influence of processing parameters and sintering...

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.

([email protected])

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



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,



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 samples. The first stage of powder densification is usually accomplished by pressing powder particles to create some initial contacts among them (green part). This process should be accompanied by particle rearrangement and plastic compaction. The green density values of 316L stainless steels are shown in Figure 3. A low compaction pressures (ranging from 150 MPa to 350 MPa) have been used to get a samples with high porosity. It is understood from this Figure that the density 316L stainless steel powder, which compressed without any addition or lubricant, increases with increasing compaction pressure, which is the normal tendency. The minimum density was reported at compaction pressure 150 MPa is 4.4 g/cm3 which represents 55% of theoretical density. The density of green samples increase with increasing the compaction pressure to be 5.12 g/ cm3 at compaction pressure 350 MPa, which represent 64% of theoretical density of 316L stainless steel. Erfan Salahinejad et. al., [16] reported that the efficiency of compaction is heavily dependent on the morphology and hardness of powder particles. The high hardness and irregular morphology of powders contribute to low green densities. Poor packing behavior of powders with irregular morphologies causes a broad pore size distribution that can inhibit sintering progress The sintered density values of 316L stainless steels are reported in Table 2. In general, it is understood from this table that the density increases with increasing compaction pressure and sintering temperature. The exception from this trend is that the density of samples which sintered in N2 atmosphere at 1250 oC, shows a density lower than the desity of samples sintered at 1200 oC. The main reason is due to the formation of Cr2N, which is absorbed from the nitrogen atmosphere which in turn reduces the diffusion rate resulting in lower sintered density. In addition, the maximum densities are reported for samples which sintered in N2 atmosphere at sintering temperature 1300 oC may be due to the formation of liquid phase sintering. On the other hand, the values of sintering density are very similar for the samples which sintered in argon and vacuum at the same conditions, (i.e., compaction pressure, and sintering temperature). Also Table 2 showing that the relative densities ranging between 60.75% (4.86 g/cm3) for samples compacted at 150 MPa, and sintered in vacuum atmosphere at 1200oC, and 88.125% (7.05 g/cm3) for samples compacted at 350 MPa and sintered in nitrogen atmosphere at 1300oC. The corresponding porosities of sintered samples are ranging from 39.25 % to 11.875%. Shrinkage of the sintered materials depends on several factors, such as green compact geometry, green density, powder composition, sintering atmosphere, and sintering temperature. For the effect of green compact geometry, shrinkage of the sintered samples along different dimensions in different sintering atmosphere were observed and reported in



Figure 4, a, b, and c. Mainly the shrinkage during sintering occurs due to reduction in size of micropores of the compacts. As can be seen, and as expected, the axial and radial shrinkage decrease with increasing compaction pressure. Also, the shrinkage grows with increasing sintering temperature. It diminishes with increasing compaction pressure and, correspondingly, with enhancement of compact density. Although the axial and radial shrinkage have the same trend, the axial shrinkage was found to be a little higher then the radial shrinkage for the most samples at the same conditions. Dimensional change is related to sintered density. It was clearly observed that dimensional change of the samples sintered in N2 atmosphere at 1300 oC was the highest. This is perhaps one of the reasons for the highest sintered density. Figure 4 a, shows that the maximum axial and radial shrinkage which occurs for samples compacted at 150 MPa, and sintered in N2 atmosphere at 1300 oC are 5.1% and 4.66% respectively. On contrast, the minimum shrinkage occurs for the samples sintered in vacuum atmosphere as shown in Figure 4 c,. In order to investigate the phase evolution of 316L stainless steel powders consolidated at various sintering temperatures, sintered specimens were analyzed by XRD patterns. Figure 5, shows the XRD patterns obtained at the surface of the samples sintered at 1200 oC 1250oC, and 1300oC in nitrogen atmosphere. 316L stainless steel presented characteristic diffraction spectra of face-centered-cubic austenite. No peak other than γ diffraction peaks (111, 200, 220) was detected for samples sintered in nitrogen atmosphere at 1200oC. For the samples sintered in nitrogen atmosphere at 1250 oC and 1300 oC besides the austenitic phase, a traces of Cr2O3 has been detected. Figure 6, shows the XRD patterns obtained at the surface of the samples sintered at 1250 oC in a different atmosphere. 316L stainless steel presented characteristic diffraction spectra of face-centered-cubic austenite. No peak other than γ diffraction peaks (111, 200, 220) was detected for samples sintered in argon and vacuum atmosphere. For the samples sintered in nitrogen atmosphere, besides the austenitic phase, a traces of Cr2O3 has been detected. Compression yield strengths obtained from compression tests for the samples which compacted at 350 MPa were shown in Figure 7. According to the data obtained the compression yield strengths of sintered samples at 1200 °C are lower than the compression strengths of the samples sintered at 1250 °C and both of them are lower than sintered samples at 1300 °C. The same trend was observed for the samples compacted at 150 MPa and 200 MPa. The main reason of low compression yield strength can be explained by insufficient sintering process at such temperatures and the porosity of the samples. Insufficient sintering depends on the low sintering temperatures. It is well known that the sintering is a process of particles bonding and consolidating into compacts through the way of “neck formation and growth”. As a result of insufficient sintering, since original powder particles cannot form exact bond for each other, weak bonds were formed among powder grain boundary. The main reason for the insufficient strength is much amount of neck regions [17]. Generally, insufficient sintered P/M materials have low mechanical properties and low corrosion strength [18]. According to Gibson-Ashby model, the most important structural characteristic of a porous compact that the influence of plateau stress is its relative density ρ/ρs, (the density of the porous compact ρ, divided by that the solid material ρs). The relationship between the relative stress and the relative density is given by [19]: σ = C σo f (ρ/ρs) where σ is the strength of compact, σo is the strength of fully dense material and f(ρ/ρs) is a fractional density dependence function. Several studies have attempted to correlate strength with density by various forms of f(ρ/ρs) [20]. The most commonly used relationship is of the form: σ = C σo (ρ/ρs)

m where C and m are empirical material constants Porous ratio, pores type, shape and size of the pores have important effects on the mechanical properties P/M materials. Powder grain shape and size are important parameters



to decrease the porous ratio of P/M materials. It is known that the porous ratio in P/M materials has serious effects on the mechanical properties of the material. Decreasing porous ratio improves all the mechanical properties of the material. In contrast, decreasing porous ratio decreases the growth of tissue in implemented parts. Shanta Raj Bhattarai et al., [21] reported that high porous materials may favor a better biological outcome of dental implants in vivo, since they seem to induce differentiation towards an osteoblastc phenotype, enhancing bone healing and long term maintenance of osseointegration. The maximum compressive yield strength has been obtained for the specimens, which sintered in nitrogen atmosphere at 1300 °C is 130 MPa (see Figure 7). As the sintering temperature increases, the porosity ratio decreases and the porosity shapes become spherical forms. Generally, increasing the sintering temperature is an important effect on mechanical properties. In all atmospheres, 316L stainless steel sintered by solid-state diffusion, therefore pore size and shape was largely dependent on compaction pressure and powder particle size and shape. Variations in compressive yield strength with sintering atmosphere were noted, particularly in the case of nitrogen atmosphere when Cr2N precipitates had formed in 316L alloy, strengthening occurred, so that compression strength values were consistently higher. Nitrogen dissolved in austenitic stainless steel acts to increase its strength [22]. The samples which sintered in vacuum atmosphere show the lowest compression strength compared with the samples sintered in nitrogen atmosphere or argon atmosphere due to the absence of solid solution strengthening which leads to low densities. Also, the results, which not plotted, show that the compression ultimate stress of the investigated porous materials is increased with increasing the compaction pressing of the compacts prior sintering. Compressive strength of cancellous bone is ranged between 1 and 100 MPa [23]. The compressive strength of the sintered porous 316L stainless steel alloy conforms to the basic compressive mechanical property requirement of cancellous bone with the different corresponding porosity. Generally, the strength of all samples is enough for fixing the implemented parts. Figure 8 illustrates the compressive stress–strain curve of the investigated 316L stainless steel porous materials sintered in argon atmosphere at 1300 oC. Three distinct regions of deformation can be depicted for all samples: linear elastic, plastic collapse, and densification. The first region is characterized by cell wall bending and stretching, whereas in the plastic collapse region the cell walls yield and buckle. Increasing the applied force beyond the plastic region (densification region) resulted in crushing the cell walls, eliminating the void space and the porous material is compressed as a solid. Porosity has a considerable contribution to the Young’s modulus. A number of micromechanical models have been developed to describe the mechanical behavior of porous materials. One of the valid equations expressing the correlation between Young’s modulus and porosity is the following relationship [24]:

E/Es = a (p/ps)2

where E is the Young’s modulus of the porous sample, Es is the Young’s modulus of the cast solid alloy, p and ps are densities of the porous sintered and cast solid alloy respectively. a constant of 1 including data of rigid polymers, elastomers, metal and glasses . Figure 9, shows the effect of sintering temperature and sintering atmosphere on the modulus of elasticity of samples compacted at 350 MPa. Although the samples which sintered in nitrogen atmosphere have a high compressive strength compared with another samples, it was found that the modulus of elasticity is lower. E. Salahinejad et. al., [25] reported that the Young’s modulus of the materials decreases by increasing the nitrogen content. That is, in this range of nitrogen concentration, the weakening effect of the metallic bonds dominates the effect of the formation of some stiff metal–nitrogen bonds on the Young’s modulus. Also, it can be seen that by increasing the sintering temperature the modulus of elasticity increases. Hardness measurements were done on the sintered samples in order to study the effect of sintering temperature and sintered atmosphere. Figure 10, shows the effect of sintering



temperature and sintering atmosphere on hardness of samples compacted at 350 MPa. It is worth to notice that the values of hardness present a tendency to increase with the sintering temperature. The hardness of samples sintered at 1300 oC was higher than those sintered at 1200 oC and 1250 oC. This can be attributed to the higher sintered density of the 1300 oC samples. In addition, the sintering in nitrogen atmosphere shows higher values of hardness than the samples that sintered in argon atmosphere, and the samples which sintered in argon atmosphere shows higher values of hardness than the samples that sintered in vacuum. The highest value (approximately 86.5 HRA) is reached at compaction pressure 350 MPa and sintering temperature 1300 oC, in nitrogen atmosphere. The main reason could be attributed to the dissolution of nitrogen into the samples during the sintering. It has been found that the nitrogen addition to austenitic stainless steels enhances their hardness and strengths. Nitrogen in austenitic stainless steels is an effective element not only in solid solution strengthening but also in grain size strengthening [26]. Under the condition of dry sliding wear with loads of 20 N, the wear loss of the 316L stainless steel sintered at 1200 oC, 1250 oC and 1300 oC in different atmosphere are listed in Table3. It is well known that the wear resistance of a material is proportional to its hardness. It can be found that the best wear resistance was obtained when samples sintered in nitrogen atmosphere at 1300 oC, while the worst were resistance was observed for samples sintered in vacuum atmosphere at 1200 oC. In general, increasing the sintering temperature leads to increasing the wear resistance. In addition, the sintering in nitrogen atmosphere shows the best wear resistance. SEM micrographs taken from the specimens sintered in nitrogen atmosphere at different sintering temperature are shown in Fig. 11. The microstructures illustrate a progression from irregular to spherical pores accompanied with a tendency for pores to be isolated in their distribution by increasing the sintering temperature. Clearly, the porosity level in the 316L stainless steels sintered at 1200 oC was higher when compared to 1300 oC sintered compacts. The solid-state sintered stainless steels contain mostly irregular, intergranular pores. In contrast, at higher temperatures microstructural coarsening is observed and the pores are more predominantly intragranular and more rounded (Fig 11 c) Also, as shown in Fig. 11 the microestructural study of 316L sintered in nitrogen atmosphere reveals that there are no chromium nitrides precipitated in the grain boundaries, and only the typical porosity of the sintered materials can be observed. As no precipitated nitrides are observed, the effect of the sintering atmosphere is due to the presence of nitrogen in the metal that promotes solid solution hardening. The solid solution of nitrogen makes stainless steels sintered in nitrogen less ductile and harder [27]. If sintering is carried out in vacuum or argon, no special feature appears in the stainless steel as shown in Fig. 12. It should be concluded that whereas conventional sintering can give rise to more densities, in some applications porous structures with adequate mechanical properties are attractive, especially in the biomaterials field. Porous materials with sufficient mechanical properties have been recognized as desired bone implants for the last decade. The porous implants provide a better fixation of implants to the bone host, via the growth of new bone tissue into the pore spaces. Furthermore, introducing pores into stainless steel parts results in a decrease in the mismatch of elastic moduli of the implant and surrounding bone, thereby improving the fixation. 4. Conclusions In this paper, the effect of sintering temperature and sintering atmosphere on densification behavior, microstructure, mechanical properties, and wear resistance were studied. The presented results in this paper derive the following: 1. The 316L stainless steel samples sintered at 1200 °C, 1250 °C and 1300 °C in different atmosphere had porosity values at the order of 39.25% and 11.87%. 2. The Mechanical properties of porous 316L stainless steel were investigated by compressive tests focusing on the effects of the porosity on the Young’s modulus and



strength. Results indicated that the Young’s modulus and peak stress decrease with an increase in porosity 3. The 316L stainless steels samples sintered in nitrogen atmosphere exhibit higher strength, wear resistance and hardness than the 316L stainless steels samples sintered in argon, and vacuum atmosphere. 4. An optimum result of the mechanical properties, density, hardness and wear resistance, of sintering in a nitrogen atmosphere at 1300 °C temperature is found to be more suitable for 316L stainless steel powder for sintering for possible hard-tissue applications. References [1] M. M. Dewidar, H. C. Yoon, J. K. Lim. "Mechanical Properties of Metals for Biomedical Applications using Powder Metallurgy Process: A Review," Met Mater Int. (2006); 12(3): pp 193-206. [2] J. J. Daniel, C. D. David. "Structure and Mechanical Properties of Ti–6Al–4V with a Replicated Network of Elongated Pores," Acta Materialia 2011; 59: pp 640–650. [3] N. S. Vandamme, L. Que, L. D. Topoleski, "Carbide Surface Coating of Co-Cr-Mo Implant Alloys by a Microwave Plasma-Assisted Reaction," Journal of Materials Science 1999; 34: pp 3525-3531. [4] J. Beddoes, K. Bucci, "The Influence of Surface Condition on the Localized Corrosion of 316L Stainless Steel Orthopaedic Implants," Journal of Materials Science: Materials in Medicine 1999; 10: pp 389-394. [5] M. U. Kamachi, T. M. Sridhar, et al. "Failures of Stainless Steel Orthopedic Devices-Causes and Remedies," Corros Rev 2003; 21: (2–3). [6] M. Sumita, M. Ikada, T. Tateishi, "Metallic Biomaterials- Fundamentals and Applications," ICP, Tokyo, 2000, pp 629-632. [7] M. Long, H. J. Rack, "Titanium alloys in total joint replacement-a materials science perspective," Biomaterials 1998; 19 : pp 1621-1639. [8] D. W. Hutmacher, "Scaffolds in tissue engineering bone and cartilage. Biomaterials," 2000; 21(24): pp 2529-2543. [9] B. S. Becker, J. D. Bolton, "Corrosion behaviour and mechanical properties of functionally gradient materials developed for possible hard-tissue applications," Journal of Materials Science: Materials in Medicine 1997; 8: pp 793 -797. [10] U. Ripamonti, A. H. Reddi, "Tissue engineering, morphogenesis, and regeneration of the periodontal tissues by bone morphogenetic proteins," Crit Rev Oral Biol Med 1997; 8: pp 154-163. [11] I. H. Oh, N. Nomura, et al.., "Mechanical properties of porous titanium compacts prepared by powder sintering," Scr Mater 2003; 49: pp 1197–1202. [12] W. Moreira-Lima, N. Candela, et al., "Corrosion and wear behaviour of AISI 316L reinforced with AlCr2 particulate composite," In: Proceedings of the Powder Metallurgy World Congress, vol. 3, Shrewsbury, UK. UK: EPMA; 1998; pp 413–418. [13] W. M. Lima, F. Velasco, J. M. Torralba, "Stainless steel matrix composites reinforced with AlCr2," Mater Sci Forum 1999; 299–300: pp 431–438. [14] I. B. Halil, "A novel water leaching and sintering process for manufacturing highly porous stainless steel," Scripta Materialia 2006; 55: pp 203–206. [15] M. M. Dewidar , K. A. Khalil, J. K. Lim, "Processing and mechanical properties of porous 316L stainless steel for biomedical applications" Trans Nonferrous Metal Soc China 2007; 17: pp 468–473. [16] Erfan Salahinejad, Rasool Amini, et al., "The effect of sintering time on the densification and mechanical properties of a mechanically alloyed Cr–Mn–N stainless steel," Materials and Design 2010; 31: pp 527–532.



[17] P. K. Ryan, S. I. Pavan, et al., "Microstructural evolution of injection molded gas- and water-atomized 316L stainless steel powder during sintering," Materials Science and Engineering A 2005; 390: pp 171–177. [18] R. M. German, "Sintering Theory and Practice," Wiley, New York, NY, 1996. [19] L. J. Gibson, and M. F. Ashby, "Cellular Solids: structure and properties," 2nd edn. Cambridge University press, 1997. [20] C. E. Wen, Y. Yamada, et al., "Processing and mechanical properties of autogenous titanium implant materials," Journal of Materials Science: Materials in Medicine 2002; 13: pp 397-401. [21] R. I. Shanta , K. A. Khalil, M. Dewidar., et al., "Novel production method and in-vitro cell compatibility of porous Ti-6Al-4V alloy disk for hard tissue engineering," Journal of Biomedical Materials Research Part A, 2007; pp 289-299. [22] M. Sumita, T. Hanawa, S. H. Teoh, "Development of nitrogen-containing nickel-free austenitic stainless steels for metallic biomaterials–review." Materials Science and Engineering 2004; C 24: pp 753–760. [23] W. Suchanek, M. Yoshimura, "Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants," Journal of Materials Research 1998; 13: pp 94-117. [24] L. Yong-Hua, C. Rui-Bo, et al., "Powder sintering of porous Ti–15Mo alloy from TiH2 and Mo powders," Journal of Alloys and Compounds 2009; 485: pp 215–218. [25] E. Salahinejad, R. Amini, M. J. Hadianfard, "Contribution of nitrogen concentration to compressive elastic modulus of 18Cr–12Mn–xN austenitic stainless steels developed by powder metallurgy," Materials and Design 2010; 31: pp 2241–2244. [26] J. Abenojar, F Velasco, et al., "Atmosphere influence in sintering process of stainless steels matrix composites reinforced with hard particles," Composites Science and Technology 2003; 63: pp 69–79. [27] E. Salahinejad, R. Amini, et al. "Microstructural and hardness evolution of mechanically alloyed Fe–Cr–Mn–N powders," Journal of Alloys and Compounds 2010; 497: pp 369–372.



List of Tables

Table 1. Chemical composition of 316L stainless steel powder.

Cr (%) Ni (%) Mo (%) Si (%) C (%) O2 (%) Fe (%) 16.4 11.4 2.5 0.50 0.02 0.18 Balance

Table 2 density of 316L stainless steel samples sintered at different temperature in different atmosphere.

Sintering Temp. oC/

Density g/cm3 in N2 atmosphere

Density g/cm3 in A atmosphere

Density g/cm3 in V atmosphere

Compaction pressure MPa



150 250 350 150 250 350 150 250 350 1200 5.09 5.352 5.858 4.8925 5.093 5.15 4.86 4.93 4.96 1250 5.057 5.336 5.644 5.223 5.397 5.39 5.2 5.21 5.32 1300 6.49 6.88 7.05 5.376 5.55 5.58 5.408 5.5 5.56

Table 3 Weight loss of 316L stainless steel samples sintered at different temperature in different atmosphere.

Sintering Temp. oC

Weight loss (mg) in N2 atmosphere

Weight loss (mg) in A atmosphere

Weight loss (mg) in V atmosphere




150 250 350 150 250 350 150 250 350 1200 13.4 13 12 14.4 13.6 13.1 14.7 13.4 12.3 1250 10.1 9.4 9 10.3 9.6 9.3 10.6 9.3 9.1 1300 9.1 8.7 8 9.4 9.1 8.8 9.3 9.0 8.8



List of Figures

Figure 1. Particle size distribution

Figure 2. SEM of 316L stainless powder

4

4.25

4.5

4.75

5

5.25

100 150 200 250 300 350 400


Gre

en d

ensi

ty g

/cm

3

Figure 3. Compressibility curve of 316L stainless steel powder.



0

1

2

3

4

5

6

100 150 200 250 300 350 400Compaction Pressure MPa

Sh

rin

kag

e %

Radial Shrinkage at 1200oC

Axial Shrinkage at 1200oC





Figure 4, a. The effect of compaction pressure on the shrinkage of samples sintered in nitrogen atmosphere.

0

1

2

3

4

5

6

100 150 200 250 300 350 400Compaction pressure MPa

Sh

erin

kag

e %

Radial shrinkage at 1200oC






Figure 4, b. The effect of compaction pressure on the shrinkage of samples sintered in argon atmosphere.



0

1

2

3

4

5

6

100 150 200 250 300 350 400

Compaction Pressure MPa

Sh

rin

kag

e %







Figure 4, c. The effect of compaction pressure on the shrinkage of samples sintered in vacuum atmosphere.

Fig. 5. XRD patterns of 316L stainless steel samples sintered in nitrogen atmosphere at different sintering temperature.

Inte

nsity

(C

PS

)

γ(1 1 1)

γ(2 0 0) γ(2 2 0)

γ(2 2 0) γ(2 0 0)

γ(1 1 1)

Cr2O3 Cr2O3

Cr2O3 Cr2O3

γ(1 1 1)

γ(2 0 0) γ(2 2 0)



Fig. 6. XRD patterns of 316L stainless steel samples sintered in different atmosphere at sintering temperature 1250oC

13001250

1200

Nitrogen atmosphere

Argon atmosphere

Vacuum atmosphere0

20

40

60

80

100

120

140

Com

pres

sive

yie

ld s

tren

gth

MP

a

Sintering temperature oC

Figure 7, Compressive yield strength of 316L stainless steel samples compacted at 350 MPa.

Inte

nsity

(C

PS

)

γ(1 1 1)

γ(2 0 0) γ(2 2 0)

γ(2 2 0) γ(2 0 0)

γ(1 1 1)

γ(1 1 1)

γ(2 0 0) γ(2 2 0)

Cr2O3 Cr2O3



0

100

200

300

400

500

600

700

800

900

1000

1100

0 10 20 30 40 50 60 70strain

Str

ess

MP

a

Figure 8, Stress-strain curve of 316L stainless steel porous sample sintered in argon atmosphere at 1300 oC

13001250

1200

Nitrogen atmosphere

Argon atmosphere

Vacuum atmosphere0

5

10

15

20

25

30

35

40

45

50

Mod

ulus

of e

last

icity

GP

a


Figure 9. The effect of sintering temperature and sintering atmosphere on the compressive modulus of elasticity of samples compacted at 350 MPa.



1300

1250

1200

Nitrogen atmosphere

Argon atmosphere

Vacuum atmosphere

60

65

70

75

80

85

90

Roc

kwel

l har

dnes

s (H

RA

)


Figure 10, effect of sintering temperature and sintering atmosphere on hardness of samples compacted at 350 MPa

Figure 11, SEM micrograph of the specimen sintered in nitrogen atmosphere at a) 1200 oC,

b) 1250 oC, and c) 1300 oC

Figure 12, SEM micrograph of the specimen sintered at 1250 oC in a) argon atmosphere, b)

vacuum atmosphere and c) nitrogen atmosphere

influence of processing parameters and sintering...

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