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  • International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 12 No: 01 10

    I J E N S IJENS 2201 yraurbeFIJENS © -IJMME 4848-103021

    Influence of processing parameters and sintering atmosphere on the mechanical properties and microstructure of porous 316L

    stainless steel for possible hard-tissue applications

    Montasser Dewidar

    Department of Materials and Mechanical Design, Faculty of Energy Engineering, South Valley University, Aswan, Egypt.

    (dewidar5@hotmail.com)

    Abstract The 316L stainless steel has been widely used in both artificial knee and hip joints in biomedical applications. The average lifetime of artificial hip joints is about 10 years due to aseptic loosening of the femoral stem attributed to polymeric wear debris; however, there is a steadily increasing demand from younger osteoarthritis patients aged between 15 and 40 years for a longer lasting joint of 25 years or more. This paper studies the properties changes of powder metallurgy 316L stainless steel, depending on the compacting pressure, sintering temperature, and the sintering atmosphere. All samples have been compacted at 150, 250, and 350 MPa, and sintered at 1200, 1250, and 1300 oC. In order to analyze the sintering atmosphere, three different media were used: nitrogen, pure argon, and vacuum. The properties of the materials are evaluated. The study covered sintering density, compressive strength, hardness, wear resistance and microstructure analysis. The results show that the porous 316L stainless steel can be used as hard tissue implant. Keywords: Biomaterials; 316L stainless steel; Processing parameters; Mechanical properties; Sintering atmosphere 1. Introduction Metallic materials are often used as biomaterials to replace structural components of the human body. Metallic biomaterials are used in many medical devices such as artificial joints, bone plates, screws, intramedullary nails, spinal fixations, spinal spacers, external fixtators, pace maker cases, artificial heart valves, wires, stents, and dental implants [1, 2]. Commercially pure titanium, Ti–6Al–4V alloys, cobalt–chromium alloys, and type 316L stainless steels are typical metallic biomaterials used for implants devices [3, 4]. In spite of the metallic biomaterials are originally developed for industrial purposes, they have been tried for biomaterial uses due to their relatively high corrosion resistance and excellent mechanical properties. However, when metallic biomaterials used as biomaterials, they pose several problems. These problems include toxicity of corrosion products and fretting debris to the human body, fracture due to corrosion fatigue and fretting corrosion fatigue, lack of biocompatibility, and insufficient affinity for cells and tissues [5, 6]. 316L stainless steel is widely used for implant devices because they are less expensive than cobalt–chromium alloys, pure titanium, and titanium alloys by a factor of one-tenth to onefifth times [7]. The main problem concerning metallic implants in orthopedic surgery is the mismatching between the modulus of elasticity of metallic 316L stainless steel implant (210 GPa) and the modulus of elasticity of bone (10- 30 GPa) [8, 9]. The strength and modulus of elasticity of 316L stainless steel can be controlled using porous material with different porosity to match the strength and the modulus of elasticity of the natural bone [10]. It is expected that the low elastic moduli of porous 316L stainless steel will reduce the amount of stress-shielding at the bone where the metallic part is implanted. The stress-shielding leads to bone resorption and then eventual loosening of the implant, and hence to prolong implant life time [11]. In

  • International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 12 No: 01 11

    I J E N S IJENS 2201 yraurbeFIJENS © -IJMME 4848-103021

    addition, by increasing the match of the strength and the modulus of elasticity between the bone and 316L stainless steel, it is expected to result in better performance of the implant bone compound. Powder metallurgical P/M processing methods have been contributed significantly in the development of more effective surgical implants during the past two to three decades. They particularly contributed in the fields of orthopedic and dentistry where load bearing ability and the need for rigid and reliable implant-to-bone fixation are paramount. Sintering process is an important procedure of P/M technology because furnace atmospheres affect the sintering process of P/M technique and the material being treated. Sintering is never performed in air or in an oxygen-rich atmosphere. The basic function of a sintering atmosphere is to protect metal parts from the effects of air contact. The final properties of the material using P/M technique are very dependent on the atmosphere where their sintering has been carried out [12, 13]. Some investigations have been carried out to fabricate porous 316L stainless steel [14, 15]. The aim of this work is to study the effect of compacting pressure, sintering temperature and sintering atmosphere on the properties of porous 316L stainless steel compacted. The mechanical properties, microstructure, hardness, and wear properties were comparatively analyzed to understand the mechanisms of sintering. 2. Experimental Procedures 316L stainless steel with average particles size 56µm supplied by (the Nilaco corporation, Tokyo) is used as the metal powder in the present study. Table 1 shows the chemical composition of the powder. Figures 1, and 2 show the distribution of the particle size of used powder, and the scanning of the electronic microscope (SEM) of the loose powder respectively. As can be seen from Figure 2, the shape of the particle is irregular which is typical for their method of production, (i.e. water atomization). Cylindrical green compacts of 12 mm diameter and 15mm height were prepared at three different pressure levels (150, 250 and 350 MPa). The green compacts were sintered at 1200, 1250, and 1300 oC for 2 h at a constant heating rate of 5 oC/min. Nitrogen (N2), argon (A), and vacuum (V) are used as different controlled atmospheres. After sintering, the 316L stainless steel compacts were cooled at the rate of 20 oC/min. The densification coefficient, ϕ, of the compacts was calculated using the following equation: ϕ = (ρs -ρg)/ (ρt -ρg) 1 where ρs, ρg and ρt are the sintered density, green density and theoretical density, respectively The green and sintered densities of samples were determined from weight and dimensional measurements, which were accurate to within ±0.001 g and ±0.001 mm, respectively. Radial and tangential shrinkage was calculated after sintering. Mechanical properties such as compaction strength, modulus of elasticity and hardness of sintered stainless steels were evaluated using universal testing machine and Rockwell hardness tester respectively. To determine the yield strength of the specimens, uniaxial compression tests were carried out at room temperature with a crosshead speed of 2×10−5 m/s using a universal testing machine. A Rigaku X-ray diffractometer was used for the XRD analysis. The glancing incidence X- ray diffraction technique was used for surface phase identification of untreated and oxidized samples. Cu Ka radiation source was used and the incidence beam angle was 2o. Diffraction angle range was between 20o and 80o, with a step increment of 0.05o and a count time of one second. The wear resistant properties of the samples were investigated using a pin-on disc type wear tester in term of weight losses. The contact surfaces of these pins were polished to 0.3 µm roughness and rubbed against a hardened stainless steel disk with the roughness of 0.3 µm and hardness of 62 HRC. All wear tests were carried out at an applied normal load of 20 N,

  • International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 12 No: 01 12

    I J E N S IJENS 2201 yraurbeFIJENS © -IJMME 4848-103021

    and a linear velocity of 0.5 m/s. The total sliding distance was 1000 m. The test was conducted at room temperature without lubricant. The sample preparation for metallographic study was performed according to the standard method of grinding on emery papers and subsequently alumina polishing. Fry’s (5 g CuCl2, 40mL HCl, 30mL H2O, and 25mL ethanol) and Marble’s (4 g CuSO4, 20mL HCl, and 20mLH2O) reagents were used for chemical etching. Scanning electron microscope (SEM) (a JSM—6400 JEOL Company, Japan) was used to study the microstructural. 3. Results and Discussion P/M techniques are very promising because it almost waste-free net-shape forming a capability of precise choice of chemical composition by using high-melting alloy to improve implant biocompatibility; and a lack of chemical inhomogeneity typical for cast as well as some of plastic formed materials, thus improving resistance to corrosion. P/M techniques have also the possibility to form various composite materials containing additions to improve the biofunctionality; and porosity that for appropriate pore geometry, improves bone tissue in-growth, thus enhancing endoprosthesis stability. Finally, P/M processing allows the flexibility to tailor the microstructures, which has been used in fabricating high porosity

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