This Small Business Innovation Research (SBIR) Phase I project aims to improve the adhesion of nanocrystalline diamond coatings to micro-scale tungsten carbide cutting tools. Nanocrystalline diamond coatings have been shown to dramatically improve the machining performance of micro end mills. However, standard diamond growth methods result in weakened tool material and the coatings suffer from premature delamination, resulting in tool failure. The improved adhesion of the nanocrystalline diamond coatings will be achieved by implementing a new surface preparation technique that eliminates the need of acid etching which weakens the tool material, and seeding of nano-diamond particles which do not bond very strongly with the tool. Machining tests will be conducted to quantify the improvement in tool life, hence coating adhesion, with the new surface preparation technique.
The broader/commercial impact of this project will be the potential to provide a new surface preparation technique that can be integrated with standard chemical vapor deposition systems to allow for high throughput and more economical diamond coatings for industrial applications. Diamond coated micro end mills are in demand to improve machining performance and enable the machining of products from otherwise un-machinable materials. The improved cutting performance of micro end mills by diamond coatings can only be realized if it is thin enough to not significantly alter the tool geometry and it strongly adheres to the substrate. The objective of this project is to address these challenges by developing continuous diamond coatings less than 100 nanometers thick for micro tools and a method to prevent premature coating delamination.
A newly developed method has been shown to improve the life of nanocrsyatlline diamond (NCD) coated tools 5-10X over tools prepared by the standard acid etch and seeding method. This preparation method was to be considered successful if the cutting distance (i.e., tool life), prior to gross coating delamination, is twice (2X) that of the standard surface preparation method (i.e., etching and seeding). The improvement in tool life is attributed to maintaining the structural integrity of the tool while creating a direct chemical bond of the coating to the tool surface. The standard surface preparation technique of acid etching WC for the removal of Co results in the embrittlement of the substrate causing it to fail, resulting in premature coating delamination. After etching, the substrate is seeded to improve the nucleation density of the surface. These seeds initially attach to the substrate via van der Waal’s forces. If there is no chemical bonding between the coating and the substrate, the adhesion strength of the coating is equal to the number of seeds multiplied by the van der Waal’s interaction force of each seed normalized by the area of the coating. The new method is very effective in preventing Co from negatively affecting the diamond deposition. This process removes Co from the substrate surface while creating a diffusion barrier preventing out diffusion during diamond synthesis. The modified layer creates high nucleation for diamond growth, eliminating the need for seeding the substrate prior to diamond deposition. Direct nucleation allows the coating to chemically bond to the surface increasing the adhesion strength of the coating to substrate. Unlike acid etching, this new method does not negatively affect the structural rigidity of WC-Co micro end mills. This results in well-adhered NCD coatings capable of prolonged, burr-free milling without significant cutting edge failure. The method is shown to be a superior surface preparation technique for synthesizing diamond coatings for WC micro end mills. Coating performance was evaluated by conducting machining tests until coating failure, resulting in increases in machining forces. These tools significantly reduce the cutting and thrust forces during machining while also producing a more predictable surface finish. Coated tools prepared via the new method experience much less wear when machining, extending the tool life much beyond uncoated tools as well as tools coated using other procedures. The new process produces tools with less embrittlement than etching and improved wear resistance than as received WC tools. These improvements result in large cost reductions for machining operations. Extending a tool’s life up to and beyond 10x not only removes the cost of 9 tools, but the time associated with set up and alignment of those 9 other tools. Maintaining the cutting edge radius allows the tool to cut with lower forces and higher tolerances, improving productivity. Lower cutting forces allow the increase of machining speeds further reducing the time required for manufacturing a part. Maintaining the tool diameter and improved surface finish require less part metrology and enable the production of higher tolerance parts.
