Biomedical Engineering Reference
In-Depth Information
This metastable behavior, however, is also at the origin of an aging
phenomena when placed in a moist atmosphere (low temperature
degradation, LTD) (Figure 9.13). As water penetrates the crystalline
structure, it generates the t-m transformation in the material surface,
and subsequently engenders surface alterations, microcracks and loss
of strength.
Figure 9.13.
Schematic illustration of the low temperature
degradation mechanism, adapted from [CHE
08]
Currently, the kinetics and impact of LTD on the life span of dental
prostheses is still unknown [LUG
10].
Today, the clinical background of zirconia-based restorations
(ZBRs) indicates high rates of short-term failures related to cohesive
fractures of the veneering ceramic (around 13% after 5 years)
[KOE
13] (Figure 9.14). This phenomenon, called chipping, is
reported to be more frequent than with PFM restorations
[HEI
10,
PEL
12, VIG
12]. The chipping mechanism remains misunderstood.
Several factors have been pointed out concerning this problem. Some
are related to the materials and to the manufacturing process,
particularly the thermal properties of zirconia and of veneering
ceramic, the cooling rate, the veneer-framework thickness ratio or the
design of the framework. It has also been shown that Y-TZP can
undergo structural changes in the surface in contact with the veneering
ceramic, due to diffusion processes or to stress development occuring
during the firing procedure [DUR
12,
MAI
13]. This highlights the
sensitivity of this sophisticated material. Besides the material-related
