Lapping is a process by which material is precisely removed from a workpiece (or specimen) to produce a desired dimension or shape. The process of lapping materials has been applied to a wide range of materials and applications, ranging from metals, glasses, optics, semiconductors, ceramics, geological samples, minerals, gems, etc. Lapping ( and polishing) techniques are beneficial due to the precision and control with which material can be removed.
Method of operation
Lapping is the removal of material to produce a smooth, flat, unpolished surface. Lapping processes are used to produce dimensionally accurate specimens to high tolerances (generally less than 2.5 µm uniformity). The lapping plate will rotate at a low speed (<80 rpm) and a mid-range abrasive particle (5-20µm) is typically used. Lapping removes subsurface damage caused by sawing or grinding and produces the required thickness and flatness. Although the lapping process is less damaging than grinding, there are two regimes of lapping: free abrasive lapping and fixed abrasive lapping. Free Abrasive Lapping is when abrasive slurry is applied directly to a lapping plate (e.g. cast iron). This is perhaps the most accurate method for producing specimens and causes the least amount of damage. Free abrasive lapping is accurate because of the rigid lapping surface which can be tailored to suit a particular material. Fixed Abrasive Lapping is when an abrasive particle in bonded to a substrate as with abrasive lapping pads. Abrasive lappingfilms have various particles bonded to a thin, uniform polyester substrate and are also capable of producing a very flat surface.
Planarazing of geological samples
Planarazing of irregular samples
Fast removal of surface layer
Removal rate is the most important figure of merit of this process. Uniformity and planarity are in many cases important as well. In order to achieve reasonable values removal rate, uniformity and planarity the following parameters should be considered.
Abrasive slurry characteristics
In general the abrasive used are in suspension (slurry) and applied to the platen during the polishing or embedded in a film that is adhered to the platen. There is a wide selection of abrasives to choose from when selecting a lapping and polishing process. Selecting an abrasive is dependent upon the specimen hardness, desired surface finish, desired removal rate, lifetime, and price. There are four basic types of abrasives that are used in lapping and polishing processes: silicon carbide (SiC), aluminum oxide or alumina (Al2O3), boron carbide (B4C), and diamond (C). All of these abrasives have distinct properties and are used for different materials and applications. SiC: SiC is hard and generally has a needle or blocky structure. SiC is used in many applications where rough lapping is required. It seldom is used for polishing or applications that require smooth surface finishes. Al2O3: Al2O3 is relatively hard and has a sharp, angular structure. Alumina is commonly used where fine surface finishes are required as it breaks down over time and gives excellent surfaces during lapping and polishing. Alumina is also relatively inexpensive. B4C: B4C is harder than most other abrasives (excluding diamond) and has a blocky crystal structure. B4C provides excellent removal rates and is typically used when fast removal with moderate surface quality is needed. Diamond: Diamond is the hardest material known and has a sharp, angular structure. Diamond is extremely useful in lapping and polishing due to it’s removal rates and surface finishing qualities. Diamond can produce excellent surface finishes combined with high removal rates.
|Material||Hardness (Knoop 100)||Density||Structure|
|SiC||2450||3.22||blocky, solid, sharp|
|Alumina||2000||3.97||blocky, solid, angular|
|BC||3000||3.97||blocky, solid, sharp|
|Diamond||6000||3.51||blocky, solid, sharp|
Equipment used for lapping and polishing can vary from application. Typically what is required for lapping and polishing are the following: 1. A lapping and polishing machine with variable speed. 2. A polishing jig for holding specimens precisely. 3. Various lapping plates for different applications. 4. Workstations for controlling lapping fixtures and conditioning equipment. 5. Conditioning equipment for maintaining plate flatness.