A process for treating the surface of articles made of titanium and alloys thereof

Abstract: A process and an apparatus for treating under a controlled atmosphere the surface of articles made of titanium and alloys thereof, particularly for prosthesis and the like in the oral cavity. All the operations required to obtain a finished article are accomplished under a neutral and controlled atmosphere so as to obtain a finished article protected by a strongly adhering film and having a fine appearance.

Description: The present invention relates to a process for treating under a controlled atmosphere the surface of pieces or articles made of titanium and alloys thereof or similar metals. The articles are preferably formed through lost-wax casting, particularly for use in prosthetic dentistry, orthodontics and the like. It is well known that titanium has peculiar properties such as the affinity with O 2 and H, and becomes easily oxidated at high temperature regardless of the O 2 partial pressure. Therefore the casting using the conventional lost wax method requires to operate under a neutral atmosphere (of argon or other noble gases). Today there are marketed automatic apparatuses equipped with an electric arc for melting the material and adapted to pour the molten titanium into a casting cylinder under a neutral atmosphere, such as an inert gas. These apparatuses are satisfactory but their diffusion is rather limited because of the plant high costs and the (currently) limited request since at present the specialized workshops only supplies "passivated" pieces, i.e. pieces that have been subjected to a passivation surface treatment that is completely inadequate. In fact such a passivation is decisely questionable from an aesthetic viewpoint since it imparts a dark and dull greysh appearance to the article; as a matter of fact it involves a deep ossidation that is without aesthetic quality. The poor biocompatibility of gold and alloys thereof, as well as that of Cr, Co, Ag and Ga alloys, and other metals presently used in the prosthetic devices and in devices for correcting defects to be used in the oral cavity has been strongly objected. It is therefore desirable to provide the medical operators with viable alternatives to the use of gold, cromium-cobalt and also the so called "surgical" steel that until today has accounted for most of the materials used in the field of dental prosthesis, orthodontic correction and implantation under the periosteum. As already mentioned there are known apparatuses for casting titanium articles under a neutral atmosphere by using an electric arc for melting the material and using zirconium oxide as a coating of the casting cylinder. Although the results are generally acceptable, such known process is carried out after the melting and with the cylinder full of molten metal which is is not recommendable. The temperature of the cylinder is high, about 800°C, and it is not convenient to open the melting and casting macchine to extract the cylinder and drop it into water, thus renouncing to the controlled atmosphere. This way one achieves a quenching of the metal with a simultaneous (deep or superficial) oxidation of the article, and a strong hydrogen contamination, which renders the article brittle and causes the formation of surface oxides of a dark grey color that are aesthetically very unpleasant (passivation). A casting process for producing castings by melting titanium with a vacuum arc in a copper, water-cooled crucible is disclosed in M.J. Donachie: "Titanium - A Technical Guide" 1988, ASM, Metals Park, Ohio, US. Oxidizing and nitriding of titanium for improving strengthening and protection against wear in known from "ASM Handbook, 10 th Ed., Vol.5, Surface Engineering" 1994, ASM International, Metals Park, Ohio, US. GB 2 244 019 and GB 2 244 019 disclose a method for casting of dental metals with a casting apparatus comprising an hermetically sealed casting chamber under inert gas atmosphere. FR 2695581 discloses a treatment of an oxidation-sensitive metal carried out in isolated surroundings. It consists of introducing the metal into these surroundings, producing a partial vacuum, melting the metal, introducing the melt into a mould, and letting it solidify. The surroundings, after they have been set under a partial vacuum, are flushed by means of an inert gas. An example of an apparatus for casting of metals such as titanium and alloys thereof is disclosed in GB 732298. It is the object of the present invention to overcome the above drawbacks by providing an improved process for the surface treatment of articles made of titanium and alloys thereof. This object is achieved through a process as claimed in claim 1. According to the invention all the operations required to obtain a finished article are accomplished under a neutral and controlled atmosphere in a single apparatus so as to obtain a finished piece that is protected by a strongly adhering film and has a fine appearence before being transferred to a normal atmosphere and then in the oral cavity environment. The piece is then subjected to anodic oxidation or nitriding in an ammonia atmosphere, which allows the properly forming of a protective coating of oxide or nitride having an adherence strength to the piece in the order of 1,400 kg/mm 2 . The process of the invention will now be disclosed in detail with particular reference to the attached drawing showing a block diagram of an apparatus adapted to carry out the process. Such apparatus substantially comprises a sealed vessel 1 provided with a transparent cover so that all the operating steps can be visually checked and controlled, and provided with introflexed rubber gloves for manipulating the articles. The apparatus is further provided with a bottle of an inert gas and an associated pressure reducing valve; it is further provided with a vacuum pump and a Getter device 3 adapted to cause a quick and complete oxidation of the metal contained within the apparatus, thus removing all the residual O 2 molecules within the sealed vessel 1. The sealed vessel 1 further houses a high temperature cylinder 5 where the article is cast, a tank 7 containing a liquid bath (e.g. oil) in which the article can be quenched, and an arm 6 for removing the article from the cylinder 5 and dipping it into the oil bath of the tank 7 where it is quenched in absence of contaminants. Within the vessel 1 of the apparatus there are further provided a finishing station 9 (cooling area), a sandblasting station 11 and a pickling station 13. Although the risk of contaminating the quenched article is very small, nevertheless according to the invention all the subsequent steps of finishing (9), sandblasting (11) and pickling (13) are accomplished under the neutral environment provided in vessel 1. Within the vessel 1 there are contained an additional (if required) station 15 for mechanically polishing the article, a station 17 for (micro) welding operations and a station (not shown) for spot-welding operations. All the above mentioned operations of polishing and welding are carried out in the neutral atmosphere. The vessel 1 further contains an anodic oxidation device 19 an a device 21 for nitriding the article in an ammonia atmosphere. Through these operations either an oxide coating or a nitride coating is formed on the surface of the article 23 or 25, such a coating having an adhesion force (cohesion) comprised between 1,000 and 2,000 kg/mm 2 , and preferably in the order of 1,400 kg/mm 2 . After such treatment(s), the article 23 (or 25) can be exposed to the atmosphere or to contaminants without the risk of being subjected to further undesided chemical or chemical-physical reactions, either on the surface or in the body of the article. This way the article can maintain indefinitely the aesthetic appearance that render such articke preferable to those made from other materials or through a different process. Such qualities are for example a pink coloring when the article has to blend with the gengival mucosa or a milk coloring when the orthodontic devices are located on the enamel of natural dental crowns.

Claims: A process for obtaining prosthetic devices in titanium or alloys thereof to be used in the oral cavity, said process comprising the steps of: providing a sealed vessel; obtaining a neutral atmosphere in said sealed vessel by means of a inert gas, a vacuum pump and a Getter device, said Getter device comprising a metal adapted to cause a quick and complete oxidation thus removing substantially all the residual O 2 molecules within the sealed vessel; forming a prosthetic device in a high temperature casting cylinder in said sealed vessel under the neutral atmosphere so obtained; dipping said cast device into a quenching bath in said sealed vessel under the same neutral atmosphere, thereby provoking the quenching of the cast device in said sealed vessel under the same neutral atmosphere; subsequently subjecting said cast device to anodic oxidation in said sealed vessel under the same neutral atmosphere, thereby forming a protective coating on the surface of said cast device. A process as claimed in claim 1, wherein said inert gas is a noble gas, preferably argon. A process as claimed in claim 1, wherein said device is formed in said high temperature casting cylinder through lost wax casting. A process as claimed in claim 1, wherein the temperature of said casting cylinder is of about 800°C. A process as claimed in claim 1, wherein said liquid bath is an oil bath. A process as claimed in claim 1, wherein such coating has an adhesion force (cohesion) comprised between 1000 kg/mm 2 and 2000 kg/mm 2 , and preferably in the order of 1400 kg/mm 2 . A process as claimed in claim 1, wherein said process further comprises the steps of finishing, sandblasting and pickling the cast device, all these further operations being carried out in said sealed vessel under said neutral atmosphere. A process as claimed in claim 1, wherein said process further comprises the steps of polishing and/or welding the cast device, all these further operations being carried out in said sealed vessel under said neutral atmosphere.

Inventors: MORENA ITALO, MORENA, ITALO
Applicants: MORENA ITALO, MORENA, ITALO
Application year: 1998
Main classification: B22D21
IPC codes: C23C8, C25D11