Effect Of Plasma Nitriding On The Surface Of Gamma Based Titanium Aluminides

A. R. Rastkar 1 and B. Shokri 1, 2
1) Laser-Plasma Research Institute, Shahid Beheshti University, Tehran, Iran
2) Phys. Dept. and Laser-Plasma Research Institute, Shahid Beheshti University, Tehran, Iran and Institute for
studies in Theoretical Phys. and Maths., P.O. Box 19395-1795, Tehran, Iran
Abstract. Gamma based titanium aluminides were plasma nitrided. The effects of nitriding temperature, nitriding time and
H2/N2 gas ratio were investigated. The microstructure and phases formed within the nitrided surface layers were
characterised using optical and electron microscopy, X-ray diffraction (XRD). The wear resistance of the plasma nitrided
samples increased considerably against ball bearing steel (AISI 52100). Experimental results indicated that time controls the
thickness and nitriding temperature and H2/N2 ratio are responsible for the crystal structure of the nitrided layers. The nitrided
compound layers consist of TiN and Ti2AlN phases.
Keywords: plasma nitriding; titanium aluminides; TiN; wear
1. INTRODUCTION
In plasma thermochemical processes such as diffusive ion nitriding/carburising, the DC glow discharge is most
widely used.
DC plasma nitriding is performed over a fairly well defined range of pressure and voltage (typically 1 to 10 Torr
and 0.3 to 0.8k V) under DC diode plasma conditions (Fig. 1a). A typical plasma nitriding system is
schematically shown in Fig. 1b.
Plasma nitriding of pure titanium and Ti-6Al-4V alloy has been investigated with DC abnormal glow discharge
(Bell-1986, Brading-1992, Lanagan-1989, Matsumoto-1982) under pure nitrogen and nitrogen-hydrogen
mixtures. Treatment temperatures were between 700°-1000°C and treatment times were between 2-25 h.
In this nonequilibrium plasma, the excitation and ionisation of neutral atoms/molecules results in enhanced
chemical reactions and heating of the cathode.
In the abnormal glow discharge energetic species hit the workpiece surface, heat it and cause sputtering, the
extent of which is controlled by the gas pressure, gas composition and the voltage. Sputtering causes
depassivation, i.e. the removal of the surface oxide film present on titanium. This enables easier absorption and
diffusion of nitrogen into the surface than is experienced using other methods [1].
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It has been suggested that the formation of nitrided layer on titanium is due to the reaction between the sputtered
titanium from the surface and nitrogen in the gas phase and deposition of TiN, and diffusion of H, NH, and N
into the treated surface. Further layer formation is then only possible by diffusion of nitrogen through this outer
nitride layer. The nitride layers of TiN, Ti2N and/or Ti2AlN are responsible for improved tribological properties.
Plasma nitriding has been successfully used to improve the tribological properties of steels and Ti alloys [1]. In
this work, the plasma nitriding of Ti–48Al–2Nb–2Mn alloy is investigated. The characteristics of the nitrided
layers and their tribological properties are discussed.
2. EXPERIMENTAL WORK
The Ti–48Al–2Nb–2Mn alloy was prepared through plasma melting in a twin torch Retech plasma furnace.
Plasma nitriding was carried out in a plasma unit with a 40kW dc power supply. Two typical plasma nitriding
treatments are shown in Table 1. The microstructures and phases of the nitrided layers were studied using
scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Sliding wear tests were conducted under atmospheric conditions using a ball-on-disc wear test system (Fig.1a).
Bearing steel balls (AISI 52100) of 10mm diameter with hardness of 720HV0.5 were run against horizontal
rotating titanium aluminides discs.5. The relative speed of the disc and rider was 100mms−1.
3. RESULTS AND DISCUSSION
3.1. Microstructure of plasma nitrided Ti–48Al–2Nb–2Mn alloy
The untreated Ti–48Al–2Nb–2Mn alloy had a lamellar structure consisting of TiAl and Ti3Al lamellae which
can be seen in Fig. 2a [2,3]. A typical cross-sectional micrograph of sample PN6 plasma nitrided at 800 C for
100 h is typically shown in Fig. 2b. By examination of Fig. 2b, two layers can be observed on the lamellar
Ti-48Al–2Nb–2Mn alloy substrate. A dense thin layer is clearly visible on the top of the sample similar to the
compound layer seen in nitrided pure titanium and titanium alloys [4], and beneath this layer, a zone of nearly
equiaxed grains has been formed, which has a different structure from the thin lamellae of the alloy. This zone is
formed due to nitrogen diffusion into the lamellae, which recrystallises (coarsens) the TiAl lamellae into the
equiaxed grains [5]. This nitrogen diffusion occurs in plasma condition, which is not possible at the temperatures
used in this research without plasma. The microstructure of the compound layer depends strongly on the H2/N2
treatment gas ratio and temperature.
Fracture sections shown in Fig. 3 represent two typical structures of the samples plasma nitrided for 100 h. The
PN3 treatment produced a columnar compound layer (Fig. 3a) without an observable diffusion layer. However,
the PN6 treatment formed a fine-grained compound layer (Fig. 3b). The compound layer of PN6 treatment is in
turn composed of at least two layers, a very fine-grained layer at the top and a nearly equiaxed coarse-grained
layer underneath the top layer. The different crystal structures are thought to correspond to the sputtering and
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redeposition rates during plasma nitriding treatment, which increase with increasing hydrogen gas ratio and
temperature [6]. In fact, this hydrogen increase causes more nuclei and hence more grains to be formed or finer
grain sizes created.
3.2. Compositional analysis of plasma nitrided Ti–48Al–2Nb–2Mn alloy
XRD results shown in Fig. 4 indicate that the nitrided layers are essentially composed of TiN and Ti2AlN. Ti2N
was not observed in plasma nitrided samples. According to Fig. 4, the relative amount of TiN to Ti2AlN varies
depending on the treatment conditions. For instance, in sample PN3, Ti2AlN is not only the main constituent of
the nitrided layer but also has a strong (110) peak, which indicates a preferential orientation of columnar grains
in that direction. However, most peaks of TiN and Ti2AlN are comparatively large in sample PN6 corresponding
to the fine-grained structure of the compound layer in that sample.
3.3. Mechanical properties of plasma nitrided Ti–48Al–2Nb–2Mn alloy
3.3.1. Effect of plasma nitriding on surface hardness
The two-phase (gamma + alpha 2) lamellar Ti–48Al–2Nb–2Mn alloy has a hardness of about 320HV. This value
is about one-tenth of the hardness value of wear resistant materials or coatings, such as TiN, TiC and Al2O3.
Plasma nitriding of Ti–48Al–2Nb–2Mn forms TiN and Ti2AlN compounds, which increases the surface
hardness significantly (Fig. 5).
3.3.2. Tribological behaviour of plasma nitrided Ti–48Al–2Nb–2Mn alloy
The volume wear loss of untreated and Plasma nitrided samples are shown in Fig.6. The steady state wear
conditions were achieved quickly, and therefore, the total volume losses are reported.
Under loads up to maximum 10N (maximum Hertzian normal contact stress of 1139MPa) [7] and sliding speeds
of 0.1 m/s, all coatings had a very high wear resistance in comparison with that of untreated material.
It has already been shown that the untreated surfaces are worn severely under these conditions through mainly
plastic deformation and ploughing [8].
It is worth nothing that the normal contact stress of 1139MPa under 10N load is much higher than the yield
strength of TiAl alloys [2] indicating the high strength of the nitrided surfaces.
Some of the nitrided surfaces were wear tested under 20N load (maximum Hertzian normal contact stress of
1436MPa) and sliding speed of 0.3 m/s to measure the load bearing capacity of the treated surfaces. Most of the
samples were worn nearly as much as those of untreated material except the two PN3 and PN6 samples after
1080m sliding distance.
The volume loss measured from sample PN6 was almost one-tenth of that of untreated material. The wear
resistance of plasma nitrided surfaces depends on the thickness and crystal structure of the compound layers.
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4. Conclusions
1. Plasma nitriding of lamellar Ti–48Al–2Nb–2Mn titanium aluminide results in a thin compound layer
composed of TiN and Ti2AlN and a diffusion zone composed of nearly equiaxed grains of recrystallised
lamellae. The diffusion layer was observed at 800 ◦C.
2. Plasma nitriding of Ti–48Al–2Nb–2Mn results in a high wear resistant compound layer, which reduces the
wear loss of the alloy up to one order of magnitude.
TABLE 1. Plasma nitriding conditions applied to Ti-48Al-2Nb-2Mn alloy.

References
1. J. Lanagan, Ph.D. Thesis, The University of Birmingham, Birmingham, 1989.
2. Y.W. Kim, in: Y.W. Kim, R.R. Boyer (Ed.), Microstructure/Property Relationships in Titanium aluminides and Alloys,
The Minerals,Metals & Materials Society, 1991, pp. 91–103.
3. Y.W. Kim, D.M. Dimiduk, JOM 43 (8) (1991) 40–47.
4. I. Yuki, N. Amano, M. Uozumi, H. Inui, M. Yamaguchi, J. Jpn. Inst. Metals 58 (5) (1994) 564–570.
5. D.S. Lee, M.A. Stucke, Dimiduk, Mater. Sci. Eng. A 192/193 (1995) 824–829.
6. B. Edenhofer, Heat Treatment of Metals, No. 1, 1974, pp. 23–28.
7. I.M. Hutchings, Edward Arnold, London, 1992, p. 4, pp. 46–47.
8. A.R. Rastkar, A. Bloyce, T. Bell, Wear 240 (2000) 19–26.

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