Novel synthesis of Al₂O₃ short fibers/Ti-12Mo-6Zr composites for cranial reconstruction applications: spark plasma sintering, microstructure and nanomechanical properties

Abstract

Ceramic-Titanium matrix composites have recently attracted significant interest as a new type of biomaterials protecting the brain from external force and infections of cranial defects due to its biocompatibility and good mechanical and corrosion properties matched with the bone tissue. Spark plasma sintering (SPS) is one of powder technology techniques that can be utilised in the fabrication of final net complex and irregular shape parts used for cranial reconstruction and maxillofacial trauma by reconstruction and cranioplasty. The present work studies the effect of alumina (Al₂O₃) short fibers reinforcement addition on the nanomechanical properties estimated by the nanoindentation measurements of the Ti-12Mo-6Zr and its correlation with the microstructure. Al₂O₃ short fibers/Ti-12Mo-6Zr of different Al₂O₃ reinforcement short fibers content up to 5 wt.% were fabricated by Spark Plasma Sintering technique. Powders of Ti, Mo, and Zr powders were mechanically wet milled with different wt.% of Al₂O₃ reinforced short fibers. The mechanically mixed Al₂O₃ short fibers/Ti-12Mo-6Zr samples of different compositions were consolidated by SPS at 1000 ^oC for 5 min under vacuum and 50 Mpa compaction pressure. Optical microscopy (OM), high-resolution scanning electronic microscopy (HRSEM) conducted with Electron dispersive spectroscopy (EDAX) unite and X-Ray Diffraction (XRD) are used to evaluate the particle size and shape, surface morphology, microstructure, the chemical compositions and the phase identifications for the investigated samples. The samples were determined by the rule of mixture (ROM) as well as the Archimedes’ principle. The nanomechanical properties were estimated by measuring the nanoindentation of the produced Al₂O₃ short fibers/Ti-12Mo-6Zr sintered samples using a Berkovich indenter with continuous stiffness measurement (CSM) method. The hardness and the Young modulus were estimated from the obtained data of the applied load-displacement in the depth curves. The obtained Al₂O₃ short fibers/Ti-12Mo-6Zr composites have good mechanical properties which revealed the efficiency of the sintering process by spark plasma sintering. Also, the estimated hardness and Young’s modulus are increased by increasing the content of the Al₂O₃ reinforcement nanoparticles from 1 to 5 wt.% in the Ti-12Mo-6Zr metal matrix. Based on our findings of the nanoindentation studies; it was expected that the produced Al₂O₃ short fibers/Ti-12Mo-6Zr new composites have appropriate physical and mechanical properties for cranial reconstruction applications.

Introduction

Traumatic injuries especially craniofacial and maxillofacial fractures are increasingly growing by ~70 million persons every year worldwide; causing disability and death which increases the need for bone tissue replacements. The statistical studies show that; trauma in the maxillofacial area represents about 45% among all the types of trauma [1]. Many challenges are reported in the literature dealing with repairing and cranial reconstruction operations [2, 3]. The osteosynthesis method in the maxillofacial field desires to regain the functionality and morphology of the bones, creating an optimal environment for the osteogenic process with the help of different biocompatible materials [4]. We are in need to develop new materials analogous to bone to protect the brain from atmospheric heat transfer, low thermal conductivity is desirable; therefore, polymers have been considered as alternative class for craniofacial repair, particularly due to their good mechanical properties to weight ratio [5]. PMMA, PEEK and PE are the most widely polymers used in cranial reconstruction operations. However, the use of alloplastics may cause an inflammatory tissue response and additional thermal and toxic damage [6, 7]. Aluminum oxide (Al₂O₃) is one of the excellent bio-inert materials possesses neither osteoconductive nor osteoproductive properties. Jahnke reported its applications in the reconstruction of the anterior skull base in cerebrospinal fluid leakage repair [8]. It can be used in the manufactures in ceramic plates. However; ceramic plates have lack of ductility which in turn does not allow any modeling of the plates to the shape of the bone.

Use of titanium and its alloys as biomaterials comes from their superior mechanical properties, biocompatibility and excellent corrosion resistance because of the presence of the other alloying elements such as V, Al, Mo, Nb, Ta, Zr and the dense thin surface oxide (such as TiO₂) layer which is formed spontaneously within milliseconds in oxidizing media preventing further transport of metallic ions and/or electrons across the film. TiO₂ is thermodynamically stable in the pH ranging from 2 to 12 [9]. The 2–6 nm thick natural oxide layer is thermodynamically stable, chemically inert, and has a low solubility in the body fluid providing favorable osseointegration. The human body fluids, which differ depending on the part of the body; e.g., blood plasma and intercellular fluids display high proteins concentrations. It was shown that the TiO₂ layers on the implants surface of Ti/Ti- based alloys effectively improve the growth of apatite [10], bone binding capacity [11], initial adhesion of osteoblast-like cells [12], bone growth of implants [13] and then the combinations of Ti plates or resorbable plates ceramic-screws might be worth taking into consideration [14].

The aim of the manufacturing of biocompatible composite materials is the synthesis of the often opposing osteoconductive and mechanical properties. There are manifold possibilities of combining materials, some examples being: ceramic such as (Al₂O₃), silicon nitride (Si₃N₄), hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), and metallic materials such as Ti, Ta, Zr and niobium (Nb). The biocompatibility and corrosion resistance of the metallic implants is an essential characteristic since the metallic materials are in contact with the aggressive constituents such as chloride ions and proteins media of the biological fluids. At the same time, the corrosion reaction and the hydrolysis reaction of the TiO₂ passivation film by the interaction with the dissolved chloride ions and proteins in the body fluids exist simultaneously, so that, the dissolution and formation of the passive film reach a dynamic balance. It was reported in previous work that; the Ti-6Al-4V alloy reaches higher stability against dissolved chloride ions than Ti itself due to the relatively stable passive film on the surface of Ti-6Al-4V alloy is postponed than that of Ti [15, 16]. Also, it was mentioned in the literature that, the biocompatibility of any implanted material is governed by the formed reactions between the implanted material and mainly with the proteins of the biological fluids which is the main reason behind the interfacial reaction between Ti and the living tissue. The conformation of proteins is changed by the adsorption on the metallic surface of the implanted materials due to the accumulated charges on its surface. The electrostatic force of proteins to the metal surface is governed by the relative permittivity of the surface oxide film such as TiO₂ on the Ti implanted surface: the larger the relative permittivity, the smaller the electrostatic force. Therefore, the structural conformational change of the adsorbed proteins on the TiO₂ thin film is possibly small (see Fig. 1) then the proteins adsorbed on the Ti surface are less susceptible [17]. Calcium and phosphate ions are the most common ions in the human body fluids that easily adsorbed on the surface of the titanium implanted parts and greatly influence on the biocompatibility of titanium. Adsorption of these ions on the protective thin layer of the TiO₂ surface causes an increase in the thickness of the film, providing better barrier and preventing aggressive ions from the solution from coming in contact with the implanted titanium. The reaction between the implanted titanium surface and the phosphate ions of the human body fluid can be represented by the following chemical equations [18, 19].

$${\mathrm{Ti}\left(\mathrm{OH}\right)}^{3+}+{\mathrm{H}}_2{\mathrm{PO}}^{4-}\to {\mathrm{Ti}}^{4+}.{\mathrm{HPO}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}$$ 1

$${\mathrm{Ti}}^{4+}.{\mathrm{HPO}}_4^{2-}+{\mathrm{OH}}^{-}\to {\mathrm{Ti}}^{4+}.{\mathrm{PO}}_4^{3-}+{\mathrm{H}}_2\mathrm{O}$$ 2

$${\mathrm{Ti}\left(\mathrm{OH}\right)}^{3+}+{\mathrm{HPO}}_4^{2-}\to {\mathrm{Ti}}^{4+}.{\mathrm{PO}}_4^{3-}+{\mathrm{H}}_2\mathrm{O}$$ 3

Fig. 1.

Schematic diagram of the change in the confirmation of the protein of the body fluid on the Ti implant

Over the past decades, titanium and its alloys have been well established as biomaterials used in reconstructing the bone skeleton, orthopedics, neurosurgery, dentistry and craniofacial prostheses. The cause of these defects could be trauma, infection, tumor, or following neurosurgical procedures. Ceramic reinforced titanium (Ti) base metal matrix composites are considered recently as suitable materials used for orthopedic and cranioplasty applications due to their excellent biocompatibility, corrosion resistance, good hardness, moderate Young’s modulus matched with human cortical bone [20, 21]. These promising properties of the ceramic/titanium base composites relative to the human cortical bone can retard and decrease the failure of the implanted parts in the fracture sites of the human bodies by retarding the osteoporosis [22].

Some crani-maxilloofacial operations were introduced to use appropriate titanium biomaterials parts in treatments of craniofacial and maxillofacial fractures have reported their usefulness due to its higher biosafety, biocompatibility, osteoconductive nature^, its mechanical stability, ease of fabrication and sterilization, orchestrate both hard and soft tissue integration and better physical properties than other metals tested for a long period of time with relatively few complication [23–25]. Among the safest alloying elements for biocompatible Ti alloys are Nb, Ta, and Zr, which are non-allergic and non-toxic [26]. To address the safety and manufacturing issues and to reduce the stress shielding effect, low modulus beta Ti alloys with non-allergic and non-toxic alloying elements have been produced. Zhang et al. [27] additively manufactured near full density (>99%) Ti–24Nb–4Zr–8 Nb (Ti2448) acetabular hip cup scaffolds using LPBF with a yield strength of 563 MPa and Young’s modulus of 53 GPa. The higher ductility of these alloys ensures the required toughness and prevents fracture. Most metallic implants such as Ti alloys are bioinert and lack sufficient osseointegration, which delays integration with host tissue and implant fixation [28].

In addition; ceramic titanium metal matrix composites have good erosion and abrasion wear resistance for a long-life span which can be considered as excellent candidate material for replacement of human bone and joints, cranial reconstruction and bone flap fixation after craniotomy [29, 30]. Unfortunately, during use of the implanted parts in the human bodies for a prolonged time an interaction between the surface of the implanted parts and the human bodies starting by the pitting corrosion process which can enhance the release and formation of some metal ions from the corroded surface such as Ti, V, Al etc, within the human bodies, resulting of acute and/or chronic inflammation of the tissues in contact with the implants, pains, infections which enhance the failure and shortened the life time of the implants [31]. Although some titanium based materials such as Ti-6Al-4V alloy were developed to improve the efficiency of the implanted parts, it is containing some cytotoxic elements, such as aluminum (Al) and vanadium (V) which may be release and electrochemically react with the media which compose the human body fluids such as water, saliva, plasma and enhancing the failure of the implanted operation. It was mentioned in the literature that; the V ions may be released from the Ti-6Al-4V implants within the human body produces an inflammatory reaction through the osteolysis process of the human bone in contact with the implanted parts by losing some essential elements like calcium [32]. Although the use of cranial titanium parts is safe and effective, some complications have been reported in previous reports include palpability, visibility, infection, exposure, pain, and implants malfunction, which in turn can necessitate implants removal [33–35].

Biocompatible ceramics particles reinforced Ti metal matrix composites such as TiO₂/Ti, SrO₂/Ti, ZrO₂/Ti, and hydroxyapatite/Ti (HA/Ti) have been developed for several cranial reconstructions and orthopedic applications [35–39]. Al₂O₃/Ti metal matrix composites are recently considered as advanced engineering materials with promising performance. The combination of the high strength and hardness of the Al₂O₃ ceramic with the high corrosion and low oxidation resistance of Ti metal beside the biocompatibility of both constituents (Al₂O₃ and Ti) enabling this composites as a candidate material for different cranial and orthopedic applications [40]. It was reported also in the literature that; Al₂O₃ and Ti have appropriate coefficient of thermal expansion which is required to increase the performance of Al₂O₃/Ti composites than the other materials. During the composite fabrication process at high temperature a diffusion metallurgical bonding in the interface between Al₂O₃ and Ti were occurred however, Al₂O₃ enhanced cracks initiations due to the different in the differences in thermal expansion coefficients and/or expansion of dissimilar materials (Al₂O₃ and Ti) which starting the early failure of the Al₂O₃/Ti implanted joints [41, 42]. The techniques for shaping sheet metals use either a press technique and/or traditional hand spacers of appropriate thickness, allowing for the forming techniques but the shaping of sheet titanium to produce parts of complex shape such as the contours of the orbital margins or facial bones requires high technology in the fabrication methods and welding of titanium, which is enabling accurate complex prostheses [43]. Using 3D computer modeling to manufacture more accurate and precise fitting patient specific titanium plates are developed and reported in the literature [44]. However, the 3D computer modeling and 3D printing are expensive and cannot provide a final net dimensions of irregular and complex shape. Numerous techniques have also been described for the reconstruction of the orbit. However, the complex contouring between the orbital roof and frontal bone creates significant design challenges for prefabricated cranio-orbital reconstructions [45].

In the current work the authors suggested that; the addition of Mo as beta phase stabilizer and Zr as alpha and beta phases strengthening to Al₂O₃/Ti composites can improve the interfacial diffusion bonding between the composite constituents during the fabrication process by SPS at high temperature and then improving its properties. Spark plasma sintering (SPS) is belonged to the powder technology processes used for the production of final net dimension and finished complex and irregular shape parts by consolidations of the desired powders and fibers constituents reinforced composites. SPS and powder technology is a promising alternative method for the manufacturing of Ti base biomaterials parts with precise dimensions and complex shape suitable for cranial reconstruction with low production coast [39, 45–47].

In the recent decades, the nanomechanical measurements such as nanoindentation, have been developed as an alternative technique to the conventional destructive testing techniques such as hardness, wear, compression and tensile test, because of it is fast evaluations dealing with specimen of small size [48–50]. It was reported in the literatures that; the mechanical response at the nano scale are varied from the micro and the bulk scale response determined by means of conventional tests such as hardness, tensile and compression tests [51]. The hardness values estimated by the nanoindentation method is closely related to the plastic deformation of the surface and other mechanical properties of materials such as ductility and fatigue resistance [52].

To decrease the expected cytotoxic effect of Al and V in Ti–6Al–4 V alloy; in the present study Mo and Zr are used to replace Al and V as low toxic metals can improve the tensile properties of the Ti biomaterial applications. In the current work, Mo as a low toxic β phase stabilizer and Zr were combined with Ti to produce new type of biocompatible material for cranioplasty and cranial reconstruction applications. This replacement was occurred to improve the nanomechanical properties of the produced composites for its application in Cranial parts. The materials in this work are designed based on the modification of the composition of the β-type Ti-12Mo-6Zr-2Fe (TMZF) for orthopedic use according to the ASTM F1813, UNS R58120 standard by replacing the iron content by the addition of up to 5 wt.% of alumina short fibers. This alloy has a desirable combination of properties, i.e., high modulus of elasticity, excellent mechanical strength, corrosion resistance, and formability, coupled with good wear and notch fatigue resistance. Also, no vanadium and aluminum in this alloy offer a biocompatibility advantage over the Ti-6Al-4V alloy. The processing development, metallurgical characteristics, physical and mechanical properties, and wear resistance of this new alloy are reported here [53]. On the other hand; the effect of addition of Al₂O₃ short fibers as a one dimension reinforcement phase on the nanomechanical properties of the sintered composites is also studied. Ti-12Mo-6Zr composites with different Al₂O₃ short fibers content up to 5 wt.% were fabricated using conventional mechanical milling of its powder constituents followed by consolidation with SPS processes to improve the composite materials properties. The microstructure, phase identifications, chemical composition and nanoindentations of the produced Ti-12Mo-6Zr as well as the Al₂O₃ short fibers/Ti-12Mo-6Zr composites are investigated to evaluate its performance as new composite materials used for orthopedic applications. The sintered density was measured by the Archimedes method to evaluate the sinterability of the produced sintered materials. The Hardness and the Young’s modulus were estimated from the load/displacement in depth curves of the obtained data of the nanoindentation measurements.

Materials and methods

Materials

Mo powder of 1–2 μm particle size (Sigma-Aldrich Chemie GmbH, Germany), Zr powder of 200–500 nm particle size (SE-Jong Materials LTD, South Korea), Ti powder of 50 μm particle size (Starmet Co.LTD, South Korea) and Al₂O₃ short fibers of ~18 μm diameter and ~600 μm length (Böhler Co. LTD, Germany) were used as powder constituents for preparing Al₂O₃ short fibers/Ti-12Mo-6Zr composite materials. Different Al₂O₃ short fibers content of 0, 1, 2.5, and 5 wt.% were reinforced the Ti-12Mo-6Zr composite to fabricate four Al₂O₃ short fibers /Ti-12Mo-6Zr sample series of different composition.

Methods

Fabrication of Al₂O₃ short fibers /Ti-12Mo-6Zr Composites by SPS

The calculated wt.% of the Al₂O₃ short fibers_, Ti, Mo and Zr powder constituents for the different samples compositions of Al₂O₃ short fibers /Ti-12Mo-6Zr were adjusted and were weighed using a four-digit balance. Table 1 listed the exact composition of all the prepared samples. Then the powder constituents undergo wet mixing with ethanol as milling processing media in a ball grinding machine of 40 rpm milling speed for 48 h using 5 mm diameter alumina ceramic balls and the powder to balls and ethanol ratio were adjusted at 1:1:1 according to our preliminary studies. The obtained powders are collected after milling then dried under vacuum at 60 ^oC for 2 h to evaporate any remained ethanol and any trapped gasses in the voids inside the powder. Figure 2 shows a schematic diagram of fabrication process of the Al₂O₃ short fibers /Ti-12Mo-6Zr Composites by SPS. The collected powder mixtures were sintered by spark plasma sintering (SPS) using Dr. SINTER.LAB apparatus to enable the formation of a fully dense products with net-shape which can be occurred by passing high pulsed electric current through the compacted powders using the joule heat effect. Highly pure graphite die of 15 mm diameter was used for uniaxial pressing of the samples to produce a 5-mm thick sample for each composition of the Al₂O₃ short fibers /Ti-12Mo-6Zr Composites with different Al₂O₃ content up to 5 wt.%. The sintering process started by passing a high pulsed electrical current through the powdered sample into the graphite die. The graphite dies and its contents are heated up due to the passed current and the sintering process is started in few seconds. The Al₂O₃ short fibers /Ti-12Mo-6Zr Composite powders of all the produced samples mixtures were sintered at 1000 ^oC, by adjusting a pulse duration of 2.8 ms, holding time of 5 min., heating rate of 100 ^oC/min, and a compaction pressure of 50 MPa simultaneously under maintaining the vacuum at 10^-3 torr during the sintering process of all the sample mixtures.

Table 1.

Composition of fabricated short fibers Al O /Ti-12Mo-6Zr composite samples

Sample No.	Sample Composition	Ti (wt.%)	Mo (wt.%)	Zr (wt.%)	Al2O3 (wt.%) Short fibers
1	Ti-12Mo-6Zr	82.00	12.00	6.00	00.00
2	1 wt.% short fibers Al2O3 /Ti-12Mo-6Zr	81.18	11.88	5.94	1.000
3	2.5 wt.% short fibers Al2O3 /Ti-12Mo-6Zr	79.95	11.70	5.85	2.500
4	5 wt.% short fibers Al2O3 /Ti-12Mo-6Zr	77.90	11.40	5.70	5.000

Fig. 2.

Schematic flowchart of the fabrication process of the Al₂O₃ short fibers/Ti-12Mo-6Zr composites

Metallographic investigations and XRD Phase identifications

The obtained Ti-12Mo-6Zr as well as the Al₂O₃ short fibers /Ti-12Mo-6Zr composites sintered samples by SPS underwent mounting in cold resin and grinding using 800, 1000, 1200, 2000, 3000, and 4000 grade silicon carbide papers respectively. The samples were polished using 3 μm diamond paste. The polished samples were etched by a freshly prepared Keller etching reagent. The microstructures of the polished samples were investigated by polaroid optical microscope and high resolution field emission scanning electron microscopy (FESEM) connected with energy-dispersive X-ray spectroscopy (EDS) unite of model SEM-XL30SFEG. The different phase identifications and chemical compositions of the obtained sintered samples were investigated by using X-ray diffractometer (XRD) of the model; D/MAX IIIC, Rigaku.

Density measurements

The theoretical densities of the Ti-12Mo-6Zr as well as the Al₂O₃ short fibers /Ti-12Mo-6Zr composites were calculated by the rule of mixtures (ROM) using the following equation,

$$\begin{array}{c}\mathrm{Density}={\mathrm{\rho }}_{\mathrm{Ti}}*{\mathrm{V}}_{\mathrm{Ti}}+{\mathrm{\rho }}_{\mathrm{Mo}}*{\mathrm{V}}_{\mathrm{Mo}}+{\mathrm{\rho }}_{\mathrm{Zr}}*{\mathrm{V}}_{\mathrm{Zr}}\hfill \\ +{\mathrm{\rho }}_{\mathrm{Al}2\mathrm{O}3}*{\mathrm{V}}_{\mathrm{Al}2\mathrm{O}3}\hfill \end{array}$$ 4

Where ρ _Ti, ρ _Mo, ρ _Zr, ρ _Al2O3 are the densities of the Ti, Mo, Zr and Al₂O₃ respectively however, V_Ti, V _Mo, V _Zr, V_Al2O3 are the volume fraction of the Ti, Mo, Zr and Al₂O₃ respectively.

The actual sintered densities of the produced Ti-12Mo-6Zr as well as the Al₂O₃ short fibers /Ti-12Mo-6Zr sintered samples by SPS, were measured by Archimedes’ principle according to MPIF standard 42 (1998) using water as a floating liquid and the actual density was calculated by the following equation,

$$\mathrm{Density}\left({\mathrm{\rho }}_{\mathrm{r}}\right)={\mathrm{W}}_{\mathrm{air}}/\left({\mathrm{W}}_{\mathrm{air}}-{\mathrm{W}}_{\mathrm{water}}\right)$$ 5

where, W_air and W_water are the weight of specimen in air and in double distilled water, respectively.

The relative densities of the sintered samples are calculated by dividing the ratio of the actual density ρ_r that obtained by Archimedes’ method on the theoretical density ρ _th by using the follows equation,

$${\mathrm{\rho }}_{\mathrm{relative}}={\mathrm{\rho }}_{\mathrm{r}}/{\mathrm{\rho }}_{\mathrm{th}}$$ 6

Nanomechanical properties and nanoindentations measurements

The produced Ti-12Mo-6Zr as well as the Al₂O₃ short fibers /Ti-12Mo-6Zr composites sintered samples underwent nanoindentation measurements by Digital Instrument of nanoindentation test system under ambient conditions using CSM (continuous stiffness measurement) methods in the national nanofab center of KAIST. Indentions were located carefully by surface imaging and the indenter was always applied at new places with no irregular deposits. A Berkovich tip of half angle 65.03^o was used in this study. The Berkovich tip penetrated deeply into the specimen under investigations and induced plastic deformation by applying a proper measuring load corresponding to the displacement in depth. The hardness and elastic modulus were estimated from the obtained loading-unloading against the displacement in depth curves. Poisson’s ratios for the samples was 0.33 and the Young’s modulus were evaluated. The applied loads were changes and adjusted varied from 2.5 mN to 560 mN with the strain rate and set at a constant value of 0.05 S^-1. of maximum varied from 2.5 mN to 560 mN.

Results and discussion

Powder Investigations

Figure 3a–h shows high resolution FESEM images with EDAX elemental composition analysis of all investigated powders used in the fabrication process of the Ti-12Mo-6Zr as well as the Al₂O₃ short fibers/Ti-12Mo-6Zr composites samples with different Al₂O₃ short fibers content. It was observed from the results that; the Al₂O₃ short fibers have a cylindrical shape of diameter 18–20 µm and 600–620 µm length (see Fig. 3a, b). Mo powder have irregular particle shape with particle size of 1–2 µm (see Fig. 3c) and the Zr particles have particle size of 200–500 nm of pseudo spherical particle shape (see Fig. 3d). The Ti particles have particle size of ~50 µm with spherical particle shape (see Fig. 3e). Figure 3f–j shows the FESEM image with low and high magnifications and the EDAX elemental compositional spot analysis of different regions for the produced Al₂O₃ short fibers/Ti-12Mo-6Zr mixture. It was observed from the results that; the Al₂O₃ short fibers are homogenously dispersed with the Ti, Mo and Zr metallic constituents (see Fig. 3g). The Ti particles are coated and encapsulated with the metallic nanoparticles of Mo and Zr alloying elements due to the particles impact and shearing during the high strain ball milling as shown in the high magnification FESEM images in Fig. 3h. The EDAX compositional analysis of the milled powder revealed that it is composed mainly of elemental Mo, Zr, Ti in addition to Al and O for the elemental constituents of the Al₂O₃ short fibers. Also, there is no foreign peaks observed owing that, the high purity of the used powders under investigations in the preparation process.

Fig. 3.

FESEM of the investigated powders; a, b Al₂O₃ short fibers, c Mo, d Zr, e Ti, f–h mechanically alloyed powdered material after ball milling, i, j EDAX elemental composition analysis produced powders of Al₂O₃ short fibers/Ti-12Mo-6Zr and Ti-12Mo-6Zr composites after wet milling in ethanol for 24 h

Phase identifications and microstructure investigations

Figure 4a–d shows XRD patterns of the as sintered Ti-12Mo-6Zr as well as the 1, 2.5 and 5 wt.% Al₂O₃ short fibers/ Ti-12Mo-6Zr composite by SPS at 1000 ^oC respectively. It was observed from the results of the XRD pattern presented in Fig. 4a that; two types of peaks are detected. The first type of peaks represented the presence of the α phase which corresponding to the Ti2Zr phase according to the # 65-4534 however, the second type of peaks indicating the formation of the Mo0.093Zr0.07 phase according to the # 00-049-1676 due to the rapid cooling of the sample by flashing with Argon atmosphere after the SPS process [39, 54]. It was reported in the literature that; the crystal structures of the Ti alloys are sensitive to the alloying elements like as well as the quenching rate after the sintering process [39, 55–57]. On the other hand, Fig. 3b–d shows XRD pattern of the as sintered 1, 2.5, and 5 wt.% Al₂O₃ short fibers/ Ti-12Mo-6Zr composite by SPS at 1000 ^oC. It was observed from the results that; additional third type of peaks corresponding to the crystalline phase composed of Al6MoTi types of peaks are indicated according to # 00-065-7596. The presence of these peaks is due to the interaction between the Al₂O₃ short fibers and the reactive metals of the Ti-Mo-Zr matrix at high temperature during the sintering process by SPS. The formation of Al6MoTi phase can help in the diffusion bonding in the interface between the Al₂O₃ and the Ti-12Mo-6Zr matrix [58, 59]. Also, another type of peaks is observed due to the contribution of the Al₂O₃ short fibers of the corundum phase according to the # 00-010-01173 [39, 58, 60]. The presence of the Al₂O₃ short fibers in the Ti matrix can improve the corrosion resistance of the produced biomaterials. It is reported in the literature that; the combination of metal oxides with the Ti matrix confers resistance to corrosion and offers acceptable biological properties. The surface oxide films are mainly consisting of stable oxides like TiO₂, Al₂O₃, Zr₂O₅ and MoO₃. These types of oxides assure the insolubility in the human body fluids, which facilitates the biocompatibility for the prepared Ti biomaterials and also associated with improved cell proliferation [61].

Fig. 4.

XRD patterns of the sintered samples by SPS samples at 1000 ^oC; where a Ti-12Mo-6Zr and b 1 wt. % Al₂O₃ short fibers /Ti-12Mo-6Zr, c 2.5 wt. % Al₂O₃ short fibers /Ti-12Mo-6Zr and d 5 wt. % Al₂O₃ short fibers /Ti-12Mo-6Zr composites

An extensive experimental work was conducted to prove the sintering mechanism during the spark plasma sintering process of the fabricated composite materials. SEM images and EDAX compositional analysis were presented in Fig. 5. It was observed from the results that; the sintering process occured by solid state sintering starting in the first stage with surface diffusion of particles and neck growth (Fig. 5a, b) followed by bulk diffusion and pore formations in the second stage (Fig. 5c, d) and finally consolidation and pore eliminations. It was also revealed from the results that the Mo and Zr metal are start to diffuse on the surface of the Ti particles during all the stages of the sintering process thus enhancing the sintering process with the spark plasma sintering.

Fig. 5.

FESEM images with secondary electron (a, c, e, f), back scattering modes (b, d) and EDAX elemental compositional analysis (g, h) of the sintering stages of the prepared Ti-12Mo-6Zr composites by SPS at 1000 ^oC and 50 MPa for 5 min

Figure 5g shows the EDAX analysis of the Ti particles during the first stage of the sintering. It was observed that the particles composition is mainly Ti metal. However, Fig. 5h shows the EDAX compositional analysis of the particles after surface diffusion; it was clear that; the surface of the particles composed of the metal matrix is mainly contains Ti, Mo and Zr metals. It is proved that; the sintering process is enhanced by the diffusion of the Mo and Zr nanoparticles into the surface of the Ti particles.

The ceramic matrix composites such as oxides, carbides and nitrides reinforced metal matrix composites are very difficult to sinter using conventional sintering technique because of its very strong covalent bonds in their crystal structures and extremely low atomic diffusivity. However, the addition of metals content as matrix and sintering by SPS is a novel sintering technique for the densification of ceramic composites. SPS unlike the conventional hot-pressing and hot-isostatic pressing, which require heat generated by an electric current passing through the heating elements, SPS does not require any heating elements. Instead, SPS generates heat by passing a high-pulsed direct current through the conductive graphite mold and the powder of the compacted sample using the joule heat effect. The SPS process heats the compacted powder by the pulse arc discharges, thus achieving very high thermal efficiency. As a result, material densification by SPS is generally is occurred very fast within a few minutes at lower sintering temperature than the conventional sintering. The sintering process is assisted by applying compaction pressure, which enhance the plastic flow of the material as well as the generated plasma, and accelerate the sintering process [62].

Figure 6a–d shows optical microscope images with high and low magnifications of the microstructure of the as prepared Ti-12Mo-6Zr and 1 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composites sintered by SPS at 1000 ^oC after etching with Keller’s reagent. It was revealed from the results that; Widmannstatten-like microstructure is observed and the Al₂O₃ short fibers reinforcement phase are homogenously distributed in the Ti-Mo-Zr metal matrix. [39, 40]. It was reported in the literature that, the α phase is initially forms at grain boundaries of the β phase. Grain boundary or triple point α is invariably related to one or more of the adjacent grains. At low levels of β stabilizer (Mo) addition and high transformation temperatures, a grows as widmanstatten sideplates from grain boundary α into the β grain with which the grain boundary α has consume the entire grain. This colony structure consists of identical, parallelly oriented variants of the α phase with thin ribs of retained β. At lower transformation temperatures or with higher β stabilizer content, transgranular α begins to dominate the structure [63–65]. Also, it is observed from the microstructure that; some Al₂O₃ short fibers are aligned vertically and some are aligned in tilted directions in addition some fibre fragments are also detected due to the milling process of the powdered samples before sintering by SPS which can shortening the Al₂O₃ short fibers [45]. Figure 7 shows the relationship between the weight % and volume % of the Al₂O₃ content in the Al₂O₃ short fibers/Ti-12Mo-6Zr composites. An increase in weight percentage led to a corresponding increase in volume percentage. The obtained data of the volume % was used to calculate the theoretical density valued as represented by the rule of mixture in Eq. (1) [60].

Fig. 6.

Optical micrographs with low and high magnifications of the prepared a, b Widmannstatten-like microstructure of Ti-12Mo-6Zr, c, d 1 wt. % Al₂O₃ short fibers/Ti-12Mo-6Zr sintered composite by SPS at 1000 ^oC and 50 MPa for 5 min

Fig. 7.

Optical image analysis and the relationship between the wt.% and vol.% of the Al₂O₃ short fibers dispersed phase (white color) in the Ti-12Mo-6Zr matrix (black color) of the Al₂O₃ short fibers/Ti-12Mo-6Zr sintered composite

Figure 8 present high resolution images by FESEM and EDAX elemental composition analysis of the as sintered Ti-12Mo-6Zr and 5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composites by SPS at 1000 ^oC after etching with Keller’s reagent. It was observed from the results that; Widmannstatten-like microstructure of the produced Ti-12Mo-6Zr biomaterial was formed (see Fig. 8a–d). Also, Fig. 8 and f shows high resolution FESEM images with low and high magnifications of the interface regions of the sintered Ti-12Mo-6Zr biomaterial. It was observed from the results that an interfacial metallurgical bonding is formed between the different constituents Ti-12Mo-6Zr biomaterial by the sintering process with the SPS in this short sintering time of 5 min. The reason behind the formation of the interface regions between the different phases in the sintered Ti-12Mo-6Zr biomaterial is the unequal diffusion current from the beta titanium phase which contains the Mo and Zr alloying elements to the commercial Ti matrix. In other words, when the Ti, Mo and Zr metals particles in contact with each other during the SPS process for a certain time, the diffusion processes will be enhanced due to the difference in chemical potential based on the mass transport in the solid state which is initiated in the grain boundaries. The activation energy for the diffusion through the grain boundaries is smaller as compared with the activation energy for bulk diffusion which will be occurred much earlier than that through the bulk of the grain [66–68]. EDAX analysis shows three type of high intense peaks represented the presence of the constituents of the sample corresponding to Mo and Zr and the balance with Ti in case of Ti-12Mo-6Zr (see Fig. 8e, f). Figure 9a–e represents the microstructures of the 5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composite with different magnifications. it was revealed from the microstructure of the prepared composites that, the Al₂O₃ short fibers are homogeneously distributed in the Ti-Mo-Zr metal matrix. Also, the microstructure of the Al₂O₃ short fibers/Ti-12Mo-6Zr composites clearly show consisting lamellar eutectics discontinuously distributed on the metal matrix (see Fig. 9a–c). The formation of the Al6MoTi dendrite phase is also observed as represented in Fig. 9f [59]. Figure 9g shows High magnification SEM images of the interface between the Al₂O₃ short fibers and the metal matrix of the 5 wt. % Al₂O₃ short fibers/Ti-12Mo-6Zr composites. It was observed from the results that, a good adhesion of interfacial bonding between the Al₂O₃ short fibers and the metal matrix. The reason behind the interfacial bonding and the adhesion is due to elemental interaction during SPS processing’ of the Al₂O₃ short fibers with the reactive Ti-Mo-Zr metal matrix during the SPS process by passing the current through the sample constituents generating the temperature to 1000 ^oC and forming the new phase of Ti2ZrAl which is useful in enhancement the driving force of the diffusion in the interface between the Al₂O₃ and the Ti-Mo-Zr matrix as shown from the diffraction pattern presented in Fig. 4d [69]. The EDAX analysis of the 5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composite sample (see Fig. 9h) shows four type of high intense peaks represented the presence of the constituents of the sample corresponding to Mo and Zr, Al and the balance with Ti. These results revealed the high purity of the powdered used in the fabrication process of the sintered materials without formation of any foreign oxides or other different phases of intermetallic particles [70–72].

Fig. 8.

a–f FESEM microstructure images with low and high magnifications, g, h EDAX elemental compositional analysis of the Widmannstatten-like microstructure of the prepared Ti-12Mo-6Zr biomaterial by SPS at 1000 ^oC and 50 MPa for 5 min

Fig. 9.

a, b FESEM microstructure images of 1 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr and 2.5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composites, (c–e) the 5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composites with different magnifications, f the Al6MoTi dendrite phase, g high magnification of the interface between the Al₂O₃ short fibers and the metal matrix and h the EDAX elemental analysis of the 5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composites prepared by SPS at 1000 ^oC and 50 MPa for 5 min

Density and shrinkage of sintered materials

The optimum sintering conditions by SPS at 50 MPa compaction pressure and 1000 ^oC sintering temperature of the prepared Ti-12Mo-6Zr as well as the Al₂O₃ short fibers/Ti-12Mo-6Zr composites samples were determined in preliminary studies. All the samples started sintering and stopped shrinking within 5 min from the beginning of the process. Table 2 lists the relative sintered density calculated from the obtained data of the theoretical density from the ROM and the actual density estimated by the Archimedes method. It was revealed that; The relative sintered density of the Ti-12Mo-6Zr material was ~99.5% and by increasing the reinforced Al₂O₃ short fibers content; the relative sintered densities decreased to 98.4%, 98.2%, and 98.1%, for Ti-12Mo-6Zr reinforced with 1, 2.5, and 5% (w/w) Al₂O₃ short fibers content respectively. The reason behind that is the combination of the low density of Al₂O₃ short fibers (3.99 g/cm³) with the higher density Ti, Mo and Zr metals decreasing the overall density of the obtained Al₂O₃ short fibers/Ti-12Mo-6Zr composites. Also, by increasing the Al₂O₃ short fibers content increasing the particle-particle interaction between the Al₂O₃ short fibers dispersed phase in the metal matrix and enhancing the formation of the pores and subsequently decreasing the sinterability and the density of the materials.

Table 2.

Theoretical, actual and relative densities of the Al₂O₃ short fibers/Ti-12Mo-6Zr sintered composites

Sample Composition	Theoretical Density, g/cm3	Archimedes Density, g/Cm3	Relative Density, %
Ti-12Mo-6Zr	5.05	5.02	99.50
1 wt.% short fibers Al2O3/Ti-12Mo-6Zr	5.03	4.95	98.40
2.5 wt.% short fibers Al2O3/Ti-12Mo-6Zr	5.02	4.93	98.20
5 wt.% short fibers Al2O3/Ti-12Mo-6Zr	4.99	4.90	98.10

Nanoindentation and nanomechanical properties

Figure 10 shows the relationship between the applied load against the displacement in depth of nanoindentation on different areas of the prepared Ti-12Mo-6Zr as well as the Al₂O₃ short fibers/Ti-12Mo-6Zr composites samples. It was observed from the obtained data that; each curve consists of three parts, the loading part, the dwell time part through the maximum loading, and the unloading part. It was reported in the literature that; the load-displacement in depth curve depends on the environmental conditions during nanoindentation testing, the applied load, the chemical composition and crystal structure of the material composed the sample under investigations. In general; materials of different chemical composition and different crystal structure possess different shapes of load/displacement in depth curves [73]. By computing of the maximum displacement in depth of the investigated samples, the zero point is determined from the position where the load increases sharply, as shown from the curves represented from Fig. 10. Also; the curve profile shows that; the material performs in both elastic and plastic mode and the maximum displacement increases gradually with the increase in depth through the sample, consistent with the observed indentation size and can indirectly reveal the decrease of hardness. The average displacement in depth of the investigated samples increases through the loading phase by increasing the applying load, which agrees with the variation of hardness as presented in Fig. 11. Also, the displacement in depth of the investigated samples reached the lowest value in case of Ti-12Mo-6Zr sample and increase gradually by increasing the Al₂O₃ content to reach the highest value in case of the 5 wt.%Al₂O₃ short fibers/Ti-12Mo-6Zr composite. The reason behind that is by increasing the Al₂O₃ content in the Ti-Mo-Zr metal matrix a decreasing in the displacement of the indenter in the depth of the sample during the loading phase is observed due to the contribution of the Al₂O₃ short fibers hard phase which reinforced the Ti-Mo-Zr metal matrix [74].

Fig. 10.

Applied Load–displacement in depth of the prepared Ti-12Mo-6Zr and Al₂O₃ short fibers/Ti-12Mo-6Zr composites by SPS at 1000 ^oC and 50 MPa for 5 min

Fig. 11.

Estimated indentation hardness values derived from the applied load–displacement in depth of the nanoindentation measurements for the prepared Ti-12Mo-6Zr and Al₂O₃ short fibers/Ti-12Mo-6Zr composites by SPS at 1000 ^oC and 50 MPa for 5 min

The estimated hardness values derived from the obtained data of the nanoindentation measurements for the fabricated Ti-12Mo-6Zr and Al₂O₃ short fibers/Ti-12Mo-6Zr composites by SPS are shown in Fig. 10. It is observed from the results that; there is a systematic increase in hardness with an increase in the content of the Al₂O₃ reinforcement short fibers in the Ti-Mo-Zr metal matrix. The 5 wt.%Al₂O₃ short fibers/Ti-12Mo-6Zr composites curve shows the maximum hardness values. The reason behind the improvement in the hardness is due to the load transfer from the Ti-Mo-Zr metal matrix to the reinforced corundum Al₂O₃ short fibers which have high hardness properties than the Ti-Mo-Zr metal matrix itself. It is also depending on the orientation and the alignment of the Al₂O₃ short fibers as one dimension particles which can adhere and increase the sinterability of the sintered materials by enhancing the formation of the new Ti2ZrAl phase (see Figs. 4b and 5c, d) in the interface rim areas between the Al₂O₃ short fibers and the Ti-Mo-Zr metal matrix which can increase the hardness by increasing the addition of the Al₂O₃ short fibers [75–78]. However, the hardness at the outermost surface increased with the addition of 1 wt.% Al₂O₃ in the Ti-12Mo-6Zr substrate. Furthermore, this phenomenon was not observed in the 2.5 wt.% and 5 wt.% Al₂O₃/Ti-12Mo-6Zr composites. It may be due to the formation of some porosity content which increase by increasing the Al₂O₃ short fibers in the Ti-Mo-Zr metal matrix. It was reported in the literature that; the mechanical strength of Al₂O₃ ceramics used for hip implants, so far, experiences with ossicular prostheses made from Al₂O₃ have shown that ceramic materials have good biocompatibility and mechanical strength [77, 79] as listed in Table 3.

Table 3.

Estimated indentation hardness and modulus of elasticity of the Al₂O₃ short fibers/Ti-12Mo-6Zr sintered composites

Samples Composition	Estimated indentation Hardness, GPa	Estimated Indentation Modulus, GPa
Ti-12Mo-6Zr	7.34	137.11
1 wt.%Al2O3 /Ti-12Mo-6Zr	8.03	138.10
2.5 wt.%Al2O3 /Ti-12Mo-6Zr	8.57	140.32
5 wt.%Al2O3 /Ti-12Mo-6Zr	9.91	159.76

The estimated elastic modulus values derived from the Load–displacement in depth of the nanoindentation measurements for the fabricated Ti-12Mo-6Zr and Al₂O₃ short fibers/Ti-12Mo-6Zr composites by SPS are shown in Fig. 12. The modulus of elasticity is a very important criterion underlying the choice of metallic materials used in orthopedics and should be as close as possible to that of the human bone (17–30 GPa). It was revealed from the results of the current work that; a progressive increase in the modulus of elasticity is observed by increasing in the Al₂O₃ short fibers reinforcement phase content in the Ti-Mo-Zr metal matrix of the produced Al₂O₃ short fibers/Ti-12Mo-6Zr composite materials. It was due to the Al₂O₃ short fibers reinforcement serve as barriers to stop any crack formation and propagation during using the composite material as implanted parts of orthopedic applications. Also, the modulus at the outermost surface increased with the addition of 1 wt% Al₂O₃ in the Ti-12Mo-6Zr substrate. Furthermore, this phenomenon was not observed in the 2.5 wt.% and 5 wt% Al₂O₃/Ti-12Mo-6Zr composites. It is may be due to the formation of some porosity content which increase by increasing the Al₂O₃ short fibers in the Ti-Mo-Zr metal matrix. In addition; it was observed that; the 5 wt.% Al₂O₃ short fibers/Ti-12Mo-6Zr composite sample shows the maximum elastic modulus among all the prepared Al₂O₃ short fibers/Ti-12Mo-6Zr. Also, the obtained modulus/displacement curves are shifted towards the left. The reason behind that is due to load sharing capability by the contributions between the Al₂O₃ short fibers reinforcement phase and the Ti-Mo-Zr metal matrix [73–81]. It was reported in the literature that; the incorporating of reinforcing materials, such as fibers, into the composite matrix can enhance its resistance to crack initiation and propagation. The reinforcements help distribute the stress and prevent cracks propagations. In laminated composite materials, introducing toughened layers between the laminae helps prevent the propagation of cracks between adjacent layers. Also; adding of metal oxide nanoparticles to the composite material can improve its mechanical properties, including resistance to crack propagation by the oxide dispersing strengthening mechanism. Then fibers and nanoparticles reinforcements can act as barriers and hinder crack growth [82, 83]. As presented in Table 4 the modulus of elasticity of the currently used orthopedic biomaterials are so far matched with the estimated indentation modulus of elasticity values of the fabricated Al₂O₃ short fibers/Ti-12Mo-6Zr sintered composites. Also, due to Mo and Zr stabilizing elements, these composites have the advantage of increasing mechanical strength and a modulus of elasticity close so fare to that of the biological bone.

Fig. 12.

Estimated indentation modulus of elasticity values derived from the applied load–displacement in depth of the nanoindentation measurements for the prepared Ti-12Mo-6Zr and Al₂O₃ short fibers/Ti-12Mo-6Zr composites by SPS at 1000 ^oC and 50 MPa for 5 min

Table 4.

Modulus of elasticity of the currently used orthopedic biomaterials and the fabricated Al₂O₃ short fibers/Ti-12Mo-6Zr sintered composites

Material	Fabrication Process	Elastic modulus (GPa)	References/Standard
Human bone	Natural	2–23	[105]
Stainless Steel 316 L	Cast & heat treated	200	[106]
Co based alloys	Cast & heat treated	240	[106]
Ti-5Al-2Sn-2Zr-4Mo-4Cr	Cast	112	[107]
Ti-11.5Mo-6Zr-4.5Sn	Cast	83–103	[108]
Ti-6Al-4V	Cast & annealed	110–114	ASTM F1472, UNS R56400
Ti-12Mo-6Zr-2Fe	vacuum melting	135	ASTM F1813, UNS R58120
Ti–15Mo	Cast & annealed	78–84	ASTM F2066, UNS R58150
Ti-15Mo-3Nb-3Al-0.2Si	Cast & annealed	72–85	ASTM F2066, UNS R58210
Ti-5Zr-3Mo-6.5Sn	Sintering	19	[108]
Ti-5Zr	Melting & swaging	86	[109]
Ti-15Zr	Melting & swaging	112	[109]
Ti-7Mn	Powder metallurgy	96	[110]
Ti-7Mn-10Nb	Powder metallurgy	87	[110]
Ti-63Zr-2Mo-2.4 Mn	Arc-melting	36	[111]
Ti–15Zr–10Cr	Arc melting	78	[112]
Ti-15Zr-15Mo	Hot-rolled	70	[113]
Ti-35Nb-7Zr-13Cu	SPS	79	[114]
Ti-35Nb-7Zr-4Cu	SPS	57	[114]
Ti-12Mo-6Zr	SPS	137	Current work
1 wt%Al2O3 /Ti-12Mo-6Zr	SPS	138	Current work
2.5 wt%Al2O3 /Ti-12Mo-6Zr	SPS	140	Current work
5 wt%Al2O3 /Ti-12Mo-6Zr	SPS	159	Current work

In addition to hardness and elastic modulus, nanoindentation is also useful to determine other parameters that can be used to predict the service life of a component [84]. Classical theories of wear show that hardness is a key parameter in controlling wear resistance of a material and usually a hard material possesses a high wear resistance [85]. Actually, it is increasingly realized that the wear of a material is related to its ability to resist elastic strain to failure which can be monitoring by the ratio of the hardness/Contact depth at the maximum load. At high hardness/Contact depth at the maximum load ratio indicates better wear resistance of the material under investigation [86]. Consequently, this parameter can be used to gauge the anti-wear ability of materials, as the wear caused by the gradual removal of material is associated with plastic deformation [85]. The higher wear resistance can lead to more reduction in release of wear debris which can prevent loosening of the implant and the subsequent need for revision surgery [87]. Also, it was reported in the literature that, Al₂O₃ is one of the excellent bio-inert materials possesses neither osteoconductive nor osteoproductive properties. It can be used in the reconstruction of the anterior skull base in cerebrospinal fluid leakage repair and in the manufactures of ceramic plates [88]. The combinations of Ti with ceramic materials like Al₂O₃ might be taking into consideration as load bearing material for orthopedic applications [26]. The aim of the manufacturing of biocompatible composite materials is the combination of the opposing osteoconductive and the mechanical properties. There are manifold possibilities of composite materials reported in the literature, some examples being: ceramic such as Al₂O₃, silicon nitride (Si₃N₄), hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), and metallic materials such as Ti, Ta, Zr and Nb which possesses non-cytotoxic, non-allergic and biocompatible properties [89]. It was observed in our previous work that; the corrosion resistance against the simulated saliva fluid was reduced significantly by the addition of 5 wt.% Al₂O₃ reinforcement particles to the Ti–12Mo metal matrix. Also by increasing the Al₂O₃ content resulted in decreased the ion concentration released in the simulated saliva fluid due to the high chemical stability of the Al₂O₃ reinforcement particles [90]. Therefore, it is necessary to conduct more thorough and uniform assessment of the corrosion and cytotoxic effects with biological body fluids with the produced Al₂O₃ /Ti-12Mo-6Zr composites in order to reach out more detailed answers of the chemical as well as biological remarks. To the best of our knowledge, our study is the first to reveal that Al₂O₃ has a strong ability to enhance the mechanical properties of the Ti-12Mo-6Zr biomaterials as load bearing materials. Furthermore, this study is the first to validate the impact of combining Al₂O₃ with Ti-12Mo-6Zr in enhancing the estimated hardness and modulus of elasticity by nanoindentation for orthopedic applications. On the other hand, We expect that designing a porous structure will improve permeability which is conducive to the transport of nutrients of the human body. By adjusting the pore size and porosity, porous scaffolds with low elastic modulus, equivalent to that of human bone, and good permeability can be expected to obtain by the SPS process at low sintering temperature and shorter sintering time [90–99].

In summary, the current study succeeded in the fabrication and characterization of new type of biomaterials composed of Ti-12Mo-6Zr as well as Al₂O₃ /Ti-12Mo-6Zr as titanium osteosynthesis material. Mo and Zr are used as additive alloying metals to modify the common used orthopedic Ti-6Al-4V alloy by replacing Al which is Alzheimer’s disease causing metal and the V which is a systemic toxic metal. Also, avoiding using Al because aluminum ions, may accumulate in cases of impaired renal function and act neurotoxic. The properties of the new materials are matched with the properties of the common used orthopedic biomaterials such as Ti-6Al-4V and Ti-15Mo which had the best resistance to corrosion towards a solution-based phosphate buffered saline, and the best alloy combination with remarkable corrosion and sliding wear resistance [97–99]. The addition of the Al₂O₃ short fibers to the titanium alloys expected to improve the surface properties of the produced biomaterials for orthopedic and cranial applications. It was reported in the literature that; the ratio of bone to implant surface contact after implantation operation can affect on the life time of the implant component. The percentage of bone tissue to the implant surface contact depends on various parameters including the quality and size of the bone area where the part is implanted, the geometric design of the implant, the loading conditions, and the implant surface properties. The topography, chemical properties, surface charge, and hydrophilic nature of implant surfaces have been found to be vital factors for optimal bone/implant contact. All these factors can affect the adsorption of proteins at the titanium implant surface, the opposition of osteoblastic cells, and the development of newly formed bone on the titanium implant surface [90–104].

Conclusions

The current work study the effect of the addition of biocompatible and none toxic Mo, Zr and Al₂O₃ short fibers of various content on the microstructure and the nanomechanical properties of Ti-12Mo-6Zr as well as Al₂O₃ short fibers/Ti-12Mo-6Zr composites fabricated by spark plasma sintering process for orthopedic and cranial applications. Ti base implants of osteosynthesis material are safe and useful for a long period of time with relatively few complications for orthopedic and cranial reconstruction applications. The microstructure as well as the phase identifications of the prepared composite materials are investigated in correlation with its estimated hardness and modulus of elasticity derived from the load/ displacement in depth relationship by the nanoindentation measurements. The Al₂O₃ short fibers are homogeneously distributed and adhered with the Ti-Mo-Zr matrix by the formation of the Ti2ZrAl phase in the interface of the core rim regions between the Al₂O₃ and the Ti-Mo-Zr matrix of the produced Al₂O₃ short fibers/Ti-12Mo-6Zr composites. The addition of Al₂O₃ short fibers up to 5 wt.% improves the properties of the fabricated Al₂O₃ short fibers/Ti-12Mo-6Zr composites. The estimated hardness and elastic modulus are increased by increasing in the Al₂O₃ short fibers reinforcement content. This improvement is correlated to the applied load transfer from the Ti-Mo-Zr metal matrix to the hard corundum Al₂O₃ short fibers reinforcement phase. It is also due to the combination with the Al₂O₃ short fibers in the Ti-Mo-Zr metal matrix which enhanced by the Al₂O₃ short fibers, which serve as barriers to any crack formation and propagation during using under applied load on the implanted or the orthopedic materials. By considering these materials and its promising properties, Al₂O₃ short fibers/Ti-12Mo-6Zr composites suggested to be one of the best composite materials for fabrications of implanted parts because of its high hardness and moderate elastic modulus and which are expected to offer interesting prospective properties for different orthopedic applications.

Author Contributions

Conceptualization, WMD, FI, HSP, BKL, and SHH; methodology, WMD, HSP, BKL, and SHH; validation, WMD, FI, HSP, BKL, and SHH; formal analysis, WMD, HSP, BKL, and SHH; investigation, WMD, FI, HSP, BKL, and SHH; resources, WMD, FI, and SHH; data curation, WMD, HSP, BKL, and SHH; writing-original draft preparation WMD, FI, HSP, BKL, and SHH; writing-review and editing, WMD, FI, HSP, BKL, and SHH; visualization, WMD, FI, HSP, BKL, and SHH; supervision, WMD, FI, and SHH; project administration, WMD, FI, and SHH. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledged the Brain Korea 21 (BK21) in South Korea for supporting this work.

Compliance with ethical standards

Conflict of interest

The authors declare no competing interests.

Informed Consent

Informed consent was obtained from all subjects involved in the study.