Paweł Sokołowski

Advanced Microscopic Study of Suspension Plasma-Sprayed Zirconia Coatings with Different Microstructures

The present paper is focused on the characterization of the differences between two microstructures that can be obtained using SPS technology, namely (i) columnar and (ii) two-zone microstructure including lamellas and fine unmelted particulates. The optimization of spray parameters was made, and the advanced microstructural studies of obtained coatings were performed. The work was focused on zirconia stabilized by yttria (YSZ, ZrO2xa0+xa014 wt.% Y2O3) and both by yttria and ceria (YCSZ, ZrO2 + 24 wt.% CeO2 + 2.5 wt.% Y2O3) which are frequently used as thermal barrier coatings. Two types of microstructure were achieved using two different plasma torches, namely SG-100 of Praxair and Triplex of Oerlikon Metco. The microstructure of prepared coatings was analyzed using scanning electron microscopy with secondary electrons detector and backscattered electrons. Energy dispersive spectroscopy was performed to analyze the chemical composition of sprayed coatings. By electron backscatter diffraction grain shape, size, and crystal orientation were determined. The analysis enabled the discussion of the coatings growth mechanism. Finally, the Shape From Shading technique was applied to recreate and to analyze 3D views of coatings’ topographies, and using laser confocal microscopy, the surface roughness was examined.

Friction Welding of Aluminium and Aluminium Alloys with Steel

The paper presents our actual knowledge and experience in joining dissimilar materials with the use of friction welding method. The joints of aluminium and aluminium alloys with the different types of steel were studied. The structural effects occurring during the welding process were described. The mechanical properties using, for example, (i) microhardness measurements, (ii) tensile tests, (iii) bending tests, and (iv) shearing tests were determined. In order to obtain high-quality joints the influence of different configurations of the process such as (i) changing the geometry of bonding surface, (ii) using the interlayer, or (iii) heat treatment was analyzed. Finally, the issues related to the selection of optimal parameters of friction welding process were also investigated.

Controlling Microstructure of Yttria-Stabilized Zirconia Prepared from Suspensions and Solutions by Plasma Spraying with High Feed Rates

Introduction of suspension and solution plasma spraying led to a breakthrough in the deposition of yttria-stabilized zirconia (YSZ) coatings and enabled preparation of new types of layers. However, their deposition with high feed rates needed, for example, for the deposition of thermal barrier coatings (TBCs) on large-scale components, is still challenging. In this study, possibility of high-throughput plasma spraying of YSZ coatings is demonstrated for the latest generation of high-enthalpy hybrid water-stabilized plasma (WSP-H) torch technology. The results show that microstructure of the coatings prepared by WSP-H may be tailored for specific applications by the choice of deposition conditions, in particular formulation of the liquid feedstock. Porous and columnar coatings with low thermal conductivity (0.5-0.6 W/mK) were prepared from commercial ethanol-based suspension. Dense vertically cracked coatings with higher thermal conductivity but also higher internal cohesion were deposited from suspension containing ethanol/water mixture and coarser YSZ particles. Spraying of solution formulated from diluted zirconium acetate and yttrium nitrate hexahydrate led also to the successful deposition of YSZ coating combining regions of porous and denser microstructure and providing both low thermal conductivity and improved cohesion of the coating. Enthalpy content, liquid-plasma interaction and coating buildup mechanisms are also discussed.

A comparison of microstructure and of thermal transport properties of yttria and ceria stabilized zirconia coatings deposited by suspension plasma spraying

The single cathode plasma torch SG-100 was used to spray suspension prepared with the use of two kinds of commercial powders ZrO2+8 wt. % Y2O3 (8YSZ) and ZrO2 + 24 wt.% CeO2 + 2.5 wt.% Y2O3 (24CeYSZ). The suspensions were formulated using finely milled solid phase, water and ethanol. The operational spray parameters being the same for each powder were modified by changing: (i) spray distance; and (ii) torch scan linear speed. The coatings microstructures were analyzed with the use of optical and scanning electron microscopes as well as by X-ray diffraction. Their porosities were found with help of the image analysis of metallographically prepared cross-sections. Thermal diffusivity was measured with the use of commercial system LFA 447 NanoFlash® working at the temperatures limited to 300°C. The measurements were made with the use of the coatings sprayed on the steel substrate and a 2-layers numerical model was applied to determine thermal diffusity of the coatings. The obtained data were used to calculate thermal conductivity of stabilized zirconia coatings.