TiC multilayered films

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Nuclear Instruments and Methods in Physics Research B 95 (1995) 323-326

RUMIB

Beam Interactions with Materials&Atoms

ELSEWIER

Structure and hardness enhancement of Fe/Tic

multilayered

films

C.H. Liu *, Wen-Zhi Li, Heng-De Li Dept. of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Received 15 June 1994; revised form received 2 November 1994 Abstract Multilayered films composed of iron metal and TIC ceramics, have been synthesized by ion beam sputtering at room temperature. Hardness testing revealed that a maximum value of HK = 4200 kg/mm2 was obtained for individual layer thicknesses of fFe = 6 nm, tTiC = 8 nm. Structural investigation by X-ray diffraction showed that these multilayers were formed with sharp interfaces. Individual layer structures were found to depend on layer thickness. Apparent compressive stress in the plane of the film and (111) texture in the TiC layers are considered as an explanation for the hardness enhancement. Annealing at 400°C for 2 h under vacuum led to the decrease of HK to 1300 kg/mm*.

1. Introduction In the last few years multilayer materials have been demonstrated to be materials of great importance for scientific study and technological applications. Studies show that when the individual layer thickness is on the nanometer scale multilayers exhibit many intriguing physical phenomena and novel material properties with respect to mechanics, magnetics, electronics, optics, etc. [l-4]. However, so far the mechanisms of the abnormal properties in the multilayers are not clear. More research work is required to determine the detailed microstructure of multilayers especially at interfaces and the relations between synthesis, resulting structure/microstructure, and properties. Interfaces have been considered to play an important role in determining the physical properties of multilayers [5,6]. However, interdiffusion at interfaces has been observed in many metal-metal multilayer systems. In fact, reacted interfaces are more common than not. In addition to the mostly studied pure metal multilayers, some metal-metalloid systems such as metal-carbon and metal-silicon have been studied by some authors. Amorphization or compound formation is found in the interfaces in these systems 171. Deterioration of sharpness in interfaces usually results in a degradation of properties of the multilayers. Titanium carbide films are well known for their good properties such as high melting point, chemical inertness, high hardness and oxidation resistance. Therefore, they are often used as wear resistant films [8]. We have been investigating multilayers where one of the constituents is TiC be-

* Corresponding 0168-583X/95/$09.50

author.

cause it facilitates formation of sharper interfaces. Doubtlessly, TiC layers in multilayer are also beneficial to the stability of the materials and to their practical applications. In the literature on multilayers or superlattices, ultrahigh vacuum evaporation with in situ characterization or conventional sputtering is often a fabrication method. Clemens and Williamson [9] concluded from Miissbauer and low angle X-ray diffraction (LXRD) that sputter-deposited Fe/Zr had flatter layers with less intermixing than electron-beam evaporated films. Here we employ ion beam sputtering because this technique operates at a lower pressure than conventional sputtering so that films of high purity are obtained. The advantages of this technique operating normally at a lower substrate temperature [lo] and quantitative control are also suitable to the formation of high-quality multilayers. In this study we select the fee TiC/bcc Fe pair, in which there is a large lattice mismatch, and describe results from structural analysis of Fe/Tic multilayered films, combined with measurements of mechanical hardness.

2. Experimental

Fe/Tic multilayers were prepared on silicon substrates by alternately depositing TiC and Fe in an ion beam sputtering system [ll]. The base pressure was 9 X lo-’ Pa, and sputtering was performed at 7 x 1O-3 Pa of argon. The TiC layers were synthesized by co-sputtering a composite target of graphite and titanium. The Ti/C component ratio in the target was adjusted to form stoichiometric TiC layers, which was confirmed in single layer TiC films

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C.H. Liu et al. /Nucl. Instr. and Meth. in Phys. Res. B 95 (1995) 323-326

by X-ray photoelectron spectroscopy, Auger electron spectroscopy and X-ray diffraction (XRD). Alternate deposition was accomplished by rotating a target holder which could contain four differen! target materials. Th,e deposition rates were kept at 0.5 A s- ’ for TIC and 2 A s- ’ for Fe. Individual component layers in the multilayers were varied in thickness from 1 to 12 nm for Fe and 2 to 10 nm for Tic. To enhance the adhesion force, the substrates were sputter cleaned with 3 keV of 50 p,A/cm2 argon ion beam for 10 min before deposition. The multilayers started with a thin layer of TIC. The substrates were cooled with water during deposition. The annealing experiment was performed in a vacuum furnace with a residual pressure of 5 X 10e4 Pa. The structure of the multilayers was investigated by reflection XRD both at low angles and at high angles. The analyses were performed on a Rigaku D/ Max-RB type of diffractometer with backmonochromized Cu K,(1..5406 A) radiation. Knoop hardness measurements were conducted using a HXD 1000 type of hardness tester. All the samples for the hardness measurements were prepared with equal thickness (about 1 pm). To avoid the indentation size effect, a small load of 1 or 2 g was applied.

3. Results and discussion 3.1. Structure

Layered structure in the films was verified by LXRD. The results confirmed that these multilayers were formed with a well-defined layered structure. Even for the multilayers with a thinner Fe individual layer thickness, the orders of satellites shown by LXRD were also large. Fig. 1 shows the LXRD pattern of a sample with t,, = 1.2 nm, tTi, = 10 nm. The periodicity is so clear that the diffraction peaks up to the seventh order can be observed. The values of modulation wave length D calculated from the corrected Bragg relation [12] were in agreement with

1

Fig. 1. Low-angle X-ray diffraction with t Fe = 1.2 nm, tT,C = IO nm.

pattern of Fe/Tic

multilayers

Fig. 2. X-ray diffraction patterns of the multilayers nm, tTIC= 8 nm (a) before, (b) after annealing.

with

tFe= 6

designed values within 5%. The sharpness of the interfaces in these multilayers was proved to be superior to that of pure metal multilayers such as Fe/MO or Fe/Ti prepared under the same conditions. High-angle XRD results showed that individual layer structures depend on layer thickness. For the multilayer with t, = 1 nm, tTi, = 2 nm, only two broad peaks TiC (200) and bee Fe (200) were shown by XRD, indicating the predominant textures in the component layers. As the TiC layer thickness was increased to 6 nm, the TIC layers showed more random texture, which varied with Fe layer thickness. For an Fe layer thickness of 1 nm, the TiC layers have (200) texture, while no visible peak of Fe was shown in the XRD pattern. The absence of Fe peaks probably implies the amorphous state of Fe layers in the multilayer. Thicker Fe layers result in the change of the multilayer to orientations of TiC (lll)/Fe (llO), indicating the close-packed planes lying in the plane of the film. That the texture of the TiC layers is influenced by the Fe layer thickness implies the growth of TIC on Fe. XRD patterns of the sample with t, = 6 nm, tri, = 8 nm are shown in Fig. 2. The visible peak at 20 = 74” was attributed to fee Fe (220). Presumably, a few monolayers of Fe close to the interface were stabilized in the fee structure due to the Tic layers. The rest of the layers converted into bee Fe. Multilayer satellite peaks are not observed in the spectra, indicating the formation of incoherent interfaces in these multilayers. It is known that multilayers with incoherent interfaces result in the crystal-coherence length being limited to the individual layer thickness [13]. Long-

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C.H. Liu et al. / Nucl. Instr. and Meth. in Phys. Rex B 95 (1995) 323-326

range coherence in the growth direction is commonly not observed in multilayer systems where there is a relatively large size mismatch [14]. Detailed structures of individual layers can be characterized using high resolution electron microscopy or the quantitative structural refinement technique of fitting the reflection XRD spectra to model calculations [15,16], and will be given elsewhere. As a description of results of preliminary work, here we present qualitative discussion, in which perpendicular lattice spacings of the constituent layers were determined directly from the central peak positions of the diffraction spectra (Si(llL) spacing was used as internal reference). Notable expansions of about 2.6% for TiC (111) and 0.9% for Fe (110) spacings were observed in the multilayer corresponding to Fig. 2a. That most of the expansion is in the TIC layers is consistent with the reports on bcc/fcc metal multilayers where it was found that the fee metal changes most drastically [13,17]. After annealing this sample at 673 K for 2 h, these lattice expansions in the layers were found to be relieved to nearly zero. Additionally, the (220) peak of metastable fee Fe disappeared (Fig. 2b). These changes probably imply stress release in the layers. The basic XRD patterns for fee TiC’and bee Fe did not change. This suggests that the stability against interdiffusion is higher in these Fe/Tic multilayers. 3.2. Hardness Fig. 3 shows the microKnoop hardness, HK, as a function of iron thickness lFe where tTiC = 8 nm. The values HK = 2300 kg/mm2 and HK = 200 kg/mm2 illustrated as dashed lines are those of a pure TiC film ( tFe = 0) and a pure Fe film ( I,~, = 0) for comparison. The hardness of the multilayers was found to increase steadily with mcreasmg tFe and reach a maximum of 4200 kg/mm2 at fFc = 6 nm. Further increase in tFe resulted in a decrease in HK. The figure shows that the layered structures can have an apparently higher hardness than that of pure TiC films, the harder constituent. Even though elastic anomalies [18] and increase of material strength [ 191 through composition modulations

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4

6

6 tt

Fig. 3. The variation of the microffioop iron thickness tFe where rTiC = 8 nm.

(nm)

hardness as a function of

Table 1 The lattice spacings of TIC (111) and Fe (110) as a function of fFe in the multilayers

with tTiC = 8 nm.

IFe bml

d T,C(Illl [Al

1 2 4 5 6 7 8

2.544 2.548 2.561 2.568 2.576 2.574 2.571 2.51 a

a Standard

d k(llll, [Al _ 2.050 2.066 2.058 2.045 2.040 2.037 2.027 ’

data from the PDF card

have been extensively studied previously, reports on hardness enhancement in multilayered films are seldom. To our knowledge, this is the first report of a remarkable increase in hardness in multilayers composed of a hard carbide and soft metal pair. The mechanical hardness, which is not a fundamental physical property, depends on both the elastic and the plastic response of the material. So, comparison of hardness results with any of the proposed theories for enhancements in the elastic constants is difficult [20]. Several years ago Koehler [21] proposed a model for increasing the strength of materials by simply inhibiting dislocation mobility through the formation of a modulated structure. Helmersson et al. 1221 interpreted their results showing an increase by a factor of 2.5 in hardness of TiN/VN superlattice structures partly with this model. However, the present system Fe/Tic is different from the ideal system discussed in the model. Lattice parameters of the fee TIC and bee Fe are quite different. Even in the fee TIC and fee Fe pair, the lattice parameter mismatch is also large (17%). Doubtlessly, smaller TIC crystal grain size and transformation of TIC layers in texture from (200) to (111) are beneficial to the increase of hardness. Additionally, apparent lattice expansions in the direction normal to the film were observed in the multilayers. On account of the Poisson effect, these expansions revealed the existence of compressive stress in the plane of the films. Metallic Ni has many similar properties to metallic Fe such as thermal expansion and elastic constants, except for its larger lattice constant which is more compatible to that of TIC. Multilayers of Ni/TiC prepared with the same parameters as that of Fe/Tic exhibit lower expansions and hardness (lower than that of TiC films). This suggests that the stress in the layers can be ascribed to lattice mismatch in the system. It are these stresses generated by interfaces in incoherent multilayers that led to hardening of the multilayers. Table 1 lists the lattice spacings of TIC (111) and Fe (110) calculated from X-ray diffraction patterns of the samples in Fig. 3. The size of difference of the lattice spacings to the standard data reflect the level of stress in

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C.H. Liu et al. /Nucl. lnstr. and Meth. in Phys. Res. B 95 (1995) 323-326

the layers. The variation of hardness observed in Fig. 3 is consistent with that of the stress in the TiC layers. Our explanation is further supported by an annealing experiment. Stress relief in the layers led to a decrease of film hardness. After annealing the multilayer with the maximum HK at 400°C for 2 h, the spacings of TiC (111) and Fe (110) from XRD were 2.511 and 2.030, which were nearly equal to their standard data from the PDF cards, respectively. The corresponding hardness of the multilayer was found to decrease remarkably to 1300 kg/mm*, which is approximately equal to the average of pure TiC and Fe hardness.

4. Summary The major conclusions from this study can be summarized as follows: (1) Fe/Tic multilayers with sharp interfaces can be formed by ion beam sputtering at room temperature. (2) Individual layer structures are dependent on layer thickness. TiC layers above 6 nm thick have a polycrystalline structure with a preferred orientation which is influenced by the Fe layer thickness. Incoherent interfaces are characterized by the loss of XRD highangle multilayer satellite peaks. (3) A maximum value of HK = 4200 kg/mm’ was obtained at t,, = 6 nm, & = 8 nm. Apparent compressive stress in the plane of the films and (111) texture in the TiC layers are considered as an explanation for the hardness enhancement. (4) After annealing the sample with maximum hardness at 673 K for 2 h, stress in the layers was found to be relieved, and the HK value decreased remarkably.

Acknowledgements This work was financially supported by Ford-China Research & Development Fund and National 863 project.

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