Modulus and Hardness Change of Silicon and Sapphire Substrates by TiC/VC Multilayer Coatings

André Belger, Marianne Reibold, Peter Paufler

Institut für Strukturphysik, Technische Universit?t Dresden, Dresden, Germany..
On Leave from Institut für Strukturphysik, Technische Universit?t Dresden, Dresden, Germany.
Triebenberg Laboratory, Institut für Strukturphysik, Technische Universit?t Dresden, Dresden, Germany.

DOI: 10.4236/msa.2012.34029                

Abstract

The nanohardness H of multilayer specimens TiC/VC@Si and TiC/VC@Sapphire prepared by Pulsed-Laser-Deposition is investigated to check the existence of a superlattice effect as known from TiN/VN multilayers. In the present work the multilayer period thickness λ varies between 1.34 nm and 24.8 nm (total layer thickness t ≈ 200 nm). Unlike Young’s modulus E, H is enhanced, regardless of t, by covering Si as well as sapphire with a TiC/VC multilayer; the relative load carrying capacity being larger for Si. The maximum value of H obtained is 38 GPa for TiC/VC@Sapphire. It is observed for a multilayer thickness of λ ≈ 10 nm. Hardness of TiC/VC@Sapphire obeys, after annealing, a Hall-Petch relation H = 35.25 + 6.945 λ^–0.5 (H in GPa und λ≥ 10 nm). From orientation dependent X-ray absorption fine structure and X-ray reflection records, short-range order and layer geometry are derived. These analyses reveal a continuous approach of interatomic distances Ti-C and V-C for deceasing multilayer periods. High-resolution transmission electron microscopy shows that multilayers are nanostructured, i.e., not only TiC/VC phase boundaries but also subgrains represent obstacles against plastic deformation. Dislocations play a major role as sources of internal stress and vehicles of plasticity.

Keywords

TiC/VC@Si or Sapphire Multilayers; Superlattice Effect; X-Ray Absorption Spectrometry; Electron Microscopy

1. Introduction

Both elastic modulus and hardness values of multilayer (MuL) thin films can exceed those of the single layers and of bulk objects. Multilayers or composition-modulated films may exhibit an increase of the elastic moduli (“supermodulus effect”) as a result of large coherency strains between the layers [1]. They are often higher than the values suggested by a simple rule of mixture between layer materials.

Unexpected values of hardness H have attracted more attention. Material with H > 40 GPa is considered superhard [2]. Getting ahead of this value became a major goal of materials research. The appearance of a hardness maximum, H_max, of a MuL@substrate system as a function of MuL period thickness, λ, was designated “superlattice hardening” [3,4]. Single crystalline TiN/VN MuLs on MgO substrate represented an early example. For λ = 5.2 nm a maximum hardness of H_max = 54.5 GPa [5] occurred.

Most models of superlattice hardening at nanoscopic scale refer to impeding of dislocation motion [6,7], because dislocations are the main vehicles of strain. Models differ in the kind of major obstacles. Also alternative mechanisms have been considered, like, for example, extension of microcracks [8].

So far, a wide variety of MuL coatings have been studied to improve the resistance of various metallic or nonmetallic bulk material (cf., e.g., Abadias et al. [9]). The aim of the present work was to study the nanohardness of TiC/VC MuLs on silicon (point symmetry group) and sapphire () substrates, respectively. These carbides have been less frequently paid attention to. Strafford [10] quoted Vickers hardness values for TiC (27.1 - 40.7) GPa and VC (18.6 - 27.9) GPa, respectively. Phani et al. [11] found the nanohardness of 800 - 900 nm TiC films to be in the range 25 - 30 GPa. Moreover, referring to their complete solid solubility thanks to the same NaCl structure type and similar lattice parameters (TiC 0.43176 nm, VC 0.41599 nm) superlattice hardening could be expected.

The present study focuses upon substrates silicon and sapphire. Both materials have become attractive as part of electronic devices. The following nanohardness values H have been reported for single crystalline silicon: H = 13 GPa for (100) and 16 GPa for (111) surfaces [12]. (0001) surfaces of sapphire exhibited H = (29.1 ± 0.1) GPa [13] at a depth of 200 nm. Improving their resistance against indentation, scratching and wear is a goal of economic importance. Preliminary work on TiC/VC@ Si and sapphire has been communicated elsewhere [14]. Special attention will be paid to the structure at nanoscale and its influence upon hardness.

2. Experimental

2.1. Preparation

TiC/VC MuLs (area 2 × 3 cm²) were prepared on sapphire (012) and silicon (001) single crystals at room temperature in vacuum (5 × 10^–6 Pa) by pulsed laser deposition (PLD) employing a Nd-YAG-Laser (Spectra Physics, GCR 5 - 20). The hardness of TiC layers can be increased essentially from about 20 to 44 GPa when applying PLD instead of magnetron sputtering [15]. Preliminary films obtained by us with the aid of solid-state reaction of PLD C-Ti-C-V MuLs exhibited strong composition gradients. Therefore, rod-shaped TiC and VC targets (hot-pressed powders) were preferred for PLD to ensure transfer of stoichiometric composition to the films. Laser parameters were chosen such that a gentle congruent ablation was going on: wavelength 355 nm, pulse duration 8 ns, pulse frequency 20 Hz, pulse energy 150 mJ, distance target-substrate 175 mm. Results of elemental analysis are given in Table 1. No contamination of the MuL with oxygen could be detected by electron probe microanalysis. X-ray reflectometry showed, however, that an oxide layer of about 1 - 2 nm thickness had formed on top of the stack after removing the sample from vacuum. An influence of this thin oxide upon properties of the stack could not be observed.

The total thickness t of the double layer package intended was 200 nm, corresponding to a range of specimens coated by 5 ∙∙∙ 100 double layers. 7 and 21 specimens were prepared on silicon and sapphire substrates, respectively. Moreover, single layers of TiC and VC on Si and Al₂O₃ substrates were also provided for comparison.

2.2. Structural Characterization

Structural characterization of MuLs was performed using X-ray reflectometry (XR), wide-angle X-ray scattering (WAXS) and extended X-ray absorption fine structure study (EXAFS) [16]. For details of the experimental procedure cf. [17]. Figure 1 shows XR curves of selected

Table 1. Chemical composition of target and films when exploiting compound targets.

Figure 1. Experimental XR data of reflected intensity versus diffraction angle q = θ of TiC/VC multilayers on (012) sapphire substrate for selected multilayer periods l = 1. The appearance of the second Bragg reflection for the 2θ- period multilayer indicates a deviation from the planned thickness ratio TiC:VC = 1:1.

MuLs on sapphire. The position of the first Bragg reflection is shifted as a function of superlattice parameter λ. The roughness of all samples amounted to 0.5 nm. Additionally, two samples (50 and 40 periods) were annealed (pressure p < 1000 - 600 Pa, 5 hours, 500˚C) to check the stability of the system. No change of the thickness but doubling of the roughness occurred due to this thermal treatment. Examples of XR analysis are summarized in Table 2.

The crystal structure of MuLs was refined by means of an X-ray diffractometer Siemens D5000 with a secondary monochromator tuned to CuK_α radiation making use of the asymmetric approach method (FPA) of the Rietveld program TOPAS 2.0. This enabled refinement of lattice parameters and crystallite sizes taking the diffracttometer device function into account. 220 reflection could be observed only, which exhibits a half width (0.55˚) smaller than the MuL Bragg reflections 220 of TiC/VC and of single layer TiC (Figure 2). Quantitative results are given in Table 3. For MuLs with smaller periods, lattice parameters could not be obtained because of poor resolution.

Figure 3 shows that, due to annealing, 220 peaks of TiC and VC sublayers were shifted to higher Bragg angles and their half widths decreased. That means cubic

Conflicts of Interest

The authors declare no conflicts of interest.

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