Papers by Keyword: Liquid Phase Sintering

Abstract: The application of “masteralloys” as alloying carriers in Powder Metallurgy (PM) steels enables the introduction of highly oxygen‑sensitive alloying elements for advanced PM‑steels, by using a tailored liquid phase. The characteristics of the liquid and its interaction with the iron base powder are determining factors for final microstructure and dimensional behaviour. In this study, theoretical calculations and experimental findings are presented for the masteralloy systems Fe_Mn_Si_C, Fe_Cr_Si_C and Fe_Mn_Cr_Si_C. Lowered melting temperatures and narrow melting temperature intervals could be achieved. The interaction between Fe base material and the masteralloys was studied by infiltration and DTA experiments. It was found that by adjusting the C and Si content in the masteralloy, liquids with widely varying properties could be obtained. This might be a key for tailoring microstructures, properties and dimensional stability of advanced PM‑steels.

Abstract: A study was carried out on the behavior during the sintering of Al₂O₃ ceramics with pre-sintering of the additives Nb₂O₅ and LiF at different temperatures. It was observed through XRD and DSC the formation of the LiNb₃O₈, Nb₃O₇F and LiNbO₃ phases during the pre-sintering of the additive powders and the formation of the LiAl₅O₈, AlNbO₄ phases during the sintering of the samples The samples showed densities around 91% of the theoretical density, and pre-sintered samples showed low growth in grain size.

Abstract: The effect of tungsten nanoparticles and microparticles on the structure and hardness of sintered Sn–Cu–Co–W alloys has been studied. Tungsten powder of 19–24 μm sized particles was milled in a planetary-centrifugal mill, after which the size of particles was 25 nm to 20 μm. The milled and non-milled tungsten was then mixed with powders of tin, copper and cobalt. The specimens were compacted in moulds and sintered in vacuum at 820°C for 20 minutes. The structure of sintered materials was studied using X-ray diffraction analysis and scanning electron microscopy. Microhardness (HV_0.01) of structural constituents and hardness of the materials were measured. It has been determined that it is alloys containing mechanically milled tungsten that have the highest hardness. The main factor influencing the rise of hardness is dispersion hardening with nanoparticles. A further factor is work hardening of tungsten microparticles during ball milling. The highest hardness of 109–111 HRB has been obtained in the Sn–Cu–Co–W alloy containing 23% wt. of milled tungsten, with the proportion of tin, copper and cobalt being 1/2.6/1.6.

Abstract: WC-Co cemented carbide is one of the widely hard materials used for cutting in machining industry, due to its microstructural and mechanical stability even at high temperature. However, diffusion wear is the most serious problem that WC-Co suffers from. One of the most applied approaches to improve the WC–Co cemented carbide performances is the addition of transition metal carbides such as: TiC, TaC and NbC which prevents diffusion wear thanks to the gamma phase (Ti,Ta,Nb,W)C which is formed during sintering. Therefore, and in order to understand the thermal metallurgical reactions occurred between WC-Co cemented carbide and (Ti, Ta, Nb)C transition carbides and theirs effects on the microstructural and mechanical properties. The WC–TiC– TaC– NbC–Co cemented carbide was elaborated by conventional powder metallurgy then thermal, microstructural and mechanical investigations were performed on the elaborated carbide. A temperature of sintering was determined to be more than 1347 oC by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Scanning electronic microscopy (SEM) coupled with energy dispersive spectrometer (EDS) observations showed that the microstructure consists in a mixture of angular WC grains and (W,Ti)C rounded grains embedded in the Co-rich binder. X-ray diffraction analysis confirmed the presence of these three phases with free carbon. The results of EDS analysis highlight the solution-reprecipitation phenomena caused by liquid phase sintering and clearly revealed the presence of small amount of free carbon. The mechanical characterizations showed that the WC–TiC– TaC– NbC–Co cemented carbide exhibits excellent hardness-fracture toughness combination.

Abstract: Si₃N₄ ceramics were sintered at relatively low temperature of 1400-1650 °C with additives from the SnO₂-SiO₂-MgO-Bi₂O₃-Y₂O₃ system. The degree of densification, α-β transformation, mass loss and microstructure were measured as a function of additives composition, sintering temperature, and nitrogen gas pressure. Specimen prepared from 3 mass% SnO₂, 3 mass% SiO₂, 3 mass% MgO, 6 mass% Bi₂O₃, and 5 mass% Y₂O₃ as the sintering additive could be sintered to almost full density at relatively low temperature as 1550 °C for 2 h under ambient pressure of nitrogen atmosphere. The α-β transformation was not completed at this condition. The mass loss was relatively high due to the evaporation of SiO₂, MgO and Bi₂O₃. However, the resulted dense materials have high bending strength.

Abstract: Graded sintering is the fundamental process of fabricating functionally gradient cemented carbide (FGCC). The diffusion-induced mass transport in cemented carbide can result in the formation of gradient microstructure and thusly lead to gradual changes in micro property. So far, several types of FGCC have been developed, and the factors that can influence the gradient formation are complex. Section 2 introduces the process of forming diffusion-controlled near-surface layer in WC-Ti (C,N)-Co hardmetal as well as the kinetic modeling work that reveals the key factors for the layer formation. Section 3 reviews the dual properties carbide produced under carburization atmosphere, for which the carbon content is a main factor of the gradient thickness. There are two models describing this process, representing different mass-transport mechanism of the so-called liquid phase migration (LPM) process. In section 4, previous and new results of modeling LPM in different dimensions and scales are presented, and the diffusion-controlled nature of LPM are discussed.

Abstract: The present work aimed to contribute to the development of high performance self-lubricating sintered composites, with low friction coefficient and high mechanical strength. Self-lubricating composites presenting embedded solid lubricants in a ferrous matrix were produced. Hexagonal boron nitride (hBN) and graphite were the solid lubricants powders added during the mixing step. The composites were processed by conventional powder metallurgy. The liquid phase sintering, by adding copper, improved the degree of continuity of the matrix by rearranging the solid lubricant particles. With this, besides the hardening effect on the matrix, the mechanical properties of the composites were improved, with tensile strength increasing when compared to the same composite without copper. By using the proposed methodologies, optimized composites presenting friction coefficient of 0.12, tensile strength of 500 MPa and scuffing resistance of 29300 N.m were obtained.

Abstract: Sintering as a technology has been followed from ancient times. However, as science it emerged in 1940s with the seminal work of Frenkel, Huettig, Kuczynski, Lenel, Kingery and Hausner. The present paper covers the historical aspects of sintering fundamentals , right from solid state sintering to liquid phase sintering, activated sintering, electronic theory of sintering, sintering with external pressure, constrain sintering etc. Various mechanisms of sintering with their microstructural relationships have been highlighted. A generalized approach to sintering is called for, which may to great extent bridge the gap between sintering theory and practice.

Abstract: BaTi₄O₉(BT4), has dielectric constant (εr)≈36 and unloaded Q-factor (Qf) >35,000GHz, but has temperature coefficient of frequency (τf~16-20ppm/°C). Ba₂Ti₉O₂₀(B2T9), very near to BT4 in the BaO-TiO₂ phase diagram, has εr~38, Qf~35,000GHz and τf<4ppm/°C. Though this phase is selected for most of the applications, there are many challenges like slow formation, decomposition, reaction with substrate etc. But, BT4 formed quickly and densified easily, but only high τf precluded it from many practical applications. Most of the literature on the modification of BT4 decreased the εr≤35. Here, the preparation and properties of ZnO/ZnTiO₃ added BaTi₄O₉ ceramics are reported. The ceramics have been prepared by the solid state method. Raw materials have been homogenized and calcined at 1125°C. Calcined powders are ball-milled, compacted and sintered in 1275−1350°C to get dense ceramics with ρ=4.38-4.44 g/cc (TD>97.5%). XRD analysis has confirmed the BaTi₄O₉ phase formation along with secondary phases. FeSEM microstructures show liquid phase assisted sintering leading to fused grains and exaggerated grain growth. The dielectric properties have been measured near 4GHz by standard rod resonator methods using cylindrical specimens. The ceramics have εr36.5−38, τf<11ppm/°C and Qf>17THz. It is possible to further improve the Qf of these ceramics by controlling the processing.

Abstract: Mg_0.8Zn_0.2TiO₃ powder was synthesized by dissolved method and calcined at 550 °C for 4 hours. The powder exhibited single phase of Mg_0.8Zn_0.2TiO₃ and nano size particle. Sintered pellet samples were prepared by compacting calcined powder which contains 4wt% B₂O₃ (MZTA), 4wt% Bi₂O₃ (MZTB) as liquid additive and non-additive sample (MZTC). Phase identification and its percentage were analyzed based on XRD pattern using Rietveld method. The result shows major phase Mg_0.8Zn_0.2TiO₃ ranging from 72.83% for MZTA, 77.9% for MZTB and 82.61% for MZTC. Furthermore, minor phases were identified as Mg₂TiO₄ and other trace compound Mg₃TiO₂(BO₃)₂ for boron additive. Sintered pellet densities were determined by Archimedes method indicate that Bi₂O₃ additive has the most effective for densification. Microstructure characterization using SEM show that MZTB possesses the largest grain size ≈3.4µm followed by MZTA 2.3µm and MZTC 1.78µm. Dielectrics characterizations within frequency 1 Hz – 32 MHz exhibited space charge polarization characteristic for frequency <1 kHz, however for frequency >1 kHz showed frequency independence of dipolar polarizations and low dielectric loss having ε_r~17.