The growth of thermally grown oxide (TGO) layers and their influence on crack formation were studied for two thermal barrier coating (TBC) systems with CoNiCrAlY bond coats produced by (i) air plasma spray (APS) and (ii) high-velocity oxy-fuel (HVOF) techniques. All samples received a vacuum heat treatment and were subsequently subjected to thermal cycling in air. The TGOs were predominantly comprised of layered alumina, along with some oxide clusters of chromia, spinel and nickel oxide. However, after extended oxidation, the alumina layer formed in the APS-CoNiCrAlY bond coat transformed to chromia/spinel, while that formed in the HVOF-CoNiCrAlY bond coat remained stable. TGO thickening in the APS-CoNiCrAlY bond coat generally exhibited a three-stage growth behavior, which resembles a high temperature creep curve, whereas growth of the alumina layer in the HVOF-CoNiCrAlY bond coat showed an extended steady-state stage. Crack propagation in these two TBCs was found to be related to the growth and coalescence of oxide-induced cracking, connecting with pre-existing discontinuities in the topcoat. Hence, crack propagation during thermal cycling appeared to be controlled by TGO growth.

TGO growth behaviour in TBCs with APS and HVOF bond coats

In thermal barrier coating (TBC) systems, spinel and nickel oxide formed in an oxidizing environment are believed to be detrimental to TBC durability during service at high temperatures. The present study shows that in an air-plasma-sprayed (APS) TBC with Co–32Ni–21Cr–8Al–0.5Y (wt.%) bond coat, pre-oxidation treatments in low-pressure oxygen environments can suppress the formation of the detrimental oxides by promoting the formation of an Al 2 O 3 layer at the ceramic topcoat/bond coat interface. The development of the thermally grown oxide (TGO) layer generally exhibits a three-stage growth phenomenon that resembles high temperature creep. The pre-oxidation treatments reduce the growth rate and extend the steady-state growth stage, leading to an improved durability. Crack propagation in the TBC proceeds via opening and growth of pre-existing discontinuities in the ceramic topcoat, assisted by crack nucleation and growth associated with the TGO. Crack propagation during thermal cycling appeared to be controlled by TGO growth, and the maximum crack length and TGO thickness generally have a power law relationship.

Pre-oxidation and TGO growth behaviour of an air-plasma-sprayed thermal barrier coating

Microstructural changes, damage initiation and spallation of thermal barrier coatings (TBC), which were deposited by air plasma spraying (APS) and consisting of ZrO2–7 wt.% Y2O3; (YSZ) thermally grown oxide (TGO); MCrAlY bond coat; and a nickel base super alloy, Hastelloy-X, were examined as a function of thermal cycles at 1020 °C. Two types of thermal shock testing were adopted. Firstly, thermal cycling consisted of a directly inserting samples to 1020 °C and then 5 min holding and water quenching. In the other cycling method, forced air was applied for quenching or cooling. Microstructural characterization was carried out by scanning electron microscopy (SEM) and optical microscopy whereas phase analysis was carried out by X-ray diffractrometry (XRD). Spallation of YSZ coatings was initiated by edge cracking after 50 cycles in water quenching and sustained upto 220 cycles, 20% spalling was adopted the criteria for this test. In case of air quenching up to 1000 cycles only 10% of the sample spalled. In both cases, no vertical cracking was observed throughout the cycling. In the water-quenched samples, after 220 cycles the thickness of the top coating was half of its original thickness, while in case of air-quenched samples no such surface deterioration was observed. Peaks for calcium zirconium oxide (CaZrO3) were found during XRD studies for water-quenched samples. Rumpling-induced micro-cracking at the YSZ/TGO interface and the formation of voids decreasing the toughness between the TGO and YSZ.

Behavior of air plasma sprayed thermal barrier coatings, subject to intense thermal cycling

Experimental results on S segregation at growing Al2O3/alloy interfaces are reviewed for binary FeAl, NiAl alloys, and ternary alloys with additions of Cr, Pt, or a reactive element, such as Zr, Hf, or Y. The segregation behavior is thermodynamic in nature, but the segregation energy can change not only with alloying elements but also with oxidation time and temperature as the oxide growth process changes. Although reactive elements are capable of eliminating interfacial S segregation, they do not stop such segregation to alloy surfaces. The segregation of a reactive element at the interface further strengthens the interfacial bonding. Cosegregation of S and Cr can occur, resulting in higher levels of S at the interface. Pt usually suppresses S segregation, but this effect can be easily overwhelmed by S-Cr cosegregation. Synergisms between alloying elements and how they affect segregation, as well as the relationship between segregation and the oxidation process, are areas that demand further investigation.

Segregation Phenomena at Thermally Grown Al 2 O 3 /Alloy Interfaces

Morphology and thermal conductivity of yttria-stabilized zirconia coatings

In order to improve the engine output and the efficiency of gas turbines, optimized thermal barrier coatings (TBCs) are required to protect the metallic components at high temperatures. In common TBC-systems, consisting of a Ni-base alloy substrate/MCrAlY-bond coat/ZrO 2 7 wt.% Y 2 O 3 top coat, an oxide layer grows at the interface bond coat/ceramic under high temperature service, which limits the life of these coatings. In this paper the oxidation resistance of a new triplex TBC-system, consisting of a CoNiCrAlY-bond coat/Pt-modified aluminide coating/ZrO 2 7 wt.% Y 2 O 3 top coat is compared with that of a common TBC-system. The as-coated Pt-aluminide coating consists of an outer region of PtAl 2 +(CoNiPt)Al followed by a single phase layer of (CoNiPt)Al. The results of the oxidation tests at 1000, 1050 and 1100°C in air show excellent oxidation resistance of the triplex TBC-system with the thickest investigated Pt-aluminide coating. In particular, a 28 μm thick Pt-aluminide coating allows the thickness of the oxide layers to be reduced up to 70% compared to the common TBC after 500 h at all examined temperatures. After heat treatment the coating systems were investigated by SEM, EDX and X-ray analysis. Annealing tests with Al 2 O 3 powder indicate which mechanism is probably responsible for the improved oxidation resistance of platinum additions. Platinum is evidently capable of decomposing aluminum oxide at temperatures above 900°C.

Improved oxidation resistance of thermal barrier coatings

To develop highly efficient gas turbines, thermal barrier coating systems with a high reliability and a long lifetime under severe operating conditions are required. The failure of TBC-systems is caused by thermal cycling conditions, oxidation attack, and insufficient adhesion at the interfaced the ceramic coating and the bond coat. Coating failure occurs mostly near the interface top coat-bond coat. Two modifications of a conventional duplex TBC-system consisting of a Ni-base alloy substrate/MCrAlY-bond coat/ZrO2 7 wt.% Y2O3-top coat, which is used as the reference system, are presented as follows. (i) By contouring the MCrAlY-bond coat with a laser, the stress distribution at the ZrO2-bond coat interface can be modified by forming folds, within the laminate structure of the ceramic top coat and increasing the bonding area. TBC-systems containing a contoured bond coat show better thermal cycling behaviour. FEM-simulation of thermally induced stress shows an alternating stress distribution which is caused by the contoured bond coat interface, (ii) High-velocity oxygen fuel (HVOF)-sprayed MCrAlY layers are a new possibility to create homogeneous bond coats. Thermal barrier coatings with LPPS-(low pressure plasma sprayed) or HVOF-CoNiCrAlY bond coats are compared by investigating their porosity, roughness, and oxidation behaviour. The porosity is proportional to the roughness of the HVOF bond coats. The oxide content was examined by TEM and EDX analysis. HVOF-CoNiCrAlY bond coats show oxidation behaviour similar to coatings produced by LPPS.

Modifications of thermal barrier coatings (TBCs)

Zeitstandversuche am Nickelbasiswerkstoff Rene-41 im JP 4-Heisgasstrom und in Luftatmosphare ergeben einen deutlichen Abfall der Zeitstandfestigkeit in korrosivem Medium. Dotierungen des Heisgases mit kunstlichem Meersalz bewirken eine weitere Verminderung der Zeitstandfestigkeit. Metallographische, rasterelektronenmikroskopische und mikroanalytische Untersuchungen ergeben vergleichbare Gefugezustande und Deckschichtausbildungen bei Beanspruchung in Luft- und JP 4-Heisgasatmosphare. Das Kriechverhalten weist jedoch spezifische Abhangigkeiten vom umgebenden Medium auf. In korrosiver Atmosphare wird der stationare Bereich der Kriechkurven stark eingeschrankt. In korrosivem Medium bilden sich auf den Korngrenzflachen duktile Phasenanteile, die vorzeitigen Bruch des Werkstoffes einleiten. Durch Auger-Mikrosonden-Untersuchungen ist der Nachweis gelungen, das Schwefelbestandteile des Treibstoffes den durch Heisgas in Fluggasturbinen spezifischen Korrosionsmechanismus auslosen.



Hot gas corrosion and creep strength of a nickel base alloy in close-to-operating conditions at 750 to 950°C



Creep rupture tests of the nickel base alloy Rene-41 in JP 4 hot gas stream and in air atmosphere show a significant decrease in the creep strength. Dotations of the hot gas with synthetic sea salt cause a more extensive decrease of the creep strength. Metallographic, scanning electron microscopic and microanalytical investigations yield comparable structure conditions and protective layer configurations under the stress of air and exhaustion gas atmosphere. The creep behaviour shows characterizing dependence of the surrounding medium. Within corrosive atmosphere there is no stationary region at the creep curves. Components of the power fuel diffuse into the material and cause ductile grain boundary coverings, which cause premature break of the material. By means of Auger microanalysis increased concentrations of sulphur could be proved in regions close to the surface.

Heißgaskorrosion und Zeitstandsfestigkeit an einer Nickelbasislegierung unter betriebsnahen Bedingungen bei 750°C bis 950°C

Failure behavior and performance analysis of hybrid-fiber reinforced PAEK composites at high temperature

Ionic contaminations, especially flux residues due to manufacturing, cause various faults and failures of printed circuits during service. The contamination (µg‐eq NaCI), surface insulation resistance (SIR), electromigration and corrosion caused by flux residues of six different fluxes (categories: F‐SW26, F‐SW32 and F‐SW34 according to DIN 8511) have been investigated. The main effort was applied to the investigation of electromigration, which is understood as the formation of short circuits by metallic bridges between two conductors of different voltage.

Kh. G. Schmitt‐Thomas

Papers

Improved oxidation resistance of thermal barrier coatings

Modifications of thermal barrier coatings (TBCs)

Heißgaskorrosion und Zeitstandsfestigkeit an einer Nickelbasislegierung unter betriebsnahen Bedingungen bei 750°C bis 950°C

Failure behavior and performance analysis of hybrid-fiber reinforced PAEK composites at high temperature

The Influence of Flux Residues on the Quality of Electronic Assemblies