Study On The Residual Stresses And Cracking Of Diamond Film Produced By DC Arc Plasma Jet

The unique characteristics of diamond film make it extremely attractive for numerous applications in machining, aerospace, microelectronics and so on. But during the process of producing diamond film by chemical vapor deposited technology, such as DC arc plasma jet, the problem of film cracking caused by excessive residual stresses has not been well solved yet. The objective of this work is to study the complicated multi-field coupling variables of electricity, magnetism, heat, mechanics inside and outside the plasma torch, and the stresses in diamond film based on thermal-mechanic coupling. The flowing and conducting heat of plasma were simulated, and the stresses existed in the various stages of self-standing diamond thick film were also simulated. Moreover, the cause and influence factors of diamond film cracking were revealed.The principal researches are as the follows:(1) A coupling magneto-hydrodynamic (MHD) model of magnetic controlled DC plasma torch, which was used for diamond film deposition, was presented. The model includes Navier-Stokes and energy equations modified by the addition of some source terms, which reflect the Lorentz force due to self-induced and external magnetic fields, radiative cooling, joule heating, and the diffusive and convective enthalpy fluxes due to the temperature, pressure, electric field, and concentration gradients of Ar-H₂ mixture plasma. In addition, the mass conservation and species conservation equation, generalized ohm's law, and Maxwell's equations were also modeled. By reprogramming the software FLUENT, the author effectively simulated the fluid field, temperature field and electromagnetic field in the MHD coupling model. These fields in the torch were obtained respectively with and without external magnetism, and the radial distributions of velocity and temperature at the outlet of the torch were also obtained.(2) Based on the results of these fields mentioned above ,a computational fluid dynamics model was established for the plasma jet spraying from the torch to the deposition chamber by taking the outlet conditions of velocity and temperature at the torch exit as their inlet conditions of the plasma jet. The flowing and conducting heat of plasma jet over the diamond film were simulated by using the reprogramming software FLUENT. The velocity field and temperature field of plasma jet were obtained, and the radial distribution of temperature on the upside of diamond film was also obtained, which laid a foundation for analyzing the thermal-mechanic coupling in the diamond film later.(3) A transient thermal-mechanic coupling model was adopted in the cooling process of film/substrate system. In this model, the coupling effect between temperature and strain was considered. A finite element simulation was made about the temperature field and thermal stresses fields during the cooling process of film/substrate system and about the thermal residual stresses fields at indoor temperature. The uneven initial temperature condition used for the simulation during the cooling period of the film/substrate system came from the calculated result of the coupling multi-field on real production condition. Both the thermal expansive coefficient difference of film/substrate material and the non-uniformity of temperature at each point in film/substrate system were reflected in the simulated results of thermal stresses and thermal residual stresses. Therefore, the simulated results of stresses are more accurate. Based on these simulated results, the cause of detachment and cracking of diamond film were analyzed, and the actual damage phenomenon was rationally explained. (4) The redistribution of thermal residual stresses in self-standing thick diamond film was studied by using "life or death element" technology while the film separated from substrate. So, the release of thermal residual stresses was obtained quantitatively. A useful suggestion about how to compute intrinsic stresses was made.(5) Some influence factors of residual stresses and cracking damage of diamond film were studied. These research achievements have considerable reference values for the control of residual stresses, the improvement of producing technology, and the increase of diamond film finished product ratio.