Effect of bondcoat roughness on lifetime of APS-TBC systems in dry and wet gases

Low pressure plasma spraying (LPPS) is a process commonly used for deposition of MCrAlY (M=Ni,Co) bondcoats for air plasma spray thermal barrier coatings (APS-TBCs). LPPS produces bondcoats with a high roughness and good oxidation resistance, which are known to play a key role for long lifetimes of APS-TBC’s. An alternative process for the bondcoat deposition is high velocity oxy-fuel (HVOF), which is substantially cheaper than LPPS but even with well optimized spraying parameters generates intrinsically lower bondcoat roughness. In the present work it is shown that a bi-layer MCrAlY-bondcoat consisting of an HVOF-base layer and an upper, thin APS-flashcoat of the same chemical composition can provide cyclic oxidation TBC-lifetimes, which are similar to those obtained with well optimized LPPS bondcoats. The key points for the extended lifetime are the specific roughness profile and microstructure of the flashcoat, which allow good adhesion of the topcoat combined with an excellent oxidation resistance. Testing of the TBC-system with the APS-flashcoat in the atmosphere with increased amount of water vapour relevant for gas-turbine operation on alternative, hydrogen rich fuels revealed some lifetime shortening with respect to the drier test gas. However, even under these more aggressive conditions, the measured cyclic furnace lifetimes of samples with APS-flashcoat are a factor of 2 to 3 longer than those of the reference TBC-system with the state of the art HVOF bondcoat. Depending on the actually prevailing coating system and test conditions, the life times of the coatings were even longer than for coating systems which were completely manufactured using LPPS. In order to correlate the bondcoat roughness profile with the APS-TBC-lifetime an alternative method based on fractal analysis is proposed. Using this method, a more accurate description of complex bondcoat surface morphologies and a better correlation with the TBC-lifetime are obtained than with the commonly used mean roughness amplitude (Ra) approach

MEM-BRAIN gas separation membranes for zero-emission fossil power plants

The aim of the MEM-BRAIN project is the development and integration of gas separation membranes for zero-emission fossil power plants. This will be achieved by selective membranes with high permeability for CO2, O2 or H2, so that high-purity CO2 is obtained in a readily condensable form. The project is being implemented by the “MEM-BRAIN” Helmholtz Alliance consisting of research centres, universities and industrial partners.\ud \ud The MEM-BRAIN project focuses on the development, process engineering, system integration and energy systems analysis of different gas separation membranes for the different CO2 capture process routes in fossil power plants

Studies of Alkali Sorption Kinetics for Pressurized Fluidized Bed Combustion by High Pressure Mass Spectrometry

This work describes the first approach to use High Pressure Mass Spectrometry (HPMS) for the quantification and analysis of alkali species in a gas stream downstream a sorbent bed of different tested alumosilicates

Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia

Yttria-stabilized zirconia powders, containing different levels of SiO2 and Al2O3, have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<_50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes

Influence of bondcoat creep and roughness on damage and lifetime of ZrO 2_{2} TBCs for gas turbines under thermocyclic loads

As a simplified model system for thermal barrier coatings (TBCs) in gas turbines, Y2O3 partially stabilized ZrO2 TBCs were applied on FeCrAlY substrates. The creep strength of the substrate was varied by using an oxide-dispersion-strengthened (ODS) alloy and a non-ODS alloy with similar chemical composition. Defined interface profiles were produced before coating. Creep properties of the oxide layer between substrate and TBC were varied by either coating the test pieces with nanocrystalline PVD-alumina or with coarser grained naturally grown Al2O3 before plasma spraying the TBC. During thermal cycling (Tmax=1050°C, Tmin=60°C, dwell of 2 hours at Tmax) periodic 2-d interfaces resulted in very low lifetime independent from substrate strength and interface oxide type. With stochastic 3-d interfaces lifetimes up to 900 cycles were reached, especially for the substrate with low creep strength combined with a coarse grained alumina interlayer