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The sliding friction for trilayer graphene is directly proportional to the applied load , but the friction for monolayer and bilayer graphene is inversely proportional to the strain in the graphene , which is in violation of Amontons ' law .
 graphene, graphene, graphene
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 graphene, graphene, graphene