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 ZrO 2 + 24 wt.% CeO 2 + 2.5 wt.% Y 2 O 3 (YCSZ) was milled the decrease the particles sizes. The yttria stabilized zirconia of composition ZrO 2 + 14 wt.% Y 2 O 3 (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.

The paper briefly describes major thermal spray techniques used to spray functionally graded coatings such as atmospheric plasma spraying, high velocity oxy-fuel spraying, suspension and solution precursor plasma spraying, and finally low and high pressure cold gas spray method. The examples of combined spray processes as well as some examples of post spray treatment including laser and high temperature treatments or mechanical one, are described. Then, the solid and liquid feedstocks used to spray and their properties are shortly discussed. The reviewed properties of functional coatings include: (i) mechanical (adhesion, toughness, hardness); (ii) physical (porosity, thermal conductivity and diffusivity, thermal expansion, photo-catalytic activity), and; (iii) bioactivity and simulated body fluid (SBF) corrosion. These properties are useful in present applications of functionally graded coatings as thermal barriers, the bioactive coatings in prostheses, photo-catalytic coatings in water treatment, coatings used in printing industry (anilox and corona rolls). Finally, some of the future possible fields of functional thermal sprayed coatings applications are discussed, e.g., to coat polymer substrates or to use the cheap technology of low pressure cold gas spray method instead of expensive technology of vacuum plasma spraying to obtain bond coatings.

The paper aims at reviewing of the recent studies related to the development of suspension plasma sprayed TiO 2 and Ca 5 (PO 4 ) 3 OH (hydroxyapatite, HA) coatings as well as their multilayer composites obtained onto stainless steel, titanium and aluminum substrates. The total thickness of the coatings was in the range 10 to 150 lm. The suspensions on the base of distilled water, ethanol and their mixtures were formulated with the use of fine commercial TiO 2 pigment crystallized as rutile and HA milled from commercial spray-dried powder or synthesized from calcium nitrate and ammonium phosphate in an optimized reaction. The powder was crystallized as hydroxyapatite. Pneumatic and peristaltic pump liquid feeders were applied. The injection of suspension to the plasma jet was studied carefully with the use of an atomizer injector or a continuous stream one. The injectors were placed outside or inside of the anode-nozzle of the SG-100 plasma torch. The stream of liquid was tested under angle right or slightly backwards with regard to the torch axis. The sprayed deposits were submitted to the phase analysis by the use of x-ray diffraction. The content of anatase and rutile was calculated in the titanium oxide deposits as well as the content of the decomposition phases in the hydroxyapatite ones. The microRaman spectroscopy was used to visualize the area of appearance of some phases. Scratch test enabled to characterize the adhesion of the deposits, their microhardness and friction coefficient. The electric properties including electron emission, impedance spectroscopy, and dielectric properties of some coatings were equally tested.

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