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An optical microscopy , a scanning electron microscopy and a laser confocal microscopy were used to observe and clarify the influence of the columnar grain boundary on the crack growth behavior and the interaction between the crack front and microstructure .
 optical microscopy, scanning electron microscopy, laser, microscopy, columnar grain boundary, crack growth, microstructure
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 optical microscopy, scanning electron microscopy, laser, microscopy, columnar grain boundary, crack growth, microstructure