ICMCTF2016 Session E2-1: Mechanical Properties and Adhesion

Zirconium alloys (Zircaloys) are chosen for use in the cores of water-cooled reactors principally due to their low neutron capture cross-section, good mechanical properties and corrosion resistance under standard operating conditions.

However, Zircaloys can undergo severe oxidation under certain adverse high temperature conditions, leading to an increased risk of cladding embrittlement and ultimately to fuel rod failure. The 2011 Fukushima nuclear disaster in Japan serves as a pertinent example whereby a loss of coolant water inside the reactor core led to chemical reactions on fuel rod surfaces in the presence of high temperature steam, resulting in an explosion of the reactor building. These catastrophic consequences have led to a renewed surge in activity to develop accident tolerant coatings for fuel cladding in nuclear reactors; the objective being under extreme or unexpected conditions to improve corrosion resistance and to prevent hydrogen ingress from catalysing the chemical reactions.

Plasma nitriding is a thermo-chemical diffusion technique used widely to improve the wear and corrosion resistance of engineering tooling and load-bearing machine parts, extending their service life. The aim of this work was to test the hypothesis that this widely used treatment technique could improve the corrosion resistance of Zircaloys via the zirconium nitride compound layer formed on the nitride surface. The diffusion treated layers were produced by Triode Plasma Nitriding (TPN) in an evacuated chamber at 700^oC under a 0.5 Pa nitrogen/argon gas mixture, for 2, 3 and 4 hours to investigate whether nitriding duration has any significant effect on the mechanical properties of the layer.

Critical work investigating the mechanical properties of the TPN layer has been carried out. Glancing-Angle XRD and Raman spectroscopy were employed to confirm the formation of a stoichiometric ZrN compound layer. The original grain size of the Zr substrate could be clearly observed to be ~12 microns, with much smaller nano-sized grains on the surface identified as a nitride compound layer from EDX analysis. GAXRD patterns confirmed the formation of cubic ZrN, with peak broadening attributed to preferential ordering of grains in the (111) crystallographic plane. SRIM simulations suggest a treatment depth of less than 1 micron.

Nanoindentation results revealed that the hardness of the TPN layer is ~17 GPa, an increase of 3.5 times compared to pure Zr, and higher than reported elsewhere. Scratch testing results show no adhesive failure detected up to a normal force of 100 N, indicating the difficulty in detecting failure modes for diffusion treated layers of less than 1 micron.

Hard coatings are extensively used to protect tools in forming, forging and milling operations. While modern deposition of hard coatings using plasma methodologies has opened up industry to a new range of potential coating technologies, challenges still exist in deeming which coating will perform better than others under specific conditions and environments.

In the presented work we have extended a previously developed contact mechanical failure map based on analytical solutions that describe stresses and strain in a coating substrate system under contact loading, and applied it for the first time to AlCrN-based ceramic coatings which have strong potential application in cutting tools, hot-forming and hot-forging technologies. Finite element simulations for spherical indentation of the coating/substrate systems were also performed in order to anticipate the mechanical response of the system, and check the validity of the proposed analytical models. We introduced a thin-plate analytical solution to predict plastic substrate yielding, which gave improved alignment with finite element solutions over the previously utilized Boussinesq solution.

The failure map was then investigated experimentally using scratch-testing, and it was found that cohesive coating failure initiated at forces predicted by the model for the ceramic coating, taking into account both the mechanical properties of the coating and substrate. We then applied the model to the case of functional hard chrome and found that cohesive failure initiated at a loading also predicted by the analytical model. We also here present recent results on the high-temperature yielding behavior of nitrided tool steel, and analyze how this effects the deformation and failure of hard-coating systems for high-temperature applications.

The results show that such analytical modelling is a quick and versatile tool for industrially-relevant hard-coating systems, and could be utilized during coating design process for parameter and material justification or combined with micro-mechanical testing including high-temperature testing to predict thin-film and coating failure in harsh industrial environments.

We present a new methodology to measure simultaneously the friction, wear, roughness evolution, hardness, elastic modulus, and plastic deformation of a surface during a tribological experiment. Based in a previous work [1], we have re-designed an experiment where a conical diamond probe of 5 µm end-diameter is permanently scanning a 10 µm track in a reciprocal movement. Different loads are applied during the running experiment in order to obtain the topographic information, while force lateral sensors register simultaneously the friction force variations. The experimental input data are information vectors that contain: load (µN), friction force (µN), vertical Z displacement (nm), lateral X displacement (nm) and time (s). The data is processed using a simple program running in MathLab®. The software output gives the resulting friction coefficient, track roughness, wear rate, thermal drift, hardness, relative elastic modulus, and plastic deformation as a function of the running cycles of the probe.

All tribological experiments have been done using a Triboindenter TI-950 from Hysitron. Examples of hard and soft coatings measured under different loads (from 3 µN to 12 mN), and different environments (dry or lubricated) are discussed to illustrate the tribomechanical and tribochemical behavior of these materials at the micro- and nano-scale. In particular, we show how the new method is able to measure the contribution to the apparent wear from the plastic deformation or material densification during tribotests for both, hard carbon and soft lead films. The new method builds a novel bridge to correlate mechanical and tribological mechanisms at different scales.

[1] E. Broitman, F.J. Flores-Ruiz, “Novel method for in-situ and simultaneous nanofriction and nanowear characterization of materials,” J. Vac. Sci. Technol. A 33 (4), (2015) 043201.

The Toolox-44 and DIN1.2769 alloys steel are widely used as die material in the cold stamping of Dual Phase alloy sheet. However, the cold forming stamping are characterized by a high plastic deformation of steel to be formed, resulting in higher friction and extreme wear conditions for the dies. Surface modification can be used to control the level of friction force, reduce the friction wear and extend the service life of dies. It is known that a pre-treatment method, which may improve the wear of PVD coated is plasma-nitriding of the substrate. In this paper, three different commercial hard coatings (Balinit® X.Cell; Balinit® Lumena; Balinit® Futura) obtained by Physical Vapor Deposition (PVD) were deposited on under nitrided substrates (Toolox-44 and DIN1.2769). The untreated substrate were coated and used as benchmarks. The main objective of this work is to evaluate and compare the wear performance of three different coatings and the effect of duplex treatment on mechanical and tribological properties of dies coated used in this work.

The Nb₂O₅ has many polymorphs and each one has interesting properties, which has been explored for biomedical, photocatalytic and optical applications, but it is not generally used for mechanical applications due to the low hardness (8 GPa). The aim of this work is to add Silicon (Si) into the Nb₂O₅ structure to increase the hardness through the formation of a nanostructured composite coating, similar to the nanocomposite coatings based on metal nitrides and amorphous silicon. In this work, we report the structure, mechanical properties and the friction coefficient in 3 different temperatures (298, 573 and 873K). The coatings were deposited on silicon and D2 steel substrates using a confocal-dual magnetron sputtering system. In order to vary the Si content, the power applied to the Si target (99.999%) was changed from 12 to 200W (radio frequency); meanwhile the power applied to the Nb target (99.95%) remained fixed at 400W (direct current). A reactive atmosphere of Argon/Nitrogen (gas flow ratio 24/6) was used and the deposition pressure was 6 x 10^-2 Pa. The coatings were deposited at room temperature and crystallization was obtained after annealing the coatings at 773K in air. The mechanical properties were obtained by nanoindentation and the friction coefficient using a pin-on-disk tribometer. The structure of the coating was characterized by X-ray diffraction; the results showed the T-Nb₂O₅ phase in all coatings, without variations as the Si content was increased. The Si content was measured by energy dispersion X-ray spectroscopy, but only for deposition powers above 75W, varying from 2.6 to 5.5 at%. Interestingly, the hardness showed a maximum of 18 GPa for the samples deposited at 50 W in the Si target.

Acknowledgement: UNAM-PAPIIT, PHOCSCLEEN-318977 and RMR thanks CONACYT for the PhD scholarship.

The effect of irradiation damage on the mechanical properties of structural nuclear materials is of key concern in both future fission and fusion nuclear reactors. In these the temperatures and levels of irradiation seen will be considerably high than those in current fission reactors and as such the effect of radiation damage much be quantified. Due to the difficulties in working with neutron irradiated samples ion irradiations are a powerful tool for simulating this damage without the added expense of working with active materials. However how successfully they perform this task is unclear.

In this talk I will summarize our key findings on relating the effects of radiation damage on new materials for nuclear reactors, in particular focusing on refractory materials and nano-structured alloys.

Tungsten is the key material for plasma facing components, but there is a lack of fundamental data regarding its behavior under neutron irradiation. This work will start out with an overview of tungsten in fusion and move on to discuss ion irradiated samples of tungsten and tungsten-rhenium-osmium alloys studied using combinations of high temperature micro-mechanical testing, atom probe tomography and transmission electron microscopy. This will include both micro-cantilever and nano-indentation experiments from -50^oC to 950^oC on self ion and helium ion irradiated samples, with changes in the hardening responses and fracture behavior related to the observed micro-structural evolution under irradiation.

The advantages and disadvantages of ion irradiation will be discussed and future research directions for developing radiation resistant refractory alloys including initial hardening responses of low activation high entropy alloys will be discussed.

Flexible electronic devices play an increasing significant role in new technical applications which include paper-like electronic displays, electronic sensitive skins and solar cells. As an elementary substructure among them, functional metallic thin films are often supported by a soft polymer substrate. The lifetime of such systems is strongly dependent on their mechanical performance since they are submitted to complex thermo-mechanical loadings in use which may affect the reliability and performance of advanced devices. These multiple strain cycles arise great interest to investigate Bauschinger effect evidenced in bulk polycrystalline where plastic deformation in one direction can affect subsequent plastic response in reverse direction. One consequence is the decrease of the yield strength of a metal when the direction of strain is changed.

In this communication, a new experimental method using uniaxial tensile testing is presented to study the Bauschinger effect in thin metal films deposited on compliant substrates. Due to the compliance of substrate and tiny thickness comparing to lateral dimension of film-substrate composite, low compressive stress could cause buckling. Thanks to our new pre-stretch setup based on previous work [1], the thin films could be deformed alternately in tension and compression within a large strain domain. The elastic intra-granular strain of polycrystalline thin films and true strain of substrates are measured in situ by X-Ray Diffraction (XRD) and Digital Image Correlation (DIC) respectively. A complete strain transfer through the interface is observed in the elastic regime when the interface is made strong enough [2]. From lattice strain-true strain curves, the mechanical response of thin film/substrate set is analyzed in view of the complete loading history and the presence of residual stresses and crystallographic texture in thin films.

[1] Renault P.-O., Faurie D., Le Bourhis E., Geandier G., Drouet M., Thiaudiere D., Goudeau P., “Deposition of ultra-thin gold film on in situ loaded polymeric substrate for compression tests”, Materials Letters 73, 99-102 (2012).

[2] Geandier G., Renault P.-O., Le Bourhis E., Goudeau Ph., Faurie D., Le Bourlot C., Djemia Ph., Castelnau O., Cherif S. M., Elastic-strain distribution in metallic film-polymer substrate composites, Applied Physics Letters 96, 041905 (2010).

Nanoscale metallic multilayers on rigid substrates are of interested for their enhanced strength, radiation damage resistance, and wear properties. However, measuring the mechanical behavior of films without substrate influence can be difficult. One way to evaluate the fracture behavior of multilayer thin films is to deposit the film system onto a polymer substrate and perform tensile straining, or fragmentation testing, of the film-substrate system. When fragmentation testing is performed in-situ, with electrical resistance measurements, with synchrotron radiation, or under a confocal laser scanning microscope, the multilayer mechanical properties can be investigated on various levels. Using confocal laser scanning microscopy and four-point-probe in-situ fragmentation methods, the initial fracture strain, crack evolution, and even the adhesion can be examined as a function of individual layer thickness and layer order. Further insight is gained with in-situ X-ray diffraction studies using synchrotron radiation to measure the residual as well as evolving lattice strains and stresses of the individual metals in the multilayer. The combined analysis of the three in-situ methods provide valuable information on the influence of layer thickness and order of multilayer films. It will be demonstrated for a Cu-Nb multilayer system on polyimide that while the layer order only influences the adhesion energy, the layer thickness greatly affects the initial fracture strain, saturation stress levels, and the type of buckle delamination that occurs.

Combinatorial materials design of thin films allows for the investigation of fundamental mechanic relationships and optimization of thin films for engineering applications. Using a newly integrated shutter controller that has been specifically designed and manufactured to allow for precise control over coating design, ternary alloys with the full compositional range can be deposited on a single wafer. By specifically programming the shutters it is possible to create multilayered thickness gradients of two or three different materials, which can then be annealed to create films with a large compositional gradient. The structural and mechanical properties of an FCC AuAlCu alloy and BCC NbWTa solid-solution alloy were investigated as a function of the changing composition. Both alloy films were deposited on (0001) sapphire wafers to a thickness of 50 nm. The composition of these films was verified using wavelength-dispersive X-ray spectroscopy (WDX), while the phases, grain size, and lattice parameter were determined using x-ray diffraction (XRD). The AuAlCu alloy consisted of multiple phases and intermetallics across the wafer which is dependent on composition; whereas the NbWTa alloy consisted of a BCC solid-solution with a range of lattice parameters also dependent on composition. Both alloys were then coated with a 500 nm thick layer of Al₂O_3, deposited using ALD, to survey the effect of composition on the interfacial adhesion. Instrumented indentation with a conical diamond tip was used to locally measure the adhesion of the coating. Indentation is carried out with a load sufficient to cause delamination at the film/PVD layer interface such that a blister of delaminated coating forms around the indent. The radius of these blisters, along with the penetration depth of the indenter tip, is then used to calculate a value for the extrinsic adhesion of the interface according to the method of Evans and Hutchinson [1]. By performing small arrays of indents over the surface of the coating, each location corresponding to a different PVD layer composition, it has been possible to test the adhesion-promoting properties of a broad spectrum of interface compositions in a single sample.

[1] Evans, A. G., Hutchinson, J.W., Int. J. Solids Structures, 20, (1984), p455-466.