Pressure and Temperature Effects in Micro-laser Assisted Machining (μ-lam) of Silicon Carbide

In the deformation and cutting process of semiconductors and ceramics, especially brittle materials such as silicon carbide (SiC), the presence of high pressure phase transformation (HPPT) is of great importance for accomplishing ductile regime machining (Patten, 2007). To augment the ductile regime machining of these nominally brittle materials, the high pressure phase can be preferentially heated and thermally softened by using concentrated energy sources such as laser beams (Dong, 2006). In this research, the effect of the pressure and temperature and the interactions of these two factors on the micro-laser assisted machining (μLAM) process are studied by using a hybrid machining system that consists of a diamond stylus (tool) and infrared (IR) fiber laser heating source. The combinations of loading (pressure) with and without laser heating prior to loading are studied at various cutting depths and speeds. The laser achieves heating and thermal softening as evidenced by the increased ductility of the material. Notably, the results of scratch tests at 1μm/sec show a doubling of the scratch depth which suggests a ~50% reduction of hardness due to thermal softening by the laser heating. The results are studied by atomic force microscopy (AFM) and confirmed with white light interferometer microscopy to verify the experimental results. INTRODUCTION Semiconductors and ceramics share common characteristics of being nominally hard and brittle, which stem from their covalent chemical bonding and crystal structure. Both types of materials are important in many engineering applications, but are particularly difficult to machine in traditional manufacturing processes due to their extreme hardness and brittleness. Ceramics have many desirable properties, such as excellent wear resistance, chemical stability, and the superb ability to retain strength at elevated temperatures. However, it is difficult to machine these materials, which remains a major obstacle that limits the wider application of these hard, but brittle, materials (Jahanmir, 1992). The plastic deformation in these brittle materials at room temperature is much less than in metals, which means they are more susceptible to fracture during material removal processes. Surface cracks generated during machining are subsequently removed in lapping and polishing processes, which significantly increases the machining cost. In ultra fine surface machining, such as precision machining the surface of an optical lens or mirror, developing a cost effective method to achieve a flawless surface poses great challenges. In many engineering applications, products require a high quality surface finish and close tolerances to function properly. This is often the case for products made of semiconductor or ceramic materials. Currently, fine grinding, lapping and polishing are typically used to obtain smooth fracture-free surfaces for semiconductor and ceramic materials. These machining methods are capable of producing satisfactory surface finish, but they are costly processes in terms of tool cost and machining time. Consequently, machining these mirror-like surface finishes contribute significantly in the total cost of a part. In some cases, grinding alone can account for 60-90% of the final product (Wobker, 1993). The real challenge is to produce an ultra precision surface finish in these nominally brittle materials at low machining cost. Current limitations for brittle material machining include the high cost of processing and product reliability. The cost is mainly due to the high tool cost, rapid tool wear, long machining time, low production rate and the manufacturing of satisfactory surface figure and form. The low product reliability is primarily due to the occurrence of surface/subsurface damage and brittle fracture. To overcome these obstacles, developing advanced precision machining techniques is required. Ductile regime machining operations, a very promising precision machining method, has been continuously studied in the last two decades (Blake, 1990, Blackley, 1994, Morris, 1995, Leung, 1998, Sreejith, 2001, Yan, 2002, 2004, Patten, 2003, 2005). Cost effective methods for precision part using laser assisted machining techniques continues to be a critical issue (Dong and Patten, 2007, Rebro, 2002). Recently Suthar (2008) analyzed the laser heating analytically and numerically with good agreement to experimental results (Dong, 2006). Scratch experiments are chosen to be the principle method in this study. Scratch testing is a better candidate for evaluating machining than indenting because the scratching parameters are more applicable to the machining process, such as depth of cut, width of cut and cutting speed parameters. The objective of the current study is to determine the effect of temperature and pressure in the μ-LAM of the single crystal 4HSiC semiconductors using scratch tests. The scratch tests examine the effect of temperature in thermal softening of the high pressure phases formed under the diamond tip. The tests also evaluate the difference with and without irradiation of the laser beam at a constant loading and cutting speed. The laser heating effect is verified by atomic force microscopy (AFM) and white light interferometric measurements of the laser heated scratch grooves. EXPERIMENTAL PROCEDURE The IR diode laser used in this investigation is a Furukawa 1480nm 400mW IR fiber laser with a Gaussian profile and beam diameter of 10μm. The IR laser beam is guided from the diode laser through a 10μm fiber optic cable to the ferrule, which is attached to the diamond stylus. A cross section of the μ-LAM operation is shown in Figure 1. In this setup, the IR laser beam passes through the diamond tip (tool) and impinges on the SiC work piece material. The laser emerges from a 90° conical single crystal diamond tip with 5μm radius spherical end, as shown in Figure 2. FIGURE 1. A SCHEMATIC CROSS SECTION OF THE μ-LAM PROCESS.