WEAR BEHAVIOUR OF BORIDED TITANIUM AND Ti-13Nb-13Zr ALLOY

WEAR BEHAVIOUR OF BORIDED TITANIUM AND Ti-13Nb-13Zr ALLOY Heck, S.C. 1 ; Fernandes, F.A.P. 1 ; Schneider, S.G. 2 ; Gallego, J. 3; Casteletti, L.C. 1 Departamento de Engenharia de Materiais, Aeronáutica e Automobilística Av. Trabalhador Sancarlense, 400, CEP:13560-970, São Carlos. castelet@sc.usp.br 1 Escola de Engenharia de São Carlos EESC-USP, São Carlos SP. 2 Escola de Engenharia de Lorena EEL-USP, Lorena SP. 3 Departamento de Engenharia mecânica DEM UNESP, Ilha Solteira - SP ABSTRACT Titanium alloys are widely used in automotive, aerospace, chemical, marine and biomedical industries due its excellent combination of mechanical properties, low density and good corrosion resistance. Ti-6Al-4V alloy represents about 50% of world production of titanium alloys, but the vanadium and aluminum of the alloy can be toxic to the human organism in biomedical use. The Ti-13Nb-13Zr alloy is a good substitute for that use of the Ti-6Al-4V alloy, however both alloys have low tribological properties. The salt bath boriding is an efficient way to improve the tribological properties of titanium alloys. For comparison of performance, in this work the titanium and the Ti-13Nb-13Zr alloy were borided in a salt bath, resulting in titanium boride layers with high hardness and wear resistant. The produced layer on the Ti-13Nb-13Zr alloy showed the better performance with respect to those properties. Keywords: Titanium alloys, thermo reactive treatment, boriding, wear. INTRODUCTION At room temperature, unalloyed (commercially pure) titanium has a hexagonal close-packed (hcp) crystal structure referred to as alpha (α) phase. At 883 C, this transforms to a body-centered cubic (bcc) structure known as beta (β) phase. The manipulation of these crystallographic variations through alloying additions and thermomechanical processing is the basis for the development of a wide range of alloys and properties. Based on the phases present, titanium alloys can be classified as either α alloys, β alloys, or α+β alloys [1]. Alpha alloys contain elements such as aluminum and tin, generally have creep resistance superior to the β alloys, and are preferred for high temperature 6055

applications. Alpha alloys are characterized by satisfactory strength, toughness, weldability, and unlike β alloys, cannot be strengthened by heat treatment. They are often used in the annealed or recrystallized condition [1]. Beta alloys contain transition elements such as vanadium, niobium, and molybdenum. They have excellent forgeability over a wide range of forging temperatures, excellent hardenability, and respond readily to heat treatment [1]. Alpha + beta alloys have compositions that support a mixture of α and β phases. The properties of these alloys can be controlled through heat treatment, which is used to adjust the amounts and types of β phase present. Solution treatment followed by aging at 480 to 650 C precipitates α, resulting in a fine mixture of α and β in a matrix of retained or transformed β phase [1]. Titanium and its alloys have been used as implant materials since the 70 s. Although the pure titanium offer better corrosion resistance and tissue tolerance than stainless steel, its comparatively low mechanical strength and unfavorable wear properties restricted its use to certain applications such as pacemaker cases, heart valve cages and reconstruction devices. There are great concerns about the potential of toxicity and potential inhibition of apatite formation and possible association with neurological disorders of the elements aluminum and vanadium in titanium alloys. To reduce this effect, two alpha-beta titanium alloys, Ti-5Al-2.5Fe and Ti-6AI-7Nb, were developed in Europe in the 80 s. Both alloys offer advantage of biocompatibility compared to the Ti-6Al-4V, due the vanadium absence and have mechanical properties quite similar to Ti-6Al-4V, but still contain the aluminum. The search for an aluminum and vanadium free biocompatible titanium alloy for orthopedic applications was initiated in the middle of 80 s in the United States. Another desired feature for the alloy would be a low modulus of elasticity since a smaller module (high flexibility), more close to that the bone, makes the prosthesis has a better distribution of natural tension adjacent to the femur bone [2-4]. New titanium alloys with higher biocompatibility were proposed and are in development. They are currently beta-type alloy composed of non-toxic elements. They also have a better balance between the strength resistance and toughness compared with that the alpha-beta alloys [5]. The beta type Ti-13Nb-13Zr alloy is a promising substitute for the Ti-6AL-4V in biomedical practice. However, both alloys have poor tribological properties [3, 6]. 6056

Boronizing is an efficient way to improve tribological properties of titanium alloys. It is a thermochemical treatment in which the material is kept in an environment of Boron donation in temperatures that range from 850ºC to 1100ºC for periods of 2 to 10h. The environment of boron donation can be solid, liquid, gas or paste. The temperature of treatment will vary depending on the type of material and thickness of layers needed. If boronizing is applied to the surface of a material, the resulting borided layer increases the wear resistance [7-9]. Surface hardening of titanium by diffusion of boron is advantageous because the process itself is simple and inexpensive, does not need complicated equipment, and because complex geometries can be coated. The diffusion leads to a dual layer coating consisting of a continuous monolithic TiB 2 outer layer and an inner layer consisting of discrete TiB whiskers, generally penetrating normal to the surface [10]. The hardness of the boride layer can varies in a broad range (in between 850HV and 3300HV), depending on the process parameters [11]. In addition to the phases TiB and TiB 2, the phases NbB 2 and ZrB 2 can be formed on Ti-13Nb-13Zr alloy due to the reaction of alloying elements with boron [12, 13]. In this work, samples of pure titanium and Ti-13Nb-13Zr alloy were borided by salt bath process and were carried out micrograph, hardness and wear tests after and before boronizing, for performance comparison. EXPERIMENTAL In the present study, samples of titanium grade 2 and Ti-13Nb-13Zr alloy were machined and ground with a 600 grit SiC emery paper. The samples were borided in borax salt bath using aluminum as activator, for 4 hours at 1000 C and then cooled in air. Borided samples were subjected to microstructural survey, hardness measurements and wear tests. Microstructural survey were conducted on the crosssection of borided samples after etching with 1% HF. Hardness measurements were conducted on the cross-section of the samples with a Vickers pyramid indenter and the test load was 25g. Wear tests were conducted in a fixed ball wear tester (Fig.1) under dry conditions. For the as-received samples the test load was 2,4N, and for borided samples the test load was 13,2N. 6057

Figure 1 - Schematic diagram of the fixed ball wear tester. RESULTS AND DISCUSSION Figure 2a shows the micrograph of borided Ti-13Nb-13Zr sample. The substrate have martensitic α microstructure, because of the relatively fast cooling, and over it can be noted a borided layer with a thickness of about 105µm and an irregular layer/substrate interface. a) b) Figure 2 Optical micrographs of borided Ti-13Nb-13Zr alloy (a) and titanium (b). Figure 2b shows a micrograph of borided titanium grade 2, were can be seen the substrate α microstructure and also a surface layer with about 12µm, with a smooth morphology in the layer/substrate interface. Both layers formed on titanium and Ti-13Nb-13Zr alloy are uniform and don t show any crack. The borided Ti and Ti-13Nb-13Zr x-ray diffraction patterns are shown in figure 3. As can be seen, only the TiB phase was formed. Also the NbB phase is formed in borided Ti-13Nb-13Zr. Others intermetallics of Ti-B system was not 6058

detected by XRD analysis. The peaks of α and α Ti also appeared on XRD patterns. Relative Intensity (A) 100 Borided Ti 80 60 40 20 1 1-Ti α 2-TiB 1 Relative Intensity,3 100 Borided Ti-13Nb-13Zr 80 60 40 20 (B) 3 2 2 2,3 1 1,3 3 2 2 2 1 3 2 2 1 1-Ti α' 2-TiB 3-NbB 0 0 10 20 30 40 50 60 70 80 90 100 2 theta (deg.) 10 20 30 40 50 60 70 80 90 100 2 theta (deg.) Figure 3 X-ray diffraction patterns of (A) borided Ti and (B) borided Ti-13Nb- 13Zr. The borided Ti-13Nb-13Zr alloy has the hardest layer compared with the titanium layer, about 1400HV and 1100HV respectively, as shown in Figure 4. There is also a great difference in hardness presented by titanium and titanium alloy Ti-13Nb-13Zr substrates. In the case of the alloy, the hardness is more than the double in comparison to that of the pure titanium. 1400 1200 Borided Ti Borided Ti-13Nb-13Zr Hardness (HV) 1000 800 600 400 200 0 0 50 100 150 200 250 Distance from surface (μm) Figure 4 Hardness profile of borided samples. 6059

About the wear behavior of as-received samples, the pure titanium and the Ti- 13Nb-13Zr alloy, have a quite similar behavior as can be seen in Figure 5a. After boronizing samples have excellent performance with a very low wear when compared with the as-received samples. A great reduction in wear coefficient occurs in both cases (Figure 6) and their wear behavior are very similar (Figure 5b) 0,4 Ti Ti-13Nb-13Zr a) 0,0060 0,0055 Borided Ti Borided Ti-13Nb-13Zr b) Wear volume (mm 3 ) 0,3 0,2 0,1 Wear volume (mm 3 ) 0,0050 0,0045 0,0040 0,0035 0,0030 200 300 400 500 600 700 800 900 1000 1100 1200 1300 Sliding distance (m) 0,0025 200 300 400 500 600 700 800 Sliding distance (m) Figure 5 Wear behavior of: a) as-received samples and b) borided samples. borided Ti-13Nb-13Zr Ti-13Nb-13Zr borided Ti 1 Ti 0.0 0.2 0.4 0.6 0.8 8 10 Wear coefficient (10-6. mm 3 m -1 N -1 ) Figure 6 Sample s wear coefficient after 798m of sliding test. 6060

CONCLUSIONS The salt bath boronizing produced high hardness borided coatings in Ti-13Nb- 13Zr alloy (1400HV) and in grade 2 Titanium (1100HV). A coating of about 100µm was formed in Ti-13Nb-13Zr alloy and in the case of pure Titanium a coating of about 12µm was formed. The wear of titanium alloys and pure titanium decrease drastically after boronizing treatment, where borided pure titanium and Ti-13Nb-13Zr alloy have performances excellent and quite similar. For as-received samples the wear behavior are similar too. Despite of similar wear resistances, the Ti-13Nb- 3Zr alloy presents a much higher hardness than the pure titanium, and therefore a higher mechanics resistance, which makes it more suitable for applications in more severe conditions of use. REFERENCES 1. DESTEFANI, J.D. Introduction to titanium and titanium alloys. In: ASM Handbook. Materials Park: ASM International, 1990, v. 2, p. 586-598. 2. YETIM, A.F.; ALSARAN, A.; EFEOGLU, I.; ÇELIK, A. A comparative study: the effect of surface treatments on the tribological properties of Ti-6Al-4V alloy. Surface and Coatings Technology, v. 202, p. 2428-2432, 2008. 3. MAJUMDAR, P.; SINGH, S.B.; CHAKRABORTY, M. Wear response of heattreated Ti-13Nb-13Zr alloy in dry condition and simulated body fluid. Wear, v. 213, p. 1015-1025, 2008. 4. WANG, K. The use of titanium for medical applications in the USA. Materials Science and Engineering, v. 213, p. 134-137, 1996. 5. NIINOMI, M. Mechanical properties of biomedical titanium alloys. Materials Science and Engineering, v. 243, p. 231-236, 1998. 6. RACK, H.J.; QAZI J.I. Titanium alloys for biomedical applications. Materials Science and Engineering, v. 26, p. 1269-1277, 2006. 7. STEWART, K. Boronizing protects metals against wear. Advanced materials and processes, v. 3, p. 23-25, 1997. 6061

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