DC pulsed plasma deposition of nanocomposite coatings for improved tribology of gray cast iron stamping dies

Automotive stamping dies are very large in size with sizes ranging up to 2 m × 2 m × 1 m. The primary material used in the manufacturing of these dies is gray cast iron that is relatively cheap with good castability, machinability and reparability. Due to its lack of adequate hardness, die wear is a major problem at highly loaded die features such as corners, bead-radii, punch-radii etc. The greater is this wear, the greater is the die-related down times and production losses. This problem of die wear has become even more significant in recent times due to the introduction of Advanced High Strength Steels (AHSS) and Ultra High Strength Steels (UHSS) in automotive stamping. Stamping of these materials is accompanied by higher contact pressures on the die surface, higher abrasion and higher surface temperature from frictional heat. This paper presents a novel DC pulsed plasma based approach to the deposition of triplex coating on cast iron substrates that can withstand high normal pressures, shears, sliding and abrasion experienced during stamping. This triplex coating consists of silicon enhanced nanocomposite a-C:H film supported by plasma nitrocarburized duplex case. This nanostructured-composite film is shown to have high toughness and extremely low coefficients of friction. A unique feature of this plasma processing is the industrial size of chambers used in the coating development. This paper also includes results of film characterization and its performance evaluation in simulated wear tests that generate stresses similar to actual stamping dies. By carefully designing the plasma processing treatment, a good hardness gradient and elastic modulus gradient is achieved that provide an improvement in life of about 5 times that of the currently used hard chrome plating.