Preparation and characterization of high-toughness ZrB2/Mo composites by hot-pressing process

ZrB₂/Mo composites with various volume fractions of Mo (0–10%) were prepared by a hot-pressing process at 1950 °C. The addition of Mo both facilitated the densification and improved the mechanical properties of the ZrB₂/Mo composites. Compared with the pure ZrB₂ ceramics, the fracture toughness of the ZrB₂/Mo (10%) composite increased from 4.52 to 7.98 MPa m^1/2 and its bending strength increased from 424 to 450 MPa. The enhancement in mechanical properties of the ZrB₂/Mo composites could be attributed to the reduced grains and the MoB phase formed in the reaction between ZrB₂ and Mo.     

Investigation on synthesis, pressureless sintering and hot pressing of chromium diboride

Experimental investigations were carried out on synthesis of chromium diboride through boron carbide reduction of Cr₂O₃. The products obtained were characterized by X-ray diffraction and the process optimized to prepare single-phase CrB₂ powder. Densification of CrB₂ was investigated by pressureless sintering and hot pressing. A maximum of 93% ρ_th was obtained by pressureless sintering at 1850 °C after a prolonged duration of 360 min. However, near theoretical density was achieved by hot pressing at 1600 °C and 35 MPa pressure for 2 h. The hardness and fracture toughness of fully dense CrB₂ was measured as 22 GPa and 3.5 MPa m^1/2, respectively. The mode of fracture in pressureless sintered samples is intergranular whereas that in hot pressed is transgranular.     

Thermo-physical and thermal cycling properties of plasma-sprayed BaLa2Ti3O10 coating as potential thermal barrier materials

Increased turbine inlet temperature in advanced turbines has promoted the development of thermal barrier coating (TBC) materials with high-temperature capability. In this paper, BaLa₂Ti₃O₁₀ (BLT) was produced by solid-state reaction of BaCO₃, TiO₂ and La₂O₃ at 1500 °C for 48 h. BLT showed phase stability between room temperature and 1400 °C. BLT revealed a linearly increasing thermal expansion coefficient with increasing temperature up to 1200 °C and the coefficients of thermal expansion (CTEs) are in the range of 1 × 10^− 5–12.5 × 10^− 6 K^− 1, which are comparable to those of 7YSZ. BLT coatings with stoichiometric composition were produced by atmospheric plasma spraying. The coating contained segmentation cracks and had a porosity of around 13%. The microhardness for the BLT coating is 3.9–4.5 GPa. The thermo-physical properties of the sprayed coating were investigated. The thermal conductivity at 1200 °C is about 0.7 W/mK, exhibiting a very promising potential in improving the thermal insulation property of TBC. Thermal cycling result showed that the BLT TBC had a lifetime of more than 1100 cycles of about 200 h at 1100 °C. The failure of the coating occurred by cracking at the thermally grown oxide (TGO) layer due to severe oxidation of bond coat. Based on the above merits, BLT could be considered as a promising material for TBC applications.     

Electrochemical performances of Ag–(Bi2O3)0.75(Y2O3)0.25 composite cathodes

The electrochemical performances of Ag–Y_0.25Bi_0.75O_1.5 (YSB) composite cathodes on Ce_0.8Sm_0.2O_1.9 (SDC) electrolytes have been investigated at intermediate temperature using AC impedance spectroscopy. The results indicated that the electrochemical performances of these composites are quite sensitive to the compositions and the microstructures of the cathode. The optimum YSB addition to Ag resulted in 10 times lower area specific resistance. The ASR of Ag-50 vol.% YSB was about 0.12 Ωcm² at 700 °C as compared to 3.9 Ωcm² for Ag cathodes. The observed high performance of Ag–YSB composite cathodes might be due to the high oxygen-ion conductivity of YSB and its high catalytic activity for oxygen reduction.     

Superplastic compressive flow in MgB2

Macroplasticity in the brittle, superconducting ceramic MgB₂ would allow for the mechanical drawing of thin, dense superconducting wires, as done for metallic superconductors. Here, we report very large uniaxial compressive deformation (engineering strain of 67% or true strain of −1.1) without fracture at 1000 °C for specimens densified from commercially available MgB₂ powders with MgO and MgB₄ second phases. Plastic flow occurs under a diffusion-controlled mechanism with activation energies of 255–447 kJ mol⁻¹ and stress exponents of 1.4–2.0, indicative of superplastic behavior.     

Combustion synthesis of TiC–TiB2 composites

An experimental study on formation of TiC–TiB₂ in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) using the reactant compacts from different combinations of Ti, B₄C, C, and B powders. Direct reaction of Ti with B₄C at stoichiometry of Ti:B₄C = 3:1 yields a TiB₂-rich composite with TiC:TiB₂ = 1:2. Formation of the products containing 20, 33.3, and 50 mol% of TiB₂ was achieved by the Ti–B₄C–C reactants. In addition, the test specimen composed of Ti, B₄C, and B was employed for the synthesis of a composite with 80 mol% TiB₂. Among three different types of the powder compacts, the boron-containing sample was characterized by the fastest combustion wave and the highest reaction temperature. The lowest combustion temperature and wave velocity were observed in the Ti–B₄C compact. When fine Ni particles were added to the Ti–B₄C reactant, it was found that the propagation rate of the reaction front was increased and the densification of the end product was enhanced significantly. This was attributed to formation of the Ti–Ni eutectic liquid during the reaction. As a result, the relative density of a TiC + 2TiB₂ composite increases from 30 to 86% with the Ni content from 0 to 20 mol%. Based upon the XRD analysis, small amounts of TiNi₃ and TiB were detected in the Ni-reinforced TiC–TiB₂ composites.     

Densification and oxidation behavior of a novel TiB2–MoSi2–CrB2 composite

Titanium diboride (TiB₂) composite with MoSi₂ and CrB₂ has been prepared and tested to possess excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM. Hardness and fracture toughness values were measured. Extensive oxidation studies of the composites were also carried out. Density of ≥ 96% ρ_th was obtained by hot pressing at 1800 °C under a pressure of 35 MPa for 1 h. Hardness and fracture toughness were in the range of 18–24 GPa and 3.5–4 MPa·m^1/2 respectively. Crack branching, deflection and bridging mechanisms were observed in the crack propagation paths. Isothermal and continuous oxidation studies of these composites up to 1000 °C showed improved oxidation resistance with the formation of protective glassy layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Diffusion controlled mechanism of oxidation was observed in the composites.     

Low temperature sintering and properties of CaO–B2O3–SiO2 system glass ceramics for LTCC applications

The P₂O₅ + ZnO, ZrO₂ + TiO₂, B₂O₃ and a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) are selected as the sintering additives, and the effect of their additions on the microwave dielectric properties, mechanical properties and microstructures of CaO–B₂O₃–SiO₂ system glass ceramics is investigated. It is found that the sintering temperature of pure CBS glass is higher than 950 °C and the sintering range is about 10 °C. With the above additions, the glass ceramics can be sintered between 820 °C and 900 °C. The dielectric properties of the samples are dependent on the additions, densification and microstructures of sintered bodies. The major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. With 10 wt% B₂O₃ and LG glass additions, the CBS glass ceramics have better mechanical properties, but worse dielectric properties. The _r values of 6.51 and 7.07, the tan δ values of 0.0029 and 0.0019 at 10 GHz, are obtained for the CBS glass ceramics sintered at 860 °C with 2 wt% P₂O₅ + 2 wt% ZnO and 2 wt% ZrO₂ + 2 wt% TiO₂ additions, respectively. This material is suitable to be used as the LTCC material for the application in wireless communications.     

Al–Ti–B grain refiners via powder metallurgy processing of Al/K2TiF6/KBF4 powder blends

The response to thermal exposure of ball-milled Al/K₂TiF₆/KBF₄ powder blends was investigated to explore the potential of PM processing for the manufacture of Al–Ti–B alloys. K₂TiF₆ starts to be reduced by aluminium as early as 220 °C when ball-milled Al/K₂TiF₆/KBF₄ powder blends are heated. The reaction of KBF₄ with aluminium follows soon after. The Ti and B thus produced are both solutionized in aluminium before precipitating out as Al₃Ti and TiB₂. All these reactions take place below the melting point of aluminium. The ball-milled Al/K₂TiF₆/KBF₄ powder blends heat treated at approximately 525 °C can be compacted to produce Al–Ti–B pellets with in situ formed Al₃Ti and TiB₂ particles. These pellets are shown to be adequate grain refiners for aluminium alloys.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min⁻¹, then to 1500 °C at 6 °C min⁻¹ and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m^1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al₂O₃ and TZ-3Y as well as Al₂O₃ and TZ-3Y in the 20 wt% Al₂O₃+80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al₂O₃ + 80 wt% ZrO₂ + 3 mol% Y₂O₃ (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m^1/2) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.     

On the properties of the eutectic alloy Al3(Nb,Cr) + Cr(Al,Nb)

The eutectic alloy Al₃(Nb,Cr) + Cr(Al,Nb) forms an in situ composite and the Al₃Nb presents high specific strength and low oxidation rate that may be improved by the combination with other phases. The purpose of this work is to investigate physical, mechanical and oxidation properties of the eutectic alloy. Therefore, Rietveld analysis was carried out for furnace cooled and water quenched samples and oxidation tests were performed on directional solidified samples. Compressive tests were performed for the eutectic alloy and also for the Nb–74.8% Cr–24.6% Al alloy in the as-cast condition. The alloy presents 12.9% Cr(Al,Nb) at room temperature, retained from the transformation Cr(Al,Nb) to Al(Nb)Cr₂. The combination of Al₃Nb with Cr(Al,Nb) and Al(Nb)Cr₂ considerably improves mechanical behaviour, leading the yield strength to 1525 MPa at 800 °C and 925 MPa at 900 °C. The oxidation tests showed the formation of several oxides at all temperatures studied and that from 900 °C on alfa Al₂O₃ is formed both in air and O₂ except under O₂ at 1000 °C. It is believed that the Cr(Al,Nb) phase acts as an Al reservoir for the formation of the various Al₂O₃ scales.     

Effects of Si, W and W–Mo on isothermal oxidation behaviors of Nb/Nb5Si3 in situ composites at high temperature

The effects of Si, W and W–Mo on the isothermal oxidation behaviors of Nb/Nb₅Si₃ in situ composites in static air at 1000 and 1200 °C for 20–100 h were investigated on as-cast materials. The results show that the oxidation kinetics of each alloy was not changed whether at 1000 or 1200 °C, and the oxidation mechanism were not changed. The oxidation resistance of Nb/Nb₅Si₃ in situ composites was sensitive to Si content, and the oxidation rate of Nb-10Si alloy was more than twice as many that of Nb–20Si alloy. By alloying of W, the oxidation resistance of Nb–20Si–10W alloy was improved significantly, because the WO₃ scale can provide the adherence for the creaked Nb₂O₅ scale and reduce the diffusion of oxygen through the scale. Comparing to alloying with W, the poor oxidation resistance of Nb–20Si–10W–10Mo alloy was attributed to the evaporation of MoO₃ and highly porous scale.     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

Structural characteristics and resistive switching properties of thermally prepared TiO2 thin films

Polycrystalline TiO₂ thin films were formed on Pt(1 1 1)/Ti/SiO₂/Si by thermal oxidation of Ti films with temperatures ranging from 600 °C to 800 °C. Results of Raman spectra testing indicate that the structure of the oxidized TiO₂ films is rutile phase. The resistance switching behaviors (RSB) have been confirmed in Pt/TiO₂/Pt structures. A stable RSB with a narrow dispersion of the resistance states and switching voltages was observed in the sample fabricated with the oxidation temperature of 600 °C. The resistance ratios of high resistance states to low resistance states are larger than 10³ with the set and reset voltage as low as 2.5 V and 0.6 V, respectively.     

Hydriding/dehydriding behavior of Mg2CoH5 produced by reactive mechanical milling

Complex Mg₂CoH₅ hydride was obtained by a combined procedure that included a milling stage of a 2Mg–Co mixture under argon followed by reactive mechanical alloying (RMA) under hydrogen, both at room temperature. During RMA, MgH₂ is produced at short milling times (10 h) and Mg₂CoH₅ (50 wt%) after 90 h. Improvement in the yield and the formation times could be associated with both refinement of microstructure and enhancement of intermixing of Mg–Co during pre-milling stage. DSC studies of Mg₂CoH₅ phase produced by RMA show that the starting decomposition temperature is about 205 °C.Absorption and desorption PCIs were determined under static (300 °C) and dynamic (230–330 °C) conditions. An important hysteresis and two plateaus were observed and correlated with formation/decomposition of Mg₂CoH₅ (high-pressure plateau) and Mg₆Co₂H₁₁ (low-pressure plateau) hydrides. For comparing hydrogen sorption kinetics, Mg₂CoH₅ (65 wt%) was also obtained by a sintering method at 410 °C and 6.0 MPa of hydrogen pressure. Absorption was very fast in the temperature range of 150–350 °C, independently of synthesis procedure. However, desorption curves showed a better behavior for RMA powders.MgCo was observed after decomposition of Mg₂CoH₅ under particular thermal treatments, while MgCo₂ phase was not detected. The results of this study reinforce the idea that kinetics factors related with atomic mobility play a key role in the formation of Mg–Co intermetallics.     

Tribological behaviour of pulsed magnetron sputtered CrB2 coatings examined by reciprocating sliding wear testing against aluminium alloy and steel

Among the number of attractive properties that transition-metal diborides (TiB₂, CrB₂, etc.) possess, high resistance to wear and chemical inertness are the most important when considering diboride coatings for dry machining of nonferrous materials, such as aluminium and its alloys. Due mostly to the problematic deposition of chromium diboride (preparation of targets, target cracking during the deposition process, control of stoichiometry etc.), these coatings remain comparatively less studied than, for example, titanium diborides, regarding their tribological performance.In this paper we report on the tribological behaviour of pulsed magnetron sputtered (PMS), smooth and fully dense, crystalline, 21-38 GPa hard CrB₂ coatings examined by reciprocating sliding wear testing in ambient air (20 ± 2 °C, 20-30% humidity) against EN AW-2017A aluminium alloy and AISI 52100 chrome steel. The results are compared to those of pulsed magnetron sputter deposited TiN and CrN coatings. It is demonstrated that pulsed magnetron sputtered chromium diboride coatings exhibit the best tribological performance, in terms of amount of aluminium adhered on the surface of the wear track, during testing against aluminium alloy. When slid against AISI 52100 steel PMS CrB₂, CrN and TiN coatings exhibited coefficients of friction of 0.6, 0.6-0.7 and 0.43-0.45 respectively. The tribological behaviour of coatings was found to be dependent on the transfer film formation and its properties. Wear rates were up to ten times lower for pulsed magnetron sputtered CrB₂ coatings, compared to DC sputtered Cr-B films.     

The study of NiAl-TiB2 coatings prepared by electro-thermal explosion ultrahigh speed spraying technology

NiAl-TiB₂ composite coatings with 0, 10 and 20 wt.% TiB₂ were synthesized on the Ni-based superalloy substrate using electro-thermal explosion ultrahigh speed spraying technology. The microstructure analysis shows that the coatings consist of submicron grains. The bond between coatings and substrate is metallic cohesion. TiB₂ as a powerful reinforcement is doped in NiAl for increasing its hardness. The isothermal oxidation test is carried out for the composite coatings at 1100 °C in air. The result shows that the oxidation resistance of NiAl coating is higher than that of NiAl-10TiB₂ and NiAl-20TiB₂ coatings. The phases of oxides on the coatings during the process at high temperature have been analyzed by X-ray diffraction. The results show that Al₂O₃ and Cr₂O₃ coexistence on surface of NiAl coating, while Al₂O₃, Cr₂O₃, TiO₂ and a small amount of NiO form on surface of NiAl-10TiB₂ and NiAl-20TiB₂ coatings after oxidation for 4 h.     

The ternary system: H2O–Fe(NO3)3–Co(NO3)2 isotherms −15 and −25 °C

The binary system H₂O–Fe(NO₃)₃ has been investigated at temperature ranging between –25 and 47 °C.The solid–liquid equilibria of the ternary system H₂O–Fe(NO₃)₃–Co(NO₃)₂ were studied at −15 and −25 °C by using a synthetic method based on conductivity measurements which allows all the characteristic points of the isotherms to be determined, and the stable solid phases which appear are respectively: ice, Fe(NO₃)₃·9H₂O, Fe(NO₃)₃·6H₂O, Co(NO₃)₂·9H₂O, Co(NO₃)₂·6H₂O and Co(NO₃)₂·3H₂O.