| The mechanism of catastrophic failures in CVD-processed diamond thick films during cooling after fabrication is investigated both theoretically and experimentally in order to develop systematic methodologies for crack prevention. Finite element calculations reveal that high thermal stresses concentrate at the round edge rim of thick film and tensile thermal stress develops in the edge region. They are thought to be the cause of crack initiation and propagation, respectively. However, the cracking as a stress relaxation mechanism is competing interface decohesion between diamond film and substrate. Hence, it is suggested that the cracking occurs conditionally although thermal stress exceeding the fracture strength of film is generated in the thick film. Stress distribution should be so configured that σ_(θθ)/ σ_(zz,i) exceeds the ratio of fracture strength of film to interface adhesion strength. On the basis of the cracking condition, three methodologies were suggested to prevent the cracking; minimization of thermal hoop stress and interface adhesion strength, and the increase of film thickness. This analysis on the cracking mechanism was verified by experimental results of diamond thick film deposited on Si, Mo and W substrates. In particular, even in high thermal stress, it is ascertained experimentally that low adhesion can prevent the cracking by introducing low adhesive interlayer. |
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