This study assesses the thermal stability of YSZ coatings produced from nanostructured feedstock by means of atmospheric and suspension plasma spraying. Free-standing YSZ coatings were isothermally treated for 24 h at different temperatures (1200-1600 °C) and at 1550 °C for 20 to 100 h. Afterwards, the coatings were examined to determine the effect of heating on phase composition, microstructure, morphology, and hardness. No evidence of tetragonal-monoclinic phase transformation was detected in the coatings that had been treated for 24 h, even at 1600 °C, but in coatings treated for different periods of time at 1550 °C, a phase transformation occurred after 40 h. Overall, the suspension plasma sprayed coating showed the greatest degree of change due to thermal aging.

This study investigates the corrosion resistance Gd 2 Zr 2 O 7 /YSZ coatings and a YSZ layer of similar thickness. All coatings were produced by suspension plasma spraying, resulting in a columnar structure. Corrosion tests conducted at 900 °C for 8 h in a molten salt bath show that Gd 2 Zr 2 O 7 is not as corrosion resistant as YSZ. Molten salts react with Gd 2 Zr 2 O 7 producing GdVO 4 along the surface as well as between the columns of the coating. The formation of GdVO 4 between the columns, in combination with the low fracture toughness of Gd 2 Zr 2 O 7 , is likely responsible for the lower corrosion resistance. Furthermore, the presence of another layer of Gd 2 Zr 2 O 7 on top of the Gd 2 Zr 2 O 7 /YSZ coating, to prevent salt infiltration, did not improve corrosion resistance.

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 .

Thermal barrier coatings (TBCs) consisting of a MCrAlY bond coat and a YSZ topcoat were air plasma sprayed onto Hastelloy X substrates. Samples were thermally cycled between 100 °C and 1100 °C and thermal fatigue failures were investigated via microstructure analyses. Final fatigue failure was caused by the formation of interface-parallel cracks in the topcoat, which was found to strongly related to the oxidation behavior of the bond coat. The development of oxide layers was therefore studied in detail and a thermo-kinetic model was used to explore the role of elemental diffusion in oxide formation.

High-entropy alloys (HEAs) are well suited for use in high-temperature environments due to their combination of strength, ductility, thermal stability, and corrosion and wear resistance. In this study, NiCoCrAlSi-based HEA coatings are deposited by HVOF and air plasma spraying (APS) and their phases, microstructure, and composition are evaluated by means of XRD, SEM, and EDS analysis. The results show that BCC/B2 phases are the main constituent in HVOF coatings that were diffusion heat treated. APS coatings of the same composition, on the other hand, exhibited a two-phase structure consisting of L12 and BCC/B2 phases. The HEA coatings produced by HVOF were tested for oxidation resistance and their morphology and oxide scales were examined with the aim of developing a high-quality bond coat for thermal barrier coating (TBC) systems.

In this study, an internal injection plasma torch is used to deposit nano-agglomerated YSZ feedstock powders on superalloy substrates at low ambient pressures ranging from 5000 Pa to 6000 Pa. Coatings with unique fully nano-equiaxed structures were obtained when operating the torch below 300 A. With increasing current, up to 700 A, coatings with mixed-grain and eventually large-grain structures were produced. Experimental results suggest that the equiaxed nanoscale structure derives from the original agglomerated nanoparticles that had undergone melting while inside the nozzle of the plasma torch and were subsequently solidified or sintered in the coating. Coating hardness and elastic modulus were also measured and are shown to correspond with microstructure.

Plasma-spray injection conditions have a major effect on particle properties and thus the characteristics of deposited coatings. In this investigation, opposing particle streams were injected into a plasma jet and the effect of different configurations and injection parameters was systematically studied. First- and second-order process maps were constructed from particle diagnostic data and in-situ beam curvature measurements, which served to guide optimization efforts.

Nano-sized, micro-sized, and bimodal Cr 3 C 2 -25NiCr feedstock powders were prepared using a spray drying and agglomeration sintering process and deposited by HVOF spraying. The microstructure and phase composition of the three powders and corresponding coatings were assessed along with coating hardness, toughness, and abrasive wear resistance. All three powders were nearly spherical in shape, but the bimodal powder had the best sphericity, the most uniform composition distribution, and the highest deposition efficiency. All three coatings were primarily composed of Cr 3 C 2 and NiCr phases, although a small amount of Cr 2 O 3 was detected in the nano- and micro-coatings due to oxidation. Compared with the micro-coating, the nano-coating has higher toughness but lower microhardness and abrasive resistance. The bimodal coating, however, integrates the advantages of the other coatings, has low porosity and a dense microstructure, and is tightly adhered to the substrate, thus displaying the best comprehensive performance.

Columnar structured coatings, due to their high strain tolerance, are well suited for thermal shock applications and thermal cycling service. In this study, plasma spray-physical vapor deposition (PS-PVD) was used to produce YSZ coatings with quasi-columnar microstructure. Thermal cycling and thermal shock tests were performed, and coating microstructure, phase composition, and residual stresses were evaluated before and after testing. Coatings obtained prior to process optimization, on average, made it to 623 cycles at 1250 °C before 10% of the coating surface showed signs of spallation.

This study evaluates the effect of Ru and Ce on the oxidation behavior of NiCoCrAl coatings deposited by HVOF spraying. Isothermal oxidation tests were conducted at 900, 1000, and 1100 °C. Test samples were also cycled between 100 °C and 1100 °C, with a dwell time of 1 h at 1100 °C. In both cases, coatings with Ru had a lower oxidation rate than those with Ce additions. β-depletion due to interdiffusion was also found to be lower when Ru was present. To help explain the findings, simulation results are presented and discussed along with observations on the influence of oxidation time on microstructure evolution.

This study investigates the correlation between thermal barrier coating (TBC) lifetime and thermally grown oxide (TGO) layer thickness. YSZ TBCs were deposited by atmospheric plasma spraying on Ni-base substrates and subjected to burner cycling tests with a thermal gradient and isothermal furnace testing. Both tests revealed that thermal cycling lifetime decreases with increasing TGO thickness, following a power law function, and for a critical TGO thickness of 5-6 μm, the failure mode changes from cracking within the YSZ layer to interface cracking around the TGO. Although either test can be used to evaluate TBC performance, burner cycling tests are better suited for evaluating ceramic topcoats, while furnace cycling test results integrate the effects of bond coat properties, especially oxidation resistance, as well as ceramic topcoat cracking resistance. The two tests can thus be used together to assess the factors that control TBC failure.

Nanostructured YSZ coatings were deposited by plasma spraying at different pressures and plasma currents. Coatings sprayed at low pressures exhibited fine equiaxed grain structures with consolidated coarser grains and loose-packed nano granules. Especially at the low end of the current range, larger amounts of crystalline grain fell into the nanoscale, significantly reducing the average grain size in the coating. At the high end of the range, the coating exhibited a uniform fine equiaxed grain structure of around 200 nm. Mechanical properties, including average microhardness and elastic modulus, were also measured and compared. LPPS YZS coatings showed slightly higher microhardness, but half the elastic modulus of APS YSZ layers.

This work investigates the fundamental mechanical properties of SPS and APS thermal barrier coatings. SPS YSZ coatings had lower Young’s modulus values and higher interfacial toughness than APS deposited layers. The low stiffness of SPS coatings limits the elastic energy that can be stored in ceramic layers. This coupled with good interfacial toughness might make SPS deposited thermal barrier coatings less prone to delamination due to thermal cycling.