Copper and Copper Alloys Sample Preparation

Introduction

Copper and its alloys (brass, bronze) are commonly analyzed materials in metallography. Common examples include electrolytic tough pitch copper,cartridge brass, and various bronze alloys. Proper preparation is essential to reveal the true microstructure without introducing artifacts such as deformation, scratches, smearing, or contamination. This guide will walk you through the complete preparation process.

Tough pitch copper, ASTM No. 30 etchant, 200X magnification. This image demonstrates the proper microstructure revealed through correct preparation techniques.

Copper alloys can be challenging due to their softness and tendency to deform and smear easily. The key is to use appropriate abrasives, maintain light pressure throughout the process, and avoid over-polishing which can introduce relief and smearing. Different copper alloys may require slight variations in technique. For example, softer alloys like electrolytic tough pitch copperrequire extra care to prevent smearing, while harder alloys like beryllium coppercan tolerate slightly higher pressures during preparation.

Sectioning

When sectioning copper and copper alloy samples, use a slow cutting speed to minimize heat generation and deformation. A cutting speed of 100-200 RPM is typically appropriate for most copper alloys. Copper's softness requires careful handling to prevent deformation and smearing. This is especially important for pure copper grades like C11000and softer brass alloys like C26000 cartridge brass.

MAX-E abrasive cut-off blades designed for soft non-ferrous materials like copper and copper alloys. Thin blades (0.5-1.0 mm) minimize heat generation and deformation.

• Use MAX-E series blades (designed for soft non-ferrous materials like copper and copper alloys)
• Use a thin abrasive cut-off wheel (0.5-1.0 mm thickness)
• Apply steady, light pressure to avoid deformation
• Use adequate coolant to prevent overheating and smearing
• Allow the wheel to do the cutting - avoid forcing
• Consider using a slower feed rate than for harder materials

Mounting

Mounting provides edge retention and easier handling. For copper and copper alloys, compression mounting with phenolic or epoxy resins works well. Cold mounting with epoxy is also suitable and avoids potential heat-related issues that could affect the microstructure.

Compression Mounting

1. Clean the sample thoroughly to remove cutting fluid and debris
2. Place sample in mounting press with appropriate resin
3. Apply pressure: 3000-4000 psi for phenolic, 2000-3000 psi for epoxy
4. Heat to 150-180°C and hold for 5-8 minutes
5. Cool under pressure to room temperature

Cold Mounting

1. Clean and dry the sample
2. Place in mounting cup with epoxy resin
3. Allow to cure at room temperature (typically 4-8 hours)
4. Cold mounting avoids heat that could affect copper microstructure

Grinding

Grinding removes sectioning damage and prepares the surface for polishing. Start with coarse grits and progressively move to finer grits. For copper and copper alloys, use lighter pressure than for harder materials to avoid deformation. We recommend starting with 240-320 grit and progressing through finer grits. This approach works well for common alloys like C26000 cartridge brassand C36000 free-cutting brass:

Silicon carbide (SiC) grinding papers in various grit sizes (240, 320, 400, 600) for progressive grinding. Rotate sample 90° between each grit to ensure complete scratch removal.

Grinding Sequence

1. 240 grit: Remove sectioning damage (20-40 seconds per step)
2. 320 grit: Remove previous scratches (20-40 seconds)
3. 400 grit: Further refinement (20-40 seconds)
4. 600 grit: Final grinding step (20-40 seconds)

Important: Rotate the sample 90° between each grit to ensure complete removal of previous scratches. Use water as a lubricant and maintain light pressure to avoid deformation. Copper alloys require less time per step than harder materials. For very soft copper alloys, you may start with 240 grit instead of 120 grit to minimize deformation.

Polishing

Polishing removes grinding scratches and prepares a mirror-like surface. For copper and copper alloys, diamond polishing followed by oxide polishing typically yields excellent results. Use softer cloths and lighter pressure to avoid smearing and relief. The recommended sequence is 6 μm → 3 μm → 1 μm diamond, followed by final polishing with 0.05 μm colloidal silica.

Polycrystalline diamond compound provides consistent cutting action for copper alloys.

Soft to medium polishing pads are recommended for copper alloys to prevent smearing and excessive relief.

Diamond Polishing

1. 6 μm diamond: 2-3 minutes on a medium-soft cloth (e.g., Microcloth)
2. 3 μm diamond: 2-3 minutes on a soft cloth
3. 1 μm diamond: 1-2 minutes on a soft cloth

Final Polishing

1. 0.05 μm colloidal silica: 30-60 seconds on a soft cloth
2. Rinse thoroughly with water and dry with compressed air

Important: Use lighter pressure than for steel. Over-polishing can introduce relief around second phases and inclusions. Monitor the surface frequently to avoid smearing, which is common with soft materials like copper. Gentle polishing is required to maintain the true microstructure.

Etching

Etching reveals the microstructure by selectively attacking grain boundaries and phases. The choice of etchant depends on the copper alloy and what features you want to reveal. Common etchants include Copper No. 1, Copper No. 2, ASTM No. 30, and various specialized solutions for specific alloys.

Tough pitch copper etched with ASTM No. 30, 200X magnification. Proper etching reveals grain boundaries and phase structure without over-etching artifacts.

Common Etchants for Copper and Copper Alloys

• Copper No. 1: Standard etchant for copper and brass. Reveals grain boundaries and twin boundaries (125ml HNO₃, 125ml H₂O). Most common for copper and brass alloys like C11000 and C26000 cartridge brass.
• Copper No. 2: Alternative to Copper No. 1. Effective for copper and brass alloys. Reveals grain structure and phases (200ml H₂O, 50ml HCl, 10g FeCl₃).
• ASTM No. 30: Standard ASTM etchant for copper and copper alloys. Reveals grain boundaries and structure. Fresh solution works best (62.5ml NH₄OH, 125ml H₂O₂ 3%, 62.5ml H₂O). Excellent for tough pitch copper.
• Ammonium Persulfate: For brasses with cobalt. Reveals grain boundaries and phase structure (50g (NH₄)₂S₂O₈, 245ml H₂O).
• Potassium Dichromate: For revealing grain boundaries in brass and bronze alloys. Produces good contrast (2g K₂Cr₂O₇, 8ml H₂SO₄, 4ml NaCl sat., 100ml H₂O). Effective for bronze alloys like aluminum bronze.
• Dichromate Etchant: For Cu-Sn (tin bronze) alloys. Reveals grain boundaries and tin phase (36ml H₂SO₄, 445ml H₂O, 18ml NaCl sat., 8.9g K₂Cr₂O₇).

Etching solutions and reagents for copper and copper alloys. Common etchants include Copper No. 1, Copper No. 2, ASTM No. 30, and specialized solutions. Etching time typically ranges from 5-45 seconds depending on the etchant and alloy.

Etching Procedure

1. Ensure sample is clean and dry
2. Apply etchant with cotton swab or immerse sample
3. Etch for 5-45 seconds (time varies by etchant and alloy - Copper No. 1 typically 5-45 sec at 20°C)
4. Immediately rinse with water, then alcohol
5. Dry with compressed air

Tip: Start with shorter etching times (5-10 seconds) and increase if needed. Over-etching can obscure fine details and create pitting. For Copper No. 1, use at room temperature (20°C) and ensure fresh solution for best results. ASTM No. 30 requires fresh solution and works well with swab application.

Troubleshooting

Common Issues and Solutions

• Scratches remaining: Insufficient grinding/polishing time or skipped grits. Ensure complete scratch removal at each step.
• Smearing: Too much pressure or too hard a cloth. Use softer cloths and lighter pressure. This is a common issue with soft copper alloys.
• Relief around second phases: Over-polishing or too soft a cloth. Reduce polishing time or use slightly harder cloth.
• Contamination: Clean between steps, use fresh abrasives, and ensure proper sample cleaning. Copper can easily pick up contamination.
• Deformation: Too much pressure during grinding or polishing. Use lighter pressure throughout the process.
• Over-etching: Reduce etching time or dilute etchant. Start with shorter times (5-10 seconds).
• Pitting after etching: Etchant too strong or etching time too long. Dilute etchant or reduce time. Ensure fresh etchant solution.
• Poor grain boundary contrast: Try different etchant or increase etching time slightly. Ensure sample is properly polished before etching.