MAT2320 Ceramic Microstructure Practical Session

Read this carefully before you start!

Samples A to F have been chosen to illustrate the importance of the processing route on the resulting microstructure and hence the properties. Many of these samples have been discussed in Prof. Reaney’s lectures. The technical support team will show you how to use an optical microscope, how to calculate the magnification and how to utilise the grating. You will need to bring your notes from Prof. Reaney’s lecture.

To complete the lab you need to:

1. Take pictures and import the images into the appropriate spaces in the worksheet.

2. Describe key aspects of the microstructure (e.g. pores, grain boundaries different phases, triple junctions) based on your notes and through discussion with the technical support staff.

3. Answer the multiple choice questions. (Underline the correct answer.)

4. Submit your worksheet online the following day.

10 marks are assigned to the images and your description of the important features and whether you have added the correct scale bar/magnification. There is one mark for each multiple choice question.

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Sample A - Unetched Alumina-Zirconia-Silica Fused Cast Refractory Block

These are used in the glass contact areas of tanks for the production of container and window glass. They are made by melting oxides in an electric arc furnace, pouring the liquid into graphite moulds and allowing to slow cool over several days. Large (up to 1m cubes) blocks are cut and positioned in the tank. The microstructure consists of white rounded zirconia, black round pores, a dark grey aluminosilicate glass, light grey corundum and a binary eutectic (Al2O3-ZrO2).

Images

AZS.

Description of key features

2

The areas with small ovoids are slightly recessed.

Marks: / 10

Questions

ZrO2 is so much brighter than the other phases because…

a. it has the highest refractive index

Marks: / 1

3

Sample B - Unetched and Thermally Etched 96% Liquid Phase Sintered Alumina

96% aluminas are used in many mechanical and electrical insulation applications including seals and bearings, shafts, thermostat washers and tap plates. Oxide liquid forming additives include MnO and TiO2. They are fired at 1420oC which is above the MnTiO3 eutectic temperature at which melting occurs. This low temperature makes them cheap to produce. They are brown due to the MnO. Unetched the microstructure reveals porosity at low magnification. A 15 minute thermal etch at 1350oC enables the individual angular grains of alumina to be seen.

Images

Unetched Alumina.20X

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Etched Alumina, focussed on protusions.

Etched Alumina, focussed on surface.

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Description of key features

Note that in the second etched image there are dark areas (aside from the protusions). These are at a slightly lower level than the surface; I believe the surface is simply uneven.

Marks: / 10

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Questions

Angular porosity similar in size to the grains often arises in an optical micrograph of a ceramic because of…

a. grain pullout during the grinding and polishing procedure

b. incomplete sintering resulting in condensation of voids at grain boundaries

c. the volatilisation of some of Al2O3 during high temperature sintering

What is the approximate grain size of the Al2O3?

a. 1 micron

b. 10 microns

c. 100 microns

Marks: / 2

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Sample C - Unetched Lightly Sintered Silica Casting Core

This component is used as a core in an investment cast Nimonic turbine blade for jet engines. It is made of fused silica grain and a small amount of clay-derived glass bond. Its low thermal expansion and subsequent good thermal shock resistance make fused silica ideal for this application. Since the core size is critical to the application it is made by a near net shape process involving injection moulding and light (viscous flow) sintering at 1050oC resulting in <0.2% shrinkage and high tolerance. The sintering treatment also causes crystallisation of cristobalite which confers creep resistance and stiffens the core.The core is then impregnated with a polymeric resin which enters the porous matrix providing handling strength. It is placed in the casting mould which is preheated to 1000oC and the Nimonic alloy at 1600oC is then cast in. The metal shrinks as it cools and the ceramic sinters and shrinks with the metal so preventing metal tearing. The core’s open pore structure also helps gas to escape. Finally, it is etched out with caustic soda.

Images

Image focussed on white grains.

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Image focussed on intergrain area.

Description of key features

2D section of a 3D structure.

Marks: / 10

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Questions

The particles are angular because...

a. the material is only lightly sintered; therefore there are only necks between particles that essentially have a geometry residual from grinding

b. the material has fractured during grinding

c. the particles naturally grow with sharp points due to their crystal growth habit

There are so few contact points between the silica particle because…

a. the ceramic has fractured during grinding

b. the material is only lightly sintered so there is insufficient time to form many bonds

c. the polymer resin has expanded and forced the particles apart

Which of the following is not an important consequence of the light sintering process in the context of the application?

a. It maintains the net shape.

b. The pores allow out-gassing.

c. The resin/silica composite is easy to machine.

Marks: / 3

10

Sample D - Barium Hexaferrite Magnetic Ceramic

Barium hexaferrite (BaO.6Fe2O3) is a hard magnetic oxide which has a strong permanent magnetisation. Some are used in the house as magnets to latch cupboards or hang memos to fridges. Others are mixed as powder with rubber or plastic and used as a flexible strip on the fridge door to act as a latch and provide sealing. The crystal structure of hard ferrites is such that the magnetically preferred (i.e. easy) orientation is the c axis. They are classified as isotropic or anisotropic depending on whether the grains are arranged randomly or aligned. BaCO3 and Fe2O3 powders are calcined to form BaO.6Fe2O3 and then ground to produce large polycrystalline aggregates. These are then spray dried to give granules to which binder is added prior to pressing. The compact is then dried and solid state sintered at about 1250oC.

Images

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Aligned Ferrite. 5X

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Aligned Ferrite. 20X

Non-aligned Ferrite. 5X

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Description of key features

>>> type here <<<

Marks: / 10

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Questions

Approximately how large are the granules?

a. 0.3 microns

b. 3 microns

c. 30 microns

Porosity surrounds each granule because…

a. the pores are interfaces between the individual granules

b. pores align to reduce free energy

c. fracture occurs between each granule

Marks: / 2

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Sample E - Barium Titanate Ag/Pd Metal MLC (Capacitor)

To maximise the capacitance high surface area is needed and this can be achieved by having lots of large, thin layers of dielectric electroceramic in contact with conducting metal.The sample is of layers of barium titanate (BaTiO3, BT3) and Ag/Pd alloy powder which have been stacked together (like a stack of sandwiches) and then co-sintered. A BaTiO3 slurry is tape cast to give a thin sheet which is dried and a paste or ink containing the metal powder is screen printed on it. The large sheets are then diced and stacked and co-sintered together.

Images

Capacitor (Face on).

Description of key features

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>>> type here <<<

Marks: / 10

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Questions

The ceramic layers are approximately…

a. 10 microns thick

b. 100 microns thick

c. 1000 microns thick

The electrode layers are approximately…

a. 10 microns thick

b. 100 microns thick

c. 1000 microns thick

The alternate electrode layers increase…

a. the area between electrodes

b. the distance between electrodes

c. the relative permittivity

Marks: / 3

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Sample F - Macor™ Glass Ceramic

This glass ceramic has the “House of Cards” microstructure consisting of randomly-oriented, flexible, mica-like flakes which arrest or deflect cracks enabling free machining with standard drills. The mica-like phase is fluorophlogopite (KMg3AlSi3O10F2).

Images

5X Zoom.

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20X Zoom.

Description of key features

>>> type here <<<

Marks: / 10

Questions

Which of the following is a second phases in Macor™ Glass ceramics?

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a. MgF2

b. residual glass

c. fluorophlogopite

Marks: / 1

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