Surface Rumpling Research Articles

A study on the thermal cyclic behavior of NiCrAlY coatings via femtosecond laser remelting with high repetition rate

NiCrAlY coatings are widely applied bond coatings in thermal barrier coating systems to prevent high-temperature oxidation of turbine blades. However, the uncontrollable microstructure design and high surface roughness seriously affect the oxidation resistance. Herein, NiCrAlY coatings fabricated through arc ion plating were subjected to high-repetition-rate femtosecond laser remelting (HRR-FLR) treatment. This study assessed the surface rumpling behavior, residual stress evolution, and thermally grown oxide (TGO) layer growth under thermal cyclic oxidation at 1100 °C to determine their impact on coating lifetime. The results indicated that the HRR-FLR treatment was effective in reducing surface roughness, restructuring microstructures, and redistributing reactive elements. These improvements facilitated the formation of a flat, continuous, and dense alumina layer, enhancing the spallation and rumpling resistance of TGO. Consequently, the efficiency of HRR-FLR technique was proven in significantly improving the lifetime of NiCrAlY coatings, which provides a new application prospect of femtosecond lasers in remelting and polishing of metallic surfaces.

Cyclic oxidation-induced deleterious effects of Ru on the surface rumpling of a Pt-modified aluminide coating

Generally, surface rumpling is detrimental to high temperature protective coatings as it leads to adhesion losses and Ru can suppress rumpling behaviour in most Pt-modified aluminide (Pt-Al) coatings during the cyclic oxidation. However, in this study, we found that this suppression is related to the cyclic oxidation cycles of Pt-Al-coated superalloys: Ru suppresses rumpling behaviour with cyclic oxidation for more than 200 cycles at 1100 °C but aggravates it with cyclic oxidation from 50 to 150 cycles. The aggravation of rumpling behaviour was turned out to be associated with coating oxidation. Microstructure observation indicated that Ru participates in oxidation and deteriorates the oxidation resistance via destroying the integrity of the Al2O3 oxide film. More Al atoms are depleted owing to oxidation degradation, attributing to partial β-NiAl phases transformation towards the L10 martensitic phases, which exacerbates volume shrinkage and the driving force of rumpling behaviour. The Ru diffusion process during cyclic oxidation has also been studied. Furthermore, the relationship among martensite transformation, rumpling behaviour and cyclic oxidation in Pt-Al coated Ru-containing superalloy was ascertained to provide a guideline for integrated design of Pt-Al coating/high-generation SX superalloys and propose an investigation strategy for rumpling behaviour of the Pt-Al coating.

Elucidating the Surface Properties of Sr3PbO Inverse‐Perovskite Topological Insulator: A First‐Principles Study

The emergence of robust surface electronic states in topological insulators such as bulk Sr3PbO inverse‐perovskite makes them suitable candidates for spintronic devices and solid‐state quantum computers. Herein, the atomic structure, surface energetics, and electronic properties of Sr3PbO inverse‐perovskite Sr2O (SO)‐terminated and SrPb‐terminated (001) surfaces are examined using density functional theory. A comparison of the computed structural properties of SO‐termianted and SrPb‐terminated (001) surfaces reveals maximum surface rumpling and changes in interlayer distances for the SrPb‐terminated surface of Sr3PbO. However, the calculated surface energies indicate that both SO‐termianted and SrPb‐terminated (001) surfaces of Sr3PbO are energetically feasible, indicating that these surfaces can coexist in a polycrystalline sample of this material. Due to the presence of Pb in Sr3PbO, a comprehensive examination of the electronic structure of bulk and supercell slab structures of Sr3PbO by taking spin–orbit coupling effects into consideration is conducted. Noninsulating nature of electronic structure for the two possible (001) terminations of Sr3PbO is found. The domination of Pb‐6p states at the Fermi energy and the hole screening observed at the SrPb‐terminated surface of Sr3PbO support the p‐type nature observed in the experiment.

Preparation and oxidation performance of a low-diffusion (Ni,Pt)Al coating with an in-situ formed α-W based diffusion barrier

A quasi-continuous α-W-based diffusion barrier (DB) forms in-situ between the (Ni,Pt)Al and the inter-diffusion zone by adding a Ni-W plating layer before the coating preparation. The oxidation behavior of the DB-contained coatings is compared with the normal (Ni,Pt)Al coating at 1100 ℃. The results indicate that the (Ni,Pt)Al with DB exhibits lower oxidation rate, slighter inter-diffusion and flatter surface. Additionally, martensite was observed in (Ni,Pt)Al-DB coatings, which contributed to lower surface rumpling for its higher stiffness and better resistance to creep. The mechanism of the DB and its effects on the oxidation performance of the (Ni,Pt)Al coating were discussed.

A comparative study on the oxidation behavior and failure mechanisms of conventional NiCoCrAl alloy and in-situ composite AlCoCrFeNi2.1 eutectic high-entropy alloy at 1300 °C

We present a comparative study on oxidation behavior and failure mechanisms of conventional NiCoCrAl alloys doped with Y and Hf (CNA) and in-situ composite AlCoCrFeNi2.1 eutectic high-entropy alloy doped with Y and Hf (ISC-EHEA) at 1300 °C. We demonstrate that the ISC-EHEA has much stronger resistance to surface rumpling and oxide spallation than CNA. Hybrid molecular dynamics (MD) and Monte Carlo (MC) simulations show that the diffusion coefficients of the metal elements in the Al-depletion layer of the ISC-EHEA are 50 % lower than those in the CNA. The low diffusion coefficients lead to low growth stress in the thermally grown Al2O3 scale on the ISC-EHEA and improve the creep resistance of the metal in contact with the scale, thus preventing the occurrence of rumpling. The oxidation rate constant of the ISC-EHEA is ∼32 % lower in comparison to those of the CNA, which is attributed to the coarser columnar Al2O3 grains that effectively mitigate grain boundary diffusion. The rumpling-free metal/oxide interface, the low residual stress and the low oxide growth rate for the ISC-EHEA result in strong resistance to scale spallation. Owing to the strong scale/alloy bonding at the interface and the build-up of strain energy during prolonged oxidation, damage in the Al2O3 scale on the ISC-EHEA is initiated as surface cracks rather than interface decohesion. When re-oxidized at 1300 °C, the ingress of oxygen along the surface cracks results in fast growth of new oxides at the metal/oxide interface, which causes local stress concentration, interface crack propagation and scale spallation.

Phase transformation-induced TGO rumpling failure of an AlCoCrFeNi high-entropy alloy after isothermal oxidation at 1200 °C

We compare the oxidation behavior of two AlCoCrFeNi-based high-entropy alloys (HEAs) at 1200 °C. The thermally grown oxide (TGO) on an AlCoCrFeNi HEA undergoes significant surface rumpling, which causes TGO cracking and spallation. However, a similar HEA with a lower Cr content, AlCoCr0.8FeNi, shows no TGO rumpling or failure even after 1000 h oxidation. Our analysis suggests that TGO rumpling on the AlCoCrFeNi HEA is predominantly driven by the body-centered cubic (BCC) to tetragonal Cr-rich σ phase transformation within the Al-depleted layer during cooling. A reduction of the Cr content in the HEA or fast water quenching prevents the phase transformation and, therefore, TGO rumpling. The findings in this study provide scientific guidance for designing more durable, oxidation-resistant AlCoCrFeNi HEAs and coatings.

On the rumpling mechanism in nanocrystalline coatings: Improved by reactive magnetron sputtering with oxygen

• The O dopped nanocrystalline coating is prepared by reactive magnetron sputtering. • The doping of O significantly improves the oxidation resistance of the coating. • The surface rumpling behaviors on coatings is retarded by oxygen. Surface rumpling is detrimental to high temperature protective coatings as it shortens their lifetime and leads to adhesion losses and unexpected corrosion degradation. The driving force and mass transport mechanism behind of rumpling remains to be clarified. In the present investigation, we subjected two types of nanocrystalline coating systems to avoid the influence of interdiffusion on rumpling study. One group was an ordinary nanocrystalline coating, and the other group was designed and prepared with trace oxygen by reactive magnetron sputtering. Systematic cyclic oxidation test at 1100 °C was also carried out. Results show the ordinary nanocrystalline coating oxidized rapidly, which leads to the fast consumption of Al and the acceleration of phase transition in the coating. Meanwhile, severe surface rumpling is observed due to the stress release of nanocrystals through plastic deformation. Besides, the reactive doping of oxygen can significantly reduce the consumption process of Al in nanocrystalline coating. The rumpling is controlled due to the improvement of coefficient of thermal expansion and Young's modulus of the coating. Thereafter, the cyclic oxidation resistance is improved.

Investigation of the structural and electronic properties and surface passivation influence on electronic properties of (001) SbNSr3 nano-surfaces: A hybrid DFT study

In this research, the (001) nano-surfaces of SbNSr3 with SbSr and NSr2 terminations are investigated using the hybrid density functional theory (DFT) calculations. The structural, energetic, and electronic properties and also surface passivation effects are studied for different thicknesses of the nano-surfaces. The calculations showed that the surface rumpling of the SbSr termination was larger than that of the NSr2 termination. The surface energy evaluation indicated that the difference in surface energy for the two terminations of (001) SbNSr3 nano-surfaces was about 0.002 eV/Å2. Therefore, it can be said that both SbSr- and NSr2-terminated (001) surfaces of SbNSr3 are stable. Both types of nano-surfaces demonstrated a metallic nature due to the surface states while the bulk has a semiconducting character. Hydrogen atoms were used for surface passivation to remove the surface states in the band gap. The hydrogen passivation opened the band gap for both types of nano-surfaces. The calculated band gap of the SbSr termination was smaller than that of the NSr2 termination. The opened band gap in the SbSr-terminated nano-surfaces with the thicknesses of 3L–9L varied between 0.672 eV, 0.415 eV, 0.840 eV, and 0.808 eV. The calculated band gaps of the NSr2-terminated nano-surfaces were 1.776, 1.033, 1.249, and 1.029 eV for the thicknesses of 3L, 5L, 7L and, 9L, respectively.

A novel single-phase γ′ nanocrystalline coating with high resistance to oxidation and scale rumpling yet mitigated interdiffusion with the alloy substrate

Novel single-phase γ′ nanocrystalline coatings with and without Pt modification were developed on René N5 single-crystal superalloy to investigate their cyclic oxidation behaviors at 1100 °C and 1150 °C. Compared with conventional β-(Ni,Pt)Al coating, the γ′ nanocrystalline coatings, whether with Pt addition or not, prevailed in terms of inhibiting elements interdiffusion and surface rumpling. More importantly, Pt introduction was observed to further enhance oxidation performances of the γ′ nanocrystalline coating. The synergistic effect of nanocrystalline structure and Pt modification promoted to develop a highly purified alumina scale, preventing the formation of extensive Y-rich oxide impurities and avoiding the premature failure of TGO.

High-temperature oxidation behaviour of refurbished (Ni,Pt)Al coating on Ni-based superalloy at 1100 °C

A single-phase (Ni,Pt)Al coating was removed by soft chemical stripping method successfully after preceding oxidation at 1100 °C for 300 h, and a fresh one was refurbished subsequently. Oxidation behaviour of the refurbished (Ni,Pt)Al coating was studied in comparison with the virgin one at 1100 °C. The results show that the refurbished coating exhibited superior isothermal and cyclic oxidation resistance than the virgin one in terms of oxidation rate and surface rumpling. The effects of surface roughness and residual elements in the refurbished coating on oxidation rate, surface rumpling and interdiffusion behaviour were discussed.

Internal Factors Affecting the Surface Rumpling of a β-NiAlHf Coating

β-NiAl coatings on a superalloy substrate will inevitably result in severe rumpling at elevated temperatures; however, the associated rumpling mechanisms are not completely understood. The scale rumpling behavior of a β-NiAlHf coating deposited by electron beam physical vapor deposition (EB-PVD) on single crystal superalloy IC21 was investigated in this work. Some internal factors, including the mismatch in the coefficient of thermal expansion and the stress induced by the growth of oxide scale and the phase transformation, were taken into consideration. The thermal mismatch stress between the coating and substrate was the main internal factor responsible for rumpling behavior during thermal cyclic loads, while the phase degradation from β-NiAl to γ’-Ni3Al in the coating played a dominant role during static thermal loads.

Surface Rumpling Behavior of Hf/Zr Single-Doped and Co-Doped β-NiAl Coatings during High-Temperature Cyclic Oxidation

Reactive elements like Hf- and Zr-doped β-NiAl are considered to be promising candidate materials for protective coatings used at ultra-high temperatures. However, the role of reactive element co-doping on cyclic oxidation behavior and the surface rumpling of β-NiAl coatings remains unclear. Thus, in this paper, Hf and Zr single-doped and co-doped β-NiAl coatings were deposited on a single crystal superalloy by electron beam physical vapor deposition and the cyclic oxidation at 1150 °C was investigated. The coatings yielded a similar oxidation rate during cyclic oxidation. Obvious surface rumpling appeared in the single-doped coatings, whereas it was effectively alleviated in the co-doped coating. Related mechanisms were discussed, including thermal expansion mismatch, martensitic transformation and phase transformation from β-NiAl to γ′-Ni3Al. The non-uniform phase transformation from β to γ′ was finally believed to be responsible for the discrepancy in the rumpling extents between the single-doped and co-doped coatings.

(Invited) Benefits of Zr Addition on Hot Corrosion and Isothermal Oxidation Resistance of Single-Phase (Ni,Pt) Al Coatings

A Zr-doped single-phase (Ni,Pt)Al coating was prepared on a Ni-based single crystal superalloy through electroplating Pt-Zr composite layer and subsequent gaseous aluminisation treatments. Hot corrosion tests of (Ni,Pt)Al and Zr-doped (Ni,Pt)Al coatings in Na2SO4/NaCl (75:25, wt./wt.) were conducted at 850 ºC in static air. The results indicated that the Zr-doped coating showed superior hot corrosion resistance to conventional (Ni,Pt)Al coatings, in which the enhanced performance is mainly attributed to the advantageous effects of Zr that interacts with S and Cl. The mechanism and role of Zr played in the corrosion test are discussed. Moreover, isothermal and cyclic oxidation behavior of the Zr-doped (Ni,Pt)Al coating samples were assessed at 1373 K in static air compared with normal (Ni,Pt)Al coatings. The results indicated that Zr-doped (Ni,Pt)Al coating demonstrated lower oxidation rate constant and reduced tendencies of oxide scale spallation as well as surface rumpling, in which the enhanced oxidation performance was mainly attributed to the favoring effects of Zr preferring to segregate at grain boundaries of oxide scale and improving Young’s Modulus of the coating. Besides, the addition of zirconium effectively delayed the oxide phase transformation from θ-Al2O3 to α-Al2O3 in early oxidation stage and the coating degradation from β-NiAl to γ'-Ni3Al in stable oxidation stage.

Thickness effect on the structural, electronic and energetic properties of the cubic KMgF3 (0 0 1) surfaces: A first-principles study

Using first-principles density functional theory (DFT) calculations, we investigate the (0 0 1) surfaces of fluoroperovskite KMgF3 with KF and MgF2 terminations. The slab layer thickness effects on the structural, electronic and energetic properties are studied. The calculations show that the MgF2 termination presents relatively larger surface rumpling than the KF termination. Both two types of surfaces retain some of the electronic structure characteristics of the bulk, while the band gaps of the surfaces are found to be reduced by about 1.1–1.5 eV with respect to that of the KMgF3 bulk. The surface stability evaluation indicates that the MgF2-terminated surface is energetically more favorable than the KF-terminated surface. Upon increasing the slab thickness from 3 to 11 atomic layers, the surface rumpling, electronic structure and surface energy are found to be slightly affected by the change of slab thickness. Especially, the predicted surface properties are almost the same when the thickness of the slab exceeds 7 atomic layers.

First-principles investigations of structural, energetic and electronic properties of (001) surfaces of cubic inverse-perovskite Sr3SnO

The structural properties, surface energies and electronic structure of SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO inverse-perovskite are investigated using first-principles calculations. Comparison of the atomic relaxations of SrSn and Sr2O layers of the (001) surfaces of Sr3SnO shows that largest atomic relaxation is achieved in the 1st layer Sn atom of the SrSn-terminated surface which dissipates as one moves toward the center of the slab. Moreover, largest surface rumpling and change in the inter-layer distances are found for SrSn-terminated surface. Cleavage, relaxation and surface energies are computed to examine stability of SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO. The electronic properties of the bulk and SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO are studied using electronic band structure and density of states. We find a conducting behavior for both SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO which is mainly caused by the Sn-5p states.

Peculiarities of application of rebars with various periodic profile as per GOST 34028–2016

In the GOST 34028–2016 “Rebar for concrete structures. Technical specifications”, forms and parameters of periodic profiles are specified, providing solving a wide range of problems from high indices of cohesion for reliable joint rebars and concrete servicing until meeting all the requirement of logistics, related to economic industrial production of reinforcing products. Application of it assumes a free choice out of four standard periodic profile configurations. Production and particularly application the periodic profiles with various configuration need additional clarification. A review of the periodic profile standard forms of rebars as per GOST 34028–2016 presented. Classification of the periodic profile standard forms and designation of rebars quoted. Optimal criteria values of cross rib relative surface rumpling of various profiles. Non-standard kinds of profiles, elaborated in Russia for industrial production, but not included into GOST 34028–2016, quoted. Recommendations on standard and non-standard forms of the periodic profile in case of concrete structures designing, including those for seismic regions, quoted.

Benefits of Zr addition to oxidation resistance of a single-phase (Ni,Pt)Al coating at 1373 K

A single-phase (Ni,Pt)Al coating with lean addition of Zr was prepared by co-electroplating of Pt-Zr composite plating and subsequent gaseous aluminization treatments. Isothermal and cyclic oxidation behavior of the Zr-doped (Ni,Pt)Al coating samples was assessed at 1373 K in static air in comparison with plain nickel aluminide (NiAl) and normal (Ni,Pt)Al coatings. Results indicated that Zr-doped (Ni,Pt)Al coating demonstrated a lower oxidation rate constant and reduced tendency of oxide scale spallation as well as surface rumpling, in which the enhanced oxidation performance was mainly attributed to the segregation of Zr at oxide scale grain boundaries and the improved Young’s modulus of the coating. Besides, the addition of Zr effectively delayed oxide phase transformation of Al2O3 from θ phase to α phase in the early oxidation stage and coating degradation of β-NiAl to γ'-Ni3Al in the stable oxidation stage.

Rumpling and Enhanced Covalency at the SrTiO3(001) Surface

The surface of SrTiO3 (001) is considered to be weakly polar, and in this work, we study the validity of this notion. It exhibits a surface structural distortion, quantified here using low energy electron diffraction at room temperature. Structural analysis shows the presence of strong surface rumpling in the TiO2 terminated surface with the oxygen atoms moving outward and Ti atoms moving inward. Density functional calculations confirm the measured rumpling, and experimental data show the distortion is localized at the surface. Angle-dependent core-level X-ray photoemission spectroscopy (XPS) shows that the surface rumpling strongly impacts the electronic structure of the surface. This observation is reinforced by density functional theory, which demonstrates that the valence state of Ti at the surface is reduced while O is enhanced, where we found the Ti–O bonds are more covalent near the surface. Our results show that surface rumpling is accompanied by a change in the bond hybridization of Ti–O at the s...

Oxidation behaviour of Pt modified aluminized NiCrAlYSi coating on a Ni-based single crystal superalloy

An aluminized NiCrAlYSi coating and Pt modified aluminized NiCrAlYSi coating were prepared on a Ni-based superalloy. Isothermal and cyclic oxidation behaviour of both coatings were investigated. The Pt modified aluminized NiCrAlYSi coating showed an enhanced oxidation resistance due to the presence of Pt decreasing the oxidation rate and improving the adherence of alumina scale. The addition of Pt in aluminized NiCrAlYSi coating inhibited the phase transformation from β to γ′. Besides, the surface rumpling of Pt modified aluminized NiCrAlYSi coating was suppressed as compared to the aluminized NiCrAlYSi coating.

First-principles calculations of different (001) surface terminations of three cubic perovskites CsCaBr3, CsGeBr3, and CsSnBr3

Three cubic bromide perovskites CsMBr3 (M = Ca, Ge, Sn) with two different surface terminations (CsBr and MBr2) were studied in this work using the first principles method. A wide range of physical properties, including electronic band structures, atom-projected density of states for each layer, surface relaxation effects, and surface energy, were evaluated for each considered surface termination. Differences between the properties of the bulk and slab models were highlighted. It was shown that surfaces with the CsBr termination have a lower energy and a more pronounced surface rumpling than those with the MBr2 termination. As a main result of this study, it was demonstrated that the CsBr-terminated surfaces appear to be energetically more stable in each of these three considered perovskites.