Ceramic Composites with Solid Lubricants Processed by Pulsed Electric Current Sintering

Friction 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.