How to increase the adhesion and toughness of hard or superhard amorphous carbon (a-C) coatings deposited on engineering substrates (steels, cemented carbide, etc.) is the subject of recent intensive study. Substrate bias grading and incorporation of metals in the coating are effective means to achieve high adhesion and high toughness yet maintain adequate hardness. This paper summarizes the newest development on this subject (bias grading and metal doping) and presents findings in adhesion and tribological studies. Four different types of magnetron-sputtered carbon-based coatings have been deposited on stainless steel substrates: (1) pure a-C deposited under constant substrate bias voltage of -140 V, (2) a-C coating deposited with substrate bias voltage graded stepwise from -20 to -150V, (3) nanocomposite coating obtained by co-sputtering of Ti and graphite targets and (4) nanocomposite coating obtained by co-sputtering of Ti, Al and graphite targets. The structure, adhesion, residual stress and tribological properties of these coatings are presented. The high adhesion strength and good tribological properties found in these coatings are highly desirable in engineering applications. (c) 2004 Elsevier B.V. All rights reserved.