Characterization
Banhart describes two dominating perspectives in which cellular metals are characterized, referring to them as atomistic and macroscopic. The atomistic (or molecular) perspective holds that a cellular material is a construction of struts, membranes, and other elements which possess mechanical properties of their bulk metal counterpart. Indeed, the physical, mechanical, and thermal properties of titanium foams are commonly measured using the same methods as that of their solid counterparts. However, special precautions must be taken due to the cellular structure of metal foams. From a macroscopic perspective, the cellular structure is perceived as a homogeneous structure and characterized by considering the effective (or averaged) material parameters.Microstructure
Titanium foams are characterized structurally by their pore topology (relative percentage of open vs. closed pores),Mechanical properties
As with other metal foams, the properties of titanium foams depend mostly on the properties of the starting material and the relative density of the resultant foam. Thermal properties in foams – such as melting point, specific heat and expansion coefficient – remain constant for both the foams and the metals from which they are composed. However, the mechanical properties of foams are greatly influenced bySensitivity to impurities
TheTheoretical models for predicting mechanical properties
Gibson & Ashby models
Gibson & Ashby micromechanical models forMinimum solid area (MSA) models
Minimum solid area models assume that the load bearing area (cross-sectional area normal to the stress) is the logical basis for modeling mechanical behavior. MSA models assume pore interaction results in reduction of stress. Therefore, the minimum solid areas are the carriers of stress. As a result, predicted mechanical properties fluctuate based on the quantification of the solid area of the foam. For titanium foams consisting of partially sintered powders, the minimum solid area consists of the neck area between powders through the cross-section of cell walls between macropores. The mathematical relationships in MSA models are relatively consistent with the Gibson & Ashby model. However, the MSA models are designed to predict mechanical property parameters over a broader range of porosity levels. Like the Gibson & Ashby models, MSA models were derived assuming idealized (defect-free) structures containing uniform pore shapes, size and distribution.Compressive properties
The most frequently reported mechanical property of titanium foams is compressive strength. It was generally accepted that the compressive properties of metal foams depended on the properties of the cell wall rather than on pore size. However, more recent research has indicated that smaller pore sizes equate to higher compressive strength. As pore sizes reach nano-dimensions, the relationship is even more clear due to changes in deformation mechanism. Tuncer & Arslan fabricated titanium foams via the space-holder method using various shaped space-holders to elucidate the effect of cell morphology on mechanical properties. They found that foams created with needle-like urea space-holders exhibited a decrease in elastic modulus and yield strength when compared to spherical pores.Processing methods
Many metal foam manufacturing techniques are accomplished by the introduction of a gaseous phase into a precursor matrix, which can occur in either molten metal or a powdered metal form. Due to titanium's high melting point (1670 °C) and high chemical affinity with oxygen, nitrogen, carbon and hydrogen (which dissolve rapidly either in liquid or solid titanium at a temperature above 400 °C), solid-state processes based on powder densification are the preferred method of fabrication. Processing methods must also be designed to avoid exposure to air or moisture; vacuum or inert gas sintering processes are usually sufficient for preventing contamination.Loose-powder (gravity) sintering
Expansion of pressurized bubbles
Superplastic expansion
Titanium undergoes allotropic transformation from its α-phase (hexagonal close-packed (hcp) structure at temperatures less than 882.5 °C) to its β-phase (body centered cubic, bcc) structure at temperatures above 882.3 °C). Alpha-phase titanium products typically exhibit medium to high strength with excellent creep strength, whereas beta-phase titanium products typically exhibit very high strength and low ductility. Foams created under thermal cycling conditions have been shown to exhibit increased porosity due to the density difference between allotropic phases. Davis et al. produced titanium foams with 41% porosity (as compared to 27% porosity through the normal HIP creep mechanism). Increases in overall ductility were also observed in foams created through thermal cycling. In a similar experiment, porosity of 44% was achieved and determined as the maximum achievable porosity under thermal cycling conditions. A later study also utilized exploitation of transformation superplasticity conditions through HIP, but in this case, theSpace-holder technique
The space-holder technique is the most commonly employed method for producing titanium foams. The space-holder technique allows for the fabrication of higher porosity foams (35–80% ) than other techniques, while also giving the engineer more control over pore fraction, shape and connectivity. Mechanical properties can be adjusted through the size, shape and quantity of space-holders employed. The space-holder technique was first demonstrated by Zhao and Sun for the fabrication of aluminum foams in a powder metallurgical method, which consisted of the incorporation of NaCl as a space-holder. The space-holder was mixed into the powder mixture and dissolved prior to sintering. The same method was used to create titanium foams for the first time when Wen et al. utilized ammonium hydrogen carbonate spacers.Powder selection
The size and shape of the metal powder has a direct impact on the stability of the precursor as well as the resulting foam. For this purpose, powders that increase packing efficiency are most advantageous. The use of spherical particles may result in less contact of particles which consequently leads to larger secondary pores and a higher probability of pore collapse prior to complete sintering. This factor can be limited through different compaction techniques that decrease the degree of interstitial sites around the titanium particles. However, this method also has limitations; for example, the powders cannot be compacted to such a degree that would promote deformation of the spacer (unless anisotropic pore shape is desired).Space-holder selection
Space-holder size and shape
The degree of homogeneity in pore distribution of the final product is primarily dependent on the adequacy of mixing of the precursor. The difference in particle size between the titanium powders and the spacers directly impacts the ability to adequately mix the preform. The greater the size difference, the more difficult it is to control this process. Nonhomogeneous mixing resulting from the use of spacers considerably larger than the titanium particles employed and has shown adverse effects in the stability of the precursor after removal of spacer and in the distribution of porosity. Spacer size has been investigated. It was shown that the use of a coarse spacer results in thicker pore walls while the use of finer spacers results in enhanced compaction, leading to increased densification. Increased densification is evidenced by a monomodal pore distribution with the employment of fine spacers and a bimodal distribution using coarse spacers. Further, finer spacers result in a more homogeneous pore distribution. Sharma et al. utilized acicular spacers and achieved porosities up to 60% where pores were undistorted. In samples employing fine particles, porosities up to 70% were achievable before noting distortion in the pores. However, the bimodal pore distribution observed in coarse-spacer samples showed to be beneficial in terms of mechanical properties in that higher compressive strengths were observed, beyond those that might exist due to the inverse relationship of porosity and compressive strength alone.Compaction
The precursor mixture of powders and space-holders are compacted into a mold under a specified pressure. This can be achieved through uniaxial or isostatic processes. The pores resulting from this method are open and interconnected via windows between neighboring pores with the size of the pores partially dependent upon the coordination number and contact area of the resulting compact. Compaction pressure must be high enough to ensure sufficient mechanical strength for retention of pore geometry specified by the space-holder, but not too high enough as to cause deformation of the space-holder.Sintering and space-holder removal
When employing dissolvable spacers, it is possible to remove the spacer after sintering, which reduces the risk of pore collapse. In most cases, foams created using space-holders contain bimodal pore distributions with macro-sized pores resulting from the space-holder particles and micro-sized pores located on the pore walls and resulting from incomplete sintering of the powder matrix. As a result, the macropores typically exhibit rough internal surfaces. In some applications, such as for the use of bio-medical implants, this is an advantageous property. Inner porosity (or micro-porosity) has been shown to reduce stiffness; thus, reduce the risk of stress-shielding effects, while also offering improved osseointegration.Space-holder materials
= Sodium chloride (NaCl)
= Sodium chloride is the most commonly chosen space-holder for titanium foams because it is highly soluble in water and inert with respect to titanium. This inertness prevents contamination and degradation of the mechanical properties of the resulting foam. Moreover, NaCl is non-toxic; any residuals are bioinert. Bansiddhi & Dunand pioneered the use of NaCl as a permanent space-holder for the fabrication of NiTi foams. The resulting foams consisted of 32-36% porosity with more complete densification than they observed when producing NiTi foams using a sodium fluoride (NaF) space-holder. However, processing parameters resulted in molten NaCl and a metal/salt blend in the cavities of the foam. Certain risks are associated with using a molten space-holder including reaction with the metal, dissolving of the space-holder in the metal and prevention of densification through the creation of a thin layer of liquid between the metal and particles. Near complete densification was achieved when NaCl was used as a permanent space-holder in pure titanium foam. In this case, a temperature below the melting point of NaCl was used; titanium is less creep resistant than NiTi, which allows for densification at lower temperatures. The resulting foams achieved porosity of 50–67% with minimal observable microporosity. Anisotropic pore shape in some areas alluded to NaCl's deformation during HIP, which is desirable for some applications. Additionally, an observed, rough inner surface of the pores holds advantages for biomedical implant applications. Jha et al. achieved 65-80% porosity through the use of NaCl as a space-holder and a cold compaction process at various pressures with two-stage sintering. In this case, NaCl was removed through dissolution after the second stage of sintering. Resulting Young's moduli (8–15 GPa) were considerably lower than the Young's modulus of 29 GPa achieved for 50% porosity foams. This illustrates the known relationship between porosity and Young's modulus wherein Young's modulus decreases linearly with increasing porosity. Achievable porosity through the space-holder method is directly related to the type and amount of space-holder utilized (up to a threshold maximum achievable porosity level).= Magnesium
= Magnesium can be removed either thermally or by reactive measures through the dissolution in acid. Esen & Bor found a critical content of magnesium as a space holder to be 55-60%, above which compacts shrink excessively during sintering. Foams ranging in porosity from 45 to 70% with a bimodal pore distribution and compressive strength of 15 MPa (for 70% porosity) were demonstrated. Kim et al. fabricated foams with anisotropic pores through the intentional deformation of Mg particles during compaction in an effort to enhance mechanical properties. A final porosity of 70% equated to a yield strength of 38 MPa for normal orientation of pores and 59 MPa when pores were aligned with the direction of compression.= Urea
= Another commonly employed space-holder for titanium foams is= Tapioca starch
== Ammonium bicarbonate
= Although ammonium bicarbonate has been utilized in the manufacturing of titanium foams, it is not an ideal spacer in that it has a low melting/dissociation point and some solubility in titanium. This results in considerable shrinkage which makes control of pore shape difficult. Furthermore, the decomposition releases environmentally harmful gases.Freeze-casting
Applications
Potential structural applications for titanium foams include their general incorporation into light-weight structures and as components for mechanical energy absorption. The most important considerations for the use of titanium foams in structural applications includes their porosity, specific strength, ductility in compression and cost. Because of low manufacturing costs, most metal foams marketed for structural applications are of a close-celled aluminum variety. In comparison, titanium foam manufacturing incurs a higher cost, but this cost is defensible in space applications where the material offers an otherwise incomparable reduction in overall weight. The lower thermal conductivity of titanium may also be appreciated in rocket construction. The specific strength, overall energy absorbing capability and high melting point all reinforce titanium's superiority to aluminum in aerospace and military applications. When used for aerospace applications, levels of porosity close to 90% are desired. Titanium foams are capable of retaining their high tensile strength at temperatures up to 400 °C; a limit imposed by the metal's low resistance to oxidation.Aerospace applications
The driving force for titanium foam's replacement of existing materials in the aerospace sector results from the following five factors: *Weight reduction: as a substitute for steels and nickel-based superalloys; *Application temperature: as a substitute for aluminum and nickel-based alloys and steels *Corrosion resistance: as a substitute for aluminum alloys and low-alloyed steels *Galvanic compatibility: with polymer matrix composites as substitutes for aluminum alloys *Space constraints: as substitutes for aluminum alloys and steels The most urgent problem of engineering and its advanced branch of aerospace engineering is the efficient use of materials as well as increased service life.Sandwich panel cores
= Auxetic structures
=Biomedical implants
Bioimplants: microstructure
Biomedical implants should have low density for patient comfort and high porosity and surface area to facilitate vascularization and the ingrowth of new bone. Ideally, the implant will allow sufficiently easy fluid flow for cell nutrition and osteoblast multiplication as well as migration for cellular colonization of the implant to become uniform. The pores contained within the foam's cellular matrix mimic the extracellular matrix of bone, allowing the body to fixate with the implant. The porosity of the implant also promotes apposition and facilitates vascularization−as cells are able to attach, reproduce and form basic functions. It has been shown that a macropore size of 200–500 µm is preferred for ingrowths of new bone tissues and transportation of body fluids. The lower bound is controlled by the size of cells (~20 µm), and the upper bound is related to the specific surface area through the availability of binding sites. Finer pores further help in tissue growth and biofluid movement. Anisotropic, elongated pores (such as those attainable via the freeze-casting technique) may be beneficial in bone implants in that they can further mimic the structure of bone. The porous surface geometry of the foam promotes bone in-growth, provides anchorage for fixation, and ensures stresses are transferred from the implant to the bone. Surface roughness in the pore can enhance bone in-growth, and coarser cell size facilitates faster tissue growth. To optimize the implant's functionality and ability to successfully fuse with bone, it may be necessary to manipulate the material's manufacturing methods in order to modify the foam's pore structure. Changes in pore structure can directly influence implant strength as well as other key properties.Bioimplants: mechanical properties
Human cancellous bone possesses a stiffness ranging from 12 to 23 GPa; careful control and modification of manufacturing parameters to achieve similar strengths is imperative for practicality of integration. Correctly predicting the Young's modulus for foams is imperative for actual biomedical integration; a mismatch of Young's moduli between the implant and the bone can result in stress-shielding effects from a disproportional handling of stress. The implant which typically exhibits a higher Young's modulus than the bone will absorb most of the load. As a result of this imbalance, the starting bone density will be reduced, there will be tissue death and, eventually, implant failure. Natural bone exhibits the ability to adjust local fiber away from the low-stress regions toward high stress regions through the distribution of porosity, thus maximizing overall comfort. Using finite element analysis, researchers examined the effect of filling pores with bone on mechanical properties. They concluded that bone ingrowth significantly improved the mechanical properties, evidenced by decreased localized plasticity and stress concentrations. In effect, the titanium foam in the study allowed the bone to exhibit its natural ability to adjust local fiber away from the low-stress regions toward high stress regions. Experiments demonstrated that random combinations of pore size and shape result in lower Young's moduli. Theoretical models for the quantification of Young's moduli do not account for random pore size and shape distribution, so experimental measurements must be conducted in the presence of heterogeneous pore size and distribution. This is a limitation of the micro-mechanical models discussed above.Osseointegration
Currently utilized implants take a great deal of time to integrate with the body after the initial surgical procedure occurs. True adhesion between the implant and the bone has been difficult to achieve and, unfortunately, success rates of implant fixation are low due to the implant's failure to achieve long-term osseointegration into the bone. With an increasing number of individuals requiring orthopedic implants, the development of materials with structural and biological potential to improve osseointegration is crucial. Utilization of titanium-based foams present one way to potentially improve the bioactivity and reduce stress-shielding effects of currently employed bioimplant materials. The problem of osseointegration is best understood by examining the process of natural bone growth. In the body, bone and tissues experience self-regeneration, and structural modifications occur normally in response to environmental stimuli. Successful osseointegration occurs in three main stages that follow a natural biologically determined procedure: 1) incorporation of the implant into the bone's formation, 2) adaption of the new bone mass to carry weight and 3) remodeling of the new bone structure. The first stage in this process is the most crucial for overall success; the implant and the bone must form a rapid connection, and this bond must be strong and enduring. Owing to its porous structure, a titanium metal foam implant may be able to achieve close fixation with the bone and will decrease patient recovery time considerably. Essentially, the foam becomes an extracellular matrix in the body as tissue is integrated into it. Today, the implants most commonly used for bone replacement lack the ability to promote these characteristics, which are found in natural bone and, as a result, the implants have limited lifetimes. This phenomenon of osseointegration works similarly to direct fracture healing. However, instead of a bone fragment-end reconnecting to bone, the fragment-end connects to an implant surface. In a study on fibroblastic interactions with high-porosity Ti6Al4V alloy, the metal foam was supportive of cell attachment and proliferation, migration through the porous network, and proved capable of sustaining a large cell population.Increasing bioactivity via coatings
Titanium's propensity to form an oxide layer on its surface prevents corrosion of surfaces that are in contact with human tissues because the surface oxides minimize diffusion of metal ions from the bulk material to the surface. When titanium gains a coating to make it more bioactive, it can turn the already biocompatible titanium surface into an interface able to enhance osteoblast adhesion and able to promote osseointegration. Today, research is heavily focused on improving the success rate of integration and uses an understanding of the natural process of bone growth and repair to create coatings that will enhance the surface finish and surface properties of the implant. These adjustments allow the artificial structure to mimic biological materials and to gain acceptance into the body with fewer negative side effects. A 3-year clinical and radiographic study found implants in humans coated by nanocrystallineAnimal studies
In the lab, synthetic nanocrystalline bone grafting material in mice has shown in-growth of vascularized fibrous tissue which resulted in improved healing. Furthermore, new blood vessels were observed at day 5 after implantation, and the implant showed a high functional vessel density. In a study examining the femoral epiphyses of rabbits in two to eight weeks of healing, bone-to-implant contact was compared to bone growth inside the chambers for four different implant surfaces. The researchers found that bone substitute materials may improve the bone apposition onto titanium.See also
*External links
* * * * *References
{{reflist Foams Metals