ICMCTF2015 Session AP: Symposium A Poster Session

The elevated temperature due to high energy efficiency requirements causes sintering and phase instability of thermal barrier coatings in the gas turbine hot parts . Thermal shock by periodic stopping of turbine system and cooling is also inevitable, but provides harmful condition to thermal barrier coatings as the coatings are to be endured thermal expansion mismatch during repetitive cycles. Therefore various materials and coating processes have been studied to overcome failure by spallation due to crack nucleation and propagation.

Spherical indentation tests are used to evaluate mechanical damage tolerance of ceramic coatings. The tungsten carbide balls are used to evaluate the damages by contact test. Prior results showed that the contact fatigue produces quasi-plastic damage of commercial thermal barrier coating (YSZ) material. Ultimately, the materials are collapsed at critical loads or cycles by radial crack propagation.

This study investigates thermal shock from 1100~1200oC to room temperature for 800 cycles in air. The substrate temperature are maintained at 900~1100oC during thermal shock test. After that, the contact fatigue tests using WC ball with a radius of 3.18mm are conducted at maximum load of 500N for 10,000 cycles. The test are applied periodically to the coating surface as a sine wave with a frequency of 5Hz.

The porosity change and crack development are observed by optical microscope during thermal shock cycles. The positions of ball indenter, the size of quasi-plastic damages are measured and compared. The possibility as a durability test of thermal barrier coating by thermal shock and post-contact fatigue will be discussed at the presentation.

The results of experimental study of microstructure changes in Zr1Nb alloy after surface modification by Low Energy High Current Pulsed Electron Beam (LEHCPEB) and hydrogenation have showed in this work. Surface treatment was carried out with energy density equal from 10 to 25 J/cm² and impulses number equal from 1 to 5. Hydrogenation was carried out at temperature equal 380 °C and constant hydrogen pressure (2 atm) until reaching the hydrogen concentration of 0.05 wt.%. The structure was analyzed by XRD, EBSD methods and by means of electron-positron techniques. The changing peculiarities of grain and subgrain structures, phase composition, modified layer depth depending on parameters of treatment and hydrogenation have been established.

The reported study was supported by Russian Foundation for Basic Research (RFBR), research project No. 14-08-31033

TiN-TiAlN nanometric multilayer coatings were deposited by cathodic arc deposition at 400°C from Al_0.5Ti_0.5 and Ti targets in the presence of a reactive Ar-N₂ gas mixture. In this work, substrate rotation speed influenced the bilayer period, resulting in different mechanical properties, and wear and oxidation resistance. SEM and XRD were employed for determining the microstructural features. Nanoindentation and ball-on-disk tribological tests were performed in order to examine the mechanical performance of the coatings. Besides, thermogravimetric analyses were conducted with the aim of monitoring their oxidation behavior.

By increasing the substrate rotation speed from 1 to 7 r.p.m., the TiN-TiAlN bilayer period was decreased gradually from 60 nm to 9 nm. When rotation speed was higher than 4 r.p.m., the superlattice structure was formed since first-order satellite peaks beside (111) reflection appeared as already reported by several authors [1]. On one hand, the bilayer period reduction allowed a hardness enhancement up to 43 ± 1.8 GPa. It is believed that interfaces play an important role since the accommodation of the lattice parameter mismatch at the interfaces takes place, thus generating coherent localized strain in a small area which impedes the dislocation motion, the so-called superlattice hardening effect [2]. On the other hand, an increasing number of interfaces in shorter bilayer period samples leads to increase the wear resistance during tribological tests.

Oxidation resistance tests were conducted at 900°C. As a result, protective alumina is formed at high temperature which limited the oxygen diffusion. The latter improved the oxidation resistance compared with single-TiN coating.

References

[1] F. Sanchette et al. Surf. Coat. Technol. 205 (2011) 5444.

[2] F. Lomello et al. Surf. Coat. Technol. 238 (2014) 216.

The effects of bond coat species on the thermal durability of thermal barrier coatings (TBCs) were investigated through cyclic thermal exposure tests in different temperature, including the microstructure evolution and thermomechanical properties. The bond coats were prepared on a Ni based single crystal substrate using three kinds of feedstock powder: Co-Ni based, Ni-Co based, and Ni based powders, which were coated by high-velocity oxy-fuel (HVOF) process. The top coat was formed by air plasma spray (APS) process using METCO 204 C-XCL powder. The cyclic thermal fatigue (CTF) tests were performed at the surface temperatures of 1100 and 1300 °C for a dwell time of 50 min, till more than 25 % of the region spalled in the top coat. In the cyclic thermal fatigue tests at 1100°C, the TBC with the Ni based bond coat showed the longest lifetime performance than those with the Ni-Co and Co-Ni based bond coats. However, when the surface temperature was increased to 1300 °C, the TBCs with Ni-Co or Co-Ni based bond coats showed a longer lifetime performance than that with Ni based bond coat. The results indicate that the bond coat species give a strong effect on the thermal durability of TBC system, dependent on temperature applied in CTF tests. The relationship between bond coat species and thermomechanical properties is extensively discussed based on microstructure evolution during cyclic thermal exposure.

The study concerns comparison of TGO formation during high temperature oxidation of EB-PVD TBCs on various bond coatings including Pt-aluminide, Pd-Pt-aluminide and Pt-diffusion γ/γ'. The bond coatings were obtained by high activity “out of pack” aluminizing and the Pt and Pd+Pt layers were deposited using PVD method prior to aluminizing. The microstructures of the coatings were compared in the initial state as well as after cyclic oxidation using FEG-SEM. High resolution S/TEM along with FIB methods were applied in order to study in detail the microstructures of Thermally Grown Oxides (TGO) that formed on the studied bond coatings. The analysis is focused on the growth of α-alumina, the behavior of Reactive Elements (RE) such as Hf, Zr and Y diffusing through the α-alumina grain boundaries during high temperature oxidation as well the formation of secondary oxides such as NiAl₂O₄.

The influence of pack siliconizing process on internal architecture and phase composition of Mo-Si protective coating on Mo and TZM alloy coupons was showed in this article as well as the oxidation resistance of those materials during quasi-static oxidation test at temperature 1200°C in laboratory air. Conventional process of diffusive coating deposition in activated powders was used to obtain the protective MoSi₂/Mo₅Si₃ layer on pure Mo and TZM alloy sheet. Four different times of process was used – from 3 to 18 hours of annealing. The microstructure of silicide coatings was characterized by SEM/EDS and EBSD methods as well as theirs phase constituent by XRD analysis. The morphology of top surface and internal architecture of coatings were described. Substrate materials with protective coatings were oxidized in air at temperature 1200° in 25 hours intervals. After each interval top surface condition and phase composition were detected by SEM and XRD analysis respectively. Presented investigations showed that both specimens revealed only short term resistance to oxidation at 1200°C. In the case of Mo specimen after ca. 100h siliconizing process partially destruction was observed. However, in the case of TZM alloy used as a substrate material, total destruction was observed after ca. 50 hours.

This work was supported by National Centre for Research and Development, project no PBS1/B5/3/2012 realized as a part of Applied Research Programme.

Thin films of Bismuth Titanium Oxides (BTO) were growth onto glass substrates in an RF magnetron sputtering system with a target of Bi4Ti3O12 and 99,999% of purity. Several thin films of bismuth titanate has been produced varying the power of the RF source. In addition to that the temperature of the substrates during the growth has been changed with the aim of evaluate the influence of this on the structural properties of the thin films. Structural analysis like X-Ray diffraction has been applied to the thin films showing the strong dependence on the temperature of growth. This results show an irregular behavior of the structure, in some cases is totally amorphous and in others is quite crystalline, but a tendency is presented as the temperature increase the crystallinity increases too.

Bismuth oxide has four main structural phases but in terms of conductivity the delta phase has values two times higher than for instance YSZ, a preferred material in the solid oxide fuel cells research field. Regarding to BiTiO is an excellent ferroelectric material and promises interesting optical properties. In that way the study of the optical properties on the thin films produced is highly interesting. Spectrophotometry UltraViolet – Visible (UV-Vis) was the technique used to explore the optical properties like transmission, absorption coefficient and refractive index in the thin films. The results of the analysis are in progress. Although studies on X-Ray diffraction and Auger electron spectroscopy has been made in order to know the structural properties of the films, further analysis are needed to comprehend the structural properties of these films.

R. Swanepoel has been an authority in terms of the determination of the optical constants based on the transmission spectrum of thin films and we are implementing a logarithm in order to program the calculus procedure and thus find the optical properties and even the thickness of the films, those analysis will help us to comprehend the behavior and the possible applications for those films.

Thermal barrier coatings (TBCs) are required in jet turbine engine environments to protect superalloy substrates from harsh temperatures. As the firing temperatures of these engines is pushed hotter, the temperature of the intermetallic bond coats is increasing to the point where platinum modified nickel aluminide (Pt,Ni)Al coatings, the industry standard, rumples under thermal cycling. Rumpling failure is a mechanism whereby undulations develop in the bond coat due to ratcheting creep caused by the thermal expansion misfit between the thermally grown oxide (TGO) and the superalloy substrate with the bond coat. As a result, the ceramic topcoat, which has low out-of-plane compliance and cannot deform with the bond coat, breaks away. Interface cracks link up and lead to buckling and spalling of the topcoat, which results in loss of thermal protection for the superalloy.

Past combinatorial experiments doping β-phase NiAl with platinum group metals found that most β-phase coatings rumple regardless of alloying additions. However, it was found that Pt or Pd additions can provide comparable TBC adhesion to (Pt,Ni)Al coatings at a significantly reduced atomic fraction. This suggests that large amounts of Pt and Cr are not needed for adequate oxidation protection in systems where the primary purpose of the bond coat is to keep the ceramic topcoat in place.

Therefore, a new strategy of improving TBC lifetimes is explored: strengthening bond coats at the expense of a decrease in oxidation behavior. Taking advantage of the inherent creep resistance of the L1₂ structure, a series of γ’ Ni₃Al coatings were designed and put through a furnace cycle test (FCT). These coatings were found to resist rumpling type failures at high temperatures and to extend the life of thermal barrier coatings. The results and failure mechanism of these coatings in an FCT will be discussed.

Grain refinement to the nanocrystalline scale has been proposed as a potential alternative to aluminum/chromium rich coatings for high temperature, oxidizing environments. It is hypothesized that the grain boundaries in nanocrystalline materials act as rapid diffusion pathways accelerating the initial and transient stages of oxidation leading to the expedited growth of protective alumina/chromia scales. This study has explored the influences of grain size on the oxidation behavior of NiAl. Nanocrystalline NiAl coatings were synthesized via direct current (DC) magnetron sputtering onto microcrystalline NiAl bulk alloy substrates. This poster reports the microstructural evolution and oxidation behavior at 900°C and 1050°C for both coated and un-coated conditions.

Characterization of molybdenum disilicide material without and with additions of modified oxides was showed in this article. As a basic material commercial, MoSi₂ powder typically applied to thermal spraying process was used. The final form of specimen was received in the process of pressure sintering in vacuum at temperature 1300°C/2h. The modification process relies on mechanical milling of MoSi₂ powder with addition of 5 % wt. of different type of oxides such as Al₂O₃, CeO₂, HfO₂, ZrO₂ etc.with next sintering process at the same conditions as basic pure material. Obtained materials were analysed from phase constituent and internal morphology point of view as well as theirs oxidation resistance at temperature 1200°C. Microstructural investigation included SEM+EDS analysis and characterization of phase composition by XRD method. Oxidation resistance was characterized by thermo-gravimetric method.

This work was supported by National Centre for Research and Development, project no PBS1/B5/3/2012 realized as a part of Applied Research Programme.

Thermal barrier coatings (TBCs) manufactured by electron beam–physical vapor deposition (EB-PVD) have been favored because their unique microstructure offers the advantage of superior to lerance to mechanical strain and thermal shock at the high temperatures which gas turbines are operated. Of various physical properties of a TBC_S material, the thermal conductivity is the most important thermal property. The purpose of the present work is the evaluation of the thermal conductivity of derived from double-layer samples (coatings and a substrate) using pulsed thermal imaging method as a function of the coatings thickness of ZrO₂-Y₂O₃ coatings deposited by EB-PVD.

2~8mol%Y₂O₃ doped ZrO₂ coatings were deposited by EB-PVD using a 45 kW electron gun. The coated samples were prepared with coatings thickness in the range of 50~500 mm. ZrO₂-Y₂O₃ coatings consist of porous-columnar grains containing nano pores. The thermal conductivity of a coatings layer can be successfully evaluated by the pulsed thermal imaging method using a combined sample of coatings and a substrate.

The thermal conductivity of the coatings decreases with increasing porosity. This result was attributed to the decrease of mean free path due to the enhancement of phonon scattering at pores. In addition, the thermal conductivity of coatings shows increasing tendency with increasing the coatings thickness.

Molten zinc is a very aggressive corrosive medium and poses a considerable problem for metallic construction materials that come into contact with it. Ceramic oxide coatings, such as Al₂O₃, can be used as a protection against the action of this medium. The problem may be damage to the ceramic coating caused by thermal shock, e.g. as a result of taking out from a molten zinc bath and rapid cooling of a coated component.

In the work, the corrosion resistance of a plasma sprayed (APS) ZrO₂-20MgO ceramic coating to the action of molten zinc was examined. The coating was applied on 22 mm diameter cylindrical specimens. Various types of bond coats were used: single-layer (NiCr) and layer graded. The latter was constituted by layers with different contents of NiCr and ZrO₂-20MgO. The tests of corrosion resistance consisted in cyclical exposure of coated specimens in a molten zinc bath for one hour and subsequently, their rapid cooling in water. In total, 41 cycles of heating and cooling were conducted. The coatings were subjected to structural examination (SEM, EPMA, XRD) before and after the test. A decrease in thickness was revealed in the ceramic coating after the corrosion test. In the case of the single-layer NiCr bond coat, local cracks formed at the bond coat/ceramic coat interface.

The so-called M_n+1AX_n – phases (n=1, 2, 3) are a group of nano-laminated materials exhibiting a high functionality. M describes a transition metal, A an A-group element and X either carbon or nitrogen. The properties of this class of materials include that of metals (electrical and high thermal conductivity, ductility, high thermal shock resistance and machinability) as well as of ceramics (low density, hardness, thermodynamic stability and oxidation resistance).

Based on the results of Al oxide/Cr thin film coatings in protecting 304L steel substrates in carburizing atmospheres¹, the present study further explores the performance of these films on 316L stainless steel substrates. Al oxide/Cr and Cr oxide/Cr coatings were deposited on 316L substrates and then both, uncoated and coated specimens were subjected to a carburizing atmosphere of CH₄+H₂+residual oxygen at 800°C for 50h. The results were compared to those obtained for similar 304L coated and uncoated substrates. For the case of the uncoated substrates, although the corrosion kinetics of 316L steel appears to be different than that of 304L, similar surface structural features developed. On the other hand, the 316L coated substrates showed very little weight gain in the carburizing atmosphere even after 50h of exposure.

1. E, Uribe et al., Surf. Eng. 2014, DOI: http://dx.doi.org/10.1179/1743294414Y.0000000322