This Small Business Innovation Research (SBIR) Phase I project aims to use nitrocarburizing plasmas to generate nanostructured layer on top of nitride case on cast iron surfaces of cast iron dies used in the stamping of high-strength-steel sheets. To deposit nanostructured coatings, reactive carbo-nitro-hydride ion species will be created by using low deposition pressures and temperatures in industrial size equipment. The deposited coatings will be evaluated for their mechanical properties and tribological performance under conditions replicating those in actual stamping operations.
The broader/commercial impact of this project will be the potential to provide high-hardness and low-friction nanostructured coatings on the nitride to significantly enhance the tribological performance of the cast iron dies surfaces, realizing significant savings for the stamping industry. Cast iron dies are extensively used in the stamping of automotive body parts. Recently, the service lives of these dies have decreased substantially with the introduction of advanced high strength steels (AHSS) and ultra high strength steels (UHSS) in stamping operations. This has led to costly downtimes and significant die repair costs. This technology is expected to dramatically improve the tribological performance of duplex surfaces, thus extending the lifetime of cast iron dies.
Automotive stamping dies are very large in size with sizes ranging up to 2m x 2m x 1m. The primary material used in the manufacturing of these dies is gray cast iron that is relatively cheap with good castability, machinability and reparability. A typical car chassis (commonly referred to as body-in-white) has around 300 stamped parts requiring around 750 stamping dies. These dies weight around 20 tons to 50 tons. If gray cast iron is used instead of steel, typical savings due to material cost, machining, polishing etc. amount to $50,000-$100,000 per die. This translates to a saving of about $2-3 million per car model. Given that a new model sells about 100,000 cars, this can result in a saving of about $200-$300 per car for the manufacturer. With the drive towards quick model changes, a high performance cast iron die can save $$ millions per year for a stamping company. Die wear is a major problem at highly loaded die features such as corners, bead-radii, punch-radii etc. Worn dies are re-welded, re-machined, reground and put back into service. The greater this wear, the greater is the die-related down times and production losses. An estimated $20,000 - $30,000 per die is spent on welding, re-machining and tryouts during its lifetime. 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. These are commonly used in modern fuel efficient vehicles like Chevy Cruze, Volt etc. Since the technological and the scientific challenges in developing a nanostructured approach for tribo-friendly treatments surfaces for the cast iron stamping dies are formidable, novel surface treatment architecture was investigated in Phase I research that employs a gradient design strategy. In this strategy the low friction thin nanostructured surface is supported by a hard compound zone that in turn is supported by a large diffusion zone. This triplex structure provides for gradual transition of hardness and elasticity gradients from the surface to the softer substrate. In the phase I research, we established the feasibility of using DC-Pulsed plasma industrial-chambers in creating C-Si based nanostructured triplex surface treatment that provide factors of magnitude (>5X) improvement in wear performance of cast iron substrates. In addition to stamping, plasma nitriding is widely used to generate a hard case on engineering surfaces for enhanced performance of many tribological components. Recently this process has found wider applications in advanced surfaces in aerospace, electronics and power systems for corrosion protection. In addition, plasma nitride surfaces are being investigated for the emerging areas of alternate power generation such as fuel cells and in human implants. In many of these emerging areas the substrate is gray cast iron: bipolar plates for PEM fuel cells, interconnect for SOFC etc. Therefore, the potential impact of this research is not only in the current fields of application but also the emerging areas vital to the energy and industrial future of our country.
