PSI - Issue 10

I. Georgiopoulos et al. / Procedia Structural Integrity 10 (2018) 280–287 I. Georgiopoulos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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aqueous solution of 0.407 M in La(NO 3 ) 3 .6H2O and 0.407 M in Al(NO 3 ) 3 .6H 2 O was formed (Solution A). In solution A citric acid was added to a molar ratio of La:Al:Citric acid = 1:1:4.5 (Solution B). Solution B was used as a feedstock material in Solution Precursor Plasma Spraying. Also development and characterization of stable YSZ water-based suspension inks suitable to be used in SPS using 7 wt% yttria stabilized zirconia fine powder (7YSZ) has been performed. The 7YSZ particles were dispersed in the water using 1.5 %wt dispersant. Suspension with 15%wt solids after several hours ball milling and pH adjustment was produced fulfilling necessary requirements such as long-term stability, re-dispersability, low viscosity (6-14 cP) and small particle size (D50 = 0.31 μm) as shown in Fig. 1.

Fig. 1. Particle size distribution of the 7YSZ stable suspensions after ball milling using different dispersants after stabilization.

2.2. TBC systems development

A Praxair SG-100 atmospheric plasma spraying gun was used for the different layers deposition. Coatings were deposited on rectangular sand blasted nimonic 901 substrates (20x25x2 mm 3 , 20x38x2 mm 3 ). During spraying, the substrates were adequately cooled. If no cooling is used the substrate temperature will increase and subsequent thermal stresses will be introduced. The substrate temperature, measured using infrared pyrometers, was kept lower than 100°C. The solutions/suspensions were fed to the plasma gun through an atomizer. 7YSZ coatings using SPS were developed using plasma power in the range of 27-33 kW while the atomization gas flow rate ranged from 15 to 20 slpm. A spraying distance of 7.5 cm was used during deposition. Deposition of LaAlO 3 perovskite-type oxide has been performed using the SPPS technique. Ar/He mixtures as plasma gases with plasma power ranging from 50 kW to 57 kW, solution feed rates of 4.50-21.8 ml/min and atomization gas flow rates 12-19 slpm were tested. In Table 1 the different TBC systems compared in this study are shown.

Table 1. Overview of the TBC systems prepared and compared. TBC System Substrate Bond Coat

Top Coat

Overlayer

YSZ( SPS ) YSZ( SPS )

1 2

Nimonic Nimonic

NiCrAlY(APS) NiCrAlY(APS)

-

LA( SPPS )

2.3. TBC systems characterization

Phase composition of powders and deposited coatings was evaluated using a Siemens D500 X-Ray Diffractometer (Cu, Ka). Microstructural analysis of the developed coatings was performed using a Jeol 6300 Scanning Electron