In situ formation of La containing dispersed phase on the sinter skin of La2O3 and Cr3C2 unitedly doped WC–Co cemented carbide

The as-sintered sinter skin of WC–11 wt% Co–0.71 wt% Cr₃C₂–0.06 wt% La₂O₃ cemented carbide was observed and analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. It was unexpectedly discovered that lanthanum which was added in the form of oxide could migrate directionally from the alloy to the sinter skin during the sintering process. As a result, La₂O₂S and LaCoO₃ phases were formed in situ on the sinter skin. It is significant that both phases are characterized with layered-structure related self-lubricating function and high melting point. Noteworthily, La₂O₂S grains preferentially grew up along a group of parallel faces with orientation index [0 0 1] in the WC + β matrix. It was revealed that the alloy had a diffusion-resulted dual structure (i.e., core and skin structure), i.e., WC + β + η three-phase structure in the inner part and WC + β + La₂O₂S + LaCoO₃ four-phase structure on the sinter skin. Cr₃C₂ with carbon-donation ability and the impurity elements from the sintering atmosphere with much bigger electronegative parameters than lanthanum are thought to be the driving force behind the directional migration.     

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.     

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.     

Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

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.     

Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide

For the production of hard, high temperature and abrasion resistant parts, like water-jet nozzles or pressing tools for forming glass lenses, binderless cemented carbide is used. In this work, the consolidation of tungsten carbide with additions of VC and Cr₃C₂ grain growth inhibitors is studied. Tungsten carbide powder dry or wet milled was consolidated by dry pressing, debindering and gas pressurized sintering and, alternatively, by spark plasma sintering. The effect of adding VC and Cr₃C₂ to binderless tungsten carbide on the grain growth was studied with contents being 0; 0.1; 0.3; 0.5; 0.7 and 1.0 wt.%. Samples with an ultrafine microstructure free of abnormal grain growth, a hardness of 25.5 GPa and a fracture toughness of 7.2 MPa·m^1/2 were archived by conventional sintering. Both carbides reduce grain growth, but with Cr₃C₂ a finer microstructure can be achieved at lower amounts. Compared to the same amount of Cr₃C₂, the addition of VC results in smaller grains but lower hardness and fracture toughness.     

VC, Cr3C2 and NbC doped WC–Co cemented carbides prepared by pulsed electric current sintering

WC–12 wt.% Co grade cemented carbides doped with 0.9 wt.% VC, NbC or Cr₃C₂ grain growth inhibitor were consolidated by pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), in the solid state at 1240 °C for 2 min. The microstructure and properties of the PECS material grades are compared with those of pressureless sintered grades, liquid phase sintered at 1420 °C for 1 h. Microstructural and hardness characterization revealed that both the chemical composition and sintering technique play an important role on the WC grain growth and final mechanical properties. To obtain a nanometer sized WC–Co microstructure, it is essential to carefully select the grain growth inhibitor in addition to the application of a fast thermal densification cycle by means of spark plasma sintering.     

Effects of additives on synthesis of vanadium carbide (V8C7) nanopowders by thermal processing of the precursor

Vanadium carbide (V₈C₇) nanopowders can be synthesized by thermal processing of the precursor of ammonium vanadate (NH₄VO₃) and nanometer carbon black. Effects of additives (CaF₂, CeCl₃·7H₂O and LaCl₃·7H₂O) on the phase composition and microstructure of the synthesized powders were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that additives (CaF₂, CeCl₃·7H₂O and LaCl₃·7H₂O) can accelerate the solid state reaction during synthesis of V₈C₇, and these additives play a vital role in determining the phase composition and microstructure. V₈C₇ powders with basically single phase can be synthesized at 1100 °C for 0.5 h with the addition of additives (CaF₂, CeCl₃·7H₂O and LaCl₃·7H₂O), and the powders show good dispersion and are mainly composed of uniformly-sized spherical particles with a mean diameter of 50 nm. Further experiment shows that V₈C₇ powders can be prepared at 950 °C for 1 h with 1.0 wt% CaF₂ as additive.     

VC, Cr3C2 doped ultrafine WC-Co cemented carbides prepared by spark plasma sintering

The influence of the addition of 0.3, 0.5 and 0.7 wt.% VC on the density, microstructure and mechanical properties of WC-Cr₃C₂-11 wt.% Co with 0, 0.2, 0.4 and 0.6 wt.% Cr₃C₂ hard metals prepared by spark plasma sintering (SPS) at a temperature of 1200 °C (5 min, 40 MPa) was investigated. Microstructure analysis revealed that the WC grain size in the sintered hard metals was strongly influenced by the VC and Cr₃C₂ content. With the addition of inhibitors and the increased amount of Cr₃C₂, the density is reduced, and on the contrary, the addition of VC as an inhibitor contributes to promoting the densification. The combined addition of Cr₃C₂ and VC could strongly reduce the WC grain growth to about 350 nm. Observation suggests that the fracture of WC-Co cemented carbide is brittle and intergranular. The amount of added VC/Cr₃C₂ should be controlled in a certain range. Samples with an appropriate proportion of VC/Cr₃C₂ added exhibit higher hardness which can be up to 1938 HV₃₀. Toughness, too, can reach 16.34 MPa m^1/2.     

The corrosion behavior of Cu–Al and Cu–Al–Be shape-memory alloys in 0.5 M H2SO4 solution

The corrosion behavior of Cu–Al and Cu–Al–Be (0.55–1.0 wt%) shape-memory alloys in 0.5 M H₂SO₄ solution at 25 °C was studied by means of anodic polarization, cyclic voltammetry, and alternative current impedance measurements. The results of anodic polarization test show that anodic dissolution rates of alloys decreased slightly with increasing the concentrations of aluminum or beryllium. Severe intergranular corrosion of Cu–Al alloy was observed after alternative current impedance measurement performed at the anodic potential of 0.6 V. However, the addition of a small amount of beryllium was effective to prevent the intergranular corrosion. The effect of beryllium addition on the prevention of intergranular corrosion is possibly attributed to the diffusion of beryllium atoms into grain boundaries, which in turn deactivates the grain boundaries.     

Synthesis of copper–niobium carbide composite powder by in situ processing

The in situ formation of niobium carbide (NbC) particles was investigated during mechanical alloying process as well as subsequent heat-treatment. Initial starting powders of Cu, Nb and C were used with a composition of Cu–40 wt% Nb–10 wt% C which was corresponding to Cu–40 vol% NbC. A mixture of this powder system was milled in an argon atmosphere for four different durations, i.e., 15 h, 31 h, 42 h and 54 h. X-ray diffraction was used to verify the formation of NbC. No NbC was formed after the milling process. The in situ formation of NbC in a copper matrix only took place after heat-treatment at 900 °C. Longer milling time resulted in the formation of NbC phase with more intense X-ray diffraction peaks due to powder refinement.     

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.     

Pulsed electric current sintered Fe3Al bonded WC composites

WC based composites with 5, 10 and 20 vol.% Fe₃Al binder were consolidated via pulsed electric current sintering (PECS) in the solid state for 4 min at 1200 °C under a pressure of 90 MPa. Microstructural analysis revealed a homogeneous Fe₃Al binder distribution, ultrafine WC grains and dispersed Al₂O₃ particle clusters. The WC-5 vol.% Fe₃Al composite combines an excellent Vickers hardness of 25.6 GPa with very high Young’s modulus of 693 GPa, a fracture toughness of 7.6 MPa m^1/2 and flexural strength of 1000 MPa. With increasing Fe₃Al binder content, the hardness and stiffness decreased linearly to 19.9 and 539 GPa, respectively with increasing binder content up to 20 vol.%, while the fracture toughness and flexural strength were hardly influenced by the binder content. Compared to WC–Co cemented carbides processed under exactly the same conditions, the WC–Fe₃Al composites exhibit a substantially higher hardness and Young’s modulus.     

Enhanced superconducting properties of bulk MgB2 prepared by in situ Powder-In-Sealed-Tube method

A systematic study on the superconducting properties of polycrystalline MgB₂ synthesized by in situ Powder-In-Sealed-Tube technique is carried out at different temperatures (750–900 °C). Both XRD and SEM results show well-crystallized MgB₂ grains in all the samples and grain size is found to be increasing with the sintering temperature. Sharp superconducting transitions are observed for all samples, irrespective of sintering temperatures, which implies the high degree phase purity and homogeneity of MgB₂ formed, while J_C(H) plot gives sample dependent critical current density. The samples heat treated at relatively low temperatures show enhanced flux pinning and hence improved J_C(H) performance. The reduced grain size and hence increased density of grain boundary pinning centers of MgB₂ bulks synthesized at low temperature are mainly responsible for the enhanced flux pinning and J_C.     

Effect of milling conditions and binder phase content on liquid phase sintering of heat treatable WC-Ni-Co-Cr-Al-Ti cemented carbides

The binder phase of WC based cemented carbides has been alloyed by adding two different aluminium compounds, AlN and TiAl₃, to mixtures comprised of WC, Ni, Co and Cr₃C₂ powders. A more efficient alloying effect is obtained by TiAl₃ additions likely due to its higher dissolution rate during liquid phase sintering. Shrinkage and melting phenomena are strongly affected by the energy of the milling process and the amount of metallic additions. The use of higher milling rotation speed induces higher oxidation of the powder mixtures and the subsequent formation of a higher volume fraction of alumina particles after sintering. Densification and WC grain growth are hindered by increasing the Al addition. Thus, full densification of alloys with higher Al additions require the use of HIP after standard vacuum sintering cycles. As-HIPed WC-Ni-Co-Cr-Al-Ti samples present a binder phase with precipitation of gamma prime similar to that found in as-cast Ni superalloys. The size, volume fraction and morphology of these precipitates has been modified by applying a standard solution treatment (1150 °C-2 h) followed by fast air cooling and subsequent aging at 600 °C and different dwelling times. Age hardening effects have been confirmed in the composition consisting of WC-12 wt% Co-12 wt% Ni-1.7 wt% Cr₃C₂-5 wt% TiAl₃ after 100 h at this temperature.     

Low-temperature fabrication of high purity Ti3SiC2

In this study, we fabricated high purity Ti₃SiC₂ ceramic by mechanical alloying (MA) and spark plasma sintering (SPS), and investigated the effect of trace amount of Al on these processes. Our results show that addition of proper amount of Al significantly increases the purity of Ti₃SiC₂ in the MA and subsequent SPS products, and remarkably reduces the sintering temperature for Ti₃SiC₂. Ti₃SiC₂ sintered compact with a purity of 96.5 wt% was obtained by 10 h of MA and subsequent SPS from a starting mixture composed of n(Ti):n(Si):n(Al):n(c) = 3:1:0.2:2 at 850 °C. At 1100 °C, Ti₃SiC₂ with a purity of 99.3 wt% and a relative density of 98.9% was obtained.     

WO3添加量对超粗硬质合金微观结构及性能影响*

本文以WC、WO₃、Co、C为原料,通过原位细晶溶解-析出长大法制备了超粗硬质合金,并分析了不同WO₃添加量对合金微观结构及性能影响规律。结果表明：初始粉末中加入的WO₃和C在烧结过程中将发生原位一步还原碳化反应转化为高活性的细WC,促进溶解-析出长大现象,使超粗硬质合金WC平均晶粒度随着WO₃含量增加而增大。同时,WO₃添加能够减少粗WC晶粒微观缺陷和曲边的阶梯状表面,平直化晶粒边界,使其形貌趋于形成完整的三角棱柱体,其(0 0 0 1)晶面占比高,能够有效提高合金硬度,阻碍裂纹扩展,增加钴相韧性断裂比例。当WO₃添加量为4.20wt.%时,制备的超粗硬质合金具有最大的硬度(1085kgf/mm₂⁾和抗弯强度(2692MPa)。     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Densification, microstructural evolution, and dielectric properties of hexagonal Ba(Ti1−xMnx)O3 ceramics sintered with fluxes

Recently, doped hexagonal BaTiO₃ (6h-BaTiO₃) ceramics have been reported as potential candidates used in microwave dielectric resonators. However, similar to other common microwave ceramics, doped 6h-BaTiO₃ ceramics require a high sintering temperature, greater than 1300 °C. In this study, the effect of sintering aids, including Bi₂O₃, B₂O₃, BaSiO₃, Li₂CO₃, CuO, V₂O₅, 5ZnO·2B₂O₃, and 5ZnO·2SiO₂, on the densification, microstructural evolution, and microwave properties of the 6h-Ba(Ti_0.85Mn_0.15)O₃ ceramics was examined. Results indicate that among the fluxes studied, Bi₂O₃, B₂O₃, and Li₂CO₃ could effectively reduce the sintering temperature of 6h-Ba(Ti_0.85Mn_0.15)O₃ ceramics through liquid phase sintering, while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 6h-Ba(Ti_0.85Mn_0.15)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 900 °C (_r: 54.7, Qf_r: 1323, and τ_f:183.3 ppm/°C), 10 wt% B₂O₃ sintered at 1100 °C (_r: 54.4, Qf_r: 3448, and τ_f: 254.5 ppm/°C), and 5 wt% Li₂CO₃ sintered at 950 °C (_r: 43.7, Qf_r: 2501, and τ_f: −29.8 ppm/°C).     

Grain growth inhibition in ultrafine hardmetals

Ultrafine and nanoscaled hardmetals show mechanical properties like hardness and bending strength which lie way above conventional fine or submicron grained hardmetals. To achieve such fine microstructures very fine WC starting powders as well as grain growth inhibitors such as Cr₃C₂ or VC are needed. To study the grain growth inhibition in ultrafine hardmetals investigations on samples made from nearly nanoscaled WC and Co starting powders with and without the addition of Cr₃C₂ were done. For studying the dissolution behaviour of Cr₃C₂ and the evolution of density, magnetic properties and lattice parameters of WC during sintering, interrupted sintering experiments were carried out. Thermal analysis techniques including TG-MS and DSC were used, to link the observed changes to expected reactions. The results show that grain growth inhibitors greatly influence the sintering behaviour already way below the eutectic melting of WC-Co. Especially dissolution of Cr₃C₂ and homogenous distribution of Cr within the samples already starts below 800 °C with the reduction of W surface oxides and the creation and spreading of Cr oxides. The findings are relevant for optimising sintering regime, composition (amount of grain growth inhibitors) as well as the microstructure and mechanical properties.