Ceramics Processing

Question 1

What are traditional ceramics?

Answer 1

Traditional ceramics are made from minerals occurring in nature. They include pottery, porcelain, bricks, and cement.

Question 2

What are new ceramics?

Answer 2

New ceramics are made from synthetically produced raw materials and cover a wide spectrum of products such as cutting tools, artificial bones, nuclear fuels, and substrates for electronic circuits.

Question 3

What is the most common raw material of traditional ceramics?

Answer 3

Silicate ceramics-clays

Question 4

What are the steps involved in traditional ceramic processing?

Answer 4

Fine ceramic powder is mixed with water.
The paste is shaped and left to dry.
The dried shape is fired turning the dried clay into fired clay.

Question 5

What are the two general categories of the comminution of lumps to fine powder.

Answer 5

Crushing and grinding.

Question 6

Name and describe four methods of crushing.

Answer 6

(a) jaw crushers, in which a large jaw toggles back and forth to crush lumps against a hard, rigid surface
(b) gyratory crushers, which use a gyrating cone to compress lumps against a rigid surface
(c) roll crushers, in which the ceramic lumps are squeezed between rotating rolls
(d) hammermills, which use rotating hammers impacting the material to break up the lumps.

Question 7

Name and describe three methods of grinding.

Answer 7

a) Ball Mill, hard spheres mixed with the stock to be comminuted are tumbled inside a rotating cylindrical container. Often water is added and powder removed as slurry.
b) Roller Mill, stock is compressed against a flat horizontal grinding table by rollers riding over the table surface.
c) Impact Grinding, particles of stock are thrown against a hard flat surface, either in a high velocity air stream or a high-speed slurry

Question 8

Name and describe the roles of the three ingredients of a clay paste.

Answer 8

(1) clay, which provides the consistency and plasticity required for shaping
(2) non-plastic raw materials, such as alumina and silica, which do not shrink in drying and firing but unfortunately reduce plasticity in the mixture during forming
(3) other ingredients, such as fluxes that melt (vitrify) during firing and promote sintering of the ceramic material, and wetting agents that improve mixing of ingredients

Question 9

What are the four types of clay shaping process (in terms of water content)?

Answer 9

(1) slip casting, 25% to 40% water
(2) plastic-forming, 15% to 25% water
(3) semi-dry pressing, 10% to 15% water
(4) dry pressing, less than 5% water.

Question 10

What are the steps in drain slip casting?

Answer 10

• Slip is poured into a plaster of Paris mold.
• Water is absorbed into the plaster mold to form a firm layer
• Excess water is poured away
• Remaining cast part is removed from the mold

Question 11

What are the steps in solid slip casting?

Answer 11

The slurry is poured into the mold and left for long enough to completely solidify. The mold must be periodically resupplied with additional slip to account for shrinkage because of absorbed water.

Question 12

What is the most common technique of plastic forming?

Answer 12

Hand molding, often with a potter’s wheel

Question 13

What is the process of semi-dry pressing.

Answer 13

The material has a low pasticity due to the low water content, it is therefor forced to flow into a die cavity with high pressure to shape it.

Question 14

What is the dry pressing process.

Answer 14

The same as semi-dry pressing however due to the low water content a binder is used to give the part sufficient strength. Dry pressing is very abrasive so the die must be made of hardened steel or cemented tungsten carbide. As there is little flow the part geometry must be simple, also no flash forms.

Question 15

What are typical products of dry pressing?

Answer 15

Typical products include bathroom tile, electrical insulators, and refractory brick.

Question 16

Describe the two stages of the shrinkage process.

Answer 16

In the first stage, the rate of drying is rapid and constant, as water is evaporated from the surface of the clay into the surrounding air and water from the interior migrates by capillary action toward the surface to replace it. It is during this stage that shrinkage occurs, with the associated risk of warping and cracking owing to variations in drying in different sections of the piece. In the second stage of drying, the moisture content has been reduced to where the ceramic grains are in contact, and little or no further shrinkage occurs. The drying process slows, and this is seen in the decreasing rate in the plot.

Question 17

When is a ceramic piece green?

Answer 17

After shaping but before firing - not fully processed or treated.

Question 18

Describe firing.

Answer 18

Firing is the heat treatment process that sinters the ceramic material; it is performed in a furnace called a kiln. In sintering, bonds are developed between the ceramic grains, and this is accompanied by densification and reduction of porosity. Therefore, shrinkage occurs in the polycrystalline material in addition to the shrinkage that has already occurred in drying. In the firing of traditional ceramics, certain chemical reactions between the components in the mixture may also take place, and a glassy phase also forms among the crystals that acts as a binder. Both of these phenomena depend on the chemical composition of the ceramic material and the firing temperatures used.

Question 19

What is the difference between glazed ceramics and unglazed ceramics?

Answer 19

Unglazed ceramic ware is fired only once; glazed products are fired twice.

Question 20

What is glazing?

Answer 20

Glazing refers to the application of a ceramic surface coating to make the piece more impervious to water and to enhance its appearance

Question 21

What are the steps involved with glazing?

Answer 21

(1) fire the ware once before glazing to harden the body of the piece, (2) apply the glaze, and (3) fire the piece a second time to harden the glaze.

Question 22

What are design considerations involved with ceramics?

Answer 22

• Ceramic materials are several times stronger in compression than in tension; accordingly, ceramic components should be designed to be subjected to compressive stresses, not tensile stresses.
• Deep holes, channels, and undercuts should be avoided, as should large cantilevered projections.
• Outside edges and corners should have radii or chamfers; likewise, inside corners should have radii.
• Part shrinkage in drying, firing and sintering may be significant and must be taken into account by the designer in dimensioning and tolerancing.

Question 23

Describe the process of a ceramic body drying and how aggregate volume changes with water content.

Answer 23

There is initially a linear decrease in the volume with a constant drying rate as the water evaporates from the surface and draws water from deeper regions via capillary action.

After particles come completely in contact with each other, the volume changes stop. Drying rate decreases as the rate of evaporation drops due to less capillary action.