Polishing Methods

Introduction

Polishing is the final step in metallographic sample preparation that removes grinding scratches and creates a mirror-like surface suitable for microstructural analysis. Proper polishing technique is essential for revealing true microstructures without introducing artifacts such as relief, contamination, or deformation.

Final polishing consumables including diamond abrasives, oxide suspensions, and polishing pads. Proper selection ensures mirror-like surfaces suitable for microstructural analysis.

The polishing process typically involves multiple stages, progressing from coarse diamond abrasives to fine oxide suspensions. Each stage must be carefully executed to ensure complete removal of previous scratches while maintaining sample integrity.

Diamond Polishing

Diamond polishing is the primary method for removing grinding scratches and preparing the surface for final polishing. Diamond abrasives are available in various particle sizes and formulations to suit different materials and applications.

Polycrystalline diamond: aggressive cutting, ideal for hard materials

Monocrystalline diamond: gentler cutting, less aggressive

Diamond paste: grease-based for manual polishing applications

Diamond Polishing Sequence

1. 9 μm diamond: Remove grinding scratches (3-5 minutes)
2. 3 μm diamond: Further refinement (3-5 minutes)
3. 1 μm diamond: Fine polishing (2-3 minutes)
4. 0.25 μm diamond (optional): Ultra-fine preparation (1-2 minutes)

Diamond Abrasive Types

• Polycrystalline Diamond: Aggressive cutting, good for hard materials
• Monocrystalline Diamond: Softer cutting action, less aggressive
• Diamond Suspensions: Water-based or oil-based suspensions
• Diamond Pastes: Grease-based for manual applications

Polishing Abrasives & Suspensions

Understanding the different types of polishing abrasives and their applications is crucial for achieving optimal results. Each abrasive type has specific characteristics that make it suitable for particular materials and polishing stages.

Diamond Abrasives

• Polycrystalline Diamond: Multiple cutting edges, aggressive removal, ideal for hard materials
• Monocrystalline Diamond: Single crystal structure, gentler cutting, less deformation
• Available Forms: Suspensions, pastes, sprays
• Particle Sizes: 15 μm down to 0.25 μm

Oxide Abrasives

• Alumina (Alpha): Standard alpha-phase alumina for general polishing
• Alumina (Deagglomerated): Processed to prevent agglomeration, more consistent
• Alumina (Low Viscosity): Lower viscosity for better flow and coverage
• Alumina (Polycrystalline): Multiple crystal structure for aggressive polishing
• Colloidal Alumina: Ultra-fine particles in suspension, excellent for final polishing
• Colloidal Silica: Very fine silica particles, produces mirror-like finishes
• Cerium Oxide: Effective for glass and some ceramics
• CMP Alumina Silica: Chemical-mechanical polishing blend

Polishing Cloths & Pads

The choice of polishing pad significantly affects polishing results. Different pad types provide varying levels of hardness, nap, and cutting action. Selecting the appropriate pad for each polishing stage is essential.

Various polishing pads and cloths for different polishing stages. Pad selection affects hardness, nap, and cutting action - choose based on material and polishing stage.

Polishing Pad (PSA) Selection Guideline

Oxide Polishing

Oxide polishing follows diamond polishing and removes fine scratches while producing a high-quality surface finish. Oxide abrasives are typically used in the final polishing stages.

Oxide Polishing Sequence

1. 0.3 μm alumina: Remove fine diamond scratches (2-3 minutes)
2. 0.05 μm colloidal silica: Final polish for mirror finish (1-2 minutes)

Oxide Selection Guidelines

• Standard Alumina: General purpose, most materials
• Deagglomerated Alumina: When consistency is critical
• Colloidal Silica: For ultra-fine finishes, minimal relief
• Low Viscosity Alumina: For better coverage and flow
• Cerium Oxide: Specialized for glass and ceramics

Important: Always use fresh oxide suspensions and clean the sample thoroughly between diamond and oxide polishing to prevent contamination.

Final Polishing

Final polishing produces the mirror-like surface required for high-quality microstructural analysis. This stage requires careful attention to detail and appropriate selection of abrasives and cloths.

Colloidal silica (0.05 μm) is the most common final polishing abrasive, producing excellent mirror-like finishes with minimal relief.

Result of proper final polishing - 431 Stainless steel, 400X. The mirror-like surface reveals true microstructure without artifacts.

Final Polishing Techniques

• Use ultra-soft cloths (silk, microcloth)
• Apply light pressure (0.5-1.5 lbs)
• Use fresh, high-quality oxide suspensions
• Monitor polishing time to avoid over-polishing
• Clean sample thoroughly after polishing

Final Polishing Abrasives

• Colloidal Silica (0.05 μm): Most common, produces excellent finishes
• Colloidal Alumina (0.05 μm): Alternative to silica, less aggressive
• Deagglomerated Alumina: For consistent, uniform polishing
• Low Viscosity Alumina: Better flow and coverage

Controlled Material Removal

Controlled removal polishing, also known as metered removal, allows for precise material removal in micron-level increments. This technique is essential for applications requiring extreme precision, such as:

• IC Flip Chip preparation
• SEM (Scanning Electron Microscopy) sample preparation
• FIB (Focused Ion Beam) sample preparation
• TEM (Transmission Electron Microscopy) sample preparation
• AFM (Atomic Force Microscopy) sample preparation
• EBSD (Electron Backscatter Diffraction) sample preparation
• Petrographic analysis
• Optical analysis requiring specific depth

How Controlled Removal Works

Controlled removal systems use precision measurement and feedback to remove material in exact increments. The system monitors removal in real-time with micrometer-level resolution, allowing operators to polish to specific depths or remove precise amounts of material.

Key Features

• Micrometer-adjustable pitch and roll control for exacting results
• Real-time removal monitoring with 0.2 micron resolution
• Precise sample load control (0-300 grams)
• Variable speed control for optimal material removal rates
• Touchscreen interface for easy operation

Recommended Equipment: ATTO-1000S Controlled Removal Polisher

The ATTO-1000S offers precision-controlled removal with micrometer-level accuracy, making it ideal for applications requiring extreme specificity for points of interest.

View ATTO-1000S Controlled Removal Polisher →

Vibratory Polishing

Vibratory polishing is a cost-effective alternative for final polishing that eliminates subsurface damage and produces superior surface finishes. This method is particularly effective for EBSD sample preparation and applications requiring minimal deformation.

Advantages of Vibratory Polishing

• Eliminates subsurface damage
• Produces superior flatness and planarity
• Reduces operator variability
• Cost-effective compared to laser removal systems
• Ideal for batch processing
• Pulse mode prevents staining

Vibratory Polishing Process

1. Prepare sample through standard grinding and initial polishing
2. Select appropriate polishing bowl size (9" or 12")
3. Add polishing suspension to bowl
4. Set frequency and power for desired polishing rate
5. Place samples in bowl and allow polishing
6. Monitor progress and adjust as needed

Applications

• EBSD sample preparation
• Final polishing for high-quality microstructural analysis
• Batch processing multiple samples
• Applications requiring minimal deformation

Recommended Equipment: GIGA-S Vibratory Polisher

The GIGA-S vibratory polisher ensures superior surface finishes with precise vibration control and interchangeable polishing bowls. A cost-effective solution for final polishing without the need for laser removal systems.

View GIGA-S Vibratory Polisher →

Material-Specific Polishing Techniques

Hard Materials (Hardened Steels, Ceramics)

• Use polycrystalline diamond for aggressive cutting
• Longer polishing times may be required
• Can tolerate higher pressure
• May benefit from controlled removal for precision work

Soft Materials (Aluminum, Copper, Lead)

• Use monocrystalline diamond for gentler cutting
• Shorter polishing times to avoid over-polishing
• Light pressure to prevent deformation
• Monitor carefully for smearing

Work-Hardening Materials (Stainless Steel, Nickel Alloys)

• Use consistent, moderate pressure
• Avoid excessive polishing time
• Progress through grits systematically
• Consider vibratory polishing for final stage

Multi-Phase Materials

• Use softer cloths to minimize relief
• Shorter polishing times to prevent over-polishing
• Monitor for relief around different phases
• May require specialized final polishing techniques

Troubleshooting Common Issues

Proper polishing technique prevents relief and surface defects. This image shows how correct polishing maintains inclusion integrity without creating artifacts around different phases.

Scratches Not Removing

• Insufficient polishing time at current stage
• Grit progression too aggressive (skipped sizes)
• Cloth too hard or too soft for current stage
• Insufficient or contaminated abrasive

Relief Around Inclusions or Phases

• Over-polishing - reduce polishing time
• Cloth too soft - use harder cloth
• Pressure too high - reduce pressure
• Consider vibratory polishing for final stage

Contamination

• Not cleaning sample between stages
• Using contaminated abrasives or cloths
• Cross-contamination from previous steps
• Dirty polishing equipment

Poor Surface Quality

• Incomplete removal of previous scratches
• Inappropriate cloth selection
• Incorrect abrasive type or size
• Insufficient polishing time

Over-Polishing

• Excessive polishing time
• Too soft a cloth for material
• Pressure too high
• Inappropriate abrasive selection