Silicon Nitride-based Materials Research Articles

Thermal Conductivity of the Materials Based on Silicon Carbide and Silicon Nitride

The temperature dependences of the thermal conductivity of silicon carbide- and silicon nitride-based materials prepared by various methods are presented. The thermal conductivity of silicon carbide–based materials prepared by reaction (SiSiC) and liquid-phase (LPSSiC) sintering and silicon nitride-based materials prepared by liquid phase sintering (SSN) is studied. The dependences of the thermal conductivity on the density, porosity, and content of oxide addition (for the LPSSiC and SSN materials) are presented.

Cation-Size-Mismatch Tuning of Photoluminescence in Oxynitride Phosphors

Red or yellow phosphors excited by a blue light-emitting diode are an efficient source of white light for everyday applications. Many solid oxides and nitrides, particularly silicon nitride-based materials such as M(2)Si(5)N(8) and MSi(2)O(2)N(2) (M = Ca, Sr, Ba), CaAlSiN(3), and SiAlON, are useful phosphor hosts with good thermal stabilities. Both oxide/nitride and various cation substitutions are commonly used to shift the emission spectrum and optimize luminescent properties, but the underlying mechanisms are not always clear. Here we show that size-mismatch between host and dopant cations tunes photoluminescence shifts systematically in M(1.95)Eu(0.05)Si(5-x)Al(x)N(8-x)O(x) lattices, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu(2+) dopant, but a blue shift when the M = Ca host is smaller. Size-mismatch tuning of thermal quenching is also observed. A local anion clustering mechanism in which Eu(2+) gains excess nitride coordination in the M = Ba and Sr structures, but excess oxide in the Ca analogues, is proposed for these mismatch effects. This mechanism is predicted to be general to oxynitride materials and will be useful in tuning optical and other properties that are sensitive to local coordination environments.

Enhanced Tribological Performance of Silicon Nitride-Based Materials by Adding Carbon Nanotubes

In the search for materials that can withstand the new demanding tribological conditions, carbon nanotubes containing ceramic composites emerge as a promising alternative in gasoline direct injection systems. To explore this prospect, the tribological properties of dense good-dispersed multiwalled carbon nanotubes (MWCNTs) containing silicon nitride (Si3N4 )n anocomposites were investigated using a reciprocating ball-on-plate configuration mated against Si3N4 balls under lubrication with isooctane. The friction coefficient and the wear resistance of the materials were evaluated as a function of the MWCNTs content (up to 8.6 vol%) and the applied load (50–200 N). For the nanocomposites both the friction coefficient and the wear volume decreased in about 40% and 80%, respectively, as compared with the monolithic material. This enhanced tribological performance of the nanocomposites was attributed to the special role played by nanotubes, acting as solid lubricant and modifying, at the same time, the stress field distribution at the contact, thus notably enhancing the wear resistance at high loads.

Spark plasma sintering: A powerful tool to develop new silicon nitride-based materials

Several key topics on the current assisted sintering of Si 3N 4-based materials are reviewed. First, different proposed mechanisms for spark plasma sintered (SPSed) ceramics are presented, discussing the electric field effect on the liquid phase behaviour and how it may influence the liquid phase sintering of Si 3N 4 ceramics. Next, we show that the SPS is a powerful tool to develop new Si 3N 4-based materials with tailored microstructures, such as functionally graded materials (FGMs) and carbon nanotubes (CNTs) containing Si 3N 4 matrix composites. Si 3N 4 FGMs are fabricated from a sole homogenous Si 3N 4 mixture just modifying the SPS system punches set-up, thus creating a temperature gradient through the specimen. Finally, the capability of SPS to get dense Si 3N 4/CNTs composites overcoming both constraint densification and nanotubes degradation is proved.

Characterization and wear behavior of modified silicon nitride

Silicon nitride-based materials are applied to many tribological components because of their superior thermal and mechanical properties and corrosion resistance. The purpose of this study is to investigate the wear performance of modified silicon nitride which contained 3 wt.% La 2O 3 and 3 wt.% Y 2O 3. The relationships between microstructure and mechanical properties have been also addressed. Vickers microhardness and toughness were measured, the later being determined by means of the direct crack measurement method (DCM). Scanning electron microscopy technique (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) was used for the morphological characterization of the samples as well as for the characterization of the wear scars. Wear properties were studied under a load of 10 N by using the ball-on-disk tribometer. AISI 52100 steel balls and balls of WC + 6% Co (6 mm diameter) were used as static counterparts, respectively. Steady state friction coefficients of 0.66 and 0.62 were measured for the tribological pairs WC + 6 % Co/Si 3N 4 and AISI 52100/Si 3N 4, respectively. The wear mechanism was determined in each case, and comparison between the wear performance of the traditional and modified silicon nitride is also presented.

Crystal growth in supercritical ammonia using high surface area silicon nitride feedstock

The use of amorphous high surface area silicon nitride is proposed as a raw material for crystallization experiments in supercritical ammonia. Compared with earlier studies, the use of highly dispersed solids results in the crystallization of inorganic nitrides under relatively mild conditions (673 K). Mineralizers such as amides (LiNH 2, NaNH 2, KNH 2) are found to be effective crystallization aids. The crystalline products, detected using powder X-ray diffraction, are either MSi 2N 3 (M=Li, Na) or Si 2N 2NH. Si 2N 2NH is also characterized using 29Si MAS NMR. The spectrum shows a narrow line located at –44.7 ppm, whereas for amorphous silicon nitride-based materials the line is broad. The ammonothermal reaction of NaAl(NH 2) 4 and high surface area silicon nitride at 673 K affords a new orthorhombic phase, isostructural with NaSi 2N 3, but with extended lattice constants ( a=9.634, b=5.643, c=5.011 Å). Effective crystallization is also achieved using fluoride mineralizers (KF, CsF) at 673 K. A new small scale autoclave, suitable for laboratory syntheses at temperatures up to 873 K, is presented that can be loaded under inert gas.

The preparation and spark plasma sintering of silicon nitride-based materials coated with nano-sized TiN

TiN coating on β-sialon ( z=2) and Si 3N 4 was performed by depositing TiO 2 on their particle surfaces through controlled hydrolysis of Ti(O–i–C 3H 7) 4, and subsequent nitridation with NH 3 gas. TiN particles covering β-sialon and Si 3N 4 were 10–20 nm in size. Spark plasma sintering of TiN/β-sialon composite particles produced ceramics with TiN networks of 0.1–0.5 μm wide at 1500 °C, but with TiN grains of 0.1–2.0 mm sizes isolated in the sialon matrix at 1600 °C. SPSed TiN/sialon ceramics at 1500 °C had >95% density of the theoretical one, irrespective of the TiN amount. TiN/sialon ceramics (25 vol.%) showed a lower electrical resistivity (∼10 −3 Ω cm) making these ceramics suitable for electric discharge machining.

High Temperature Mechanical Behavior of Silicon Nitride Ceramics.

The temperature dependences of Young's modulus (E) and fracture toughness (K1c), as well as the flow behavior, including stress relaxation, creep and superplasticity of several silicon nitride-based materials, monoliths and composites, are reviewed. A transition range between a low softening rate and a higher one, which coincides with the onset of creep ductility, is observed between 1080 and 1150°C on the E(T) curves and is attributed to the behavior of the secondary glassy phases. The higher the Y/Al ratio or the SiC content, the higher the transition temperature. The K1c(T) curves exhibit four different stages which were discussed and interpreted through a theoretical analysis, warning against the frequent confusion between the intrinsic and the apparent (experimentally accessible) toughness. Reliable creep resistant- or inversely superplastic-ceramics are now available. However, the high temperature deformation mechanisms are not well understood yet. Non-Newtonian flow regimes and the pronounced tension/compression flow asymmetry are still intriguing. Today, it can be anticipated that with the development of materials containing low amounts of highly refractory grain boundary phases, ceramists are facing a situation that places them closer to metallurgists, and which should allow them to derive more benefit from the exceptional intrinsic properties of covalent crystals such as Si3N4. Consequently, grains should play a more and more important role on the high temperature mechanical behavior.

The effect of ELID grinding on the flexural strength of silicon nitride

Advanced structural ceramics, such as silicon nitride-based materials, are of significant interest due to their excellent physical and mechanical properties. However the cost of grinding these ceramics, an integral part of their fabrication, is very high and can result in surface and subsurface damage to the material. These defects can significantly reduce the strength and reliability of the finished component and are sensitive to grinding parameters. In this investigation, the effect of finish electrolytic in-process dressing (ELID) grinding on the flexural strength of silicon nitride specimens were studied. Kyocera's silicon nitride SN 235, in the form of Modulus of Rupture (MOR) specimens, were ground using the application of ELID grinding with a # 6000 grit sized cast iron bonded diamond (CIB-D) grinding wheel. A significant improvement in the strength of the Si 3N 4 specimens was noted when finish ELID grinding was performed. This was the result of ductile regime grinding using the application of finish ELID grinding. Another method to improve the flexural strength of silicon nitride specimens will also be addressed in this paper.

Synthesis and Pyrolysis of Polyborosilazane with Low Oxygen Content

Polyborosilazane with a low oxygen content was synthesized by reacting perhydrosilazane with tris (dimethylamino) borane. The chemical structure of this polymer was investigated by infrared and nuclear magnetic resonance spectroscopies. The borazine ring structure was observed in the polymer. Amorphous silicon nitride was obtained by pyrolysis of this polymer in a stream of N2 at 1000°C with about 80mass% ceramic yield. The pyrolysis product remained amorphous after further heating to 1700°C under N2. The oxygen content in the pyrolysis product was 1.1mass%. Crystallization to β-Si3N4, β-SiC and α-Si3N4 took place on annealing in N2 at 1800°C for 1h. These results indicate that this polyborosilazane is a good precursor for amorphous silicon nitride-based materials with low oxygen content.

Studies of ceramic-liquid metal reaction interfaces

An investigation has been made of the nature and extent of chemical reactions between various liquid metals and a range of engineering-grade ceramics typically used as cutting tool inserts. Such possible reactions are relevant to chemical wear effects during metal cutting but also relate to liquid metal containment by ceramics and ceramic-metal joining. The experimental procedure has involved immersing pre-polished ceramic sections in liquid metals for controlled times with subsequent sectioning and examination of the reaction interface. The ceramics studied were two alumina-based materials and five silicon nitrides and sialons. The metals were pure iron, pure nickel and four iron-nickel alloys (a mild steel, a stainless steel and two nickel-based superalloys) and span a range of Fe-Ni compositions. The reaction rates of the alumina materials were found to be much lower than those of the silicon nitride-based materials and reflect the chemical stability of the Al-O bond array. Zirconia-toughened alumina showed little evidence of reaction with clean iron alloys but substantial attack by oxygen-containing iron-based materials was found resulting in the formation of iron-aluminium spinel reaction products. Al2O3-TiC/N exhibited preferential metal attack of the carbonitride phase with dissolution and/or replacement of the TiC/N dispersion. Within the silicon nitride-based group, ferrous alloys were found to be more damaging than mainly nickel alloys and silicon nitrides were more readily attacked than sialons. The difference in behaviour between the sialons and silicon nitrides is attributed to alumina additions in the former group of materials increasing resistance to attack by molten metals. A detailed mechanism of attack for these mixed-phase ceramics is proposed whereby a silicon concentration gradient is established from the crystalline ceramic phases, through the glassy binding phase, to the metal. The result is dissolution of the crystalline phase and an increase in volume fraction of the glassy binder at the metal-ceramic interface with concomitant progressive disruption of the ceramic microstructure.

Corrosion of hot pressed silicon nitride-based materials by molten copper

The behaviour of several types of silicon nitride-based materials in contact with molten copper has been evaluated. Tests were performed at a temperature of 1400 K for a holding time of 260 h in fluido-dynamic conditions.

Characterization of hot-pressed silicon nitride-based materials by microhardness measurements

The microhardness A Si3N4-based materials may be related to their phase and chemical compositions and to microstructural parameters such as porosity, grain size and secondary phases. By reducing the amount of intergranular phase, the microhardness may reach values up toHV 500 = 3000 kg mm−2. Moreover, a comparison among different materials must include the microhardness tests in a wide range of values of the applied load. The relationd n =B 1 +B 2 d 2, obtained from those of Meyer and Kick, gives two constants:B 1 may represent one class of materials andB 2 is specific for each single material.