Metastable Tetragonal Phase Research Articles

Alternative Treatments for Zirconium Oxide to Compare Commonly Used Surface Treatments to Determine Which Has the Least Effect on the Phase Transformation.

Traditional mechanical processing of zirconium leads to an unfavorable transformation, from a metastable tetragonal phase to a monoclinic phase (t→m), which weakens the structure of the material and subsequently leads to damage to the prosthetic restoration. The aim of this research is to compare commonly used surface treatments to determine which has the least effect on t→m. Thirty cylindrical samples made of sintered zirconium were divided into six groups based on the following treatments: polishing, grinding, sandblasting, chemical etching, laser structuring or dry plasma etching. After surface treatment, the samples were subjected to the following tests: X-Ray Diffraction, microscopic examination, surface wettability and surface roughness measurements. Chemical etching, laser structuring and plasma etching significantly reduce the content of the monoclinic phase. All surface treatments significantly reduced the final amount of the monoclinic phase. However, chemical etching did not provide sufficient surface roughness. Both laser and plasma processing offer the advantage of creating structural patterns on the surface of elements. However, as plasma etching requires a mask to obtain the appropriate pattern on the surface, it seems that laser processing offers more and varied structuring possibilities. Laser structuring is easier to control and more economical than the other methods.

Effect of T’-YSZ@Al2O3 core-shell structure on fracture toughness of YSH24 ceramics

To meet the demands of higher operating temperatures in aero-engines, yttrium oxide fully stabilized cubic (C) phase hafnium oxide (YFSH) has emerged as a promising candidate for thermal barrier coatings due to its ultra-high melting and phase transition temperatures. However, its application is constrained by its inherently low fracture toughness. To enhance the fracture toughness of C-HfO2 phase ceramic, the metastable non-equilibrium tetragonal (T’) phase of Y0.08Zr0.92O1.96 (YSZ8) was synthesized using the sol-gel method. An Al2O3 layer was then coated on YSZ, forming a T’-YSZ@Al2O3 core-shell structure (with Al2O3 comprising 20 % of the YSZ volume). The phase organization stability and diffusion resistance of this T’-YSZ@Al2O3 core-shell structure were investigated. Composite ceramic bulks, with compositions x T’-YSZ@Al2O3/(1-x) YSH24 (where x is the volume fraction, x = 0, 0.1, 0.2, 0.3, 0.4), were prepared using spark plasma sintering (SPS). The impact of the T’-YSZ@Al2O3 addition on the fracture toughness of the YSH24 matrix and the toughening mechanisms were analyzed. The results indicate that the introduction of the T’-YSZ@Al2O3 core-shell structure with good structural integrity into YSH24 ceramics prevents phase transitions. As the volume fraction of the core-shell structure increases from x = 0–0.4, the fracture toughness initially rises and then decreases, reaching a maximum of 2.36 MPa·m1/2 at x = 0.3. This value represents a 38.8 % improvement compared to that of the single C-HfO2 phase YSH24 bulks. The primary toughening mechanism is attributed to the ferroelastic domain switching of T’-YSZ within the T’-YSZ@Al2O3 core-shell structure.

Contribution of oxygen vacancies to phase transition and ferroelectricity of Al:HfO2 films

We investigate the effects of oxygen vacancies on the ferroelectric behavior of Al:HfO2 films annealed in O2 and N2 atmosphere. X-ray photoelectron spectroscopy results showed that the O/Hf atomic ratio was 1.88 for N2-annealed samples and 1.96 for O2-annealed samples, implying a neutralization of oxygen vacancies during O2 atmosphere annealing. The O2-annealed films exhibited an increasing remanent polarization from 23 μC cm−2 to 28 μC cm−2 after 104 cycles, with a negligible leakage current density of ∼2 μA cm−2, while the remanent polarization decreased from 29 μC cm−2 to 20 μC cm−2 after cycling in the N2-annealed films, with its severe leakage current density decreasing from ∼1200 μA cm−2 to ∼300 μA cm−2. A phase transition from the metastable tetragonal (t) phase to the low-temperature stable orthorhombic (o) phase and monoclinic (m) phase was observed during annealing. As a result of the fierce· competition between the t-to-o transition and the t-to-m transition, clear grain boundaries of several ruleless atomic layers were formed in the N2-annealed samples. On the other hand, the transition from the t-phase to the low-temperature stable phase was found to be hindered by the neutralization of oxygen vacancies, with almost continuous grain boundaries observed. The results elucidate the phase transformation caused by oxygen vacancies in the Al:HfO2 films, which may be helpful for the preparation of HfO2-based films with excellent ferroelectricity.

THE EFFECT OF THE CHOICE OF THE STARTING MATERIAL ON THE PHASE COMPOSITION AND PHASE STABILITY OF ZRO<sub>2</sub> PARTICLES SYNTHESIZED BY THE HYDROTHERMAL METHOD

In this work, the phase composition, microstructure and phase stability of zirconium dioxide samples obtained by hydrothermal synthesis from various starting materials were investigated. It was found that when using ZrOCl2·8H2O as a starting material, zirconium dioxide particles containing monoclinic and tetragonal (cubic) phases are formed, at the same time, when using ZrO(NO3)2·2H2O as a starting material, only the monoclinic phase was identified in the samples. The CSR dimensions calculated using the Scherrer equation are in the range from 9 to 40 nm. Analysis of SEM images of experimental samples showed that nanoparticles form conglomerates with sizes of several microns. A study of the phase stability of the t, c – ZrO2 phase from temperature exposure showed that t, c – ZrO2 is a metastable phase with CSR sizes up to annealing of 10 nm. With an increase in the annealing temperature, the metastable tetragonal (cubic) phase of ZrO2 gradually transforms into a monoclinic one, due to the processes of minimizing surface energy and particle proliferation, as well as sintering conglomerates into larger monolithic particles.

D0 ferromagnetic display of zirconia nanocrystallites and the impact of gold doping on their structural, optical, and magnetic properties

[Formula: see text] ferromagnetism is currently under extensive investigation as an alternative to transition metal-doped dilute magnetic semiconductors in the continuously evolving field of spintronics and optoelectronic applications. The literature contains numerous conflicting sources regarding its origin, specifically in materials such as oxides, making it a subject of ongoing controversy. This work mainly centers on the interplay among gold doping, oxygen concentration, and resulting changes in the structural, optical, and magnetic characteristics, with a specific focus on [Formula: see text] ferromagnetism and room temperature ferromagnetism in sol–gel synthesized zirconia nanocrystals. The powder samples crystallized in a mixed monoclinic-tetragonal phase. The weak absorbance band around 280[Formula: see text]nm and the O-1s peak in X-ray photoelectron spectroscopy provide insights into the metastable tetragonal phase’s stability, attributed to oxygen vacancies. The bandgap slightly increases from 4.6[Formula: see text]eV to 5.0[Formula: see text]eV due to the transformation from a tetragonal to a monoclinic phase. Magnetic studies reveal room-temperature ferromagnetism induced by oxygen vacancies in undoped zirconia, while a shift from ferromagnetic to paramagnetic behavior occurs in gold-infused zirconia samples.

Tailoring of Colloidal HfO2 Nanocrystals with Unique Morphologies and New Self‐Assembly Features

Advancing the synthesis of HfO2 nanocrystals, a refined anhydrous protocol that enables kinetic trapping of the metastable tetragonal phase and modulates twinning defects in anisotropically grown monoclinic nanoprisms is presented. This evolved sol–gel approach examines the role of the capping agent tri‐n‐octylphosphine oxide (TOPO) and introduces a novel heating strategy with sequential growth stages. Replacement of TOPO with triphenylphosphine oxide (TPPO) leads to the formation of prismatic hafnia nanocrystals exhibiting pristine {011}‐facets of the monoclinic phase. Furthermore, a hot‐injection inspired heating approach yields sub‐4 nm isotropic HfO2 nanocrystals in the tetragonal phase, bypassing the need for aliovalent cation species. In contrast, a heat‐up approach culminates in the generation of well‐characterized HfO2 nanorods. With sophisticated transmission electron microscopy analysis and the Wulff construction method, insights into the structural nucleation of nanoparticle growth are provided. This synthesis offers exceptional control and facilitates the formation of self‐assemblies akin to liquid crystals, opening the door for new applications with nanocolloidal HfO2.

AC Conduction Mechanism and Impedance Analysis of Conventional and Plasma‐Sintered Yttria‐Stabilized ZrO2@Mullite Composite

AbstractThe addition of 8 mol% of Y2O3 to the ZrO2@mullite composites stabilizes the metastable tetragonal phase of ZrO2 in conventional and plasma‐sintered Y2O3‐stabilized ZrO2@mullite composites. A fused, highly dense, and interconnected microstructure is formed in the case of plasma‐sintered Y2O3 stabilized ZrO2@mullite composite. At 1 MHz frequency, the room temperature and tanδ of (10.37 and 2.06) is obtained for the conventional Y2O3 stabilized ZrO2@mullite composite and that of (38.8 and 4.02) is observed for the plasma sintered Y2O3 stabilized ZrO2@mullite composite. The substitution of Y3+ in place of Zr4+ creates sufficiently large oxygen vacancies at the grain boundary regions of these composites endowed mostly with the ZrO2 phase. In conventional Y2O3‐ ZrO2@mullite composites, both dielectric relaxation as well as conductivity relaxation are observed due to the dynamic motion of dipoles and relaxation due to the presence of defect states and porous structures. This leads to higher grain boundary conductivity than the grain conductivity of these composites. Thermally activated conduction mechanisms exhibited in these composite materials are projected from the complex impedance and modulus analysis. The conventional and plasma sintered Y2O3‐ ZrO2@mullite composite has an optical bandgap of 3.26 and 3.58 eV, respectively.

Effect of annealing on the mechanical properties and cytocompatibility of CoCrMo alloy with tantalum coating

Cobalt–chromium–molybdenum (CoCrMo) alloy is frequently utilized to replace artificial joints and dental components. Tantalum (Ta) coating is used to improve the osteogenic property of CoCrMo alloy. Annealing treatment can increase the Ta coating adhesion and further improve wear resistance and biocompatibility. In this study, two kinds of thicknesses of Ta coatings were applied on CoCrMo alloy substrates via magnetron sputtering technology. The average thicknesses of the Ta coatings were 4.3 and 9.5 μm. The Ta coatings were annealed for 5 h at 800 °C, 900 °C, and 1000 °C, respectively. Results indicated that heat treatment improved the adhesion of the coatings. The Ta coatings were composed of stable body-centered cubic phase (α-Ta) and metastable tetragonal phase (β-Ta). The Ta coating with a thickness of 4.3 μm exhibited excellent wear resistance after being annealed at 800 °C, with little variability in the friction curve and a low average coefficient of friction. The adhesive force of the two coated samples increased initially and subsequently dropped as the annealing temperature increased. At 900 °C, the Ta-coated samples exhibited the highest adhesive force; after being annealed, the rate of cell proliferation increased. At 900 °C, samples with Ta coatings exhibited the highest rate of cell multiplication and the best cell compatibility compared with the other samples. Ta coatings' adhesive force, wear resistance, corrosion resistance, and cytocompatibility were all significantly enhanced at various annealing temperatures. Thus, the results of this work are a useful benchmark for the surface modification of metal implants used in medical procedures, and Ta coatings are a practical orthopedic material for the replacement of joints and bones.

Effect of superspeed sintering on translucency, opalescence, microstructure, and phase fraction of multilayered 4 mol% yttria-stabilized tetragonal zirconia polycrystal and 6 mol% yttria-stabilized partially stabilized zirconia ceramics

Statement of problemThe optical properties of recently developed multilayer zirconia have mainly been studied for the effects of conventional sintering and speed sintering but not as much for the effect of superspeed sintering. As superspeed sintering protocols typically require a higher sintering temperature and higher heating and cooling rates than speed- and conventional sintering protocols, the optical properties of superspeed sintered zirconia may be affected differently. PurposeThe purpose of this in vitro study was to investigate the effect of superspeed sintering on the optical properties, microstructure, and phase fraction of multilayered 4 mol% yttria-stabilized (4Y-) and 6 mol% yttria-stabilized (6Y-) zirconia. Material and methodsMultilayered 4Y- and 6Y-zirconia were sectioned. After conventional and superspeed sintering, the translucency parameter (TP), and opalescence parameter (OP) were measured with a spectrophotometer (n=10). To obtain the grain sizes from the field emission scanning electron microscopy (FE-SEM) images for each layer (n=2), more than 500 (6Y-zirconia) and 800 grains (4Y-zirconia) were measured by linear intercept methods. The phase fractions were obtained through X-ray diffraction (XRD) analysis by using the Rietveld method (n=1). The results were analyzed by 3-way ANOVA and post hoc Tukey honest significant difference tests (TP and OP) and by 3-way ANOVA and post hoc Scheffé tests (grain size) (α=.05). ResultsNo layers exhibited a significant difference in TP after superspeed sintering, except the dentin layer (DL) and transition layer 2 (T2) of 4Y- and 6Y-zirconia, respectively. The TP increased (P<.05) in DL for superspeed sintered 4Y-zirconia and decreased (P<.05) in T2 for the superspeed sintered 6Y-zirconia. However, the difference in TP by superspeed sintering was lower than the perceptibility thresholds of 50:50%. The OP decreased (P<.05) in the DL and T2 of 4Y-zirconia after superspeed sintering. For 6Y-zirconia, the OP decreased (P<.05) in all layers except for the transition layer 1 (T1) after superspeed sintering. However, the difference in OP values was minimal, with only a 1.1 difference observed for Zolid Gen-X (4Y) and a range of 1.22 to 1.62 for Katana UTML (6Y) when using superspeed sintering. No significant change was found in the grain size after superspeed sintering of either zirconia. Regardless of the sintering speed, the average grain size of the 6Y-zirconia (conventional: 2.09 to 2.21 μm; superspeed: 2.11 to 2.20 μm) was larger than that of the 4Y-zirconia (conventional: 0.50 to 0.52 μm; superspeed: 0.52 to 0.54 μm). Owing to superspeed sintering, the metastable tetragonal (T′) phase content increased while the tetragonal (T) phase decreased in 4Y-zirconia; in 6Y-zirconia, the cubic (C) phase content increased, while the T′-phase content decreased. ConclusionsSuperspeed sintering did not result in any clinically significant changes in the translucency and opalescence of 4Y- or 6Y-zirconia.

Thermal cycling and interface bonding performance of single phase (Ni,Pt)Al coating with and without pure metastable tetragonal phase 4YSZ

The thermal cycling behavior of single-phase β-(Ni,Pt)Al coated with and without an APS-4YSZ ceramic layer was evaluated at 1,100 °C. The results showed that the 4YSZ/TGO interface was composed of intermixed and non-intermixed zones, which implied a better bonding force to resist interface determination. The APS-4YSZ ceramic layer applied on (Ni,Pt)Al significantly compressed the TGO fluctuation height during the thermal cycling test, which eventually led to TGO curling.

Ag-induced Phase Transition of Bi2 O3 Nanofibers for Enhanced Energy Conversion Efficiency towards Formate in CO2 Electroreduction.

Bi-based electrocatalysts have been widely investigated in the CO2 reduction reaction (CO2 RR) for the formation of formate. However, it remains a challenge to achieve high Faradaic efficiency (FE) and industrial current densities at low overpotentials for obtaining both high formate productivity and energy efficiency (EE). Herein, we report an Ag-Bi2 O3 hybrid nanofiber (Ag-Bi2 O3 ) for highly efficient electrochemical reduction of CO2 to formate. Ag-Bi2 O3 exhibits a formate FE of >90% for current densities from -10 to -250 mA ⋅ cm-2 and attains a yield rate of 11.7 mmol ⋅ s-1 ⋅ m-2 at -250 mA ⋅ cm-2 . Moreover, Ag-Bi2 O3 increased the EE (52.7%) by nearly 10% compared to a Bi2 O3 only counterpart. Structural characterization and in-situ Raman results suggest that the presence of Ag induced the conversion of Bi2 O3 from a monoclinic phase (α-Bi2 O3 ) to a metastable tetragonal phase (β-Bi2 O3 ) and accelerated the formation of active metallic Bi at low overpotentials (at > -0.3 V), which together contributes to the highly efficient formate formation.

Structure, performance regulation and typical device applications of HfO&lt;sub&gt;2&lt;/sub&gt;-based ferroelectric films

The rapid developments of big data, the internet of things, and artificial intelligence have put forward more and more requirements for memory chips, logic chips and other electronic components. This study introduces the ferroelectric origin of HfO&lt;sub&gt;2&lt;/sub&gt;-based ferroelectric film and explains how element doping, defects, stresses, surfaces and interfaces, regulate and enhance the ferroelectric polarization of the film. It is widely accepted that the ferroelectricity of HfO&lt;sub&gt;2&lt;/sub&gt;-based ferroelectric film originates from the metastable tetragonal phase. The ferroelectricity of the HfO&lt;sub&gt;2&lt;/sub&gt;-based film can be enhanced by doping some elements such as Zr, Si, Al, Gd, La, and Ta, thereby affecting the crystal structure symmetry. The introduction of an appropriate number of oxygen vacancy defects can reduce the potential barrier of phase transition between the tetragonal phase and the monoclinic phase, making the monoclinic phase easy to transition to tetragonal ferroelectric phase. The stability of the ferroelectric phase can be improved by some methods, including forming the stress between the substrate and electrode, reducing the film thickness, constructing a nanolayered structure, and reducing the annealing temperature. Compared with perovskite oxide ferroelectric thin films, HfO&lt;sub&gt;2&lt;/sub&gt;-based films have the advantages of good complementary-metal-oxide-semiconductor compatibility and strong ferroelectricity at nanometer thickness, so they are expected to be used in ferroelectric memory. The HfO&lt;sub&gt;2&lt;/sub&gt;-based 1T1C memory has the advantages of fast reading and writing speed, more than reading and writing 10&lt;sup&gt;12&lt;/sup&gt; times, and high storage density, and it is the fast reading and writing speed that the only commercial ferroelectric memory possesses at present. The 1T ferroelectric field effect transistor memory has the advantages of non-destructive reading and high storage density. Theoretically, these memories can achieve the same storage density as flash memory, more than reading 10&lt;sup&gt;10&lt;/sup&gt; times, the fast reading/writing speed, low operating voltage, and low power consumption, simultaneously. Besides, ferroelectric negative capacitance transistor can obtain a subthreshold swing lower than 60 mV/dec, which greatly reduces the power consumption of integrated circuits and provides an excellent solution for further reducing the size of transistors. Ferroelectric tunnel junction has the advantages of small size and easy integration since the tunneling current can be largely adjusted through ferroelectric polarization switching. In addition, the HfO&lt;sub&gt;2&lt;/sub&gt;-based field effect transistors can be used to simulate biological synapses for applications in neural morphology calculations. Moreover, the HfO&lt;sub&gt;2&lt;/sub&gt;-based films also have broad application prospects in antiferroelectric energy storage, capacitor dielectric energy storage, memristor, piezoelectric, and pyroelectric devices, etc. Finally, the current challenges and future opportunities of the HfO&lt;sub&gt;2&lt;/sub&gt;-based thin films and devices are analyzed.

A Perspective on ferroelectricity in hafnium oxide: Mechanisms and considerations regarding its stability and performance

Ferroelectric hafnium oxides are poised to impact a wide range of microelectronic applications owing to their superior thickness scaling of ferroelectric stability and compatibility with mainstream semiconductors and fabrication processes. For broad-scale impact, long-term performance and reliability of devices using hafnia will require knowledge of the phases present and how they vary with time and use. In this Perspective article, the importance of phases present on device performance is discussed, including the extent to which specific classes of devices can tolerate phase impurities. Following, the factors and mechanisms that are known to influence phase stability, including substituents, crystallite size, oxygen point defects, electrode chemistry, biaxial stress, and electrode capping layers, are highlighted. Discussions will focus on the importance of considering both neutral and charged oxygen vacancies as stabilizing agents, the limited biaxial strain imparted to a hafnia layer by adjacent electrodes, and the strong correlation of biaxial stress with resulting polarization response. Areas needing additional research, such as the necessity for a more quantitative means to distinguish the metastable tetragonal and orthorhombic phases, quantification of oxygen vacancies, and calculation of band structures, including defect energy levels for pure hafnia and stabilized with substituents, are emphasized.

RealizingzT&gt;2 in Environment‐Friendly Monoclinic Cu2S—Tetragonal Cu1.96S Nano‐Phase Junctions for Thermoelectrics

AbstractPhase‐junction nanocomposites, made of nanograins with the same composition but different phases, offer a platform to optimize the physiochemical performance of materials. Herein, we demonstrate a straightforward strategy to synthesize Cu2−xS phase‐junction nanocomposites by retaining surface 1‐dodecanethiol (DDT) ligands, in contrast to the traditional method that strips the ligands. As a result, phase junctions between a conventional monoclinic (m) phase and an unconventional metastable tetragonal (t) phase are obtained. The significantly improved power factor is obtained due to the doping of the t‐phase. The phase‐junction interfaces reduce thermal conductivity. Finally, surface regulation of phase junctions pushes the peakzTto 2.1 at 932 K, being the highest reported for environment‐friendly metal sulfides. This work provides a paradigm to optimize thermoelectric performance by controlling phase junctions through surface‐ligand tuning.

Realizing zT>2 in Environment-Friendly Monoclinic Cu2 S-Tetragonal Cu1.96 S Nano-Phase Junctions for Thermoelectrics.

Phase-junction nanocomposites, made of nanograins with the same composition but different phases, offer a platform to optimize the physiochemical performance of materials. Herein, we demonstrate a straightforward strategy to synthesize Cu2-x S phase-junction nanocomposites by retaining surface 1-dodecanethiol (DDT) ligands, in contrast to the traditional method that strips the ligands. As a result, phase junctions between a conventional monoclinic (m) phase and an unconventional metastable tetragonal (t) phase are obtained. The significantly improved power factor is obtained due to the doping of the t-phase. The phase-junction interfaces reduce thermal conductivity. Finally, surface regulation of phase junctions pushes the peak zT to 2.1 at 932 K, being the highest reported for environment-friendly metal sulfides. This work provides a paradigm to optimize thermoelectric performance by controlling phase junctions through surface-ligand tuning.

Improved Ferroelectricity in Cryogenic Phase Transition of Hf0.5Zr0.5O2

Cryogenic transition from metastable tetragonal phase (t-phase) to orthorhombic phase (o-phase) is crucial in achieving the desired ferroelectric characteristics. Observing the reversible transition from anti-ferroelectricity (AFE) to ferroelectricity (FE) in electrical characteristics, the cryogenic phase transition is experimentally analyzed in Hf0.5Zr0.5O2 alloys. Furthermore, the stabilized o-phase formation is more favorable when applying cryogenics to superlattice Hf0.5Zr0.5O2, manifesting a 23% increase in remanent polarization (Pr) at 77K. To theoretically clarify the emergence of phase transition with decreasing temperatures, Landau-Ginzburg-Devonshire theory and firstprinciple study are combined in this work. Based on the detailed calculation, the increasing relative free energy of the t-phase contributes to lowering the energy barrier when decreasing the temperature, making the convenient transitional pathway from metastable t-to o-phase. This work exhibits the cryogenic phase transition model involving t-and o-phases in Hf0.5Zr0.5O2 and presents a method to boost ferroelectricity for emerging HfO2-based cryo-device.

Effect of Deposition Conditions on Phase Content and Mechanical Properties of Yttria-Stabilized Zirconia Thin Films Deposited by Sol-Gel/Dip-Coating

The effect of yttria concentration (0-33.4 mol%), extraction rates (0.17, 0.33, 0.50, and 0.67 mm s-1), and the number of layers (up to four) on the phase content, surface defects, thickness, hardness, adhesion strength, and wear rate of yttria-stabilized zirconia coatings produced by sol-gel/dip-coating were studied for its use on thermolabile substrates. At 700°C, a metastable tetragonal phase ( t ″ ) was obtained even with 33.4 mol% yttria when heat treated for 24 hours; however, a fully cubic structure was attained by extending the heat treatment up to 48 hours as confirmed by Raman spectroscopy. Furthermore, it was necessary to use withdrawal speeds of at least 0.67 mm s-1 to produce defect-free coatings. Although the coatings were produced at low temperature, they showed 41% lower wear rate than steel and an adhesion strength of 30 MPa. Our work stresses the importance of the heat treatment history on the stabilization of the cubic phase in sol-gel YSZ coatings.

Experimental and Simulation Analysis of the Evolution of Residual Stress Due to Expansion via CMAS Infiltration in Thermal Barrier Coatings

The failure behavior of thermal barrier coatings (TBCs) involves multilayered systems infiltrated with calcium–magnesium–alumino-silicates (CMAS). The metastable tetragonal phase is mainly composed of 7YSZ (7 mol.% Y2O3-stabilized ZrO2), and it destabilizes into the Y-lean tetragonal phase, which may be induced by CMAS infiltration, and transforms into a monoclinic phase during cooling. The phase transformation leads to volume expansion around the CMAS-rich layer. Furthermore, it is shown that the spalling of the coating system emerges when the surface of the coating system is subjected to significant residual stress. In this study, a double-cantilever beam model is established to describe the macroscopic phenomenon of thermal buckling induced via CMAS. The result of the buckle height is used to demonstrate the consistency of the experiment and finite element simulation. The experimental parameters are imported into a multilayer cantilever beam model to analyze the interfacial stresses due to CMAS infiltration. The finite element results indicate that the phase transformation leads to damage in the coating system wherein the interfacial stresses due to phase transformation are 27% higher than those in the model without phase transformation.

Development of deposition beam current dependent microstructure and nanomechanical properties in ZrO2-8wt%Y2O3 thermal barrier coatings produced by electron beam-physical vapor deposition technique

8 wt% yttria stabilized zirconia (8YSZ) coatings have been produced by changing the beam currents of electron beam-physical vapor deposition. The phase composition, microstructure, roughness, nanomechanical properties and residual stress were characterized and analyzed. The as-deposited 8YSZ coating was mainly composed of the metastable tetragonal phase. The enhancement of deposition beam current caused larger crystallite sizes and root mean square roughness, which confirmed by microscopic observations and atomic force microscopy technique. The nanomechanical properties were evaluated by an instrumented nanoindentation test. It was found that Young's modulus and hardness increase with the improvement of deposition beam current, which is due to the lower porosity. The thermal residual stresses generated in the deposition process was measured by two-dimension X-ray diffraction using sin2ψ technique. The measurement result indicated that residual stresses were dependent on the coating process.

Tribological properties and microstructure of monolayer and multilayer Ta coatings prepared by magnetron sputtering

To improve the wear resistance of medical titanium alloy, Ta coatings were prepared by magnetron sputtering method on Ti6Al4V substrates. Phase structure and surface morphology of Ta coatings were characterized by X-ray diffraction and atomic force microscope, respectively. The properties of Ta coatings, including adhesion, nano-hardness, and wear resistance, were tested using nano-scratch, nano-indentation and tribometer, respectively. The results show that both stable body-centered cubic (α-Ta) phase and metastable tetragonal phase (β-Ta) coexist in a multi-layer structure, and it shows a non-columnar feature. Interestingly, only a pure phase with columnar crystals is found in monolayer Ta coating. A body-centered cubic (α-Ta) phase is found in a monolayer prepared under the pulse power bias voltage with low deposition temperature. A pure metastable tetragonal phase (β-Ta) is prepared under the direct current bias voltage, which has the best adhesion and wear resistance compared with other Ta coatings.