Mario Rudolphi

Prediction of Mechanical Scale Failure – Current Status and Perspectives

A new model concept for predicting mechanical oxide scale failure is applied to Al2O3, Cr2O3, Fe3O4 and NiO. The calculated critical strain values are plotted versus the physical defect size using a simplified version of the original h-w-concept. A limited number of experimental data existing in the literature were entered into the plots and yield satisfactory agreement with the model data. Future efforts should focus on extending the experimental data basis and converting these data into h-values for the model.

Investigations for the Validation of the Defect Based Scale Failure Diagrams—Part II: Extension of the Concept and Application to Nickel Oxide, Titanium Oxide and Iron Oxide

The chemical stability of oxide scales and the oxide growth kinetics are important factors to consider when choosing a material for high temperature application. Low oxide growth rates and good chemical stability are, however, not the only aspects to be taken into account. The mechanical stability of the oxide scale formed can also play a significant role, especially when external loads or fast heating or cooling rates come into play. In this work, experimental data on oxide scale failure and a defect based scale failure model are used to calculate mechanical stability diagrams for titanium oxide and iron oxide. For these diagrams the original η-c-approach is extended by a term characterizing the level of residual strains in the scale. In addition to titanium and iron oxide this extended approach is also applied to former measurement data on nickel oxide. With the stability diagrams developed it is possible to estimate the maximum tolerable strain for the oxide scale as a function of the physical defect situation in the scale. Metallographic inspection and 4-point bending tests are used to derive the mechanical stability parameter η and the parameter εr for the residual strain. Once these parameters are known, metallographic inspection alone is sufficient to estimate the remaining tolerable load or strain limit after a certain oxidation period.

The effect of moisture on the delayed spallation of thermal barrier coatings: VPS NiCoCrAlY bond coat+APS YSZ top coat

Abstract A fundamental understanding of failure mechanisms for thermal barrier coatings (TBC) is important for accurate life-time prediction and hence of much interest for industry. Failure (i.e. spallation or cracking) of the TBC usually occurs immediately upon cooling the specimen. However, in some cases spallation of the TBC is observed with a delay of several hours or even days after cooling, when the specimen is at ambient temperature and exposed to laboratory air. Because laboratory air contains water vapour, one hypothesis is that water plays a role in delayed failure of TBCs. This hypothesis is strongly supported by experiments in which the application of liquid water to a pre-oxidized TBC leads to spontaneous spallation/delamination at room temperature. The aim of this work is to study the effect of moisture on TBC systems in more detail. A series of experiments including acoustic emission techniques for in situ detection of cracking within the specimen and nuclear reaction analysis to determine hydrogen concentration depth profiles support the proposed hypothesis. Optical micrographs of APS TBCs isothermally oxidized at 1100°C show increased inward growing oxidation in cauliflower-like structures for specimens oxidized in moist atmospheres.

Fireside Corrosion of Chromium- and Aluminum-Coated Ferritic–Martensitic Steels

In modern fossil power plants, biomass is used more and more as secondary fuel in addition to coal. This leads to a significant decrease of the carbon footprint of such power plants. However, the demands on the corrosion resistance of the materials in the boilers increase because of chlorine in the atmosphere and salt-containing sulfides and chlorides. Heat-resistant ferritic–martensitic steels such as P91 are of great interest as superheater material. However, their corrosion resistance has to be improved for an application in modern fossil power plants with biomass combustion. For this purpose, chromium and aluminum diffusion coatings were developed and applied on P91 steel. The uncoated and coated material was investigated in a simulated biomass–brown coal ash with CaSO4, Na2SO4, K2SO4, KCl, and Al2O3 deposits and an atmosphere containing nitrogen with H2O, CO2, O2, SO2, and HCl. The improvement of the corrosion resistance is illustrated using metallographic methods such as electron probe micro-analysis.

Nitrogen Self-Diffusion in Polycrystalline Si3N4 Films: Isotope Heterostructures vs. Gas-Exchange

The self-diffusion of nitrogen is investigated in polycrystalline thin silicon nitride films using a gas-exchange method (14N2/Si3 15N4) in comparison to Si3 14N4/Si3 15N4/Si3 14N4 isotope heterostructures. The films are produced by reactive r. f. magnetron sputtering. Depth profile analysis is carried out with secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS), and nuclear resonant reaction analysis (NRRA). The nitrogen diffusivities determined with the use of isotope heterostructures follow an Arrhenius law in the temperature range between 1200 and 1700 °C with an activation enthalpy of DH = 4.9 eV and a pre-exponential factor of D0 = 1 x 10-6 m2/s, indicating a conventional diffusion mechanism via localized point defects. Using the gas-exchange method, the nitrogen diffusivities could be obtained only in the temperature range between 1600 and 1700 °C. This is due to the fact that at temperatures below 1600 °C the surface exchange process with its high activation enthalpy (about 10 eV) is rate limiting, leading to non detectable diffusion profiles. The application of the different methods of depth profiling leads to the same diffusivities within estimated errors.

Hydrogen Detection in Buried Layers of Thermal Barrier Coatings

Thermal barrier coatings used in airplane engines or land-based gas turbines can show catastrophic failure (i. e. spallation) typically during cooldown due to thermal expansion mismatch stresses. However, it is also often noted that spallation occurs minutes, hours, or even days after the sample is cold. This type of delayed failure, called “desk top spallation” is, up to now, not fully understood and therefore a field of great interest. Because desk top failure occurs in ambient air, the working hypothesis is that water vapor from the office environment plays a role. Consequently, a number of experiments have been designed to verify this hypothesis. The experiments include more traditional approaches like acoustic emission measurements during cyclic oxidation, but also innovative new approaches like acoustic emission during water drop testing, and hydrogen detection at the interface to the thermally grown oxide using ion beam techniques.