Evaluation of Fracture Toughness of (Ti,Nb)N Coatings by Spherical Indentation Test

Spherical indentation tests were conducted to evaluate fracture toughness of (Ti,Nb)N coatings deposited on high-speed steel substrate by using multi-arc ion plating. Under the normal load of 600N, only radial cracks were formed around the indentation. The number and length of the cracks around the indentation were analyzed statistically, then the crack density function β was put forward to evaluate the fracture toughness of (Ti,Nb)N coatings. The experimental results show that fracture toughness of the (Ti,Nb)N homogenous coatings decreases with increase of microhardness for their high britlleness. Ti xNb 1-xN gradient coating exhibits better fracture toughness even though microhardness of the coating is high. The crack density β of the pre-nitrided coating is the lowest and the fracture toughness of the coating is the best. The crack density β can be used to quantitatively or half-quantitatively evaluate the fracture toughness of the coatings by spherical indentation test.