Advanced ceramics have found their place in a number of applications, largely due to their superior mechanical properties. Ceramics are unrivaled in their ability to withstand extreme amounts of heat and abrasion, and very high levels of electrical interaction. Although advanced ceramics exhibit many appealing properties, designers and engineers are sometimes deterred from choosing these materials because of two crucial attributes of any project: cost and lead time.

The cost of traditional ceramics can sometimes be a difficult balancing act. Deviating from typical shapes (e.g., round, square, etc.) can cause the price of the components to become extraordinarily high, as this may require the use of CNC machining equipment with four or five axes. Injection molding of ceramics is another way to reduce costs in high volumes, but the initial investment for tooling can make this prohibitive unless very large quantities are required. Once again, the complexity of the part being manufactured dictates the cost.

Lead time is another important aspect of any project. Lead times for both traditional manufacturing of ceramics as well as injection molding are typically between 8-14 weeks. Some so-called “rapid” ceramic manufacturers claim to deliver parts in two weeks, but this is rarely nothing more than marketing hype.

Manufacturing Processes

Today’s advanced ceramics are largely manufactured using traditional means. The manufacturing process can be broken down into a number of steps:

• Raw material processing through milling
• Isostatic pressing to desired blank shape
• Machining of the blank shape through use of machine tools (e.g., lathe, mill, grinder) to form the green body
• Sintering of the green body to enhance mechanical properties
• Optional: hot isostatic pressing (HIP) or cold isostatic pressing (CIP) to boost mechanical properties after sintering
• Final grinding using diamond tooling to achieve exacting dimensions
• Inspection for defects such as surface cracks, spalling, etc.

In contrast, the additive ceramic manufacturing process can be broken down into the following steps:

• Raw material processing
• Addition of binders, processing aides and sintering aides
• Rheological check of material to ensure quality of formed parts
• Forming of ceramic green body, including use of support materials to allow for complex geometry
• Pre-processing of green body to boost mechanical performance
• Sintering of the green body to enhance mechanical properties
• Optional: HIP or CIP to boost mechanical properties after sintering
• Final grinding using diamond tooling to achieve exacting dimensions
• Inspection for defects such as surface cracks, spalling, etc.

Many of the processes used during additive ceramic manufacturing are identical to those of the traditional ceramic process. Because of the reduction in time required for the formation of the ceramic green body, however, the time required for the additive process is drastically reduced. Further, due to the high mechanical properties that can be achieved with additive ceramics without the use of HIP, the lead time and cost can often be significantly less than that of the traditional manufacturing method.

Mechanical Properties

Many industry sectors find advanced ceramics to be extremely useful, if not a necessity. With this necessity comes compromise, as many geometries are simply impossible to form using traditional means. The nozzle in Figure 1, for example, has a 0.75 in. outside diameter, 0.25 in. inside diameter, and 1.50 in. total length.

The surfaces that make the “U” profile and the undercut geometry toward the bottom of the cavity would be unsuitable for both traditional manufacture as well as ceramic injection molding. The profile, coupled with the small size of this part, would not allow for traditional manufacturing using a CNC mill/lathe. If there were a feasible way to form this part, the setup costs and programming time required would be substantial, and special tooling would likely be necessary.

When comparing the mechanical properties of traditionally formed advanced ceramic components to the additive forming method, many similarities are apparent. In Table 1, the various mechanical properties are listed for the traditional forming method as well as the additive forming method.

Although the mechanical properties of the additive ceramics are slightly less than that of the traditional ceramics, overall they are very comparable, considering the advantages that can be realized over ordinary manufacturing techniques. The porosity of the additive ceramics has been found to be less due to the nature of the fabrication process, which uses a slurry-based technique. This method allows the particles to arrange themselves in a more compact manner, while the traditional ceramics are typically formed using a dry compaction method such as uniaxial or isostatic pressing operations.

Forming Process

Both traditional and additive ceramic forming methods offer some advantages. Table 2 lists the various forming process attributes.

Traditional ceramics have the advantage of build size, as the large machines used to fabricate the parts are typically found in ceramic fabrication shops. High production volumes are also a prominent feature of traditional ceramics, especially when it comes to geometry that can be injection molded. Cycle times for injection molding machines may be as little as 10 seconds when less complex geometry is being molded.

The common pitfall of traditional ceramic methods is that it is typical for lead times to exceed 10 weeks, especially if the geometry to be machined requires the use of 5-axis CNC machining. If the manufacturer chooses to form the part in a series of operations, such as turning and then milling, the process times are increased due to the necessary manual setup for each part or set of parts. The cost of the parts reflects this increase in processing time, and more often than not, the lead time.

Ceramic Design

Design for manufacturing is another important comparison between traditional and additive ceramics. Often, when the engineer is tasked with designing a ceramic component into a system, strong consideration must be given to the capabilities of traditional machine tools and what operations can be performed on the equipment. The cognizant engineer will attempt to form all of the features on a single machine to reduce cost and lead times. With these constraints in place, however, the overall design loses fidelity. In contrast, the additive ceramic process allows for the design to be completed based on its functionality, not its manufacture.

Although additive ceramics allow for a more open design process, limitations that are identical to those of traditional ceramics still exist. These limitations are present due to the nature of the sintering process, where a large amount of heat is applied to the ceramic green body. It is still necessary to adhere to guidelines, such as minimizing the tensile and shear stresses that are placed on the component, reducing stress concentrations such as sharp internal corners, avoiding sudden changes in cross-sectional area, and minimizing the use of threads on the ceramic body.

Post-Processing Techniques

Post-processing techniques found in traditional ceramic manufacturing, such as hot isostatic pressing, can also be used with additive ceramics to boost mechanical performance. When tight tolerances are required, hard grinding operations using diamond tooling can be employed with additive ceramics in the same way they are currently used with traditional ceramics.

Grinding is commonly used with traditional ceramics, as the shrink rate of the materials is between 19-22%. While the features of the component can be scaled to account for this shrinkage, grinding is often necessary because target tolerances are difficult to reach in the as-fired state. One unique feature of the additive ceramic process is that because of its lower shrink rate (between 6-12%), grinding operations can be bypassed; the target tolerances are more achievable in the as-fired state. While this is not always true, this reduction in shrinkage makes the additive process more appealing in terms of cost.

Evaluating Results

In order to fully evaluate any advanced ceramic, it is helpful to analyze the microscopic level for the structural suitability of the ceramic and study any flaws that may have occurred during its manufacture. In Figures 2 and 3, microscopic images at the 100 micron level were taken of both the traditionally manufactured ceramic (Figure 2) and of the additively manufactured ceramic (Figure 3).

Both samples were fired and ground in the same fashion. From this level of magnification, the surface of the samples looks almost identical. When measuring the density of the samples, the traditional ceramic measured 97.5% (2.5% porosity), while the additive sample measured 98.5% (1.5% porosity).

Increasing Confidence

With the excitement surrounding additive manufacturing today, many companies are familiar with the processes of commercially available equipment. This familiarity is often accompanied by hesitation, however, mainly because of previous disappointing experiences using new technologies. Many technologies claim to produce parts that will be able to supplant traditionally manufactured components, but what is often produced is hardly suitable for functionality testing. Manufacturers hope to eventually find confidence in a proven replacement technology.

The up-and-coming additive ceramics process does not aim to completely fill the advanced ceramic market, but rather endeavors to complement existing manufacturing processes. The technology’s purpose is to fill a gap in terms of lead time and geometric complexity, and to remove the design barriers that prevent cutting-edge technologies from being developed and ultimately released into the market.  

 For additional information, contact the author at (440) 787-3181 or benjamin.becker@hotendworks.com, or visit www.hotendworks.com.