Aspects regarding the tribological behaviour of some magnetron sputtered Ti-Si-C coatings.

Ionescu, Cristian ; Munteanu, Daniel ; Munteanu, Alexandru 等

1. INTRODUCTION

The subject of the deposition of Ti-Si-C type thin films presents a wide interest at world level, fact revealed by the numerous articles and monographs published in the specialty literature (Barosum, 2000; Lopez et al., 2007; Emmerlich et al., 2004; Barosum, 2001; Eklund et al., 2005; Hogberg et al., 2005; Veprek, 1997). Despite of the fact that the available data regarding Ti-Si-C system (MAX phase, generally) have an increasing trend, the most of the researchers encourage the further investigation on this system.

The research regarding the Ti-Si-C system has drawn toward a new family of materials, i.e. MAX phases, [Ti.sub.3]Si[C.sub.2] being the most investigated, (Barosum, 2000). The successful synthesis of MAX phases in the form of thin films has attracted the attention over these unique materials. The ternary compound known as the MAX phase ([M.sub.n+1] A [X.sub.n], where M = transition metal; A = A group element; X = C and/or N; n=1-3) and, particularly, [Ti.sub.3]Si[C.sub.2] phase, has attracted a considerable attention lately (Lopez et al., 2007; Emmerlich et al., 2004; Barosum, 2001) because of the extraordinary properties. In these materials, (Lopez et al., 2007; Barosum, 2001), the metallic properties, such as electrical and thermal conductivity and thermal shock resistance, are combined with ceramics properties, such as good oxidation resistance, a refractivity and high decomposition temperature (1800[degrees]C). MAX phase materials are interesting from the technological point of view, because of ductility and workability, (Emmerlich et al., 2004). This material presents, also, a high rupture resistance and a very low friction coefficient. According to (Emmerlich et al., 2004), thin films containing MAX phases, particularly [Ti.sub.3]Si[C.sub.2], can become essentials in applications such: electrical contacts and coatings with wear protection role. The big disadvantage is the fact that the thin films containing MAX phases can be obtained at deposition temperatures higher than 700[degrees]C, (Lopez et al., 2007; Emmerlich et al., 2004; Eklund et al., 2005). Because of this high temperature, the choice of the substrates becomes very important, because these do not have to change the composition and especially the structure at the deposition temperature. In this way, (Lopez et al., 2007), the research programmes should be developed in order to solve this big disadvantage. On the other hand, lower deposition temperatures lead to the formation of some nanocomposite materials, considered promising for protective coatings from the electrical contacts, because of the high wear, corrosion resistance and high conductibility, (Eklund et al., 2005; Hogberg et al., 2005).

The technique used for Ti-Si-C films deposition on high-speed steel substrates is the one of sputtering. The sputtering technique is (Munteanu et al., 2007) a deposition method of thin films in vacuum, where the deposition particles in the form of neutral atoms or neutral atoms groups, which have energy between 10 eV and 40 eV, are obtained by vacuum sputtering of the solid-state deposition material.

In the last ten years, a relevant number of papers regarding the deposition of nanostructured films by sputtering were published (Lopez et al., 2007; Emmerlich et al., 2004; Veprek, 1997). The goal of these researches is the obtaining of hard films, but also tenacious, thermal stable and witch should present low friction coefficients. In the most of the cases (Lopez et al., 2007), the motivation of the researches was directly correlated with the influence that the crystalline grains have over the properties, so over the performance of these multifunctional coatings, knowing the fact that the final effects will be as relevant as the grain size is smaller. For example, from the mechanical behaviour point of view, (Lopez et al., 2007), a decrease of the grain size leads to the improvement of the mechanical resistance and toughness, by the process of blocking and stopping the dislocation pinning. Nanostructured polycrystalline materials were obtained, consisting of atoms with different chemical reactivity, witch then form different types of phases that do not exist naturally. During the deposition operation, phase segregation take place, creating multiphase materials, in witch, for example, crystalline phases are encircled by other types of phases, in the crystalline grain lattice. A nanocomposite material is created this way, such the case of the well-known Me-Si-N system (where Me = Ti, W, V), in which Me-N nanocristallites are incorporated in a Si-N amorphous matrix, (Veprek, 1997).

2. EXPERIMENTAL DATA

The Ti-Si-C thin films studied in the present paper were deposited on AISI M2 high-speed steel substrates, using the magnetron sputtering technique. The principal parameters of the sputtering operation were: substrate temperature in the deposition chamber was maintained at 300[degrees]C, substrate bias voltage -50V, argon flow 100 sccm, target--substrate distance 65 mm, Ti target current varied between 0,25-0,5A.

Table 1 presents the composition of the deposited films. The highest Ti content and the lowest C content were registered in the case of sample A15, while the highest C content corresponds to the sample A 4.

The wear behaviour was appreciated using a CSM Instruments Tribometer. The tribometer also gives information regarding the friction coefficients. The principal parameters of the wear test were: method--rotative "ball on disk", wear radius 5 mm, rotation speed of the disk 4,8 cm/s, normal force 5 N. The working principle of the tribometer is the following: the static partner (the ball) is loaded over the sample with a constant force. While the disk is rotating with the specified speed, the friction forces that appear at the ball--sample contact are measured using a sensor. The wear for ball and sample is calculated on the base of the volume of lost material during test. This simple method facilitates the study of the friction and of the wear behaviour of almost any type of solid-state material, by changing time, load, speed, temperature, humidity.

3. RESULTS AND DISCUSSIONS

Table 2 presents the results (friction coefficients, wear) obtained after the wear test. The A2 and A6 samples presented an insignificant wear, which could be considered equal with zero. This insignificant wear is a result of the high hardness registered in the case of the two samples, from figure 1 resulting clearly the strong correlation between the wear behaviour and the hardness of the analyzed coatings.

Figure 2 presents the correlation between the wear behaviour and the residual stress state. It can be observed that a very good wear behaviour is associated with low values of the residual stress.

Regarding the friction coefficients, figure 3 shows that the lowest value of the friction coefficient was registered in the case of sample A4, sample for witch it was registered the highest value of mass loss during wear.

4. CONCLUSIONS

The paper presented some general aspects regarding the deposition of Ti-Si-C films on high-speed steel substrates by magnetron sputtering. Taking into account the tribological and the mechanical characteristics of the Ti-Si-C coatings synthesized in this research, it is possible to conclude the fact that there is a strong correlation between the tribological properties (wear, friction) and hardness, residual stress state. The further research should follow the obtaining of coatings with different chemical composition, in order to see how it changes the tribological behaviour of the coatings, if compare with the coatings from this paper.

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5. REFERENCES

Barsoum, M. (2000): The [M.sub.n+1] A [X.sub.n] phases: A new class of solids: Thermodynamically stable nanolaminates, Progress in Solid State Chem., vol. 28, no. 1, pp. 201-281

Barsoum, M. (2001): The MAX Phases: Unique New Carbide and Nitride Materials, American Scientist, vol. 89, no. 4, (Jul.-Aug. 2001) pp. 334-343

Eklund, P. et al. (2005): Structural, electrical and mechanical properties of nc-TiC/a-SiC Nanocomposite thin films, Journal of Vac. Science and Technol., B.23, (Nov. 2005) pp. 2486-2495

Emmerlich, J. et al. (2004): Growth of [Ti.sub.3]Si[C.sub.2] thin films by elemental target magnetron sputtering, Journal of Applied Physics, vol. 96, no. 9, pp. 4817-4826

Hogberg, H. et al. (2005): Growth and characterization of MAX phase thin films, Surface Coat. Technol. vol. 193, issues 13, pp. 6-10

Lopez, C. et al. (2007): Magnetron sputtered Ti-Si-C thin films prepared at low temperatures, Surface and Coatings Technology, doi: 10.1017/j.surfcoat.2007.01.025

Munteanu, D. et al. (2007): Ti-Si-C and Ti-O-C type coatings obtained by reactive magnetron sputtering, Transilvania University of Brasov Publishing House, ISBN 978-973635-931-6, Brasov, Romania

Veprek, S. (1997): Conventional and new approaches towards the design of novel superhard materials, Surface Coat. Technol. vol. 97, issues 1-3, pp. 15-22

IONESCU, C[ristian]; MUNTEANU, D[aniel] & MUNTEANU, A[lexandru] *

* Supervisor, Mentor

Tab. 1. The composition of the coatings

 Composition, at.%
Sam
No. Ti Si C O Atomic formula

A2 51.14 2.62 43.09 3.25 Ti[Si.sub.0.05][C.sub.0.84]
A4 27.19 5.29 65.32 2.2 Ti[Si.sub.0.19][C.sub.2.40]
A6 48.79 5.01 44.24 1.96 Ti[Si.sub.0.10][C.sub.0.90]
A15 78.63 8.63 2.65 10.09 Ti[Si.sub.0.10][C.sub.0.03]

Tab. 2. The thickness and tribological characteristics of the
coatings

 Medium
Sample Thickness Friction Wear rate
 no. [micro]m C/Si coeff. mm3/N/m

 A2 0.9 16.44 0.45797 [congruent to]0
 A4 0.3 12.34 0.23547 1.38 x 10-5
 A6 0.6 8.83 0.54169 [congruent to]0
 A15 2 0.31 0.56149 1.02 x 10-5