Reducing Chemical Wear of Single Crystal Diamond Tools Cutting Alloys

University Of North Carolina At Charlotte, Charlotte NC

Investigators

Abstract

Next-generation freeform optical components are often made from metal molds which themselves are cut on high quality machines using synthetic single crystal diamonds (SCD). Despite being the hardest known tool material, diamond wears surprisingly quickly when cutting some metals due to chemical interactions. This research will investigate aspects of those interactions, such as temperatures in cutting which are extremely difficult to measure at the length scale of the diamond/workpiece interaction. The temperatures can be predicted, using models with some embedded assumptions; methods developed for microsystems manufacturing will be used to deposit temperature sensors on the diamond tool surfaces and measure temperature distributions. These measurements will be used to validate the models. Chemical wear rate as a function of diamond orientation will be measured using specially manufactured tools and as a function of oxygen concentration by machining in a controlled-atmosphere chamber. Quantitative prediction of diamond wear rates could enable cost effective production of longer life optical molds in alloys not currently considered "diamond machinable". Such molds will enable manufacturing of next generation structured and freeform optical surfaces for applications ranging from energy efficient illumination to compact imaging systems for virtual reality, heads-up displays, and night vision. Improvements in the scientific understanding of diamond machining can positively impact a number of manufacturing sectors, including automotive, aerospace, consumer electronics and defense. Technology dissemination will be through publications, professional societies, industry consortia such as the NSF I/UCRC Center for Freeform Optics, and through educational activities. UNC Charlotte is a minority serving institution, and therefore outreach to underrepresented minorities for undergraduate and graduate research opportunities should be fruitful. Results will be integrated into graduate and undergraduate courses. In sophomore manufacturing, "cool" applications such as heads-up displays and future generations of virtual reality spark interest, while the example that the hardest material cannot cut one of the softest metals (cerium) helps keep the students engaged. This project will seek an improved fundamental understanding of the thermal, chemical and mechanical conditions leading to single crystal diamond tool wear during metal alloy machining, with the goal of allowing process improvement. Diamond tool tip temperatures are a significant factor in chemical tool wear, but they are notoriously difficult to measure during cutting. Validated thermal models are required. Validated temperatures will be used to determine activation energies for the chemical wear reaction using the Arrhenius equation, providing predictive understanding of tool wear under different conditions. The high thermal conductivity of diamond significantly affects modelled temperature distributions and should therefore affect the reaction rates. Synthetic diamonds are now being produced with thermal conductivities around 30% higher than previously available. Wear of tools made from diamonds with measured, different thermal conductivities will be tested by using diamonds sourced from different suppliers and produced through different means. The crystallographic orientation of a diamond affects mechanical wear and diamond etching rates by some molten metals. To determine the effect of diamond orientation on chemical wear, the wear rates for SCD tools of varying orientation when cutting reactive alloys will be tested. An environmental chamber enclosing a turning operation with a diamond tool will address the importance of oxygen on diamond wear. Freeform optics are the next revolution in optical capabilities. Their use in illumination improves energy efficiency and reduces light pollution. More compact, higher performance imaging systems enable smaller, lighter systems ranging from multispectral military systems to CubeSat missions. The scientific understanding of the manufacturing process developed here will enable an ever broadening range of cost effective applications. The results will be disseminated through industry interactions and publications/presentations at the Optical Society of America, SPIE (the international society for optics and photonics), the American Society for precision Engineering, and CIRP (The International Academy for Production Engineering). UNC Charlotte is dedicated to recruitment and retention of under-represented groups who will encounter the results of this research in undergraduate (Manufacturing Systems (core ME) and Metrology and Precision Engineering (elective)) and graduate classes (Introduction to Optical Fabrication and Testing, Advanced Surface Finish).