Increasing demands for high performance materials leads to a sustained pressure for the development of new materials, pushing physical and mechanical limitations to new levels. Parts with locally differentiated material properties allow a tailored adaptation to the desired application profiles. Designs may require components to be fixed in specific locations or combined with other parts; but joining these dissimilar materials, for instance ceramics with metals, for further use in assemblies is challenging. In this research work a new low-cost, reliable and robust coating technique for ceramics based on friction surfacing was developed. A standard milling machine was adapted to apply metal coatings onto ceramic substrates. Suitable material combinations for oxide and non-oxide ceramic groups were identified with their bonding mechanisms and relationship to their coating parameters investigated. A parametric study for the oxide ceramics group was conducted using a substrate of Al2O3, namely alumina, and AlMg4.5Mn0.7 (EN AW-5083) as a coating material. Because of the low thermal shock resistance of the ceramic substrate preheating was used. Despite this approach, crack formation in the alumina substrate coupled with a decrease of substrate strength in that area was observed. Main bonding mechanisms for the Al/Al2O3 combination were identified as van der Waals forces and mechanical interlocking. For the non-oxide ceramics group Si3N4, silicon nitride, was used as a substrate and AlMgSi0.5 (EN AW-6060) as a coating material. Due to the high thermal shock resistance of the ceramic substrate no detrimental effects were observed. Analysis of the interface revealed formation of a glassy phase consisting of predominantly amorphous SiAlON, which was attributed to oxide-free coating material (Al) coming into contact with the substrate surface leading to a reduction reaction of the ceramic substrate. Recreating the bonding mechanisms through simplified experiments that model the essential process parameters, two main mechanisms required for the durability of the bond were confirmed which can act independently or in combination. Firstly, the aluminium coating needs to flow into the pores of the substrate forming anchor points, allowing van derWaals force to persist. Secondly, oxide-free coating material needs to be exposed to the substrate surface leading a chemical reaction and forming of compounds.
| Date of Award | 31 May 2023 |
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| Original language | English |
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| Supervisor | Simon Barrans (Main Supervisor) |
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