Alumina thermal shock cracks

Suppose further that at first we have suspension dip-coating in mind for the preparation of the layer to be obtained. The coating should be suitable as a substrate for a microfiltration membrane (layer 3) with a pore diameter of 200 nm. Which coating compoimd material is most appropriate This depends on the application and on the substrate material. When there is no reason not to use alumina, this is the best choice because thermal shock cracks can then be avoided during heat treatment (sintering) of the coating. 

On the other hand, in weakly bonded particulates such as alumina, thermal shock resistance decreases with increasing filler content. In this case, interfacial debonding occurs because of the difference in thermal expansion, and then the crack propagates through the interface preferentially. 

Figure 12.3 Thermal shock cracks in alumina ceramics, (a) In quenched specimens (b) In a paper machine foil after severe operation. In this case the thermal shock was caused by water cooling after...

LGMs of the AT/alumina and AT/ZTA displayed some very interesting properties which include excellent machinability, low thermal expansion coefficient, improved thermal shock resistance, low hardness (about 5 GPa), low Young s modulus (E) (250 GPa) and excellent flaw tolerance [Pratapa, 1997 Pratapa Low, 1998 Skala, 2000 Manurung, 2001], These materials appeared to display a large degree of near-surface quasi-plasticity under the Hertzian or the Vickers indenter which effectively inhibits the formation and propagation of cracks. The ductile behaviour of these materials was... 

Thermal shock testing of an alumina/20 vol.% SiC whisker composite showed no decrease in flexural strength with temperature transients up to 900°C.33 Monolithic alumina, on the other hand, shows significant decreases in flexural strength with temperature changes of >400°C. The improvement is a result of interaction between the SiC whiskers and thermal-shock induced cracks in the matrix, which prevents coalescence of the cracks into critical flaws. 

There is the possibility to make substrates in various materials Alumina is an obvious possibility, but monoliths formed from alumina are particularly susceptible to thermal shock problems, and they readily crack during rapid temperature excursions. Silicon carbide and boron nitride are other possible materials having good properties, but they are expensive. 

Alumina-SiC whisker Higher toughness from crack bridging by whiskers, higher thermal shock resistance Chemical affinity of SiC for Fe... 

An SEM picture of the collected chips from ductile grinding of alumina oxide shows a spiraled deformed shape like a turning chip (Figure 11.3). Because of this thermal shock, even the groimd surface can be affected and some cracks can be observed. To avoid this, some Japanese researchers used a low-power laser to close the cracks after grinding. 

All tested brick samples show crack formation due to thermal shock during kiln operation (Fig. 3). In case of the magnesia-chromite brick brands there is either minimal or no wear visible. The high wear observed for the alumina-chromia brick RESISTAL RK30SR can be explained by crack formation and loss of the brick parts due to high thermal shock from the beginning of testing. 

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