The research programme was implemented through two main phases. The former, based on multi-stage experimental activity and theoretical studies, was aimed to identify the best joining process parameters, to orient and assess the actions performed by a loop procedure. The latter was aimed to produce demonstrative components and test them under actual operating conditions. Specific reference power plant components in which metal to ceramic joined parts shall be mounted, were selected. In the gas turbine application, they consist of combustion chamber thermal shields and nozzle gas vanes of an improved Ansaldo engine. In the boiler application, protective modules for sensors are expected to be applied a re-heater section of a Finnish coal fired power plant. Metal to ceramic joints such as Al2O3 to IN738, Al2O3 to Nimonic PK33 and SiCp/Al2O3 (CMCp) to Nimonic PK33, were obtained pursuing different approaches under different experimental stages, namely Preliminary, First, Second and Intermediate Round. Pre-treatments of the surfaces to be joined were included to improve material compatibility and minimise effects from critical formation of brittle surfaces. Two different coating methods were used in the programme combined with HTB and DB methods: Physical Vapour Deposition (PVD), for thin coatings and Plasma Spraying, atmospheric/vacuum (APS,VPS) for thick coatings. Bonding activities were supported by chemical, micro-structural, mechanical considerations and finite element analyses. A characterisation campaign was performed on samples through ultrasonic inspections (US), surface analyses and mechanical tests to evaluate joints soundness and strength both at room and high temperature. Only commercially available base materials, active metal brazes and compliant interlayers were considered, as final components are intended to be produced using materials directly obtainable from the market. A Third Round was added to study the transferability of bonding technologies to a novel ceramic matrix composite material i.e. SiCf/Al2O3 (CMCf). Alumina tiles and sensor shields were produced and subjected to functional testing such as thermal fatigue, thermal shock and endurance test.
The developed bonding techniques showed that: 1) appropriate interlayers are necessary to compensate the mismatch of dissimilar materials properties, hence to reduce the building up of residual stresses; 2) preparation of surfaces to be joined and parameters of thermal treatments are key factors for the joint soundness; 3) the statistical behaviour of ceramic parts requires geometry of test pieces alike those of final components; 4) the results obtained in CMCf to CMCf joints are highly encouraging. Further development of joining procedures for CMCp and CMCf seems to be a realistic aim to be pursued in future R&D projects to fully benefit these materials in real components.
The proposed research is intended to develop new metal/ceramic joining techniques for the manufacturing of components working at temperatures up to 1500 C in corrosive environments. Such techniques are based on high quality pre-treatments of the surfaces to be joined, prior to the actual joining process.
By means of tailored and well characterised treatments (such as surface activation, metallization by sputtering or PVD, plasma coating, etc.) it is expected that problems of material compatibility and interface brittleness, encountered in present state-of-the-art technologies, be strongly reduced. The interest of the research originates from the need of producing high strength and reliable joints, to enable the use of ceramics as base materials in the manufacturing of gas turbines hot parts. In the first phase, the research will consist of a development activity, in which the process parameters are varied in order to identify the optimum joining parameters. This phase will be supported by extensive characterisation activity, by theoretical studies of the interface phenomena and by finite element analysis.
In the second phase, the best performing techniques will be used to manufacture demonstrative components which will be tested under conditions representative of the operating conditions of industrial gas turbines.
Funding SchemeCSC - Cost-sharing contracts