- 217008~
Use of gold alloys for precision attachments in dental
technology
5 Description:
The invention relates to the use of gold alloys as material
for precision attachments in dental technology.
Precision connecting elements, such as for example
attachments or joints, are frequently used as so-called
precision attachments especially in the manufacture of
removable dental appliances. Such precision attachments
are nowadays usually offered in prefabricated form. By
15 reason of the individual problems, numerous different
constructions exist. Thus, already in 1989 about 290
different systems were on the market. A large number of
different alloys based on noble metals are used for their
manufacture.
These precision attachments are usually very expensive to
fabricate. This is due to the smallness of the parts,
which is coupled with complicated geometries and close
tolerances in order to withstand the high demands in
25 clinical use. In addition, by reason of the high strength
requirements, high-strength alloys must be used, which as a
rule have difficult forming behaviour, owing to the
elevated hardness. Usually only those alloys are used
which have Vickers hardnesses above 200 and yield points of
30 more than 450 MPa.
In addition, high requirements are set for these alloys
with regard to the corrosion resistance, in order to
guarantee the biocompatibility of corresponding dental
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constructions. In this connection the requirements with
regard to corrosion resistance are more likely to have to
be rated higher than in the case of cast dental appliances,
since in the case of precision attachments, crevices, for
example between the attachment surfaces, are unavoidable.
All preconditions for a situation of intensified corrosion
by crevice corrosion are thereby met. An optimum bio-
compatibility is obtained by the use of alloys of the
highest resistance to corrosion, for which the least
10 possible alloying elements suffice.
The precision attachments are fastened to the parts of the
prosthesis by brazing or by direct casting-on of the dental
alloy to the precision attachments. For these precision
15 attachments, alloys are required that contain no base
metal, so that during the preheating before the casting-on,
no interfering oxide layer is formed which would prevent a
bond of self-substance between precision attachments and
dental alloy. The compositions of these alloys are usually
20 based on gold-platinum-palladium or also platinum-iridium.
Especially high-strength alloys of this type are described
for example in DE-PS 35 42 641.
Especially in the case of yellow dental alloy systems
capable of being fused to, which can be veneered with
special low-melting ceramics, precision attachments of
yellow alloys are desirable, in order that these do not
stand out in colour from the base material. These yellow
alloys have previously all been alloyed with base metals
and so are not capable of being cast on. Their composition
is generally based on gold-platinum-silver-copper and they
owe their mechanical strength largely to the silver-copper
miscibility gap. As a result of the relatively high
proportion of copper required thereby, there is potentially
2170Q8~
a tendency to discoloration, especially when a crevice
corrosion situation exists. Recently, therefore, the goal
has been pursued of further reducing the copper content of
these alloys. In order to be able to guarantee high
5 mechanical strengths, further steps of alloy technology are
required, which, though, result in a reduction of the
ductility and, accompanying this, a still greater
fabrication expenditure.
It was therefore the object of the present invention to
discover gold alloys for precision attachments in dental
technology, which have a golden yellow colour and are
sufficiently hard and well formable. They should also be
extremely corrosion-resistant, have an optimum bio-
15 compatibility, and therefore have no toxically questionableconstituents.
This object is achieved according to the invention by the
use of gold alloys with 1.2 to 2.3 wt.% titanium, the
remainder being gold.
Preferably, alloys are used that contain 1.6 to 1.8 wt.%
titanium, the remainder being gold.
Surprisingly, alloys can be produced on the basis of gold
with titanium additions of 1.2 to 2.3 wt.% which have a
considerably more favorable forming and corrosion behaviour
than the previously-known alloys for constructional
elements. The high reactivity of the element titanium can
30 be controlled as regards melt technology by melting under
inert gas in suitable crucibles. Binary alloys of gold and
1.6 to 1.8 wt.% titanium, which have optimum properties
with regard to colour and corrosion resistance as well as
forming behaviour, have especially proved their worth.
217008~
Listed in Table 1 by way of example are three yellow alloys
of the conventional type of alloy (alloys 1 to 3) in
addition to some alloys according to the invention (alloys
4 to 6). The accompanying mechanical characteristic data
can be found in Table 2.
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Table 1: Alloy compositions (wt.~)
Alloy Au Ag Pt Pd Cu Zn In Ti Other
1 13 111.5 Ir 0.1
65.5 8.9 0.5
2 9 - 4.4 2 1.5 Ir 0.1
73.8 9.2
3 Zr 3.0
97.0
4 1.4 Ir 0.1
98.6
1.7
98.3
6 2.1
97.9
Table 2: Mechanical characteristic data
HV-H HV-W Rp-H Rp-W A-W A-H (%)
Alloy (MPa) (MPa) (~)
1 275180 710 420 28 11
2 230150 500 310 14 6
3 23090 600 247 21 <1
4 210145 420 82 71 12
24055 480 95 61 9
6 25095 500 170 45 6
HV-H: Vickers hardness in hard condition (optimum heat
treatment temperature in each case)
HV-W: Vickers hardness in soft condition (quenched in
water)
Rp-H: Yield point in hard condition
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Rp-W: Yield point in soft condition
A-H: Ductility for hard condition in tensile test
(technical elongation)
A-W: Ductility for soft condition in tensile test
(technical elongation)
While in the hard condition the known alloys and the alloys
according to the invention have no serious differences in
the mechanical characteristic data, in the soft-annealed
state the alloys according to the invention have clearly
better values. The hardness values, in contrast to the
conventional alloys, can fall considerably further, in some
cases to only about one-third of the values that the
conventional alloys possess. This is accompanied by a
large reduction of the yield point and a massive rise of
the ductility to values of about 60 ~. Both the rise of
the ductility and the clear reduction of hardness and yield
point lead to a striking improvement of the forming
behaviour. As a result of the higher ductility, greater
20 degrees of deformation and a reduction of the number of
intermediate annealings can be achieved and as a result of
the lower strength, lower deformation forces are required.
Both factors together lead to clearly reduced fabrication
costs and times and also permit novel, more efficient
fabrication methods, as for example impact extrusion.
After the deformation processes, high strengths and lower
ductilities can again be set through suitable heat
treatments. From now on the hard state has a very
favourable behaviour during subsequent shape cutting. In
the use as precision attachments also the high strengths
are again necessary.
Surprisingly, these alloys have also proved to be
distinguished by an extraordinary corrosion resistance.
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The characteristic data on corrosion resistance are
assembled in Table 3. The corrosion resistance was
determined according to DIN E 13927 in a one-week immersion
test in 0.1 molar lactic acid - common salt solution on the
5 basis of the liberated corrosion products. Listed in Table
3 are the totals of the liberated corrosion products per
cm2 of sample surface. The investigations were carried out
once on samples with freshly ground surface and once on
samples that were subsequently subjected to a heat
treatment in addition. An ageing of 1 h at 500 C in air
was selected in each case as heat treatment. In this heat
treatment the alloys according to the invention reach the
highest strengths. Such a treatment may therefore be
carried out after the last shaping. While with the
15 conventional alloys, the corrosion rate rises as a result,
so that the annealing must be carried out under inert gas,
this annealing treatment surprisingly leads in the case of
the alloys according to the invention to a further
improvement of the corrosion resistance. In the case of
the binary alloys, the corrosion rate even sinks below the
limit of detection
Alloy Total corrosion rate Total corrosion rate
(~g/cm2) of ground (~g/cm2) after additional
surface heat treatment
1 6 43
2 8 32
0.3 <DL*
6 0.5 <DL*
* DL = detection limit = 0.13 ~g/cm2
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The following examples are intended to illustrate the
alloys according to the invention in more detail:
S 1. For the manufacture of an attachment, an initial
material with a rectangular cross-section of 3.3 x
6 mm must be produced from a continuously cast
cylindrical rod of 9 mm diameter. For alloy 1
(Table 1), 8 intermediate annealings as well as 9
reduction passes followed by 3 drawing steps are
required.
For alloy 2 (Table 1), 11 intermediate annealings as
well as 10 reduction passes followed by 3 drawing
steps are required for this.
The alloys nos. 5 and 6 according to the invention
(Table 1), on the other hand, require only 3 or 4
intermediate annealings respectively as well as 6
reduction passes followed by 3 drawing steps. The
fabrication of the initial material is therefore
considerably less time-consuming.
2. For the manufacture of a cylindrical root pin,
conventional alloys are machined on a lathe. With the
alloy no. 6 according to the invention, on the other
hand, a wire of appropriate diameter, after previous
soft annealing, can be pressed directly by means of an
impact extrusion operation into a final shape. For
this application a finishing operation is not
required.
