The key process parameters influencing formation of columnar microstructure in suspension plasma sprayed zirconia coatings

The paper discusses the screening of experimental variables leading to formation of a columnar microstructure in suspension plasma sprayed zirconia coatings. These variables tested in 12 experimental runs included: (i) 2 types of zirconia powder; (ii) 4 concentration of solids in suspensions; (iii) 4 substrate preparation methods; and (iv) 2 plasma spray setups. Two different, commercially available, powders were used to formulate the suspensions. Yttria and ceria stabilized zirconia of composition ZrO2 + 24 wt.% CeO2 + 2.5 wt.% Y2O3 (YCSZ) was milled the decrease the particles sizes. The yttria stabilized zirconia of composition ZrO2 + 14 wt.% Y2O3 (14YSZ) was used as received. The coatings were deposited on 304L stainless steel substrates which had the surface prepared by: (i) grid blasting; (ii) grinding; (iii) turning; and (iv) laser treatment. The 3D topographies of substrates' surfaces were characterized and their roughnesses were measured. The suspensions were plasma sprayed using the following plasma torches: SG-100 of Praixair and Triplex of Sulzer-Metco. The microstructure of powders and coatings was analyzed by optical microscopy, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) as well as by X-ray diffraction. The columnar microstructure was formed in coatings sprayed with both plasma setups sprayed using finer 14YSZ powder suspensions. The substrate surface preparation as well as low concentration of solids in suspension promoted their formation. Rietveld method was applied to determine the quantity of different phases in the structure of coatings and to calculate the lattice parameters. The YCSZ coatings crystallized in mainly tetragonal phase with a small content of monoclinic phase. The 14YSZ crystallized in cubic phase. Finally, the thermal diffusivity of coatings was characterized up to 523 K with the use of laser flash method and thermal conductivities of coatings were determined. The conductivities were in the range from 0.6 to 1.1 W/(mK) depending on temperature for YCSZ and 14YSZ coatings.