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Mechanical properties of new bone substitute materials in dependence of degradation time
WEAR
RESISTANCE OF THE NITROGEN DIFFUSION HARDENING OF THE Ti-6AI-4V ALLOY D. Rodriguez, F.J. Gil, and J.A. Plane11 Dept. Ciencia de 10s Materiales e lngenierfa Metalnrgica. E.T.S.Ingenieros lndustriales de Barcelona. Universidad Politecnica Cataluna.
MECHANICAL MATERIALS
de
INTRODUCTION: The titanium based materials are highly corrosion resistant and well tolerated by the human body due to a complex titanium oxide film consisting mainly of TiOz and also TizOs and TiO up to 5 nm thickness which forms rapidly in ambient air on the surface. The mechanical interaction during articulation leads to disruption of the non shear resistance oxide layer mainly of TiOz. This results in depassivation and accelerated wear of the metallic surface by three-body abrasion due to the presence of particles. In modem surface engineering there are many possibilities to increase the hardness and the stability of the surface of Titanium and its alloys for use as articulating components The purpose of this paper is to present a new surface modification: nitrogen diffusion hardening and to determine its abrasive wear resistance. MATERIALS AND METHODS: Cylinders of 15.4 mm in diameter and 7 mm height were machined with a chemical composition shown in the next Table. The alloy presents “mill annelaed” microstructure. AlIV)FeI 6.1 I 4.0
1 0.11
C 1 0.021
1 02 1 0.09
N2
1 0.010
H2
1 0.003
1 Ti I bal.
The specimens were introduced in a tubular furnace with high purity nitrogen atmosphere at a fixed temperature for each experiment. The temperatures used were 700, 900 and 950°C for Ti-6AI-4V and the times of heat treatment were 1.4, 8 and 16 hours. The microstructures obtained were studied by optical, scanning and transmission electron microscopy. The Vickers hardness measurements were done by applying 500 g during 15 s. The wear study was carried out according to the standard DIN-535 16 for abrasive wear. A linear speed of v = 0.3 m/s and a load of 1 kg which were used at all times, being sufficiently low as to avoid the adhesive wear effect by seizure. These conditions guarantee that only abrasive wear is measured. The tests were carried out at 37°C. The weight loss was evaluated for different testing times: 5, 10, 20, 30.40. 50 and 60 min. RESULTS AND DISCUSSION: The maximum hardness value obtained in Ti-6AI-4V was 1100 HV after 16 hours at 950°C. After 1 hour at the same temperature the hardness value was 1050, being 315 HV for the as received material. In this alloy, the nitrogen stabilizes the hexagonal a-phase and reduces the volume fraction of the high temperature B-phase, leading to hard, nitrogen enriched a grains, which contribute to the enhanced hardness of the wrought Ti alloy. The depth of the nitride coating was between 60 and 1ooplll. From the results of weight loss obtained for Ti-6Al-4V alloy it can be observed that the weight loss is linearly dependent on the testing time, being the slope a measurement of the abrasive wear speed. These results show that the nitruration treatment produce an important decrease of abrasive wear. The abrasive wear speed for the nitrided samples is about 75% lower than for the non-treated samples for Ti-6Al-4V. CORRESPONDENCE: D. Rodriguez Dept. Ciencia de 10s Materiales e Ingenieda Metalurgica. E.T.S.Ingenieros Industriales de Barcelona. Univetsidad Politecnica de Catalmia, Av. Diagonal 647. 0802%Barcelona. Spain.
I I”’ Conference
PROPERTIES OF NEW BONE SUBSTITUTE IN DEPENDENCE OF DEGRADATION TIME S. Wolf’, A. Ignatius’, D. Reif’, L. Claes’ ‘Abteilung Unfallchirurgische Forschung und Biomechanik, Universitiit Ulm, Germany, ‘Biovision GmbH. Ilmenau, Germany INTRODUCTION: A long resorption period of bone replacement materials such as tricalicum phosphates has led to the development of new biocompatible glass ceramics. The disadvantage of sintered glass ceramics is their brittleness, which results in a low strength. For this reason an attempt was started to improve the material compound and therefore, to enhance the strength by the addition of polymers. The aim of the present study was to evaluate the mechanical strength of new glassceramic composites with respect to their degradation time. METHODS: The materials investigated were three different composites existing of tricalcium phoshate (alpha-TCP), glass ceramics with neutralized surface treatment (GB 9N, GB 14N), and polylactide with a mixing ratio of 45:55. The material compounds were sintered to cylindric specimens and gamma sterilized. The in virro degradation was performed in a Sorensen puffer (pH 7) at 37’C. At eight different dates within 78 weeks five specimen per material were extracted. Using a compression test the yield strength was determined. The molecular weight of the polymer was determined at two specimens per material using gel-permeation chromatography. The statistical analysis included the Kruskal-Wallis test. RESULTS: The strength of all three materials decreased significantly with the degradation period (p
.-.. GB
0
Figure
20 deQmdatlon
40 perlcd
to
14NlPolyloctid
SO
(weeks)
1.
DISCUSSION: With the compound of ceramics and polymers a strength can be reached, which corresponds to that of cancellous bone’. The nearly complete reduction of the mechanical strength takes place for all three composites at the same time in the in vitro investigation. This led to the assumption that the strength is predominantly dependant of the degradation of the polymer, which is supported by the high correlations between mechanical properties and the molecular weights. CONCLUSIONS: The newly developed glass ceramic composites showed similar mechanical properties compared to the compounds of tricalicum phosphate. Due to the faster resorption of the glass ceramics, which was demonstrated in previous in vitro experiments, an advantage over the TCP is expected in viva. REFERENCES: I. Linde F et al., J Biomech., 24.803-809, 199 I CORRESPONDENCE: DipI.-lng. Steffen Wolf Abt. Unfallchirurgische Forschung und Biomechanik, Universitaet Ulm, Helmholtzstrage 14, 89081 Ulm, Germany, Phone: +49-731-5023496, Fax: +49-731-5023498, [email protected]
of the ESB, July 8-I I 98, Toulouse,
France
49
RESISTANCE OF THE NITROGEN DIFFUSION HARDENING OF THE Ti-6AI-4V ALLOY D. Rodriguez, F.J. Gil, and J.A. Plane11 Dept. Ciencia de 10s Materiales e lngenierfa Metalnrgica. E.T.S.Ingenieros lndustriales de Barcelona. Universidad Politecnica Cataluna.
MECHANICAL MATERIALS
de
INTRODUCTION: The titanium based materials are highly corrosion resistant and well tolerated by the human body due to a complex titanium oxide film consisting mainly of TiOz and also TizOs and TiO up to 5 nm thickness which forms rapidly in ambient air on the surface. The mechanical interaction during articulation leads to disruption of the non shear resistance oxide layer mainly of TiOz. This results in depassivation and accelerated wear of the metallic surface by three-body abrasion due to the presence of particles. In modem surface engineering there are many possibilities to increase the hardness and the stability of the surface of Titanium and its alloys for use as articulating components The purpose of this paper is to present a new surface modification: nitrogen diffusion hardening and to determine its abrasive wear resistance. MATERIALS AND METHODS: Cylinders of 15.4 mm in diameter and 7 mm height were machined with a chemical composition shown in the next Table. The alloy presents “mill annelaed” microstructure. AlIV)FeI 6.1 I 4.0
1 0.11
C 1 0.021
1 02 1 0.09
N2
1 0.010
H2
1 0.003
1 Ti I bal.
The specimens were introduced in a tubular furnace with high purity nitrogen atmosphere at a fixed temperature for each experiment. The temperatures used were 700, 900 and 950°C for Ti-6AI-4V and the times of heat treatment were 1.4, 8 and 16 hours. The microstructures obtained were studied by optical, scanning and transmission electron microscopy. The Vickers hardness measurements were done by applying 500 g during 15 s. The wear study was carried out according to the standard DIN-535 16 for abrasive wear. A linear speed of v = 0.3 m/s and a load of 1 kg which were used at all times, being sufficiently low as to avoid the adhesive wear effect by seizure. These conditions guarantee that only abrasive wear is measured. The tests were carried out at 37°C. The weight loss was evaluated for different testing times: 5, 10, 20, 30.40. 50 and 60 min. RESULTS AND DISCUSSION: The maximum hardness value obtained in Ti-6AI-4V was 1100 HV after 16 hours at 950°C. After 1 hour at the same temperature the hardness value was 1050, being 315 HV for the as received material. In this alloy, the nitrogen stabilizes the hexagonal a-phase and reduces the volume fraction of the high temperature B-phase, leading to hard, nitrogen enriched a grains, which contribute to the enhanced hardness of the wrought Ti alloy. The depth of the nitride coating was between 60 and 1ooplll. From the results of weight loss obtained for Ti-6Al-4V alloy it can be observed that the weight loss is linearly dependent on the testing time, being the slope a measurement of the abrasive wear speed. These results show that the nitruration treatment produce an important decrease of abrasive wear. The abrasive wear speed for the nitrided samples is about 75% lower than for the non-treated samples for Ti-6Al-4V. CORRESPONDENCE: D. Rodriguez Dept. Ciencia de 10s Materiales e Ingenieda Metalurgica. E.T.S.Ingenieros Industriales de Barcelona. Univetsidad Politecnica de Catalmia, Av. Diagonal 647. 0802%Barcelona. Spain.
I I”’ Conference
PROPERTIES OF NEW BONE SUBSTITUTE IN DEPENDENCE OF DEGRADATION TIME S. Wolf’, A. Ignatius’, D. Reif’, L. Claes’ ‘Abteilung Unfallchirurgische Forschung und Biomechanik, Universitiit Ulm, Germany, ‘Biovision GmbH. Ilmenau, Germany INTRODUCTION: A long resorption period of bone replacement materials such as tricalicum phosphates has led to the development of new biocompatible glass ceramics. The disadvantage of sintered glass ceramics is their brittleness, which results in a low strength. For this reason an attempt was started to improve the material compound and therefore, to enhance the strength by the addition of polymers. The aim of the present study was to evaluate the mechanical strength of new glassceramic composites with respect to their degradation time. METHODS: The materials investigated were three different composites existing of tricalcium phoshate (alpha-TCP), glass ceramics with neutralized surface treatment (GB 9N, GB 14N), and polylactide with a mixing ratio of 45:55. The material compounds were sintered to cylindric specimens and gamma sterilized. The in virro degradation was performed in a Sorensen puffer (pH 7) at 37’C. At eight different dates within 78 weeks five specimen per material were extracted. Using a compression test the yield strength was determined. The molecular weight of the polymer was determined at two specimens per material using gel-permeation chromatography. The statistical analysis included the Kruskal-Wallis test. RESULTS: The strength of all three materials decreased significantly with the degradation period (p
.-.. GB
0
Figure
20 deQmdatlon
40 perlcd
to
14NlPolyloctid
SO
(weeks)
1.
DISCUSSION: With the compound of ceramics and polymers a strength can be reached, which corresponds to that of cancellous bone’. The nearly complete reduction of the mechanical strength takes place for all three composites at the same time in the in vitro investigation. This led to the assumption that the strength is predominantly dependant of the degradation of the polymer, which is supported by the high correlations between mechanical properties and the molecular weights. CONCLUSIONS: The newly developed glass ceramic composites showed similar mechanical properties compared to the compounds of tricalicum phosphate. Due to the faster resorption of the glass ceramics, which was demonstrated in previous in vitro experiments, an advantage over the TCP is expected in viva. REFERENCES: I. Linde F et al., J Biomech., 24.803-809, 199 I CORRESPONDENCE: DipI.-lng. Steffen Wolf Abt. Unfallchirurgische Forschung und Biomechanik, Universitaet Ulm, Helmholtzstrage 14, 89081 Ulm, Germany, Phone: +49-731-5023496, Fax: +49-731-5023498, [email protected]
of the ESB, July 8-I I 98, Toulouse,
France
49