الفهرس 
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Abstract
Summary and Conclusions
The chemical durability of Duceram Low Fusing Ceramic has been studied in different solutions of different pH values in the range of 1.70 to 11.72.The samples throughout this study were grouped as follows:
1. Group (1): is the control group in which the samples were fired according to the manufacturer instructions. The starting temperature is 460C and the final temperature is 660C.
2. Group (2): is the controlled heat treatment in which the starting temperature is 460C and the final temperature is 660C. The firing cycle takes place within 20 minutes whereas the cooling period is about 15 min. This slow heating and slow cooling led to a densification of the ceramic.
3. Group (3): frequent firing at the sintering temperature 3-5 times. This firing stimulates the circumstances usually followed in the porcelain laboratories.
4. Group (4): the discs were soaked for 4-hours in silane (3- methacry-oxypropyltri-methoxysilane , 97 %) as a moisture barrier. Silanes are coating materials which cause changes in the surface free energy and hence change the surface wettability.
The chemical durability was studied through several methods:
I) Corrosion rate determination by weight loss.
The samples of the four groups were shacked in corroding solutions of pH = 2.40 and pH = 9.18 for 18 hours at 80C and the weight loss (mg/L) was calculated. The results revealed the following:
a) The alkaline solution is more corrosive to Duceram LFC than the 4 % acetic acid, solution, pH = 2.40. The weight loss recorded for group (3) samples was the largest, while the weight loss for group (2) samples. controlled heat treatment, was the least. The results of group 4 (silane coated) were in between the results of group (1). Control group, and group (2), controlled heated group.
b) The effect of addition of fluoride ions, added as potassium fluoride, on the rate of corrosion of Duceram LFC was studied. The results showed that corrosion was diminished after the addition of fluoride ions. This was attributed to the reinforcement of the ceramic network by fluoride ions which substitute the leached Na+, K+ and Ca2+ ions from the surface layer of the ceramic.
c) The effect of the ion, chloride and acetate, on the rate of corrosion of the ceramic was studied at a constant pH = 2.4. Two corrosive solutions ,hydrochloric acid and acetic acid of the same pH, were prepared and samples of Duceram LFC were shacked in for 40 hours at 80C and the value of the pH was recorded through out the periode of the experiment. The change in the pH is an indicator of the corrosion of the samples. The results showed that acetate ions are more corrosive than the chloride ions. The acetate ion is a strong chelating agent whereas the chloride ion is not, chelation increases the dissolving ability of the ion.
II) Corrosion rate determination by electrochemical methods.
The studies were performed using the electrolytic cell, Fig (7). Variation of the pH during the corrosion process leads to variation of the EMF of the cell. The values of E¬¬¬¬¬¬¬¬¬¬¬¬¬¬corr were measured using a combined glass electrode as well as a quinhydrone electrode coupled with a saturated calomel electrode (as a reference electrode). The results showed that the change in potential of the cell is high in the first ten hours then a state of equilibrium is reached at the end of the experiment (28 hours). This result indicates that the rate of corrosion is high at first then diminishes as a result of the formation of a protective layer.
The porosity of Duceram LFC was studied before and after corrosion for the four studied groups. The porosity % is a good indication for the extent of corrosion. The maximum porosity % was found in group (3) samples. This is due to the formation of cracks on the surface of the studied ceramic as a result of frequent firing and the glassy nature of Duceram LFC. Non-crystalline nature enhances cracks propagation specially post corrosion in the aggressive alkaline medium.
III) Flexure strength of duceram LFC:
Flexure strength was studied pre- and post- corrosion in acid and alkaline media. The results showed that flexure strength increased post corrosion in acidic medium, pH = 2.40. In this medium the leached layer, the super facial layer, consists mainly of alkali and earth alkaline elements. Leaching this layer exposes the stronger silicate-based matrix on the surface; the result is an increase in the flexure strength.
The alkaline medium, pH = 9.18, had an adverse effect on the flexure strength. The aggressive nature of the medium leads to the hydrolysis of the silicate rich matrix (break the Si-O bond) and the result is a decrease in the flexure strength of Duceram LFC.
The flexure strength increased in group (2) samples as a result of the densification of the ceramic which leads to a reduction in the points of defects. After corrosion in acid medium, the value of the flexure strength increased as a result of leaching of the weak super facial layer leaving the dense and strong silica rich layer.
The flexure strength decreased in group (3) samples. The lowest flexure strength was recorded for this group, frequent firing. Frequent firing led to formation of cracks which were deepened on corrosion either in acid or in alkaline media. Hence, the lowest values of flexure were recorded for this group. Silane did not affect the flexure strength, it delays corrosion rather than increases the strength of the ceramic.
IV) Scanning electron microscope:
Scanning electron microscopy was used to study the topography of Duceram LFC before and after corrosion. The SEM images revealed that corrosion takes place as surface irregularities and roughness. Generally the defects were in the form of minute pitting of the surface except the SEM images for post corrosion in alkaline medium where pores increased in size and depth.
The scanning images of group (3) showed minute cracks and pores specially at the meeting point of three crack lines. The surface showed an evidence of a true breakdown of Duceram LFC post corrosion in both acid and alkaline media. The SEM images showed clearly that the alkaline medium is a stronger corroding medium than the acid medium for all the studied groups.
V) Colour measurements.
The colour coordinates: x, y, and z were calculated for the four studied groups of Duceram LFC. The values of L*, a*, and b* as well as the colour difference ∆E were calculted for all groups before and after corrosion in both acid and alkaline media. The colour difference ∆E was found to be within the clinically invisible range (∆E must be more than 3.7 to be noticed by human eye). The colour difference calculated for all groups of Duceram LFC, before and after corrosion, did not exceed 3.7 except for group (3) samples after corrosion in alkaline medium where ∆E = 3.9. The colour difference is high as a result of crack formation, leaching of most of the elements of the surface layer and break down of the silicate matrix.
The L* values were calculated, L* is related to the luminance of the sample. The highest values are of group (2) samples. Densification and reduction of surface defects led to the increase in L*. After corrosion in both acid and alkaline media, L* values did not decrease significantly as a result of the low corrosion rate of this group , surface properties were not significantly affected even after corrosion.
In group (4) any changes in colour is due to the flaw of silane into the surface of the ceramic, light reflection is changed. However, the colour difference in this group, ∆E, is very low.
VI) Spectroscopic measurements.
Spectroscopic methods are among the accurate methods used to investigate the elemental composition, base unit structure, base unit geometry and phase structure of a ceramic. Of these methods are X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and diffuse reflectance. All these methods were used in the present work.
XRF is a multi element method of analysis that does not require the dissolution of the sample. A fluorescence band characteristic of each element appears in the spectrum of the sample. In this work, XRF of Duceram LFC showed the existence of the elements constituting the ceramic even those with minute concentrations and are not reported by the manufactrer as Ni, Cr, and S. After corrosion, the XRF spectra, of the different groups studied, proved that none of the elements was completely leached by corrosion in acid or alkaline media.
XRD-spectra of the studied groups indicated clearly: the glassy natured of Duceram LFC, the built up of crystals (order) on regulated heat treatment, destruction of crystals on frequent firing and a detectable decrease in crystals on corrosion specially in alkaline medium.
FTIR spectra identify clearly the functional groups in a molecule. The spectrum is a finger-print of the molecule. The FTIR spectra carried out in this work proved:
a) The existence of –OH group (not H2O) in the blank sample.
b) The basic structural unit of Duceram LFC is SiO44- and HO -Si-O33-.
c) The bands of the fundamental vibrations of: Si-O, O-Si-O and Si-O-H are strong.
d) On corrosion the Na+, K+, and Ca2+ ions in the ceramic are exchanged by H+ adds to the O2-.
e) No chemical reaction occurs between silane and Duceram LFC after immersion for four hours.
Diffuse reflectance spectra were studied before and after corrosion. There is a weak colour change. The extent of colour change is dependent on:
i) The way of treatment of the sample.
ii) The corrosive medium.
Conclusions and Recommendation
1. Mass loss % can be used to estimate the corrosion rate of Duceram-LFC.
2. The corrosion rate of Duceram-LFC at 90C depends on pH and it is a minimum at pH 4. The order of corrosion rate is pH 2.40 > pH 9.18 > pH 1.70 > pH 11.72.
3. The corrosion rate increases with rising temperature.
4. The firing procedure has a pronounced effect on both the porosity and the corrosion rate of Duceram: Frequently heated > untreated (manufacture firing procedure) > controlled heated. The suggested firing procedure improves the corrosion resistance of Duceram-LFC over the procedure recommended by the manufacturer. The firing procedure usually used in practice should be avoided.
5. The soaking of Duceram-LFC with silane leads to decreases in porosity and corrosion rate. It is recommended to coat the studied ceramic with silane used in this study.
6. The presence of fluoride ions in the corrosive solution leads to a decrease in the corrosion rate.
7. The corrosion rate showed a slight dependence on the corrosive anion at one and the same pH.
8. The mass loss %, and hence the corrodibility, of Duceram-LFC in the present study is higher by more than one order of magnitude than that given by the manufacturer under similar conditions.
Finally, it is recommended to follow the firing procedure in the present study and avoid the frequent firing procedure which usually followed in the dental laboratories.