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.