Hardness Testing Sample Preparation

Introduction

Preparing samples for hardness testing requires specific considerations that differ from standard metallographic preparation. While the basic steps (sectioning, mounting, grinding, polishing) remain similar, hardness testing has unique requirements for surface finish, edge retention, and sample geometry that must be carefully addressed.

The quality of hardness test results depends directly on sample preparation quality. Poor preparation can lead to inaccurate readings, inconsistent results, and invalid test data. This guide covers the specific requirements for preparing samples for various hardness testing methods.

Hardness Test Types and Requirements

Different hardness testing methods have varying requirements for sample preparation. Understanding these requirements helps you prepare samples appropriately for each test type.

Rockwell Hardness Testing

Rockwell testing uses relatively large indenters and loads, making it less sensitive to surface finish than microhardness tests. However, proper preparation is still essential.

• Surface finish: Typically 0.8 μm Ra or better (600 grit finish is usually sufficient)
• Sample thickness: Must be at least 10 times the indentation depth
• Flatness: Surface must be flat to prevent anvil effects
• Edge distance: Indentation must be at least 2.5 times the indentation diameter from any edge
• Parallelism: Top and bottom surfaces should be parallel

Vickers Microhardness Testing

Vickers testing uses a diamond pyramid indenter and is more sensitive to surface finish. Higher quality preparation is required.

• Surface finish: 0.1-0.2 μm Ra or better (polished to 1 μm diamond minimum)
• Sample thickness: At least 1.5 times the indentation diagonal
• Flatness: Critical for accurate measurements
• Edge distance: At least 2.5 times the indentation diagonal from edges
• Scratch-free: Scratches can interfere with indentation measurement

Knoop Microhardness Testing

Knoop testing uses an elongated diamond indenter, making it ideal for thin samples and case depth measurements. Requires excellent surface finish.

• Surface finish: 0.1 μm Ra or better (polished to 0.05 μm colloidal silica)
• Sample thickness: Can test thinner samples than Vickers
• Flatness: Extremely critical due to elongated indenter shape
• Edge retention: Essential for case depth measurements
• Scratch-free: Even minor scratches can affect measurements

Brinell Hardness Testing

Brinell testing uses a large ball indenter and heavy loads, making it less sensitive to surface finish but requiring adequate sample thickness.

• Surface finish: 1.6 μm Ra or better (400-600 grit finish usually sufficient)
• Sample thickness: Must be at least 10 times the indentation depth
• Flatness: Important to prevent anvil effects
• Edge distance: At least 2.5 times the indentation diameter from edges

Sectioning for Hardness Testing

Sectioning samples for hardness testing requires careful attention to prevent work hardening, overheating, or other damage that could affect hardness measurements.

Key Considerations

• Minimize damage: Use appropriate cutting speeds and adequate coolant to prevent work hardening or phase transformation
• Cut location: Section through the area of interest (e.g., through case-hardened layer, weld zone, or specific microstructure)
• Sample size: Ensure adequate size for mounting and testing (typically 25-32 mm diameter mounts work well)
• Orientation: Consider the direction of hardness testing relative to material structure (e.g., perpendicular to case-hardened surface)

Cutting Parameters

Use slower cutting speeds and adequate coolant to minimize thermal and mechanical damage:

• Cutting speed: 100-200 RPM for most materials (slower for hard materials)
• Coolant: Use adequate coolant flow to prevent overheating
• Feed rate: Steady, moderate pressure - let the wheel do the cutting
• Blade selection: Choose appropriate abrasive blades for the material

Mounting for Hardness Testing

Mounting provides edge retention, easier handling, and creates a flat surface for testing. For hardness testing, mounting is often essential, especially for case depth measurements and edge testing.

Mounting Material Selection

Choose mounting materials that provide adequate support and edge retention:

• Phenolic: Hard, provides excellent edge retention. Good for most hardness testing applications. Preferred for case depth measurements.
• Epoxy: Softer than phenolic but still provides good edge retention. Suitable for most applications.
• Conductive mounting: Required for some automated hardness testers that use electrical contact for positioning
• Transparent mounting: Useful when you need to see sample edges or features

Mounting Considerations

• Edge retention: Critical for case depth measurements and edge hardness testing. Use harder mounting materials and proper mounting techniques.
• Sample orientation: Mount sample so the test surface is perpendicular to the mounting surface
• Mount size: Standard 25-32 mm diameter mounts work well for most hardness testers
• Parallelism: Ensure mounted sample surface is parallel to the mount base for consistent testing
• Mounting pressure: Use appropriate pressure to ensure good adhesion without damaging the sample

Grinding for Hardness Testing

Grinding removes sectioning damage and creates a flat surface. For hardness testing, grinding must be thorough to remove all damage while avoiding work hardening.

Grinding Sequence

Use a progressive grinding sequence to remove damage:

1. 120 grit: Remove sectioning damage (30-60 seconds)
2. 240 grit: Remove previous scratches (30-60 seconds)
3. 400 grit: Further refinement (30-60 seconds)
4. 600 grit: Final grinding step (30-60 seconds)

Grinding Techniques

• Rotate 90°: Rotate sample 90° between each grit to ensure complete scratch removal
• Consistent pressure: Use moderate, consistent pressure - avoid excessive pressure that could cause work hardening
• Adequate time: Spend sufficient time at each step to remove all previous scratches
• Water lubrication: Use water as lubricant to prevent overheating
• Check progress: Visually inspect to ensure all previous scratches are removed

Polishing for Hardness Testing

Polishing creates the smooth, scratch-free surface required for accurate hardness testing. The polishing quality required depends on the hardness test method.

Polishing Sequence

For most hardness testing applications, use the following sequence:

1. 9 μm diamond: 3-5 minutes on hard cloth (e.g., Texmet)
2. 3 μm diamond: 3-5 minutes on medium-hard cloth
3. 1 μm diamond: 2-3 minutes on soft cloth
4. 0.05 μm colloidal silica: 1-2 minutes on soft cloth (required for Vickers and Knoop testing)

Polishing Requirements by Test Type

Polishing Techniques

• Consistent pressure: Use moderate, consistent pressure throughout polishing
• Adequate time: Spend sufficient time at each step to remove all previous scratches
• Clean between steps: Thoroughly clean sample between polishing steps to prevent contamination
• Check surface: Inspect surface under microscope to ensure scratches are removed
• Avoid over-polishing: Over-polishing can create relief and affect flatness

Surface Requirements

Hardness testing requires specific surface characteristics that differ from standard metallographic preparation. Understanding these requirements ensures accurate test results.

Surface Finish

Surface roughness (Ra) requirements vary by test method:

• Rockwell: ≤ 0.8 μm Ra (typically achieved with 600 grit finish)
• Vickers: ≤ 0.1-0.2 μm Ra (requires diamond polishing to 1 μm minimum)
• Knoop: ≤ 0.1 μm Ra (requires final polishing with 0.05 μm colloidal silica)
• Brinell: ≤ 1.6 μm Ra (typically achieved with 400-600 grit finish)

Flatness

Surface flatness is critical for accurate hardness measurements:

• General requirement: Surface should be flat within 0.05 mm over the test area
• Microhardness: Even stricter flatness requirements (0.01-0.02 mm)
• Measurement: Use a straightedge or optical flat to check flatness
• Correction: If surface is not flat, re-polish with attention to maintaining even pressure

Scratch-Free Surface

For microhardness testing (Vickers and Knoop), the surface must be completely scratch-free:

• Visual inspection: Examine at 200-500x magnification
• No visible scratches: Any scratches can interfere with indentation measurement
• Clean surface: Remove all polishing residue and contaminants
• Proper lighting: Use appropriate illumination to reveal scratches

Surface Cleanliness

Clean surfaces are essential for accurate hardness testing:

• Remove polishing residue: Thoroughly clean after final polishing
• No contaminants: Ensure no oil, fingerprints, or other contaminants
• Proper cleaning: Use water, alcohol, or appropriate solvents
• Dry surface: Ensure surface is completely dry before testing

Edge Retention

Edge retention is critical for case depth measurements, edge hardness testing, and testing near sample boundaries. Poor edge retention can lead to inaccurate measurements and invalid test results.

Why Edge Retention Matters

During grinding and polishing, edges can become rounded or damaged, making it difficult or impossible to make accurate hardness measurements near edges. This is especially important for:

• Case depth measurements: Hardness must be measured starting at the surface
• Edge hardness: Testing hardness near sample edges or boundaries
• Coating thickness: Measuring hardness through thin coatings
• Weld zones: Testing hardness across weld boundaries

Techniques for Edge Retention

• Hard mounting materials: Use phenolic or other hard mounting resins that provide better edge support
• Reduced pressure: Use lighter pressure during grinding and polishing, especially near edges
• Edge protection: Consider using edge protection techniques or fixtures
• Progressive refinement: Use finer abrasives and lighter pressure as you approach final polish
• Shorter polishing times: Avoid over-polishing which can round edges

Verifying Edge Retention

Before testing, verify that edges are properly retained:

• Visual inspection: Examine edges under microscope at 100-200x
• Sharp edges: Edges should appear sharp and well-defined
• No rounding: Check for edge rounding or chamfering
• Consistent edge: Edge should be consistent along the length

Case Depth Testing Preparation

Case depth measurements require special preparation techniques to ensure accurate hardness profiles from the surface into the core material.

Sample Orientation

Mount samples so the case-hardened surface is perpendicular to the mount surface:

• Perpendicular mounting: Case surface should be at 90° to mount base
• Consistent orientation: Ensure all samples are mounted consistently
• Marking: Mark the case surface to identify it during preparation

Preparation Requirements

• Excellent edge retention: Critical for accurate surface hardness measurement
• Flat surface: Surface must be flat to allow accurate depth measurements
• Scratch-free: Required for microhardness testing
• Proper polishing: Polish to 0.05 μm colloidal silica for Knoop testing

Measurement Considerations

When preparing for case depth measurements:

• Test direction: Hardness measurements are made perpendicular to the case surface
• Starting point: First measurement should be at or very near the surface
• Measurement spacing: Typically 0.05-0.1 mm spacing for detailed profiles
• Depth range: Prepare surface to allow testing through the entire case depth

Troubleshooting

Common problems encountered when preparing samples for hardness testing and their solutions:

Common Issues and Solutions

Quality Verification

Before hardness testing, verify sample quality:

• Visual inspection: Examine surface at 200-500x magnification
• Scratch-free: No visible scratches (critical for microhardness)
• Flat surface: Check flatness with straightedge or optical flat
• Edge retention: Verify edges are sharp and well-defined
• Clean surface: Ensure no contamination or residue
• Proper finish: Surface finish meets requirements for test method