DUCTLE REGIME MACHINING OF CERAMICS
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HIGH PRESSURE INDUCED DUCTILE REGIME MACHINING OF SILICON NITRIDE Advanced structural ceramics have been increasingly used in automotive, aerospace, military, medical and other applications due to their high temperature strength, low density, thermal and chemical stability, and good wear resistance. Due to high hardness and brittleness of ceramics, machining techniques such as diamond grinding, polishing result in surface cracking and subsurface damage. Under certain controlled conditions,it is possible to machine brittle materials using single point diamond tooling so that material removal is by plastic deformation, leaving a damage free surface. This process is termed as 'ductile regime machining'. The goal of present work is to explore this ductile regime machining of ceramics,silicon nitride in particular,as a potential method of ceramic finish machining. It is believed that high pressure induced at the cutting tool and workpiece interface zone causes ductile mode material removal. To understand this phenomenon, research is being carried out at The University of North Carolina at Charlotte (UNCC), North Carolina State University, Western Michigan University, Oak Ridge National Laboratory and University of Tennessee. At UNCC, diamond turning experiments are carried out on Diamond Turning Machine using single and polycrystalline diamond tools. Silicon nitride sample were machined at depths ranging from 250nm to 10microns. Force data is collected and surface roughness characteristics are analyzed for ductile regime machining.Chip morphology is also studied for the machined depths using Scanning Electron Microscopy. In addition, cutting tools are inspected using Optical Microscope to correlate tool wear to forces and ductile/brittle behavior. Experimental studies indicate that there is a possibility of ductile material removal for micron and submicron level depth of cuts. The machining process is numerically modeled using commercial metal cutting FEA software AdvantEdge.The conditions conducive to ductile/brittle transition are dealt in the simulations. Pressure induced phase transformation is a possible mechanism for ductile behavior at low depths of cut. Simulations are run for depths ranging from 40microns to 500nm and the pressure distribution at the tool and workpiece interface isstudied. A parametric study is carried out in the simulations to study the effect of speed,depth of cut and pressure sensitivity of Silicon Nitride, on theductile behavior. The experimental results indicate that brittle-to-ductile transformation is possible at a depths of 1-5micrometers and the material behaves in ductile fashion at depths less than 1micrometer. Numerical results show that high pressure phase transformation is likely to take place in the interface zone at about the hardness of Silicon Nitride at 5microns and 1 micron depths of cut . The force data from the experimental and numerical analyses are correlated by taking the ratio of Cutting Force to Chip Area into consideration.
ME&ES	College of Engineering	UNC-Charlotte

DUCTLE REGIME MACHINING OF CERAMICS

HIGH PRESSURE INDUCED DUCTILE REGIME MACHINING OF SILICON NITRIDE

  Advanced structural ceramics have been increasingly used in automotive, aerospace, military, medical and other applications due to their high temperature strength, low density, thermal and chemical stability, and good wear resistance. Due to high hardness and brittleness of ceramics, machining techniques such as diamond grinding, polishing result in surface cracking and subsurface damage. Under certain controlled conditions,it is possible to machine brittle materials using single point diamond tooling so that material removal is by plastic deformation, leaving a damage free surface. This process is termed as 'ductile regime machining'. The goal of present work is to explore this ductile regime machining of ceramics,silicon nitride in particular,as a potential method of ceramic finish machining.

  It is believed that high pressure induced at the cutting tool and workpiece interface zone causes ductile mode material removal. To understand this phenomenon, research is being carried out at The University of North Carolina at Charlotte (UNCC), North Carolina State University, Western Michigan University, Oak Ridge National Laboratory and University of Tennessee. At UNCC, diamond turning experiments are carried out on Diamond Turning Machine using single and polycrystalline diamond tools. Silicon nitride sample were machined at depths ranging from 250nm to 10microns. Force data is collected and surface roughness characteristics are analyzed for ductile regime machining.Chip morphology is also studied for the machined depths using Scanning Electron Microscopy. In addition, cutting tools are inspected using Optical Microscope to correlate tool wear to forces and ductile/brittle behavior. Experimental studies indicate that there is a possibility of ductile material removal for micron and submicron level depth of cuts. The machining process is numerically modeled using commercial metal cutting FEA software AdvantEdge.The conditions conducive to ductile/brittle transition are dealt in the simulations. Pressure induced phase transformation is a possible mechanism for ductile behavior at low depths of cut. Simulations are run for depths ranging from 40microns to 500nm and the pressure distribution at the tool and workpiece interface isstudied. A parametric study is carried out in the simulations to study the effect of speed,depth of cut and pressure sensitivity of Silicon Nitride, on theductile behavior.

  The experimental results indicate that brittle-to-ductile transformation is possible at a depths of 1-5micrometers and the material behaves in ductile fashion at depths less than 1micrometer. Numerical results show that high pressure phase transformation is likely to take place in the interface zone at about the hardness of Silicon Nitride at 5microns and 1 micron depths of cut . The force data from the experimental and numerical analyses are correlated by taking the ratio of Cutting Force to Chip Area into consideration.

ME&ES

College of Engineering

UNC-Charlotte