O-Sialon production
O-Sialons are a unique group of advanced ceramic. This interactive details the laboratory production steps and product testing being conducted by Dr Ian Brown’s team at IRL in Wellington.
Transcript
Sourcing the component materials
The component materials are silicon dioxide (silica), halloysite clay, elemental silicon and the solvent isopropyl alcohol. They need to be carefully selected for purity, fineness of powder particle size, and availability.
Acknowledgement:: Alan Wright, IRL (now Callaghan Innovation)
Weighing out
Working from a predetermined recipe, scientist Bill Owers is carefully weighing out each of the powder components and placing them in a beaker prior to solvent addition.
Suspension of the components
Isopropyl alcohol is added to the beaker to create a suspension of the component particles prior to milling. Isopropyl alcohol is used because it does not react chemically with the raw materials and is easily recovered by solvent evaporation.
Bill transfers the suspension to the ball-milling container full of small ceramic milling balls.
Ball-milling
In the milling room, Bill places the ball-milling container on the rotating rack where it will sit for an extended period of time – typically overnight.
This step in the process is essential. It ensures that the very fine component particles are very thoroughly inter-blended such that they sit together as near to a molecular level as possible.
At the end of the mixing phase, excess solvent is evaporated off using a rotary evaporator.
Making the ‘green body'
The milled powder is now able to be shaped and compacted. Bill is using a standard hydraulic press to prepare small cylindrical test specimens of 25mm diameter. Further compressing is needed in order to force the grains of material as close together as possible. An isostatic press capable of developing very high compressive forces is used for this.
Isostatic pressing involves transmission of pressure to the encapsulated sample using a fluid medium.
The effectiveness of this press can be seen in the example of a polystyrene cup before and after isostatic pressing.
Sintering the ‘green body’
The ‘green body’ compact is now loaded into the specially designed high-temperature furnace. The compact is heated to 1800°C in an atmosphere of pure nitrogen. In this step, the fine, thoroughly blended powder grains react with the nitrogen from the furnace atmosphere and coalesce to form a solid material. This process of reaction sintering allows nitrogen from the furnace atmosphere to be incorporated into the chemical makeup of the O-Sialon.
Analysis of the product
The type of O-Sialon produced is then analysed using a technique called X-ray diffraction. This type of analysis gives information about the structure and identity of the chemical phases formed in the fired ceramic. It enables a check on the phase purity of the product and an assessment as to whether the ceramic reactions have gone to completion.
Testing the product
The O-Sialon produced is then subjected to a series of tests to measure it’s physical properties and assess it’s worth as a ‘new’ advanced ceramic. Here, the sample is being tested for thermal shock resistance by heating it up and then plunging it in cold water. Other tests include porosity, density, hardness and fracture toughness.
Acknowledgement:: Alan Wright, IRL (now Callaghan Innovation)
