Effect of the Cu-substrate thickness on the hardness variation of flame-heated Cr-C deposits

Cr-C deposits were electroplated on a pure Cu substrate in a Cr+3-based plating bath. After electroplating, the Cr-C deposits with and without a Cu substrate were flame-heated for 2 s to increase their hardness. Experimental results show that the hardness and crack density of a flame-heated Cr-C deposit were strongly affected by the Cu-substrate thickness. The hardness of as-plated Cr-C deposit with a Cu substrate increased from 750 to 1600 Hv after flame heating, while a low hardness value of 970 Hv was detected from the flame-heated Cr-C deposit without a Cu substrate. Increase in hardness values of flame-heated Cr-C deposits is attributed to precipitation of crystalline diamond membranes. Fully crystalline diamond-like membranes were found in the flame-heated Cr-C deposit with a Cu substrate; while semi-crystalline diamond-like membranes were detected in the flame-heated deposit without a Cu substrate. The constraint effect of the Cu substrate induced a high internal tension stress in the Cr-C deposit during and after flame heating, leading to widening through-deposit cracks and increasing the deposit hardness.