Mechanical properties and fracture behavior of interfacial alumina scales on plasma-sprayed thermal barrier coatings

Abstract

The mechanical properties and fracture behavior of Y-doped Al₂O₃ scales were investigated by furnace thermal cycling (to 1,150°C) of plasma-sprayed thermal barrier coatings (TBCs) with vacuum plasma-sprayed (VPS) or air plasma-sprayed (APS) Ni–22Cr–10Al–1Y bond coatings. No significant alterations in Al₂O₃ hardness or Young’s modulus (as measured by mechanical properties microprobe) were detected as a function of bond coat type, exposure time, or number of thermal cycles. The interfacial Al₂O₃ scales on VPS NiCrAlY exhibited progressive increases in localized fracture, buckling, and delamination during thermal cycling. The concentration of arrayed lenticular voids in the columnar Al₂O₃ grain boundaries significantly increased during cyclic oxidation (as compared to isothermal oxidation), but only in scales which formed on convex surfaces, suggesting internal void growth was stress-related. The amount and frequency of scale damage was higher on convex surfaces with a relatively large radius of curvature as compared to convex surfaces with a very small radius of curvature. Although the thermo-mechanical fracture resistance of Al₂O₃ scales on APS NiCrAlY was superior to scales on VPS NiCrAlY, TBC lifetimes on VPS NiCrAlY were greater by a factor of 2. Apparently, severe interfacial scale damage did not rapidly degrade the adherence of the ceramic top coatings.     

Microstructural evidence for counter-diffusion of aluminum and oxygen during the growth of alumina scales

Abstract

A novel, two-stage oxidation experiment is described that enables the outward diffusion of cations in alumina scales during high-temperature oxidation to be analyzed on the basis of microstructural changes in the surface morphology of the scale. Using this technique, observations of aluminum out-diffusion along α-Al₂O₃ grain boundaries during oxidation of Fe–Cr–Al alloys, nickel aluminides and platinum-modified NiAl bond-coats are made. Although microstructural evidence for the inward grain boundary diffusion of oxygen is more difficult to obtain, it still can be demonstrated by the growth of the oxide above interface cavities on nickel aluminides and inside internal cracks in the alumina scales during cyclic oxidation of zirconia top-coated material. SEM examination of the crack surfaces after scale spallation provides a vivid illustration of two simultaneous processes, aluminum outward and oxygen inward diffusion along grain boundaries in the scale.     

Characterization of alumina interfaces in TBC systems

Interfacial segregants in thermally grown α-Al₂O₃ scales formed during high temperature exposure of thermal barrier coating systems reflect the oxygen-active dopants present in the bond coating and substrate, such as Y and Hf. These dopants diffuse outward and segregate to the substrate-alumina interface and the alumina grain boundaries. Related studies suggest that these segregants affect the growth and mechanical properties of the alumina-scale; however, the characterization of segregation in alumina formed on coated superalloy systems has been limited. Segregation examples evaluated using analytical transmission electron microscopy are given from traditional Pt-modified aluminide coatings and newer Pt diffusion coatings. Model systems are used to illustrate that grain boundary segregants on the columnar alumina boundaries are not because of the reverse diffusion of cations from the Y₂O₃-stabilized ZrO₂ top coating, and that interstitial elements in the substrate likely affect the outward flux of cation dopants. The dynamic nature of this segregation and oxygen-potential gradient-driven diffusion is discussed in light of observations of substrate dopant and interstitial contents affecting coating performance.     

Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders

Abstract

The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O₂ and Ar-4%H₂-2%H₂O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H₂–H₂O than in Ar–O₂. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H₂/H₂O containing gas.     

Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: A novel approach

Al₂O₃–ZrB₂ in situ composites of 97% of theoretical density were successfully fabricated by a novel self-propagating high temperature synthesis (SHS) dynamic compaction, using less expensive raw materials zirconium oxide, boron oxide, and aluminium. The process is fast, energy efficient, where no furnace sintering is required. The process inhibits and controls the grain growth and microstructure. The densification behaviour and correlation with microstructure of the SHS dynamic compacts were compared with the furnace sintered composite samples where the composite powder was prepared by SHS process. The furnace sintered samples showed coarser grain growth and maximum density of 94.5% of theoretical density was achieved. The SHS dynamic compacted in situ composite had much finer grains in the range of 0.5–3 μm with density 95.5% of the theoretical value. The average grain size was found to decrease from 10 μm to 1.4 μm for alumina and from 5.4 μm to 1.0 μm for zirconium diboride from furnace sintering to SHS dynamic compaction, respectively. Addition of Al₂O₃ as a diluent during SHS reaction enhanced the density to 97%. During SHS dynamic compaction, the amount of liquid and the time interval at which the sample stays at high temperature are the controlling factor of the final microstructure and the densification of the composite.     

Effect of cerium zirconate (Ce2Zr2O7) on microstructure and mechanical properties of Ce-ZTA

Powders for ZrO₂ toughened Al₂O₃ (ZTA) composites containing 8, 11, 13.8 and 16.5 vol% ZrO₂ (stabilized with 11.5 mol% CeO₂) are prepared by a hybrid sol-gel method using Al₂O₃ powders and a sol formed from Zr-alkoxide and cerium nitrate. Besides ZrO₂, a small amount of a Ce-zirconate phase (Ce₂Zr₂O₇) forms when the powders are calcined in air. The zirconate phase persists in the sintered specimens and its amount increases from surface to centre of the specimen. Presence of higher amount of Ce₂Zr₂O₇ promotes exaggerated grain growth of Al₂O₃. Particles of Zr rich phases are found to be trapped inside the Al₂O₃ grains. Composites exhibit higher fracture toughness despite lower transformability of t-ZrO₂ during fracture when they contain high amount of zirconate. Crack bridging is shown to be an important mechanism contributing to enhancement in fracture toughness in these composites.     

TEM study of the surface morphology of extracted ZrO2-Al2O3 fibres

The surface morphology and microstructure of a series of melt extracted ZrO₂-Al₂O₃ based fibres (ZA, ZAT and ZAS) have been imaged at the nanometre scale by transmission electron microscopy (TEM) using an advanced Pt/C replica technique. Growth characteristics of ZrO₂, Al₂O₃ and other crystalline phases formed upon heating up to 1550 °C are illustrated and described. Several grain morphologies including spherical and polygonal grains, as well as grains with rounded plate-like growth were observed indicating different active growth mechanisms. ZrO₂ particles on the surface of the fibres were almost spherical with some facetting and rounding of corners. These grains were very fine (< 50 nm) in the ZA and ZAS fibres while they were several microns in size in the ZAT fibres. Al₂O₃ grains were generally much larger (up to several microns) and exhibited two distinct growth morphologies of layered and rhombohedral type. Different grain morphologies of the ZA and ZAS fibres have been correlated to the phases identified by X-ray diffraction.     

Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

Abstract

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.     

Characterization of Al2O3 composites with fine Mo particulates,I. Microstructural development

Alumina based composites containing nano- or submicron-meter Mo grains in the amounts of 20 vol% or less were prepared through a dissolution of molybdenum oxide in ammonium solution, followed by spray-drying, hydrogen reduction and sintering with or without hot-pressing. The properties of alumina/molybdate solutions and the ζ-potential of alumina particles in the solution were measured. By using electron microscopic and quantified X-ray diffraction techniques, the microstructural features and the evolution of Mo paniculate in spray-dried powder and sintered bodies were analyzed. The time dependent exponent and activation energy of grain growth of Mo between 600 to 900 °C were determined. There is no glassy phase or reaction at the interfaces between Mo/Al₂O₃ of dense composites. Only one coherent interface was found, and the others are incoherent. The results reveal that submicrometric Mo grains may grow by surface diffusion in reduction stage (≤ 900 °C) and greatly retard the densification and reduce the grain size of alumina matrix in sintering stage.     

Preparation of ZrO2-Al2O3 powder by thermal decomposition of gels produced from an aluminium chelate compound and zirconium butoxide

Organic precursors containing Al and Zr atoms were synthesized from an aluminium chelate compound and zirconium n-butoxide. A ZrO₂-Al₂O₃ composite powder was prepared by the thermal decomposition of these precursors. An amorphous phase exists to higher temperatures for this ZrO₂-Al₂O₃ powder than for a comparable powder prepared from aluminium sec-butoxide and zirconium n-butoxide. In addition the tetragonal ZrO₂ phase was stabler in this ZrO₂-Al₂O₃ powder than in a comparison powder. The ZrO₂ grains were 50–500 nm in diameter and were homogeneously dispersed in the Al₂O₃ matrix after heating at 1400 °C.     

In situ formation of Al2O3–ZrO2–Y2O3 composite ceramic coating by plasma electrolytic oxidation on ZAlSi12Cu3Ni2 alloy

In situ formation of Al₂O₃–ZrO₂–Y₂O₃ composite ceramic coating on ZAlSi12Cu3Ni2 aluminum alloy was successfully prepared by plasma electrolytic oxidation (PEO) technology in a zirconate electrolytic solution. The morphologies, phase components, the thermal diffusion coefficient and thermal conductivity of the composite coatings were investigated by scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction and laser pulse tester. The results indicate that the composite coatings are relatively dense and uniform in thickness, and predominantly composed of Al₂O₃, c-Y0.15Zr0·85O1·93Vo0·07(Vo-oxygen vacancies), monoclinic ZrO₂ (m-ZrO₂) and littleY₂O₃. The composite coatings exhibit a gradient distribution in phase component from the surface to the inner part. With the increase of the applied voltage, the micropores, the discharges products, thickness and the ZrO₂ content of the composite coatings increase. With the oxidation time increasing, the surface of coating generates oxide ceramic particles around the holes and accumulates repeatedly. The content of zirconium is the higher on the surface and interface. The content of Al is less and it shows that the ceramic coating contains mainly the zirconium oxide. This is attributed to the presence of micropores and microcracks, plus the extremely fine grain size and the presence of an amorphous phase. When considered in conjunction with the possible thickness range, it’s clear that this PEO coatings offer considerable promise as thermal barriers.     

Hydrothermal deposition and characterization of ultrafine ZrO2 thin films

Ultrafine ZrO₂ thin films (0.6 μm) have been prepared by a hydrothermal method on polycrystalline alumina substrates (α-Al₂O₃). The hydrothermal treatment of zirconium hydroxide sol in the temperature range of 100–240 °C for 6–72 h results in the formation of monoclinic single-phase films. The resulting films have a white appearance and are homogeneous, without visible pores and defects, and the average grain size in the film is 18 nm.     

The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000

Abstract

Preliminary studies have been undertaken on cyclic and isothermal oxidation at 1,300°C of thin (125 μm) samples of commercial ODS alloy PM2000 for up to 350h in two different oxidising environments; dry and moist air. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) have been used to study the influence of such environments on alumina scale formation and growth. Initial mass gain observations showed that the alumina scale, which formed on the samples oxidised in air+2.5vol% H₂O grew faster in the early stages of oxidation than in the case of dry air. However the SEM analysis revealed that the scale morphologies in both dry air and air+2.5vol% H₂O were similar. In both cases the scales consisted of equiaxed grains at the scale–gas interface with Ti-rich particles in the outermost part of the scale. The major factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in moist air than in dry air     

Preparation of Y2O3 stabilised ZrO2 ceramic nanopowders by surface doping

Abstract

Weakly agglomerated nanocrystalline Y₂O₃–ZrO₂ powder was prepared by dispersion Y₂O₃ on the surface of ZrO₂ nanopowder (7·3 nm) that was derived from gas phase synthesis. The utmost dispersion capacity of Y₂O₃ on the surface of ZrO₂ was determined to be 0·16 gY₂O₃ /gZrO₂ (or 8·7 mol.-%Y₂O₃–ZrO₂ ) which suggests that 3 mol.-%Y₂O₃ would be homogeneously dispersed on ZrO₂ and no phase segregation would occur during surface doping. The results show that the tetragonal phase content in surface doped ZrO₂ increased with grain growth or heating temperatures, unlike the undoped and the bulk doped ZrO₂ . The stabilisation of the tetragonal phase resulted from the incorporation of Y ³⁺ cations from the surface into the grains of ZrO₂ . This conclusion is supported by the X-ray photoelectron spectroscopy and X-ray diffraction evidence. Surface doped powders have a strong tendency to improve the anticoarsening ability and suppress grain growth, especially at higher doping levels and at lower heating temperatures.     

Zirconia-based nanocrystals in the ZrO2-In2O3 system

Zirconia-based ZrO₂-In₂O₃ nanocrystals are prepared by hydrothermal treatment of coprecipitated zirconium oxyhydroxide and indium hydroxide. Indium oxide is shown to dissolve predominantly in cubic zirconia nanocrystals. Its solubility in nanocrystalline zirconia notably exceeds the equilibrium In₂O₃ solubility in ZrO₂ single crystals.     

Application of TEM and SNMS in the study of thermally grown alumina scales

Abstract

The oxidation behaviour of several alumina-forming alloys containing reactive elements was investigated at 1100°C in air under atmospheric pressure. Analytical techniques were applied to characterise the corrosion products and to understand the role of reactive elements on the alumina scale growth. Secondary neutral mass spectrometry was used to determine the in-depth compositional profile of the element composing the oxide scale, as well as the oxygen isotope, after sequential oxidation experiments using ¹⁶O₂ and ¹⁸O₂ enriched environments. Transmission electron microscopy revealed the microstructure of the oxide scale and located the reactive elements within the Al₂O₃ scale or at the metal–oxide interface. The combination of both techniques led to a better understanding of the oxide scale growth mechanism.     

Diffusion-preventive Al2O3 layer growth at the interface of plasma-sprayed niobium and FeCrAlY in a tungsten fibre-reinforced high-temperature superalloy composite

The growth mechanism of a diffusion-preventive layer formed at elevated temperatures (∼1500 K) between the plasma-sprayed niobium layer and the plasma-sprayed FeCrAlY matrix in a tungsten fibre-reinforced high-temperature superalloy composite (Nb-coated W fibre/FeCrAlY composite) was studied. The diffusion-preventive layer was identified as α-Al₂O₃ by scanning electron microscopy/electron probe microanalysis, energy dispersive X-ray spectrometry analysis, and X-ray diffraction. Heat-treatment experiments were implemented systematically and it was found that the Al₂O₃ layer was also formed at the plasma-sprayed Nb/rolled FeCrAl interface. No Al₂O₃ layer, however, was formed either at the rolled Nb/plasma-sprayed FeCrAlY interface or at the rolled Nb/rolled FeCrAl interface. In these cases, an intermetallic compound layer was formed instead. A growth mechanism is proposed in which the Al₂O₃ is attributed to a chemical reaction between the residual oxygen in the plasma-sprayed niobium and aluminium in FeCrAlY or FeCrAl. The magnitude of the driving force was evaluated by a new model based on thermodyanamics. Numerical calculations have shown that the proposed growth mechanism is thermodynamically reasonable.     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

Effect of V<Subscript>2</Subscript>O<Subscript>5</Subscript> content on the optical,structural and electrochromic properties of TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> thin films

Vanadium oxide (V₂O₅) mixed titanium oxide (TiO₂) and zirconium oxide (ZrO₂) thin films were fabricated on glass substrates (corning 2947) and on indium tin oxide (ITO) coated glass substrates by sol gel spin coating process. Their optical, structural and electrochromic properties were investigated. The results were compared with pure TiO₂ and ZrO₂ thin films. Mixture of V₂O₅ with both types of film reduces the transmittance at the higher wavelengths. The refractive index of the V₂O₅ mixed TiO₂ and ZrO₂ films increases when compared with pure TiO₂ and ZrO₂ films. AFM images demonstrate no significant topographical changes for V₂O₅ mixed TiO₂ whereas for V₂O₅ mixed ZrO₂ films a topographical change is observed. V₂O₅ mixed TiO₂ showed slight increase in their charge capacity.     

FEG-SEM examination of alumina scales formed on FeCrAl alloys

Abstract

Field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) has been used to investigate the microstructure and the oxidation behaviour of ultra-high purity Fe–20Cr–5Al model alloys and a commercial Fe–20Cr–5Al alloy. The model alloys contain controlled additions of phosphorus and carbon impurities and increased levels of more beneficial elements including yttrium, hafnium and titanium. The samples studied were oxidised at 800°C and 1200°C in humidified air for up to 3100 h, and 900°C and 1000°C for 1 h in laboratory air. At the higher temperature, well-adhered, compact and highly protective α-alumina scales formed, whereas at the lower temperature the scales formed were a less protective type of metastable alumina.

Preliminary examination showed that the texture of the formed alumina scale was unaffected by the texture of the underlying substrate and the substrate compositions. At the higher temperature, the study revealed that the alumina scale comprised two distinct regions; the outer region at the scale/gas interface contained small, equiaxed (0.5–1 micron) grains and the inner region at the scale/metal interface contained, columnar grains that are 2–3 times larger than the equiaxed ones. However, at the lower temperature these two distinct regions were not apparent. Instead, grains of predominantly metastable alumina were observed. The links between texture morphology and oxide growth mechanisms will be discussed in this paper.