Hybrid Processing of Ti-6 Al-4 V Using Plasma Immersion Ion Implantation Combined with Plasma Nitriding

Divisão de Engenharia Mecânica-Aeronáutica, Instituto Tecnológico de Aeronáutica, Praça Mal. Eduardo Gomes, 50, 12228-900 S. J. Campos SP, Brazil Laboratório Associado de Plasma, Instituto Nacional de Pesquisas, S. J. Campos SP, Brazil Departamento de Física, Instituto Tecnológico de Aeronáutica, S. J. Campos SP, Brazil Institute of Ion Beam Physics and Materials Research, Center Rossendorf, Dresden, Germany Departamento de Física Universidade Federal do Paraná, Curitiba PR, Brazil


Introduction
The Ti-6Al-4V alloy, is one of the most used titanium alloys in aeronautical industries and in biomedical applications, due to its excellent combination of mechanical resistance, toughness and excellent corrosion resistance 1,2 being considered chemically stable 3 .The excellent corrosion resistance and biocompatibility can be attributed to the formation of a passive titanium oxide film on titanium and its alloys avoiding further oxidation of the bulk [4][5][6][7][8][9][10][11] .The Ti-6Al-4V alloy is classified as α + β alloy presenting specific weight of 4.43 g/cm 3 , therefore 56% of the corresponding value to the steel and approximately double to the aluminum density 12,13 .However, the alloy presents inadequate tribological properties, with high friction coefficient of around 0.8 14 .In wear absence, the titanium and its alloys possesses excellent corrosion resistance in many enviroments including saline solutions, similar to corporeal fluids.However in the presence of localized wear, intense corrosion could occur 15,13 .To improve corrosion and wear resistance, as well as fatigue strength, surface treatments such as cold plasma plating and ionic implantation have been investigated 16,13 .
The present work aims to improve the surface mechanical properties of the Ti-6Al-4V alloy by the combination of two plasma assisted processes: Plasma Nitriding and Plasma Immersion Ion Implantation or the combination of both processes to obtain thicker nitrogen implanted layer.

Experimental Procedure
The experiments were carried out at Laboratórios Associados de Plasma -LAP of Instituto Nacional de Pesquisas Espaciais -INPE.The samples with 10 mm in diameter by 1 mm in thickness were polished to mirror like surface, mounted in a sample holder and then introduced to the reactor chamber (Figure 1).The plasma chamber was initially evacuated down to ~ 2.1 x 10 -3 Pa, and then filled with argon gas for approximately 10 minutes.After this purging stage, the argon gas was replaced by nitrogen.The experimental conditions are presented in Table 1.The ion bombardment heats the sample up to around 400 °C.The surface hardness was analyzed by nanoindentation technique, the composition profile by Auger Electron Spectroscopy (AES) and the surface topography by Atomic Force Microscopy (AFM).

Results and Discussions
The Figure 2 presents the hardness profile obtained by nanoindentation technique for the specimens prepared according to Table 1.The hardness varied from 3.8 to 4.7 GPa and 4.3 to 6.1 GPa respectively for untreated (reference) sample and sample #4 treated for process PIII during 150 minutes resulting in an hardness increase of approximately 29.8% in the maximum hardness values.For the two step process sample PIII (75 minutes) + PN (240 minutes) the hardness values varied from 4.0 to 6.6 GPa, that is, around 40.4% increase.The highest hardness values was presented by the sample #1 treated according to PIII (75 minutes) + PN (75 minutes), with an increase of about 59.6%, with values ranging from 4.2 to 7.5 GPa.
The Figure 3a presents the composition profile obtained by AES of untreated Ti-6Al-4V alloy used as a reference showing thin layer of carbon and oxygen, probably, TiC and TiO 2 .The Figure 3b shows   3c, presented the highest nitrogen layer thickness of 65 nm, for maximum atomic concentration of nitrogen of 40% while the lowest value was presented by sample #6, Figure 3d, with 15 nm of depth, with maximum atomic concentration of nitrogen of only ~ 7%.All the analyzed samples presented thin layer of oxygen as expected for this material, concerning to the passive layer that protects the material bulk of corrosion.
Through AFM technique, it was obtained the surface topography of samples #4 and #5, treated according to PIII (150 minutes) and PIII (75 minutes) + PN (240 minutes), respectively, which are presented in Figures 3a and 3b.The AFM results confirm the AES data in the sense that the sample #4 (which is the counterpart of sample #2) presented larger RMS rugosity of 6.9 nm compared to 1.8 nm of sample #5 (counterpart of sample #6).Also the values of the maximum depth of the rugosity, R y , were 53.4 and 18.0 nm respectively for the same samples.These results can also be seeing in the Figures 4a and 4b.
The above results evidenced the surface hardness increase of all the samples compared to reference sample as expected that could improve the wear resistance of Ti-6Al-4V titanium alloy.

Conclusions
The nanoindentation results confirm the increase in surface hardness for both, single (PIII) and two steps (PIII + PN) processes being about 29.8% for the first and 59.6%, for the last compared to untreated sample.
The increase in surface hardness is due to nitrogen implanted layer confirmed by the nitrogen profile measurente by AES technique.
The thickest layer of 65 nm was presented by single step processed sample, PIII ion implanted during 150 minutes.
The thinnest layer of 15 nm was presented by two step processed sample (PIII during 75 minutes + PN during 240 minutes).This result   could be attributed to the nitrogen sputtering during the second step PN processing.It was also evidenced that among the several procedures, the best result was obtained for the material processed according to PIII during 150 minutes.
New combinations of processing parameters are being carried out aiming thicker layer of implanted nitrogen.

Table 1 .
Experimental parameters adopted in the tests.