Ceramics and Glasses

Question 1

What are general mechanical properties of ceramics?

Answer 1

Despite having a low toughness, ceramics have high hardness and excellent high temperature and corrosion resistance and are thermal and electrical insulators.

Question 2

Why is the stress-strain behavior of brittle ceramics not usually ascertained by a tensile test?

Answer 2

1. It is difficult to prepare and test specimens having the required geometry
2. It is difficult to grip brittle materials without fracturing them
3. Ceramics fail after only about 0.1% strain, which necessitates that tensile specimens be perfectly aligned to avoid the presence of bending stresses, which are not easily calculated

Question 3

What is a suitable transverse bending test for ceramics?

Answer 3

A three or four point loading technique.

Question 4

What does the three point bending point measure?

Answer 4

The stress at fracture is known as Flexural Strength (Modulus of rupture, fracture strength, bend strength)

Question 5

Why does flexural strength depend on specimen size?

Answer 5

With increasing specimen volume there is an increase in the probability of the existence of a crack producing flaw.

Question 6

What are the six types of ceramic?

Answer 6

Glasses
Clay Products
Refractories
Abrasives
Cements
Advanced Ceramics

Question 7

What is crystallization?

Answer 7

The process of transforming non-crystalline inorganic glasses to a crystalline state by a high temperature treatment. These are known as glass ceramics.

Question 8

What are characteristics of glass ceramics

Answer 8

Glass-ceramic materials have been designed to have the following characteristics: relatively high mechanical strengths; low coefficients of thermal expansion (to avoid thermal shock); relatively high temperature capabilities; good dielectric properties (for electronic packaging applications); and good biological compatibility.

Question 9

What are the two main types of clay products, give examples.

Answer 9

Structural Clay Products, bricks and tiles

Whitewares, porcelain and china

Question 10

What are the desirable properties of refractories?

Answer 10

Capacity to withstand high temperatures and the capacity to remain unreactive and inert when exposed to severe environments. In addition, the ability to provide thermal insulation is often an important consideration

Question 11

Name the four main categories of refractories

Answer 11

Fire Clay
Silica
Basic
Special

Question 12

What are typical applications for refractories?

Answer 12

Furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation

Question 13

What are mechanical properties of abrasives?

Answer 13

They are very hard particles that do not easily fracture

Question 14

What are common uses of abrasives.

Answer 14

Abrasives are used in several forms-bonded to grinding wheels, as coated abrasives, and as loose grains. Sandpaper or grind wheels are used for grounding and polishing.

Question 15

Give an overview of cement.

Answer 15

The most common type is Portland Cement, it is produced by mixing clay and lime bearing materials and heating to up to 1400C in a process called Calcination. The resulting Clinker is ground to a powder which is a hydraulic cement. When mixed with water and left to set it goes solid.

Question 16

Give some examples of advanced ceramics

Answer 16

Optical fibres (high purity silica), ceramic ball bearings (Lighter and Harder than steel so higher speeds are achieved with less vibration), piezoelectric materials.

Question 17

If you wanted to increase the mechanical strength of your glass how might you achieve this and what is the physical mechanism which causes the strength increase?

Answer 17

Inorganic glasses can be made to transform from a non-crystalline state to one which is crystalline by the proper heat treatment. This process is called crystallisation, and the product is a fine-grained polycrystalline material which is often called a glass-ceramic. Due to the change in the structure this imparts superior mechanical strength.

Question 18

Cite 2 properties that may be improved by crystallization of glasses. [

Answer 18

(1) a lower coefficient of thermal expansion, and (2) higher strengths.

Question 19

Why may a ceramic make a good insulator

Answer 19

Ceramics have no free electrons and may be designed to be porous

Question 20

Describe the crystal structure of a ceramic in terms of crystallinity and composition

Answer 20

Ceramics form highly ordered (crystalline) structures and are generally more complex than metals as they are composed of at least two elements. Positively charged cations bond with negative charged anions to form crystalline structures.

Question 21

Why are ceramics considered resistant to corrosion

Answer 21

As ceramics contain metallic and non-metallic atoms they can be considered to have already eroded.