Suspension Plasma Spraying is one of the newest thermal spray processes. It uses liquid feedstock instead of dry powders, as it is in case of conventional Powder Plasma Spraying. The suspension preparation process should be optimized individually depending on the powder material to avoid poor suspension stabilization, fast sedimentation of solids or too high viscosity. However, regardless of the liquid feedstock preparation, each kind of suspension causes some technological problems during spraying. In this study the possibility of using high power ultrasonic waves to support the suspension feeding and injection was investigated. The idea was to break the powder agglomerates before the suspension goes through the injector (in the pipeline system). The in-line ultrasonic dispersion system was designed. The geometry of all important components (transducer, booster and horn) was analyzed and calculated depending on the frequency and power of ultrasounds. By using modal analysis and Finite Element Method (FEM) the propagation of ultrasonic wave was simulated for different sonotrode geometries, called: (i) stepped, (ii) exponential, (iii) catenoidal and (iv) tapered. The in-house system was initially tested in order to see if all elements were designed and manufactured properly. Finally, the ultrasonic cavitation was observed inside the feeding pipeline system, which should allow dispersing the suspension prior to its injection into the plasma jet.

In this study, 8YSZ and 24CeYSZ coatings were deposited on stainless steel by suspension plasma spraying. The suspensions were formulated using finely milled powder, water, and ethanol. Spraying parameters were modified by changing spray distance and torch scan speed and were the same for each material. Coating microstructure, phase composition, and porosity were assessed and thermal diffusivity was measured and used to calculate thermal conductivity.

Plasma generated by an SG-100 torch was applied to spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) and ZrO 2 +24 wt% CeO 2 +2.5 wt% Y 2 O 3 (24CeYSZ) as solid phases. The solids have the mean size of about 4.5 µm for 8YSZ and 3.9 µm 24CeYSZ and were obtained by milling of commercial powders Metco 204 NS and Metco 205NS, respectively. The suspensions were formulated with the use of 20 wt% solid phase, 40 wt% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (i) the electric power at 40 kW (ii) the working gases composition 45 slpm of Ar and 5 slpm of H 2 . On the other hand, the spray distance was varied from 40 to 60 mm and torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates to reach the thickness ranging from 70 to 110 µm (8YSZ) and about 70 µm (24CeYSZ). The coating microstructures were analyzed with the use of a scanning electron microscope. Mechanical properties were tested with the use of indentation and scratch tests. The indentation test was carried out with various loads ranging from 100 to 10,000 mN to determine elastic modulus and Martens microhardness. Young’s modulus of the coatings was in the range 71 to 107 GPa for 8YSZ and 68 to 130 GPa for 24CeYSZ. Scratch tests were conducted to determine the scratch macrohardness.

Titanium oxide coatings were suspension plasma sprayed onto different substrates. The suspension was formulated using fine rutile pigment in the mixture of water with ethanol. The zeta potential of the suspension was determined. The spray process parameters were designed using a full factorial plan using spray distance and torch scan velocity as the variables. The temperature at spray process was monitored using a pyrometer. The X-ray diffraction (XRD) analysis enabled to find out the crystalline phases in sprayed deposits and, in particular, the anatase content. Scanning electron microscope (SEM) enabled to characterize the coatings’ microstructure. The coatings included well molten lamellas and zones of loosely agglomerated and sintered grains. The scratch test of the coatings enabled to determine their mechanical properties such as critical load and scratch hardness.

Suspension plasma spraying is a process which enables production of finely grained nanometric or submicrometric coatings. The suspensions were formulated with the use of fine powder of ceramic particles of yttria stabilized zirconia in water with alcohol. The present paper focuses on the theoretical analysis of the formation process of sintering of fine solids impacting the growing coating’s surface. The heat flux input to the coatings was estimated and their surface temperature at spraying was measured. The theoretical analysis of sintering during the coating’s growth was carried out. The different models of sintering were analyzed and adapted to the suspension plasma spraying conditions. The model of surface diffusion was found to be the most appropriate to describe the sintering during suspension plasma spraying. The formation of the necks having the relative size equal to 10 % of the particle diameter was found to be possible during the coatings deposition.

In this investigation, titanium dioxide and hydroxyapatite (HA) suspensions are plasma sprayed onto stainless steel, titanium, and aluminum substrates and the structure and properties of the resulting layers are correlated with spraying conditions. The suspensions were formulated with fine TiO 2 pigment and HA milled from spray-dried powder or synthesized from calcium nitrate and ammonium phosphate. In some experiments, an atomizer was used to inject the suspensions into the plasma jet, and in others, the suspensions were fed into the jet using continuous stream injection. The deposits are characterized on the basis of morphology, chemical and phase composition, scratch hardness, and dielectric strength.