Sintering behavior and phase transformation of YSZ-LZ composite coatings

Lyu, Guanlin,Kim, In-Soo,Song, Dowon,Park, Hyeon-Myeong,Kim, Jun Seong,Song, Taeseup,Myoung, Sangwon,Jung, Yeon-Gil,Zhang, Jing Elsevier 2020 CERAMICS INTERNATIONAL Vol.46 No.2

<P><B>Abstract</B></P> <P>Sintering behavior and phase transformations in yttria-stabilized zirconia (YSZ)-based thermal barrier coatings (TBCs) control their applications in gas turbines operated at high working temperatures required for improved fuel efficiency. In this work, to control the sintering behavior and reduce phase transformations in YSZ-based TBCs, lanthanum zirconate (LZ) powder was blended with the YSZ feedstock powder, and YSZ-LZ composite coatings were fabricated using the air plasma spraying method. The influence of mixture weight ratio of YSZ to LZ (75:25, 50:50, and 25:75) on the sintering behavior and phase stability of the composite coatings was investigated through the isothermal exposure test at 1100, 1300, and 1400 °C. The as-coated composites showed the pyrochlore and tetragonal phases, indicating that the phases are LZ and YSZ, respectively. As the exposure temperature was increased, the phase transformation of YSZ from the tetragonal phase to the monoclinic phase was accelerated. The content of monoclinic phase was changed with the increasing LZ content after thermal exposure at 1300 and 1400 °C. In addition, the composites showed different sintering and bridging behaviors at the adjacent splats with the LZ content. The composites prepared with the blended feedstock powders of LZ and YSZ produced an obvious effect on the phase stability and mechanical properties.</P>

Effect of thermal cycling frequency on the durability of Yb-Gd-Y-based thermal barrier coatings

Lyu, Guanlin,Choi, Baig-Gyu,Lu, Zhe,Park, Hyeon-Myeong,Jung, Yeon-Gil,Zhang, Jing Elsevier 2019 Surface & coatings technology Vol.364 No.-

<P><B>Abstract</B></P> <P>The effects of thermal cycling frequency and buffer layer on the crack generation and thermal fatigue behaviors of Yb–Gd–Y-stabilized zirconia (YGYZ)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) test. TGMF tests with low- (period of 10 min) and high-frequency (period of 2 min) cycling were performed at 1100 °C with a 60 MPa tensile load. Different cycling frequencies in TGMF test generate two kinds of crack propagation modes. The sample with low-frequency cycling condition shows penetration cracks in the YGYZ top coat, and multiple narrow vertical cracks are generated in high-frequency cycling. To enhance the thermomechanical properties, different buffer layers were introduced into the TBC systems, which were deposited with the regular (RP) or high-purity 8 wt% yttria stabilized zirconia (HP-YSZ) feedstock. The purity of the feedstock powder used for preparing the buffer layer affected the fracture behavior, showing a better thermal durability for the TBCs with the HP-YSZ in both frequency test conditions. A finite element model is developed, which takes creep effect into account due to thermal cycling. The model shows the high stresses at the interfaces between different layers due to differential thermal expansion. The failure mechanisms of YGYZ-based TBCs in TGMF test are also proposed. The vertical cracks are preferentially created, and then the vertical and horizontal cracks will be propagated when the vertical cracks are impeded by pores and micro-cracks.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Yb-Gd-Y-stabilized zirconia (YGYZ) based TBCs with buffer layer was well prepared. </LI> <LI> Thermal cycling frequency caused different crack growth behavior in TGMF test. </LI> <LI> HP-YSZ buffer layer showed a better thermal durability in low-frequency TGMF test. </LI> <LI> A finite element model showed the high stresses at the interfaces between different layers. </LI> <LI> The failure mechanisms of YGYZ-based TBCs in TGMF test were proposed. </LI> </UL> </P>

Fracture behavior and thermal durability of lanthanum zirconate-based thermal barrier coatings with buffer layer in thermally graded mechanical fatigue environments

Lyu, Guanlin,Kim, Bong Gu,Lee, Seoung-Soo,Jung, Yeon-Gil,Zhang, Jing,Choi, Baig-Gyu,Kim, In-Soo Elsevier 2017 Surface & coatings technology Vol.332 No.-

<P><B>Abstract</B></P> <P>The effects of buffer layer on the fracture behavior and lifetime performance of lanthanum zirconate (La<SUB>2</SUB>Zr<SUB>2</SUB>O<SUB>7</SUB>; LZO)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) tests, which are designed to simulate the operating conditions of rotating parts in gas turbines. To improve the thermal durability of LZO-based TBCs, composite coats consisting of two feedstock powders of LZO and 8wt% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing different volume ratios (50:50 and 25:75, respectively). The composite coat of 50:50 volume ratio was employed as the top coat, and two types of buffer layers were introduced (25:75 volume ratio in LZO and 8YSZ, and 8YSZ only). These TBC systems were compared with a reference TBC system of 8YSZ. The TGMF tests with a tensile load of 60MPa were performed for 1000cycles, at a surface temperature of 1100°C and a dwell time of 10min, and then the samples were cooled at room temperature for 10min in each cycle. For the single-layer TBCs, the composite top coat showed similar results as for the reference TBC system. The triple-layer coating (TLC) showed the best thermal cycle performance among all samples, suggesting that the buffer layer was efficient in improving lifetime performance. Failure modes were different for the TBC systems. Delamination and/or cracks were created at the interface between the bond and top coats or above the interface in the single-layer TBCs, but the TBCs with the buffer layer were delaminated and/or cracked at the interface between the buffer layer and the top coat, independent of buffer layer species. This study allows further understanding of the LZO-based TBC failure mechanisms in operating conditions, especially in combined thermal and mechanical environments, in order to design reliable TBC systems.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Multilayered LZO-based TBCs were well prepared using composite powders. </LI> <LI> The lifetime performance of LZO-based TBCs was investigated through TGMF test. </LI> <LI> TBC with triple-layered top coat showed the most outstanding thermal durability. </LI> <LI> Crack initiation and its growth behavior were affected by the direction of applied load. </LI> </UL> </P>

Thermal Durability of Thermal Barrier Coatings in Furnace Cyclic Thermal Fatigue Test: Effects of Purity and Monoclinic Phase in Feedstock Powder

Park, Hyun-Myung,Jun, Soo-Hyk,Lyu, Guanlin,Jung, Yeon-Gil,Yan, Byung-Il,Park, Kwang-Yong The Korean Ceramic Society 2018 한국세라믹학회지 Vol.55 No.6

The effects of the purity and monoclinic phase of feedstock powder on the thermal durability of thermal barrier coatings (TBC) were investigated through cyclic thermal exposure. Bond and top coats were deposited by high velocity oxygen fuel method using Ni-Co based feedstock powder and air plasma spray method using three kinds of yttria-stabilized zirconia with different purity and monoclinic phase content, respectively. Furnace cyclic thermal fatigue test was performed to investigate the thermal fatigue behavior and thermal durability of TBCs. TBCs with high purity powder showed better sintering resistance and less thickness in the thermally grown oxide layer. The thermal durability was found to strongly depend on the content of monoclinic phase and the porosity in the top coat; the best thermal fatigue behavior and thermal durability were in the TBC prepared with high purity powder without monoclinic phase.

Non-invasive biomarkers for liver inflammation in non-alcoholic fatty liver disease: present and future

Terry Cheuk-Fung Yip,Fei Lyu,Huapeng Lin,Guanlin Li,Pong-Chi Yuen,Vincent Wai-Sun Wong,Grace Lai-Hung Wong 대한간학회 2023 Clinical and Molecular Hepatology(대한간학회지) Vol.29 No.-

Inflammation is the key driver of liver fibrosis progression in non-alcoholic fatty liver disease (NAFLD). Unfortunately, it is often challenging to assess inflammation in NAFLD due to its dynamic nature and poor correlation with liver biochemical markers. Liver histology keeps its role as the standard tool, yet it is well-known for substantial sampling, intraobserver, and interobserver variability. Serum proinflammatory cytokines and apoptotic markers, namely cytokeratin-18, are well-studied with reasonable accuracy, whereas serum metabolomics and lipidomics have been adopted in some commercially available diagnostic models. Ultrasound and computed tomography imaging techniques are attractive due to their wide availability; yet their accuracies may not be comparable with magnetic resonance imaging-based tools. Machine learning and deep learning models, be they supervised or unsupervised learning, are promising tools to identify various subtypes of NAFLD, including those with dominating liver inflammation, contributing to sustainable care pathways for NAFLD.

A review on the failure behavior and countermeasures of thermal barrier coatings

Jiahang Liu,Zhe Lu,Yanwen Zhou,Jing Zhang,Guanlin Lyu 한양대학교 청정에너지연구소 2023 Journal of Ceramic Processing Research Vol.24 No.2

Thermal barrier coatings are widely used in high-temperature components in aircraft thrusters, power generation, and marineengines, enabling gas turbines to operate at elevated temperatures for extended periods by reducing the superalloys' surfacetemperature. During service, high-temperature oxidation, hot corrosion, and sintering occur inside the thermal barriercoatings, resulting in changes in the macro and microstructure of the coatings, and thermal-mechanical propertiesdegradation, eventually leading to coating failure. The main factors that lead to the failure of thermal barrier coatings andaffect the life of thermal barrier coatings are reviewed, including the formation of thermally grown oxides on the surface ofthe bond coat, the corrosion caused by the deposits on the surface of the coating and the sintering of the high-temperatureceramic layer, and the failure mechanism of the coating is analyzed. The countermeasures to prolong the service life of coatingsare reviewed from thermal barrier coating materials, coating structure, coating preparation methods, and post-treatment.