Corrosion resistance of TiN coated implant alloys in artificial physiological solution

The corrosion behavior of TiN coated NiTi-Shape Memory Alloy, Ti6Al4V and 316L Stainless Steel implant alloys in artificial physiological solutions were investigated by means of electrochemical test. The cathodic arc physical vapor deposition method was used for film deposition. Surface morphology and surface composition were studied using SEM/EDAX and XRD. ICPAES was used to determine ions leached from each specimen when kept immersed in artificial physiological solution for sixteen weeks. The corrosion stability, elemental out-diffusion resistance, microhardness were found to be increased by this treatment.


INTRODUCTION
Nickel-Titanium (NiTi), shape memory alloy is useful in medical applications due to their superelastic properties and shape memory effect [1][2][3][4][5]22 . NiTiis safe and bio-compatible in many in vitro and vivo studies due to its surface oxide film, which is mainly titanium oxide and it prevents from corrosion as well Ni leaching out from surface.The mechanical damage of superficial oxide film is noted to leach out nickel as it has high content of Ni 13 .Nickel is noted to induce hypersensitive reactions and tissue necrosis.The induction of cancer by nickel ions is suspected 14 .The toxicity of NiTi corrosion products has been noted in vitro 15 and increase of Nickel in blood has been found after NiTi implanted into animals 16 .Ti6Al4V alloy is also driving attention due to its excellent mechanical properties, corrosion resistance and super plasticity.Ti6Al4V alloy have a strong affinity for oxygen due to their high titanium content and promoting the formation of a stable and tightly adherent protective oxide layer on their surface.This is the reason for the good corrosion properties of this material.In biomedical applications, the chemical composition and stability of this surface oxide layer is very important because the surface of a biomaterial is in direct contact with biological tissues.Recently, some studies have shown that the possibility of vanadium release from Ti-6Al-4V in vitro and vivo, which is considered as a toxic element, may give rise to biocompatibility problems, altering the stability of this alloy and its viability as a biomaterial [17][18][19][20][21] .It is also noted that NiTi and 316L stainless steel fall in higher grade in terms of anticorrosion 6 .However, physicians frequently still have concerns against 316L stainless steel because of the contents Ni and Cr, which are both toxic.NiTi, Ti6Al4V and 316L stainless steel has proven safe and bio-compatible in many vitro and vivo studies but the leaching out of the harmful ions to the body tissues and fluids inside the human body has been raising much safety concerns.Their medical applications noted various medical complications such as allergic reactions, metallogenic and electrochemical reactions, metal toxicities and carcinogenicity [7][8][9][10][11][12] .Therefore, proper passivity to prevent surface layer degradation and ions release into the environment is considered crucial for medical applications of implant alloys.In this study, cathodic arc physical vapor deposition technique is used for TiN coating to improve corrosion resistance of medical implant alloys.Samples are immersed in the plasma produced by the vacuum cathodic arc technique and negative bias is applied to them.The energetic ions having energy equal to neV (where n is the charge state of the ions and V is the applied bias voltage) are extracted from the plasma and are bombarded on the samples at nearly perpendicular direction.Nitrogen pressure during the deposition is maintained in the range of 10 -3 torr.As the sheath of plasma takes the shape of the samples, samples having any complicated geometry can be uniformly bombarded by these ions.

MATERIALS AND METHODS
Samples in the sheet form of NiTi, Ti6Al4V and 316L stainless steel implant alloys were used in the present study.NiTi-shape memory alloy was procured from Johnson Metthey, USA.Chemical composition of NiTi, Ti6Al4V and 316L stainless steel implant alloys are shown in Table 1, Table 2, and Table 3 respectively.
The sheets were cut into 12mm X 12mm square samples each having thickness 0.5mm.Samples were mechanically polished using SiC paper in successive grades from 200 to 2000 grit followed by a final mirror polish with a 0.5µm grade diamond-lapping compound.Samples were ultrasonically cleaned with acetone before coating.Cathodic arc physical vapor deposition technique was used to produce TiN coating on samples with process condition of bias voltage -200V, gas pressure 8 mtorr and deposition time 20 minutes.To get good adhesion of the deposited film with the samples, the samples were prior to deposition first sputter cleaned with Argon ions at pressure of 10 -4 torr and then heated to temperature of 400 0 C. Increase in surface temperature helps in better bonding of film with the samples.This was done using radiant heater mounted in the chamber.The sample surface was also sputtered by titanium ion bombardment for three minutes.For this bombardment the titanium ions coming from arc source were accelerated towards the sample surface with the bias voltage of -1000V.Because of this Ti ions clean the substrate as well as they get implanted in the sample surface.The projected length of these ions is of the order of few nm.These implanted ions help in the formation of good adherent film.Six Ti cathodes with purity 99.95% were used.All coated samples were ultrasonically cleaned again before electrochemical test.Corrosion resistance of implant alloys were investigated by Tafel Extrapolation method.The electrochemical tests were carried out at room temperature, in Normal saline solution and Ringer Lactate solution.The concentration of electrolytes in Normal saline solution and Ringer Lactate solution is shown in Table 4 and Table 5 respectively.
In dissolution test, six samples of surface area 1 cm 2 of each type coated and bare were immersed in 50ml of Ringer Lactate solution in polypropylene bottles.The bottles were evacuated and closed tightly and incubated in thermostatic chamber at 37±1 0 C for sixteen weeks.All bottles were shaken and rotated with a speed of 72 rpm.After 4, 8 and 16 weeks the solutions were analyzed by Inductive Coupled Plasma Atomic Emission Spectroscopy (ICPAES) to determine amount of Ni from NiTi, Al and V from Ti6Al4V and Cr from 316L stainless steel, leached out from samples.Microhardness measurements were performed on six different places of a sample by means of a microhardness tester of indenter type Vickers, duration time 5 seconds and test load of 10 g.Film thickness was determine by optical microscope examination of cross section through the coating.Thickness of TiN coating on all samples was found to be 1.5µm.Surface morphology and surface composition were studied using SEM/EDAX and XRD.

RESULTS AND DISCUSSION
The essential readings acquired from the electrochemical tests are summarized in Table 6 and Table 7.The Tafel plots for alloys have been presented in Fig. 1 and Fig. 2.     Corrosion resistance of bare NiTi and Ti6Al4V is found to be comparable but lower than that of 316L stainless steel in Normal Saline solution.The order of corrosion resistance is 316LSS>> NiTi>>Ti6Al4V.For TiN coated NiTi, Ti6Al4V and 316LSS the order of corrosion resistance is 316LSS>>NiTi>>Ti6Al4V.Reduction in I corr value due to TiN coating, the order is 316LSS>>Ti6Al4V>> NiTi>>.TiN coating is best coating technique for reduction in I corr for 316LSS rather than for NiTi and Ti6Al4V in Normal Saline solution.TiN coating shows improvement in corrosion resistance for NiTi, Ti6Al4V and 316LSS in Normal Saline.
The Ringer Lactate solution is more corrosive medium than Normal Saline solution for all alloys considered here for study.Bare NiTi shows good corrosion resistance in Normal Saline solution than Ringer Lactate solution.But TiN coated NiTi shows almost equal I corr value in both electrolyte solutions.
The dissolution of Ni from NiTi, Al and V from Ti6Al4V and Cr from 316L stainless steel from bare and TiN coated samples is shown in Table 8.The Cr and V were not detected (ND) even after 16 weeks dissolution test of bare and coated 316L stainless steel to the limit of 50 ppb.Al was detected but coating reduces the leaching out from surface by one third of its bare samples in sixteenth week for Ti6Al4V alloy.For bare NiTi, Ni leached out from surface was detected in sixteenth week and found to be 60ppb.But for TiN coated NiTi it was not detected.
Wt.% ratio N/Ti of TiN film prepared under the deposition conditions, nitrogen gas pressure of 8 mtorr, bias voltage -200V and deposition time 20 minutes is shown in Table 9.The presence of small amount of oxygen in the film was noted.This could be due to contamination of film while handling in open atmosphere before EDAX.Equal wt.% of Nitrogen and Titanium in the film stoichiometry was observed in 316L stainless steel.In case of NiTi and Ti6Al4V alloy wt.% of Titanium is more than Nitrogen in film composition.SEM micrograph of samples deposited at -200V bias voltage is shown in Fig. 3

Fig. 1 :
Fig.1: Tafel plots for Bare and TiN coated NiTi, Ti6Al4V and 316L Stainless Steel in Ringer Lactate Solution at room temperature