Modified surface properties of percussion tools used in downhole drilling

A downhole percussion tool, comprising:

• a casing comprising a top end, a bottom end, and an internal surface extending from the top end to the bottom end, the internal surface defining a casing passageway extending longitudinally therethrough;
• a top sub comprising a top end coupleable to a drill string and a bottom end coupled to the top end of the casing;
• a mandrel being supported within a lower portion of the casing;
• a piston slidably mounted within the casing passageway above the mandrel and moveable to deliver an impact force onto the mandrel, the piston comprising:
• an interior wall extending from an upper surface of the piston to a lower surface of the piston and defining a piston passageway extending longitudinally therethrough;
• and
• an exterior wall surrounding the interior wall and extending from the upper surface of the piston to the lower surface of the piston, the exterior wall and the casing being positioned in close fitting relationship;
• one or more portions of the internal surface of the casing or the exterior wall of the piston has been modified using a ferritic nitrocarburization process;
• a rotary bit coupled to the mandrel and extending outwardly from a bottom portion of the mandrel, and
• wherein:
• each of the top sub and the mandrel has a respective passageway extending longitudinally therethrough and in fluid communication with the piston passageway for supplying pressurized fluid from the drill string to the rotary bit,
• the rotary bit has a bit passageway extending therein and in fluid communication with the mandrel passageway,
• the piston further comprises:
• an inlet port formed in the exterior wall of the piston and an inlet passage extending from the inlet port to the lower surface of the piston, and
• an exhaust passage extending from the piston passageway to the exterior wall of the piston,
• the downhole percussion tool further comprises a tube extending from an upper portion of the mandrel,
• a base portion of the piston passageway is enlarged to receive the tube when the piston is engaged with the mandrel,
• the exhaust passage extends from the base portion of the piston passageway, and
• one or more portions of the internal wall of the piston or the exterior wall of the tube has been modified using a ferritic nitrocarburization process.