Introduction
In advanced ceramic manufacturing, the “core-shell” structure is a subtle but critical defect. It refers to a significant difference in density, grain size, or even phase composition between the interior (core) and the surface (shell) of a sintered ceramic component. This inhomogeneity directly degrades mechanical properties such as flexural strength and fracture toughness, and often leads to cracking or distortion during subsequent processing or service.
For structural ceramics like Si₃N₄, SiC, Al₂O₃, ZrO₂, as well as MIM (metal injection molding) ceramic parts, eliminating core-shell inhomogeneity is essential for producing high-reliability products. This article analyzes the physical origins of the defect and provides systematic solutions – from process principles to equipment selection.

1.Why Does Core-Shell Structure Occur?
The core-shell structure arises from mismatched sintering kinetics. During heating and soaking, the surface and core of a ceramic green body may experience completely different thermal histories, stress states, or atmosphere exposure. The main causes include:

• Temperature gradient: Especially in large or thick-walled parts. The surface heats up faster and densifies earlier; the core lags behind. Once the surface becomes dense, it blocks gas escape and further shrinkage of the core, creating a density gradient.
• Binder residue: For parts containing binders (e.g., MIM feedstocks), incomplete debinding leaves carbon or organics in the core. At high temperatures, these residues decompose and produce gas, inhibiting grain boundary migration – leading to fine grains in the core and coarse grains in the shell.
• Uneven pressure distribution: In gas pressure sintering or hot pressing, if pressure transfer is affected by mold or part geometry, the effective pressure on the core can be much lower than on the shell, resulting in inhomogeneous densification.
• Surface reactions: In reactive atmospheres (e.g., N₂, CO), the surface may form a dense layer or reaction layer prematurely, sealing porosity and preventing further densification of the core.

2. How to avoid a "chip-shell" structure? Six major process strategies.

2.1 Methods for homogenizing temperature gradients in ceramic sintering: Multi-zone temperature control, heat spreaders, and segmented heating to eliminate temperature field inhomogeneity.

Temperature gradient is the most common culprit in core-shell structures. Solutions:

• Use multi-zone independently controlled heating to ensure axial and radial temperature difference < ±5°C.
• Install heat shields and radiators to reduce thermal radiation shadows.
• Use heat-conducting setters to avoid direct contact between parts and heating elements or cold walls.
• For thick-walled parts, apply ramp-and-hold heating to allow the core to “catch up” with the surface temperature.

2.2 Controlling the heating rate during thick-walled ceramic sintering: Allow sufficient time for the core to catch up with densification

An excessively rapid heating rate will amplify the temperature gradient. Recommendation:

• In the 200–800°C range (debinding and pre-sintering), use 0.5–2°C/min to ensure complete decomposition and diffusion of organics.
• In the main densification zone, keep heating rate ≤5°C/min and add intermediate soaking steps (e.g., 1–2 hours at 1200°C) to promote uniform core densification.

2.3 Hot pressing (HP) is suitable for thick-walled silicon nitride: how axial pressure breaks the core-shell imbalance
Hot pressing (HP) and gas pressure sintering (GPS) can effectively suppress core-shell structures:

• Hot pressing: Axial mechanical pressure applies compressive stress to both surface and core, greatly reducing sensitivity to temperature gradients. Ideal for large Si₃N₄ or B₄C parts.
• Low pressure reaction sintering: For SiC ceramics, using low pressure (<0.1MPa) silicon vapor or silicon-containing atmosphere slows premature surface densification, allowing molten silicon to infiltrate uniformly.

Note: Pressure should be applied in stages (e.g., after reaching a threshold temperature) to avoid gas entrapment from early surface densification.

2.4 Integrated debinding and sintering furnace reduces carbon residue: How to solve the problem of black core in MIM ceramic parts?
For MIM ceramic parts or injection-molded ceramics, incomplete debinding is a primary cause of core defects. The solution is a vacuum debinding-sintering integrated furnace, which enables:

• Slow heating under vacuum to 400–600°C, continuously removing binder decomposition products.
• Use of slight overpressure carrier gas (N₂ or Ar) to promote convective transport of residues from the core pores.
• Direct ramp-up to sintering temperature after debinding, avoiding cooling and reheating that could solidify residual carbon.

This integrated process eliminates the interruption between debinding and sintering, significantly reducing the probability of carbon residue in the core.

2.5 Low-pressure reactive sintering of silicon carbide: partial pressure control delays premature surface densification
In reaction sintering or non-oxide ceramics (Si₃N₄, SiC), the atmosphere directly affects surface densification rate:

• Vacuum sintering (<10⁻² Pa) inhibits surface oxide film formation, allowing volatiles to escape and delaying surface pore closure.
• For atmosphere-protected systems (e.g., Si₃N₄ with N₂), use partial pressure control – heat under vacuum to an intermediate temperature, then backfill with high-purity N₂ to 0.1–0.5MPa, avoiding premature dense nitrided layer formation.
• Near the end of sintering, reducing gas pressure (dynamic vacuum) can help open residual pores in the core.

2.6 Conduct preliminary process research using an laboratory sintering furnace, and rapidly screen in small batches to avoid core-shell structures
Core-shell structure is highly sensitive to part size and shape. Before mass production, use a laboratory sintering furnace (small, fast-cooling, multi-atmosphere) to perform gradient process tests:

• Design matrix experiments with different heating profiles, soaking times, and atmosphere combinations.
• Evaluate density distribution and microstructure (SEM) on small cross-sections to quickly identify optimal parameters.
• Then scale up to production equipment.

This is the most cost-effective and time-efficient approach.

3. Sintering Equipment Selection Guide: Characteristics of Different Furnace Types

Vacuum sintering furnaces, MIM vacuum debinding and sintering integrated furnaces, ceramic low-pressure reaction sintering furnaces, Hot press furnaces, Laboratory sintering furnaces… How do different furnace types help eliminate core-shell defects? The table below summarizes the applicability of common furnace types in avoiding core-shell structures (technical reference only, no brand endorsement):

When selecting equipment, consider the ceramic material system, part geometry, batch size, and cost. For example, small ZrO₂ parts (<30mm diameter) can be well sintered in a high-quality vacuum furnace, while Si₃N₄ rollers >20mm thickness would benefit from hot pressing or gas pressure sintering.

4. How to confirm core-shell suppression after ceramic sintering? Density distribution, hardness indentation, and SEM observation methods.

After sintering, use the following methods to verify uniformity:

1. Density distribution test: Cut the part into sections along the central axis and measure Archimedes density for each section. A core-to-shell density difference <0.5% indicates success.
2. Microhardness indentation: Measure Vickers hardness every 1mm from edge to center. Hardness fluctuation <5% suggests good uniformity.
3. SEM / optical microscopy: Check for abrupt grain size changes or residual porosity bands.
4. Ultrasonic C-scan: For batch production, non-destructive ultrasound can map core porosity distribution.

Conclusion: Refinement Direction of Advanced Ceramic Manufacturing from the Perspective of Core-Shell Control
The core-shell structure is the result of unbalanced heat, mass, and stress fields during sintering. Eliminating it does not rely on a single “magic” parameter, but on systematic optimization of temperature uniformity, atmosphere control, pressure application, and debinding kinetics. From proper equipment selection (e.g., vacuum furnaces with integrated debinding, hot pressing with external pressure) to precise process design (ramp-and-hold, partial pressure control), each step reflects the evolution of advanced ceramic manufacturing from experience-based to science-based.
For process engineers, every success in suppressing core-shell defects not only improves yield, but also deepens the understanding of sintering physics. We hope the strategies outlined here provide practical help for your new product development or process improvement.