UCI: Solving Hardness Tests on Tricky Surfaces

Many conventional hardness testing methods are not suited for specialized applications.

We're talking about the significant challenge of getting an accurate reading on thin materials, and the specific knowledge needed for how to do hardness testing for coatings or for hardness testing for case-hardened surfaces.

For professionals who are responsible for quality control, this is a known source of frustration. Anyone who has seen a standard indenter damage a finished component knows exactly what we are referring to.

From our experience, these applications require a more intelligent approach. This is why we so strongly recommend our UCI Series of Ultrasonic Portable Hardness Testers as the proper tool for the job.

A More Effective Approach to Hardness Testing

The fundamental issue with older methods is that they rely on heavy force, which is often too aggressive for delicate work. This makes a reliable method for how to do hardness testing for coatings or on thin sheets seem impossible with old equipment.

This is why we stand behind the practical design of the Ultrasonic Contact Impedance (UCI) method.

The operational principle is straightforward and effective. The tester uses a probe with a vibrating rod, which is pressed onto the component with a light, predetermined force. The rod creates a micro-indentation, and the change in its vibration frequency indicates the material's hardness.

A harder surface results in a greater frequency shift. The machine instantly provides a value you can depend on. This entire concept is a superior way to operate, offering clear, practical advantages:

- It Leaves a Minimal Impression: We believe this is a critical benefit. The micro-indentation is typically invisible to the naked eye, preserving the component's surface integrity and appearance.

- It Offers Complete Portability: Quality checks should not be restricted to a lab. These lightweight units are easy to carry, enabling you to perform tests on the production floor, in the field, or on large-scale fabrications.

- It Allows for Pinpoint Accuracy: The small probe tip enables you to get a reading on the precise area you need to measure, such as a weld, a heat-affected zone, or a component with a complex shape.

- It Delivers Immediate Results: We understand that efficiency is key. Obtaining a reliable reading in seconds significantly accelerates your quality control process.

Getting Accurate Results: How to Do Hardness Measurements on Thin Materials

When a client needs to know how to do hardness measurements on thin materials, for instance, checking the consistency of stamped automotive body panels or verifying the temper of thin-walled stainless steel tubing, we always caution them against using high-load equipment. 

A versatile, general-purpose unit like our Portable Ultrasonic Hardness Tester - UCI-3000H is ideal here.

From a technical standpoint, this aligns with industry standards like ASTM A1038, which stress the importance of proper sample fixation to avoid any influence from the substrate or edges (Burik & Pešek, 2014; Mihaliková et al., 2011). It's a precise method, but factors like how the sample is mounted can affect the final reading (Burik & Pešek, 2014).

For a valid test, we advise focusing on these key considerations:

- Provide a Solid Base: The material must be placed on a hard, flat surface. This prevents any flexing during the test, which is essential for a correct reading.

- Apply the Correct Test Load: Always start with the lightest appropriate force setting on the tester. This ensures the measurement is non-destructive and avoids influence from the support anvil.

- Confirm Your Material's Thickness: It's important to verify that the material meets the minimum thickness for the UCI method. This check ensures the reading reflects the material itself.

Achieving True Hardness Values: How to Do Hardness Testing for Coatings

For many components, the coating provides the essential performance characteristics. Think of a wear-resistant thermal spray coating on an aircraft landing gear component, or a hard chrome plating inside a hydraulic cylinder.

The success of these parts depends on proper hardness testing for coatings. This is a critical point because while the UCI method is excellent for this task, the results can be influenced by the substrate if the coating isn't thick enough (Gogolinskii et al., 2019).

For reliable results, it's essential to consider the properties of both the coating and the base material and use proper calibration protocols (Burik & Pešek, 2014). For maximum repeatability, a model with a motorized probe, like our UCI-3000M, ensures the test load is applied identically every time.

To correctly verify your coating hardness, we suggest this procedure:

- Maintain Probe Perpendicularity: Ensure the probe is held at a 90-degree angle to the surface. This is vital for creating a proper indentation and achieving a repeatable, accurate result on the thin layer.

- Select a Load Appropriate for the Coating: The test load must be light enough to contain the indentation entirely within the coating layer. This precision is the key to successful hardness testing for coatings.

- Prepare the Test Location: A smooth, clean surface is required for reliable results. If the surface is overly rough, readings can be inconsistent. A light polish of the test area can improve consistency.

Verifying Your Process with Hardness Testing for Case-Hardened Surfaces

In our professional opinion, hardness testing for case-hardened surfaces is a critical verification step. A perfect real-world scenario is ensuring the teeth of an industrial gearbox or the surface of a camshaft have been hardened correctly.

Research confirms that the UCI method is highly effective for these surfaces, as long as the hardened layer is deep enough to prevent influence from the softer core (Junek et al., 2017).

Recent advancements in UCI technology, such as probes with integrated load sensors, have further improved accuracy and reduced user-dependent variables, making them even more reliable for this specific application (Frehner, 2017; Frehner et al., 2017). 

For this, a specialized instrument like our Elongated Probe Ultrasonic Hardness Tester - UCI-3300C can be invaluable. To confirm your heat treatment was successful, follow these steps:

- Develop a Hardness Profile: Do not rely on a single reading. Take multiple measurements from the outer surface inward to map the hardness gradient and verify the effective case depth.

- Target High-Stress Areas: Use the pinpoint probe to test critical, high-wear areas—such as the tips of gear teeth—to ensure they have achieved the specified hardness.

- Isolate the Hardened Case: Always use a light test force. This ensures the indentation is made only in the hardened layer and prevents the softer core from causing a false, lower reading.

Your Partner for Advanced Hardness Testing

At Qualitest, we provide solutions to these specific industry challenges.

We focus on supplying tools our customers can depend on, and we know these tools must be cost-effective. Our range of Ultrasonic Portable Hardness Testers, from the workhorse UCI-3000 to specialized probe models, is engineered to solve these exact testing problems—from how to do hardness measurements on thin materials to verifying a hardened case.

We believe these user-friendly instruments are the optimal way to empower your team to conduct tests wherever they are needed.

We are committed to helping you improve your workflow and guarantee the integrity of your products. If you are looking for a better method for your hardness testing applications, we encourage you to review our collection of ultrasonic portable hardness testers.

Visit our product page or contact our team for more information.

References:

1. Burik, P., & Pešek, L. (2014). Effect of Experimental Factors on Hardness Measurement Using the UCI Technique. Materials Science Forum, 782, 61 - 64. https://doi.org/10.4028/www.scientific.net/msf.782.61

2. Frehner, C. (2017). NEW GENERATION OF UCI PROBES WITH ACCURATE LOAD DETECTION: OVERVIEW AND PRACTICAL BENEFITS.

3. Frehner, C., Mennicke, R., Gattiker, F., & Chai, D. (2017). Advancements of ultrasonic contact impedance (UCI) hardness testing based on continuous load monitoring during the indentation process, and practical benefits.

4. Gogolinskii, K., Syasko, V., Umanskii, A., Nikazov, A., & Bobkova, T. (2019). Mechanical properties measurements with portable hardness testers: advantages, limitations, prospects. Journal of Physics: Conference Series, 1384. https://doi.org/10.1088/1742-6596/1384/1/012012

5. Junek, M., Janovec, J., & Ducháček, P. (2017). Dependence of ultrasonic contact impedance hardness on Young’s modulus of elasticity of creep-resistant steels. ACTA IMEKO, 6, 27-32. https://doi.org/10.21014/acta_imeko.v6i1.325

6. Mihaliková, M., Német, M., & Zubko, P. (2011). USING THE VIDEOEXTENSOMETRY AND UCI - HARDNESS FOR MONITORING OF AUTOMOTIVE STEEL SHEETS.