Crystalline/amorphous nanocomposite coatings have a number of unique mechanical characteristics. For example, a super-hardness effect was reported for TiN/Si3N4 composites¹. More recently, a super-toughness effect was discovered for composites, consisting of 10-50 nm nc:TiC nanocrystals encapsulated in an amorphous diamond-like carbon (a:DLC) matrix^{2, 3}. Carbides and DLC are widely used in wear resistant and low friction coatings. Improvement of their toughness by designing crystalline/amorphous nanostructures is considerd. These novel structures permit initiation of crack deflection on grain boundaries, crack energy dissipation in the amorphous phase, "quasi-plasticity" with nano-grain shifts, etc. The mechanical and tribological properties of the nc:TiC/a:DLC and nc:WC/a:DLC nanocomposites are described, including hardness, scratch toughness, wear rate coefficients, and friction coefficients. Attention is also given to friction mechanisms for crystalline/amorphous coatings. To provide lower friction coefficients, doping with solid lubricants (MoS₂ and WS₂) was performed. This generated three phase coatings consisting of nanocrystalline carbide, amorphous DLC, and nanocrystalline dichalcogenides. Results are correlated with a review of recent publications, and general concepts for tough, wear resistant crystalline/amorphous nanocomposite coatings are proposed.

¹ S. Veprek, S. Reiprich, Tthin Solid Films 268 (1995) 64.

² A. A. Voevodin, S. V. Prasad, and J.S. Zabinski, J. Appl. Phys., 82 (1997) 855-858.

³ A. A. Voevodin and J. S. Zabinski, J. Mat. Sci. , 33 (1998), 319-327.

Nanocomposite a-Si₃N₄ / nc-TiN films 400nm thick were synthesised in various compositions by using ion beam sputtering with concurrent nitrogen and argon ion bombardment. The substrate temperature was varied from 80 C to 750 C. The ion assist beam energy was varied from 300 eV to 700 eV for various ion/atom arrival ratios ranging from 0.5 to 3. The film composition was controlled by varying the area of a silicon strip overlayed on a Ti target and determined by RBS. Film morphology was characterised by glancing incidence X-ray diffraction (GXRD) and TEM; hardness was determined by nanoindentation.

The hardness was found to depend upon film morphology, which in turn depended on the film composition and deposition parameters. The average TiN crystal size varied from 1.6nm to 8.6nm as determined by analysis of peak broadening of GXRD data, which was confirmed by TEM. The crystal size was seen to increase with ion/atom arrival ratio, reach a maximum, and then decrease at a given substrate temperature. Hardness varied with composition and a maximum of 48 GPa was achieved with a 9.25% Si content, at a substrate temperature of 500 ^C.

Nanocomposite thin films consisting of both nanosized solid solutions or nanosized polycrystalline materials embedded in various amorphous matrix materials thus provide a great potential for future mechanical devices. In this respect, films resulting from additions of Si to TiN matrix and prepared by r.f. reactive magnetron sputtering, will be prepared and characterised in this paper.

Structural properties such as growing characteristics (type of matrix, texture, grain size) and mechanical properties such as hardness and adhesion were analysed. Both conventional transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM) were used for the structural characterisation, while ultarmicrohardness test and scratch test were chosen for the mechanical characterisation. The atomic composition of the samples was obtained by Rutherford Backscattering Spectrometry (RBS). X-ray diffraction (XRD) experiments were used in order to obtain texture factors (rocking curves) and microstress states by the sin²Ψ method. The deflection method was used to evaluate the macroresidual stress states.

All properties will be characterised and discussed as a function of the Si content in the Ti_1-xSi_xN_y matrix and several relations will be made regarding important parameters such as texture factor, grain sizes, stress states and specially the type of matrix developed. Regarding the preliminary results, the samples with a composition of Si between 5.9 and 10.6 at. % show higher hardness values (about 50 GPa). This region is also the one where the most oriented films grow (220 cubic fcc orientation) together with the smallest grains observed. The highest critical adhesion loads are also obtained in this region. Relations within these results, together with those of residual stresses will be made and discussed in detail.

Recently, new superhard (> 40 GPa) coatings in a form of superlattices of metal nitrides , e.g. TiN/VN, and nanocomposite coatings composed of nanocrystalline metal nitrides embedded in hard amorphous matrix, e.g. nc-TiN/a-Si₃N₄, were developed. These materials exhibit very high hardness up to 50 GPa but they are also very brittle due to their low plasticity at loads exceeding their elastic limit. However, in many applications hard coatings with higher toughness are needed. Such a requirement meet nanocomposite coatings of the type nc-MeC/a-C and also nanocomposites of the type nc-MeN/soft metal matrix (e.g., Cu, Ni, Mo, Co, Ag, etc.)

The paper reports on properties of ZrN/Cu films, i.e. on a new class of hard coatings based on nitrides of copper based alloys. The ZrN/Cu films were prepared by d.c. reactive magnetron sputtering of an alloyed target ZrCu (70/30 wt.%) in Ar+N₂ gas mixture at the total pressure of 0.7 Pa using a circular planar magnetron 100 mm in diameter. Mechanical properties of the ZrN/Cu film, i.e. its hardness, elasticity and plasticity, can be controlled by substrate temperature T_s, substrate bias U_s, substrate ion current density i_s and partial pressure of nitrogen p_N2. The film hardness can be continuously increased from low values of about 10 GPa to very high values up to 55 GPa. While the ZrN/Cu film with a relatively low hardness of about 10 GPa exhibits a high (40%) plasticity and low (60%) elastic recovery, the superhard ZrN/Cu film with hardness of about 50 GPa, on the contrary exhibits high (80%) elastic recovery and low (20%) plasticity.

The mechanical properties of ZrN/Cu films are closely connected with their structure. XRD patterns from superhard (> 40 GPa) ZrN/Cu films show a well developed high intensity ZrN(111) reflection only; no reflection from Cu is observed. It means that the superhard films are composed of several nm ZrN grains embedded in amorphous Cu matrix. On the contrary, the ZrN/Cu films with a lower (20-30 GPa) hardness and higher (40-30%) plasticity are two phase nanocomposite films composed of small ZrN and Cu grains. The XRD patterns from these films exhibit reflections from two crystalline phases ZrN(111) and Cu(111); the intensity of ZrN(111) reflection is considerably lower than that from the superhard ZrN/Cu film.

A phase composition of the ZrN/Cu film can be controlled by energy delivered to the film during its growth at a constant substrate temperature T_s. The superhard ZrN/Cu films are formed at T_s = 400 °C and p_N2 = 0.05 Pa when larger amount of energy is delivered to the growing film by bombarding ions, i.e. when i_s ≥ 1 mA/cm² is used. To prepare hard ZrN/Cu films with higher toughness the substrate ion current densities i_s < 1 mA/cm² are sufficient. These films can be produced even in absence of both the substrate heating and ion bombardment, i.e. at T_s = RT and U_s = U_fl, but higher values of p_N2 = 0.3 Pa has to be used. The XRD patterns from these films exhibit broad, very low intensity ZrN(200) and ZrN(111) reflections. It means that hard ZrN/Cu films sputtered on unheated substrate at U_s = U_fl have a very fine grained structure with the grain size below 5 nm.