ICMCTF 2023 Session EP-ThP: Tribology and Mechanical Behavior of Coatings and Engineered Surfaces (Symposium E) Poster Session

The continuous development of tribological systems to increase their energy efficiency and sustainability is of increasing social and economic interest. The chain drive as a common machine element offers open potential in this respect. The service life of a chain is determined by its wear-related elongation. The wear of chain pins in the chain joint can be reduced by CrAlN coatings. This component also leads to most frictional losses in a chain drive. When lubricating with greases, friction- and wear-reducing reaction layers are formed in interaction with the steel surface. Interactions of CrAlN coatings with the lubricating greases, however, are limited due to their chemically inert properties. Triboactive CrAlMoN coatings offer a promising solution. In this study CrAlMoN coatings deposited by magnetron sputtering physical vapor deposition (MS-PVD) were tribologically investigated via pin-on-disc tribometer (PoD) and compared to a CrAlN coating and an uncoated reference. All systems were tested with two fully additivated grease references and two basic greases, which exclusively contain a S- or P-additive. To approach the application, selected combinations were then tested in a chain-joint tribometer (CJT), a component test bench for chain drives. The coatings were successfully deposited on chain pins. The incorporation of the triboactive element Mo leads to a lower indentation hardness H_IT of the coatings. The CrALMoN coating with a high Mo content achieved a low total wear volume and lead to a stable and relatively low coefficient of friction (CoF) in contrast to the other tested coatings. This coating leads to low levels of friction and total wear in combination with a basic grease +S equivalent to a steel surface in combination with a fully additivated grease lubricant. The results show the high potential of triboactive coatings for wear and friction reduction in chain drives. Additionally, expensive and environmentally dangerous anti-wear and extreme pressure additives can be avoided using the triboactive CrAlMoN coating.

Barrier coatings are applied to many machine elements that are exposed to aggressive/hardservice conditions to prevent corrosion, oxidation and wear at high temperatures. These coatings are widely used to protect structural components of gas/steam turbines in the energy and aerospace industries against aggressive operating conditions. Looking at the usage areas in general terms, it is seen that thermal barrier coatings (TBC) are of vital importance in terms of environmental and socio-economic aspects. With the increase in technological developments, gas/steam turbines and high temperature components such as aircraft/jet engines are expected to operate under supercritical conditions and with minimum losses. In this study, Nb and Ta doped CrYN film was coated on 316L stainless steel (SS) using CFUBMS (Closed Field Unbalanced MagnetronSputtering) technique. Then, the corrosion resistance, structural and mechanical properties of CrYN:Nb/V solid thin films were investigated. Microhardness device was used for mechanical properties. SEM and XRD techniques were used to study the microstructural characteristics of the coatings on the silicon wafer. Corrosion properties were investigated using potentiostat test unit NaCl solution. The highest hardness was found as 38.60 GPa for CrYN:Nb film and 26.47 GPa for CrYN:Ta. The results show that the coated samples have higher corrosion resistance than the uncoated samples. In addition, it has been observed that the corrosion resistance of Ta doped CrYN thin films is relatively better than Nb doped thin films.

The design of metallic thin film with controlled composition and microstructure has become increasingly important for industry applications, involving strong mechanical solicitations. Specifically, metallic glasses (MGTFs) and high entropy alloys thin films (HEATFs) have shown a great potential due to their unique combination of large mechanical properties such as yield strength (3 GPa) and ductility (10%) [1,2]. However, the relationship microstructure-mechanical properties are not fully understood since the morphological control is often limited by the most employed sputtering deposition. In this field, Pulsed Laser Deposition (PLD) offers the possibility to widely control the morphology of the films by simply changing the process parameters, affecting the growth mechanisms from atom-by-atom to cluster-assembled growth regimes. Recently, PLD has shown a large potential for the deposition ZrCu MGTFs and CoCrCuFeNi HEATFs reporting large and tunable mechanical properties such as an elastic modulus and hardness of 175 and 11 GPa [2].

Here, we explore the possibility to further nanostructuring PLD deposited ZrCu compact and nanogranular MGTFs by performing annealing treatments from 300 up to 550°C, while investigating the devitrification process and the evolution of the mechanical properties.
Structural characterization shows that compact films remain amorphous up to 420°C, while the crystallization process of nanogranular films is completed at 420°C due to the combination of high interface density, free volume and O content [3]. We show that the mechanical properties increase with the annealing temperature due to the progressive crystallization reaching a plateau upon complete crystallization with elastic modulus and hardness up to 180 and 14 GPa, respectively. Furthermore, we show that compact films have residual tensile stress from 169 to 691 MPa whose magnitude increase as a function of the temperature due to nanocrystalline phase nucleation followed by grain growth. On the other hand, nanogranular films show a maximum residual stress of 1.1 GPa at 420°C followed by a decrease at higher annealing temperatures, indicating a complete crystallization.

Overall, we show that PLD in combination with post-thermal annealing can generate different families of metallic films with varying nanoscale morphologies, resulting in tunable mechanical properties and thermal stability with potential as structural coatings.

References:
1. Y. Zou et al., Nat. Commun., 6, 7748, 2015.
2. M. Ghidelli et al., Acta Mater., 213, 116955, 2021.
3. F. Bignoli et al., Mater. Des., 221, 110972, 2022.

Measuring the elastic modulus of novel metallic thin films, such as intermetallics or high entropy alloy (HEA) films, can be challenging. Nanoindentation is the most used method to determine the hardness, but has recently been shown that nanoindentation is not a reliable method to measure elastic modulus of films less than 5 µm thick without having an influence from the substrate [1] (especially in case of stiff thin film on compliant substrate). Therefore, another method should be utilized to accurately measure thin film elastic modulus. One approach that has not been considered is to use in-situ stress measurements while heating the film. From the initial slope of the heating and cooling stress-temperature curves, a substrate-free elastic modulus can be calculated. In order to determine if this technique is viable, known metal films of Mo and Al on silicon substrates will first be evaluated and compared to literature and nanoindentation of the same film systems. Then, more complex, metastable MoAg alloys and refractory HEA films, with applications in power electronics, will be evaluated.

[1] S. Zak, C.O.W. Trost, P. Kreiml, M.J. Cordill, J. Mater. Res. (2022).

Adhesion of metal films to polymers is an increasing area of research. Not only are metal-polymer interfaces found in flexible and foldable electronics, but also in rigid microelectronics. While several methods to quantify the metal-polymer interface adhesion are available, a direct comparison of the available methods on the same interface has not been performed yet. In this work, the adhesion of the WTi-Polyimide (PI) interface was evaluated with methods, using spontaneous buckles and the tensile induced delamination model. One unique aspect of the samples is that all fabrication processes were performed at the wafer level and testing performed on released WTi-PI dogbone shaped tension samples. After being released from the rigid wafer, spontaneous cracking and buckling occurred due to the release of residual stresses in the WTi film. The results show that both models lead to similar adhesion energies. The similarity of adhesion results indicate that both methods are suitable to assess interface strength and that the two methods can be confidently compared.

Coatings bring advantages for functional surfaces on different relevant mechanical performances: higher wear resistance for a longer lifetime or lower friction coefficient for a reduced energy consumption. The physical phenomena involved at the surfaces under stress conditions represent a key part in understanding the performances of some coatings. Scratch testing characterizes the scratch resistance and adhesion of coatings. Tribology testing characterizes the wear resistance and friction of surfaces in relative contacts. The unique combination of mechanical testing with 3D-profilometry (confocal microscopy and white-light interferometry) provides new insights into the visualization of surface damages occurring in scratch testing (cracks, plastic deformation and delamination of coatings) and tribology (modes of wear and deformation). Examples of characterization with 3D imaging on PVD coatings are shown.

Nowadays, nanoindentation has become a routinely technique for the mechanical characterization of thin films and small-scale volumes. Thanks to the development of friendly analysis software and advances in high sensitive instrumentation, it feels like the measurement and calculation of hardness and elastic modulus can be easily done by just “the pushing of one button.” However, the consequences of easy procedures have led many researchers to multiple publications with erroneous data.

Recently, we have reviewed the nanoindentation hardness of materials at macro, micro, and nanoscale (E. Broitman, Tribology Letters, vol. 65, 2017, p. 23). Some misconceptions in the nanoindentation technique were highlighted, and solutions to errors were proposed. In this paper, five typical mistakes in the measurement and data analysis during the nanoindentation of thin films will be critically reviewed, and the possible ways to correct them will be discussed: (1) the wrong area selection to calculate instrumented indentation hardness; (2) the wrong data conversion from Vickers microindentation to Berkovich nanoindentation; (3) the confusion of thermal drift with creep and viscoelastic effects; (4) the wrong correlation of hardness with tip penetration; (5) the preconceptions about a direct relationship between elastic modulus and hardness.

The origins of the aforementioned mistakes will be elucidated from the lack of understanding on contacts mechanics theory, the limits and validation of the Oliver and Pharr’s method, and preconceptions transmitted from generation to generation of nanoindenter users. At the whole, it will be stressed that it is not enough to know “how to push the button” in order to measure the nanoscale mechanical properties of coatings.