Papers by Keyword: Sintering Atmosphere

Abstract: This study aims to equip orthodontic bracket SS 17-4 PH fabricated using metal injection molding with antibacterial properties. This can be achieved by applying TiO₂ coating on the surface of brackets using magnetron sputtering PVD method. This method is chosen due to its compatibility to be used on bulk metal and its ability to control thin-film stoichiometry. Samples were prepared using the series of following steps which comprised of metal injection molding, binder elimination with solvent and thermal debinding, sintering in vacuum and argon atmosphere, electropolishing, and magnetron sputtering PVD coatings as the final stage. Negative bias, sputtering power, and partial pressure on vacuum chamber were set as the constant parameters. The atmosphere inside the PVD chamber was controlled using oxygen and argon gases. XRD and SEM observations were carried out to obtain the information on the phase and morphology of the films. Rutile and anatase crystalline structures with 2,27 nm and 9,78 nm crystal size were measured in as-deposited PVD TiO₂ respectively. The deposition films were achieved in the range of 3 μm-8 μm.

Abstract: Powder metallurgy products may be started from powders with widely varying oxygen affinity. Thus the natural oxygen content of the powder compacts also varies in reducibility in the early stages of sintering. Here it is shown that prealloyed powders containing Cr require higher temperatures for oxygen removal than e.g. unalloyed or Ni-Cu alloyed grades. In case of powder mixes of base iron powder with Cr, Mn or Si, oxygen transfer from Fe to the additive powders may occur during heating up to sintering temperature, the “internal getter effect”. A similar effect can be observed in Cr prealloyed powders in which iron oxides initially present on the powder surfaces are transformed to more stable oxides in a fairly early stage of heating. Finally, also the formation of CH₄ observed when sintering alloy steels containing Si, Mn or Cr in H₂ can be attributed to an oxygen transfer effect.

Abstract: Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD) test were utilized to detect the phase transformation of a HA/Ti Functionally Graded Material (FGM) prepared via Powder Metallurgy (PM) technique. The effects of oxygen (O₂), nitrogen (N₂), forming (N₂+H₂) and Argon (Ar) sintering atmospheres on the FGM specimens were examined by considering the gas flowing duration. It was found that the original metallurgical profile of pure Ti in HA/Ti FGM sintered under N₂ atmosphere was almost preserved. However the carburization of the pure Ti was observed on the specimen. Medium alkyl halides (C-Br) and alkenes (-C=C-) stretches were detected, producing the dominant elements in the pure Ti layer of the specimen. The almost stable constituent element remains in the specimen sintered under flowing N₂+H₂ atmosphere as detected by XRD result. This proved the significance of controlling the sintering atmosphere during the entire sintering process. The results achieved reveal the high tendency of Ti and HA elements to react with the sintering environment, thus very precise furnace with controllable atmosphere is crucial for the fabrication of the HA/Ti FGM.

Abstract: Attempts have been made to describe the influence of the process parameters, such as compaction pressure and sintering atmosphere on the microstructure and properties of PM Al4Cu alloy. Homogenous mixtures of Al4Cu elemental powders were achieved by tumbling powders for 30 minutes in the Turbula T2F mixer. The powders were subsequently cold compacted under pressures of: 200MPa, 300MPa and 400MPa in a rigid die on a single action press. The green compacts were sintered in two different atmosphere - nitrogen and vacuum at 600°C for one hour. After that, the samples sintered in nitrogen atmosphere were re-pressed and re-sintered (2p2s) under the same conditions. The green compact and as-sintered densities were measured using the geometric method. Additionally, the Brinell hardness and the bending strength in three point bend test were determined. The microstructure of the samples was also analyzed using both the light microscopy (LM) and scanning electron microscopy (SEM).The obtained results show, that optimal pressing pressure is 300MPa. Increasing pressure to 400 MPa has not a substantial effect on increase of the final sample density. Therefore applying higher compaction pressure (over 300MPa), from the economical point of view, is unnecessary.

Abstract: Their excellent properties, such as corrosion resistance, fatigue strength and bio-compatibility, made Cobalt-chromium-molybdenum (CoCrMo) were used in total hip and knee replacements and dental devices. The green CoCrMo compacts specimens in rectangle shape were fabricated by powder pressing technique. The effects of sintering temperature and atmosphere on the mechanical properties and microstructure of the CoCrMo compacts which is sintered at 1300°C-1400°C under two different inert gases sintering atmosphere (Ar₂/N₂H₂) were investigated. The experimental results show that the grain boundaries sizes of CoCrMo compact sintered specimen were increased with increasing sintering temperature. The CoCrMo compacts specimens sintered at 1350°C under inert gases N₂H₂ atmosphere possess highest density (8.096 g/cm³) and hardness (327.1Hv). However, when the compacts specimens are sintered at 1400°C, the density (7.596 g/cm³) and hardness (320 Hv) properties of sintered compact were decreased.

Abstract: A one-step carbon thermal method was used to prepare LiFePO4/C particles by using normal Fe₂O₃, LiH₂PO₄ and sucrose as raw materials. The effect of H₂ content in the sintering atmosphere of N₂ on the morphology and the electrochemical performance were investigated. LiFePO₄/C materials were characterized by X-ray diffraction, scanning electron microscopy and the elemental analyzer. The results show that the precursor sintering under the atmosphere of 8%H₂+N₂ exhibits the highest electrochemical capacity (162.3 mAh/g at 0.1C) .

Abstract: For the sintered materials, the mechanical properties are strongly dependent on the density of the final product. A substantial reduction of the porosity can be achieved using additives in the powder mixture which promote the formation of a liquid phase during sintering. Boron is a potential liquid phase promoter in ferrous alloys, when sintering is carried out using hydrogen or argon atmospheres. These atmospheres, however, are costly, and the use of nitrogen containing low content of hydrogen could be beneficial. In this study the effects of 10 to 50% hydrogen in nitrogen atmosphere on the microstructure and mechanical properties of a Fe-0.3%C-0.1%B alloy sintered at 1120 and 1250°C were investigated. Boron addition increased the sintered densities, but lowered the transverse rupture strength and hardness in relation to the control alloy (Fe-0.3%C). No significant differences were observed among the samples sintered in different atmospheres for each alloy. Nitrogen containing up to 50% hydrogen atmosphere is not suitable to sinter Fe-C alloy containing boron since it lowers the mechanical properties with the formation of fragile boron nitride precipitates at the grain boundaries and lower perlite fraction.

Abstract: Microstructure of the powder metallurgy (PM) steels and especially mechanism of its formation differs significantly from the microstructure of the conventional steels even if composition will be exactly the same. The difference is not only connected to the presence of the pores, which are inalienable feature of the PM parts. Presence of the prior inter-particle boundaries, which can be contaminated by residual oxides, as well as microstructure heterogeneity are another characteristic features of the microstructure of PM steels. Microstructure heterogeneity is connected to the PM manufacturing process: powder mix, consisting of the base powder and additional alloying elements is compacted and then sintered. Fully prealloyed powder is not always possible to use in standard press & sintering route due to the solid solution strengthening of the ferrite resulting in bad powder compressibility. Hence, in order to provide good powder compressibility only pure iron or low-alloyed (typically <3 wt.%) powders are used. Required alloying elements and carbon (added as graphite) are further admixed in the powder form and are distributed during sintering by diffusion into iron particles at high temperatures. To assure satisfactory distribution of alloying elements, oxide layer, covering surface of the powder particles and hindering mass-transfer of the alloying elements, has to be removed first. This can be done by gaseous reducing agents as hydrogen and carbon monoxide. However, their cost and/or purity are of issue for modern alloyed PM steels. Admixed carbon, additionally to its function as alloying element, plays a role of effective reducing agent at higher temperatures. Paper summarizes the main features of microstructure formation during the whole sintering cycle (heating and isothermal sintering) and effect of alloying additives (different carbon sources, alloying elements) and processing parameters (sintering atmosphere composition, temperature profile) on the microstructure formation during conventional sintering process. Results indicate that for successful sintering of alloyed PM steels with homogeneous defect-free microstructure, hydrogen-rich atmospheres and high-temperature sintering are required.

Abstract: In this work, the effect of sintering atmosphere on the corrosion resistance of sintered titanium has been evaluated in 0.9 % aqueous NaCl solution to simulate physiological environment. Corrosion tests were performed on titanium porous sintered under vacuum and vacuum plus dynamic argon. The results showed better passive properties associated to the titanium sintered under argon plus vacuum atmosphere than to the vacuum sintered titanium. The better corrosion resistance of the argon plus vacuum sintered titanium was attributed to the formation of a thin passive film on the titanium surface during sintering due the low oxygen content present in this atmosphere.

Abstract: The microstructure and mechanical property of hot-pressed Al2O3/Cu nanocomposites with a different temperature for atmosphere changing from H2 to Ar have been studied. When the atmosphere changed from H2 to Ar gas at 1450°C, the hot-pressed composite was characterized by inhomogeneous microstructure and low fracture strength. On the contrary, when the atmosphere changed at a lower temperature of 1100°C, a more homogeneous microstructure and higher fracture strength was observed.