A Polishing-free Etching Method for Microstructure Observation of Fine-grained Ceramics

Microstructural studies are very important because of their decisive role in the properties of advanced ceramics. During the sample preparation process, it is critical to grind, polish, and etch the fracture surface of the ceramic for effective microstructure observation. Here, a sample preparation process is proposed by directly etching the fracture surface of the ceramic without the time-consuming grinding and polishing. We used this method to create micrographs for MgAl 2 O 4 , Al 2 O 3 , and ZrO 2 ceramics with an average grain size less than 1 μm; these were clearly resolved by SEM. More importantly, the damage resulting from grinding or polishing is minimized and SEM images taken from samples prepared via this new method are closer to the original morphology of the microstructures. This method also greatly simplies the sample preparation process and is especially suitable for ne-grained ceramics.


Introduction
The microstructure of ceramics includes pores, inclusions, grains, and grain boundaries [1,2]. To obtain ceramics with excellent properties, the rst goal is to determine the relationship between the microstructure and properties of advanced ceramics. The observation and interpretation of the microstructure are not new subjects in ceramic science. To effectively resolve the microstructure of ceramics, high-quality sample preparation processes before SEM observation are important.
Traditionally, the fracture surface or the polished surface after etching was chosen for microstructure observation. The contrast between grains and grain boundaries is easily distinguished for a ceramic that fractures in a transgranular fracture mode. If the roughness of the fracture surface is relatively low, then observing the fracture surface is enough to obtain the microstructure morphology [3,4]. However, if the roughness of the fracture surface is high-especially when the grain size is larger than the depth of eld (DOF) of the microscope-the microstructure morphology cannot be imaged successfully. In this case, post-processing steps including grinding, polishing, and etching must be carried out on the fracture surface to obtain a at surface with su cient contrast between the grains and grain boundaries.
For a ceramic that fractures in the intergranular fracture mode, the contrast between the grains and grain boundaries cannot be easily distinguished on the fracture surface due to the existence of glassy phases and impurities segregated on grain boundaries. In this case, similar post-processing steps, including grinding, polishing, and etching, are needed for successful imaging [5][6][7]. However, during the grinding and polishing processes, the practical morphology of the microstructure could be somehow destroyed [8][9][10]. Lateral cracks and grain pullout can result from the machining processes and are barriers to detailed information on the microstructure. Although newly developed advanced polishing technologies, such as mechanical polishing [9], chemical polishing [11], or ion-beam polishing [12], can sharply minimize the destruction of the original microstructure of ceramics, extensive hands-on experience is still needed to properly prepare high-quality samples.
During the machining process of ne-grained ceramics, surface aws such as lateral cracks or grain pullout are much more serious than in coarse-grained ceramics. The residual polishing particles will even hide the practical information of grains or pores. The methods mentioned above for observing the microstructures of ceramics on the fracture surface or the polished and etched surface are not suitable for ne-grained ceramics. Advanced ceramics with grain sizes on the submicron scale or lower can be easily fabricated. As a result, the surface roughness that resulted from the fractured surface of the negrained ceramics is small enough to be ignored for microscopic imaging on the submicron scale. In other words, the grain size of advanced ceramics is much smaller than the the DOF of microscopes. Highquality micrographs can thus be obtained successfully. Hence, the fracture surface of ne-grained ceramics is su cient to be effectively imaged after proper etching.
In this paper, we report a novel strategy of sample preparation for observing the microstructure of ceramics with ne grains. A thermal etching process was applied directly on the fracture surfaces of  Results And Discussion Figure 1 shows the microstructure of the MgAl 2 O 4 ceramic sintered at 1420 °C for 3 h. Panels include micrographs of fracture surface (a) and thermal etched fracture surfaces (b) and (c). In Fig. 1 (a), the fracture surface is almost at with no contrast between the grains and grain boundaries. Obviously, the as-sintered MgAl 2 O 4 ceramic fractured in an intergranular fracture mode. Traditionally, polishing and etching processes are required for ceramics fractured in this mode before observing the microstructure. However, in this process, machining through grinding or polishing were not needed due to the previously formed at surface after fracturing. In Fig. 1 (b) and (c), clear grain boundaries can be distinguished after directly thermally etching the fractured surface. Grain sizes were distributed on a scale of 50-500 nm; all were imaged successfully. The original pores after etching are still similar as that in Fig. 1 (a). In summary, the as-prepared ne-grained MgAl 2 O 4 ceramic described here has grains small enough to be totally imaged on the micrometer scale by SEM. As a result, the machining step was diminished, and the etching process proceeded directly on the fracture surface. The simpli ed sample preparation method is not only time-saving but can also protect the original microstructure from damage during machining.

Materials And Method
On the contrary, if the machining processes such as grinding or polishing were followed, then all destructions such as scratches and pullout grains may occur. Figure 2 shows the three typical destructions on the microstructure of the MgAl 2 O 4 ceramic that resulted from polishing and etching.
Surprisingly, Fig. 1 and Fig. 2 were SEM pictures taken from the same sintered MgAl 2 O 4 ceramic, but they appear completely different. In Fig. 2 (a) and (b), many grains as small as 30 nm or less are shown. However, the actual grain sizes are distributed between 50 nm and 500 nm. As against the same ceramic sample shown in Fig. 1, one sees that the machining and etching processes on the ceramic surface in Fig. 2 resulted in some misleading phenomena such as ultra-small grains. The abrasive particles might be left on the ceramic surface after polishing. Fig. 2 (b) shows obvious scratches that resulted from grinding. These affect the imaging quality and the calculated average grain size. Fig. 2 (c) shows a single pore with a size of about 1 μm. This is several times bigger than the average grain size. During grinding and polishing, some grains may be pulled out due to the weak grain boundaries; the pore-like information is then detailed on the SEM picture. The large pore in Fig. 2 (c) may result from the grain pullout during machining. These types of damage (false ultrasmall grains, scratches, etc.) are barriers for researchers and obscure the true features of the ceramic microstructure.
For a ceramic that fractures in the transgranular fracture mode, observing the fracture surface is preferred for obtaining the actual original morphology of the microstructure. No etching process is usually necessary due to the contrast between the grains and the grain boundaries that already exist because the fracturing behavior occurs just along the grain boundaries. Glassy phases and impurities may exist on the grain boundaries of these ceramics. This will also affect the imaging quality of SEM. To increase contrast, one can groove the grain boundaries by an etching process on the fracture surface before observation. The microstructure can be of higher quality after etching.  Figure 4 (b) shows better contrast between the grains and grain boundaries due to the glassy phases or impurities that were erased by etching.

Conclusions
A novel method that directly etching the fracture surfaces instead of etching the polished surfaces is more convenient for SEM sample preparation. More important, the microstructure close to the original fracture surface can be detected instead of the possible misguiding information due to the destruction that occurred during the machining process. This proposed method is especially suitable for ne-grained