Papers by Keyword: Powder Processing

Abstract: As a potential candidate for the top coating in novel Environmental Barrier Coating systems, one representative of a Barium-Magnesium-Aluminium-Silicate family was produced in the form of the powder. Initial compounds were heat-treated to synthesize and the product was crushed in the ball mill device down to the fraction of 20 micrometers. In the next step, the atmospheric plasma spray (APS) technique was used to form a coating on a steel sheet substrate. The aim of this study was to obtain the most favorable technological parameters for the thermal spraying process and therefore two plasma spraying parameters for pure alumina or yttria stabilized zirconia, and other three experimentally designed ones were tested. Despite the same stand-off distance used for coatings manufacturing, thickness and porosity differed in order of tens of micrometers and several percent, respectively. Resulting coatings consisted of a mixture of amorphous and crystalline Al₂O₃, SiO₂ and MgO phases.

Abstract: Barium-Magnesium-Aluminium-Silicate (BMAS) powder was produced from a mixture of initial compounds BaO–MgO–Al₂O₃–SiO₂ by means of solid state synthesis at the temperature of 1200 °C for 3 hours in a laboratory furnace. Synthetized powder was crushed into the fraction of 15-45 μm in a planetary ball mill. Thermal barrier coating system consisting of CoNiCrAlY (bond coat) and BMAS (top coat) was sprayed by atmospheric plasma spray technique onto the polycrystalline nickel-based superalloy substrate. During plasma spraying process, the BMAS underwent phase transformation and the amorphous phase within the top coat was produced. Therefore, after the spraying, several samples were crystallized via annealing in a furnace (4 hours at 1200 °C or 24 hours at 1000 °C) or by subjecting them to several passes of plasma jet. Both samples with an amorphous phase and fully-crystallized samples were subjected to the fire in a burner-rig test (propane-oxygen flame, single 3 + 3 minute cycle), where the top coat reached the temperature of 1150 °C. Top coat failure occurred during the cooling period due to the transformation of the amorphous phase into the crystalline one and/or due to the difference in thermal conductivity and expansion between the top coat and the bond coat.

Abstract: CNTs-Al matrix composites are considered to be promising heat dissipating materials because thermal conductivity can potentially be improved whilst their density is reduced. Although casting has many advantages in the fabrication of large, complex components, this process cannot be easily employed when manufacturing CNTs-Al composites. In order to produce CNTs-Al matrix composites by casting a CNTs-Al powder precursor was manufactured using mechanical milling and electroless plating processes. Aluminium powder with CNTs of 10 wt.% and 20 wt.% were mixed and ball milled using a horizontal mill. After milling for 3 hrs., the milled powder exhibits a flattened morphology with a band-type distribution of CNT clusters observed within the aluminium particles. Prolonged milling of up to 24 hrs. introduces an equiaxed particle shape for the milled powders with a uniform distribution of CNTs within the aluminium particles. However, as milling time increases, the CNTs become fractured by ball-to-ball collisions. There was no reaction evident between the aluminium and the CNTs at milling times up to 24 hrs. In order to improve the wettability between the CNTs-Al powder precursor and Al melt during the casting process, electroless Ni plating was performed. The processing time for the Ni-plating affects the uniformity of the coating layer. For a uniform coating condition, the average thickness of the coating was ~1.87 μm, with no evidence of gaps between the milled powders and coatings observed.

Abstract: This study considers the weldability of Ti-6Al-4V alloys formed by various powder consolidation methods. Samples were prepared from commercially sourced pre-alloyed Ti-6Al-4V powder using both conventional press-and-sinter (PS) and the new novel resistance-heated pressing (RHP) methods. Fusion welding was executed by the gas tungsten arc (GTA) process with arc stability assessed in-situ by observations of the arc as well as monitoring of transient arc voltage. Results indicated equivalent arc stability between samples of RHP and commercially sourced wrought material while samples formed by PS showed high instability in arc initiation, attributed to high levels of porosity. Post weld analysis of mechanical hardness in powder based samples revealed no significant deviation in weld metal properties from welds conducted on commercially sourced wrought material. In all cases weld microstructures typical of Ti-6Al-4V alloys were observed with significant grain growth in the fusion and heat affected zones. Samples prepared by PS methods showed internal porosity due to gas evolution upon solidification, which may again be attributed to the highly porous initial microstructure.

Abstract: Sintered ZTA(zirconia toughened alumina) which has good mechanical properties at a low temperature was produced by milling and mixing with Al₂O₃ and ZrO₂(3Y-TZP). In order to examine the effect of sintering aids on the mechanical properties of ZTA, fracture toughness and hardness of the produced ZTA were observed in accordance with change of the added quantity of ZrO₂ Scanning electron microscopy and X-ray diffraction technique were applied to observe microstructural change and phase transformation during the process. Experimental results showed that the addition of sintering aids in ZTA at a low temperature induced densification and adding SiO₂ and talc lowered sintering temperature and promoted crystallization process of the compound. The mechanical strength of ZTA added ZrO₂ showed higher mechanical strength and SEM analysis revealed that Al₂O₃ and ZrO₂ during the sintering process restrained the grain growth each other. Especially, the 92% Al₂O₃ added sintering aids showed more than 98% of the theoretical density and more than 1500 Hv of hardness value at a low temperature of 1400. It was also showed that the fracture toughness is gradually increasing first and decreasing later in accordance with the quantity of ZrO₂.

Abstract: The ceramic-metal composites that have all phases continuous throughout the structure are known as Interpenetrating Phase Composites (IPCs) and they have many applications in various fields. In this investigation ZrO₂-Ni IPCs of varied compositions were synthesized using tubular furnace and microwave furnace routs. Samples were Characterized using BET surface area, Pycnometer density, dilatometry and scanning electron microscopy. The Electrical parameters of the composites were measured using impedance spectrometer. Results indicate that threshold percolation reached at 40 volume percent of Ni in both cases. Moreover, no significant difference was observed in BET surface area and CTE of composites prepared by conventional and microwave sintering processes.

Abstract: This study investigates the effect of powder processing on powder flowability, compact ability, and the heat treatment of the resulting coatings on the mechanical properties of cold gas dynamic sprayed Ti-6Al-4V alloy. Nitrogen gas was used throughout the coating deposition process. Propellant gas temperature and pressure were attuned to maximize particle impact velocity. Three powder processing conditions were used in this study: as received (AR), low-energy ball milled (BM), and argon atmosphere heat treated (HT). Results showed coating porosities of around 6 to 7%, regardless of the feedstock powder used or the heat treatment performed. It was observed at 600 and 800°C anneals that a coating hardness reduction occurred, possibly due to static recovery and recrystallization, with minor sintering possibly occurring at the 800°C anneals. In addition, micro tensile tests showed an increase in cohesion strength at higher heat treatment temperatures.

Abstract: This paper discusses the anti-explosion control system project for the fine powder processing. To prevent explosion, safety interlock control system is brought up. The article describes the hardware configuration and software establishment of the project, and makes suggestions for the project implement about sensor installing and program debugging.

Abstract: The lanthanum-silver doped hydroxyapatite (La/Ag/HAP) powder was prepared by a coprecipitation way. Its microstructure, phase composition and thermostability were investigated by means of X-Ray diffraction, scanning electron microscopy, and Fourier Transform Infrared. The antibacterial properties against E.coli and S.Aureus were detected using a minimum inhibitory concentration method. The biocompatibility was confirmed by cell culture of osteoblasts of rat cranium. Results show that the investigated La/Ag/HAP powder exhibited the features of typical HAP phase in the experimental temperature, and its shape was needle-like with a diameter of 50~80 nm and a length of 300~500 nm. It is of a good biocompatibility and is more stable at high temperature, and is also a stronger antibacterial agent than pure HAP powders.

Abstract: In this work percolation range for Al₂O₃-Ni interpenetrating phase composite (IPC) was studied to find the optimal combination of electrical conductivity and coefficient of thermal expansion (CTE). The impedance spectroscopy and scanning electron spectroscopy were used to study the percolation limit.