Proto-TGO formation in TBC systems fabricated by spark plasma sintering

Thermal barrier coatings (TBC) are commonly used in modern gas turbines for aeronautic and energy production applications. The conventional methods to fabricate such TBCs are EB-PVD or plasma spray deposition. Recently, the spark plasma sintering (SPS) technique was used to prepare new multilayered coatings. In this study, complete thermal barrier systems were fabricated on single crystal Ni-based superalloy (AM1®) substrate in a one-step SPS process. The lifetime of TBC systems is highly dependent on its ability to form during service a dense, continuous, slow-growing alumina layer (TGO) between an underlying bond coating and a ceramic top coat. In the present paper, we show that such kind of layer (called proto-TGO in the following) can be in situ formed during the SPS fabrication of TBC systems. This proto-TGO is continuous, dense and its nature has been determined using TEM-EDS-SAD and Raman spectroscopy. This amorphous oxide layer in the as-fabricated samples transforms to α-Al₂O₃ during thermal treatment under laboratory air at 1100 °C. Oxidation kinetics during annealing are in good agreement with the formation of a protective α-Al₂O₃ layer.     

High sintering resistance of a novel thermal barrier coating

Sintering resistance of a novel thermal barrier coating Nd_xZr_1 − xO_y with Z dissolved in, where 0 < x < 0.5, 1.75 < y < 2 and Z is an oxide of a metal selected from Y, Mg, Ca, Hf and mixtures thereof, was studied. The coatings of Nd_xZr_1 − xO_y and typical 7YSZ were deposited by electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS). The samples with the coating system of EB-PVD Nd_xZr_1 − xO_y or 7YSZ overlaid onto a MCrAlY bond coat were cyclically sintered at 1107 °C for 706 hours. The freestanding coatings of EB-PVD Nd_xZr_1 − xO_y and 7YSZ were isothermally sintered at 1371 °C for 500 hours. The microstructure of EB-PVD Nd_xZr_1 − xO_y before and after the sintering was evaluated and compared with EB-PVD 7YSZ. The sintering resistance of freestanding APS Nd_xZr_1 − xO_y coating was also investigated after isothermal sintering at 1200 °C for 50 and 100 hours. The results demonstrated that the new coatings of Nd_xZr_1 − xO_y applied with both EB-PVD and APS have higher sintering resistance than EB-PVD and APS 7YSZ, respectively.     

Investigation of interfacial properties of atmospheric plasma sprayed thermal barrier coatings with four-point bending and computed tomography technique

A modified four-point bending test has been employed to investigate the interfacial toughness of atmospheric plasma sprayed (APS) yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal heat treatments at 1150 °C. The delamination of the TBCs occurred mainly within the TBC, several to tens of microns above the interface between the TBC and bond coat. X-ray diffraction analysis revealed that the TBC was mainly tetragonal in structure with a small amount of the monoclinic phase. The calculated energy release rate increased from ~ 50 J/m^− 2 for as-sprayed TBCs to ~ 120 J/m^− 2 for the TBCs exposed at 1150 °C for 200 h with a loading phase angle about 42°. This may be attributed to the sintering of the TBC. X-ray micro-tomography was used to track in 3D the evolution of the TBC microstructure non-destructively at a single location as a function of thermal exposure time. This revealed how various types of imperfections develop near the interface after exposure. The 3D interface was reconstructed and showed no significant change in the interfacial roughness after thermal exposure.     

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y₂O₃ stabilized ZrO₂ (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.     

Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate

Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5-40 μm). Young's modulus E_f of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300-600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo E_f = 40 GPa and for 32 wt.% of Mo E_f = 23 GPa.     

Tensile fracture behavior of thermal barrier coatings on superalloy

Tensile fracture behavior of thermal barrier coatings (TBCs) on superalloy was investigated in air at room temperature (RT), 650 °C and 850 °C. The bond coat NiCrAlY was fabricated by either high velocity oxygen fuel (HVOF) or air plasma spraying (APS), and the top coat 7%Y₂O₃-ZrO₂ was deposited by APS. Thus two kinds of the TBC system were formed. It was shown that the coating had little effect on tensile stress-strain curves of the substrate and similar tensile strength was obtained in two kinds of the TBC system. However, the cracking behavior in the two kinds of TBC system at RT was different, which was also different from that at 650 °C and 850 °C by scanning electron microscopy. The interface fracture toughness of the two kinds of TBC system was evaluated by the Suo-Hutchinson model and the stress distribution in the coating and substrate was analyzed by the shear lag model.     

Effect of morphology on thermal conductivity of EB-PVD PYSZ TBCs

Partially yttria stabilized zirconia (PYSZ) based thermal barrier coatings (TBC) manufactured by electron beam-physical vapour deposition (EB-PVD) protect turbine blades, working under severe service conditions in aero engines and stationary turbines. These coatings show a high strain tolerance relying on their unique morphology which is comprised of weakly bonded, preferred-oriented columns, voids between feather-like sub-columns and, finally, of intra-columnar closed pores.The results obtained in this work demonstrate that variation of the EB-PVD process parameters alters the resulting columnar morphology and porosity of the coatings. The physical properties and, most importantly, thermal conductivity, are greatly affected by these morphological alterations. This study investigates three morphologically different EB-PVD PYSZ TBC top coats in terms of the spatial and geometrical characteristics of their porosity and correlates those with the thermal conductivity values measured in as-coated state and after heat treatment at 1100 °C for 1 h and 100 h. Changes in the open and closed porosity caused by heat-treatment are characterized by small-angle neutron scattering (SANS), Brunauer-Emmett-Teller Method (BET) and scanning electron microscope (SEM). Correlation of shape and surface-area changes in all porosity types of the analysed coatings revealed that the thermal conductivity of these coatings is influenced primarily by size and shape distribution of the pores and secondarily by the pore surface-area available at the cross section perpendicular to the heat flux.     

Effect of heat treatment at 900 °C on microstructural and mechanical properties of thermal barrier coatings

The effect of heat treatment at 900 °C in air on the microstructural and mechanical properties of electron beam physical vapour deposited thermal barrier coatings (TBCs) was investigated in this work. To clarify this effect, the phase and the microstructure of the zirconia ceramic top coat (TC) before and after the treatment were first characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. The changes in Young's modulus and hardness of the TC before and after the treatment were then measured by indentation tests at room temperature. The delamination behavior, especially the evolution of the interfacial load capacity between the ceramic TC and the thermally grown oxide (TGO), was finally examined by the finite element analysis with surface-based cohesive behavior. Correlations between the changes in microstructure and mechanical properties were also discussed. Results showed that after the treatment neither the phase transformation from the tetragonal zirconia to the monoclinic nor the transformation to the cubic was detected, whereas the TC sintered. The combined effect of the sintering related densification and the cracks formed during the treatment led to an initial increase and a subsequent decrease in Young's modulus and hardness of the TC with increasing treatment time. The interfacial load capacity increased with increasing Young's modulus of the TC and increasing thickness of the TGO.     

Microstructural evolution of CoCrAlY bond coat on Ni-based superalloy DZ 125 at 1050 °C

CoCrAlY alloy has been widely used as metallic protective coatings or the bond coats in thermal barrier coatings (TBCs) to protect the underlying superalloy from oxidation and hot-corrosion. In this paper, the TBC consisting of yttria stabilized zirconia (7YSZ) ceramic top coat and CoCrAlY bond coat was deposited onto directionally solidified nickel based superalloy DZ 125 by electron beam physical vapor deposition (EB-PVD). The microstructural evolution of the bond coat on this superalloy was investigated after thermal exposure for 100 h at 1050 °C. Due to a significant inward diffusion of Al, Co and Cr from the coating and outward diffusion of Ni, Hf, W and Ti from the substrate, the phase transformation from the Co-based Al-rich β-CoAl phase to the Al-deficient γ-CoNi solid solution phase occurred in the bond coat. Simultaneously, a large amount of Ni-based β-NiCoAl phase was present in the bond coat. In addition, the particles containing substrate strengthening elements Hf and/or W are abundant in the thermally grown oxides (TGO) and within the bond coat. The mechanism for the microstructural evolution is discussed.     

Atmospheric plasma sprayed thermal barrier coatings with high segmentation crack densities: Spraying process, microstructure and thermal cycling behavior

Thermal barrier coatings (TBCs) with high strain tolerance are favorable for application in hot gas sections of aircraft turbines. To improve the strain tolerance of atmospheric plasma sprayed (APS) TBCs, 400 μm-500 μm thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9 cracks mm^− 1, and porosity values lower than 6% were obtained. The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding.Thermal cycling tests yielded promising strain tolerance behavior for the manufactured coatings. Samples with high segmentation crack densities revealed promising lifetime in burner rig tests at rather high surface (1350 °C) and bondcoat temperatures (up to 1085 °C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC.     

金属基陶瓷涂层弹性模量和界面断裂韧度

对双面涂层三点弯曲试验方法进行了改进(两面涂层不必等厚度),基于该理论对一种铁基陶瓷涂层进行了试验研究,并与单面涂层的三点弯曲试验结果进行了比较.结果表明,随涂层厚度的增大,弹性模量增大.分析认为,一方面涂层越厚,测量精度越高;另一方面涂层厚度增大,孔隙率降低,使得弹性模量增大.研究发现涂层弹性模量与喷涂方向有关,呈现了各向异性的特性.文中还介绍了测量涂层/基体界面断裂韧度的研究进展,着重介绍了测量界面临界应变能量释放率的四点弯曲试验、改进的拉伸试验方法以及采用威布尔应力评价界面强度的局部法.     

Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings

The effects of thermal annealing at 1000 °C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4 mol% Y₂O₃ partially stabilized ZrO₂ top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed.     

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti-Al-Cr-Y-N and Ti-Al-Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti-45Al-8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD).Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti-Al-Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti-Al-Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti-Al-Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.     

An investigation into the correlation between nano-impact resistance and erosion performance of EB-PVD thermal barrier coatings on thermal ageing

Nanomechanical testing (nano-impact and nanoindentation mapping) has been carried out on the top surfaces of as-received and aged 8 wt.% yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) produced by electron-beam physical vapour deposition (EB-PVD). The correlation between the nanomechanical test results and the previously reported erosion resistance of the TBCs has been investigated. The experimental results revealed that aged TBCs on zirconia for 24 h at 1500 °C or on alumina for 100 h at 1100 °C resulted in large increases in their hardness (H), modulus (E), H/E and H³/E² ratios but their erosion resistance was reduced. Nano-impact tests showed a dramatic decrease in impact resistance following the ageing of these TBCs, which is consistent with the erosion results. The strong correlation between the nano-impact and erosion resistances has confirmed the premise that rapid laboratory impact tests must produce deformation with similar contact footprint to that produced in the erosion tests.     

Oxidation resistance of YSZ-alumina composites compared to normal YSZ TBC coatings at 1100 °C

In the present work oxidation behavior of plasma sprayed YSZ-alumina composite TBC coatings on Ni-base (IN-738LC) super alloy substrate was studied and compared to normal YSZ. Cyclic oxidation process in 4 h intervals was performed in an air electrical furnace at 1100 °C and the specimens were cooled in the furnace during each cycle. Preliminary checking was done with naked eye and further investigation was achieved using scanning electron microscopy. If there were any cracks or spallation in the coating's edge, the tests were stopped, the time was recorded and coating microstructure was studied. YSZ-alumina composites were made by applying alumina layer at the top of YSZ or mixed with YSZ as a TBC layer on the bond coat. Composite coatings of YSZ-alumina having alumina as a top coat and the mixed YSZ-alumina layer, showed better resistance than normal YSZ in oxidation test. It was observed that alumina overlay on YSZ has promoted the oxidation resistance of the coatings for longer times by preventing infiltration of oxygen through YSZ layer.     

Reaction, transformation and delamination of samarium zirconate thermal barrier coatings

The rare earth zirconates have attracted interest for thermal barrier coatings (TBCs) because they have very low intrinsic thermal conductivities, are stable above 1200 °C and are more resistant to sintering than yttria-stabilized zirconia (YSZ). Samarium zirconate (SZO) has the lowest thermal conductivity of the rare earth zirconates and its pyrochore structure is stable to 2200 °C but little is known about its response to thermal cycling. Here, columnar morphology SZO coatings have been deposited on bond coated superalloy substrates using a directed vapor deposition method that facilitated the incorporation of pore volume fractions of 25 to 45%. The as-deposited coatings had a fluorite structure which transformed to the pyrochlore phase upon thermal cycling between 100 and 1100 °C. This cycling eventually led to delamination of the coatings, with failure occurring at the interface between the TGO and a “mixed zone” that formed between the thermally grown alumina oxide (TGO) and the SZO. While the delamination lifetime increased with coating porosity (reduction in coating modulus), it was significantly less than that of similar YSZ coatings applied to the same substrates. The reduced life resulted from a reaction between the rare earth zirconate and the alumina-rich bond coat TGO, leading to the formation of a mixed zone consisting of SZO and SmAlO₃. Thermal strain energy calculations show that the delamination driving force increases with TGO and mixed layer thicknesses and with coating modulus. The placement of a 10 μm thick YSZ layer between the TGO and SZO layers eliminated the mixed zone and restored the thermal cyclic life to that of YSZ structures.     

Mechanical properties of thermal barrier coatings after isothermal oxidation.: Depth sensing indentation analysis

Thermal barrier coatings (TBC) are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. Isothermal oxidation degrades the performance of these coatings, so this work analyses the mechanical properties (Young's modulus, E, and hardness, H) of TBC and its evolution after thermal exposure in air. ZrO₂(Y₂O₃) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The TBC were isothermally oxidized in air at 950 °C and 1050 °C for 72, 144 and 336 h. Depth sensing indentation tests were carried out on the ceramic coating to evaluate E and H in the as-sprayed materials and after isothermal oxidation. An approach based on multiple tests at different loads was used to determine size independent apparent E an H. These mechanical properties, measured perpendicular to the surface, clearly decreased after isothermal oxidation as a consequence of microcracking within the ceramic coating.     

Erosion-oxidation behaviour of pack-aluminized 9% chromium steel under fluidized-bed conditions at elevated temperature

Pack-aluminized 9% chromium steel specimens were exposed to angular silica sand particles in a fluidized-bed erosion-oxidation rig for 200 h. The exposures were conducted in air at temperatures of 550 °C to 700 °C for particle impact angles of 30° and 90°, at speeds of 7.0-9.2 m s⁻¹. Subsequently, the mean thickness changes of the specimens were determined and the specimens were examined and analyzed by scanning electron microscopy and X-ray diffraction. The specimens experience only slight thickness changes for 30° angle impacts but significant material loss for 90° angle impacts, typical of a brittle erosion process. Under 30° angle impacts, the coatings were mostly retained on the substrate surface and slightly deformed. Thin oxide scales were detected on the surface at all test temperatures. Under 90° angle impacts, thickness losses increased with increase in speed and temperature up to 650 °C, resulting in complete loss of the coating in the test period. A porous, cracked, but continuous, oxide scale was observed on the surface of the exposed substrate. At 700 °C, the coating was partially retained on the substrate, with the residual coating thickness decreasing with increase in speed. Explanations for these observations are presented, the interactions between the erosion and oxidation processes for the specimens are discussed and the degradation mechanisms for the coatings under the test conditions are described in this paper.     

The effect of partially carbonized fibers on the mechanical properties of carbon/carbon composites

The carbon/carbon (C/C) composite with satisfactory mechanical properties were obtained through introduction of partially carbonized fibers as a precursor. Applying this procedure the production cost of C/C composites may be significantly reduced. Stabilized PAN fibers were partially carbonized at temperatures ranging from 400 to 1000 °C and reinforced with phenolic resin, resole type. Cured composite were carbonized up to 1000 °C in an inert atmosphere. Monofilament tensile test strength, Young's modulus and tensile strength of partially carbonized fibers were determined. Mechanical properties of carbon/carbon composites (flexural strength and flexural modulus) determined by using three-point bending test. The effect of partially carbonized fibers on the mechanical properties of C/C composites was examined by scanning electron microscope (SEM) through analysis of the fracture surface. The C/C composite reinforced with partially carbonized fibers at 600 °C showed quite satisfactory flexural strength. This confirms assumptions that through co-carbonization of partially carbonized fibers and resin C/C composite with suitable mechanical properties could be obtained.     

Microstructural evolution of 7 wt.% Y2O3-ZrO2 thermal barrier coatings due to stress relaxation at elevated temperatures and the concomitant changes in thermal conductivity

The purpose of this study was to evaluate the combined effect of stress and temperature on the microstructure of air plasma-sprayed 7 wt.% Y₂O₃-ZrO₂ thermal barrier coatings, and relate microstructural changes to the thermal conductivity, k_th. To simulate TBC service conditions, stand-alone tubes of YSZ were stress relaxed, starting from a compressive stress of 60 MPa, at temperatures of 1000 °C or 1200 °C. The duration of the stress relaxation test was either 5 min or 3 h. Detailed scanning electron microscopy (SEM) and Porod's specific surface area (SSA) analysis of small angle neutron scattering (SANS) results were used to determine which void systems, either interlamellar pores or intralamellar cracks, contributed to the observed relaxation of stress in the coatings. SEM investigations revealed closure of intralamellar cracks located perpendicular to the stress direction. For thinner YSZ coatings, SANS measurements indicated a statistically significant reduction in the total SSA and SSA associated with intralamellar cracks after stress relaxation at the times, temperatures, and stress investigated compared to those samples that were exposed to identical times and temperatures, but no stress. The SSA associated with the interlamellar pores was not significantly smaller in YSZ coatings stress relaxed from 60 MPa at 1200 °C for 3 h compared to as-sprayed coatings. The thermal conductivity of the coatings was strongly influenced by stress, with increases in k_th observed after only 5 min at 60 MPa and 1200 °C. Reductions in the total SSA were directly linked to increases in k_th.