Pulsed Plasma Nitriding of Stainless Steel Nitriranje nerjavnega jekla v pulzirajoči plazmi
Torkar M.,1 V. Leskovšek, IMT Ljubljana
B. Rjazancev, Department of Orthopaedics, Hospital Jesenice
A use could be found for pulsed-plasma nitrlded AISI 316L stainless steel in a wlde range of biomedlcal appllcatlons, e.g. femoral and biartlcular heads of joint prostheses. Forged samples of steel vvere pulsed-plasma nitrided at a temperature of 540°C for 24 hours to increase the hardness of the surface. During nitriding, the hardness of the 70 pm thick layer uniformly increased to 958 HV 0.1. The hardness of the steel belovv the nitride layer remained unehanged, 210 HV 0.1. The thickness of the layer depended on the process parameters. The research shovved that nitriding of stainless steel for medical implants in pulsed plasma is feasible.
Key vvords: stainless steel, pulsed plasma nitriding, biomedical applications
Nerjavno jeklo AISI 316L, nitrirano v pulzirajoči plazmi, bi bilo mogoče uporabiti za biomedicinske namene, npr. za femoralne in biartikularne glave kolčnih vsadkov. Kovani vzorci jekla so bili nitrirani v pulzirajoči plazmi 24 ur na temperaturi 540°C. Med nitriranjem je trdota 70 pm debele plasti enakomerno narasla do 958 HV 0.1. Trdota jekla pod nitrirano plastjo je ostala nespremenjena, 210 HV 0.1. Debelina nitrirane plasti je odvisna od parametrov procesa. Raziskava je pokazala, da je nitriranje medicinskih implantatov v pulzirajoči plazmi izvedljivo.
Ključne besede: nerjavno jeklo, nitriranje v pulzirajoči plazmi, uporaba v biomedicini
1. Introduction
An increase of the wear resistance and surface strength of different steels and alloys vvith nitride forming elements can be obtained by pulsed plasma nitriding'. Usually, also the corrosion resistance of the surface is increased, vvhile the corrosion resistance of stainless steel is sometimes slightly dimin-ished. Modem nitriding devices and technology, hovvever, maintain or even increase the corrosion resistance2.
Nitriding in pulsed plasma is beneficial due to the low temperature (betvveen 350 C and 660" C) of the process and the possibility of influencing the compo-sition of the nitrided layer (y, e, y+e, and diffusion layer)3. The low temperatures of the process maintain the mechanical properties of the material belovv the layer unehanged. The process is ecological friendly and nontoxic.
Gases in normal state are nonconductive for elec-trical current. This property is changed by high volt-
Dr. Matjaž TORKAR IMT. Lepi pot 11 61000 Ljubljana
age or at low pressure when lightning or glovv dis-charging appears, respectively. In both cases, the nonconductive gas is transformed to an ionized plasma vvith a sufficient electrical conductivity. Nitriding in pulsed plasma is based on glovv diseharging pulsed current in a lovv pressure chamber. Electrons are released on the cathodic surface of the sample, sputtering off the surface atoms, and nitrogen ions migrate into the specimen.
At a distance of some millimeters above the cathodic surface of the specimen, the ions accelerate and hit the surface vvith high kinetic energy. About 90 % of this energy is transformed to heat, vvhich vvarms the surface up to the nitriding temperature. The heat is controlled by electric povver, and no additional heating is required.
Nitrogen ions are highly reactive in the plasma, and iron nitrides start to form on the sputtered surface. Because of the lovv temperature, FeN mole-cules on the surface of the tool or sample decom-pose into lovver nitrides. At the decomposition FeN-^Fe2N^Fe3N^Fe4N, nitrogen is released. A part of the released nitrogen diffuses into the sample and the rest is returned into the plasma. The process
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Figure 1:
Schematic presentation of the ion nitriding process Slika 1:
Shematski prikaz postopka ionskega nitriranja
of ion nitriding is shovvn schematically in Fig. 1. In principle, ali materials based on iron can be nitrided, since vvith glow discharging, ions of nitrogen are ac-tive enough to recombine on the surface. In some minutes of treatment, the nitride layer is formed and the steep gradient of concentration accelerates the diffusion of nitrogen into the specimen4.
The objective of the present research vvas to check the nitriding of AISI 316L stainless steel in pulsed plasma.
2. Experimental work and results
2.1. Experimental procedure
The samples for nitriding vvere machined from the AISI 316L stainless steel bars vvith diameter of 30 mm and the composition of the steel vvas: 0.049 % C, 17.9 % Cr, 13.1 % Ni, 2.5 % Mo, 0.03 % Al and vvere pulsed-plasma nitrided at 540 C for 24 h.
After nitriding, the samples vvere prepared for microstructure examination and hardness tests. The hardness vvas measured vvith a Vickers indenter, us-ing a 100 g load and a 10 s load duration. The microstructure vvas examined, after etching vvith
Marble's reagent, by optical microscopy, vvhile the fracture surfaces of the nitride layer vvere examined by scanning eleetron microscopy (SEM).
2.2. Experimental results
In Fig. 2 a and b, the optical micrographs of unetehed and etehed nitrided layers are shovvn. The nitrided layer is light-gray in an unetehed condition, and only inclusions are found by optical or scanning microscopy.
After etching in Marble's reagent, the nitride layer is darker and it looks homogeneous.
The optical micrographs (Fig. 3 a and b) of the transverse seetion through the base steel and nitride layer shovv Vickers imprints and a needle serateh, both of vvhich confirm the increased hardness of the nitride layer. The needle serateh, easily visible in the base steel, disappeares vvhen crossing the harder nitride layer. The increased hardness of the nitride layer is connected to the reduetion of its fracture toughness and the appearance of a brittle fracture, vvhereas the fracture of the steel belovv the layer remains duetile. In Fig. 4 a the cracks in the nitride layer, formed at bending the nitrided surface to an angle 180 are shovvn. The fracture is brittle and the crack stops in the interface of the nitride layer-steel, because the base material has a higher fracture toughness. The topology and morphology of the nitrided surface vvith a bending crack is shovvn in Fig. 4 b. The nitride surface can be polished to a high brilliancy, vvhich is important for medical use. as i.e. for femoral and biarticular heads of joint pros-theses.
The modification of the surface morphology and topology may be beneficial for an improved biologi-cal performance or improved bone-bonding5 at the uncemented modular stem or cement-bonding at the cemented stem.
The base steel shovvs after nitriding a duetile trans-granular fracture (Fig. 5).
Hardness measurements shovv an average and homogeneous hardness of 958 HV 0.1 in the layer (Fig. 3 a) and a hardness of 210 HV 0.1 in the base material. The usual hardness transition zone. found in nitrided steels, vvas not found. The results confirm that the process of ion nitriding is suitable for inereasing of the relatively soft AISI 316L stainless steel surface. It is regarded as promising for inereasing the vvear resistance of such materials.
3. Conclusions
Ion nitriding of AISI 316L steel in pulsed plasma in-creases the surface hardness from 210 HV 0.1 to 958 HV 0.1, vvhile the hardness of the base alloy is unehanged.
(a)	'	'	100/im (b)	- - . 1 OO^ni
Figure 2 a and b: Optical micrograph of the nitrided layer after nitriding in pulsed plasma for 24 hours at 540 C,
(a) - unetched, (b) - etched vvith Marble's reagent Slika 2 a in b: Optični mikroposnetek nitriranega sloja po nitriranju v pulzirajoči plazmi, 24 ur na temperaturi 540 C,
(a) - nejedkano, (b) - jedkano v Marble jedkalu
Figure 3 a and b: Optical micrograph of the nitride layer, (a)- Vickers indentions and (b)- a needle scratch Slika 3 a in b: Optični mikroposnetek nitriranega sloja, (a)- odtisi merjenja trdote po Vickersu in (b)- raza preko
nitriranega sloja, napravljena z iglo
The thickness of the layer after 24 hours of nitriding at 540 C is up to 70 pm.
The propagation of the crack opened in the nitride layer stopps at the interface nitrided layer-base steel due to higher ductility of the base steel. This indi-cates to a difference in fracture toughness betvveen the nitrided layer and the matrix.
The modification of the surface morphology and topology may be beneficial for an improved surface bonding vvith bone or cement.
Acknovvledgement
The authors are grateful to the Ministry of Science and Technology, Slovenia for supporting this vvork.
References
1 H. Hornberg; Glimm-Nitrieren: ein Verfahren zum Nitrieren von Stahloberflachen mit Hilfe einer Glimmentladung, Harterei Tech. Mit., 17, 1962, 2, 82.
Figure 4 a and b: SEM micrographs. Nitrided surface vvith bending cracks (a)- bending angle 180 and
(b)- detail of the surface
Slika 4 a in b: SEM posnetek nitrirane površine z razpokami, nastalimi pri upogibu, (a)- kot upogiba 180 in
(b)- detajl površine
Figure 5: SEM micrograph. Ductile fracture of the base material
Slika 5: SEM posnetek žilavega preloma jekla pod nitriranim slojem
2	R. Chaterjee-Fischer, VVarmebehandlung von Eisen-vverkstoffen, Expert Verlag, Sindelfingen, 1986, 125
3	M. Hempel, A. Kochendorfer and E. Hillnhagen: Gefiigeveranderungen in u- eisen durch lonen-bestrahlung, Arch. Eisenhuttenvv., 33, 1962, 6, 504
4	H. Knupell, K. Brotzmann and F. Eberhard, Nitrieren von Stahl in der Glimmentladung, Stahl u. Eisen, 78, 1958. 26, 1871
5	C. P. A. T. Klein, J. G. C. VVolke, R. C. Vriesde, J. M. A. De Blieck-Hoger Vorst: Cortical bone ingrovvth in grooved implants vvith calcium phosphate coatings: a gap model study, J. Mater. Sci. Mater. Med., 5, 1994, 9&10, 569-574