Fabrication of high strength β-sialon by reaction sintering

The reaction sintering of β-sialon (Si₄Al₂O₂N₆) from a powder mixture of Si₃N₄, Al₂O₃ and AIN was studied to clarify factors affecting the densification. The presence of sufficient SiO vapour on the compact and excess oxide with respect toβ-sialon composition was the most important factor. High densityβ-sialon was fabricated by heating the compact at 1800° C under 1 atm. N₂. The sintering was carried out with sufficient SiO vapour pressure to prevent thermal decomposition of sintered sialon by packing the compact with a powder mixture of Si₃N₄ and SiO₂. Care was taken to minimize the amount of excess oxide in the starting composition to obtain a high density sialon with a small amount of intergranular X-phase. The maximum density of 3.04 g cm⁻³ was obtained from the compact with 2 wt % excess Al₂O₃ in the composition. The strength of the sinteredβ-sialon was 490 MN m⁻² at room temperature and 480 MN m⁻² at 1200° C. The values are the best among those so far published for sinteredβ-sialons.     

AEM investigation of ceramic/lncoloy 909 diffusional reactions after joining by HIP

Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si₃N₄ and either 60 vol% TiN or 50 vol% TiB₂, while three were monolithic materials, namely Si₃N₄ with 2.5 wt% Y₂O₃ as a sintering additive, Si₃N₄ without additives, and Si₂ N₂O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 m in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.     

Liquid-Phase Sintering and Properties of Si3N4–TiN Composites

Densification during liquid-phase sintering of Si₃N₄–TiN was studied in the presence of Y₂O₃. The content of TiN was varied from 0–50 mass%. During the densification Y-silicate was formed. The amount of silicate increased with both decreasing fraction of TiN and increasing isothermal heating time. Density, fracture toughness, and electrical resistivity were measured as a function of TiN content. It was found that the density and fracture toughness increased with increasing TiN content. The electrical resistivity drops drastically, from 10¹⁰ m for sintered Si₃N₄ to 10^–3 m for sintered Si₃N₄–TiN composite containing 30 vol% TiN.     

Property-microstructure correlation in in situ formed Al2O3, TiB2 and Al3Ti mixture-reinforced aluminium composites

The in situ formed Al₂O₃, TiB₂ and Al₃Ti mixture-reinforced aluminium composites were successfully fabricated by the reaction sintering of the TiO₂-B-Al system in a vacuum. With increasing boron content in the TiO₂-B-Al system, the amount of generated TiB₂ in the composites increased and Al₃Ti content decreased. At the same time the distribution uniformity of the in situ formed Al₂O₃ and TiB₂ particulates was obviously improved, and the size of the Al₃Ti particles was reduced. The in situ Al₂O₃ and TiB₂ particulates had sizes from 0.096–1.88 m. The interface between the in situ formed particulates and the aluminium matrix was clean, and no consistent crystallographic orientation relationship was found. The strength and elastic modulus of the composites was significantly improved by lowering the Al₃Ti content. When the boron content in the TiO₂-B-Al system rose, the morphology of the tensile fracture surface of the composites was changed from large fractured Al₃Ti blocks and fine dimples, to fine dimples and pulled-out particulates. The strengthening and fracture of the composites have been modelled.     

Microstructure,chemical reaction and mechanical properties of TiC/Si3N4 and TiN-coated TiC/Si3N4 composites

Silicon nitride containing various compositions of as-received TiC and TiN-coated TiC, were hot pressed at 1800 °C for 1 h in a nitrogen atmosphere. In TiN-coated TiC/Si₃N₄ composites, TiC reacted first with the TiN coating to form a titanium carbonitride interlayer at 1450 °C, which essentially reduced further reactions between TiC and Si₃N₄ and enhanced densification. TiN-coated TiC/Si₃N₄ composites exhibited better densification, hardness, flexural strength and fracture toughness than those of as-received TiC/Si₃N₄. The toughening mechanisms for as-received TiC/Si₃N₄ and TiN-coated TiC/Si₃N₄ composite were attributed to crack deflection, load transfer and crack impedence by the compressive thermal residual stress.     

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.     

High Temperature Construction Materials on the Basis of Nanodisperse Silicon Nitride

Si₃N₄‐Al₂O₃‐Y₂O₃, Si₃N₄‐TiN and Si₃N₄‐AIN‐Al₂O₃‐Y₂O₃ (β‐sialon) nanopowders with the specific surface area of 60–70 m²/g and average particle size of 30–50 nm have been prepared by plasmachemical synthesis. By means of the hot pressing method at 1850°C compact materials with fine‐grained structure were prepared from this powders as well as from mixture of Si₃N₄‐Al₂O₃‐Y₂O₃ with the second phase (10 wt.% SiC‐Si₃N₄, ZrO₂, TiN nanopowder). Addition of the second phase to silicon nitride improves material strength.     

In situ reacted TiB2-reinforced alumina

In situ formation of TiB₂ in Al₂O₃ matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed samples, in addition to TiB₂, TiC and Al₂TiO₅ were also found to be dispersed phases in Al₂O₃ matrix. However, in the case of pressureless-sintered samples, pure Al₂O₃/TiB₂ composite with > 99% relative density can be obtained through a preheating step held at 1300°C for longer than 30 min and then sintering at a temperature above 1500°C. Pressureless-sintered composite containing 20vol% TiB₂ gives a flexural strength of 580 MPa and a fracture toughness of 7.2 MPa m^1/2.     

Thermal expansion coefficient of Y-α′-sialon

A sialon composite composed of Y-α′-sialon and β′-sialon has been fabricated by hot pressing mixtures of Si₃N₄, Y₂O₃ and AlN powders. Thermal expansion coefficients of the Y-α′-sialon and β′-sialon were determined by the high-temperature X-ray diffraction technique. The thermal expansion coefficient of Y-α′-sialon depended on the composition, being minimum at x=0.3 in the formula Y_x(Si_12-4.5x, Al_4.5x)(O_1.5x, N_16-1.5x). The coefficient of β′-sialon increased with increasing lattice constant, that is, the z value in the formula Si_6-zAl_zO_zN_8-z. The thermal expansion coefficient of sialon composites determined by a differential dilatometer increased with increasing amount of Y-α′-sialon. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Mechanical properties of Al2O3 particle-Y-TZP matrix composite and its toughening mechanism

Dense Al₂O₃ particle-Y-TZP matrix (Al₂O₃<40 vol%) composite was prepared by pressureless sintering at 1550°C. Composites with 10–30 vol% Al₂O₃ particles showed enhanced fracture toughness, bending strength and Vicker's hardness as compared to single-phase Y-TZP. The highest strength (1150 MPa) and highest toughness (12.4 MPa m^1/2) were obtained for the composite containing 10 vol% Al₂O₃. It was found that, in addition to the contribution by the crack-deflection effect, the enhanced phase transformation from tetragonal to monoclinic during fracture was the main toughening mechanism in operation in the composites.     

Effect of cutting speed on the performance of Al2O3 based ceramic tools in turning nodular cast iron

This paper presents the results of an experimental investigation on the effect of cutting speed in turning nodular cast iron with alumina (Al₂O₃) based ceramic tools. Three different alumina based ceramic cutting tools were used, namely TiN coated Al₂O₃ + TiCN mixed ceramic, SiC whisker reinforced Al₂O₃ and uncoated Al₂O₃ + TiCN mixed ceramic tool. Turning experiments were carried out at four different cutting speeds, which were 300, 450, 600 and 750 m/min. Depth of cut and feed rate were kept constant at 1 mm and 0.1 mm/rev, respectively, throughout the experiments. Tool performance was evaluated with respect to tool wear, surface finish produced and cutting forces generated during turning. Uncoated Al₂O₃ + TiCN mixed ceramic was the worst performing tool with respect to tool wear and was the best with respect to surface finish. SiC whisker reinforced Al₂O₃ exhibited the worst performance with respect to cutting forces. If tool wear, surface finish and cutting force results are considered together, among the three tools studied, TiN coated Al₂O₃ + TiCN mixed ceramic tool is the most suitable one for turning nodular cast iron, especially at high cutting speeds (V_c > 600 m/min).     

Crystallisation of M-SiAlON glasses to I w -phase glass-ceramics: preparation and characterisation

The crystallisation of M-SiAlON glasses (M = Y, Ln) is particularly sensitive to small variations in composition and heat treatment temperature. The formation of I_w-phase in the YSiAlON system has been studied but little information concerning its nucleation, thermal stability, mechanical properties and extension into the lanthanide sialon series is available. In order to better understand the synthesis and to characterise more fully the I_w-glass-ceramic materials, a series of glass compositions have been prepared and characterised. These include M₃Si₃Al₂O_12.15N_0.90 (M = Y, Er, Ce, Yb); M_3.45Si₃Al₂O_12.76N_0.95 (M = Y, Er); Y₄Si₃Al₂O_13.50N and Y₄Si_3.37Al_1.50O_13.50N. Heat treatments were performed on these glasses and the resulting crystalline products have been studied by X-ray diffraction (XRD). Differential Thermal Analysis (DTA) in combination with XRD analysis of Y_3.45Si₃Al₂O_12.76N_0.95 and Ce₃Si₂Al₂O_12.15N_0.90 parent glass-compositions was used to investigate crystal phase formation. Young's modulus (E), hardness (H _v and fracture toughness (K _IC) of the glass-ceramics were measured. Glass-ceramics containing I_w-phase as the only detectable crystalline phase have been produced from M₃Si₃Al₂O_12.15N_0.90 and M_3.45Si₃Al₂O_12.76N_0.95 (M = Y and Er) compositions. No equivalent phase was found in the Ce- and Yb-sialon compositions.     

Self-monitoring of fracture and strain in titanium carbide reinforced silicon nitride ceramics

Si₃N₄ Ceramic Composites with TiC powder have been fabricated by gas-pressure sintering and their electrical conductivity has been investigated. The ceramic composites with different electrical resistivity consist of Si₃N₄ powder as an insulating matrix, and TiC as electrically conductive additive. Under tensile loading or compressive unloading, the R/R of TiC/Si₃N₄ composites reversibly increased. Under compressive loading, the R/R decreased gradually with the increasing of loading up to fracture. The results suggest the possibility of self-monitoring fractures and strains in the composites under tensile and compressive loading.     

Static fatigue and creep resistance of a commercial sialon

The static fatigue and creep resistances of a commercial sialon material were evaluated by elevated temperature flexural stress rupture testing in air. The material is a ceramic alloy with a solid solution sialon phase, Si_6-z Al _z O _z N_8-z , with a substitution levelz less than 1. The utilization of yttria sintering aids and post-sintering heat treatments leads to a fully crystalline grain-boundary phase. Creep and static fatigue resistances were excellent at temperatures up to 1200° C.     

Effect of nano-CeO2 on microstructure properties of TiC/TiN+nTi(CN) reinforced composite coating

TiC/TiN+TiCN reinforced composite coatings were fabricated on Ti?C6Al?C4V alloy by laser cladding, which improved surface performance of the substrate. Nano-CeO₂ was able to suppress crystallization and growth of the crystals in the laser-cladded coating to a certain extent. With the addition of proper content of nano-CeO₂, this coating exhibited fine microstructure. In this study, the Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coatings were studied by means of X-ray diffraction and scanning electron microscope. The X-ray diffraction results indicated that the Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coating consisted of Ti₃Al, TiC, TiN, Ti₂Al₂₀Ce, TiC_0·3N_0·7, Ce(CN)₃ and CeO₂, this phase constituent was beneficial to increase the microhardness and wear resistance of Ti?C6Al?C6V alloy.     

High-temperature oxidation of silicon nitride-based ceramics by water vapour

Hot-pressed Si₃N₄, sintered Si₃N₄ and three kinds of sialon with different compositions were oxidized in dry air and wet nitrogen gas atmospheres at 1100 to 1350° C and 1.5 to 20 kPa water vapour pressure. All samples were oxidized by both dry air and water vapour at high temperature, and formed oxide films consisting of SiO₂, Y₂Si₂O₇ and Y4A1209. The oxidation rate was in the order sialon > sintered Si₃N₄ > hot-pressed Si3N4. The oxidation rate of sialon increased with increasing Y₂O₃ content, and oxidation kinetics obeyed the usual parabolic law. The oxidation rates in dry air and wet nitrogen were almost the same: the rate in wet nitrogen was unaffected by water vapour pressure above 1.5 kPa. The activation energy was about 800 kJ mol^–1.     

Effects of Al2O3 addition on the sinterability and ionic conductivity of nasicon

The effects Al₂O₃ doping on the sintering behaviour and ionic conductivity of nasicon have been investigated using formulations of Na_3+2x Zr₂Si₂Al _x P_1–x O₁₂ where Al³⁺ ions were substituted to P⁵⁺ sites within the range 0x0.2. The density of Na_3+2x Zr₂Si₂Al _x P_1–x O₁₂ was increased with increasing amount of Al₂O₃ doping due to the enhanced liquid-phase sintering. The relative density of Na_3.4Zr₂Si₂Al_0.2P_0.8O₁₂ reached a maximum value of 96% when sintered at 1200°C for more than 7 h. The maximum conductivity of 0.24 ^–1 cm^–1 at 300°C was obtained for the composition with x=0.1 when sintered at 1200°C for 10 h.