EXAMINATION OF CERTAIN PROPERTIES OF DENTAL CERAMICS
Bella Udvardi1, Róbert Géber2, László A. Gömze3
1 undergraduate student, 2 assistant professor, 3 professor 1,2,3 University of Miskolc, Institute of Ceramics and Polymer Engineering,
Hungary, Miskolc – Egyetemváros
ABSTRACT
The aim of our research was to examine the properties of dental ceramics. The main
goal was to make a mixture which is easy to reach and can be produced from low-priced
raw material, and at the same time the product has adequate mechanical properties.
Combination of the recipe was made without any strengthening and stabilizing
substances. Using conventional mineral materials test samples were prepared and
mechanical tests were made.
INTRODUCTION
During the mixture preparation it was very important to reach a particle size of 10
microns by grinding [1]. It was quite difficult, therefore grinding time, rotation speed
(rpm) were optimized and wet grinding method was also used. Particle size distribution
was observed by scanning electron microscopy (SEM).
In order to define the melting properties heating microscopy was used and the results of
this test helped us to plan the firing cycle. Casting moulds from gypsum were also made
to prepare test samples by gravitational casting. These samples were used to determine
the shrinkage in a laboratory furnace. After firing, shrinkage and hardness of samples
were determined.
SAMPLE PREPARATION
In order to create raw mixtures, it was very important for the Authors to use only low
cost raw mineral materials instead of expensive dental raw materials.
These minerals were:
orthoclase (K-feldspar)
quartz
kaolinite
albite (Na-feldspar)
SEM micrographs were taken by Hitachi TM-1000 scanning electron microscope in
order to give information about morphology and particle size distribution. SEM
micrographs were taken in 250x and 1000x magnifications.
MultiScience - XXXI. microCAD International Multidisciplinary Scientific ConferenceUniversity of Miskolc, Hungary, 20-21 April 2017
ISBN 978-963-358-132-2
DOI: 10.26649/musci.2017.032
Figure 1. SEM micrographs of the raw materials
Particle size of K-feldspar is varied, particles with 10 microns and also 30-70 microns
can be observed. Kaolinite is the coarsest mineral because there is a high amount of
particles with 100 microns.
All raw materials were ground together by PM 400 Planetary Ball Mill (manufactured
by Retsch)
Grinding was started with dry grinding for five minutes at 200 rpm. SEM micrographs
were taken after each grinding process in order to observe how the size of the particles
changed.
Figure 2. Dry grinding for 5, 15, 20 and 30 minutes
The effect of milling time on the particle size is clearly observable on the micrographs.
The size of the particles was coarser than 10 microns, so additional change was
necessary. Therefore, kaolinite was wet ground separately for 30 minutes by planetary
ball mill. This grinding was very effective.
After reaching the right particle size, four different mixtures were made based on former
measurements of dental powders and with the help of Ref. [2]. Mixtures were then used
for making test samples.
47,5
𝝁𝒎
65,1 𝝁𝒎
K-feldspar Kaolinite
Quartz Na-feldspar
18,4
𝛍𝐦
Table 1. Mixtures
Mixture
number
K-feldspar
[m/m %]
Kaolinite
[m/m %]
Quartz
[m/m %]
Albite
[m/m %]
1. 65 0 25 10
2. 70 1 20 9
3. 72 2 20 6
4. 75 3 20 2
These mixtures were wet ground again together with the pre-ground kaolinite for 30
minutes at 300 rpm. After that, SEM test was done on the mixtures. Figure 3. shows the
result of scanning electron microscope tests.
Mixture 1.
Mixture 2.
Mixture 3.
Mixture 4.
Figure 3. Results of wet grinding after 30 minutes
It can be seen on the micrographs that most particles are under 10 micrometers. It can
be stated that the right particle size was reached to produce dental ceramics.
EXAMINATION OF SHRINKAGE
The knowledge of change in the dimensions (shrinkage or swelling) is important during
firing of dental ceramics, because accuracy of dimensions is essential to fit the porcelain
tooth well into the patient’s mouth.
Shrinkage of samples fired in a furnace
The amount of shrinkage (length change) was observed on the test samples. These
materials, which contained feldspar in a higher amount than kaolinite, shrinkage was
more considerable.
Figure 4. Firing curve
Figure 5. Length change of products after firing
Shrinkage of samples fired in a heating microscope
The test was performed on a MicroVis heating microscope (manufactured by Camar
Elettronica). The used firing curve can be seen on Fig. 6, marked by a red dashed line.
It can be stated that the height of the sample is growing for the effect of firing (so
swelling). When the maximum temperature is reached, the height of the sample starts to
shrink.
1 2 3 4
0
5
10
15
20
25
30
Sh
rin
ka
ge
[%]
Mixture [-]
Length change of products
Figure 6. Results of the shrinkage by heating microscopy
During the examination a digital camera continuously takes photos of the sample. From
the real picture the controller software generates a binary picture. An accurate shrinkage
value can be determined by the height and width values of the sample.
Sample 1.
(initial)
Sample 1.
(after cooling)
Sample 2.
(initial)
Sample 2.
(after cooling)
Sample 3.
(initial)
Sample 3.
(after cooling)
Sample 4.
(initial)
Sample 4.
(after cooling)
Figure 7. Binary images of the samples (initial and after cooling)
Comparing the results of shrinkage performed by both methods (firing in a furnace;
firing by heating microscope) it can be stated that shrinkage was higher using a furnace
than using a heating microscope.
It may have two reasons:
Samples which were made by casting, therefore contained water, were fired in a
laboratory furnace. Samples for heating microscope were prepared only from
dried raw mixtures. Shrinkage was higher when furnace was used, because of
evaporation of water.
On the other hand, the software of the heating microscope was capable to set only
one dwell time. Therefore, there may be a difference among the shrinkages.
MICRO VICKERS HARDNESS TEST
Micro hardness tests were made using a Vickers hardness tester (load=1 kg, t=15 sec)
[3]. Figure 8. shows the results of the hardness test.
1 2 3 4
6,0
6,1
6,2
6,3
6,4
Har
dn
ess
[GP
a]
Mixture [-]
Vickers Hardness
Figure 8. Result of the Vickers hardness
Hardness values of the measured test samples were compared to the hardness values of
dental feldspar porcelain tooth found in Ref. [4]. Table 2. shows the results of the
hardness comparison.
Table 2. Comparison of hardness of dental porcelains
Dental porcelain HV
[GPa]
Samples
number
Own samples
hardness
[GPa]
VMK (Vita VMK 68 (body)-
feldspar porcelain 6.56 ±0.32 1.1 6.24
OP Optec HSP-feldspar porcelain 7.9±0.56 2.1 6.24
3.1 6.08
4.1 6.39
It can be stated that the difference between the hardness of feldspar porcelain in Ref. [4]
and our samples is insignificant.
The aim was to make products with nearly equal hardness on favorable price, so only
raw materials were used. During the make of the recipe no strengthening and stabilizing
substances were used. It can be clearly seen from the table that the hardness of the
samples was suitable.
CONCLUSION
According to the examination results the largest difference was observed in shrinkage.
It can be stated that the quantity of the kaolinite and the feldspar significantly influenced
the size change under firing. With the use of accurate recipe and firing curve the final
size of the finished products can be well planned.
During the research a recipe was successfully made with which hardness of feldspar
porcelain available commercially could be reproduced, from low-cost raw materials,
without any strengthening material.
ACKNOWLEDGEMENT
The described article was carried out as part of the EFOP-3.6.1-16-00011 “Younger and
Renewing University – Innovative Knowledge City – institutional development of the
University of Miskolc aiming at intelligent specialisation” project implemented in the
framework of the Szechenyi 2020 program. The realization of this project is supported
by the European Union, co-financed by the European Social Fund.
REFERENCES
[1] MSZ EN ISO 6872 standard
[2] Richard van Noort: Introduction to dental materials, Fourth edition 2013
Elsevier, p. 421
[3] Sharifa Al-Shehri: Relative fracture toughness and hardness of dental ceramics,
2002
http://repository.ksu.edu.sa/jspui/bitstream/123456789/7163/1/Relative%20frac
ture%20toughness%20and%20hardness%20of%20dental%20ceramics.pdf
Date of access: 2016. 10. 27.
[4] http://www.gordonengland.co.uk/hardness/hvconv.htm
Date of access: 2016. 11. 04.