Characterization of borided AISI 316L stainless steel implant

Characterization of borided AISI 316L stainless steel implant

Vacuum 65 (2002) 521–525 Characterization of borided AISI 316L stainless steel implant a . I. Ozbek , B.A. Kondukb, C. Bindala,*, A.H. Ucisikb a Eng...

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Vacuum 65 (2002) 521–525

Characterization of borided AISI 316L stainless steel implant a . I. Ozbek , B.A. Kondukb, C. Bindala,*, A.H. Ucisikb a

Engineering Faculty, Department of Metallurgical and Materials Engineering, Materials Engineering, Sakarya University, 54187 Esentepe Campus-Sakarya, Turkey b Department of Prostheses, Materials and Artificial Organs, Institute of Biomedical Engineering, Bogazic-i University, 80815 Bebek-Istanbul, Turkey

Abstract The present study reports on characterization of borided AISI 316L stainless steel implant. Boronizing heat treatment was performed on a cylindrical bar of AISI 316L austenitic surgical stainless steel with a diameter of 2 mm and a length of 10 mm using slurry salt bath consisting of borax, boric acid and ferro-silicon. The susbstrate AISI 316L was essentially containing 0.022 wt% C, 0.79 wt% Si, 1.6 wt% Mn, 0.25 wt% P, 0.002 wt% S, 15.30 wt% Cr 14.09 wt% Ni, 2.63 wt% Mo and 0.05 wt% Cu, respectively. Boronizing treatments were conducted at 8501C, 9001C, 9501C, and 10001C, for 2, 4 and 6 h, respectively. Depending on process time and temperature, the thickness of boride layer formed on substrate ranged from 12 to 40 mm. The hardness of borides formed on the surface of substrate was over 1500 VHN. The presence of borides (e.g. Fe2B, CrB, Ni3B) formed on the surface of borided AISI 316L stainless steel was confirmed by classical metallographic technique combined with X-ray diffraction analysis. The distribution of alloying elements was determined by means of energy dispersive X-ray spectroscopy spectrum from surface and line-scan analysis from surface to interior. r 2002 Published by Elsevier Science Ltd. Keywords: Boronizing; Stainless steel; Borides; Slurry salt; Hardness

1. Introduction In orthopaedic surgery and, particularly in total hip replacement, metals are the most favored materials because of their good mechanical stability. On the other hand, metals corrode in contact with aggressive body fluids or tissue [1]. Therefore, the designer must be careful when selecting materials of this type. Austenitic stainless steels, as their name implies, have an austenitic microstructure (FCC) at room temperature and cannot be hardened to any great extent by heat treatment, *Corresponding author. Fax: +90-264-346-0351. E-mail address: [email protected] (C. Bindal).

although they can be appreciably strengthened by cold-work. They are, however, usually quenched, not to produce martensite but to minimize the formation chromium carbide as this causes a reduction in the corrosion resistance of the alloy. These steels are austenitic and low carbon, chromium–nickel–molybdenum stainless steels. The presence of higher molybdenum enhances the corrosion resistance. AISI 316L stainless steel has low carbon and high nickel and chromium. Low carbon content does not cause intercrystalline corrosion. But this type of steels may corrode inside the body under certain conditions such as that in a highly stressed and oxygen-depleted region. Mainly, these steels are used as implant

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materials in biomedical applications, for example, in temporary devices such as fracture plates, screws, and hip nails [2–5]. Boronizing is a thermochemical diffusion surface treatment in which boron atoms are diffused into the surface of work piece to form hard borides with the base material [6–10]. The aim of the present study is an attempt to characterize borided AISI 316L austenitic stainless steel implants in conventional slurry salt bath, e.g. hardness, distribution of alloying elements, etc. Specifically, we used a Vickers indenter, an optical microscope, scanning electron microscope (SEM) to determine mechanical and microstructural properties of boronized 316L stainless steels. To determine distribution of alloying elements from surface to interior energy dispersive X-ray spectroscopy (EDS) was used.

2.3. Film characterization, coating layer and hardness The nature and type of borides formed in coating layer are closely related to chemical composition of substrates concerned. The presence of borides formed in coating layer was confirmed by means of X-ray diffraction (XRD), SEM and optical microscope. The distribution of alloying elements was confirmed via EDS from surface to interior. The microhardness of borides formed on the surface of AISI 316L austenitic surgical stainless steel was measured using a Vickers microhardness tester and a load of 0.5 N. Vickers hardness values of borides formed on the surface of AISI 316L stainless steel and matrix were 22 and 6 GPa, respectively. The thicknesses of borides were measured by means of a digitial thickness measuring instrument attached to optical microscope.

2. Experimental details 2.1. Substrate materials

3. Results

The substrate material used for this study was AISI 316L austenitic surgical stainless steel. AISI 316L austenitic surgical stainless steel test piece had a cylindrical shape and was 10 mm in length and 2 mm in diameter. The susbstrate AISI 316L was essentially containing 0.022 wt% C, 0.79 wt% Si, 1.6 wt% Mn, 0.25 wt% P, 0.002 wt% S, 15.30 wt% Cr, 14.09 wt% Ni, 2.63 wt% Mo and 0.05 wt% Cu, respectively.

3.1. Microstructure

2.2. Boronizing Boronizing was carried out using a slurry salt bath consisting of borax, boric acid and ferrosilicon. Boronizing treatments were performed at 8501C, 9001C, 9501C, and 10001C, for 2, 4 and 6 h, respectively. Test materials to be boronized were immersed in a slurry salt bath using a sealed container then they were sealed including test materials and they were placed in an electrical resistant furnace. Test materials were heated to desirable temperature under atmospheric pressure and held for a predetermined amount of time. This is followed by quenching in air.

Both optical and SEM cross-sectional examinations of the borided AISI 316L austenitic stainless steel implants revealed a compact and smooth morpholgy to a depth ranging from 12 to 40 mm. Coating layer formed on the stainless steel substrate essentially have three distinct regions which are; (i) layers having borides (i.e. Fe2B, CrB, Ni3B), (ii) the region below boride layers, where boron makes solid solution, which has hardness less than that of borides and higher than that of original alloy, and (iii) steel matrix, which is not affected by boron. Fig. 1 shows an optical cross-sectional view of AISI 316L stainless steel borided in slurry salt bath medium. As can be seen in Fig. 1, borides formed on the stainless steel substrate had a compact and smooth morphology compared to borides formed on the surface of plain carbon steels. The distribution of alloying elements was determined by means of EDS spectrum from surface and linescan analysis from surface to interior (Figs. 2 and 3).

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3.2. Hardness, film characterization and boride layer

Fig. 1. The optical cross-sectional view of borided AISI 316L stainless steel sowing morphology of borides, 850  .

The hardness of borides formed on the surface of borided AISI 316L austenitic surgical stainless steel is much higher than that of substrate. It is possible to claim that these are a consequence of presence of hard Fe2B, CrB, and Ni3B (see Fig. 4). Fig. 4 shows X-ray diffraction patterns of AISI 316L austenitic surgical stainless steel borided in slurry salt bath medium. Vickers hardness values of borides formed on the surface of AISI 316L stainless steels and matrix were 22 and 6 GPa, respectively. The depth values of borides are given in Table 1 as a function of process time and temperature.

Fig. 4. X-ray diffraction pattern of borided AISI 316L austenitic surgical stainless steels for slurry salt bath medium.

Fig. 2. The distribution of alloying elements from surface to interior confirmed by EDS from surface to interior.

Table 1 The variation of depth of boride layer thickness as a function of process temperature and time Steel AISI 316L

Temperature (1C) 850

900

950

1000 Fig. 3. EDS spectrum of borided AISI 316L stainless steel.

Time (h)

Thickness of boride layer (mm)

2 4 6 2 4 6 2 4 6 2 4 6

5 8 10 7 9 12 14 25 32 15 30 40

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4. Discussion The above results demonstrated that it is possible to characterize some properties of stainless steel by using conventional slurry salt bath boriding technique. Optical and SEM examinations of borides formed on the surface of AISI 316L austenitic surgical stainless steel substrates revealed a smooth and compact morphology and good bonding of the film to the steel substrates. As is well-known, alloying elements by changing the diffusivity of boron atoms modify coatingsubstrate interface. The higher the alloy, the smooth the compact interface. The diffusivity of boron at 9501C is 1.82  10 8 cm2 s 1 for boride layer and 1.53.10 7 cm2 s 1 for diffusion zone. As a consequence, the boron containing diffusion zone is extended several fold the depth of nonoxide ceramic boride layer into the steel substrates. Depending on boronizing time and chemical compositions of steel substrates, the depth of borides ranged from 10 to 40 mm (Fig. 5). It was observed that increase in temperature results in thick coating layer. There is nearly a parabolic relationship between depth of borides and diffusion time. Previous studies done by Pelleg [11] and Ucisik et al. [12] showed that boronizing of carbon steels usually leads to formation of two borides, FeB and Fe2B, FeB near the surface and Fe2B in the vicinity of steel matrix.

Fig. 5. The variation depth of coating layer as a function of process time and temperature.

In the present study, the presence of borides was identified via XRD analysis (see Fig. 4). EDS revealed that nickel concentrates in the base metal beneath the coating, chromium and manganese preferentially enter the coatings by substituting for iron in the Fe2B and FeB. Silicon, which is insoluble in iron borides, concentrates strongly at the interface with the coatings. The boriding coatings grown on the ternary alloys generally constituted an inner Fe2B single-phase and an outer FeB-base polyphase region containing FeBx with x > 1 and, external surface, an iron in boron solid solution. With increasing contents of the third alloying element in the alloys, both the depth of FeB-base region and FeB/FeBx ratio increases. Hardness measurements showed that the hardness of borides are much higher than that of base steel. We believe that this is a consequence of the presence of hard Fe2B, CrB, and Ni3B in the coating layer. In thermo-chemical boronizing treatments, high hardness is attained directly through formation of borides during boronizing and does not require quenching. Vickers hardness values of borides and AISI 316L stainless steels substrate were 22 and 6 GPa, respectively.

5. Conclusion The results obtained from present study can be summarized as follows: (a) Optical and SEM examinations of boride types ceramics formed on the surface of AISI 316L surgical stainless steel has a compact and smooth morphology. (b) Depending on holding time and process temperature the layer thickness of boride types of ceramics ranged from 5 to 40 mm It was found that increase in temperature results in thick coating layer. (c) EDS revealed that nickel concentrates in the base metal beneath the coating, while chromium and manganese preferentially enter the coatings by substituting for iron in the Fe2B and FeB. Silicon, which is insoluble in iron borides, concentrates strongly at the interface with the coatings.

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