post-HIP Treatment Research Articles

Effect of Reducing Atmosphere on AlN Ceramics Sintered by Millimerter Wave-HIP Combined Sintering

The effect of reducing atmosphere on yttria dispersed AlN ceramics sintered by millimeter wave-HIP combined sintering was investigated. The specimens were sintered by millimeter wave (MMW) heating under nitrogen reducing atmosphere containing 3 vol% hydrogen followed by post-HIP treatment at 1700°C for 0.5 h under nitrogen pressure of 190 MPa. The relative densities to the theoretical one saturated at the first MMW heating over 1700°C. Thermal conductivities of AlN ceramics sintered by MMW heating under heavily reduced atmosphere with nitrogen were higher than that through MMW heating under normally reduced atmosphere. The thermal conductivity increased with decreasing the yttria additive amount and with increasing the sintering temperature. The optimized yttria dispersion amount was 1 wt% in the MMW heating at 1800°C for 0.5 h. Relative density and thermal conductivity increased by post-HIP treatment. The optimized yttria dispersion amount shifted from 1 wt% to 3 wt% after post-HIP treatment.

Millimeter Wave-HIP Combined Sintering of AlN Ceramics and their Thermal Conductivity

With the aim to improve the thermal conductivity of yttria dispersed AlN ceramics through a short time and low temperature treatment under nitrogen atmosphere, millimeter wave (MMW) heating was combined with post-HIP treatment. Relative density increased by post HIP treatment over 1600°C on previously MMW heated samples even with the same temperature. The thermal conductivity exceeded over 170 W/m·K when treated through MMW-HIP combined sintering with 1700°C + 1700°C treatment by 0.5 h + 0.5 h. This valued had been attained through the MMW treatment alone at 1800°C for 2 h under 3 wt% hydrogen mixed nitrogen atmosphere. The optimal dispersion amount was 3 wt% in the MMW-HIP combined sintering while that was 5 wt% in the MMW sintering alone. The relative density has been saturated at the first MMW heating over 1700°C. On the other hand, thermal conductivity increased with the treatment temperature both in first MMW and post HIP. No significant pressure dependence can be seen in the thermal conductivity with this experimental condition.

Synthesis and optical characterizations of Yb-doped CaF 2 ceramics

Nanopowders of Yb-doped calcium fluoride were obtained by three different ways: mechanical alloying, reverse micelle method and co-precipitation. Transmission electron microscopy showed that the particle sizes were monodispersed for the powders obtained from soft chemistry with an average size around 20 nm and polydispersed for the powder from the chemical alloying. An annealing step at 400 °C is needed to remove impurities adsorbed at the particles surface for powders obtained from the reverse micellar and the co-precipitation methods. Transparent ceramic with a grain mean size of 24 μm was obtained from the co-precipitation synthesized powder after a vacuum sintering followed by a HIP post-treatment. Loss of transmission can be attributed mainly to the presence of a residual closed porosity. The best transmission value is 55% reached in the infrared range at 1.2 μm.

Improvement of piezoresistance properties of silicon carbide ceramics through co-doping of aluminum nitride and nitrogen

The piezoresistance coefficient was measured on co-doped silicon carbide ceramics. Evaluation samples of α-silicon carbide ceramics were first fabricated by glass capsule HIP method using powder mixture of silicon carbide and aluminum nitride with various ratios. The resultant aluminum nitride added silicon carbide ceramics were doped with nitrogen by changing the post-HIP nitrogen gas pressure. The lattice parameter increased with the amount of adding aluminum nitride indicating that the incorporated aluminum substituted smaller silicon atoms. After post-HIP treatment, lattice parameter then decreased with nitrogen gas pressure. The piezoresistive coefficient increased with the addition of aluminum nitride, it further increased with the nitrogen doping pressure.

The Influence of Sintering Process on the Microstructure of Submicron Alumina Fabricated by Gelcasting

Using ultra-fine alumina powders as raw materials, the submicron transparent alumina with relative density more than 99.9% was fabricated by gelcasting and then sintering-HIP process. It was found that only the samples with relative density more than 95% after presintering could obtain fully densification by post-HIP treatment. The final grain sizes increased after post-HIP treatment and were decided by either HIPing temperature or presintering temperature, depending on which one is higher. The maximal strength can reach about 650MPa when grain size is about 1 μm.

Study of Ti(C,N)-Based Cermets Fabricated by Pressureless-HIP Sintering

In this paper, the Ti(C,N)-based cermets were fabricated with ultrafine Ti(C,N) powders and metal binders Mo and Ni, in which the ratio of Mo to Ni was 1:1 and total metal content ranged from 10wt% to 20wt%. The specimens were first CIP and pressureless sintered at various temperatures ranged from 1450°C to 1600°C and then treated at 1350°C for 20mins by HIP at the pressure of 140MPa. Results revealed that Post-HIP treatment could dramatically improve the density of specimens if their densities after pressureless sintering could reach the required standard. At the same sintering temperature, the grains became finer with increasing metal contents. The grain sizes increased dramatically with increasing sintering temperature from 1550°C to 1600°C. Compared to those obtained by hot pressing, the cermets obtained by pressureless-HIP sintering had better mechanical properties.

Ce and Y Local Structures of High-performance ZrO2 Composites Codoped with Ce and Y for Implant Applications

Abstract 12Ce–ZrO2 (containing 12 mol %CeO2)-based composites with the addition of 3Y–ZrO2 (containing 3 mol %Y2O3) were fabricated by the sintering at 1400 °C and post-HIP treatment at 1350 °C. These composites possessed the high strength and fracture toughness. Their local structures of Ce-K, Y-K, and Zr-K edges for these 12Ce–ZrO2/3Y–ZrO2 composites were examined by XAFS measurements.

Fabrication and Evaluation of High Performance 12Ce-ZrO2/3Y-ZrO2 Composites for an Implant

12Ce-ZrO2 based composites reinforced with the addition of 3Y-ZrO2 as a second phase were fabricated by the pressureless sintering at 1400°C and subsequently post-HIP treatment at 1350°C. These 12Ce-ZrO2 based composites with 3Y-ZrO2 possessed the high strength over 1000 MPa and also maximum fracture toughness of 8 MPa•m1/2. Furthermore, the significant inhibition of phase transformation of tetragonal to monoclinic phase under an aqueous hydrothermal condition at 150°C was obtained for these 12Ce-ZrO2/3Y-ZrO2 composites. Their microstructural observations and evaluation of some properties were carried out for these composites. Especially, their local structures of Ce-K, Y-K and Zr-K edges for these 12Ce-ZrO2/3Y-ZrO2 composites were examined by XAFS measurements.

Ｃｅ‐ＺｒＯ２／Ｙ‐ＺｒＯ２複合材の機械的特性

In order to improve the strength, fracture toughness and thermal stability of 12Ce-ZrO2 ceramics, Zirconia composites consisting of 12Ce-ZrO2 and 3Y-ZrO2 were fabricated by normal sintering at 1450°C and post HIP treatment at 1350°C. Some mechanical properties were evaluated for these composites. The bending strength and fracture toughness were measured by three-point bending test and indentation fracture (IF) method, respectively. The microstructures of the composites were characterized by SEM observations and TEM-EDS analysis. The phase stability in these composites were also evaluated by hydrothermal aging test at 150°C and 0.48 MPa. The addition of Y-ZrO2 into Ce-ZrO2 suppressed grain growth of 12Ce-ZrO2 matrix and improve the mechanical properties of 12Ce-ZrO2. Ce-ZrO2/Y-ZrO2 composites exhibited higher bending strength above 1000 MPa and higher fracture toughness above 8 MPa · m1/2 in the composition region of 80∼90 wt% 12Ce-ZrO2. The tetragonal→monoclinic phase transformation under hydrothermal conditions was inhibited in this composition region. It was found that the distribution of CeO2/Y2O3 in the ZrO2 matrix has a great influence on the mechanical properties and phase stability.

Powder sintering and shape-memory behaviour of NiTi compacts synthesized from Ni and TiH2

The use of TiH 2 to produce NiTi shape-memory alloys from elemental powders is investigated. Mixtures of Ni and TiH 2 powders are cold-pressed and sintered in vacuum or under protective atmosphere. Some of the sintered NiTi compacts are subjected to an additional post-HIP treatment to further increase their density. Post-HIPped NiTi compacts with approximately 7% porosity are obtained. A detailed characterization of the microstructure of the compacts is carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The phase transformation behaviour is studied by differential scanning calorimetry (DSC). The specimens present significant shape-memory effects. A recovery strain of up to 1.6% is reached under a stress of 120 MPa.

Powder sintering and shape-memory behaviour of NiTi compacts synthesized from Ni and TiH2

Abstract The use of TiH 2 to produce NiTi shape-memory alloys from elemental powders is investigated. Mixtures of Ni and TiH 2 powders are cold-pressed and sintered in vacuum or under protective atmosphere. Some of the sintered NiTi compacts are subjected to an additional post-HIP treatment to further increase their density. Post-HIPped NiTi compacts with approximately 7% porosity are obtained. A detailed characterization of the microstructure of the compacts is carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The phase transformation behaviour is studied by differential scanning calorimetry (DSC). The specimens present significant shape-memory effects. A recovery strain of up to 1.6% is reached under a stress of 120 MPa.

Herzian Contact Fatigue Test of Y-PSZ and Y-PSZ/Al2O3 Ceramics in Hot Water.

A Hertzian contact fatigue test was proposed to simulate conventional rolling wear experiments, by which the wear behavior and phase transformation of 3mol%Y2O3-partially stabilized zirconia (3Y-PSZ) and 3Y-PSZ/20 mass%Al2O3 ceramics in water at 90°C were investigated. The Hertzian stress contact fatigue property was found to be greatly dependent on fracture strength and remarkably improved by the post-HIP treatment. The Hertzian contact stress was revealed to have minor influence on the water-assisted tetragonal-to-monoclinic phase transformation in Y-PSZ-based ceramics at temperatures as low as 90°C. The addition of Al2O3 into Y-PSZ effectively suppressed the low-temperature phase transformation, but it did not lead to the improvement in the contact fatigue property probably because the strength of the resultant composite was lower than the monolithic Y-PSZ; however, more work is required to completely understand the Hertzian stress contact fatigue behavior of composites.

Superplasticity of HIP MgO · Al 2O 3 spinel: Prospects for superplastic forming

Polycrystals of hot-isostatically pressed MgO · Al 2O 3 spinel, having a fine-grained size of 610 nm, have been deformed in uniaxial compression. The results have been analysed by taking into account grain growth. The variation in stress exponent n suggests a transition from interface reaction ( n = 2) at low stresses to diffusional mechanism ( n = 1) at high stresses, but cavitational creep was responsible for the non-linear behaviour at stresses above 60 M Pa. The grain size exponent found at 60 M Pa ( p = 3), the absence of dislocations within the fine grains and the similarity of equiaxed grain shapes before and after large flow (≈ 40%) resemble the characteristics of micrograin superplastic flow. The value of the grain size exponent suggests that grain-boundary diffusion makes a significant contribution at medium stresses to the diffusion-accommodated grain-boundary sliding. Post-HIP treatment of superplastically deformed samples results in a general density increase and improved transparency for the less stressed sample.

常圧焼結とポストＨＩＰ処理により合成されたＡｌＮ‐ＳｉＣ複合セラミックスの機械的性質

The pressureless-sintering behavior of AIN-SiC ceramics with 2mass% Y2O3 as a sintering aid was investigated. The AIN-SiC composites containing up to 30vol% SiC (two kinds of SiC sources: equiaxed α-SiC and rod-shaped β-SiC particles) were sintered to about 97% of the theoretical density at 2073K in Ar atmosphere of 0.1MPa, and fully densified further by cladless post-HIP-sintering. The mechanical properties including Young's modulus, bending strength and fracture toughness were evaluated for the densely HIPed materials. The Young's modulus was measured from room temperature to 1723K, and the AIN and AIN-SiC composites showed no decrease in Young's modulus at temperatures up to 1473K. The AIN-SiC composites showed increasing bending strength with increasing SiC content, and the highest bending strength was obtained in the AIN-30%SiC composite with rod-shaped β-SiC particles. The fracture toughness of the AIN increased about 60% after the post-HIP treatment due to the toughening effect of elongated grains which was found to be preferably formed in the AIN and AIN-rich composites. The AIN-SiC composites showed no remarkable increase in the fracture toughness with increasing SiC additions, as a result of the decrease in the amount of the elongated grains.