ICMCTF2014 Session B4-1: Properties and Characterization of Hard Coatings and Surfaces

Eventough approximately 80% of all cutting tools applied in industrial shop floors are coated, the tools’ cutting performance very often is far behind performance potentials promoted by many researchers. A reason for this is that in demanding applications (e.g. turbomachinery manufacturing) the coating properties have to be exactly customized to cope with the individual machining conditions. If this is not the case, a reliable improvement in the tool performance cannot be guaranteed. For instance, PVD coatings’ performance varies depending on the temperature in the cutting zone. In turn, the temperatures generated during the machining process are mainly influenced by the applied machining conditions, in special the cutting velocity, as well as the definition of the tool and coating materials. In case of the cutting process milling the challenges even itensify owing to the repetitive and discontinuous tool engagement. Therefore, the objective must be to ensure the reliability of (newly) developed coatings in a production environment, without resorting to the extensive and expensive practical testing that is currently used for evaluating proper tool specifications and machining conditions.

Against this background, in this paper a systematical methodology based on analytical, experimental and numerical investigations for predicting coated tools' efficiency in milling is introduced. In the frame of the investigations different types of coatings are examined regarding their adherent cutting wedge thermal and mechanical loadings during the material removal process. The Ti-based PVD coatings are deposited using the standardized DC and the innovative HIPIMS process. After a metallographic analysis of the coatings, nano-indentation and impact tests at ambient and elevated temperatures are performed. On this base the strength properties as well as the brittleness and fatigue behaviour of the coatings can be modeled. Moreover, inclined impact tests for evaluating the films’ adhesion are applied. These results, along with FEM-based calculations of the cutting wedge thermo-mechanical loads facilitate the prediction and explanation of the coated inserts’ cutting performance at various conditions. The findings are validated by dint of endurance testing.

CVD hard coatings deposited on cemented carbide substrates typically exhibit tensile residual stresses, which are mainly due to differences in the thermal expansion coefficients of coating and substrate material. However, CVD TiB₂ coatings have been reported to show high compressive residual stresses, although their thermal expansion coefficient is larger than that of the cemented carbide substrate. Within this work, TiB₂ coatings were deposited on TiN base-layers using thermally activated CVD. The coating composition was examined using wave-length dispersive X-ray spectroscopy and electron energy-loss spectroscopy, yielding compositions close to stoichiometry. Slices of the coating were prepared and synchrotron X-ray nanodiffraction experiments were performed in transmission geometry to illuminate the origin of the compressive stresses. Thus, the gradients of stresses and texture could be determined as a function of the coating thickness with a resolution of 100 – 200 nm. Coating microstructure as well as mechanical and tribological properties were investigated by scanning and transmission electron microscopy, the sin²ψ method, nanoindentation and ball-on-disc tests. Pole figure measurements revealed no pronounced texture for the TiN base-layer and a (111) texture for the TiB₂ top-layer, evidencing that there is no epitaxial relation between both layers. Hardness and compressive residual stresses of ~45 GPa and ~2.4 GPa, respectively, could be determined for the morphologically almost featureless TiB₂ coatings. The combination of high hardness and compressive residual stresses results in excellent wear properties.

Cr-based nanocomposite coatings are attracting increasing attention as wear protection coatings in dry machining and on aero-engine compressor blades where the components are consistently subjected to high loading, high impact or heavy erosive environments. Although generally attributed to the high coating hardness, this can also be influenced by the fracture toughness, particularly in extreme applications. In this paper, we study the fracture toughness and cracking behaviour of the CrAlN/Si₃N₄ nanocomposite coatings and demonstrate how these properties depend on the coating composition. The nanocomposite coatings were deposited with different silicon contents using a lateral rotating cathode arc technique. Their composition, microstructure, chemical bonding states and mechanical properties were characterized using EDS, XRD, XPS and nanoindentation respectively and compared with CrN and CrAlN coatings deposited under the same conditions. The fracture toughness of the coatings was characterized by an in-situ double cantilever beam (DCB) compression method. It is found that the CrAlN/Si₃N₄ nanocomposite coatings are significantly tougher than the CrN and CrAlN coating, and the fracture toughness values increase with the coating’s Si content. The in-situ observations also demonstrated that the cracking of the coatings was influenced by the microstructural variability of the coatings.

Moderate temperature (MT)-TiCN is one of the most important components in modern coating systems for cutting tools. Hardness, adhesion and wear resistance can be affected strongly by the controlled adjustment of structure and chemical composition. The aim of this study is to investigate the effect of MT-TiCN coated carbide tools, as a function of carbon content, on mechanical properties and cutting performance. Deposition of high carbon contented MT-TiCN was conducted by doping a hydrocarbon gas for the CVD process with varied hydrocarbon gas to CH₃CN mole fraction from 0 to 30 at 840°C and 9kPa. The carbon content x in TiC_xN_1-x measured from change in lattice constant, increased with hydrocarbon gas to CH₃CN mole fraction and saturated at x=0.73. Crystalline size derived from XRD showed a minimum size of 39nm when carbon content x varied from 0.66 to 0.69. Hardness and adhesion was evaluated by indentation and scratch tests. Indentation hardness increased according to the increase of carbon content and maximum hardness reached 2920mgf/µm² when carbon content x =0.69. The adhesion of film to carbide substrate showed a decrease of critical load when carbon fraction was increased. Furthermore, a cutting test for ductile cast iron (AISI:100-70-03) was performed with different kinds of carbon content x (x =0.55 to 0.73) in MT-TiC_xN_1-x(10µm)/Al₂O₃(5µm) coated carbide tools. TiC_0.69N_0.31/Al₂O₃ coated carbide tools showed significant increase in tool-life by 30-50% compared to commercially available MT-TiC_0.55N_0.45/Al₂O₃ coated carbide tools.

Recent investigations showed that even small amounts of impurities, such as oxygen, affects the growth process, the crystal structure, and the grain size of ceramic-like hard coatings. Ti_1-xAl_xN is due to the excellent mechanical and thermal properties a well-established hard coating system, used for various industrial applications. We have used Ti_1-xAl_xN as a model system to study the influence of oxygen impurities on growth and film properties. By using Ti_0.50Al_0.50 targets with different oxygen contents (< 950, 950-2000, > 2000 ppm) and base pressures of either 10^-5 mbar or 10^-8 mbar, we are able to identify the major oxygen impurity source for reactively sputtered Ti_1-xAl_xN coatings. Furthermore, the coatings were prepared at 500 and 800 °C and when using the target with the highest O-content we also varied the sputtering power for 2 selected deposition conditions. Consequently, 14 different Ti_1-xAl_xN coatings with a similar Al content of x~0.6 have been prepared.

The crystallite sizes of the coatings, which vary between 20 and 80 nm, are strongly determined by the O content as well as deposition temperature, as proven by x-ray diffraction, transmission electron microscopy, and atom probe tomography. Coatings prepared from the highest purity target (O < 950 ppm) exhibit hardness values between 29 and 34 GPa, depending on the substrate temperature or base pressure used. A much higher variation in hardness between 18 and 35 GPa is obtained when using the target with the highest O-content of > 2000 ppm.

But not only the as-deposited structure and properties of Ti_1-xAl_xN are strongly influenced by the oxygen impurities, also their age-hardening behaviour and hence thermal stability strongly depends on the purity of the target as well as the base pressure and deposition temperature used.

The objective of this study was to evaluate the fracture toughness and to investigate the effect of texture on the fracture toughness of ZrN hard coatings. We employed a recently developed energy-based method named internal energy induced cracking (IEIC) method to evaluate the fracture toughness. ZrN film was continuously deposited until reaching the thickness at which the film was fractured due to stored energy accumulation. The residual stress before crack initiation was used to calculate the stored energy (Gs), from which fracture toughness (Gc) can be derived. The residual stress of the ZrN coating was measured by laser curvature method, the Young’s modulus of ZrN coatings was determined by nanoindentation, and the film thickness was determined from SEM cross-sectional image. The results showed that for a ZrN coating with (111) texture coefficient of 0.71, the fracture toughness was estimated to be ranged from 23.9 to 42.4 J/m². The hardness of the ZrN coatings was not changed with increasing film thickness. Stress gradient may play an important role in the fracture mode. Fracture of the ZrN coating may be initiated at a position of local maximum stress gradient accompanying with defects. The stress gradient in the ZrN coating may be originated from the competitive stress generation mechanisms. As the film thickness was above 4.2 μm, the (200) orientation increased in the strongly (111) textured ZrN coatings, which may be related to the stress relief . When the stored energy in the ZrN coating was higher than 31.6 J/m², it was partly released accompanying with the stress relief. The release of stored energy was considered to be the driving force of texture inversion. The advantages of this method are without externally applying stress and special sample preparation, and the substrate effect can be avoided. However, there are some disadvantages in applying the IEIC method. To insure that the fracture process is within the film, good adhesion and high residual stress are required. In addition, this method can only applied to hard coatings due to the requirement of small plastic zone size.

Rubber seals are commonly used in lubrication systems and bearings in order to avoid contamination of the systems and leakage of lubricants. However in dynamic contact where sliding contact conditions are present, rubber seals are easily damaged due to the high friction coefficient developed between the rubber and the seal.

The surface protection of rubber seals by applying a low friction coefficient coating is being studied in the last years; however this application requires specific materials properties either in terms of thermo-mechanical or tribological behaviours that are not yet developed.

Due to their layered structure and weak inter-layer bonding, transition metal dichalcogenides (TMD) has been studied in last decades in the field of low and super low friction coatings. Alloying TMD sputtered coatings has been the most attractive solution for improving and optimizing their friction behavior in a large range of loading environments, in order to overcome the two main drawbacks which have impeded their widespread application for reducing friction: the low loading bearing capacity and the high sensibility to moisture.

The aim of this work was to develop TMD+C coatings by PVD, in order to lowering the friction in mechanical contacts and studying the thermo-mechanical and tribological behavior of the system when applied on rubbers.

For these purposes, W-S thin films were alloyed with different C contents using magnetron sputtering and deposited on nitrile butadiene rubber (NBR) substrates.