hydroxyapatite composite with different powder mixing techniques

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

Available online at www.sciencedirect.com

Journal of Magnesium and Alloys xxx (xxxx) xxx www.elsevier.com/locate/jma

Full Length Article

Mechanical and degradation behaviour of biodegradable magnesium–zinc/hydroxyapatite composite with different powder mixing techniques Siti Nur Hazwani Mohamad Rodzi a, Hussain Zuhailawati a,∗, B.K. Dhindaw b a Biomaterials

Niche Area Group, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia b School of Minerals, Metallurgical and Materials Engineering, Indian Institute of Technology Bhubaneswar, Bhubaneswar 751007, India Received 13 September 2019; accepted 1 November 2019 Available online xxx

Abstract Magnesium-based biomaterials have recently gained great attention as promising candidates for the new generation of biodegradable implants. This study investigated the mechanical performance and biodegradation behaviour of magnesium-zinc/hydroxyapatite (Mg–Zn/HA) composites fabricated by different powder mixing techniques. A single step mixing process involved mechanical alloying or mechanical milling techniques, while double step processing involved a combination of both mechanical alloying and mechanical milling. Optimum mechanical properties of the composite were observed when the powders were prepared using single step processing via mechanical alloying technique. However, Mg–Zn/HA composite fabricated through single step processing via mechanical milling technique was found to have the most desirable low degradation rate coupled with highest bioactivity. The composite achieved the lowest degradation rate of 0.039 × 10−3 mm/year as measured by immersion test and 0.0230 mm/year as measured by electrochemical polarization. Ca:P ratio of the composite also slightly more than enough to aid the initial bone mineralization, that is 1:1.76, as the required Ca:P ratio for initial bone mineralization is between 1:1 and 1:1.67. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Magnesium-based composite; Biodegradable implant; Powder metallurgy; Mechanical alloying; Mechanical milling.

1. Introduction Magnesium (Mg) and its alloys have recently gained much attention owing to its mechanical compatibility resembling the mechanical properties of natural bone. As biodegradable implant, Mg-based alloys possess unique characteristics not present in Zn-based alloy and Fe-based alloys. Although iron (Fe), zinc (Zn) and magnesium (Mg) are all nutritional requirements for a healthy body, the amount of each element still must be taken into consideration as the recommended daily intake of Mg (240–420 mg/day) is up to 52.5 times more than that of iron (8–18 mg/day) and zinc (8–11 mg/day) ∗

Corresponding author. E-mail address: [email protected] (H. Zuhailawati).

[1]. Considering the daily needs for all the elements, higher dosage of pure zinc or iron-based implants might induce health problems in the patient. Furthermore, Mg-based alloys used as biodegradable implant is a promising candidate to avoid the stress shielding problem arising from high Young’s modulus metals such as Zn (∼90 GPa) or Fe (∼211.4 GPa), since Mg displays Young’s modulus (41–45 GPa) that is closest to that of natural bone (∼5–23 GPa) [2]. Several in vitro studies exploring the corrosion behaviour of several Mg alloys have suggested that Ca, Zn and Sn would be appropriate alloying candidates for the Mg-based biomaterials [3,4]. Like Mg, Zn is an essential nutrient for the human body, playing an important role in various biological functions. Alloying Mg with Zn can improve the strength of the alloy via solid solution strengthening mechanism. The

https://doi.org/10.1016/j.jma.2019.11.003 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

JID: JMAA

2

ARTICLE IN PRESS

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

strengthening mechanism associated with Zn achieves two important objectives. First, the atomic size of Zn (0.133 nm) is smaller than that of Mg (0.160 nm), enabling the atoms of Zn to occupy the interstitial sites in the Mg lattice, which then causes disturbance or distortion in the well-mannered order of the Mg atoms. This disturbance renders the sliding of the layers more difficult, leading to improvement in mechanical strength. A study [5] found that the addition of Zn beneficially refine the grain size of the Mg matrix, which grain refinement is widely known to be very preferable for reduction of corrosion behaviour of Mg. The smaller size of grains induced by alloying of Zn into Mg would increase the grain boundary areas, which the grain boundaries is known as the least susceptible area to be attacked by corrosion medium. As the area of grain boundary increases by alloying, the corrosion rate is decreases [6,7]. Referring to the binary phase diagram of the Mg–Zn alloy system, the maximum ability of Zn to be solid-solved in Mg is 6.2 wt% (i.e. 2.5 at%) at 341 °C. As alloying of Zn in Mg can improve the corrosion/degradation rate, increasing the weight fraction of Zn potentially reduces the corrosion rate. However, the literature records disagreements regarding the optimal addition of Zn, which was either as low as 1 wt% or as high as 6 wt% [5,8–10]. To improve the bioactivity of Mg–Zn alloy synthesized in this work, hydroxyapatite (HA) was chosen to be secondary particles in the Mg–Zn-based composite. Incorporating HA particles in the Mg-based composite is well-acknowledged to impart valuable bioactivity properties as HA possesses excellent biocompatibility and bioactivity due to its close resemblance to the chemical and structural properties of human bone and tooth minerals. Furthermore, the calcium component in HA (Ca10 (PO4 )6 (OH)2 ) also decrease the tendency of Mg to corrode, owing to its low solubility in the body environment [11–13]. Amongst the wide options available in the fabrication of Mg-based composites by powder metallurgy, mechanical alloying (MA) facilitated in a high energy ball milling and mechanical milling (MM) in low energy ball milling are frequently used to develop powder of alloy and composite. MA and MM techniques offer distinct degrees of success in gaining homogeneity of the elements being dispersed in the matrix [14]. What differentiates these techniques is materials transfer. MA has been widely utilized in the fabrication of composite materials due to its ability to incorporate the reinforcement particle into the metal matrix in a close distance, involving the material transfer that takes place due to numerous processes of cold welding, fracturing and re-welding of the milled powder particles in a highly energetic ball mill, rendering the powder more homogeneous [14–16]. Hence, MA appears to be better for achieving homogeneity of the alloy and for refining the microstructure, which is desirable for enhancing the alloy’s mechanical performance and corrosion behaviour. On the other hand, MM is utilized to reduce the size of particles or grains while increasing the surface area [15] and does not involve material transfer that oftenly promote the formation of a homogeneous alloy. While MM’s benefits include low cost and simplicity, MM cannot guarantee uniformity in

distribution of the milled particles, since the produced fine powder is often found to be agglomerated [14]. Even though both involve mixing of powder, their mechanisms are different, thus the properties of the resultant composites are expected to show a variation. This work was conducted to investigate the differences in outcomes resulting from use of the different mixing techniques (MA, MM and combination of MA and MM) as little appears in the literature concerning the effect of powder mixing method on mechanical and biodegradation properties of Mg-based composite. 2. Materials and methods Mg–Zn/HA composite was the test material of this study. Mg (≥98.5% purity), Zn (>99.9% purity) and HA (≥90.0% purity) powders were supplied by Merck (Germany), Alfa Aesar (England) and Sigma-Aldrich (USA), respectively. Size of particles of raw Mg, Zn and HA averaged 680 μm, 49 μm and 5 μm, respectively. The composites of Mg–Zn/HA were fabricated through three different milling techniques and Mg–Zn alloy was used as a reference material. 2.1. Fabrication of Mg–Zn/HA composites through various powder mixing techniques This work studied the effects on the resultant Mg–Zn/HA composite of three mixing techniques: two single step processing (SSP) techniques – mechanical alloying in a high energy planetary mill (PM) and mechanical milling in a low energy ball milling (BM), respectively – and a double step processing (DSP) technique, which is a combination of both MA and MM. Single step processing (SSP) mixes the Mg, Zn and HA either by mechanical alloying or mechanical milling, while in double step processing (DSP), the matrix of Mg–Zn alloy is first alloyed in PM with subsequent dispersion of HA powders using BM. To investigate the effect of HA incorporation in the Mg–Zn alloy, an alloy of Mg–Zn without addition of HA was fabricated as the control sample using MA. As the Mg–Zn was used as matrix of the composite, the weight fraction of Mg to Zn was 94:6. To fabricate the Mg–Zn/HA composite, the weight fraction of Mg–Zn to HA powder was 92:8 wt.%. The ball-to-powder (BPR) ratio for both milling processes [planetary mill (PM) and ball milling (BM)] was fixed to 8.75:1. Balls used to mix the powder in MA and MM processes were stainless steel balls. In PM, the powders were loaded into 250 ml stainless steel vial with pre-determined numbers of 20-mm stainless steel balls (dependent on the BPR) with 3 wt.% addition of nheptane and purged with argon (Ar). The mixture of the powders was then mechanically alloyed at the pre-set milling time of 4 h and a milling speed of 220 rpm. The milling speed was selected based on the findings of a previous study [17] which reported that the compressive strength of the composite was optimum and lay within the limits of the mechanical strength required by natural bone when the milling speed was in the region of 220 rpm. In MM, a pre-determined weight of powder

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

was loaded into 500 ml polyethylene jar with a certain number of 20-mm alumina balls (dependent on the BPR). After sealing the jar tightly, the jar was placed on the rotating ball mill at a speed of 20.0 Hz for 4 h. To consolidate the milled powders, compaction formed green bodies with pellet height of 5-mm and 10-mm diameter. The powders were pressed under 400 MPa, followed by sintering at 300 °C for 1 h under argon flow. Both heating and cooling rates were set at 10 °C/min. 2.2. Phases identification and microstructure evaluation Phases and compound identification of raw materials and sintered pellets were investigated using XRD. The general overview of the microstructure of the sintered composite was observed under optical microscope (OM) Meiji, while the microstructure of as-received powder materials and immersed samples was observed using the Field Emission Scanning Electron Microscope (FESEM) ZEISS SUPRA 35VP equipped with energy dispersive x-ray (EDX). Prior to microstructural assessment of the sintered composite, the pellets were ground using silicon carbide (SiC) papers up to 1200 grit and subsequently polished by alumina slurry of 1 μm, 0.3 μm and 0.05 μm. The surface of the sintered pellets was then etched with a solution of ethanol, distilled water and picric acid. 2.3. Density measurement The Archimedes’ principle was employed in density measurement of the sintered pellets. Five readings were taken for each sintered Mg–Zn alloy (control sample) and Mg– Zn/HA composites. The samples were first balanced using an electronic balance (Precisa XB220A) in dry state inside a metal cone, before being immersed in water and the suspended weight was recorded. The values of sintered density were then recorded and compared to theoretical density of the composite to get the relative density. The theoretical density of the composite was computed according to the rule-ofmixture (ROM) equation as in Eqs. (1) and (2) below, while the relative density (%TD) was calculated as in Eq. (3) below. The densities of Mg (ρ Mg ), Zn (ρ Zn ) and HA (ρ HA ) were taken to equal 1.74 g/cm3 , 7.14 g/cm3 and 3.156 g/cm3 , respectively. The weight fraction of Mg to Zn alloy was 94:6, while that of Mg–Zn to HA composite was 92:8. The weight fraction of each component (Mg, Zn and HA) was converted into volume fraction and symbolized as VMg , VZn and VHA , respectively. T heoretical Density = ρMgVMg + ρZnVZn + ρHAVHA

(1)

VMg + VZn + VH A = 1

(2)

%T D =

Sintered Density × 100 % T heoretical Density

(3)

3

2.4. Microhardness and mechanical testing Prior to Vickers microhardness measurement using LECO Microhardness Tester LM 248AT, the sintered pellets were ground using 1200 grit SiC paper to remove the oxide scale formed on the surface. A 10-mm-diameter sample was placed under the diamond indenter with 300 g force (gf) of indentation load and a dwell time of 10 s. Ten readings were taken for each sintered compact, and the average was recorded. Mechanical performance of Mg–Zn/HA composite prepared for bone implant application was evaluated by a compression test. The compression test was performed according to ASTM E9-89a standard test methods of compression testing of metallic materials at room temperature. A resized cylindrical sintered compact with 10 mm height and 10 mm diameter (1:1 ratio) was tested using a universal testing machine (UTM) Instron 5982. The compression test of sintered composites was replicated five times, and the average was recorded. Constant crosshead speed was set at 0.5 mm/min. 2.5. Immersion test Samples for the immersion test were prepared by grinding with up to 1200 grit and subsequently polished by alumina slurry of 1 μm and 0.3 μm. The samples were then ultrasonically cleaned in ethanol and finally dried under warm airflow. Prior to immersion test in Hank’s balanced salt solution (supplied by Gibco, Life Technologies (USA)), the sintered pellets with 10-mm-diameter and 5 mm height were weighed using a 4 decimal point electronic balance (Sartorius) to get the initial weight, MO of the pellet. The amount of Hank’s balanced salt solution (HBSS) to be put into the Falcon tube was calculated according to the following equation [18]: VS = Sa /10

(4)

where Vs is the volume of HBSS required to be poured into the tube (ml) while Sa represents the apparent surface area of pellets (mm2 ). Since the pellets were cylindrical, the surface area was calculated as in the following equation: Sa = 2 π r 2 + 2 π rt

(5)

where r represents the radius (mm) of the pellets and t is the height (mm) of the pellets. The amount of HBSS as calculated in Eq. (4) was then poured into the tube and the sample was submerged into the bottom of the tube. The tubes containing the submerged pellets were then loaded into a water bath heated to 37 °C prior to loading the tubes. The immersion test for weight loss measurement was performed in the HBSS for 24 h. After removal from the HBSS, samples prepared for weight loss measurement were dipped in chromic acid to remove the degraded layer by dissolving Mg(OH)2 and rinsed first with ethanol and then with de-ionized water. The samples were put in the dryer for 24 h. After 24 h, samples were taken out from the dryer and balanced by electronic balance with accuracy to four decimal points to weigh the final weight, MF . Weight loss of the immersed sample was calculated as in Eq. (6). Degradation rate of the samples was calculated

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

4

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx 18000

α

14000 12000

Intensity (a.u.)

α-Mg

α

α

16000

10000

α

α

α

α

α

α α

α α

(iv)

8000

(iii) 6000 4000

(ii)

2000

(i) 0 20

30

40

50

60

70

80

90

2θ (degrees) Fig. 1. XRD diffractograms of sintered (i) Mg–Zn alloy (PM) and Mg–Zn/HA composites fabricated through (ii) SSP PM (iii) SSP BM and (iv) DSP.

according to Eq. (7). WL = MO − MF Degradation Rate (mm/year ) =

(6) WL A ×ρ ×t

(7)

where WL is the weight loss of the sample (g), A is the original surface area of the sample (cm2 ), ρ is the density of the sample (g/cm3 ) and t is the exposure time of the sample in the solution (hours). Samples immersed in artificial body fluid are likely to experience shrinkage and collapse of surface structure due to the effects of surface tension if directly air dried after being removed from the solution media. Therefore, a series of dehydration and fixative steps were employed to clean the samples to maintain the morphology of the immersed samples. Dehydration of the samples was performed in steps using five series of ethanol concentration while the fixative step was performed using hexamethyldisilizane (HMDS) as promoter of adhesion on the surface of samples. Each sample was submerged in a series of five ethanol concentrations (30%, 50%, 70%, 90% and absolute ethanol) for 5 min in each concentration. The sample was then immersed in HMDS for 10 min and left for 4 h to allow the HMDS to evaporate. All the dehydration and fixative steps were carried out in a fume hood to avoid any risk of exposure to highly toxic chemicals. All the immersed samples were then kept in a refrigerator at 4 °C to avoid any contamination from air and moisture. 2.6. Electrochemical polarization test The working surface of the samples was ground with abrasive SiC paper up to 1200 grit and polished with alumina slurry of 1 μm. Electrochemical measurement was conducted at 37 °C in a glass beaker containing HBSS on the laboratory corrosion measurement equipment (PGSTAT30). The tested

sample was attached as the working electrode (WE), while a saturated calomel electrode (SCE) and a platinum (Pt) electrode were used as reference electrode (RE) and counter electrode (CE), respectively. The curve of potentiodynamic polarization was measured at a scan rate of 0.5 mV/s initiated at −250 mV below the open circuit potential. The Tafel region was extrapolated to the corrosion potential for the purpose of determining the corrosion rate from the polarization measurements. Data was analysed using AutoLAB software. 3. Results and discussion 3.1. Phase analysis by XRD XRD diffractograms of the sintered alloy and composites are presented in Fig. 1. Only single phase α-Mg solid solution was detected in the diffractograms of Mg–Zn alloy, Mg– Zn/HA (SSP PM), Mg–Zn/HA (SSP BM) and Mg–Zn/HA (DSP), indicating the formation of a homogeneous solid solution of α-Mg which is caused by reduction in distances between milled particles [19] that promote dissolution of Zn alloying elements into the host Mg metal lattice structure. The diffractograms indicate that the composites of Mg–Zn/HA (ii– iv) experienced slightly greater peak broadening than did that of the Mg–Zn alloy (i). The peak broadening was associated with the refinement of crystallite size and/or accumulation of internal strain in the milling process [20]. Peak broadening of the composite fabricated by planetary mill (SSP PM) and double step processing (DSP) became obvious as the crystallite size reduced to 46.82 nm and 39.14 nm, respectively (Table 1). Smaller crystallite size tends to promote the diffusivity of Zn into the Mg lattice to form α-Mg solid solution, which is a desired enhancement of the properties of the composite. Mg–Zn/HA (SSP BM) peaks showed a drastic reduction in intensity, mainly caused by reduction in crystallinity of the composite. Since highly crystalline materials possess very

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

5

Table 1 Lattice parameter, crystallite size and internal strain of sintered Mg–Zn alloy and Mg–Zn/HA composites fabricated via different powder mixing techniques. Sample/processing

˚ a (A)

˚ c (A)

c/a

Crystallite size (nm)

Internal strain (%)

Mg–Zn (PM) Mg–Zn/HA(SSP PM) Mg–Zn/HA(SSP BM) Mg–Zn/HA(DSP)

3.2041 3.2048 3.2047 3.2039

5.2018 5.2021 5.2140 5.2010

1.6235 1.6232 1.6270 1.6233

59.03 46.82 52.90 39.14

0.18 0.22 0.19 0.28

Fig. 2. Diffraction patterns of (i) as-milled and (ii) sintered Mg–Zn/HA composite fabricated through ball milling (SSP BM) mixing technique.

high intensity and sharp peaks under x-ray diffraction, reduction in intensity of the particular composite indicate a reduction in crystallinity in the composite fabrication. In order to investigate the factor of reduction in crystallinity of the SSP BM composite, the diffraction pattern of as-milled powder of the composite was compared with the diffraction pattern of sintered composite (Fig. 2). The diffraction pattern showed no Zn peaks in the as-milled powder, indicating that Zn had already been dissolved into the Mg lattice within 4 h of ball milling time. However, the impact between the ball and the powders containing hard and brittle HA might lead to the reduction in crystallinity of Mg and Zn metal powders. Since the bioactive ceramic HA powders are known to have amorphous characteristics, inhomogeneous dispersion of the powders in the Mg–Zn alloy matrix within the ball mill could lead to reduction in crystallinity as well, which may cause a drop in mechanical performance. The diffraction patterns of Mg–Zn/HA (SSP BM) and Mg–Zn/HA (DSP) shifted to the lower Bragg’s angle, a shift which is associated with the shrinkage of the lattice parameter of HCP crystal structure of Mg-based composites. The lattice parameter (a and c) of Mg–Zn/HA composite ˚ and 5.2010 A ˚ when the composite was reduced to 3.2039 A was double-step processed with a combination of MA and MM (Table 1). Double step processing induced additional impacts on the composite powders, causing the maximum reduction in lattice parameter in the composite powders as well as the smallest size of crystallite (39.14 nm) with the highest internal strain (0.28%) of all the composites (Fig. 3).

To investigate the degree of refinement effected by each powder mixing technique, crystallite size and internal strain of the alloy and composites were studied. All the composites showed remarkable refinement in crystallite size over that of the Mg–Zn alloy. Among the three composites, double step processed Mg–Zn/HA (DSP) showed the smallest crystallite size (39.14 nm), followed by Mg–Zn/HA (SSP PM) (46.82 nm) and Mg–Zn/HA (SSP BM) (52.90 nm). The smallest crystallite size of Mg–Zn/HA (DSP) was contributed by the secondary stage of HA dispersion incorporated into Mg–Zn alloy after the alloy was synthesized using planetary mill. The ductile and flaky shape of the Mg–Zn alloy powder was continually refined in the secondary mixing step, thus producing the composite powder with the smallest crystallite size. Furthermore, hard HA ceramic powders also exerted a shearing cutting action during milling together with the Mg–Zn alloy matrix, thereby contributing to refinement of the size of the powders. The Mg–Zn/HA composite synthesized using SSP PM produced the compact with a crystallite size of 46.82 nm, a result of the repeated fracturing and re-welding of the powder while milling at high energy. On the other hand, the composite that was subjected to ball milling (SSP BM) produced the composite with the largest crystallite size (52.90 nm). The large crystallite size of the particular composite with the lowest internal strain (0.19%) hindered the dissolution of Zn into the Mg lattice since refinement of crystallite size promotes the kinetic energy sufficient for Mg to react with Zn and form α-Mg phase due to ball-to-powder collision impact during milling [21].

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

Crystallite size

65

Internal strain (%)

Crystallite size (nm)

60

0.30 0.25

55 50

0.20

45

0.15

40

Internal strain (%)

6

0.10

35 30

0.05 Mg-Zn (PM)

Mg-Zn/HA Mg-Zn/HA (SSP PM) (SSP BM) Processing

Mg-Zn/HA (DSP)

Fig. 3. Crystallite size and internal strain of sintered Mg–Zn alloy and Mg–Zn/HA composites fabricated through various powder mixing techniques.

3.2. Microstructures of Mg–Zn/HA composites Optical micrographs of Mg–Zn alloy without HA and of Mg–Zn/HA composites fabricated through the three various powder mixing techniques are given in Fig. 4. The lamellae layer microstructure of α-Mg solid solution are clearly observable in Fig. 4(a) and (b), but those lamellae are absent in the micrographs of Mg–Zn/HA fabricated through ball mill and double step processing (Fig. 4(c) and (d)). The composite fabricated through planetary mill (SSP PM) underwent more intensive grain refinement than did those fabricated through SSP BM and DSP, with the formation of elongated grains caused by high impact ball-to-powder collision. The lamellae of Mg–Zn solid solution clearly formed in the composite fabricated through SSP PM indicates that the highly energetic ball-to-powder collision induced by the planetary mill successfully diffused the Zn atoms into the Mg lattice by reducing diffusion distances. Reduction in diffusion distances due to application of continuous high impact energy and the temperature increment during milling process provides energy for the diffusion to take place, which has also been reported by Raghu and co-workers [21,22]. Concerning the microstructure of the Mg–Zn alloy, it is noticeable that the lamellae are formed in the matrix alloy, before subsequent dispersion of HA into the matrix by DSP, thus inducing the changes in microstructure. High energy milling effected by SSP PM caused the formation of microstructure with the finest size of grains and elongated grains. The finer grains of Mg–Zn/HA (SSP PM) had a higher area of grain boundaries. Higher plastic deformation of composite powders milled by SSP PM induced greater residual stress and dislocation [13], which then contributed to the well-homogenized structure of the Mg–Zn/HA composites. Composites fabricated through BM and DSP demonstrated grain coarsening, forming large equiaxed grains and less area of grain boundary. In ball milling (SSP BM), the elemental powders of Mg and Zn which are soft and ductile were

mixed together with hard bioceramics of HA in a low energy ball mill. It seems that the powders of HA agglomerated at the grain boundaries of Mg–Zn (Fig. 4(c)). Formation of the large equiaxed grains and the lower area of grain boundary correlate with low energy during milling, which low milling energy is insufficient to shear the powders finely and homogeneously like those of the composite milled using planetary mill. Meanwhile, the low milling energy consumed to disperse HA particles in the secondary mixing process of Mg–Zn/HA (DSP) continuously broke down the particles to a smaller size. The main difference between the resulting composite lay in the mechanisms of ball-powder and powder-jar collisions in the planetary mill (during mechanical alloying) and in the ball mill (during mechanical milling) is the presence of repeated cold welding and fracturing during mechanical alloying. Mechanical milling in SSP BM only applies a low level of strain to facilitate powder mixing without subsequent cold welding of the milled powders, leading to agglomeration of powder particles without any reaction such as alloying. Abundant porosity in Mg–Zn/HA (SSP BM) and Mg–Zn/HA (DSP) led to deterioration of mechanical performance. The abundant porosity observed in both composites is probably due to the absence of materials transfer between the milled particles in the milling process. 3.3. Density, microhardness and mechanical performance As shown in Fig. 5, sintered density of the composite was slightly higher than that of the Mg–Zn alloy, in compliance with the density distribution according to rule-of-mixture (ROM) formula [23]. The theoretical density of Mg–6 wt%Zn alloy is 1.8228 g/cm3 , while 8 wt% of HA additions into the Mg–Zn alloy matrix caused the theoretical density to increase up to 1.8863 g/cm3 , due to the high density of HA (3.156 g/cm3 ). Single step processed (SSP) Mg–Zn/HA composite fabricated with the planetary mill (SSP PM) possessed the highest sintered density (1.811 g/cm3 ) as well

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

JID: JMAA

ARTICLE IN PRESS

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

7

Table 2 Correlation between average grain size and compressive strength of sintered Mg–Zn alloy and Mg–Zn/HA composites fabricated via different powder mixing techniques.

(a)

Lamellae of α-Mg solid solution

(b) Lamellae of α-Mg solid solution

Sample/processing

Average grain size (μm)

Mg–Zn (PM) Mg–Zn/HA(SSP PM) Mg–Zn/HA(SSP BM) Mg–Zn/HA (DSP)

104.89 56.14 100.54 93.89

Compressive strength (MPa) 171.96 156.45 107.75 128.00

posites was slightly lower as HA, possessing the properties of hard and brittle ceramic particles, was incorporated into the Mg–Zn matrix. That the composite fabricated through PM exhibited the highest compressive strength can be explained by the Hall–Petch law, as the law describes the relationship between mechanical strength and grain size. The law can be described as σs = σ0 + k d − 2

1

(c)

Pore

(d) Pore

Fig. 4. Optical micrographs of sintered (a) Mg–Zn alloy (PM) (b) Mg– Zn/HA (SSP PM) (c) Mg–Zn/HA (SSP BM) and (d) Mg–Zn/HA (DSP).

as the greatest microhardness (56.83 HV). Single step processing (SSP PM) underwent greater refinement than did single step processing in ball milling (SSP BM) and double step processing (DSP), which caused the composite to have finer particles’ size, as shown in Fig. 4(b). Consequently, finer powder eased the process of compaction and properly reduced the pores formed in the compact. Finer particle size resulting from high energy milling provided sufficient impact to gradually refine the powders, thus forming composites with uniform and finer grains. The compressive strength of the Mg–Zn alloy and its composites is shown in Fig. 6. The strength of Mg–Zn/HA com-

where σ s represents compressive strength, σ 0 is the friction stress for movement of dislocations on the slip plane, k is the stress concentration factor and d represents the average grain size [24–25]. Mg–Zn/HA composite fabricated through SSP PM showed the finest grain size (56.14 μm), followed by that fabricated by DSP (93.89 μm) and by SSP BM (100.54μμm) (Fig. 4). Comparing the grain size and mechanical strength of the composites, the composites clearly obeyed the Hall– Petch law, as the composite with the finest grain size, Mg– Zn/HA (SSP PM) also had the highest compressive strength (156.45 MPa) (Table 2). Mg–Zn/HA mixed in SSP PM was observed to have much smaller grains of α-Mg solid solution in the form of lamellae that contributed to the increment in the composites’s compressive strength. Higher strain level exerted in the planetary milled powder (SSP PM) through the numerous collisions between powders and ball induced thorough cold welding and fracturing in the milled powders. This process also enhances the homogeneity of HA dispersion in the matrix of the Mg–Zn alloy. These factors contributed to the formation of composites which have strong interfacial bonding between the matrix (Mg–Zn) and the reinforcement (HA) phase, thus enhancing the strength of the composite. The Mg– Zn/HA composite fabricated through planetary mill (PM) had the most outstanding compressive strength (156.45 MPa). 3.4. Electrochemical polarization study To investigate the issue of rapid corrosion rate of Mg, an electrochemical polarization study of Mg–Zn alloy and Mg– Zn/HA composite that were fabricated through three different mixing methods was conducted. Electrochemical data for the Mg–Zn alloy and the Mg– Zn/HA composite are presented in Fig. 7 and Table 3. The alloy and the composites corroded differently, since the mixing techniques were distinctive. The addition of HA particles into the matrix of Mg–Zn alloy did not effectively reduce the corrosion rate for the composite fabricated through DSP; the

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

Sintered Density

Sintered Density (g/cm3)

1.82

Microhardness

70

1.81

60

1.80

50

1.79

40

1.78

30

1.77

20

1.76

10

1.75

Microhardness (HV)

8

0

Mg-Zn (PM)

Mg-Zn/HA Mg-Zn/HA (SSP PM) (SSP BM) Processing

Mg-Zn/HA (DSP)

Compressive Strength (MPa)

Fig. 5. Sintered density and microhardness of Mg–Zn alloy (PM) and Mg–Zn/HA composites fabricated through various powder mixing techniques.

200 180 160 140 120 100 80 60 40 20 0 Mg-Zn (PM)

Mg-Zn/HA Mg-Zn/HA Mg-Zn/HA (SSP PM) (SSP BM) (DSP) Processing

Fig. 6. Compressive strength of Mg–Zn alloy and Mg–Zn/HA composites. Table 3 Corrosion behaviour of Mg–Zn alloy and Mg–Zn/HA composites as shown by electrochemical polarization. Processing

Corrosion current density, Icorr (A/cm2 ) × 10−4

Corrosion potential, Ecorr vs SCE (V)

Mg–Zn Mg–Zn/HA (SSP PM) Mg–Zn/HA (SSP BM) Mg–Zn/HA (DSP)

3.21 1.16 0.18 4.68

−1.549 −1.590 −1.665 −1.581

Corrosion rate (mm/year) 0.4250 0.1529 0.0230 0.6129

corrosion rate of that particular composite is even higher than that of the Mg–Zn alloy itself. Composites fabricated through SSP PM and SSP BM successfully reduced the corrosion rate in the biological environment, except for Mg–Zn/HA synthesized through DSP. Mg–Zn/HA (SSP BM) had the lowest corrosion rate (0.0230 mm/year), while Mg–Zn/HA (DSP) exhibited the highest corrosion rate in Hanks’ balanced salt solution (HBSS) (0.6129 mm/year). Therefore, Mg–Zn/HA (SSP BM) exhibited the most desirable corrosion resistance

among the composites, along with the smallest corrosion current density (0.18 × 10−4 A/cm2 ) and noblest corrosion potential (−1.665 V). The DSP composite was subject to more severe deformation as compared to those fabricated through SSP BM and SSP PM, because the double step processing of the composite involved a higher amount of strain during the first (in SSP PM) and secondary (in SSP BM) processes. This higher amount of strain induced during the powder mixing process of DSP composite produced the smallest crystallite size (39.14 nm) with the highest internal strain (0.28%). This highest internal strain exerted on the DSP composite caused the residual stress inside the microstructure to be severe, thus exerting a detrimental effect on its corrosion resistance. This finding is consistent with that proposed by Wu et al. [26] in their studies on the effects of residual stress on the corrosion of a biodegradable surgical staple. This study has reported that the presence of residual stress in the Mg-based surgical staple accelerated the corrosion rate of the sample, because it accelerated pre-crack propagation. On the other hand, the SSP BM composite was the only composite fabricated via the process (BM) with the lowest internal strain (0.19%). The low internal strain induced during the powder mixing process tends to reduce the residual stress in the composite, thus slowing crack propagation along the grain boundaries while the sample was electrochemically polarized. In general, grain boundary areas often act as physical corrosion barriers. A reduced grain size significantly increases the area of grain boundaries, which then decreases the corrosion rate [6,7]. In the current work, the SSP PM composite was the only composite with a microstructure of small and elongated grain boundaries, while SSP BM and DSP composites showed enlarged and irregularly shaped grain boundaries. It was observed that the refined grain size of SSP PM composite (56.14 μm) did not contribute to a reduction in corrosion resistance, since the SSP BM composite showed the most excellent corrosion resistance behaviour despite its enlarged grains (100.54 μm). As mentioned earlier, the corro-

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

Immersion Test

0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

5.0E-03 4.0E-03 3.0E-03 2.0E-03 1.0E-03 0.0E+00

Corrosion Rate (mm/year)

Corrosion Rate (mm/year)

Electrochemical Polarization

9

-1.0E-03

Mg-Zn (PM)

Mg-Zn/HA (SSP PM)

Mg-Zn/HA (SSP BM)

Mg-Zn/HA (DSP)

Processing Fig. 7. Corrosion rate of Mg–Zn alloy and Mg–Zn/HA composites based on tests of electrochemical polarization and immersion in HBSS.

-0.5

Mg-Zn

Mg-Zn/HA (SSP PM)

Mg-Zn/HA (SSP BM)

Mg-Zn/HA (DSP)

Potential (V) vs SCE

-1.0

-1.5

-2.0

-2.5

-3.0 1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

Current Density (A/cm2) Fig. 8. Electrochemical polarization curves of Mg–Zn alloy and Mg–Zn/HA composites.

sion resistance of the composites was related to the internal strain induced during the powder mixing techniques, therefore the internal strain can be associated with the residual stress contained in the composite. It is generally agreed that residual stresses are dangerous and may lead to stress corrosion cracking (SCC) [27]. The level of residual stress of the DSP composite was higher than that of the other two composites, due to the secondary steps of milling process. The first milling process already induced a certain amount of stress in the resultant powders via the repeated fracturing and re-welding of the particles at high milling energy, subsequently followed by secondary milling which introduced further stress that continuously broke the powder particles at low milling energy. On the other hand, the SSP BM composite was subjected only to the lowest magnitude of energy during the mixing process, causing the residual stress of the powder particles to be low and reducing the tendency to corrosion attacks. As depicted in Fig. 8, the corrosion behaviour of the alloy and composites was evaluated according to potentiodynamic

polarization curves. Table 3 shows that the icorr values rise in the order of SSP BM composite SSP PM composite < Mg– Zn alloy < DSP composites. Thus, the polarization curves indicate that the corrosion resistance of the alloy and composites reduced in the order of SSP BM composite  SSP PM composite > Mg–Zn alloy > DSP composites, with the Mg–Zn/HA composite fabricated through SSP BM showing the best corrosion resistance while the composite fabricated through DSP was the most susceptible to corrosion attacks. The DSP composite demonstrated the least corrosion resistance of the three composites due to the effect of cold working in its powder mixing techniques. The powders were plastically deformed into a flaky shape, leading to the formation of highly deformed grains, which then exerted a detrimental effect on its resistance to corrosion. Double step processing of the composite powders introduced additional residual stresses that make the composite highly susceptible to stress-corrosion cracking [28]. Furthermore, inhomogeneity of HA dispersion in the matrix of Mg–Zn alloy during the secondary milling

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

JID: JMAA

10

ARTICLE IN PRESS

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx

process also caused agglomeration of the HA fine powders, thus destroying the strength of interfacial bonding between matrix and reinforcement, leading to formation of pores [29]. The pores quickly became the primary site of initial attack by the corrosion medium when the material was exposed to the corrosive environment. The combination of high residual stress and high numbers of pores due to agglomeration of HA severely deteriorated the corrosion resistance of the DSP composite. 3.5. Weight loss measurement by immersion test The weight loss measurement of the composites fabricated via three distinct powder mixing techniques showed good agreement with the corrosion assessment by potentiodynamic polarization. Interestingly, adding HA particles into the Mg– Zn alloy was observed to effectively reduce the degradation rate of the alloy in artificial body fluid (HBSS). As described in Fig. 7, the degradation rate of Mg–Zn alloy was as high as 4.67 × 10−3 mm/year, but addition of HA to the composites effectively reduced the degradation rate. HA particles are widely used in orthopaedic applications to promote bone growth, but, since the degradation initiates from the surface of Mg upon contact in the biological environment, introducing HA into the Mg can give better protection as well as enhance the bioactivity of the Mg. The DSP composite degraded the most (2.14 × 10−3 mm/year), followed by the SSP PM composite (0.662 × 10−3 mm/year), while no weight loss was observed for the SSP BM composite. The SSP BM composite surprisingly gained a small amount of weight (0.039 × 10−3 mm/year) after being immersed in HBSS with an immersion time of 24 h. As portrayed in Fig. 9(c), the morphology of the SSP BM composite formed an apatite layer along the grain boundaries and the formation of the apatite layer was confirmed by EDX. The SSP BM composite showed the most exciting formation of apatite layer, which is very desirable to induce bone mineralization, with a Ca:P ratio of 1.76 [30]. (Note: Required Ca:P ratio to initialize bone mineralization is in the range of 1:1 to 1:1.67). Fig. 9 shows the surface morphologies of the Mg–Zn alloy, Mg–Zn/HA (SSP PM), Mg–Zn/HA (SSP BM) and Mg– Zn/HA (DSP) composites after immersion in HBSS for 24 h. Surface cracks were observed quite intensely in the Mg– Zn alloy and DSP composites, suggesting dehydration of the corrosion products and subsequent differential shrinkage [31]. The surface of DSP composite was covered with layered corrosion products, along with large observable pits. As for the SSP PM composite, the surface had tiny cracks and small agglomerated spherical-like bulges. The EDX analysis (Table 4) confirms the presence of Mg, O, and small atomic fraction of Ca and P, suggesting the formation of an apatite layer. A small fraction of Zn was detected in the EDX analysis for the SSP BM composite, suggesting that Zn had been leached from the solid solution of Mg, possibly due to weak metallic bonding between Mg and Zn during sintering.

Fig. 9. Micrographs of (a) Mg–Zn alloy (PM) (b) Mg–Zn/HA composite (SSP PM) (c) Mg–Zn/HA composite (SSP BM) and (d) Mg–Zn/HA composite (DSP) after immersion in HBSS for 24 h.

Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003

ARTICLE IN PRESS

JID: JMAA

[m5+;November 14, 2019;3:10]

S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw / Journal of Magnesium and Alloys xxx (xxxx) xxx Table 4 EDX of Mg–Zn alloy and Mg–Zn/HA composites fabricated through various powder mixing techniques after immersion in HBSS for 24 h. Processing

Mg–Zn alloy Mg–Zn/HA(SSP PM) Mg–Zn/HA(SSP BM) Mg–Zn/HA (DSP)

Atomic percent (at%)

Ratio Ca:P

Mg

Zn

O

Ca

P

44.67 41.74 57.67 33.48

– – 1.46 –

55.33 57.06 30.25 60.84

– 0.51 6.77 2.82

– 0.69 3.85 2.87

– 0.74 1.76 0.98

4. Conclusion Biodegradable Mg–Zn/HA composite fabricated through single step processing in planetary mill (SSP PM) possessed the best mechanical compatibility with human cortical bone (compact bone) of all composites studied. However, the Mg– Zn/HA composite fabricated through single step processing with ball mill (SSP BM) demonstrated the best degradation behaviour under physiological conditions, despite its poor mechanical properties. To conclude, single step mixing in planetary mill (SSP PM) is suggested as the most suitable powder mixing technique to produce Mg–Zn/HA composite with the combination of best mechanical performance and tolerable degradation behaviour. The density of the composite (1.811 g/cm3 ) lies within the range of density of human bones (1.8–2.0 g/cm3 ), while the microhardness of the composite fabricated through SSP PM was much higher (56.83 HV) than that of composites fabricated through SSP BM (40.90 HV) and DSP (55.88 HV). SSP PM also displayed the highest compressive strength (156.45 MPa) compared to SSP BM and DSP composite. In terms of biodegradation of the Mg– Zn/HA composite fabricated through the various powder mixing techniques studied, the composite fabricated through SSP PM exhibited a tolerable corrosion rate (through electrochemical polarization) of 0.1529 mm/year and a tolerable degradation rate (through immersion test) of 0.662 × 10−3 mm/year, which is much better than those of Mg–Zn without HA addition (0.4250 mm/year and 4.67 × 10−3 mm/year, respectively). Declaration of Competing Interest None. Acknowledgments The authors would like thank to Universiti Sains Malaysia for FRGS Grant No. 203/PBAHAN/6071386 and financial scholarship from Ministry of Higher Education of Malaysia. The authors also would like to thank Madam Alena Lee Sanusi for her helpful comments and language editing work for this manuscript. References

11

[2] M. He, X. Hua, F. Xue, P. Deng, J. Alloys Compd. 663 (2016) 156–165, doi:10.1016/j.jallcom.2015.12.120. [3] A. Mohamed, A.M. El-Aziz, H.-.G. Breitinger, J. Magn. Alloys 7 (2019) 249–257, doi:10.1016/j.jma.2019.02.007. [4] W. Jiang, J. Wang, W. Zhao, Q. Liu, D. Jiang, S. Guo, J. Magn. Alloys 7 (2019) 15–26, doi:10.1016/j.jma.2019.02.002. [5] S. Cai, T. Lei, N. Li, F. Feng, Mater. Sci. Eng. C 32 (8) (2012) 2570– 2577, doi:10.1016/j.msec.2012.07.042. [6] P. Saha, M. Roy, M.K. Datta, B. Lee, P.N. Kumta, Mater. Sci. Eng. C 57 (2015) 294–303, doi:10.1016/j.msec.2015.07.033. [7] H. Waizy, J.-.M. Seitz, J. Reifenrath, A. Weizbauer, F.-.W. Bach, A. Meyer-Lindenberg, B. Denkena, H. Windhagen, J. Mater. Sci. 48 (2013) 39–50, doi:10.1007/s10853- 012- 6572- 2. [8] S. Zhang, X. Zhang, C. Zhao, J. Li, Y. Song, C. Xie, H. Tao, Y. Zhang, Y. He, Y. Jiang, Y. Bian, Acta Biomater. 6 (2010) 626–640, doi:10. 1016/j.actbio.2009.06.028. [9] Y. Song, E.-.H. Han, D. Shan, C.D. Yim, B.S. You, Corros. Sci. 65 (2012) 322–330, doi:10.1016/j.corsci.2012.08.037. [10] X. Xia, J.F. Nie, C.H.J. Davies, W.N. Tang, S.W. Xu, N. Birbilis, Mater. Des. 90 (2016) 1034–1043, doi:10.1016/j.matdes.2015.11.040. [11] D. Ahmadkhaniha, M. Fedel, M.H. Sohi, A.Z. Hanzaki, F. Deflorian, Corros. Sci. 104 (2016) 319–329, doi:10.1016/j.corsci.2016.01.002. [12] R. del Campo, B. Savoini, A. Munoz, M.A. Monge, G. Garces, J. Mech. Behav. Biomed. 39 (2014) 238–246, doi:10.1016/j.jmbbm.2014.07.014. [13] A. Arifin, A.B. Sulong, N. Muhamad, J. Syarif, M.I. Ramli, Mater. Des. 55 (2014) 165–175, doi:10.1016/j.matdes.2013.09.045. [14] Z. Hussain, L.C. Kit, Mater. Des. 29 (7) (2008) 1311–1315, doi:10. 1016/j.matdes.2007.07.003. [15] C. Suryanarayana, N. Al-Aqeeli, Prog. Mater. Sci. 58 (4) (2013) 383– 502, doi:10.1016/j.pmatsci.2012.10.001. [16] C. Suryanarayana, Prog. Mater. Sci. 46 (1–2) (2001) 1–184, doi:10. 1016/S0079- 6425(99)00010- 9. [17] L.L. Soon, Z. Hussain, S. Ismail, B.K. Dhindaw, Acta Metall. Sin. (English Lett.) 29 (5) (2016) 464–474, doi:10.1007/s40195- 016- 0410- 5. [18] T. Kokubo, H. Takadama, Biomaterials 27 (15) (2006) 2907–2915, doi:10.1016/j.biomaterials.2006.01.017. [19] A. Nouri, X. Chen, Y. Li, Y. Yamada, P.D. Hodgson, C. Wen, Mater. Sci. Eng. A 485 (2008) 562–570, doi:10.1016/j.msea.2007.10.010. [20] T. Raghu, R. Sundaresan, P. Ramakrishnan, T.R. Rama Mohan, Mater. Sci. Eng. A 304–306 (2001) 438–441, doi:10.1016/S0921-5093(00) 01444-1. [21] E.M. Salleh, H. Zuhailawati, S. Ramakrishnan, M.A.-H. Gepreel, J. Alloys Comp. 644 (2015) 476–484, doi:10.1016/j.jallcom.2015.04.090. [22] M. Rabiee, H. Mirzadeh, A. Ataie, Adv. Powder Technol. 28 (2017) 1882–1887, doi:10.1016/j.apt.2017.04.023. [23] E.M. Salleh, H. Zuhailawati, S. Ramakrishnan, B.K. Dhindaw, Metall. Mater. Trans. A 48 (5) (2016) 2519–2528, doi:10.1007/ s11661- 017- 4028- 7. [24] Y. Wang, H. Choo, Acta Mater. 81 (2014) 83–97, doi:10.1016/j.actamat. 2014.08.023. [25] W. Yuan, S.K. Panigrahi, J.-.Q. Su, R.S. Mishra, Scr. Mater. 65 (11) (2011) 994–997, doi:10.1016/j.scriptamat.2011.08.028. [26] H. Wu, C. Zhao, J. Ni, S. Zhang, J. Liu, J. Yan, Y. Chen, X. Zhang, Bioact. Mater. 1 (2) (2016) 122–126, doi:10.1016/j.bioactmat.2016.09. 005. [27] Z. Ahmad, Principles of Corrosion Engineering and Corrosion Control, Elsevier Science and Technology, United Kingdom, 2006. [28] J.R. Davis, Corrosion: Understanding the Basics, ASM International, United States of America, 2000. [29] A. Atrens, M. Liu, N.I.Z. Abidin, Mater. Sci. Eng. B 176 (20) (2011) 1609–1636, doi:10.1016/j.mseb.2010.12.017. [30] A.A. Zadpoor, Mater. Sci. Eng. C 35 (2014) 134–143, doi:10.1016/j. msec.2013.10.026. [31] H.R. Bakhsheshi-Rad, E. Hamzah, S. Farahany, M.P. Staiger, J. Mater. Eng. Perform. 24 (2) (2015) 598–608, doi:10.1007/s11665- 014- 1271- 6.

[1] Q. Chen, G.A. Thouas, Mat. Sci. Eng. R Rep. 87 (2015) 1–57, doi:10. 1016/j.mser.2014.10.001. Please cite this article as: S.N.H. Mohamad Rodzi, H. Zuhailawati and B.K. Dhindaw, Mechanical and degradation behaviour of biodegradable magnesium– zinc/hydroxyapatite composite with different powder mixing techniques, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.003