Sputter-deposited nitrides for oxidation protection in a steam environment at high temperatures

The oxidation behaviours of ZrN, TiN and TiSiN in a steam environment in the high temperature range of 600 – 900ºC have been studied and compared. Nitride coatings were deposited by reactive magnetron sputtering onto Zirc-alloy and silicon wafer substrates. The steam oxidation test was performed in order to investigate oxidation resistance in the Loss–of–Coolant Accident (LOCA) scenario in Light Water Reactor applications. It was found that TiSiN showed better oxidation resistance in a steam environment than ZrN and TiN. Coatings in the as-deposited state and after thermal exposure were characterised using focused ion beam, transmission electron microscopy and X-ray diffraction to evaluate microstructure and phases present in the coatings

A Conformable High Temperature Nitride Coating for Ti Alloys

There are many applications including aeroengine design where one would like to operate Ti or its alloys at higher temperatures, but the threat of oxidation or fire remains a longstanding challenge. Here, we have designed a bilayer nitride coating for Ti and its alloys produced by magnetron sputter deposition of a SiAlN coating (1.2 μm thick) with a Mo interlayer. We have taken advantage of interdiffusion and inter-reaction at the interface during cyclic oxidation at 800°C to form a layered nitride coating system comprising: a SiAlN top layer, a TiN0.26 and Ti5Si3 mixed phase interlayer, and a Ti-Mo solid solution. The novel TiN0.26 interlayer exhibits adaptive conformability via mechanical twinning, thereby accommodating the thermal mismatch strain between the coating and substrate. This, along with high adhesion, confers excellent thermal cycling life with no cracking, spallation and oxidation of the coating evident after hundreds of hours of cyclic oxidation (>40 cycles) in air at 800°C. This work provides a design pathway for a new family of coatings displaying excellent adhesion, adaptive conformability and superior environmental protection for Ti alloys at high temperature

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films : Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures

Donor-doped TiO 2-based materials are promising thermoelectrics (TEs) due to their low cost and high stability at elevated temperatures. Herein, high-performance Nb-doped TiO 2 thick films are fabricated by facile and scalable screen-printing techniques. Enhanced TE performance has been achieved by forming high-density crystallographic shear (CS) structures. All films exhibit the same matrix rutile structure but contain different nano-sized defect structures. Typically, in films with low Nb content, high concentrations of oxygen-deficient {121} CS planes are formed, while in films with high Nb content, a high density of twin boundaries are found. Through the use of strongly reducing atmospheres, a novel Al-segregated {210} CS structure is formed in films with higher Nb content. By advanced aberration-corrected scanning transmission electron microscopy techniques, we reveal the nature of the {210} CS structure at the nano-scale. These CS structures contain abundant oxygen vacancies and are believed to enable energy-filtering effects, leading to simultaneous enhancement of both the electrical conductivity and Seebeck coefficients. The optimized films exhibit a maximum power factor of 4.3 × 10 -4 W m -1 K -2 at 673 K, the highest value for TiO 2-based TE films at elevated temperatures. Our modulation strategy based on microstructure modification provides a novel route for atomic-level defect engineering which should guide the development of other TE materials

Strain tolerance evolution of EB-PVD TBCs after thermal exposure or CMAS attack

The microstructural evolution and Young’s modulus evolution of EB-PVD TBCs upon thermal exposure and separately after CaO-MgO-Al2O3-SiO2 (CMAS) attack have been compared and investigated. Moduli measured by four methods all show an increase due to sintering whereas their rates of increase are different. On finer scale (i.e. nano indentation), modulus increases from 87.3 GPa in as-deposited coatings to 198 GPa after sintering at 1400 °C for 100 h. While on global scale, the modulus increases from below 10 GPa to153 GPa after identical exposure. For the CMAS attacked TBCs at 1300 °C for 0.5 h, modulus values acquired by different methods are much closer. The effect of sintering and CMAS infiltration on coating’s structural integrity is discussed in terms of elastic strain energy available for driving edge delamination. The energy release rate of CMAS attacked TBCs at 1300 °C for 0.5 h is ∼1200 J/m2, which is equivalent to that of TBCs exposed at 1400 °C for 250 h (no CMAS)

Precipitation and its effect on age-hardening behavior of as-cast Mg-Gd-Y alloy

Microstructures evolution of Mg–7Gd–3Y–0.4Zr (wt.%) alloy during aging at 200 °C was investigated by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results showed that the alloy could exhibit remarkable age-hardening response by optimum solid solution and aging conditions. Especially, the highest Vickers hardness (HV) of this alloy was obtained when it was aged at 200 °C for 120 h, which was mainly attributed to a dense distribution of β′ precipitation in the matrix

A New Al2O3‐Protected EB‐PVD TBC with Superior CMAS Resistance

Abstract Calcium‐magnesium‐aluminium‐silicate (CMAS) attack is a longstanding challenge for yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) particularly at higher engine operating temperature. Here, a novel microstructural design is reported for YSZ TBCs to mitigate CMAS attack. The design is based on a drip coating method that creates a thin layer of nanoporous Al2O3 around YSZ columnar grains produced by electron beam physical vapor deposition (EB‐PVD). The nanoporous Al2O3 enables fast crystallization of CMAS melt close to the TBC surface, in the inter‐columnar gaps, and on the column walls, thereby suppressing CMAS infiltration and preventing further degradation of the TBCs due to CMAS attack. Indentation and three‐point beam bending tests indicate that the highly porous Al2O3 only slightly stiffens the TBC but offers superior resistance against sintering in long‐term thermal exposure by reducing the intercolumnar contact. This work offers a new pathway for designing novel TBC architecture with excellent CMAS resistance, strain tolerance, and sintering resistance, which also points out new insight for assembly nanoporous ceramic in traditional ceramic structure for integrated functions

Unveiling the protection mechanism of LaPO₄ against CMAS attack

This work evaluated the resistance of LaPO4 ceramics to CMAS attack at 1250 °C and explored their interaction mechanisms. The chemical reaction between molten CMAS and LaPO4 involves two stages leading to formation of a continuous inner layer and outer polyhedron grains, which effectively prevents the infiltration of CMAS. The reaction layer in this study is formed by inter-diffusion between LaPO4 and CMAS melt in the interface region, which involves a series of events including solution, substitution and recrystallization. The mitigating mechanism differs significantly from the typical 'dissolution-precipitation' process observed in rare-earth zirconate or alumina-based materials during CMAS attack. Because of this novel mechanism, the reaction layer has a crack-free microstructure and seals the surface of the underlying LaPO4 ceramic, which consequently results in enhanced resistance to CMAS attack.</p