Question

a) Explain why ceramics that are bonded covalently exhibit: i) the highest stiffness ii) highest melting points iii) lowest thermal expansion coefficients

b) why does water expand during cooling

c) what are the main differences between the two mechanical properties: stiffness and strength?

d) the stress necessary for plastic deformation of Nickel is far lower compared to Tungsten even though both have the same number of slip systems. Why?

Answer 1

(a)Answer:- Ceramic materials are held together by covalent bond. With these bonding type the valence electrons are either shared between neighbour(ion). The electrons are not free to move through out structure in the way that they are for materials with metallic bonding, and that means that ceramics are poor conductors of heat. Bonds are very strong and stiff resulting in ceramics that exhibit very high stiffness and very high melting point.

It is very difficult for atoms to slide each other in the way they do in metals under stress. Their inability to do this means that when ceramics are put under mechanical stress, rather than deforming they are susceptible to failure in a fracture.

(b)Answer:- When liquid water is cooled, it contracts until a temperature 4 degree Celsius is reached. After that it expands slightly until it reaches the freezing point, and then when it freezes it expands by 9 %.

Ice has more extra space in structure than water. Due to this reason water expands after cooling.

(c) Answer:- Difference between stiffness and strength:-

Stiffness:- stiffness is an indicator of the tendency for an element to return to its original form after being subjected to a force.

Strength:- Strength measures how much stress can be applied to an element before it deforms permanently or fractures.

(d) Answer:-

Tungsten is body centered cubic. Slip in this crystal system usually occurs in the

Perpendicular family of planes. A plane will experience no shear stress if their Schmid factor is zero . This corresponds to the plane normal being either perpendicular or parallel to the applied stress . Of the perpendicular family.

But Ni has fcc slip plane .