Effect of environment atmosphere on thermal shock resistance of the ZrB2–SiC–graphite composite

The thermal shock resistance of the ZrB₂–SiC–graphite composite was evaluated by measuring the retention of the flexural strength after the electrical resistance heating to the temperature ranging from 1000 °C up to 2500 °C. The experiment was operated in two different environment atmospheres (pure oxygen and low oxygen partial pressure which mixed O₂ and Ar with 1:9) at total pressure 2000 Pa. The residual strength for the specimen decreased gradually as the temperature increased up to 2200 °C, and it was slightly higher when heated in low oxygen partial pressure environment than in pure oxygen. In contrast to the specimen heated in low oxygen partial pressure environment, the residual strength for the specimen in pure oxygen increased steeply as the temperature increased from 1600 °C up to 1800 °C. The analysis of the SEM observations combined with EDS confirmed that the surface oxidation played a positive role in the thermal shock resistance of the ZrB₂–SiC–graphite composite with different environment atmospheres. The results here pointed out a potential method for charactering the effect of environment atmosphere on thermal shock resistance of the ZrB₂–SiC–graphite composite.     

In situ joining of titanium to SiC/Al composites by low pressure infiltration

A method of in situ joining of titanium to SiC/Al composites by low pressure infiltration was proposed. The effect of infiltration temperature on microstructure and bending strength of in situ joining composites was investigated and the best infiltration temperature was confirmed to be 710 °C. The interfacial region of SiC/Al/Ti composites was consisted of Ti substrate, Al–Ti interfacial layer, Al layer and SiC/Al composite. The bending strength of SiC/Al composites kept nearly constant as the infiltration temperature changed while that of SiC/Al/Ti composites was influenced significantly by the infiltration temperature. The fracture occurred at the Al–Ti and Al–SiC/Al interfaces alternately as infiltrated at 670 °C. But as the infiltration temperature was increased to 710 °C, the fracture occurred only at the Al–SiC/Al interface which shows a great interfacial bonding at the Al–Ti interface. The formation of Al–Ti brittle intermetallics and the effect of crystallization and grain coarsening are two possible reasons which lead to the decrease of bending strength when the infiltration temperatures were increased from 710 °C to 730 °C.     

The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints

Silica carbide modified carbon cloth laminated C–C composites have been successfully joined to lithium–aluminum–silicate (LAS) glass–ceramics using magnesium–aluminum–silicate (MAS) glass–ceramics as interlayer by vacuum hot-press technique. The microstructure, mechanical properties and fracture mechanism of C–C/LAS composite joints were investigated. SiC coating modified the wettability between C–C composites and LAS glass–ceramics. Three continuous and homogenous interfaces (i.e. C–C/SiC, SiC/MAS and MAS/LAS) were formed by element interdiffusions and chemical reactions, which lead to a smooth transition from C–C composites to LAS glass–ceramics. The C–C/LAS joints have superior flexural property with a quasi-ductile behavior. The average flexural strength of C–C/LAS joints can be up to 140.26 MPa and 160.02 MPa at 25 °C and 800 °C, respectively. The average shear strength of C–C/LAS joints achieves 21.01 MPa and the joints are apt to fracture along the SiC/MAS interface. The high retention of mechanical properties at 800 °C makes the joints to be potentially used in a broad temperature range as structural components.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrB2 composites

C/SiC–ZrB₂ composites prepared via precursor infiltration and pyrolysis (PIP) were treated at high temperatures ranging from 1200 °C to 1800 °C. The mass loss rate of the composites increased with increasing annealing temperature and the flexural properties of the composites increased initially and then decreased reversely. Out of the four samples, the flexural strength and the modulus of the specimen treated at 1400 °C are maximal at 216.9 MPa and 35.5 GPa, suggesting the optimal annealing temperature for mechanical properties is 1400 °C. The fiber microstructure evolution during high-temperature annealing would not cause the decrease of fiber strength, and moderate annealing temperature enhanced the thermal stress whereas weakened the interface bonding, thus boosting the mechanical properties. However, once the annealing temperature exceeded 1600 °C, element diffusion and carbothermal reduction between ZrO₂ impurity and carbon fibers led to fiber erosion and a strong interface, jeopardizing the mechanical properties of the composites. The mass loss rate and linear recession rate of composites treated at 1800 °C are merely 0.0141 g/s and 0.0161 mm/s, respectively.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.     

High-velocity impact damage behavior of plain-woven SiC/SiC composites after thermal loading

This study investigates characteristics of foreign-object damage in plain-woven SiC/SiC composites after thermal loading. High-speed impact tests were conducted on virgin specimens, thermally exposed specimens, and thermally shocked specimens, in which the maximum temperature during thermal loading was 600 °C or 1000 °C. An oxide layer was generated on the specimen surface by thermal loading at 1000 °C. Damaged areas on the front and back surfaces induced by particle impact were independent of thermal loading. However, in specimens thermally loaded at 1000 °C, brittle failure, i.e. cone cracking without fiber pull-out, occurred due to oxidation of the fiber/matrix interfaces, and the ballistic limit velocity significantly decreased. Finally, the ballistic limit is predicted using static strength properties, and the effect of thermal loading on impact resistance is discussed.     

Analysis of tensile strength and microstructure of Ni–SiC composites prepared by electroforming

Ni–SiC metal matrix composites with two kinds of SiC content were prepared by electroforming in a nickel sulphamate bath. Tensile strength and microstructure of the composites before and after heat treatment were investigated. The maximum of tensile strength was obtained after heat treatment at 300 °C × 24 h. The values were 641 N/mm² and 701 N/mm² respectively. The complete reaction between nickel and SiC particles can produce shrinkage pores in the interface. The volume of shrinkage pores was equal to 8% of the volume of SiC particles in the composites. The interfacial reaction products were composed of Ni₃Si and a little amount of Ni₃₁Si₁₂ after heat treatment at 600 °C × 24 h. The fracture evolution went though microcracks initiation, growth and coalescence. Cracking of the matrix, debonding of Ni–SiC interfaces and cracking of particles were three types of cracking modes for Ni–SiC composites.     

Effects of copper addition on the in-situ synthesis of SiC particles in Al–Si–C–Cu system

In this research work, SiC particles have been successfully in-situ synthesized in Al–Si–Cu matrix alloy utilizing a novel liquid–solid reaction method. The effect of copper addition on the synthesis of SiC in Al–Si–C–Cu system was investigated. The composites mainly contain spherical SiC particles and θ-Al₂Cu eutectic phases, which are embedded in the α-Al matrix. Results indicated that the temperature for forming in-situ SiC particles significantly reduced from 750 °C to 700 °C with the copper addition. The size of in-situ synthesized SiC particles can be as low as 0.2 μm. Further study found that the addition of 10 wt.% copper into Al–Si–C alloy causes its solidus temperature to decrease by about 65 °C. Additionally, the Rockwell hardness value of SiCp/Al–18Si–5Cu composites has an average of 92, which is 50% higher than that of the sample without copper addition.     

Tyranno ZMI fiber/TiSi2–Si matrix composites for high-temperature structural applications

A Tyranno ZMI fiber/TiSi₂–Si matrix composite was fabricated via melt infiltration (MI) of a Si–16at%Ti alloy at 1375 °C under vacuum. The Si–Ti alloy was used as an infiltrant to conduct MI processing below 1400 °C and inhibit the strength degradation of the amorphous SiC fibers. The alloy matrix formed was dense and comprised primarily of TiSi₂–Si eutectic structures. The TiSi₂–Si matrix composite melt-infiltrated at 1375 °C showed a pseudo-plastic tensile stress–strain behavior followed by final fracture at ∼290 MPa and ∼0.9% strain. When the MI temperature was increased to 1450 °C, however, substantial reduction in the stiffness and ultimate strength occurred under tensile loading. Microstructural observations revealed that these degradations were attributed to the damages that occurred on the reinforcing fibers and pyrolytic carbon interfaces during the MI process. The present experimental results clearly demonstrated the effectiveness of the low-temperature MI process in strengthening Tyranno ZMI fiber composites and reducing the processing cost.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.     

Effect of supercritical water applied to reactor on microstructure and flexural strength of ZrB2–SiC–Graphite ceramic

The ZrB₂–SiC–Graphite ceramic was immerged in the supercritical water for different times. The microcracks appeared on the surface of the specimen and the composition of the microcracks was confirmed by EDS analysis to be ZrO₂. The obvious corrosion of SiC and graphite flake on the surface of the specimen was not observed, which was attributed to the dissolution of the oxides of silicon and carbon (graphite) into the supercritical water. The corrosion of the specimen was accelerated as the pressure and temperature of the water increased. XPS analysis was carried out on the specimen corroded in water of 40 ± 1 MPa and 500 ± 10 °C for 75 min, and the significant peak of B 1s was also measured, indicating the presence of the ZrB₂ phase on the surface of the specimen. For the specimen immerged in all conditions, the Vickers’ hardness did not reduce, the fracture toughness was improved and the minimal strength of the immerged specimen was still higher than 90% of the original strength of 480 MPa, which indicated that the ZrB₂–SiC–Graphite ceramic has excellent resistance to corrosion of the supercritical water applied to reactor.     

Sintering and technological properties of alumina/zirconia/nano-TiO2 ceramic composites

The present work focuses on studying the effect of nano TiO₂ (0.0–25 mass%) on the sintering behavior and mechanical properties of alumina/zirconia ceramic composites. Al₂O₃–ZrO₂–TiO₂ oxides mixture was sintered at 1600 °C to obtain the desired composites. The sinterability and the technological properties of these ceramic composites, i.e. the sintering parameters and microhardness as well as thermal shock resistance were investigated. Moreover, phase composition and microstructure of the sintered bodies were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results revealed that nano TiO₂ is a beneficial component for alumina/zirconia ceramic composites. The batch containing 20 mass% TiO₂ exhibited the highest sintering and mechanical properties as well as resistance to thermal shock. The obtained microstructure exhibited high compacted ceramic matrix composites.     

Thermal expansion of hydroxyapatite between − 100 °C and 50 °C

Hydroxy apatite (HAp) ceramic was synthesized using traditional sintering. Dilatometric and lattice thermal expansion properties of a HAp ceramic were evaluated at temperatures of ? 100–50 °C. In that temperature range, the dilatometric thermal expansion coefficient and the lattice thermal expansion coefficient of the HAp ceramic were, respectively, 10.6 × 10^? 6/°C and 9.9 × 10^? 6/°C. Furthermore, thermal expansion properties of a human tooth were measured. The thermal expansion coefficient of the horizontal direction perpendicular to the growing direction of a tooth was 15.5 × 10^? 6/°C; that of the vertical direction along with the direction of tooth growth was 18.9 × 10^? 6/°C at the temperature range described above.     

Rapidly cured epoxy/anhydride composites: Effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite’s short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.     

Novel method to synthesize SiC nanowires and effect of SiC nanowires on flexural strength of Cf/SiC composite

The graphite fiber toughened silicon carbide (C_f/SiC) composite was annealed at 1500, 1600, 1700, 1750, 1800 and 1850 °C for 30 min and the flexural strength increased first and then decreased as annealing temperature increased. The maximum flexural strength was obtained for the sample annealed at 1750 °C. The microstructure observation indicated that the amount and length of in situ formed SiC nanowires increased as the annealing treatment time increased, which was very favorable to the improvement of flexural strength. The formation of SiC nanowires was attributed to the presence of the metal La and Al. The vapor–liquid–solid growth mechanism of the SiC nanowires along 〈1 1 1〉 direction is proposed. Furthermore, the purpose of this paper is to report novel method to synthesize SiC nanowires.     

Properties of polypropylene composites containing aluminum/multi-walled carbon nanotubes

Polypropylene/aluminum–multi-walled carbon nanotube (PP/Al–CNT) composites were prepared by a twin-screw extruder. The morphology indicates that the CNTs are well embedded or implanted within Al-flakes rather than attached on the surface. During preparation of composites, the CNTs came apart from Al–CNT so that free CNTs as well as Al–CNT were observed in PP/Al–CNT composite. The crystallization temperatures of PP/CNT and PP/Al–CNT composites were increased from 111 °C for PP to 127 °C for the composites. The decomposition temperature increased by 55 °C for PP/CNT composite and 75 °C for PP/Al–CNT composite. The PP/Al–CNT composite showed higher thermal conductivity than PP/CNT and PP/Al-flake composites with increasing filler content. PP/Al–CNT composites showed the viscosity values between PP/CNT and PP/Al-flake composites. PP/Al–CNT composite showed higher tensile modulus and lower tensile strength with increasing filler content compared to PP/CNT and PP/Al-flake composites.     

Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)_n composites with a density of 1.43 g/cm³ are 204.4 MPa and 3028 kJ/m³ respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm³. The enhanced mechanical properties of C/(PyC–SiC)_n composites are attributed to the presence of multilayered (PyC–SiC)_n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)_n composite is a promising structural material with low density and high flexural strength and toughness.     

A new hydroxyapatite-based biocomposite for bone replacement

Since the 1970s, various types of ceramic, glass and glass–ceramic materials have been proposed and used to replace damaged bone in many clinical applications. Among them, hydroxyapatite (HA) has been successfully employed thanks to its excellent biocompatibility. On the other hand, the bioactivity of HA and its reactivity with bone can be improved through the addition of proper amounts of bioactive glasses, thus obtaining HA-based composites. Unfortunately, high temperature treatments (1200 °C ÷ 1300 °C) are usually required in order to sinter these systems, causing the bioactive glass to crystallize into a glass–ceramic and hence inhibiting the bioactivity of the resulting composite. In the present study novel HA-based composites are realized and discussed. The samples can be sintered at a relatively low temperature (800 °C), thanks to the employment of a new glass (BG_Ca) with a reduced tendency to crystallize compared to the widely used 45S5 Bioglass®. The rich glassy phase, which can be preserved during the thermal treatment, has excellent effects in terms of in vitro bioactivity; moreover, compared to composites based on 45S5 Bioglass® having the same HA/glass proportions, the samples based on BG_Ca displayed an earlier response in terms of cell proliferation.     

Preparation and thermal performance analysis of novel multilayer cladding structure composites

With regard to the adiabatic principle of insulation, a novel multilayer cladding structure composites (MCSC) with vacuum inside was put forward, which could be used in high temperature insulation field. In the composites, SiO₂ was used to fill the microcracks and protect the carbon matrix from oxidizing. This novel material was composed of two parts, one was the core material consisted of SiC foam ceramic, the other was the flawless outer shell consisted of carbon fiber reinforced composites with vacuum inside that produced by Chemical Vapor Infiltration (CVI) Pyrolytic Carbon (PyC) and silicasol-infiltration–sintering methods. Material density was 0.81 g/cm³. The effective thermal conductivity of MCSC ranged from 0.193 W/m · K to 0.721 W/m · K within the temperature from 303 K to 703 K, which was 13.5–23.3% lower than the value of SiC ceramic foam core materials. However, at 1473 K, the measured data of MCSC and SiC foam were 1.815 W/m · K and 1.911 W/m · K, respectively. It was only 5.02% lower than that of SiC foam.