Gas nitriding and subsequent oxidation of Ti-6Al-4V alloys
© Lee et al; licensee Springer. 2012
Received: 7 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Ti-6Al-4V alloys consisting of α-Ti grains and intergranular β-Ti islands were nitrided at 850°C for 1 to 12 h under a nitrogen pressure of 1 Pa. With increasing nitriding time, the Ti-N compound layer became thicker, and the α-Ti diffusion zone containing dissolved nitrogen became wider. In the Ti-N compound layer, the initially formed Ti2N became TiN as the nitriding progressed. The nitride layers were oxidized to rutile-TiO2 after oxidation at 700°C for 10 h in air.
Titanium alloys are widely used in the aircraft, automobile, chemical, and biomedical industries due to their high specific strength, good corrosion resistance, and biocompatibility. However, their main drawbacks are their low hardness and poor wear resistance. In order to overcome these problems, various nitriding techniques including diffusion, ion-plasma, detonation, laser, and high-energy methods have been applied to synthesize TiN surface layers [1–7]. TiN films are the most widely used films in such industrial applications as cutting tools, die molds, mechanical parts, diffusion barriers in microelectronics, and decorative items [8–10]. In this study, the gas nitriding technique, a type of thermodiffusion treatment, was utilized to synthesize TiN films on the Ti-6Al-4V alloy. It takes advantage of the high reactivity of titanium with nitrogen to produce hardened surface layers that are well bonded to the tough matrix, without deteriorating the mechanical properties. For industrial application, a full understanding of the gas nitriding technique and high-temperature oxidation behavior of the nitrided Ti alloys is necessary because these wear-resistant, hard TiN films are frequently exposed to oxidative atmospheres during their service life. Since TiN films begin to oxidize at temperatures as low as 550°C, their thermal stability is important [11, 12]. However, the effect of oxidation on nitrided Ti alloys is not well established. The diffusion of oxygen from the atmosphere to the reaction interface or the desorption of nitrogen from the reaction interface to the atmosphere was proposed as the main factor governing the oxidation of TiN films [12–14]. The purpose of this study is to investigate the nitride layers that formed on Ti-6Al-4V alloys under controlled gas nitriding conditions and their high-temperature oxidation characteristics.
Ti-6Al-4V alloy was used as the substrate as it is the most widely used titanium alloy. The substrates were cut into dimensions of 15 × 10 × 1 mm3, polished with a 0.1-μm diamond paste to reduce the maximum value of the roughness, Ra, to 0.4 μm, degreased in benzene, washed with deionized water, and nitrided via the following gas nitriding technique. The substrates were placed in the reaction chamber inside the furnace in a vacuum of 10-3 Pa, heated to 850°C at a heating rate of 0.04°C/s, held at this temperature for 1, 6, or 12 h at PN2 = 1 Pa, cooled to 500°C at a cooling rate of 0.03°C/s at PN2 = 1 Pa, and further cooled to room temperature in a vacuum of PN2 = 10-3 Pa. Nitrogen was deoxygenated by filtering the moisture and oxygen through silica gel and titanium chips at 1, 000°C. Oxidation tests on the nitrided specimens were conducted at 700°C in atmospheric air for 10 h.
The nitrided and subsequently oxidized specimens were investigated by scanning electron microscopy [SEM], electron probe microanalysis [EPMA], X-ray diffraction [XRD] with CuKα radiation at 40 kV and 300 mA, and transmission electron microscopy [TEM] (operated at 200 keV) in conjunction with EDS with a 5-nm spot size. The TEM sample was prepared by milling in a focused ion beam system after carbon coating.
Results and discussion
Concentration of spots 1 to 13 shown in Figure 4 (at.%)
Concentration of spots 1 to 9 shown in Figure 5 (at.%)
The nitriding of Ti-6Al-4V alloys at 850°C at PN2 = 1 Pa resulted in the dissolution of the interstitial nitrogen and the formation of nitrides. When nitrided for 1 h, a 0.8-μm-thick Ti-N compound layer that consisted of Ti2N and a 2.5-μm-thick α-Ti(N) diffusion zone that consisted of Ti having dissolved nitrogen were formed. When nitrided for 6 h, a 2.3-μm-thick compound layer consisting of Ti2N and TiN and a 4.6-μm-thick α-Ti(N) diffusion zone were formed. When nitrided for 12 h, a 5-μm-thick compound layer consisting of Ti2N and TiN and a 14-μm-thick α-Ti(N) diffusion zone were formed. Aluminum tended to be depleted at the Ti-N compound layer. The nitrides that were formed were oxidized to rutile-TiO2 during oxidation at 700°C in air.
This work was supported by the Human Resource Development Project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government's Ministry of Knowledge Economy (no. 20114010203020).
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