Thermal shock behaviour of paper‐derived alumina ceramics
Authors: Kluthe C. , Kollenberg W. 
Published: December 2013

Micro‐scaled alumina filled preceramic papers are used to produce paper‐derived alumina ceramics. The thin (0.6–0.7mm) and porous ceramics fabricated by this process can be, besides other possible applications, potentially utilized as setter plates on cordierite kiln furniture to avoid contact between the cordierite and powder metal substrate during sintering. The SiO2 of the cordierite causes objectionable reactions with the powder metal. For this application it is important to investigate the thermal shock behaviour of the paper‐derived alumina ceramics especially focusing the residual strength, because fast cooling is a common technique in the metal sintering industry to avoid cost and save production time. Two differently processed types of paper were investigated: calendered (additionally roll‐pressed) and uncalendered paper. Their remaining strength has been measured by the B3B‐ test after thermal shocks of a temperature difference ΔT = 100 K up to ΔT = 1000 K to evaluate the critical temperature difference ΔTC and the type of crack growth according to the method of Hasselman. In order to determine the cyclic thermal shock behaviour, at a temperature difference ΔT of 400 K and 600 K the ceramics have been quenched up to 5 times. The results of these investigations have been compared to the properties of tape casted alumina ceramics, a material already commercially used as setter plates. The initial strength of calendered paper‐derived ceramics was 240 MPa. After quenching at a temperature difference ΔT = 600–700 K, 50% strength decrease was observed. Temperature differences of more than 800 K caused >90% strength reduction. Uncalendered paper‐derived ceramics have 185 MPa strength. Between thermo shocks of temperature differences ΔT = 700–800 K, 50% of the strength reduction was measured. With thermo shocks of a temperature difference ΔT = 800 K only 10% of the initial strength remains. Generally the uncalendered ceramics showed a more stable crack growth than the calendered samples. Cyclic shocking at 400 K causes strength losses for both kinds of paper. But they differ in magnitude. While uncalendered paper only lost 7% strength, calendered paper strength was reduced by 20%, compared to their initial strength. This observation gets even more significant by shocking about 600 K. Calendered ceramics start with a higher initial strength and end up with a lower residual strength than the uncalendered ceramics. Only 30 MPa remain after five cycles of shocking, while the uncalendered paper‐derived ceramics have 90 MPa remaining strength. Uncalendered ceramics show a better thermal stress resistance, which can be correlated to their higher porosity and therefore their increased crack deflection in the microstructure. Tape‐casted alumina ceramics show a lower initial strength and lesser thermal shock resistance against simple and cyclic shocking. SEM pictures of the tape‐casted alumina ceramics show large amounts of small, spherical shape pores, while paper‐derived alumina microstructures show long cylindrical pores left by pyrolized cellulose fibres. Considering these results, paper‐derived ceramics are a serious alternative to tape casted ceramics for an application as setter plates.