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(11) | EP 0 938 940 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A process and an apparatus for treating the surface of articles made of titanium and alloys thereof |
| (57) 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. |
[0001] 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. The invention further relates to an apparatus for carrying out such process.
[0002] It is well known that titanium has peculiar properties such as the affinity with O2 and H, and becomes easily oxidated at high temperature regardless of the O2 partial pressure.
[0003] Therefore the casting using the conventional lost wax method requires to operate under a neutral atmosphere (of argon or other noble gases).
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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).
[0010] It is the object of the present invention to overcome the above drawbacks by providing an improved process and a new apparatus for the surface tratment of articles made of titanium and alloys thereof.
[0011] This object is achieved through a process and an apparatus as claimed in claims 1 and 8, respectively.
[0012] 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.
[0013] 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/mm2.
[0014] 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.
[0015] 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.
[0016] 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 O2 molecules within the sealed vessel 1.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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/mm2, and preferably in the order of 1,400 kg/mm2. 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.
1. A process for treating the surface of an article made of titanium and alloys thereof
that has been formed in a casting cylinder under a neutral atmosphere, characterized
in that it comprises the steps of:
quenching the cast article;
subjecting the quenched article to a treatment for forming a protective coating on its surface;
all the above steps being carried out under the same neutral atmosphere in which the article was formed.
2. A process as claimed in claim 1, characterized in that said treatment for forming
a protective coating comprises the anodic oxidation of said article.
3. A process as claimed in claim 1, characterized in that said treatment for forming
a protective coating comprises forming a nitride coating on the surface of said article
in an ammonia atmosphere.
4. A process as claimed in claim 2 or 3, characterized in that the adhesion force of
said coating is in the order of 1,400 kg/mm2.
5. A process as claimed in the preceding claims, characterized in that said quenching
of the cast article comprises moving of the cast article from said cylinder and dipping
it into a tank (7) containing a quenching liquid.
6. A process as claimed in the preceding claims, characterized in that said article is
obtained through lost wax casting.
7. A process as claimed in the preceding claims, characterized in that said article is
a article to be used in dental prosthesis, orthodontic correction and implantation
under the periosteum, reconstructive surgery.
8. An apparatus for treating the surface of articles made of titanium and alloys thereof
characterized in that it comprises a sealed vessel (1) housing:
a bottle of inert gas and an associated pressure reducing valve;
a vacuum pump and a Getter device (3);
a high temperature casting cylinder (5);
a tank (7) containing a queching bath;
a movable arm (6) for moving the article from said cylinder (5) to said tank (7);
a cooling area (9);
a station (19, 21) for forming a protective coating on the surface of said article.
9. An apparatus as claimed in claim 8 characterized in that said station for forming
a protective coating comprises an anodic oxidation device (19)
10. An apparatus as claimed in claim 8, characterized in that said station for forming
a protective coating comprises a nitriding device (21).
11. An apparatus as claimed in claim 8, characterized in that it further comprises introflexed
rubber gloves to manipulate said articles.