Friction and wear are systemic properties that extend beyond specific material characteristics, playing a key role in determining the longevity, durability, and overall functionality of dental implants. The interplay of these factors can affect the ability of the implant to withstand mechanical stress, resist degradation, and maintain its structural integrity over time. Therefore, a comprehensive understanding of these interactions is essential for optimizing implant design, improving material choices, and ensuring the long-term success and reliability of dental implants. Figure 1 illustrates the various tribological contacts present in dental implants, including those between the abutment and the restoration (1), between the implant and the abutment (2), within the structure of the implant itself (3), and at the interface between the implant and the surrounding bone tissue (4). In this study, the tribo- Science and Research 12 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 1 Introduction Dental implants play a crucial role in restoring oral function and must meet specific requirements to ensure their effectiveness and longevity. To successfully replace missing teeth, the materials used for dental implants must possess high strength in combination with a moderate Young’s modulus, ideally ranging from 70 to 80 GPa. This balance helps prevent the overloading of newly formed bone cells while simultaneously minimizing stress shielding over the long term, which could otherwise compromise the stability of the implant. Furthermore, dental implant materials must provide biocompatibility and corrosion resistance to reduce the risk of implant rejection. In addition, the materials must exhibit sufficient wear resistance to prevent metal particle formation and the accumulation of wear debris, which in turn can result in material degradation and inflammatory reactions in adjacent tissues. This raises the risk of complications such as peri-implantitis and bone loss, potentially resulting in complete implant loss.[1] Understanding tribological interactions, particularly friction and wear, is crucial for evaluating the performance of dental implants within the dynamic and complex conditions of the oral environment. These interactions are significantly influenced by various factors such as material pairings, loading conditions, environmental variables as well as the presence and type of lubrication. Tribological Investigation of the Novel Titanium Alloy TNTZ-O for Dental Implant Applications: Subsequent Results of a Comparative Study Alexander Roegnitz, Elias Merz, Hubert Mantz, Carsten Siemers, Andreas Haeger* submitted: 27.09.2024 accepted: 16.04.2025 (peer review) Presented at GfT Conference 2024 To date, the titanium alloy Ti-36Nb-2Ta-3Zr-0.3O (TNTZ-O) has not been used in class III medical products, despite its favorable properties for implant applications, including high strength, low stiffness, high elastic strain, and good biocompatibility. To assess the performance and longevity of dental implants within the dynamic oral environment, it is vital to understand its tribological interactions. This study examines the frictional behavior and wear characteristics of TNTZ-O in artificial saliva against 100Cr6 and compares it to established titanium alloys, specifically CP-Ti Grade 4 and Ti-6Al-4V ELI. The findings indicate that TNTZ-O exhibits higher wear volume and frictional instability, indicating the need for further optimization for dental implant applications. Keywords TNTZ-O, Tribology, Dental implants, Wear, Biomaterials, Titanium alloys Abstract * Alexander Roegnitz, M. Eng. 1 (corresponding author) Elias Merz, M. Sc. 2 Prof. Dr. rer. nat. Hubert Mantz 1 Carsten Siemers 2 Prof. Dr.-Ing. Andreas Haeger 1 1 Ulm University of Applied Sciences, Institute for Manufacturing Technology and Materials Testing, 89075 Ulm, Germany 2 Technische Universität Braunschweig, Institute for Materials Science, 38106 Braunschweig, Germany logical aspects related to the contact area between the implant and the abutment (2) are investigated. For the last three decades, “Commercially Pure Titanium” (CP-Ti) has been extensively used in dental implants due to their favorable corrosion resistance and good mechanical properties. In particular, CP-Ti Grade 4 has been preferred due to its superior strength and reliability. However, the mechanical properties of CP-Ti are still limited and its relatively poor tribological performance highlights the need for more advanced alternatives. [2] To address these limitations, various alloying elements have been incorporated into titanium-based materials to improve their mechanical and tribological properties. For instance, Ti-6Al-4V ELI (Titanium Grade 23) is highly valued for its enhanced strength and corrosion resistance. However, the potential cytotoxic effects associated with aluminum and vanadium ions in this alloy raise concerns about its long-term safety and suitability for implants [3]. Recent advancements have led to the development of low-modulus Ti-based alloys with higher contents of β-stabilizing elements, free of aluminum and vanadium, such as Ti-13Nb-13Zr (TNZ), Ti-15Mo and Ti-36Nb-2Ta-3Zr-0.3O (TNTZ-O). These alloys show considerable promise due to their high strength, low Young’s modulus, and absence of cytotoxic elements, making them an attractive alternative for dental applications [4]. Among these so-called second and third generation medical titanium alloys, the latter exhibits the lowest Young’s modulus as well as high strength. While these properties are promising, further research is necessary to fully evaluate the tribological behavior and overall suitability of TNTZ-O for dental implants and other medical applications. This study investigates the frictional behavior and wear characteristics of TNTZ-O and compares it to established titanium alloys, specifically CP Ti Grade 4 and Ti-6Al-4V ELI. 2 Experimental methods 2.1 Materials TNTZ-O is classified as a third-generation titanium alloy specifically designed for class II medical applications, i.e. orthopedic wires. Depending on its specific chemical composition, the microstructure can comprise a range of phases, including α-, β-, α"and ω-phases [4, 5]. Currently, this alloy is manufactured by powder metallurgy and sintering, with subsequent rotary swaging, and wire drawing. This manufacturing process restricts its applications to wires with diameters less than 2 mm. To enable future dental implant applications as a Class III medical product, a conventional manufacturing route involving ingot production followed by deformation processes is currently under development by one of our project partners. The TNTZ-O material used in the present study was obtained in the “as received” (AR) state from Toyota Tsusho Material Incorporated, where it had been roll-leveled from a 7.3 mm diameter coil. The material exhibits a β-phase microstructure, which is a critical factor in its performance characteristics. For comparative Science and Research 13 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 Figure 1: Relevant tribological contact areas in dental implants Material Ti Nb Ta Zr O Al V Fe CP-Ti Grade 4 bal. - - - 0,30 - - 0,08 Ti-6Al-4V ELI bal. - - - 0,11 6,08 3,99 0,14 TNTZ-O (AR) bal. 36,2 ± 0,1 1,9 ± 0,1 3,0 ± 0,1 0,21 ± 0,01 - - - Table 1: Chemical composition of the tested materials (in wt.%) Material Tensile strength (MPa) Young’s Modulus (GPa) Vickers Hardness (HV5) Phases TNTZ-O (AR) 896 ± 23 65 ± 3 253 ± 7 β CP-Ti Grade 4 835 ± 8 105 ± 7 255 ± 7 α Ti-6Al-4V ELI 1057 ± 9 110 ± 6 312 ± 9 α + β Table 2: Mechanical properties and phases of the tested materials β-phase regions concentrated at the grain boundaries. The microstructural features are illustrated in Figure 2. 2.2 Tribological testing and characterization The tribological behavior of the materials was evaluated utilizing an oscillating ball-on-disc SRV ® III-Tribometer from Optimol, conducting tests in artificial saliva at a controlled temperature of 37 °C to simulate the conditions in the oral environment. The Aldiamed mouth spray, produced by Certmedica Int. GmbH was selected as the saliva substitute due to its viscosity and lubricating properties, which closely resemble those of natural saliva. The tribological samples were initially sectioned into disks with a height of 3 mm. These disks underwent a preparation process involving grinding and polishing to ensure comparable and uniform surface properties. The final surface finishing was conducted using a 50 nm alkaline colloidal silica suspension from Cloeren Technology GmbH, which provided a mirror-like surface and minimized the impact of surface roughness (Ra: 0,013 - 0,049 µm) on the measurements of friction and wear. A hardened 100Cr6 - G20 steel ball (HRC: 62) with a diameter of 10 mm was selected as the counter body for the tests. The specific load parameters and experimental conditions that were used during the tests are summarized in Table 3. Science and Research 14 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 analysis, the chemical composition and mechanical properties of TNTZ-O, CP-Ti Grade 4, and Ti-6Al-4V ELI are detailed in Tables 1 and 2. The CP-Ti Grade 4 and Ti-6Al-4V ELI alloys, used as reference materials in this study, were supplied by from Klein SA. These reference alloys have a diameter of 8 mm and conform to ASTM F-67-13 and ASTM F-136 standards, respectively [6, 7]. The microstructures of TNTZ-O, CP-Ti Grade 4, and Ti-6Al-4V ELI were analyzed using optical microscopy (OM) and scanning electron microscopy (SEM). Prior to imaging, the samples were ground and polished to obtain smooth surfaces. TNTZ-O and CP-Ti Grade 4 were etched with a custom solution composed of 86 ml H 2 O, 12 ml H 2 O 2 , 5 ml HF and 4,5 ml HNO 3 while Ti-6Al-4V ELI was etched using Kroll’s reagent. After etching, the samples were examined with a Zeiss Axio Imager.M2m optical microscope and analyzed using AxioVision 4.8 software. Additionally, the Hitachi TM3000 desktop SEM was used to acquire images with backscattered electron (BSE) contrast. TNTZ-O exhibited large, globular β-phase grains, with apparent deformation features, presumably twinning. CP-Ti Grade 4 is characterized by a uniform structure consisting mostly of equiaxed α-phase grains. In contrast, Ti-6Al-4V ELI revealed a fine-grained, globular microstructure, with a duplex arrangement of equiaxed α-phase grains and Figure 2: Microstructural images of TNTZ-O, CP-Ti Grade 4, and Ti-6Al-4V ELI. The top row includes optical microscopy (OM) images: TNTZ-O (A), CP-Ti Grade 4 (B), and Ti-6Al-4V ELI (C). The bottom row displays scanning electron microscopy (REM) images with backscattered electron (BSE) contrast: TNTZ-O (A'), CP-Ti Grade 4 (B'), and Ti-6Al-4V ELI (C'). Load (N) Duration (s) Frequency (Hz) Stroke (mm) Temperature (°C) 20 2520 1 2 37 Table 3: Load collective parameters for tribological testing The coefficient of friction (COF) was recorded, providing a detailed analysis of frictional behavior and tribological stability during reciprocating sliding in an artificial oral environment. For each material, three comparable test runs were conducted to ensure reliable and consistent results. Post-test examination of the wear tracks was performed using the μsurf #1018 confocal microscope from NanoFokus, equipped with an Olympus LMPLFL20x/ 0.40 objective. The captured images were then processed and analyzed using Mountain 8 software from DigitalSurf to determine the wear volumes. Further investigations of the present wear mechanisms involved SEM using a Zeiss Sigma 300 VP. Elemental analysis of the wear tracks was conducted using Energy Dispersive X-ray Spectroscopy (EDX) with a Bruker XFlash 6|60 detector, identifying the elemental composition and distribution within the wear tracks. 3 Results and discussion 3.1 Friction behavior The coefficient of friction graph, as depicted in Figure 3, illustrates the distinct tribological behavior of TNTZ-O, CP-Ti Grade 4 and Ti-6Al-4V ELI throughout the 42-minute test duration, revealing their varying frictional responses and wear characteristics. After an initial run-in phase, the friction values of TNTZ-O stabilize within the range of 0,25 to 0,28, reaching COF comparable to those observed in the reference materials. However, these stabilized values are intermittently disrupted by significant friction peaks, with COF values occasionally exceeding 0,8, indicating periods of instability during sliding. Although the material’s friction behavior intersects with that of the other reference materials at various points, it ultimately diverges, displaying a slight upward trend toward the end likely due to the accumulating friction fluctuations. In contrast, CP-Ti Grade 4 and Ti-6Al-4V ELI exhibited stable COF trends throughout the test. Both materials settled quickly after a brief run-in phase of approximately 180 to 240 seconds, with Grade 4 maintaining values around 0.21 to 0.23. Meanwhile, the COF of Ti-6Al-4V ELI gradually decreased, ultimately finishing slightly lower than Grade 4 by the end of testing. The low standard deviation (SD COF ) values in Table 4 demonstrate the consistent and stable frictional performance of CP-Ti Grade 4 and Ti-6Al-4V ELI, especially when compared to the fluctuating and unstable frictional behavior observed in TNTZ-O. 3.2 Wear behavior TNTZ-O exhibited the highest wear volume at 0,098 mm 3 after the full test duration of 42 minutes, which corresponds with significant fluctuations in its COF. This indicates increased adhesive wear and delamination of the forming oxide layer. The observed peaks in the COF suggest stick-slip behavior, where temporary adhesion or interfacial locking contributes to the wear mechanisms. An example of a wear track for TNTZ-O is shown in Figure 4A, featuring an oval-shaped pattern that illustrates the observed wear. The increased wear standard deviation (SD wear ) for TNTZ-O of 0,0164 mm 3 , reflects the unstable friction behavior of TNTZ-O. These fluctuations in friction lead to inconsistent sliding con- Science and Research 15 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 Figure 3: Exemplary COF curves for TNTZ-O, CP-Ti Grade 4 and Ti-6Al-4V ELI tested in reciprocating ball-on-disc tribometer for 42 min in artificial saliva against hardened 100Cr6. within the contact area, as illustrated in Figure 5A, supports this observation. The oxidation of the surface and subsequent delamination of the oxide layer seems to be the primary causes for the COF fluctuations and the pronounced friction peaks observed during testing. This adhesive wear mechanism is further verified by the EDS measurements of the 100Cr6 steel balls, as depicted in Figure 6B. These analyses reveal the presence of titanium (Ti) and niobium (Nb) from the TNTZ-O alloy and provide evidence of notable material transfer between the disc and the steel balls. While the precise wear volume of TNTZ-O on the ball surface could not be accurately quantified, the detected elements strongly suggest substantial material detachment and re-adhesion during sliding. In contrast, the SEM images of CP-Ti Grade 4 and Ti-6Al-4V ELI, presented in Figure 5B and Figure 5C respectively, reveal wear tracks with fine grooves and ridges. These features are indicative of abrasive wear, suggesting that the wear process for these alloys primarily involves micro-cutting or micro-plowing of the surface material, with significantly less material transfer and delamination compared to TNTZ-O. This results in a more uniform and consistent friction behavior, characterized by fewer fluctuations or peaks in friction and minimal adhesive wear. Consequently, these alloys exhibit stable tribological performance, highlighting their robustness and reliability under the tested conditions. This stable performance is consistent with existing literature, Science and Research 16 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 ditions, resulting in increased variability in wear volume. Conversely, CP-Ti Grade 4 and Ti-6Al-4V ELI demonstrated considerably lower wear volumes, recorded at 0,012 mm 3 and 0,017 mm 3 , respectively. Despite Ti-6Al-4V ELI having a lower final COF than CP-Ti Grade 4, it exhibited a slightly higher wear volume. Nevertheless, both materials displayed overall comparable tribological performance. Table 4 presents a summary of the tribological data, including the average COF after a 10-minute running-in phase, the standard deviation (SD) of COF, minimum (Min COF ) and maximum (Max COF ) COF values, and the corresponding wear volumes. To further investigate the observed wear behavior, a detailed SEM analysis of the wear tracks was conducted, as illustrated in Figure 5. This investigation revealed distinct wear mechanisms for each alloy, providing additional insight into their tribological performance. In the case of TNTZ-O, the SEM images (Figure 5A) illustrate a wear track characterized by larger flakes of material compared to those observed in CP-Ti Grade 4 (Figure 5B) and Ti-6Al-4V ELI (Figure 5C). This indicates a dominant adhesive wear mechanism, where material is torn from the surface, re-adheres, or undergoes delamination. The presence of a heavily oxidized region Figure 4: Exemplary wear track on TNTZ-O disc using confocal microscopy (A). Average wear volumes for TNTZ-O, CP-Ti Grade 4 and Ti-6Al-4V ELI (B) tested in reciprocating ball-on-disc tribometer for 42 min in artificial saliva against hardened 100Cr6. Material COF (Avg. after 10 min) SD COF Min COF Max COF Wear volume (mm³) SD wear (mm³) CP-Ti Grade 4 0,23 0,01 0,22 0,28 0,012 0,0003 Ti-6Al-4V ELI 0,23 0,01 0,20 0,27 0,017 0,0011 TNTZ-O (AR) 0,32 0,07 0,25 0,68 0,098 0,0164 Table 4: Friction and wear characteristics of the tested materials which suggests that under low load and temperature conditions a transition from adhesive wear to a combination of abrasion and oxidative wear occurs [8]. Additionally, the presence of artificial saliva provides lubricating effects that further reduce the primarily oxidative and abrasive wear mechanisms affecting Ti-6Al-4V ELI and CP-Ti [9, 10]. Although EDS results for the counter-bodies of these alloys are not presented, preliminary observations suggest reduced material transfer to the steel balls due to the smaller contact area and wear scar. A homogeneously distributed titanium signal is detected in both CP-Ti Grade 4 and Ti-6Al-4V ELI counter-bodies, with a reduced presence of aluminum and vanadium specifically identified on the Ti-6Al-4V ELI counter-body. In contrast, the TNTZ-O counter-body exhibits distinct areas of material detachment, characterized by pronounced saturated regions. 4 Conclusion and outlook TNTZ-O exhibits the highest wear volume and significant fluctuations in the COF among the tested materials. This increased wear and friction instability can be attributed to its β-phase body-centered cubic microstructure, which exhibits lower hardness and a reduced Young’s modulus compared to CP-Ti Grade 4 and Ti-6Al-4V ELI. The inherent properties of the β-phase microstructure contribute to greater elastic deformation and adhesive wear, which lead to higher wear volumes and inconsistent friction behavior. In contrast, CP-Ti Grade 4 and Ti-6Al-4V ELI, with their hexagonal close-packed α and α+β microstructures respectively, demonstrate more stable frictional behavior and lower wear volumes. The increased stiffness and hardness of these materials contribute to their superior performance in terms of wear resistance and friction stability. To enhance the wear behavior of TNTZ-O, future studies will focus on optimizing its microstructure through advanced thermo-mechanical processing. Specifically, promoting the transformation of the β-phase to the αor ω-phases could potentially improve its tribological performance by increasing hardness and reducing elastic deformation, thus mitigating wear and friction instability. Further research is required to gain a comprehensive understanding of the wear mechanisms of TNTZ-O. In order to provide a more detailed analysis of the wear processes and material interactions, it would be beneficial to complement the existing detailed SEM and EDS analyses of the wear tracks with a cross-sectional examination. Additionally, utilizing more application-specific Science and Research 17 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 Figure 5: Exemplary wear track of TNTZ-O (A), CP-Ti Grade 4 (B) and Ti-6Al-4V ELI (C) tested in reciprocating ball-on-disc tribometer for 42 min in artificial saliva against hardened 100Cr6. Images are captured in SE contrast at 1000: 1 magnification. Figure 6: Exemplary SEM images with subsequent EDS analysis of a wear track on a TNTZ-O disc A, and the corresponding 100Cr6 ball (B) tested in reciprocating ball-on-disc tribometer for 42 min in artificial saliva. Images are captured at 130: 1 magnification. [5] Zhang J-L, Tasan CC, Lai MJ, et al. Partial recrystallization of gum metal to achieve enhanced strength and ductility. Acta Mater 2017; 135: 400 - 410. [6] American Society for Testing and Materials (ASTM). Specification for Unalloyed Titanium, for Surgical Implant Applications (ASTM F-67-13; UNS R50250, UNS R50400, UNS R50550, UNS R50700). [7] American Society for Testing and Materials (ASTM). Specification for Wrought Titanium-6Aluminum-4Vanadium ELI (Extra Low Interstitial) Alloy for Surgical Implant Applications (ASTM. F-136; UNS R56401). [8] Mao YS, Wang L, Chen KM, et al. Tribo-layer and its role in dry sliding wear of Ti-6Al-4V alloy. Wear 2013; 297: 1032-1039. [9] Lee Y-S, Niinomi M, Nakai M, et al. Differences in Wear Behaviors at Sliding Contacts for β-Type and (α + β)- Type Titanium Alloys in Ringer’s Solution and Air. Mater Trans 2015; 56: 317-326. [10] Cvijović-Alagić I, Cvijović Z, Bajat J, et al. Electrochemical behaviour of Ti-6Al-4V alloy with different microstructures in a simulated bio-environment. Mater Corros 2016; 67: 1075 -1087. [11] Lee Y-S, Niinomi M, Nakai M, et al. Predominant factor determining wear properties of β-type and (α + β)-type titanium alloys in metal-to-metal contact for biomedical applications. J Mech Behav Biomed Mater 2015; 41: 208 -220. Science and Research 18 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0008 counter body materials in testing could offer a more accurate evaluation of TNTZ-O’s wear resistance in practical dental applications. Despite its promising mechanical properties for dental implants, TNTZ-O’s current tribological performance indicates challenges that must be addressed. Enhancing its wear behavior is crucial to ensure long-term durability and reliability in dental applications, which highlights the need for continued development and optimization. References [1] Abd-Elaziem W, Darwish MA, Hamada A, et al. Titanium-Based alloys and composites for orthopedic implants Applications: A comprehensive review. Mater Des 2024; 112850. [2] Breme J, Eisenbarth E, Biehl V. Titanium and its Alloys for Medical Applications. In: Titanium and Titanium Alloys. John Wiley & Sons, Ltd, pp. 423-451. [3] Willis J, Li S, Crean SJ, et al. Is titanium alloy Ti-6Al-4 V cytotoxic to gingival fibroblasts - A systematic review. Clin Exp Dent Res 2021; 7: 1037-1044. [4] Gordin DM, Ion R, Vasilescu C, et al. Potentiality of the ‘Gum Metal’ titanium-based alloy for biomedical applications. Mater Sci Eng C Mater Biol Appl 2014; 44: 362- 370.