Synthesis of Porous Silicon Nitride-Boron Nitride Composites by Gel-Casting and PIP

The Si₃N₄-BN composites were prepared by gel-casting and precursor infiltration and pyrolysis route using borazine as the precursor. The composition, mechanical, microstructural, and dielectric properties of the composites were investigated. The composites are composed of h-BN, α-Si₃N₄, and β-Si₃N₄. The typical density, porosity, flexural strength, elastic modulus, and fracture toughness of the composites are 2.21 g/cm³, 17.8%, 185.59 MPa, 69.13 GPa, and 2.47 MPa m^1/2, respectively. The dielectric constant and loss tangent of the composites are 4.507-4.635 and 1.06 × 10^?3-1.97 × 10^?3 at the frequency of 7-18 GHz. Desirable properties of the composites have been achieved.     

Formation of high-density TiN/Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> ceramic composites using preceramic polymer

The formation, microstructure and properties of high-density TiN/Ti₅Si₃ ceramic composites created by the pyrolysis of preceramic polymer with filler were investigated. Methylpolysiloxane was mixed with TiH₂ as filler and ceramic composites prepared by pyrolysis at 1200°C to 1600°C under N₂, Ar and vacuum were studied. When a specimen with 70 vol.% TiH₂ was pyrolyzed up to 1600°C in a vacuum after a preheat treatment at 850°C in a N₂ atmosphere and subsequently heat-treated at 1600°C for 1 h under Ar at a pressure of 2 MPa, a ceramic composite with full density was obtained. The microstructure of the ceramic composite was composed of TiN and Ti₅Si₃ phases. Under specific pyrolysis conditions, a ceramic composite with a density of 99.2 TD%, a Vickers hardness of 18 GPa, a fracture toughness of 3.5 MPam^1/2, a flexural strength of 270 MPa and a electrical conductivity of 6200 ohm⁻¹·cm⁻¹ was obtained.     

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

Synthesis of fibrous β-Si3N4 structured porous ceramics using carbothermal nitridation of silica

Porous Si₃N₄ ceramics with a fibrous interlocking microstructure were synthesized directly by carbothermal nitridation (CT/N) of SiO₂. Carbon black was used as the carbon source and α-Si₃N₄ was used as seed. The cold-pressed samples of powder mixtures were heated at nitrogen pressures above 0.6 MPa and temperatures exceeding 1600 °C. The addition of the α-Si₃N₄ to the initial powder mixtures had important effects on the microstructure and mechanical properties of the porous Si₃N₄. Fine elongated fibrous β-Si₃N₄ grains were developed in the seeded samples with stoichiometric C–SiO₂ ratio and Y₂O₃ as the sintering additive, when sintered at 1700–1750 °C. A sample of outstanding strength resulted, five times stronger than the seed-free samples. Such a technique offers the possibility of synthesizing highly porous and strong Si₃N₄ materials at considerably lower cost than at present.     

Growth Kinetics and Microstructure Evolution of Intermediate Phases in MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-WSi<Subscript>2</Subscript>/Mo Diffusion Couples

The growth kinetics and silicon diffusion coefficients of intermediate silicide phases in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple prepared by spark plasma sintering were investigated in temperatures ranging from 1200 to 1500 °C. The intermediate silicide phases were characterized by x-ray diffraction. The microstructures and components of the MoSi₂-Si₃N₄-WSi₂/Mo composites were investigated using scanning electron microscope with energy-dispersive spectroscopy. A special microstructure with MoSi₂ core surrounded by a thin layer of (Mo,W)Si₂ ring was found in the MoSi₂-Si₃N₄-WSi₂ composites. The intermediate layers of Mo₅Si₃ and (Mo,W)₅Si₃ in the MoSi₂-Si₃N₄-WSi₂/Mo diffusion couples were formed at different diffusion stages, which grew parabolically. Activation energy of the growth of intermediate layers in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple was calculated to be 316 ± 23 kJ/mol. Besides, the hindering effect of WSi₂ addition on the growth of intermediate layers was illustrated by comparing the silicon diffusion coefficients in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo and MoSi₂-3.5 vol.% Si₃N₄/Mo diffusion couples. MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂ coating on Mo substrate exhibited a better high-temperature oxidation resistance in air than that of MoSi₂-3.5 vol.% Si₃N₄ coating.     

High α-β phase transition and properties of YbF3-added porous Si3N4 ceramics obtained by low temperature pressureless sintering

In this paper, we used YbF₃ as a sintering additive to get a high α-β phase transition in porous Si₃N₄ ceramics. The mechanism of YbF₃ as sintering additives as well as the relationship between microstructure and mechanical properties have been investigated in detail. In addition, we used pressureless sintering to lower the temperature to 1550 °C. YbF₃ makes α-Si₃N₄ completely transform to β-Si₃N₄, whereas only 41.1% β-Si₃N₄ could be obtained with Yb₂O₃. This process yielded ceramics with more flexural strength and increased fracture toughness using less energy. In addition, using YbF₃ substituted for part Yb₂O₃ could promote sintering behaviors of Si₃N₄ ceramics at low temperature to increase α-β phase transition rate and improve the properties of silicon nitride ceramics significantly. In particular, when we used YbF₃-Yb₂O₃ as additives, we obtained a flexural strength of 269.87 MPa and a fracture toughness of 4.59 MPa·m^1/2.     

Superplasticity in a fine-grained beta-silicon nitride ceramic containing a transient liquid

Compressive deformation in a dense fine-grained β-Si₃N₄ sintered with 7 wt% cordierite was investigated over a wide range of temperatures (1450–1650°C) and strain rates (1×10⁻⁵–1×10⁻³/s). The superplastic characteristics and the results of microstructural examination are presented. These indicate that: (a) fine-grained β-Si₃N₄ ceramics are capable of high rates of deformation (about 10⁻⁵–10⁻⁴/s) at 1550°C without strain hardening occurring; (b) remarkable phase and microstructural evolutions, such as formation of elongated and strongly textured Si₂N₂O grains occurred during deformation; (c) a transient liquid phase present in this material system which facilitated the formation of Si₂N₂O grains enhanced the superplastic flow, and well-aligned Si₂N₂O grains formed in situ did not have a detrimental impact on the superplasticity; and (d) the mechanism of deformation is grain boundary sliding accommodated by diffusion-controlled solution–precipitation creep.     

Morphology and ablation mechanism of Cf/Si3N4 composites ablated by oxyacetylene torch at 2200°C

Abstract

Si₃N₄ ceramic matrix composites reinforced by carbon fibres (C_f/Si₃N₄) were prepared by low pressure chemical vapour infiltration at 1250°C using SiCl₄ and NH₃ as precursor. The as prepared C_f/Si₃N₄ composites were ablated to determine the mechanism of the ablation resistance and oxidisation resistance by oxyacetylene torch at 2200°C. The morphology and microstructure of the composites were examined by scanning electron microscopy. The phase compositions of the composites were confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction. The results indicated that the matrix of the C_f/Si₃N₄ composites was composed of the amorphous Si₃N₄ and nanometre α-Si₃N₄. A central ablation region and a ring oxidisation region appeared on the surface of the as ablated C_f/Si₃N₄ composites. Sublimation of the Si₃N₄ matrix and oxidation of the carbon fibres are the main ablation behaviours in the central region. Oxidation of the Si₃N₄ matrix and deposition of SiO₂ particles are the main ablation behaviour in the ring region. A large number of SiO₂ liquid droplets produced during ablation were retained and formed spherical solid particles on the surface of the ring region after ablation. For the mismatch of the coefficient of thermal expansion of the carbon fibres and the Si₃N₄ matrix, Si₃N₄ matrix was cracked under the thermal impact of the oxyacetylene flame. With the passive oxidation of the as cracked surface, the continuous SiO₂ liquid was formed in the ring region. Subsequently, some residual Si₃N₄ particles were covered by transparent SiO₂ layer to form an amber-like microstructure.     

The Sliding Wear and Friction Behavior of M50-Graphene Self-Lubricating Composites Prepared by Laser Additive Manufacturing at Elevated Temperature

M50 steel is widely applied to manufacture aircraft bearings where service lives are mainly determined by the friction and wear behaviors. The main purpose of this study is to investigate the tribological behaviors and wear mechanisms of M50-1.5 wt.% graphene composites (MGC) prepared by laser additive manufacturing (LAM) (MGC-LAM) sliding against Si₃N₄ ball from 25 to 550 °C at 18 N–0.2 m/s. XRD, EPMA, FESEM, and EDS mapping were conducted to understand the major mechanisms leading to the improvement in the sliding behavior of MGC-LAM. The results indicated that MGC-LAM showed the excellent friction and wear performance at 25-550 °C for the lower friction coefficient of 0.16-0.52 and less wear rate of 6.1-9.5 × 10^?7 mm³ N^?1 m^?1. Especially at 350 °C, MGC-LAM obtained the best tribological performance (0.16, 6.1 × 10^?7mm³ N^?1 m^?1). It was attributed to the dense coral-like microstructure, as well as the formed surface lubricating structure which is composed of the upper uniform lubricating film with massive graphene and the underneath compacted layer.     

Fabrication of Y2Si2O7 coating and its oxidation protection for C/SiC composites

Yttrium silicate (Y₂Si₂O₇) coating was fabricated on C/SiC composites through dip-coating with silicone resin + Y₂O₃ powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y₂Si₂O₇ coating were in–estigated. Y₂Si₂O₇ can be synthesized by the pyrolysis of Y₂O₃ powder filled silicone resin at mass ratio of 54.2:45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and fa–orable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%–140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y–Si–Al–O glassy phase between Y₂Si₂O₇ and Al₂O₃ sample bracket at 1500 °C is disco–ered.     

Densification, microstructure, and fracture behavior of TiC/Si3N4 composites by spark plasma sintering

TiC/Si₃N₄ composites were prepared using the β-Si₃N₄ powder synthesized by self-propagating high-temperature synthesis (SHS) and 35 wt.% TiC by spark plasma sintering. Y₂O₃ and Al₂O₃ were added as sintering additives. The almost full sintered density and the highest fracture toughness (8.48 MPa·m^½) values of Si₃N₄-based ceramics could be achieved at 1550°C. No interfacial interactions were noticeable between TiC and Si₃N₄. The toughening mechanisms in TiC/Si₃N₄ composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si₃N₄ matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.     

Effects of α- and β-Si3N4 as precursors on combustion synthesis of (α + β)-SiAlON composites

Preparation of (α + β)-SiAlON composites from the powder compacts of Si, Si₃N₄, SiO₂, Al, and AlN was investigated by self-propagating high-temperature synthesis (SHS) in nitrogen of 2.17 MPa. Test samples adopted not only pure α- and β-Si₃N₄, but also a mixture of (α + β)-Si₃N₄. The combustion temperature and flame-front propagation velocity decreased with increasing ratio of Si/Si₃N₄, but they increased with proportion of α/β-Si₃N₄. For the sample containing pure α-Si₃N₄, the synthesis reaction yielded only α-SiAlON with various morphologies including equiaxed crystals, elongated grains, and fine whiskers. As a mixture of (α + β)-Si₃N₄ was employed, the resulting products were (α + β)-SiAlON composites, within which the content of β-SiAlON increased with increasing β-Si₃N₄. For the sample adopting 100% β-Si₃N₄, comparable amounts of α- and β-SiAlON were produced. Additionally, the morphology of (α + β)-SiAlON composites was dominated by elongated grains with a high aspect ratio.     

Effect of Ti3SiC2 Content on Tribological Behavior of Ni3Al Matrix Self-Lubricating Composites from 25 to 800 °C

The self-lubricating composites of Ni₃Al-Ti₃SiC₂-TiC-C (NMC) with varying Ti₃SiC₂ contents were fabricated by spark plasma sintering technique. Dry sliding pin-on-disc friction and wear tests of NMC against Si₃N₄ ceramic ball were undertaken at 25, 200, 400, 600, and 800 °C in air, respectively. The results showed that NMC with 15 wt.% Ti₃SiC₂ lubricant owned the excellent tribological properties over a wide temperature range from 25 to 800 °C, whose friction coefficients and wear rates were about 0.17-0.58 and 0.31-4.2 × 10^?5 mm³/N/m, respectively. A possible explanation for these results was that the subsurface microstructure self-refinement and the special stratification morphology of the tribo-layer were beneficial to the reduction of friction coefficient. Meanwhile, the protective action of the tribo-layer for the frictional surface could also decrease the wear rate.     

Direct bonding of silicon paris with heterogeneous insulator using different annealing methods

We prepared SOI (silicon-on-insulator) wafer pairs of 2000 Å-SiO₂/Si(100) and 560 å-Si₃N₄/Si(100) by CFA (Conventional electric Fumace Annealing), RTA (Rapid Thermal Annealing), and FLA (Fast Linear Annealing) at different annealing temperatures for each annealing process. We measured the bonding area and the bonding strength for the respective processes. It was demonstrated that the measured bonding area was close to 100% above 450°C for RTA, and 400°C for CFA. The maximum bond strength of the SiO₂/Si₃N₄ wafer pair was 2344, 2300, and 195 mJ/m² for CFA, FLA, and RTA, respectively. We clearly demonstrated that the FLA method is far superior in producing high-quality directly bonded Si wafer pairs with SiO₂ and Si₃N₄films compared to the CFA and RTA methods.     

Effect of Pyrolysis Temperature on the Properties of Three-Dimensional Silica Fiber Reinforced Nitride Matrix Composites

Three-dimensional silica fiber reinforced nitride matrix composites (3D SiO_2f/nitride composites) were prepared through four cycles of vacuum infiltration of a hybrid precursor and pyrolysis under ammonia atmosphere. The effects of pyrolysis temperature on densification behavior, mechanical properties, and microstructures of the composites were investigated. With the increase of pyrolysis temperature from 800 to 1300 °C, the density of SiO_2f/nitride composites increased from 1.83 to 1.88 g/cm³, while the flexural strength decreased from 148 to 53 MPa, and the elastic modulus decreased from 41.5 to 25.1 GPa. The higher temperatures cause serious damage to silica fibers. The properties of silica fibers and the reinforcing mechanism determine the mechanical properties of the composites.     

Oxidation behavior of molybdenum silicides and their composites

A key materials issue associated with the future of high-temperature structural silicides is the resistance of these materials to oxidation at low temperatures. Oxidation tests were conducted on Mo-based silicides over a wide temperature range to evaluate the effects of alloy composition and temperature on the protective scaling characteristics and pesting regime for the materials. The study included Mo₅Si₃ alloys that contained several concentrations of B. In addition, oxidation characteristics of MoSi₂–Si₃N₄ composites that contained 20–80 vol.% Si₃N₄ were evaluated at 500–1400°C.     

Thermal properties of pressureless melt infiltrated AlN–Si–Al composites

Thermal properties of AlN-Si-Al composites produced by pressureless melt infiltration of Al/Al alloys into porous α-Si₃N₄ preforms were investigated in a temperature range of 50-300 °C. SEM and TEM investigations revealed that the grain size of AlN particles was less than 1 μm. In spite of sub-micron grain size, composites showed relatively high thermal conductivity (TC), 55-107 W/(m.K). The thermal expansion coefficient (CTE) of the composite produced with commercial Al source, which has the highest TC of 107 W/(m.K), was 6.5×10^?6 K^?1. Despite the high CTE of Al (23.6×10^?6 K^?1), composites revealed significantly low CTE through the formation of Si and AlN phases during the infiltration process.     

Thermal shock and thermal fatigue resistance of Sialon–Si3N4 graded composite ceramic materials

Sialon–Si₃N₄ graded composite ceramic materials were fabricated by hot press sintering. The mechanical properties and microstructure of the composites were examined. The thermal shock and thermal fatigue resistance of the Sialon–Si₃N₄ graded composite ceramic materials were investigated by means of the water quenching method. The microstructure of the composites was characterized with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results showed that the graded ceramic exhibited higher retained flexural strength under all thermal shock temperature differences compared to the homogeneous reference one, indicating the higher thermal shock resistance of the graded ceramic. The highest critical temperature difference of the graded composites was 600 °C. The crack growth (∆c) of graded ceramic materials was much lower than that of homogeneous ceramic materials, which revealed the higher thermal fatigue resistance of the graded ceramics. The improvement of the thermal shock and thermal fatigue resistance was attributed to the formation of compressive residual stress in the surface layer and the enhanced mechanical properties induced by the graded compositional structure.     

Thermal Shock Resistance of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/h<Emphasis Type="Italic">-</Emphasis>BN Composites Prepared via Catalytic Reaction-Bonding Route

Si₃N₄/h-BN ceramic matrix composites were prepared via a catalytic reaction-bonding route by using ZrO₂ as nitridation catalyst, and the water quenching (fast cooling) and molten aluminum quenching tests (fast heating) were carried out to evaluate the thermal shock resistance of the composites. The results showed that the thermal shock resistance was improved obviously with the increase in h-BN content, and the critical thermal shock temperature difference (ΔT _c) reaches as high as 780 °C when the h-BN content was 30 wt.%. The improvement of thermal shock resistance of the composites was mainly due to the crack tending to quasi static propagating at weak bonding interface between Si₃N₄ and h-BN with the increase in h-BN content. For the molten aluminum quenching test, the residual strength showed no obvious decrease compared with water quenching test, which could be caused by the mild stress condition on the surface. In addition, a calculated parameter, volumetric crack density (N _f), was presented to quantitative evaluating the thermal shock resistance of the composites in contrast to the conventional R parameter.     

Direct measurement of residual stresses and their effects on the microstructure and mechanical properties of heat-treated Si3N4 ceramics II: With CeO2 as a single additive

The heat treatment of silicon nitride ceramics with only CeO₂ as a sintering additive was carried out at different temperatures. Large residual stress was induced only by the non-relaxed volume changes and not by the relaxed volume changes (i.e. the crystallized phases with cavities or microcracks). The fact that the liquid formation temperature of the Si–Ce–O–N system (∼1470 °C) is much lower than that of the Si–Yb–O–N system (∼1650 °C) is the reason why the residual stress is comparable almost to each other in CeO₂-doped Si₃N₄, but increases steadily with heat-treatment temperature in Yb₂O₃-doped Si₃N₄. The large residual stress and the induced cavities and microcracks are the dominant factors for the reduction in room-temperature strength of the heat-treated samples. The defects in β-Si₃N₄ grains of heat-treated samples were caused by the large residual stress, and may lead to the reduction of both room- and high-temperature strengths. These results significantly extends our previous study (Guo GF, Li JB, Yang XZ, Lin H, Liang L, He MS, Tong XG, Yang J. Acta Mater 2006;54:2311) on heat-treated Si₃N₄ ceramics with only Yb₂O₃ (one of the heavier lanthanide oxides) as a sintering additive.