The core of achieving "non-destructive" microscopic exposure in metallographic polishing machines lies in the precise control of the material removal process, gradually eliminating surface damage layers while avoiding the introduction of new defects, ultimately obtaining a flat and smooth surface suitable for microscopic observation. The scientific steps and material removal mechanism can be divided into the following stages:
1、 Scientific steps: staged control from rough grinding to fine polishing
Rough grinding stage
Use coarse-grained sandpaper (such as 180-400 mesh) or diamond grinding discs to remove the cutting damage layer, oxide scale, and macroscopic unevenness on the surface of the sample under high pressure (1-2N/cm ²) and medium speed (150-300rpm). At this stage, the sample is rapidly thinned through mechanical cutting, but pressure and time need to be controlled to avoid surface overheating or plastic deformation caused by excessive cutting. For example, aluminum alloy samples need to be cooled by a water cooling system during rough grinding to prevent recrystallization of grains due to thermal effects.
Precision grinding stage
Gradually switch to finer sandpaper (600-2000 mesh) or silicon carbide grinding discs, reduce the pressure to 0.5-1N/cm ², and increase the speed to 200-400rpm. At this stage, the surface roughness is refined through micro cutting to eliminate scratches left by rough grinding, while the residual stress layer is removed by the friction between the abrasive and the material surface. For example, anhydrous ethanol should be used as a lubricant during precision grinding of steel samples to reduce iron chip adhesion and prevent surface scratches.
Polishing stage
Use polishing cloth (such as silk or velvet cloth) combined with alumina or silica polishing solution (particle size 0.05-1 μ m) to perform chemical mechanical polishing (CMP) at extremely low pressure (0.1-0.5N/cm ²) and low speed (50-150rpm). The particles in the polishing solution are mechanically rubbed to remove surface micro protrusions, while the polishing agent reacts chemically with the material surface to form a soft layer that is easy to remove, accelerating the leveling process. For example, copper alloy samples need to be polished with a polishing solution containing citric acid to remove the surface oxide layer through chelation and avoid scratches.
2、 Material removal mechanism: mechanical chemical synergistic effect
Mechanical cutting
The rough and fine grinding stages are mainly mechanical cutting, where abrasive particles are embedded on the surface of the sample under pressure, and the material is removed through sliding, rolling, or plowing. This process requires controlling the abrasive particle size and pressure to avoid excessive single particle cutting force that may cause grain peeling or microcrack propagation.
chemical corrosion
The polishing stage softens the surface layer and reduces cutting resistance through chemical reactions between the polishing agent and the material surface. For example, during polishing of stainless steel samples, nitrate ions in the polishing solution can selectively corrode the iron substrate, retaining the chromium oxide protective layer and achieving selective removal.
synergy
The synergy between mechanical action and chemical corrosion is the key to "non-destructive" exposure. Mechanical cutting provides surface leveling power, while chemical corrosion reduces mechanical damage by softening the surface layer. For example, in the polishing of semiconductor materials such as silicon, potassium hydroxide solution reacts with the silicon surface to form soluble silicates, and the mechanical action of the polishing cloth accelerates the peeling of reaction products, achieving atomic level flatness.
3、 Key control parameters
Pressure and speed: Excessive pressure leads to surface plastic deformation, while insufficient pressure results in low efficiency; The speed should match the pressure to avoid local overheating.
Abrasive particle size: The gradient selection from coarse to fine can gradually refine the surface and reduce residual damage.
Cooling and lubrication: Water based or oil-based coolant can lower temperature, reduce friction, and prevent surface oxidation or adhesion of debris.
Polishing time: Real time monitoring of surface quality through a microscope is required to avoid excessive polishing leading to surface roughness rebound.
Through precise control of the above steps and mechanisms, the metallographic grinding and polishing machine can remove surface defects while preserving the original microstructure of the material, providing high-quality samples for metallographic analysis, microhardness testing, etc. For example, in the metallographic inspection of aircraft engine blades, the fully automatic grinding and polishing machine can control the sample flatness error within ± 0.01mm, ensuring clear exposure of microscopic features such as grain boundaries and phase boundaries.