Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce Inconel 718 coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled Inconel 718 powders, as well as the HVOF Inconel 718 coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth as evidenced by a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good correspondence with the experimental results.

LCO (La2Ce2O7), a solid solution of La2O3 in CeO2, is a promising top coating in TBCs. However, it is generally necessary to use LCO as a top coat on YSZ coating to construct a multilayer TBC due to poor mechanical properties of LCO. Therefore, the thermal and chemical stability of LCO/YSZ at high temperature environment become important issue. In this paper, the 50LCO-50YSZ composite coating was deposited using blend powders of YSZ and LCO. The LCO powders used have a nominal particle size range from 10 to 44μm. The LCO/YSZ deposits were exposed at 1300 ¢XC for different durations. The microstructure evolution at the LCO/YSZ interface was investigated by quasi-in-situ SEM and EDS examination. At an exposure temperature of 1300°C, it was observed that some LCO splats in contact with YSZ splats experienced the grain morphology change from columnar one to quasi-axial grains with interface healing and however some grains tended to disappear with thermal exposure. Results indicated that LCO-YSZ composite coating is not phase stable at 1300 °C. Diffusion of La element from LCO splat towards the adjacent YSZ splat occurred during sintering, leading to the formation of La2Zr2O7 within YSZ splat.

In this paper, nanostructure Lanthanum zirconate thermal barrier coatings (MCrAlY+ La 2 Zr 2 O 7 ) were prepared by atmospheric plasma sprayed (APS). The microstructures and thermal stability properties were systematically studied by Scanning Electric Microscopy (SEM), transmission electron microscope (TEM) and X-Ray diffraction(XRD). The results showed that the nanostructured lanthanum zirconate coatings were typical lamellar structure which was composed of columnar grains about 90nm in diameter. A large quantity of micro-cracks and homogeneous distributed fine pores formed in the nanostructured zirconia coating. After ablation at 1300 °C for 24 h, no apparent phase transformation was observed in lanthanum zirconate coating. The growth mechanism of the grains was subsequently discussed.

While depositing Fe-Al intermetallic powders applying a gas detonation spraying a certain coating structures containing oxide ceramics are created. These structures exhibit both extreme mechanical resistance and unusual thermophysical properties (TP) also. One of such property is relatively low thermal conductivity. A possible application as thermal barrier coatings needs precise determination of TP dependence on temperature and resistance of the coating structure to temperature exposition. At present study TPs were investigated for a coating produced from Fe-Al intermetallic powder in a course of complex measurements including DSC analyses, laser flash thermal diffusivity measurements, dilatometric studies complemented with microstructural analyses. The study resulted in full characterization of the investigated structure TPs: density, thermal expansivity, heat capacity and thermal conductivity. During thermal analyses interesting phenomena concerning thermal resistance to the temperature exposition of the investigated coating were revealed. The obtained results complement rather sparse literature data on TPs in that subject and contribute to better understanding of gas detonation spraying (GDS) process technology and intermetallic/oxide structures property understanding.

cBN/NiCrAl nanocomposite coating was deposited by cold spraying using mechanically alloyed composite powders. To examine the thermal stability of coating microstructure, the nanocomposite coating was annealed at different temperatures from 750 to 1000°C. The microstructure of composite coatings was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the nanostructure was retained in the coating when the annealing temperature was lower than 825°C which is 0.7 times of the melting point of NiCrAl matrix. The dislocation density significantly reduced when annealing temperature was higher than 750°C. The cBN particle growth became significant when the annealing temperature was higher than 825°C. The effects of crystal grain refinement and work-hardening strengthening mechanisms were quantitatively estimated as the function of annealing temperature. The effect of annealing temperature on the contribution of different strengthening mechanisms to coating hardness was discussed.

Cold gas dynamic spraying has shown to be a promising approach for fabricating titanium structures directly from powder in the absence of a controlled atmosphere. This study investigates the effect of annealing on the microstructure, hardness, and tensile properties of commercial purity titanium deposited under cold spray conditions. Equiaxed grains with ultrafine grain structure are observed in the annealed samples. A physical based model is proposed for recrystallization of cold-sprayed titanium deposits and the results are compared with conventional cold-rolled and annealed titanium products.

In this work, one new technique named as supersonic suspension plasma spraying (SSPS) is applied to deposit quasi-columnar scandia-yttria co-doped zirconia (ScYSZ) and yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The phase composition, microstructural evolution, fracture toughness and failure behavior of both TBCs before and after thermal cycling tests at 1300 °C were systematically studied. It was found that both as-sprayed TBCs were fully non-transformable tetragonal (t’) phase. After the thermal cycling test, tetragonal (t) phase and cubic (c) phase formed for the SSPS-YSZ TBC, while single t’ phase retained for the SSPS-ScYSZ coating. The fracture toughness of the ScYSZ coating was comparable or superior to that of the YSZ coating. As for the thermal cycling behavior, the lifetime of the ScYSZ coating was better than that of the YSZ coating, which confirmed that ScYSZ was a promising alternative material for YSZ.

The thermal phase stability of plasma sprayed TBC is presented and discussed. TBC phase transformation after various isothermal heat treatment processes was studied via Xray diffraction (XRD) and Raman Spectroscopy methods. The YSZ-Gd-Yb and 20YSZ TBCs demonstrated superior thermal phase stability as compared with the high and standard purity 8YSZ TBCs, although higher purity also helps delay the tetragonal to monoclinic phase transformation. The phase transformation appears to be suppressed by cooling at higher rates. This data presents a qualitative phase stability comparison between the various coatings. However, cooling rate has to be taken into consideration in determining the extent of phase instability during the coating design of aero turbine applications.

In suspension plasma spraying (SPS); the use of water based suspensions provides a cheaper; safer and more environmentally friendly alternative to organic liquids. However; due to the physical properties of water; producing a water based SPS coating with desirable microstructure has so far been elusive. In this study; the effects of pH and dispersant on the rheology and stability of YSZ water based suspensions were investigated. PEI; PBTCA and α-Terpineol were used as dispersant polymers. The stabilized suspensions were deposited by Axial III plasma spray system and the relationship between suspension parameters and the atomized droplet size and the final coating microstructure was studied. The results showed that a combination of Terpineol dispersant with pH adjustment to 2.5; could lead to a SPS coating with columnar microstructure having 17.4 vol.% porosity.

Perovskites are considered as potential materials in solid oxide fuel cells (SOFC) for different reasons at different parts of the fuel cells. Perovskites such as La 0.8 Sr 0.2 MnO 3 (LSM) and other compositions are electrically conductive which is necessary for SOFC applications. One possible application is protection coating for interconnect plates (bipolar plate) to avoid chromium oxide evaporation from the surface of ferritic stainless steel. Different commercial and experimental perovskite powders were sprayed by plasma and HVOF spraying under different spray conditions. Spraying of pervoskites was found to be challenging and required careful parameter optimization in both spray methods. Microstructure and phase structure of the coatings were investigated. A very fine crack structure, possibly caused by low mechanical strength and low ductility of the compounds, was easily formed in coatings prepared by plasma and HVOF spraying. Spraying method, parameters and spraying atmospheres were found to affect the stability of the perovskite compounds due to low chemical stability at high spray temperatures. Oxygen deficiency or oxygen surplus was concluded to cause distortion of the compounds crystal structure, causing thus shifting of XRD-peaks due to change of lattice parameters. Electric conductivity was affected by the crystal structure.

In the past 25 years, we have seen thermal spray processes such as Atmospheric Plasma Spray (APS), High Velocity Oxygen Fuel Spray (HVOF) and Suspension Plasma Spraying (SPS) transition from research and development towards mainstream production technology. Since thermal spraying is a multi-parameter and multi-response process, it is susceptible to process instabilities. Consistency and repeatability are of utmost importance in production lines that spray high volumes of coatings. Since process parameters such as feed rate, particle velocity, temperature, electrode wear and nozzle blockages can dramatically affect the coating properties, monitoring and controlling these parameters can significantly improve the process repeatability. This study presents an improvement in the online process monitoring sensor, the Accuraspray 4.0, to achieve repeatable coatings. The Accuraspray 4.0 introduces a new process stability measurement in order to quantify and control the variability in thermal spray processes and to make process monitoring more production friendly. The sensor includes a stability analysis tool which compares the real-time standard deviations of nominal plume parameters against bench-marked values found using statistical principles. This study details the theory behind the process stability measurement and presents a case study conducted in collaboration with an automotive production facility employing this new feature. It was found that monitoring the standard deviations of parameters being measured can allow the user to control stability in their thermal spray process to spray large volumes with higher confidence in the consistency of their coatings. This upgrade brings the Accuraspray 4.0 one step closer to becoming the industry standard for thermal spray production facilities.

Zirconia stabilized with a combination of scandia and yttria (ScYSZ) powder for plasma spraying was synthesized by chemical coprecipitation process in the experiment, and the ScYSZ powder contains 7.0mol% scandia and 1.5mol% yttria. The microstructure, phase stability and thermal conductivity of plasma sprayed ScYSZ thermal barrier coatings (TBCs) were investigated. The results revealed that the ScYSZ TBCs had excellent stability to retain single metastable tetragonal t′phase even after high temperature (1500 °C) exposure for 300 hours and did not undergo a tetragonal-to-monoclinic phase transition upon cooling. Furthermore, the ScYSZ TBCs had lower thermal conductivity than 3.5-4.5mol% yttria-stabilized zirconia TBCs currently used in gas turbine engine industry. ScYSZ TBCs could be developed as a novel TBCs for advanced gas turbine engines.

Gadolinium zirconate (Gd 2 Zr 2 O 7 , GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3mol% yttria partially-stabilized zirconia (3YSZ) incorporation. The fracture toughness of the composite of 90mol%GZ-10mol% 3YSZ (GZ-YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ-YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock resistance of GZ-YSZ composite coating was improved significantly, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of nanostructured 3YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ and GZ-YSZ composite coatings were also discussed.

Phase stability of the thermal barrier deposits made from yttria-partially-stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000 °C, 1200 °C, and 1400 °C with times from 1 to 1000 hours has been evaluated using Rietveld analysis of neutron diffraction data. Results show that yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, the tetragonal phase transformed into monoclinic phase on cooling. An increase in monoclinic phase content was clearly observable after annealing 24 hours at 1400 °C and was nearly 35 % after 100 hours at 1400 °C. A similar trend was observed at 1200 °C for longer annealing times, with monoclinic phase formation beginning after 400 hours. At 1000 °C experimental times were not sufficient for monoclinic phase to form although a decrease in the yttria concentration in the tetragonal phase was observed. Keywords: neutron scattering, yttria-stabilized zirconia, phase composition, Rietveld analysis

In this work, CeO 2 -G d2 O 3 co-stabilized ZrO 2 (CGZ) thermal barrier coatings are deposited by solution precursor plasma spraying and the microstructure, phase stability, thermophysical properties, and thermal cycling behaviors of the resulting coatings are investigated and discussed in comparison to conventional 8YSZ coatings.

Residual stresses are inherent in thermal barrier coatings (TBC's) and can influence in-service performance and life of the coatings. Therefore, the effective design and processing of TBC's requires knowledge about residual stress generation and the effect of residual stresses on TEC life. Understanding residual stress generation and the effects on thermal barrier coating life are formidable tasks that have received little attention in the literature. This work addresses the first task. Specifically, the objectives of this work were to better understand how processing and post-processing residual stresses are generated in TBC's. The approach was to evaluate the effect of substrate temperature during processing and the effect of post-processing thermal cycling on the generation of coating residual stresses. Residual stress measurements were conducted using an experimental residual stress evaluation technique called the "Modified Layer Removal Method." Results showed residual stresses could be changed both by controlling the substrate temperature during processing and by thermal cycling after processing. Residual stresses in specimens with a higher substrate temperature during processing were found to be more compressive than residual stresses in specimens with a lower processing substrate temperature. Post-processing thermal cycling caused the residual stresses to become more compressive for specimens with both the higher and lower substrate processing temperatures. Residual stresses for one and ten post-processing thermal cycles were evaluated. For both substrate processing temperatures, the change in TBC compressive residual stresses for the first cycle was more than three times the total residual stress change that occurred in cycles two through ten. Interestingly, the increase in residual stresses in cycles two through ten for the higher substrate processing temperature was greater than that for the lower processing substrate temperature. In other words, based on results obtained here, compressive residual stresses generated during thermal cycling appear to depend on the existing processing residual stress. For these conditions, higher processing compressive residual stresses lead to higher post-processing changes in compressive stresses per thermal cycle.

The Young's modulus of the ceramic top coat of a plasma sprayed thermal barrier coating (TBC) has been reported to vary by as much as a factor of three with changes in processing parameters and by as much as a factor of four due to prolonged thermal exposure. Since the residual stress is expected to vary directly with the modulus of the ceramic layer, it follows that a change in modulus will change the residual stresses in the ceramic layer. The objective of this study was to evaluate the modulus of plasma sprayed coatings as a function of thermal cycle exposure and silica content of the ceramic. The study employed the Cantilever Beam Bending Method to examine Young's modulus for an yttria stabilized zirconia TBC applied by plasma spraying, for zero and ten thermal cycles and for silica contents of 0.1% and 1.0%. Results are discussed in terms of mechanisms that may affect modulus and the effect of modulus variations on residual stresses.

In this study, two types of thermal barrier coatings (TBC); duplex and functionally graded coatings were deposited on superalloy Nimonic 263 substrates using air plasma spray process. The duplex coating consists of YSZ top coat and NiCrAlY bond coat. The functionally graded coating consists of five layers with GZ as top layer, GZ+YSZ and YSZ+NiCrAlY as intermediate layers. The TBC samples were subjected to isothermal heat treatment at 1100 °C for 100 hours before undergoing thermal cyclic tests at 1200 °C up to 20% spallation to evaluate the oxidation and thermal fatigue resistance of the coatings. Results indicate that the functionally graded GZ TBC has a better cyclic life than the duplex YSZ TBC after isothermal heat treatment. The isothermal heat treatment also improved the thermal cyclic lifetime of the functionally graded GZ TBC by more than threefold in comparison to the as-sprayed GZ TBC.

Plasma sprayed coatings, thanks to their specific structure, are considered for applications with thermal shocks. Alumina, Alumina-Titania as well as less common spraying materials like mullite, forsterite and ilmenite were selected for testing. Carbon steel coupons were used as substrate. Flame-sprayed bond coats of Ni-Cr and Ni-Al under plasma sprayed ceramic coating were applied for comparison in selected cases. Two different regimes were used to simulate high and low temperature thermal cycling. Before the test and after given number of cycles coating's adhesion was tested by a standard tensile test (EN 582). After the test also the microstructure of samples was observed to evaluate the damage mechanism. It has been proved that Titania addition to Alumina improves the adhesion of coatings under thermal cycling, especially for the coatings without any bond coat. All materials had higher adhesion after each given number of cycles with bond coat than without. Cracking occurred dominantly on the boundary with the base in the case of pure Alumina and mullite while a cohesive cleavage is preferred in ilmenite and combination of both types is typical for other tested materials.

Stresses developed within a thermal barrier coating (TBC) due to the mismatch in thermal expansion of different coating components can cause coating failure. Nanostructured materials have an increased volume fraction of grain boundaries and this microstructural attribute may allow coatings to relieve the strain in the coating structure thereby improving the effectiveness and the lifetime of the TBC. Multi – layered TBCs were prepared using two techniques; atmospheric pressure plasma spray (APS) using a commercial system, and reduced pressure plasma spray using the Triple Torch Plasma Reactor (TTPR). The coatings were deposited on mullite and on NiCrAlY-coated steel substrates, and consisted of an inter – layer of nano-phase partially stabilized zirconia (n – PSZ) and a layer of conventional partially yttria stabilized zirconia coating (c – YSZ) as the top thermal barrier coat. The coatings were heat treated at various temperatures and the microstructural changes analyzed using scanning electron microscopy (SEM) images. Mechanical properties of the coating were studied using four point bend testing to better understand the effect of the n-PSZ inter-layer on the strain relief mechanisms that may be operative within the TBC.