A Comprehensive Guide to Hardness of Materials

Understanding the hardness of materials is crucial for various industries, from manufacturing to materials science. It determines how resistant a material is to deformation, scratching, or indentation, and plays a significant role in choosing the right material for specific applications. Therefore, this article will explore the definition of material hardness and types of hardness. Furthermore, it will delve into different measurement methods, and provide a material hardness chart for easy reference. Whether you’re new to the topic or looking to deepen your knowledge, this guide will give you the essential insights to understand material hardness effectively.

What is the Hardness of Materials?

Hardness refers to a material’s ability to resist surface deformation, such as scratching, indentation, or abrasion. It is a key property that helps determine how well a material can withstand wear and tear in various environments. The hardness of a material is often used as an indicator of its strength, durability, and suitability for specific applications. Materials with high hardness tend to be more resistant to damage. However, those with low hardness may be more prone to scratching or denting.

Furthermore, hardness is not a single value but can vary depending on the type of material and the testing method used. Different testing techniques, such as Brinell, Rockwell, and Vickers, measure hardness in specific ways.

What are Five Hardness Examples?

1. Diamond

With a Mohs scale hardness of 10, diamond is the hardest naturally occurring material. As a result, its unmatched resistance to scratching and wear is suitable for tools used in cutting, grinding, and drilling.

2. Steel

Steel generally falls between 6 and 8 on the Mohs scale, depending on its alloy and treatment. Due to its strength and resilience, steel is commonly utilized in construction and manufacturing. In addition, its hardness can be adjusted through heat treatment, offering versatility across various applications.

3. Glass

Glass, which typically ranges from 5 to 6 on the Mohs scale, is prone to breaking upon impact. However, its strong resistance to scratches is fit for use in products such as windows, screens, and containers.

4. Copper

With a hardness of around 3 on the Mohs scale, copper is relatively soft and malleable, allowing for easy shaping. Although it is widely employed in electrical wiring and plumbing, copper is susceptible to scratching and, over time, may corrode.

5. Talc

Talc, rated 1 on the Mohs scale, is the softest known mineral. Its low hardness makes it easily scratched, so it is generally used in products like talcum powder. Additionally, this softness is well-suited for fields where minimal friction is required.

Material	Hardness (Mohs Scale)	Description
Diamond	10	The hardest material, used for cutting, grinding, and drilling.
Steel	6-8	Varies with alloy and treatment, widely used in construction and manufacturing.
Glass	5-6	Brittle but resistant to scratches, used in windows and screens.
Copper	3	Soft and malleable, used in electrical wiring and plumbing.
Talc	1	The softest mineral, used in talcum powder and applications requiring minimal friction.

Types of Material Hardness

Material hardness involves several types, each reflecting different methods of measurement and the material’s ability to resist various forms of deformation. The main types of hardness include:

1. Scratch Hardness

Scratch Hardness measures how well a material resists scratching by a harder object. It is often applied to assess hardness when other hardness tests are not suitable. In this test, a harder substance scratches the material, and if a scratch appears, it indicates that the material is softer. This method commonly aims to evaluate minerals, coatings, and small components. A typical example is the Mohs hardness scale, which ranges from 1 to 10. For example:

• Diamond (Mohs hardness 10) can scratch all other materials.
• Quartz (Mohs hardness 7) can scratch most common materials but is not as hard as diamond.
• Talc (Mohs hardness 1) is the softest mineral and is easily scratched by other materials.

In general, scratch Hardness provides an intuitive and convenient way to compare a material’s resistance to scratching, especially for materials with relatively close hardness differences.

2. Indentation Hardness

Indentation Hardness differs from scratch hardness in that it measures a material’s resistance to deformation, rather than just scratching. While scratch hardness tests how well a material resists being scratched by a harder substance, indentation hardness, on the other hand, provides more detailed insights into a material’s response to load. Specifically, it focuses on how a material deforms under pressure. Furthermore, the depth or size of the indentation offers a more precise measure of hardness, particularly when resistance to load is a crucial factor.

Types of Indentation Hardness tests include:

• Brinell Hardness: Press a steel or tungsten carbide ball into the material’s surface. Measure the size of the indentation to determine the material’s hardness.
• Rockwell Hardness: Use a diamond cone or steel ball to create an indentation. Convert the depth of the indentation into a hardness number, using various scales for different materials.
• Vickers Hardness: Apply a set force with a diamond pyramid indenter to measure the size of the indentation. Use this test for thin materials or small samples.
• Knoop Hardness: Use an elongated diamond indenter, similar to Vickers. This method is ideal for testing thin coatings or small areas.

3. Rebound Hardness

Rebound Hardness measures a material’s resistance to deformation by assessing how high a ball rebounds after striking its surface. Unlike indentation hardness, which measures the depth of deformation, rebound hardness focuses on the material’s elasticity. In this test, a ball is dropped onto the material, and the rebound height is used to determine its hardness. Common methods include the Leeb Rebound Test, where the ball’s rebound velocity is measured, and Shore Hardness, typically employed for softer materials like rubber. Moreover, rebound hardness tests are quick, non-destructive, and ideal for testing materials with varying thicknesses

4. Compression Hardness

Compression Hardness evaluates a material’s resistance to deformation under compressive forces. In this test, a sample between two parallel plates is placed, and a compressive force is applied. The material’s resistance is determined by how much it deforms or displaces. This method is commonly used for materials like rubber, foams, and plastics, which undergo compressive stress in real-world applications. By assessing the material’s ability to resist permanent deformation, the test provides valuable insights into its durability. Overall, compression hardness is crucial for evaluating how materials perform under heavy loads in various industrial contexts.

Types of Hardness Unit

Here are some common hardness units used to measure the hardness of materials:

1. Brinell Hardness Number (BHN): The Brinell number is calculated by measuring the diameter of the indentation left by a ball pressed into the material’s surface. The value depends on the applied load and the size of the indentation.
2. Rockwell Hardness Number (HR): The Rockwell scale determines hardness by measuring the depth of the indentation created after applying a specific load. Different Rockwell scales are selected based on the material’s hardness.
3. Vickers Hardness (HV): You calculate Vickers hardness by measuring the diagonal length of the indentation made by a diamond pyramid indenter. The value is derived by dividing the applied force by the surface area of the indentation.
4. Knoop Hardness (HK): Knoop hardness is measured using an elongated diamond indenter. You calculate it by measuring the length of the long diagonal of the indentation, which is often used for small or thin materials.
5. Mohs Hardness: This qualitative scale, ranging from 1 (softest) to 10 (hardest), evaluates hardness based on the ability of one material to scratch another. It is primarily used for minerals.
6. Shore Hardness: The Shore scale measures the depth of an indentation created by applying a specified force, typically on elastomers and soft materials. The Shore A scale measures softer materials, while the Shore D scale measures harder materials.

Methods to Measure Hardness

1. Brinell Hardness Test (BHN)

It is a widely used method for measuring the hardness of materials, particularly metals. This test assesses a material’s resistance to indentation, and here’s a detailed explanation of how it works:

The procedure involves pressing a steel ball, typically 10 mm in diameter, into the material under a load ranging from 500 to 3000 kgf for 10 to 30 seconds. Afterward, the diameter of the indentation is measured using a microscope, and the BHN is calculated based on the indentation’s size.

Formula

Where:

P = Applied load (kgf)

D = Diameter of the ball (mm)

d = Diameter of the indentation (mm)

Advantages

• One of the main advantages of the Brinell Hardness Test is its suitability for testing materials with coarse or uneven grain structures.
• Moreover, it is especially effective for materials with a thickness greater than 10 mm.
• In addition, the test is relatively simple and quick to perform, making it convenient for routine quality control.

Limitations

• However, this method is not ideal for testing very thin materials or those with exceptionally high hardness, as the indentation may not be accurate.
• Furthermore, the accuracy of the test can be influenced by the surface finish of the specimen, so careful surface preparation is necessary.
• Finally, compared to other hardness tests, it requires larger and more expensive equipment.

2. Rockwell Hardness Test

It’s another widely used method for determining the hardness of materials. This test evaluates a material’s resistance to indentation by measuring the depth of the indentation made by a hard indenter under a specific load. Here’s how the test works in detail:

The procedure begins by selecting the appropriate indenter, a steel ball, or a diamond cone, and applying a preload to the material to ensure uniform contact. Then, a main test load is applied, typically for 1-2 seconds. After the load is removed, the depth of the indentation is measured, and the Rockwell Hardness Number (HR) is calculated based on the depth of indentation.

Formula

Where:

N = Scale constant (130 for Rockwell B, 100 for Rockwell C, etc.)

h = Depth of the indentation (in millimeters)

The depth of indentation h is the difference between the indenter’s position under preload and the position after the full load is applied.

Advantages

• Speed: The Rockwell Hardness Test is fast and can be completed in seconds, making it ideal for high-throughput testing.
• Versatility: It offers multiple scales (e.g., Rockwell B, Rockwell C) that cater to different material types, from soft to hard materials.
• Minimal Sample Preparation: The test requires minimal surface preparation, making it suitable for irregularly shaped or small materials.
• Non-Destructive: The test is non-destructive, so the material remains largely unaffected after testing.

Limitations

• Surface Quality: The accuracy of the test can be affected by surface roughness or unevenness, requiring smooth, uniform surfaces for reliable results.
• Material Type: It may not be suitable for very hard materials, as the diamond cone may cause too much damage. Other tests like Vickers or Knoop may be more appropriate in these cases.
• Thin or Coated Materials: For thin materials or coatings, the indentation depth may not represent the overall material hardness effectively.

3. Vickers Hardness Test (HV)

The procedure involves pressing a diamond pyramid indenter with a square base and a 136° apex angle into the material’s surface under a known load. The load typically ranges from 1 to 100 kgf (kilogram-force). The indenter is applied for a specified duration (usually 10-15 seconds). After the load is removed, the diagonals of the indentation left in the material are measured using a microscope. These diagonal measurements are used to calculate the Vickers Hardness Number (HV).

Formula

Where:

F = Applied load (kgf or N)

θ = Angle of the diamond pyramid (136°)

d = Average length of the diagonals of the indentation (in millimeters)

In practice, for a 136° indenter, the formula simplifies to:

Where:

F is the applied load in kilograms force

d is the average diagonal length of the indentation in millimeters.

Advantages

• High Precision: The Vickers test provides a very precise hardness value, especially for small or thin samples.
• Universal: The Vickers Hardness Test can be applied to a wide range of materials, from very soft to very hard.
• Small Indentations: Due to the small size of the indentations, the test is ideal for microhardness testing, thin coatings, or small components.
• Uniformity: The Vickers test offers a consistent result across different materials, as it uses a single indenter geometry and load.

Limitations

• Surface Finish: Like many hardness tests, the surface finish of the material can influence the accuracy of the results. A smooth, polished surface is recommended.
• Indentation Size: For very soft materials, the indentation can be relatively large, which may not provide a meaningful result for very thin or small specimens.
• Time-Consuming: The measurement process can be slower than other Rockwell, as it requires precise measurement of indentation diagonals under a microscope.

Material Hardness Chart

A material hardness chart is a quick reference to determine the hardness of various materials. The chart below lists different materials, alloys, and tempers, along with their Rockwell hardness values, helping you easily assess their suitability for specific applications.

Conclusion

In summary, measuring the hardness of materials is crucial for choosing the right material for specific tasks. Different methods, like Brinell, Rockwell, and Vickers, help assess a material’s resistance to scratches, indentation, and wear. Each method is suited to different materials and applications. By understanding hardness, manufacturers can ensure that the right materials are used for better performance and durability in their products.

Additionally, at Runsom Precision, we provide expert machining and material selection services, ensuring that your projects are built with the right materials for optimal results. Please feel free to contact our experts or ask for an instant quote.

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