Manufacturing Silicon Carbide Microrotors by Reactive Hot Isostatic Pressing within Micromachined Silicon Molds

A novel ceramic microfabrication process—based on the idea of silicon carbide (SiC) reaction sintering within a micromachined silicon mold—has been developed to produce a SiC microroter for miniaturized gas turbines. The new process involves the micromachining of silicon molds; filling the molds with powder mixtures of α-SiC, graphite, and phenol resin; bonding the molds with an adhesive; reaction sintering by hot isostatic pressing (HIP); and the releasing of a reaction-sintered workpiece from the mold by wet etching. Using this process, we have successfully fabricated SiC microrotors with a diameter of 5 mm, whose complicated geometry was well transferred from the negative shape of the micromachined silicon mold. The reaction-HIPed SiC ceramics within Si molds showed reasonably good mechanical properties, which are comparable to those of the commercialized reaction-sintered SiC ceramics.     

Reactive Hot Pressing of Alumina-Molybdenum Disilicide Composites

Al₂O₃-MoSi₂ composites were prepared by reactive hot pressing using molybdenum, aluminum, and mullite powders as precursors. The Gibbs free energy was highly negative for the composite-forming reaction, which indicated that the products were stable relative to the reactants. After the reaction, the composites had high relative density, ∼96%. Based on the composite-forming reaction, the composites should have contained 18 vol% MoSi₂ in an Al₂O₃ matrix. Scanning electron microscopy revealed that the MoSi₂ inclusions were elongated, with an average thickness of ∼5 μm and inclusion lengths that ranged from 5 to 50 μm. Average composite strength was 467 MPa, and toughness was 3.7 MPa·m^1/2.     

Hot Isostatically Pressed Alumina-Silicon Carbide-Whisker Composites

Alumina-silicon carbide-whisker composites were hot isostatically pressed at 1550°C and 200 MPa for 1 h. The silicon carbide whiskers were treated in different acid and gas environments before they were pressed. All samples exhibited linear elastic behavior with no ductility tendency. Improved strength and fracture toughness were obtained compared with unreinforced alumina. Mechanisms for the improved mechanical properties are discussed. These include grain growth control, whisker encapsulation of defects, and related stress relief at the defect.     

Compression Deformation Mechanism of Silicon Carbide: I, Fine-Grained Boron- and Carbon-Doped β-Silicon Carbide Fabricated by Hot Isostatic Pressing

The deformation behavior of boron- and carbon-doped β-silicon carbide (B,C-SiC) with an average grain size of 260 ± 18 nm containing 1 wt% boron was investigated by compression testing at elevated temperatures. Extensive grain growth during deformation was observed. The stress–strain curves were compensated for grain growth by assuming power-law type of dependence on grain size and strain rate. The stress exponent n was ∼1.3 and the grain size exponent p was ∼2.7 at temperatures ranging from 1593° to 1758°C. The apparent activation energy of deformation Q _d was ∼760 kJ/mol, which was lower than the activation energy for lattice diffusion of silicon and carbon in SiC and higher than that for grain-boundary diffusion of carbon in SiC. These results suggest that the deformation mechanism of the fine-grained B,C-SiC is grain-boundary sliding accommodated by the grain-boundary diffusion.     

Hot Isostatic Pressing of Alumina and Examination of the Hot Isostatic Pressing Map

The hot isostatic pressing (HIP) of four alumina powders is studied in the temperature range 1100° to 1400°C, at 5- to 200-MPa applied pressure, and for times ranging from 0.5 to 4 h. Density increases with increasing HIP temperature, pressure, and time; decreasing grain size results in increased density after HIP. An empirical relation is derived for grain growth during HIP, and the HIP map proposed by Helle et al. is found applicable to the present results. Densification is governed by the grain-boundary diffusion of aluminum ions; with the transport coefficient and the grain-growth values found in the present study, the map can be used to express experimental results to within a factor of 4 for all densification stages except near full density.     

Mechanical Properties of Submicrometer‐Grained Transparent Yttria Ceramics by Hot Pressing with Hot‐Isostatic Pressing

Here, we report the fabrication and mechanical properties of submicrometer‐grained (0.29–0.58 μm) transparent yttria ceramics by hot pressing combined with hot‐isostatic pressing. The effects of the grain size on the microhardness and the fracture toughness were studied. An unusual decrease of the fracture toughness with an increase in the grain size was revealed, which may be attributed to the different grain size dependence of the fracture behavior of the ZrO₂‐doped yttria ceramics compared to that of other yttria ceramics. The microhardness and fracture toughness of the transparent yttria ceramics were found to be better than those of the large‐grained yttria ceramics.     

Reactive Hot Pressing of Titanium Nitride–Titanium Diboride Composites at Moderate Pressures and Temperatures

Dense composites in the Ti-B-N system have been produced by reactive hot pressing of titanium and BN powders. The effect of the addition of a small amount of nickel (1–3 wt%) on the reaction kinetics and densification of TiN–TiB₂ (40 vol%) composite has been studied. Composites of ∼99% of theoretical density have been produced at 1600°C under 40 MPa for 30 min with 1% nickel. The hardness and fracture toughness of these composites are 24.5 ± 0.97 GPa and 6.53 ± 0.27 MPa·m^1/2, respectively. The microstructural studies on samples produced at lower temperatures indicate the formation of a transient liquid phase, which enhances the kinetics of the reaction and densification of the composite.     

Threshold Stress in Creep of Alumina-Silicon Carbide Nanocomposites

Tensile creep and the creep ruptures of an alumina-17 vol% silicon carbide nanocomposite at 1200°C were investigated, with a particular interest in the behaviors at low applied stresses. At stresses of <50 MPa, the creep rates were remarkably decayed and the creep lives were substantially prolonged, which suggested the presence of a threshold stress, below which creep stopped, in the creep of the nanocomposite. The stress was estimated to be in the range of 20–35 MPa. This threshold stress was in agreement with that predicted from another model where the motion of grain-boundary dislocations that were responsible for vacancy nucleation and annihilation was considered to be pinned by hard particles.     

Tensile Creep of Alumina-Silicon Carbide "Nanocomposites"

The tensile creep behavior of an (Al₂O₃-SiC) nanocomposite that contains 5 vol% of 0.15 μm SiC particles is examined in air under constant-load conditions. For a stress level of 100 MPa and in the temperature range of 1200°–1300°C, the SiC reduces the creep rate of Al2O3 by 2–3 orders of magnitude. In contrast to Al₂O₃, the nanocomposite exhibits no primary or secondary stages, with only tertiary creep being observed. Microstructural examination reveals extensive cavitation that is associated with SiC particles that are located at the Al₂O₃ grain boundaries. Failure of the nanocomposite occurs via growth of subcritical cracks that are nucleated preferentially at the gauge corners. A modified test procedure enables creep lifetimes to be estimated and compared with creep rupture data. Several possible roles of the SiC particles are considered, including (i) chemical alteration of the Al₂O₃ grain boundaries, (ii) retarded diffusion along the Al₂O₃-SiC interface, and (iii) inhibition of the accommodation process (either grain-boundary sliding or grain-boundary migration).     

Hot Isostatic Pressing of Sintered Silicon Nitride

Pressureless-sintered silicon nitride with varying additives was hot isostatic pressed under 150 MPa of nitrogen at 1800°C. Moderate increases in densities were observed when sintered densities exceeded 90% of theoretical. However, density changes became insignificant as the amount of additives exceeded 12 wt%; moreover, density reduction was occasionally observed. Microstructural analysis, after the silicon nitride was reannealed at 1650°C under 0.1 MPa of nitrogen, revealed that intergranular glass was supersaturated with nitrogen and "bloated" as a result of nitrogen evolution. This result suggested that the effectiveness of container-free hot isostatic pressing of silicon nitride was severely limited by enhanced solubility of nitrogen gas in the glassy phase under high pressure.     

Translucent α-Sialon Ceramics by Hot Pressing

The single-phase α-sialon ceramics with high optical transmittance have been prepared by hot pressing. The maximum transmittance reached 65.2% and 52.2% in the infrared wavelength region, 58.5% and 40% in the visible region for the samples 1.0 and 1.5 mm thickness, respectively. The material also exhibited good mechanical properties of high hardness (20 GPa) and better fracture toughness (5.1 MPa·m^1/2). Both high optical transmittance and improved toughness of α-sialon ceramics were attributed to the less-grain-boundary glassy phase and the homogeneous microstructure, which was obtained by a proper process and confirmed by SEM and TEM observation, compared to that prepared by ordinary sintering. It is, therefore, expected that the translucent α-sialon ceramics could be a promising optical window material.     

Analysis of Hot Isotatic Pressing of Presintered Zirconia

Hot isostatic pressing (HIP) of presintered Y-TZP was studied at 1100° to 1400°C, 5 to 200 MPa, for 0.5 to 4 h. The effects of process variables of HIP and the characteristics of the presintered specimens on the densification behavior of HIP were examined. The microstructural development after HIP was also examined. Grain growth occurred during the densification in HIP. Empirically, a linear relation having a rather constant slope was found between the logarithms of porosities and grain sizes, for each starting condition. Provided this relationship was taken into account, the Ashby's model for HIP could express the densification process for this system satisfactorily.     

Densification of Precursor-Derived Si-C-N Ceramics by High-Pressure Hot Isostatic Pressing

Densification behavior of precursor-derived Si-C-N ceramics by hot isostatic pressing (HIP) has been investigated to obtain dense ceramics derived from polymer precursor. An as-pyrolyzed ceramic monolith, which had a porosity of about 17%, could be deformed up to a strain of 8% in preliminary uniaxial compression tests. The flow stress of the material was much higher than 200 MPa at 1600°C; thus high stress was necessary for densification by HIP. The density of the monolith increased from 1.9 to 2.4 g/cm³ by HIP at 1600°C and 980 MPa. Although the number of pores decreased, large pores were formed in the hot isostatically pressed monolith. On the other hand, denser ceramics, in which pores were not observed by optical microscopy, were obtained by hot isostatically pressing the pyrolyzed powder compact.     

Hot Isostatic Pressing of a Presintered Yttria-Stabilized Zirconia Ceramic

The effect of hot isostatic pressing on the bend strength of ZrO₂–3 mol% Y₂O₃ was critically dependent on the presintering process. Optimally sintered bodies contained no open porosity and exhibited large increases in strength following hot isostatic pressing. When open porosity of as little as 0.3% persisted after sintering, hot isostatic pressing increased the bulk density, but little or no increase in strength was realized. Two-parameter Weibull analysis of the strength data was used to quantify the strength improvement obtained following hot isostatic pressing. Typical fracture-initiating flaws were identified through optical and electron microscopy.     

氮化铝陶瓷低温真空热压烧结研究

以自蔓延高温合成的AIN粉体为原料,Y2O3、Dy2O3、La2O3为添加剂,采用真空热压烧结工艺,实现了含有添加剂的AIN陶瓷体的低温烧结;研究了烧结温度对AIN烧结性能的影响。用XRD、SEM对AIN高压烧结体进行了表征。研究表明：粉体粒径、烧结工艺、烧结助剂对AIN陶瓷低温烧结真空热压烧结性能有很大影响;含烧结助剂的真空热压烧结能够有效降低AIN陶瓷的烧结温度并缩短烧结时间,使烧结体的结构致密。烧结温度1550℃条件下,真空热压烧结90min时,得到的AIN陶瓷的致密度最高。     

Yttrium α-Sialon Ceramics by Hot Isostatic Pressing and Post-Hot Isostatic Pressing

Dense α-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Y _x (Si_{12–4.5 x} , Al_{4.5 x} )-(O_{1.5 x} ,N_{16–1.5 x} ) with 0.1 ≤ x ≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of β-sialon ( x = 0.1) to large amounts of α-sialon ( x = 0.4) and increasing amounts of mellilite and sialon polytypoids ( x = 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α-silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructrual analyses showed that the post-HIPed samples had the highest α-sialon content. A higher amount of α-sialon and subsequently a lower amount of intergranular phase were detected at x = 0.3 and x = 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness ( K _IC) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values.     

Transparent Hydroxyapatite Prepared by Hot Isostatic Pressing of Filter Cake

Hydroxyapatite powder was synthesized and formed into a compact in an aqueous medium using a filter-cake method. The compact was hot isostatically pressed at 700° to 1000°C and 100 MPa for 2 h. Fully dense, transparent materials were obtained above 800°C. Both forming and densification methods were found to be important in obtaining transparent materials.     

Grain Growth During Hot Isostatic Pressing of Presintered Alumina

Grain growth during hot isostatic pressing is examined at 1373 to 1673 K at 5 to 200 MPa for 0.5 to 4 h on alumina presintered at 1723 to 1873 K. A linear relation is found between logarithms of grain size and porosity. To account for this result, the development of similar microstructures is suggested regardless of the histories of the starting powder and process. Temperature dependences of grain growth and densification must also be equal during hot isostatic pressing. Both grain growth and densification were controlled by the grain-boundary diffusion of the aluminum ions.     

Preparation of High-Strength and Translucent Alumina by Hot Isostatic Pressing

A vacuum-pressure slip-casting technique and hot isostatic pressing (HIPing) were used to prepare high-strength and translucent alumina ceramics. A low-viscosity and high-solids-content slurry (46 vol% solids) was prepared, and a dense green compact was formed. The samples were sintered and subjected to capsule-free HIPing. Extremely high-density (99.9%) and fine-grained (0.7 to 15 μm in diameter) alumina ceramics were obtained. The HIPed samples showed high bend strength and translucency with in-line transmittance of 30% to 46% (1 mm thick).     

Rheology of Porous Alumina and Simulation of Hot Isostatic Pressing

Rheology of a porous alumina was studied using sinter-forging, hot-pressing, and sintering tests. The results are analyzed using constitutive equations for porous materials. The deformation and densification rates are found to follow Coble creep behavior with an eventual control by interface reactions. The effect of porosity on shear and densification behavior is studied and compared to results obtained on metal powders showing similar trends for shear and a stronger influence in the case of densification. Large pores are likely to buckle at low densities when external forces are applied. The sintering pressure is also estimated and lies in the range 0 to 3 MPa. Finally, the constitutive equations are used to simulate hot isostatic pressing of test shapes, showing that the proposed model correctly predicts the deformation of the ceramic preforms.