Assessing Surface Roughness During the Production of Critical Ceramics in the Aerospace and Defence Industries

Advanced Ceramics are being extensively used in the defense and aerospace industries. As the production of critical ceramic components require a high degree of engineering expertise and manufacturing know-how, the collective abilities of engineers and manufacturers to define, specify, and produce the “correct” surface roughness for a particular application may determine whether the component will perform to specifications. Figure 1 shows an optically polished silicon carbide mirror.

Figure 1. A lightweight silicon carbide (SiC) mirror has been optically polished to deliver superior field performance.

Standard uses of ceramics within the aerospace and defense industries include:

• Transparent Windows and Domes
• Rocket Components
• Nuclear Components
• Mirrors (Lasers, Telescopes)
• Radomes
• Gas Turbine Components (Shrouds, Vanes, Blades, Blisks).

Application Considerations

In order to understand how to use a component in a particular application, familiarity with optical considerations and strength is necessary. Diamond wheel grinding is the main way of machining advanced ceramic components to meet tight dimensional and surface roughness requirements.

Sub-surface damage is created by the diamond wheel grinding process and sub-surface damage is created that can impact directly component strength. A number of parameters such as bond type, wheel grit, depth of cut, feed rate and speed can impact the sub-surface damage created. The strength of the component and grind line directionality impacts component strength in high stress areas.

Strict guidelines are followed by which test specimens are machined to ASTM standards that define the wheel grit, speeds, and feed rates.

In optical applications where the main consideration is minimal sub-surface damage, surface roughness is a critical parameter. Optical components have very tight surface roughness specifications, often in the sub-nanometer Ra range, which require lapping and polishing with increasingly finer abrasives to eliminate the subsurface damage.

There is a wide range of advanced technical ceramics, and each has unique properties with regards to roughness. There are infiltrated materials, monolithic, composites and porous materials. One must be knowledgeable about the potential for pullout, reaction layers, porosity, and substitution of “lower grade” materials as one’s choice of material can have a significant impact on surface roughness.

Cost is always an essential factor along with material. The cost and lead times can be significantly increased by vague or unnecessary surface roughness requirements.

Manufacturing Considerations

Machining of advanced ceramics is normally done by diamond wheel grinding, lapping, and polishing. There are several types of grinding wheels. These include:

• Resin-bonded
• Metal-bonded
• Vitrified
• Electroplated.

Grinding considerations along with bond type include abrasive size and concentration. Grinding process parameters such as depth of cut, orientation of grind lines, plunge vs. reciprocating, wheel truing/dressing, and spark outs can also greatly impact surface roughness. It is important to weigh all these considerations while grinding to obtain the correct surface roughness while ensuring the part will function correctly in its application.

Lapping involves using a loose abrasive between two surfaces that are rubbed together, thus abrading the part. Lapping is used to achieve a surface roughness and flatness beyond the capabilities of standard grinding. The difference between a ground part and a lapped one is clear as shown in Figure 2.

Figure 2. The ground finish has very evident grind lines with a distinct orientation. In contrast,the lapped finish has lines that are hard to resolve by visual inspection, and has no definite orientation.

The type of abrasive and lap material is based on the type of material being lapped and is critical because the choices will affect material removal rates, surface roughness, and how well the lap holds its form. Along with the changing of laps, abrasive size is decreased accordingly to yield better finishes.

Removal rate is also increased with automated machinery or is specifically designed for accelerating the process with pneumatics and faster spinning speeds. Even though the conventional method of hand lapping can produce precise and repeatable results, it is a time-consuming process. Polishing and lapping are similar, however, the process of removing microscopic surface roughness is complex. Polishing can involve the removal of molecule clusters at an atomic level.

With the use of zirconia-toughened alumina (ZTA substrates), one ZTA piece is polished with aluminum oxide (Al₂O₃) and the other with a diamond abrasive, giving both surfaces a mirror finish. Nevertheless, a surface roughness profile shows that there are major differences between the two surfaces as shown in Figure 3.

Figure 3. Although the diamond abrasive is harder than aluminum oxide (Al₂O₃), the diamond produces a better surface roughness.

Metrology Considerations

Metrology is critical for quantifying and inspecting a specific surface roughness while dealing with advanced technical ceramics. There are two fundamental ways of verifying a surface roughness: contact and non-contact. Both methods need technical expertise to understand when and how to use them.

The most fundamental method is visual comparison to a known standard. This is quite crude and errors are inevitable. A Perthometer is a contact method of checking surface roughness for values greater than 2µin. This method comprises of a diamond-tipped stylus that measures surface variations based on vertical displacement of the stylus as a function of position.

Various parameters that can affect measurements are the size of the radius on the stylus, traverse length, and what filters are set. The benefits of a Perthometer include the fact that it is insensitive to surface contamination, color of the material, or the reflectance of the material.

This method is mostly portable enabling in-process inspection on the machine, a flexible and low-cost option. Precautions to keep in-mind are that the diamond-tipped stylus can wear quickly on ceramic materials and the stylus can scratch some parts. The roughness of a ground finish is shown in Figure 4.

Figure 4. The roughness of a ground finish is measured with different orientations and methods.

White light interferometry is an optical, non-contact method used when specifications require accuracy that cannot be achieved by a Perthometer. Data is collected by an optical profiler over a field of view instead of a line negating grinding orientation.

Also there is no surface damage risk and data gathered may be filtered or transported to software for interpretation. The optical profiler also enables surface examination from different perspectives and can make measurements in the sub-nanometer range (<0.04µin. Ra). Drawbacks include the inability to measure work in-process on the machine and surfaces with low reflectivity. Data is also collected over a considerably small area that may not be representative of the entire surface.

Conclusion

Typically, most detailed drawings lack the specifications required for producing and inspecting a successful ceramic component. Hence it is critical to have open communication. On-going interaction among individuals who understand the application and those who produce the part is critical to the success of the final component. Also, effective lapping, polishing, grinding and metrology require a combination of experienced engineering and manufacturing professionals with outstanding judgment and state-of-the-art equipment.

This information has been sourced, reviewed and adapted from materials provided by PremaTech Advanced Ceramics.

For more information on this source, please visit PremaTech Advanced Ceramics.