Dental casting alloy

Abstract

In order to improve the fracture strength of a dental casting alloy and to prevent it from becoming excessively hard, a dental casting alloy is provided substantially comprising: from 25 to 32 wt % of Cr from 8 to 12 wt % of W, from 0.05 to 0.4 wt %, in each case, of one or more elements in Group 4a and/or Group 5a of the Periodic Table, production-related impurities, the rest being cobalt.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International Application No. PCT/EP2003/012465, filed Nov. 7, 2003, which claims Priority of German Application No. 102 52 776.8, filed on Nov. 7, 2002, which are incorporated herein by reference in their entirety and for all purposes.

BACKGROUND OF THE INVENTION

The invention relates to a dental casting alloy for the production of dental prosthetic frameworks.

Casting alloys for dental prosthetic frameworks, in particular for so-called prosthetic model casts, have been known since 1935.

Such model casting alloys have, along with chromium and molybdenum, high carbon contents in order to achieve both the rigidity required in dental technology and a low viscosity of the melt, which facilitates accurate casting of the model.

Manganese and silicon are used in these alloys in order to improve the fluidity thereof.

The use of an alloy for the production of castings for dental applications is disclosed in DE 36 09 184 C2. This alloy contains from 4.5 to 5.5 wt % of molybdenum.

An alloy for dental castings is disclosed in DE 198 15 091 C2, which specifies a molybdenum content of from 4 to 8 wt %. In addition, a fraction of from 0.05 to 1.2 wt % of tantalum, niobium, and/or tungsten is required, the fraction of each individual element tantalum, niobium, or tungsten being less than 0.5 wt %.

In general, special demands are imposed on alloys to be used in dental technology. For example, alloys to be fused with porcelain or ceramics materials must be compatible with commercial dental ceramics in terms of thermal expansion and contraction. Furthermore, these alloys must form a thin surface layer of oxide to ensure the formation of a bond between the metal and the ceramics material. Also, for aesthetic reasons, the color of the oxide should not show through the opaque ceramics material. A certain activation capacity and spring hardness are required for dental castings that are not to be fused with porcelain, for example, removable prostheses with clasps. Furthermore, it is particularly important in dental technology for the alloys used to be capable of being processed with the facilities available in the dental laboratory, ie they should be capable of being cast in conventional casting centrifuges. Therefore, the alloys commonly used in dental technology as model casting materials have a much higher carbon content than is tolerable under various other standards. Furthermore, dental casting alloys are to be preferred that have a hardness in their cast state that does not differ substantially from the hardness of natural tooth enamel, so that there is no significant abrasive wear of the tooth arising from contact between the dental casting alloy and the tooth surface. Furthermore, it is advantageous when the alloy can be produced with a low nickel content so that patients allergic to nickel can also be fitted with such prostheses.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide, within the limitations mentioned above, a casting alloy of the type described above that exhibits a high elongation at break and a Vickers hardness number (HV 10) that is not excessively high.

This object is achieved by the invention in respect of the dental casting alloy defined above, in that this alloy is composed substantially of

25-32 wt % of chromium

8-12 wt % of tungsten,

one or more elements selected from elements in Group IVa and/or Va of the Periodic Table as a fraction of 0.05-0.4 wt % in each case, the rest being cobalt.

In addition, production-related impurities are present in amounts of less than 0.1 wt %, more preferably less than 0.05 wt % in each case. Preferably the total amount of such impurities amounts to not more than 0.4 % by weight, more preferably to not more than 0.2 % by weight. The amount of an alloy element included in preferred dental alloys as described below are of course to be ignored when calculating the total amount of impurities even if its amount is small, comparable with or even lower than the amount allowed for an impurity. Also the upper limit for each of the impurities given above are not applicable to those elements.

In a preferred dental casting alloy of the invention, the total content of elements in Group IVa and/or Group Va of the Periodic Table is ca 0.05-0.4 wt %.

It is further preferred that the elements in Groups IVa and Va of the Periodic Table be selected from the group consisting of titanium, zirconium, niobium, and tantalum.

Furthermore, alloys of the invention are preferred that are substantially free from the elements iron, vanadium, nickel, molybdenum, and carbon.

Other preferred alloys contain

• from 0 to 1 wt % of Mn
• from 0.8 to 1.6 wt % of Si and/or
• from 0.1 to 0.35 wt % of N.

The addition of Mn has a deoxidizing effect on the melt and binds oxygen with slag formation.

The silicon fraction serves to lower the viscosity of the melt with the result that very fine details can be reproduced well in the cast.

The nitrogen fraction increases the ductility. If the nitrogen content is too low, there are obtained a poorer elongation at break and a lower elastic limit.

Highly corrosion-resistant prosthetic frameworks can be made with the dental casting alloys of the invention; furthermore, these alloys exhibit good working properties, in particular a low hardness number and a lower thermal expansion coefficient, and they are also easily laser-welded.

The low thermal expansion coefficients simplify fusing of the framework with ceramics material/porcelain.

DETAILED DESCRIPTION OF THE INVENTION

A particular composition of the alloy of the invention is given below (Table 1) by way of example, and it is emphasized that the invention is not, of course, confined to this particular alloy. The mechanical properties of the alloy depicted in Table 1 are summarized in Table 2 below.

Table 3 summarizes Comparative Examples 1 through 6 and, by comparing them with the above example of an alloy of the invention, illustrates the importance of complying with the recipe specified in order to obtain the well-balanced, beneficial properties of the dental casting alloy.

TABLE 1
Elementary  Percent by
Ingredients weight (wt %)
Co          60
Cr          28
Mo          —
W           9.8
Mn          0.3
Si          1.5
N           0.2
Nb          0.2

TABLE 2
Mechanical/Physical Properties
Elastic Limit Rp 0.2 (MPa)       620
Tensile Strength Rm (MPa)        845
Modulus of Elasticity (GPa)      190
Elongation at Break A5 (%)       10.2
Vickers Hardness Number HV 10    300
Thermal Expansion Coefficient (TEC) within the 14.1
range of 20-500° C. (10−6K−1)

	TABLE 3
	Comparative Examples
	#1	#2	#3	#4	#5	#6
Co	70	68	59.5	68	61	65
Cr	20	20	20	20	24	28
Mo	4.5	5	5	—	7	4.5
W	5	5	5	10	5	—
Mn	0.3	0.3	0.3	0.3	0.25	0.25
Si	0.1	1.5	—	1.5	1.5	1.6
N	0.1	0.2	0.2	0.2	0.25	0.2
Fe	—	—	10	—	—	0.35
other					Ce, C	Al, La, Ce
HV 10	289	297	280	291	340	310
TEC	15.05	14.95	15.43	15.4	14	14.7
(10−6 K−1)
RP 0.2 (MPa)	380	480	467	457	700	520
Rm (MPa)	630	636	698	675	900	730
A5 (%)	17.5	10.5	18.1	15.7	7	11

The alloys summarized in the table above depicting the comparative examples are all suitable for framework material and for fusing it to ceramic or porcelain, but they all exhibit one or more of various drawbacks when compared with the alloy of the invention.

Although the alloy of Comparative Example 5 has a low thermal expansion coefficient, it nevertheless has a relatively high hardness number.

The alloy of Comparative Example 6 exhibits a relatively low hardness number of 3 10 HV 10, but it has the disadvantage of having a high thermal expansion coefficient, which could possibly cause difficulties during ceramic fusing.

The Comparative Examples 1 through 4 relate to alloys having low hardness numbers, but they exhibit much higher thermal expansion coefficients than the alloy of the invention and are therefore difficult to process with ceramic materials.

This also holds true for the molybdenum-free alloy of Comparative Example 4, which, compared with the alloy of the invention, has a lower thermal expansion coefficient and surprisingly, in spite of its high tungsten content, exhibits a low hardness number and an astonishingly high elongation at break of more than 10%.

The addition of a Group IVa or Group Va element selected, in particular, from the group consisting of the elements titanium, zirconium, niobium, and tantalum, as proposed herein, improves corrosion resistance and increases thermal resistance when the alloy is subjected to fusing to ceramic materials or porcelain.

Moreover, the element niobium is comparatively biocompatible, as is titanium or tantalum or zirconium, and causes no tissue reactions.

Consequently, the alloy of the invention is highly biocompatible.

The preferred elements in Group IVa and Group Va of the Periodic Table, that is, niobium, titanium, tantalum, and zirconium, also bind the residual carbon in the alloy and thus inhibit the formation of chromium carbides. This also makes it easy to laser-weld the alloy of the invention.

Moreover, the presence of the elements niobium, titanium, tantalum, and/or zirconium provides for a strong passivation of the alloy of the invention. As a result, the alloy shows a very high resistance to corrosion.

Finally, it is emphasized that the alloy of the invention contains no precious metal fractions, as is the case in some conventional low-hardness alloys containing up to 2% of gold in order to reduce the hardness of the alloy to a manageable level.

Claims

1. A dental casting alloy, for the production of dental prosthetic frameworks, substantially comprising:

from 25 to 32 wt % of Cr,

from 8 to 12 wt % of W,

from 0.05 to 0.4 wt %, in each case, of one or more elements in Group 4a and/or

Group 5a of the Periodic Table,

production-related impurities,

the rest being cobalt.

2. The alloy as defined in claim 1, having a total content of Group 4a and/or Group 5a elements of from about 0.05 to about 0.4 wt %.

3. The alloy as defined in claim 1, wherein the Group 4a and/or Group 5a elements are selected from the group consisting of titanium, zirconium, niobium, and tantalum.

4. The alloy as defined in claim 3, wherein the alloy is substantially free from the elements iron, vanadium, nickel, molybdenum, and carbon.

5. The alloy as defined in claim 1, wherein the alloy additionally contains a fraction of Mn in the range of from 0 to 1.0 wt %.

6. The alloy as defined in claim 1, wherein the alloy additionally contains a fraction of Si in the range of from 0.8 to 1.6 wt %.

7. The alloy as defined in claim 1, wherein the alloy additionally contains a fraction of N in the range of from 0.1 to 0.35 wt %.