Effect of thickness of polycrystalline diamond layer and diameter of compact on residual thermal stress of PDC

Objectives: Polycrystalline diamond compact (PDC) is an ultra-high hardness composite material made from polycrystalline diamond (PCD) and cemented carbide through high temperature and high pressure sintering. PDC is widely used in oil and gas extraction, geothermal development, and coal field drilling. As drilling goes deeper, the formation pressure increases, and the geological rock layers become denser, harder, and more abrasive. Therefore, the comprehensive performance requirements for the drill bit and PDC are increasingly high. The presence of residual thermal stress can significantly weaken the performance of PDC, causing the PCD layer to fracture and detach, which is one of the important factors leading to the premature failure of PDC. Previous research methods for determining the residual thermal stress in PDC have certain limitations. Finite element simulation calculations can effectively compensate for these shortcomings. Therefore, this paper uses the ANSYS Workbench software to calculate and analyze the influence of the thickness ratio of the PCD layer to the cemented carbide layer in PDC and the diameter of PDC on residual thermal stress, providing references for the optimization of PDC design and performance improvement. Methods: ANSYS is one of the most commonly used finite element analysis software, which can effectively integrate multiple disciplines such as structural dynamics, thermodynamics, and fluid dynamics for simulation calculations. Using ANSYS, the model of PDC (assuming that temperature does not affect the physical properties of materials) can be established, and the residual thermal stress of PDC can be analyzed using the thermo-mechanical coupling method (which solves the impact of the temperature field on stress, strain, and displacement in the structure). The specific calculation process in this study is as follows: (1) Select the calculation method for steady-state temperature field and structural mechanics field coupling. (2) Establish the geometric model of PDC. According to the axisymmetric characteristics of PDC, establish its 1/4 structure to save computational space. (3) Define the physical and mechanical properties of the PCD layer and the cemented carbide layer, and then perform mesh division. (4) Set boundary conditions and loads, using the temperature difference as the load, setting the reference temperature including the stress relaxation temperature of PDC, the axisymmetric boundary conditions of the model, as well as the heat convection boundary conditions between the outer surface of PDC and the air. (5) Perform result calculation and analysis. Results: Using the software to simulate the calculation of the residual thermal stress value and distribution during the unloading and cooling process of PDC, the following conclusions can be drawn: (1) The ANSYS simulation calculation shows that when the diameter of PDC is 16 mm and the total thickness is 13 mm, the optimal thickness of the PCD layer is 2.0 mm; (2) When the PCD layer thickness is 2.0 mm, the diameter of the composite piece can be selected as 18 mm, and the residual thermal stress of this specification of PDC is at the best value in the calculated range. When the PCD layer thickness is 3.0 mm, it cannot be decided based on a single residual thermal stress influence; this must rely on the specific application situation and the load condition, with a comprehensive consideration of the influence of the four residual thermal stresses; (3) The point of PDC diameter of 17 mm is one of the many fluctuation points, which may be a critical point. At this time, the radial displacement of the interface far from the center axis of PDC changes abruptly, causing the deflection of the entire PDC to change, and the axial tensile stress at the edge of the interface changes. Conclusion: The finite element calculation method can intuitively and clearly simulate the value and distribution of residual thermal stress during the unloading and cooling process of PDC, and effectively avoid the shortcomings of other experimental tests. It can provide useful ideas and suggestions for the design of PDC by analyzing the influence of the two appearance sizes, composite layer thickness, and diameter on the residual thermal stress of PDC, and drawing relevant conclusions. Based on the outstanding results in the simulation calculation, the conclusions obtained are tested by experiments to ensure the reliability of the final results.