Planar Machining

Planar machining of brittle-hard materials usually requires a multi-stage process control: Grinding processes are used for preliminary shaping, the final machining is then carried out with lapping and polishing processes. The following processes are used at the IMPT.

SURFACE GRINDING

For the machining of planar functional surfaces with high metal removal rates, surface grinding processes are used. Plane grinding is based on a track-bound operating principle, since the rigidly bound grain penetrates the workpiece on a predetermined track. Due to the high hardness and wear resistance of the brittle-hard materials (ceramics, single crystals), diamond is mainly used as cutting material. Machining-related component damage such as chipping, cracks and residual stresses in the edge zone are critical, and reworking of the components is often necessary. At the IMPT, surface grinding is being researched for machining metals and for coarse thickness reduction of ceramic substrates.

FINE LAPPING

Lapping is defined as cutting with largely loose cutting grains dispersed in a liquid or paste, which performs a rolling motion due to the relative motion between the lapping wheel and the workpiece. The lapping grains move in the space between the workpiece and the tool, so lapping is based on a space-dependent principle. In addition to grinding, lapping of ceramics is a manufacturing process with which high surface qualities and shape accuracy can be achieved economically. Diamond is mainly used as lapping grain (abrasive grains) for machining ceramics. SiC grains are preferred for machining ductile materials (metals). Due to the relatively large abrasive grains and hard lapping wheel, component damage such as chipping, cracks and residual stresses in the edge zone can be reduced but not completely avoided. If instead of a typically hard lapping wheel (cast iron) a lapping wheel made of softer material (Cu, Al, Zn) is used, damage to the edge zone can be significantly reduced. In this case, one speaks of fine lapping or "loose abrasive grinding". At the IMPT, fine lapping is mainly used for coarse planarization of ceramic substrates (wafers)

NANO GRINDING

An alternative to the finishing processes mentioned above is the nano grinding process, an ultra-precision process for the production of flat or slightly curved surfaces. While in lapping, the grain performs a rolling motion between the workpiece and the tool, in nano grinding the cutting grain is bound in a soft, metallic working disc. In comparison to the spatially bound operating principle in lapping, the operating principle in nano grinding is force bound due to the constant surface pressure acting on the workpiece. Nano grinding is also known in literature as "fixed abrasive grinding". Nano grinding is a two-phase ultra-precision manufacturing process for the production of flat or slightly curved surfaces, consisting of a generation and machining phase. In the generation phase, the nano grinding tool is created by embedding diamonds of 0.5 to 1 µm in a tin working disc. In the work phase, the workpiece is machined with this tool. The process is suitable for finishing high-strength ceramics. Compared to competing processes, nano grinding achieves both high surface qualities and relatively high cut-off rates. At the IMPT, nano grinding is used for fine planarization of ceramic substrates (wafers) as well as for surface processing (angle, bevel, side walls) of small chips.

POLISHING (MECHANICALLY)

Polishing is a smoothing finishing process for various materials. The smoothing effect is usually achieved with two mechanisms. Firstly, roughness peaks of the surface structure are plastically and partially plastically deformed and thus levelled. On the other hand, depending on the type of polishing, there is very little to little material removal, as well as filling of depressions. The surface thus achieved is often shiny due to its smoothness. During polishing, the roughness of the workpiece is reduced, but the shape accuracy generally decreases. Polishing cloths (textile, felt, polymer) are mainly used as polishing medium carriers. Stiffer polishing media carriers are better suited for maintaining shape accuracy, as softer ones round off the edges of the components. As abrasive grains, besides diamond, also other nano-grained particles such as Al2O3, SiO2, CeO2 are used. At the IMPT, plane polishing is used for the surface finishing of ceramics or ductile substrates (metals)