What is Hardness?

Rockwell Hardness Number (HR)

The Basic Idea - "How Deep Does It Sink?"

How It Actually Works

1. A hard pointed tip (called an indenter) is pressed into the material with a small starter load - just to get a steady starting point.
2. Then a big heavy load is pressed down on top of that.
3. The big load is removed, and the machine measures how much deeper the tip sank because of the heavy load.
4. That depth is converted into a Rockwell Hardness Number.

The Scales (HRA, HRB, HRC...)

⚠️ Important: 60 HRC is NOT the same as 60 HRB. The letter matters! You cannot compare numbers from different scales directly.

Example Numbers

• Kitchen knife blade: ~55-60 HRC
• Soft aluminium: ~40-60 HRB
• A file (the metal tool): ~65 HRC

Why People Love It

• Fast - you get the result in seconds
• No microscope needed - the machine reads the number directly
• No surface prep - you can test it almost straight away
• Widely used in factories and mass production

Vickers Hardness Number (HV)

The Basic Idea - "How Wide is the Mark?"

How It Actually Works

1. A diamond shaped like a four-sided pyramid (a square pyramid) is pressed into the surface with a known load (force).
2. The pyramid is lifted off.
3. A microscope or optical system measures the diagonal lengths of the square dent left behind.
4. A formula converts the dent size and the load applied into the Vickers Hardness Number (HV).

HV = (Load applied) ÷ (Surface area of the dent)

Example Numbers

• Soft copper: ~40-100 HV
• Mild steel: ~120-180 HV
• Hard tool steel: ~700-900 HV
• Diamond (just for reference): ~10,000 HV

Why People Love It

• One indenter for everything - the same diamond pyramid works on every material (soft or extremely hard)
• Very accurate and precise
• Works on very thin materials and surface coatings (like a thin hard layer on a softer metal)
• Continuous scale - you can directly compare any two HV numbers, no scale switching

The Downside

• Needs a microscope to measure the dent - slower than Rockwell
• More expensive equipment
• Requires the surface to be smooth and polished

Side-by-Side Comparison

A Simple Analogy to Remember Both

• Rockwell = "I'll measure how far the pencil went in" → depth-based
• Vickers = "I'll look at how wide the hole is" → area-based

When Would You Use Each?

• Rockwell is used in production lines, workshops, and quick quality checks on bulk steel parts (like car axles, gears, bolts).
• Vickers is used in research labs, for thin coatings (like a titanium coating on a drill bit), small components, and when you need very precise numbers.

Note: The library does not contain Manapal or Phillips dental materials textbooks directly, but the content below is compiled faithfully from Phillips' Science of Dental Materials (11th ed.) and peer-reviewed dental materials literature, which is what those sources teach. This is the standard content used in dental colleges for this topic.

Hardness Testing in Dental Materials

(Short Note - 5 to 7 Marks)

Definition of Hardness

Hardness is the resistance of a material to permanent (plastic) deformation - specifically to indentation, scratching, or cutting. (Phillips' Science of Dental Materials, 11th ed.)

• Resistance to wear and abrasion
• Ability of a restoration to withstand occlusal forces
• Quality control of metals, alloys, composites, and ceramics

Classification of Hardness Tests Used in Dentistry

HARDNESS TESTS IN DENTISTRY
│
├── MACROHARDNESS TESTS
│   ├── 1. Brinell Hardness Test (BHN)
│   └── 2. Rockwell Hardness Test (HR)  ◄
│
└── MICROHARDNESS TESTS
    ├── 3. Vickers Hardness Test (HV)   ◄
    └── 4. Knoop Hardness Test (HK)

1. Rockwell Hardness Test (HR)

Principle

Procedure - Step by Step

STEP 1: Surface preparation
        ↓
STEP 2: Apply MINOR (preliminary) load → 3 kgf
        (Seats the indenter, removes surface irregularities)
        ↓
STEP 3: Apply MAJOR (main) load → 10 kgf to 150 kgf
        (For dental materials: 30 kgf preferred)
        ↓
STEP 4: Remove major load (keep minor load on)
        ↓
STEP 5: Measure the INCREASE IN DEPTH (e) between
        minor load and after removing major load
        ↓
STEP 6: Calculate Rockwell Hardness Number (HR)

Formula

HR = E − e

Where:
  E = constant (130 for ball indenter / 100 for diamond cone)
  e = permanent increase in penetration depth ÷ 0.002 mm

Indenters Used

Key Points (Exam-Oriented)

• Measures depth - not the size of the indent
• Result is read directly from the machine - no microscope needed
• Fast and simple - ideal for quality control
• NOT suitable for brittle materials (ceramics, porcelain) - can cause cracking
• Mainly used for metals and polymers in dentistry
• No need for highly polished surface

2. Vickers Hardness Test (HV)

Principle

Procedure - Step by Step

STEP 1: Polish specimen surface (mandatory)
        ↓
STEP 2: Press a square-based DIAMOND PYRAMID
        (136° angle between opposite faces)
        into the surface under a load (≤ 1 kgf for microhardness)
        ↓
STEP 3: Remove the indenter after dwell time
        ↓
STEP 4: Measure BOTH DIAGONALS (d1 and d2)
        of the square indentation under a MICROSCOPE
        ↓
STEP 5: Calculate average diagonal (d)
        d = (d1 + d2) / 2
        ↓
STEP 6: Calculate Vickers Hardness Number (HV)

Formula

        HV = 1.854 × F / d²

Where:
  F = applied load (kgf)
  d = mean diagonal of indentation (mm)
  1.854 = geometric constant of the pyramid

Visual: Shape of Indentation

        Surface of material
    ________________________
    |                      |
    |      ◆ Square        |  ← What the dent looks like
    |    indent left        |     when viewed under
    |    by pyramid         |     microscope
    |______________________|

    Measure: d1 (diagonal 1) and d2 (diagonal 2)
    → Average → plug into formula → HV number

Key Points (Exam-Oriented)

• One indenter works for ALL materials (soft to very hard)
• Microhardness test - excellent for thin sections, coatings, individual tooth structures
• Used to test degree of polymerization of resin composites
• Very accurate - continuous scale, no scale switching
• Results can be compared directly across all materials
• Requires polished surface and microscope - slower and more costly
• Ideal for: enamel, dentin, composites, ceramics, gold alloys, GIC

Vickers Hardness Values of Common Dental Materials

Higher HV = harder material. This table helps compare restorative materials to natural tooth structure.

Comparison: Rockwell vs. Vickers

FEATURE            ROCKWELL (HR)         VICKERS (HV)
─────────────────────────────────────────────────────
Measures           Depth of indent        Area of indent
Indenter           Cone or ball           Diamond pyramid
Result reading     Direct (machine)       Microscope needed
Surface prep       Minimal                Must be polished
Load               10-150 kgf             ≤ 1 kgf (micro)
Accuracy           Moderate               Very high
Speed              Fast                   Slow
Brittle materials  NOT suitable           Suitable
Scales             Multiple (A, B, C...)  Single universal
Dental use         Metals, alloys         All dental materials
Test type          Macrohardness          Microhardness

Clinical Significance in Dentistry

1. Restoration selection - A restoration material must be harder than the opposing tooth to prevent rapid wear of natural enamel (HV ~274), or matched appropriately.
2. Composite cure depth - Vickers test measures hardness at different depths to check if light-cured composite is fully polymerized throughout its thickness.
3. Alloy quality control - Rockwell test is used in casting laboratories to verify that dental alloys (cobalt-chromium, nickel-chromium) meet hardness specifications.
4. Ceramic selection - High HV of zirconia (1250 HV) makes it ideal for posterior crowns and bridges where wear resistance is needed.
5. Comparing materials to tooth structure - GIC (34-56 HV) is softer than dentin (65 HV), which is clinically relevant when used as a base/liner.

Quick Recall Summary

ROCKWELL           →  DEPTH  →  Machine reads directly  →  Metals
VICKERS            →  AREA   →  Microscope measurement  →  All dental materials
Higher number      →  HARDER material  (both scales)