Effects of Binders and Internal Lubricant Compositions on Mechanical Properties of Dry Pressed Sintered Alumina

Introduction

Alumina, which is also known as aluminum oxide (Al₂O₃) has good chemical and thermal stability, relatively good strength and well electrical insulation characteristics This material is popular and has very wide range applications such as high temperature and electrical insulators, filter, abrasive, catalysis and biomaterials [1].

The steps of alumina processing are preparation of dried alumina powders, mixing ceramic powder with binders, forming and sintering [2,3]. In alumina processing, binder plays an important roles to improve the mechanical properties of green bodies before sintering [4]. Binders provide strength to the green ceramic body by forming bridges between the particles [2]. Binder content would be such that it will give adequate strength and not to create microstructural defect in green body during binder burnout step. Hence, selection and optimization of binder plays an important role in shape forming of ceramic products [4]. Most common binders used are long chain polymers such as polyvinyl alcohol (PVA) [5] and polyethylene glycol (PEG) [6]. Besides binders, internal lubricant is also important to reduce the friction between particles and die wall during fabrication of green body. It plays an important role in the closure of large inter-granular pores after sintering and reduces the shear between two surfaces of particles during the forming process. The optimised binder-lubricant composition is one of important parameter to obtain uniformly shaped ceramic granules [3].

The most common technique to mix binder with ceramic powder is by physical mixing. However, this conventional technique has disadvantages as the ceramic powder did not homogenously mixed with binders and subsequently will cause microstructural defects which lead to poor mechanical properties of the sintered ceramic. Spray dry technique is another alternative technique that can be used to produce ceramic-binder granules. It is one of several versatile processing methods for the manufacture of ceramic-binder granules [3]. This technique will produce the low shear strength of granules ceramic powder and the process may contribute to a novel and industrially viable approach to the manufacturing of ceramic in which the number of processing steps and overall processing time will be reduced. The process is also economical, convenient and beneficial with producing high quality components for advanced application [3].

The aim of this work is to study the effects of polymeric binders composition, sintering temperature and internal lubricant on the mechanical properties and microstructure of sintered alumina. The ceramic-binder granules were prepared by spray dried method.

Materials and Methods

Materials and sample preparation

The ceramic materials used in these studies was commercial alumina with 0.6 micron average particle sizes. The two different polymeric binders were polyethylene glycol (PEG) and polyvinyl alcohol (PVA). Table 1 illustrates four different batches of alumina-binder formulations as prepared by spray dried method. The composition used was a mixture of fixed amount of PEG (3 wt%) and varies amount of PVA from 1 to 3 wt%. The stearic acid was used at 3 wt% as internal lubricant.

Table 1: Alumina-binder formulations

PVA was dissolved in distilled water at 80°C and PEG was added into the water and stirred for 30 minute before added with alumina and continue stirring process for 2 h to form alumina slurry. In the cases of adding internal lubricant into the solution, 3 wt% of steric acid was added and stirred to form alumina slurry (sample V3G3S3). Finally, the alumina slurry was dried using spray dryer technique. The nozzle of the spray drier was operated at a set atomizing pressure and feed rate at 20°C/min. The inlet and outlet temperatures were set at 190°C and 90°C respectively. The constant weight of dried alumina-binder granules were pressed by Carver hydraulic machine, operated at 4.5 tonnes pressure into circular disk shape to ensure the alumina disk achieved 60% relative density.

A disk with diameter of 15 mm and 5 mm thickness were prepared for all alumina-binder formulations. Debinding operation at 450°C for 3 h at the heating rate of 3°C min^-1 in the electrical furnace was done to remove the binders. The sintering process was eventually conducted in air using furnace (Model Carbolite HTF 17/10) at temperatures of 1200°C, 1400°C, 1600°C and 1700°C.

Characterization

Thermogravimetri analysis (TGA) (Model Mettler Toledo TGA/SDTA851/SF) was used to determine the degradation temperature of binders in the alumina-binder samples. The heating condition was from room temperature up to 900°C at a heating rate of 10°C min^-1 in atmosphere of nitrogen. The mechanical strength of sintered alumina was evaluated using the diametral compression method known as “Brazilian test” [7,8]. The disk-shaped samples were inserted in the universal testing machine (Model Instron 3382) and the load cell applied was 100 kN. The load, F applied radially at a speed rate of 0.5 mm min^-1 until the specimen fractured. The tensile strength of the sample (σ) was calculated using the formula [2]:

Where, F_max is the load applied at fracture, D is the diameter and e is thickness of the sample.

The morphology of spray dried alumina-binder particles was observed under field emission scanning electron microscopy (FESEM: Model Hitachi SU8020). The microstructure of tensile fracture surfaces of selected sintered samples were also carried out using the FESEM.

Conclusion

The composition of binders gave significant effects on the mechanical properties. Alumina sample containing 2 wt% PVA and 3 wt% PEG binders and sintered at 1400°C shows the highest tensile strength. The presence of 3 wt% internal lubricant in alumina with 3 wt% PVA and 3 wt% PEG sample increased the porosity in the microstructure, thus reduced the mechanical properties of sintered alumina. Therefore, results obtained in the present work suggest that the composition of internal lubricant need to be controlled below 3 wt% in order to produce uniform alumina compacts with homogenous grain distribution and dense microstructure, hence good mechanical properties.

References

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