Friction and wear mechanisms of high performance polyetheretherketone and silicone

This work examines two high performance polymer tribology systems. Polyetheretherketone (PEEK) is a high temperature, low wear thermoplastic that has potential for several modern industries, but the understanding of its wear mechanisms in relation to transfer film is not well understood. An investigation into these mechanisms would benefit the utility of PEEK in several applications. The second polymer system investigated is high performance silicone used in implantable cardiac devices (ICDs). Understanding the wear mechanisms of silicone in-vivo is challenging, and examining the fundamental causes of wear would benefit device design in surgical implantation methods. First, the viability of using finite element analysis as a way to understand fundamental contact behavior is investigated. It was found that for high-level contact models, average roughness is a weak sole descriptor of contact behavior. Next, two PEEK studies in dry sliding were conducted. The first study examined multi-linear and reciprocating sliding in relation to roughness orientation, while proposing hypotheses to explain transfer film behavior. The second PEEK study, examined the development of transfer film and wear with respect to roughness orientation for a variety of sliding distances. From these studies, it was found that frictional heating affects the volume of transfer film, multi-directional sliding and reciprocation play a role in wear and transfer film development, and roughness orientation can greatly impact both wear and transfer film of PEEK. Lastly, a silicone lead in implantable cardiac devices was studied by using three key parameters thought to affect its wear: load, albumin protein, and silica abrasive. It was found that none of these parameters greatly impacted the wear scar metrics, but silica and albumin can lead to wear mechanisms that might impact long-term wear or other wear modes

Evolution des propriétés mécaniques de surface suivie par spectrométrie Raman sur des couples en carbure de silicium et en carbones imprégnés

Les garnitures mécaniques en carbure de silicium (SiC) sont généralement utilisées en milieu lubrifié. Toutefois afin d'accéder à de nouvelles applications technologiques, il peut être nécessaire de les employer en contact sec. Cependant, l'emploi du couple SiC/SiC dans les conditions de frottement sec conduit à une usure sévère des garnitures, voire à leur rupture. Afin de développer cette application une des bagues est remplacée par un matériau en carbone-graphite (CG) dont les propriétés mécaniques et tribologiques sont renforcées par le procédé d'imprégnation qui consiste à introduire dans les porosités du CG un polymère ou un métal ductil à basse température. Dans cette étude, nous avons choisi l'imprégnation soit par une résine phénolique, du PTFE, ou de l'antimoine. Ces ajouts ont pour effet respectif de renforcer la structure de la bague en CG, de diminuer son énergie de surface et de permettre l'accomodation par déformation du film interfacial. Nous proposons d'étudier les mécanismes de dégradations tribologiques qui surviennent à l'interface entre le couple SiC/CG au moyen d'un tribomètre rotatif réalisant un contact conforme anneau/anneau, à température ambiante, avec une vitesse de glissement de 0,5 m/s et une pression de contact de 0,1 MPa. Les faciès d'usure et les tribofilms sont examinés par microscopie électronique à balayage et par spectroscopie de dispersion des rayons X. La spectrométrie Raman vient compléter l'identification chimique des films tribologiques. Cette technique permet la mesure et la cartographie de la taille des cristallites de graphite et la répartition des contraintes résiduelles à la surface des bagues en SiC ou en CG.La description des mécanismes de dégradation des matériaux s'appuient sur le concept du circuit tribologique qui exprime un bilan des débits de matière dans le contact. Dans le cas des couples SiC/CG, les débits sources proviennent principalement des bagues en CG. Les bagues en SIC sont préservées de l'usure. Pour tous les couples de frottement, le troisième corps contient du carbone amorphe et des cristallites de grandes dimensions dont le maintien dans le contact est favorisé par la dynamique du tribomètre. L'atmosphère continue à jouer un rôle dans l'interface car, bien que le contact soit conforme, l'oxydation de l'antimoine est détectée à la surface des bagues qui en sont imprégnées. Les tribofilms les plus stables réduisent le débit d'usure et cette stabilité est fonction de l'imprégnant employé

Effect of Initial Conditions on the Compound Shear- and Buoyancy-driven Mixing

The effect of initial conditions in combined shear- and buoyancy- driven mixing was investigated through the use of an implicit large eddy simulation code under active development at Los Alamos National Laboratory and Texas A&M University. Alterations were done over several months both at Los Alamos National Laboratory and at the Texas A&M University campus, and include a transition from tilted rig to convective channel arrangement, introduction of an inertial reference frame, alteration of boundary conditions, etc. This work resulted in the development of a numerical framework with the capability to model various shear and Atwood number arrangements such as those seen in an inertial confinement fusion environment. In order to validate the code, it was compared to three published experiments, one with Atwood number 0.46 (White et al. 2010), one with high Atwood number 0.6 (Banerjee et al. 2010), and one with very low Atwood number 0.032 (Akula et al. 2012). Upon validating the code, pure Rayleigh-Taylor and pure Kelvin-Helmholtz instabilities were modeled along with five intermediate cases of increasing shear and constant density gradient. Plots of mixing width, Richardson number, growth parameter, and molecular mixing were compared in order to determine at what level of shear the minimum amount of mixing occurs. The results of height gradient and Reynolds number were to previous experiments and theory. The least amount of molecular mixing at the centerline was found to be when the system had a low Atwood number (0.032) and a multimode initial interface perturbation. While the increase in modes of the interface perturbation did not result in a significant change in the growth parameter, the level of molecular mixing at the centerline substantially decreased. As shear was increased in the system, the mixing width and molecular mixing subsequently increased. For this reason, the shear in the system should be eliminated, or at least minimized, if at possible so as to prevent any additional amalgamation in the system. Analysis of the Reynolds number revealed that with an increase in velocity difference between the fluid layers, the value consequently increased. This trend matches with theoretical results as the value is a function of the mixing width and velocity, thus further validating the code. Analysis of the transitional Richardson number revealed that it had a smaller value in the computational case over the experiment, but this fact can be attributed the difference in mixing width between the two methods. The development of the numerical framework with the capability to model various shear and Atwood number arrangements offers the platform for future study of hydrodynamic instabilities

Thermal behavior of silicon carbide/carbon tribological tests

During friction, the materials in contact undergo a thermal field that can accelerate their deterioration or promote the creation of protective layers. Silicon carbide/carbon dry contact often experiences these phenomena through an oxidation process and a material transfer. In this study, three carbon samples with different impregnation conditions have been considered. Dry friction experiments using a ring-on-ring tribometer were carried out and thermal fields induced were followed by means of Infrared Thermography. Wear volumes and rates were analyzed as well as the third body generated during the tests. Energetic flows were identified and a wear mechanism for the silicon carbide/carbon dry contact is proposed, with respect to the concept of the third body approach

Tribological behavior of a silicon carbide/carbone dry contact

The development of new high-performance mechanical seals working in severe conditions requires higher material performances. Sintered silicon carbide (SSC), widely used as a hard mating material, is a potential candidate but its friction and wear properties need to be investigated in the scope of these new applications. Silicon carbide offers good mechanical properties (high hardness, high Young modulus), good corrosion resistance and good thermal conductivity, that make it suitable for tribological applications in different atmosphere (in air, argon or vacuum) and in dry or lubricated sliding. Combined with a counter-face ring made of a softer carbon-graphite, the dry sliding of SSC can be sustained even under severe conditions of pressure and speed. Graphite has been intensively studied in tribology since Bragg first described its lamellar structure. It has been thought during many years that graphite could act as a solid lubricant thanks to this structure. In fact, the environmental conditions strongly influence its tribological behavior. The hardness of the ceramic facing the carbon seal has also an impact on its friction properties. A transfer layer of carbon is generally found on the ceramic surface. In this study, a first experiment assesses the tribological behavior of SSC sliding against itself and three different carbon-graphite materials. Dry friction and ring-on-ring configuration are considered. A second test uses an infrared camera to estimate the temperature variations of a SiC/C couple during sliding, which determines relationship between displacement resistance and the heat generation

On the origin of the widespread self-compatible allotetraploid Capsella bursa-pastoris (Brassicaceae)

Polyploidy, or whole-genome duplication, is a common speciation mechanism in plants. An important barrier to polyploid establishment is a lack of compatible mates. Because self-compatibility alleviates this problem, it has long been hypothesized that there should be an association between polyploidy and self-compatibility (SC), but empirical support for this prediction is mixed. Here, we investigate whether the molecular makeup of the Brassicaceae self-incompatibility (SI) system, and specifically dominance relationships among S-haplotypes mediated by small RNAs, could facilitate loss of SI in allopolyploid crucifers. We focus on the allotetraploid species Capsella bursa-pastoris, which formed similar to 300 kya by hybridization and whole-genome duplication involving progenitors from the lineages of Capsella orientalis and Capsella grandiflora. We conduct targeted long-read sequencing to assemble and analyze eight full-length S-locus haplotypes, representing both homeologous subgenomes of C. bursa-pastoris. We further analyze small RNA (sRNA) sequencing data from flower buds to identify candidate dominance modifiers. We find that C. orientalis-derived S-haplotypes of C. bursa-pastoris harbor truncated versions of the male SI specificity gene SCR and express a conserved sRNA-based candidate dominance modifier with a target in the C. grandiflora-derived S-haplotype. These results suggest that pollen-level dominance may have facilitated loss of SI in C. bursa-pastoris. Finally, we demonstrate that spontaneous somatic tetraploidization after a wide cross between C. orientalis and C. grandiflora can result in production of self-compatible tetraploid offspring. We discuss the implications of this finding on the mode of formation of this widespread weed

Hybrid seed incompatibility in Capsella is connected to chromatin condensation defects in the endosperm

Hybridization of closely related plant species is frequently connected to endosperm arrest and seed failure, for reasons that remain to be identified. In this study, we investigated the molecular events accompanying seed failure in hybrids of the closely related species pair Capsella rubella and C. grandiflora. Mapping of QTL for the underlying cause of hybrid incompatibility in Capsella identified three QTL that were close to pericentromeric regions. We investigated whether there are specific changes in heterochromatin associated with interspecific hybridizations and found a strong reduction of chromatin condensation in the endosperm, connected with a strong loss of CHG and CHH methylation and random loss of a single chromosome. Consistent with reduced DNA methylation in the hybrid endosperm, we found a disproportionate deregulation of genes located close to pericentromeric regions, suggesting that reduced DNA methylation allows access of transcription factors to targets located in heterochromatic regions. Since the identified QTL were also associated with pericentromeric regions, we propose that relaxation of heterochromatin in response to interspecies hybridization exposes and activates loci leading to hybrid seed failure.Author summarySeed failure in response to interspecific hybridizations is a well-known reproductive barrier preventing interbreeding of closely related species and thus maintaining species boundaries. This reproductive barrier is established in the endosperm, a nourishing tissue supporting embryo growth. In this study, we discovered that the endosperm of interspecific hybrids between the recently diverged species Capsella rubella and C. grandiflora suffers from mitotic abnormalities and random chromosome loss. We found that the endosperm has reduced levels of DNA methylation and chromatin condensation, likely accounting for the chromosome loss. Importantly, we found that genes located in pericentromeric regions were preferentially deregulated, suggesting that reduced DNA methylation exposes transcription factor binding sites in pericentromeric regions, leading to hyperactivation of genes and seed arrest. In support of the relevance of pericentromeric regions for hybrid seed arrest, we identified three QTL connected with the phenotype that were all located in pericentromeric regions. These results link epigenetic changes in hybrid endosperm with distinct genetic loci underpinning hybrid seed failure

How impregnation can modify tribological performances of a pair of rings in silicon carbide and carbon-graphite during dry sliding

Tribological behavior of silicon carbide (SiC) had been intensively studied in lubricated condition. However, in dry sliding condition, the degradation of a pair of SiC rings became catastrophic [1-2]. To avoid this impracticable situation, we selected two candidates for the replacement of one counter-face with impregnated carbon-graphite (CG) ring. In this study, we had examined the effect of the antimony and PTFE impregnation in CG rings. SEM coupled with energy dispersive X-ray spectroscopy (EDX) and Raman spectroscopy were used to identify the chemical composition of tribofilm, show the evolution of the particle size and the evolution of the mechanical stress at the surface of the rings