Search

The CaZrO3 ceramic core materials with excellent mechanical and chemical properties were successfully prepared using single‐phase CaZrO3 powders. Effects of particle size ratio and sintering temperature on the mechanical and chemical properties of CaZrO3 ceramic core materials were researched. The chemical property was analyzed by leaching research of core materials in 10 wt% and 20 wt% HNO3 solution at the boiling point. Results showed that the suitable particle size ratio was important for the preparation of CaZrO3 ceramic core materials with excellent comprehensive properties. The addition of fine particles in ceramic core materials promoted the densification process owing to the framework formed by coarser particles and sintering neck formed by fine particles between coarse particles, which was beneficial for further improving their bending strength. When the content of particles with 200 mesh size was 80wt%, the highest bending strength was obtained, 54.38 ± 5.28 MPa. The porosity was 17.45% and the volume density was 3.86 g/cm3. The increasing sintering temperature increased the densification of CaZrO3 ceramic core materials by offering the sintering driving force, further leading to the improvement of bending strength. When the temperature was 1650℃, at the 20% content of particles with 200 mesh size, the highest bending strength of CaZrO3 cores reached 51.01 ± 5.18 MPa. Meanwhile, the porosity was 18.65% and the volume density was 3.83 g/cm3. Additionally, the CaZrO3 samples could be effectively leached in 10 wt% HNO3 solution. Therefore, CaZrO3 materials with good mechanical and leaching properties were believed to be a suitable candidate for ceramic core materials in the investment casting of alloys with high melting point. [ABSTRACT FROM AUTHOR]

C-axis textured α-SiC ceramics were prepared by gel-casting in a strong magnetic field of 6 T and subsequent sintering at 1950 °C for 2 h. The effects of sintering aids on the microstructure and properties of textured SiC ceramics were discussed. Results showed that highly textured α-SiC green bodies were obtained by gel-casting in a strong magnetic field of 6 T. The sintering process slightly suppressed the texture development of α-SiC ceramic. The degree of texture in the α-SiC green bodies and sintered bodies both showed a decreasing trend with the increase of sintering aids content. And the increase of the sintering aids content promoted the densification and improved the bending strength of α-SiC ceramics. Besides, the bending strength of textured SiC ceramics showed an anisotropy based on the grain orientation. [ABSTRACT FROM AUTHOR]

Al2O3‐based ceramic cores with a uniform microstructure were fabricated successfully by a traditional pressing forming method, in which Al2O3 powders were used as matrix and yttrium oxide as additive. The influences of yttrium oxide content and sintering temperature on properties of ceramic cores were studied carefully. Results indicated that a higher sintering temperature benefited the preparation of ceramic cores with excellent properties. As the temperature was above 1400°C, the reaction of Al2O3 and yttrium oxide occurred, leading to the formation of YAG phases. And, YAG was uniformly adhered on the surface of Al2O3 particles, exerting a good role in connecting Al2O3 particles. Based on XRD analyses, it was found that the increase in the sintering temperature could promote the formation of more YAG phases. When sintering temperature was adjusted to 1600°C, with the increase in the yttrium oxide content, their relative density developed a trend of decreasing first and then increasing, while the apparent porosity had an opposite change tendency. With the increase in the sintering temperature, the line shrinkage and bending strength of Al2O3‐based ceramic cores both increased gradually. In our research, their bending strength reached to 53.5 MPa and apparent porosity was 33.9% when the ceramic cores were prepared with 9 wt% yttrium oxide at 1600°C. [ABSTRACT FROM AUTHOR]

Textured porous Si3N4‐TiB2 composites were prepared by gel‐casting–assisted magnetic field alignment technology and subsequent pressureless sintering in the N2 atmosphere. The effects of TiB2 content on grain orientation and properties of composites were studied. Results showed that using the magnetic field of 6T, Si3N4 grains exhibited a high a‐ or b‐axis orientation and TiB2 grains exhibited a high c‐axis orientation. In textured green bodies, the increase in TiB2 content from 0 to 20 wt%, the degree of texture of Si3N4 grains showed a slight decrease from 0.78 to 0.71. However, the degree of texture of TiB2 grains was maintained at about 0.98 regardless of TiB2 content. After sintering, except for the Si3N4 and TiB2 phases, new TiN and BN phases also appeared in the samples. Additionally, the sintering process promotes grain orientation of Si3N4 and TiB2. Herein, the degree of texture of TiB2 reached to about 1 regardless of TiB2 content, meaning that the complete grain orientation formed for TiB2. Owing to the reconstruction reaction between TiB2 and N2, new TiN and BN phases did not form the orientation. The increase in TiB2 contents gradually increased the apparent porosity of the textured samples from 27.9% to 39.4%, thereby leading to the decrease in bending strength from 126.8 ± 18.8 MPa to 60.3 ± 11.2 MPa and from 105.3 ± 19.8 MPa to 53.6 ± 13.0 MPa in two directions, respectively. [ABSTRACT FROM AUTHOR]

Abstract Al 2 O 3 -based ceramic cores with uniform microstructure were fabricated by sintering ball-shaped Al 2 O 3 powders as the matrix and silicone resin as the precursor and binder in air atmosphere. Effects of silicone resin content and sintering temperature on phase composition, microstructure and properties of the Al 2 O 3 -based ceramic cores were investigated. Results showed that a low sintering rate was beneficial for preparing high-quality Al 2 O 3 -based ceramic cores with no defects and cracks. Silicone resin was uniformly adhered on the surface of the ball-shaped Al 2 O 3 particles, playing a well role in connecting Al 2 O 3 particles. Sintering process made the organic-inorganic transformation process of silicone resin happen, leading to the formation of amorphous silica at 1300 °C, followed by cristobalite appeared by the crystallization of amorphous silica as the temperature was higher than 1400 °C. The sintering necks between ball-shaped Al 2 O 3 grains were formed by the pyrolyzed products of silicone resin. As the temperature was lower than 1500 °C, the phase composition consisted of Al 2 O 3 and silica. However, at 1600 °C, new mullite phases were in situ formed on the surface of ball-shaped Al 2 O 3 particles. The increasing silicone resin content promoted the formation of more mullite phase. Al 2 O 3 -based ceramic cores prepared had well dimensional stability during sintering, and the linear shrinkage rate reached the minimum value of 0.23% in the ceramic cores with silicone resin content of 20 wt% at 1400 °C. The open porosities of Al 2 O 3 -based ceramic cores prepared were 24.2~30.0% and relative densities were 63.9~70.2%. The bending strength increased with the increase in the sintering temperature, and the maximum value was obtained in the ceramic cores with silicone resin content of 15 wt% at the same sintering temperature. In our research, the highest bending strength reached to 77.06 MPa in the ceramic cores with silicone resin content of 15 wt% at 1500 °C. [ABSTRACT FROM AUTHOR]

Three kinds of high-entropy metal diboride composites were successfully prepared by pressureless sintering. The as-synthesized (Hf 0.2 V 0.2 Ta 0.2 Ti 0.2 Nb 0.2)B 2 (HEB-Hf), (Zr 0.2 V 0.2 Ta 0.2 Ti 0.2 Nb 0.2)B 2 (HEB-Zr) and (Cr 0.2 V 0.2 Ta 0.2 Ti 0.2 Nb 0.2)B 2 (HEB-Cr) powders were used as the matrix, and Ni metal was used as the binder phase. The results demonstrated that three kinds of single-phase high-entropy metal diboride powders (HEB-Hf, HEB-Zr, and HEB-Cr) were successfully synthesized by a facile high-energy ball milling-assisted boro/carbothermal reduction method, at 1600 °C for 2 h. Each HEB powder had a hexagonal structure, and the five main elements were evenly distributed. Further, Ni as the binders effectively improved the density and fracture toughness of the sintered HEB composites. In the case of maintaining high hardness, the fracture toughness of the HEB composites was greatly improved. Meanwhile, The composites exhibited excellent properties when the heat-treating temperature was increased to 1600 °C. The hardness of the HEB-Hf, HEB-Zr, and HEB-Cr composites were 24.2 ± 0.6, 23.8 ± 1.2 and 21.8 ± 0.8 GPa, and their fracture toughness were 9.2 ± 0.3, 7.1 ± 0.7 and 7.3 ± 0.5 MPa∙m1/2, respectively. This manuscript reports a novel method for future commercial production of high-entropy diboride. Further, there will be more applications for high-entropy diborides because of their increased toughness when prepared by this method. [Display omitted] • Three single-phase high entropy diboride powders were successfully prepared by boro/carbothermic reduction reaction. • High-dense high-entropy boride composites were successfully prepared using Ni metal as the additive by pressureless sintering at 1600 °C. • The Vickers hardness of HEB-Hf composites, HEB-Zr composites and HEB-Cr composites are 24.2 ± 0.6, 23.8 ± 1.2 and 21.8 ± 0.8 GPa, respectively and the fracture toughness values are 9.2 ± 0.3, 6.1 ± 0.7 and 7.3 ± 0.5 MPa∙m1/2, respectively. [ABSTRACT FROM AUTHOR]

Textured TiB 2 -(Ti,Zr)B 2 composites were prepared by gel-casting in 6 T and subsequent pressureless sintering without the magnetic field. These results showed that c-axis of TiB 2 and ZrB 2 grains were oriented to parallel to the direction of the magnetic field in textured green bodies. Separately, the higher c-axis orientation was obtained for TiB 2 and the lower c-axis orientation was obtained for ZrB 2 . After sintering, ZrB 2 phases disappeared and new (Ti,Zr)B 2 solid solutions appeared by the reaction of ZrB 2 and TiB 2 . The degree of texture of TiB 2 increased compared with that in green body. (Ti,Zr)B 2 solid solutions also showed high c-axis orientation. With the increase of sintering temperatures from 1800 °C to 1950 °C, the degree of texture of TiB 2 slightly decreased from 0.95 to 0.92 and the degree of texture of (Ti,Zr)B 2 showed the big decreasing from 0.80 to 0.62. [ABSTRACT FROM AUTHOR]

Abstract: Preferred orientation of porous Si3N4 ceramics with β‐Si3N4 powders as raw material is obtained by gel‐casting in a longitudinal rotating magnetic field and subsequent pressureless sintering. The effects of rotational speed on crystal orientation and properties of porous Si3N4 ceramics are analyzed. The results show that the c‐axis of β‐Si3N4 grains tended to align along the direction of the rotation axis. The degree of orientation in the samples increases with the increasing rotational speed. The textured porous samples show higher bending strength than that of untextured porous samples owing to the integrative effect of the grain orientation and pores. [ABSTRACT FROM AUTHOR]

Highly c-axis textured TiB 2 ceramics were prepared by slip-casting in a strong magnetic field of 6 T and subsequent pressureless sintering. By analyzing the zeta potential and viscosity, the optimal slurries with good dispersibility and stability were prepared when the pH was about 10.5. In 6 T, c-axis of TiB 2 grain was oriented to parallel to the direction of magnetic field and its a or b-axis was perpendicular to the direction of magnetic field in TiB 2 ceramics. The degree of texture was 0.80 on the top surface perpendicular to the direction of magnetic field in the green body. After sintering without the magnetic field, the degree of texture reached 0.94. Sintering process promoted the improvement of degree of texture in TiB 2 ceramics by grain growth. [ABSTRACT FROM AUTHOR]

C -axis textured SiC ceramics were prepared by a strong magnetic field of 6 T assisted gel-casting and subsequent pressureless sintering. The optimal suspension parameters for gel-casting were determined by analyzing the influences of pH value and dispersant content on the stability and dispersibility of suspensions. The effect of sintering conditions on the texture development and properties of SiC ceramics was discussed. It was found that the increasing sintering temperature or holding time promoted the densification process of SiC ceramics. The c -axis of SiC grain was aligned parallel to the magnetic field by applying a strong magnetic field of 6 T. The degree of texture of SiC ceramics showed a slightly increasing trend with the increase of sintering temperature or holding time. When the samples were sintered at 1950 °C for 4 h or 6 h, the large elongated grains were formed in the samples, leading to the extremely evident anisotropic microstructure on different planes. Textured SiC ceramics exhibited the anisotropic bending strength. [ABSTRACT FROM AUTHOR]

The textured Si 3 N 4 ceramics with β -Si 3 N 4 powders as raw material were fabricated by gel-casting under a strong magnetic field of 6 T and subsequent pressureless sintering. The results show that the a or b -axis of β -Si 3 N 4 crystal is oriented parallel to the direction of magnetic field. The degree of texture in the green body reaches 0.72. With the increase of sintering temperature, the degree of texture in the sample increases from 0.74 at 1700 °C to 0.81 at 1800 °C. The bending strength of the Si 3 N 4 ceramic depends on the orientation of grains. [ABSTRACT FROM AUTHOR]

In this paper, SiO 2 -based ceramic cores were prepared successfully by the preceramic polymer technique. Effects of sintering temperatures and silicone resin contents on the microstructure and properties of SiO 2 -based ceramic cores were investigated in N 2 atmosphere. Results showed that in N 2 atmosphere, the silicone resin transformed into the amorphous SiOC and C-nanowires. SiOC phases mainly adhered on the surface of ball-shaped SiO 2 grains and C-nanowires were mainly located in the gaps between ball-shaped SiO 2 grains. Meanwhile, no crystallized phases formed when the sample was sintered at 1300 °C. The sintering temperatures had an important effect on the formation of C-nanowires. When the temperature reached 1300 °C, a lot of C-nanowires were formed. The method provided a pathway to prepare the SiO 2 -based ceramic cores with high properties by in-situ formed C-nanowires. • C-nanowires were in situ formed between SiO 2 particles. • The existence of C-nanowires increased the bending strength of porous ceramic cores. • This process offered a pathway to prepare the ceramic cores with in situ grown nanowires. [ABSTRACT FROM AUTHOR]

The orthogonal test method was adopted to investigate the effects of ternary sintering aids and sintering temperature on properties of alumina ceramics. Based on results from statistical analysis, the important order of ternary sintering aids and sintering temperature on the properties of alumina ceramics was achieved. Results showed that the important order of parameters in the response of high bending strength was as follows: SiO 2 content, MnO 2 content, sintering temperature and MgO content. And the important order in the response of densification was as follows: sintering temperature, MnO 2 content, SiO 2 content and MgO content. By optimizing the parameters, when the MnO 2 content was 2 wt%, SiO 2 content was 0.5 wt%, MgO content was 0.3 wt% and the sintering temperature was 1550 °C, the highest bending strength of 458.1 MPa was obtained, which was attributed to the combined effect of fine grains, close contact among grains and high relative density. This investigate showed an available method about the effects of multiple processing parameters on properties of ceramics, which contributed to the preparation of materials with optimal properties. • Orthogonal test with high efficiency was used to study the multiple factors effects. • Important order of parameters in the response of bending strength and density was obtained. • Combined effects of fine grains, close contact among grains and high density were beneficial for high strength. [ABSTRACT FROM AUTHOR]

Porous Al 2 O 3 ceramics with different SiC contents were successfully prepared through preceramic polymer process following by sintering at 1500 °C at N 2 atmosphere, where ball-shape Al 2 O 3 powders and preceramic polymer silicone resin were used as the matrix and the binder. According to the SEM, XRD and elemental analysis, it was found that C-nanowires were formed in the gap between grains in sintered samples, which was formed by the organic-inorganic transformation of silicone resin at N 2 atmosphere. The existence of SiC was beneficial for the formation of C-nanowires. With the increase of SiC content in the samples, the porosity had no evidently change, however, the bending strength increased from 23 MPa to 50 MPa. [ABSTRACT FROM AUTHOR]