Biomaterials Volume 24, Issue 20 , September 2003, Pages 3377-3381
Wear resistance of experimental Ti–Cu alloys
C. Ohkubo, , a, I. Shimuraa, T. Aokia, S. Hanatania, T. Hosoia, M. Hattorib, Y. Odab and T. Okabec a Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, 2-1-3 Tsurumi Tsurumi-ku, Yokohama 230-8501, Japanb Department of Dental Materials Science, Tokyo Dental College, 1-2-2, Masago Mihama-ku, Chiba 261-8502, Japanc 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
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.
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.
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.
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.
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
Because there were no significant differences (p...
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