Fatigue Endurance and Hardness Characterization of DLC (DIAMOND-LIKE CARBON) Coating On HQ 805 Substrat

Abstract

Machinery steel HQ (High Quality) 805 is a group of machinery steels that are widely used as machinery components such as long shafts, crankshafts, gears, rods and pins. HQ 805 steel is a low alloy steel and has chemical composition (wt %) of 0.3673 C, 0.2273 Si, 0.296 S, 0.0093 P, 0.7136 Mn, 1.3080 Ni, 1.4031 Cr, 0.0099 Ti, 0.1585 Mo, 0.2184 Cu, 0.0073 W, 0.0142 Sn, 0.0045 Ca, 0.0127 Zn, 0.0142 Al and 95.48 Fe. Hardness and fatigue endurance of HQ 805 are necessary to be improved so that the lifetime of the components can increase or endure. These properties can be obtained by performing an appropriate surface treatment to HQ 805 material. DLC (Diamond-Like Carbon) is a new type of coating and it has high mechanical properties such as hardness (like diamond) and Young modulus. This DLC coating can be deposited on a substrate by CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition) technique. The objective of this research is to study and characterize the effect of parameters process of DLC coating on hardness, fatigue endurance and microstructure of HQ 805 material. The surface treatment with DLC coating was selected as a surface coating of HQ 805 in order to obtain the properties as needed. DLC coating was deposited on surface of HQ 805 by CVD plasma discharge technique. DLC coating derived from a mixture of argon gas (Ar) and methane (CH )has a ratio of 76 : 24 by volume. Parameters process of DLC coating were variation of pressure and deposition time. Pressure variations used were 1.2, 1.4 and 1.6 mbar with deposition time of 2, 4, and 6 hours. Operating temperature was maintained at 400 o C. The tests have been done on raw material specimens (HQ 805 without coating) and HQ 805 with coating. Surface hardness was characterized by Vickers micro hardness test with indentation load of 10 grams and indentation time of 10 minutes and fatigue endurance was obtained by rotary bending test machine with load variations. Microstructure analysis and composition were performed by SEM (Scanning Electron Microscopy)and EDS (Energy Dispersive X-Rays Spectroscopy) respectively. The results show that the parameters process of DLC coating such as pressure and time deposition is capable to modify the hardness and fatigue endurance significantly. The deposition pressure of 1.4 mbar results higher hardness compared to its hardness of coating obtained with deposition pressure of 1.2 and 1.6 mbar. The increasing of deposition time gives the increment of hardness. The highest hardness of 665 VHN (Vickers Hardness Number) is produced by a DLC coating with the pressure of 1.4 mbar and deposition time of 4 hours, while the base metal HQ 805 (without coating) has the hardness of 327 VHN. The surface roughness of specimen tested increases with increasing of deposition time. DLC coating with a deposition pressure of 1.4 mbar gives the smallest surface roughness compared with 1.2 and 1.6 mbar deposition pressure. HQ 805 substrate (without the DLC layer) has a fatigue endurance of 400 MPa. DLC coating on the substrate can increase or decrease the fatigue endurance depending on pressure and time deposition of coating used. DLC coatings with 1.2 and 1.4 mbar pressure produce the same fatigue endurance of 450 MPa for 2 hours time deposition. DLC coating with the pressure of 1.4 mbar and deposition time of 4 hours results a highest fatigue endurance of 512 MPa, and otherwise fatigue endurance drops dramatically (to 395 MPa) for deposition time of 6 hours. The general conclusion can be stated that the DLC coating on the substrate HQ 805 will provide the best fatigue endurance if the substrate is coated with a pressure of 1.4 mbar and deposition time of 4 hours.