ICMCTF2010 Session B5-2: Properties and Characterization of Hard Coatings and Surfaces

Nitrides of the group IVb-VIb transition metals have been well known for their unique combination of exceptional mechanical and electrical properties sharing high hardness values and electrical conductivity. Ternary transition metal nitrides (TMN) have lately gained special attention (with Ti_xZr_1-xN being the most widely studied) in an effort to improve further these properties. Another pathway in tailoring the structure and properties of TMNs is by alloying with noble metals (Cu, Ag), which do form bonds with the transition metal but not with N. In this case nanocomposite films are produced.

In this work we present a comparative study of a very wide range of ternary transition metal nitrides of the form: Ti_xMe_1-xN (Me=Zr,Hf,Nb,Ta,Mo,W) and Ta_xMe_1-xN (Me=Zr,Hf,Nb,Mo,W) over the whole x range (0<x<1) grown by Pulsed Laser Deposition (PLD), Dual-Cathode Confocal Magnetron Sputtering (MS) and Dual Ion Beam Sputtering (DIBS). The density, chemical composition and structure are investigated using X-Ray Diffraction and Reflectivity (XRD/XRR), in-situ Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). We study the stability of the rocksalt structure in all these films using and map the lattice constant determined from XRD and ab-initio calculations. We consider the electron hybridizations and the bond nature of the ternary nitrides based on the ab-initio calculation results. Despite the possible different valence electron configuration of the constituent elements, (e.g. Ta-d³s² and Ti-d²s²), we show that TMNs are completely soluble to each other due to the hybridization of the d and sp electrons of the metals and N, respectively. Optical absorption bands are manifested due to the N-p→Me-d interband transition and the t_2g→e_g transition due to splitting of the metals’ d band, proving the partial ionic character of the bonds in TMNs. The texture development mechanisms are considered and a critical comparison of ternary films grown by different techniques is presented. The key thermodynamic and kinetic factors that determine the structure of such films are investigated.

Finally, we consider the growth of Ti-Cu-N nanocomposites by PLD and MS. We investigate the kinetic and energetic effects that determine the microstructure and the atomic configurations in the films; we identify the conditions for forming intermetallic phases within the nanocomposite and study the bonding and coordination number of the atoms at the interfaces.

TiC coatings are technologically important materials given their high hardness and good wear resistance. Addition of Si has been seen as a promising way to further improve the properties of TiC_x. For example Ti-Si-C nanocomposites have been demonstrated as suitable coatings in electrical contacts [1-2]. However, to develop multifunctional Ti-Si-C coatings requires increased understanding on how Si is incorporated in the NaCl-structure TiC_x phase as well as how Si segregates to form a composite material at low temperatures. In this study we investigate these phenomena by growth of Ti-Si-C thin films using low temperature deposition by dc magnetron sputtering. The effect on microstructure, mechanical and electrical properties was studied by incorporating different quantities of Si in crystalline TiC_x thin films. The Ti-Si-C films were deposited onto Al₂O₃ (0001) and Si (100) substrates kept at 350°C. During the process three elemental targets were used in an Ar discharge with an Ar pressure of 4 mTorr. XRD results show that the growth of pure TiC_x onto Al₂O₃ was epitaxial. By increasing the Si content from 5 to 20 at.%, the structure of the films becomes nanocrystalline. SEM reveals a segregation of Si on the surface. Initial TEM results shows that the epitaxial growth is retained up to approximately 50 nm in films containing 5 at.% Si, while thicker films exhibit nanocolumnar grains.

[1] Eklund,P., Surface Engineering, 2007, 23, 406

T_hkl = [I_m(hkl)/I₀(hkl)]/{ 1/n * sum [I_m(hkl)/I₀(hkl)]_1..n}

Hard coatings in the Ti-B-C-N system possess very interesting properties for cutting applications. They combine the advantages of TiCN with the potential of improved wear resistance due to increased coating hardness.

A series of TiB_xC_yN_z (x+y+z=1) coatings deposited on cemented carbide by chemical vapor deposition (CVD) has been investigated. The choice of precursors (CH₄/CH₃CN) and deposition temperature (850-1000°C) influenced the microstructure, crystallinity, microhardness and chemical composition. The B:C:N ratio could easily be changed by varying the gas precursor flow rates in the CVD process.

Laser Raman Spectroscopy was used to study the coatings since it has been shown to be a very sensitive technique regarding composition changes. Even small changes in the B:C:N ratio resulted in systematical shifts of the Raman peaks.

Low-boron containing coatings are known to possess improved thermal stability. Therefore, Raman spectroscopy was further used to investigate the oxidation resistance of Ti-B-C-N coatings in comparison with TiCN. Depending on the annealing temperature, different oxidation products like anatase and rutile could easily be distinguished.

Cr-Si-O- N hard coatings 5 micrometer thick, with excellent oxidation resistance, phase and nanocrystalline stability were deposited by reactive arc evaporation on onto cemented carbide substrates . The systematic variation of the O₂/N₂ flow ratios resulted in a oxygen atomic concentration C_O2 = O/(N + O) between 0 and 100 at. %. For C_O2 < 80 % the films exhibit a cubic fcc crystallographic structure, while at higher oxygen content the structure of the samples is rhombohedral. The crystallite size of about 20 nm remains approximately constant for the cubic Cr-Si-O-N phase in the 0 – 80 % C_O2 range. An enhancement of a (002) preferred orientation is observed for samples with the fcc structure with increasing the oxygen concentration. The progressive decrease of the stress-free lattice parameter of the films with the cubic structure with increasing oxygen content suggests the formation of a Cr_0.92Si_0.08O_xN_1-x solid solution. The microstructure of the samples, studied by scanning electron microscopy (SEM), is dense with smooth fracture faces for all samples. The hardness H of the coatings increases with increasing oxygen content up to a value of 27 GPa for C_O2 = 44 %. Higher oxygen contents result in a reduced hardness of the coatings. Conversely, the elastic modulus E decreases with increasing C_O2. As a consequence, the coatings exhibit a resistance to plastic deformation H³/E² up to 0.19 for C_O2 = 44 %. X-ray powder diffraction (XRD) studies were performed in situ at high temperatures up to 1000°C, in high-vacuum (HV) and in air. In general, the crystallite growth, occurring at elevated temperatures both in HV and in air, is hindered considerably by the presence of oxygen and silicon. The grain growth is hardly observable up to 1000°C. The Cr-Si-O-N coatings with the cubic structure, annealed both in HV and in air, provide an exceptional thermal stability and oxidation resistance, with no evidence of oxidation or the formation of the Cr₂N phase up to 1000°C.

The recent discovery of the metastable γ’’’-FeN phase has opened a new field of investigation for the thin films community. Indeed, this phase crystallizes with the same cfc structure as TiN and CrN. However, the literature also reports the existence of another cubic phase with a ZnS-like structure: γ’’-FeN. Recently, we succeeded in the magnetron sputtering deposition of γ’’’-FeN free from its cubic-centred twin. By introducing silicon, the formation of a nanocomposite structure can be expected.

This study is focused on the evolution of the structure of γ’’’-FeN thin films as a function of the amount of silicon introduced. Within this objective, thin coatings were deposited on glass, mirror-polished silicon and steel substrates by magnetron co-sputtering of distinct iron and silicon targets. The films were then analysed by means of various spectroscopic measurements. A large range of wavelengths was used in order to register information on every aspect of the structure, from infra-red to gamma rays.

FTIR spectroscopy gave information on the linking bonds inside the material, showing proofs of Si-N bondings. Shifting in UV-visible spectra traduced the disturbances in the grains induced by the introduction of silicon. X-ray diffractograms showed only peaks of γ’’’-FeN whatever the Si content. EDS measurements indicated that the silicon content was proportional to the current applied on the Si target and was ranging between 0 and 13 at. %.

Finally, the structure was assessed by Mossbaüer spectroscopy in order to evaluate the chemical environment of iron atoms, and thus the main locations of the Si atoms in the complex FeSiN nanostructure.

The main purpose of this research was to investigate the effect of oxygen content and film thickness of zirconium oxynitride (ZrN_xO_y) on the structure and properties. By introducing the oxygen flow during coating, ZrN_xO_y thin films shows good mechanical properties, chemical inertness, and have various colors. ZrN_xO_y thin films were deposited on P-type Si(100) wafer using hollow cathode discharged ion-plating (HCD-IP) by changing O₂ flow rate from 0 to 8 sccm and deposition time from 14 to 56 min. Effect of thickness and other composition, structure, and properties of ZrN_xO_y thin films were characterized. The thickness of ZrN_xO_y films, measured by scanning electron microscopy, increased with increase of deposition time, and increased with increase of oxygen flow rate. Phase separation of ZrN_xO_y to ZrN and monoclinic ZrO₂ was identified by X-ray diffraction pattern when the oxygen content was higher than 11 at.%. Hardness of ZrN_xO_y thin films increased with increasing thickness and packing density of film and with crystallinity of ZrN as 0 and 5 sccm O₂ flow rate. No significant change of the overall and ZrN residual stress with thin thickness was found while they were decreased with the decrease of oxygen content. The resistivity of ZrN_xO_y thin films increased with decreasing thickness.