Semiconductor TiO2Gas Sensor for Controlling Nitrocarburizing Processes

Abstract

TiO2 films can serve as oxygen sensors for controlling the nitrogen potential in the process of nitrocarburizing. In contrast to conventional semiconductor-base oxygen sensors that lose stability under extreme thermal and chemical conditions in nitriding and carburizing atmospheres, TiO2 films obtained by the sol-gel method offer a promising alternative. In the present work TiO2 films with a density of about 80% and a grain size of 30 – 50 nm are deposited by the sol-gel technology. Steel substrates and commercial substrates from Al2O3 aluminum oxide with platinum electrodes are coated and calcinated. In order to preserve the sensitive element of the sensor a special casing is constructed for operation under conditions very close to the atmosphere of nitrocarburizing. The sensitive element consists of an Al2O3 substrate with built-in Pt-electrodes, which is coated by a thin sol-gel TiO2 film. The direct current in the furnace is measured for nitrocarburizing in various mixtures (N2, O2, H2, and NH3) at a temperature of 500 – 600°C. A linear dependence log[R] – log[p O2], where R is the electrical resistivity, is obtained for the films in the studied range of partial pressure of oxygen independently p O2 of the presence of NH3 or H2 in the atmosphere. The dependence log[R] – log[p O2] for a nitriding furnace with a capacity of 90 liters is shown to be stable with 3% scattering per month.