Supersonic Microparticle Impact Experiments at Temperatures Approaching 2000 °C

This paper presents a modified laser-driven particle impact platform integrated with a resistive heating system and a portable vacuum chamber, enabling supersonic microparticle impact experiments on materials at temperatures approaching 2000 °C to study extreme condition behaviors such as the temperature-dependent cratering of POCO graphite.

The Gist

Imagine you are trying to understand how a car's paint job holds up when hit by tiny pebbles while driving at supersonic speeds through a scorching desert. That is essentially what this research is about, but instead of a car, they are testing advanced materials used in rockets and jet engines, and instead of pebbles, they are shooting microscopic particles at them.

Here is a breakdown of the paper using simple analogies:

The Big Problem: The "Double Whammy"

Materials in high-speed machines (like hypersonic jets or rocket engines) face a "double whammy." They are being hit by debris at incredibly high speeds (high strain rate) while simultaneously being baked by extreme heat (up to 2000°C).

• The Analogy: Think of a piece of chocolate. If you hit it with a hammer while it's cold, it shatters. If you hit it while it's warm, it squishes. But what happens if you hit it with a hammer while it's melting? Scientists need to know exactly how materials behave in this chaotic "melting-and-hitting" zone to build safer engines.

The Old Way vs. The New Way

Previously, scientists had two main ways to test this, but both had flaws:

1. The "Big Gun" (Gas Guns): These shoot large, millimeter-sized rocks. It's like trying to test how a car paint holds up against a baseball, when the real problem is dust and sand. Also, these machines are huge, expensive, and slow to use.
2. The "Microwave" (Heating): You could heat a material, but you couldn't easily shoot tiny particles at it at the same time without the heat ruining the equipment.

The New Solution: The "Laser Slingshot"

The researchers built a new machine called LIPIT (Laser-Induced Particle Impact Test). Think of it as a high-tech slingshot powered by a laser.

• How it works: They place a tiny particle (like a grain of sand) on a special metal foil. They zap the foil with a super-fast laser pulse. The laser vaporizes a tiny layer of the foil, creating a mini-explosion that catapults the particle forward at supersonic speeds (faster than sound).
• The Upgrade: The team modified this slingshot to work in a super-hot oven. They replaced the old rubbery parts (which would melt) with thin metal foils (aluminum and copper) that can handle the heat.

The "Vacuum Oven"

One major issue with heating things up is oxidation (rusting/burning). If you heat a piece of metal in the air, it gets covered in a layer of oxide, which changes how it breaks.

• The Analogy: It's like trying to study how a fresh apple bruises, but someone keeps spraying it with brown paint while you do it. You can't tell if the bruise is from the hit or the paint.
• The Fix: They built a portable vacuum chamber (a clear box with no air inside). This allows them to heat the target to 2000°C and shoot particles at it without the air interfering. It's like studying the apple in a clean, paint-free room.

The Experiment: The "Graphite Test"

To prove their machine works, they tested a material called POCO Graphite (used in rocket nozzles).

1. Room Temp: They shot particles at cold graphite. The dents were small and neat.
2. Hot in Air: They heated the graphite to 1040°C in the air. The surface got rough and pitted (like rust), and the dents were twice as deep. The heat made the material softer and more vulnerable.
3. Hot in a Vacuum: They heated the graphite to a blistering 1740°C inside their vacuum box. The surface stayed smooth and pristine (no rust). When they shot the particles, they saw a unique type of damage that only happens at these extreme temperatures without the "rust" interference.

Why This Matters

This new machine is like a high-speed, high-heat microscope for engineers.

• Speed: It can run hundreds of tests quickly (high-throughput).
• Precision: It uses tiny particles (microns) that match the real debris found in engines.
• Control: It separates the effects of "heat" from the effects of "rusting," giving scientists pure data.

The Bottom Line:
The team built a "laser slingshot" inside a "vacuum oven" that can shoot tiny rocks at materials while they are glowing hot. This helps engineers design better, safer rockets and jets that won't fall apart when flying through the fiery skies of the future.

Technical Summary

1. Problem Statement

Materials used in high-speed applications (e.g., hypersonic flight, gas turbines, rocket engines) are subjected to extreme, simultaneous mechanical and thermal loads. These conditions involve high-strain-rate deformations (exceeding $10^5/s$) and elevated temperatures (often $>1000^\circ\text{C}$).

• The Gap: Existing experimental techniques have limitations:
  • Split-Hopkinson Pressure Bars (SHPB): Limited to strain rates up to $10^4/s$ and temperatures up to $1600^\circ\text{C}$.
  • Gas Guns: Typically accelerate millimeter-sized particles, whereas erosion and Foreign Object Damage (FOD) often involve micron-sized particles. They also suffer from low throughput and high cost.
  • Centrifugal/Sandblasting Rigs: Provide bulk erosion data but fail to resolve microscale mechanisms of individual particle impacts.
  • Standard Laser-Induced Particle Impact Testing (LIPIT): Previously limited to room temperature or low temperatures ($\le 300^\circ\text{C}$) because traditional elastomeric expansion layers (e.g., PDMS) degrade at high heat. Furthermore, conducting these experiments in air at ultra-high temperatures causes rapid oxidation of the target, masking the true thermal-mechanical response.

Objective: Develop a high-throughput experimental platform capable of accelerating microparticles to supersonic velocities while impacting targets heated to near $2000^\circ\text{C}$ in a controlled (vacuum/inert) environment to isolate thermal effects from oxidation.

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2. Methodology

The authors developed a modified Laser-Induced Particle Impact Test (LIPIT) system integrated with a resistive heating system and a custom vacuum chamber.

A. Laser-Driven Launch System

• Driver: A Q-switched Nd:YAG laser (1064 nm, up to 3 J, 4–9 ns pulses) with a homogenized "top-hat" beam profile.
• Launch Pad Innovation: Replaced traditional elastomeric layers with ductile metal foils (100 $\mu$m Aluminum or 50 $\mu$m Copper) bonded to a glass substrate with an epoxy ablation layer.
  • Mechanism: The laser ablates the epoxy, generating high pressure that rapidly expands the metal foil, accelerating the microparticle to supersonic speeds.
  • Benefit: Metal foils withstand extreme temperatures without degrading, unlike polymers, and can launch denser/larger particles (e.g., Tungsten Carbide) without fragmentation.

B. Target Heating and Mounting

• Resistive Heating: Targets are held between two tungsten rods acting as electrodes. A programmable DC power supply (up to 80 V, 60 A) passes current through the target, heating it via Joule heating.
• Temperature Monitoring: Dual Infrared (IR) cameras (Optris Xi400 for $<900^\circ\text{C}$ and PI05M for $900\text{--}2450^\circ\text{C}$) monitor surface temperatures. Calibration confirmed that lateral surface measurements accurately reflect the impact zone temperature.
• Sample: Ultra-fine grain POCO graphite (ZXF-5Q) was used as the primary case study material.

C. Vacuum Chamber Integration

• Design: A custom-machined Aluminum 6061 chamber with optical windows (Borosilicate or Potassium Bromide) for IR imaging and laser access.
• Function: Achieves high vacuum ($5 \times 10^{-4}$ mbar) to prevent oxidation/nitridation of the target at temperatures $>1300^\circ\text{C}$.
• Alignment: The entire chamber is mounted on XYZ stages to align the particle launch site with the laser focus and high-speed imaging optics.

D. Diagnostics

• High-Speed Imaging: A Shimadzu HPV-X2 camera (up to 10 million FPS) with a long-working-distance microscope lens captures impact and rebound velocities.
• Post-Mortem Analysis: Laser confocal microscopy (Keyence VK-X3000) and 3D profilometry measure crater morphology and dimensions.

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3. Key Contributions

1. High-Temperature LIPIT Platform: Successfully adapted LIPIT for temperatures approaching $2000^\circ\text{C}$ by replacing elastomeric launch pads with metal foil expansion layers.
2. Vacuum Integration: Developed an optically accessible vacuum chamber that allows for particle impact experiments in high-vacuum environments, eliminating oxidation artifacts at ultra-high temperatures.
3. Direct Resistive Heating: Implemented a robust, direct-contact resistive heating method using tungsten electrodes, capable of heating millimeter-scale conductive targets rapidly (reaching $1740^\circ\text{C}$ in $<6$ seconds).
4. High-Throughput Capability: The system retains the high-throughput nature of LIPIT, with a single launch pad accommodating up to 25 experiments, enabling the generation of large statistical datasets required for predictive modeling.

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4. Results

The system was validated using POCO graphite targets under three conditions:

• Room Temperature (RT): Baseline experiments confirmed standard cratering behavior with alumina particles ($\sim 60 \mu$m) impacting at $>500$ m/s.
• High Temperature in Air ($1040^\circ\text{C}$):
  • The heated target showed significant surface roughening due to oxidation.
  • Crater Depth: Increased to 15.2 $\mu$m (more than double the RT depth of 7.3 $\mu$m).
  • Morphology: Significantly reduced material piling at the crater rim compared to RT, indicating thermal softening and oxidation-induced weakening.
  • Limitation: Experiments above $1300^\circ\text{C}$ in air resulted in excessive oxidation/pitting, making post-mortem analysis impossible.
• Ultra-High Temperature in Vacuum ($1740^\circ\text{C}$):
  • Successfully achieved impact at $1740^\circ\text{C}$ under vacuum.
  • Surface Integrity: The impact site remained pristine and polished, confirming the absence of oxidation.
  • Morphology: Revealed a unique crater morphology distinct from both RT and atmospheric high-temperature impacts, attributed purely to thermal softening and strain-rate effects.
  • Velocity: Particles were accelerated to supersonic speeds (e.g., 465 m/s) with rebound velocities measured (185 m/s).
• Limits: Temperatures exceeding $2000^\circ\text{C}$ were approached, but the system was limited by the thermal softening and deformation of the tungsten electrodes, which caused misalignment and loss of contact.

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5. Significance

• Material Modeling: This platform provides critical data on the coupled effects of high strain rates and high temperatures, which are essential for developing accurate constitutive models for materials in hypersonic and propulsion systems.
• Decoupling Effects: By enabling vacuum experiments, researchers can now distinguish between damage caused by thermal softening and damage caused by chemical oxidation/nitridation.
• Scalability: The system bridges the gap between low-throughput gas guns and bulk erosion rigs, offering a high-throughput method to study micron-scale Foreign Object Damage (FOD) mechanisms under realistic extreme conditions.
• Future Applications: The methodology supports the investigation of advanced materials (ceramics, composites, refractory metals) for next-generation aerospace applications where current testing capabilities are insufficient.