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Ongoing & Future Work

Subsurface Damage Analysis for Single Point Diamond Turned CVD-Silicon Carbide and Quartz

Project Overview: The main idea of this topic of study is to investigate the subsurface damage of CVD coated SiC and Quartz after a Single Point Diamond Turning (SPDT) operation. The samples were investigated under an optical microscope after the machining operation and no signs of surface damage were identified. The surface roughness of all samples continued to improve after SPDT which further confirms ductile regime machining. However, at that point, no attempts were made in order to observe the subsurface conditions of the machined regions. The aim of this study is to make sure that ductile regime machining has not caused any subsurface damage such as voids and micro-cracks. If no surface damage is observed then this would confirm our initial assumption that since no surface damage is seen therefore subsurface damage will not occur. This would prove that the term/theory “ductile regime machining” is true for both surface and subsurface conditions. A total of three samples were investigated: (1) single point diamond turned CVD coated SiC, (2) single point diamond turned Quartz, and (3) laser ablated and single point diamond turned CVD coated SiC. Subsurface damage analysis was carried out on the machined samples using non-destructive techniques (NDT) such as Optical Microscopy, Raman Spectroscopy and Scanning Acoustic Microscopy (SAcM) to show evidence that the chosen material removal method leaves a damage-free surface and subsurface.


Determining the Significance of Crystal Orientation for Single Point Diamond Turned CVD 3C-Silicon Carbide

Project Overview: The main goal of this research was to determine the effect of crystal orientation relative to fracture damage, tool wear and friction in the manufacturing process. The proposed experimental program aims to quantify these observations, and result in a substantial increase in our knowledge relative to the effect of the chemically vapor deposited (CVD) crystal orientation on the ductile and brittle behavior of the material. Single point diamond turning (SPDT) was performed on a CVD coated silicon carbide (SiC) disk to improve its surface roughness.  The outcome of this project is an evaluation of the effects of crystal orientation in determining the resultant surface roughness, tool wear (due to friction) and material removal rate. Two non destructive techniques were used to study the crystal orientation of the material: (1) Orientation Imaging Microscopy (OIM) and (2) X-ray Diffraction (XRD). The results from XRD were preferred as it yielded in a stronger diffraction signal compared to OIM.

 


Analytical Modeling for Ductile Mode Machining of Brittle Materials

Project Overview: In this study, the mechanism of ductile chip formation in the ductile mode cutting of brittle materials is analyzed theoretically. It is understood that under the conditions where the undeformed chip thickness is at the micrometer or nanometer scales, extremely large compressive stress and shear stress is generated in the chip formation zone. The compressive stress largely reduces the stress intensity factor in the chip formation zone in such a way that the stress intensity factor is smaller than the fracture toughness of the workpiece material in the chip formation zone, such that crack propagations don’t occur in the chip formation zone. At the mean time, the dislocations occur in the material due to the large shear stress generated (due to the high pressures). The chip formation is thus dominated by dislocation rather than fracture. For the experimental verification of the proposed analytical model, cutting results of Si and SiC at various conditions are compared. The main objective of the proposed analytical model is to be able to accurately predict the correlation between the applied load (thrust force) and the resulting depth of cut for brittle materials (Si and SiC will be studied initially). The analytical model will assume a fully ductile material removal process (fracture and brittle mode machining will not be incorporated) where the elastic and plastic deformation in the material is theoretical studied.


The Effects of Laser Heating on the Material Removal of Silicon Carbide (4H, 6H and 3C)


Project Overview: A study is done thus far to compare the results of scratch tests done on single crystal 4H-SiC, with and without laser heating. The effects of laser heating were studied by verifying the depths of cuts for scratch tests carried out on single crystal SiC with increasing loads (thrust force) to  wherein the scratch shows both ductile and brittle response (with a ductile to brittle transition (DBT) region within the scratch). Optical microscopy, force data (cutting and thrust) and cross-sectional cutting profiles using a white light interferometer were correlated in this study. The remaining experiments proposed will be done using a single crystal diamond cutting tool (1mm nose radius with a -45 degree rake and 5 degree clearance angle) to optimize the machining (depth of cut, feed and cutting speed) and laser heating (laser power) parameters. Once the material removal experiments are completed, characterization analysis will be carried out using SEM, TEM, and Micro Raman spectroscopy. In this study, three different polytypes of SiC will be experimented on: single crystal 4H, single crystal 6H and polycrystalline 3C.


Acoustic Emission to Detect the Onset and Position of Cracks and Fracture


Project Overview: In this study, AE is proposed as a method for monitoring the onset of brittle fracture with respect to the tool position. The purpose of this study is to examine the onset of fracture using AE signal processing during micro-scratching tests of single crystal Si and SiC. However, this study is not limited to only identifying the onset of fracture but also correlating the AE spectrum to the location/position of fracture/cracks (i.e., leading edge, trailing edge or beneath the cutting stylus/tool). The onset of brittle fracture/cracks during the material removal process will be studied using scratch tests as it best resembles the single point diamond turning (SPDT) machining operation. All scratch tests will be conducted on the Universal Micro-Tribometer (UMT) by CETR. The current version of the UMT is equipped with an AE sensor coupled with a band pass filter with a frequency range of 500 KHz – 5 MHz.
 

The Effects of Laser Heating on the Material Removal of Single Crystal Silicon

Project Overview: A study is done to compare the results of scratch tests done on single crystal Silicon, with and without laser heating. The effects of laser heating were studied by verifying the depths of cuts for scratch tests carried out on single crystal Silicon with increasing loads (thrust force) to  wherein the scratch shows both ductile and brittle response (with a ductile to brittle transition (DBT) region within the scratch). Optical microscopy, force data (cutting and thrust) and cross-sectional cutting profiles using a white light interferometer were correlated in this study. The remaining experiments proposed will be done using a single crystal diamond cutting tool (1mm nose radius with a -45 degree rake and 5 degree clearance angle) to optimize the machining (depth of cut, feed and cutting speed) and laser heating (laser power) parameters. Once the material removal experiments are completed, characterization analysis will be carried out using SEM, TEM, and Micro Raman spectroscopy.  


Ductile to Brittle Transition of Ceramics and Semiconductors 

Project Overview: This study discusses results from experimental work carried out to determine the DBT depth of several ceramics/semiconductors such as Silicon Carbide (SiC), Silicon (Si), Quartz (SiO2), AlON(Al23O27N5), Sapphire (Al2O3), Spinel (MgAl2O4) and Aluminum Titanium Carbide (AlTiC). Since SiC is the main emphasis in this study, three different polytypes will be evaluated: single crystal 4H, 6H and CVD 3C. Scratch tests are carried out to determine the DBT depth of the materials. Scratch tests were chosen to be the principle method of investigation in this study as it is a better candidate for evaluating machining conditions than indenting because the mechanics during scratching are more applicable to the machining process such as single point diamond turning (SPDT).The scratch tests are performed within a load range (thrust force) that is expected to exhibit the ductile, DBT and brittle regimes. Once the scratch tests are completed, optical microscope images, force data and interferometric profilometer analysis are carried out and correlate in order to determine the DBT depth (with its corresponding cutting and thrust forces). The results from the depth analysis is finally correlate to the mechanical material properties to identify the property, if any, that is most influential in the DBT depth.
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