This work focuses on the processing and deposit by suspension plasma spraying (SPS) of ZrO 2 -based ceramic materials for Thermal Barrier Coatings (TBC's) applications. The system of interest is ZrO 2 -16mol%Y 2 O 3 -16mol%Ta 2 O 5 (16YTZ). This ceramic has been reported to keep a non-transformable tetragonal phase (t'-phase), suitable to overcome the thermodynamic limits of the mostly used conventional 7-8wt.% yttria stabilized zirconia (YSZ). The research consists into evaluate the t'-phase stability and performance of the 16YTZ SPS coating. Synthesis of 16YTZ and, the evolution of the resulting microstructure in the dense ceramic and in the coating are a central part of the study. Sintering behavior in dense ceramics prepared from both precursor derived and milled powders is evaluated. Microstructural characterization by XRD, SEM and RAMAN spectroscopy of the as-deposited ceramic coating is presented and discussed.

During the impact and solidification of thermal spray droplets on a substrate, the density increases when the droplet solidifies. Depending on the material, the changes in density could be significant. For example, aluminum oxide's density changes by 66%, while the changes are 12% and 19% for nickel and copper, respectively. For zirconia, this change is 24%. The effect of such densification on the dynamic of the droplet impact and the formation of porosity could be dramatic. In this study, the effect of shrinkage of a molten droplet during solidification on droplet impact is numerically investigated for several materials. Results for the impact of molten alumina, nickel, copper, and zirconia droplets on both smooth and rough surfaces are presented. The results of variable density cases are compared with those assuming constant density. The effect of thermal shrinkage is particularly vital in the interaction of two impacting droplets. The shrinkage promotes the formation of additional pores.

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.

The abradable coatings had significantly enhanced turbomachinery performance by acting as a sacrificial seal between rotating blades and stationary casing. Further improvement in seal design to meet the higher energy demand and increase the service time has been the key challenges to solve in the gas turbine industry. Honeycomb seals have become the industry standard clearance seal technique due to their unique design and high structural strength with minimum weight. The present study proposes a concept to form a thermal shock resistance structure to achieve higher temperature capability and improve the reliability of abradable seal structures. A cavity layer of honeycomb seal structure made of SS 321 alloy was coated with advanced high-temperature ZrO 2 +7.5%Y 2 O 3 +4% polyester seal material using TriplexPro-210 plasma spray system. The integrity of a seal structure was assessed by a cross-sectional analysis and evaluation of the coating microstructure. Additionally, the microhardness test was performed to estimate coating fracture toughness, and Object-Oriented Finite Element analysis was used to assess its thermo-mechanical performance. The concept proposed in this study should be further validated to develop the most capable innovative technology for advanced gas turbine abradable seal structures.

The effect of deposition pressure on the microstructure and ablation behavior of ZrB2 coatings deposited by very low pressure plasma spraying is investigated. The results show that under a chamber pressure less than 50 kPa, as the spray chamber pressure decreases, the porosity of the coating deposited at the same distance decreases, and the coating prepared under 100 Pa presents the lowest porosity of 1.79 %. Furthermore, among the ZrB2 coatings deposited at 100 Pa, 5 kPa, 10 kPa and 50 kPa, the dense coating deposited at 100 Pa showed the lowest ablation rate of 0.33 μm/s, 0.75±0.08 mg/s.

Coatings applied on steel molds used for casting aluminum parts have two main purposes: avoid mold metal reaction and control thermal transfer to obtain directional solidification. The coatings widely known to foundry operators are water-based sodium-silicate bonded ceramic suspensions; they are air sprayed and cured on mold surfaces and typically last for 100 casting cycles. Although thermal sprayed coatings have been shown to last more than 5000 casting cycles, they are not yet the preferred mold protection method. This study addresses the issue by developing a knowledge base of thermal transfer properties that can be achieved with air plasma sprayed magnesium zirconate powders. The properties are assessed on an instrumented mold using the inverse technique for different coating compositions.

This study assesses the potential of scandia-stabilized zirconia (ScSZ) produced by very low-pressure plasma spraying (VLPPS) for metal-supported solid oxide fuel cell (MS-SOFC) applications. To investigate the microstructure of ScSZ, coating samples were deposited at spraying distances of 150, 250, 350 mm. The fragile nature of coating cross-sections suggests that the typical lamellar structure of zirconia is replaced by a transgranular structure. Nonetheless, apparent porosity, ionic conductivity, open circuit voltage, and ohmic resistance measurements indicate that VLPPS is a viable method for producing MS-SOFCs.

Enhancing mechanical and chemical properties of ceramic coatings is a perpetual objective in the thermal spray community. Utilizing either novel feedstock or thermal spray process modifications are the most plausible ways to achieve enhancements such as improved corrosion resistance or damage tolerance. Quasi-plasticity and ultrahigh yield strength can be obtained with ceramic coatings by increasing the number of nanoscale features in the as-sprayed coating. Currently, thermal spraying using liquid feedstock can produce coatings with very fine microstructures, either by utilizing submicron particles in the form of a suspension or through in-situ synthesis. The focus of this work was to obtain a bimodal microstructure by using simultaneous powder-precursor HVOF spraying. Nanostructure was achieved from YSZ and ZrO 2 solution-precursors and the microstructural features were obtained from using a conventional Al 2 O 3 spray powder. The microstructure of the coatings revealed some clusters of unmelted nano-sized ZrO 2 embedded in a generally dense matrix of Al 2 O 3 . The phase compositions consisted of γ- and α-Al 2 O 3 and tetragonal and monoclinic ZrO 2 with the former being dominant. The mechanical strength of the coatings was evaluated by Vickers hardness test and cavitation erosion. The addition of the nanostructured ZrO 2 into the coating was found to deteriorate the structural cohesion of the coatings, leading to degraded durability when compared to a conventional HVOF-sprayed Al 2 O 3 -coating.

In the Plasma Spray-Physical Vapor Deposition (PS-PVD) process, the vapor atom of feedstock material is one deposition unit of the columnar structure coating. It is reported that the gas phase may be transformed into cluster when the powder feeding rate increases from small to large or the sedimentation distance increases from a certain distance to another distance. In order to understanding the variation of vaporized coating material in free plasma jet, the gaseous material capacity of plasma jet must be fundamentally understood. In this work, the thermal characteristics of plasma were firstly measured by optical emission spectrometry (OES). The results show that the free plasma jet is in the local thermal equilibrium due to a typical electron number density from 2.1×1015 to 3.1×1015 cm -3 . In this condition, the temperature of gaseous zirconia can be equal to the plasma temperature. A model was developed to obtain the vapor pressure of gaseous ZrO 2 molecules as a two dimensional map of jet axis and radial position corresponding to different average plasma temperatures. The overall gaseous material capacity of free plasma jet was further established. At a position of plasma jet, clusters may form when the gaseous material exceeds local maximum gaseous material capacity.

The ability of suspension plasma spraying (SPS) to overcome difficulties associated with feeding of fine (submicron or nano-sized) powders and achieve more refined microstructures than possible in atmospheric plasma spraying (APS) is well established. In recent times, the use of axial injection plasma spray systems has yielded substantial enhancement in deposition rates/efficiencies due to improved thermal exchange between the plasma plume and injected feedstock. The present paper describes utilization of both the above advances in plasma spraying to create various function-dependent coating architectures through simultaneous and/or sequential spraying of hybrid powder-suspension feedstock. A specific variant of such hybrid axial plasma spraying that enables deposition of composite coatings by simultaneous injection of a powder and a suspension is discussed in particular detail. Results obtained using an Al 2 O 3 -ZrO 2 material system as a case study reveal that composite coatings combining the micron-size features arising from the spray-grade Al 2 O 3 powder and submicron or nano-sized features attributable to the ZrO 2 suspension can be conveniently realized. The surface morphology, microstructure, and composition of these coatings, as well as their tribological behaviour determined using scratch and ball-on-disc tests, are presented herein. The utility of this method to develop a wide array of composite coatings is also discussed.

Thermally sprayed ceramic coatings are used in environments requiring good wear- and corrosion resistance among others. However, a typical issue with ceramic coatings is their low impact resistance and tendency to fail catastrophically by cracking. In bulk ceramics, the Al 2 O 3 -ZrO 2 –composition has been of interest for long since already small additions of ZrO 2 into Al 2 O 3 have shown improvements in fracture toughness compared to pure Al 2 O 3 . Efforts are being made to induce this increased resistance to fracturing in thermally sprayed coatings as well, resulting in higher wear resistance due to a more predictable behavior and damage-tolerance. In this work, Al 2 O 3 -ZrO 2 -coatings have been deposited by atmospheric plasma spray (APS) and high-velocity oxy-fuel spray (HVOF) processes. The wear characteristics of the coatings were evaluated with cavitation erosion, delving into the mechanics of the erosion and the resulting microstructural changes in the coatings. Evidence of phase transformation of t-ZrO 2 to m-ZrO 2 was found during the erosion. The HVOF-sprayed coating exhibited greater wear resistance against the cavitating bubbles due to its finer microstructure.

In this work, ZrB 2 -MoSi 2 (ZM) composite coatings are fabricated using two plasma spraying techniques: VPS and APS. Phase composition and coating microstructure were assessed and microstructural changes at 1500 °C were investigated along with corresponding oxidation behaviors. The results show that the VPS composite coatings have lower oxide content, surface roughness, and porosity and much higher oxidation resistance than coatings produced by atmospheric plasma spraying. Possible reasons for the differences observed are presented.

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.

The main purposes of applying the coatings on the surface of different components include the improvement of their functional and decorative properties. The functional properties of coating are strongly dependent on its microstructure. The main goal of the paper is to show the differences of two microstructures that can be obtained using suspension plasma spraying technology: (i) columnar one; and, (ii) lamellar, two-zone microstructure. Initially, the optimization of spray parameters was made and then the microstructural studies were performed. The work was focused on zirconia stabilized by yttria (YSZ) and both by yttria and ceria (YCSZ) which are most frequently used as thermal barrier coatings (TBC). Moreover, two types of microstructure were achieved using two different plasma torches, namely SG-100 of Praxair and Triplex of Sulzer Metco. After optimizing the spray parameters the microstructure of prepared coatings was analyzed using electron microscopy (SEM). The conventional SEM microscopy with secondary electrons detector (SE) and back scattered electrons one (BSE) were used. The energy dispersive spectroscopy (EDS) was performed to analyze the chemical composition. Finally, by electron backscatter diffraction (EBSD) grain shape, size and texture were determined for discussion of the coatings growth mechanism.

In this study, lanthanum (LZ) and gadolinium zirconate (GZ) powders were prepared by spray drying and deposited on graphite substrates by air plasma spraying. Free-standing 1-1.5 mm thick LZ and GZ coating samples were obtained by mechanical delamination and subjected to long-term annealing in air at 1250 °C. Significant changes occurred during heat treating in the form of microstructure evolution, crystal structure ordering, and phase transitions as observed via SEM, XRF, and XRD analysis. These changes affected a number of coating properties, including hardness, porosity, fracture toughness, and thermal conductivity. In the first 20 h, thermal conductivity increased in gadolinium zirconate from 0.5 to 1.0 W/m·K and in lanthanum zirconate from 0.5 to 1.5 W/m·K.

This study investigates a new zirconia-based ceramic for potential use in thermal barrier coatings. In the experiments, Sc 2 O 3 -Gd 2 O 3 -Yb 2 O 3 -ZrO 2 (SGYZ) powder was synthesized by coprecipitation and calcination, then agglomerated and sintered to facilitate spraying. The structure, morphology, and phase stability of the powder and plasma-sprayed SGYZ coatings were analyzed and thermal conductivity was measured. Test results show that the powders and coatings have good phase stability even after 500 h at 1400 °C and do not undergo tetragonal-to-monoclinic phase transition upon cooling. Plasma-sprayed SGYZ also has a lower thermal conductivity than YSZ, which is currently used in gas turbine engines.

This work evaluates the potential of using a plasma spray process to introduce SiC into zirconia diboride ceramic coatings. Controlling the spraying of the ultra-refractory compound ZrB 2 is the first challenge as it represents the matrix in which SiC particles will reside. To that end, the experiments focus on spraying parameters that influence the plasma jet and the nature of the precursor feedstock. The results show that ZrB 2 coatings containing controlled amounts of SiC can be obtained through high-energy suspension plasma spraying. The ZrB 2 -SiC coatings will be evaluated in a high-temperature oxidative environment in future work.

Dysprosia stabilized zirconia coatings with large globular pores have good potential as TBC topcoats. In previous work, such coatings have been produced by air plasma spraying with the aid of a polymer pore former. The aim of this work is to optimize the spraying parameters. A design of experiments approach was used to create a two-level full factorial test matrix based on spray distance, powder feed rate, and hydrogen flow. An agglomerated and sintered dysprosia stabilized zirconia (DySZ) powder mixed with polymer particles was sprayed on Hastelloy X substrates that had been prepared with NiCoCrAlY bond coats. The coatings obtained were evaluated based on thermal conductivity, thermocyclic fatigue life, and morphology, which are shown to correlate with spray parameters and in-flight particle properties.

Adhesive strength of the plasma-sprayed thermal barrier coating (TBC) is one of the most important parameters which influence the reliability during service. In the past, numerous test methods were reported to measure the coating adhesion. However, most of them require careful and time consuming preparation. Consequently, limited information could be obtained to establish the relationship between the processing conditions and the adhesive property. To produce more measurements using a simpler procedure, the interfacial indentation test and the modified tensile adhesive test are examined. In this paper, the interfacial fracture toughness of the plasma-sprayed ZrO 2 coatings, deposited on Al substrates, were evaluated by these two tests. In order to study the effects of the powder injection, samples were sprayed with various carrier gas flow rates. The test results show a certain correlation between the melting index and the interfacial fracture toughness. In addition, variations between the results obtained from the two different methods are discussed.

Nanostructured zirconia coatings were prepared by solution plasma spraying. The phase composition and microstructure of the coatings and powders dried from solution were observed by XRD and SEM. Fine grained nanostructured zirconia coatings can be obtained. Nano-zirconia coatings adhered well with the substrate, and exhibited few vertical cracks.