Biomedical Engineering Reference
In-Depth Information
WC substrates. They found that the adhesive toughness of diamond on WC to be in the range of
20-37 J m 2 . Commercial burs exhibited much higher adhesive toughness than flat substrates due to
the large surface roughness and the absence of interfacial voids. This factor needs to be investigated
in detail for nonplanar dental burs.
The effectiveness of using HFCVD-coated dental burs was measured by comparing uncoated
burs, HFCVD dental burs, and PCD sintered diamond burs when drilling and machining extracted
human teeth, acrylic teeth, and borosilicate glass.
15.4.6 Drilling Experiments
A sequence of fifty drillings was employed in each drilling experiment. The sharpness and initial con-
dition of the burs were inspected visually after the burs had drilled 10 holes in sequence. An abrading
coefficient of drilling, C a , has been defined as a quality criterion for small drilling tools. It is defined
as the ratio between the bur's total abraded area, S , and the effective coated area of the bur used during
the drilling process. The effective coated area is given by the difference of the nominal coated bur area,
D b , and the area of the bur consumed during drilling, W b . C a is defined by the following equation:
(15.1)
/( )
A high-quality-coated dental bur is one that produces accurate drilling that has an area S close
to D b and does not lose its coating during the machining process, i.e., W b ≈0. The cutting will there-
fore have an abrading coefficient close to unity. One must remember that the quality of machining is
dependent on the cutting speed, V c , defined by the ratio of the drilling depth by the drilling duration.
A comparative factor of merit for the dental bur is defined as
C
S D
W
a
b
b
(15.2)
/
where F , the figure of merit, is directly related to the lifetime of the dental bur for a specific drilling
process.
F
a
C V
c
15.4.7 Performance Results
Figures 15.23-15.25 show the results of drilling the dental materials with the three types of burs on
borosilicate glass, acrylic tooth materials, and human teeth, respectively.
For all the materials investigated including acrylic, borosilicate glass, and human tooth material,
it is evident that the best-performing bur in term of figure of merit were the HFCVD diamond-coated
burs on the WC-Co substrates and TiN.
Figure 15.26 shows a micrograph of uncoated WC-Co dental bur tested on the borosilicate glass
using the same machining conditions. The uncoated WC-Co burs displayed flank wear along the cut-
ting edge of the bur. The areas of flank wear were investigated at the cutting edge of the dental bur.
Figure 15.31 shows flank wear as a function of cutting time when drilling borosilicate glass. It is
evident that the action of machining causes high rates of flank wear on the cutting edge of dental bur.
Therefore, the cutting edges of WC-Co dental burs should have a minimum thickness of CVD dia-
mond, which will enhance not only quality of cutting but also prolong the life of the bur.
Figure 15.27 shows the surface of a sintered diamond bur after being tested on borosilicate
glass at a cutting speed of 30,000 rpm for 5 min with an interval at every 30 s. It is clear that there is
Search WWH ::




Custom Search