The thermomechanical properties of thin DLC films on steel substrates have been measured and related to their structure. The modulus and thermal expansion coefficient have been determined from beam curvature measurements conducted on Si and steel substrates. The fracture toughness and the adhesion have been characterized by using probes that introduce controlled strains into the DLC, through the substrate. Multistrain and sphere impression methods have been used for this purpose. The strains cause multiple cracking and local spalling events that can be analyzed to obtain the toughnesses. The DLC toughness is found to be similar to that for typical carbides. The interfaces have a much greater toughness, attributed to the use of a thin Cr adhesion layer.