RESUMO
Recent engine design and emission trends have led to the commercial use of Atmospheric Plasma Spray (APS) coatings for cylinder liner applications like the TiO2 APS coating. It was shown in our previous work that this type of coating showed better friction results compared to steel lubricated with MoDTC. To further investigate this feature, a parametric study was carried out involving the effect of MoDTC concentration, test temperature, Hertzian contact pressure and the change of counterpart materials from steel balls to ceramic balls (Al2O3 and ZrO2). Ball-on-flat tribotests were carried out on a reciprocating (ball-on-flat) tribometer lubricated with base oil containing MoDTC. Results show that for all the test conditions used including the concentration of MoDTC, test temperature and the contact pressure, lower friction and wear is observed for the TiO2 APS coating compared to reference steel. To explain the low friction behavior, tribofilm compositions were investigated and it was observed that MoS2 is always formed in the case of TiO2 APS with no oxysulphide species. For the reference steel, MoO x S y species are mainly detected in the tribofilms. XPS analyses performed on TiO2 APS flats when the counterpart material was changed from steel balls to ceramic balls suggested the formation of MoS2 (Mo in +iv oxidation state) and Mo-C (Mo in +iv or +ii oxidation state) species with a negligible amount of MoO3 (Mo in +vi oxidation state). It was also shown that a significant amount of molybdenum atoms inside the tribofilm, originating from MoDTC (Mo in +v oxidation state) were reduced in the tribological contact. A mechanism for the decomposition of MoDTC on the basis of tribocatalytic behaviour hypothesized in our previous work was proposed and discussed.
RESUMO
Amide molybdate has been recently introduced as a friction modifier for tribological applications. Combined with zinc dithiophosphate (ZDDP) and fatty amines, it provides an ultralow friction coefficient. The ultimate product of Mo compound transformations in tribological contact, due to frictional heating and shearing, as well as chemical interactions with oil additives, is molybdenum sulfide (MoS2). Understanding the decomposition of amide molybdate leading to MoS2 is of primary importance to the optimization of the design of lubricant formulations. This study focuses on the investigation by Raman spectroscopy of amide molybdate decomposition intermediates. Raman spectra of tribofilms, obtained after friction tests under different temperatures and pressures, revealed the formation of an amorphous MoS3 intermediate coexisting with MoS2. However, under severe conditions, the tribofilms are mostly composed of MoS2.
RESUMO
Variation at 12 polymorphic isozyme loci was studied in the European beech on the basis of an extensive sample of 389 populations distributed throughout the species range. Special emphasis was given to the analysis of the pattern of geographic variation on the basis of two contrasting measures of genetic diversity, gene diversity (H) and allelic richness, and to their relationship. Measures of allelic richness were corrected for variation in sample size by using the rarefaction method. As expected, maximum allelic richness was found in the southeastern part of the range (southern Italy and the Balkans), where beech was confined during the last ice age. Surprisingly, H was lower in refugia than in recently colonized regions, resulting in a negative correlation between the two diversity measures. The decrease of allelic richness and the simultaneous increase of H during postglacial recolonization was attributed to several processes that differentially affect the two diversity parameters, such as bottlenecks due to long-distance founding events, selection during population establishment, and increased gene flow at low population densities.
