The X-ray diffraction(XRD) pattern of the synthesized LZA (LaZnAl11O19) powders is shown in Fig.1a. It can be observed that theacquired XRD pattern of the as-synthesized LZA powders matches the standard Joint Committee on Powder Diffraction Standards (JCPDS) card (JCPDF#88-2135) well, indicating pure LZA powders were obtained by the solid-state reaction at 1600℃ for 12h.
According to the XRD results, the lattice parameters of LZA are calculated to be a=b=5.588 Å,c=22.01Å,β=120°. The crystallographic density was determined to be 4.26g cm-3,nearly 29% lower than that of YSZ(6.0g cm-3). LZA exhibits a hexagonal crystal structure with the space group of P63/mmc,which can be regarded as a derivative of magnetoplumbite-type oxide whose general formula is A2+B123+O19.
Fig.1 Characterization of LaZnAl11O19 powders:(a) XRD pattern;(b) morphology of the as-synthesized LaZnAl11O19 powders;(c) morphologies of powders after spray drying.
As illustrated in Fig.2, the large La3+ion is coordinated by twelve O2- ions and presents as a mirror plane, while the Zn2+ occupies the tetrahedral sites in the unit cell which is energetically favorable as in ZnAl2O4. The two spinel blocks are separated by two intermediate mirror planes containing a large La3+ion, and then grain growth along the c-axis is strongly suppressed. As shown in Fig.1b, typical platelet-like hexagonal grains can be detected in the SEM micrograph of LZA powders. After spray drying, the morphology of free flowing LZA powders presented in Fig.1c is near-spherical with a hollow structure, which ensures good flow characteristic for plasma spraying and deposition efficiency. The magnified morphology of LZA powders is presented in Fig.1c as well.Numerous LZA grains in anomalous shapes agglomerate through the adhesion agent, and those irregular shapes of LZA particles are caused by the crushing process during ball-milling.
Fig.2 Crystalline structure of the magnetoplumbite-type LaZnAl11O19.
Detailed microstructures of the as-sprayed LZA coating are presented in Fig.3. The surface microstructure of the as-sprayed LZA coating is shown Fig.3a. LZA has a rough surface with well-molten, semi-molten "splats"and un-molten par-ticles. As shown in Fig.3b, layered cleavage characteristics canbe observed from the fractured cross-section micrograph of the LZA coating, which is very common for hexaaluminates with a magnetoplumbite-type structure and impart this coating excel-lent fracture toughness.37 It should be noted that the LZA coating does not exhibit a lamellar structure, which is commonly found in plasma sprayed YSZ coating. The cross-section micrograph shown in Fig.3c indicates that the LZA powder has a good melting condition and the as-sprayed coating is homogeneous apart from a few voids and pores. The porosity of the LZA coating was detected by image analysis. The cumulative porosity of the as-sprayed LZA coating is found to be about 9.8%, which is relatively lower than the porosity level of the traditional YSZ coating(14.1%).
Fig.3 Characterization of the as-sprayed LaZnAluO1g coating:(a)-(b) SEM images of the surface and fractured cross-section of the as-sprayed coating;(c) microstructure of the polished cross-section, and the associated binary image;(d) XRD pattern of the as-sprayed LaZnAl11O19coating;(e) TG-DSC curves of the as-deposited LaZnAl1O19 and LaMgAl11O19 coatings.
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