Research Paper
Mechanical and physical behavior of newly developed functionally graded materials and composites of stainless steel 316L with calcium silicate and hydroxyapatite

https://doi.org/10.1016/j.jmbbm.2015.05.020Get rights and content

Abstract

This study aimed to investigate the structural, physical and mechanical behavior of composites and functionally graded materials (FGMs) made of stainless steel (SS-316L)/hydroxyapatite (HA) and SS-316L/calcium silicate (CS) employing powder metallurgical solid state sintering. The structural analysis using X-ray diffraction showed that the sintering at high temperature led to the reaction between compounds of the SS-316L and HA, while SS-316L and CS remained intact during the sintering process in composites of SS-316L/CS. A dimensional expansion was found in the composites made of 40 and 50 wt% HA. The minimum shrinkage was emerged in 50 wt% CS composite, while the maximum shrinkage was revealed in samples with pure SS-316L, HA and CS. Compressive mechanical properties of SS-316L/HA decreased sharply with increasing of HA content up to 20 wt% and gradually with CS content up to 50 wt% for SS-316L/CS composites. The mechanical properties of the FGM of SS-316L/HA dropped with increase in temperature, while it was improved for the FGM of SS-316L/CS with temperature enhancement. It has been found that the FGMs emerged a better compressive mechanical properties compared to both the composite systems. Therefore, the SS-316L/CS composites and their FGMs have superior compressive mechanical properties to the SS-316L/HA composites and their FGMs and also the newly developed FGMs of SS-316L/CS with improved mechanical and enhanced gradation in physical and structural properties can potentially be utilized in the components with load-bearing application.

Introduction

Metallic biomaterials such as titanium alloys, stainless steel and chrome cobalt are most widely used as orthopedic implants because of their superior mechanical properties, formability, durability, and resistance to corrosion (Liu et al., 2002, Sutha et al., 2013). Biomaterial grade stainless steel type 316L (SS-316L) has shown many advantages of availability and easy processing at low cost compared to the titanium alloys and chrome cobalt alloys (Chew et al., 2013, Gopi et al., 2014). It also has fair biocompatibility (Frutos et al., 2012, Oberringer et al., 2013) and readily been utilized as implants in bone screw/plate, intra-medullary rod, fixation wire, femoral stem in hip joint, and knee joint for orthopedic surgery (Gopi et al., 2012, Marcomini et al., 2014). In addition, the low carbon content (<0.03 wt% (Ramakrishna et al., 2010)) in the SS-316L provides an excellent improved corrosion resistance in simulated physiological environments (Gopi et al., 2012). The bioinert property of SS-316L increased risk of implant loosening due to the lack of firm bonding between bone and implant. Therefore, the SS-316L implants were coated with bioactive ceramics such as hydroxyapatite (HA) and calcium silicate (CS) in order to improve bonding between bone and implants.

HA and CS are bio-ceramic materials with excellent bioactivity and biocompatibility properties (Oshkour et al., 2014) and due to these promising properties they are widely considered as bone graft materials (Schumacher et al., 2014). However, they have low fracture toughness and load bearing capacity, thus limiting their application in the human body in monolithic form (De Aza et al., 2014, Long et al., 2006, Sprio et al., 2009). Therefore, numerous studies have endeavored to enhance the load bearing capacity and toughness of HA and CS by reinforcing them with other biocompatible materials such as polymers (Pramanik and Kar, 2009, Shirazi et al., 2015), carbon nano-tubes (Borrmann et al., 2004), graphene oxide (Mehrali et al., 2014), ceramics (Schumacher et al., 2014, Shirazi et al., 2014) and metals or alloys (Oshkour et al., 2014). The composite materials have shown superior properties by overcoming the weaknesses of their constituents׳ phases. Meanwhile, functionally graded materials (FGMs) due to their unique properties such as gradient change in mechanical and biocompatibility properties attracted much interest for biomedical application. FGMs are referred to as a class of advanced composites (Askari et al., 2012). FGM is such a mixture of two distinct materials (e.g., ceramics with different ceramics, polymers, metals or alloys) whose composition varies gradually along a gradient direction (Babaei and Lukasiewicz, 1999, Matějíček et al., 2014). FGMs may be able to discard the disadvantages of metal matrix composites (MMCs) and ceramic matrix composites (CMCs) easily. In this context, it has been found that the natural bone has properties like FGMs (Pramanik et al., 2015, Pramanik et al., 2014).

Although based on the widely accepted biocompatibility of the SS-316L, HA and CS they have been trying to use as implant materials and many other biomedical applications, several problems in their monolithic form have made themselves as large barrier in load bearing applications (De Aza et al., 2014, Long et al., 2006, Oshkour et al., 2014, Pramanik et al., 2015, Sprio et al., 2009). Thus, the present study aimed to investigate the composites and FGMs composed of SS-316L/HA and SS-316L/CS materials developed by pressureless solid state sintering to achieve good load-bearing capacity for biomedical applications. Therefore, the present work investigated the structural, physical and compressive mechanical properties of the SS-316L/HA and SS-316L/CS bio-composites and FGMs on load-bearing applications.

Section snippets

Powder preparation

The analytical graded CS, HA, and SS-316L raw powders were purchased from Sigma Aldrich. Initially, wet ball milling was performed on each raw material separately to obtain uniform particle sizes using zirconia ball to powder ratio of 5:1 (w/w) in 100 ml ethanol medium in the zirconia jar at a speed of 300 rpm for 6 h using planetary ball mill (PM200, Retsch, Germany). Then, the powders were dried overnight (16 h) in an oven at 110 °C. The metallic phase was mixed with the two ceramic phases

Structural characterization

Fig. 2, Fig. 3 illustrate the XRD patterns of different composites of SS-316L/HA after and before 3 h solid state sintering at 1200 °C, respectively. The XRD was conducted on the raw mixture (before sintering) of SS-316L with 20, 40, and 60 wt% of HA just to show the comparison with sintered products and to confirm the reactions occurred between the SS-316L and HA after sintering. It indicates clearly that the mixture powders of raw materials were unreacted and similar to as-received SS-316L and

Conclusion

The increase in the sintering temperature induces two different effects into the SS-316L/HA and SS-316L/CS composites. The increase in the sintering temperature led to reduction in the compressive mechanical properties of the 316L/HA composites and FGM while it helps to improve the compressive mechanical properties of SS-316L/CS composites and FGM. The sintering process at 1000 °C, 1100 °C and 1200 °C causes expansion in the samples composed of SS-316L and HA with weight percentage in the range of

Acknowledgment

This study was supported by "UM/MOHE/HIR" Project no. D000014-16001.

References (38)

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