Adhesion is one of the most important properties in the field of ceramics coating. To evaluate adhesion, the scratch test is widely used because of its easy operation. Adhesion is generally evaluated by measuring the critical load Lc
, but it is well known that Lc
depends on many factors such as substrate hardness. However, the dynamical meaning of Lc
and the mechanism of coating failure have not been sufficiently clarified.
In this study, scratch tests were carried out on TiN films applied to SKH 51 by ion plating to clarify the relationship between the modes of coating failure and the frictional force, and also some factors that influence the critical load were investigated from the point of plastic deformation. The experimental results obtained were as follows: (1) In the scratch channel, three types of cracks, partial coating loss, and overall coating loss were observed with the increase of vertical load. Partial coating failure involved oblique cracks extending from the edge of the channel. (2) Frictional force changed similar to vibration corresponding to the partial coating loss and increased rapidly corresponding to the overall coating loss. These changes of frictional force enabled the determination of critical load (the load at which partial coating loss starts to develop: Lc1
, and the overall loss load: Lc2
depended on the substrate hardness, while the widths of channels Wc1
were almost same regardless of the substrate hardness. (3) Lc1
depended on coating thickness, while the critical contact angle θci
, was almost same regardless of coating thickness. Furthermore, Lc1
depended on stylus tip radius R
and was proportional to R1.4
, while the critical contact angle θci
was almost same.