Dynamic behavior of selected ceramic powders

An experimental setup has been developed on the continuous recording of the stress profiles in ceramic powders subject to shock loading with manganin gauges. A series of plate impact experiments on highly porous ceramic powders such as Al₂O₃, SiC and B₄C were conducted at the laboratory's single stage powder gun facility. The relationship between shock wave velocity and particle velocity was measured to obtain the Hugoniot data. Plate impact onto powder sample experiments were conducted at loading stresses ranging from 1.6 to 4.2 GPa. The experimental results show that the shock wave speeds in various ceramic powders vary between 1 and 2 km/s. Linear Hugoniot relations between shock velocity (D) and particle velocity (u) are observed. The loading stress–specific volume form of Hugoniot relations (P–V) was constructed using the data from quasistatic compression test results, Hopkinson bar dynamic compression test results and powder gun plate impact experiment results. The P–V diagram shows that the crush strength of ceramic powders is comparable to the loading stress level. The B₄C and SiC powders with bigger particle size more easily reach the solid state Hugoniot than the powders with smaller particle size at the same loading condition. In the case of Al₂O₃, the material shows less sensitivity to particle size difference at the same level of loading rate as compared to B₄C and SiC.     

Shock Wave Fabricated Ceramic-Metal Nozzles

Shock compaction was used in the fabrication of high temperature ceramic-based materials. The materials' development was geared towards the fabrication of nozzles for rocket engines using solid propellants, for which the following metal-ceramic (cermet) materials were fabricated and tested: B₄C-Ti (15 vol.-%), B₄C-Al, and TiB₂-Al, with an Al content typically between 15–20 vol.-%. Here, the B₄C-Ti was only shock-compacted, while the other two cermets were shock compacted followed by melt infiltration with Al.The materials were subjected to gradually more severe testing conditions. Slabs of the materials were first tested for thermal shock resistance in an acetylene flame, followed by testing in the exhaust gas stream of a rocket propellant, and thereafter as a cylindrical insert in a nozzle of TZM alloy. The B₄C-Ti composite showed erosion and cracking after the first test in the propellant flame, while the B₄C-Al composite failed the insert tests. The TiB₂-Al composite performed well under all conditions. A venturi nozzle of that material was formed during compaction. This real, shaped nozzle was shown to function well, even during repeated 3–6 s tests. This could be explained by the resistance of TiB₂ to molten Al, the high thermal conductivity of the TiB₂-Al cermet and the in situ formation of a protective layer, consisting mainly of Al₂O₃.     

Shock and release of polycarbonate under one-dimensional strain

The behaviour of the thermoplastic polycarbonate has been investigated using manganin stress gauges in both longitudinal and lateral orientations. These have been used to determine the shock stress, shock velocity, particle velocity, release velocity and shear strength. The relationship between shock velocity and particle velocity has been shown to be linear, with the value of c₀ (the zero particle velocity intercept of shock velocity) equating to the measured bulk sound speed. This behaviour is more commonly observed in metals. Shear strength has been observed to increase behind the shock front, a feature observed in other polymers such as PMMA or PEEK. It also increases with stress amplitude, although the projected intercept with the calculated elastic response indicates that the Hugoniot elastic limit (HEL) is lower than in other polymers, for example PMMA (ca. 0.75GPa) or PEEK (ca. 1.0GPa). This further suggests that the yield strength of polycarbonate does not obey a Mohr-Coloumb criterion, and hence is not as strongly pressure dependent as other polymers.     

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.     

Prediction of mechanical and wear properties of copper surface composites fabricated using friction stir processing

Friction stir processing (FSP) has evolved as a novel solid state method to prepare surface composites. In this work, FSP technique has been successfully applied to prepare copper surface composites reinforced with variety of ceramic particles such as SiC, TiC, B₄C, WC and Al₂O₃. Empirical relationships are developed to predict the effect of FSP parameters on the properties of copper surface composites such as the area of the surface composite, microhardness and wear rate. A central composite rotatable design consisting of four factors and five levels is used to minimize the number of experiments. The factors considered are tool rotational speed, traverse speed, groove width and type of ceramic particle. The effect of those factors on the properties of copper surface composites is analyzed using the developed empirical relationships and explained in this paper taking into account the microstructural characterization of the prepared copper surface composites. B₄C reinforced composites have higher microhardness and lower wear rate.     

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.     

Novel ceramic–polymer composites synthesized by compaction of polymer-encapsulated TiO2-nanoparticles

A novel processing route for producing composites from ceramic particles and a thermoplastic polymer with high ceramic content was developed. Via a radical emulsion polymerization reaction in an aqueous suspension, titanium dioxide is encapsulated by a thin layer of poly(methyl methacrylate). Subsequently, the coated particles are compacted by applying high pressure (∼1 GPa) at a temperature above the glass transition temperature of the polymer (∼160 °C). This technique enables producing dense, hard and stiff composites at low processing temperatures. Microstructural investigations of composites by scanning electron microscopy confirm successful coating of titanium dioxide particles by polymer. Compositions were estimated from thermogravimetric measurements. A maximum TiO₂ volume content of almost 70% was achieved. For characterizing mechanical properties, Vickers microhardness as well as flexural strength and elastic modulus were determined. With respect to pure PMMA, composites exhibit a 10-fold increase in microhardness. Furthermore, a strong increase in elastic modulus with TiO₂ contents, up to 40 GPa at 66 vol.% TiO₂ was observed. These moduli are among the highest found in literature for ceramic polymer composites. However, bending strength of the material is still low.     

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.     

Effect of titanium on microstructure and fluidity of Al–B<Subscript>4</Subscript>C composites

The effect of Ti on the interfacial reactions, microstructural characteristics, and the related fluidity of Al–12%B₄C composites has been investigated. Without Ti addition, B₄C decomposed heavily during holding, and a large quantity of reaction-induced compounds, Al₃BC and AlB₂, was generated. When Ti was added, a TiB₂ layer was built surrounding B₄C particle surfaces, which acted as a diffusion barrier to separate B₄C from liquid aluminum. Thus, the decomposition of B₄C slowed down remarkably. The fluidity of the composite without Ti was the shortest of all composites and deteriorated quickly during the holding time. The fluidity of the composite melt was improved significantly with increased Ti levels. The optimum Ti level for the best fluidity results lied between 1.0 and 1.5%. The solid particle volume and the particle agglomeration are the two main factors influencing the fluidity.     

Interface interaction in the (B4C + TiB2)/Cu system

Interface phenomena in the TiB₂/(Cu–B) and (B₄C + TiB₂)/(Cu–B) systems were investigated in order to determine conditions for cermet preparation by free infiltration. The wetting behavior of the two-phase ceramic substrate may be accounted for as a superposition of behavior patterns characteristic of the two ceramic phases. The relatively low wetting angles of the liquid Cu on the titanium diboride substrate is attributed to a minor departure of the ceramic phase composition from stoichiometry. Copper alloyed with above 8 at.% B yields contact angles less than 20° sufficient for fabrication of cermets based on the two-phase ceramic by free infiltration. The enhanced wetting and the absence of a new phase formation were confirmed by SEM and TEM analysis of the infiltrated cermets.     

B4C/Al Composites Processed by Metal-assisted Pressureless Infiltration Technique and its Characterization

In order to improve the wettability between Al melt and B₄C ceramic preform during fabricating B₄C/Al composites by pressureless infiltration technique, trace amount of Ti particulates with high melting point was added into the starting materials as infiltration inducer. A simple and cost-effective method, metal-assisted pressureless infiltration technique, was developed to fabricate light-weight B₄C/Al composites. The microstructure, phases, and mechanical behavior of B₄C/Al composites were characterized by SEM, XRD, and mechanical property test. The density of the as-fabricated B₄C/Al composites was about 2.75 g/cm³ and the relative density of this kind of composites was over 97%. The as-fabricated B₄C/Al composites exhibited rather well wear resistance. The flexural and compressive strengths of the as-fabricated B₄C/Al composites were about 200 MPa and 670 MPa, respectively.     

Nacre-inspired lightweight and high-strength AZ91D/Mg<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>w composites prepared by ice templating and pressureless infiltration

We developed a facile and low-cost approach to prepare lightweight and high-strength magnesium–matrix composites with a nacre-inspired laminated structure. First, lamellar Mg₂B₂O₅ whisker (Mg₂B₂O₅w) scaffolds with initial solid loadings of 10, 15 and 20 vol% were prepared by ice templating. The wettability between a molten AZ91D alloy and the Mg₂B₂O₅w scaffold was greatly improved by the incorporation of nano-SiO₂ sol in the aqueous slurry, making the preparation of nacre-mimetic AZ91D/Mg₂B₂O₅w composite by way of pressureless infiltration feasible. The SiO₂ content in the Mg₂B₂O₅w scaffold has a significant effect on the processing and the microstructure and properties of the composites. The optimum SiO₂ content was about 6–8 wt% of the total ceramic loading. A lower SiO₂ content resulted in incomplete infiltration, while a higher content led to the formation of a large quantity of Mg₂Si in the composite. The flexural strength of the composites seemed independent of the initial ceramic loading (10–20 vol%), whereas the compressive strength and elastic modulus increased considerably and the crack-growth fracture toughness decreased with increasing ceramic content. The mechanism for such variations was addressed.     

Properties of B4C/Al–B4C composite with a two-layer structure

In order to obtain a material with a promising bulletproof performance, a two-layer structure composite consisting of B₄C/Al-B₄C was obtained using a two-step method for both hot pressing and infiltration aluminum in vacuum. Before aluminum infiltration the B₄C porous layer of the two-layer preform looked like a three-dimensional network of interconnected capillaries. For the B₄C ceramics layer the microstructure showed no apparent change before and/or after aluminum infiltration. The two-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C-Al material.     

Synthesis,microstructure, and mechanical properties of boron suboxide materials doped with chromium boride

Improvement on the densification and fracture toughness of ceramic materials based on boron suboxide (B₆O) has been of great importance. The mechanical properties of B₆O with and without chromium boride addition, hot pressed at a temperature range of 1850–1900°C and pressures of 50–80?MPa for 20 minutes, were investigated. The relative density, phase relationship, microstructures and mechanical properties of the processed ceramics were examined. More than 96% of the theoretical density was obtained for both ceramic systems. A good combination of mechanical properties was obtained with the B₆O-CrB₂ material (H_V?=?32.1?GPa, K_IC?=?4.5?MPa?·?m^0.5) compared to the pure B₆O material (H_V?=?30.5?GPa, K_IC?=?brittle). The addition of 1.7?wt.% CrB₂ resulted in a pronounced improvement in both the hardness and fracture toughness values. Crack bridging and deflection are some of the toughening mechanisms liable for the enhanced fracture toughness of the sintered ceramic materials.     

Response of boron carbide subjected to high-velocity impact

This article presents an evaluation of the response of boron carbide (B₄C) subjected to impact loading under three different conditions. Condition A is produced by plate-impact experiments where the loading condition is uniaxial strain and the stresses and pressures are high. Under plate-impact loading the material fails at the Hugoniot Elastic Limit (HEL) and the failed material undergoes high confining pressures and relatively small inelastic strains. Condition B is produced by projectile impact onto thick targets where the stresses and pressures are dependent on impact velocity, but they are generally lower than those from plate impact. Under thick-target impact/penetration most of the material fails under compression, the inelastic strains are large and the material appears to exhibit more ductility than under condition A. Lastly, condition C is produced by projectile impact and perforation of thin targets where the stresses and pressures are a combination of compression and tension. Under thin-target perforation the material fails in both tension and compression. The Johnson–Holmquist–Beissel (JHB) constitutive model is used to evaluate the material behavior for each of the three conditions, but it is not possible to accurately reproduce the experimental results of the three conditions with a single set of constants. Instead, three different sets of constants are required to accurately model the three impact conditions. These three models/constants are used to provide insight into the complex response of B₄C, and to identify possible mechanisms that are not included in the JHB model.     

Influence of B4C on the tribological and mechanical properties of Al 7075–B4C composites

In the present investigation, the influence of B₄C on the mechanical and Tribological behavior of Al 7075 composites is identified. Al 7075 particle reinforced composites were produced through casting, K₂TiF₆ added as the flux, to overcome the wetting problem between B₄C and liquid aluminium metal. The aluminium B₄C composites thus produced were subsequently subjected to T6 heat treatment. The samples of Al 7075 composites were tested for hardness, tensile, compression, flexural strengths and wear behavior. The test results showed increasing hardness of composites compared with the base alloy because of the presence of the increased ceramic phase. The wear resistance of the composites increased with increasing content of B₄C particles, and the wear rate was significantly less for the composite material compared to the matrix alloy. A mechanically mixed layer containing oxygen and iron was observed on the surface, and this acted as an effective insulation layer preventing metal to metal contact. The coefficient of friction decreased with increased B₄C content and reached its minimum at 10 vol% B₄C.     

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.     

Sintering behaviour and mechanical properties of Cr3C2–NiCr cermets

Cr₃C₂–NiCr cermets are used as metal cutting tools due to their relatively high hardness and low sintering temperatures. In this study, a powder mixture consisting of 75 wt% Cr₃C₂–25 wt% NiCr was sintered at four different temperatures and characterized for its microstructure and mechanical properties. The highest relative density obtained was 97% when sintered at 1350 °C. As the relative density increased, elastic modulus, transverse rupture strength, fracture toughness and hardness of the samples reached to a maximum of 314 GPa, 810 MPa, 10·4 MPa·m^1/2 and 11·3 GPa, respectively. However, sintering at 1400 °C caused further grain growth and pore coalescence which resulted in decreasing density and degradation of all mechanical properties. Fracture surface investigation showed that the main failure mechanism was the intergranular fracture of ceramic phase accompanied by the ductile fracture of the metal phase which deformed plastically during crack propagation and enhanced the fracture toughness.     

Preparation of B4C-ZrB2-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Preparation of B<Subscript>4</Subscript>C-ZrB<Subscript>2</Subscript>-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.