Tech Talk
What is the "Grindability of Ceramics vs. Metals"?
Ceramics include a wide range of materials with their thermophysical properties dependent on the type of, composition, microstructure, and the processing methods. In this regard, ceramics are a family of materials, just as we recognize metals as a family of materials. Once we recognize this commonality, many of the principles applicable to abrasive finishing of metals – an industry with over 100 years of history – became applicable to the abrasive machining of ceramics. However, for the same properties considered (thermal conductivity, Young’s modulus, high temperature resistance, fracture toughness, etc.), metals and ceramics fall into distinctly different ranges. If we recognize these differences, then the principles of abrasive machining of metals can be modified for successful abrasive machining of ceramics.
The strength of ceramic material varies widely, depending on the material chosen. Even for a given material, like silicon nitride, for example, the strength depends on the sintering aids used and the sintering methods (pressureless sintering, hot pressing, hot isostatic pressing (HIP), etc.) applied. In this regard, the ceramic materials are analogous to metals in which the composition, microstructure, and strength influence their grindability.
In general, ceramic materials have higher stiffness than metals. This would imply that elastic deformation of ceramic materials during grinding would be less than metals for the same applied grinding forces. This is indeed true, and as a result, ceramic materials can be machined to closer tolerances, more precise geometry, superior finish, and better flatnesses and parallelism than is possible for metals. This principle is the basis for choosing ceramic materials over metals in precise instrument parts, machine tool beds or ways, as well as for gage blocks.
Ceramics are generally more chemically stable than metals. Hence the burn sometimes observed on metals during grinding is rarely observed on ceramic materials. Furthermore, the hot hardness and recovery hardness of metals are much lower than those of ceramics. In other words, any large scale thermal softening that aids in the grinding of metals may be rarely counted on in the grinding of ceramics.
Thermal conductivity of ceramic materials varies widely. This property determines the ability to conduct heat away from the grinding zone. Poor thermal conductivity in metals such as Titanium and Inconel can lead to high temperatures and great difficulty in grinding. Similarly, it is more difficult to grinding poorly conducting ceramics and the problem becomes worse when they are also poor in thermal shock resistance.
One characteristic, that significantly distinguishes ceramics from metals is their low fracture toughness. With all the above similarities between metals and ceramics in mind, it would appear that it is possible to achieve successful grinding of ceramics if the generation and propagation of cracks during the grinding process can be minimized.
Mechanisms of Material Removal in the Grinding of Ceramics. Recognizing the low fracture toughness of ceramics, models have been proposed which describe initiation and propagation of microcracks during the grinding of ceramics. These models are generally described as “Indentation Fracture” models. It has also been proposed that ductile deformation of ceramics takes place under extremely small depth of cut. This model described as “ductile regime grinding” assumes occurrence of plastic deformation only and seeks to achieve this condition using extremely low material removal rates. Under practical production grinding conditions, it would appear that both these models operate, dependent on the force per grain, volume removed per abrasive particle, size of the abrasive particle, etc. Several of the features of input elements, i.e., machine tool, diamond wheels, work material, and operational factors influence these process interactions, i.e., brittle fracture vs. plastic deformation. It would appear that the mechanisms of material removal in the grinding of ceramic material are associated with both plastic deformation and brittle fracture. It may be nearly impossible to isolate one mechanism or the other unless extreme grinding conditions are chosen. These are identified as “coarse grinding” (where brittle fracture mechanism dominates) and “fine or ductile regime grinding” (where plastic deformation mechanism dominates).The consequence of such choices are low strength and poor reliability in the case of “coarse grinding” and extremely small material removal rate, high forces and specific energy in the case of “fine or ductile regime grinding”. Referring to the process interactions, it may be envisioned that plastic deformation is desirable to increase the cutting component. However, excessive deformation may result in inefficient cutting process accompanied by excessive ploughing and rubbing/sliding components. While brittle fracture may be a means of generating surfaces at the lowest forces and energy, it is certainly not desirable where highest strength and reliability are required. Thus optimizing the ceramic grinding system becomes a process of maximizing the cutting component through plastic deformation, while minimizing the grinding forces and energy input. This can be accomplished through a careful and simultaneous selection of input parameters. While this approach is common with metal grinding, the additional factor in ceramics grinding will be the control of force per grain. Under such process interactions, selection of suitable grinding cycle including rough and finish grinding steps is a matter of trade off between process economics and surface characteristics desired. There are several examples of the successful use of this systems approach for grinding of ceramics.
Applications of the Systems Approach for Grinding of Ceramics. Production Viable Grinding: Material removal rates (MRR) of 1-30mm3/mm/sec (.1-3 in3/min/in) are now routinely achieved; the higher MRR are used for rough grinding processes while the lower values are used for finish grinding. Such MRR, if they are achieved using coarse abrasive particles, predominantly result in brittle fracture with poor surface quality (low strength and poor surface finish). Finer abrasive grains can be successfully used to achieve the high MRR mentioned earlier, if the machine tool has rigidity and power to transmit the forces and energy required. This may, on occasion, also require unique wheel design to overcome the limitations of part geometry and fixturing.
Mirror Finish Grinding: Plastic deformation dominated grinding of ceramics using fine abrasive particles often permit controlled finishing of ceramic surfaces. As a result, extremely smooth surfaces have been produced in various grinding modes (e.g.) surface grinding, cylindrical grinding, creep feed grinding. For example, mirror finish grinding of ceramics in external cylindrical grinding has resulted in finish of the order of 7nm in hot pressed Silicon Nitride. Such superior finish is often dependent on the work material grain size, uniformity and strength.
Grinding from Solid: When the grinding system is integrated as described earlier, with attention to process interactions that minimize the force per grain, high MRR grinding conditions can be used to grind components of complex geometry from simple solid shapes.
Microgrinding of Ceramics: The principles of controlled chip formation in finishing of ceramics can be extended to emulate precision finishing processes used in metal components. Hyperlap is a fixed abrasive finishing process in which grinding wheels operating at low velocities replace loose abrasive lapping. This results in higher MRR achievable in the grinding processes while maintaining flatness and parallelism achievable in lapping processes. This technology has been extended in the finishing of ceramics using a process called Microgrinding. Sometimes this application is also described as Flat Honing. There are different kinds of results achieved in microgrinding of ceramics on a variety of work materials. In addition to the obvious productivity and quality advantages, this process is environmentally benign as simple coolants or water can be used without the need for lapping fluids. Also oil bearing abrasive slurry need not be disposed of in microgrinding processes as none is required.
