Processing and surface modification of beta-titanium alloys produced by powder metallurgy for biomedical applications
- Autores: Caterina Del Carmen Chirico Rodríguez
- Directores de la Tesis: Sophia Alexandra Tsipas (dir. tes.), Elena Gordo Odériz (codir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Alexandra Amherd Hidalgo (presid.), Carlos Romero Villarreal (secret.), Isabel Montealegre Melendez (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
This doctoral thesis under the title "Processing and Surface modification of Ti-Nb alloys produced by powder metallurgy for biomedical applications" was carried out by Caterina Chirico Rodríguez and supervised by Prof. Elena Gordo and Prof. Sophia Tsipas of the University Carlos III of Madrid (UC3M). It has been developed in the research group Powder Technology Group (GTP), in the framework of the doctoral programme Ciencia e Ingeniería de Materiales of UC3M (Real Decree 99/2011). This thesis has been financed in the frame of BIOHYB project (Ref. PCIN-2016-123) in collaboration with University of Minho (Portugal) and University Estadual Paulista and University of Sao Paulo (Brazil).
Total hip arthroplasty (THA) surgery improves the patients' quality of life, relieving their pain and recovering their mobility almost immediately. THA surgery only represents the beginning of the treatment since, over time, revision surgery is required to bring back the patient's quality of life.
More than one million THA surgeries are worldwide performed every year. According to the Organisation for Economic Co-operation and Development (OECD), the rate of primary THA surgeries increased by 25% between 2000 and 2009. The OECD also reported that this percentage is expected to continue to rise over time. Moreover, the revision surgeries rate is also growing; it is expected that the total number of revision surgeries will rise by 137% between 2005 and 2030.
During the last decades, the research and development activities concerning hip implants have gained attention. A proof of that is the large variety of materials and designs of implants available nowadays. In addition, the increasing revision surgeries rate (where the prosthesis must be replaced before than expected) has motivated the development of novel and more suitable biomedical alloys that prolong the durability of the implant.
Orthopaedic implants present long-term issues associated with unsuitable material properties that reduce their lifespan. The implant material, its design, the surgeon's expertise, surgical technique, the health status of the patient, patient age, obesity, bone density,gender,the recovery process and the post-operative activities are factors involved in the long-term clinical success of Total Hip Arthroplasty (THA) procedures. This work is limited to evaluate aspects related to the implant material.
Implant loosening, low fatigue strength, high wear level and lack of bioactivity are the leading causes of hip implants failure. Implant loosening is a consequence of the stress-shielding phenomenon, which is caused by the mechanical mismatch, due to the significant difference in elastic modulus between bone and implant. The bone exhibits elastic modulus from 4 to 30 GPa, while the current Ti alloys employed in hip implants, such as Ti6Al4V and Ti6Al7Nb, reach much higher values, about 100-120 GPa.
Stress-shielding involves a non-homogeneous stress transfer between the implant and bone. In a healthy femur, the load is transmitted from the femur head to the cortical bone. When the bone is subjected to stress, it is continuously remodelled by the osteoclasts cells that dissolve the bone and osteoblasts cells, which allow the ossification forming new bone. Hence, the load transfer helps maintain the cellular balance between the osteoblasts and osteoclasts. On the contrary, when there is a metallic implant, the elastic modulus difference produces a heterogeneous stress transfer. The upper femur part withstands a small load fraction, whereas the section close to the stem tip is overloaded. Since regular load transfer has been altered, the cellular activity changes and bone resorption may occur in the surrounding region to the implant. Therefore, over time, implant gradually losses fixation to the bone. When the bone resorption is too high, implant loosening is produced, and the patient would require revision surgery to replace the damaged prosthesis.
Another issue related to the metallic prosthesis is the wear and its consequences. Wear damage causes the release of metal ions and wear debris from the implant to the surrounding tissue or bloodstream. Wear debris size is a factor to take into account since, in the nanometric range, it can have nanotoxicity effects. The smaller is the particle size of wear debris, the higher the risk that these particles can be introduced into the cells, changing their biological effect. For instance, wear debris can migrate to the phagosome of macrophages, which activates the osteoclast cells promoting bone resorption.
Excessive wear may produce adverse tissue reaction. It has been reported that high concentration of alloying elements, frequently used in hip implant material, such as aluminium (Al), vanadium (V), in Ti-6Al-4V alloy, are associated with adverse effects concerning cell viability; and long-term health issues, like neurotoxic effects and local inflammation. Besides, Al delays bone mineralisation and has been associated with Alzheimer's disease. Hence, a suitable implant material must improve wear resistance in Ti alloys, incorporating biocompatible alloying elements so that, if these particles are released, they do not involve toxic effects.
The lack of bioactivity may promote implant failure in the early stage after surgery, as well as, lead to gradual implant loosening in the long term. Even though Ti is the most biocompatible metal, it cannot induce or promote bone formation itself, since it cannot form a direct bond with the bone. This fact causes fixation issues that compromise the THA success. Hence, implants must be subjected to surface treatments that modify the topography, incorporating bioactive agents that enhance the osseointegration process between the implant and bone tissue.
Considering the main issues of implant materials, an ideal material for this application should have the following requirements: 1) high biocompatibility, 2) high corrosion resistance, 3) good mechanical properties (low Young's modulus, high fatigue resistance, high ultimate strength), 4) high wear resistance and 5) osseointegration ability.
Research on the development of novel Ti-based alloys designed for biomedical applications is focused on obtaining a material that combines mechanical properties and low elastic modulus, using biocompatible and non-toxic alloying elements. In the bibliography, two main strategies can be found in order to achieve this. The first strategy is the development of porous materials, since elastic modulus decreases by increasing the amount of porosity. However, given that strength also decreases with increasing porosity, this must be carefully controlled in order to guarantee an adequate mechanical performance of the material. On the other hand, porous structure enhances the stress distribution along the implant and provides channels suitable for the growth of bone tissue, promoting the osseointegration process.
A second possibility is the development of β-Ti alloys, which are considered promising materials for hip implants since they exhibit lower Young's modulus than biphasic (α + β). Several studies have reported on the development of biocompatible and low modulus β-phase Ti alloys, using different stabilisers such as Nb, Mo, Zr and Ta. Among these, Nb is considered the most biocompatible β-stabilising element. Overall, the higher the β-stabiliser elements amount, the lower the elastic modulus values are achieved, due to increased β-phase stability.
This PhD work aims to develop a low-cost, low-modulus and biocompatible β-Ti alloy suitable for implant material, produced by conventional powder metallurgy (press and sinter). To achieve this general goal, the work was divided into three sections focused on: (1) designing and developing β-Ti alloys, which exhibit low Young’s modulus, for orthopaedic implants; (2) enhancing wear properties of the β-Ti substrates; and (3) adapting surface features to induce bioactivity, improving the biological response between the implant surface and the bone.
Reducing elastic modulus:
Elastic modulus was reduced by developing of β-Ti alloys, using Nb and Fe as alloying elements and titanium hydride (TiH2) as Ti source. Based on the thermodynamic design, using Thermocalc software, three compositions were selected: Ti-12Nb, Ti-40Nb and Ti-5Fe-25Nb (compositions are indicated in weight per cent).
Ti-Nb alloys, developed for biomedical applications, require at least 27.5 Nb wt.% to begin retaining the β-Ti phase. As Nb is an expensive metal, large amounts of Nb additions increase the alloy cost. One way to reduce the processing cost of Ti alloys is employing low-cost alloying elements, provided that these allow achieving materials with good properties.
In this regard, Fe is a suitable alternative, since it is an abundant and low-cost material. Also, Fe is a strong β-stabiliser element that diffuses fast into β-Ti, and favours its quick dissolution during sintering. Small Fe additions increases the β-Ti stability; hence, they will lead to a reduction in the Nb content required to stabilise the β phase. Therefore, substituting Nb with Fe can contribute to the cost reduction of the alloy.
In addition to reducing the alloy cost, Fe addition has several advantages for Ti alloys processing. Fe provides a strong solid-solution strengthening effect on Ti alloys and enhances the sinterability of Ti alloys. Since Fe accelerates the mobility of Ti atoms by its rapid diffusion, and increases the Ti self-diffusion coefficient, it enhances the diffusion process of slower diffuser alloying elements like Nb or Mo, promoting a homogeneous microstructure.
TiH2 is used since it provides several advantages over Ti powders: (1) TiH2 as raw material is cheaper because it is an intermediate product in HDH-Ti powder production. (2) It achieves higher densification compared to Ti sintered under the same conditions. (3) The brittle behaviour of TiH2 cuases particles to fragment during pressing, improving the compressibility of the powder. (4) The lattice defects generated by decomposition reactions of TiH2 activate the diffusion process, which leads to pore healing and accelerates the chemical homogenisation of the final product. (5) Hydrogen released during the transformation to Ti, through the reaction〖 TiH〗_2→Ti +H_2, provides a protective atmosphere for the Ti surface that reduces contamination risk.
The success of TiH2 use as a Ti substitute highly depends on the dehydrogenation process, that is, how hydrogen is released when sample is heated. Hence, this thesis includes a detailed study about the dehydrogenation process and how alloying elements (Nb and Fe) influence this process, considering the effect that they have, when they are incorporated individually and in a combination form. This study revealed that alloying elements (Fe and Nb) do not interfere with the dehydrogenation process, and a complete transformation from TiH2 to β-Ti is acheived. Nevertheless, it was found that alloying elements accelerate the onset temperature of TiH2 decomposition by about 50-95 °C, while the offset temperature of the final dehydrogenation stage is delayed between 15-50 °C compared to unalloyed TiH2 powder. From this study, relevant principles were established to define the appropriate consolidation conditions that promote a controlled and complete transformation from TiH2 to Ti.
Sintering conditions were evaluated, looking to obtain dense and homogeneous alloys, following two criteria: (1) the heating rate while the dehydrogenation process takes place was varied to achieve a complete and controlled transformation from TiH2 → Ti, and (2) the sintering temperature was modified from 1200 °C to 1450 °C to promote the diffusion process of the alloying elements in order to stabilise the highest β-Ti fraction possible.
Results point out that biphasic Ti-12Nb alloy reach Young's modulus values similar to Ti-6Al-4V, while it is decreased for the two β-Ti alloys, reaching 73±12 GPa for Ti-40Nb and 95±14 GPa for Ti-5Fe-25Nb. Moreover, biocompatible tests indicated that the three alloys exhibit similar biological response to Ti samples, so all processed samples would be suitable for biomedical applications.
Improving wear resistance:
Two strategies were proposed to improve wear resistance. Both of these approaches attempt to increase the sample hardness, since harder surfaces typically exhibit improved wear properties. First, it was proposed to modify the bulk properties of the β-Ti alloys by developing titanium matrix composites (TMC), incorporating 5 vol.% of TiB2 and TiN ceramic particles as reinforcement. The second strategy consisted on performing surface modification by gas and plasma nitriding treatment to obtain a wear-resistant TiN layer on the surface.
The selection of the reinforcement type is an important aspect to consider to produce TMC materials successfully. An appropriate reinforcement compound should have good interfacial bonding between the metal matrix and reinforcement, avoiding the formation of reaction products at the interface. In addition, it should exhibit high thermodynamic stability in the Ti matrix at the sintering temperature, high hardness, and a low difference in thermal expansion coefficient, in order to avoid residual thermal stress generation, which might compromise mechanical properties. TiB2 and TiN could be candidates to produce TMC for biomedical applications, since they comply with reinforcement requirements and show similar cytotoxicity to CP-Ti.
Overall, TMC materials improve mechanical properties, including wear resistance. The reinforcement fraction incorporated in TMC must be carefully controlled, since a relative high reinforcement amount could bring elastic modulus increases, which would be unfavourable for load-bearing implants. Alternatively, to avoid an excessive increase in elastic modulus of TMC materials for orthopaedic and dental implants, these can be used as a coating or a functionally graded material could be developed. In this way, surface wear resistance will be improved, maintaining part of the internal properties of the base alloy, such as low elastic modulus.
Nitriding treatment is one of the most popular thermochemical treatments employed to enhance the surface properties of Ti and its alloys. Nitriding treatment involves interactions between nitrogen and the metal surface, to produce a hard layer composed of TiN and Ti2N phases that protects Ti substrate. Nitride coatings show high adhesion to the metal substrate and present good wear and corrosion resistance. In addition to protecting Ti surface against wear and having high hardness, nitride coatings exhibit high corrosion resistance, good biocompatibility, haemocompatibility, chemical stability and good interfacial bonding between the coating and Ti substrates.
Moreover, because of the high affinity of Ti with nitrogen, nitrogen reacts with the internal matrix to form internal nitride precipitates beneath the compound layer. The high nitrogen solubility in α-Ti leads to the formation of α-Ti(N), which strengthens the alloy by interstitial solid solution. Hence, hardness values decrease from the surface inward, since to nitrogen concentration in the metal matrix decreases, producing a hardness gradient in the diffusion zone.
In this work, dry sliding tests against an alumina ball, applying 10 N and 20 N of load, were performed to evaluate the improvement of wear resistance in terms of microstructure, wear track characteristics and evolution of coefficient of friction during wear test. Wear resistance was evaluated for untreated β-Ti substrates (Ti-40Nb and Ti-5Fe-25Nb), TiN reinforced and gas nitrided β-Ti alloys.
Results suggest that modified β-Ti alloys, either by reinforcement addition to produce a composite material or by deposition of nitriding coatings, showed an increase in the hardness values compared to untreated alloys. Among these, gas nitrided (GN) samples exhibited better wear resistance than TiN reinforced alloys. This is because the wear resistance of TiN reinforced alloys is strongly influenced by the consolidation characteristics and homogeneity of the reinforcement phase within the matrix. Hence, detached TiN reinforcement particles worsen wear, acting as a third body.
GN treatment allows enhancing wear resistance of the two studied Ti alloys at both load conditions. At 10 N, wear rate was 86% and 43 % lower than untreated alloys for GN-Ti-40Nb and GN-Ti-5Fe-25Nb, respectively. At 20 N, lower wear rate reduction was obtained than at 10 N, achieving about 4% and 15 % lower wear rate than untreated samples, for GN-Ti-40Nb and GN-Ti-5Fe-25Nb, respectively.
TiN reinforced alloys present the highest elastic modulus values, reaching 114 GPa for Ti-40Nb-TiN and 150 GPa for Ti-5Fe-25Nb-TiN. In contrast, gas nitrided samples, despite achieving an improved surface hardness, show elastic modulus values similar to untreated alloys, reaching about 71 MPa for GN-Ti-40Nb and 102 MPa for GN-Ti-5Fe-25Nb. The balance between high hardness and low young modulus suggest GN treatment could be a promising alternative to enhance the wear resistance of biomedical Ti alloys. Overall, samples of Ti-Nb-Fe system show better wear resistance than samples of Ti-Nb system. This is attributed to the strengthening effect of Fe and higher hardness and lower porosity of Ti-5Fe-25Nb alloys compared to Ti-40Nb.
Inducing bioactivity:
Surface modification techniques are usually employed to adapt the implant surface features, to mimic the surrounding bone tissue, providing a better biological response. These include ion implantation, sol-gel method, thermal spraying, chemical vapour deposition, physical vapour deposition, and micro-arc oxidation (MAO). Among these surface treatments, MAO has gained attention because of its versatility to produce bio-functionalised coatings that improve cell adhesion, proliferation, and osseointegration by incorporating bioactive elements found in natural bone, like calcium (Ca), phosphorous (P) and magnesium (Mg), as well as antimicrobial agents as Ag, Zn and Cu, which could inhibit the adhesion of bacteria on the material surface, reducing the infection risk. Moreover, it has been reported that the corrosion and wear behaviour of MAO treated samples could be also improved due to the formation of TiO2.
TiN reinforced β-Ti alloys were MAO treated using an electrolyte containing 0.02 M of β-GP and 0.35 of CaA. This electrolyte was modified by the addition of 0.02 M, 0.05 M and 0.07 M of ZnO NPs. This work presents a brief analysis of the electrical response of MAO treated samples, in order to understand the growth mechanisms of the oxide layer. How ZnO NPs affect the porous surface morphology and how these NPs are incorporated into the coating is also evaluated . In addition, the Ca/P ratio, Zn amount and anatase and rutile presence are determined, to evaluate the potential use of MAO treatment to provide bioactivity to the β-Ti alloys developed in this work.
Results indicate that MAO treated TiN reinforced alloys (MAO-Ti40Nb-TiN and MAO-Ti5Fe25Nb-TiN) show suitable features, related to improved biological response. Both alloys achieved well-adhered coatings with a thickness between 5-10 μm and exhibited multiscale surface porosity and high apparent roughness. In addition, coatings have a high Ca/P ratio, between 2.7 and 3.2, which will promote the osseointegration process, and Zn content that provides antibacterial effects. All these suggest that MAO treated samples could provide a suitable environment to promote improved biological behaviour.
In summary, the objectives established in this thesis have been successfully achieved, reaching reduced elastic modulus, improved wear properties and bio-functionalised surfaces. It can be concluded that the materials developed are attractive and promising alternatives to be considered for use as implant materials, especially gas nitrided samples (GN-Ti-40Nb and GN-Ti-5Fe-25Nb), that combine low elastic modulus and higher wear resistance. Moreover, it was confirmed that TiH2 is a viable alternative for the processing of Ti alloys by powder metallurgy. It allows the reduction of production costs without compromising the alloy performance.
