Papers by Author: John R. Nicholls

Abstract: Automotive exhaust gas temperature sensors are fitted to monitor the performance of a vehicle emission control system. The aggressive working environment is a big challenge in sensor design. This paper introduces an FEA simulation model developed to support the mechanical reliability of new sensor designs. The simulation model was validated by laboratory tests. Suggestions for optimising sensor reliability are given based on the simulation results.

Abstract: Thermal barrier coatings (TBCs) are an enabling materials technology to improve the efficiency and durability of gas turbines and thus through such efficiency improvements offer reduce fuel usage and an associated reduction in CO2 emission. This commercial drive is pushing both aero- and industrial turbines to be lifetime dependent on TBC performance – the TBC must be “prime reliant”. However, the prediction of the durability of the TBC system has proved difficult, with lifetimes varying from sample to sample and component to component. One factor controlling this is the inability to measure accurately the bondcoat/ceramic interface temperature when buried under a TBC. In operating engines this is further exacerbated by the fact that such TBC systems operate in strong temperature gradients due to the need to cool aerofoil components. This research examines the design and manufacture of self diagnostic thermal barrier coatings capable of accurately measuring the interface temperature under the TBC, whilst providing the requisite thermal protection. Data on the temperature sensing capability of various rare earth doped EB-PVD thermal barrier coatings will be reported. It will be shown that systems exist capable of measuring temperatures in excess of 1300oC. Details of the measurement method, the compositions and the thermal stability of such systems will be discussed in this paper. The ability to produce a sensing TBC capable of measuring interface temperature, surface temperature and heat flux will further be discussed permitting the design of thermal barrier protected components capable of in-situ performance monitoring.

Abstract: Gas turbines are critical components in the combined cycle power systems being developed to generate electricity from solid fuels, such as coal and biomass. The use of such fuels to produce fuel gases introduces the potential for significant corrosive and erosive damage to gas turbine blades and vanes. Single crystal superalloys have been developed for use with clean fuels but are now being deployed in industrial gas turbines. The performance of these materials, with coatings, has to be determined before they can be used with confidence in dirtier fuel environments. This paper reports results from a series of laboratory tests carried out using the ‘deposit replenishment’ technique to investigate the sensitivity of candidate materials to exposure conditions anticipated to cause type I hot corrosion in such gas turbines. The materials investigated have included the single crystal nickel-based superalloys CMSX-4 and SC2-B, both bare and with Pt-Al coatings. The exposure conditions within the laboratory tests have covered ranges of SOx (50 and 500 volume parts per million, vpm) and HCl (0 and 500 vpm) in air, as well as 4/1 (Na/K)2SO4 deposits, with deposition fluxes of 1.5, 5 and 15 5g/cm2/h, for periods of up to 500 hours at 900°C. Data on the performance of materials has been obtained using dimensional metrology: pre-exposure contact measurements and post-exposure measurements of features on polished cross-sections. These measurement methods allow distributions of damage data to be determined for use in the development of materials performance modelling. In addition, the types of damage observed have been characterised using standard optical and SEM/EDX techniques. The damage rates of the single crystal materials without coatings are too high for them to be used with confidence in gas turbines fired with gases derived from ‘dirty fuels’. Under the more severe combinations of gas composition, deposition flux and metal temperature, the corrosion rates of these materials with Pt-Al coatings are also excessive. The data produced from these tests has allowed the sensitivity of hot corrosion damage to changes in the exposure environment to be determined for the single crystal alloys and coating systems examined.

Abstract: This paper examines the erosion and cyclic oxidation performance of novel thermal barrier coatings produced via the sol-gel route. The ceramic top coat, with a thickness of 5-80 m, was deposited via a sol-gel route onto standard MCrAlY and PtAl bond coats. In both the erosion and the cyclic oxidation tests it was found that the bond coat had a profound affect on the results. The erosion of the sol-gel coatings were compared to standard EB PVD and PS TBCs and were found to be significantly higher. The effect of aging (100 h at 1100°C) on the erosion rates was also evaluated and was found to increase the erosion rates. The information obtained from the erosion and cyclic oxidation tests have highlighted the need to develop and optimise the parameters for producing thicker coatings