425
Nanohardness tests show that the top of layer n is harder than the bottom of layer n+1 due to grain boundary strengthening .
 process parameter: layer; process parameter: layer; concept or principle: grain boundary
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: 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:
 process parameter: layer; process parameter: layer; concept or principle: grain boundary