Search Results (18)

Oxidation of the bond coat during turbine operation leads to additional stresses in the thermal barrier coating (TBC) system that promotes spalling of the thermal insulation. Therefore, the oxidation behavior of a TBC system plays an important role in the thermal cycling of a TBC system. To delay the loss of thermal insulation, research has typically focused for a long time on the composition and microstructure of the ceramic topcoats and metallic bond coats. More recently, heat treatment for the diffusion annealing of the bond coat has also become a focus of research. Several studies have shown that pre-oxidation of the bond coat prior to the application of the ceramic topcoat slows down the subsequent oxidation of the bond coat in service. The improved thermal cyclability has been demonstrated in studies for systems with atmospheric plasma-sprayed (APS), suspension plasma-sprayed (SPS) or electron beam physical vapor deposition (EB-PVD) top coatings. However, no study has directly compared the effects of pre-oxidation on different topcoats. Therefore, this study compared the effect of pre-oxidation on APS and SPS coatings with the same bond coat. For both topcoats, pre-oxidation slowed the subsequent TGO growth and thus increased the lifetime of the coatings. The improvement in lifetime was particularly pronounced for the systems with an SPS topcoat. Overall, the lifetime of the coatings with an APS topcoat was higher as the critical energy release rate within the coating was not exceeded in these coatings. Full article

Columnar structured thermal barrier coatings (TBCs) have been intensively investigated due to their potential to enhance the durability and reliability of gas turbine engine components. These coatings consist of vertically aligned columns that provide excellent resistance to thermal cycling. In this study, the lifetime of columnar suspension-plasma-sprayed (SPS) TBCs was evaluated using burner rig tests. The tests were carried out under high-temperature conditions. Significantly, the pre-oxidation of the bondcoat during diffusion bonding treatment was found to have a substantial impact on the performance of the SPS TBCs. The optimized treatment resulted in columnar SPS TBCs demonstrating excellent thermal stability and resistance under the test conditions. The lifetime of the coatings was significantly extended compared to conventional TBCs by pre-oxidation of the CoNiCrAlY bondcoat in argon, which suggests that columnar SPS TBCs have great potential for use in gas turbine engines. Full article

Solid particle erosion (SPE) is a common phenomenon observed in gas turbine engines. Particles entrained in the gas flow impact engine hardware, resulting in micro-scale damage that leads to deleterious effects such as material removal. For protective coatings, damage due to SPE is a key concern, since it can negatively affect the durability of the coating and subsequently the life of the underlying component. In this work, the high-temperature SPE behavior of two state-of-the-art environmental barrier coatings (EBCs) deposited via air plasma spray (APS) is investigated using alumina erodent to understand the effect of particle kinetic energy, impingement angle, and temperature. The SPE behavior of the EBCs is also compared to APS and electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) to elucidate similarities and differences in the erosion response. The EBCs were more susceptible to SPE than the EB-PVD TBC but had greater SPE resistance compared to the APS TBC. Coating microstructure and porosity were shown to have a strong influence on the observed behavior. Full article

A FIB/STEM interfacial study was performed on a TBC/Ti₂AlC MAX phase system, oxidized in an aggressive burner rig test (Mach 0.3 at 1300 °C for 500 h). The 7YSZ TBC, α-Al₂O₃ TGO, and MAXthal 211^TM Ti₂AlC base were variously characterized by TEM/STEM, EDS, SADP, and HRTEM. The YSZ was a mix of “clean” featureless and “faulted” high contrast grains. The latter exhibited ferro-elastic domains of high Y content tetragonal t″ variants. No martensite was observed. The TGO was essentially a duplex α-Al₂O₃ structure of inner columnar plus outer equiaxed grains. It maintained a perfectly intact, clean interface with the Ti₂AlC substrate. The Ti₂AlC substrate exhibited no interfacial Al-depletion zone but, rather, numerous faults along the basal plane of the hexagonal structure. These are believed to offer a means of depleting Al by forming crystallographic, low-Al planar defects, proposed as Ti_2.5AlC_1.5. These characterizations support and augment prior optical, SEM, and XRD findings that demonstrated remarkable durability for the YSZ/Ti₂AlC MAX phase system in aggressive burner tests. Full article

During coal-based power generation, fuel oil is used to assist with ignition of pulverised coal. Fuel oil passes through several pieces of equipment on its way to the burner section of the boiler. In this article the focus is on the lubricity behaviour of three representative fuel oil types and on the potential blocking of filters and nozzles caused by the presence of unwanted components in these fuel oils. The high frequency reciprocating rig (HFRR) (ISO 12156-1) was used to determine the lubricity of these fuel oils at different temperatures. Results indicate that the presence of asphaltenes (components of heavy fuel oils with complex aromatic structures) changes the viscosities of fuel oils, which, in turn affect their lubricity behaviour. Medium wax-blend fuel oil (MFO) containing high molecular weight paraffins (wax), low concentrations of asphaltenes and solid particles caused less friction and wear (with coefficient of friction (COF) values below 0.1) and good high temperature performance. Crude-derived heavy fuel oil (HFO), containing high concentrations of asphaltenes and solid particles caused very high coefficients of friction (COF peaks above 0.3) and severe abrasive wear at high temperatures. Although the third fuel oil tested was a light cycle oil (LFO) and did not contain any asphaltenes, results indicated a sensitivity to oxidation, increasing with temperature, which can have an adverse effect on in situ performance. Full article

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd₂Zr₂O₇, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd₂Zr₂O₇ TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation. Full article

High-temperature operation service conditions can be used to evaluate the durability of Atmospheric Plasma-Sprayed Thermal Barrier Coating systems (APS-TBCs). To evaluate the durability of TBCs within their life span, two different thermal cycling testing results, i.e., isothermal furnace cycling and burner rig cycling tests, are utilized to numerically investigate possible crack driving forces that might lead to the failure of TBCs. Although there are many studies on failure and life prediction, there is still a lack of quantitative evaluation and comparison on the crack driving forces under these two different thermal cycling schemes. In this paper, by using modified analytical models, strain energy release rates (ERRs) are estimated and compared between these two testing approaches using experimental data. A new residual stress model was developed to study the position where the maximum residual stress occurs due to coefficient of thermal expansion (CTE) mismatch at different thermally grown oxide (TGO) thicknesses. The main crack driving forces are identified for two types of thermal cycling. A possible cracking route is found based on the calculated equivalent ERRs with respect to distance from the interface between the topcoat (TC)/TGO layers. The relationship between crack driving force of isothermal furnace and burner cycling tests is also elaborated. Full article

Thermal barrier coatings (TBCs) are commonly used to protect gas turbine components from high temperatures and oxidation. Such coatings consist of ceramic top coats and metallic bond coats. The mismatch in thermal expansion of the top coat, the bond coat and the component material is one main factor leading to the failure of the coating system. Columnar-structured top coats offer an enhanced tolerance to the strain during thermal cycling. On a flat bond coated surface, these TBCs reach higher thermal cycling performance. However, on rough surfaces, as used for thermal spray coatings, the performance of these thermal barrier coatings seems to be restricted or even stays below the performance of atmospheric-plasma-sprayed (APS) thermal barrier coatings. This low performance is linked to out-of-plane stresses at the interface between the top coat and the bond coat. In this study, a thin additional oxide-dispersion-strengthened (ODS) bond coat with high alumina content provides a reduced mismatch of the coefficient of thermal expansion (CTE) between the top coat and the bond coat. Columnar suspension plasma sprayed (SPS), yttria-stabilized zirconia (YSZ) TBCs were combined with low-CTE ODS bond coats. The behavior of these TBCs was characterized with respect to thermal cycling performance and degradation in a burner-rig facility. The comparison showed an up-to-four-fold increase in the performance of the new system. Full article

A fiber-coupled, compact, remotely operated laser absorption instrument is developed for CO, CO₂, and H₂O measurements in reactive flows at the elevated temperatures and pressures expected in gas turbine combustor test rigs with target pressures from 1–25 bar and temperatures of up to 2000 K. The optical engineering for solutions of the significant challenges from the ambient acoustic noise (~120 dB) and ambient test rig temperatures (60 °C) are discussed in detail. The sensor delivers wavelength-multiplexed light in a single optical fiber from a set of solid-state lasers ranging from diodes in the near-infrared (~1300 nm) to quantum cascade lasers in the mid-infrared (~4900 nm). Wavelength-multiplexing systems using a single optical fiber have not previously spanned such a wide range of laser wavelengths. Gas temperature is inferred from the ratio of two water vapor transitions. Here, the design of the sensor, the optical engineering required for simultaneous fiber delivery of a wide range of laser wavelengths on a single optical line-of-sight, the engineering required for sensor survival in the harsh ambient environment, and laboratory testing of sensor performance in the exhaust gas of a flat flame burner are presented. Full article

Effective testing of ceramic matrix composites (CMCs) and CMC/coating systems for high temperature, high stress, high velocity and/or severe oxidation/corrosion environments is a critical need in materials/coatings evaluation for extreme environments of hot section parts in jet engine and hypersonic applications. Most current technology can evaluate two or three of the extreme conditions for a given application; however, incorporating as many of the extreme thermo-mechanical-environmental factors is highly advantageous to understand combinatorial effects. A high velocity oxygen fuel (HVOF) burner rig offers an excellent platform to evaluate many of these extreme conditions. In this work, the following three different thermo-mechanical-environmental test conditions using an HVOF rig on SiC-based CMCs are highlighted: (1) fatigue at temperature for >Mach 1 velocity and high temperature compared to typical stagnant air test environment, (2) high temperature hard particle erosion at temperature for ≤Mach 1 conditions and (3) ~Mach 5 near-hypersonic velocity conditions at very high temperature exposure. Full article

Turbine environments may degrade high temperature ceramics because of volatile hydroxide reaction products formed in water vapor. Accordingly, the volatility of transient TiO₂ and steady-state Al₂O₃ scales formed on the oxidation-resistant Ti₂AlC MAX phase ceramic was examined in 1300 °C high velocity (Mach 0.3, 100 m/s) and high pressure (6 atm, 25 m/s) burner rig tests (BRT). Unlike metals, the ceramic was stable at 1300 °C. Unlike SiC and Si₃N₄, neither burner test produced a weight loss, unless heavily pre-oxidized. Lower mass gains were produced in the BRT compared to furnace tests. The commonly observed initial, fast TiO₂ transient scale was preferentially removed in hot burner gas (~10% water vapor). A lesser degree of gradual Al₂O₃ volatilization occurred, indicated by grain boundary porosity and crystallographic etching. Modified cubic-linear (growth-volatility) kinetics are suggested. Gas velocity and water vapor pressure play specific roles for each scale. Furthermore, a 7YSZ TBC on Ti₂AlC survived for 500 h in the Mach 0.3 burner test at 1300 °C with no indication of volatility or spalling. Full article

The oxidation and corrosion behavior at elevated temperatures of a SiC_F/SiC(N) composite with two plasma-sprayed environmental barrier coating (EBC) systems were studied. After both processes, the formation of a silica-based thermally grown oxide (TGO) layer was observed. The formation of this TGO caused two principal failure mechanisms of the EBC. Firstly, spallation of the EBC induced by stresses from volume expansion and phase transformation to crystalline SiO₂ was observed. Water vapor corrosion of the TGO with gap formation in the top region of the TGO was found to be a second failure mechanism. After a burner rig test of the Al₂O₃-YAG EBC system, this corrosion process was observed at the TGO surface and in the volume of the Al₂O₃ bond coat. In the case of the second system, Si-Yb₂Si₂O₇/SiC-Yb₂SiO₅, the formation of the TGO could be delayed by introducing an additional intermediate layer based on Yb₂Si₂O₇ filled with SiC particles. The SiC particles in the intermediate layer were oxidized and served as getter to reduce the permeation of oxidants (O₂, H₂O) into the material. In this way, the formation of the TGO and the failure mechanisms caused by their formation and growth could be delayed. Full article

Ceramic matrix composites (CMCs) are promising materials for high-temperature applications. Environmental barrier coatings (EBCs) are needed to protect the components against water vapor attack. A new potential EBC material, YAlO₃, was studied in this paper. Different plasma-spraying techniques were used for the production of coatings on an alumina-based CMC, such as atmospheric plasma spraying (APS) and very low pressure plasma spraying (VLPPS). No bond coats or surface treatments were applied. The performance was tested by pull–adhesion tests, burner rig tests, and calcium-magnesium-aluminum-silicate (CMAS) corrosion tests. The samples were subsequently analyzed by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Special attention was paid to the interaction at the interface between coating and substrate. The results show that fully crystalline and good adherent YAlO₃ coatings can be produced without further substrate preparation such as surface pretreatment or bond coat application. The formation of a thin reaction layer between coating and substrate seems to promote adhesion. Full article

Higher durability in thermal barrier coatings (TBCs) is constantly sought to enhance the service life of gas turbine engine components such as blades and vanes. In this study, three double layered gadolinium zirconate (GZ)-on-yttria stabilized zirconia (YSZ) TBC variants with varying individual layer thickness but identical total thickness produced by suspension plasma spray (SPS) process were evaluated. The objective was to investigate the role of YSZ layer thickness on the durability of GZ/YSZ double-layered TBCs under different thermal cyclic test conditions i.e., thermal cyclic fatigue (TCF) at 1100 °C and a burner rig test (BRT) at a surface temperature of 1400 °C, respectively. Microstructural characterization was performed using SEM (Scanning Electron Microscopy) and porosity content was measured using image analysis technique. Results reveal that the durability of double-layered TBCs decreased with YSZ thickness under both TCF and BRT test conditions. The TBCs were analyzed by SEM to investigate microstructural evolution as well as failure modes during TCF and BRT test conditions. It was observed that the failure modes varied with test conditions, with all the three double-layered TBC variants showing failure in the TGO (thermally grown oxide) during the TCF test and in the ceramic GZ top coat close to the GZ/YSZ interface during BRT. Furthermore, porosity analysis of the as-sprayed and TCF failed TBCs revealed differences in sintering behavior for GZ and YSZ. The findings from this work provide new insights into the mechanisms responsible for failure of SPS processed double-layered TBCs under different thermal cyclic test conditions. Full article

A combustion facility which includes uniaxial mechanical loading was implemented that enables environmental conditions more akin to jet engine environments compared to conventional static environment tests. Two types of woven SiC/SiC ceramic matrix composites (CMCs), melt-infiltrated (MI) and chemical vapor infiltrated (CVI), were subjected to fatigue loading in the combustion facility and under isothermal furnace conditions. Some CVI test coupons were coated with a multilayer environmental barrier coating (EBC) of mullite + ytterbium monosilicate using slurry infiltration process to demonstrate the performance with a coating. Combustion conditions were applied using a high velocity oxy fuel gun on the front side of the specimen and mechanical loading was applied using a horizontal hydraulic MTS machine. All the specimens considered were subjected to tension-tension fatigue loading at 100 MPa, stress ratio of 0.1 and specimen front-side surface temperature of 1200 ± 20 °C. Nondestructive evaluation (NDE) methods, such as electrical resistance (ER), was used as an in-situ health monitoring technique. Similar fatigue tests were performed in an isothermal furnace for comparison. A much lower fatigue life was observed for the uncoated specimens tested under combustion conditions in comparison to isothermal furnace condition. This difference in fatigue life was attributed to damage associated with added thermal stress due to the thermal gradient and higher rate of oxidative embrittlement due to the presence of high velocity combustion gases in the combustion environment. EBC coating increased the fatigue life in combustion environment. However, EBC coated specimens experienced spallation in the high-velocity flame due to the presence of micro cracks in the coating surface. Fracture surfaces of the failed specimens were investigated under the scanning electron microscope (SEM) to determine the extent of oxidation and damage. Full article