harshen vs toughen up

harshen

verb
  • To make, or to become harsh; render hard and rough. 

  • To render peevish, morose, or austere. 

toughen up

verb
  • To make or become mentally tougher. 

How often have the words harshen and toughen up occurred in a corpus of books? (source: Google Ngram Viewer )