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Study of Mechanical Properties of Cr-C Coatings by Using the Unbalanced Magnetron Sputtering

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[[abstract]]The chromium carbide (Cr-C) films have excellent properties including chemical stability, oxidation resistance, low coefficient of friction and wear resistance. In this study, the Cr-C films are deposited on high speed tool steel (SKH51) by the reactive unbalanced magnetron sputtering process. Meanwhile, a gradient interlayer consisted of Cr / CrN/ CrCN between the Cr-C layer and the substrate is prepared for increasing the adhesion strength. In experimental design, the L18(21×37) orthogonal array experiments are used. Meanwhile, the effects of the eight control factors on wear resistance and anti-sticktion properties of the Cr-C films are also investigated. The results indicate that optimization experiments of the coefficient of friction could be confirmed and decreased from 0.557 to 0.254, compared to the undeposited substrate.. Compared to the initial condition, the maximum gain was 7.45 dB. Optimization experiments of the water contact angle experiments can be progressed and that the average water contact angle increased from 73.40° to 109.05°, compared to the undeposited substrate. It could effectively reduce the surface tension, and increased anti-stickting properties of the surface. Effects of significant factors (Cr target current, C2H2 flow rate ratio) on structure and properties of the films are investigated by one-by-one factorial experiments. When the target current is below 2 A, poisoning of the target increases, the films start to become the amorphous diamond-like carbon (DLC) with carbides structures. However, performed the lowest coefficient of friction and wear rate were 0.222 and 0.06×10-4×mm3×N-1×m-1, respectively. It could be effectively reduce the coefficient of friction ,and enhance the wear resistance property. When the target current increased, the water contact angle increased. and performed the highest water contact angle was 98.30°, and this result was opposite to the results of tribological properties. The coefficient of friction and wear rate linearly decrease with the increase of C2H2 flow rate ratio. The coefficient of friction decreased from 0.630 to 0.285, and the wear rate decreased from 2.73 to 0.03 (10-4×mm3×N-1×m-1). These results showed that film substrate transforms to amorphous DLC, the phenomenon of graphite increased with the increase of C2H2 flow rate ratio. When the films structures were the DLC with mixed carbides Cr3C2+Cr7C3, it could be effectively reduced the wear condition, and improved wear resistance of properties. The water contact angle was more than 93° when C2H2 flow rate ratio below 30 %. In the final step, the multi-objective experiments for “low coefficient of friction and high water contact angle” were carried out. By calculated of the gray relational grade, it could be get the best experimental parameters for multi-objective responses, which were argon cooling time after deposition at 20 min, bias of substrate at -100 V, chromium target current at 2 A, C2H2 flow rate ratio at 30 %, ion bombardment voltage at -500 V, work distance at 13 cm, frequency of substrate at 100 KHz, and rotation speed of substrate holder at 7 rpm. However, the percentage contributions of the sum of the work distance, chromium target current and C2H2 flow rate ratio is performed as 66 %. The performance results of the verification experiment for the multi-objective process parameters were shown as follows: the friction coefficient decreased from 0.656 to 0.234, and the water contact angle increased from 83.17 to 96.25˚, compared to the undeposited subtrates. In addition, the multiple performance index gain was 49% positive as compared to the initial condition. Compared all the properties data, this method can balance the requirement of each property of the films, and saved the cost and time of experiments. And design of the Cr / CrN / CrCN intermediate layer configuration can enhance the adhesion ability between the Cr-C films and the substrates, and the critical load (Lc) is higher than 60 N, which confirms that this method can effectively increase the interface bonding strength.

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