WO2011038702A1

WO2011038702A1 - Method of production of nano-structural titanium semis for implants

Info

Publication number: WO2011038702A1
Application number: PCT/CZ2010/000071
Authority: WO
Inventors: Miroslav Greger; Ladislav Kander; Miroslav Kursa
Original assignee: Vysoká Škola Báňsá-Techniká Univerzita Ostrava
Priority date: 2009-10-01
Filing date: 2010-06-15
Publication date: 2011-04-07
Also published as: CZ302421B6; WO2011038702A8; EP2483020A1; EP2483020B1; CZ2009640A3

Abstract

Coarse-grained titanium powder of high purity and not containing toxic ingredients is placed in to the one-sidedly closed casing (1), the casing (1) is placed into the forming matrix (4) and the powder is pressed with use of the extruder (5) to a density of at least 4050 kgm^-3. The loose part of the casing (1) is removed, the casing (1) is closed and the powder is sintered, advantageously at the temperature from 1250 to 1450°C for 1.5 to 2.5 hours. The sintered material is repeatedly extruded, advantageously at least ten times, through the narrowed space in the matrix (4) using the CEC method of cyclic compression, advantageously at the temperature of 220 to 250°C. By this method a nano- titanium is obtained, which is then processed by drawing through the die (7), advantageously at the temperature from 160 to 200°C to form a body, advantageously a wire, which is the semis for manufacturing of a dental implant.

Description

Method of Production of Nano-structural Titanium Semis for Implants Technical field

The invention relates to the field of metal forming and metallurgy, for which it resolves the method of production of nano-structural titanium semis suitable for use for implants, particularly for dental applications. Background art

At present the dental implants based on titanium are produced either from titanium alloys or from castings made by melting of commercially pure coarsegrained titanium. Materials for the manufacture of implants must meet high requirements concerning their strength, chemical purity, bio-compatibility, conditions for osteo-integration, wetting, adherence of tissue cells, etc.

Metallic alloys for implants and methods of their production are described for example in EP 2003-2905. For example the document CZ U 17408 describes the use of titanium alloys for dental implants. Titanium alloys have high strength, but their disadvantage is the presence of impurities, which act in human body as allergens. For example a super-plastic duplex alloy of a and β titanium Ti-6AL-4V is known, which was previously considered as suitable for implants. Later on it was, however, established that vanadium in these alloys is toxic, and also aluminium is also considered as potentially toxic element. Other known titanium alloys also contain elements that are toxic for human organism. For example the alloys used in dental applications Ti-20Cr0.2Si, Ti-20Pd-5cr, Ti-13Cu-4.5Ni contain elements that are classified as allergens.

The development is therefore focused on the effort to replace in titanium alloys toxic and potentially toxic elements by non-toxic elements, such as tantalum, molybdenum, niobium or zirconium. There are also known and used alloys of β titanium, containing the elements of very different densities and melting temperatures. Both a and β and also β alloys require, however, special production technology, which significantly increase production costs and price of the product. Due to the fact that sensitivity of the population to allergies continually increases, the titanium alloys are no longer preferred for use in the implants.

The disadvantage of the presence of undesirable substances at use of the implants made of alloys is partially eliminated by the fact that special surface finish is applied on them. To that end, the manufactured implant must undergo specific surface treatment, which however substantially increases their price and complexity of production. For example CZ U 17408 describes the problems arising from the use of titanium alloys for the implants caused by the fact that due to allergies it is recommended to perform a specific surface treatment on the implant, which creates a modified surface layer with qualitatively more satisfactory mechanical properties, and removes part of undesirable ingredients. The cited source for example recommends application of surface treatment by effects of an aqueous solution of hydrofluoric acid, or with addition of nitric acid. These chemical techniques create a compact surface layer, which has significantly better qualities than the original alloy. The surface treatment, however, makes production of implants both more expensive and more complicated, and it is also obviously not the most suitable solution. For example right in the case of the dental implants it is possible that there may occur mechanical damage of the modified surface layer on the implant, and thus creation of an access to chemically non-modified allergenic material.

Coarse-grained commercially pure titanium, cpTi, is currently considered to be the most suitable material for dental and other implants. Dental and other implants are formed from the semis, which is at present being prepared as a casting that is cast from commercially pure titanium powder. In order to ensure sufficient bio- compatibility of material and refining of its structure, the casting is then subsequently processed by forming processes known as ECAP. ECAP, it means the Equal Channel Angular Pressing, is method of angular extrusion through the channel of the same cross-section. This is forming, in which the titanium casting in the form of wire is repeatedly extruded by extruder through the matrix. A matrix in a form of a casing containing a cavity is used, in which the cavity form a through channel of circular cross-section, into which a wire is inserted wire and here a pressure from two sides is applied to it by mechanical devices called extruders. Pressure from two sides is achieved by the fact that the channel is bent at an angle and/or it is tapered, and the extruder is pushed in direction of the channel taper. Arrangement of matrices is described for example in CZ U 18713. Disadvantage of that solution, in comparison with dental metals in the form of alloys, consists in low values of strength properties, normally around 650 MPa. It is therefore desirable, in order to maintain low modulus of elasticity, to increase the strength of the final product without the use elements that may be only potentially toxic or allergenic.

For the preparation of nano-materials the CEC method of cyclic extrusion is generally known. This method of forming also uses a matrix with a through channel, through which the repeated extrusion of material is conducted by an extruder, while this material is usually based on metal casting. Preparation of nano-materials from any types of metals or metal alloys is not yet sufficiently known. Successful implementation depends on choosing the appropriate method and specific equipment, on the structure, chemical composition, purity and size of material and on many other circumstances. Possible success or failure depends on the findings and compliance with specific conditions for each material, which is still under investigation.

Shortcomings of the existing techniques of production of implants are reflected in particular in dental technology. Mini-implants that are commercially available on the market, made from titanium alloy, with diameter of approximately 2 mm, due to their material properties do not allow a full load and are suitable rather for supporting function, for example for hybrid prosthesis or for interstitial pillar. For example their use for the pillar in the area of insufficient thickness of the alveolus is questionable.

Disclosure of invention

The above mentioned disadvantages are eliminated in great extent by the invention. New method of production of titanium semis for implants has been resolved, which involves first production of titanium powder of at least technical purity by powder metallurgy, which is, however, not melt but compressed, the titanium powder in the compressed is sintered, and afterwards the sintered material is processed by repeated extrusion by the CEC technology to a compact poly-crystalline nano-titanium, from which the implant is shaped by commonly used methods.

The method according to the invention has resolved and explains also the specific conditions enabling production of nano-structural titanium implant. Titanium powder without toxic ingredients is advantageously used, with the grain size up to 100 nm and titanium content of 98 weight %, is placed into the casing closed from one side, the casing with the powder is placed into the forming matrix and the powder in the casing is pressed using push of extruder on the powder in the casing to a density of at least 4050 kgm ³, which corresponds approximately to 90% of theoretical density of compact titanium.

Titanium powder is advantageously placed into the channel of circular cross-section situated in a metallic casing, where this channel has advantageously a height to diameter ratio of 2.5 : 1 and it is closed at one end by a bottom. Compression of titanium powder is made by pushing of the extruder through the channel open end inside on the contained powder in direction against the bottom of the channel. At the same time casing with the powder may be extruded through the matrix.

When the titanium powder is compressed to a specified density, it is appropriate to remove the excess part of the casing with free space above the compressed powder. The open end of the casing is then hermetically closed and the compressed powder is sintered.

Sintering creates is from the powder a compact material. If the casing according to the invention is used a titanium body of cylindrical shape is formed form the powder. Before repeated extrusion, between individual extrusions or after extrusion of the sintered material through the matrix by the CEC method it is appropriate to remove the casing from the sintered material.

Sintering of the compressed titanium powder is advantageously carried out at the temperature from 1250 to 1450°C, which corresponds to approximately 0.8 of the melting temperature of pure titanium, for a period of 1.5 to 2.5 hours

The sintered material is then repeatedly extruded through the matrix by the CEC method of cyclic compression, advantageously at the temperature from 220 to 250°C, which corresponds to a homologous temperature of approximately 0.25 to 0.27. By homologous temperature we understand the ratio of the forming temperature to the melting temperature.

The sintered material according to the invention is extruded repeatedly through the matrix by the CEC method of cyclic compression until it achieves the specified average grain size of at least 100 nm.

Refinement of the structure is satisfactory if extrusion of the sintered titanium material through the matrix by the CEC method of cyclic compression is performed under the conditions according to this invention at least five times, advantageously at least ten times.

When by the repeated extrusion by the CEC method a body is made of nano-titanium with grains of satisfactory structure, advantageously with an average size of 100 to 150 nm, thus produced nano-titanium body is further processed by drawing through a die, advantageously at the temperature from 160 to 200^°C, which corresponds approximately to homologous temperature of 0.22 to 0.25.

The invention is usable in the field of human and veterinary medicine. It makes it possible to produce material, which is a semis suitable for implants, especially for dental implants. By application of the procedure according to the invention a compact, polycrystalline, semis is obtained, from which it possible to produce with use of common machining technologies for example dental implants, which in comparison to classical coarse-grained titanium are characterised by high strength Rm 1070 to 1200 MPa and high yield strength, an optimal elongation of approximately 12%, better bio-compatibility in comparison with titanium alloys, and in comparison with other implants, such as dental metallic implants, it has low value of modulus of elasticity in tension E, 100 GPa or less. Strength of nano- titanium produced according to the invention is approximately three times higher than strength of normal coarse-grained titanium, which will allow use of implants of a smaller diameter with preservation of comparable load capacity as that of standard implant of larger diameter, made of coarse-grained titanium. Material produced by the method according to the invention is usable in particular for dental technology. The implant thus prepared can be used for example as a pillar in the area of insufficient alveolar thickness. Nano-structural titanium retains all the important properties from the viewpoint of medicine, thanks to which pure titanium became the preferred material also for dental implants. At the same time nano- titanium outperforms other materials used in this application by its specific mechanical properties that are important for maintaining long-term safety function of the implant. For example a nano-implant made from material nTiGr4 has 2.25 times higher ultimate tensile strength than the coarse-grained titanium cpTiGr4, 1.35 times higher breaking strength than the implants made of alloy Ti-6 AI-4V, which contains potentially toxic elements, 1.37 times and 1.44 times higher breaking strength than the alloy Ti-6AI-7Nb and Ti-15Mo-SZr. Brief description of drawings

The disclosure of invention is clarified by the drawings, where the figures 1 to 8 illustrate progressively various stages of production of nano- structural semis according to the invention, and namely Figure 1 shows a casing with titanium powder closed from one side, Figure 2 shows process of compression of the titanium powder in the casing situated in the matrix by action of extruder, Figure 3 shows the casing with the compressed powder and free space above the powder after compression, Figure 4 shows the compressed powder in the rest of the casing after cutting of the open part of the casing along the line AA shown in the previous figure, Figure 5 shows the process of refining the material structure by illustration of three stages A, B and C of the condition of the casing with the sintered material during extrusion through a matrix by the CEC method, Fig. 6 A, B shows a compact body of nano-titanium, where A shows the casing with nano-titanium, and B shows the same after removal of the casing, Figure 7 shows processing of nano-titanium by its drawing through the die to a wire, and Figure 8 shows the final product in the form of wire made of nano-titanium.

Carrying out the invention An illustrative example of carrying out the invention is the following procedure for the production of nano-structural titanium semis for implants, the individual production stages of which are shown progressively, as they should follow each other, in the Figures 1 to 8 Production of nano-structural titanium semis for implants starts by manufacture of titanium powder of at least technical purity by using known methods of powder. For the purpose of clarifying the disclosure of the invention a powder with grain size up to 100 nanometers, nm and the following composition was used: element weight % in titanium powder

C max. 0.008

Fe max. 0.030

O max. 0.060

N max. 0.004

Cr max. 0.01

Ni max. 0.01

V max. 0.005

Al max. 0.006

Ti rest to 100

Titanium powder was poured from one side of the closed casing 1, as it is shown in Figure 1. The casing 1 , made of stainless steel, contains for this purpose the channel 2 of circular cross-section. The channel 2 has a height to diameter ratio of 2.5:1 and it is closed at the lower end by the bottom 3. Compression of the powder is shown in Fig.2. It was carried with use of methods of forming, namely by extrusion through the matrix 4, by pushing the forming extruder 5 against the bottom 3 of the channel 2, as indicated by an arrow in Figure 2. The extrusion represents such a pressure, that the casing 1 deforms under it adequately to the space in the matrix 4. The powder has been compressed after one extrusion to a specified density of 4050 kgm^"3.

Compression increased the free space above the powder in the casing 3, as shown in Fig.3. As the next step the part of the casing 1 with free end and free space above the compressed powder was cut along the line AA, shown in Figure 3, and then the open end of the casing 1 was hermetically sealed by welding the solid lid 6, as shown in Figure 4 The titanium powder was afterwards sintered, i.e. it underwent the process of sintering. Sintering was achieved by insertion of the casing with compressed titanium powder into a furnace heated to the temperature of approx. 1.450°C and left here at this temperature for 2 hours. Sintering created a compact polycrystalline structure of material.

The casing1 containing the sintered titanium material was removed from the furnace and inserted into the thermostat, where it was cooled down to 250°C.

The casing 1 containing the sintered material was then repeatedly extruded through the matrix 4 by the extruder 5 by the CEC method of cyclic compression at temperatures ranging from 220 to 250°C, as shown in Fig.5. In Figure 5 the images marked as A, B, C illustrate various positions of extruded material and method of material compression in the casing 1 during the extrusion process, with the arrows indicating the direction of pressure forces F1 and F2 on exerted on the extruders 5 in the matrix 4. During application of the CEC material in the matrix 4 is pushed by two extruders 5 acting against each other, while magnitude of the forces F and F2 exerted on the material from the top and bottom alternates in such a manner, that the force of the desirable movement of material prevails. The material is repeatedly extruded, with simultaneous exertion of the force F1 from the top and force F2 from below through the tapered diameter of passage in the matrix 4, by which the structure is refined. Figure 5 shows in positions A, B and C the movement and progressive compression of the material during the phase of extrusion, at which in Figure 5 A, B the force F1 exerted from the top is greater than the force F2 exerted from below, while Figure 5 C shows the contrary. The extrusion was carried out repeatedly through the tapered part of the matrix 4 by alternation of the dominance of the force exerted on the upper or lower extruder 5, until the limit of the specified average grain size of 100 nm was reached. As it is also shown in Figure 5 repeated extrusion by the CEC method under the conditions according to the invention resulted in changes of the structure of the processed material to nano-titanium. At this temperature and with observation of the optimal conditions according to the invention this happens after approximately ten extruions. In this particular case, the extrusion was performed twelve times, by which an average grain size from 100 to 150 nm was achieved. Due to the fact that sintering creates a compact body, it is possible after the sintering process, at the stage before the repeated extrusion, between individual extrusions or after extrusion, to remove the casing 1 from the processed material. Fig. 6 illustrates at the position A the encapsulated body made of nano- titanium material, and at the position B the same body after removal of the original elements and elements added later of the casing 1. This concerned specifically removal of the original part of the casing 1 with the bottom 3, and cutting-off of the lid 6.The body thus obtained had a shape of a cylinder.

After the procedure according to the invention when the body made of sintered material achieved by repeated extrusion by the CEC method a structure of nano-titanium with the grains of average grain size ranging from 100 to 150 nm, the nano-titanium body thus produced after removal of the casing was processed by drawing through the die 7 at the temperature of approx. 180°C to a wire with diameter of 6 mm, as shown in Figure 7 and 8. Figure 7 shows drawing through the die 7, i.e. the next step at processing of nano-titanium realised by method of unidirectional drawing through the die 7 with use of the force F, where the arrow shows the direction of drawing of the nano-titanium wire in the die 7, and Figure 8 illustrates schematically the produced semis.

The above procedure according to the invention produced material with the following mechanical properties. Strength Rm was 1070 MPa, modulus of elasticity in tension E was 100 GPa, elongation A was 2%, limit size of contained grain d_z was 100 to 150 nm. This material could already be processed by common methods of shaping, e.g. by machining, to produce an implant. For example procedure according to the invention produced from titanium powder poured into the channel 2 with diameter of 26 mm in the casing 3 a nano-titanium wire with diameter of 6 mm, suitable for the manufacture of dental mini implants.

Claims

C L A I M S

1. Method of production of nano-structural titanium semis for implants, which involves first production of titanium powder of at least technical purity by powder metallurgy techniques, characterised by that the titanium powder is compressed, it is then sintered in the compressed state, and the sintered material is then processed by repeated CEC extrusion technology to a compact polycrystalline nano-titanium, and nano-titanium produced by this method is subsequently processed by drawing through a die (7) to the semi, from which the dental implant is finally shaped.

2. Method of production of nano-structural titanium semis for implants according to claim 1 , characterised by that titanium powder without toxic ingredients, with grain size up to 100 nm and titanium content of 98 weight %, is placed into the casing closed from one side (1), the casing (1) with the powder is placed into the forming matrix (4) and the powder in the casing (1) is pressed using push of extruder (5) on the powder in the casing (1) to a density of at least 4050 kgm^"3.

3. Method of production of nano-structural titanium semis for implants according to claim 2, characterised by that titanium powder is placed into the channel (2) of circular cross-section situated in a metallic casing (1), where this channel (2) has a height to diameter ratio of 2.5 : 1 and it is closed at one end by a bottom (3), and compression of the powder is made by pushing of the extruder (5) into the channel (2) in direction against the bottom (3) of the channel (2) with simultaneous extrusion of the casing (1) through the matrix (4).

4. Method of production of nano-structural titanium semis for implants according to claims 2 and 3, characterised by that after the powder is compressed to a specified density, the excess part of the casing (1) is removed with free space above the compressed powder, the open end of the casing (1) is closed and subsequently the compressed powder is sintered.

5. Method of production of nano-structural titanium semis for implants according to claims 2 to 4, characterised by that the casing (1) is removed from the sintered material before its repeated extrusion by the CEC method, between extrusions by the CEC method, or after extrusion by the CEC method.

6. Method of production of nano-structural titanium semis for implants according to claims 1 to 5, characterised by that sintering of the compressed powder is made effects of temperature ranging from 1250 to 1450°C for 1.5 to 2.5 hours.

7. Method of production of nano-structural titanium semis for implants according to claims 1 to 6, characterised by that the sintered material is repeatedly extruded in the forming matrix (4) by the CEC method of cyclic compression at the temperature of 220 to 250°C.

8. Method of production of nano-structural titanium semis for implants according to claims 1 to 6, characterised by that the sintered material is extruded repeatedly in the matrix (4) using by the CEC method of cyclic compression until it achieves the specified average grain size of at least 100 nm.

9. Method of production of nano-structural titanium semis for implants according to claims 1 to 8, characterised by that the sintered material is extruded in a matrix using the CEC method of cyclic compression for at least ten times.

10. Method of production of nano-structural titanium semis for implants according to claims 1 to 6, characterised by that after a body with the grains of an average grain size from 100 to 150 nm is made from nano-titanium by repeated extrusion by the CEC method, the nano-titanium body thus produced is processed by drawing in a die (7) at the temperature of 160 to 200°C.