Microstructure and mechanical properties of silicon nitride-titanium nitride composites prepared by spark plasma sintering

Si₃N₄-TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si₃N₄ matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si₃N₄ matrix results in the highest fracture toughness (8.39 MPa m^1/2) of sample SPS2 sintered at 1300 °C.     

Creep of silicon nitride

General features of silicon nitride based ceramics, which may well influence their creep behavior are presented. Then, the most commonly invoked models for the microscopic mechanisms assumed to take place during creep (viscous flow, solution-precipitation, cavitation and shear thickening) are analyzed. Finally, the very numerous macroscopic and microscopic experimental findings about the plastic deformation of silicon nitride based ceramics at high temperatures, such as the fundamental role played by the secondary phases, the essential compressive-tensile asymmetry, and the microstructural evolution accompanying creep are summarized and discussed in terms of those models.     

Creep of reaction-bonded silicon nitride

The high temperature mechanical properties, mainly the creep behaviour of reaction-bonded silicon nitride (RBSN), a new engineering ceramic for the gas turbine, have been a point of considerable interest. During the recent development a remarkable increase of the creep resistance of RBSN has been reached and the latest data show creep rates of below 10^–6 h^–1 at 1300° C and 70 to 100 MM m^–2. Activation energies between 540 and 700 kJ mol^–1 and stress exponents of 1in vacuo. Methods to determine the amount of internal oxidation,namely X-ray diffraction analysis, electron microprobe analysis and Rutherford backscattering of -particles were used. The deleterious effects of the internal oxidation are explained in terms of the microstructure, mainly porosity and pore size distribution, and ways to avoid this effect are discussed.     

Creep of hot-pressed silicon nitride

Creep tests were undertaken on hot-pressed silicon nitride in the temperature range 1200 to 1400° C. The activation energy for creep was determined to be 140 kcal mol^?1 and the stress exponent of creep rate was 1.7. The creep behaviour is ascribed to grain-boundary sliding accommodated by void deformation at triple points and by limited local plastic deformation. Electron microscopic evidence supporting this mechanism is presented.     

Creep behavior in SiC-whisker reinforced silicon nitride composite

Tensile and flexural creep tests of 20 vol % SiC whiskers reinforced Si₃N₄ composite processed by gas pressure sintering have been carried out in air in the temperature range of 1000–1300°C. The stress exponent for flexural creep is 16 at 1000°C. However, at 1200 and 1250°C the stress exponents for both tensile and flexural creep vary with increasing stress. In the low stress region, the activation energy for creep is 1000 kJ/mol. In the high stress region, it is 680 kJ/mol. The different creep mechanisms dominate in the low and high stress regions, respectively. This revised version was published online in September 2006 with corrections to the Cover Date.     

Silicon carbide platelet-reinforced silicon nitride composites

Different grades of high aspect ratio SiC platelets were used to reinforce Si₃N₄. Dispersion of additives (4 wt % Y₂0₃ and 3 wt % Al₂0₃) was achieved by ball milling in ethanol using alumina balls, while dispersion of platelets was done by ball milling using plastic balls. Consolidation of the composites was carried out by uniaxial hot pressing. A slight decrease in flexural strength was measured, while significant increases in elastic properties, fracture toughness and Weibull modulus were noted. Microstructure and crack-propagation studies as well as reinforcement mechanisms are presented.     

Creep and strain recovery in hot-pressed silicon nitride

It is observed that creep response in hot-pressed silicon is characterized by two parallel phenomena; one accounts for a persistent non-recoverable plastic deformation and the other for a transient viscoelastic recoverable deformation. The persistent creep component is time-dependent, and apparently follows parabolic time kinetics. It is further observed that creep is characterized by a power law stress exponent of about 4 and an activation energy of 848 kJ mol^–1. The viscoelastic recoverable component of strain is found to be independent of the total plastic strain in the material. The recovery rate at any given time is directly proportional to the preceding creep stress and therefore can be considered linear viscoelastic. The creep compliance of the viscoelastic transient is temperature-dependent with an activation energy of about 722 kJ mol^–1. It is further observed that the viscoelastic recovery is characterized by a spectrum of retardation times and can be modelled by a series of Kelvin analogue models. Finally, the viscoelastic recovery and the viscoelastic component of subsequent creep appear to be inversely related and apparently obey Boltzman superposition. A model is developed for the creep and recovery behaviour of hot-pressed silicon nitride consistent with all experimental observations and based in relative grain motion accommodated by the fluid grain-boundary glass liquid flow, cavitation and wedge opening.     

Hi-Nicalon reinforced silicon nitride matrix composites

The present paper reports on the fabrication and the mechanical properties of SiC (Hi-Nicalon) fibres reinforced Si3N4 matrix composites. The composite was fabricated by liquid infiltration of an aqueous Si3N4 slurry followed by hot-pressing. The effect of fibre coating layers was investigated with a 400 nm thick pyrolytic carbon. The fibre coating was found to have a significant effect on the frictional stress of the fibre-matrix interface and consequently on the fracture behaviour of the composite.     

Preparation and characterization of carbon nanotube reinforced silicon nitride composites

Multiwall carbon nanotube (MWNT) reinforced silicon nitride ceramics matrix composites have been prepared. The hot isostatic press (HIP)-sintering method was used for composite processing. Bending strength and elastic modulus of MWNT-silicon nitride composites showed a considerable improvement compared to matrices with added carbon fiber, carbon black or graphite. However, the silicon nitride samples without any carbon addition, because of higher densities present an even higher value. In the case of carbon fibers addition, a deterioration during sintering has been observed. The increasing pressure and sintering time resulted in carbon nanotube free structures.     

Silicon nitride matrix composites with unidirectional silicon carbide whisker reinforcement

SiC whisker reinforced Si₃N₄ was fabricated by fiber extrusion and hot pressing. SiC whiskers were unidirectionally oriented in a carrier fiber. The fibers containing the oriented whiskers were hot pressed in Si₃N₄ powder to form a SiC_w/Si₃N₄ composite with approximately 5 volume% whiskers. SEM micrographs were image processed to quantify whisker orientations in the extruded fiber and the composite. Oriented whiskers contributed to nominal increase in fracture strength over monolithic samples before and after thermal shock testing from 500, 600 and 700°C.     

Multi-scale electrical response of silicon nitride/multi-walled carbon nanotubes composites

Dense silicon nitride (Si₃N₄) composites with various amounts (0-8.6 vol%) of multi-walled carbon nanotubes (MWCNTs) are electrically characterised by combining macroscopic dc-ac and nanoscale conductive scanning force microscopy (C-SFM) measurements. In this way, a coherent picture of the dominant charge transport mechanisms in Si₃N₄/MWCNTs composites is presented. A raise of more than 10 orders of magnitude in the electrical dc conductivity compared to the blank specimen is measured for MWCNTs contents above 0.9 vol%. Semiconductor and metallic-like behaviours are observed depending on both the temperature and the MWCNTs content. Macroscopic measurements are further supported at the nanoscale by means of C-SFM. The metallic-type conduction is associated to charge transporting along the nanotube shells, whereas the semiconductor behaviour is linked to hopping conduction across nanotube-nanotube contacts and across intrinsic defect clusters within the nanotubes.     

Structural transformations of the wurtzitic boron nitride-titanium nitride composition at high pressure and temperature

The results of the electron microscopy and XRD examination of structural transformations of the wurtzitic boron nitride-titanium nitride powder composite obtained by plasmachemical synthesis under a pressure of 8 GPa and temperature of 1100–1700°C have been considered. The interaction in this system has been shown to yield Ti-B-N solid solutions. It has been found that at 1300°C continuous BN-TiN interfaces form and at T ≥ 1500°C a monolithic layer, which is a nanodispersed TiN-TiB₂ composite, is formed between nitride boron grains.     

Corrosion protection of aluminium-matrix aluminium nitride and silicon carbide composites by anodization

Anodization is an effective surface treatment for improving the corrosion resistance of aluminium-matrix composites. For SiC particle-filled aluminium, anodization was performed successfully in an acid electrolyte, as usual. However, for AlN particle-filled aluminium, anodization needed to be performed in an akaline (0.7 N NaOH) electrolyte instead of an acid electrolyte, because NaOH reduced the reaction between AlN and water, whereas an acid enhanced this reaction. The concentration of NaOH in the electrolyte was critical; too high a concentration of NaOH caused the dissolution of the anodizing product (Al2O3) by the NaOH, whereas too low a concentration of NaOH did not provide sufficient ions for the electrochemical process. The corrosion properties and anodization characteristic of pure aluminium, Al/AlN and Al/SiC were compared. Without anodization, pure aluminium had better corrosion resistance than the composites and Al/SiC had better corrosion resistance than Al/AlN. After anodization, the corrosion resistance of Al/AlN was better than Al/SiC and both composites were better than pure aluminium without anodization, but still not as good as the anodized pure aluminium.     

Fabrication and oxidation resistance of silicon nitride fiber reinforced silica matrix wave-transparent composites

Wave-transparent ceramic matrix composites for the high temperature use should possess excellent oxidation resistance. In this work, Si_3N_(4f)/SiO_2 composites with different fiber content were fabricated by filament winding and sol gel method. The oxidation resistance was investigated by tracking the response of flexural strength to the testing temperature. The results show that the flexural strength and toughness of the composites with fiber content of over 37% can reach high levels at around 175.0 MPa and 6.2 MPa m1/2, respectively. After 1 h oxidation at 1100?C, the flexural strength drops a lot but can still reach 114.4 MPa, which is high enough to ensure the safety of structures. However, when the oxidation temperature rises to 1200–1400?C, the flexural strengths continue to fall to a relatively low level at 50.0–66.4 MPa. The degradation at high temperatures is caused by the combination of over strong interfacial bonding, the damage of fiber and the crystallization of silica matrix.     

Effects of interface coating and nitride enhancing additive on properties of Hi-Nicalon SiC Fiber reinforced reaction-bonded silicon nitride composites

Strong and tough Hi-Nicalon SiC fiber reinforced reaction-bonded silicon nitride matrix composites (SiC/RBSN) have been fabricated by the fiber lay-up approach. Commercially available uncoated and PBN, PBN/Si-rich PBN, and BN/SiC coated SiC Hi-Nicalon fiber tows were used as reinforcement. The composites contained 24 vol% of aligned 14 m diameter SiC fibers in a porous RBSN matrix. Both one- and two-dimensional composites were characterized. The effects of interface coating composition, and the nitridation enhancing additive, NiO, on the room temperature physical, tensile, and interfacial shear strength properties of SiC/RBSN matrix composites were evaluated. Results indicate that for all three coated fibers, the thickness of the coatings decreased from the outer periphery to the interior of the tows, and that from 10 to 30 percent of the fibers were not covered with the interface coating. In the uncoated regions, chemical reaction between the NiO additive and the SiC fiber occurs causing degradation of tensile properties of the composites. Among the three interface coating combinations investigated, the BN/SiC coated Hi-Nicalon SiC fiber reinforced RBSN matrix composite showed the least amount of uncoated regions and reasonably uniform interface coating thickness. The matrix cracking stress in SiC/RBSN composites was predicted using a fracture mechanics based crack bridging model.