Spontaneous inversion of the submicron ceramic layer deposited on steel and the copper droplet positioned on their top (case of ceramic poorly wetted by liquid Cu)

In this paper, 50 … 680 nm thick AlN-Al2O3 coatings are deposited by magnetron sputtering on the surface of a steel substrate and a piece of copper is melted on top of the ceramic. Upon heating the ceramic layer is cracked, and the phase inversion of the two top phases from steel/ceramic/copper configuration to the steel/copper/ceramic configuration takes place within 30 s of liquid time of copper. This phase inversion process is accompanied by a Gibbs energy change of about − 1.78 J/m2, due to good wettability of solid deoxidized steel by liquid copper in contrary to poor wettability of the ceramic by the copper. When copper is melted on AlN-Al2O3 coating with its thicknesses smaller than a critical value of about 170 ± 60 nm, liquid copper droplets hanging down into the cracks within the ceramic reach the solid steel surface at the bottom of the cracks, thus the flow of Cu down along the cracks is enabled. However, when copper is melted on AlN-Al2O3 with its thickness larger than the critical value of 170 ± 60 nm, Cu first forms a non-wetting droplet on top of the ceramics, and only after a certain incubation time it starts flowing down the cracks. This incubation time was found to depend linearly on the thickness of the ceramic, as cracks are filled from the bottom upwards by liquid copper via the evaporation–condensation mechanism. By the end of the process, the steel/copper/ceramic configuration is further stabilized by gravity. Graphical abstract

Understanding the Failure Mechanism of Thermal Barrier Coatings Considering the Local Bulge at the Interface between YSZ Ceramic and Bond Layer

The TC/BC interface morphology in APS TBC is one of the important factors leading to crack propagation and coating failure. Long cracks are found near the bulge on the TC/BC interface. In this study, the TBC model with the bulge on the interface is developed to explore the influence of the bulge on the coating failure. Dynamic TGO growth and crack propagation are considered in the model. The effects of the bulge on the stress state and crack propagation in the ceramic layer are examined. Moreover, the effects of the distribution and number of bulges are also investigated. The results show that the bulge on the interface results in the redistribution of local stress. The early cracking of the ceramic layer occurs near the top of the bulge. One bulge near the peak or valley of the interface leads to a coating life reduction of about 75% compared with that without a bulge. The increase in the number of bulges further decreases the coating life, which is independent of the bulge location. The results in this work indicate that a smooth TC/BC interface obtained by some possible surface treatments may be an optional scenario for improving coating life.

Preparation and Characterization of NiCr/NiCr-Al2O3/Al2O3 Multilayer Gradient Coatings by Gas Detonation Spraying

This paper investigates the influence of the technological parameters of detonation spraying on the phase composition of NiCr- and Al2O3-based coatings. It was determined that the phase composition of Al2O3 coatings during detonation spraying strongly depends on the barrel filling volume with the gas mixture. The acetylene–oxygen mixture, which is the most frequently used fuel in the detonation spraying of powder materials, was used as a fuel gas. To obtain a ceramic layer based on Al2O3, spraying was performed at an acetylene–oxygen O2/C2H2 mixture ratio of 1.856; the volume of filling of the detonation gun barrel with an explosive gas mixture was 63%. To obtain a NiCr-based metallic layer, spraying was performed at the O2/C2H2 ratio of 1.063; the volume of filling of the detonation gun barrel with an explosive gas mixture was 54%. Based on a study of the effect of the detonation spraying mode on the phase composition of NiCr and Al2O3 coatings, NiCr/NiCr-Al2O3/Al2O3-based multilayer coatings were obtained. Mixtures of NiCr/Al2O3 powders with different component ratios were used to obtain multilayer gradient coatings. The structural-phase composition, mechanical and tribological properties of multilayer gradient metal–ceramic coatings in which the content of the ceramic phase changes smoothly along the depth were experimentally investigated. Three-, five- and six-layer gradient coatings were obtained by alternating metallic (NiCr) and ceramic (Al2O3) layers. The phase composition of all coatings was found to correspond to the removal of information from a depth of 20–30 μm. It was determined that the five-layer gradient coating, consisting of the lower metal layer (NiCr), the upper ceramic layer (Al2O3) and the transition layer of the mechanical mixture of metal and ceramics, is characterized by significantly higher hardness (15.9 GPa), wear resistance and adhesion strength.

Preparation and Thermal Shock Resistance of Gd2O3 Doped La2Ce2O7 Thermal Barrier Coatings

As one of the promising thermal barrier coating (TBC) candidates, stoichiometric (La0.8Gd0.2)2Ce2O7 (LGC) coatings were prepared by atmospheric plasma spraying (APS), using (La0.8Gd0.2)2Ce2.5O8 as a spray powder and optimized spray parameters. It was found that spray distance and spray power both play an important role in the phase composition and microstructure of the coating. The LGC coating exhibited lower thermal conductivities than that of La2Ce2O7 (LC) coating, which is ~0.67 W/m·K at 1200 °C. Double-ceramic-layer (DCL) optimum (La0.8Gd0.2)2Ce2O7/YSZ (LGC/YSZ) thermal barrier coating was prepared and its thermal shock behavior was investigated. The LGC/YSZ DCL TBCs had better thermal shock resistance ability than that of LC/YSZ TBCs, which was around 109 cycles at 1100 °C. However, the failure mode was similar to that of LC/YSZ DCL TBCs, which was still layer-by-layer spallation in the top ceramic layer due to the sintering of the ceramic coating.

Preparation and properties of ZrO2-5CrMnMo composites by ceramic injection molding

ZrO2-5CrMnMo composites were fabricated by ceramic injection molding in this research. The hardness and wear properties of ZrO2 ceramic layer and 5CrMnMo substrate were investigated. Moreover, physical properties and microstructures of ZrO2 ceramic coatings were studied and the interfaces of composite samples were observed. The results illustrated that the interface was smooth and properly bonded, and it was concluded that the 5CrMnMo substrate ceramic layer could be provided effectively by ZrO2 ceramic coating. Thermal insulation and thermal shock cycle tests were carried out. The heat insulating property of ZrO2 ceramic coating was remarkable, and even better at a high temperature. The composite samples prepared at 1200 °C did not failed until after more than 68 thermal shocks. The main reasons of limiting the application of this composites so far were still the physical and thermodynamic mismatch between ceramics and steel. But the composite samples fabricated by ceramic injection molding showed excellent thermal shock resistance and high bonding strength in this work.

LC/8YSZ TBCs Thermal Cycling Life and Failure Mechanism under Extreme Temperature Gradients

The purpose of this paper is to study the thermal shock resistance and failure mechanism of La2Ce2O7/8YSZ double-ceramic-layer thermal barrier coatings (LC/8YSZ DCL TBCs) under extreme temperature gradients. At high surface temperatures, thermal shock and infrared temperature measuring modules were used to determine the thermal cycling life and insulation temperature of LC/8YSZ DCL TBCs under extreme temperature gradients by an oxygen–acetylene gas flame testing machine. A viscoelastic model was used to obtain the stress and strain law of solid phase sintering of a coating system using the finite element method. Results and Conclusion: (1) Thermal cycling life was affected by the surface temperature of LC/8YSZ DCL TBCs and decreased sharply with the increase of surface temperature. (2) The LC ceramic surface of the failure coating was sintered, and the higher the temperature, the faster the sintering process. (3) Accelerated life test results showed that high temperature thermal cycling life is not only related to thermal fatigue of ceramic layer, but is also related to the sintering degree of the coating. (4) Although the high temperature thermal stress had great influence on the coating, great sintering stress was produced with sintering of the LC ceramic layer, which is the main cause of LC/8YSZ DCL TBC failure. The above results indicate that for new TBC ceramic materials, especially those for engines above class F, their sinterability should be fully considered. Sintering affects the thermal shock properties at high temperature. Our research results can provide reference for material selection and high temperature performance research.

Microstructure and Oxidation Resistance of Thermal Barrier Coatings with Different Ceramic Layer

Thermal barrier coatings are widely used for protection of gas turbine parts against high temperature oxidation and hot corrosion. In present work the microstructural assessment of TBCs produced by atmospheric plasma spray (APS) method was conducted. Three types of ceramic powders were used: magnesia- stabilized zirconia oxide (Metco 210), yttria stabilized zirconia oxide (YSZ -Metco 204) and fine-grained YSZ – Metco 6700. As a base material the Inconel 713 was used as well and CoNiCrAlY was plasma sprayed (APS) as a bond coat. The thickness of all ceramic layers was in range 80 – 110 μm. The elemental mapping of cross-section of magnesia-stabilized zirconia showed the presence of Mg, Zr and O in outer layer. In the YSZ ceramic layer the Y, Zr and O were observed during elemental mapping. The isothermal oxidation test was conducted at 1100 °C for 500 h in static laboratory air. On all samples the delamination and spallation of ceramic layers was observed. Chemical composition analysis of coatings showed the presence of two areas: the first one contained elements from bond coats: Ni, Cr, Al, Co and second area contained O, Cr Co and O that suggest the scale formation. The obtained results showed the total degradation of all ceramic layers as a result of internal stresses in bond-coat. Microscopic analysis showed the areas with complete degradation of bond coats and formation of thick oxides layer.