Mechanical and tribological study of DLC coatings deposited by filtered cathodic vacuum arc

• Main Author: Choo, Chee Wee.
• Other Authors: Sam Zhang Shanyong
• Format: Final Year Project
• Language: English
• Published: 2011
• Subjects: DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films
• Online Access: http://hdl.handle.net/10356/45620
• Institution: Nanyang Technological University

Diamond-like carbon (DLC) coating is a well know material for its high hardness, good wear resistance and low friction when sliding against most engineering materials. On the other hand, DLC has weakness like high residual stress, low toughness, bad thermal stability and poor oxidation resistance. Weaknesses like high residual stress and low toughness limits the thickness of the coatings. This indicates that the working life of DLC coating is short. With their desirable properties, DLC coating is used as protective coating for cutting tools and mechanical components. Therefore, multilayer architecture coating is introduced to improve the adhesion, reduce the residual stress, increase wear resistance, and working life of DLC coatings. This study consists of monolayer DLC coatings deposited by filtered cathodic vacuum arc under different substrate bias voltage and multilayer DLC coatings with different sublayer thickness under alternating substrate bias voltage. Their mechanical properties like hardness, Young’s modulus, fracture toughness, residual stress and tribological properties like friction coefficient, wear rate and bonding structure evolution have been examined. The Id/Ig ratio of the monolayer DLC coatings increased from 0.63 to 1.22 due to the increase of substrate bias voltage from −102 V to −1200 V. This indicated an increase of graphite-like (sp2) content in the coating, and caused the drop in hardness from 35.73 GPa to 24.49 GPa and Young’s modulus from 340.33 GPa to 281.02 GPa. Residual stress also decreased from 4.84 GPa to 1.02 GPa due to annealing effect from the high substrate bias voltage. As the hardness and Young’s modulus decreased, the fracture toughness decreased from 1.7 MPa.m1/2 to 0.8 MPa.m1/2, coating wear rate and friction coefficient increased. With the structure of soft and hard sublayers, the residual stress of the multilayer coatings were kept around 2 GPa but have a reduced hardness and Young’s modulus around 25 GPa and 280 GPa respectively. It also slightly increased the fracture toughness to 1.8 MPa.m1/2, decreased coating wear rate and friction coefficient As the Id/Ig ratio of the multilayer coatings with different sublayer thickness varies from 0.77 to 1.05, indicating the difference in graphite-like (sp2) content in the coatings was not as large as monolayer coating. Therefore, from the above mentioned, i can said that they have similar properties. In overall, multilayer DLC coating has a better performance as a protective coating than monolayer DLC coating when the requirement of hardness and Young’s modulus are not very high. As multilayer DLC coating has lower residual stress, coating wear rate and friction coefficient therefore the working time of the protective coating will be longer and cost saving.