Abradable Seal Coatings Research Articles

Electrochemical noise characterization of corrosion behavior for NiCrAl-NiC coating

The electrochemical noise analysis (including frequency domain and wavelet analyses) method is used to characterize the corrosion behavior of NiCrAl-NiC Abradable Seal Coating (ASC) at different immersion times. By converting EN data to frequency domain analysis, the slope (k values) of current power spectral density (PSD) curves of NiCrAl-NiC ASC increases with prolonged immersion time, suggesting a transition from localized corrosion to pitting. Wavelet analysis results indicate that the D7-D8 detail coefficients control the wavelet energy throughout the immersion process, and with increasing immersion time, the electrochemical activity on the NiCrAl-NiC ASC surface decreases. The electrochemical noise analysis can effectively explain the corrosion behaviors of NiCrAl-NiC ASC.

Fabrication, mechanical properties and abrasibility of YSZ/BN@ZrO2 coating with core-shell-structured feedstocks by atmospheric plasma spraying

The surface bonding technology is used to manufacture the BN@ZrO2 feedstock, and the YSZ, YSZ/BN, and YSZ/BN@ZrO2 abradable seal coatings are prepared to investigate the mechanical properties and abrasibility at room temperature, 800 ℃, and 1000 ℃. Results show that the porosity of YSZ/BN@ZrO2 is reduced to 12.24±1.25 %, the h-BN content is increased to 3.35±1.42 %, and the hardness is reduced to 3.24 GPa. The wear debris of YSZ/BN@ZrO2 is compressed to form a smooth “glaze” layer at the friction temperature reaching 1000 ℃, B2O3 produced by h-BN oxidation acts as a “sintering agent”, which improves the strength and stability of the “glaze” layer and dramatically improves the abrasibility of the YSZ/BN@ZrO2 coating (coefficient of friction: 0.3; incursion depth ratio: −64 %). Thus, YSZ/BN@ZrO2 coating with core-shell structured feedstocks has significant prospects in high-temperature abradable seal coating.

Porous Yb2Si2O7 coating: A new‐type high‐temperature‐resistant abradable seal coating

AbstractTraditional abradable seal coatings (ASCs) face huge challenges for the requirements of next‐generation aero engines due to high inlet temperature (>1300°C) and non‐matched coefficients of thermal expansion for SiC based composites. Herein, a novel Yb2Si2O7 seal coating was developed to meet the demands of high‐temperature resistance and match the coefficient of thermal expansion with SiC. The prepared Yb2Si2O7 seal coating has excellent properties, including high porosity of 35.5%–23.3%, moderate hardness of 77.9–82.5 HR15Y, and high bond strength of 7.8–9.0 MPa. Adjusting spray current could effectively control the thickness, porosity, and hardness of the coating. More importantly, the coating also has good erosion resistance. Our results show that the fabricated porous Yb2Si2O7 coating would have promising applications in the field of high‐temperature ASCs.

Response of NiCrAl-bentonite abradable seal coatings to thermal/salt corrosion

The growing need for engine services in marine environments has resulted in increasing salt and coupled corrosion of abradable seal coatings (ASCs). In this study, the response of NiCrAl-bentonite ASCs to thermal/salt corrosion was assessed. The results indicated that NiCrAl-bentonite ASCs could provide excellent protection for TC4 alloys in marine environments. NiCrAl-bentonite ASCs retained their low hardness and high strength and exhibited good abradability in chloride-rich environments. Compared with the Al-, Ni-, and YSZ-based coatings, the proposed coating demonstrated improved synergy between strength and hardness/abradability. This research provides valuable insights into the reliable use of NiCrAl-bentonite ASCs in marine environments.

Simultaneously improving strength and abradability for abradable seal coatings by localized bonding hollow spheres

To solve the long-standing contradiction between the bonding strength and abradability of an abradable seal coating (ASC), a novel spherical closed-pore structure was proposed. The new structure consisted of three discontinuous phases, including locally enriched metals, hollow thin-shell spheres, and pores. The results show that the bonding strength of the abradable seal coating reaches 25.4 MPa, and the abrasion ratio between the coating and blade tip, as a characteristic of the abradability of ASCs, is > 5900. The dewetting kinetics of the metal film on the ceramic sphere were calculated. The size of the shrink metal hemisphere is 3.6 times the metal film thickness. The locally enriched metal enables a high bonding strength, while the fragmentation of the hollow thin-shell spheres enhances the abradability, thus simultaneously improving the strength and abradability. The coating is expected to be used in next-generation turbines to improve efficiency and stability.

Influence of Ni Content on Microstructure and Hardness of Nickel-Graphite Abradable Seal Coatings Produced by Plasma Spraying

The influence of the initial shape of graphite powder and Ni content on the structure and properties of the nickel-graphite powders and plasma-sprayed coatings is investigated. The irregularly and spherically shaped graphite powders were produced by mechanical crushing and mechanical crushing with rolling, respectively. It is shown, that spherically shaped graphite powder has advantages in flowability and better deposition of Ni layer. The nickel-graphite core-shell powders with 50 and 75 wt.% Ni content were used to deposit coatings by plasma spraying. The composition, structure, and hardness of the nickel-graphite coatings were studied.

Porous spheres enabling excellent high-temperature abradability and long lifetime for abradable seal coating

Low hardness and high abradability are often required for abradable seal coating (ASC) to protect the blade tip of gas turbines and aeroengines. However, the low bonding strength often lead to spallation failure. In this paper, a novel abradable seal coating with thin-wall porous spheres was designed and prepared. Results show that this coating has both a low hardness of 48.6 HR15Y and a high bonding strength of 25.4 MPa. The abradability and erosion resistance of this coating are 4 times and 63 times higher than reported metal-based coatings. In addition, no surface densification occurs after incursion due to the novel abradable mechanism of crushing of thin-wall sphere. This demonstrates the coating can retain excellent abradability during service and have a long lifetime. The combination of durable excellent abradability and adhesive strength contribute to a long lifetime required by next-generation gas turbines and aeroengines towards higher efficiency and reliability.

Microstructure and phase correlation of DySZ-Polyester abradable seal coating after thermal exposure

Advanced gas turbine blade/shroud clearance control system requires abradable seal coatings with a high-temperature capacity above 1000 ℃ to accommodate the increasing turbine inlet temperature requirements. Dysprosia-stabilized-zirconia (DySZ) based abradable seal Coatings were fabricated on the Ni-based single-crystal superalloy via the atmospheric plasma spraying technique. The microstructure of the DySZ-based coating before and after thermal ageing was characterized using high-contrast SEM. The phase composition and distribution of the coatings were measured by XRD and Raman spectroscopy, respectively. Focused ion beam (FIB) and Scanning transmission electron microscope (STEM) technologies were employed for the phase confirmation. The results indicated that phase transformation from tetragonal to monoclinic ZrO2 was observed in the DySZ-Polyester after 300 h thermal exposure. Based on the different concentrations of Dy element in the monoclinic and tetragonal phases, the morphology and distribution of the monoclinic phase in the coating were observed intuitively using high-contrast BSE images and further corroborated by the Raman spectra mapping results. During heat treatment, the polyester added to the feedstock was oxidized to form amorphous carbon, which is mainly distributed in the gaps of interlamellar cracks and big pores. In addition, numerous Dy2O3 precipitates added to the feedstock were detected in the as-sprayed coating, and some reacted with ZrO2 to form the Dy2Zr2O7 phase during thermal spraying and thermal ageing. The correlation between Dy concentration and phase structure was then discussed.

Modeling deformation and failure in AlSi-polyester abradable sealcoating material using microstructure-based finite element simulation

A plasma-sprayed aluminum-silicon (AlSi)/polyester coating is applied in modern gas turbine engines as an abradable sealcoating to maintain tight clearances between the rotating blades and the static casing. While running the engine, the rotating blades “rub” with the abradable coating, which results in extreme strain rate (up to 106) dynamics and a high-temperature environment. Due to the difficulty of collecting direct measurements, predictive computational models are important for analyzing the deformation and failure of the abradable material, to help meet the design target of avoiding damage to the blade tip and maintaining high fuel efficiency. In this research, a microstructure-based finite element (FE) computational model was developed to capture the complex mechanical behavior of the AlSi/polyester microstructure. The model is based on a virtual representative-volume-element (RVE) of a metal-polymer microstructure, reconstructed from x-ray computed tomography. It models the plastic deformation of, and damage to, each AlSi and polyester constituents, as well as the failure at their interface. The model was calibrated and validated with uniaxial tension and compression experiments, conducted at two temperatures (298 K and 533 K) at an applied strain rate of 103-104s-1. The material exhibited strongly asymmetric tension-compression behavior and a sensitivity to temperature, which was well captured by the model. The model was further applied to investigate changes in mechanical behavior due to variations in constituents’ volume fractions, which provides guidance to the microstructural design of AlSi/polyester abradable materials. The model is expected to facilitate the development of improved abradable materials by bypassing the conventional trial-and-error approach and extensive testing requirements.

Self-lubricating epoxy-based composite abradable seal coating eliminating adhesive transfer via hierarchical design

Aluminum-based composite abradable seal coatings are pivotal to improving the efficiency of aero engines or gas turbines. However, the adhesive transfer frequently occurs between metallic blade tips and aluminum-based composite coatings, resulting in engine vibration and even jam. Many past studies had tried to solve this problem by reducing coating hardness, improving lubrication, or strengthening blade tips, but all had failed. In this paper, we proposed a novel epoxy-based composite abradable seal coating, eliminating adhesive transfer by changing metal-to-metal scraping pair to metal-to-polymer scraping pair. The coating was developed via a hierarchical structure design. Large spherical pores were uniformly distributed in the continuous epoxy matrix with fine graphite dispersion. By adding 20 vol.% graphite and 50 vol.% hollow microspheres, a self-lubricating epoxy-based coating of 0.26 friction coefficient with thermal conductivity of 0.28 W/(m·K), coating HR15Y hardness at 54.8, and bonding strength at 18.7 MPa can be reached. When the metallic blades scrape the epoxy-based composite coating, no adhesive transfer occurs. Besides, a smooth scraped surface is formed by pseudoplastic deformation. This epoxy-based composite abradable seal coating opens a new way to improve the efficiency and reliable operations of air engine compressors.

Excellently abradable seal coating with spherical closed-pore for next-generation aero-engine application

To improve the aero-engine compressor efficiency and reliability, abradable seal coatings need to have good abradability resulting from reduced hardness. However, low hardness is often coupled with low bonding strength, resulting in coating spalling off failure during service. To defeat the conflicts between low hardness and high bonding strength, a new abradable seal coating with a spherical closed-pore structure was prepared in this study. Compared to conventional coatings, the new coating has both lower hardness and higher bonding strength. In the high-speed scraping test, the coating presented excellent abradability without any coating spalling. The new abradable seal coating is promising to improve the efficiency and reliability of the next-generation aero-engine.

Dynamic Hardness of MCrAlY Abradable Seal Coating

The purpose of this investigation is to study the dynamic hardness of MCrAlY abradable coatings under different strain rates. A dynamic indentation device based on the split Hopkinson pressure bar system (SHPB) was used. The results show that the hardness of MCrAlY coating increased with the increase of the strain rate, which has a positive strain rate effect. In addition, the difference of the static hardness of MCrAlY coating prepared by HVOF and LPPS was only 4%, while the difference in dynamic hardness was 16%.

A Study on YSZ Abradable Seal Coatings Prepared by Atmospheric Plasma Spray and Mixed Solution Precursor Plasma Spray

The yttria-stabilized zirconia (YSZ) ceramic has been proved to be a promising material for abradable seal coatings. In this research, abradable seal coatings named C-YSZ and M-YSZ were prepared by applying the conventional plasma spraying method with improved nanostructured powders and by the mixed solution precursor plasma spraying technique, respectively. A study on the microstructure, mechanical properties, oxidation resistance and hot corrosion resistance was conducted on both C-YSZ and M-YSZ coatings by using scanning electron microscopy, transmission electron microscopy, muffle furnace and x-ray diffractometer. The results show that the C-YSZ coating presents larger-size pores with denser YSZ structures and grains showing size larger than 100 nm distributed around the pores compared with M-YSZ, in which the pores are much smaller and the grain size is 20-30 nm. As a result, the M-YSZ coating displays lower hardness and bond strength compared with C-YSZ. The oxidation tests show that the M-YSZ coating possesses better oxidation resistance due to the nanostructure topcoat compared with C-YSZ. However, the hot corrosion tests indicate that both the C-YSZ coating and the M-YSZ coating fail to present ideal hot corrosion resistance.

Galvanic Corrosion Performance of an Al–BN Abradable Seal Coating System in Chloride Solution

In this study, we investigated the galvanic corrosion performance of an Aluminum–Boron Nitride (Al–BN) abradable seal coating system (with a Ni5Al bond layer and a 0Cr17Ni4Cu4Nb substrate) in chloride solution by electrochemical methods. The results indicated a three-stage process occurred during the anodic dissolution of the coupled coating system, consisting of a spontaneous pitting stage I under charge transfer control with a decreasing rate, a corrosion developing stage II under mass transfer control with an increasing rate, and a final steady stage III. Precipitation of Al(OH)3 restricts the oxygen transport process to the cathode and induces localized acidification of the occluded pores of the Al–BN layer, which was the mechanism that could explain the changes of corrosion performance during the three immersion stages of Al–BN coating system. The study suggests that galvanic corrosion of the porous multi-layer Al–BN abradable coating system is mostly influenced by its corrosion product deposition.

Lightweight epoxy-based abradable seal coating with high bonding strength

To gain high efficiency and low fuel consumption, aluminum-based abradable seal coatings had been widely used in the compressor casing of aero engines or gas turbines owing to the low elastic modulus. However, the adhesive transfer phenomenon frequently occurs when the radial rubbing generates between titanium alloy blade tips and aluminum-based coating. It tends to increase scratch force and results in blades vibration and even engine jam. To eliminate this problem, a new lightweight epoxy-based abradable seal coating with high bonding strength was developed by an effective and porosity controllable mixing process that distributes spherical pores uniformly in the continuous matrix. A lightweight coating of 63% porosity with a hardness of 33.1(HR15Y) can be reached when the content of hollow microspheres is 31 wt.%. The coating density is 0.5 g/cm3 and the bonding strength is as high as 18 MPa. The performance of the epoxy-based coating is comprehensively better than aluminum-based coatings in five essential indices. This study is expected to provide a new technical path for obtaining high-quality abradable seal coatings to guarantee the efficient and safe operation of compressors.

A novel method to evaluate the densification of abradable seal coatings

In order to further accurately evaluate the densification degree and investigate the densification mechanism of the abradable seal coating, X-ray Tomography (XRT) was used to quantitatively and qualitatively characterize the 3-D distributions of pores in the Al/hBN abradable coating before and after rubbing. Moreover, through comparing with the calculation of the densification factor, it was found that the experimental results of the two were consistent, which indicated that this novel method is proved to be an effective criterion. Furthermore, it was found that the densification of the Al/hBN abradable coating mainly occurred in the subsurface layer below the worn surface, and the effect of the high-speed rubbing on deeper areas was inconspicuous.

Fabrication and characterization of 8YSZ ceramic based abradable seal coatings by atmospheric plasma spraying

8 wt% yttria-stabilized zirconia (8YSZ) powders are fabricated as high-temperature based materials via a solid-state reaction method and ground into spheres in this paper. Following that, 4 wt% Nickle (Ni), 4 wt% Hexagonal Boron Nitride (hBN) and 4 wt% PHB (Polyphenyl ester) are added to 8YSZ for getting 8YSZ ceramic-based abradable seal powders (8YSZ CASp). Then, the 8YSZ CASp are sprayed on the stainless steel substrate with a NiCoCrAlY transition layer by an atmospheric plasma spraying (APS) technology. The phase structure, surface morphology and the cross-section topography of the fabricated are analyzed, the indentation hardness and nano-indentation test are conducted. The experiments of 8YSZ ceramic-based abradable seal coatings (8YSZ CASc) show that the deposition efficiency and porosity are respectively 78.5% and 21.8%, the bond strength is 4.6 MPa, the cycle number of thermal shock resistance is 37 times, those parameters prove that the fabricated 8YSZ CASc are promising abradable seal coatings.

Investigation on high-speed rubbing behavior between abrasive coatings and Al/hBN abradable seal coatings

The tribological behaviors and wear mechanisms between Al/hBN coatings and Ni/cBN or Ni/Si3N4 abrasive coatings were investigated. Results shown that, after a period of efficient cutting, bare blade and Ni/cBN coating were covered by different degrees of transfer materials, while no obvious transfer layer was detected on Ni/Si3N4 coating. Overall, the abrasive coatings inhibited the materials transfer of Al/hBN coating, and the Al/hBN coating exhibited excellent abradability when rubbed against abrasive coatings. The failure mechanism of Ni/cBN coating was that some particles were pulled out or covered by the transfer materials, while the Ni/Si3N4 coating failed due to the fracture of particles during rubbing.

Effects of blade material characteristics on the high-speed rubbing behavior between Al-hBN abradable seal coatings and blades

Abradable seal coatings have been widely used in aero engines to reduce gas path clearance. Research has been performed to evaluate the abradability of the seal coating. In addition, the high-speed rubbing test rig has been employed to simulate the interaction between the seal coating and blade. However, almost all previous studies tested blades made of one type of material. In real engines, the blade material for a seal coating is changeable. In this work, blades made of different titanium alloys were tested and rubbed against the same Al-hBN seal coating. Macro-morphology and micro-morphology observations with scanning electron microscopy (SEM) were employed to examine the wear behavior of the blade-coating couple. The results indicated that the Ti6Al4V blade exhibited the most severe coating adhesions and almost no blade wear; the Ti-8Al-1Mo-1V blade experienced less coating adhesions; and the Ti-5Al-2Sn-2Zr-4Cr-4Mo blade exhibited the least coating adhesions. The mechanical strengths at room and high temperatures and the rub performances during the single pendulum scratch test were checked to find the relationship between the mechanical properties and high-speed rub performances. Unfortunately, high mechanical strength did not appear to guarantee the absence of blade wear, and the thermal properties of the different materials blades were identified as the main reason for the different wear behaviors.

Effect of APS process parameters on high-temperature wear behavior of nickel–graphite abradable seal coatings

It has been a while since abradable coatings have been used in turbine engines to seal the gap between rotating and stationary components to protect these parts and increase the efficiency of engines. The properties of these coatings should satisfy the need of low wear resistance to seal the gap without any damage to the parts involved, and high erosion resistance against compressed air trying to pass through the gap. Many parameters are involved in order to tailor the coatings' properties to the needs; in this work, the effect of atmospheric plasma spraying (APS) process parameters such as hydrogen flow rate as the secondary plasma gas and spraying distance on wear behavior of nickel-graphite (Ni-G) abradable seal coatings at 400°C have been investigated. The results showed these parameters have a direct impact on the hardness, adhesion strength, coefficient of friction and consequently wear resistance of the coatings.