MATS3002: A Cylindrical Component of Zirconia (ZrO2) Component – Fundamentals of Ceramic Processing Assignment, UNSW, Singapore

MATS3002 FUNDAMENTALS OF CERAMIC PROCESSING ASSIGNMENT: THE UNIVERSITY OF NEW SOUTH WALES (UNSW)

Question 1) A cylindrical component of zirconia (ZrO2) component (nominally 20 mm long by 20 mm diameter) is formed by single-ended uniaxial dry-pressing of zirconia powder. The starting powder consists of uniaxial particles approximately 0.05 µm in size and the powder is significantly agglomerated. During pressing, the compaction pressure is applied from the top.

After pressing and firing, it is found that the component has the following significant defects: (i) the diameter at the top of the cylinder is significantly larger than the diameter at the bottom of the cylinder (as shown schematically below left), and (ii) the microstructure of the component (see example shown below right) contains a significant amount of porosity which is relatively large in size with this porosity being more pronounced towards the bottom of the component.

a) With the aid of a sketch, briefly describe the process of single-ended uniaxial dry-pressing.
b) What are the likely causes of the as-described defects in the component?
c) Explain what could be done in terms of the starting powder and/or dry-pressing process to try to minimise/eliminate the as-described defects.

Question 2) Despite what was proposed in your answer in Part 1)c), it is found that the defects still occur and so it is proposed to fabricate the zirconia component instead by a process involving slip casting. The casting slip is to be prepared by dispersing the same starting zirconia powder in water, with an acid or alkali to be used to control pH.
a) With the aid of a sketch, briefly describe the process of slip casting and the mechanism by which the cast material forms.

b) Given that the isoelectric point of the zirconia is ~6.4, explain the likely effect of pH (and why) on each of the following characteristics:
i) tendency for particle settling in the casting slip
ii) viscosity of the casting slip
iii) solids content of the casting slip
iv) casting rate
iv) bulk density of the cast layer after drying

c) Propose an approximate pH value that the casting slip could be adjusted to so as to optimise the slip casting process and quality of the resultant zirconia component. Justify your answer with reference to your answers in Part 2)b).

Question 3) A structural clay product is to be made from a naturally occurring clay by a process involving the mining, crushing, and grinding of the clay followed by extrusion (using an auger extruder), drying (in a tunnel dryer) and firing (in a tunnel kiln). The clay deposit is in the form of separate, approximately horizontal strata with the strata having distinctly different chemical and physical characteristics. The product has a complicated cross-sectional shape and so the clay needs to have high plasticity for forming.

a) Discuss the significance/implication of the clay existing as distinctly different strata for the overall manufacturing operation and for the operation of the manufacturing operation in the long-term.
b) Define the term plasticity and explain the mechanism by which plasticity is achieved in a clay material.
c) Specify and explain three aspects of the clay body which could be controlled so as to maximise the plasticity.
d) With the aid of a sketch, briefly explain the principle of operation of the auger extruder.
e) In drying the extruded ceramic product in the tunnel dryer, it is found that the product is more prone to warping and cracking during the first one third (approximately) of the drying cycle. With reference to the mechanism(s) of drying explain:
i) the underlying causes(s) of the warping and cracking.
ii) why the cracking occurs in the first one third (approximately) of the drying cycle.
iii) what modifications or changes could be made to the control of the drying conditions in an attempt to reduce or eliminate this problem.

Question 4) A cylindrical engineering alumina component is to be made from a commercial high-purity α-Al2O3 powder refined from mined bauxite by the Bayer Process. A sample of the supplied powder is shown in the adjacent SEM micrograph. The powder contains particles ~0.1-0.2 µm in size with some being present in the form of large agglomerates up to ~4 µm in size. The fabrication route involves single-ended uniaxial dry-pressing in cylindrical die followed by pressureless sintering in air. The intended nominal dimensions of the final sintered component are 30.0 mm long and 30.0 mm diameter.
a) Explain why having a small particle size is a desirable characteristic of the powder.
b) Explain the likely behaviour(s) of the agglomerates during the fabrication process and comment on the effect(s) that they would likely to have on the final sintered microstructure of the alumina.
c) Explain a likely cause of the agglomerates arising from the Bayer Process and specify what could be done to the powder to minimise/eliminate the agglomerates.
d) The dimensions of the component after sintering are found to be non-uniform in that the diameter at one end is close to the nominal value of 30.0 mm but is 32.2 mm at the other end. Propose a feasible cause of this non-uniformity and specify what could be done to the fabrication route to improve the component’s dimensional accuracy.

Question 5) Whatever attempts to improve the component’s dimensional accuracy specified in Question 1)d) were unsuccessful in practice and so it was decided to make the alumina component instead by a process involving slip casting. The casting slip is to be prepared using the same powder as that used in Question 1) plus water and an acid or alkali to control pH. The isoelectric point of the alumina is ~8.2.

With reference to the electrical characteristics of alumina particles in water (DLVO theory), explain the likely way in which each of the following characteristics is affected by pH and how it should be optimised for good performance of the alumina casting slip. On the basis of your answers, propose a recommended pH value for the casting slip.
a) tendency for particle settling in the casting slip
b) viscosity of the casting slip
c) solids content of the casting slip
d) bulk density of the cast layer after drying

Question 6) A clay product is to be made by a fabrication route involving forming by extrusion and drying in a tunnel dryer. An attempt is to be made to improve the plasticity of the starting clay (so as to make it better for forming) by adding to it an amount of a second, more plastic, clay.
a) Explain three possible characteristics of the second clay which make it, in relative terms, a more plastic clay compared with the starting clay.
b) Draw a schematic diagram showing the layout and main features of a tunnel dryer. Explain briefly the principle of operation of the dryer.
c) It is found that the resultant product (i.e, made from the mixed clays) tends to suffer significantly more cracking during drying compared with a product made from just the starting (original) clay. Explain a reason for this.
d) The cracking occurs during the first one third (approximately) of the length of the tunnel dryer (from where the product enters the dryer). Explain the reason for this and explain how the operation of the dryer could be changed so as to minimise/eliminate the cracking.

Question 7) An alumina femoral head component (shown in the figure on the right) is to be fabricated from a commercially-available alumina powder by a processing route involving ceramic dry forming. The component is to have the following specifications:
Outside diameter = 25.4 ± 0.01 mm
Relative bulk density >99.95%
Phase Composition = single-phase (α-Al2O3) alumina
Grain size (<1.2 µm)
a) Specify three characteristics that the starting powder should have in order to achieve the above specifications. For each characteristic, explain its significance/importance in the fabrication process.
b) Using the as-supplied powder described above as the starting point, propose a feasible processing route to fabricate the component and specify the main equipment used.
c) Define the term relative bulk density. Explain how the forming process will influence and determine the relative bulk density and dimensional tolerances attained in the final component.

Question 8) A structural clay product is to be made from a naturally occurring clay by a process involving the mining, crushing, grinding of the clay followed by extrusion (using an auger extruder), drying and firing. The product has a complicated cross-sectional shape and so the clay needs to have high plasticity for forming.
a) Define the term plasticity and explain how plasticity is achieved in a clay material.
b) Specify and explain three aspects of the clay material which could be controlled so as to increase plasticity.
c) Briefly explain the principle of operation of the auger extruder.
d) As a consequence of the two aspects described in part b) it is found that the product tends to suffer an increased incidence of cracking during drying. Explain the reason for this and prescribe a possible solution to the problem.

Question 9) An alumina component is manufactured by a process involving slip casting and drying in an intermittent (batch) dryer.
a) With reference to the electrical characteristics of ceramic particles in water (DLVO theory), explain how pH and/or deflocculant addition should be manipulated in order to control the viscosity and casting rate of the casting slip and the bulk density of the cast layer after drying.
b) Draw a schematic diagram showing the layout and main features of an intermittent (batch) dryer. Explain briefly the principle of operation of the dryer.
c) Define the term relative humidity. Draw a schematic graph showing the relative magnitudes of temperature and relative humidity as a function of the drying cycle.
d) It is found that the product tends to crack early in the drying cycle. Explain a likely cause of this cracking and, with reference to your answers in parts a) and b), propose a potential solution to the problem.

Question 10) A structural clay product is to be made from a naturally occurring clay by a process involving the mining, crushing, grinding of the clay followed by extrusion (using an auger extruder), drying and firing.
a) Describe briefly a method of mining and stockpiling that could be used to obtain the clay raw material and explain any advantages and/or disadvantages of this method.
b) Define “plasticity” and with reference to the electrical characteristics of ceramic particles in water (DLVO theory) explain the mechanism by which it is generated in the clay material.
c) With reference to the electrical characteristics of ceramic particles in water (DLVO theory), explain how each of the following characteristics will influence the plasticity of the clay:
i) mineralogical content of the clay
ii) particle size(s) of the clay and non-clay minerals
iii) water content of the plastic body
d) With the aid of a sketch, briefly explain the principle of operation of the auger extruder. Explain the importance of desiring of the clay.

Question 11) A cylindrical zirconia component (20 mm diameter, 20 mm long) is formed by single-ended uniaxial pressing. During pressing, the compaction pressure is applied from the top. After firing, it is found that the component is not within dimensional tolerances and, in particular, the diameter at the top of the cylinder is significantly smaller than the diameter at the bottom of the cylinder (as shown schematically at left). Explain the likely cause(s) of this defect and propose ways to minimise or eliminate this defect.

Question 12) An alumina component is manufactured by a process involving slip casting. The casting slip is to be prepared using a purchased commercial high-purity alumina powder and water, with an acid or alkali used to control pH.
a) Describe the process of slip casting and the mechanism by which the cast material forms.
b) Given that the isoelectric point of the alumina is ~8.2, propose an approximate pH value that the casting slip should be so as to optimise the characteristics listed below.
c) With reference to the electrical characteristics of ceramic particles in water (DLVO theory), explain how pH affects each of the characteristics below and justify your pH value proposed in Part b).
i) tendency for particle settling in the casting slip
ii) solids content of the casting slip
iii) viscosity of the casting slip
iv) casting rate
v) bulk density of the cast layer after drying

Question 13) An extruded ceramic product is dried in a continuous (tunnel) dryer.
a) Draw a schematic diagram showing the layout and main features of this type of dryer. Explain briefly the principle of operation of the dryer.
b) Draw a schematic diagram showing the magnitudes of temperature and relative humidity as a function of position along the length of the dryer.
c) With reference to the main stages of drying and your diagrams in Parts a) and b), explain the way in which the temperature and relative humidity should be controlled so as to give fast but safe drying of the product.