Combustion Test Rig Research Articles

Tailored microstructures of gadolinium zirconate/YSZ multi-layered thermal barrier coatings produced by suspension plasma spray: Durability and erosion testing

This work employed an axial suspension plasma spray (SPS) process to deposit two different gadolinium zirconate (GZ) based triple layered thermal barrier coatings (TBCs). The first was a ‘layered’ TBC (GZ dense/GZ/YSZ) where the base layer was YSZ, intermediate layer was a relatively porous GZ and the top layer was a relatively dense GZ. The second triple layered TBC was a ‘composite’ TBC (GZ dense/GZ + YSZ/YSZ) comprising of an YSZ base layer, a GZ + YSZ intermediate layer and a dense GZ top layer. The as sprayed TBCs (layered and composite) were characterized using SEM/EDS and XRD. Two different methods (water intrusion and image analysis) were used to measure the porosity content of the as sprayed TBCs. Fracture toughness measurements were made on the intermediate layers (GZ + YSZ layer of the composite TBC and porous GZ layer of the layered TBC respectively) using micro indentation tests. The GZ + YSZ layer in the composite TBC was shown to have a slightly higher fracture toughness than the relatively porous GZ layer in the layered TBC. Erosion performance of the as sprayed TBCs was evaluated at room temperature where the composite TBC showed higher erosion resistance than the layered TBC. However, in the burner rig test conducted at 1400 °C, the layered TBC showed higher thermal cyclic lifetime than the composite TBC. Failure analysis of the thermally cycled and eroded TBCs was performed using SEM and XRD.

The influence of air-stage method on flameless combustion of coal gasification fly ash with coal self-preheating technology

As the by-product of circulating fluidized bed (CFB) coal gasification, coal gasification fly ash (CGFA) contains low volatile matter (lower than 6 wt%) and high ash matter (higher than 40 wt%), which cannot be burnt steadily and effectively with conventional combustion technologies. However, the annual output of CGFA is enormous in China and the available energy is considerable. Therefore it is necessary to achieve a highly efficient and clean combustion of CGFA. As a novel combustion technology, coal self-preheating combustion has proven its potential to utilize semi-coke, anthracite and other flammable solid fuels, which can also be applied for CGFA. In this paper, flameless mode of CGFA was achieved in a 30 kW coal self-preheating combustion test rig and the influence of air-stage method on image luminosity, temperature profiles and NOX emissions was studied. The results show that stable flameless combustion of CGFA can be easily established, and in the combustion zone, the image luminosity is uniform and no obvious flame fronts can be observed. Compared with flame mode, the combustion zone is enlarged in flameless mode and the temperature profile is more uniform. The effect of the tertiary air nozzle position on temperature profiles is greater than that of the secondary air equivalence ratio. And with the increase of λ2, the temperatures of zone close to the preheated fuel nozzle decrease. The oxygen concentration along the axis of the down-fired combustor is uniform with the peak value less than 6 vol% for all cases. The exit NOX emissions can be lowered down a lot for flameless combustion, and the general emissions are less than 100 mg/Nm3 (@ 6% O2) with the lowest value even less than 50 mg/Nm3 (@ 6% O2).

Flicker measurement of burner flames through electrostatic sensing and spectral analysis

Measurement of the flicker frequency of a burner flame is desirable to achieve an in-depth understanding of the dynamics of combustion processes. This paper presents a novel method based on electrostatic sensing to measure the flicker frequency of a hydrocarbon flame. The flame flickering behaviour is determined by measuring the concentration changes of ions in the flame and characterized by processing the signals from an electrostatic sensor. Power spectral analysis of the sensor signals is performed to obtain their frequency components. Experimental tests were conducted on a laboratory-scale combustion test rig for different equivalence ratios. Flame images were simultaneously acquired using a high-speed camera to provide a direct comparison between the results from the electrostatic sensors and those from the imaging approach. It is found that the flickering frequency of the burner flame decreases with the equivalence ratio. The results obtained demonstrate that the electrostatic sensing method can measure the flickering frequency of the flame and is useful for the assessment of flame stability.

Experimental study on combustion, flame and NOX emission of pulverized coal preheated by a preheating burner

Experimental researches on combustion characteristics of pulverized Ningdong bitumite preheated by a preheating burner were carried out in a 0.2 MW coal preheating combustion test rig, and the effects of preheating temperature, secondary air equivalence ratio and the positions of tertiary air nozzles on combustion, flame and NOX emissions of preheated fuels were studied. The results showed that the 0.2 MW coal preheating combustion test rig can operate stably and the combustion efficiency can be higher than 97% while the NOX emissions are lower than 100 mg/Nm3. With the increase of preheating temperature, NOX emissions increase, and the combustion efficiency does not change obviously. With the increase of secondary air equivalence ratio, the combustion efficiency increases, and NOX emissions increase as well. With the increase of the distance between the tertiary air nozzle and the preheated fuel nozzle, NOX emissions decrease, and the combustion efficiency decreases as well. The flame characteristic is not sensitive to preheating temperature, but closely related to the air distribution of down-fired combustor. With reasonable air supply positions, flameless combustion can be realized.

Evolution of Residual Stresses in PS-PVD Thermal Barrier Coatings on Thermal Cycling

Plasma spray-physical vapor deposition (PS-PVD) is an advanced technique to fabricate quasi-columnar structured thermal barrier coatings (TBCs) with excellent thermal cyclic lifetime. In this study, PS-PVD TBCs were investigated via burner rig test. The residual stresses in both of the topcoat layer and the thermally grown oxide (TGO) scale were measured non-destructively using Raman spectroscopy and Cr3+ photoluminescence piezo-spectroscopy, respectively. Evolution of the microstructures and distribution of residual stresses in such kind structured TBCs before and after thermal cycling test were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly and then essentially maintained at a relatively stable state with further testing. Moreover, the pre-heating of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat.

Investigation of Combustion of Emulated Biogas in a Gas Turbine Test Rig

Combustion of biogas in gas turbines is an interesting option for provision of renewable combined heat and power from biomass. Due to an increasing share of fluctuating renewable energies in the power grid (especially from wind and solar power), flexible power generation is of increasing importance. Additionally, with an increasing share of agricultural and municipal waste in biogas production, biogas composition is expected to be within a broader range. In this paper, the combustion of synthetic biogas (carbon dioxide and methane) in a combustion test rig with a swirl burner and a high pressure optical chamber is researched at different conditions. Results are compared to a CHEMKIN-PRO simulation using a detailed reaction mechanism. The results show that within the researched experimental matrix, stable biogas combustion for gas turbines can be achieved even with significantly changing gas composition and nominal power. Carbon dioxide concentration is varied from 0 to 60%. CO concentrations (normalized to 15% O2) in the flue gas do not change significantly with increasing carbon dioxide in the fuel gas and, for the researched conditions, stayed below 10 ppm. NOx concentration is below 10 ppm (normalized to 15% O2) for pure methane, and is further decreasing with increasing carbon dioxide share in the fuel gas, which is mainly due to changing reaction paths as reaction analysis showed. Thermal load of the combustor is varied from 100% to 20% for the reference gas composition. With decreasing thermal load, normalized carbon monoxide flue gas concentration is further reduced, while NOx concentrations are remaining at a similar level around 5 ppm (normalized to 15% O2).

Experimental Research on Flameless Combustion with Coal Preheating Technology

As a novel combustion technology, flameless combustion is attractive to achieve highly efficient and low NOX utilization of fuels. In this paper, a new method to realize flameless combustion of pulverized coal based on coal preheating technology was adopted. Experimental research on flameless combustion was implemented with pulverized coal in a 30 kW coal preheating and combustion test rig. In the study, coal preheated characteristics, combustion behavior, and the formation mechanism of NOX are discussed. The results show that the temperature of pulverized coal can be preheated up to above 850 °C, and most of the volatiles are released during the preheating process. Only 33.90% of the fixed carbon in pulverized coal is converted into coal gas and most remains in the high temperature coal char, and 91.14% of the fuel-N released from pulverized coal is converted to N2. Stable flameless combustion can be achieved with preheated fuels in the down-fired combustor, and no obvious flame front can be observed thr...

Structure-property relationship and design of plasma-sprayed La2Ce2O7/8YSZ composite coatings for gas turbine blades

Novel lanthanum-cerium oxide/8 wt% yttria partially stabilized zirconia (LC/8YSZ) thermal barrier coatings (TBCs) were deposited by supersonic atmospheric plasma spraying. The thermal insulation temperature and thermal shock resistance of LC/8YSZ double-ceramic-layer TBCs (DCL-TBCs) were quantitatively evaluated by a burner rig test. The results showed that the thermal insulation temperature increased with the increase of LC layer thickness in DCL-TBCs. When the thickness ratio between LC layer and 8YSZ layer was close to 1:1, the DCL-TBCs had the highest thermal shock resistance. LC/8YSZ thickness ratio significantly affected the energy release rate and the stress induced by thermal gradient or sintering. The sintering stress was found to be the main reason that caused the delamination of LC layer, however, the stress induced by thermal gradient resulted in the spallation of YSZ layer.

Combustion characteristics of syngas on scaled gas turbine combustor in pressurized condition: Pressure, H2/CO ratio, and N2 dilution of fuel

To facilitate fuel interchangeability for industrial gas turbines, various kinds of fossil fuels including syngas from coal gasification and steel mill gases including Blast Furnace Gas (BFG) have been investigated by previous researchers to determine their viability as alternative fuels for power generation. In the present study, various syngas components produced from coal gasification were combined to produce a simulated mixed gas, which was fed into a pressurized gas turbine combustion test rig to determine the combustion characteristics. A scaled model of the 7EA combustor from GE was used to conduct a basic comparative study on the emission trend (NOx), exhaust gas temperature distribution according to the combustor pressure, syngas composition and dilution ratio at the fuel side. The major findings were that modification of the empirical formula for pressure affects NOx emission and nitrogen dilution ratio affects the temperature profile uniformity on the fuel side.

Towards acoustic condition monitoring for detection and characterisation of laser-induced breakdown in a gas turbine laser ignition system

Acoustic detection and characterisation of laser-induced breakdown is an attractive proposition in laser ignition systems in which condition monitoring is necessary but where optical access for monitoring purposes is impractical. This paper presents a signal processing method based on wavelet decomposition for the non-invasive detection of acoustic emissions resulting from laser-induced breakdown in an atmospheric pressure combustion test-rig, representative of a single combustion chamber in a sub-15 MW industrial gas turbine. The probability and consistency of laser-induced breakdown is determined from the acoustic signal and used to characterise the operating conditions and identify abrupt and incipient or slowly developing faults.

Assessment of gas radiative property models in the presence of nongray particles

ABSTRACTIn this study, a radiation code based on the method of lines solution of the discrete ordinates method for the prediction of radiative heat transfer in nongray gaseous media is developed by incorporation of two different spectral gas radiative property models, banded spectral line-based weighted sum of gray gases (banded SLW) and gray wide band (GWB) approximation in the presence of nongray absorbing–emitting–scattering particles. The aim is to introduce an accurate and CPU efficient spectral gas radiation model, which is compatible with spectral fuel/ash particle property models. Input data required for the radiation code and its validation are provided from two combustion tests previously performed in a 300 kWt atmospheric bubbling fluidized bed combustor test rig burning low calorific value Turkish lignite with high volatile matter/fixed carbon (VM/FC) ratio in its own ash. The agreement between wall heat fluxes and source term predictions obtained by global and banded SLW models reveal that global SLW model can be converted to an accurate wide band gas model (banded SLW) which can directly be coupled with spectral particle radiation. Furthermore, assessment of GWB approximation by benchmarking its predictions against banded SLW model shows that GWB gives reasonable agreement with a higher CPU efficiency when the particle absorption coefficient is at least in the same order of magnitude with the gas absorption coefficient.

Assessment of primary air on corn straw in a fixed bed combustion using Eulerian-Eulerian approach

In this paper, mathematical modelling is conducted on the combustion of corn straw in a one-dimensional bench combustion test rig, and the effects of the primary air flow rate are assessed over a wide range. Due to complex solid combustion mechanisms and inadequate knowledge of the process, the development of such combustion system is limited. Numerical modelling of this combustion system has some advantages over experimental analysis, although the development of a complete model for this type of combustion system remains a challenge. Due to its characteristic properties, modelling of biomass combustion has to overcome many difficulties. One such problem is displaying the process of initiating the combustion in numerical modelling. This study finds that the volatile release and combustion of char increases, thus increasing the amount of primary air up to a critical point, where the starting time of ignition becomes shorter as the primary air flow rate increases. The peak concentration of NO decreases with the increase of primary air, whereas with the increase in the amount of air, there is a reduction in the release of SO2 as well as a reduction in CO emissions in the bed.

Performance and Steady State Heat Transfer Analysis of Functionally Graded Thermal Barrier Coatings Systems

Thermal barrier coatings (TBC’s), typically 8 wt.% Yttria Stabilized Zirconia (8YSZ), in single layered configuration have been traditionally used in aerospace components to protect them from degradation at high temperatures and to improve the thermal efficiency of the system. This paper compares the performance of two types of TBC configurations: single layered and multilayered functionally graded materials (FGM). Aerospace alloy, Inconel 718 substrates, NiCrAlY bond coat (BC) and 8YPSZ top coat (TC) were the materials used. FGM configuration was used to improve the durability and life of the conventional TBC system by reducing the coefficient of thermal expansion (CTE) mismatch. The TBCs were subjected to thermal fatigue (thermal shock and thermal barrier test) in laboratory scale burner rig test and oxidation stability test in high temperature furnace upto 1000°C. The as-sprayed and thermal fatigue tested specimen were characterized by X-ray diffraction (XRD) analysis and Scanning Electron Microscope (micro-structure). Results are discussed in the light of suitability of coating configuration, thermal fatigue and spalling characteristics with reference to aerospace applications at temperatures in the 900°C to 1500°C range. Computational work was carried out comprising a simulation model involving the developed TBCs..

Improving Flexibility and Performance of Staged Pressurized Oxy-Combustion

Staged pressurized oxy-combustion (SPOC) is a novel coal combustion power technology being developed by Washington University in St. Louis (WUSTL) that offers the potential to deliver low-carbon power at a reduced cost. Oxy-combustion plants operate by removing most of the nitrogen contained in air prior to combustion, thereby burning fuel in a near-pure oxygen stream instead of air. This combustion produces a flue gas containing primarily carbon dioxide (CO2) and water vapor that allows relatively straightforward CO2 capture. First-generation, atmospheric oxy-combustion technology relies on a flue gas recycle (FGR) diluent to reduce the peak temperatures achieved and the resulting high thermal radiation levels that would otherwise occur in a fuel/oxygen only flame. In contrast, by staging the combustion in several steps, SPOC reduces the peak temperatures of combustion without resorting to a high degree of FGR to control flame temperatures. The SPOC process operates by utilizing two or more pressurized boiler modules connected in series to produce fuel staging; hence, only a portion of the fuel is combusted in any given combustion module. Subsequently the thermal energy released at each stage can be captured and removed from the gases prior to subsequent stages, when more fuel is introduced. This allows the SPOC process to operate with minimal FGR, avoiding the associated efficiency losses and additional costs. While the staging controls the heat release profile in SPOC, operating in a pressurized condition yields a more compact system, reducing modular boiler size and delivering enhanced heat transfer. Additionally, operating at elevated pressures ensures that the latent heat of moisture available from moisture in the fuel and generated by the combustion of the hydrogen content in the fuel can be captured as useful thermal energy for steam turbine integration. This is possible due to elevated dew point from the high partial pressure of the moisture generated in this process, which allows substantial feedwater heater bypassing to be achieved. The inclusion of this thermal energy, normally lost to the atmosphere in traditional combustion processes or to cooling systems in atmospheric oxy-combustion, allows the resultant net efficiency of the SPOC system to be up to 6 percentage points greater than first-generation systems. To begin the process of developing this technology for full-scale application, the Electric Power Research Institute, Inc. teamed up with WUSTL, American Air Liquide, Doosan Babcock, and the U.S. Department of Energy to investigate a practical and workable boiler design concept that would minimize process risks where possible. The SPOC system offers unique process flexibility opportunities due to the staged nature of the combustion, making it a prime candidate for future primary energy delivery where the ability to operate at reduced load in an efficient manner and ramp rapidly will be valuable for balancing intermittent renewable electricity generation. The efficiency of traditional coal-fired power plants is lower at reduced loads below the steam temperature control range due to the challenges in balancing the heat pickup between radiative and convective superheater and reheater surface. Introducing additional heating surface for optimal extended controlled low load operation will likely lead to excessive attemperation at high loads, ultimately leading to a poorer overall cycle efficiency. The SPOC process can facilitate the adjustment of fuel firing rates between stages, targeting heat pickup to where it is most needed and potentially can shut down stages for low-load operation, facilitating an increased steam temperature control range on turndown. The boiler design concept assessment is being carried out to identify the maximum permissible compactness of the SPOC boiler heating surfaces. The assessment will determine the minimum overall height that will deliver appropriate tube operating metal temperatures at full load and achieve rated reheat steam temperatures at low operating loads, balanced against the needs of efficient coal combustion, and the potential resultant slagging and ash environments. The design parameters are being validated by combustion testing in the 100-kWth pressurized combustion test rig at WUSTL. Parameters being investigated include flame stability, fuel burnout, ash composition, radiative heat flux, and temperature profiles. These are being carried out for a range of inlet gas conditions and compositions and at multiple load conditions to investigate combustion stability at reduced load. The results of this testing inform the modeling of the large-scale system, that was sized at 550 MWe to allow direct comparison with the National Energy Technology Laboratory baseline cases for atmospheric oxy-combustion, air firing, and air firing with post-combustion capture.

Experimental research on combustion characteristics of coal gasification fly ash in a combustion chamber with a self-preheating burner

Coal gasification fly ash (CGFA) is the by-product of coal gasification in a circulating fluidized bed (CFB) and is characterized by its low volatile-content, low carbon-content, and high ash-content, which can be used as a secondary fuel. In China, the annual output of CGFA is enormous; therefore, the realization of a highly efficient CGFA combustion method would be beneficial to achieve better overall coal utilization. Nevertheless, it is challenging to burn CGFA steadily with conventional technologies because of its low volatile content. However, the coal preheating combustion technique has been proven to be an effective method to burn semi-coke and anthracite, both of which are poorly flammable, and could also be applied to CGFA. We have used an advanced preheating combustion technique with a self-preheating burner and carried out experimental investigations into CGFA combustion in a 0.4-MW CGFA preheated combustion test rig. The results show that the preheated combustion system ran smoothly and the CGFA combustion efficiency reached 98.6% with NOX emissions of 155 mg/m3 (6% O2). Therefore, the highly efficient and clean combustion of CGFA was realized, and the combustion of poorly flammable fuels using a self-preheating burner was shown to be feasible. The effects of the primary air ratio, secondary air ratio, and the position of the tertiary air inlet on the combustion characteristics and NOX emissions of CGFA were also studied.

NOx-Formation and CO-Burnout in Water-Injected, Premixed Natural Gas Flames at Typical Gas Turbine Combustor Residence Times

With the transition of the power production markets toward renewable energy sources, an increased demand for flexible, fossil-based power production systems arises. Steep load gradients and a high range of flexibility make gas turbines a core technology in this ongoing change. In order to further increase this flexibility research on power augmentation of premixed gas turbine combustors is conducted at the Lehrstuhl für Thermodynamik, TU München. Water injection in gas turbine combustors allows for the simultaneous control of NOx emissions as well as the increase of the power output of the engine and has therefore been transferred to a premixed combustor at lab scale. So far stable operation of the system has been obtained for water-to-fuel ratios up to 2.25 at constant adiabatic flame temperatures. This paper focuses on the effects of water injection on pollutant formation in premixed gas turbine flames. In order to guarantee for high practical relevance, experimental measurements are conducted at typical preheating temperatures and common gas turbine combustor residence times of about 20 ms. Spatially resolved and global species measurements are performed in an atmospheric single burner test rig for typical adiabatic flame temperatures between 1740 and 2086 K. Global measurements of NOx and CO emissions are shown for a wide range of equivalence ratios and variable water-to-fuel ratios. Cantera calculations are used to identify nonequilibrium processes in the measured data. To get a close insight into the emission formation processes in water-injected flames, local concentration measurements are used to calculate distributions of the reaction progress variable. Finally, to clarify the influence of spray quality on the composition of the exhaust gas, a variation of the water droplet diameters is done. For rising water content at constant adiabatic flame temperature, the NOx emissions can be held constant, whereas CO concentrations increase. On the contrary, both values decrease for measurements at constant equivalence ratio and reduced flame temperatures. Further analysis of the data shows the close dependency of CO concentration on the equivalence ratio; however, due to the water addition, a shift of the CO curves can be detected. In the local measurements, changes in the distribution of the reaction progress variable and an increase of the flame length were detected for water-injected flames along with changes of the maximum as well as the averaged CO values. Finally, a strong influence of water droplet size on NOx and CO formation is shown for constant operating conditions.

Overall cooling effectiveness of effusion cooled annular combustor liner at reacting flow conditions

A three-sector annular combustor test rig is built to carry out experimental investigation on the liner effusion cooling performance using steady state infrared radiation thermography method at reacting flow conditions. 7 rows of effusion holes in a staggered layout are distributed on both inner and outer liner bended testing plates, where the temperature field is captured and overall cooling effectiveness is analyzed. Coolant-to-main air flow rate ratio and equivalence ratio are varied to evaluate their effects on the cooling effectiveness. Three axial nozzles generating representative high swirling flows are fueled by methane-air mixture to perform premixed combustion. Besides, results for effusion holes in different inclination angle (90° and 30°) are compared. The results indicate that a region with low overall cooling effectiveness on the both liners for normal effusion holes due to the swirling flame impingement effect. Because of double length-to-diameter ratio, inclined holes at 30 degree perform better cooling protection than the normal holes. The variation of equivalence ratio induces the change of the flame pattern, temperature field and flow field, which will have an impact on the liner effusion cooling in consequence.

Convective Scaling of Intrinsic Thermo-Acoustic Eigenfrequencies of a Premixed Swirl Combustor

Spectral distributions of the sound pressure level (SPL) observed in a premixed, swirl stabilized combustion test rig are scrutinized. Spectral peaks in the SPL for stable as well as unstable cases are interpreted with the help of a novel criterion for the resonance frequencies of the intrinsic thermo-acoustic (ITA) feedback loop. This criterion takes into the account the flow inertia of the burner and indicates that in the limit of very large flow inertia, ITA resonance should appear at frequencies where the phase of the flame transfer function (FTF) approaches −π/2. Conversely, in the limiting case of vanishing flow inertia, the new criterion agrees with previous results, which state that ITA modes may arise when the phase of the FTF is close to −π. Relying on the novel criterion, peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. Various combustor configurations are investigated over a range of operating conditions. It is found that in this particular combustor, ITA modes are prevalent and dominate the unstable cases. Remarkably, the ITA frequencies change significantly with the bulk flow velocity and the position of the swirler but are almost insensitive to changes in the length of the combustion chamber (CC). These observations imply that the resonance frequencies of the ITA feedback loop are governed by convective time scales. A scaling rule for ITA frequencies that relies on a model for the overall convective flame time lag shows good consistency for all operating conditions considered in this study.

Failure Mechanism of Yttria Stabilized Zirconia Atmospheric Plasma Sprayed Thermal Barrier Coatings Subjected to Calcia-Magnesia-Alumino-Silicate Environmental Attack

The contribution focuses on the description of failure mechanism of atmospheric plasma sprayed multilayer thermal barrier coatings subjected to calcia-magnesia-alumino-silicate (CMAS) environmental attack. To identify exothermic and endothermic reactions which occurred during heating/cooling by means of calorimetry was also utilized initial yttria stabilized zirconia (YSZ) powder subsequently used for thermal spraying of multilayer thermal barrier coating system (TBCs), CMAS powder later on utilized for thin layer deposition and its mixture. Atmospheric plasma spray technique was used to produce the TBCs on a grit blasted nickel-based superalloy substrates, where CoNiCrAlY powder was used for deposition of a bond coat and YSZ powder was sprayed as a top coat. In accordance to the aerospace standard the thin layer of CMAS was deposited on as sprayed TBCs samples surface from its colloidal solution by paint brush method. Burner-rig test, utilizing direct propane-oxygen flame, was used for thermal cyclic exposition of the multilayer coated samples at the temperature of 1150 °C. Samples after thermal cyclic exposure test were investigated by means of materialographic analysis approaches. The significant reduction in life-time of CMAS coated YSZ top coat was observed due to lower melting point phase formation and molten silicate crystallization within the pores providing the spallation identified as a major mechanism of TBCs failure.

Oxidation of Al2O3 Scale-Forming MAX Phases in Turbine Environments

High temperature oxidation of alumina-forming MAX phases, Ti2AlC and Cr2AlC, were examined under turbine engine environments and coating configurations. Thermogravimetric furnace tests of Ti2AlC showed a rapid initial transient due to non-protective TiO2 growth. Subsequent well-behaved cubic kinetics for alumina scale growth were shown from 1273 K to 1673 K (1000 °C to 1400 °C). These possessed an activation energy of 335 kJ/mol, consistent with estimates of grain boundary diffusivity of oxygen (~375 kJ/mol). The durability of Ti2AlC under combustion conditions was demonstrated by high pressure burner rig testing at 1373 K to 1573 K (1100 °C to 1300 °C). Here good stability and cubic kinetics also applied, but produced lower weight gains due to volatile TiO(OH)2 formation in water vapor combustion gas. Excellent thermal stability was also shown for yttria-stabilized zirconia thermal barrier coatings deposited on Ti2AlC substrates in 2500-hour furnace tests at 1373 K to 1573 K (1100 °C to 1300 °C). These sustained a record 35 µm of scale as compared to 7 μm observed at failure for typical superalloy systems. In contrast, scale and TBC spallation became prevalent on Cr2AlC substrates above 1423 K (1150 °C). Cr2AlC diffusion couples with superalloys exhibited good long-term mechanical/oxidative stability at 1073 K (800 °C), as would be needed for corrosion-resistant coatings. However, diffusion zones containing a NiAl-Cr7C3 matrix with MC and M3B2 particulates were commonly formed and became extensive at 1423 K (1150 °C).