ICMCTF2010 Session B1-2: Sputtering Coatings and Technologies

Ternary transition metal nitride (TMN) films are offering superior mechanical properties, oxidation resistance or tribological performance compared to their binary counterparts. Among them, Ti-Al-N and Cr-Al-N systems are intensively studied. Their deposition by PVD techniques results in the formation of (meta)-stable solid solutions or nanocomposites, depending on growth conditions, concentration of substituting elements, or subsequent thermal treatments.

In the present work, we investigate the growth, structure, thermal stability and oxidation resistance of Ti-Zr-N and Ti-Ta-N systems, which represent archetypes of iso-structural (B1-type) and non-isostructural TMN systems. These coatings, with thickness up to 300 nm, were grown by dc reactive magnetron co-sputtering in Ar/N₂ plasma discharges at 300°C on Si substrate. The growth parameters were optimized from the hysteresis curves of N₂ partial pressure vs. N₂ flow. X-ray Diffraction patterns reveal the stabilization of solid solutions with B1-type structure in the whole composition range for Ti_1-xZr_xN and up to y=0.75 for Ti_1-yTa_yN. Due to the difference in the average deposited energy, the two systems exhibit distinct texture and surface morphology. Post-growth thermal annealing (up to 850°C under vacuum and 600°C under air) were performed to study their structure stability and stress relaxation. In situ temperature XRD experiments were also implemented on selected samples to investigate the onset of oxidation process.

Coatings like TiN or TiAlN are well established as hard and wear resistant tool coatings. These coatings often will be prepared by PVD techniques like arc evaporation or d.c. magnetron sputtering. Typical micro hardness values of such hard coatings are in the range of 30 GPa. As a clear advancement compared to d.c. magnetron sputtering processes the pulsed magnetron sputter deposition technique could be shown. TiAlN hard coatings as well as modified TiAlN coatings with additional elements like Si, Cr, W and C were prepared using the pulsed magnetron sputter technique in a CC800/9 batch coater equipped with 4 targets. Coatings prepared with the pulsed sputter process showed both high hardness and high wear resistance. Beside hardness and wear other properties like adhesion or friction coefficient were determined. Cross sectional SEM images revealed the growth structure in dependence of the applied substrate bias and of the added elements. The chemical composition of the coatings was investigated by electron microprobe analysis and the phase and crystal size were determined by X-ray diffraction. Using the pulsed magnetron sputter process the coating properties, especially the hardness could be significantly improved. With HU_plast > 40 GPa the range of superhard materials could be reached The application potential of these coatings will be demonstrated by turning and milling tests results of coated cemented carbide cutting inserts.

[TiCN/TiNbCN]_n multilayer coatings were grown onto Si (100) and steel substrates by reactive r.f. magnetron sputtering technique using two targets (TiC and Nb) and alternating deposition conditions. The bilayer period (Λ) was varied from the micrometric to the nanometric range, maintaining the total thickness of the coatings in the range of a few microns by depositing a suitable number of bilayers (n). The structure of the coatings was characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The stress was calculated by measuring the curvature of the films onto Si (100) substrates with a profilometer. Hardness measurements were performed by using nanoindentation. The tribological properties were determinate via dynamic contact test using a Microtest MT 4001‑98 tribometer and Scratch Test Microtest MTR2 system; from them, the friction coefficient and critical load for the different samples were measured. An enhancement of both hardness and elastic modulus was observed when the bilayer period (Λ) in the coatings was reduced. Sample with the smallest bilayer period (Λ = 15 nm, n = 200 bilayers) showed the lowest friction coefficient (~0.1) and the highest critical load (80 N), corresponding to 2.2 and 1.6 times better than those values for the coating with n = 1, respectively. The enhancement effects in the [TiCN/TiNbCN]_n multilayer coatings can be attributed to the Hall Petch effect in multilayered coatings, in which the interfaces act as a barrier against the movement of the dislocations and the bilayers of materials having different mechanical properties generate an inhomogeneous system prohibiting the advancement of potential micro-cracks.

This work was supported by the Center of Excellence for Novel Materials (CENM) under Colciencias/CENM contract # RC-043-2005. Colciencias-Univalle and J. C. Caicedo thank Colciencias for the doctoral fellowship.

In film growth, the competition between interface roughening and smoothening essentially determines the nano-/microstructure and consequently the properties of a deposited film. This paper reports several new findings on the breakdown of dynamic roughening in thin film growth. With increasing energy flux of concurrent ion impingement during pulsed DC magnetron sputtering, a transition from dynamic roughening to dynamic smoothening is observed in the growth behavior of TiC/a-C nanocomposite films. The nanocomposite films show a negative growth exponent and ultra-smoothness (RMS roughness ~0.2 nm at film thickness of 1.5 µm). Based on high resolution cross-sectional transmission electron microscopy observations, we conclude that an amorphous front layer 2 nm thick covers the nanocomposite film during growth and suppresses the influence of nanocrystallites on the roughness evolution of the nanocomposite films, which is a solid experimental proof of the impact-induced downhill flow model [1] and subplantation model [2-4]. We were able to predict the evolution of surface roughness based on a linear equation of surface growth which contains two diffusivity parameters that control the atomic mobility along the growing outer surface. The model is in good agreement with atomic force microscopy measurements of roughness evolution [5, 6].

[1] M. Moseler, P. Gumbsch, C. Casiraghi, A.C. Ferrari, J. Robertson. Science 309 (2005), 1545.

[2] Y. Lifshitz, S.R. Kasi, J.W. Rabalais. Phys. Rev. Lett. 62 (1989), 1290.

[3] D.R. McKenzie, D.A. Muller, B.A. Pailthorpe. Phys. Rev. Lett. 67 (1991), 773.

[4] J. Robertson. Philos. Trans. R. Soc. London. A 342 (1992), 277.

[5] Y.T. Pei, K.P. Shaha, C.Q. Chen, R. van der Hulst, A.A. Turkin, D.I. Vainshtein, J.Th.M. De Hosson. Acta Mater. 57(2009), 5156.

[6] A.A. Turkin, Y.T. Pei, K.P. Shaha, C.Q. Chen, D.I. Vainshtein, J.Th.M. De Hosson. J. Appl. Phys. 105 (2009), 013523.

Metallic glasses are a well-known group of materials with interesting physical and chemical properties. The glass forming ability (GFA) during solidification of alloys have been studied by Inoue and certain rules for high GFA have been formulated [1]. These rules can also be partly applied on thin film deposition of e.g. metal carbides. Sputtering of amorphous metal carbides films have previously been observed in some systems such as Cr-C, W-Ni-C and W-Fe-C (see e.g. ref. [2]). In the present study we have investigated magnetron sputtering of Zr-Si-C thin films. Krzanowski et al. [3] have found evidence for GFA in this ternary system but no systematic study has been performed before.

The coatings were deposited with non-reactive DC magnetron sputtering with elemental targets. The silicon content was varied between 0-30 at% and the zirconium content between 30-60 at%. Chemical composition and bonding was analyzed using XPS while the microstructure was studied using XRD, SEM and TEM. Mechanical and electrical properties were analyzed using nanoindentation, resistivity measurements and contact resistance measurements. We have found that Zr-Si-C thin films deposited at 250 °C are amorphous in a large composition range and that the GFA is highly depending on the Si content in the film. Initial results of mechanical and electrical properties show a resistivity of 300-400 µΩ cm, a hardness of 9-16 GPa and a Young’s modulus of 200-300 GPa. The GFA as well as the influence of the structure on the chemical and physical properties of the coatings will be discussed.

Ion energy distributions and plasma parameters for plasmas used to deposit low-temperature Al₂O₃ without a chrome template layer were investigated as a function of oxygen partial pressure in the vicinity of the substrate using a Langmuir probe and energy-resolved mass spectroscopy (EQP). The thin films were deposited using a mid-frequency AC reactive magnetron sputtering system operating at 40kHz. In this work, 4kW power, and total flow rates of 40 and 70sccm were used. The gate valve was set such that the pressure in the system reads 2 and 5mTorr for a pure argon discharge for each flow rate. Oxygen partial pressures ranging from 0-70% were investigated. Monitored species include Ar⁺, O⁺, O₂⁺, and Al⁺. The growth rate and the elemental composition of the deposited film was investigated

EQP results show that at constant chamber pressure changes in the ratio of the partial pressures of Ar/O₂ affects the high energy tail of ions in the discharge. At high oxygen partial pressures the high energy peak diminishes and all the sputtered alumina atoms are consumed. Deposition with low oxygen results in aluminum thin film growth while alumina was deposited with high concentrations of oxygen. Similarly, changes in pressure influence the characteristics of the Al₂O₃ films deposited. Preliminary studies show that the introduction of oxygen results in the formation of plasma layers which in turn affect the ion energies. Correlations between the ion energy distribution of the discharge particles, plasma layers and the elemental composition in the films deposited are analyzed.