Biomaterials
Volume 24, Issue 20 , September 2003, Pages 3377-3381

Wear resistance of experimental Ti-Cu alloys

C. Ohkubo^{, , a}, I. Shimura^a, T. Aoki^a, S. Hanatani^a, T. Hosoi^a, M. Hattori^b, Y. Oda^b and T. Okabe<sup>c

a</sup> Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, 2-1-3 Tsurumi Tsurumi-ku, Yokohama 230-8501, Japan
^b Department of Dental Materials Science, Tokyo Dental College, 1-2-2, Masago Mihama-ku, Chiba 261-8502, Japan
^c Department of Biomaterials Science, Baylor College of Dentistry, Texas A&M University System Health Science Center, 3302 Gaston Ave., Dallas, TX 75246, USA

Received 10 October 2002;  accepted 9 March 2003. ; Available online 13 May 2003.

1. Abstract

After using cast titanium prostheses in clinical dental practice, severe wear of titanium teeth has been observed. This in vitro study evaluated the wear behavior of teeth made with several cast titanium alloys containing copper (CP Ti+3.0 wt% Cu; CP Ti+5.0 wt% Cu; Ti-6Al-4 V +1.0 wt% Cu; Ti-6Al-4 V+4.0 wt% Cu) and compared the results with those for commercially pure (CP) titanium, Ti-6Al-4 V, and gold alloy. Wear testing was performed by repeatedly grinding upper and lower teeth under flowing water in an experimental testing apparatus. Wear resistance was assessed as volume loss (mm³) at 5 kgf (grinding force) after 50,000 strokes. Greater wear was found for the six types of titanium than for the gold alloy. The wear resistance of the experimental CP Ti+Cu and Ti-6Al-4 V+Cu alloys was better than that of CP titanium and Ti-6Al-4 V, respectively. Although the gold alloy had the best wear property, the 4% Cu in Ti-6Al-4 V alloy exhibited the best results among the titanium metals. Alloying with copper, which introduced the
Ti/Ti₂Cu eutectoid, seemed to improve the wear resistance.

Author Keywords: Wear; Wear resistance; Titanium casting; Titanium alloys

2. Article Outline

1. Introduction

2. Material and methods

2.1. Specimen preparation

2.2. Wear test

2.3. Assessment of wear

2.4. Microhardness measurements

2.5. SEM observations of wear surface

3. Results

3.1. Volume loss

3.2. Alloy structure

3.3. Observation of worn specimens

4. Discussion

5. Conclusion

Acknowledgements

References

3. 1. Introduction

Commercially pure (CP) titanium has been increasingly used for some dental appliances because of its excellent biocompatibility, corrosion resistance, and light weight [1]. Through a considerable amount of research performed to solve casting problems, the quality of cast titanium prostheses has improved; however, there are still some obstacles to be overcome for the application of titanium to dentistry to be completely successful.

One of the disadvantages of titanium for structural applications is its poor tribological characteristics [2]. In their review of titanium alloys for orthopedic applications, Long and Rack [3] presented the complications of the wear phenomenon of titanium and indicated that overall alloy composition, which controls the surface oxide composition and subsurface deformation behavior, is a critical factor in titanium wear. A dentist observed the in vivo wear of cast CP Ti dental prostheses [4] and found that it underwent the greatest amount of wear, compared to conventional dental alloys. Shimura et al. [5] reported the greatest wear was found when the same grade of cast CP Ti teeth was used for both upper and lower teeth. Similarly, Kawalec et al. [6] observed the most severe wear when pieces of wrought Ti-6Al-4 V specimens fretted against themselves compared to a Co-Cr-Mn alloy. Despite the importance of wear resistance, detailed studies on the friction and wear performance of titanium alloys are sparse. We previously compared the wear of teeth made from various titanium alloys [7]. Among the different types of titanium alloys,
+
alloys exhibited the best results, which were consistent with a report by Khan et al. [8]. Better wear resistance of the
+
alloys was considered to be mainly due to the increased resistance to plastic deformation that is attributable to the existence of
needles in the retained
matrix. On the other hand, the higher ductility of the
titanium seemed to be a cause for its poor wear resistance. Thus, we thought that by adding elements to titanium, we should be able to make an alloy with an increased resistance to plastic deformation. One way to improve strength is to introduce a eutectoid constituent to the alloy structure, as is the case for carbon steels [9].

The objective of this in vitro study was to use a two-body wear testing apparatus that simulated chewing function to evaluate the wear behavior of teeth made from
CP titanium and
+
Ti-6Al-4 V, both alloyed with copper.

4. 2. Material and methods

2.1. Specimen preparation

The titanium alloys used in this study (Table 1) were ASTM grade 3
CP titanium, one
+
alloy [Ti-6Al-4 V (64)], and four experimental alloys [CP Ti+3.0% Cu (CP-3.0); CP Ti+5.0% Cu (CP-5.0); Ti-6Al-4 V+1.0% Cu (64-1.0); and Ti-6Al-4 V+4.0% Cu (64-4.0)] (the copper concentrations in the latter two alloys were based on the concentration of titanium in these alloys) (percentages given here are all weight percent). The concentrations we chose for making the Ti-Cu alloys (3% and 5%) fall in the hypoeutectoid range [10]. We chose these copper concentrations since we found earlier that the ductility of the hypereutectoid Ti-Cu alloys (>7.1%Cu) is much reduced [11].

Table 1. Metals tested in this study

Denture tooth patterns of upper and lower teeth were duplicated from artificial first molars (Livdent FB30, GC, Japan) with auto-polymerizing resin (pattern resin, GC). After the patterns were invested with a phosphate-bonded, Al₂O₃/LiAlSiO₆ investment material (T-INVEST, GC), they were burned out, and all tooth specimens were cast with all six titanium alloys using a one-chamber, gas pressure casting unit (Autocast HC-III, GC), in accordance with the manufacturer's recommendations. As a control, Type IV gold alloy (70.0Au-6.0Pt-4.7Ag-19.0Cu-0.3other) (PGA-3, Ishifuku, Japan) (Control) was also cast in a conventional centrifugal casting machine.

According to dental laboratory practice, the surfaces of the titanium castings were treated as follows: (1) surfaces were sandblasted with 50 
m grain-sized aluminum oxide (Al₂O₃) powder for 30 s; and (2) chemical polishing was performed by immersing the castings in 31.3%HNO₃-4.5%HF solution (Chemi-Polish, Shofu, Japan) at room temperature for 5 min and then they were ultrasonically washed. For the Type IV gold alloy teeth, acid cleaning was performed with 10%HCl for 1 min. Five pairs of cast upper and lower teeth were made for each alloy, making a total of 70 teeth.

2.2. Wear test

The same in vitro two-body wear testing apparatus as used in our previous study [7] was used in this study ( Fig. 1). Upper and lower teeth were mounted on the apparatus so that they simulated tightly fitting occlusion on each surface. The wear test was performed by repeated grinding using a load of 5 kgf (60 cycles/min; grinding distance: 2 mm). As the upper teeth contacted the lower teeth and the vertical load was applied, the lower teeth also moved horizontally, causing a sliding motion to occur in the occlusal relationship between tooth cusps and cusp to fossa of the teeth. As the load was lifted by the camshaft rotation, the lower teeth returned to their original position. These actions constituted one cycle. A total of 50,000 cycles were carried out for each set of teeth.

(15K)

Fig. 1. Wear testing apparatus.

2.3. Assessment of wear

After 50,000 cycles, the teeth were removed from the testing device. Before the wear test, the weight of each tooth was measured on an electrical balance (accuracy: 0.1 mg; AFG-45SM, Shimazu, Japan). The wear resistance was assessed as volume loss calculated using the total weight loss after 50,000 cycles and the density of each specimen. The density (
) of each tooth was calculated based on Archimedes' principle using the two weights measured. The results were analyzed using ANOVA followed by Fisher's test at a significance level of
=0.05.

2.4. Microhardness measurements

After surface cleaning, the Vickers microhardness of the surfaces of the cast metals was measured with a load of 100 g and a loading time of 30 s (Hardness tester MVK-E, Akashi, Japan). The hardness numbers were obtained from five specimens of each kind of metal at three arbitrarily chosen sites per specimen.

2.5. SEM observations of wear surface

The contact area of the tested teeth and the polished cross sections near the surfaces where the opposing teeth contacted each other was examined using either an optical microscope (Epiphot 200, Nikon, Japan) or a scanning electron microscope (SEM) (JSM-6300, JEOL, Japan). In addition to microscopic observation, X-ray diffractometry was carried out (Miniflex, Rigaku Denki, Japan) using K-alpha radiation at 30 kV and 10 mA at a scan rate of 0.5 E/min. Phase identification was performed by matching each characteristic peak with the JCPDS file (1998).

5. 3. Results

3.1. Volume loss

Because there were no significant differences (p>0.05) in volume loss between upper and lower teeth for each metal tested, the wear resistance was assessed as the total volume loss of the sum of upper and lower teeth in this study. Table 2 presents the total volume loss (mm³) for the teeth of each metal, the existing alloy phase, and the density (g/cm³) for each metal [mean (SD)].

Table 2. Volume loss, structure, and density of metals tested

Identical letters: no statistical difference.

The total volume loss ranged from 0.55 to 1.89 mm³. Fig. 2 compares the volume loss for each metal. Greater wear was found for the six types of titanium than for the gold alloy. Of the titanium teeth, CP titanium showed higher (p<0.05) volume loss than did either of the two metals alloyed with copper. The Ti-6Al-4V-4%Cu alloy exhibited the best results among the titanium metals. As seen in Table 2, there was no correlation between volume loss and hardness among all the metals tested.

(9K)

Fig. 2. Volume loss for metals tested.

3.2. Alloy structure

The combined results of the X-ray diffractometry and microscopy of the cast metals revealed that CP titanium and Ti-6Al-4V have polycrystalline
and
+
structures, respectively. Both the 3% and 5% Cu titanium alloys consist of a hypoeutectoid structure. This structure was made up of the primary precipitates of
Ti and the eutectoid (lamellae of
Ti and Ti₂Cu) that were transformed from
Ti at the eutectoid temperature. Ti-6Al-4V with both 1% and 3% Cu had structures similar to Ti-6Al-4V without Cu, which has a Widmanstätten structure with acicular
delineated by the
between them. Ti-6Al-4V with Cu showed the existence of Ti₂Cu, indicating that the alloys contained the eutectoid constituent. It was also notable that the
platelets contained more Cu than did the
phase region.

3.3. Observation of worn specimens

SEM observation of the occlusal areas in the metal teeth specimens indicated that all the worn surfaces were very similar to one another. Judging from their appearance, it seems that these surfaces were mainly worn out from the abrasive and adhesive wear process [2]. Similar to what we found for various titanium metals [7], metal was gouged out of the surface; some of this metal completely detached from the surface, and some was scraped to the side of the chewing area and adhered to the surface. Examination of cross sections near the worn surfaces of all the titanium specimens showed that grains in the subsurface areas were damaged by shear fracture and/or plastic deformation, revealing the plastic flow of
platelets or acicular
in the grains.

The worn areas of the gold alloy teeth indicated that the wear process was carried out plastically through abrasion and adhesive wear. The appearance of the worn areas was similar to what we found for titanium.

6. 4. Discussion

When the clinical longevity of a restoration is considered, its wear resistance is one of the important characteristics that must be studied. However, assessment of wear testing of restorative materials is not easy either in vitro or in vivo. Wear testing procedures are not standardized; many types of wear testing have been used [12, 13 and 14]. Generally, a round or cone-shaped specimen acting as a stylus opposes a flat specimen [14]. In the present in vitro investigation, the wear resistance of six titanium alloys (four of them containing copper) was evaluated by repeatedly grinding upper tooth specimens against opposing lower tooth specimens. This type of wear testing produced generalized and localized wear and could simulate the clinical situation well.

In general, metals with low theoretical tensile and shear strengths exhibit higher coefficients of friction than higher-strength materials [2]. Titanium has relatively low values for these properties, so we could assume that titanium would exhibit high friction values and an increased tendency for material transfer and adhesive wear. However, in our previous studies on CP titanium and some titanium alloys [7 and 15], we were not able to find any correlations between their amount of wear and their strength data [16]. In a previous similar study [7], we also found that there was no correlation between their bulk hardness and their wear.

The microstructure of all the titanium specimens is generally similar in that they all exhibit a Widmanstätten structure based on
platelets or
acicular needles. Both the
Ti-Cu alloys and
+
Ti-6Al-4V+Cu contained eutectoid
Ti/Ti₂Cu. In spite of some differences in the phases existing among the titanium examined, the microstructures in the worn surfaces are similar: the plastic deformation of grains was evident from the flow in the Widmanstätten structure. No notable differences were seen in the microstructures examined using optical microscopy or SEM metallography up to 1000×. However, there is a trend toward improvement in the wear resistance (Table 1) with increasing Cu concentration in both CP titanium and Ti-6Al-4V. These improvements in wear resistance must be the result of the inclusion of the eutectoid in their structure. For example, the wear resistance of high carbon steels having optimal carbon concentration was improved by the presence of Fe₃C particles, although their ductility decreased [17]. Our recent studies of Ti-Cu alloys [11] and Ti-6Al-4V with Cu [18] showed that with increasing Cu concentration, the tensile and yield strength increased, whereas the ductility decreased. In our previous study [7], we found that ductile metastable BCC
titanium alloys exhibited more wear than HCP
titanium or two-phase
+
titanium alloy. It appears that the alloy formulation, through the introduction of the eutectoid structure, improves the wear resistance, and also the two-phase microstructure makes the material more wear-resistant. These phenomena occur because the type of microstructure is more resistant to plastic deformation.

The wear phenomenon is complicated. There are many types of wear processes, including abrasive, fretting, corrosive, fatigue, galling, and adhesive wear [2]. In our study, the main wear processes were abrasive, adhesive wear, and possibly galling. In both our previous study [7] and the present study, the gold alloy used as the control consistently exhibited the best wear resistance. This result could be attributed to the difference in the ease of movement of dislocations during plastic deformation, which affects the galling process: metals such as pure copper and gold with low stacking-fault energy have a high number of impeding stacking faults and a decreased tendency to cross slip and therefore, are less prone to gall [19]. This may explain why the gold alloy had the best wear resistance. It should also be pointed out that the grain diameter of the gold alloy tested in this study was approximately 1/5 that of the cast titanium, which may have contributed to the better wear resistance of the gold. On the other hand, pure titanium has a high dislocation cross slip rate and tends to gall. Alloying copper to titanium may reduce the stacking-fault energy, making the alloys wear-resistant.

7. 5. Conclusion

Inclusion of the eutectoid
Ti/Ti₂Cu in
titanium, which increases the resistance to plastic deformation, improves the wear characteristics of these alloys. Alloying copper to Ti-6Al-4V further improves the wear resistance of this alloy with the added benefit of the two-phase
+
structure.

8. Acknowledgements

This research was partially supported by NIH/NIDCR grant DE11787. The authors acknowledge the assistance of Mrs. Lucyna Carrasco and Mr. Mikhail Brezner with the metallography and X-ray diffractometry, and Mrs. Jeanne Santa Cruz with the English editing of this paper.

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