Titanium alloys are widely used in various industrial fields because they exhibit high specific strength and corrosion resistance. However, their tribological properties are poor compared with other industrial materials. To improve the tribological properties of titanium alloys, surface modifications can be effectively applied. Nitriding is especially widely applied to increase the surface hardness and improve the tribological properties of titanium alloys. General nitriding techniques such as gas or plasma nitriding normally require several hours to achieve the desired degree of surface modification, although some previous studies have reported the formation of nitrided layers on titanium alloys by plasma nitriding in less than 1 h [1, 2]. Therefore, it would be beneficial to reduce this processing time span to improve productivity.
A new nitriding technique has been developed which is referred to as gas blow induction heating (GBIH) nitriding. In this process, nitrogen (N2) gas is blown onto a metal heated by high frequency induction in N2 atmosphere. Using this technique, a nitrided layer, composed of both a nitrogen compound layer and a nitrogen diffusion layer, was formed on commercially pure (CP) titanium [3] and a Ti-6Al-4V alloy [4, 5] for a treatment time of approximately 3 min. It was also revealed that the wear resistance of the GBIH nitrided Ti-6Al-4V alloy processed at temperatures above 973 K was improved [4]; however, the fatigue strength of the GBIH nitrided Ti-6Al-4V alloy processed at 1173 K was decreased [5]. One reason for this is grain coarsening during the treatment. Therefore, this decrease in fatigue strength could potentially be avoided by lowering the treatment temperature.
Generally, peening creates a surface layer having a high dislocation density and fine grains . For example, Wu et al. [6] reported that a nanocrystalline layer was formed by ultrasonic peening on the surface of CP titanium, and concluded that this was because dynamic recrystallization occurred by sequential introduction of dislocations. Unal et al. [7] performed shot peening for CP titanium, and revealed that ultrafine grains with sizes below 100 nm were formed on the surface.
It was also reported that when nitriding is performed for peened surfaces, nitrogen diffusion is facilitated compared with untreated surfaces [16–23]. For example, Hassani-Gangaraj et al. [8] revealed that ultrafine-grained or nano-structured layers were created at the surface of AISI 4340 steel by the pre-treatment with shot peening, which facilitated nitrogen diffusion during subsequent nitriding. Farokhzadeh et al. [9] also reported that shot peening for a Ti-6Al-4V alloy created a severe plastic deformation layer with microstructural defects at the treated surface, and that the pre-treatment with shot peening enhanced nitrogen diffusion during subsequent plasma nitriding. Sun et al. [10] revealed that the hardness and wear resistance of CP titanium were improved by low-temperature plasma nitriding after a pre-treatment with ultrasonic peening. As mentioned above, it is revealed that surface layers with fine grains and high density dislocations are created by peening, which facilitated nitrogen diffusion during subsequent nitriding.
Fine particle peening (FPP), which applies smaller particles (less than 200 µm in diameter) than conventional shot peening, is one of the effective surface modification processes which also creates a surface layer with a high dislocation density and fine grains. Particle velocity of FPP is higher than that of shot peening, and corresponding to this, a higher strain rate is introduced at the peened surface. This results in the formation of finer grains at the surfaces of S45C steel and CP titanium by FPP than by shot peening. It has been reported that pre-treatments with FPP, which create fine grains and dislocations with high density, accelerate elemental diffusion during subsequent nitriding or oxidation. For example, FPP prior to gas nitriding enabled the formation of a thicker compound layer with higher hardness on AISI 4135 steel, leading to a lower friction coefficient and less wear loss [11]. Moreover, Hirota et al. [12] revealed that FPP treated surface possesses a superior oxygen diffusion capacity. These results suggest that a pre-treatment with FPP allows GBIH nitriding to be achieved at a lower temperature due to the acceleration of nitrogen diffusion.
In this study, a rapid nitriding process for a titanium alloy at a low temperature is developed combined with FPP as a pre-treatment of GBIH nitriding. The effect of the proposed treatment process on the surface characteristics and wear resistance of a titanium alloy was investigated.
The results are summarized as follows:
1. Nitrogen diffusion into titanium during GBIH nitriding was accelerated by a pre-treatment of FPP. This was because fine grains were created near the surface by FPP. This resulted in the formation of a nitrided layer composed of a nitrogen compound layer and a nitrogen diffusion layer on the titanium alloy, even when employing a relatively low treatment temperature during GBIH nitriding.
2. GBIH nitriding at a low temperature with a pre-treatment of FPP improved the wear resistance of the titanium alloy. This was due to the formation of a hard nitrogen compound layer at the surface.
3. The proposed surface treatment process, which was a combination of GBIH nitriding and pre-treatment with FPP, was effective at modifying the surface properties of the titanium alloy within a short span of time while avoiding grain coarsening and transformation from an equiaxed α phase to an acicular α phase. List of references
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