Non-precious metal alloys have been known as substitutes for the gold alloys used in fabricating dental prosthetic appliances for many years. However, despite the fact that gold is much more expensive than these alloys, dental technicians appear to prefer the gold alloys. Gold is found to exhibit many desirable physical characteristics including low melting point, ductibility, ability to form impact resistant porcelain to metal bonds, ease of polishability and solderability, etc. Therefore, in order for the economic advantage of the non-precious metal alloys to be considered by the dental technician most of the desirable physical characteristics of gold must be matched.
U.S. Pat. Nos. 2,156,757; 2,162,252; 2,165,793; and 2,165,849 teach the use of non-precious metal alloys in place of gold for dentals. These alloys include Ni, Co and Cr, to which boron is added to reduce the propensity of Cr to form a high melt slag. It is noted by the patentee that boron increases castability and fluidity of the alloys. However, the alloys taught in these patents all contain at least 1 and preferably 5% boron to achieve improved castability and fluidity. Such amounts of boron as taught by the patentee would be detrimental in that ductility and ease of polishability would be poorer. This may explain why the alloys taught in these patents (which were invented prior to 1939) have not achieved commercial success as dental alloys.
At about the same time other patentees were teaching the addition of zirconium to nickel-chromium alloys to improve their use as a dental alloy (see U.S. Pat. No. 2,180,288) and adjusting the amounts of carbon and Mo in cobalt chromium alloys for the same purpose. (See U.S. Pat. No. 2,246,288). Neither of these patents suggest means to improve the castability of the rather high melting alloys disclosed therein.
Other patentees have added various other elements in minor amounts to the well known Co, Ni and Cr alloys to make them suitable for dental applications. See for example, U.S. Pat. No. 3,756,809 and 3,837,838 wherein tantalum and niobium are added to form intermetallics with cobalt and thereby strengthening the alloy. Again, there is no teaching as to providing good castability to these rather high melting alloys.
In U.S. Pat. No. 2,309,136 a Ni-Cu alloy is added to a Co, Cr, and Mo alloy to form a product useful in dental applications. The patentee states that it is critical to his invention to have copper in this alloy to insure that the alloy does not phase separate. The instant novel alloys specifically exclude copper which is known to cause hot shortness in alloy casting and to cause discoloration during the firing of porcelain on gold restoration.
U.S. Pat. No. 3,865,585 teaches a dental alloy having a critical concentration of nitrogen to achieve the patentee's goal of having improved ductility without the loss of tensile strength in a dental alloy. The instant novel alloy does not require nitrogen although some nitrogen may be incorporated during the melting process used to prepare the alloy.
The patentees, of U.S. Pat. Nos. 3,907,555 and 3,841,868 add gallium and tin or tin alone, respectively to a Ni, Cr, and Mo dental alloy to improve castability. The instant novel alloys show improved castability without the use of gallium or tin. Moreover, tin and gallium are low melting elements and would probably be difficult to control during the initial formulation of the alloy.
U.S. Pat. No. 3,716,418 teaches the use of an 80-20 Ni-Cr alloy as a dental alloy. The teaching of this patent is limited to adding very slight amounts of Fe and Mn to the basic alloy to form a special oxide surface which improves bonding of the alloy to porcelain. This modification is outside the scope of the instant invention.
In U.S. Pat. No. 3,749,507 the patentee adds Mo, Be, and Al to the 80-20 alloy mentioned above. Be and Al are necessary ingredients of this alloy; Al improving the strength and Be increasing the fluidity. However, neither of these elements are desirable in the alloys of the instant invention in that Al tends to decrease the castability of remelted alloys and Be is extremely hazardous to health in its oxide form.
In U.S. Pat. No. 3,464,817 the patentee found that maintaining the cobalt level below 22% and the aluminum level below 6% resulted in dental alloy having polishability without the loss of necessary physical characteristics such as yield strength, etc. As in the previously mentioned patent, the alloy disclosed in this reference excludes Al and Be neither of which are desirable in the instant novel alloys.
In U.S. Pat. No. 3,544,315, the patentee finds that maintaining the Mo and C in critical amounts advantageously limits the hardness of a Ni, Co, and Cr alloy while maintaining its strength. The critical amounts defined therein are not necesssary in the instant alloys. Furthermore, although the patentee suggests incorporating silicon in his alloy, the silicon is taught to function as a deoxidizer and therefore must be below 1%. The patentee states that the boron content must be maintained below 0.01%. Both of these limitations are unnecessary and/or undesirable in the instant alloys.
U.S. Pat. No. 3,914,867 teaches a dental alloy which includes niobium, tin and Al as well as the Ni, Cr, Mo, Si and B combination claimed herein. The patentee points out that all of the ingredients disclosed are critical for his purpose of providing a dental alloy having the properties of low melting temperature and outstanding physical properties. There is no suggestion that excluding Nb, Sn and Al would result in the desirable physical properties (from the standpoint of dental use) of the novel alloy disclosed herein.
Other references which teach alloys which are similar to those disclosed herein include: U.S. Pat. Nos. 3,922,168; 3,713,175; 3,658,515; 2,868,667; 970,364; 2,875,043; 2,714,760; 3,930,848; 3,925,072; 4,012,227; 3,900,316 and 3,892,541. These alloys are not suggested for use as dental alloys and therefore because of the very stringent requirements for such dental alloys it is believed that this art is not relevant in that the same objects are not being sought in these references.