New approach to MoSi2/SiC intermetallic-ceramic composite with B4C

The effects of SiC and B₄C additives in the MoSi₂ matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200°C by a thermal mechanical analysis (TMA). The experimental results show that the Mo₂B₅ reinforced phase was formed in situ in the hot-pressed MoSi₂/SiC/B₄C composites. Both the Mo₂B₅ phase and the SiC phase significantly improved the mechanical behavior of MoSi₂. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi₂ composite with the B₄C additive. As a result, a dense and homogenous MoSi₂/SiC/B₄C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi₂/SiC/B₄C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200°C in comparison with the pure MoSi₂ when adding 40 vol% SiC. This MoSi₂/SiC/B₄C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

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.     

Superconductivity in carrier-doped silicon carbide

Abstract

We report growth and characterization of heavily boron-doped 3C-SiC and 6H-SiC and Al-doped 3C-SiC. Both 3C-SiC:B and 6H-SiC:B reveal type-I superconductivity with a critical temperature T_c=1.5 K. On the other hand, Al-doped 3C-SiC (3C-SiC:Al) shows type-II superconductivity with T_c=1.4 K. Both SiC:Al and SiC:B exhibit zero resistivity and diamagnetic susceptibility below T_c with effective hole-carrier concentration n higher than 10²⁰ cm^?3. We interpret the different superconducting behavior in carrier-doped p-type semiconductors SiC:Al, SiC:B, Si:B and C:B in terms of the different ionization energies of their acceptors.     

Sliding wear of self-mated Al2O3-SiC whiskerreinforced composites at 23–1200°C

Microstructural changes occurring during sliding wear of self-mated Al₂O₃-SiC whiskerreinforced composites were studied using optical, scanning electron microscopy and transmission electron microscopy. Pin-on-disc specimens were slid in air at 2.7 m s^–1 sliding velocity under a 26.5 N load for 1 h. Wear tests were conducted at 23, 600, 800 and 1200°C. Mild wear with a wear factor of 2.4 x 10^–7–1.5 x 10^–6 mm³ N^–1 m^–1 was experienced at all test temperatures. The composite showed evidence of wear by fatigue mechanisms at 800°C and below. Tribochemical reaction (SiC oxidation and reaction of SiO₂ and Al₂O₃) leads to intergranular failure at 1200°C. Distinct microstructural differences existing at each test temperature are reported.Resident Research Associate at NASA Lewis Research Center.     

Microstructure and mechanical properties of silicon carbide fibre-reinforced aluminium nitride composite

SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)₂ · Al₂O₃ by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test.     

Oxidation resistance of carbon-ceramics composite materials sintered from ground powder mixtures of raw coke and ceramics

Carbon-SiC-B₄C composite materials were prepared from ground powder mixtures of petroleum raw coke, SiC and B₄C by powder sintering, without the use of any special binder and hot-pressing process. Dense composites with a fine microtexture were obtained. Oxidation tests were carried out on the composites at temperatures from 1000 to 1300° C under an air flow. The oxidation resistance depended strongly on the SiC/(SiC + B₄C) ratio and total contents of SiC and B₄C in the composites, which determined the compositions of B₂O₃ and SiO₂ in the protective film formed at the surface of the composite block during oxidation. In optimum ratios, from 63 to 87%, the composites showed such a high oxidation resistance that they were comparable with Si₃ N₄ at 1200° C.     

Wear resistance of silicon infiltrated B4C/BN composites

The B₄C/BN composites were fabricated by hot-pressing process. In this research, the silicon infiltration process was applied to improve the surface hardness and wear resistance of the B₄C/BN composites. The phase composition, microstructure, Vickers hardness and wear resistance of the silicon infiltrated B₄C/BN composites were investigated and compared with the hot-pressed B₄C/BN composites. XRD analysis results of the silicon infiltrated specimens showed that the resultant coating was mainly composed of silicon carbide and silicon. The Vickers hardness of the silicon infiltrated B₄C/BN composites was significantly improved in comparison with the hot-pressed B₄C/BN composites. The Vickers hardness of the silicon infiltrated B₄C/BN composites achieved to 12-16 GPa. The wear resistances of the silicon infiltrated B₄C/BN composites were also significantly improved in comparison with the hot-pressed B₄C/BN composites. SEM micrograph of silicon infiltrated specimens showed that the thickness of silicon carbide and silicon coating was about 200-300 μm, which significantly improved the surface hardness and wear resistance of the B₄C/BN composites.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Strength,toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol% SiC whisker-reinforced Si₃N₄ material were determined as a function of temperature from 25 to 1400°C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100°C; however, appreciable strength degradation started at 1200°C and reached a maximum at 1400°C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPam^1/2. It was also observed that the composite material exhibited no rising R-curve behaviour at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si₃N₄ matrix did not provide any favourable effects on strength, toughness and R-curve behaviour.     

Mechanical and thermal properties of silicon-carbide composites fabricated with short Tyranno® Si-Zr-C-O fibre

Silicon carbide (SiC) composites reinforced with 10–50 mass% (10.5–51.2 vol%) of short Tyranno® Si-Zr-C-O fibre (average length 0.5 mm) and 0–10 mol% of Al₄C₃as a sintering aid were fabricated using the hot-pressing technique. Firstly, the effect of Si-Zr-C-O fibre addition on the relative density (bulk density/true density) of the SiC composite hot-pressed at 1800 °C for 30 min was examined by fixing the amount of Al₄C₃to be 5 mol%. Although the relative density was reduced to 87.4% for 10 mass% of Si-Zr-C-O addition, further increases in the amount of Si-Zr-C-O fibre increased density to a maximum of 92.8% at 40 mass% of fibre addition. Secondly, the effect of varying the amount of Al₄C₃addition on the relative density was examined by fixing the amount of Si-Zr-C-O fibre to be 40 mass%. The optimum amount of Al₄C₃addition for the fabrication of dense SiC composite was found to be 5 mol%. The fracture toughness of the hot-pressed SiC composites with 20–40 mass% of Si-Zr-C-O fibre addition (amount of Al₄C₃: 5 mol%) was 3.2–3.4 MPa · m^1/2and approximately 1.5 times higher than that (2.39 MPa · m^1/2) of the hot-pressed SiC composite with no Si-Zr-C-O fibre addition. SEM observation showed evidence of Si-Zr-C-O fibre debonding and pull-out at the fracture surfaces. The hot-pressed SiC composite with 5 mol% of Al₄C₃and 40 mass% of Si-Zr-C-O fibre additions showed excellent heat-resistance at 1300 °C in air due to the formation of a SiO₂layer at and near exposed surfaces.     

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.     

Preparation, Properties and Application of C/C-SiC Composites Fabricated by Warm Compacted-in situ Reaction

Carbon fibre reinforced carbon and SiC dual matrices composites (C/C-SiC) show superior tribological properties, high thermal shock resistance and good abrasive resistance, and they are promising candidates for advanced brake and clutch systems. The microstructure, mechanical properties, friction and wear properties, and application of the C/C-SiC composites fabricated by warm compacted-in situ reaction were introduced. The results indicated that the composites were composed of 50-60 wt pct carbon, 2-10 wt pct residual silicon and 30-40 wt pct silicon carbide. The C/C-SiC brake composites exhibited good mechanical properties. The value of flexural strength and compressive strength could reach 160 and 112 MPa, respectively. The impact strength was about 2.5 kJ·m^-2. The C/C-SiC brake composites showed excellent tribological performance, including high coefficient of friction (0.38), good abrasive resistance (1.10 μm/cycle) and brake steadily on dry condition. The tribological properties on wet condition could be mostly maintained. The silicon carbide matrix in C/C-SiC brake composites improved the wear resistance, and the graphite played the lubrication function, and right volume content of graphite was helpful to forming friction film to reduce the wear rate. These results showed that C/C-SiC composites fabricated by warm compacted-in situ reaction had excellent properties for use as brake materials.     

Fabrication of particulate aluminium-matrix composites by liquid metal infiltration

Aluminium-matrix composites were fabricated by liquid metal infiltration of porous particulate reinforcement preforms, using AlN, SiC and Al₂O₃ as the particles. The quality of the composites depended on the preform fabrication technology. In this work, this technology was developed for high-volume fraction (up to 75%) particulate preforms, which are more sensitive to the preform fabrication process than lower volume fraction whisker/fibre preforms as their porosity and pore size are much lower. The technology developed used an acid phosphate binder (with P/Al molar ratio=23) in the amount of 0.1 wt% of the preform, in contrast to the much larger binder amount used for whisker preforms. The preforms were made by filtration of a slurry consisting of the reinforcement particles, the binder and carrier (preferably acetone), and subsequent baking (preferably at 200 °C) for the purpose of drying. Baking in air at 500 °C instead of 200 °C caused the AlN preforms to oxidize, thereby decreasing the thermal conductivity of the resulting Al/AlN composites. The reinforcement-binder reactivity was larger for AlN than SiC, but this reactivity did not affect the composite properties due to the small binder amount used. The Al/AlN composites were superior to the Al/SiC composites in the thermal conductivity and tensile ductility. The Al/Al₂O₃ composites were the poorest due to Al₂O₃ particle clustering.     

Erosion mechanisms of aluminium nitride ceramics at different impact angles

In this paper, erosion wear behaviour of aluminium nitride (AlN) ceramics is studied. The influence of particle hardness and shape on erosion of the AlN surface is examined. The effect of varying the impingement angle on the weight loss and the roughness parameters of AlN ceramics testing sample is also determined. Therefore, erosive wear behaviour of AlN ceramics was investigated using SiC and SiO₂ particles as erodents, at following impact angles: 30°, 45°, 60°, 75° and 90°. Scanning electron microscopy (SEM) was used to analyze the eroded surfaces in order to determine erosion mechanisms. The roughness parameters (R_a, R_z and R_max), before and after erosion with SiO₂ and SiC particles at 30° and 90° angles of impingement, respectively, were determined using a profilometer. It was found that the impact angle is influencing the erosion wear of the AlN ceramics and maximum erosion takes place at impact angle of 90°. The results indicate that hard, angular SiC particles cause more damage than softer, more rounded SiO₂ particles.     

Study of aeroabrasive wear of hot-pressed materials of the B4C-TiB2 system

The results of the studies of aeroabrasive wear of hot-pressed materials of the B₄C-TiB₂ system at different angles of abrasive particles attacks have been considered. It has been shown that the materials wear resistance is essentially affected by the relation of phases in the composite. The formation of 5–10 wt % titanium diboride in a B₄C composite have been defined to provide a high wear resistance because of the increase of the fracture toughness K _Ic from 3.8 to 4.4 MPa·m^1/2 on retention of the high hardness (H _V = 20–23 GPa).     

Microstructure and properties of an electroconductive SiC-based composite

In this work, an SiC-based electroconductive composite is obtained through simultaneous addition of MoSi₂ and ZrB₂ particles. The composite material is fully densified by hot pressing at 1860 °C and the microstructure is investigated by SEM-EDS analysis. Microstructural features and mechanical properties are compared to those of a monolithic hot-pressed SiC material. The MoSi₂ and ZrB₂ particles, besides increasing the electrical conductivity of the silicon carbide matrix, also act as reinforcement for the material. Room-temperature strength reaches the value of 850 MPa and the fracture toughness is 4.2 MPa m^0.5. The composite electrical resistivity is of the order of 10⁻³ Ω cm.     

Schwingungsverschleiß von ungeschmeierten,faserkeramischen C/C-SiC-Gleitpaarungen

Unlubricated oscillating sliding wear of C/C-Sic fibre reinforced composites C/C-SiC fibre reinforced composites were produced by liquid infiltration technique. Porous C-fibre laminates containing carbonic resin were infiltrated with liquid silicon, leading to a SiC-matrix. Mechanical and thermal properties of the composites were measured. Tribological tests were carried out on self-mated C/C-SiC and C/C-SiC mated with ZrO₂ and steel, respectively, in unlubricated oscillating sliding contact using a ring-on-block tribometer. Environmental conditions such as relative humidity and testing temperature were varied. Microstructures of the composites as well as the worn surfaces were systematically analysed using scanning electron microscopy. Experimental results showed a significant influence of the relative humidity and the testing temperature on tribological properties. Self-lubricating effects due to carbon films occurred at sufficient humidity, contact temperatures < 90°C and below a critical surface pressure.     

Fabrication of silicon carbide composites with carbon nanofiber addition and their fracture toughness

The authors have examined the fabrication conditions of SiC composites containing carbon nanofiber, i.e., vapor-grown carbon nanofiber (VGCF), to enhance the fracture toughness. Commercially available ultrafine SiC powder (specific surface area: 47.5 m² g⁻¹) was mixed with VGCF and sintering aid in the Al₄C₃–B₄C system. Approximately 1.5 g of the mixture was uniaxially pressed at 50 MPa to obtain a compact with a diameter of 20 mm and a thickness of approximately 1.5 mm. The resulting compact was hot-pressed at 1800 °C for 1 h in Ar atmosphere under a pressure of 62 MPa. The relative density of hot-pressed SiC composite decreased from 98.0 to 96.3%, whereas the fracture toughness was enhanced from 3.8 to 5.2 MPa m^1/2, as the amount of VGCF increased from 0 to 6 mass%. Furthermore, an acid treatment of VGCF was conducted to enhance its dispersibility within the SiC matrix, owing to the formation of COO⁻ groups on the VGCF surface. As a result of this treatment, the relative density and fracture toughness of hot-pressed SiC composite with 6 mass% acid-treated VGCF addition increased to 99.0% and 5.7 MPa m^1/2, respectively.     

Fabrication and thermal shock resistance of HfB2-SiC composite with B4C additives

A HfB₂ based ceramic matrix composite containing 20 vol.% SiC particles with 2 vol.% B₄C as sintering additives was fabricated by hot-pressed sintering. The microstructure and properties, especially the thermal shock resistance of the composite were investigated. Results showed that the addition of B4C improved the powder sinterability and led to obtaining nearly full dense composite. The flexural strength and fracture toughness of the composite were 771 MPa and 7.06 MPam^1/2, respectively. The thermal shock resistance tests indicated that the residual strength decreased significantly when the thermal shock temperature difference was higher than 600 °C. The large number of microcracks on the sample surface was the main reason for the catastrophic failure.