Biomedical Engineering Reference
In-Depth Information
Feldspar becomes the bond when melted between 1,250 °C and 1,500 °C, that can
tightly integrate quartz and kaolin clay. Kaolin clay (K
2
O
3
· 2SiO
2
· 2H
2
O) has a
certain plasticity that benefi ts the shape when making porcelain products. It has the
advantage of being easy to combine with feldspar and brings toughness and opacity
to the material; however, it shrinks a lot after dehydration. Fluxing agents, like
borax, are also added to lower the melting temperatures, thereby making feldspar-
based porcelains easier to handle in the laboratory. Quartz (Si
2
O) can increase the
strength of porcelain material, while the fracture strength can also be raised by
adding crystals, which additionally helps modify the coeffi cient of thermal expansion
to comply with that of the metal match to which it is fused. Potassium oxide is used
to give the porcelain a coeffi cient of thermal expansion that can match the metal
alloy very closely [
47
].
Changing the proportion of the components of feldspathic porcelain can affect
the mechanical characteristic of the material. Feldspathic porcelain is innoxious and
nonirritating to the body; furthermore, it can tolerate the effect of microbes,
enzymes, in the saliva. With good biocompatibility, biological safety, and biofunc-
tionality, this traditional biphasic feldspathic veneer porcelain is widely and
commonly used to be fi red onto a metal substructure to obtain the natural appear-
ance and function of teeth.
Feldspathic porcelains are brittle in nature and subject to premature failures
under repeated occlusal contact in vivo. Their wear resistance is infl uenced by the
material property and oral environments.
H. Y. Yu et al. used a small-amplitude reciprocating apparatus to explore the
wear behavior of two distinguished dental feldspathic porcelains under simulated
oral conditions [
48
]. They used Cerec Vitablocs MarkII and Vita VMK 95 as the
samples of industrially prefabricated feldspathic porcelain and traditional biphasic
feldspathic veneer porcelain, respectively. The two samples exhibited different
microhardnesses and toughnesses. Many voids appeared on the surface of Vita 95,
with maximum diameters of tens of microns. Cerec Vitablocs MarkII porcelain had
a more uniform microstructure, with a higher toughness but lower microhardness
than those of Vita 95. The results showed that the wear depth of Vita 5 (52
μ
m
maximum) was much lower than that of Cerec Vitablocs MarkII (93
m maximum)
under the same conditions. It indicates that the differences in hardness and tough-
ness may clearly explain both an increase in wear depth and the different wear
mechanisms from Vita 95 to Cerec MarkII because wear behavior is closely associ-
ated with such mechanical properties as the hardness and toughness of materials
tested. Also, the microstructural differences may contribute to the wear behavior. In
addition, the load effect was prominent to the friction coeffi cient and wear depth to
both porcelains.
Much research has been conducted to improve the mechanical properties of
porcelains by means of surface treatments and internal enhancements. However,
internal change often leads to deterioration of the optical characteristics of porce-
lain, such as color and transparency, and then signifi cantly infl uences the aesthetic
effect of porcelains. Hence, surface treatments are widely focused on in current
research. Auto-glazing, surface ion exchanging, and polishing are three surface
μ
