Search

Zirconium diboride (ZrB2) and silicon carbide (SiC) composites have long been of interest since it was observed that ZrB2 improved the thermal shock resistance of SiC. However, processing of these materials can be difficult due to high and different sintering temperatures and differences in the thermodynamic stability of each material. ZrB2–SiC composites have been processed in a variety of ways including hot-pressing, spark-plasma sintering, reactive melt infiltration, pack cementation, chemical vapor deposition, chemical vapor infiltration, stereolithography, direct ink writing, selective laser sintering, electron beam melting, and binder jet additive manufacturing. Each manufacturing method has its own pros and cons. This review serves to summarize more than 60 years of research and provide a coherent resource for the variety of methods and advancements in development of ZrB2–SiC composites.

Materials capable of oxidizing in a protective manner at ultrahigh (>1700 °C) temperatures are needed to push beyond this barrier defined by SiC. Although possessing attractive mechanical properties and oxidation resistance, SiC-based materials are ultimately temperature limited by the melting point of SiO2. The vast array of ultra-high and high temperature ceramic literature indicates the majority of these materials, like borides, carbides, MAX-phases, and high-entropy ceramics, fall woefully short regarding oxidation resistance. However, for specific applications, like low-orbit aeropropulsion, high ballistics coefficient atmospheric re-entry, and hypersonic cruise, there are a few promising materials. In the present review, oxidation criteria are gathered to build application specific heuristics and are then applied to a multitude of ultra-high temperature ceramics to gauge material efficacy. Discussion of oxidation kinetics, mechanisms and reaction products is offered for each material, identifying strengths, weaknesses, and the remaining gaps in our knowledge.

This paper reports on the translocation of 5.6 ha of a 9.1 ha dry mesotrophic grassland in 1998. For 4 years before and 5 years after translocation, the vegetation was monitored annually by randomised quadrat sampling and compared with an area of the site which remained untranslocated. Associated soil and climatic variables were also monitored. The hypothesis to be tested was that changes in the vegetation ascribable to translocation were of a similar amplitude to those ascribable to other causes. Soil monitoring showed that it had been possible to reconstruct both vegetation and substrate with only minor changes in soil compaction and soil fertility. The frequencies of 28 species in the source meadow in the 4 years before translocation varied significantly. Three areas of the translocated vegetation monitored separately showed significant changes in frequency in a similar number of species, as did the untranslocated area. Species density gradually declined on the translocated turf for the first 4 years after translocation but recovered in the fifth year, although at least one species has so far failed to recover. Multivariate analysis showed that the translocated vegetation diverged from the untranslocated portion for the first 4 years but that this divergence ceased and partially reversed in the final year. The divergence involved an increase in the cover-abundances of several grasses and a parallel fall in a range of forbs. Weather records suggest that this pattern correlates with 4 years without water stress followed by a much drier growing season in the fifth year. It is concluded that there was a definite response to the 1998 translocation which appears to be comparable in severity with the pre-translocation changes in terms of the effects on species frequency. The untranslocated area also changed suggesting that the changes were not all the result of translocation, and the results for the last year of monitoring imply an element of recovery. The results support the use of turf translocation as a restoration technique where the alternative is the complete loss of the site and where the process of reconstructing the functioning of the ecosystem meets certain standards. Monitoring is continuing for 5 more years.

Due to reductions in domestic usage, legislation and changes in fuel use, coal derived particulates in the UK urban atmosphere have been significantly reduced. However, a large increase in road usage and an expansion in the use of diesel engines, has meant that the majority of particulates, now present in the urban atmosphere, originate from vehicle exhausts. Particulate matter, resulting from coal combustion, has been recorded as being present in black patinas observed on some historic stone buildings and monuments and has been associated with accelerated weathering of stone surfaces as a result of enhanced gypsum formation. In contrast, the effects of particulates resulting from vehicle exhaust on stone are much less understood. To investigate this, a comparative study was undertaken using the technique of microcatchments under ambient atmospheric conditions. This compared the elemental composition and volume of precipitation runoff from Portland Limestone coated with three different particulate treatments. Treatments consisted of coal and diesel particulates, both separately and in combination. Combining these treatments attempts to investigate any synergistic effects that may occur when coal derived particulates are overlain by layers formed by particulates from more contemporary sources. It was found that diesel-coated samples were much darker in appearance and showed a significant reduction (P

This study demonstrates significant correlations between the organic matter content of urban street dust and certain mineral magnetic properties, which accords with previous work that indicates magnetic parameters offer potential as a proxy for organic content. However, site-specific data demonstrate the relationship can be different for particular roads, even within the same area. This indicates the association may be more complex than previous work proposes and a cautionary note is required. It is recommended that the nature of the relationship between magnetic and organic properties should be fully explored for particular urban environments and individual field settings, before using magnetic measurements as a proxy for organic matter content. Furthermore, whilst soil is believed to significantly contribute to urban street dust, magnetic values in this study are much higher than those previously reported for top-soils and indicate the influence of other sources, such as anthropogenic pollutants. This suggests that using magnetic measurements to discriminate sources of urban particulates has considerable potential for development.

Ball-on-rod rolling contact fatigue experiments were performed using M50–M50 all-steel and Si 3 N 4 –M50 hybrid components, respectively. Experiments were performed using a jet turbine engine lubricant in the as-received condition or with particulate contaminants added. The added particulate contaminants were Arizona test dust or glycothermally synthesized α-Al 2 O 3 . Wear performance of the all-steel and hybrid rolling systems was evaluated with and without particulate contaminants in the lubricant. Wear surfaces were analyzed using optical microscopy, scanning electron microscopy, surface profilometry, and atomic force microscopy. Wear mechanisms, volume specific wear rates and modes of failure were determined for all systems. The primary wear mechanism produced in rolling contact fatigue experiments is shown to be contact stress fatigue, with secondary mechanisms of indenting, cutting, plowing and embedding occurring as particulate debris passes through the elastohydrodynamic lubricant layer between the ball and rod surfaces.

In order to gain a greater insight into the mechanisms responsible for the deterioration of sandstone buildings, chemical analysis of runoff water from sandstone and glass (control) surfaces has been carried out. Results indicate that the blocking of the stone's pores by atmospheric particulates, soluble salts and algae may be involved in this decay. Evidence is presented which indicates that pressures created by salts and trapped moisture may cause the detachment of surface layers of sandstone from buildings. Weakening by kaolinite formation is identified as a potential factor in sandstone weathering.