This guide is for technical ceramic products (such as alumina, zirconia, silicon nitride, silicon carbide) produced by “conventional” forming techniques, meaning ceramic powders that are pressed into a form, machined in a “green” or pre-fired state, fired, and then finished ground.
This guide is not intended for ceramic products which are produced by other methods such as extrusion, additive manufacturing (3D printing), slip casting, injection moulding, tape casting, or lamination. Because these methods differ, additional or different design considerations and limits can apply.
Ceramic Physical Properties
Ceramic’s set of physical properties differ greatly from those of metals and polymers. These differences should be considered when designing a ceramic part to replace a metallic or polymeric part to assure the design is both economical and fit for the intended purpose.
For example, when designing a metallic part, the average of measured mechanical strength properties is often used in design calculations, because the distribution of measured strengths is typically narrow. However, strength distributions of ceramics can be wide. If mechanical reliability of the part is critical, then understanding the ceramic’s Weibull modulus in combination with its measured average strength is important.
- Because fired ceramics are very hard, grinding is the usual machining method for a post-fired ceramic part. However, hard grinding is a slow operation compared to turning or grinding the part in its pre-fired state (its “green” or “brown” state). Consequently, it is preferable to machine a part before it has been fired. This minimises the amount of post-fired grinding time needed to obtain the final tolerances of the part’s dimensional features.
- Likewise, when designing the configuration of a ceramic part, the more basic or simple the shape is (e.g., a straight cylinder versus a ribbed cylinder), the more likely the shape can be pressed directly to its final form, minimising the need to machine the part after pressing.
- Similarly, part features with larger tolerances may be able to be formed without any machining after pressing, or at least after firing, reducing costs. For example, features with tolerance of +/- 0.005 inch (0.127mm) or +/- 1% of the feature dimension, whichever is larger, can often be produced in the as-fired condition without subsequent grinding.
- If the part needs to be glazed (for appearance, to ease cleaning such as for labware, or to control reflectivity in laser chambers) added tolerance allowance to accommodate the glaze thickness and specifying adjacent surfaces where the glaze can be incomplete or absent avoids unnecessary cost.
- Internal as-fired through-holes, blind holes, and counter-bore diameters should accommodate a draft angle of 2 degrees, or a camber of 0.006 inch per inch of length (0.1524mm per mm of length), otherwise machining of the diameters will be required.
- Round parts are more economical than oval, square or rectangular parts because the press tool cavities will cost less to fabricate and the pressed parts will be less costly to machine.
- Wall thicknesses or sections should be uniform throughout the part as much as possible to reduce the potential for density gradients and stress risers or stress concentrations within the part.
- If uniform sections are not possible, then provide gradual wall thickness transitions from section to section.
- A design that consists of an assembly of multiple simple shapes of uniform wall thickness may be more economical and effective than a single complex configuration of varying wall sections.
- Thick components will not necessarily have greater strength than thinner components, because thick sections have a greater probability of a large flaw than thinner components. If the design must be relatively thick, then selecting a ceramic with superior crack resistance such as zirconia may be appropriate.
- Apply chamfers (or radii) on edges and corners to avoid stress concentrations and reduce potential for chipping from handling damage.
- Avoid designs that subject the part to tensile loading. Ceramics perform best under compression.