Erosive wear occurs when a surface is exposed to a flow of material in a fluid. Particles within the fluid impact on the exposed surface and impart some of their kinetic energy into the exposed surface. If sufficiently high, the kinetic energy of the impacting particles creates significant tensile residual stress in the exposed surface, below the area of impact. Repeated impacts cause the accumulation of tensile stress in the bulk material that can leave the exposed surface brittle and lead to cracking, crack linkage and gross material loss.
Erosive wear is a cause for concern in applications as diverse as hydroelectric turbines, jet engine turbine blades, aircraft surfaces and wellbore drilling and stimulation environments. Each situation has its own particular challenges in mitigating erosive wear. Hydroelectric turbines are subject to high velocity flows of water mixed with various amounts of silt and sand. Jet engine turbine blades are subject to flows of superheated, high velocity gases. Aircraft surfaces must withstand high speed movement through air particulates such as rain, ice, dirt, and acidic pollution. Tools and equipment for wellbore exploration, including drilling and formation stimulation, are subject to a constant flow of mud and sand.
Typically, components that are exposed to erosive flow are subject to various hardfacing treatments to improve erosion resistance. Such treatments often include either surface preparations that harden and smooth the base material itself or bonding erosion resistant materials to the surface of the base material. Surface preparations can often make the base material more resistant to impact from particles with low kinetic energy, but these same preparations can leave the base material more brittle and thus susceptible to cracking as a result of impacts from high kinetic energy particles. Also, such surface preparations are usually applied using high-temperature processes, thus limiting their applicability only to high-temperature resistant materials such as metals and ceramics. Bonding of erosion resistant materials is typically performed using thermal spray techniques such as High Velocity Oxy-Fuel (HVOF) or Air Plasma Spray (APS). These techniques use a fuel/oxygen mixture or a DC arc to melt a metal powder and spray it onto the surface to be coated. As such, high-heat bonding techniques are amenable only for use on high-temperature resistant materials. Further, in highly erosive environments, the residual tensile stress that results from multiple impacts can accumulate at the junction of a base material and its bonded coating, leading to delamination of the coating material.
An addition issue arises when components of a device are difficult to access once put in place. For example, many devices are manufactured to be replaceable (e.g., the device may be welded, snapped or riveted together) rather than serviceable. In other instances, a device may be permanently placed in an inaccessible location, being intended to serve reliably for the lifetime of the structure (e.g., devices cemented into structure walls). In such cases, it is common to “over-design” the component such that it can reliably perform its function for the life of the device, even if the component is badly eroded. As a result, the cost of design and manufacture of such components may be significantly increased, along with their size and strength.
Erosion control is of particular concern in wellbore operations. During wellbore drilling, a drilling mud, usually consisting of significant amounts of solids such as sand, chert or other rock suspended in water, is constantly pumped into the wellbore at velocities that can exceed 50 meters per second. The drilling mud provides cooling to bottomhole assemblies, hydraulic horsepower to mud motors that rotate the drill bit, and a medium for removing the cuttings. In this environment, the mud motor rotor and stator are subject to significant erosive forces, as are the drill bit and particularly the shirttails (the exposed outer face of the roller cone-bearing journal).
Likewise, in wellbore completions, such as gravel packing or fracturing operations, a slurry of particles suspended in a liquid are pumped under high pressure into the wellbore. In gravel packing, gravel of various sizes is pumped into an angular flow diverter to pack the annulus between the wellbore and the casing with gravel, to prevent the production of formation sand. In fracturing, the slurry includes a propant, typically sand, that is pumped into the formation to stimulate low-permeability reservoirs. Here, the angular flow diverters are subject to erosive wear.
Because of the harshly erosive environment of wellbore operations, significant effort and expense is expended to mitigate erosive loss and improve wellbore tool and equipment life. Hardfacing treatments, as described above, are used extensively to protect a wide array of wellbore tools. Also, wellbore tools and equipment are often over-designed to provide adequate service life. Additional steps are often taken to treat the fluids to make them less erosive. However, all of these steps routinely prove inadequate to provide sufficient protection from erosion, and wellbore operations are often interrupted to replace broken tools that were unable to withstand the prolonged stress.
From the foregoing it will be apparent that there is a need for an improved method of providing erosion resistance to components exposed to a flow of erosive material.