Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Effect of Carbon-Containing Conducting Fillers on the Properties of Epoxy Coatings

We have established that antistatic coatings based on EP-0010 putty and graphite have low adhesion to metal and breaking strength but are resistant to the action of an operating environment. The use of carbon black as a conducting additive increases 1.5–3 times the strength of free films obtained on the basis of the given epoxy composition as compared with graphite and decreases their mass losses in an operating environment. The adhesion of epoxy coatings based on the EP-0010 composition modified by carbon black is 2–2.5 times as great as that of coatings with a graphite filler. The use of a plasticizer based on epoxidated vegetable oil instead of an inert phthalate plasticizer in a carbon-containing epoxy composition is promising and, in the case of optimal content of the components, enables one to enhance significantly the physicomechanical characteristics of antistatic epoxy coatings and their resistance to the action of the operating environment. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 2, pp. 109–113, March–April, 2005.     

Effect of mechanical activation procedure on the phase composition of gypsum

This paper examines the effect of mechanical activation by impact and attrition milling in planetary and ball ring mills on the phase composition of gypsum. Partially dehydrated powders obtained without heat treatment contain up to 60–70% β-CaSO₄ · 0.5H₂O (impact milling) and up to 40–47% α-CaSO₄ · 0.5H₂O (attrition milling). On the addition of a set retarder (maleic acid + poly(vinyl alcohol)), such materials acquire high strength during hardening: up to 11–17 MPa in compression and 7–12 MPa in bending.     

Dependence of the structure and properties of unfilled and filled compositions based on binary mixtures of hardenable thermoreactive resins on their formula

Crosslinked materials with different phase structures can be produced in hardening of the binary mixtures of unsaturated polyester resins or mixtures of epoxy resins. The dependences of the mechanical and electrophysical properties on the formula of such mixtures have been determined. The properties of filled compositions based on these mixtures with a two-phase structure, which have been produced by selective introduction of the filler only into one phase of the composition, significantly differ from the properties of filled single-phase mixtures and from the properties of two-phase mixtures produced with a uniform distribution of the filler in both phases. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 5, pp. 93–100, September–October, 2005.     

Effect of treated filler loading on the photopolymerization inhibition and shrinkage of a dimethacrylate matrix

This study shows how treated filler loading influences the photopolymerization of a dimethacrylate comonomer mixture, regarding, in particular, shrinkage and inhibition under atmospheric oxygen, present in oral environment. Bis-GMA/TEGDMA (75/25 wt.%) resins were loaded with hybrid filler (Ba aluminosilicate glass and pyrogenic silica), treated with γ-methacryloxy(propyl)trimethoxysilane, at 0–50 wt.% and light cured over a total of 30 s (45 mW/cm²). Degree of double-bond conversion (DC), obtained using FTIR, decreased with filler content. ¹H MAS spectra (293–340 K) and STRAFI images (293 K) were obtained as a function of irradiation time and filler concentration. ¹H signals of unreacted methacrylate groups were more intense for higher loaded resins and resonances from –CH₂SiO₂(OH) (T²) and –CH₂SiO₃– (T³) units, also observed by ²⁹Si NMR, were resolved and suggest the presence of T²–resin bonds. 1D images show a reduction on polymerization contraction and reaction inhibition at the composite resin surface with filler loading. 2D resin images present a highly mobile surface layer, hence with lower DC.     

Effect of acid and TETA modification on mechanical properties of MWCNTs/epoxy composites

Acid treatment and triethylene-tetramine (TETA) modification of multi-walled carbon nanotubes (MWCNTs) purposing to attain better dispersibility and stronger interfacial bonding between MWCNTs and epoxy matrix have been carried out in this paper. The epoxy and MWCNTs/epoxy composites were produced by cast molding method. Stress–strain curves show that TETA-MWCNTs/epoxy hold the greatest toughness of all samples with 0.5 wt.% nanoparticles. The Young’s modulus of TETA-MWCNTs/epoxy has a significant increase about 38% compared to the neat epoxy, while the Young’s modulus of unmodified MWCNTs/epoxy or acid-modified MWCNTs/epoxy has a bit of decrease. Tensile and impact strength tests reflect that TETA-MWCNTs reinforced epoxy composites have an obvious improvement of tensile strength about 30% and an enhancement of impact strength over 34% compared to the pure epoxy composites with only 0.5 wt.% loading of TETA-MWCNTs. Scanning electron microscopy images of fractured surface of MWCNTs/epoxy indicate homogeneous dispersibility of TETA-MWCNTs and strong interfacial adhesion between the TETA-MWCNTs and the epoxy in the MWCNTs/epoxy composite.     

Precipitation of MC phase and precipitation strengthening in hot rolled Nb–Mo and Nb–Ti steels

Hot rolled Nb–Mo steel of yield strength 600 MPa and Nb–Ti steel of yield strength 525 MPa with polygonal and acicular ferrite microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (<10 nm in size) of Nb–Mo steel was 58.4%, and higher than that of Nb–Ti steel with 30.0%. Therefore, the results of strengthening mechanisms analysis showed the higher strength of Nb–Mo steel than that of Nb–Ti steel is attributed to its relatively more prominent precipitation strengthening effect. The yield strength increments from precipitation hardening of Nb–Mo steel attained 182.7 MPa and higher than that of Nb–Ti steel.     

Influence of Surface-Modified Conducting Fillers on the Properties of Epoxy Coatings

We have developed a new epoxy composition for antistatic coatings on metallic surfaces, filled with technical carbon and talc, which had been surface-modified by paranitroaniline and phosphoric acid. It has been shown that, with the introduction of modified fillers, the content of gel fraction in an epoxy coating increases by 5–6%, its adhesion becomes 2.5–2.7 times higher, and the amount of organic solvents decreases. We have also established that the electric resistance of the coating is more stable in time as compared with the commercial antistatic ground-coat enamel, and the frequency dependence of the resistance is more clearly pronounced after exposure to a chloride-containing medium, which corroborates the better protective properties of antistatic coatings with modified current conductive fillers. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 53–58, July–August, 2005.     

Development in high-grade dual phase steels with low C and Si design

Cold rolled dual phase steels with low C and Si addition were investigated in terms of combination of composition and processing in order to improve mechanical properties and workability including welding and galvanizing. Mo and Cr could be used as alloying elements to partially replace C and Si to assure enough hardening ability of the steels and also give solute-hardening. Mo addition is more effective than Cr addition in terms of obtaining the required volume fraction of martensite and mechanical strength. The ferrite grain was effectively refined by addition of Nb microalloying, which gives optimized mechanical properties. The experimental results show that it is possible to obtain the required mechanical properties of high grade 800 MPa dual phase steel, i.e., tensile strength>780 MPa, elongation>15%, and yield/tensile strength ratio<0.6 in the condition of low carbon (C<0.11 wt.%) and low silicon design (Si<0.05 wt.%) through adequate combination of composition and processing.     

The effect of multiwall carbon nanotubes on the in-plane shear behavior of epoxy glass fiber reinforced composites

In this study, we investigate how multi-wall carbon nanotubes (MWCNTs) affect the in-plane shear mechanical behavior of glass fiber/epoxy composite. These multi-scale composites are fabricated using vacuum infusion: pristine MWCNT and amino-functionalized MWCNT are incorporated into epoxy resins at concentrations of both 0.1 and 0.3 wt.% and are subsequently evaluated. The MWCNT are mixed into the resins by mechanical stirring and sonication prior to resin infusion, and the MWCNT distribution in the cured laminate is then evaluated by performing a heat conduction assessment. Monotonic and cyclic quasi-static room temperature in-plane shear tests are performed following the ASTM D 4255 standard. The initial shear modulus, the deterioration of the shear modulus during plastic deformation and material hardening are evaluated. Incorporating MWCNT into the resins did not affect the parameters investigated under the imposed conditions.     

Synthesis and properties of sandwiched films of epoxy resin and graphene/cellulose nanowhiskers paper

Graphene (GN)-based composite paper containing 10 wt.% cellulose nanowhiskers (CNWs) exhibiting a tensile strength of 31.3 MPa and electrical conductivity of 16 800 S/m was prepared by ultrasonicating commercial GN powders in aqueous CNWs suspension. GN/CNWs freestanding paper was applied to prepare the sandwiched films by dip coating method. The sandwiched films showed enhanced tensile strength by over two times higher than the neat resins. The moduli of the sandwiched films were around 300 times of the pure resins due to the high content of GN/CNWs paper. The glass transition temperature of the sandwiched films increased from 51.2 °C to 57.1 °C for pure epoxy (E888) and SF (E888), and 49.8 °C to 64.8 °C for pure epoxy (650) and SF (650), respectively. The bare conductive GN/CNWs paper was well protected by the epoxy resin coating, which is promising in the application as anti-static materials, electromagnetic interference (EMI) shielding materials.     

Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interface applications

Graphite nanoplatelet (GNP)/rubbery epoxy composites were fabricated by mechanical mixer (MM) and dual asymmetric centrifuge speed mixer (SM). The properties of the GNP/rubbery epoxy were compared with GNP/glassy epoxy composites. The thermal conductivity of GNP/rubbery epoxy composite (25 wt.% GNP, particle size 15 μm) reached 2.35 W m⁻¹ K⁻¹ compared to 0.1795 W m⁻¹ K⁻¹ for rubbery epoxy. Compared with GNP/rubbery epoxy composite, at 20 wt.%, GNP/glassy epoxy composite has a slightly lower thermal conductivity but an electrical conductivity that is 3 orders of magnitude higher. The viscosity of rubbery epoxy is 4 times lower than that of glassy epoxy and thus allows higher loading. The thermal and electrical conductivities of composites produced by MM are slightly higher than those produced by SM due to greater shearing of GNPs in MM, which results in better dispersed GNPs. Compression and hardness testing showed that GNPs increase the compressive strength of rubbery epoxy ∼2 times without significantly affecting the compressive strain and hardness. The GNP/glassy epoxy composites are 40 times stiffer than the GNP/rubbery epoxy composites. GNP/rubbery epoxy composites with their high thermal conductivity, low electrical conductivity, low viscosity before curing and high conformability are promising thermal interface materials.     

Effect of silica nanoparticles on compressive properties of an epoxy polymer

The effect of nanosilica on compressive properties of an Epikote 828 epoxy at room temperature was studied. A 40 wt% nanosilica/epoxy masterbatch (nanopox F400) was used to prepare a series of epoxy based nanocomposites with 5–25 wt% nanosilica content. Static uniaxial compression tests were conducted on cubic and cylindrical specimens to study the compressive stress–strain response, failure mechanisms and damage characteristics of the pure and nanomodified epoxy. It was found that the compressive stiffness and strength were improved with increasing nanosilica content without significant reduction in failure strain. The presence of nanosilica improved ductility and promoted higher plastic hardening behaviour after yielding in comparison with the unmodified resin system. This result suggested that nanoparticles introduced additional mechanisms of energy absorption to enhance the compressive properties without reducing the deformation to failure.     

Microstructures and mechanical properties of Mg–8Gd–0.6Zr–<Emphasis Type="Italic">x</Emphasis>Nd (<Emphasis Type="Italic">x</Emphasis> = 0, 1, 2 and 3 mass%) alloys

Mg–8Gd–0.6Zr–xNd (x = 0, 1, 2 and 3 mass%) alloys were prepared by metal mould casting method, and the microstructures, age hardening responses and mechanical properties have been investigated. The microhardness of the as-cast alloys is increased with increasing Nd content. The age hardening behavior and mechanical properties are enhanced significantly by adding Nd element. The peak ageing hardness of the Mg–8Gd–0.6Zr–3Nd alloy is 103, it is about 1.3 times more than that of the Mg–8Gd–0.6Zr alloy. The aged Mg–8Gd–0.6Zr–3Nd alloy exhibits maximum ultimate tensile strength and yield strength, and the values are 271 and 205 MPa at room temperature, 205 MPa and 150 MPa at 250 °C, respectively. Which are about 2 times higher than those of Mg–8Gd–0.6Zr alloy. The improved hardness and strength are mainly attributed to the fine dispersiveness of Mg₅RE and Mg₁₂RE precipitates in the alloy.     

Structure and properties of high-modulus alloys of the Ti-B system

We study the influence of alloying with Al, Zr, and Si on the structure and mechanical properties (including the modulus of elasticity) of cast and deformed alloys of the Ti-B system. It is shown that, by optimizing the compositions of titanium alloys subjected to combined silicoboride hardening and additionally alloyed with aluminum, it is possible to get the modulus of elasticity E as high as 160 MPa with a strength σ of 1500 MPa and a level of plasticity δ of 2–5% at room temperature. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 27–32, May–June, 2006.     

Synergistic effects of oxidized CNTs and reactive oligomer on the fracture toughness and mechanical properties of epoxy

The soft modifiers added to improve the fracture toughness of epoxies generally deteriorate their mechanical properties. Hence, oxidized multi-walled carbon nanotubes (O-CNTs) were added to the epoxy modified with reactive oligomer. The NCO terminated reactive oligomer acts as a cross-linker between the O-CNTs and the OH groups of the epoxies. The impact strengths of the 15 wt.% oligomer modified epoxy containing 0.5 wt.% of O-CNTs at room temperature (RT) and cryogenic temperature (CT) are enhanced by 23.6% and 69.5% compared to that of the unmodified epoxy. In addition to increasing fracture toughness, the tensile strength (TS) of the modified epoxy/O-CNTs at CT is found to be 91.7 MPa, which is comparable to that of the unmodified epoxy (92.1 MPa). Hence, the attachment of O-CNTs to the reactive oligomer modified epoxy can be an efficient approach to toughen epoxy resins without compromising their tensile properties.     

Production of conversion oxide-ceramic coatings on zirconium and titanium alloys

We consider the possibilities of surface hardening of zirconium and titanium alloys with conversion oxide-ceramic coatings. These coatings have been produced by the method of plasma-electrolytic treatment in alkaline solutions. We have established that the plasma temperature in discharge spark channels reaches (6–9) · 10³ K. The thickness of the coatings is 100 to 120 and 30 to 40 μm, and their microhardness is ∼ 800 and ∼ 1000 MPa for zirconium and titanium alloys, respectively. The functional properties of the coatings depend on the synthesis conditions, including the electrolyte composition, the cathode and anode current densities, and also the treatment time. We have evaluated the thickness, microhardness, and wear resistance of the coatings under conditions of dry friction and cavitation as well as their fatigue strength and corrosion resistance. We have established that this treatment provides a high wear and corrosion resistance of the alloys under study with insignificant decrease in their fatigue strength. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, pp. 117–124, March–April, 2006.     

Incorporation of radioactive ion-exchange resins in a slag binder

The possibility of solidification of radioactive ion-exchange resins by their incorporation into a slag alkaline binder to obtain 200–l cement blocks was examined. The solidified products with the bentonite clay sorption additive with respect to their mechanical strength and radionuclide leachability meet the requirements to safe storage in both standard and simplest repositories, valid in Russia and other countries. The admissible degree of matrix filling with the resin is 18.2 wt %, and the diffusion leach rate of ¹³⁷Cs after the lapse of 30 days decreases to 1 × 10⁻³ cm day⁻¹ and lower values.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.