Spectroscopic analysis of structural and chemical properties of solid thin film lubricants

Upon tribological loading, especially at elevated temperatures or in humidity, chemical transitions and structural processes are known to take place in solid thin film lubricants like amorphous carbons (a-C) and molybdenum disulphide (MoS2) and to critically limit their performance. To accommodate those restrictions, a selection of element-modifications is applied on a regular basis. General aim is to analyse structural and chemical properties in a-C and MoS2 thin films. The understanding of those is a prerequisite for the scientific conception around tribo-film formation, which is identified as critical part of the tribological system. For this, Raman microscopy is augmented by a setup for optical temperature tuning via a pulsed laser. This work summarises the Raman-accessible film properties and the changes after element-modification and thermal impact. In a-C studies of optical temperature tuning, two ordering mechanisms are found, which are affected by element-modification. As initial reaction upon heating, graphitic clusters within an amorphous matrix increase in number and, subsequently, in size upon further heating. A Five Stage-model of structural relaxation in a-C is proposed. For MoS2, it is found that the formation of tribo-films is strongly dependent on the working environment, the tribo-film gains temperature resistance over untouched thin film material, and tribo-film formation is affected by element-modification. In studies of optical temperature tuning, an initial ordering process of amorphous MoS2 and subsequent chemical reaction to distinct oxides were found. The results are useful for identification of defect mechanisms and possibly for gauging the status of wear in a-C and MoS2 thin films. Element-modification in a-C with previously unknown effects may now be interpreted within a found Five Stage-model; similarly, the behaviour of MoS2 spectra upon heating can now be evaluated

Structure and tribo-mechanical properties of MoSx:N:Mo thin films synthesized by reactive dcMS/HiPIMS

Modifying MoS2 thin films by additional elements shows great potential in order to adjust the property profile and to meet the increasing requirements regarding high wear resistance and low friction properties of industrial components. Within that context, MoSx:N:Mo thin films were deposited by a reactive hybrid dcMS/HiPIMS process. By systematically increasing the Mo target cathode power, an investigation of the structural and the mechanical properties was conducted to understand the evolution of the tribological behavior. A low Mo target cathode power of 1 kW is related to the formation of the preferential (002) MoS2 basal-plane and thus a low friction with µ = 0.2. With an increasing amount of Mo, the film loses its solid lubricant MoS2 properties and a nitride constitution of the thin film is developing due to the formation of crystalline Mo and MoN phases. Related to this transformation, the hardness and elastic modulus are increased, but the adhesion and the tribological properties are impaired. The film loses its plasticity and the generated film material is directly removed from the contact area during the sliding contact

MoS2-Dünnschichten für unsynchronisierte, trockenlaufende Schraubenmaschinen

Unsynchronisierte, trockenlaufende Schraubenmaschinen weisen ein großes Potential auf, eine ressourcenschonende Alternative zu konventionellen Bauformen von Schraubenmaschinen zu liefern. Durch die Einsparung eines Synchronisationsgetriebes und den Verzicht auf ein flüssiges Schmiermittel ergeben sich ökonomische und ökologische Vorteile, da der Material- und Energieeinsatz reduziert werden. Die Reinheit des Prozessgases wird erhöht und die Umweltbelastung wird somit minimiert. Vakuumbeschichtungsverfahren stellen eine der Schlüsseltechnologien zur Realisierung einer unsynchronisierten, trockenlaufenden Schraubenmaschine dar. Durch die endkonturnahe Beschichtung und die Möglichkeit, die strukturellen Eigenschaften der Beschichtung gezielt zu verändern, lassen sich die Dünnschichten optimal konditionieren. Durch den Einsatz von MoS2-Dünnschichten als Vertreter der Festschmierstoffe können die Reibeigenschaften der Schraubenmaschinen verbessert werden, sodass auch an dieser Stelle ein großes Potential vorliegt, die Reibung und somit den Energieeinsatz zu reduzieren

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Structure and tribo-mechanical properties of Si- and W-containing amorphous carbon based multilayers

Designing the film architecture of amorphous carbon based systems is effective in tailoring the tribo-mechanical properties. For this purpose, alternating a-C and a-C:X layers, with X = Si or W, were grown with a layer ratio of 1, a bilayer period of ~200 nm, and a bilayers number of 5 in a magnetron sputtering process. By comparing with a-C(:X) monolayers, the structure and tribo-mechanical properties of alternating a-C/a-C:X films were evaluated.Although the bonding state of the a-C network of a-C:X in a-C/a-C:X is comparable to monolayered a-C:X, the multilayer design significantly affects the tribo-mechanical properties. The a-C/a-C:X multilayers exhibit a higher hardness compared to a-C:X. With a coefficient of friction of 0.12 ± 0.01, a-C/a-C:Si shows a low friction as a-C:Si with 0.09 ± 0.01, but the wear rate is significantly lower for a-C/a-C:Si with (3.4 ± 0.7) × 10-7 mm³/Nm than a-C:Si with (8.3 ± 1.0) × 10-7 mm³/Nm. Contrarily, a-C/a-C:W and a-C:W provide similar wear rates of 1.2 to 1.4 × 10-7 mm³/Nm, but the coefficient of friction is lower for a-C/a-C:W with 0.29 ± 0.02 than a-C:W with 0.36 ± 0.01. Hence, the multilayer design is efficient in improving the tribo-mechanical properties of a-C:X based films

Tuning of solid-to-solid structural transitions in amorphous carbon films by optical pumping and chemical modification

Amorphous carbon (a-C) attracts great attention in tribology research and thin film technologies due to its versatile properties. However, high temperatures and mechanical stresses may cause significant changes in the structural ordering of the a-C network. We present an optical method to initiate structural ordering and to probe solid-to-solid structural transitions of element modified a-C films. A pulsed pump laser introduces heat into the film in a controlled manner, while a second laser probes confocally the first- and second-order Raman scattering signatures of the a-C network. For low pump power, the number of defects and non-sixfold aromatic rings is reduced. A further increase in the laser power leads to sharply evolved changes in the Raman scattering features, indicating a transition from a-C to defected graphite and an effusion of hydrogen. Moreover, graphite-dominant defect relaxation and an enhancement in hexagonal lattice areas occur and, in turn, activate second-order Raman scattering lines. A rising laser power subsequently results in progressive graphitization. Chemical modification of the films with Si or Cu enhances their thermal stability and even shifts the upper thermal limit of the film ablation, while the a-C:W film demonstrates a more efficient enrichment of nanocrystalline graphitic clusters

Investigation of the tribofilm formation of HiPIMS sputtered MoSx thin films in different environments by Raman scattering

Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoSx thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoSx films, the environment and the conditions during the Ball-on-disk tests affect tribofilms and thus the friction. Therefore, the influence of the surface pressure and sliding velocity in air, argon and nitrogen environments on the generation of the third body particles and the tribofilm formation of randomly oriented MoSx films is investigated. A high surface pressure is one major factor to achieve low friction, especially under humid conditions, which is important considering the use in industrial applications, for example dry-running screw machines. However, the mechanisms leading to that frictional behavior are still affected by the surrounding environment. While low friction is caused by a more extensive tribofilm formation in air, in argon and nitrogen, large size third body particles dispensed all over the contact area contribute to a lower friction. Raman scattering reveal a different chemistry of these particles reflected in the absence of laser- or temperature-induced surface oxidation compared to the as-deposited film and the wear track. The Raman scattering results are discussed with respect to the wear particle size, its chemical reactivity and strain-induced bonding changes