ICMCTF2010 Session B5-1: Properties and Characterization of Hard Coatings and Surfaces

The strength and ductility of thin films are important material parameters owing to their significant influence on lifetime and structural integrity of components. Such intrinsic parameters of the coatings are difficult to determine due to the small dimension, the defects, the brittleness, the residual stress, etc.

The purpose of this presentation is to present a new technique for the evaluation of intrinsic strength and intrinsic mode I fracture toughness of thin films based on the fabrication of microcantilevers by means of a focused ion beam workstation and the subsequent testing of these cantilever beams in an in situ microindenter mounted in a scanning electron microscope. The advantages and the limitations of the method will be presented. The measured strength and fracture toughness of a TiN coating and different types of interfaces are analysed. Besides the determination of strength and toughness, the residual stress distribution can be evaluated and the effect on fracture behaviour is investigated.

In this work, TiAlN/CrN multilayer thin films were deposited on an AISI 403 stainless steel substrate by cathodic arc deposition (CAD). The effect of substrate orientation (vertical or horizontal to the arc) and substrate rotational speed were investigated in detail. Microstructure characterizations of the as-deposited thin films were done by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. Additionally, nanoindentation was conducted on the thin film multilayers. The experimental results indicate that the TiAlN/CrN multilayer films prepared with a vertical substrate orientation and a substrate rotational speed of 4 rpm possessed superior coating hardness and hardness/elastic modulus (H/E) ratio. The microstructure and interfacial structure of the indented TiAlN/CrN multilayer thin films were investigated by transmission electron microscopy and the deformation mechanism will be discussed.

Erosion-resistant coatings are used to protect compressor components of land and airborne gas turbines from destructive effects of hard particles suspended in the inflowing air. To meet demands of the aerospace industry, new high performance erosion-resistant coatings are being developed. In particular, nano-structured Physical Vapour Deposition (PVD) coatings are among the most promising materials for this application. Nano-structured TiN/VN coatings with a range of bilayer periods were deposited on Ti-6Al-4V alloy and 410 stainless steel substrates by Unbalanced Magnetron Sputtering (UMS) and evaluated for microstructural and mechanical properties. A nanomechanical indentation technique was used to determine hardness H, Young’s modulus E and H³/E² ratio. The H³/E² ratio was used to evaluate potential erosion-resistance properties of the TiN/VN coatings. It was found that the obtained values of hardness and H³/E² ratio varied widely with deposition conditions and appeared to be dependent on the substrate material. Further investigations were carried out using an XRD technique to evaluate bilayer period (from satellite peak position) and preferred orientation of the coatings. The XRD measurements were complemented by texture evaluation through pole figures. It was concluded that the preferred orientations of the coatings on different substrates evolved differently with rotation speeds, which might be caused by the difference in magnetic properties of the substrate materials. Contrary to 410 stainless steel which is martensitic, titanium alloy Ti-6Al-4V is non-magnetic, and thus the ion bombardment during the UMS deposition process was different in both cases. The bi-layer period of the TiN/VN coatings varied between 2.47 and 11.4 nm and was practically independent of the substrate materials. Thus, the hardness variations of the coatings on different substrates could not be attributed to the bilayer period. In the next step, the effect of preferred orientation on hardness enhancement was analysed and positively linked with observed hardness changes in tested coatings. It was found that for the 410 stainless steel substrate, coatings with the (111) preferred orientation had the lowest hardness in the series as opposed to coatings with a (200) or (220) orientation that showed higher hardness. For the Ti-6Al-4V substrate, the hardest coatings were those with a strong (220) orientation. However, these observations did not explain all variations in the coatings’ hardness, and the differences were attributed to residual stress in the coating/substrate systems. Further investigations will be carried out in this direction.

Titanium nitride coatings (TiN) are used amongst other applications as wear-protective coatings or as diffusion barriers in IC technology. In these applications the film stress is a key factor determining the performance of the coating.

Two series of TiN films with thickness up to 100 nm have been deposited on Si wafers by reactive DC magnetron sputtering from a Ti target in an AJA sputtering system at a temperature of 360 °C and a RF substrate bias voltage of -150 V. The deposition rate was approximately 100 nm/h. The nitrogen flow was kept constant; the argon flow was varied to obtain deposition pressures of 0.4 and 0.6 Pa, respectively. With a two-laser beam set-up we measured the change in wafer curvature and hence the film force during film growth. From the film force the average stress and the instantaneous stress are derived.

We found a distinctly different film force development for the films grown at different deposition pressures. At equal total thickness the 0.6 Pa film shows a small average film stress of -0.6 GPa, the 0.4 Pa film a more compressive film stress of -5.6 GPa. The 0.6 Pa film revealed a linear increase of film force for the first 30 nm; then the film force remains constant, indicating no stress in the higher up parts of the film. In contrast, the film force of the 0.4 Pa film showed linear increase over the whole film thickness. X-ray diffraction on both series revealed a different texture evolution as the films grow thicker: the 0.6 Pa film has a 111 texture, the 0.4 Pa film a 001 texture.

At 0.6 Pa the bombardment of the growing film with energetic particles will be less intense than at 0.4 Pa. An intense film bombardment will lead to 001 texture [Greene et al. APL 67 (1995) 2928]. By this mechanism we explain the difference in texture. The difference in stress we explain by a difference in the susceptibility to compressive stress generation by energetic particle bombardment. Crystals with their 001 direction parallel to the growth direction are more open than crystals with their 111 direction parallel to the growth direction.

With our in-situ measurements we are able to correlate film stress and film texture development of TiN films. We show that for TiN the film texture is an important parameter for controlling film stress, energetic particle bombardment generating more compressive stress in crystals with their 001 direction parallel to growth direction than crystals with their 111 direction parallel to growth direction.

The thickness dependent residual stress state in thin films is analyzed in magnetron sputtered Cr and CrN zone-T structured layers with thicknesses ranging from 100 nm to 3 µm. The relationship between layer thickness on silicon (100) substrates and both intrinsic and thermal stresses was investigated in detail by means of X-ray diffraction and substrate curvature methods and correlated with the film microstructure. Due to competitive growth, the grain size was found to increase with increasing layer thickness. The development of both stress components strongly depends on the grain size. The ion-irradiation induced compressive stresses were observed to increase with decreasing grain size, whereas the thermal stresses were tensile and highest for small grain sizes. The thus conclusive relation between the thermal expansion coefficient and grain size results from a more pronounced volume expansion of fine grained structures when compared with coarser ones. This is related to the increasing volume fraction of disordered grain boundaries with decreasing layer thickness, representing the interface between mutually misorientated crystallites where the thermal stress is redistributed. This phenomenon is described by a model that explains the size effect of the thermal expansion and the origin of internal stresses in thin polycrystalline films. The model is further extended for a CrN/Cr dual-layer system where a 100 nm to 2.9 µm thick CrN toplayer is epitaxially aligned with the underlying highly (200)-oriented Cr interlayer.

In this research work different designs of Ti- and Cr-based multilayers were deposited onto substrates pretreated by polishing and different etching. The influence of multilayer design and substrate pretreatment on residual stress of these multilayers and the subsurface substrate were analyzed by means of synchrotron X-ray diffraction. Furthermore multilayers adhesion and hardness were systematically analyzed and correlated with residual stresses and designs. Tribological tests were carried out to study the effect of residual stresses on wear and friction of the multilayers.

Epitaxial Al-Cr-N thin films are deposited on c-cut sapphire substrates using reactive magnetron sputtering in Ar and N₂ gas mixture at 500°C. The samples are annealed in the temperature range of 900-1100°C for 2 hours in order to induce thermal decomposition of cubic Al-Cr-N. X-ray diffraction and transmission electron microscopy are used to characterize the influence of the heat treatment on phase composition, epitaxial orientation relationships and mechanical state of individual phases with respect to the substrate. The pole figure analysis shows that cubic (111) oriented Al-Cr-N turns into hexagonal (0002) AlN and cubic (111) CrN whereby the in-plane orientation can be correlated with the substrate. The cubic phases show always two variants. The residual stresses in the phases are quantified using sin²ψ method.

New material analysis strategies using radio frequency (RF) plasma source atomic spectroscopy and mass spectrometry have been developed to address the increasingly complex characterization needs from the semiconductor, solar, disk drive, and metallurgical industries. The specific techniques that were studied include glow discharge optical emission spectroscopy (RF GD-OES) and laser ablation ICP mass spectrometry (LA ICP-MS). The findings reveal that these techniques provide distinct applications and data where traditional analyses incorporating electron beam, ion beam, and x-ray techniques are not sufficient. By using RF plasma or laser ablation for material sputtering, excitation or ionization, many intrinsic limitations associated with traditional techniques are avoided.

RF GD-OES has been used to examine surface stoichiometry of metallurgical coatings and assist advanced surface modification processes designed for chemical corrosion and mechanical wear resistance. Its excellent depth resolution and simultaneous multi-element profiling capability also enable the examination of both physical and chemical deposition processes by studying the vertical elemental distributions in coatings from the surface to the interface region. LA ICP-MS has been used for microscopic defect identification and determination of trace impurities in sputtering targets, hard coatings and multi-layer thin films. These experiments and data collection is unaffected by any surface equilibrium issues or changes in ion yield at the film interface regions as with other techniques. Examples of the materials analyzed to date include anodized alumina, ceramic, quartz, polymer, diamond and diamond like carbon (DLC), various metal and alloy films, silicon carbide (SiC), silicon nitride (Si₃N₄) and a wide range of thin film materials used in the semiconductor and photovoltaic manufacturing process. The signal intensities produced by these techniques all have simple and well-defined mathematical (linear) relationships with elemental concentrations in metallurgical materials. Their wide linear dynamic ranges coupled with various NIST traceable material standards developed in our laboratory have made accurate and precise analysis possible, especially for those challenging and difficult advanced metallurgical materials and multifunctional thin films emerging from high-tech manufacturing processes.