Research Progress in Residual Stress Modulation of Diamond-like Carbon Film
Diamond-like carbon(DLC)films are widely recognized for their outstanding properties and exhibit significant potential in various fields.Nevertheless,the presence of high residual stresses in DLC films weakens their adhesion to the substrate,thus resulting in film cracking or spalling.This limitation severely affects their durability and reliability,which is a significant challenge in DLC film research.Moreover,this issue must be addressed to enable the practical application of DLC films.In this paper,stress-generation sources in DLC films are discussed and research progress pertaining to stress modulation is summarized.In particular,elemental doping,transition layers,and process adjustments are elucidated in addition to the current global research landscape.Elemental doping is classified into single-and multi-element doping,which can be further categorized as strong and weak/non-carbide-forming elements,depending on their bonding strength with carbon atoms.The primary objective is to mitigate residual stress by reducing the proportion of distorted C-C bond lengths and C-C-C bond angles in the film,as well as moderating the extent of distortion in the bond lengths and angles.Notably,weak/non-carbide-forming elements,despite significantly reducing internal stresses,deteriorate the mechanical properties owing to their weaker bonding energies.Multi-element doping leverages the complementary properties of diverse elements,thereby reducing the stresses in DLC films significantly while maintaining robust mechanical properties and satisfying the demands of complex operating conditions more comprehensively.Metal doping primarily reduces residual stresses within the structure of DLC films.Nonetheless,owing to the dissimilar thermal-expansion coefficients between the substrate and film,high stresses can persist at the interface.Hence,a transition layer(monolayer,multilayer,gradient,etc.)is introduced between the DLC film and substrate to effectively mitigate residual stresses by alleviating internal stresses caused by mismatches in thermal-expansion coefficients.Furthermore,various deposition parameters,such as the substrate bias pressure,gas-source flux ratio,deposition temperature,deposition pressure,and carbon-source incidence angle,exert different effects on the intrinsic structure of the film.Different parameter combinations result in distinct residual-stress states.More importantly,these process parameters function synergistically,and their effects on the residual stress of the film varies under different conditions.Consequently,a comprehensive consideration of these parameters and their optimization based on specific application requirements is essential during DLC film deposition.Notably,the relationship between microstructural evolution and stress in the same elemental doping system under different preparation methods or in different elemental systems under the same preparation method varies.Hence,a refined computer-simulation technique at the atomic scale is proposed to investigate the effects of various preparation methods on the intrinsic structures of DLC films and to elucidate their stress-evolution patterns.In the future,the integration of advanced materials science and technology,such as machine learning and artificial intelligence,can be considered to further investigate DLC film preparation and stress-control challenges.
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