212-225 INGO eng.qxq

Dr. Gabriel M. Ingo
CNR-ISMN, Monterotondo (Rome), ITALY
Gabriel M. Ingo is Senior Researcher of the National
Research Council. Currently, he heads the Laboratory
of Surface Analysis of the Institute for the Study of
Nanostructure Materials, Monterotondo (Rome).
Over the past few years, there has been an increase of
research activity on refractory materials for jewellery
production. This activity has been followed by a
remarkable improvement of the quality of these
materials and consequently of the final product. This
paper will present some of the most interesting results
recently obtained on this subject.
Authors:
Dr. Gabriel M. Ingo, Dr. Cristina Riccucci
CNR-ISMN, Monterotondo (Rome) - Italy
Prof. Gualtiero Gusmano - Dept. of Science and
Chemical Technology, University of Tor Vergata, Rome, Italy
Prof. Giampiero Montesperelli - Dept. of Chemical
Sciences and the Earth. University of Ancona - Italy
Dr. Patrizio Sbornicchia - Dept. of Science and Chemical
Technology, University of Tor Vergata, Rome, Italy
Thermal Stability and Mechanical Properties of
Gypsum Bonded Investment with Regard to Burnout Cycle
Summary
Thermal stability, mechanical properties and chemical and physical characteristics of
a type of investment commonly used for the preparation of moulds for investment
casting have been studied, with regard to the burnout cycle. Scanning electron
microscopy coupled with microchemical analysis by X-ray energy dispersion
(SEM+EDS), X-ray diffractometry (XRD), differential thermal analysis coupled with
thermogravimetry (DTA-TG), porosimetry and compressive strength evaluation have
been used for this research.
The results show a remarkable effect of burnout cycle temperature and length on
mechanical properties and thermal stability of the investment.
These variations have been correlated with thermal expansion and changes of
mechanical, chemical and physical characteristics produced by the parameters of
the burnout cycle in the components of the material.
From the point of view of the technology, the results show the possibility of managing
the end properties of gypsum based investment in the best way, by suitable
adaptation of burnout cycle time, maximum temperature, heating and cooling rate.
Introduction
A commercial investment type widely used in the jewellery production industry has
been used to investigate the effect of maximum burnout temperature and holding
time at maximum temperature on thermal stability, mechanical properties and
chemical and physical characteristics of the materials used for the production of
moulds for investment casting.
The investment we used is formed by a mixture of 25-30% calcium sulphate
hemihydrate (2CaSO4.H2O), that acts as a binder, and 70-75% silica, that is the
actual refractory material. Silica is present as quartz and α-cristobalite. The typical
shape of these materials is shown in Figure 1: the fibrous, elongate crystals are
calcium sulphate hemihydrate (EDS spectrum B), while the chipped particles with
conchoidal fracture are silica (EDS spectrum A).
Analyses carried out in the past showed that the quartz/α-cristobalite ratio can vary,
affecting the final properties of the mould, because thermo-mechanical
characteristics of quartz and α-cristobalite, like transition temperature and thermal
expansion coefficient, are different [1-5].
Therefore special care has been devoted to obtain a starting material with very
homogeneous chemical and structural composition, in order to get homogeneous
samples for performing our experiments. Producer recommendations have been
followed for the method and time of mixing the water with the investment powder.
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Figure 1 - Shape of investment powder particles. EDS microanalysis enables to distinguish
silica particles (spectrum A) from calcium sulphate hemihydrate crystals (spectrum B)
The heating cycle recommended by the producer for the burnout, Figure 2, has also been
followed, to obtain the material we used as reference. Different burnout cycles, with different
time and temperature, have been experimented, to study thermal stability and physical and
mechanical characteristics. In particular, a wide range of maximum burnout temperature
has been selected, including the following values: 500°C, 600°C, 720°C, 800°C, 900°C and
1000°C. The holding time at maximum temperature has been 1, 5 and 10 hours.
The evaluation of the results of the different burnout cycles has been carried out by
means of a scanning electron microscope with X-ray energy dispersion analysis (SEM
+ EDS), X-ray diffractometry (XRD), porosimetry, mechanical tests and differential
thermal analysis coupled with thermogravimetry (DTA-TG).
Figure 2 Burnout cycle recommended by the producer
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Results and discussion
In particular, the TG technique has been used to evaluate the thermal stability of the
materials i.e. the temperature at which the TG curve deviates from the straight line,
due to calcium sulphate decomposition that occurs at temperature higher than 980°C,
Figure 3. It is well known that this decomposition causes a typical defect present in
investment cast jewellery, i.e. gas porosity [3-7].
The typical thermal behaviour of the hydrated gypsum bonded investment
(CaSO4.2H2O + SiO2) is shown in Figure 3 and these data evidence the chemicalphysical reactions experienced by the investment during the burnout process. These
reactions could also take place at the casting temperature.
Figure 3 - DTA-TG thermograms for the hydrated gypsum bonded investment
(CaSO4.2H2O + SiO2), the so called “green” investment, obtained by mixing the
investment powder with distilled water and waiting the setting time.
The measurement was carried out under static air at a heating rate of 50°C/min.
Hydrated gypsum bonded investment (CaSO4.2H2O + SiO2) undergoes a twostage dehydration process, with peak temperatures at 104°C and 246°C, during
the water removing step. The commonly used temperature for removing water
ranges from 130°C to 180°C, therefore, the first dehydration step can take place
at 104°C without problems, if the investment is slowly heated to this
temperature. On the contrary, the second dehydration step occurs at about
240°C, when the further heating of the investment to higher temperature takes
place. In this case the surface of the investment could be modified by violently
boiling water and a rough surface could be generated. Therefore, a holding
time at about 240°C should be included in the burnout process for a complete
and gentle removal of water.
The small endothermic peak occurring at 332°C could be due to the
transformation of unstable forms of cristobalite that induces a consequent
volume change [1,2].
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As pointed out by D. Ott, it is advised that this temperature should be passed
through gently to avoid cracking [2]. The small exothermic peak occurring at
466°C has been attributed to a γ−CaSO4
β-CaSO4 phase transition that
does not remarkably affect the final properties of the investment. At 607°C the
α-quartz to β-quartz transformation takes place with the well known expansion
phenomenon. Above about 980-1000°C, the gypsum bonded investment
exhibits endothermic dissociation with the production of calcium silicates and
sulphur compounds according to the following reaction:
2CaSO4 + SiO2
Ca2SiO4 + 2SO3
DTA-TG techniques have been also used for measuring the variation of the
temperature of CaSO4 thermal decomposition as a function of the maximum
temperature reached by the investment during the burnout process and of the
duration of the isothermal step at maximum temperature.
The results are shown in Figure 4 and evidence a remarkable variation of the
CaSO4 thermal decomposition temperature as a function of the temperature
and duration of the burnout process.
Figure 4 - Variation of the temperature of CaSO4 thermal decomposition as a function of
the duration and of the maximum temperature reached during the burnout process.
Indeed, the temperature of CaSO4 thermal decomposition changes slightly with
respect to the value for the investment powder, when the maximum
temperature of the investment burnout process is 500°C or 600°C. On the
contrary, the temperature of the CaSO4 thermal decomposition drops down
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when the investment is isothermally treated at a temperature varying from
720°C to 800°C. 720°C is the temperature commonly adopted for burnout.
Furthermore, in the burnout temperature range from 900°C to 1000°C, the DTATG data evidence that CaSO4 thermal decomposition occurs at a temperature
higher than the decomposition temperature of the investment fired according
to the specifications of the producer.
These latter thermal results could be explained considering that calcium
sulphate decomposition is induced by the presence of silica and is strictly
related to the surface acid-base interaction between SiO2 and CaSO4 particles.
Indeed, at a molecular level, the interaction between the active electron
donor/acceptor sites slowly occurs at temperature lower than 980°C-1000°C
and gives rise to the formation of a thin layer of reaction products on the CaSO4
particles, that act as a barrier against the further thermal decomposition [3,5].
As a consequence of the presence of this thin layer, the temperature of the
CaSO4 thermal decomposition could be slightly increased with a longer
holding time at maximum temperature. However, when both treatment
temperature and time are increased, the surface of the CaSO4 particles
becomes more thermally stressed and reactive and therefore, the thermal
decomposition occurs at a lower temperature.
From a thermal point of view, the results reported in Figure 4 confirm that 600°C
is the best maximum burnout temperature for the investment. This burnout
temperature enables to obtain an investment with CaSO 4 thermal
decomposition temperature of about 1020°C, that is the same value obtained
for the starting powder and is about 30°C higher than the value obtained from
the investment fired at 720°C. On the contrary, the producer specifications
recommend a burnout temperature ranging from 700°C to 720°C, in order to
remove the wax from the investment completely.
The structure of the different materials has been examined by means of XRD,
to show the modifications caused by the different burnout cycles. This
enabled us to identify the sequence of transformations taking place during
the burnout of the set of samples to be submitted to porosimetry and
mechanical tests.
In the first step - the preparation of the samples - the transformation of the
calcium sulphate hemihydrate (2CaSO4.H2O) contained in the starting
powder into gypsum (hydrated calcium sulphate, CaSO4 .2H 2 O) is the
principal reaction taking place in the binder. This is practically the only
structural transformation taking place during setting, because silica stays
unchanged.
The gypsum is then transformed into anhydrite (CaSO4) by means of
isothermal - or nearly isothermal - dehydration in the first step of burnout.
The results of XRD are shown in Figure 5 and show that the duration of the
burnout cycle affects the crystal structure of the different phases, particularly at
relatively low burnout temperature (500-600°C).
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Figure 5 - XRD spectra of the material heat treated at 600°C for one, five or ten hours.
The peak A (200) comes from anhydrite, while peaks αC (101) and αQ (101)
come from α-quartz and α-cristobalite respectively
At 500°C a longer burnout time causes an appreciable increase of the perfection of
the crystal structure of anhydrite, as can be seen from the reduced widening of the
main X-ray diffraction peak (200). In other words, a longer burnout time favours
atoms diffusion to repair the lattice damage caused by the dehydration of gypsum.
At 500°C this lattice restoration is slow, but at 600°C it is considerably faster. In fact
the width of the (200) diffraction peak decreases with increasing burnout time. The
apparent increase of the height of the peak can be ascribed to natural heterogeneity
of the material.
However the interpretation of the behaviour at 600°C is more complicated, because
it is affected by two concomitant phenomena, i.e. the recrystallization of the binder,
faster at 600°C, and the rapid transformation of α-quartz (less symmetrical crystal
structure, stable at lower temperature) into the more symmetrical ß structure, taking
place at 572°C.
The results of XRD for the samples heated to 720°C maximum burnout temperature
do not show appreciable difference from the samples heated to 600°C.
The results of XRD for the samples heated to 800°C show a higher degree of crystal
perfection of anhydrite, in comparison with the samples heat treated at 600°C. At
800°C, time does not affect the perfection of crystal structure appreciably: it means
that 1 hour is sufficient to obtain full restoration of the crystal lattice of CaSO4.
However the width of the (200) X-ray diffraction peak of anhydrite is slightly wider for
the samples heat treated at 800°C for 10 hours rather than 1 hour: it changes from
0.1861° to 0.1865°.
This phenomenon can be explained as the beginning of thermal decomposition of
CaSO4. The emission of gaseous sulphur trioxide (SO3) from the crystal lattice
causes some degree of disorder in the surface of the lattice, that is ascribed to
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molecular vacancies.
Figure 6 shows that this behaviour is more evident at 900°C: the XRD data show a higher
level of restoration of the crystal lattice, but the effect of the length of the heat treatment
is no more so evident as for the samples heat treated at lower temperature, because
decomposition of the surface of calcium sulphate crystals could already be occurring.
Figure 6 - XRD spectra of the material heat treated at 900°C for one, five or ten hours.
The peak A(200) comes from anhydrite, while peaks αC (101) and αQ (101)
come from α-quartz and α-cristobalite respectively
The effect of temperature on the recrystallization process can also be observed for
short heat treatments. In the case of 1 hour treatment the restoration of the crystal
structure of the anhydrite is more evident for the samples heat treated at temperature
higher than 800°C, in comparison with the samples heat treated at lower
temperature. On the contrary, with long heat treatment at high temperature another
process of crystal degradation takes place, caused by the decomposition of CaSO4,
that induces disorder in the crystal lattice.
Probably the temperature of 800°C could represent a compromise with maximum
restoration of defects induced by dehydration and minimum crystal lattice
degradation by the decomposition of CaSO4.
The modifications of the structure of the material as a function of heat treatment
temperature and length have been observed under the SEM and some results are shown
in Figure 7. In this figure, the change of the morphology of the investment is shown by the
micrographs made after the burnout process carried out in air for 5 hours at 600°C, 720°C
and 800°C peak temperature: micrographs A, B and C, respectively.
SEM micrographs show a variation of size and shape of the acicular anhydrite crystals
that appear smaller and less interconnected in the samples treated at higher temperature
than in the material treated at lower temperature with the same heat treatment time.
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Figure 7 - SEM micrographs of the investment after burnout carried out in air for
5 hours at 600°C, 720°C and 800°C maximum temperature: A, B and C, respectively
The above briefly discussed variations of the morphological and structural
aspects as a function of the burnout parameters are confirmed by the porosity
measurements, carried out for determining the size distribution of the pores. This
is an important parameter that could affect the permeabilty of the investment and
therefore the quality of the products.
The porosity measurements were carried out by mercury intrusion on a
Porosimeter 2000 (Carlo Erba Instruments) equipped with macropores unit. Five
specimens for each experimental condition were tested and the mean value has
been reported in Figures 8 and 9. The standard deviation of the data was about
±1.0%, so the results are sufficiently reliable to detect the changes induced by
the burnout parameters.
As a general trend, the porosimetry data evidence that the maximum value of the
pore size distributions is shifted toward higher values of diameter as a function of
firing time, even if this effect has not been observed on samples fired at the lower
temperature (500°C), that show a quite constant pore size distribution and total
porosity.
The maximum shift is due to ripening of the pores, that induces pore growth by
atomic diffusion. At 600°C, pores coalescence is sufficient to cause an appreciable
shift of the maximum of pore size distribution with increasing burnout time, Figure
8. In the material fired for ten hours pore diameter is larger than in the material fired
for one or five hours.
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Figure 8 - Pore size distributions of the material fired at 600°C for different times [8]
Furthermore, porosimetry data reported in Figure 9 show that total porosity
increases as a function of temperature and firing duration, except for short burnouts
carried out at 600°C, where total porosity decreases as a function of the burnout time
[8]. These results indicate that, as a function of maximum temperature and burnout
duration, the chemical-physical reactions play a compacting role, inducing an
increase of pore size and total porosity, that could affect gas permeability during
casting and therefore the quality of products.
Figure 9 - Total porosity of the different materials as function of maximum
firing temperature and duration of isotherm stage at the firing temperature [8].
Also mechanical tests have been performed to study the role played by the above
cited burnout parameters, and the results are reported in Figures 10 and 11. The
measurements have been carried out with a MTS 858 Minibionix (Instron) press
machine under strain control (0.5 mm·min-1) according to the ASTM (C133)
standard [8].
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Figure 10 - Compressive strength of traditional investment as a function of maximum
burnout temperature and holding time at maximum temperature. The effect of burnout at
600°C is shown on the right side [8]
Figure 11 - Elasticity modulus of the materials as a function of maximum burnout
temperature and holding time at maximum temperature. The effect of burnout at 600°C is
shown on the right side [8] – Indicative data
The combination of porosity, mechanical and XRD data enabled us to clarify some
interesting phenomena occurring in the investment, when it is subjected to heat
treatment with different temperature and time.
The results indicate that at the lowest burnout temperature (500°C) the pore
distributions is nearly constant with time. The CaSO4 structural restoration is poor and
other chemical-physical reactions, such as thermal decomposition, do not occur at all.
Total porosity, nature of the species and residual stresses induced by cooling, mainly
due to SiO2 contraction, can play a constant role on determining structure defects,
thus providing very similar mechanical properties.
Burnout at 600°C can be divided into two cases, both affected by quartz
transformation, occurring at 572°C with a relevant volume expansion [1]. Short
treatments, of about one hour, lead to the formation of relatively high porosity due to
the role played by the quartz phase change, but the compressive strength is
practically similar to that of the investment fired at 500°C, because pore size and
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distribution are very similar to this latter case.
Firing treatments carried out at 600°C for longer time induce pore ripening and
therefore porosity decreases, but pore size is increased and bigger defects are formed,
even though less abundant, thus reducing the investment compressive strength.
The mechanical and structural properties of materials heat treated at 730°C are
comparable to the materials fired at 600°C for longer time, even though the thermal
stability is decreased. Burnout at 800°C provides slightly more porous and stronger
investment, even if there is an incipient decomposition of CaSO4, thus inducing
structural defects and decreasing thermal stability. At higher temperature, CaSO4
dissociation generates a very thin film of silicates via surface interaction, which
increase the compressive strength of the mould [8]. In this case, the weakening
caused by the shift of pore distribution is compensated by the increase of the amount
of silicate as a function of firing duration. Therefore mechanical properties appear
almost constant as a function of heat treatment time. Furthermore, at high burnout
temperatures, the synthesis of silicates is very efficient and can extensively defeats
the weakening effect derived by the increase of porosity and pore diameter [8].
Conclusions
On the basis of the above shown results, it is possible to draw the following conclusions.
Burnout at low temperature, i.e. 500°C, leads to the production of moulds with relatively
low strength and porosity. This could make difficult complete wax combustion and gas
suction from mould channels during investment casting, so that high porosity and other
defects can often be found in the products.
Short burnout carried out at 600°C gives the highest thermal stability and mechanical
properties relatively better than those measured on investment fired at 730°C for five
hours, according to the standard cycle. Unfortunately this temperature could be too low
to ensure full wax elimination, if a large amount of oxygen is not present in the
atmosphere of the burnout oven. It is worth noting that wax residues or carbonaceous
products catalyse during casting, thus inducing gas porosity. These short burnout
cycles could be preferred only if selected waxes are used that can burn at 600°C
completely, thus reducing firing and production costs. The results indicate that longer
burnout at 600°C reduces considerably the compressive strength of the investment.
Burnout at 730°C and 800°C decreases thermal stability. At this latter temperature
porosity and compressive strength of the mould increase, but this firing temperature is
quite high and makes possible lattice damaging of CaSO4. This phenomenon could
facilitate CaSO4 decomposition during casting.
At higher burnout temperature, i.e. 900°C and 1000°C, the compressive strength of the
moulds increases with respect to the low temperature fired moulds and pore diameter
is also increased. Concerning CaSO4 thermal stability, long-term (10 hours) burnout at
900°C or short-term burnout at 1000°C produce investments characterised by good
properties, even though the materials are stressed and production costs are increased.
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References
[1] Phase diagrams for Ceramists Ed. R.L. Stone, Am. Ceram. Society (1947) p. 19.
[2] D. Ott, “Handbook on casting and other defects in gold jewellery manufacture, Ed C. Corti,
Published by World Gold Council, Ind.Division, London 1997, p.17-24.
[3] G.M. Ingo, C. Riccucci and Gianni Chiozzini, Journal of American Ceramic Soc. 84 (2001) p.
1839-43.
[4] G.M. Ingo, V. Faccenda, G. Chiozzini, C. Riccucci and C. Veroli, Proc. of the Santa Fe
Symposium on Jewellery Manufacturing Tecnology 1999, p. 163.
[5] G.M. Ingo, G. Chiozzini, V. Faccenda, E. Bemporad, C. Riccucci, Thermochimica Acta, 321
(1998) p. 175-183.
[6] E. Bell, “Wax elimination, burnout and the mold’s effect on porosity in casting” Gold
Technology, n° 11, Nov. 1993, p. 21-7.
[7] I. Colussi and V. Longo, “La decomposizione termica del solfato di calcio-Thermal
decomposition of calcium sulphate”; Il Cemento, 2, p. 75-98 (1974) and references therein.
[8] P. Sbornicchia, Doctorate Thesis, “Sviluppo e caratterizzazione di refrattari per la microfusione
a cera persa” Rome University, Tor Vergata (2004).
[9] G. Montesperelli, G.M. Ingo, C. Riccucci, P. Sbornicchia, Proc. of the VI Convegno Nazionale
AIMAT (2002) S3-112.
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