Hardness Testing: Definition, How It Works, Types, and Benefits

Hardness refers to the resistance of a material to deformation, particularly when subjected to various forms of external forces like compression, indentation, scratching, or abrasion. It’s an essential mechanical property that indicates the material’s ability to withstand such forces without undergoing permanent deformation or damage. Hardness is influenced by factors such as the material’s composition, microstructure, and processing methods. It plays a crucial role in determining the suitability of materials for specific applications, such as in engineering, manufacturing, construction, and geology. Hardness testing methods are commonly employed to quantify and compare the hardness of different materials.

What Is Hardness Testing?

Hardness testing assesses a material’s ability to resist permanent deformation at its surface by applying pressure with a harder material. It finds applications across various industries for comparing and selecting materials, as well as ensuring quality control in manufacturing or hardening processes. The test involves using a specifically designed indenter with predetermined dimensions to apply force onto the material under examination. The duration of the pressure application is also a critical factor in the hardness test.

While hardness isn’t an intrinsic physical property of a material, it serves as a measurable characteristic. Nonetheless, it offers valuable insights into the strength and durability of a material, tailored to its intended use.

How Does Hardness Testing Work?

The mechanism of hardness testing involves applying a standardized, rigid tool known as an indenter onto the material under examination. This indenter is pressed into the sample for a predetermined duration (typically measured in seconds) and with a specified load (force) applied behind it. As the indenter penetrates the material, it causes measurable surface deformation. This deformation is quantified, and the resulting measurement is utilized to determine the material’s hardness value based on the scale specific to the test being conducted.

What Type of Materials Undergo Hardness Testing?

Hardness testing can be applied to various types of materials, some of which are listed below:

Metals and alloys.

Ceramics.
Elastomers.
Polymers and plastics.

Films.
Rocks and minerals.

What Is the Importance of Hardness Testing?

Hardness testing holds significant importance for various reasons. The hardness of a material often serves as a critical parameter in its intended application. For instance, in the realm of metal components utilized for cutting and grinding, or in rubbers employed for shock absorption, hardness plays a crucial role. Objective measurement and comparison of material hardness are essential aspects.

Establishing an objective and standardized hardness value for a material is vital for ensuring quality control throughout the manufacturing process. Each unit must fall within an acceptable range of the specified hardness to meet quality standards. Moreover, these standardized hardness values facilitate comparisons among different materials, aiding in material selection tailored to meet specific application requirements.

What Are the Benefits of Hardness Testing?

Hardness testing offers a significant measure of material characteristics with several advantages:

Simplified Measurement: Automated hardness testing machines streamline the process, making it relatively straightforward to obtain hardness measurements.

Versatility: Hardness testing can yield meaningful readings across a broad spectrum of materials, ranging from soft to hard.
Direct Measurement: In certain applications, the material’s hardness stands as a crucial and directly measurable characteristic, eliminating the need for inference from physical properties.

What Are the Challenges of Hardness Testing?

While hardness testing is a valuable tool, it also presents several challenges:

Surface Condition: Surface irregularities or roughness can affect the accuracy of hardness measurements, as the indenter may not make consistent contact.
Sample Preparation: Proper preparation of the sample surface is crucial for accurate results. Improper preparation, such as uneven polishing or contamination, can lead to erroneous readings.

Indenter Selection: Choosing the appropriate indenter for the material being tested is critical. Using an incorrect indenter may result in inaccurate hardness values.
Scale Variation: Different hardness scales exist for various materials and testing methods. Understanding which scale to use for a specific material can be challenging.
Interpretation: Interpreting hardness values requires expertise, as they may not always directly correlate with material properties such as strength or wear resistance.

Edge Effects: When testing near the edge of a sample, edge effects can distort hardness readings due to the indenter’s proximity to the sample’s boundary.
Sample Size: The size and shape of the sample can influence hardness measurements, particularly for non-homogeneous materials or those with varying microstructures.

What Are the Different Types of Hardness Test Methods?

Certainly! Hardness testing encompasses various methods to measure a material’s resistance to deformation when subjected to an applied force. Here are the different types of hardness test methods explained in more detail:e:

Rockwell Hardness Test: This method assesses the depth of indentation caused by a specific load on the material’s surface. It’s highly versatile and straightforward, making it widely used across industries.

Brinell Hardness Test: Using a spherical indenter, this test determines the hardness by measuring the diameter of the indentation created by a known force. It’s suitable for materials with coarse grain structures or rough surfaces.

Vickers Hardness Test: Utilizing a square-based pyramidal diamond indenter, this test calculates hardness based on the indentation’s surface area. It’s applicable to a wide range of materials and provides precise results.

Knoop Hardness Test: Similar to the Vickers test, but with a more elongated diamond-shaped indenter. It’s preferred for thin materials or small parts due to its ability to generate tiny indentations.

Shore Hardness Test: Specifically designed for elastomers and plastics, this test measures the material’s resistance to indentation using a standardized indenter. It’s crucial for assessing materials like rubber or silicone.

Mohs Hardness Test: This qualitative test ranks minerals based on their scratch resistance against standard materials with known hardness values. It’s commonly used in geology and mineralogy for mineral identification.

These hardness testing methods serve various purposes, from quality control in manufacturing processes to material selection for specific applications. By understanding a material’s hardness, engineers and manufacturers can ensure product durability, performance, and suitability for intended uses.

How Do Hardness Testing Loads Work?

Each hardness testing method has a range of loads that are applicable to the use of the test. Some tests, like the Vickers hardness test, can be used on a macro scale as well as a micro scale. The loads required are listed in Table 1 below:

Method	Load Range	Standard
Brinell	1 kgf–3,000 kgf	ASTM E10, ISO 6506
Rockwell	15 kgf–150 kgf	ASTM E18, ISO 6508
Knoop	1 gf–1 kgf	ASTM E384, ISO 4545
Vickers	1 gf–1 kgf 1 gf–120 kgf 10 gf–100 kgf	ASTM E384 ASTM E384 ISO 6507

Table 1: Different Hardness Test Methods and Their Corresponding Loads

How Is a Hardness Test Different From a Ductility Test?

A hardness test employs a controlled compressive force to induce slight deformation on a material’s surface, thereby determining its relative hardness value. Conversely, a ductility test utilizes a destructive tensile force to quantify the material’s ability to endure plastic deformation before fracture.

While both tests involve applying a load to induce plastic deformation, they differ in the direction of force (compressive versus tensile) and the magnitude of applied force (focused on surface indentation versus inducing complete yield).

What Does Indentation Mean in Hardness Testing?

In hardness testing, indentation refers to the permanent deformation on the surface of the sample material caused by the testing instrument. The resulting indentation exhibits measurable geometrical characteristics based on the shape of the instrument used. These measurable features of the indentation, such as its depth or variations in the size of its surface diagonal, are then translated into a hardness value.

How Is Hardness Testing Measured?

Hardness testing is assessed using various scales, which are contingent upon the specific hardness testing method employed. Across these scales, the hardness value of a material correlates directly with its hardness—a material with greater hardness will yield a higher hardness value. The selection of a scale can be somewhat arbitrary in certain hardness tests. For instance, in the Rockwell hardness test, hardness values range from 0 to 100 based on indentation depth, but each test method defines its scale uniquely.

Hardness values themselves lack a specific unit and solely represent a value on the scale corresponding to a particular test. However, they are accompanied by a designation indicating the scale and test utilized to derive them. For example, HR denotes Rockwell hardness, HB signifies Brinell hardness, HV represents Vickers hardness, and so forth.

What Are the Hardness Testing Standards?

There are various standards available for the different hardness testing methods. Generally, the internationally accepted standards for hardness testing are those issued by ASTM (American Society for Testing Materials) and ISO (International Organization for Standardization). The two standards are very similar for each corresponding type of hardness test but have small differences between them for details such as the minimum thickness of the sample. The common ASTM and ISO standards for the four most popular hardness tests in metallic materials are listed below:

ASTM E10: This standard covers Brinell hardness testing in metallic samples.

ASTM E18: This standard includes the various ways that Rockwell hardness can be measured by standard methods, specifically for metallic materials.
ASTM E384: This standard is for hardness testing on a micro-scale, and therefore includes the Vickers and Knoop hardness tests.
ISO 6506: This standard mirrors ASTM E10, as it contains the standard method for measuring the Brinell hardness of metals.

ISO 6507: This standard contains the details for the Vickers hardness test in metallic materials.
ISO 6508: This standard is very similar to ASTM E18, as it deals with the Rockwell hardness tests, both superficial and regular, for metallic samples.
ISO 4545: This standard covers Knoop hardness testing for metals.

What Is the Static Hardness Testing Method?

Static hardness testing involves pressing and maintaining an indenter against the surface of the material under examination. Throughout most of the test duration, the indenter remains stationary, inducing permanent surface deformation on the material. In this method, the indenter navigates through the material’s elastic deformation, ultimately compelling plastic deformation. Static hardness testing represents the predominant approach, with widely utilized tests like Rockwell, Brinell, Vickers, and Knoop employing this method.

What Is the Dynamic Hardness Testing Method?

Dynamic hardness testing quantifies the rebound velocity of a mobile indenter upon impacting the material sample. Hardness values are derived from either the rebound distance or velocity. This method finds utility in scenarios where conducting a static hardness test (typically performed on a testing machine) might be impractical, such as field operations or when dealing with sizable specimens. However, dynamic hardness testing tends to be less precise compared to static methods.

Can Hardness Testing Be Conducted on Any Type of Material?

No, hardness testing is not universally applicable to all materials. Generally, hardness testing can be conducted on a wide range of materials, but there are exceptions where it’s not feasible. For instance, the material must be solid, or at least have a gel-like consistency. Even among solid materials, certain factors like highly irregular surfaces or non-homogeneity can impede accurate hardness testing. For further insights, refer to our guide on Steel Mechanical Properties.