Catalytic Converter

High‑Reliability Catalytic Substrates for Aircraft Exhaust Systems – Solving Extreme Environment Pain Points

This one came from a Tier 1 aerospace supplier. They made exhaust components for regional jets. Not the engine itself – the exhaust system downstream of the turbine. They had a problem with a catalytic converter that kept failing.

The converter was supposed to clean up emissions from an auxiliary power unit. The APU runs when the plane is on the ground. It provides power and air conditioning. But the exhaust from an APU is nasty – high heat, high vibration, and the space is crammed into the tail cone, where everything is hot and nothing has room to breathe.

Their existing substrate from another supplier would crack after about 500 operating hours. Sometimes less. They'd pull the converter, see the honeycomb broken into pieces. They tried thicker foil. Tried different coatings. Nothing worked.

They came to us because they heard we did custom work for tough environments.

The Pain Points

We flew out to their facility – my first time in an aerospace plant. Everything was clean. Too clean. But the problem was real.

They laid out three big issues.

Heat. The APU exhaust hit 650 degrees Celsius continuous. Spikes to 750 during certain cycles. Most catalytic substrates start to soften around 600. Aluminum would turn to putty. Even standard stainless would creep over time.

Vibration. The tail cone of a jet is not a smooth place. The APU shakes. The airframe shakes. The exhaust system shakes with everything. Their old substrate was cracking along the brazed joints – classic vibration fatigue.

Space. The converter had to fit into a cylindrical can that was only 4 inches in diameter and 6 inches long. Small. That meant the cell density had to be just right – too many cells and backpressure would spike, too few and the emissions wouldn't clean up.

Also, weight mattered. Not as much as on a wing, but every pound still counted.

What We Did Differently

We didn't just send them a sample. We spent a week going through their data. Vibration logs. Temperature profiles. Backpressure requirements. Emissions targets.

Then we built a prototype batch of 20 substrates.

Material. We used Inconel 625. Not stainless. Not aluminum. Inconel. It's a nickel‑chromium alloy that stays strong at 750 degrees. Expensive as hell. But it doesn't creep, doesn't oxidize, doesn't crack. For an APU that runs thousands of hours, it was worth it.

Cell density. 300 cpsi. Lower than standard automotive. Bigger cells meant less backpressure through that small can. And the bigger cells were less likely to plug from any soot the APU made.

Foil thickness. 0.08 mm. Thicker than automotive, but not as thick as industrial. We balanced durability with light‑off time. The APU runs continuous, so fast light‑off wasn't critical.

Brazing. We used a high‑temperature nickel‑based brazing filler. Melting point over 1,000 degrees. It would never soften in the APU exhaust.

Mounting mat. We used a dense, high‑temp mat that doesn't take a permanent set. Designed for vibration. We also added a secondary retention ring inside the can – a metal lip that held the substrate in place even if the mat lost tension.

The whole thing weighed about 30% more than their old aluminum substrate. But it was still under their weight budget.

Testing – Real World, Not Just Lab

They didn't just put our substrates on a shaker table. They put them on an actual APU, in a test cell, running real cycles.

Four hundred hours of continuous operation, with temperature spikes to 750 every few hours. Then they shut it down, let it cool to ambient, and started again.

We did 2,000 hours of testing. That's about five times their old substrate's lifespan.

After 2,000 hours, they cut the converter open. The Inconel substrate looked almost new. No cracking. No sagging. The brazed joints were solid. The mat was still tight.

They did a second test on a different APU. Same result.

After that, they ordered 100 pieces for fleet trials. Those have been running for two years now. No failures.

What We Learned

Inconel works. It's expensive – about four times the cost of stainless. But for an aerospace application where a failure means a plane on the ground, that cost is worth it.

Brazing Inconel is different than brazing stainless. The filler has to match the alloy. We had to adjust our furnace cycle – higher temperature, longer soak. The first few samples had incomplete braze flow. We tweaked the cycle and got it right.

The retention ring was a good addition. The mat alone might have held, but with the ring, there's no chance of the substrate moving. Over‑engineered? Maybe. But in aerospace, that's the standard.

Also, documentation. Aerospace customers want a paper trail for everything. Foil certs, braze logs, test reports, dimensional data. We had to upgrade our record‑keeping. But now we have it, and it helps with all our customers.

What the Customer Said

Their project engineer told me: "We've tried four other suppliers. Your Inconel part is the only one that survived our full test cycle. We're spec'ing it into our new platform."

The purchasing guy said: "The price made me choke. But the zero failures made it worth it. We haven't had a single warranty claim on these."

Aircraft exhaust systems are not forgiving. High heat. High vibration. Tight spaces. Long operating hours. Most standard substrates – even good stainless ones – can't handle it.

But Inconel can. With the right cell density, the right brazing, and a mounting system that won't let go.

We built that substrate for one aerospace customer. Now we've made it for two others. The tooling is dialed in. The process is repeatable.

If you have an extreme environment – aviation, military, high‑temp industrial – talk to us. We'll tell you if Inconel is the answer. And if it's not, we'll find something that is. We've done it before.