Root associated fungi respond more strongly than rhizosphere soil fungi to N fertilization in a boreal forest.

Nitrogen (N) fertilization is a routine practice in boreal forests but its effects on fungal functional guilds in Pinus sylvestris forests are still incompletely understood. Sampling is often restricted to the upper organic horizons and based on DNA extracted from mixtures of soil and roots without explicitly analysing different spatial niches. Fungal community structure in soil and roots of an 85-y-old Pinus sylvestris forest was investigated using high throughput sequencing. Fertilized plots had been treated with a single dose of N fertilizer, 15 months prior to sampling. Species richness of fungi colonizing roots was reduced in all horizons by N fertilization. In contrast, species richness of soil fungi in the organic horizon was increased by N fertilization, but unaffected in the mineral horizons. Community composition of fungi colonizing roots differed from that of soil fungi, and both communities were significantly influenced by soil horizon and N. The ectomycorrhizal community composition in both roots and soil was significantly affected by N fertilization but no significant effect was found on saprotrophic fungi. The results highlight the importance of analysing the rhizosphere soil and root compartments separately since the fungal communities in these two niches appear to respond differently to environmental perturbations involving the addition of nitrogen.

Wear and friction properties of experimental Ti-Si-Zr alloys for biomedical applications.

Titanium alloys are widely used in biomedical applications due to their higher biocompatibility in comparison to other metallic biomaterials. However, they commonly contain aluminum and vanadium, whose ions may be detrimental to the nervous system. Furthermore, they suffer from poor wear resistance, which limits their applications. The aim of this study was to evaluate the tribological performance of experimental Ti-1.25Si-5Zr, Ti-2.5Si-5Zr, Ti-6Si-5Zr and Ti-2.5Si-5Zr-0.2Pd alloys as compared to that of control Ti-6Al-4V, CoCr F75 and CoCr F799 alloys. Friction and wear tests were performed using a standard ball-on-disc rig in serum solution at ambient temperature with Si3N4-balls as counterparts. The alloys microstructure and hardness were investigated using optical microscopy, XRD, scanning electron microscopy (SEM) and Vickers indentation. The coefficients of friction of the experimental Ti-Si-Zr alloys were generally lower than the commercial ones with Ti-6Si-5Zr presenting the lowest value (approx. 0.1). Their wear rates were found to be 2-7 times lower than that of the commercial Ti-6Al-4V alloy, but still higher than those of the CoCr alloys. SEM analysis of worn surfaces showed that abrasion was the predominant wear mechanism for all studied materials. Wear and friction were influenced by the formation and stability of transfer layers, and while commercial Ti-6Al-4V as well as the experimental Ti-Si-Zr alloys demonstrated extensive material transfer to the ceramic counterparts, the CoCr alloys did not show such material transfer.

Evaluation of silicon nitride as a wear resistant and resorbable alternative for total hip joint replacement.

Many of the failures of total joint replacements are related to tribology, i.e., wear of the cup, head and liner. Accumulation of wear particles at the implants can be linked to osteolysis which leads to bone loss and in the end aseptic implant loosening. Therefore it is highly desirable to reduce the generation of wear particles from the implant surfaces. Silicon nitride (Si(3)N(4)) has shown to be biocompatible and have a low wear rate when sliding against itself and is therefore a good candidate as a hip joint material. Furthermore, wear particles of Si(3)N(4) are predicted to slowly dissolve in polar liquids and they therefore have the potential to be resorbed in vivo, potentially reducing the risk for aseptic loosening. In this study, it was shown that α-Si(3)N(4)-powder dissolves in PBS. Adsorption of blood plasma indicated a good acceptance of Si(3)N(4) in the body with relatively low immune response. Si(3)N(4) sliding against Si(3)N(4) showed low wear rates both in bovine serum and PBS compared with the other tested wear couples. Tribofilms were built up on the Si(3)N(4) surfaces both in PBS and in bovine serum, controlling the friction and wear characteristics.

Complex frictional analysis of self-lubricant W-S-C/Cr coating.

Transition metal dichalcogenides belong to one of the most developed classes of materials for solid lubrication. However, one of the main drawbacks of most of the self-lubricating coatings is their low load-bearing capacity, particularly in terrestrial atmospheres. In our previous work, alloying thin films based on tungsten disulfide with non-metallic interstitial elements, such as carbon or nitrogen, has been studied in order to improve tribological performance in different environments. Excellent results were reached with the deposited coatings hardness, in some cases, more than one order of magnitude higher than single W-S films. In this work, W-S-C films were deposited with increasing Cr contents by co-sputtering chromium and composite WS2-C and targets. Two films were prepared with approx. 7 and 13 at.% of Cr. Alloying with chromium led to dense films with amorphous microstructure; the hardness and adhesion was improved. Sliding tests were carried out in dry and humid air using a pin-on-disc tribometer with 100Cr6 steel balls as a counterpart. To analyse the sliding process, the surfaces in the contact were investigated by X-ray photoelectron spectroscopy (bonding), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Surface and sub-surface structural modification of the coating and composition of the transferred tribolayer are discussed in detail. High friction in humid air was attributed to the absence of a well-ordered WS2 sliding interface. On the other hand, the existence of such an interface explained the very low friction observed in dry air.