Wear Behavior of Ti-xNb Biomedical Alloys by Ball Cratering

Abstract

Ti alloys have been developing through the years, aiming the biomedical application since it has suitable properties. Among Ti alloys, the Ti-Nb systems are a pronounced group to biomedical applications due to its low elastic modulus, good corrosion resistance, and mechanical properties. Although this system is quite well-known regarding its phases, structure and properties, there is not plenty of information about wear available in the literature. To investigate the wear resistance, the samples were submitted to x-ray diffraction (XRD) and scanning electron microscopy (SEM) to analyze the phases formed. Hardness and elastic modulus were measured by microhardness Vickers and dynamic Young modulus by excitation impulse. Additionally, wear volume, wear resistance, and H/E ratio were calculated to understand the wear material’s performance. This study aims to investigate the wear resistance of Ti-xNb (x = 15, 25 and 40wt.%), one of each type of Ti alloys and phases formed: Ti-15Nb (α´), Ti-25Nb (α”) and Ti-40Nb (β) and the influence of cooling rate after solution heat treatment on wear properties through ball cratering. It was possible to find that the harder the alloy, the higher the wear resistance. Thus, in the case of Ti-xNb (x = 15, 25 and 40wt.%), alloys the hardness plays a significant role in wear resistance. Besides that, the samples that have presented the α´ or α”phase have the lowest wear resistance. Therefore, not only the hardness influences the wear resistance but also the combination of phases formed.

Keywords:
Ti alloys; low elastic modulus; wear resistance; micro abrasive ball cratering; structural characterization

1. Introduction

Ti and its alloys have been acclaimed for biomedical applications since they present suitable properties such as good corrosion resistance, excellent biocompatibility, and low elastic modulus¹-³. Low elastic modulus is an expected and essential characteristic because of the mismatch of this property inducing the implant to stress shielding in the bone, which can lead to bone resorption and implant loosening (stress shielding)⁴,⁵. Therefore, alloys with elastic modulus nearer the human bone’s elastic modulus are desirable. Among some possibilities of Ti alloys, β Ti alloys have been developed to produce a suitable alloy with an elastic modulus nearer the bone’s modulus, thus increasing the implant service life. For instance, Ti-25Ta-Zr-Nb, Ti-35Nb-7Zr-5Ta, Ti-15Zr-10Mo and Ti-Nb systems⁵,⁶, with good biocompatibility and low elastic modulus (50-80 GPa) have been developed for biomedical application. Ti-Nb system presents low elastic modulus, which can be explained by the presence of β phase in some alloys. β phase presents a less dense packing of atoms in its body-centered cubic (bcc) structure in comparison with α phase, hexagonal close-packed (hcp). Yet, the presence of α´ and α″ also have been associated to the decreasing of elastic modulus⁷-⁹. In addition, Nb is an excellent β phase stabilizer, plays a significant role in reducing the elastic modulus improving Ti-based alloys' corrosion resistance¹⁰,¹¹. Besides that, it is essential to say that Nb has a relatively lower melting temperature than other β phase stabilizers, such as V, Ta, and Mo, resulting in less energy consumption. Furthermore, it was shown through invitro and vivo tests that adding Nb may improve bone tissue adhesion and proliferation.

Several studies devoted to Ti-Nb system has been published in recent past years. These studies proposed to investigate different aspects of this system, such as characterizing the structure and phase composition¹², lattice parameters¹³, mechanical properties¹⁴ and, biocompatibility. Although a considerable number of studies have been conducted to understand the relations between structure and properties, only a few studies devoted themselves to investigating the wear behavior of the Ti-Nb system. Wear behavior is an essential material property since it plays an important role in the time life of the material, which is crucial in biomedical applications.

The wear mechanism identified in orthopedic implant can be abrasive and might be related with material composition, hardness and surface properties¹⁵ Ureña et al.¹⁶ studied surface modification and showed that Ti-Nb coating improve wear behavior of Titanium. Alberta et al.¹⁷ studied tribocorrison of a β- Ti alloy (Ti-Nb-Ga), they showed Ga addition increase wear resistance of Ti-45Nb. Besides that, authors have reported a better understanding about tribocorrosion mechanism in a simple simulated body fluid. Asl et al.¹⁸ showed that an anodized coating may improve wear behavior of Ti-Nb alloys. Gonçalves et al.¹⁹ showed that tribocorrosion properties can be improved when a β-Ti alloy is reinforced with NbC addition.

This study aims to analyze the wear behavior through micro-ball cratering of Ti-xNb (x = 15, 25 and 40wt%) and compare this processing with two different heat treatments conditions regarding cooling rate (slowly and rapid cooled) imposed from the solid state. The characterization was conducted by the XRD patterns, followed by SEM analysis, hardness, elastic modulus tests, and wear resistance. In the end, it is shown, even in the as-cast state, it is possible to obtain an alloy with an elastic modulus as low as 53 GPa and good wear behavior.

2. Experimental Procedure

The ingots of Ti-xNb (x = 15, 25 and 40wt%) were obtained by copper mold suction casting in a compact arc melting furnace, Edmund-Bühler MAM-1, under an inert atmosphere with argon. These samples were named CM (copper mold). Besides that, some samples of these alloys were submitted to different heat treatment when it comes to cooling rate, rapid cooling (RC) upon water quenching and slow cooling (SC) under furnace cooling. Both treatments had a heating rate of 10 ºC/min and treatment temperature of 1000 °C but one was cooled in furnace (SC) and the other in water (RC). It was used an suction cast arc melting furnace EDG under vacuum atmosphere with 10^-7 torr. After suction cast and heat treatment the samples were submitted to XRD, Bruker D8 Advanced-ECO Diffractometry, to analyses the phases formed. In addition, the hardness was measured by Shimadzu microhardness HMV G2 while to measure the elastic modulus dynamic young’s modulus through ATCP Sonelastic equipment were used. For the hardness test the ASTM E384 was used and ASTM E1876 was adopted for the elastic modulus measurement. Finally, Optical Microscopy (OM), scanning electron microscopy (SEM) equipped with an energy dispersive X-ray detector (EDS) Phillips XL30 were used to observe the microstructure and homogeneity of the alloying elements. For the wear tests under dry system, it was used steel balls (AISI 52100, diameter of 25.4 mm and hardness 818 HV) rotating at 200 revolutions per minute (rpm) along 60 min, producing wear craters. For such tests, a 200 g load was used. The wear volume (V) in μm³ and wear coefficient (K) are calculated derived from Archard's law. For such analysis, measurement through image analysis (ImageJ) is done on the inner diameter of the crater.

3. Results and Discussion

Table 1 depicts the chemical composition through EDS analysis of all alloys in CM condition. It has shown that the melting process was satisfactory, once Ti and Nb elements are in the expected concentration. In the production of titanium alloys, there is an ASTM standard for the commercial Ti-15Mo alloy, which stipulates that an experimental alloy with 14 and 16% Mo by weight can be classified as Ti-15Mo, that is, in the production of a binary alloy containing molybdenum an error of approximately 1% is acceptable. In light of the norm and performing a slight extrapolation for the Ti-Nb alloys, it is observed that all the alloys developed in this work are within the error stipulated by the norm, where the amount of experimental niobium is less than 1%, indicating that all mergers were carried out satisfactorily²⁰.

Table 1
- Semi-quantitative chemical composition from EDS analysis of all Ti-xNb alloys (weight%).

	Ti-15Nb	Ti-25Nb	Ti-40Nb
Ti (wt.%)	85 ± 1	77 ± 1	60 ± 1
Nb (wt.%)	15 ± 1	23 ± 2	40 ± 1

The XRD patterns and SEM images corresponding to all alloys and conditions are shown in Figure 1. Regarding Ti-15Nb, CM sample presents an almost completely α″ structure, while SC is showing a structure nearer to equilibrium condition α + β, which is expected since the imposed cooling condition, and RC presents α’ and β phases. It was possible to see that heat treatment induces alloys to different phases, so it plays a significant role in phase transformation and in the XRD patterns. Besides that, it was observed that SC could separate the phases clearly and form finer peaks compared to sample RC and the one without heat treatment.

Figure 1
- XRD patterns and SEM micrographs of (a) Ti-15Nb (b) Ti-25Nb and (c) Ti-40Nb in three different conditions: CM (copper mold), SC (slowly cooled), RC (rapidly cooled).

Regarding Ti-25Nb, it was noticed similar behavior to 15 Nb. The RC sample was induced to α″ + β phase whereas SC was induced to α + β and CM was induced to α″ + β phases. Nevertheless, the SC sample has presented finer and clearer peaks than others. Finally, in the XRD pattern of 40Nb, only bcc structure was present in all conditions. The MC sample has presented only the β phase, whereas the SC and RC have presented not only the β phase but also a β’ phase The high content of the β-stabilizer element may explain it; when there is a high quantity of a β-stabilizer solute, it is possible to form a new β phase, which is called β’, with different lattice parameter and chemical composition through spinodal decomposition. These results are in agreement with literature²¹,²².

The microstructure of all Nb content and alloy conditions are also shown in Figure 1, and it can confirm the XRD patterns. Firstly, it is possible to observe some needles and grains 15 CM, only needles due to rapid cooling in 15 RC and grains and a lamellar structure in 15 SC, which is expected since the cooling is nearer equilibrium. Secondly, in the 25 CM is noticed some α″ needles and β grains, while in RC condition was possible to notice more grains than in 25 MC and some tiny needles; In the SC condition, it was found that the α and β structures, very similar to the 15 SC conditions (due to the conditions nearer equilibrium). Finally, the 40 RC and SC showed a similar granular structure due to the β-stabilizer element content. Differently from them, 40 MC has presented an utterly dendritic structure, which is common in processing with mold. The results found here are in coherence with the ones found in the literature²¹,²³-²⁵.

Figure 2 shows the variation of hardness, elastic modulus, wear coefficient and volume according to Nb content and sample condition. In Figure 2a is presented the results of Ti-15Nb and it is noticed that wear volume varies according to hardness in a clear tendance. The lower hardness the higher wear volume. The stablished phases in condition SC were α + β and the decrease of hardness is related with α phase as showed by Lee et al⁸. Therefore, the found results points to an increase of wear volume when α phase is stablished. When it comes to wear coefficient (K) the tendence is different. The higher elastic modulus, the higher K and the highest K is presented by SC condition. In both cases, wear volume and coefficient, the results presented by CM and RC conditions did not vary significantly since it has similar hardness and elastic modulus although it has presented different phases. The hardness and elastic modulus results are in accordance with literature²¹-²⁵

Figure 2
- Wear volume x Vicker microhardness and Wear coefficient x Elastic modulus of Ti-15Nb (a), Ti-25Nb (b) and Ti-40Nb (c) in three different conditions: CM (copper mold), SC (slowly cooled), RC (rapidly cooled).

In Figure 2b it is presented the results of Ti-25Nb. It was observed a similar result that 15Nb and the wear volume varies according to hardness. Besides that, the highest wear volume value is found in the samples with the lowest hardness value. It is also related with the α phase stablished in this samples since the SC condition leads to the presence of this phase. The hardness value is in accordance with literature. Otherwise, the elastic modulus showed by CM and RC showed a lower value than the related in literature. It might be explained by the suppression of ω phase, which increase the elastic modulus of Ti alloys and the phase was not found in XRD patterns²⁶,²⁷. Yet, the highest elastic modulus the highest K as found as in Ti-15Nb.

In Figure 2c is presented the results of Ti-40Nb. It was observed the same tendence that it was observed in 15 and 25 Nb and the found results are in accordance with literature²⁸. However, it may be explained by other mechanism since the 40Nb did not present α phase. The heat treatment is able to explain these differences since cooling rate plays a significate role in materials properties. SC condition might stablish a big gran size while RC stablish a smaller one²⁹. Besides that, the internal stress can change the mechanical properties and tends to be higher in RC than SC³⁰.

Regarding the wear volume the authors observed a clear relation between wear behavior and hardness as presented in Figure 2 and there is no significative difference between CM and RC in all conditions. CM and RC lead the alloys to similar hardness and because of that the route processing did not affect the wear behavior since they are inducing to α´ or α″ phases. On the other hand, the SC condition induced a pronounced difference on wear volume of the samples. SC condition presented the lowest value of hardness and in these samples were find the highest wear volume values, which suggest the combination of α + β, principally α presence, are easier to be worn than others. Different from the others, 40 Nb has presented a different rule. Since it is completely β structure there is no huge difference between the samples independent of the condition, although this fact the tendence presented is the same that one found in 15 and 25 Nb. Besides, was noticed that Nb content reduce the wear volume, which was also related in literature³¹,³².

There is a correlation between wear behavior and mechanical properties know as H/E ratio (Hardness/ Elastic modulus) and the classical theories of wear points to hardness as a key parameter in wear controlling³³,³⁴. The harder material the best wear resistance and it is related that if H/E > 0.04 the alloy exhibits a good wear resistance²⁴,²⁵. H/E was calculated in this study and is shown in Figure 3. It is clear that there is a clear tendence and the authors observed that H/E is inverse to wear coefficient. The worst result of H/E is presented by SC condition, which is smaller than 0.04 and it is in accordance with literature since the SC sample induce the lowest hardness value. Therefore, the SC condition is not convenient to be applied when high wear resistance is required. The CM and RC condition are similar and both of them presented satisfactory results of H/E, being RC lightly higher than CM. Additionally, the results of K and H/E are also presented in Table 2.

Figure 3
- H/E ratio x Wear coefficient of Ti-15Nb, Ti-25Nb, Ti-40Nb and its marks caused by ball cratering in three different conditions: CM (copper mold), SC (slowly cooled), RC (rapidly cooled).

Table 2
- Wear coefficient (K) and H/E ratio results of Ti-xNb alloys in all conditions.

	Ti-15Nb		Ti-25Nb		Ti-40Nb
	K (µm3/N.µm)	H/E	K (µm3/N.µm)	H/E	K (µm3/N.µm)	H/E
SC	1.0E-2	0.03	2.0E-2	0.03	9.0E-4	0.03
CM	9.6E-5	0.05	8.5E-4	0.05	4.5E-4	0.04
RC	1.1E-4	0.06	2.7E-4	0.06	3.0E-4	0.05

4. Conclusions

The CM process was enough to ensure chemical homogeneity to all Nb content alloys. It was noticed that CM condition led to α″ martensite, RC lead to α´ martensite phase for 15Nb alloy and SC lead to α + β phases. In 25Nb alloy the α″ martensite is formed in CM and RC conditions, whereas SC stablish α + β phases closer to equilibrium condition. The 40Nb formed β phase in all conditions (SC and CM) and when submitted to heat treatment followed by and RC lead to the formation of β + β´ phase separation. Hardness and elastic modulus are associated with phases formed for each alloy. It is confirmed that α + β structure in 15Nb and 25Nb alloys lead to the lowest hardness and highest elastic modulus due to α phase fraction. The CM and RC conditions presented similar values for mechanical properties in 15 and 25Nb alloys. The 40Nb alloy has presented similar results in CM and RC samples and also presented the lowest hardness and highest elastic modulus in SC condition as 15 and 25Nb alloys. However, it is associated with greater grain size since 40Nb alloy has presented only β or β + β´ phase. Regarding wear behavior, it was possible to conclude that it follows an inverse tendency to wear coefficient and hardness. CM and RC samples did not present a significant difference regarding H/E ratio since it presents similar phases and hardness values. The lowest hardness values are associated with α phase in 15% and 25%Nb alloys and with a greater β grain size in 40%Nb alloy. Therefore, the greater the α phase fraction and the higher the grain size lead to a decreasing in the wear resistance of the alloys.

5. Acknowledgments

This study was financed partly by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior of Brazil (CAPES), Finance Code 001. The authors would like to thank the "IF Sudeste MG - Campus Juiz de Fora". The authors wish to thank the Brazilian agencies: the CNPq Universal Project #422015/2018-0 (C.R.M.A.) and the FAPESP (São Paulo Research Foundation) for financial support through “Projeto Temático” #2018/18293-8 and FAPESP post-doc grants #2019/26517-6 (P.A.B.K.), #2021/03865-9 (M.C.R) whom the authors thank.

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  Weng W, Biesiekierski A, Lin J, Li Y, Wen C. Impact of rare earth elements on nanohardness and nanowear properties of beta-type Ti-24Nb-38Zr-2Mo alloy for medical applications. Materialia. 2020;12:100772. http://dx.doi.org/10.1016/j.mtla.2020.100772
  » http://dx.doi.org/10.1016/j.mtla.2020.100772

Publication Dates

• Publication in this collection
  26 Feb 2024
• Date of issue
  2024

History

• Received
  13 Nov 2023
• Accepted
  21 Dec 2023