ICMCTF2016 Session D3: Bio-Corrosion, Bio-Tribo-Corrosion and Bio-Tribology

On one-side the need for orthopedic implants has sky rocketed (572000 hip implants and 3.48 million Knee implants by 2030) due to ageing population and failure of joints in active young patients and on the other-side, the failure of implants within short span is of serious concern. This has imposed a stringent requirement for implants with longer service period as apposed to conventional implants which serves only for 15-20 years. The failure of metallic implants (Co-Cr) due to poor wear resistance and release of nano sized debris and associated concerns such as genotoxicity, pain and cost for revision surgery has resulted in search for new materials and also appropriate testing protocols. Despite the fact that ceramic materials Alumina and Zirconia and ceramic coatings such DLC, TiN exhibited lower wear resistance in invitro conditions, these materials also fail suddenly in invivo conditions as the simulators used for testing does not reproduce all conditions experienced in invivo.

When two surfaces are in contact and slide in aggressive environment as in human body, the mechanical, chemical or mechanical-electrochemical processes involves multi-scale phenomena such as mechanical wear, fatigue, corrosion ,depassivation/repassivation, adsorption, embrittlement and few other processes. In order to understand the effect of all these to the total wear at different time scales it is important to develop an appropriate testing method. Tribocrrosion is one of the techniques which is very sensitive to various factors and can provide some insights on contribution of individual factors to the total wear.

In this talk, an introduction to various orthopedic implants and the challenges posed by them will be presented. This will be followed by brief description on various testing protocols adopted as per standards and their limitations. The talk will address how the tribocorrosion studies are superior and makes an attempt to address various challenges posed by conventional techniques. This will be discussed based on various results obtained by our research group on tribocorrosion behavior of nanoalumina , nanozirconia and nanocrystalline diamond coatings on Ti based alloys.

Introduction: Modern hip implants feature a modular design. Increased reported failure rates associated with the utilization of modular junctions have raised many clinical concerns. Typically, these modular interfaces contain circumferential machining marks (threads). To date, no study has examined the effect of modular junction-associated surface topography on the fretting-corrosion behavior of total hip implant materials. This study reports the effects of machining marks on the mechanical and corrosion behavior of hip implant modular junctions.

Methods: The flat portions of two cylindrical, machine polished, CrCrMo alloy pins were loaded horizontally against one rectangular Ti alloy rod. Two surface preparation groups were used for the Ti6Al4V rod (Machine-Polished (MP) and Machine-Lined (ML)). The MP group was prepared by the same methods as the CoCrMo pins. The ML surfaces were prepared with a diamond blade fly cutter. Bovine Calf Serum (30 g/L protein content; 37^oC) at a pH of 7.6 was used to simulate the joint fluid environment. The samples were mounted into the fretting corrosion apparatus with a normal force of 200N and a sinusoidal fretting motion was applied onto the rod at a frequency of 1Hz and a displacement amplitude of 25µm. All electrochemical measurements were performed with a potentiostat in a three electrode configuration with the metals as the working electrode, a graphite rod as the counter electrode, and a saturated calomel reference electrode. Fretting-corrosion tests were conducted under free-potential mode (no applied potential) and potentiostatic mode (applied potential of -250mV).

Results: The results show that there are significant differences between Machine-Lined (ML) samples and Machine-Polished (MP) samples in both electrochemical and mechanical response. In all cases, the magnitude of the drop in potential was greater in the ML group compared to the MP group. The dissipated friction energy, The ML group shows a lower total dissipated friction energy for the entire test compared to MP group. Additionally, the potentiostatic test measurements revealed a higher evolved charge in the ML group compared to the MP group.

Discussion: The ML surfaces increased the mechanical stability, but also compromised electrochemical performance in the implant. In summary, the findings correlate with increased reports of corrosion present in the modular/taper joints.

The laser power interaction effects on the evolving properties of commercially pure titanium during laser metal deposition were analysed. The optimized processing parameters obtained for this research study were, spot size of 4 mm, powder flow rate of 2 g/min, gas flow rate of 2 l/min, and the scanning speed set at 0.002 m/s. A total of seven samples were fabricated with the optimised settings by depositing titanium powder onto a Ti-6Al-4V base metal; An Nd-Yag laser was used by varying the laser power from 400 to 1600 watts while keeping all the other parameters constant. The deposited samples were characterised through the microstructure, microhardness, wear and the corrosion behaviour. The microstructural evaluation revealed that the ratio of dilution increased with an increase in the laser power. Furthermore, it was found that as the dilution increased, the wear resistance of the deposits decreased due to the increased foreign elements (Al and V) from the substrate which inhibited smooth fusion as the molten deposit cooled. Also, microstructural evaluations showed that finer martensitic microstructures were obtained at lower laser power rating which was associated to intra-layer porosity and due to the low laser-material interaction. However, Widmanstätten structures were observed at higher laser power settings together with the presence of interlayer porosity which is desirable for osteointegration.

For biocompatibility, immersion tests in the Hank’s solution were conducted for 14 days. The atomic absorption spectroscopy analyses conducted showed that no leaching happened during the immersion process for all the samples hence, confirming the desirable properties expected of biomedical implants. An overall overview on the effects of the laser power which has a significant effect on the evolving properties is essential in order to know how this process parameter can be controlled to attain certain properties of the material for special, specific and tailored functions.

Hip replacement is one of the most common orthopedic surgical procedures in the United States. However, FDA issued a recall on metal-on-metal (MOM) hip implant in February 2011, leading to the urgency to look for the modification of current design. One study reported that graphitic materials were observed on metal-on-metal hip implants retrieved from patients. Even though the exact mechanism for how the graphitic material is formed has not been determined, the observation supports the hypothesis that the presence of graphitic materials lowers the friction and wear on the implant. Since this tribolayer has the similar Raman spectrum as carbide-derived carbon(CDC), we propose CDC can provide the similar tribological performance as the tribolayer. Due to the necessity of the carbide precursor, silicon carbide's superior mechanical and chemical properties are qualified for the application. The objective of this study is to develop a CDC layer, study the tribological performance with CDC processing conditions and CDC's biocompatibility.

Production of CDC: CDC specimens were produced by heating SiC with 3.5% Cl₂-Ar gas in a fused silica reaction tube inside a resistance-heated electric furnace under ambient pressure. The processing conditions studied include chlorination temperature (1000^oC and 1100^oC), chlorination time (8 hours and 19 hours) and post chlorination treatments (with Hydrogen, ammonia and vacuum for 3, 8 and 24 hours). Tribological Testing: sliding wears of the CDC specimens were determined using a linear reciprocating tribometer (ball-on-flat sliding wear) made by Ducom Instruments. High temperature alumina balls were used in all the sliding tests. Sliding parameters studied include sliding frequency (1, 5, and 10 Hz), sliding time and cycles (0.1, 0.2, 1, 4, and 8 hours). Wear volumes were determined using a 3-D non-contacting optical surface profiler. Bio-compatibility: Cell Counting Kits-8(CCK-8) and live/dead cell viability kit were performed to evaluate whether CDC is applicable to implant application.

The findings show that CDC has superior tribological properties (significantly lower wear rate) as compared to UHMWPE, CrCoMo and Ti6Al4V alloys. Additionally, the study results indicate that chlorination time has an impact on the CDC microstructure and has the most impact on wear rate among the variables studied. Meanwhile, the experimental results show CDC is bio-compatible, indicating the potential of CDC in biomedical field.

Metals and their alloys are very important for orthopedic applications, and the basic requirements for sucessful application of an implant are: chemical stability, mechanical behavior, and biocompatibility in body fluids and tissues [1,2]. The body is an aqueous environment containing ions and organic substances, which form an electrolyte solution. For prosthesis application, the corrosion resistance of metals is one of the major prerequisites to avoid impairment of the material properties due to degradation[3]. The combined action of corrosion and wear on a material is called tribocorrosion[4]. DLC (Diamond-Like Carbon) films have been extensively studied due to their properties that can increase biocompatibility and protect the prosthesis from corrosion. Additionally, DLC coating can be used on Ti6Al4V prosthesis to prevent the substrate from eluting Al and V by plastic deformation and corrosion. This paper is about tribocorrosion studies on DLC and DLC-Ag (DLC containing silver nanoparticles) on Ti6Al4V substrates. DLC and DLC-Ag films were obtained by PECVD by using hexane as precursor. The tribocorrosion behavior of Ti6Al4V covered with DLC and DLC-Ag was investigated in reciprocating mode in Ringers solution. From polarization test results, the protective efficiency of the film was calculated. Silver nanoparticles improved the corrosion resistance of the films. The protective efficiency was 15 and 19% for DLC and DLC-Ag films, respectively.

Acknowledgments: This work was supported by CNPq, CAPES, and FAPESP

References:

[1] U. Kamachimudali, T.M. Sridhar, B. Raj, Corrosion of bio implants, Sadhana. 28 (2003) 601–637.

[2] C. Fleck, D. Eifler, Corrosion, fatigue and corrosion fatigue behaviour of metal implant materials, especially titanium alloys, Int. J. Fatigue. 32 (2010) 929–935.

[3] S. Bauer, P. Schmuki, K. von der Mark, and J. Park, “Engineering biocompatible implant surfaces: Part I: Materials and surfaces,” Progress in Materials Science , vol. 58, pp. 261–326, 2013.

[4] D. Landolt, Corrosion and Surface Chemistry of Metals. Lausanne: EPFL Press, 2007.

The progress that is observed in implant technology requires new materials with enhanced properties in respect to broadly understood mechanical, tribological and biological features. Reducing the wear of implants is one of the important aspects of their functioning in the human body . Wear debris of small size and large surface development can cause inflammations and allergies, e.g. due to increased release of metal ions to the tissue . The studies under dry friction conditions often do not correspond to the working environment of implants. Thus, it is advisable to test new materials under conditions similar to the natural, where during friction and wear the components of body fluids, such as proteins are involved, what may lead to enhanced wear loss [1].

SiO_x incorporated carbon coatings with different Si/O surface concentrations ratio were synthesized on AISI 316L steel with use of the hybrid RF PACVD/MS technique [2,3]. Ball on disk tests were carried out under ambient and simulated body fluids conditions against ZrO₂ and AISI 316L balls. As the body fluids bovine serum albumin and 0.9% NaCl test solution were used. Studies of tribological properties under ambient conditions were supplemented by measurement of friction properties in nanoscale with the use of LFM technique. Additional adhesion and reciprocating sliding wear tests were performed on a nanoindenter equipped with a diamond conical tip. Surface topography and mechanical properties of the coatings were measured using AFM and nanoindentation, respectively.

[1] D. Sun, J.A. Wharton and R.J.K. Wood, Wear, 267, 1845 (2009).

[2] D. Batory, A Jedrzejczak, W. Szymanski, P. Niedzielski, M. Fijalkowski, P. Louda, I. Kotela, M. Hromadka, J. Musil, Thin Solid Films 590 (2015) 299-305.

[3] D. Batory, A. Jedrzejczak, W. Kaczorowski, W. Szymanski, L. Kolodziejczyk, M. Clapa, P. Niedzielski, Surf. Coat. Technol. 271 (2015) 112–118.

Dental implants, inserted into the oral cavity, are subjected to a synergistic interaction of wear and corrosion (tribocorrosion), which may lead to implant failures. The objective of this study was to investigate the tribocorrosion behavior of Ti oxide films produced by micro-arc oxidation (MAO) under oral environment simulation. MAO was conducted under different conditions as electrolyte composition: Ca/P (0.3 M/0.02 M or 0.1 M/0.03 M) incorporated with/without Ag (0.62 g/L) or Si (0.04M); and treatment duration (5 and 10 min). Non-coated and sandblasted samples were used as controls. The surfaces morphology, topography and chemical composition were assessed to understand surface properties. Data were analysed by ANOVA and Tukey's HSD tests (α=.05). Biofunctional porous oxide layers were obtained. Higher Ca/P produced larger porous and harder coatings when compared to non-coated group (p<.001), due to the presence of rutile crystalline structure. The total mass loss (Kwc), which includes mass loss due to wear (Kw) and that due to corrosion (Kc) were determined. The dominant wear regime was found for higher Ca/P groups (Kc/Kw ≈ 0.05) and a mechanism of wear-corrosion for controls and lower Ca/P groups (Kc/Kw ≈ 0.11). The group treated for 10 min and enriched with Ag presented the lowest Kwc (p<.05). Overall, MAO process was able to produce biofunctional oxide films with improved surface features, working as tribocorrosion resistant surfaces.