Materials Performance

JUN 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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37 NACE INTERNATIONAL: VOL. 55, NO. 6 MATERIALS PERFORMANCE JUNE 2016 sive and sliding wear resistance, spallation resistance, and indentation crack resis- tance obtained by controlled deposition. 10 Al 2 O 3 /TiO 2 coatings obtained by APS have been extensively investigated. 11-12 The effect of TiO 2 content on the properties of Al 2 O 3 / TiO 2 nanostructured ceramic coatings pre- pared by the APS technique, however, has been largely overlooked. Experimental Procedures Materials The substrate material used in this study was AISI H13 (UNS T20813) steel. Rectangular specimens, 60 by 40 by 5 mm, were cut from a H13 steel sheet. All of the specimens were first ground with alumina waterproof abrasive paper up to 1200 grit, thoroughly immersed in carbon tetrachlo- ride (CCl 4 ) for degreasing, then cleaned in distilled water and dried in ambient air. Coating Deposition The nanostructured Al 2 O 3 (α-Al 2 O 3 ) and TiO 2 (rutile-TiO 2 ) powders were acquired from Shanghai Zhuerna High Technology Powder Materials Co., Ltd. The average di- ameters for Al 2 O 3 and TiO 2 particles were 60 and 10 nm, respectively. First, the nano- structured powders were dispersed by ul- trasonic vibration to avoid agglomeration; then they were mixed with a polyvinyl alco- hol aqueous solution to produce powder mixtures with the following compositions: Al 2 O 3 -3 wt% TiO 2 (AT3), Al 2 O 3 -13 wt% TiO 2 (AT13), Al 2 O 3 -20 w t% TiO 2 (AT20), and Al 2 O 3 -40 w t% TiO 2 (AT40). Finally, th e mixed powders were reconstituted to form agglomerates that were large enough (40 to 60 μm) to spray with APS. The reconstitu- tion process consists of spray-drying the slurry that contains Al 2 O 3 and TiO 2 parti- cles, followed by heat treating at a high temperature (1,123 °K) for 1 h. High-veloc- ity oxygen fuel (HVOF) spraying was used to spray a Ni/Al bond coat onto the plate- shaped substrate. APS was carried out with a DH-1080 † plasma-spraying machine. The AT3, AT13, AT20, and AT40 agglomerates † Trade name. FIGURE 1 XRD patterns of AT13 powder and its nanostructured ceramic coating at different levels of spraying power. were sprayed in turn onto the substrate samples. The velocity of the spray gun was 7 m/min. Characterization Phase compositions of the coatings were analyzed by ARL X'TRA † x-ray diffrac- tion (XRD) using Cu Kα radiation between 2θ values of 10 and 80 degrees with a step length of 0.02 degree at a scanning rate of 1 degree/min. The x-ray generator settings were 45 kV and 40 mA, respectively. Oxidation tests were performed to in- vestigate the oxidation resistance of the coatings. Specimens were heated in an air temperature of 1,323 °K in a muffle furnace. After oxidizing for several hours, the speci- mens were cooled to room temperature and weighed with an electrical balance with a sensitivity of 0.01 mg. Then the spec- imens were put into the furnace again for the next oxidizing period. The cumulative weight changes of the coated specimens were calculated and reported as a function of oxidation time. Thermal shock tests were conducted in a programmable furnace. First, the air in the furnace was heated to 1,373 °K, and the specimens were inserted into the furnace for 10 min. Then, the specimens were re- moved and water-quenched at tempera- tures maintained between 293 and 303 °K. The lifetime of the coating was defined as the number of thermal shock cycles re- quired to produce a spallation region of ~5% of the coating surface area. A group of three parallel samples was used for each test. Results and Discussion X-Ray Diffraction Analysis Figure 1 shows the XRD patterns of AT13 nanostructured ceramic coatings ap- plied at different levels of spraying power. Some α-Al 2 O 3 transformed into γ-Al 2 O 3 , while rutile-TiO 2 present in the coatings in- creased som e with increased spraying power. The intensity of the dif fraction peaks corresponding to γ-Al 2 O 3 slightly in- creased and the α-Al 2 O 3 marginally de- creased with increased spraying power. This appears to be related to the transfor- mation of α-Al 2 O 3 to γ-Al 2 O 3 at high tem- peratures. In addition, AlNi 3 and AlN were also present in the coatings. Oxidation Resistance Figure 2 shows the isothermal oxida- tion curves of Al 2 O 3 /TiO 2 nanostructured ceramic coatings at dif ferent level s of spraying power. Generally, the weight gain increased rapidly in the first 20 h and then slowly increased with increasing oxidation

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