The influence of environment on lifetime decay and structure evolution of thermal history paint

Abstract

In this work, the influence of environmental factors, including atmosphere compositions, substrate materials and exposure durations, on the lifetime decay and the material structural evolution of a temperature sensor for combustion environment, Thermal History Paint (THP), is investigated. Heat treatments from 700 °C to 900 °C for 2 hours and 20 hours were conducted on THP samples with sapphire/metal substrates. The luminescence Lifetime Decay (LTD) of THP was measured with a specially made benchtop instrument. Luminescence spectroscopy, photography, FTIR, XRD, DSC and SEM/EDS/FIB were used to characterize the material structural evolution of THP. The results showed that THP on sapphire samples can maintain temperature measurement capability (a monotonic LTD increase with temperature) for 2 h exposures when heated in atmosphere with elevated H₂O and/or CO₂ concentrations and the simulated combustion atmospheres of a natural gas turbocharger and a gasoline engine. A specific set of calibration data need to be established for each atmospheric condition to achieve the optimised accuracy. The LTD calibration curve moves to low temperature range with elevated H₂O or H₂O + CO₂ level or longer exposures, which generally leads to higher LTD readings. A concept of nano-sized luminescent Y based crystals/crystal nucleus doped with Eu³⁺ evenly embedded in the amorphous potassium silicate binder is established as the true functional structure of THP. Contaminants from atmosphere and metal substrates can take potassium away from THP, leading to the breakdown of THP functional structure, of which the process is investigated thoroughly.