Key Engineering Materials Vols. 512-515

Abstract: To investigate the bond strength of different adhesive luting materials bond to zirconia ceramics. 40 (30mm×5mm×5mm) Cercon Smart Ceramics samples were divided into groups: (1) glass-ionomer bonded (control group), (2) Clearfil™ Repair bonded, (3) Panavia F bonded, (4) Variolink N bonded. Tensile bond strength and shear bond strength were evaluated after 24h storage at 37°C in distilled water. Each group had 10 samples for different test. Data were evaluated using ANOVA analysis (α=0.05). The experiment groups are statistically significant differences with control group (P<0.05). The shear bond strength of the Glass ionomer FX Ι, the Clearfil™ Repair, Panavia F and Variolink N groups were 14.23±4.48 MPa, 21.95±1.32 MPa, 31.16±1.50 MPa and 43.54±1.97 MPa, separately. Comparisons among groups were significant differences (P<0.001). Variolink N bonded ceramics achieved highest tensile and shear bond strength. Different adhesive materials significantly influence the bond strength of zirconia ceramics.

Abstract: The erosion wear resistance of YSZ ceramics is worth studying because solid particle erosion is one of the main causes of destroying the materials and apparatuses. In this paper, 8YSZ ceramics reinforced by NiCr alloy and Al2O3 particles were pressureless sintered at an optimized sintering condition. The facture toughness reached a maximum value of 4.6 MPa•m ^1/2 when the addition of NiCr alloy is 12 vol.%, which was much higher than that of the pure 8YSZ ceramic (2.0 MPa•m ^1/2) fabricated in the same condition. Solid particle erosion wear behavior of NiCr-Al₂O₃-ZrO₂ (8Y) ceramics composites was performed by self-designed sand blasting type solid particle erosion wear equipment, using 36# SiC particles as abrasive particles with the 90o erosion angle. The effect of the volume addition of NiCr alloy on the erosion wear of NiCr-Al₂O₃-ZrO₂ (8Y) ceramic composites at room temperature was investigated. The results show that the volume erosion rate of the ceramic composites decreased as the volume addition of the NiCr alloy increasing. Crossing cracks, plastic deformation and minor chipping are the major erosion mechanisms.

Abstract: 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•m^½ with the LMA addition of 2wt%.

Abstract: Sm₂Zr₂O₇ is one of the promising candidate materials for the next generation thermal barrier coatings because of its excellent thermal properties. But at high temperature the thermal conductivity of rare-earth zirconate increases because of radiation conduction. NiCr₂O₄, which can absorb the infrared photons intensely, was introduced to reduce the radiation conduction of rare-earth zirconate. NiCr₂O₄ powder was prepared by coprecipitation. The bulks of Sm₂Zr₂O₇— NiCr₂O₄ with different content of NiCr2O4 were prepared by using non-pressure sintering. The phase and microstructure of the samples were characterized by XRD and SEM. The optical absorption was also investigated. The absorptivity of the composite, which was generally higher than that of pure Sm₂Zr₂O₇ prepared by coprecipitation and non-pressure sintering, was enhanced with the content of NiCr₂O₄ increasing. The enhancement of absorptivity will reduce the radiation conduction of rare-earth zirconate potentially.

Abstract: For low thermal conductivity and high corrosion resistance, yttria stabilized zirconia (YSZ), as a top coat (TC), is widely used in thermal barrier coatings (TBCs), and the micro-structure of the TC has significant effects on it thermal shock resistance. Combining digital image processing technique with finite element mesh generation methods, finite element (EF) models based on actual microstructures of plasma sprayed YSZ thermal barrier coatings are built in this paper, so as to simulate the coating’s dynamic failure process when suffering thermal shocking loads. The cracking process is revealed by calculating both the stress and strain evolutions within the coating. Based on the proposed method, the effects of porosity and distribution are further studied. The simulation results agree well with the experimental observation, indicating that the cracks are mainly caused by pore connectivity, which promotes the growth of cracks. This work is expected to be helpful to establish the quantitative relationship between the TBCs porosity and the coating’s service performance.

Abstract: According to the theory of phonon transport and thermal expansion, a new complex rare-earth zirconate ceramic (La0.4Sm0.5Yb0.1)2Zr2O7, with low thermal conductivity and high thermal expansion coefficient, has been designed by doping proper ions at A sites. The complex rare-earth zirconate (La_0.4Sm_0.5Yb_0.1)₂Zr₂O₇ powder for thermal barrier coatings (TBCs) was synthesized by coprecipitation-calcination method. The phase, microstructure and thermal properties of the new material were investigated. The results revealed that single phase (La_0.4Sm_0.5Yb_0.1)₂Zr₂O₇ with pyrochlore structure was synthesized. The thermal conductivity and the thermal expansion coefficient of the designed complex rare-earth zirconate ceramic is about 1.3W/m•K and 10.5×10^-6/K, respectively. These results imply that (La_0.4Sm_0.5Yb_0.1)₂Zr₂O₇ can be explored as the candidate material for the ceramic layer in TBCs system.

Abstract: Due to extremely low oxygen transportation at elevated temperature, samarium zirconate (Sm₂Zr₂O₇) with pyrochlore structure was introduced into ZrB₂-based functional gradient composites as oxygen insulation. The elastic properties of composites with different phase ratio were investigated in this study. A series of nearly-fully dense ZrB₂-based composites with different Sm₂Zr₂O₇ content was consolidated by spark plasma sintering (SPS) technology. The microstructure and phase compositions were characterized by scanning electronic microscope (SEM) and X-ray diffraction respectively. The dynamic elastic properties were obtained by resonance method at room temperature. The results revealed that the addition of Sm₂Zr₂O₇ can significantly improve its densification for the refractory diboride, which mainly caused by the high sintering activity of Sm₂Zr₂O₇. Young’s modules of the series of composites obviously decreased with the increasing content of Sm₂Zr₂O₇. The low Young’s modulus of Sm₂Zr₂O₇ was the main reason. The Young’s modules of the series of composites were calculated theoretically. The experimental results matched the theoretical data well. A little deviation appeared due to the continuous structure of Sm₂Zr₂O₇ in the composites.

Abstract: Sublattice model is used to describe the complex system, Neodymium-doped yttrium aluminum garnet (Nd:YAG), and then Gibbs energy function of Nd:YAG system is seriously evaluated, mainly focusing on the instability of NAG, which has a direct effect on the Gibbs energy of NAG as well as Nd:YAG. Incorporating the selected literature thermochemical data of Nd:YAG system with the reevaluated parameters in Gibbs energy function according to a method utilized for defining the instability of NAG, the Gibbs energy function is well described. Trying to be more convincible, the method utilized for defining compounds stability and reevaluating the parameters has been test in Al₂O₃-Nd₂O₃, Al₂O₃-Y₂O₃, Al₂O₃-Gd₂O₃, Al₂O₃-Sm₂O₃ systems, achieving a satisfying agreement.

Abstract: A kind of laminated composite named Al₂O₃/Nylon/Al with high work of fracture was prepared by a simple process using epoxy resin adhesives as binder in a leaky mold at a pressure of 5 MPa. Light microscopy and scanning electron microscopy were employed to observe the microstructures and crack propagation of the laminated composites. The flexural strength and fracture toughness were measured through three-point bending test, and the work of fracture of the laminated composite was calculated from load-displacement curves of three-point bending test. The experimental results show the composite have low Young's modulus and flexural strength, however, the work of fracture of the laminated composite appears to be high of 2850 J/m², and the fracture toughness reaches about 11 MPa•m^1/2. Analysis of microstructure and crack propagation reveals that the failure of the laminated composite exhibit distinctive characteristic.

Abstract: First principles calculations were run on bulk and the （110）（001）（012）（113） surfaces of α-Al₂O₃ in order to examine the growth habit of α-Al2O3 crystals.The Materials Studio package was used, specifically the program CASTEP, utilizing Perdew Burke Ernzerh of exchange-correlation pseudo-potentials. The calculation results shows that the ranking of the face energy on different crystal face are E(001)‹ E(113) ‹ E(012) ‹E(110) which is in good agreement with experiment phenomenon observed that the ranking of the growth rates of different crystal faces are V(001) ‹ V(113) ‹ V(012) ‹ V(110).