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Tribocorrosion Performance of TiAlN and AlTiN Coatings on Ti6Al4V in Artificial Saliva: Influence of Mechanical and Electrochemical Responses

Although AlTiN coatings demonstrated superior mechanical properties, TiAlN coatings exhibited better overall tribocorrosion resistance in artificial saliva due to lower electrochemical activation and more stable friction behavior, resulting in the lowest wear rate.

Original authors: Thiago de Lima Gontarski, Aline Leal, Bruno Leandro Pereira, Carlos Augusto Henning Laurindo, Luís César Fontana, Lin Wu, Richard Chromik, Ricardo Diego Torres, Paulo Soares

Published 2026-10-12

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Thiago de Lima Gontarski, Aline Leal, Bruno Leandro Pereira, Carlos Augusto Henning Laurindo, Luís César Fontana, Lin Wu, Richard Chromik, Ricardo Diego Torres, Paulo Soares

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Inside the human mouth, a complex chemical and mechanical drama plays out every time we speak, chew, or swallow. The materials used to replace damaged teeth or repair jawbones must survive a harsh environment: a warm, wet space filled with salt, acids, and constant friction. Among the most common materials for these medical implants is a specific titanium alloy known as Ti6Al4V. It is strong, lightweight, and generally friendly to the human body, but it has a weakness. Like many metals, it can wear down when rubbed against other surfaces, and in the presence of saliva, this wear accelerates. The saliva acts as an electrolyte, a liquid that conducts electricity, which means that as the metal surface is scratched and damaged, it also begins to corrode. This dual attack, where mechanical wear and chemical corrosion feed into each other, is called tribocorrosion. It can cause implants to degrade faster than expected, releasing particles into the body and potentially leading to failure. To stop this, scientists often coat these metal parts with thin, hard layers designed to act as a shield. But a new study suggests that the hardest shield is not always the best one when the environment is wet and salty.

Researchers set out to test two popular types of protective coatings on the titanium alloy: one rich in titanium and aluminum, and another with a higher proportion of aluminum. They wanted to see which coating would hold up better in a liquid that mimics human saliva. The team prepared smooth, polished discs of the titanium alloy and covered them with these thin films using a high-energy process that vaporizes material and deposits it onto the surface. They then placed these samples into a machine that rubbed a small ceramic ball back and forth across the coating while it was submerged in the artificial saliva. Throughout the test, they measured how much force was needed to slide the ball, how the electrical potential of the surface changed, and how much material was eventually worn away. They also used powerful microscopes to look at the scars left behind on the surface after the rubbing stopped.

The results began with a surprise that challenges a common assumption in materials science. The aluminum-rich coating proved to be the superior mechanical shield, being significantly harder than the other coating and able to withstand greater pressure before cracking or peeling off. However, the story changed completely once the samples were submerged in the salty liquid. Under these wet, corrosive conditions, the harder aluminum-rich coating actually wore away faster than the slightly softer titanium-rich coating. The titanium-rich coating, despite being mechanically weaker in standard tests, lost the least amount of material overall.

The key to this difference lay in how the coatings reacted to the combination of rubbing and chemical attack. As the ceramic ball slid across the surface, it scraped away the protective layer, exposing fresh metal to the saliva. This exposure caused a sudden drop in the surface's electrical stability, a sign that corrosion had begun. The aluminum-rich coating suffered a larger drop in stability and struggled to keep a protective film intact during the sliding. In contrast, the titanium-rich coating maintained a more stable electrical state, suggesting it was better at keeping a protective layer in place even while being rubbed. When the researchers examined the worn tracks under a microscope, they found that the titanium-rich surface had retained a continuous, chemically modified film that seemed to protect the metal underneath. The aluminum-rich surface, however, showed signs that this protective film was discontinuous and largely stripped away, leaving the metal vulnerable to the aggressive ions in the saliva.

This discovery highlights a crucial lesson for designing medical implants. The study demonstrates that simply making a coating harder does not guarantee it will last longer in the human body. In the wet, salty environment of the mouth, the ability of a surface to maintain its chemical stability and repair its protective film while being rubbed is more important than raw hardness. The titanium-rich coating succeeded because it could sustain a resilient barrier that resisted both the physical scraping and the chemical attack of the saliva. The aluminum-rich coating, while tougher in the dry sense, failed to maintain this barrier once the corrosion process was triggered. The researchers concluded that for applications in artificial saliva, the titanium-rich coating offers the best protection against material loss, proving that the best shield for the human body is not necessarily the hardest one, but the one that can best survive the chemical battle of the wet environment.

Technical Summary: Tribocorrosion Performance of TiAlN and AlTiN Coatings on Ti6Al4V in Artificial Saliva

Problem Statement
Ti6Al4V is a standard material for biomedical components due to its biocompatibility and mechanical strength; however, its susceptibility to mechanical wear in physiological environments remains a critical limitation. In oral and physiological settings, wear occurs simultaneously with electrochemical reactions, a phenomenon known as tribocorrosion. This interaction accelerates material degradation beyond the sum of separate wear and corrosion processes. While surface engineering strategies like TiAlN and AlTiN coatings are widely used to enhance dry-wear resistance, the specific influence of coating composition (Ti-rich vs. Al-rich) on tribocorrosion behavior in chloride-containing artificial saliva is insufficiently understood. A key scientific gap exists regarding whether the superior mechanical properties (hardness, elastic strain tolerance) typically associated with Al-rich AlTiN coatings translate into improved resistance against tribocorrosion-induced material loss, or if electrochemical activation and tribofilm stability become the dominant controlling factors.

Methodology
The study investigated Ti6Al4V substrates coated with TiAlN and AlTiN films deposited via cathodic arc evaporation (arc-PVD). The coatings were characterized using:

Structural and Chemical Analysis: Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), and X-ray Diffraction (XRD) to determine morphology, thickness, crystalline structure, and elemental composition.
Mechanical Testing: Nanoindentation (quasi-continuous stiffness measurement) to evaluate hardness ( $H$ ), elastic modulus ( $E$ ), and derived parameters ( $H/E$ and $H^3/E^2$ ). Scratch testing was employed to determine critical loads for cohesive and adhesive failure.
Tribocorrosion Testing: A ball-on-plate reciprocating tribometer was used in a three-electrode electrochemical cell with artificial saliva (pH 7.0, chloride-containing) as the electrolyte. Tests involved a 6-mm alumina counter-body under a 10 N normal load over 10 m of sliding distance.
In Situ Monitoring: Open-Circuit Potential (OCP) was continuously monitored to track electrochemical activation and repassivation kinetics during sliding.
Post-Test Analysis: Wear volume was quantified via White Light Interferometry (WLI), and wear tracks were analyzed using SEM/EDS to assess tribofilm formation and chemical composition.

Key Results

Mechanical Properties: AlTiN coatings demonstrated superior mechanical performance compared to TiAlN. AlTiN exhibited higher hardness (~31.7 GPa vs. ~28.9 GPa), higher $H/E$ and $H^3/E^2$ ratios, and higher critical loads in scratch testing, indicating better resistance to plastic deformation and adhesion.
Friction Behavior: Both coatings reduced the coefficient of friction (COF) relative to the uncoated Ti6Al4V substrate. AlTiN achieved the lowest average COF, while TiAlN exhibited the most stable friction response with minimal fluctuation.
Electrochemical Response: During sliding, the uncoated substrate showed the largest OCP drop (most negative potential), indicating severe depassivation. AlTiN showed an intermediate drop, while TiAlN exhibited the smallest potential perturbation, suggesting a lower degree of electrochemical activation and better retention of a protective surface layer during sliding. Post-sliding, AlTiN showed the fastest OCP recovery, followed by TiAlN.
Wear Rates: Contrary to the mechanical rankings, TiAlN exhibited the lowest wear rate ( $1.288 \times 10^{-6} \text{ mm}^3/\text{N}\cdot\text{m}$ ), followed by AlTiN ( $3.035 \times 10^{-6} \text{ mm}^3/\text{N}\cdot\text{m}$ ), with the uncoated substrate showing the highest wear rate ( $4.281 \times 10^{-4} \text{ mm}^3/\text{N}\cdot\text{m}$ ).
Tribofilm Analysis: SEM/EDS analysis revealed that the TiAlN wear track retained a more continuous, chemically modified tribofilm enriched with oxygen and electrolyte-derived species. In contrast, the AlTiN track showed localized film removal, with electrolyte-derived elements concentrated primarily at the track edges, suggesting less effective film retention in the central sliding zone.

Significance and Claims
The paper concludes that under the specific conditions of artificial saliva (near-neutral, chloride-containing), the tribocorrosion performance of TiAlN and AlTiN coatings is not dictated solely by mechanical hardness or elastic strain tolerance. While AlTiN possessed superior mechanical indicators, TiAlN provided the highest resistance to tribocorrosion-induced material loss.

The authors assert that the superior performance of TiAlN is attributed to its ability to maintain a more stable and resilient passive/tribochemical film during sliding, which minimizes galvanic coupling between depassivated wear tracks and the surrounding passive surface. This electrochemical stability effectively suppresses the corrosion component of the wear-corrosion synergy. Conversely, the higher hardness of AlTiN did not prevent significant material loss, likely due to less effective film retention and stronger galvanic coupling during sliding.

The study emphasizes that rankings established for dry-wear applications cannot be directly extrapolated to tribocorrosion environments. In chloride-containing physiological analogues, the stability and reparability of the surface film become the rate-controlling factors for material degradation. Therefore, for applications in oral environments, TiAlN coatings may offer superior protection against tribocorrosion compared to Al-rich AlTiN coatings, despite the latter's mechanical advantages.

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