By: Dr. Lim Gnian Cher
Laser is a potential tool for machining structural profiles on hard and brittle materials such as ceramics and carbides that traditional tools and methods find them near impossible to do. EDM (electro-discharge machining) may be used for substrates that are conductive, such as tungsten carbide. However, drilling small holes or machining channels and other features that are less than 100 um in dimensions cannot be achieved even with EDM.
Ultra-short pulse lasers in the range from nano-second to femto-second pulse width have the potential advantage of removing materials efficiently while at the same time minimizing the thermal effect if the pulse energy is optimised to be just above the threshold of material decomposition. The ICALEO 2010 paper on “Effects of Laser Fluence on the Micro-Structure Formation and Material Removal Rate in Ablation of Silicon Carbide (SiC)” investigated the use of a nano-second pulse laser to process SiC. The investigation used a galvanometer scanner with an f-theta focusing lens to raster scan SiC substrate surfaces with a 20 nano-second, 355 nm wavelength laser beam. The SiC substrates were positioned with their surfaces at the focus position of the scanning beam. A wide range of laser parameters were experimented with different average power, scanning speed and raster line separation in order to cover a square area of 1mm x 1mm for each parameter combination. It demonstrated that, depending on the total laser fluence (energy deposited per unit areas, J/cm2) used, the structure of the laser scanned surfaces may be classified into four categories, namely material melting, removal or ablation, oxide nodule formation, and oxide flack formation. Material removal was achieved at a very low fluence of 50 J/cm2. At high laser fluence, hard porous oxide would form and continued to build-up with increasing fluence until a height exceeding 0.5 mm above the workpiece surface. The formation of these features depended only on the laser fluence deposited on the SiC surface irrespective of the average laser power, speed, hatching line separating, and overall process time. The low fluence required for material removal of the SiC substrate signified that a low power, low cost laser may be used. Accurate micro-structure profiles could be achieved by using the laser to carve layer-by-layer but only with laser fluence below about 300 J/cm2 in order to ensure suppression of oxide growth. The carved surfaces have roughness Ra of 0.2 to 0.3 um at low fluence and increased to 0.8 um at higher fluence. Secondary surface finish may be required for most applications. However, it is expected that further research would greatly improve the surface roughness. Oxide formation and debris redeposition may be avoided if the process is carried out under appropriate atmosphere such as inert gas or flowing water. Under such environment, carving could be achieved even at a high fluence of 73 kJ/cm2.
