CHARACTERIZATION OF POROUS NITRIDED Ti6Al4V BIOMEDICAL ALLOYS PRODUCED BY SOLID STATE SINTERING

Relative high elastic Modulus in materials such as 316L, CoCrMo and Ti6Al4V in long term implant devices (<10 years), can lead to stress shielding effect, inducing a bone–implant mismatch. A

reduction in elastic modulus was possible using the space holder method, achieving a maximum total porosity of 48% and a total reduction of Young modulus 47.8 times in comparison with

wrought Ti6Al4V. However, induced porosity implied a substantial increase of specific area, accelerating the corrosion rate. Hence, nitrogen was flowing during 0.5 h and 2h in the sintering

processes at 1250 °C. The corrosion resistance of nitrided sintering was a minimum two times higher in comparison with sintering compacts.

Keywords: Powder metallurgy, biomaterials, corrosion , Ti6Al4V

Farías Velázquez, Iván Gerardo, Jiménez Alemán Omar, Olmos Navarrete Luis, Alvarado Hernández Francisco, Vergara Hernández

Héctor Javier, Cabezas Villa José Luis.

MATERIALS AND METHODS

The sintering compacts were fabricate using prealloyed powders of Ti6Al4V,according to ASTM B348 standard grade 23.

The space method was carried out according to follow diagram.

Shape, distribution and particle size.

Porous Ti6Al4 fabrication by means space holder method.

It was two finalsproducts, where one wassintered under argonatmosphere, the otherone was sintered firstunder argon during 1hour and after nitrideduring 0.5 and 2 hoursrespectively at 1250 °C.In each case, the coolingwas under argonatmosphere. All thesintering cycles aresummarizing.

Sintering cycles.

RESULTS

a) b)

Mechanic behavior in of different sintering samples.

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José Guadalupe Zuno #48, los Belenes, Zapopan,

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The SEM micrographs shown thedifference between shrinkage of particlesand the macropores yielded by the spaceholder.

Porous samples sintered: a) 1100° C and b) 1300°C.

Two pairs of samples are shown: Sample WithoutInduced Porosity (SWIP) and Induced PorositySample (IPS). Sintered at 1100°C and 1300° Crespectively.

Pore volume fraction on samplesranges from 27 to 50%. Largerinduced pores increased around20% of the volume fraction.

Sample E(GPa) σ (MPa) Yield strength

(MPa, off set =0.2%)

IPS1300 6.657 551.3 237.5

IPS1100 4.484 418.6 108

SWPI1300 3.894 335.4 79.3

SWPI1100 2.608 138.1 58.7

Elastic modulus was determined from the graph stress vs deformation withuniversal testing machine, in compression mode. Using cylindrical specimenwith dimensions of 0.8 in diameter by 1 cm in height.

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

Ti6Al4V Ti6Al4V, Nitrided0.5h

Ti6Al4V, Nitrided2h

Sintered 1100°C Sintered 1300°C Sintered, Nitrided0.5h

Sintered, Nitrided2h

Corrosion Rate

Icorr (A/cm²)

0

1

2

3

4

5

6

7

IPS1300 IPS1100 SWPI1300 SWPI1100

Elastic Modulus

E(GPa)

CONCLUSIONS

In the present work samples with different values of porosity from 27 to 50% werestudied in terms of their mechanic and corrosion properties. In order to enhance thecorrosion properties a film of TiN was developed during the sintering process by passingN2 gas at high temperature. The following conclusions can be drawn:

1. Compression tests showed that increasing sintering temperature increased the elasticmodulus. This is mainly because the size and number of necks increased during theprocess. It was also identified that porosity diminished the mechanical properties.

2. The corrosion rate in sintering Ti6Al4V were until 254 times higher in the worst casescompared with wrought Ti6Al4V. However, it was found a enhancement reducing thatpath at 45 times by means the used of nitrogen atmosphere during the sinteringprocess.

3. Compression tests were evaluated by Computed microtomography showing that largepores drives the deformation mechanism.

Potentiodinamic polarization curves

Element mapping by EDS of in situ nitrided samples

Compacts sintered and nitrided in situ.

Plastic deformation of thesample during compression iscontrolled by the large pores.First pores close to the punchwere deformed until they areclose, with small changes inthe lower part of the sample.this behavior prevents thefissure propagation on thedirection of charge.

Fissures go in perpendiculardirection to the charge. Largerpores appeared aftercompaction and the volumefraction of small pores issignificantly reduced.