Revolutionary High-entropy Zr-Y-Yb-Ta-Nb-O Oxides for Next Generation Thermal Barrier Coating Applications

We report a revolutionary ceramic material with exceptional high temperature stability and superior thermo-mechanical properties for next generation thermal barrier coatings (TBCs) for aeroengines. The multicomponent oxides (Zr1 − 4xYxYbxTaxNbxO2) designed via a high entropy concept could exhibit a double tetragonal phase. The optimized composition breaks the limitation of intrinsic brittleness in previously reported TBC candidate materials and shows a superior toughness up to ~ 4.59 MPa m1/2 due to ferroelastic and phase transformation toughening mechanisms. It also shows a remarkable high temperature stability at 1600 ºC, which is almost 400 ºC higher than the state-of-the-art yttria stabilized zirconia TBC material. In addition, it also exhibits a significantly lower thermal conductivity (~ 1.37 W∙m− 1∙K− 1 at 900 ºC) and a higher coefficient of thermal expansion (~ 11.3 × 10− 6 K− 1 at 1000 ºC), as well as excellent corrosion resistance to molten silicate (~ 2.9 µm/h at 1300 ºC). This work provides a new approach to design ceramics by extending the high-entropy concept to both medium-entropy and high-entropy compositions searching for multifunctional properties.

Two Morphologies of Tetragonal Phase in the MgO-PSZ Ceramics

The relationship between microstructures and sintering, heat treatment processes have been investigated in 8mol% MgO-0.5mol% Y2O3-PSZ ceramics. The result show that the T phase exhibits two different morphologies during sintering below 1800°C and isothermal holding or aging anew at 1500°C. Their shape, size, distribution and stability all have obvious difference, so they have different effects on the phase transformation toughening.

Mechanical Properties of ZrO2–LaMgAl11O19 Ceramics

ZrO2 ceramics have been widely used to many fields with its excellent physical and chemical properties, but the mechanical properties of YSZ ceramics, especially the fracture toughness, decline caused by the failure of the phase transformation toughening at high temperature. In this investigation, plate-like LaMgAl11O19 toughened ZrO2 ceramics were prepared by pressureless sintering at 1550 °C for 3h in air . The bulk density of the sintered samples are between 5.5 to 6.0 g/cm3, and the relative density are above 93%. The mechanical properties of the ZrO2-LaMgAl11O19 ceramics were studied systematically at room temperature. The flexure strength and fracture toughness of ZrO2-LaMgAl11O19 ceramic are 811.8 MPa and 13.9 MPa•m½ with the LMA addition of 2wt%.

Fracture Mechanics Study on Porosity Zirnocia/hydroxyapatite Coating Obtained through Microarc Oxidation on Medical Titanium Alloy Surface

In electrolytes TiO2-based complex ceramic coatings containing Ca and P on titanium alloy were ultimately formed containing zirconia/hydroxyapatite by Micro-arc oxidation process. As well as the biomechanics measure experiment of speciments had been done, and the datas were analysised and researched. In addition, the toughening mechanism of reinforced coating was studied in detail, and consists of the second phase toughening, phase transformation toughening and microcrack toughening.The results show that the zirconia /hydroxyapatite coating has advanced biomechanics propertys compared to the individual hydroxyapatite one.

Ceramics Toughening Mechanism Study of Mixed-Mode I-III Cracks with a New Yield Criterion

ABSTRACTConsidering the SD (strength differential) effect on compressive strength and tensile strength in zirconia ceramic material, a yield criterion with a special parameter is introduced. In addition, by analogy with associated flow rule, the constitutive model of phase transformation ceramic material has been established. Under generalized plane strain condition, the theoretical toughening expressions of mixed-mode I-III stationary cracks and steady-state growing cracks have been developed with the constitutive model. The crack toughening effect has been discussed in detail with the Poisson ratio, parameters k / α (the ratio of nominal yield strength and SD effect factor) and ω (the scale factor of mode I crack and mode III). The integral calculation shows that phase transformation toughening of stationary cracks is negative shielding effect and the toughening effect of the steady-state growing cracks change obviously with the increase of parameter k / α. Comparison between experimental data and theoretical data indicates that the yield criterion is in accord with the actual characteristics of the zirconia ceramic, when the expression of mixed-mode I-III crack is reduced to mode I crack. The results obtained in present paper can provide the useful theoretical reference for the research of phase transformation toughening in ceramic materials.

Toughening Analysis of Mixed Cracks in Transformation Toughened Ceramics on Influence of SD Effect

Considering the SD effect, the parabolic-type yield criterion is obtained by using a new parameter. And by analogy with associated plastic flow rule, the ceramic phase transformation constitutive model is established. Under plane strain condition, the theoretical toughening expressions of mixed-mode I-II stationary cracks and steady-state growing cracks are developed by applying the weight function method. And the toughening effect is discussed under the influence of Poisson ratio, parameter and . The simulation results show that these phase transformation toughening effects are in good agreement with experimental results. And comparing with other yield criterions, it is more in line with actual characteristics of zirconia ceramic materials, when the expression of mixed I-II crack is reduced to mode I crack. And it also could provide theoretical support and reference for the further research of ceramic phase transformation toughening.

Effects of CeO2 on the Mechanical Property and Microstructure of 3Y-TZP Nanocomposite Ceramic Die Material

Thermal stability and wear resistance of nanoceramic materials are excellent, so it is an ideal die material and has become one of the increasingly important topics in the field of die research. A new 3Y-TZP nanocomposite die material was prepared with hot pressing technique by using 3Y-TZP, CeO2, Al2O3 and nanometer sized TiC as raw materials. Effects of CeO2 on the mechanical property and microstructure of 3Y-TZP nanocomposite die material were investigated. The addition of CeO2 has obvious effect on the flexural strength and fracture toughness. When the content of CeO2 is 2mol%, the fracture toughness of the composite material reaches 11.22MPa•m1/2. With the increase of CeO2 content, the fracture toughness becomes lower, the flexural strength and hardness first increase and then decrease. When the CeO2 content is 6mol%, the maximum flexural strength reaches 866MPa. The reason for the improvement of mechanical property can be attributed to the effect of CeO2 on the phase transformation of t-ZrO2 and microstructure and the resulted phase transformation toughening mechanism.

Effect of Superplastic Deformation on the Properties of Zirconia Dispersed Alumina Nanocomposite

In this paper constrained extrusion of the zirconia dispersed alumina nanocomposite under superplastic conditions was conducted. The mechanical properties of deformed material were studied and its results were compared with those of the initial materials. The microstructure evolution during superplastic deformation was also analyzed. The results demonstrated that after superplastic extrusion the flexural strength, relative density, Vickers hardness as well as fracture toughness of the material increased noticeably. The flexural strength of the deformed composite even retained at a high value of 310MPa at 800°C. The fracture toughness of the material increased from 6.92 MPa·m1/2 to 8.87 MPa·m1/2 after deformation. After superplastic extrusion due to grain boundary sliding and the compressive stress state, the internal porosities in as-sintered materials were eliminated. During extrusion with grain coarsening the effect of t-ZrO2 to m-ZrO2 transformation toughening increased because more zirconia grains reached the critical dimension. Although grain coarsening may cause the decrease of the fracture toughness in some extent, the phase transformation toughening and strengthening dominated. As a result, the mechanical properties of the deformed material were improved.

Effect of Second Sintering on Intragranular Structure and Mechanical Properties of 10vol%Al2O3/3Y-TZP Composites

To study the effect of second sintering on intragranular structure and mechanical properties of 10vol%Al2O3/3Y-TZP composites, the composites were prepared by ball-milling nano-γ-Al2O3 and nano-ZrO2(3mol%Y2O3), pressing unidirectionally, cold-pressing isostatically, then first sintering at 1400°C for 2h and second sintering at 1600°C for 5h in air. The composition and microstructure of the composites were examined by X-ray diffractometer and scanning electron microscope, transmission electron microscope. The results showed that the second sintering changed the microstructure and fracture mode. After second sintering, the structure of fine grains was changed into the mixed one of large grains and small grains . Many large grains contained small grains, forming intragranular stucture that could be produced by the interblending of not only Al2O3 and ZrO2 but also ZrO2 itself. The intergranular fracture after first sintering was changed into transgranular fracture and intergranular fracture after second sintering . The second sintering led to grinds growing up and intragranular forming, which enhanced the contribution of the phase transformation toughening. Although the second sintering resulted in abnormal increase of grain size, the fracture strength and the fracture toughness of the composite were 667 MPa and 12.9 MPa⋅m1/2, respectively, due to the intragranular structure and the enhanced phase transformation toughening.