Introduction

Thermal shock—the stress created when a material experiences rapid temperature change—is a common cause of mechanical seal failure. When hot seal faces suddenly contact cool liquid, or when equipment starts up after being cold, thermal gradients can crack brittle materials. Silicon carbide is renowned for its excellent thermal shock resistance. This article explains why.

What Causes Thermal Shock? 

When one part of a component heats or cools faster than another part, differential expansion creates internal stress. If this stress exceeds the material's strength, cracking occurs. For seal rings, thermal shock often occurs during emergency shutdowns, cold starts, or when process upsets cause sudden temperature changes.

The severity of thermal shock depends on three material properties: thermal expansion coefficient, thermal conductivity, and fracture strength. Materials with low expansion, high conductivity, and high strength resist thermal shock best.

Why SiC Excels 

Silicon carbide combines all three favorable properties. Its coefficient of thermal expansion is very low at 3.7 × 10⁻⁶ K⁻¹, meaning it expands and contracts very little with temperature changes. This minimizes the strain generated by thermal gradients.

Its thermal conductivity is exceptionally high at 160 W/m·K, allowing heat to flow quickly through the material. This prevents large temperature differences from developing across the seal ring. Heat spreads uniformly rather than creating hot spots.

Finally, its flexural strength of 450 MPa means it can withstand significant stress before failing. The combination of low expansion, high conductivity, and good strength gives SiC outstanding thermal shock resistance.

Testing Thermal Shock Resistance 

Standard thermal shock testing involves heating samples to a set temperature and then plunging them into cold water. The temperature difference that causes cracking is recorded. Alumina typically cracks at a ΔT of 200–280°C. Silicon nitride withstands ΔT of 750°C. Silicon carbide performs even better, with some grades surviving ΔT over 1,000°C.

This exceptional performance explains why silicon carbide seal rings are specified for applications involving steam, thermal cycling, and emergency shutdowns.

Applications That Benefit

Any application with frequent starts and stops benefits from SiC's thermal shock resistance. Steam systems that experience condensation shocks are prime candidates. Processes with emergency shutdown requirements need the reliability that SiC provides. Furnaces and high-temperature reactors that cycle between ambient and operating temperatures also benefit.

Need Seal Rings for Thermal Cycling Applications? 

At BSTEC, we specialize in silicon carbide seal rings for demanding thermal applications. Our engineering team can help you select the right SiC grade for your specific temperature profile.