Abstract:
Castings made of boron-containing alloys based on at least one platinum group metal are treated by thermal ageing in the presence of oxygen and at temperatures below the melting point of the alloy. This enables the alloys to be processed at temperatures customary in the jewelry industry. The treated castings can also be processed into medical technology products.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Section 371 of International Application No. PCT/EP2010/001182, filed Feb. 26, 2010, which was published in the German language on Sep. 16, 2010, under International Publication No. WO 2010/102726 A1 and the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to the treatment of boron-containing, platinum group metal-based alloys, in particular in order to improve their properties for utilization in the jewelry industry and medical technology. 
         [0003]    The melting point of platinum metals and alloys thereof should be as low as possible to allow them to be cast more easily into pieces of jewelry and other objects. At the same time, the material should be hard enough so that expensive jewelry stones set therein are not lost too easily. 
         [0004]    Japanese patent application publication (Kokai) JP 56-081646A describes an alloy made of 80-95% Pt, 1-15% of at least one metal from the group, Pd, Ir, Ru, Rh, Au, Ag, Cu, Ni, and Co, and 0.01-3% B or calcium boride, which is especially hard, possesses good mechanical properties at high temperatures, and has good casting properties due to its melt being very fluid. The alloys are well-suited as ornamental material for setting jewelry stones. The advantageous properties have been related to the boron and/or calcium boride. 
         [0005]    In particular, adding approximately 2% by weight boron to platinum-based alloys (e.g., jewelry alloy Pt95.2 Ru4.8) allows the melting point to be lowered from approximately 1,800° C. to approximately 800° C. This enables the processing of platinum alloys with melting furnaces and casting devices customary in the jewelry industry and used to produce gold-based jewelry. 
         [0006]    Despite these advantages that have already been achieved, efforts are being made to further improve the properties of platinum metal alloys. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Surprisingly, a so-called ageing at approximately 750° C.—i.e., below the melting point—allows the B content to be reduced substantially. It has proven to be advantageous to keep removing from the surface of the Pt alloy, by rinsing in warm water, the boron oxide produced on the surface. This treatment is continued until the casting shows a sufficient toughness for use as a piece of jewelry. Due to the residual boron content of the alloy, the casting has substantially increased toughness as compared to the untreated Pt alloy, which has a beneficial effect on the wear resistance of the piece of jewelry. 
         [0008]    Slight modification of the method also allows castings made from palladium-based alloys to be produced. 
         [0009]    The invention therefore relates to a method for the treatment of castings made of an alloy based on at least one platinum group metal and containing 1-3% by weight of boron. The method comprises multiple thermal ageing procedures carried out in the presence of oxygen and at temperatures below the melting point of the alloy, between each of which procedures the boric oxide thus generated on the surface is removed by treatment with water. The invention further relates to castings treated according to this method. Advantageous embodiments are evident from the further description and illustrations provided in the following. 
         [0010]    As mentioned above, the addition of 1.5-2.5% by weight boron is used to lower the melting point of Pt and alloys thereof to approximately 800° C. In this temperature range, the alloy can be melted in furnaces and processed with casting devices that are customary in the production of pieces of jewelry made from Au alloys. After casting, the castings are characterized by very high hardness (a Vickers hardness HV1 of approximately 500-600 is typical) and are very brittle—the cast-on sections are easy to break off; the casting crumbles when cut with guillotine shears; a fall from a height of 1.5 m onto a concrete floor causes it to shatter into small fragments (see  Platinum Metals Review  22(3):78-87 (1978)). 
         [0011]    Ageing in an air atmosphere at approximately 750° C. causes the boron to oxidize on the external surface of the casting to form a molten boric oxide coating (melting point 450° C.) without the metallic casting starting to melt. The depletion of the boron content can be tracked, for example, by determining the hardness (including according to Vickers) and/or by chemical analysis (preferably ICP—inductively coupled plasma). Regarding the use of the casting as a piece of jewelry, it has proven to be advantageous to continue the oxidation treatment until the hardness has reached approximately HV1=250. This is equivalent to a boron content of approximately 0.2% by weight. In this condition, the piece of jewelry still possesses sufficient hardness to be easy to polish, and shows good resistance to wear. On the other hand, the material is sufficiently ductile to allow it to be cut with guillotine shears and can be dropped onto a concrete floor from a height of 1.5 m without being damaged. It has proven to be advantageous in the oxidation treatment to occasionally remove the boric oxide thus generated by rinsing in warm water. 
         [0012]    The castings treated according to the method of the invention have a Vickers hardness which decreases from the core to an external surface of the casting. The Vickers hardness preferably decreases by more than 50% from the core to the external surface of the casting. The casting also has a microstructure in which the number of metallic phases decreases from the core to an external surface of the casting. 
         [0013]    Aside from determining the hardness, the progress of the oxidation treatment can often be followed by a color change. Alloys containing non-precious metal components often form a colored tarnish layer as a sign that the boron content is down to the range of a few tenths of a percent. 
         [0014]    The following alloys, for example, can be processed according to the method according to the invention:
       Pt96Cu4   Pt95.2Ru4.8   Pt95W5   Pt80Ir20   Pt95Co5   Pt95Rh5       
 
         [0021]    Pieces of jewelry can be produced from palladium alloys in an analogous manner. Due to the higher eutectic temperature of the Pd—B alloy system as compared to Pt—B, minor adaptations of the process need to be made, which can be determined through simple laboratory experiments. Typical Pd-based alloys are:
       Pd95.2Ru4.8   Pd95(InGa)5.       
 
         [0024]    The invention enables the simplified production of pieces of jewelry, watch housings, etc., from Pt and Pd alloys. Although they are not customary metals in the jewelry industry, the method according to the invention can also be applied to alloys of other metals from the platinum group, as for example, iridium or rhodium, which are known to form eutectic mixtures with boron at 1046 and 1143° C., respectively (see  Platinum Metals Review  1(4):136-137 (1957)). 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0025]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
           [0026]      FIG. 1  is a photograph showing the hardness profile of the sample described in Example 7 below, after determination of the Vickers hardness. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In the remainder of the description, the numbers given for the alloy components are in units of % by weight unless stated otherwise. 
       Example 1 
       [0028]    A total of 5 kg of the alloy PtRu4.8, pre-melted by conventional means, were poured off, then rolled to form a rod having a diameter of 10 mm, and then cut into pieces of approximately 30 mm in length. The sections were subsequently heated slowly by inductive means in a zirconium oxide crucible in an argon atmosphere, wherein 3.0% by weight boron granulate material were added to the melt. After brief melting, heating of the melt above 1000° C. was carefully avoided in order to minimize the risk of the boron and the zirconium oxide of the crucible reacting with each other. The alloy thus produced was then cast into a water bath to form granulate material with a grain size of 1-5 mm. 
         [0029]    After drying, 120 g of the granulate material were melted in the zirconium oxide crucible of a conventional centrifugal casting plant, such as is customarily used in the production of jewelry and dental components, and cast at approximately 1000° C. into a nest of moldings having 20 blanks for wedding rings. The mold used was a commercial casting mold made of gypsum-bound investment mass, produced according to the lost wax casting procedure. The maximal wall thickness of the rings was 2.2 mm. Cleared from the casting mold, the cast-on sections could be broken off without applying any force due to the extreme brittleness of the alloy. Measurement of the hardness according to Vickers showed the hardness to be HV1=520. 
         [0030]    Once the surface of the blanks was cleaned by blasting with glass beads, the blanks were placed in a chamber furnace and aged at 750° C. in an air atmosphere. The surface was wetted with molten boric oxide. After ageing for 3 hours, the blanks were taken out of the furnace and rinsed with warm water in order to remove the boric oxide. The oxidation treatment including subsequent rinsing was repeated a total of 8 times until the hardness was reduced to HV1=280. It was then feasible to drop the blanks onto a concrete floor from a height of 1.5 m without damage. The remainder of the cast-on sections was ground off and the surface of the rings was polished. 
       Example 2 
       [0031]    Similar to Example 1, a casting alloy containing 3.0% by weight of boron was produced by melting from the commercial jewelry alloy PtCo4.8 and then cast into granulate material. The centrifugal casting procedure was used to cast four blanks for wrist watches housings. The cross-section of the housings at their thickest sites was 3.2 mm. The hardness was measured after casting to be HV1=560. 
         [0032]    The blanks were cleaned following the process described in Example 1 and treated in an air atmosphere, whereupon, in addition to the hardness measurement, the boron content was determined through ICP analysis. After just 10 treatment cycles (oxidation for 3 hr at 750° C./rinsing in warm water), the first signs of slightly reddish tarnish discoloration were observed on the surface of the castings, which was presumably related to oxidation of the non-precious component, cobalt. After two more treatment cycles, the hardness was measured to be HV1=240, and the residual boron content was 0.18% by weight. The hardness ensures good wear resistance of the watch housing made from the blank. 
       Example 3 
       [0033]    Similar to Example 1, an alloy consisting of pure platinum and 3.0% by weight of boron was melted, granulated, and cast into a nest of moldings having 20 blanks for wedding rings. The hardness was measured after casting to be HV1=480. 
         [0034]    The blanks were cleaned according to the process specified in Example 1 and treated in an air atmosphere. After 12 treatment cycles (oxidation for 3 hr at 750° C./rinsing in warm water), the residual boron content was measured to be 0.08% by weight. Accordingly, the platinum meets the commercial “999 platinum” specification. The hardness was HV1=150, which ensures sufficient wear resistance for wedding rings. 
       Example 4 
       [0035]    The conventional vacuum die casting procedure was used to cast two blanks for brooches based on the granulate material from Example 3. The brooches were very fine-structured and had fin widths between 1.5 mm and 0.1 mm. The mold filling capability of the Pt—B alloy was excellent; no casting faults were detected. Simultaneously, a cuboid-shaped plate having dimensions of 10 mm×10 mm×1.5 mm was cast. The blanks and the plate were aged for 3 hours at 750° C. in a chamber furnace in an air atmosphere, and subsequently the boric oxide thus generated was rinsed off in warm water. This process was repeated a total of 7 times until the hardness of the plate was determined to be HV1=140. The boron content of the plate was 0.075% by weight. Accordingly, the platinum meets the commercial “999 platinum” specification. After cleaning and polishing, it was feasible to set diamonds in the brooches. 
       Example 5 
       [0036]    Analogous to the preceding Examples, an alloy made of pure palladium with 3% by weight of boron was melted in a zirconium oxide crucible and granulated. However, due to the higher eutectic temperature of the Pd—B alloy system (1065° C.), as compared to the Pt—B system (790° C.), the melt had to be heated to approximately 1100° C. in this case. Despite the melting temperature being higher, the alloy took up only 60 ppm zirconium by reacting with the melting crucible, which is non-objectionable for jewelry applications. 
         [0037]    The granulate material was used analogously to Example 1 to cast 20 blanks for wedding rings. Cleared from the casting mold, the cast-on sections could be broken off without applying any force, due to the extreme brittleness of the alloy. The hardness of the castings was HV1=520. Due to the eutectic temperature being higher, it was feasible to carry out the heat treatment at 800° C. for reduction of the boron content, whereupon the boric oxide thus generated was also rinsed off in warm water after 3 hours of ageing. After just 5 cycles of treatment, the hardness was reduced to HV1=130. The ICP analysis showed the residual boron content to be 0.09% by weight. It was feasible to polish and engrave the blanks without any difficulty. 
       Example 6 
       [0038]    Analogous to Example 1, 2 kg of the common medical implant alloy PtIr10 were alloyed with 3.0% by weight of boron and cast to form granulate material. Based on the granulate material, the centrifugal casting procedure was used to cast a nest of moldings having 100 blanks for head electrodes, which are used for tissue stimulation in cardiac pacemakers. The electrode head had a diameter of 1 mm and a thickness of maximally 0.1 mm, while the shaft had a diameter of 0.2 mm and a length of 5 mm. The complex shape of the parts, including bore holes and undercuts, was reproduced without any difficulty. After 12 cycles of ageing in an air atmosphere, as illustrated in Example 1, the electrode blanks had a hardness of HV1=125, which is indicative of a very low residual boron content. The ICP analysis showed the residual boron content to be 0.0015% by weight. 
       Example 7 
       [0039]    Similar to Example 1, a granulate material was produced by melting from the alloy Pt95Rh5 containing 2.5% by weight of boron. 
         [0040]    Approximately 9 grams of the granulate material were heated to approximately 900° C. in a graphite mold having an internal diameter of 30 mm using a hydrogen/oxygen flame until the alloy liquefied within just a few seconds. Upon cooling, the alloy solidified to form a disc having a thickness of approximately 0.7 mm. 
         [0041]    The disc was aged for 3 hours at 700° C. in a chamber furnace in an air atmosphere, and subsequently the boric oxide thus generated was rinsed off in warm water. The process was carried out a total of 5 times. Finally, the disc was split along the diameter. One half was processed as a metallographic section. 
         [0042]    Clear structural differences between a dendritic two-phase structure on the inside and a brightly-shiny one-phase structure of the platinum mixed crystal near the edge were observed across the cross-section. 
         [0043]    Commencing approximately 30 μm from one of the external surfaces, the hardness was measured across the entire thickness in increments of approximately 85 μm (Vickers micro-hardness in accordance with DIN EN ISO 6507-1, HV 0.05). The measured hardness values were as follows: 170, 483, 537, 554, 571, 581, 402, 167. 
         [0044]    Both the appearance of the microstructure and the hardness measurements show a clear gradient to exist from the core of the sample towards the two external surfaces, which is caused by depletion of the boron content of the material toward the surface. 
         [0045]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.