precipitated.

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纳米Al₂O₃-13%TiO₂粉末及其等离子喷涂涂层的相变

路学成^1,2，阎殿然¹，杨 勇¹，董艳春¹，何继宁¹，张建新¹

1. 河北工业大学 材料科学与工程学院，天津 300130；

2. 军事交通学院 军事物流系，天津 300161

摘  要：将商用纳米Al₂O₃与TiO₂粉末混合制浆，通过喷雾造粒技术重构成大颗粒纳米Al₂O₃-13%TiO₂团聚粉体，然后采用不同加热温度进行烧结，制备用于热喷涂的纳米喂料。采用等离子喷涂技术沉积形成纳米涂层。利用XRD和SEM对纳米粉末和涂层的形貌、显微结构和晶相进行表征。结果表明，在烧结过程中纳米氧化铝没有发生相变，而纳米氧化钛则由锐钛矿相转变为金红石相；在等离子喷涂过程中，部分α-Al₂O₃发生熔化转变为γ-Al₂O₃，且转变量与喷涂工艺参数有关；而TiO₂相只在较低喷涂工艺参数下存在衍射峰，在较高喷涂工艺参数下则以χ-Al₂O₃·TiO₂固溶体存在；涂层在热处理过程中会发生γ-Al₂O₃向α-Al₂O₃的转变，且有TiO₂相析出。

关键词：Al₂O₃-13%TiO₂；纳米粉末；纳米涂层；相变

(Edited by Hua YANG)

Foundation item: Projects (51072045, 51102074) supported by the National Natural Science Foundation of China

Corresponding author: Dian-ran YAN; Tel: +86-22-26564581; E-mail: yandianran@126.com

DOI: 10.1016/S1003-6326(13)62819-8