Deposition of hybrid plasma-sprayed coatings employing both dry powder and liquid feedstocks enables preparation of innovative coating architectures. Using this technique, miniature domains of additional (secondary) material may be introduced via the liquid feedstock route into the more conventional powder-deposited coating, providing potential benefits for the coating functionality. In this contribution, we have explored the tribological properties of hybrid coatings sprayed from alumina powder with additions of chromia (Cr 2 O 3 ), zirconia (ZrO 2 ), yttria-stabilized zirconia (YSZ), and titania (TiO 2 ) delivered from liquid feedstocks. The coatings were subjected to dry sliding wear testing and a subsequent analysis of the wear tracks to determine their wear resistance and coefficient of friction, as well as a qualitative assessment of the wear mechanisms. The hybrid coating doped with the chromia addition matched the remarkable wear resistance of highly-dense suspension-sprayed coatings. This is a significant result, especially when considering the order of magnitude better production efficiency of the hybrid coatings.

Recent development of plasma spraying with liquid feedstocks enabled deposition of novel thermal barrier coatings (TBCs) with top-coats incorporating various desirable features, such as columnar microstructure or ultrafine porosity. Moreover, different materials may be relatively easily combined, e.g., by alternating feedstocks in the feed line or using feedstock mixtures. Coatings with gradient change of chemistry/microstructure towards the free-surface can be also prepared by gradual change of the feedstock composition, which may be potentially beneficial for example to mitigate stresses at the macroscopic interfaces between TBC layers (typically bond-coat/top-coat or within layered top-coat). In this study, three experimental TBCs with gradient top-coats were successfully deposited by hybrid water/argon-stabilized WSP-H plasma torch, i.e., one version of coating with 8 wt.% yttria stabilized zirconia transitioning into gadolinium zirconate Gd 2 Zr 2 O 7 (YSZ→GZO) and two versions of Al 2 O 3 transitioning into YSZ (Al 2 O 3 →YSZ). Thermal cycling fatigue (TCF) test with peak temperature of 1100 °C showed outstanding thermal shock resistance of the YSZ→GZO coating and mediocre to poor resistance of both Al 2 O 3 →YSZ coatings.

In this paper, yttria-stabilized zirconia (YSZ) coatings were prepared by plasma spraying of ready-to-spray suspensions provided by three different manufacturers. High-enthalpy hybrid water-argon plasma torch WSPH 500 was successfully used for deposition of coatings with porous and columnar microstructure consisting of tetragonal non-transformable phase. Sensitivity of the deposition process to variation of deposition conditions was also evaluated by the change of suspension injection point position. Slight differences in the microstructures of the deposited coatings (in particular character of porosity and mutual bonding of the microsplats) were reflected in slight but measurable differences in hardness and wear resistance of the coatings indicating changes in the coating cohesion. Tensile adhesion/cohesion strength of the coatings was found to be in the range of 9 to 15 MPa. High coating porosity desirable for low thermal conductivity combined with high suspension feed rate (from about 100 to 120 ml/min in this study) makes the WSP-H coatings promising for further development for example in thermal barrier coatings applications.

This paper presents the results of three experiments in which coatings are produced by suspension plasma spraying using a water-stabilized hybrid torch. Shadowgraphy is used to optimize the injection of the suspension and visualize liquid fragmentation in the plasma jet. Deposition efficiency is evaluated and the production of coatings with different microstructures is demonstrated for YSZ, YAG, and Al 2 O 3 .

This paper examines the microstructure and morphology of zirconia coatings and demonstrates the calculation of elastic modulus and Martens hardness based on instrumented indentation test results. Coatings samples varying in microstructure, phase content, and chemical composition were deposited by suspension plasma spraying using different torches and different suspension formulations. Coatings produced from low-concentration suspensions with submicron-size powders had a columnar structure with long vertical pores between the columns and fine spherical pores within the columns. Coatings made from suspensions with high concentrations of solids and coarser, more irregular powders, on the other hand, were more uniform and their surfaces smoother. They are also shown to be harder and have higher elastic modulus based on indentation test results.

Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the spatiotemporal localization of crack initiation and the dynamics of crack propagation are studied. The resonance bending fatigue test is employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical TBC YSZ/NiCoCrAlY composites were tested. The strain distribution on the coating surface is evaluated by the digital image correlation method (DIC) through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e. the changes of material properties, crack initiation, and crack propagation are identified. The tested coatings strongly influenced the first two phases, the influence on the crack propagation was less significant.

Alumina-zirconia ceramic material has been plasma sprayed using a water stabilized plasma torch (WSP) to produce free standing coatings. The as-sprayed coatings have very low porosity and are mostly amorphous. The amorphous material crystallizes at temperatures above 900 °C. A spark plasma sintering apparatus has been used to heat the as-sprayed samples to temperatures above 900 °C to induce crystallization while at the same time a uniaxial pressure of 80 GPa has been applied to the their surface. After such post-treatment, the ceramic samples are crystalline and exhibit very low open porosity. The as-sprayed amorphous materials also exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated samples whose microstructure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix.

The fatigue performance of conventional structural steel with an applied thermal barrier coating (TBC) was evaluated via cyclic bending. Tests were carried out for as-received and grit-blasted substrates as well as for samples with thermally sprayed bond coats and topcoats. Failure mechanisms were identified and changes in fatigue resistance were assessed based on results obtained for different loading amplitudes supplemented by fractographic analysis.

Adhesion/cohesion testing represents one of the most common methods for benchmarking and optimization of thermal spray coatings. However, due to the inhomogeneous coating microstructure, such testing may be quite troublesome. In this study, adhesion/cohesion strength of representative metallic and ceramic coatings deposited by Water Stabilized Plasma (WSP) spraying was evaluated by different methods, namely Tensile Adhesion Test (TAT), newly utilized pin test and Tubular Coating Tensile (TCT) test. Combination of various methods enabled the evaluation of the splat bonding quality in different loading modes. Limitations and benefits of each method for testing of WSP coatings are demonstrated. Dominating failure micromechanisms were determined by supplementary fractographic analysis.

Mechanical and thermal properties of thermally sprayed coatings, especially ceramics, are strongly influenced by cracks and pores that are present in the coating microstructure. In the recent past, there have been efforts to find an analytical model describing the coating properties based on the microstructural characteristics. Various analytical models were developed and published in the literature. In this study, several major models were applied to ceramic and metal coatings to describe their elastic modulus and thermal conductivity. The sensitivity of the models to the variations in the microstructure and relevancy of their use in specific cases were examined. The results were compared with those obtained by FEM modeling and experimentally measured values.

The effect of grit blasting exposure time on the adhesion of plasma sprayed Al 2 O 3 and 316L stainless steel coatings was studied in the present work. The steel substrates were grit blasted prior to the coating deposition. Two sets of substrates with exposure time of 1 and 4 seconds were prepared. Both types of coatings were deposited using Water Stabilized Plasma (WSP) torch. Adhesion strength was evaluated using standardized pulloff test. The obtained results showed a slight improvement in the adhesion strength for the blasting time of 4 s. Failure processes taking place in the coatings during the pull-off tests were described based on the detailed fractographic analysis.

The thermo-mechanical properties of a thermal barrier bond coat (BC) play an important role in governing the life-time of a coating system. The presented work aims to determine these properties for NiCoCrAlY coatings sprayed on Hastelloy X substrates sprayed under different process conditions. Temperature dependent Young’s modulus values are determined for both Atmospheric Plasma Sprayed (APS) and HVOF sprayed coatings using the four-point bending test. Particular attention is paid to microstructure-property relationships during heating. Young´s modulus was determined up to 950°C and evaluated for coatings loaded in both tension and compression. Results are discussed in the context of the effect of feedstock material, process conditions and microstructure characteristics. The methods and results presented are attractive, particularly for the thermal spray industry, since these properties are a prerequisite when the BC is to be considered in component design.

Pores and cracks have significant influence on the structural rigidity and mechanical behavior of the thermally sprayed materials. For some applications dense coatings are needed, while for others (e.g. thermal barriers) some level of porosity is desirable. Recent development in thermal spraying focuses on the tailored design of pores and cracks for specific applications. In this project, ceramic coatings with different level and morphology of pores and cracks were plasma and HVOF sprayed on titanium alloy substrates. Coating microstructures were observed using scanning electron microscopy. Mechanical behavior of the prepared coatings was evaluated using four-point bending test in terms of changing coating stiffness with increasing mechanical load both in compression and tension. Significant level of coating non-linearity and hysteresis were observed. Tests carried out for coatings with the same chemical composition but different microstructure proved strong dependency of coatings mechanical properties on pores and cracks morphology. Microstructure features relevant for the applied loading are discussed.

This study investigates the mechanical response of plasma-sprayed ceramic coatings to different levels of mechanical and thermal loading. Test samples were subjected to four-point bending and thermal cycling loads. Nonlinear behavior and significant hysteresis were observed, indicative of inelastic phenomena. Previous tests were complemented by structural examinations and bonded-interface testing. Relevant structural features and possible mechanisms underlying this behavior are discussed.

Tungsten-copper composites and FGMs can find applications in various thermal management systems. One example is plasma facing components for nuclear fusion devices, where tungsten provides the heat-resistant plasma facing armor, copper provides the highly conductive heat sink, while the composite or FGM can reduce the stress concentration at the interface. In this study, W+Cu composites of various compositions were produced by water-stabilized plasma spraying. With the help of in-flight particle and plume diagnostics, the powder injection was optimized for each material, and the feed rates were adjusted to account for different deposition efficiencies. The composition, structure, thermal and mechanical properties of the coatings were characterized.