RU2732716C1 - Method of producing porous material based on titanium nickelide - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to metallurgy, specifically to the technology of producing porous metal materials, and can be used in medical implantology. Method of producing porous material based on titanium nickelide involves two-step sintering of a mixture containing titanium nickelide powder with intermediate exposure between stages. Initial titanium nickelide powder is sifted into fractions and a fraction with particle size of 100 to 150 mcm is selected for sintering, into the mixture is added from 5 to 7.5 % of titanium powder with particle size from 100 to 150 mcm, wherein in the process of two-stage sintering, the first step is carried out at temperature of 1200 ± 5 °C with holding for 15 ± 5 minutes, and second stage at temperature of 1240 ± 5 °C with holding for 15 ± 5 minutes.EFFECT: higher porosity of produced material and higher deformation resistance.1 cl, 5 dwg

Description

The invention relates to metallurgy, specifically to a technology for producing porous metallic materials, and can be used in medical implantology.

Porous alloys based on titanium nickelide (TiNi) have a set of unique structural and functional properties. The combination of a developed three-dimensional porous structure with a surface oxide layer with the possibility of implementing thermoelastic martensitic transformations in the TiNi compound makes an alloy based on titanium nickelide preferable for creating implants [1]. Currently, implantable devices based on titanium nickelide have been developed for use in various fields of surgery, traumatology, and oncology. One of the directions for optimizing the properties of a porous material is to achieve structural conformity of the TiNi alloy to the bone tissues of the human body.

Biocompatible porous materials based on titanium nickelide are obtained by self-propagating high-temperature synthesis (SHS) and sintering. It is noted that the method of sintering TiNi intermetallic powders makes it possible to obtain porous materials with a more uniform phase-chemical composition than in the case of the SHS method [2, 3].

In addition, in porous materials obtained by sintering, the highest content of the TiNi phase is up to 90%, which makes it possible to fully manifest the effects of superelasticity and shape memory, which are responsible for the deformation life of the implant.

The possibility of creating by sintering porous materials with the required geometric dimensions allows you to minimize production waste in the manufacture of implants.

However, along with these advantages, there is the problem of obtaining a material with a porosity coefficient approaching the porosity of natural bone, reaching 65–70%. Typical values of the porosity coefficient for titanium nickelide obtained by diffusion sintering are at the level of 55–60%. A number of methods have been proposed to increase porosity. For example, there is a known method of increasing porosity by introducing foaming additives [4]. However, this results in a material with an inhomogeneous phase-chemical composition and an irregular structure, which is expressed in the formation of cracks between layers of porous material.

There is also known a method of obtaining a material with a porosity index increased to 60-65% by choosing a temperature-time regime at a temperature of 1220-1240 C and a holding time of 15 minutes [5]. However, such a material is characterized by incomplete sintering and poor quality interparticle contacts, which limits its mechanical stability.

Thus, the known methods of obtaining porous titanium nickelide from TiNi powder allow, with satisfactory mechanical characteristics, to achieve porosity values of no more than 55–60%, which is inferior to that for cancellous bone tissue 60–75%.

The closest in technical essence can be considered a method for producing a porous material based on titanium nickelide according to RF patent 2651846 [6], taken as a prototype. It includes two-stage sintering of titanium nickelide powder with intermediate holding between stages. The first stage includes heating to a temperature of (1200 ± 5) ºС for 5-6 minutes, holding includes natural cooling to normal temperature, the second stage includes heating to a temperature of (1250 ± 5) ºС and holding for 40 ± 5 minutes, followed by natural cooling. Two-stage sintering with intermediate holding provides:

- at the first stage - the primary fixation of the structural elements of the charge (grains),

- in the process of aging - stabilization of the structure in a given shape and size,

- at the second stage - the formation of stable bridges between the grains, as well as the structuring of the grain surface. The porous alloy obtained by the known method has a high coefficient of porosity - up to 55%, making it suitable for use as incubators for cell cultures.

At the same time, the known method has the aforementioned disadvantages, that is, the coefficient of porosity is still low in comparison with natural bone, as well as low resistance to deformation due to the insufficiently uniform structure and the presence of thick bridges on which deformation stresses are concentrated.

The technical result of the proposed method consists in increasing the porosity of the resulting material and increasing deformation stability.

The technical result is achieved by the fact that when implementing the method for producing a porous material based on titanium nickelide, including two-stage sintering of a charge consisting of titanium-nickelide powder, with intermediate holding between the stages, the difference is that the original powder is separated by sieving into fractions, choosing for sintering a fraction with a particle size of 100 to 150 microns, 5 to 7.5% of titanium powder with a particle size of 100 to 150 microns is added to the charge composition, and in the process of two-stage sintering, the first stage is carried out at a temperature of 1200 ± 5 ° C with holding for 15 ± 5 minutes, and the second stage - at a temperature of 1240 ± 5 ºС with exposure for 15 ± 5 minutes.

The implementation of the technical result is determined as follows.

1. Adding titanium powder to the charge provides a shift in the stoichiometry of the resulting material in the region of such a ratio of titanium and nickel concentrations, which corresponds to the temperature range of martensitic transformation in the temperature range typical of the human body. Under these conditions, an implant made of a porous material has the greatest mechanical stability.

2. The choice of the titanium particle size range is due to the optimization of their specific surface area, which determines the reagent properties. The range is limited from below due to the fact that fine particles have an excessively large specific surface area, which promotes the intensive formation of a passive oxycarbonitride film, which prevents the diffusion of titanium into the melt. The range is limited from above due to the fact that large particles do not completely diffuse into the TiNi matrix, remaining foreign inclusions that violate mechanical homogeneity.

3. Separation of nickel-titanium powder particles by size ensures their separation according to their characteristic structure and morphology. This makes it possible to obtain a more homogeneous semifinished product and to carry out sintering in a temperature-time regime, which ensures the formation of contact areas close in size between the particles.

4. Isolation of particles of titanium nickelide powder with an average size fraction (100–150) µm is due to the structural features of powder particles of different sizes revealed as a result of the study:

- Particles larger than 150 µm generally have a pronounced compact structure. These particles are in the form of products of crushing of fused monoliths or flattened, highly compacted spongy powder particles. After sintering, the porosity of the resulting material does not exceed 55%, and its coarse-grained structure contributes to a wide variation in the size of the bridges and, as a consequence, reduces the mechanical stability of the material.

- Particles with sizes from 50 to 100 and from 100 to 150 microns differ from larger particles by the preserved spongy structure. This indicates that they were formed in the process of synthesis in more benign conditions, without being exposed to extreme influences. Their initial porosity is 71% for the fraction with sizes (50–100) µm and 75% for the fraction with sizes from 100 to 150 µm. This fact shows that it is promising to select a medium-sized fraction (100–150) µm for obtaining a material with the highest porosity.

- The exclusion from the composition of the charge of a fine fraction with particle sizes from 50 to 100 μm is also due to the experimentally revealed features of its mixing with titanium powder, namely, the volumetric heterogeneity of the mixture, which is not eliminated with an increase in the mixing time of the charge. In addition, when sintering in a mixture with a fine fraction, titanium particles are not completely assimilated by the matrix.

5. The temperature-time features of two-stage sintering in the production of a porous material have been worked out experimentally in relation to new stoichiometric and morphological characteristics of the charge. They were obtained in the course of optimization of the obtained characteristics of porosity and mechanical stability of the obtained material. Temperature setting intervals ± 5 ° C are determined by the accuracy of the furnace thermostats and temperature gradients in its volume. The holding accuracy of ± 5 min is determined by the temperature inertness of the furnace heating elements.

A method for producing a porous material based on titanium nickelide includes a two-stage sintering of a charge consisting of nickel-titanium powder with intermediate holding between stages. When implementing the proposed method, the initial titanium nickelide powder is separated by sieving into fractions, choosing for sintering a fraction with a particle size of 100 to 150 microns, 5 to 7.5% of titanium powder with a particle size of 100 to 150 microns is added to the charge composition, and In the process of two-stage sintering, the first stage is carried out at a temperature of 1200 ± 5 ° C with holding for 15 minutes, and the second stage - at a temperature of 1240 ± 5 ° C with holding for 15 minutes.

The invention is illustrated in figures 1-5.

FIG. 1 illustrates the effect of adding titanium on the deformation curve of a titanium nickelide-based material.

FIG. 2 shows micrographs of coarse and fine fractions of nickel-titanium powder of PN55T45S grade.

FIG. 3 shows photographs of samples of a porous material based on titanium nickelide obtained by mixing a charge based on a fine fraction of nickelide-titanium powder and on the basis of an intermediate fraction.

FIG. 4 shows micrographs of thin sections of the cross-section of samples that were sintered at different temperatures a) 1230 ºС, b) 1240 ºС, c) 1250 ºС.

FIG. 5 shows photographs of samples of a porous material based on titanium nickelide obtained with an optimal and non-optimal choice of the temperature-time regime.

The novelty and inventive level of the proposed method are determined by original experimental studies of the morphology and structure of the charge components, the peculiarities of their interaction in the sintering process, as well as the selection of optimal compositions and modes of obtaining a porous material.

The introduction of titanium powder into the charge is due to the chemical composition of the feedstock, represented by titanium nickelide powder, obtained by the method of calcium hydride reduction. It was found experimentally that this powder, predominantly consisting of a TiNi compound, is enriched in nickel due to the transfer of a certain amount of titanium to a Ti ₂ Ni compound. An excess of nickel shifts the temperature range of manifestation of the effects of superelasticity and shape memory to the region of low temperatures, which does not correspond to the conditions for the functioning of porous implants in the human body. It is not possible to correct the proportion of titanium and nickel in the process of calcium hydride reduction, since it is set by thermodynamic equilibrium between different fractions in the course of a chemical reaction. Therefore, it is necessary to correct the proportion by adding titanium powder to the charge used in the diffusion sintering process.

In the course of experiments, it was found that such a correction is achieved when titanium powder is added to the finished titanium nickelide powder in a proportion of 5–7.5%. The initial temperature range of martensitic transformations of the order of -100 ... -20 ° С is shifted to the region of + 10 ... + 60 ° С. At the same time, the porous material fully exhibits superelastic properties at temperatures typical of the human body, respectively, an implanted implant made of such a material has the greatest mechanical stability. FIG. Figure 1 shows the deformation curves of the sintered initial TiNi powder without the addition of titanium powder (curve 1) and with the addition of titanium powder (curve 2). It is seen that the deformation limit with the addition of titanium increases from 6% to 9%. Without the addition of titanium, the shape of curve 1 corresponds to deformation, which is characterized by the accumulation of dislocations and relatively rapid fracture during multiple cycles. With the addition of titanium, the shape of curve 2 is determined by thermoelastic deformation due to reversible martensitic transformations, for which the number of cycles before fracture reaches 10 ⁶ and more. This significantly increases the deformation stability of bone-replacing implants made from the resulting porous material. The shape of curve 2 is retained when the concentration of titanium powder changes within the above range of 5–7.5%.

To effectively realize the positive effect of adding titanium - the shift in the temperature range of martensitic transformation to the area inherent in the human body - it is necessary to minimize the associated side effects. They consist in the fact that the presence of titanium powder particles limits the number of contacts between the particles of the useful titanium nickelide fraction and prevents them from sintering. This is due to the fact that titanium belongs to refractory materials, having a melting temperature of 1668 ° C, and certain conditions are required for its diffusion into the main component of the charge - titanium nickelide.

The particle size of the titanium powder is in the range from 5 to 200 μm. The addition of titanium powder particles with different sizes to the charge manifests itself in different ways. A decrease in the average particle size of titanium powder, on the one hand, facilitates its diffusion into the nickel-titanium matrix, but, on the other hand, leads to an increase in the specific surface area and active formation of an oxycarbonitride film, which inhibits the diffusion process. An increase in the average particle size of titanium powder, on the one hand, slows down the formation of a passivating oxycarbonitride film, but, on the other hand, complicates contacts between titanium nickelide particles, since titanium particles play the role of a refractory interlayer between them. In addition, large particles of titanium powder do not completely diffuse into the matrix, remaining granular inelastic inclusions that differ sharply in mechanical properties from the nickel-titanium matrix. A compromise between the choice in favor of a coarser or finer fraction of titanium powder was found by the use of particles with a size of 100 to 150 μm for adding to the charge.

The quality of the porous material depends to a large extent on the morphology and structure of the particles of the main component of the charge - titanium nickelide powder.

The analysis of the structural features of the TiNi powder intermetallic compound showed that the powder particles based on titanium nickelide have a double morphology - compact and spongy. A sieve analysis of the TiNi powder intermetallic compound was carried out, which made it possible to isolate fractions with different behavior characteristics with sizes of 50–100 µm, 100–150 µm, and 150–200 µm.

The coarse fraction (Fig. 2a) is mainly represented by particles of compact morphology in the form of solid bodies of irregular shape, on the surface of which traces of destruction and cracks are observed after mechanical processing of the powder material. The spongy powder particles found among them have a deformed compacted structure, which can be described by flattening the body of the powder particle perpendicular to the plane with the maximum area.

Fractions with sizes 50–100 µm and 100–150 µm (Fig. 2b) do not contain compact particles, which reduce the porosity index, but consist of developed undeformed spongy particles of complex shape.

In connection with the revealed features of the structure of various fractions for each of them, sintering was carried out separately.

As expected, from a charge based on large, mainly compact powder particles, the material was obtained with a low porosity index - less than 55%.

Samples of porous materials with a porosity index of about 65–70% and with a high quality of interparticle contacts providing satisfactory strength properties were obtained from a charge based on a fraction of nickel-titanium powder with a particle size in the range of (100–150) microns, mainly of spongy powder particles. porous material based on TiNi. A material with a lower porosity, about 60%, was obtained from a charge based on a fraction of 50–100 µm.

Obviously, in order to obtain a homogeneous porous material, it is preferable to separate two radically different fractions and use one that has a structure favorable for achieving high porosity. In this regard, it is proposed to exclude the larger fraction from the composition of the charge. The boundary value between the particle sizes of the compact and spongy fractions is set to be of the order of 150 μm; separation is performed using a sieve with appropriate cells.

Further, in the process of obtaining the charge and its sintering, a difference was established in the behavior of the finest spongy fraction of titanium nickelide powder - from 50 to 100 μm - and the intermediate spongy fraction - from 100 to 150 μm. It was revealed that the finer fraction is mixed with great difficulty with titanium powder to an acceptable homogeneity. This can be explained by the fact that when mixing powder components consisting of particles significantly different in size, these two components form weakly interacting clusters, and the mixture remains inhomogeneous, despite a significant increase in the mixing time. Uniform mixing is obtained when the titanium powder particles are comparable in size to the particles of the main material - titanium nickelide powder. Since the quality of the resulting product largely depends on the homogeneity of the mixture of powders, the fine fraction of TiNi powder, as well as the largest, was excluded from the composition of the charge.

The inconsistency of using a fine fraction of nickel-titanium powder from - 50 to 100 μm to obtain a porous material is illustrated in Fig. 3a, which shows the sample obtained by sintering with signs of uneven mixing of the charge: the presence of dark and light areas and inhomogeneity of the cross section. FIG. 3b shows a sample obtained on the basis of an intermediate fraction of 100–150 µm, characterized by uniformity of color and shape.

The temperature and time characteristics of two-stage sintering were established experimentally. The first stage includes the preliminary formation of bridges at the contacts between the porous particles due to the low-melting fraction Ti ₂ Ni. For the selected fraction of 100–150 µm, the optimal mode was found to include heating to 1200 ± 5 ° C and holding for 15 minutes.

The second stage is carried out after natural cooling of the pre-sintered billet, accompanied by mutual fixation of particles and relaxation of internal stresses. For the second stage, the mode is optimized, including heating up to 1240 ± 5 ° С and holding for 15 minutes. In the second stage, additional melting of the bridges between the porous particles occurs, also mainly due to the fusible Ti ₂ Ni compound and, at the same time, due to the diffusion of TiNi in them. Upon cooling, the final consolidation of the porous structure, composed of porous particles welded together, occurs. Under optimal conditions, a porous material is obtained with a developed system of bridges between particles that retain the original spongy structure, as shown in Fig. 4a.

Deviations from the optimized modes in the implementation of both stages degrade the quality of the resulting porous material. With a decrease in the temperature and sintering time, the interparticle bridges remain insufficiently formed, as shown in Fig. 4b, which manifests itself in the increased brittleness of the material. An increase in sintering temperature and time leads to the formation of massive bridges and increased particle melting as shown in FIG. 4c, which is accompanied by material shrinkage and a decrease in its porosity.

The effect of temperature on the properties of the samples obtained is illustrated by photographs of their appearance. FIG. 5a shows the appearance of a sample sintered at temperatures below 1230 ° C. The sample has no strength and falls apart. FIG. 5b shows the appearance of the sample sintered at 1240 ° C. It retains its predetermined shape well and is uniform in color, which indicates a uniform structure. FIG. 5c shows the appearance of a sample sintered at a temperature above 1250 ° C. The sample has signs of shrinkage associated with increased melting of contacts between particles and a corresponding compaction of the structure.

In samples obtained at temperatures above 1250–1260 ° C, individual powder particles are combined into conglomerates by enhanced wetting with a melt. As a result, the homogeneity of the macrostructure of the porous body and the strength directly related to homogeneity are violated. As indicated in the publication [Methodological features of the deformation behavior of metallic medical materials and implants: Methodological manual. / V.E. Gunther. - Tomsk: Izd-vo MITs, 2013. –32 p.], In the presence of enlarged interpore bridges in the material, they are the first to be destroyed during deformation. The maximum level of breaking stresses in a porous material is achieved when the structural elements are close in size, due to which they are simultaneously involved in the deformation process, simultaneously reaching the maximum critical stresses of crack initiation. As a result, a homogeneous porous material based on TiNi can be up to 3.5 times stronger than an alloy with a heterogeneous structure.

Thus, the experimental results confirm that the proposed method for preparing a porous material based on titanium nickelide by two-stage sintering of a medium-sized fraction of titanium nickelide powder with the addition of titanium powder with exposures of 15 min at 1200 ° C and 15 min at 1240 ° C provides previously unattainable for similar processes indicators of porosity and deformation stability.

Literature

1. Gunther V. E., Khodorenko V. N., Chekalkin T. L. et al. Medical materials with shape memory. Medical materials and implants with shape memory. Vol. 1. / Ed. VE Gunther - Tomsk: Publishing house "NPP" MIC ", 2011. - 534 p.

2. V.N. Khodorenko, S.G. Anikeev, V.E. Gunter / Structural and strength properties of porous titanium nickelide obtained by SHS and sintering // Izvestiya Vuzov. Physics. 2014. T. 57, No. 6, pp. 17–23.

3. S. G. Anikeev, N. V. Artyukhova, V. N. Khodorenko, A. S. Garin, V. E. Gunther / Structural features of biocompatible porous materials based on titanium nickelide with a terrace-like morphology of the surface of the pore walls // Izvestia Universities. Physics. 2018.Vol. 61, No. 6, pp. 34–41.

4. Abidi, I. H., & Khalid, F. A. (2012). Sintering and Morphology of Porous Structure in NiTi Shape Memory Alloys for Biomedical Applications. Advanced Materials Research, 570, 87–95.

5. S. G. Anikeev, N. V. Artyukhova, V. N. Khodorenko, A. S. Garin, V. E. Gunther / Structural features of biocompatible porous materials based on titanium nickelide with a terrace-like morphology of the pore wall surface // Izvestia Universities. Physics. 2018.Vol. 61, No. 6, pp. 34–41.

6. RF patent No. 2651846, IPC C22C 1/08, C22C 19/03, C22C 14/00, B22F 3/16, publ. 04.24.2018.

Claims (1)

A method for producing a porous material based on titanium nickelide, including two-stage sintering of a charge containing titanium nickelide powder with intermediate holding between stages, characterized in that the initial titanium nickelide powder is sieved into fractions and a fraction with particle sizes from 100 to 150 microns is selected for sintering, 5 to 7.5% of titanium powder with particle sizes from 100 to 150 μm is added to the charge, and in the process of two-stage sintering, the first stage is carried out at a temperature of 1200 ± 5 ° C with holding for 15 ± 5 min, and the second stage at temperature 1240 ± 5 ° С with exposure for 15 ± 5 min.

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