ICMCTF2012 Session B4-2: Properties and Characterization of Hard Coatings and Surfaces

Although plasma-assisted processing techniques are now widely established for the production of wear-resistant coatings and treatments on machine tools and engineering components manufactured from traditional (mainly ferrous), metallic substrate materials, there remain considerable scientific and technical challenges in applying such techniques effectively to address the increasing demand for tribological functionality on articles made from non-ferrous and composite materials. Although such materials in general tend to offer (amongst many desirable engineering properties) high specific strength - and excellent static and/or bulk-material load-bearing properties - the opposite is often true of their surface wear and corrosion behaviour when subjected to dynamic contact forces. In principle, techniques such as plasma-assisted physical vapour deposition (PVD) are an ideal choice to apply hard, wear-resistant coatings to - for example - light alloys (which the aerospace automotive and biomedical sectors of industry would ideally choose to use more widely); however, in practice the relatively low yield strength and compliant nature of many such materials is not conducive to optimising the performance of stiff ceramic thin films.

Taking the example of titanium alloy substrates, one efficient approach that can be adopted to improve the performance and durability of PVD ceramic thin films is to diffusion pre-treat the substrate material prior to coating. In this regard, nitrogen and oxygen diffusion treatments are promising candidates – particularly if they can be performed by plasma-assisted techniques which may be i) more rapid, flexible and controllable than other available methods and ii) possible to integrate seamlessly and cost-effectively within the PVD coating process cycle. Despite the fact that such ‘duplex’ plasma diffusion/coating techniques have been commonly investigated (and reported in the literature) over the last 15-20 years, their commercial exploitation remains somewhat lower than might be expected. One widely known issue in the duplex treatment/coating of ferrous materials (where nitriding is frequently the substrate pretreatment of choice) is that of surface nitride compound layer control and/or elimination – although the means to achieve this are now quite well understood.

For titanium alloys however, the situation is considerably more complex and less well resolved; the response of different alloy materials to diffusion treatment by either nitrogen or oxygen is critically dependent on both alloy composition and microstructure (the latter also being, in some cases, very sensitive to treatment temperature). Furthermore, the nature and thickness of the surface compound layers produced during such treatments can also strongly influence the growth characteristics of the underlying diffusion zone – which (on the one hand) needs to be sufficiently deep to provide the necessary load support to a PVD coating subsequently applied, but (on the other) also needs to be produced as rapidly as possible, to make the duplex process cost-effective (whilst at the same time not compromising significantly the substrate bulk properties in terms of mechanical strength and/or fatigue resistance).

Here we present details of a hot-filament enhanced (triode) plasma processing technique, which can be used both to triode-plasma diffusion treat and (by electron-beam evaporative PVD) to duplex treat/coat selected titanium alloys. The diffusion treatments comprise triode plasma nitriding (TPN), triode plasma oxidation (TPO) and a sequential ‘oxy-nitriding’ (TPON) approach, each of which can be optimised to suit different substrate materials and applications. Structural, mechanical and tribological properties provided by such treatments are discussed in the context of two Ti-alloys; the (ubiquitous) α/β alloy Ti-6Al-4V and a metastable β alloy, Ti-15Mo – each of which illuminate particular (and different) treatment design and processing considerations, both for the diffusion treatment efficiency/performance and for their receptiveness to subsequent duplex PVD coating.

We have been deposited CrN, TiAlN and TiN films by reactive magnetron sputtering to evaluate the tribological properties in different environment conditions (vacuum, room and nitrogen) using the pin on disc test. The aim of the study was to investigate the effect of the presence of oxide wear particles on the friction and wear coefficient. The formation of such wear debris is known to be important but the details of the effect on the tribological behavior of coatings are not completely clear. Our results show that under room conditions the CrN films has an initial stable friction coefficient, due to the formation of roll-shaped wear particles that can easily support the load. Meanwhile, the TiN film showed the formation of a higher and unstable friction coefficient due to generation of angular particles that causes surface ploughing because of its greater hardness than the coating; this is what directly produces the higher wear coefficient. The TiAlN coating shows a similar wear mechanism to the TiN but the friction coefficient is more stable because the material has a higher plastic deformation and oxidation resistance allowing for distortion of the wear particles.

Despite of its poor tribological properties (low hardness and low resistance to friction wear and abrasion) and a poor seizure resistance, aluminium has become a potential material in the automotive lightweight engineering. To overcome these weaknesses and increase the engine lifetime, a surface treatment can be one of the best options. In this work, Cu-Ni alloy that has such excellent properties such as; ductility, corrosion and wear resistance, good electrical and thermal conductivity as well as it can be easily joined or fabricated into useful shapes was chosen. The work aims to study the wear behaviour of Cu-Ni coatings deposited on Al7075 substrates using an atmospheric plasma spray (APS) with different plasma powers. Lubricated wear test were carried out on a pin-on-disc tester under an applied load of 100 N with fixed sliding speed of 0.851 ms^-1 at room temperature (23 °C). It was found that a decrease in plasma power from 40 to 30 kW promoted finer microstructures and higher hardness of the coatings, up to 39%. At 30 kW plasma power, the formed splats presented a high degree of flattening and solidification without many splashes. At greater power of 40 kW, the velocity and temperature of the droplets were increased, resulting rough coating structures that most likely caused by overlapping splats. This, however, weakened the bonding between splats. In general, an increase in the wear resistance of Cu-Ni coatings on Al7075 was found from ~ 6 x 10^-5 mm³/Nm to ~18 x 10^-5 mm³/Nm, indicating a mild wear regime, as expected, was attributed to the increase of the coating hardness.

Amorphous B-C and Si-B-C ceramics were deposited by CVD from BCl₃-CH₄-H₂ and BCl₃-CH₃SiCl₃-H₂ mixtures, respectively, at temperatures around 1000°C and reduced pressure. All the as-deposited (Si)-B-C ceramic coatings are nearly amorphous and consist of a common very disordered boron carbide phase (B_xC) and, in the Si-B-C coatings, sub-nanometric SiC crystals. The proportions of the B_xC and the SiC phases vary with the deposition conditions. A SiC grain growth is observed after heat treatment beyond 1200°C, while the amorphous B-C-rich phase is gradually transformed into free aromatic carbon and B₄C nanocrystals, in agreement with the thermodynamic equilibrium. This particular crystallization process is expected to lead to a significant time/temperature-dependent change of the density and the mechanical properties. It has to be considered to better understand and predict the behavior of the ceramic matrix composite parts in future aeronautic engines.

The crystallization behavior in inert atmosphere of the (Si)-B-C ceramics is investigated in situ, by differential scanning calorimetry analyses and X-ray diffraction. Ex-situ analyses are also conducted by heat-treating the specimens under high vacuum at different temperatures/durations, and by characterizing the resulting microstructures at short and long range, by solid MAS-NMR, Raman microspectroscopy, neutron diffraction, X-ray absorption, X-ray diffraction and transmission electron microscopy.

Indentation tests are achieved at room temperature on the pristine and annealed coatings, to evaluate their stiffness, hardness and toughness. High temperature tensile tests are also performed on model 1D composites consisting of (Si)-B-C coatings deposited on soft carbon monofilaments. These micro tensile tests allow the evaluation of the changes of (i) the volume (or density), (ii) the Young’s modulus, (iii) the creep rate and (iv) the thermal expansion of the coatings. We expect the mechanical behavior and the structural ordering of the (Si)-B-C ceramics to be strongly interrelated. The results obtained on the various specimens will be discussed on the basis of their elemental composition, initial structure and processing conditions.

Physically vapor deposited Ti_1-xAl_xN thin films are well acknowledged in various industrial applications due to their beneficial effect on lifetime and performance of e.g. cutting or milling tools. The excellent thermal stability of Ti_1-xAl_xN is determined by the incorporation of Al on Ti lattice sites in a cubic (c) supersaturated metastable solid solution of Ti_1-xAl_xN after deposition. Thermally activated diffusion processes lead however to a series of decomposition phenomena, involving first recovery of deposition induced defects and then the spinodal decomposition process which accounts for a hardness increase with increasing isostructural decomposition state into c-TiN- and c-AlN-enriched domains [1, 2]. The subsequent recrystallisation and development of a dual phase structure composed of cubic TiN and hexagonal AlN initiates deteriorating mechanical properties. Earlier investigations [3] have proven the big potential of the age hardening phenomenon for high temperature applications in cutting applications, however the development of thermal, electrical and optical properties has not yet been investigated as a function of temperature. Hence, the present study focuses on the thermal evolution of heat conductivity, electrical resistivity and optical reflectance from room temperature up to 1400 °C and relates to decomposition induced structural and chemical changes of Ti_1-xAl_xN. This contribution aims for a deeper understanding of Ti_1-xAl_xN thin films under thermal load, which is necessary for the development of e.g. state-of-the-art protective coatings for cutting tools or thermal barrier coatings for electronic devices.

References:

[1] P. H. Mayrhofer, A. Hörling, L. Karlsson, J. Sjölen, T. Larsson, C. Mitterer, L. Hultman, Self-organized nanostructures in the Ti-Al-N system, Appl. Phys. Lett. 83 (2003) 2049-2051

[2] R. Rachbauer, S. Massl, E. Stergar, D. Holec, D. Kiener, J. Keckes, J. Patscheider, M. Stiefel, H. Leitner, P. H. Mayrhofer, Decomposition pathways in age hardening of Ti-Al-N thin films, J. Appl. Phys. 110 (2011) 023515

[3] A. Hörling, L. Hultman, M. Odén, J. Sjölen, L. Karlsson, Mechanical properties and machining performance of Ti_1-xAl_xN-coated cutting tools, Surf. Coat. Technol. 191 (20005) 384-392

This study focuses on the coherent and isostructural decomposition of cubic c-(Ti,Al)N thin films during metal machining, i.e. under the influence of large normal stress and elevated temperature typical for the selected cutting application. It is well known that c-(Ti,Al)N decomposes in two steps at elevated temperatures. In the first step, c-(Ti,Al)N decomposes spinodally into coherent isostructural cubic TiN- and AlN-enriched domains and in the second step a transformation from c-AlN to the stable wurtzite AlN occurs. Associated with the first step is an increase in hardness due to a coherency strain between the domains whereas the second step introduces large incoherent precipitates, thus rapidly lowering the hardness. However, the insight on the decomposition process during the application of variable stresses and heat, as in the case of metal cutting is scarce.

Here we have deposited thin films of Ti_0.6Al_0.4N by cathodic arc evaporation onto tungsten carbide (WC-Co) cutting tool inserts and studied the phase and microstructure evolution after a continuous turning operation for 10 minutes in carbon steel (C45E). For comparison, reference samples have been heat treated with a heating rate of 20 Kmin^-1 followed by an isothermal step of 10 minutes at 900 °C and 1000 °C respectively.

During turning at variable cutting speeds, v_c , between 100 m/min and 400 m/min, the peak temperature at the cutting edge was between 750 °C and 950 °C as measured by an IR-CCD camera. The peak normal stress, evaluated from measured contact forces during cutting, was about 2 GPa and independent of the range of cutting speeds investigated. The deduced stress distribution also shows that the normal stress along the rake face is decreasing with an increasing distance from the cutting edge towards the end of the tool-chip contact. Transmission electron microscopy (TEM), including analytical scanning TEM (STEM) has been used for detailed characterizations of the tool-chip interface along the cutting edge. The progression of the spinodal decomposition is determined by the domain size of the TiN and AlN enriched domains and is measured from STEM micrographs. After heat treatments, the results show an increased domain size from 2.8 to 5.0 nm with increasing annealing temperature from 900 to 1000 °C which is expected. After cutting tests however, an increase in domain size from 3.2 to 5.6 nm is seen with the increasing normal stress along the cutting edge. This implies that even relatively small pressures promote coherent isostructural decomposition, which is in line with theoretical studies but has previously not been shown experimentally.

This work presents a comparative study of the influence of temperature on the mechanical behaviour of bulk single crystal (001) TiN (SC-TiN), magnetron sputtered single crystal TiN and magnetron sputtered Ti_1-xAl_xN films (x=0.34,0.52) deposited on (001) MgO substrates. High temperature nanoindentation (room temperature to 350 ^oC) was used to extract fundamental deformation rate-controlling parameters such as activation volume, activation energy and Peierls stress, demonstrating the usefulness of the indentation as an effective experimental technique for investigating kinetics of plastic deformation in ceramics at moderately elevated temperatures.

This analysis showed that the hardness of bulk single crystal TiN dropped from 21±0.5 GPa at 22°C to 10.7±0.45 GPa at 350°C. Interestingly, the hardness of sputtered single crystal TiN and Ti_1-xAl_xN with low aluminium concentration (x= 0.34) also dropped by a similar magnitude although from a higher starting values. However, the x=0.52 film exhibited a remarkable hardness stability with the temperature, showing a slight drop in values from 30±1.3 GPa at 22°C to 26±2.6 GPa at 300°C. This suggests that increase in Al addition improved not only the room temperature hardness but also lead to an increase in the activation energy for slip from 0.75 eV for SC-TiN to 1.26 eV for x=0.52 film. A heat treatment of this film at 600°C for 24 hours resulted in a slightly lower hardness and a reduction of the activation energy of slip to similar values seen in pure TiN.

The increase of the life span and thus a more efficient use of tools are important goals related to industrial applications. Owing to this purpose, friction as well as wear should be reduced significantly. PVD (Physical Vapour Deposition) coatings such as Ti / TiAlN or Cr / CrAlN meet these requirements and possess a high wear resistance. Due to the high heat resistance of titanium aluminum nitride, these coatings have a great potential for the use in forming tools employed at elevated temperatures . However, the different thermoelastic properties of the multilayer system and substrate can lead to delamination and spallation of the layers after the deposition process due to critical residual stress states. Therefore, the influences of the bias voltage on the residual stresses, as well as the tribological properties, were investigated within the scope of this work in order to understand the operational behavior of multilayer systems. The production parameter should be varied to favorably adjust the residual stress states selectively and to produce a sound coating system. By means of a magnetron sputtering device both multilayer systems were deposited on hot work steel substrates AISI H11. In addition to the metallurgical phase analysis, residual stress measurements were performed using X - ray diffractometry . The structure and the morphology investigations of the coatings were determined by scanning electron microscopy and energy dispersive X-ray spectroscopy. Furthermore, a nanoindenter and a ball-on-disk device were employed in order to characterize the mechanical and tribological properties and to clarify the effect of the process parameter on the service behavior of the PVD coating.