We study rubber sliding friction on hard lubricated surfaces. We show that even if the hard surface appears smooth to the naked eye, it may exhibit short wavelength roughness, which may give the dominant contribution to rubber friction. That is, the observed sliding friction is mainly due to the viscoelastic deformations of the rubber by the substrate surface asperities. The presented results are of great importance for rubber sealing and other rubber applications involving (apparently) smooth surfaces.

In this work, the friction and wear behaviour of acrylonitrile butadiene rubber (NBR), hydrogenated acrylonitrile butadiene rubber (HNBR), acrylate rubber (ACM), and fluoroelastomer (FKM) against steel surfaces under unidirectional dry sliding conditions have been studied. The influence of surface roughness of the steel counterface on friction and wear was studied using a block-on-ring test configuration. At low load, the friction coefficient decreased after a runningin period and the wear was insignificant, especially for the ACM and FKM. The running-in time in terms of achieving a stable dry friction for the different elastomers, from longest to shortest, is in the order HNBR, NBR, FKM, and ACM, with an exception in case of FKM sliding against a smooth steel counterface. At higher contact pressure, powdery worn particles on the ACM and a decrease in friction coefficient were observed, but for FKM and HNBR, worn particles with roll shapes were produced. The worn particles of FKM were significantly larger than those of the other tested materials, and a considerably higher wear in FKM was observed.

Since abrasion is a common cause of seal failures, understanding the mechanisms of abrasion of an elastomer in the presence of lubricants is of importance in sealing applications. In this research a block on ring configuration was used to study the influence of lubrication on the two body abrasion of several commonly used sealing elastomers (two acrylonitrile butadiene rubbers, an acrylic rubber and a fluoro rubber). The friction force and the abrasive wear of the samples were measured and the worn surfaces and wear particles were investigated using an optical microscope. The tear strength of the elastomers before and after immersion in monoester oil as well as the oil absorption has been measured. Both scratches (parallel to the direction of sliding) and ridges (perpendicular to the direction of sliding) were observed on the worn surfaces of nitrile rubbers but the surfaces of acrylic and fluoro rubber were characterised by scratches only. The worn surfaces of nitrile rubbers were defined with more continuous ridges at lower sliding velocity and the presence of a lubricant in the contact reduced the continuous ridges. Examination of the wear particles shows that the wear particles (particularly for acrylic rubber) under dry sliding condition were aggregated, but the lubricant dispersed the wear particles and prevented aggregation. In most cases, abrasive wear of the elastomers under lubricated condition is higher than that under dry condition. The results show that the friction coefficient increased with increasing sliding velocity and decreased with contact pressure. Apart from the fluoro rubber, the friction coefficient as well as the tear strength of the elastomers decreased significantly in the presence of lubricant, particularly for acrylic rubber.

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Introduction Coronavirus disease 2019 (COVID-19) is a viral infection caused by a novel coronavirus known as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The disease raises an enormous public health challenge for the international community. Liver enzymes have been reported to be frequently elevated in hospitalized patients with severe COVID-19 disease. Materials and Methods This article is a narrative review of abnormal liver tests and liver injury as a manifestation of progression to severe pneumonia. We collected data from the PubMed database (National Library of Medicine, Bethesda, Maryland, United States). We used the search term “abnormal liver test” and relevant records were measured. The review article was organized thematically. Results This narrative review aims to summarize the available clinical data on abnormal liver enzymes in coronavirus infection and its association with the risk of mortality, severer pneumonia, and systemic inflammation. Some clinical studies refer to abnormal liver tests and liver injury as a manifestation of progression to severe pneumonia. Recent research verified the relationship between hepatic liver enzyme activities and liver damage in patients with COVID-19, which suggested that it might reflect the infection severity and the mortality risk. Thus, this review investigated the correlation between liver serum enzymes level and the severity of COVID-19 patients, by reviewing investigating the relationship between the illness severity in COVID-19 patients with abnormal liver tests, liver pathology, and markers of inflammation. Conclusion In the current pandemic of SARS-CoV-2, abnormalities of liver enzyme tests were commonly observed in patients with COVID-19. However, because of multiorgan damages that observed in COVID-19 patients, various issues should be considered such as the pathology and pathophysiology of the liver tissue, especially on the activation process of the immune response and cytokine storm to prevent the severity of the disease.