Polycrystalline ultra hard material cutting elements include an ultra-hard cutting layer, such as polycrystalline diamond (“PCD”), polycrystalline cubic boron nitride (“PCBN”) or thermally stable polycrystalline ultra hard material (“TSP”), formed over a substrate such as a cemented tungsten carbide substrate. The substrate is typically a body of cemented tungsten carbide with the tungsten carbide particles cemented together with cobalt. Such cutting elements have well-known applications in the industry.
Many commercially available polycrystalline ultra hard cutting elements, such as PCD cutting elements, are formed in accordance with the teachings of U.S. Pat. No. 3,745,623, the contents of which are fully incorporated herein by reference, whereby a relatively small volume of ultra hard particles is sintered in a thin layer of approximately 0.5 to 1.3 mm onto a cemented tungsten carbide substrate. While the teachings of U.S. Pat. No. 3,745,623 utilize a belt press in the disclosed sintering process, it is also known that a cubic press or a Piston-Cylinder (PC) press may also be used.
Generally speaking, the process for making an ultra hard cutting element includes placing the carbide substrate adjacent a layer of ultra hard material particles, as for example diamond, and subjecting the substrate and the ultra hard layer to high temperature and high pressure conditions where diamond is thermodynamically stable. These temperatures are in the range of 1300°-1600° C. and the pressures are in the range of 5.5 GPa. This process is also known as a high pressure, high temperature “HPHT” sintering process. This process results in re-crystallization and the formation of a polycrystalline ultra hard material layer on the surface of the tungsten carbide substrate.
When the ultra hard material layer is a PCD layer, the layer may include tungsten carbide particles and/or small amounts of cobalt. The cobalt particles may be mixed with the diamond particles prior to sintering, and/or may infiltrate the diamond layer from the cemented tungsten carbide substrate during sintering. Cobalt is used as a catalyst material to promote the formation of polycrystalline diamond during the HPHT sintering process.
After sintering, the PCD layer has a structure of individual diamond crystals that are interconnected to define a lattice structure. Cobalt particles from the pre-sintered diamond mixture and/or from the substrate are often found within the interstitial spaces in the diamond lattice structure, between the bonded diamond crystals. However, cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Heating of the polycrystalline diamond, such as by frictional heating during operation of the cutting element, causes the cobalt to expand more than the diamond. This relative expansion can cause cracks to form in the diamond lattice structure, resulting in the deterioration of the PCD layer. TSP can be created by removing the cobalt from the diamond lattice structure, such as by leaching. After leaching, the TSP layer becomes more heat resistant. However, the TSP layer also becomes more brittle. Accordingly, in certain cases, only a select portion, measured in depth and/or width, of the PCD layer is leached to form TSP, in order to gain thermal stability without losing impact resistance.
TSP material may also be formed by forming polycrystalline diamond with a thermally compatible silicon carbide binder instead of cobalt. “TSP” as used herein refers to either of the aforementioned types of TSP materials. Use of TSP cutting layers in cutting elements is described in U.S. Pat. No. 7,234,550 and in application Ser. No. 10/967,584 filed on Oct. 18, 2004, which are fully incorporated herein by reference.
The current methods of forming ultra hard material cutting elements generate significant residual stresses at the interface between the ultra had material layer and the substrate. These stresses are caused by the mismatch between the coefficient of thermal expansion of the ultra hard material layer and that of the substrate. The higher the heat that the ultra hard material and the substrate are exposed to during sintering, the higher the residual stresses, as the relative expansion of the ultra hard material and the substrate is exaggerated by the higher heat.