AVS2013 Session SE-ThP: Poster Session

In this study, the tribological and thermal properties of TiAlCN coatings were investigated to evaluate their feasibility in automobile applications. TiAlCN coatings with carbon compositions between 25 and 65 at.% were prepared by inductively coupled plasma (ICP) assisted sputtering and were annealed at 400, 500, and 600°C in air. The structures and compositions of the coatings were studied by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and Auger electron spectroscopy. The hardness and tribological performance were evaluated by micro-indentation and ball-on disk tests. The wear rates were calculated using a non-contact 3D surface profiler. At a carbon composition > 50 at.%, the coatings featured a mixture of amorphous and crystalline phases and exhibited superior tribological performance compared to that of commercialized carbon or nitride coatings. Raman spectroscopy revealed that even when the amorphous carbon phase has a relatively high sp² to sp³ ratio, there are sufficient amounts of TiAlN grains to support the hardness in the coatings. The coating hardness decreased slightly after high-temperature annealing due to graphitization in the amorphous carbon phase, however, the friction coefficients and wear rates were reduced with the annealing temperature.

The challenging environment associated with a fusion reactor will require the utilization of advanced materials in order to enable successful development of fusion energy for the future. Tungsten(W)-based materials have been considered for nuclear reactor applications for its outstanding properties such as high melting point, low vapor pressure, high thermal conductivity, and low thermal expansion coefficient. However, pure W exhibits low fracture toughness at all temperatures and a high ductile to brittle transition, which depends on the chemical and microstructure. The present work was focused on the W-Y based alloy coatings grown by sputter-deposition. The sputtering was performed using a W-Y target to fabricate coatings on to MgO(100) substrates. W-Y coatings were made at various growth temperatures (T_s) ranging from room temperature to 500 ^oC. The structural and mechanical properties of the coatings were evaluated as a function of T_s. While the ultimate goal is to investigate the performance of W-Y coatings as a structural material in the next generation nuclear reactors, preliminary results obtained on the crystal structure, composition, stress evolution and mechanical properties of the coatings are presented and discussed.

There are a large number of factors involved in wear processes (e.g. mechanical, physical and chemical properties, surface topography, loading), making the precise theoretical and quantitative approach of wear a challenge even for “simple” tribo-systems. Many of these factors are hard to measure, may vary with time and space, and there is not yet a general theory available of how to link the basic properties with the tribological response. Several well established testing methods (e.g. pin-on-disk, fretting and impact tests) have been widely used to study treated surfaces and coatings on various substrates. However, many of these existing techniques have limitations in their ability to characterize materials, since they mainly focus on a single mode of loading and wear (e.g. only impact or sliding).

In this study, a new Dynamic Impact and Sliding Test (DIST) for the tribo-mechanical evaluation of surfaces under complex loading conditions is presented, where the surfaces are simultaneously subjected to sliding and impact loading. Such modes exist in many critical applications, from biomedical (e.g. hip/knee implants) to automotive applications (e.g. diesel injectors, engine valves, cam shafts), in cutting tools, general machine parts and systems, etc. Instruments and techniques for combined loading situations (such as the proposed DIST) are a feasible way for fast, economical and reliable evaluation of complex tribo-systems with high practical and industrial interest. Expected benefits include the time and cost effective evaluation of various surfaces and the better understanding of their peculiarities under such multi mode loading conditions. Some of the unique characteristics of the DIST (e.g. combined impact and sliding testing; wear area in a single point; pre-setting of desired maximum wear depth possible; evaluation of materials’ properties and behavior in a single run) are presented.

Characterized as exhibiting water droplet contact angles > 150° and sliding angles < 5 °, superhydrophobic films have attracted considerable research attention as a result of their remarkable non-wetting properties and potential applications in self-cleaning, anti-fouling and anti-icing. The combination of hydrophobic chemistry and surface roughness necessary for imparting such non-wetting characteristics presents a challenge towards industrial applicability due to the intrinsic frail nature of highly rough surfaces.[1] Sol-gel synthesis offers a versatile and scalable means for producing superhydrophobic films. However, traditional sol-gel approaches are often reliant on ‘needle-like’ aggregations of nanoparticles to impart surface roughness. This surface structure, whilst ideal for minimizing solid-water interactions is inherently fragile.[2, 3] Upon contact, high aspect-ratio asperities experience excessive pressures usually exceeding the mechanical properties of the material[4], consequently such superhydrophobic films are very easy to abrade and damage. To overcome this challenge a templating method was used to engineer more robust structures. Discrete polymer spheres were embedded within an alkoxysilane sol-gel to form a continuous, robust, thin film. Roughness was then engineered into the film by thermally degrading the polymer spheres within the gel network, leaving behind crater-like structures with durability far exceeding its predecessor’s. The resultant crater-like films exhibited pencil hardness exceeding 4H, eclipsing traditional films’ pencil hardness, typically of the order 8B – HB. This avenue may provide a scalable approach for controlling roughness features in durable superhydrophobic films and allow for large scale application in areas of self-cleaning and anti-fouling.

References

1. Verho, T., et al., Advanced Materials, 2011. (5): p. 673-678.

2. Nakajima, A., K. Hashimoto, and T. Watanabe, Monatshefte für Chemie/Chemical Monthly, 2001. (1): p. 31-41.

3. Nakajima, A., et al., Thin Solid Films, 2000. (1–2): p. 140-143.

4. Bhushan, B. and M. Nosonovsky, Acta materialia, 2003. (14): p. 4331-4345.

SiOC(-H) films were synthesized on polycarbonate substrates from mixture of trimethylsilane (TrMS) and O₂ gases by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method to improve the abrasion resistance of polycarbonate substrates.

In order to improve the adhesion to polycarbonate substrates, we applied the acrylic intermediate layer prepared by ultraviolet curing method between SiOC(-H) films and polycarbonate substrates. The nanosilica particles were mixed with pentaerythritol triacrylate and pentaerythritol tettraacrylate at various concentrations to control the hardness of the intermediate layer.

The scratch resistance of SiOC(-H) films deposited on polycarbonate substrates with the intermediate layer was improved as the pencil hardness of the intermediate layer increased. After the taber abrasion tests of the SiOC(-H) films deposited on polycarbonate substrates with the intermediate layer, the occurrence of delamination was inspected by digital microscope. The delamination was confirmed to be markedly suppressed as the hardness of the intermediate layer increased.

The haze difference (ΔHaze) between before and after the abrasion tests exhibited sufficient abrasion resistance of 3.5% after 1000 revolutions when the 500 nm thick-SiOC(-H) film was deposited on polycarbonate substrates with the intermediate layer contained 25 wt.% nanosilica particles.

By combining organic and inorganic nanomaterials using two different material growth techniques (self assembly and atomic layer deposition), we demonstrate a facile and reliable fabrication method for TiO₂ and ZnO semiconductor nanonetworks. Self-assembled peptide-amphiphile nanofibers are used as three-dimensional organic nano-templates, whereas subsequently atomic layer deposited metal-oxide films formed the conformal inorganic functional nano-coatings. Apart from the traditional organic templates, we used a fully dried, three-dimensional (cm-scale), highly interconnected peptide nanofibrous network template, which enabled atomic layer deposition (ALD) precursors to be homogenously deposited with exceptional conformity. The wall thickness of the inorganic nanotubes can be precisely controlled by simply altering the number of ALD cycles. TiO₂ and ZnO nanonetworks demonstrated superior performance compared to the unstructured TiO₂ and ZnO substrates in photocatalytic activity because of the enhanced specific surface area of the photocatalysts with nanostructured morphology. Importantly, immobilization of the photocatalysts on a solid support enabled recycling of the material, which can dramatically reduce the treatment cost and prevent secondary contamination of the water sources with inorganic materials. Furthermore, we discovered that there is an optimal wall thickness for gaining photocatalytic advantage through nanostructuring for both TiO₂ and ZnO. This optimum nanotube wall thickness was found to be around ~8 nm for both TiO₂ and ZnO. These results demonstrate significant potential of using peptide-based organic templates to fabricate high-quality TiO₂ and ZnO nanostructures not only for photocatalysis, but for several applications where increased surface area plays a crucial role: chemical/gas sensing, dye synthesized solar-cells , etc. Further studies can be extended to other transition-metals and their compounds, such as oxides, nitrides, and sulfides. As a result of the rapid and convenient scaling of the peptide nanofibers into macro-size networks, new opportunities could be available for fabrication of a wider range of inorganic materials.

For microfocusing x-ray mirrors, an elliptical shape is essential for aberration-free optics. However, it is difficult to polish elliptical mirrors to x-ray quality smoothness. Previously we have invented and succeeded a profile coating technique to make elliptical Kirkpatrick-Baez (KB) mirrors using flat Si substrate [1]. We found that it is advantageous to pre-figure the flat surface so that less coating is needed [2]. In this presentation we describe our attempt to pre-figure the flat Si substrates using a profile etching technique with a commercial broad beam ion source.

In profile etching, a contoured graphite mask placed above the ion source and right below a moving substrate determines the desired surface profile along a direction perpendicular to the substrate moving direction after etching. The shape of the contour is calculated point by point on the profiling direction from the ion beam distribution and the desired profile. When the substrate is moving across the mask during ion etching, the etching depth is proportional to the length of the opening of the mask along the moving direction and the weight of the ion beam distribution along the path. By equating the summation of relative weighting to the required etching depth at the same point, the length of the opening at that position can be determined. Repeating this process for the whole length of the profile, a contour can be determined. The substrate can be moved at a constant speed and multiple moving passes are used until the desired etching depth is reached.

Initial design of the profile-etching setup and primary results are presented. Major challenges and obstacles will be discussed.

[1]. “From flat substrate to elliptical KB mirror by profile coating”, C. Liu, R. Conley, L. Assoufid, Z. Cai, J. Qian, and A. T. Macrander, AIP Conf. Proc. 705, 704, 2004.

[2]. “Plastic Deformation in Profile-Coated Elliptical KB Mirrors,” C. Liu, R. Conley, J. Qian, C. M. Kewish, W. Liu, L. Assoufid, A. T. Macrander, G. E. Ice, and J. Z. Tischler, ISRN Optics, vol. 2012, Article ID 151092, doi:10.5402/2012/151092, (2012).

* This work is supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.