Thermal shock behaviour of paper‐derived alumina ceramics

Abstract

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 SiO 2 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 rollpressed) and uncalendered paper. Their remaining strength has been measured by the B3B-test after thermal shocks of a temperature difference DT = 100 K up to DT = 1000 K to evaluate the critical temperature difference DT C 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 DT 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 DT = 600 -700 K, 50% strength decrease was observed. Temperature differences of more than 800 K caused A90% strength reduction. Uncalendered paper-derived ceramics have 185 MPa strength. Between thermo shocks of temperature differences DT = 700 -800 K, 50% of the strength reduction was measured. With thermo shocks of a temperature difference DT = 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...

Highly Filled Papers, on their Manufacturing, Processing, and Applications

“…Another physical property that has been determined for paper‐derived alumina platelet samples is the critical temperature corresponding to the thermal shock resistance of the material, as defined by the decrease of the residual strength to its half‐value after a thermal shock. A value of 600–800 °C for the critical temperature was found at an in‐plane fracture toughness of the paper‐derived ceramic of 1.9 ± 0.1 to 2.1 ± 0.1 MPa * m 1/2 . In contrast to paper‐derived alumina, paper‐derived hydroapatite‐type ceramic layers, which had been employed in the LOM process, had a range of Young's moduli from 0.33 to 1.53 GPa and a range of B3B flexural strengths from 7.6 to 33.1 MPa .…”

“…An overview of the different process routes for the heat treatment of highly filled papers is shown in Figure . The final sintering products or reaction products generated can be divided in the following groups: Oxide Ceramics Non‐Oxide Ceramics Ceramic‐Based Composites Metal‐based Composites …”

“…Calendering alumina preceramic papers led to increases in density of the paper‐derived alumina from 2.4 g cm −3 up to 3.0 g cm −3 . Also decreases in the total porosity from 30 to 22 vol% were observed for the paper‐derived alumina, when the preceramic papers had been calendered before . Multilayers manufactured by the LOM method and the resulting alumina multilayer had an average total porosity of 31 vol% …”

“…Paper‐derived alumina or cordierite ceramics have been considered for applications as thermal shielding layers within high‐temperature furnaces or as thermal insulators. The addition of alumina or zirconia fibers helped to further increase the thermal shock resistance of the paper‐derived ceramics . In comparison to setter plates made from tape‐casted alumina ceramics, the setter plates from paper‐derived alumina ceramics were mechanically more stable.…”

“…When the oven reaches a sufficiently high temperature, the filler materials either undergo a sintering process or chemical reactions are initiated, yielding paper‐derived ceramics, ceramic‐, or metal‐based composites. Optionally, the green body can be infiltrated by a hot melt, such as liquid silicon or a liquid Cu–O alloy …”