Carbon/ceramics composites — Preparation and properties

Fabrication methods for carbon/ceramics composites were established by using two different processes of hot-pressing and pressureless sintering without any binder phase. In the hot pressing method, some boron compounds were found to be an effective aid for sintering and graphitization of coke powder above 2000°C under some pressure. When the content of boron compound such as B₄C was high, graphite/B₄C composites could be fabricated. If some other ceramic powder such as NbC, TiC or TaC was mixed in addition to the B₄C, three component composites with graphite matrix could be obtained. In pressureless sintering method, raw coke carbon powder was ground for a long time to be transformed in to a sinterable and non-graphitizing-type carbon powder. From a mix of ceramic powders such as SiC or B₄C with the ground coke powder, the composites of carbon/SiC or carbon/SiC/B₄C systems could be fabricated by heat-treatment under normal pressure.Some properties of the graphite samples and carbon/ceramic composites were investigated. It was found that their mechanical properties were much better than those of conventional graphite samples and the resistance to oxidation and corrosion was also excellent. It is suggested that the composites could be applied as bearing or mechanical seals both for use in high temperature environments and as machine parts in contact with some molten metals.     

The microstructure and mechanical properties of the B4C/Cu matrix composite fabricated by SPS-HR

Copper matrix composites containing different volume fractions of B₄C particles (0–15%) were first fabricated by spark plasma sintering followed by hot rolling in atmospheric environments, then their microstructures, phase compositions, mechanical properties and sintering mechanism were investigated. It was found that B₄C particles distributed relatively homogeneously in the copper matrix. Reaction products of CuC₈ and B were observed and identified in the composite. Under increasing B₄C particle content, the ultimate tensile, yield strength and elongation to fracture of the composites decreased. Failure mode of composites included: (1) the interfacial debonding and (2) the cleavage fracture of copper. Moreover, micro-discharge between the adjacent particles occurred, and its led to local high temperature at the interface.     

Hybrid aluminium matrix composites containing boron carbide and quasicrystals: manufacturing and characterisation

Hybrid composites of boron carbide (B₄C) and Al_62.5Cu₂₅Fe_12.5 quasicrystals (QCs) were prepared by ball milling and pressureless sintering in aluminium matrix to investigate their individual and hybrid effects on microstructural and mechanical properties. Hybrid composite contained B₄C and QCs in 3?wt-% each, making a total of 6?wt-%. For reference, specimens of pure aluminium and two composites containing 6?wt-%B₄C and 6?wt-% QCs were prepared. Microstructural characterisation was performed using optical, scanning electron microscopy and X-ray diffraction, while evaluation of mechanical properties was carried out by hardness and compression tests. Uniform dispersion of reinforcements in composites was observed along with significant increase in the mechanical properties. The composite containing 6?wt-% QCs demonstrated the highest hardness, while the hybrid composite showed better compressive properties.     

Bio-inspired structured boron carbide-boron nitride composite by reactive spark plasma sintering

Nature creates composite materials with complex hierarchical structures that possess impressive mechanical properties enhancement capabilities. An approach to improve mechanical properties of conventional composites is to mimic the biological material structured ‘hard’ core and ‘soft’ matrix system. This would allow the efficient transfer of load stress, dissipation of energy and resistance to cracking in the composite. In the current study, reactive spark plasma sintering (SPS) of boron carbide B₄C was carried out in a nitrogen N₂ gas environment. The process created a unique core-shell structured material with the potential to form a high impact-resistant composite. Transmission electron microscopy observation of nitrided-B₄C revealed the encapsulation of B₄C grains by nano-layers of hexagonal-boron nitride (h-BN). The effect of the h-BN contents on hardness were measured using micro- and nano-indentation. Commercially available h-BN was also mechanically mixed and sintered with B₄C to compare the effectiveness of nitrided B₄C. Results have shown that nitrided B₄C has a higher hardness value and the optimum content of h-BN from nitridation was 0.4%wt with the highest nano-indentation hardness of 56.7 GPa. The high hardness was attributed to the h-BN matrix situated between the B₄C grain boundaries which provided a transitional region for effective redistribution of the stress in the material.     

Production and characterization of AA6061–B4C stir cast composite

This work focuses on the fabrication of aluminum (6061-T6) matrix composites (AMCs) reinforced with various weight percentage of B₄C particulates by modified stir casting route. The wettability of B₄C particles in the matrix has been improved by adding K₂TiF₆ flux into the melt. The microstructure and mechanical properties of the fabricated AMCs are analyzed. The optical microstructure and scanning electron microscope (SEM) images reveal the homogeneous dispersion of B₄C particles in the matrix. The reinforcement dispersion has also been identified with X-ray diffraction (XRD). The mechanical properties like hardness and tensile strength have improved with the increase in weight percentage of B₄C particulates in the aluminum matrix.     

Sintering and characterization of Al2O3-B4C composites

The effect of B₄C on the densification, microstructure and mechanical properties of pressureless sintered Al₂O₃-B₄C composites have been studied. Sintering was performed without sintering additives with varying B₄C content from 0–40 vol %. Up to 20 vol % B₄C, more than 97% theoretical density was always obtained when sintered at 1850 °C for 60 min. On increasing the sintering time from 30–120 min, there was no change in density. The result of X-ray diffraction analysis showed that no reaction occurred between Al₂O₃ and B₄C. The grain growth of Al₂O₃ was inhibited by B₄C particles pinned at the grain boundary and the grain-boundary drag effect. The critical amount of B₄C to drag the grain boundary migration effectively was believed to occur at 10 vol % B₄C sintered at 1850 °C for 60 min. The maximum three-point flexural strength was found to be 550 MPa for the specimen containing 20 vol % B₄C, and the maximum microhardness was 2100 kg mm^–2 for 30 vol % B₄C specimen.     

Pressureless-sintered and HIPed SiC-TiB2 composites from SiC-TiO2-B4C-C powder compacts

Dense SiC-TiB₂ composites with prescribed compositions were obtained through pressureless sintering of SiC-TiO₂-B₄C-C powder compacts. During the process, TiO₂, B₄C and C reacted to form TiB₂, followed by the consolidation of SiC matrix with the aid of excess B₄C and C. The effects of the composition of the starting powders on the final density were investigated and the mechanical properties of the composite were evaluated. The sintered body with additional HIPing at 1900 °C exhibited the average four-point flexural strength of more than 700 MPa at both 20 and 1400 °C.     

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.     

Microstructure and mechanical properties of B6O-B4C sintered composites prepared under high pressure

Sintered composites in the B₆O-xB₄C (x = 0–40 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1500–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available B₄C powder. Relationship among the formed phases, microstructures and mechanical properties of the sintered composites was investigated as a function of sintering conditions and added B₄C content. Microhardness of the sintered composite was found to increase with treatment temperature up to 1800°C, while fracture toughness decreased slightly. Maximum microhardness of Hv 46 GPa was obtained from B₆O-30vol%B₄C sintered composite under the sintering conditions of 4 GPa, 1700°C and 20 min.     

The effect of Fe addition on processing and mechanical properties of reaction infiltrated boron carbide-based composites

Dense metal-ceramic composites based on boron carbide were fabricated using boron carbide and Fe powders as starting materials. The addition of 3.5–5.5 vol% of Fe leads to enhanced sintering due to the formation of a liquid phase at high temperature. Preforms, with about 20 vol% porosity were obtained by sintering at 2,050 °C even from an initial boron carbide powder with very low sinterability. Successful infiltration of the preforms was carried out under vacuum (10⁻⁴ torr) at 1,480 °C. The infiltrated composite consists of four phases: B₁₂(C, Si, B)₃, SiC, FeSi₂ and residual Si. The decrease of residual Si is due to formation of the FeSi₂ phase and leads to improved mechanical properties of the composites. The hardness value, the Young modulus and the bending strength of the composites fabricated form a powder mixture containing 3.5 vol% Fe are 2,400 HV, 410 GPa and 390 MPa, while these values for the composites prepared form iron free B₄C powder are 1,900 HV, 320 GPa and 300 MPa, respectively. The specific density of the composite was about 2.75 g/cm³. The experimental results regarding the sintering behavior and chemical interaction between B₄C and Fe are well accounted for by a thermodynamic analysis of the Fe–B–C system.

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Al-5Cu/B4Cp composites: The combined effect of artificially aging (T6) and particle volume fractions on the corrosion behaviour

In this study, the Al–5Cu matrix composites reinforced with different boron carbide (B₄C) particle volume fractions have been successfully produced by the hot-pressing method. Then, the artificially aging (T6) was applied to the composites for increasing their mechanical properties. The combined effect of the T6 heat treating and the B₄C particle volume fraction on the corrosion behaviour of the composites were investigated by potentiodynamic scanning (PDS) technique under aerated and deaerated 3.5% NaCl marine environments. The effect of the T6 treating on the hardness and corrosion susceptibilities of the composites were also evaluated microstructurally to contribute to their industrial use and production processes. The microstructural characterization of the composites was carried out by using a scanning electron microscope (SEM) with an attached energy dispersive spectrum (EDS) and X-ray diffraction (XRD). It was found that the corrosion susceptibilities of the composite have been interestingly decreased with increasing the B₄C particle volume fraction in the matrix while the T6 treatment enhances the pitting susceptibility of the composites. The reason of the behaviour has been discussed in details the text.     

In-situ synthesis of TiC/Fe alloy composites with high strength and hardness by reactive sintering

Fe alloy composites reinforced with in-situ titanium carbide(Ti C) particles were fabricated by reactive sintering using different reactant C/Ti ratios of 0.8,0.9,1 and 1.1 to investigate the microstructure and mechanical properties of in-situ Ti C/Fe alloy composites.The microstructure showed that the in-situ synthesized Ti C particles were spherical with a size of 1–3 μm,irrespective of C/Ti ratio.The stoichiometry of in-situ Ti C increased from 0.85 to 0.88 with increasing C/Ti ratio from 0.8 to 0.9,but remained almost unchanged for C/Ti ratios between 0.9 and 1.1 due to the same driving force for carbon diffusion in Ti Cxat the common sintering temperature.The in-situ Ti C/Fe alloy composite with C/Ti = 0.9 showed improved mechanical properties compared with other C/Ti ratios because the presence of excess carbon(C/Ti = 1 and 1.1) resulted in unreacted carbon within the Fe alloy matrix,while insufficient carbon(C/Ti = 0.8)caused the depletion of carbon from the Fe alloy matrix,leading to a significant decrease in hardness.This study presents that the maximized hardness and superior strength of in-situ Ti C/Fe alloy composites can be achieved by microstructure control and stoichiometric analysis of the in-situ synthesized Ti C particles,while maintaining the ductility of the composites,compared to those of the unreinforced Fe alloy.Therefore,we anticipate that the in-situ synthesized Ti C/Fe alloy composites with enhanced mechanical properties have great potential in cutting tool,mold and roller material applications.     

Effect of fabrication process on the microstructure and dynamic compressive properties of SiCp/Al composites fabricated by spark plasma sintering

The effect of fabrication process on the microstructure and dynamic properties of SiC_p/Al composites was studied in this paper. Pure Al matrix composites reinforced with 20 vol.% SiC particles were fabricated by spark plasma sintering, and the pre-blended powders were prepared by two different processes. One was to mix the powders in conical flask by using a mechanical stirrer, and the other was the mechanical alloying process by using a planetary ball mill. The sintering temperature was also explored. The conventional split Hopkinson pressure bar was used to test the dynamic properties of these composites. The results show that the sintering temperature significantly affects the consolidation of the composites. The composites, which have not been fully densified, have very loose microstructure and poor mechanical properties. Mechanical alloying process can improve the microstructure and mechanical properties of the composites. These composites are rate dependent, their strengths increase with increasing strain rates.     

In situ synthesis of titanium diboride composites through volume combustion

Abstract

Hard in situ synthesis of TiB₂–Fe₂B metal matrix composite (MMC) has been synthesised by volume combustion synthesis (VCS) reactions of Fe–FeTi–FeB system. VCS samples were characterised by SEM, EDX, XRD and DTA. Results show that it is possible to synthesise in situ structured MMC samples (with TiB₂ and Fe₂B phases) by VCS. Metallographic investigations show that Fe₂B and TiB₂ are found dispersed throughout the metal matrix, and other borides are present in microlevel patches dispersed in a eutectic matrix. The Fe–TiB₂ composites sintered at temperature of 1200°C consist of three different regions, i.e. α-Fe, TiB₂ and Fe₂B regions. The increase in sintering temperature to 1400°C leads to a hypereutectic microstructure of the Fe–B binary system having TiB₂ grains uniformly distributed throughout the matrix. A semiliquid phase sintering occurred by increasing eutectic phase transformation temperatures to 1400°C, which increased the efficiency of VCS. On the other hand, increasing sintering time from 1 to 3 h decreased the volume fraction of α-Fe and increased the volume fraction TiB₂ phase.     

Improvements in mechanical properties of TiB2 by the dispersion of B4C particles

Densities up to 99% of the theoretical value were achieved by hot-pressing of TiB₂-B₄C composites at 1700° C for 1 h using 1 vol % Fe as a sintering aid. The microstructure consists of dispersed B₄C particles in a fine-grained TiB₂ matrix. Addition of B₄C particles increases the fracture toughness of TiB₂ (to 7.6 MPa m^1/2 at 20 vol % B₄C) and yields high fracture strength (to 700 MPa at 10 vol % B₄C). Microstructural observations indicate that the improved strength is a result of a higher density, smaller grain size and intergranular fracture, and the toughness increase is a result of crack deflection around the B₄C particles.     

Effect of the filament nature on fatigue crack growth in titanium based composites reinforced by boron,B(B4C) and SiC filaments

Crack propagation testing has been applied to synthetic metal matrix composites (MMC) in order to compare failure mechanisms in Ti-6Al-4V alloy reinforced by uncoated boron, B(B₄C) and chemical vapour deposition (CVD) SiC filaments. The impeding effect of the fibres leads to low crack growth rates, compared to those reported for the unreinforced Ti-6Al-4V alloy and to higher toughness despite the presence of the reinforcing brittle phases. After long isothermal exposures at 850° C, the MMC crack growth resistance is reduced mainly due to fibre degradation, fibre-matrix debonding and an increase in matrix brittleness. However, for short-time isothermal exposures (up to about 10 h for B/Ti-6Al-4V, 30 h for B (B₄C)/Ti-6Al-4V and 60 h for SiC/Ti-6Al-4V) the crack growth resistance is significantly increased. This improvement is related to the build up of an energy-dissipating mechanism by fibre microcracking in the vicinity of the crack tip. This damaging mechanism allowing matrix plastic deformation is already effective for boron and B(B₄C) in the as-fabricated state, but occurs only after 10 h of thermal exposure at 850° C in the case of SiC/Ti-6Al-4V composites.     

Fabrication and microstructural evaluation of ZrB2/ZrC/Zr composites by liquid infiltration

The microstructure of ZrB₂/ZrC/Zr composites was examined using scanning electron microscopy, optical microscopy, and X-ray diffraction techniques. Dense ZrB₂/ZrC/Zr composites could be fabricated by the reaction sintering of molten zirconium with ZrB₂ preform. The composites were made by infiltration of molten zirconium into ZrB₂ preform, which contained 0–40 vol% B₄C, at 1900C for 10 min. The average grain size of ZrB₂ in the reaction-sintered composites decreased slightly with an increase in the volume fraction of the B₄C. The volume fraction of the solid increased with further increase of B₄C contents. The mechanical properties were measured in accordance with B₄C contents. The composites exhibited a four-point bending strength of up to 570 MPa and a fracture toughness of up to 11.5 MPa m^1/2.     

Effects of Manufacturing Processes on Microstructure and Properties of Al/A356–B4C Composites

Liquid and semi-solid stir casting processes were applied to fabricate B₄C particles-reinforced aluminum–matrix composites. The effects of manufacturing processes on particle distribution, particle/matrix interface, and mechanical properties of the prepared composites were studied. The results show that particle distribution can be significantly improved by using K₂TiF₆–flux and Ti powders in the liquid stir casting process, whereas in the semi-solid stir casting process it could be improved by decreasing the temperature of the slurry. With additions of Ti, the decomposition of B₄C was prevented, and the interfacial bonding strength was significantly improved due to the fact that a TiB₂ layer formed at the particle/matrix interface. Compared to the matrix, the hardness and tensile strength of the Al–B₄C composite fabricated by the liquid stir casting process were increased by 89.6% and 128.8%, respectively; those of the A356–B₄C composite fabricated by the semi-solid stir casting process had no significant improvement due to the weak particle/matrix interface and the presence of particle porosity clusters.     

Microstructure and mechanical properties of pulsed electric current sintered B4C-TiB2 composites

Monolithic B₄C, TiB₂ and B₄C-TiB₂ particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B₄C-TiB₂ composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000 °C. Fully dense B₄C-TiB₂ composites were achieved by PECS for 4 min at 2000 °C when applying the maximum external pressure of 60 MPa after volatilization of the oxide impurities, whereas a relative density of 95-97% was obtained when applying the external pressure below 2000 °C. Microstructural analysis showed that B₄C and TiB₂ grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60 vol% TiB₂ composite, combining a Vickers hardness of 29 GPa, a fracture toughness of 4.5 MPa m^1/2 and a flexural strength of 867 MPa, as well as electrical conductivity of 3.39E+6 S/m.     

Mechanical properties of three-dimensional interconnected alumina/steel metal matrix composites

Three-dimensional interconnected alumina/steel metal matrix composites (MMCs) were produced by pressureless Ti-activated melt infiltration method using three types of Al₂O₃ powder with different sizes and shapes. By partial sintering during infiltration an interpenetrating ceramic network was realised. The effect of the ceramic particle size and shape on the resulting ceramic network, volume % fraction and the MMC properties is presented. The MMCs were characterised for mechanical properties at room temperature and elevated temperature. An increase in flexural strength and Young’s modulus with decreasing particle size has been observed. In addition, the effect of the volume of ceramic content and the surface finish of the MMCs on the wear behaviour is shown.