Scratch hardness of diamond-like carbon (DLC) coatings deposited by closed-field unbalanced magnetron sputtering

Thin coatings are subject to scratch and wear damage in tribological applications. Scratch hardness numbers are used to evaluate scratch and abrasive wear resistance. Few studies have measured the scratch hardness of thin films, and a proper scratch hardness measurement method for thin coatings is still lacking. In this study, the scratch hardness of these DLC coatings was measured at both micro-scale and nano-scale, respectively. The micro-scratch test using a 200 µm tip is an inapplicable method because the substrate effect has dominated the scratch hardness measurement. On the other hand, a nano-scratch hardness test using a sharper tip is an effective method to measure the scratch hardness of thin coatings. Although the scratch hardness introduces a lateral force compared with the indentation hardness, it well correlates with the nano-indentation hardness. The ratio of the scratch hardness to the indentation hardness is close to 1 for DLC coatings due to the amorphous structure. It proves that the tangential force induced by scratching does not influence the hardness property. In addition, a simple approach is proposed to obtain the scratch hardness of DLC coatings qualitatively without width measurement.