Single Point Diamond Turning Effects on Surface Quality and Subsurface Damage in Ceramics

Advanced ceramics, such as Silicon Carbide (SiC) and Quartz, are increasingly being used for industrial applications. These ceramics are hard, strong, inert, and light weight. This combination of properties makes them ideal candidates for tribological, semiconductor, MEMS and optoelectronic applications respectively. Manufacturing these materials without causing surface and subsurface damage is extremely challenging due to their high hardness, brittle characteristics and poor machinability. Often times, severe fracture can result when trying to achieve high material removal rates during machining of SiC or quartz due to their low fracture toughness. This research demonstrates that ductile regime Single Point Diamond Turning (SPDT) is possible on these materials to improve its surface quality without imparting subsurface damage. Machining parameters, such as depth of cut and feed, used to carry out ductile regime machining will be discussed. Subsurface damage analysis was carried out on the machined samples using non-destructive methods such as Optical Microscopy, Raman Spectroscopy and Scanning Acoustic Microscopy to show evidence that the chosen material removal method leaves a damage-free surface and subsurface. Optical microscopy was used to image the improvements in surface finish whereas Raman spectroscopy and scanning acoustic microscopy was used to observe the formation of amorphous layer and subsurface imaging in the machined regions. All three techniques complement the initial hypothesis of being able to remove a nominally brittle material in the ductile regime. INTRODUCTION Silicon carbide is used in specialized industries due to its excellent mechanical properties such as extreme hardness, high wear resistance, high thermal conductivity, high electric field breakdown strength and high maximum current density. The fully dense cubic (beta) polycrystalline silicon carbide (manufactured by POCO Graphite) CVD coating (≈250μm thick) is a potential candidate to be used as mirrors for surveillance, high energy lasers (such as airborne laser), laser radar systems, synchrotron x-ray, VUV telescopes, large astronomical telescopes and weather satellites. The primary reasons CVD coated silicon carbide is preferred for these applications is that the material possesses high purity (>99.9995%), homogeneity, density (99.9% dense), chemical and oxidation resistance, cleanability, polishability and thermal and dimensional stability. Machining silicon carbide is extremely challenging due to its extreme hardness (≈27 GPa) and brittle characteristics. Besides the low fracture toughness of the material, severe tool wear of the single crystal diamond tool also has to be considered. Quartz, also known as silicon dioxide (SiO2), is the most abundant nonmetallic mineral on earth. There are several forms of quartz such as quartz crystals, natural fused silica (amorphous form of SiO2) and synthetic fused silica (polycrystalline). For this research experiment, a synthesized fused silica (Spectrosil 2000.) was used. Spectrosil 2000 is an ultra pure synthetic fused silica manufactured by Saint-Gobain Quartz PLC. This quartz material has a wide optical range from 180nm in the deep ultra violet transmission through to 2000nm in the infrared (IR). This material possesses a chemical purity of 99.999% and is manufactured using an environmentally friendly process, which results in a material that is both chlorine-free and bubble-free. A 6” diameter round disk was obtained in order to carry out the SPDT experiments. The mechanics of material removal in SiC and glass (Quartz) can be classified in two categories: brittle fracture and plastic deformation. Good optical quality surfaces can be achieved by removing the material in a ductile manner. The work of past researchers suggests that glasses do not necessarily behave as brittle material (even at room temperature) especially in the nanometric scale. The strength, hardness and fracture toughness of the work piece material are the governing factors that control the extent of brittle fracture. Some studies include detailed observations of a small amount of plastic deformation in brittle materials during a precision machining operatio 8 n. Previous researchers have successfully been able to precisely grind CVD-SiC (using high precision grinding) but this process is very expensive and the fine abrasive wheels often result in an unstable machine/process. Single point diamond turning (SPDT) was chosen as the material removal method as SPDT offers better accuracy, quicker fabrication time and lower cost when compared to grinding and polishing. Although SiC and Quartz are naturally brittle, micromachining these materials are possible if sufficient compressive stress is generated to cause a ductile mode behavior, in which the material is removed by plastic deformation, instead of brittle fracture. This micro-scale phenomenon is also related to the High Pressure Phase Transformation (HPPT) or direct amorphization of the material. 12 The plastic deformation or plastic flow of the material, at the atomic to micro scale, occurs in the form of severely sheared machining chips caused by highly localized contact pressure and shear. EXPERIMENTAL METHOD The equipment used to carry out all of the machining experiments was the Micro-Tribometer (UMT) from the Center for Tribology Research Inc. (CETR). This equipment was developed to perform comprehensive micro-mechanical tests of coatings and materials at the micro scale. Figure 1 shows the equipment setup for the 6” CVD coated SiC disk. A similar setup was used to carry out SPDT experiments on the 6” Quartz disk. A single crystal diamond tool with a 3mm nose radius, 45 degree rake angle and 5 degree clearance angle was used for the cutting tests. The MASTERPOLISH 2 Final Polishing Suspension (contains alumina and colloidal silica with a pH ~9) from Buehler, Inc. was used as the cutting fluid for all experiments involving diamond turning SiC. Likewise, all machining for the Quartz piece was done under wet conditions using the MASTERMET 2 Colloidal Silica Suspension from Buehler Inc. This cutting fluid (obtained from Buehler, Inc.) contains fine, noncrystallizing (0.02μm) SiO2 particles in an aqueous base that is suitable for machining quartz/glass. Figure 1: Machining Setup for the 6” CVD coated SiC disk SPDT of the 6” CVD Coated SiC The preliminary single point diamond turning experiments were successful in reducing the surface roughness of a CVD coated silicon carbide disk. The Ra was brought down by over one order of magnitude (from 1158nm to 83nm). Since the goal of this research was to develop machining parameters appropriate for ductile mode machining of CVD SiC, there were several additional steps (machining passes) that had to be carried out in order to confirm or verify the processing parameters. For the actual manufacturing process, many steps (passes) were eliminated to make the actual production process more cost and time efficient. Pass Programmed Depth of Cut Actual Depth of Cut Feed (μm/rev)