A Comparison in Mechanical Properties of Cermets of Calcium Silicate with Ti-55Ni and Ti-6Al-4V Alloys for Hard Tissues Replacement

This study investigated the impact of calcium silicate (CS) content on composition, compressive mechanical properties, and hardness of CS cermets with Ti-55Ni and Ti-6Al-4V alloys sintered at 1200°C. The powder metallurgy route was exploited to prepare the cermets. New phases of materials of Ni16Ti6Si7, CaTiO3, and Ni31Si12 appeared in cermet of Ti-55Ni with CS and in cermet of Ti-6Al-4V with CS, the new phases Ti5Si3, Ti2O, and CaTiO3, which were emerged during sintering at different CS content (wt%). The minimum shrinkage and density were observed in both groups of cermets for the 50 and 100 wt% CS content, respectively. The cermets with 40 wt% of CS had minimum compressive Young's modulus. The minimum of compressive strength and strain percentage at maximum load were revealed in cermets with 50 and 40 wt% of CS with Ti-55Ni and Ti-6Al-4V cermets, respectively. The cermets with 80 and 90 wt% of CS showed more plasticity than the pure CS. It concluded that the composition and mechanical properties of sintered cermets of Ti-55Ni and Ti-6Al-4V with CS significantly depend on the CS content in raw cermet materials. Thus, the different mechanical properties of the cermets can be used as potential materials for different hard tissues replacements.


Introduction
Calcium silicate (CS, CaSiO 3 ) and titanium (Ti) and Ti alloys have widely been used in implants especially for bone hard tissue due to their unique bioactivity and biocompatibility properties [1]. CS is a highly bioactive ceramic, while it is a brittle material with low fracture toughness [2][3][4]. Therefore, many studies tried to increase the load bearing capacity and toughness of CS by reinforcing it with other materials such as alumina [5], carbon nanotube [6], graphene oxide [7], reduced graphene oxide [8], and polymers [9]. On the other hand, Ti and Ti alloys are biocompatible and have excellent corrosion resistance [1]. However, they are bioinert materials without having bioactivity to create a strong bonding with host bone as implant [10]. As a result, the Ti alloys implants encapsulated by fibrous tissue cause clinical failure due to isolation from the surrounding tissue after implantation into a living body [10,11]. In this context, monolithic ceramics, particularly CS [2,5] and synthetic or natural hydroxyapatite (HA) [12][13][14][15], are known as excellent biocompatible materials. Therefore, the metallic implants have been coated with some bioactive materials such as HA and CS to ensure a good bonding between implant and host bone [16][17][18][19][20]. However, coating could be flaked off from metals substrate because of weak bond between ceramic and metallic phases [21]. In addition, our several theoretical studies have shown that stress intensity factors at the external surface of the cement layer of the cement coated metallic implants are higher which may be the primary cause of easy flaking off of the coatings from the metallic surfaces [22,23]. Composites of many ceramics, including HA [13,24], CS [14,25], and alumina (Al 2 O 3 ) [5,26], have been used in different biomedical applications successfully. Therefore, composites or cermets of Ti alloys with CS or HA ceramics could be employed to 2 The Scientific World Journal resolve the problems related to the coating and brittleness of pure ceramics.
The composite material made of the ceramics and metals is called cermet. Based on the matrix and the reinforcing materials, composite materials are categorized into the ceramic matrix-metal composites (CMMCs) and metal matrix-ceramic composites (MMCCs). A dramatic change in mechanical properties in cermets is a result of the ceramic or metal particles inclusion into the metal or ceramic matrix [27,28]. Thus, due to the overlapping strength and weakness of the ceramics and metals, the CMMCs and MMCCs possess superior stiffness, fracture, fatigue, and tribological and thermal properties in comparison with the monolithic ceramic and metals counterparts [29]. According to the suitable properties of conventional and monolithic materials, the monolithic ceramics and metals are rapidly changed with these composites in various engineering applications from biomedical [24,26] to the aerospace or automobile fields [30].
Despite many works on Ti, Ti alloys, and CS in different applications according to their individual important properties, to the best of our knowledge there has been no work on composites of CS with Ti-55Ni (TN) or Ti-6Al-4V (TAV) reported. Therefore, in the present study, first time we investigate the effect of CS content on the mechanical properties and microstructural fracture surfaces in the TN/CS and TAV/CS cermets for hard tissues replacement in biomedical applications.

Materials and Methods
The procedure employed in the present work to study the cermets of calcium silicate with Ti-55Ni and Ti-6Al-4V is illustrated as a schematic flowchart in Figure 1.

Sample Preparation.
Powder metallurgy technique was employed to fabricate the different novel cermets of TN and TAV with variation of CS contents in two different cermets in the form of ceramic/metal matrix composites and vice versa. All the chemicals were purchased from Sigma-Aldrich and used without any further purification. Briefly first, the raw powders were mixed in different weight ratios as presented in Table 1. Then wet ball milling was carried out in a planetary ball mill (PM 200, Retsch) to blend the raw powders at a ball to powder weight ratio of 5 : 1 in 75 mL ethanol medium at a speed of 300 rpm for 6 h to get a homogeneous mixture. The mixture was dried overnight in an oven at temperature of 110 ∘ C for 16 h. Following that, the dried milled powders were compacted under 100 MPa by using manual hydraulic press (GS15011, Graseby Specac) to form green samples of fixed diameter of 6.35 mm and average compact height of 13 mm. Afterwards, the green compacted samples were placed in cold isostatic pressure (CIP) machine (Reiken Seiki Japan) at room temperature under pressure of 250 MPa to achieve further densification and a uniform compacting pressure distribution. Then pressureless sintering was performed on samples at 1200 ∘ C for 3 h in inert argon gas environment using a vacuum atmosphere furnace (XY1600, Nanyang Xinyu Furnaces) to prevent oxidation of metal phases. Sintering cycle exploited in this study is presented in Figure 2. The temperature was increased gradually at rate of 5 ∘ C/min from 50 ∘ C to 500 ∘ C. Then the temperature was kept at a constant holding temperature of 500 ∘ C for 2 h to improve the sintering ability in addition to removing the moisture and some impurities of the samples. After that the temperature was again increased at rate of 3 ∘ C/min to 1200 ∘ C and was soaked for 3 h. The ramp rate was decreased in the second step to prevent the crack formation in the samples owing to the thermal shock as well as difference in the thermal expansion coefficients of the phases present in the cermets. The 3h soaking time at 1200 ∘ C was used to complete the sintering process to the whole sample. Afterwards, the temperature was cooled to room temperature with two steps of 3 ∘ C/min and 5 ∘ C/min ramp rate, respectively, to avoid the crack formation as well as thermal stress in the samples.

2.2.
Characterization. X-ray diffraction (XRD) was conducted on sintered samples by using X-ray diffractometer (Empyrean, PANalytical) to determine and analyze the phase constitution. The density ( , g/cc) was measured by employing Archimedes' principle explored elsewhere [25] according to (1) and volume shrinkage ( %) of sintered samples was computed following (2). The materials were assumed to be perfectly solid and completely insoluble in water at the measured condition. At least five identical specimens were carried out at 25 ∘ C, where water density was considered as 0.99704 g/mL, to compute the average as well as standard deviation (SD) for each sintered sample: The Scientific World Journal  The compressive test conducted on samples using universal testing machine (4469, Instron) at a constant cross-head speed of 1 mm/min to determine compressive elastic modulus, compressive strength, and strain percentage at maximum load is shown in Figure 3. At least three identical samples were performed to take the mean and SD of mechanical properties.
The Vickers hardness test was conducted on the polished surfaces of sintered samples using micro-Vickers hardness tester (AVK-C200, Mitutoyo) as shown in Figure 4. The pyramid shaped diamond indentor was used at constant load of 5 N for 10 seconds to take at least 5 indentations from each sample.
Finally, the microstructure of sintered samples was examined by scanning electron microscope (TM3030, Hitachi) as shown in Figure 5.  after sintering at 1200 ∘ C, respectively. In sintering process, three new phases of nickel silicon titanium (Ni 16 Ti 6 Si 7 ), calcium titanium oxide or calcium titanate (CaTiO 3 ), and nickel silicon (Ni 31 Si 12 ) appeared in XRD patterns of TN/CS cermets (see Figure 6). These phases were the results of reaction between compounds of TN and CS. The volume fraction of new phases in sintered cermets was a function of CS wt%. With increasing of CS content in TN/CS cermets, the crystalline peaks of Ni 16 Ti 6 Si 7 clearly appeared in the cermets from 10 to 80 wt% CS, while the CaTiO 3 phase appeared from 20 to 80 wt% and the Ni 31 Si 12 phase was found mainly from 40 to 80 wt% CS in XRD patterns of Figure 6.

Results and Discussion
On the other hand, three new phases of titanium silicon (Ti 5 Si 3 ), titanium oxide (Ti 2 O), and CaTiO 3 were also developed in another sintered cermet (TAV/CS) comprised TAV and CS as shown in Figure 7. It has also been found that few of the XRD peaks in pure TAV (i.e., 0.0 wt% CS) also match the titanium oxide (Ti 3 O, ICSD reference code: 01-072-1806). This trace amount of Ti 3 O is produced due to the oxidation of a small part of the surface. The Ti 5 Si 3 phase was found in all cermet groups of TAV with 10 to 90% wt of CS content. CaTiO 3 was revealed in sintered cermets with more than 30 wt% CS except in group of 40 wt% CS which showed only phase of Ti 5 Si 3 . On the other hand, Ti 2 O only was found in groups of TAV with 10 wt% and 20 wt% CS. The different extra phases developed in both the cermets are mainly responsible for the mechanical properties, such as compressive strength and microhardness, and density or shrinkage of the sintered cermets.

Physical Characterization. Figures 8(a) and 8(b) represent shrinkage and density of different cermets of TN
and TAV with CS after sintering process. The variation of shrinkage as a function of CS wt% shows a concave plot with the maximum value of shrinkage in the pure metallic and ceramic phases and a minimum value at around 50 wt% of CS in both groups of cermets (see Figure 8(a)). The percentage of shrinkage is nearly constant in 40 to 60 wt% CS contents for both cermets, while a dramatic increase in shrinkage has been found below 40 wt% CS and a gradual increment is noticed for the cermets above 60 wt% CS content. On the other hand, increase of CS content in cermets leads to gradual reduction in density of sintered cermets samples.
Therefore, pure metallic phase showed the maximum density and pure ceramic phase showed minimum density (Figure 8(b)). In contrast, the reduction in density with increase of CS content was consistent with a study of Chenglin et al. [21] for Ti-HA functionally graded materials. In addition, the density of pure CS used in the current study is in the range of density reported by Liu et al. [31].

Mechanical Characterization.
In order to investigate the relationship between CS content and the mechanical properties of sintered specimens, compressive properties were measured and the results were presented as a function of CS content (wt%) in Figure 9. Figure 9(a) depicts variation of Young's modulus in different cermet groups of TN/CS and TAV/CS. The elastic modulus of cermet materials in both groups gradually decreased with CS content up to 40 wt% and then it followed an increasing trend up to pure CS (100 wt% CS). The TN/CS cermets group showed higher modulus elasticity than the cermets of TAV/CS in any CS content. An insignificant effect was noticed in the change in Young's modulus owing to the increase of CS content more than 70 wt% in TN/CS cermets. Based on the XRD patterns (Figure 6), the reason for limited change in modulus of elasticity in TN/CS cermet group with more than 70 wt% CS could be explained by dominance of unreacted CS on other phases. Figure 9      The Scientific World Journal with 60 wt% metallic phase and 40 wt% CS showed minimum ultimate compressive strength (UCS), while both the cermets with 70 wt% metallic phase and 30 wt% CS showed high UCS, excellent modulus of elasticity and maximum strain at UCS. In this study, it has been found that the main responsible phases for increasing the mechanical properties in case of TN/CS and TAV/CS cermets are Ni 16 Ti 6 Si 7 and Ti 5 Si 3 , respectively. In a special case of TAV/CS cermet at 30 wt% CS, the UCS is significantly lower compared to 30 wt% TN/CS which is a result of the development of more amount of a new CaTiO 3 phase. The amount of increase in mechanical properties of above 50 wt% CS cermets is lower than that of the cermets below 50 wt% CS. The micro-Vickers hardness test result portrayed in Figure 9(d) also showed similar pattern as found in compressive properties. Results of Vickers hardness test showed that the pure metallic phases of each group of cermets had maximum hardness and gradually it decreased with CS content till 60 wt% in TN/CS and 50 wt% in TAV/CS and following that hardness of both cermets was found to be increased till 100 wt% CS. The hardness behaviour in study is in agreement with hardness trend in Ti-HA functionally graded material in works conducted by Watari et al. [32] and Chenglin et al. [21]. The Vickers hardness of 100 wt% CS is comparable with reported hardness for pure CS by Liu et al. [31]. Since the two new phases such as Ni 16 Ti 6 Si 7 and Ti 5 Si 3 were significantly found on the surface for both the TN/CS and TAV/CS cermets between 40 and 60 wt% CS, respectively, in XRD patterns, the overall mechanical properties were found to be significantly detracted to those cermets.

Scanning Electron Microscope.
The SEM was conducted on samples after compressive test and some important specimens are selected to explore the fracture surfaces in Figure 10. The SEM images displayed the uniform distribution of CS in cermets. The sintered samples of TN/CS showed less porosity than the TAV samples. The second phase particles and the pores at the fracture surface of the materials are indicated by black and white coloured arrows, respectively, on the SEM images in Figure 10. The fractured surfaces became rougher with increase in CS content up to 50 wt% CS in both groups of cermets and then the roughness of fractured surfaces gradually declined with more increase in CS content. The smooth surface for fractured surfaces reveals brittle facture, more bonds near to the pure phases of cermets in comparison with the cermets close to the 50 wt% CS.

Conclusion
During the sintering process at 1200 ∘ C based on the weight ratio of raw powders, different new phases were produced in final product during sintering process. The maximum shrinkage was revealed in pure metallic phases (TN and TAV) and ceramic phase (CS), while the minimum shrinkage was revealed at 50 wt% CS in different cermets of TN/CS and TAV/CS. On the other hand, density of cermets declined by CS weight percent increase and 100 wt% CS showed the minimum density. The new phases of Ni 16 Ti 6 Si 7 , CaTiO 3 , and Ni 31 Si 12 appeared in TN/CS cermets and Ti 5 Si 3 , Ti 2 O, and CaTiO 3 in TAV/CS cermets are mainly responsible for their mechanical properties. The compressive mechanical properties and hardness decreased with CS content up to 50 wt% and then followed an increasing up to 90 wt% CS. The amount of increasing in overall mechanical properties of above 50 wt% CS cermets was lower than that of the cermets below 50 wt% CS. The TN/CS illustrated better compressive properties than TAV/CS. In addition, the strain percentage at the maximum load for cermets with 80 wt% and 90 wt% CS was more than pure ceramic phase of CS. Since the mechanical properties of the above cermets are substantially higher than that of the natural bones [25,33], the different mechanical properties of the above cermets, therefore, can be used as per desired strength of the different hard tissues in biomedical applications [8,33].