Microstructure and fracture-mechanical properties of carbon derived Si3N4 + SiC nanomaterials

The microstructure and basic mechanical properties, as hardness, fracture toughness, fracture strength and subcritical crack growth at room temperature were investigated and creep behavior at high temperatures was established. The presence of SiC particles refined the microstructure of Si₃N₄ grains in the Si₃N₄ + SiC nanocomposite. Higher hardness values resulted from introducing SiC nanoparticles into the material. A lower fracture toughness of the nanocomposite is associated with its finer microstructure; crack bridging mechanisms are not so effective as in the case of monolithic Si₃N₄. The strength value of the monolithic Si₃N₄ is higher than the characteristic strength of nanocomposites. Fractographic analysis of the fracture surface revealed that a failure started principally from an internal flaw in the form of cluster of free carbon, and on large SiC grains which degraded strength of the nanocomposite. The creep resistance of nanocomposite is significantly higher when compared to the creep resistance of the monolithic material. Nanocomposite exhibited no creep deformation, creep cracks have not been detected even at a test at 1400 °C and a long loading time, therefore the creep is probably controlled mainly by diffusion. The intergranular SiC nanoparticles hinder the Si₃N₄ grain growth, interlock the neighboring Si₃N₄ grains and change the volume fraction, geometry and chemical composition of the grain boundary phase.     

Fracture toughness of Si3N4 and its Si3N4 whisker composite without sintering aids

Si₃N₄ without sintering aids is studied with special interest to the fracture behaviour and its relation to microstructure. Cracks propagated almost transgranularly and no rising R-curve behaviour was found, because crack-wake region gave no contribution on toughening due to very high grain-boundary bonding strength. Microstructure with highly elongated grains was obtained by addition of 20%Si₃N₄ whisker, but fracture toughness was found to be similar to that of the monolithic Si₃IM₄ with equiaxed grains. It is recognized that fracture toughness is not determined simply by apparent microstructural parameters such as mean aspect ratio of grains when grain-boundary bonding is sufficiently strong. Detailed examination of microfracture behaviour is, therefore, necessary for the analysis of toughening in this kind of composites.     

Mechanical properties of Si3N4/BN fibrous monolithic ceramics at elevated-temperature

The mechanical properties at high temperature of Si₃N₄/BN fibrous monolithic ceramics were tested. The flexural strength of SiC whisker reinforced Si₃N₄/BN fibrous monolithic ceramics from 25°C to 1200°C were investigated. The strength degraded slowly from 1000°C to 1200°C which was different to Si₃N₄ monolithic ceramics. The creep behaviors of the material at different temperatures were characterized. Si₃N₄/BN fibrous monolithic ceramics possess high creep resistance. The chemical composition and microstructure of the composites were analyzed by XRD and SEM.     

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.     

Effect of sintering additives on the behaviour of SiC whisker-reinforced Si3N4 composites

The effects of sintering additives on the microstructural development, whisker stability, oxidation resistance and room-temperature mechanical properties of the SiC whisker-reinforced Si₃N₄ matrix composites were investigated. Seven different combinations of Y₂O₃ and Al₂O₃ were used as sintering additives. The composites containing 20 vol % SiC whiskers were densified by hot pressing. The microstructure of the resulting composites was characterized using X-ray diffraction, scanning and transmission electron microscopy. Oxidation testing of the composite at 1400 °C was conducted to investigate the relationship between matrix compositions and oxidation resistance. The flexural strength, fracture toughness and crack propagation patterns were also characterized and correlated with the microstructural features.     

Si3N4 rodlike crystal-reinforced MoSi2 matrix composites

A MoSi₂-based composite reinforced with 20 vol.% Si₃N₄ rodlike crystals was fabricated by Spark Plasma Sintering (SPS) process. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite were above 95%. No reactions between Si₃N₄ and MoSi₂ were observed. The composite containing Si₃N₄ rodlike crystal had higher hardness than monolithic MoSi₂. The room-temperature flexural strength increased 60% compared to that of pure MoSi₂. Especially the room-temperature fracture toughness of the composites was higher than that of MoSi₂, from 3.6 MPa m^1/2 for MoSi₂ to 5.1 MPa m^1/2 for composites with 20 vol.% Si₃N₄, respectively. Besides, the yield strength at elevated temperature and the low-temperature oxidation resistance for 20 vol.% Si₃N₄–MoSi₂ composite exhibited considerable improvement over that of monolithic MoSi₂. These results showed that Si₃N₄ rodlike crystal is an effective reinforcement for MoSi₂.     

Preparation of Si3N4-SiC composites by microwave route

Si₃N₄-SiC composites have been microwave sintered using β-Si₃N₄ and β-SiC as starting materials. Si₃N₄ rich compositions (95 and 90 vol.% Si₃N₄) have been sintered above 96% of theoretical density without using any sintering additives in 40 min. A monotonic decrease in relative density is observed with increase in SiC proportion in the composite. Decrease in relative density has manifested in the reduction of fracture toughness and microhardness values of the composite with increase in SiC content although the good sintering of matrix Si₃N₄ limits the decrease of fracture toughness. Highest value of fracture toughness of 6.1 MPa m^1/2 is observed in 10 vol.% SiC composite. Crack propagation appears to be transgranular in the Si₃N₄ matrix and the toughening of the composites is through crack deflection around hard SiC particles in addition to its debonding from the matrix.     

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.     

Mechanical properties and microstructure in sintered and HIPed SiC particle/Si3N4 composites

Pressureless sintered (PLS) and gas-pressure sintered (GPS) Si₃N₄, PLS and GPS SiC particle/Si₃N₄ composites, and PLS + HIP and GPS + HIP SiC particle/Si₃N₄ composites were produced. Investigation of their mechanical properties showed that PLS + HIP and GPS + HIP composites, containing SiC particles in the beta-silicon nitride grains, yield higher bending strength, although its fracture toughness remains at the same level. This is attributed to the fact that the added SiC particles inhibit excessive growth of beta-Si₃N₄ grains without changing the fracture behaviour. However, this investigation also found precipitation during the reaction between SiC and nitrogen in gas pressure sintering, resulting in a low Young's modulus and low density in the GPS composite.     

Effect of size and morphology of particulate SiC dispersions on fracture behaviour of Si3N4 without sintering aids

The relation between microstructural characteristics and fracture behaviour of Si₃N₄/SiC-particle composites were evaluated for a series of materials containing a 25 vol% dispersion, with mean size in the range 7–106m. All the composites were fabricated by hot isostatic pressing without external addition of sintering aids via glass encapsulation. Quantitative image analysis techniques were employed to assess the microstructural parameters, dealing with morphology and distribution of the SiC particles. A fracture mechanics analysis based on the determination of fracture strength, toughness, work of fracture and rising R-curve behaviour provided the basis for discussion of the effectiveness of the SiC dispersions.The results of mechanical tests are compared with those obtained on the monolithic material fabricated by the same process. The microfracture mechanisms in composites are discussed by relating microstructural data, obtained by image analysis, to toughness data.     

Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

Surface strengthening and toughening of Si3N4/TiC layered composite by slip casting

Layered composite using monolithic Si₃N₄ as outer layers and Si₃N₄-15v/o TiC as inner core was fabricated by slip casting and pressureless sintering. As the composite is cooled from high sintering temperature, the difference in coefficients of thermal expansion between the constrained inner core and outer layer is expected to establish a compressive surface stress. The existence of this residual stress was verified by theoretical analysis and Vicker's indentation for the samples with various outer layer thickness. The layered composites exhibited greater strength, apparent fracture toughness and damage resistance due to the presence of compressive surface stresses in the layer.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

The direct bonding between copper and MgO-doped Si3N4

An application of direct bonding method for copper to silicon nitride (Si₃N₄) joining was investigated. Si₃N₄ was sintered with 5wt% MgO at 1700 ° C for 30 min in nitrogen atmosphere, and oxidized at various temperatures. The bonding was performed at 1075 ° C in nitrogen atmosphere with low oxygen partial pressure. The direct bonding was not achieved for the Si₃N₄ oxidized below 1200 ° C or nonoxidized. During oxidation, magnesium ion added as sintering aids, diffused out to the surface of Si₃N₄ and formed MgSiO₃, which seemed to have an important role in the bonding. Fracture of the bonded specimen under tensile stress took place within the oxide layer of Si₃N₄. The bonding strength was decreased with oxidation temperature and time. Maximum strength was found to be 106 kg cm^–2 for the Si₃N₄ oxidized at 1200 ° C for 1 h.     

Effect of particulate additions on the contact damage resistance of hot-pressed Si3N4

Silicon nitride matrix composites containing particles of SiC, TiC, and BN were fabricated and tested for improved contact damage resistance at 900° C in an oxidizing atmosphere. Contact damage resistance was characterized with profilometry, scanning electron microscopy, bend tests, and coefficient of friction measurements. The results of these tests indicated that composites containing TiC particles showed dramatically improved friction and wear behaviour compared to the SiC- and BN-containing composites, as well as the monolithic Si₃N₄. Auger spectroscopy indicated that the improved behaviour was due to the formation of a lubricious oxide containing both titanium and silicon on the surface of the composite and the transfer of some of the oxide to the rider.     

Pressureless sintering and mechanical properties of alumina-sialon composites

Compositions of Al₂O₃, Si₃N₄ and AlN were sintered to produce six different alumina-sialon composites. The composites reached the highest density at 1700 °C in a nitrogen atmosphere. Sialon acted as a binder, promoting the densification and suppressing the grain growth of alumina. The composites had a bending strength of 450 MPa at room temperature and maintained high strength over 300 MPa at 1400 °C. The composite with 30 wt % sialon had the maximum fracture toughness of 4.3 MPa·m^1/2 and exhibited good oxidation resistance even at 1400 °C. Mullite was formed in the oxidation layer and little degradation in strength was observed after oxidation.     

Effect of whisker aspect ratio on the density and fracture toughness of SiC whisker-reinforced Si3N4

Controlling the suspension properties prior to slip casting optimizes the homogeneity, density and fracture toughness of silicon carbide whisker reinforced silicon nitride (SiC_w/Si₃N₄). Further improvements in the mechanical properties are realized by combining ball milling with ultrasonic dispersion of the composite suspension. Ball milling reduces the SiCw aspect ratio from 25 to 15 which in turn increases the dispersion of the whiskers within the suspension, resulting in increases in the green and sintered density, along with the fracture toughness. In a binderless process, 20 volume% reduced aspect ratio (r = 15) SiC_w/Si₃N₄ can be densified to 95% theoretical density by pressureless sintering using 8% Y₂O₃ and 2% Al₂O₃ by weight as sintering aids. These composites had measured values of fracture toughness from 9–10.5 MPa · m^1/2, representing an average increase of approximately 30% over the fracture toughness for monolithic Si₃N₄ processed under identical conditions.     

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

A high density Ti₃SiC₂/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti₃SiC₂/SiC composite was finer than that of monolithic Ti₃SiC₂, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti₃SiC₂ matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m^1/2 and 1080 kg mm^–2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti₃SiC₂ was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti₃SiC₂.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Processing and characterization of an amorphous Si–N–(O) fibre

Si–N–(O) fibres were grown according to a high temperature vapour–solid process involving the reaction between SiO and NH₃ on a substrate. The oxygen concentration of the fibres is related to the partial pressures of SiO and NH₃ during fibre growth, depending respectively, on the processing temperature and the ammonia flow rate. The fibres consist of amorphous silicon oxynitride of composition Si0_2x N_4(1–x)/3 (0.1 < x < 0.2). They exhibit a large spread in tensile strength. The lowest values (about 1 GPa) correspond to large surface defects caused by intergrowth while the highest values reach 5 GPa for perfect fibres. The fibres are stable in nitrogen up to 1450 °C (10 h) in terms of composition, structure and mechanical behaviour owing to their high processing temperature (1450 °C) and the nitrogen pressure preventing decomposition. A superficial crystallization into Si₃N₄ is only observed at 1500 °C inducing a moderate decrease of strength. In argon, decomposition starts at 1400 °C yielding gaseous species (SiO and N₂), crystalline Si₃N₄ and free silicon beyond 1400 °C and induce a catastrophic drop of strength. Annealing in oxygen results in a growth of a protective SiO₂ scale, amorphous or partially crystalline at 1400 °C.