A METHOD FOR COATING TITANIUM AND TITANIUM ALLOYS WITH CERAMICS

Abstract: The invention relates to a method for coating medical as well as dental titanium or titanium alloy implants, prosthesis and their parts with ceramics. It is characteristic for the invention that metallic surfaces of implants, prosthesis and their parts are coated with a thin layer of electrochemical silver (Ag). It is also in accordance with the invention to overcoat the silver layer with one of the following metals: gold, platinum, palladium, tin or indium.

Description: A method for coating titanium and titanium alloys with ceramics The invention relates to a method for coating titanium or titanium alloy based medical and dental implants, prostheses and their parts with ceramics. Metal-ceramic restorations have been increasingly used in dentistry since the 1960s. The metal framework for this kind of dental crowns and bridges has usually been made of gold alloys containing different amounts of platinum and palladium. The results have been good regarding the durability and biological combatibility of the framework material. Recently it has been reported sensitivity and allergic reactions caused by these metals, and even gold has been pointed out to be a possible cause for these reactions. A significant drawback has also been the high price of the noble metals used. Therefore transition metal alloys like Co- Cr- and Ni-Cr-alloys have been used. During the last two decades titanium has made a breakthrough in odontology especially as regards the success of clinical application of artificial tooth roots, i.e. dental implants. Reasons for that have been the good strength- and corrosion properties of titanium and its excellent compatibility with the living tissues. There are no reports of sensitivity or allergic reactions between oral cavity tissues and titanium. Titanium's good corrosion properties and excellent biocompatibility are due to the thick and very stable oxide layer on the surface of the bulk metal. The good biocompatibility of titanium has led to several attempts to develop methods to apply the use of titanium also to the production of dental prostheses, above all crowns and bridges. The low price of titanium, one percent of that of gold, has encouraged these efforts. The framework of crowns and bridges will be coated with several layers of porcelain giving natural tooth outlook and a hard functional surface which tolerates high biting forces. The coating with porcelain occurs by sintering suitable mineral powders, eg. of three layers of different powders, onto the surface of the given metal framework. The first layer forms an opaque layer on the surface ofthe metal, the second layer gives to the metal framework the desired dental bone hue and the outmost transparent layer after glazing gives the impression of tooth luster The sintering occurs around temperatures of 750 °C The ceramic powders contain usually mainly silicon dioxide (Siθ2) and/or aluminium oxide (AkOs) Besides the good properties of titanium, its weakness is the tendency to dissolve great amounts of several gaseous species, mainly oxygen, hydrogen and nitrogen First of all titanium ' s great tendency to dissolve great amounts of oxygen (ab 33 %) and its affinity to silicon in the firing temperatures generates, when sintering with Siθ2 -based ceramics, a proportionally thick reaction layer composed of titanium and solid solution of titanium and oxygen α-Ti(O)3, and above of it a layer composed of titanium and silicon, i.e TisSi3(O) It is well known, that a few percent of dissolved oxygen makes titanium very brittle. Titanium silicides are also brittle compounds These brittle interlayers have the effect that the joint between titanium and porcelain is mechanically weak and during the cooling, thermal stresses induce crack formation in the reaction layer parallel to the metal surface Attempts have been made to avoid the weaknesses related to the coating of titanium with ceramics by using vacuum during sintering, using coating ceramics with low sintering temperatures, and by using metallic interlayers Eg in dentistry the durability of the ceramic coating on titanium has clearly been improved by using gold or a special ceramics as an interlayer The application of gold or special ceramics as an interlayer causes several drawbacks, which tend to weaken the bond between the porcelain and the titamiun framework In the case of gold the main reason for these drawbacks is the tendency of gold and titanium to form brittle intermetallic compounds with each other and the high diffusion rate of titanium in gold This leads to formation of several layers of brittle chemical compounds of variable compositions The most of the benefits aimed at with the use of gold has thus been lost The use of the ceramic interlayers leads also to the formation of brittle compounds and thus to the problems described above The aim of the present invention is to bring into use a method to coat titanium and especially dental crowns and bridges of titanium with ceramics, e.g with Siθ2-based porcelain, which lacks of the drawbacks related to the previous technics The method according to the invention is characterised by, what is presented in the first claim. The following presents the benefits of the method according to the present invention compared with the methods of the known technics. As it has become obvious from the above the weaknesses of the known technics in the coating of titanium with eg. Si-based porcelain powders are due to the ability of titanium to dissolve great amounts of oxygen at the sintering temperatures and to the activity of titanium and silicon to form brittle intermetallic compounds. To avoid these difficulties, the natural oxide layer of titaniun should be removed, and access of oxygen to the surface of titanium and the chemical reactions between titanium and silicon hindered as far as possible. The solution according to the present invention is to coat titanium before sintering with a thin layer of metal which accomplishes these requirements. The metal in question should be soft enough to relax the thermal stresses which due to the differences in thermal expansion coefficients of titanium and porcelain are created during the cooling phase after sintering. Rapid temperature changes are possible also when using these prosthetic devices. The interlayer composed of gold or ceramics according to the known tehnics does not fulfil these requirements. The present invention is based on extensive research, which for the first time revealed that the soft nobel metal silver (Ag) is the only metal that fulfils all the requirements needed for a well functioning interlayer. Compared eg with gold silver reacts very sluggishly with titanium at the sintering temperatures. This is caused by the comparatively low driving force ofthe reaction between silver and titanium. The reaction rate between titanium and silver is, however, enough to quarantee good attachment of silver layer to the surface of titanium Another good property of silver is titanium ' s low diffusion rate in it. During sintering titanium is diffused through the silver layer just enough to create preconditions for good attachment of porcelain onto the silver layer. Being a soft and having the best thermal conductivity, silver metal is suitable to relax effectively temperature variation dependant stresses in the porcelain-titanium joint. The following presents a detailed description of one method according to the present invention for coating titanium with silicon based porcelain powders and comparative test results obtained with test specimen according to ISO 9693 - and DIN 13279 standards coated with the method of the present invention and compared with the methods of known technics For the coating of titanium it is important that the interlayer of silver is deposited onto the pure metallic titanium surface The coating method of titanium according to the present invention contains, eg. in the manufacture of metalloceramic dental crown, the following stages Stage 1 The surfaces to be coated will be roughened with sandblasting in order to obtain a bigger, mechanically better interlocking surface which improves the strength of the joint Stage 2 The oxide layer is removed from the surface ofthe titanium by treating titanium to be coated about 5 minutes in concentrated hydrochloric acid in the temperature of +95 °C Stage 3 : Electrolytic presilvering in silver-kaliumsyanide water solution in the temperature of +18 °C during 2 minutes. Stage 4 The actual silvering occurs in the same solution as the presilvering, the silver syanide concentration of the solution is only increased The thickness of the silver layer is adjusted by current density and time to ab. 10 μm. In both cases a 99% pure silver plate is used as an anode Stage 5: The sintering of porcelain layers onto the silver plated titanium surface occurs in several stages First is to be coated the opaque layer, then the dental bone imitating layer and finally the luster layer which will be glazed. The sintering ofthe ceramic layers occurs at temperatures around 750°C It is also accordig to the present invention to use metals like gold, platinum, palladium or tin as coating material above the silver layer. It is also according to present invention to vary the composition of the electrolytic bath used in the coating processes, the current rate and the holding time depending on the size and form ofthe item to be coated. Table 1 presents the results of the bending tests obtained with the specimen coated by the method of the present invention and by the methods of the known technics. The bending tests have been carried out according to standards ISO 9693 and DIN 13267 The results are averages of 10 tests. The standard deviations of the test results are presented in brackets Table 1. Specimen Breaking force, MPa Coating according to inv 55,4 (3,2) Ceramic interlayer 42,4 (6, 1) No interlayer 36,4 (4,4) The bending test results obtained with the specimen coated according to the present invention fulfil the requirements of the above mentioned standards and are clearly and significantly (p<0.001) better than the results obtained by the specimen coated according to the known technics.

Claims: Claims 1. A method for coating titanium or titanium alloys with ceramics wherein the method comprises that a silver layer is deposited on the titanium or titanium alloy surface before coating it with ceramics. 2. A method according to 1 wherein the silver layer is deposited on the titanuim or titanium alloy surfac by using an electrochemical method 3. A coating method according to claim 1 wherein the thickness of the silver layer is 1-50 μm, more favorably 1 -20 μm. 4. A coating method according to claim 3 wherein another metal layer consisting of one of the following metals: gold, platinum, palladium, indium or tin is deposited onto the silver layer. AMENDED CLAIMS [received by the International Bureau on 16 September 1997 ( 16.09.97 ) ; origi nal claims 1 -4 replaced by amended claims 1-4 ( 1 page) ] 1. A method for coating protheses used in medicine or dentistry with ceramics wherein the method comprises that a silver layer is deposited chemically or electrochemically on titanium or titanium alloy surface before coating them with ceramics. 2. A coating method according to claim 1 wherein the thickness of the silver layer is 1-50 μm, preferably in the range of 1-20 μm. 3. A coating method according to claim 1 wherein another metal layer composing of one of the following metals: gold, platinum, palladium, indium or tin is deposited onto the silver layer. 4. A coating method according to claim 1 wherein the method contains the following stages: Stage 1 : The surfaces to be coated will be roughened by sandblasting in order to obtain a larger, mechanically better interlocking surface which increases the strength of tiie joint. Stage 2: The surface oxide layer of titanium is removed immersing the titanium into the concentrated hydrochloride acid (HCI) at the temperature of +95 °C for about 5 minutes. Stage 3: Electrolytic presilver-coating occurs in water-based silver- and potassiumcyanide solution at the temperature of +18 °C for 2 minutes. Stage 4: The actual silver-coating is produced in the presilver-coating solution after increasing its silvercyanide concentration. About 10 μm thick silver layer is fabricated by adjusting the current density and deposition time. In both cases 99% pure silver is used as an anode. Stage 5: The sintering of porcelain layers onto the silver- plated titanium surface is performed in several stages: firstly the opaque layer, secondly, the dental bone imitating layer and finally the luster layer which is glazed. The sintering of the ceramic layers is carried out at the temperatures of about 750°C.

Inventors: KIVILAHTI JORMA, KOENOENEN MAUNO, KIVILAHTI, JORMA, KOENOENEN, MAUNO
Applicants: KIVILAHTI JORMA, KOENOENEN MAUNO, KIVILAHTI, JORMA, KOENOENEN, MAUNO
Application year: 1997
Main classification: C23C28
IPC codes: A61C8, A61C13, A61K6, A61L27, C23C28, C25D5