Biomedical Engineering Reference
In-Depth Information
A miniature compressed air or electric motor is used to drive low-speed hand pieces. They oper-
ated with rotational speeds of up to 4,000 rpm. The burs used in low-speed operations are used to trim
dentures and remove decayed tissue and can be made of stainless steel.
Air turbine hand pieces can run up to 500,000 rpm, although they are rarely used at such high
speeds. General operating speeds are
∼
20,000-50,000 rpm varying with the diameter of the bur. The
burs used at high speeds are mainly diamond coated or made of tungsten carbide. High speeds are
used in the removal of enamel, dentine, and old fillings. When operating at such high speeds, a built
in water spray is used as a coolant, protecting both the bur and tooth.
Conditions in the surgery are different from the laboratory. The purpose of coating the bur with
diamond is to lengthen the working life of a bur so that it can be used for multiple patients and be
able to withstand repeated sterilization
[21,22]
. This process can increase the risk of instrument frac-
ture and also has the potential to be corrosive and may affect the cutting surface or coating of the bur.
15.3
CHEMICAL VAPOR DEPOSITION OF DIAMOND FILMS
ONTO DENTAL BURS
In order to increase the life and performance of dental burs, they have been coated with diamond.
Diamond is an attractive material
[12,15,22-47]
for this type of application because it has a high
hardness value, high thermal conductivity, chemical inertness, and high wear resistance (
Table 15.1
).
Normally natural diamond is obtained from mines, and it has formed over many years under extreme
conditions of high-temperature and -pressure underground. Diamond needs to be formed under condi-
tions that are thermodynamically favorable. From the carbon phase diagram, it is evident that heating
carbon under pressure can form diamond. Hence, the high-pressure, high-temperature (HPHT) growth
technique
[24-26,28]
was developed to produce “industrial” diamond. However, this is not suitable
for dental burs and therefore a chemical vapor deposition (CVD) process has been developed to grow
diamond onto various substrate materials in thin film form
[22,24-33]
. This process is conceptually
very simple and require thermal decomposition of carbon containing precursors, such as acetylene, at
low pressure. The films deposited have many of the properties associated with natural diamond.
Diamond films have extreme properties and have been deposited onto a range of substrates. Their
structure ranges from single crystals, polycrystalline to amorphous materials depending on the type of
Table 15.1
Physical Properties of Diamond
[22,24,28]
Property
Value
Hardness (kg mm
2
)
10,000
Strength, tensile (GPa)
1.2
Strength, compressive (GPa)
110
Density (g cm
3
)
3.52
Young's modulus (GPa)
1.22
Thermal expansion coefficient (K
1
)
0.0000011
Thermal conductivity (W cm
1
K
1
)
20.0
