Ceramic Composites with Solid Lubricants Processed by Pulsed Electric Current Sintering

Defence is held on 20.8.2021 12:00 – 15:00 https://aalto.zoom.us/j/63661889988Friction is a system response to the interaction between moving surfaces and wear is the costly outcome due to material degradation. Their elimination constitutes a technological challenge, for which ceramic matrix composites with solid lubricants present a potential solution. This thesis focuses firstly on studying the tribological and mechanical properties of ceramic matrix composites modified with solid lubricants and synthesised by pulsed electric current sintering (PECS), and then on developing a new self-lubricating ceramic composite with W20O58, while exploiting unique advantages of PECS.Nanostructured composites of alumina hardened zirconia with various high temperature solid lubricants were synthesised by PECS, and their tribological performance was studied. Formation of oxygen deficient lubricious metal oxides were first observed during wear of Al2O3 + Mo composites at 400 °C. Tungsten oxide of the same type was then implemented as a solid lubricant additive in ZrO2 + W20O58 composites. Oxygen deficient metal oxides, i.e., Magnéli oxides of molybdenum and tungsten with easy shear planes were synthesised by vacuum annealing. Alumina hardened zirconia composites with solid lubricants had similar overall hardness as pure alumina, owing to preserved nanosized grain structure of the matrix. The CoF of the hard matrix was reduced by 20 – 24 % at 25 °C and by 60 – 65 % at 400 °C. Improvement in friction properties of Al2O3 matrix was limited at 25 °C when modified by Mo. At 400 °C, the CoF of the composite, when compared to alumina, was dropped by 60 – 65 %, and wear rate by two orders of magnitude with the addition of Mo. At high temperature tests, formation of Mo4O11 was observed, when Mo content was below 10 wt%. The lowest CoF was measured from Al2O3 + 5 vol% Mo against alumina at 400 °C. Low friction was explained by in-situ formation of Magnéli oxide phases as a result of surface temperature and applied normal load. Hard ZrO2 matrix was modified by W20O58 and the composites were tested against alumina at 25 °C. Relatively high pressure was applied during consolidation of the composites for preventing undesired phase transformations. The lowest CoF and wear rate was obtained for n-ZrO2 + 10 vol% WO2.9 under 10 N normal load. Lastly, Al2O3 + cBN composites were studied with both PECS and an ultrahigh pressure method. Despite the lower applied pressure, PECS produced composites with better fracture toughness and significantly higher wear resistance. Overall, PECS proved to be a versatile tool for synthesis of self-lubricating and wear resistant ceramic matrix composites. Formation of Magnéli oxide phase, as well as using one from the start as a solid lubricant, improved the friction properties of ceramic matrix significantly. Implementation of PECS resulted in achieving better mechanical properties

Persistan karın ağrısı: Familyal Akdeniz Ateşi ve poliarteritis nodosa birlikteliği

Familial Mediterranean Fever is an autosomal recessive disease, characterized by recurrent and self-limited bouts of fever and polyserositis. Association of Familial Mediterranean Fever and other vasculitic diseases such as polyarteritis nodosa and Henoch-Sch;ouml;nlein vasculitis is uncommon but has been increasingly reported in the literature. Special clinical findings, genetic variation and treatment of Familial Mediterranean Fever associated vasculitis was not adequately determined. Hereby, we report a case of Familial Mediterranean Fever associated with polyarteritis nodosa presenting persistent abdominal pain.Familyal Akdeniz Ateşi, tekrarlayıcı ateş ve poliserozit atakları ile seyreden kendi kendini sınırlayan otozomal resesif geçişli bir hastalıktır. Hastalığın Henoch-Schönlein vasküliti ve poliarteritis nodosa gibi vaskülitlerle birlikteliği nadir olmakla birlikte artan sıklıkta literatürde bildirilmektedir. Familyal Akdeniz Ateşli hastalarda vaskülit için belirleyici olabilecek klinik bulgular, genetik özellikler ve bu hastaların tedavisi henüz kesin olarak belirlenememiştir. Bu makalede genetik olarak tanı konmuş ve persistan karın ağrısı yakınması ile vaskülit düşünülerek poliarteritis nodosa saptanmış bir olgu sunulmuştur

Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear tests

Funding Information: This work was supported partially within the framework of the SINTERCER project (REGPOT-2012-2013-1 EUFP7, project no. 316232 ) by the European Commission under the FP7 Specific Programme ‘Capacities’), and by Academy of Finland via Graduate School for Advanced Materials and Processes (Grant 118728 ). Dr. Ajai Iyer is acknowledged for his help in Raman measurements and Mr. Andreas Friman for his help with tribology tests. Mr. Joonas Lehtonen is acknowledged for helpful discussions. Publisher Copyright: © 2021 The Authors Copyright: Copyright 2021 Elsevier B.V., All rights reserved.Oxygen vacancies in WO2.9 yield to formation of easy shear planes and they can potentially be applied in boundary lubrication conditions for reducing friction. Mechanical and tribological properties of pulsed electric current sintered monolithic WO2.9 were studied by nanoindentation and nanoscratch, and the composites of ZrO2 + WO2.9 with reciprocating wear tests. Hardness of WO2.9 at 25 °C was ~11 GPa and reduced elastic modulus was ~150 GPa. The ploughing dominated coefficient of friction as between 0.09 and 0.24 when measured against a Berkovich diamond tip. The composite n-ZrO2 + 10 vol% WO2.9 presented the lowest CoF, and wear rate 10−10 mm3/Nm measured under 10 N load against alumina ball (6 mm diameter) due to WO2.9 acting as a solid lubricant.Peer reviewe