Metals in the mouth

Question 1

What bonding occurs in metals?

Answer 1

Metallic bonding

Question 2

Give 3 reasons why metals and alloys are used extensively in dentistry.

Answer 2

1. They have a high corrosion resistance (due to their pH range and chemical composition)
2. They are biologically inert.
3. They are strong and ductile

Question 3

What is the objective of the lost wax casting technique?

Answer 3

Take a wax pattern and reproduce it in metal.

Question 4

What are the steps in lost wax casting?

Answer 4

1. Tooth wax pattern.
2. Put on a sprue with a sprue base.
3. The whole structure is then packed around it with sand.
4. The sand dries and sets.
5. The sprue base is taken out and the structure is heated to melt the wax out.
6. The void left is filled with metal and broken out of the set sand.
7. It is then pickled (to remove oxides)
8. Polished and then delivered to the patient

Question 5

What does the CAD/CAM production of prostheses pertain?

Answer 5

using digital technology to design and manufacture dental prosthetic devices.

It involves cutting or grinding a tooth/restoration shape from a solid block of material. It is a subtractive process: material is taken away

Question 6

What does additive manufacturing from CAD/CAM pertain and why is it weaker than the subtractive process?

Answer 6

1. A layer by layer process which includes the laser sintering of powdered metal/alloy.
2. It is weaker as each interface created by the layering technique has a potential zone of weakness.
3. Therefore these interfaces are more vulnerable to shear forces.

Question 7

Define the word ‘phase’ in relation to metals.

Answer 7

Define the word ‘phase’ in relation to metals.

Question 8

What is occurring at thermal arrest of a metal?

Answer 8

1. The metal is transitioning from a liquid to a solid.
2. As it crystallises, heat is released.
3. This is at equilibrium with the temperature decrease hence the term ‘thermal arrest’:

Question 9

What is the energy released during thermal arrest termed as?

Answer 9

latent heat of fusion.

Question 10

• How does the microstructure of metals change from liquid to solid state?

Answer 10

1. Starts of as a melt with a few nuclei of crystallisation.
2. As freezing occurs,
3. A multitude of small crystals called grain form.
4. The end result is a polycrystalline material with grain boundaries

Question 11

Define a unit cell in metals.

Answer 11

The smallest repeating array of atoms in a crystal.

Question 12

What is the solid crystalline phase defined as?

Answer 12

The local atomic arrangement is repeated at regular intervals millions of times in 3D.

Question 13

What are the 7 3D crystal lattice systems?

Answer 13

1. Cubic
2. Tetragonal
3. Orthorhombic
4. Rhombohedral (trigonal)
5. Hexagonal
6. Monoclinic
7. Triclinic

Question 14

What is the arrangement of atoms in a body centered cubic cell?

Answer 14

1. Overall 2 atoms
2. Atom in the centre, and a quarter of an atom on each corner:

Question 15

What is the arrangement of atoms in a face centered cubic cell?

Answer 15

1. 5 atoms overall
2. Atoms packed
3. Atom in centre
4. Half atoms sit on each face of cube:

Question 16

What does the packing factor tell you?

Answer 16

The percentage of the cube that is filled with atoms.

Question 17

What is the equation for the packing factor of a lattice?

Answer 17

(Volume of atoms inside the cube) / (Volume of the cube)

Question 18

• Is a high or low packing factor preferred and why is this?

Answer 18

high As metals seek the lowest energy state.

Question 19

Give binary and ternary systems metal alloys can have.

Answer 19

look at notes

Question 20

How are combinations of the metals used to make alloys investigated?

Answer 20

Study of cooling curves.

look at notes for cooling curves

Question 21

what does a phase diagram show?

Answer 21

1. Map of phases
2. In other words, shows the state of the alloy with different compositions of the 2 metals used, at different temperatures:

Question 22

What are phase diagrams constructed from?

Answer 22

cooling curves

Question 23

What are the 3 types of alloys?

Answer 23

1. Solid solutions
2. Intermetallic compounds
3. Eutectic mixtures

Question 24

What are 2 properties of solid solutions?

Answer 24

1. Mix of 2 elements
2. There is no way of predicting where the atoms will sit

Question 25

What are the 2 types of solid solutions and their conditions?

Answer 25

1. Substitutional solid solution Atoms have similar valency Atoms have similar crystal structure **Atom sizes are within 15% of each other.** 2. Interstitial solid solution **Solute atoms much smaller than the solvent atoms**

Question 26

What are the conditions of inter-metallic compounds?

Answer 26

1. Formed when 2 or more metals react with each other to form a new component at a stoichiometric ratio. 2. In other words the chemical align into non-mixing layers. 3. E.g. amalgam where:

Question 27

What are 3 properties of inter-metallic compounds?

Answer 27

1. Higher melting point 2. Usually more brittle 3. Clear distinct phases.

Question 28

What are the conditions of a eutectic alloy?

Answer 28

1. Where metals are completely insoluble in the solid phase. 2. They crystallise at a very precise temperature : they behave like a pure metal in this sense (point C on this diagram):

Question 29

Why are eutectic alloys avoided?

Answer 29

1. Low strength 2. Corrode easily

Question 30

How is a wrought structure created from a cast?

Answer 30

1. Cold working/strain hardening = mechanically deforming a metal at a relatively low temperature 2. The deformation in the grains leaves residual stress from the distortion of the lattice structure.

Question 31

What can the creation of a wrought structure cause to the lattice structure?

Answer 31

Distortion of the lattice in the form of vacancies:

Question 32

What can be done to adjust atoms dislocating in the lattice?

Answer 32

Slip caused by shear stress - can use slip planes to avoid fracture or failure of crystal lattice:

Question 33

How can slip from shear stress lead to plastic deformation?

Answer 33

When stress exceeds the elastic region it enters the plastic region, where metals cannot go back to their original shape:

Question 34

How can deformed lattices from wrought structures be healed?

Answer 34

Annealing: 1. Heating the diffusion of atoms 2. Causes recrystallisation 3. New grains form. The more extensively wrought the structure (the more stress in grains) the lower the temperature at which recrystallisation occurs:

Question 35

What is lost and what is gained when annealing wrought structures?

Answer 35

1. Tensile strength lost 2. Ductility gained

Question 36

Why is the outer layer of an endodontic file heat treated?

Answer 36

Allows a hard surface for wear resistance - higher tensile strength.

Question 37

Why is the inner layer of an endodontic file not heat treated?

Answer 37

Less wrought so more ductile and flexible.

Question 38

Why is it only advised to turn the endodontic file 3x then throw it away?

Answer 38

1. Turning causes distortions. 2. Every turn the file becomes more brittle and more likely to snap in the canal.