Preparation of zirconia-toughened bioactive glass-ceramics

Bioactive glass-ceramics toughened by tetragonal zirconia polycrystal (TZP) were prepared by hot-pressing mixed powders of the MgO-CaO-P₂O₅-SiO₂ glass and TZP containing 20 to 80% alumina. The bending strength and the fracture toughness of the composite materials were improved compared with those of the material without TZP. These composites showed high bending strengths (400 to 500MPa) and high fracture toughness ( 2.8MPa m^1/2). The existence of a crack deflection mechanism was observed by scanning electron microscopy. After soaking in simulated physiological solution at 100 °C, no phase transformation from tetragonal to monoclinic of TZP in the composites and no degradation in bending strength occurred.     

Mechanical and thermal properties of Si–C–N material from polyvinylsilazane

Commercial polyvinylsilazane was crosslinked and then crushed to powder. The powder was compacted by cold isostatic pressing at 630 MPa and pyrolysed at 1050 °C in flowing argon. Crack-free Si–C–N material was obtained. Bulk density of the material was 1.95 Mg m–3. Open porosity was 9.6%. The material was amorphous as a result of X-ray diffraction analysis. Elastic modulus measured by pulse–echo method was 105 GPa. Vicker's hardness calculated from indentation at 98 MPa was 6.1 GPa. Fracture toughness measured by indentation fracture method was 2.1 MPa m1/2. Average bending strength was 118 MPa. The material shrank 1.9% in length during heating up to 1400 °C in nitrogen. The thermal expansion coefficient of the material heat treated up to 1400 °C increase from 3.08 × 10–6 °C–1 at 100 °C to 3.96 × 10–6 °C–1 at 1200 °C.     

Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina

Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 m. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions.     

Novel Zirconia–Alumina Nanocomposites Combining High Strength and Toughness

Alumina toughened zirconia (ATZ) composites, combining a matrix of tetragonal zirconia polycrystals (TZP) and an alumina dispersion, are known for their high strength, toughness, and abrasion resistance. Depending on the type of stabilizing oxide in TZP the materials can be tailored to be either extremely strong (Y–TZP) or extremely tough (Ce–TZP) with a considerable cutback in the complementary property. Novel nanocomposite ATZ materials consisting of TZP co‐stabilized by neodymium and yttrium oxide (Nd–Y–TZP) combined with a reinforcement of 0–40 vol% alumina were produced to achieve high bending strength of 1200–1500 MPa at a toughness of 8–12 MPa·√m. TZP was produced by coating monoclinic nanopowder with 1.5 mol% yttria and 1.5 mol% neodymia. The powder was then blended with submicron size alumina and consolidated by hot pressing. Mechanical and microstructural properties were investigated.     

Investigation on crystalline phases and mechanical properties of TZP ceramics prepared from sol-gel powders

Y₂O₃-stabilized tetragonal zirconia polycrystalline (TZP) ceramics containing 1–5 mol% Y₂O₃ were prepared by hot pressing and pressureless sintering of sol-gel-derived powders. Sintered ceramics were evaluated for their density, grain and crystallite size, width of transformation zone, crystalline phases and mechanical properties. Variation in the values of fracture toughness and flexural strength has been explained on the basis of crystallite size and proportion of transformable tetragonal phase, which are influenced by the concentration of Y₂O₃ in TZP ceramics. Correlation of the data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics.     

Mechanical and thermal expansion properties of β-eucryptite prepared by sol–gel methods and hot pressing

The microstructures, mechanical properties and thermal expansion behavior of monolithic lithium aluminosilicate glass–ceramics, prepared by sol–gel method and hot pressing, were investigated by using X-ray diffraction, scanning and transmission electron microscopies, three-point bend tests and dilatometry. β-eucryptite appeared as main phase in the monolithic lithium aluminosilicate glass–ceramics. The glass ceramics exhibited high relative densities and the average flexural strength and fracture toughness values were 154 MPa and 2.46 MPa m^1/2, respectively. The lithium aluminosilicate glass–ceramics hot pressed 1300 and 1350 °C demonstrated negative coefficient of thermal expansion, which was affected by amount and type of crystalline phases.     

等静压处理对磷酸钙骨水泥结构和性能的影响

采用冷等静压技术,在150MPa的压力下对磷酸钙骨水泥粉体进行等静压处理,然后使其在一定的条件下进行水化.采用扫描电镜(Scanning electronmicroscope,SEM)、红外光谱(Fourier transform ininfrared spectrome-try,FTIR)、X射线衍射(X-ray diffraction,XRD)和压汞仪等测试手段对其水化产物的结构和性能进行研究.结果表明,经过等静压处理,磷酸钙骨水泥粉体水化环境被改变,部分水化产物的晶体形貌由细小的短棒状变为六棱柱状,但主要物相仍为羟基磷灰石;水化固化体的孔隙率由未经等静压处理时的(46.32±2.70)%降到(24.75±1.15)%,抗压强度由(12.62±2.70)MPa提高到(43.05±2.08)MPa.     

Preparation and mechanical properties of SiC-AlN ceramic alloy

Fine-grained SiC-AlN ceramic alloys were synthesized by the reaction of silicon nitride and aluminium nitride powders with additions of carbon and CaO. X-ray diffraction and STEM-EDX analysis showed that they are single phase solid solutions. The SiC-AlN alloys were sintered without additives to full density by hot isostatic pressing at 2173 K and 200 MPa for 2 h. The flexural strength, microhardness and fracture toughness were measured and it was found that SiC-AlN alloys have double the strength and slightly higher fracture toughnesses than conventional monolithic SiC.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Effect of initial microstructure on mechanical properties in warm caliber rolling of high carbon steel

In this study, the effect of initial microstructure on change of mechanical properties was investigated by warm caliber rolling (WCR) of high carbon steel. Experiments were carried out with two different kinds of initial microstructures of pearlite and tempered martensite at the temperature of 500 °C. For comparison, the microstructure of austenite phase obtained from the conventional hot rolling at the temperature of 900 °C up to about 83% of the accumulative reduction in area was assumed to be a reference case. It was found that the WCR provided better mechanical properties in terms of strength and toughness compared to the conventional hot rolling based on experimental results of micro-hardness, tension, and Charpy impact tests. The improvement of strength and toughness was attributed to smaller ferrite grain and dispersed cementite particles with smaller interspacing aligned to the rolling direction after the WCR owing to field emission scanning electron microscopy. The investigated WCR might be useful in obtaining the high strength material with better toughness without adding new alloying elements for industrial applications according to the present investigation.     

热处理对选区激光熔化17-4PH不锈钢力学性能的影响

对选区激光熔化成形的17-4PH不锈钢分别进行真空热处理、热等静压高压淬火处理和组合热处理(热等静压固溶后快淬和马弗炉时效后水冷),在1040℃固溶处理2 h和在480℃时效4 h,观察其显微组织并研究了热处理对其力学性能的影响。结果表明,17-4PH不锈钢由回火马氏体和淬火马氏体组成,热处理后沉淀相弥散分布于晶粒内部,其颗粒尺寸为100~150 nm。真空热处理使合金内部孔隙的尺寸减小到3~7 μm,而热等静压使内部孔隙几乎完全闭合,使钢的密度基本上达到理论值。真空热处理+水淬使沉积态17-4PH不锈钢的抗拉强度和硬度都显著提高(分别提高到1300 MPa和448.5HV);热等静压在提高沉积态17-4PH不锈钢抗拉强度的同时使其延伸率显著提高到22.4%。断口分析结果表明,沉积态和热等静压样品的断口形貌为典型的韧性断裂,热等静压样品的韧窝更深、尺寸更大。真空热处理和组合热处理样品的断口形貌具有部分脆性断裂的特征且出现裂纹,与沉积态相比塑性略有降低。     

Silicon carbide-toughened zirconia ceramics

Tetragonal zirconia polycrystalline (TZP) ceramics containing SiC reinforcement in the form of fine particles (nano-scale), particles (micro-scale), whiskers and platelets were synthesized by hot-pressing. The effects of morphology and grain size of SiC reinforcement on the strength and fracture toughness at room temperature were investigated. The addition of SiC (in whatever form) caused decreases in strength and toughness at room temperature with the exception of whisker-reinforced materials. Toughness fell off with increasing temperature, but nevertheless retained about one-half of the room-temperature value for that particular SiC reinforcement. However, the whisker- and particle-reinforced materials had higher K _lc values at high temperature than fine particle- or platelet-reinforced materials, with values in excess of 7 MPa m^1/2 at 1000 °C. The microstructure was examined for SiC whisker-reinforced/TZP materials by TEM and HREM, to examine the nature of the whisker/zirconia interface.     

Thermomechanically induced embrittlement in hot isostatically pressed Si3N4/SiC composites

The macroscopic fracture properties of an Si₃N₄/SiC-platelet composite fabricated by hot isostatic pressing (HIP) without sintering aids were measured by the chevron-notch technique in bending and related to micromechanisms of fracture by means of a quantitative profilometric analysis of the fracture surfaces. Compositional and processing parameters were varied systematically in order to maximize both the fracture toughness and the work of fracture of the composite. Data were compared with those of monolithic Si₃N₄ fabricated by the same process. Cooling-rate from the HI Ping temperature was indicated as a critical parameter especially when cooling was performed under high pressure. A marked embrittlement of the composite body was found by cooling at around 650 °C h^–1 and it could not be completely recovered by successive annealing even up to temperatures above 1700 °C. The highest fracture toughness and work of fracture in the composite (obtained at a cooling rate of about 100 °C h^–1), were measured as 4.6 MPa m^1/2 and 58.6 J m^–2, respectively. In agreement with fractal analysis results, they were estimated to be about 60%–70% of the maximum values, respectively, obtainable in the present composite system, provided that a complete debonding at the platelet/matrix interface can occur.     

Hot pressing of SiC-TiC composites

Sintering behaviour during hot pressing of SiC-TiC composite ceramics has been investigated with special emphasis on the effect of various processing parameters on the density and mechanical properties of the sintered body. At hot pressing temperatures greater than 2000° C, significant densification occurred in SiC-50 wt%TiC (–0.5 wt%B-1 wt%C) composites. The room temperature flexural strength of the sintered body increased with the hot pressing temperature up to 2000° C and reached a highest value of 710 MPa in accordance to the variation of density with temperature. In sintering of composites without additives, densification was enhanced with the addition of up to 25 wt%TiC, with relative densities higher than 98% observed when hot pressing at 2150° C for 2h.     

Preparation and critical current density of hot pressed Bi2Sr2Ca2Cu3Ox/Ag tapes

Bi2Sr2Ca2Cu3Ox(Bi-2223)/Ag tapes have been prepared by hot pressing performed at 800–840 °C for 12–96 h under 6 or 12 MPa in air. The highest transport critical current density, Jc, is 3900 A cm–2 at 77 K and 8800 A cm–2 at 65 K under zero magnetic field, which is observed in tape hot pressed at 820 °C under 12 MPa for 24 h twice. The tape has undergone a cold pressing under 260 MPa between hot pressings. Jc is limited to 120 A cm–2 in tape hot pressed for 48 h continuously, in spite of total hot pressing time, temperature and pressure all being the same as for tape hot pressed for 24 h twice. It is found that alternate hot pressing and cold pressing is effective in the preparation of Bi-2223 tapes with high Jc, which is determined by the strength of grain coupling. Grain coupling is strengthened in tape hot pressed with an intermediate cold pressing. © 1998 Chapman & Hall     

Coprecipitation-derived mullite and mullite-zirconia composites

Precursor powders of mullite-zirconia (0–40 wt% ZrO₂) were prepared by a hydroxide coprecipitation method and their behaviour during calcination between room temperature and 1500 °C was studied using thermal analysis, X-ray diffraction and electron microscopy. The only crystalline phases present in the precalcined powders were bayerite and gibbsite, and these were stable up to 250 °C. Powders containing ZrO₂ were initially amorphous, but on calcination between 250 and 850 °C produced different crystalline phases at temperatures which depended on the amount of zirconia present. Thus in the case of mullite-40 wt% ZrO₂, zirconia crystallized at about 850 °C and was stable up to 1200 °C, when it reacted with free silica to form zircon (ZrSiO₄). Mullite formed above 1250 °C at the expense of zircon and remained stable at higher temperatures. The oxide powders were very homogeneous, and on sintering produced ceramics with a fine-grained uniform microstructure. The powders were very reactive and could be sintered conventionally to near-theoretical density at 1600–1700 °C without sintering aids. The fracture strength of mullite was about 275 MPa, and this could be improved to 350 MPa by hot isostatic pressing the presintered bodies. Addition of zirconia enhanced the sintering kinetics as well as the fracture strength of mullite.     

TiC/Ni3Al Composites Manufactured by Self-Propagating High-Temperature Synthesis and Hot Isostatic Pressing

TiC–20 wt% Ni₃Al and TiC–40 wt% Ni₃Al composite materials were produced by self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP). In the SHS method the reacted powders were compacted by uniaxial pressing immediately after the reaction. The microstructure of the materials produced by SHS consisted of spherical carbides embedded in the Ni₃Al matrix, whereas the microstructure of the materials produced by HIPing was more irregular. A maximum hardness of 2010 HV₁ was measured for the material produced by HIP and a maximum fracture toughness of 10.5 MPa m^1/2 was measured for materials produced by SHS. High-temperature resistance was investigated by exposing the materials to 800°C in air for 110 h. The results obtained showed that the TiC + Ni₃Al composite materials can be recommended for use in environments consisting of oxidizing atmosphere at temperatures around 800°C where high wear resistance is required.     

Zirconia toughened cordierite

Zirconia-cordierite ceramic composites were fabricated by the hot pressing of cordierite and zirconia powders at 1400 ° C/20 MPa for 10 min. Both the fracture toughness and fracture strength have been enhanced by the addition of zirconia. Such improvements are due to stress-induced transformation toughening and to the generation of residual stresses. Micro-crack toughening has been found not to occur in the composite as a consequence of the large thermal expansion mismatch. The transformability of zirconia inclusions in the cordierite matrix is higher than in the case of an alumina matrix, although the inclusions have a rounded morphology, resulting from the presence of a liquid phase at the hot-pressing temperature.     

High-strength biofunctional zirconia: mechanical properties and static fatigue behaviour of zirconia-apatite composites

Zirconia and zirconia-apative (hydroxyl- and fluor-) composites were sintered at 1225°C by glass-encapsulated hot isostatic pressing. In addition to the general assessment of the mechanical properties of the materials, the static fatigue behaviour of the zirconia-hydroxylapatite composite was evaluated by measuring the slow crack propagation at different loads in a circulating 0.9% NaCl solution at 37°C. The mean fracture strengths of the materials in three-point bending mode were 770, 860 and 910 MPa for zirconia-fluorapatite, zirconia-hydroxylapatite and zirconia, respectively. The high value of the slow crack growth stress exponent, calculated from the average strength at the different loading rates, indicates that the zirconia-hydroxylapatite has excellent fatigue resistance in addition to high strength. The even distribution of the apative phases as islets in the zirconia matrix may contribute to mineralization and direct bone apposition to this type of ceramic composites. The machinability of zirconia materials is discussed.     

石墨烯增强镍基粉末高温合金复合材料的力学性能

采用湿混法将石墨烯纳米片分散到高温合金粉末中,并采用热等静压+热挤压+等温锻造+热处理的方法制备出FGH96镍基粉末高温合金。结果表明:石墨烯纳米片在高温合金中分散均匀,初步发现在后续的热工艺过程中并未发生变性;添加0.1%(质量分数)的石墨烯后,室温抗拉强度和屈服强度分别提高了58MPa和43MPa,塑性从21.0%提高到37.3%;650℃条件下抗拉强度和屈服强度分别提高了58MPa和28MPa,塑性从18.5%提高到26.5%。此外对石墨烯增强FGH96镍基高温合金力学性能的作用机制也进行了进一步分析。