Effects of aluminum phosphate sealant containing al2o3 nanoparticles on the hardness and wear resistance of the al2o3-Tio2 plasma sprayed coating

Vietnam Journal of Science and Technology 55 (6) (2017) 698-705 DOI: 10.15625/2525-2518/55/6/9026 1 EFFECTS OF ALUMINUM PHOSPHATE SEALANT CONTAINING Al2O3 NANOPARTICLES ON THE HARDNESS AND WEAR RESISTANCE OF THE Al2O3-TiO2 PLASMA SPRAYED COATING Pham Thi Ha1, *, Pham Thi Ly1, Nguyen Van Tuan1, Vo An Quan1, Le Thu Quy2 1Institute for Tropical Technology, VAST, 18, Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam 2National Key Laboratory for Welding and Surface Treatment Technologies, NARIME 4 Pham Van Dong, Cau Giay, Ha Noi, Viet Nam *Email: hapham205@gmail.com Received: 16 December 2016 ; Accepted for publication: 8 October 2017 Abstract. In the present study, Al2O3-40 wt.% TiO2 composite coatings were fabricated on medium carbon steel substrate by plasma spraying technique. The coatings were sealed with aluminum phosphate solution containing 5 wt.% Al2O3 nanoparticles and then heat treated at 400 oC. The phase composition, structure morphology, microhardness and wear resistance of the coating were studied. The study results phase composition of the coatings showed that the coatings were composed γ-Al2O3 and Al2TiO5 phase. The compounds AlPO4 and Al(PO3)3 were found in the coating sealed with aluminum phosphate. The coatings sealed with aluminum phosphate containing Al2O3 nanoparticles have lower porosity density, higher hardness and higher wear resistance than the coating sealed with aluminum phosphate uncontain Al2O3 nanoparticles and the unsealed coating. Keywords: Al2O3-TiO2 coating, aluminum phosphate, plasma spraying, Al2O3 nanoparticles. 1. INTRODUCTION Plasma spraying is a versatile technology to produce protective coatings for facilities in extensive application. Many studies indicate that Al2O3-TiO2 plasma sprayed coatings have good corrosion, wear and erosion resistance [1 - 11]. However, owing to their lamellar structure and rapid solidification, plasma sprayed coating usually have plenty of cracks and voids. These structural defects provide paths for corrosive species to corrode the substrate, leading to a deleterious effect when these coatings have to perform in an aggressive environment (e.g., seawater). Thus, post-treatments of the as-sprayed coatings are necessary to reduce porosity and enhance the corrosion resistance of the coatings [12]. Laser remelting and high temperature treatments have been applied, but the high thermal inputs may induce microstructural changes of the substrate material, in some cases even lead to cracking of the coatings due to the large residual stress [12]. On the other hand, these methods are very expensive and need very sophisticated equipment. In consideration of the economic and Effects of aluminum phosphate sealant containing Al2O3 nanoparticles on the hardness and . 699 technical benefits, sealants are widely used, in there, solutions aluminum phosphate was selected and used in many studies. It has been reported that aluminum phosphate sealant could pores decrease obviously and improve wear resistance of the plasma sprayed coatings [12 - 14]. The results of previous studies by the authors have shown that the aluminum phosphate has contributed to reduce the porosity and enhanced protection against corrosion and abrasion of the thermal sprayed coating [15 - 21]. With the desire to improve the properties of Al2O3-TiO2 coating particularly wear resistant, in this study, we will fabricate Al2O3-TiO2 coating by plasma spraying technology, then the coating is impregnated with aluminum phosphate containing Al2O3 nanoparticles and heat treated at 400 oC. Some properties of this coating will be studied and discussed. 2. EXPERIMENTAL 2.1. Materials The main chemical used in the study include NiCr powder size of 15 ÷ 45 µm (China), Al2O3-40 wt.% TiO2 powder size of 15 ÷ 45 µm (China), α-Al2O3 powder size approximately 150 nm (USA), 85 % phosphoric acid H3PO4 (Germany), aluminum hydroxide Al(OH)3 powder (Germany), medium carbon steel substrate of composition 98,36 % Fe - 0,13 % C - 0,24 % Si - 1,15 % Mn - 0,12 % (S, P, Ni, ) by weight. 2.2. Preparation of samples The coatings were prepared by plasma spraying equipment Tafa 3710-PRAXAIR (US) at National Key Laboratory for Welding and Surface Treatment Technologies (NARIME), with the optimized parameters as shown in Table 1. A bond coating of NiCr (thickness about 120 µm) was deposited between the steel substrated and the top Al2O3-40 wt.% TiO2 composite coating (thickness about 300 µm). Table 1. Plasma-spraying operating parameters. Parameters Bond coating NiCr Al2O3-40 wt.% TiO2 coating Arc current (A) 380 550 Arc voltage (V) 24 24 Flow rate of primary plasma gas Ar (ml/m) 50 60 Flow rate of secondary plasma gas H2 (ml/m) 8 8 Spraying distance (mm) 100 100 Powder feed rate (g/m) 30 50 Aluminum phosphate solution was prepared from phosphoric acid (85 % H3PO4) and aluminum hydroxide Al(OH)3 powder. The molar ratio P:Al was 3:1. The solution was mixed and slightly heated on a magnetic stirrer until it became clear. Then, 5 wt.% Al2O3 nanoparticles were added to the solution and constantly stirred in 24 hours to get a well dispersed sealant. Aluminum phosphate solution was stood statically at room temperature for 12 hours and then was slowly scanned on Al2O3-40 wt.% TiO2 coating surface. The sealed coating was stood Pham Thi Ha, Pham Thi Ly, Nguyen Van Tuan, Vo An Quan, Le Thu Quy 700 at room temperature for 12 hours before heat treatment. The coatings were heat treated according to the process given in Fig. 1. Samples denotions are presented in Table 2. Figure 1. Heat treatment diagram. Table 2. Samples denotion. No. Type of sample Denote 1 The coating unsealed with aluminum phosphate AT 2 The coating unsealed with aluminum phosphate, heat treated AT- T 3 The coating sealed with aluminum phosphate – 0 wt.% Al2O3 nanoparticles, heat treated AT-P0 4 The coating sealed with aluminum phosphate – 5 wt.% Al2O3 nanoparticles, heat treated AT-P5 2.3. Research Methods The microstructure was characterized using a scanning electron microscope SM-6510LV (Japan). The microhardness was measured from their cross-sections using AVK-C0/Mitutoyo equipment to TCVN 258-1:2007 standard. The phase composition was done using X-ray diffraction (XRD, X-RAY D5005/ SIEMENS, Gemany) with Cu-Kα radiation and the scan step for 2θ at 0.02o/s. The wear resistance of the coatings were determined by using a pin-on-disk UMT – 3MT - CETR (US) to ASTM G99:2010 standard. The hardness of Cr pin is 63 HRC. The test was conducted under following conditions: applied constant load of 10 N, radius from the center of the sample to pin of 6 mm, rotational speed of 60 rpm, duration of 1000s. 3. RESULTS AND DISCUSSION 3.1. The phase composition of the coating Figure 2 shows the X-ray diffraction patterns of Al2O3-40 %TiO2 powder and the Al2O3-40 wt.% TiO2 coating. The results show that, the phase composition of Al2O3-40 wt.% TiO2 powder before plasma spraying include α-Al2O3 and rutile-TiO2 phases. The XRD patterns of the coatings has the appearance of γ-Al2O3 and Al2TiO5 phases. Thus, the missing of α-Al2O3 phase could be because of during the plasma spraying, α-Al2O3 phase transforms into γ-Al2O3 phase. During plasma sprayed, the α-Al2O3 and rutile-TiO2 phases react together to form Al2TiO5 phases. AT-P0 and AT-P5 were composed of γ-Al2O3 and Al2TiO5 phases, along with compounds of AlPO4 and Al(PO3)3. However, the characteristic peaks of α-Al2O3 phase do not appear in XRD spectrum of AT-P5. This is explained by the content of α-Al2O3 nanoparticles in aluminum phosphate solution is too small. It is difficult to detect by XRD method (its content is below the detection limit of the XRD equipment). Effects of aluminum phosphate sealant containing Al2O3 nanoparticles on the hardness and . 701 (a) (b) Figure 2. XRD patterns of Al2O3-40 wt.% TiO2 powder (a) and Al2O3-40 wt.% TiO2 coating (b). 3.2. Microstructure and microhardness of the coating Cross-sectional structure of the Al2O3-40 wt.% TiO2 coating is shown in Figure 3. The thickness of the top Al2O3-40 wt.% TiO2 coating and the bond NiCr coating are about 300 µm and 120 µm, respectively. Figure 3. Cross-sectional of plasma sprayed Al2O3-40 wt.% TiO2 coating. Cross-sectional structure of the plasma sprayed coatings is given in Fig. 4. Cross-sectional surface image of AT-T and AT showed heat treatment to 400 oC without significantly affect to the structure of Al2O3-40 wt.% TiO2 coating. This two coatings were still high porosity and some micro cracks. Meanwhile, the sealed coatings (AT-P0 and AT-P5) have lower density of pores than the unsealed coatings (AT and AT-T). Adding 5 wt.% Al2O3 nanoparticles to aluminum phosphate solution was made the structure of this coating becomes more packed than the remaining coating. The reason is that, after heat treatment to 400 oC, the water in aluminum phosphate solution was evaporated whole, Al2O3 nanoparticles and aluminum phosphate binder into a solid mass fill the pores. Meanwhile, for AT-P0, because of lower specific weight, Pham Thi Ha, Pham Thi Ly, Nguyen Van Tuan, Vo An Quan, Le Thu Quy 702 aluminum phosphate was shrinkage after the water evaporates. This will creat more space in the pores, leading to effective reduction of porous cover. The microhardness of the coatings was measured from their cross-sections. The measured region were permeable by aluminum phosphate (for AT-P0 and AT-P5 ). Largest distance from the sample surface to the measured position is 30 µm. Results of the microhardness measurement are presented in Table 3. Figure 4. Cross-sectional of the plasma sprayed coatings. Table 3. Microhardness of the plasma sprayed coatings, scale HV0.2. Sample Position 1 Position 2 Position 3 Average AT 483,2 487,9 478,5 483,2 AT-T 477,8 503,3 483,2 488,1 AT-P0 635,9 644,3 632,4 637,5 AT-P5 713,0 718,6 703,3 711,6 The results showed that the microhardness of the coatings are range from 483.2 to 711.6 HV in ascending order AT, AT-T, AT-P0 and AT-P5. Thus, heat treatment at 400 oC and impregnation with aluminum phosphate were increased the microhardness of the coating. However, if only the heat treatment, the hardness of the coating was not significantly increased (about 1 %). Meanwhile, the hardness of the sealed with aluminum phosphate containing 5 wt.% Al2O3 nanoparticles and heat treated coating increases about 47 %. The presence of Al2O3 nanoparticles have also improved the hardness of the coating (increase about 11 %). This is the Pores AT AT-T AT-P0 AT-P5 100 µm Effects of aluminum phosphate sealant containing Al2O3 nanoparticles on the hardness and . 703 result of reduce the porosity of the coating. Moreover, α-Al2O3 nanoparticles very hard and durable also contributed to increase the hardness of the coating. 3.3. The wear resistance of the coating The wear depth is used to evaluate the wear resistant ability of the coatings. The test results of wear resistance are given in Table 4. Table 4. The wear depth of the coatings. The coatings The wear depth, µm AT 158,8 AT-T 127,8 AT- P0 64,9 AT- P5 53,2 The results in Table 4 showed that, AT-P5 had the lowest wear depth corresponding to the highest wear resistance. It was suitable for the results of hardness and porosity above. The wear resistance ability of AT-P5 is about 18 % and 58 % higher than AT-P0 and AT-T, respectively. Thus, the presence of 5 % Al2O3 nanoparticles by weight in aluminum phosphate sealant improved coating’s wear resistant ability. The reason is after sealing with aluminum phosphate containing nanoparticles and heat treatment the coating becomes harder and denser. 4. CONCLUSION Al2O3-40 wt.% TiO2 coatings had the appearance of γ-Al2O3 and Al2TiO5 phases. In the plasma spray process, one part of α-Al2O3 phase changed to γ-Al2O3 phase. The α-Al2O3 phase interacted with rutile-TiO2 phase to form Al2TiO5 compound. The sealed coatings with aluminum phosphate also appeared aluminum phosphate compounds of AlPO4 and Al(PO3)3. After sealing with aluminum phosphate and heat treatment, the porosity of the coating decreased. The sealed coating with aluminum phosphate containing Al2O3 nanoparticles has the lowest porosity. The hardness of heat treated and sealed coating with aluminum phosphate containing Al2O3 nanoparticles increased about 47 %. 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