Mechanical behaviors and porosity of porous Ti prepared with large-size acicular urea as spacer
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Porous Ti scaffolds with average pore size of 600 µm and porosity ranging from 31.1 to 61.2% were successfully prepared by powder metallurgy technology employing large size acicular urea as space holder. Results indicated that the porosity (P) of porous Ti could be determined accurately by added spacer content (Sc) in the green-pressing because the relationship could be formulated as P = aSc + b, where a and b are constants. The compressive strength and structural stiffness were in the range of 50–332 MPa and 0.6–3.7 GPa, respectively. Moreover, the dependence between relative porosity and relative strength, along with the dependence between relative density and relative Yong’s modulus were formulated. Our research can provide helpful information to fabricate porous Ti with desirable structure and mechanical properties by spacer method.
KeywordsPorosity Porous Ti Mechanical behavior
Porous Ti and its alloys have received considerable attentions in recent years for applications in the field of biomedical engineering such as dental implants and artificial joints due to their unique structure, high chemical resistance and remarkable biocompatibility [1, 2, 3, 4]. There are a large number of published reports investigating the preparation and characterization of Ti foams employed in the biomedical industry [5, 6, 7, 8, 9]. According to the study of Oh , the produced Ti foams with porosity of 30% possess an elastic modulus very close to that of cortical bone. In practical, these novel materials with combined advantages of structural foams and metallic property have many other promising applications. For example, they can be employed as functional materials in terms of heat adsorption given the superior heat conductivity for Ti metal and good energy consumption behavior for foams, if their pore structures are artificially tuned to meet specific demands . In addition, the applications of Ti based foams in many potential engineering community such as load-bearing sandwich cores in aerospace, naval, and ground transportation industries have already been in focus of attention [12, 13], which requires materials that can work at high temperature or in harsh environment.
Corresponding to different applications, the pore structure including pore size, pore distribution and porosity should be precisely characteristic. Since the space holder technique which can manufacture desirable porous structure without any impurity has been first introduced by Zhao and Sun , numerous research have been carried out to prepare porous metal with various spacers such as Mg [15, 16, 17], NaCl , NH4HCO3  and urea [15, 20]. In the work of Lee et al. , four types of spacers, namely irregular-shaped urea, spherical urea, sodium chloride, and starch powders, were applied to fabricate porous Ti, and they found that desirable strength and elastic modulus can be obtained by selecting a proper space holder. Tuncer et al.  studied the relationship between porosity of Ti foams and size of spacer, and found that strength and stiffness of products tend to increase with the increasing of pore size. However, they have little investigation on the relationship between mechanical properties of Ti foams and porosity induced by spacer (urea) with large size (> 1750 μm). This is important because porous Ti with large pores and relatively high strength can be applied not only as energy adsorbent that can reduce the noise or heat but also as load-bearing sandwich cores in aerospace that can lower weight for supporter. In terms of porous Ti prepared by space holder technology, the porosity is gained by adding needed volume fraction of space holder namely spacer content. Literature [15, 21, 23, 24, 25, 26, 27] reported that the porosity might be less than, equal to or greater than spacer content. The main reason of this interesting phenomenon could be attributed to the decreased volume of macro-pores is larger, equal or less than increased volume of micro-pores during sintering. But once the large size spacer is employed to prepare foams, the volume of micro pores would be reduced to a large extent, leading to the obtained porosity more desirable and related mechanical behavior much closer to what the manufacturer want. Therefore, the relationship should be further clarified.
This work attempts to shed light on the effect of large size spacer on porosity and mechanical behaviors of prepared porous Ti, with the size of space holder over 1750 μm. Compared with other spacers, urea has the advantages of rapid dissociation, easy removal, little residual and low cost, thus it is selected as the space holder in our work. Our experiment would be meaningful not only lying in the systemically analyzed the effect of this urea on the porosity and mechanical properties for obtained Ti foams, but also giving a supplement for predicting these parameters of Ti foams prepared by spacer method.
2 Materials and methods
Chemical components of commercial Ti power
The commercial acicular urea has the average particle size of 2 mm with content of CO(NH2)2 > 99%, indicating that the spacer can be removed with little impurity. One thing should be noted that the Ti powders were pretreated by the high energy ball-milling machine for 25 min in order to reduce the particle size and simultaneously improve the fluidity between powders.
To investigate the compressive behavior, the obtained foams were compressed by a Universal Testing Machine at a displacement rate of 1 mm/min to guarantee a stable stress transfer through the pressuring samples. All the compression processes are performed through the longitudinal direction. Considering the reliability of compression data and repeatability of experiment, the porosity, compressive strength and structural stiffness were finally determined by calculating the average value of five simples.
3 Results and discussion
Mixture ratio, porosity and compressive data of porous Ti
3.10 ± 0.04
31.1 ± 0.8
331.84 ± 7.5
3.69 ± 0.08
2.66 ± 0.01
40.9 ± 0.2
193.25 ± 6.6
2.87 ± 0.10
2.19 ± 0.05
51.3 ± 1.1
108.67 ± 4.2
1.68 ± 0.06
1.75 ± 0.03
61.2 ± 0.6
50.34 ± 3.3
0.64 ± 0.04
Based on this table, one can find that the porosity of forms witnesses a rising tendency with the increasing content of added spacer from 30 to 60%, and the values for calculated porosities are all larger than those of added volume of spacer, which will be further analyzed in the following section. On the other hand, the compressive strength along with the density of obtained products are decreased sharply as the increasing content of urea. This is understandable because if the adding content of spacer becomes larger, the volume fraction for pores would get large accordingly after sintering, thereby leading to the plummet in material density and strength. Detailed analysis would be mentioned in the following sections.
3.1 Pore structural analyses
It has been reported  that pores in porous metals could be divided into two types: the micro-pores and the macro-pores. The former is generated due to the partial sintering of Ti powers on the sidewall of pores, which frequently has the size only several micrometers (in Fig. 2a). On the other hand, the latter that has much larger size are obtained by the decomposition of spacer particles. To meet the requirement of practical applications, the kinds of macro-pores should be distinguished as well. The first type is unconnected pores as depicted in Fig. 2b, in which the pores stand individually and solely due to the homogeneous distribution of a small fraction of urea particles on Ti powers. In other words, this kind of pores are likely to be formed in the foams with small porosity lower than 30%. Whereas, the second one is called interconnected pores as seen in Fig. 2c, which are formed because of the large fraction of spacer in a form, leading to a large number of vacancies after sintering. This pores are also named opening pores, which usually can be found in a high porosity foam larger than 50%.
3.2 Dependence between porosity and spacer content
3.3 Mechanical behaviors analyses
The previous discussions in the basis of Table 2 have concluded that both compressive strength and structural stiffness are showed a continuous decrease as the porosity increased, and here we give some detailed expressions. It has been reported [16, 38] that the stress–strain curves exhibits three distinct regions in the compressing process: (1) linear elastic region, (2) plateau region, and (3) densified region. In the plateau region, stress oscillates around an average stress value and remains relatively flat even with increase in strain value (the average stress in this region is termed as plateau stress). In densified region, stress increased drastically with slight increase in strain. In this work, the compressive stress–strain curves are presented in a similar way, although ill-defined densified regions. From Fig. 5, one can deduce that with the increase of porosity, not only the compressive stress becomes gradually decrease, but related strains drop when the stress grows up to the largest. This could be attributed to the increasing porosity with large pore size of average 600 µm induced by the large size of acicular urea in the foams. In other words, these formed large pores are likely to be collapsed in the compression process, thus accelerating the linear elastic region to a large extent. Apart from that, stable plateau regions can be found when the foams suffered the compressive strength. This is in agreement with the property for porous material, which can be explained by the uniform stress undergone by the foams with the gradually collapsed pores if the prepared foams possess the homogenous pore structure. In the third region, the compacting specimens were completely collapsed and crumbled into a pitch of small foams, instead of a dense bulk. This would be the reason why there has no evident densified region of our prepared specimens during compression. In practical, the behavior of stress–strain curve is closely related to the property of the spacer holder; while the main purpose of this work is to investigate the effect of large size spacer on the porosity and mechanical properties of Ti foams. In this regard, one could comprehend that these large size of pores are vulnerable to be crushed once the compacting stress is beyond their topmost strength, resulting in the breakage of Ti walls between pores, and then collapsing the foams. Even so, these materials possessing relatively high strength and good yield performance would be suitable for energy cushioning and consuming material given their uniform distribution of pore structure.
After obtaining these two formulations, it would be more comprehensive to understand the porosity and mechanical properties of Ti foam prepared by urea, through analyzing the effect of large size urea on these parameters. In addition, it would be more convenient to predict the physical properties of Ti foams prepared by spacer method.
The porosity, compressive strength, and structural stiffness for obtained foams are in the range of 31.1–61.2%, 50–322 MPa and 0.6–3.7 GPa, respectively.
The average pore size of 600 μm is generated and distributed uniformly inner the foams.
The relationship between porosity (P) and spacer content (Sc) is formulated by P = 1.007Sc + 0.7255, which is helpful to obtain certain porosity of a foam by removal of the spacer.
Additionally, the relation between relative compressive strength and density obeys a power law relation; while the relationship between relative stiffness and porosity are described as a linear dependence. In this way, the mechanical properties of foams could be predicted by the models when using the space holder technology.
This study was supported by the Fundamental Research Funds for the Central Universities (No. 106112017CDJPT130004) and by the Natural Science Foundation of China (No. 51174243) and (No. 51674055).
GQ conceived and designed the research, HC performed the research and wrote this manuscript while JW and TL helped analyze the data. GQ and HC contribute equally towards this work.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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