Patent Application
20070251604
As used herein, “a” or “an” means one or more. Unless otherwise indicated, the singular contains the plural and the plural contains the singular.
As used herein, “inert gas” is defined as a gas which does not substantially react with the substrate materials described herein. Oxygen and nitrogen, among other gases, are not inert gases under this definition.
The instant invention relates to surface oxidized and/or nitrided compositions that are rejected for non-conformance to specification. Such components have found extensive application in the field of medical implants and the present invention is expected to be particularly usefully applied in this field. After the oxidation and/or nitridation process, rejected workpieces comprising the compositions are typically scrapped because the oxide and/or nitride does not allow for ease of rework. The objective of this invention is to completely dissolve the surface oxide and/or nitride and drive the resultant oxygen or nitrogen homogeneously throughout the composition. Therefore, the workpiece comprising the compositions can be re-oxidized and/or re-nitrided. In the case of the use of the compositions for medical implants, the reworked workpiece and be re-formed (re-oxidized/re-nitrided) into a medical implant.
In general terms, the instant invention relates to the reworking of a composition comprising a substrate and an oxidation and/or nitridation layer on the surface of the substrate. In some embodiments, the composition forms a portion of, or the entirety of, a workpiece. In some embodiments, the workpiece is an orthopaedic implant, such as, but not limited to hip implant, a knee implant, a dental implant, a vertebral implant, a shoulder implant, an ankle implant, an elbow implant, and a temporo-mandibular implant. In this way, the term medical implant includes dental implants.
While most of the remaining discussion focuses on the application of the instant invention where the substrate is zirconium or zirconium alloy and the surface comprises oxidized zirconium such as that described in U.S. Pat. No. 2,987,352 to Watson and U.S. patent U.S. Pat. No. 5,037,438 to Davidson, among others, it should be understood that the instant invention is broader in scope and can be applied to any composition comprising a metallic substrate and an oxidized and/or nitrided surface. Presently, the manufacturing yield of oxidized zirconium components is typically less than 95%. The components that are rejected after the oxidation treatment cannot be reworked due to the oxide on the surface. The removal of oxide with machining or grinding leads to the embedding of the oxide particles on the surface and such components having embedded oxide particles can not be re-oxidized. The general approach had been to scrap these components entirely which leads to reduced output of the product line and increased cost of the final products.
The re-work process involves treating the oxidized zirconium components in vacuum (less than 10−2 torr) or in reduced pressure (less than about 10 torr) at elevated temperatures (preferably greater than 500° C.) for extended periods. The time, temperature and pressure can be decided based on the starting oxide thickness. Diffusion of oxygen through the entire cross-section will increase the oxygen concentration of the alloy marginally, still keeping it in specification.
In the present method, a vacuum process is used to dissolve the surface oxide and/or nitride. The vacuum or inert gas heat treatment is applied in such a way as to dissolve/diffuse the oxygen and or nitrogen substantially homogeneously throughout the entire substrate until the oxide and/or nitride is substantially completely removed from the surface. For best results, the oxide is preferably completely dissolved such that the oxygen concentration is uniform or near-uniform through the entire substrate.
Broadly, the oxidized and/or nitrided composition is placed in an elevated temperature environment and in a vacuum or a reduced pressure environment (typically less than about 10−2 torr, although the pressure may be higher if a longer treatment time is used). Alternatively, the oxidized and/or nitrided composition is placed in an inert gas environment in an elevated temperature environment. The treatment under reduced pressure/vacuum environment or under inert gas at elevated temperatures dissolves the oxide and/or nitride. Preferably, the temperature is about 650° C.
Thus, in the manufacture of an oxidized and/or nitrided composition, components that would otherwise be scrapped are reworked following dissolution of the oxide/nitride. A schematic flow diagram of the manufacturing process incorporating this rework procedure is provided in
However, by application of elevated temperature and substantially depriving the composition of oxygen or nitrogen and other non-inert gases causes any surface oxides or nitrides to be dissolved and diffused into the bulk substrate. One means to substantially depriving the composition of oxygen or nitrogen is to treat the composition is by placing it in a low pressure or vacuum environment (typical 10−2 torr or less). At such low pressures, concentrations of oxygen and nitrogen as sufficiently low as to preclude further oxide and/or nitride growth as the oxide and/or nitride is dissolved/diffused from the surface into the bulk substrate.
The general method to dissolve the surface oxide and/or nitride from the surface and dissolve the oxygen and/or nitrogen into the bulk substrate comprises the oxidized and/or nitrided composition under reduced pressure or vacuum, or under inert gas the above said implant at a temperature of about 500° C. or greater. The exact temperature is chosen will depend upon the time of treatment and the thickness of the surface layer to be removed. Use of low temperatures will generally require longer treatment times. A thicker surface layer will also require longer treatment times which may be made shorter through the use of higher dissolution/diffusion temperatures. In the case of an oxide, the oxygen atoms thus released are driven deeper into the alloy substrate, hardening the material. The time required is deduced from the diffusion coefficient and desired hardened depth. The approximate relationship is x2=D′t, where x is diffusion layer thickness, t is time and D′ is an effective diffusion coefficient.
Preferably, the dissolution/diffusion treatment should be performed at the lowest temperature which is feasible under the conditions of oxide thickness and the required time for completion. In this way, a more complete preservation of the microstructure of the substrate is promoted. The oxide is completely dissolved until the oxygen concentration is near uniform through the entire substrate. Preferably, surface hardness of the alloy after re-work and before re-oxidation/re-nitridation is kept below 4.5 GPa.
It should be understood that although the preferred temperature is about 500° C. or greater, the temperature used can be varied, particularly in the case of different substrate compositions. As indicated, it is preferable for optimal results to perform the treatment at the lowest practical temperature.
The re-oxidized compositions can then be used as stock in a finished product manufacturing process. As oxidized and nitrided surfaces have shown particular applicability in the field of medical implants, the present invention is expected to be particularly useful in the manufacture of medical implants. Such medical implants are formed using the reworked composition as stock. Forming the medical implant using the reworked composition as stock may be performed by any suitable method, including those known or obvious to those of ordinary skill in the art.
The present invention is applicable to all oxidized and nitrided surfaces. It is particularly useful in oxidized or nitrided zirconium or zirconium alloy surfaces. It is, for example, applicable to the oxidized zirconium surfaces of Davidson as described in U.S. Pat. Nos. 5,037,438 and 5,180,394, those of Watson as described in U.S. Pat. No. 2,987,352, those of Hunter et al. in U.S. Pat. Nos. 6,447,550; 6,585,7723, and also to a new composition of oxidized zirconium described in copending U.S. utility application Ser. No. 11/558,756 filed Nov. 10, 2006 and corresponding international patent application PCT/US2006/043838 filed Nov. 10, 2006. The foregoing is a list of non-limiting examples, as the present invention is applicable to reworking of all oxidized and nitrided surfaces.
In the case of medical implants having oxidized and/or nitrided surfaces, in some embodiments, the present invention is useful in reclaiming out-of-specification samples. In practice, a medical implant is formed from a metal or metal alloy stock and is then oxidized and/or nitrided to form an oxidized and/or nitrided medical implant. The medical implant may (or may not) be then subjected to various finishing steps. At this point, the implant is tested for conformance to manufacturing specifications. One such specification is the oxide thickness. If the oxide thickness of the implant is less than the specification, such implant will not be accepted. If the oxide thickness is greater than specification, then also the component will not be accepted. Historically, if such implants were deemed to be outside of specifications, it would be discarded as scrap. In one embodiment of the present invention, the medical implant, or a portion thereof, is treated according to the present invention such that the oxidized and/or nitrided surface is dissolved to rework the implant and the implant can thereafter be either re-oxidized and/or re-nitrided, or otherwise finished into a useful medical implant. Finishing steps include, but are not limited to mass-finishing to obtain a desired final finish, laser marking to identify the implant and packaging, sterilization of the implant as a final step, etc. The mass-finish can typically be employed using abrasive media. The other techniques of polishing such as that using diamond media may also be employed. Sterilization may be carried out using gamma rays, or other methods found in the art.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
This application claims priority to U.S. provisional application Ser. No. 60/795,102 filed Apr. 26, 2006. U.S. provisional application Ser. No. 60/795,102 filed Apr. 26, 2006 is incorporated by reference herein as though fully disclosed herein.
| Number | Date | Country | |
|---|---|---|---|
| 60795102 | Apr 2006 | US |
