Introduction
This article is based on a case study of 99.5% alumina setter plates that cracked in the middle at 1050°C.
Through investigation and failure mechanism analysis, we identified several key factors that led to the failure, including microcracks, residual stress, and process deviations. Following this, we proposed improvement measures for both the production and end-user sides. After making these improvements, no similar problems occurred in subsequent batches.
We hope this case study will be a useful guide to failure prevention for technical personnel in the electronic ceramics industry.
A Failure Case Worth Examining
The following case is based on feedback from one of our customers during use. The customer reported that while using a batch of 99.5% alumina setter plates, they discovered cracks in some of the plates (accounting for about 3% to 4% of the total). These plates were used for firing operations at 1050℃.
It is worth noting that, based on the material’s standard properties, this operating temperature is well below the load-softening temperature of alumina (> 1700℃). Therefore, this is not a typical failure mode for the material, prompting us to conduct a further analysis.
Preliminary Investigation
Below are our preliminary findings:
| Item | Description |
| Material | 99.5% alumina |
| Length × Width | 150 × 180mm |
| Thickness | 2mm |
| Operating Temperature | 1050℃ |
| Items Loaded | Ceramic components, with a total weight of about 80g. |
| Symptoms of Failure | Ceramic setter plate cracked in the middle. The failure occurs irregularly. |
| Failure Rate | 3-4% |
| Features of the Defective Batch | The cracking issue occurred only in a single batch; no failures were reported in previous batches. |
After analysis, we believe we should further explore the following aspects:
- Temperature: A temperature of 1050℃ is sufficient to activate the microstructure inside the material.
- Location of the Crack: The cracks all occur in the middle of the ceramic setter plates, not at the edges.
- Quality Issue in a Single Batch: This indicates that a deviation has occurred in one of the variables of the production process.
Failure Mechanism Analysis
Microcracks: The Hidden Defect in the Forming Process
The alumina setter plate is formed by dry pressing. During this process, if the pressure distribution is uneven or the temperature control is inadequate, the ceramic body may develop microcracks invisible to the naked eye.
Residual Stress: The Freezing Effect in the Cooling Process
After sintering, alumina setter plates will have residual stress inside. Under normal circumstances, the stress will be gradually released during the cooling process. However, if the cooling speed is too fast, the stresses may be “frozen” within the material.
When the product is reheated to 1050℃, the material expansion activates the residual stresses, causing them to release from the weakest points and ultimately leading to a crack in the middle.
Process Deviations: Cumulative Effect of Multiple Variables
A minor deviation in a single process parameter usually does not cause failure. But when multiple parameters deviate from the standard range at the same time, a cumulative effect will occur.
Summary of Root Causes: Ultimately, we concluded that the failure was not a single material problem. It was a structural sensitivity issue caused by the combined effects of microcracks, residual stress, and process deviations.
Improvement Measures
Production-Side Control
Ceramic manufacturers must strictly control the production process in three key areas: pressing, sintering, and kiln cooling. As shown in the table below:
| Control Process | Key Parameter | Control Objective | Test Method | Purpose |
| Pressing | Pressure Fluctuation | ≤±2% | Online pressure monitoring | To avoid uneven density caused by pressure fluctuations. |
| Sintering | Heating Rate | ≤5℃/min | Process Review | Gradually increase the temperature to avoid thermal stress concentration caused by significant temperature differences between the inside and outside of the ceramic body. |
| Kiln Cooling | Time required to cool from 1000℃ to 500℃ | ≥300min | Cooling Curve Recording | Extend the cooling time to allow sufficient stress release. |
End-User Operation Suggestions
The customer should note the following points when using this 2mm thick alumina setter plate:
1. Preheating treatment for ceramic setter plates: Before loading into the furnace, the cold ceramic setter plate should be slowly heated (2-3℃/min) to 200-250℃, and maintained at this temperature for 45 minutes to 1 hour.
2. Maintain the temperature at 550-600℃ for 30 minutes, with a heating rate not exceeding 5℃/min.
3. Make sure the bottom of the furnace is flat during use.
4. After use, products fired on the ceramic setter plate must also cool down with the kiln through a step-down cooling method. We recommend a slow cooling rate of 2-3℃/min when cooling from 1050°C to 700℃. This is also the stress-release zone where products dissipate heat slowly, continuing to transfer heat to the ceramic setter plate.
As the plate edges cold-shrink while the central area supporting the product remains hot, the plate becomes prone to cracking. Once the kiln has cooled slowly to 150–80°C, it’s time to open the kiln and remove the products.
In addition to the usage suggestions above, we also have some suggestions regarding material selection.
Based on our extensive experience in advanced ceramic production, we believe that the selection of ceramic setter plates must be based on real applications. The following is a summary of key considerations:
| Operating Temperature | Recommended Plate Thickness | Application |
| ≤800℃ | 2-3mm | Medium-temperature sintering and rapid heating applications |
| 800-1200℃ | 3-4mm | Conventional electronic ceramic sintering |
| ≥1200℃ | 4-6mm | High-temperature, large-sized products |
Verification Results
After adjusting the corresponding processes and operating conditions, we conducted continuous monitoring and verification on subsequent production batches.
Under the same 1050℃ firing conditions and similar operating conditions, the optimized products no longer exhibited concentrated failures with intermediate cracks. The overall structural integrity was significantly improved compared to previous batches.
It is important to note that the actual performance of this type of ceramic setter plate remains closely tied to the loading method, thermal field distribution, and operating conditions. Therefore, the relevant validation results should be continuously monitored and verified under specific application conditions.
Conclusion
This case demonstrates that the failure of alumina setter plates is typically caused by a combination of factors, including microcracks, residual stress, and process deviations, rather than a single material deficiency.
In actual applications, relying solely on theoretical material properties is insufficient to fully assess reliability. A comprehensive analysis combining process and operating conditions is necessary. Targeted optimization can effectively improve product stability under high-temperature conditions.
Should you have similar technical issues regarding ceramic setter plates, feel free to reach out to us.
Frequently Asked Questions
Q1: Is the material quality bad if the alumina setter plate cracks at 1050℃?
A1: Not necessarily. This temperature point is prone to thermal stress issues, typically due to the activation of microscopic defects or residual stresses, rather than inherent material limitations.
Q2: Can microcracks in ceramic setter plates be detected?
A2: Generally, microcracks in ceramic setter plates are difficult to observe directly with the naked eye.
For mass production, ultrasonic testing can be used for sampling, but its ability to identify small or closed cracks is limited. Dye penetration testing is suitable for detecting surface-open cracks, but it cannot identify internal defects.
Q3: Which is more durable, 2mm or 4mm thick alumina setter plate?
A3: 4mm thick alumina setter plate usually exhibits high strength and excellent thermal shock resistance, while 2mm thick alumina setter plate heats up quickly but is sensitive to stress. We need to choose based on the application.
Q4: Are there significant differences in performance between 99.5% alumina setter plates from different manufacturers?
A4: Yes, the performance of 99.5% alumina setter plates from different manufacturers varies greatly.
Purity is merely one of the indicators, while factors such as microstructure, grain size, and residual stress levels have a greater impact on practical performance. This is why, even when the specifications are the same, some setter plates are durable, while others crack during use.
