Daniel Renusch

Bond coat oxidation and its significance for life expectancy of thermal barrier coating systems

Abstract The time and temperature dependent evolution of the microstructure of thermal barrier coating systems under isothermal conditions between 950 and 1100 ° C up to 5000 h are investigated for two APS - TBC systems and two EB-PVD systems. Kinetics for the thermally grown oxide thickness values, the β phase depletion underneath the oxide scale and the physical defects in and around the thermally grown oxide are determined by extensive SEM and subsequent interactive image analysis. In the case of physical defects, the size of pores, interacting pore populations and maximum crack lengths are measured. Additionally, the latter are classified with respect to their local orientation in the thermally grown oxide or its vicinity. Finally, the results are discussed with regard to their significance in lifetime modelling of gas turbine components.

Progress in life time modeling of APS-TBC Part I: residual, thermal and growth stresses including the role of thermal fatigue

Abstract For about the last two decades there has been an effort to produce a reasonable life time model for the thermal barrier coating systems (TBC) that are used in the gas turbine industry. However, recent advances in testing technology, namely acoustic emission (AE) analysis and Raman spectroscopy, have provided many new insights into the life of TBCs. This new technology used in conjunction with more traditional testing, such as the four point bend mechanical test, has provided much needed data for the development of a life time model. Part I of this paper is devoted to the stress situation that exists in isothermally and cyclically oxidized TBC systems. The focus of the current TBC research is on the air plasma sprayed (APS) systems. However, some results for the electron beam physical vapor deposited systems (EB–PVD) are presented for the purpose of providing insight into the role of interface roughness and the stresses in the thermally grown oxide (TGO). AE analysis of isothermally and cyclically oxidized samples is used to address the role of “desk top failure” and of thermal fatigue. The possible role of intrinsic growth stress is addressed by presenting the measurement data from the oxidation of freestanding NiCoCrAlY foil. Findings are supported by thermal stress calculations and micrographs. Part II of this paper is devoted to the development of a life time model. Here TBC top coat failure and life time is considered as a two-step process, where step 1 is time to delamination macro-cracking and step 2 is time to through macro-cracking. This two-step failure mechanism comes directly from critical strain measurement data and traditional coating failure theory. Included in the model is damage accumulation due to bond coat oxidation and due to thermal fatigue. The damage terms in the model have their origins in AE data from cyclic oxidation samples. The life time model is then used in concert with measured micro-crack lengths. Finally, a full model is presented for isothermal and cyclic oxidation of APS–TBC.

Chemical-mechanical failure of oxide scales on 9% Cr steels in air with H2O

Abstract The oxidation behaviour of 9% Cr steels P91 and Nf616 has been investigated at 650°C in dry air and in air with water vapour, where particular attention was given to breakaway failure. Additional emphasis was given to the quantitative characterisation of the kinetics of chromium depletion in the metal subsurface zone resulting from scale growth, CrO2H4 evaporation, and scale cracking and healing, with scale cracking being monitored by acoustic emission measurements. While in dry air the steels show protective oxidation behaviour up to 10000 h, breakaway oxidation may occur already after 100 h in humid environments, which was correlated with the stronger Cr-depletion and the development of intrinsic oxide scale growth stresses exceeding a critical value, in the case of water vapour containing air. In the paper the different parameters that are responsible for breakaway oxidation were identified and discussed with regard to the role of water vapour in the environment. As a conclusion it turns out that breakaway is not a consequence of Intrinsic Chemical Failure (InCF) but of a Mechanically Induced Chemical Failure (MICF).

Mechanical behaviour of as sprayed and sintered air plasma sprayed partially stabilised zirconia

Abstract The mechanical behaviour of free standing air plasma sprayed (APS) partially yttria stabilised zirconia (P-YSZ) thermal barrier coatings (TBC) at room temperature and at elevated temperatures up to 1050°C have been investigated. Creep tests under constant compressive load have been conducted as well as cyclic measurements of compressive stress – strain hysteresis loops with increasing maximum load, yielding Youngs moduli of the porous partially yttria stabilised zirconia. Both mechanical parameters are needed for accurate modelling of the local stress fields of, for example, airfoils to identify critical regions where damage or even failure of the component may occur. Specimens in the as sprayed and sintered state were tested. The microstructural changes caused by sintering and mechanical loading at high temperature of the thermal barrier coatings have been characterised by porosity measurements made from metallographic cross-sections.

Progress in life time modeling of APS-TBC Part II: critical strains, macro-cracking, and thermal fatigue

Abstract Here TBC top coat failure and life time are considered as a two-step process, where step 1 is time to delamination macro-cracking and step 2 is time to through macro-cracking. This two-step failure mechanism comes directly from critical strain measurement data and traditional coating failure theory. The critical strain data was measured by using a four point bend test on preoxidized samples. Included in the model is damage accumulation due to bond coat oxidation and thermal fatigue. The damage terms in the model have their origins in acoustic emission data from cyclic oxidation samples. The life time model is then used in concert with measured thermally grown oxide micro-crack lengths from isothermal oxidation samples. Finally a full model is presented for isothermal and cyclic oxidation of APS–TBC.

Experience with the use of the acoustic emission technique for monitoring scale failure in wet and dry environments

Abstract A brief survey of the acoustic emission technique for monitoring scale cracking and failure on 2.25–24% Cr steels in wet and dry environments is given. A number of acoustic emission test rigs are described. Some of the more simple test rigs are used for testing small oxidation coupons during isothermal oxidation. More sophisticated rigs have been used for testing full size heat exchanger tubes during thermal cycling. Most acoustic emission measurements in a wet environment come from testing at temperatures below 650°C. There are examples from Alloy 800 and thermal barrier coatings that were tested at higher temperatures, 900°C and 1100°C, respectively. Through the years acoustic emission tests have been performed in dry air, dry air+10%H2O, dry air+0.5%SO2, and Ar+5%H2+50%H2O. Consequently, a wide variety of exposure temperatures and atmospheres can be investigated using acoustic emission techniques. Qualitative acoustic emission results can detect when scale cracking occurs at exposure temperature, where such cracks are produced by growth stress. Acoustic emission signals have been measured during sample cooling, where the signal arises from scale cracking that is caused by the thermal expansion mismatch stress. Measured results have clearly shown that scale cracking caused by both growth stress and thermal expansion mismatch stress are affected by water vapor in the exposure environment. Post-test metallographic investigations show that crack orientation and the oxide scale phases are also affected by the gas composition in the test rig. Additionally the sample mass gain and scale thickness is affected by water vapor content. Finally, acoustic emission techniques are helpful for understanding the phenomena of breakaway oxidation and spallation/exfoliation.

A new concept of oxidation protection of Ni-base alloys by using the halogen effect

Abstract The Ni-base alloys with Al-contents of less than 10 wt% are widely used in high temperature technology due to their beneficial mechanical properties. However, their oxidation resistance may be insufficient at temperatures above 1000°C. Oxidation of these Ni-base alloys does not form a pure continuous Al2O3 protective scale on the surface, but rather a complex layer structure. This structure is characterized by inward growing oxides showing a discontinuous alumina scale. A new method for the formation of a dense protective alumina scale on the surface is now presented. The method is based on the halogen effect, which was successfully applied for TiAl-alloys. Thermodynamic calculations show the preferred formation of gaseous Al-halogenides within a certain region of fluorine partial pressures. The fluorine treatment is performed by ion implantation. The implantation parameters are defined by using Monte Carlo simulations. Following these results fluorine implantations of the Ni-base alloy IN 738 are performed to meet the required fluorine content near the surface. After oxidation at 1050°C a thin continuous external alumina scale is formed on the surface, whereas the untreated alloy shows a mixed oxide scale with significant inward growing oxides. The results offer a new and innovative way to protect the Ni-base alloys against oxidation.

The effect of water vapor in increasing growth stresses in the oxide scale on martensitic steam plant alloys

Abstract Investigation of the oxidation behavior of 9% Cr steels at 650°C in dry air and air + 10% H2O have shown that oxide scale growth stresses may play a significant role in breakaway. For this reason, preliminary studies on the development and characteristics of growth stresses in oxides on these materials have begun. These studies include in situ acoustic emission (AE) for monitoring scale cracking, deflection testing in monofacial oxidation (DTMO) and in situ X–ray measurements. The measurements were complemented by detailed post-experimental metallographic investigations. From the DTMO measurements the stresses in the oxide on the specimens tested in humid environment are increased by about a factor of 5 compared to dry air at the beginning of oxidation for P91. In a humid environment the stresses decrease with oxidation time while in dry air they remain almost constant. Significant acoustic emission (AE) occurs in humid air for oxidation coupons while virtually no AE is observed for thin foils in a humid environment and for coupons in dry air. These first results seem to be in good agreement with weight gain data characterizing the breakaway behavior in these environments, indicating that, indeed, oxide growth stresses play a key role for oxidation resistance and, thus, component lifetimes of such steels. The first X–ray results indicate that for E911 the scale structure and composition changes completely between the two environments. Furthermore, in humid air, a breakaway effect is observed with a change from protective spinel type oxide to locally non-protective Fe–rich oxide.

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.