Hardness testing has become an indispensable material testing technique across various industries. It’s a convenient, rapid, and direct method for obtaining material properties. So, what information does hardness testing actually provide? What specific methods exist? And why is it so important? This article will unravel this mystery for you.
What is Hardness Testing
About Hardness
German metallurgist Martens defined hardness as an object’s ability to resist indentation by another (harder) object. Simply put, hardness refers to a material’s resistance to localised plastic deformation, such as indentation or scratching. Hardness is not a fundamental physical property of materials; its specific value depends on the testing method used. Many confuse strength with hardness; though closely related, they are distinct concepts. We could change the material hardness through hot or cold working.
About Hardness Testing
Hardness is a comparative value that cannot be directly measured. Therefore, specific testing methods are required to convert it into measurable variables for indirect representation. The most prevalent hardness testing method—the indentation method—operates on the principle of pressing a harder material into the surface of the tested material under a specific load and maintaining it for a period of time. Subsequently, the measured hardness value is obtained based on the permanent plastic indentation formed on the surface of the tested material. Depending on the specific hardness testing method, the measured hardness value can be obtained based on parameters such as indentation depth or indentation length.
In different hardness tests, the measured hardness value of the material primarily depends on:
The load applied to the indenter is used to resist the material
The duration the indenter remains on the material under full load (load holding time)
The geometric shape of the test indenter
Hardness testing applies to a wide range of materials, including metals, plastics, minerals, ceramics, and elastomers. Different materials require different hardness testing methods. The primary hardness testing methods include Rockwell hardness testing, Brinell hardness testing, Vickers hardness testing, and Knoop hardness testing.
Why Conduct Hardness Testing
Verifying the Effectiveness of Heat Treatment Processes
This is one of the most important and classic applications of hardness testing in industrial fields. The results of hardness testing can directly determine whether heat treatment processes such as quenching and tempering have enabled the material to achieve the expected performance metrics.
Assessing Material Strength
This is the most direct application of hardness testing. For many metallic materials, hardness and tensile strength exhibit a strong positive correlation. Consequently, hardness testing serves as an excellent method for rapidly evaluating metallic materials. Additionally, it aids in assessing properties such as ductility and wear resistance.
Quality Control
Hardness testing is a critical step in quality control before manufacturing parts. Testing enables rapid verification of whether materials meet standards or part specifications. A non-compliant hardness value may indicate heat treatment failure, incorrect material selection, or underlying material defects.
Detection of Potential Material Defects
Hardness testing aids in identifying surface defects such as decarburization, quenching soft spots, overheating/overburning, and grinding burns.
Material Comparison and Selection
This is another vital function of hardness testing. It allows for direct comparison of hardness-related properties between two or more materials, enabling more comprehensive consideration when selecting materials for your projects.
Measuring a Material’s Resistance to Deformation
The principle of hardness testing involves calculating hardness values based on the plastic deformation of the material’s surface. Thus, it reveals a material’s ability to resist deformation.
Fundamental Tool for Failure Analysis
When components experience damage or failure during use (e.g., wear, fracture, deformation), hardness testing typically serves as the first step in failure analysis. Comparing hardness values between the failed area and undamaged regions allows rapid identification of failure causes. These may include excessive wear or deformation due to insufficient hardness, brittle fracture caused by excessive hardness, or premature failure resulting from uneven hardness distribution.
Material Identification
Hardness testing enables rapid differentiation and identification of various materials.
Compliance with Industry Standards and Specifications
Many industries (such as aerospace, automotive, petrochemical, etc.) impose mandatory hardness requirements on materials. To ensure the reliability of final products, 100% hardness inspection is necessary, making hardness testing the fundamental means to meet these standards and specifications.
Common Hardness Testing Methods
Numerous hardness testing methods are available today, which can be categorised into static and dynamic methods based on the state of the applied test force.
Static hardness testing involves slowly and steadily applying the test force, which is then maintained for a period of time. This category includes depth measurement methods represented by the Rockwell hardness test, as well as optical measurement methods encompassing the Brinell hardness test, Vickers hardness test, and Knoop hardness test. Optical measurement methods utilise optical microscopes to measure indentation dimensions, thereby calculating material hardness values.
Dynamic hardness measurement methods involve testing hardness by applying dynamic impact forces, primarily including Leeb hardness testing and Shore hardness testing.
The following explains each hardness testing method.
The Rockwell Hardness Testing
The Rockwell hardness measurement method primarily employs two types of indenters: diamond cones for hard materials and small-diameter steel balls for soft materials. The measured hardness value is directly displayed on the dial gauge by reading the depth of the indentation or penetration on the surface of the tested material.
The Rockwell hardness measurement method is commonly used to rapidly obtain hardness values for metals. Its most prominent feature is operational convenience. It offers fast measurement speeds and high precision in results, making it the most widely applied method on industrial production lines.
It is primarily divided into Rockwell surface hardness testing and Rockwell conventional hardness testing. The latter is more suitable for testing thin specimens, but for extremely thin materials or surfaces requiring extremely high precision, Vickers or Knoop hardness testing would be better choices.
Symbols: HRA, HRB, HRC, etc. (Different scales correspond to different indenters and loads).
The Brinell Hardness Testing
The indenter for Brinell hardness testing is typically a large-diameter carbide ball. The measured hardness value is calculated based on the diameter of the indentation measured under a microscope. The load applied to the indenter usually ranges from 1 kgf to 3000 kgf.
A characteristic of Brinell hardness testing is that it produces a relatively large indentation, and the measured hardness value represents a broad average. This method is insensitive to local hardness variations (non-uniformity) within materials and does not accurately reflect overall material properties. Consequently, it is suitable for softer materials (such as copper, aluminium, and their alloys) or materials with coarse-grain structures and inherent non-uniformity. Compared to other hardness testing methods, this process is relatively time-consuming.
Due to the large indenter size, this method imposes specific requirements on the thickness and surface preparation of the test specimen. It is classified as a micro-destructive test.
Symbol: HBW
The Vickers Hardness Testing
The indenter used in Vickers hardness testing is a diamond pyramid with a 136° apex angle, resulting in a square indentation. The measured hardness value is calculated by precisely measuring the length of the indentation’s diagonals under a microscope.
This method is suitable for both macro and micro hardness testing, meaning it can assess both very thin surface layers and large workpieces. With exceptional precision and a wide load range (from approximately 10 kgf to 100 kgf), the Vickers hardness test is an excellent tool for studying microstructural hardness.
Note that to obtain accurate results, especially at lower applied loads, the specimen surface must be polished to ensure sufficient surface finish.
Symbol: HV
The Knoop Hardness Testing
Vickers hardness testing employs a long diamond pyramid with a rhombic base as the indenter. The material’s hardness value is calculated by measuring the diagonal length of the indentation.
The most significant advantage of the Knoop test is its ability to produce extremely shallow indentations, making it particularly suitable for testing the hardness of thin or brittle materials (glass, ceramics, etc.). Additionally, due to the asymmetry of the indentation shape, this method is also well-suited for measuring the hardness of samples with directional properties.
It is important to note that to accurately measure such minute indentations, this method places high demands on the surface finish of the test specimen.
Symbol: HK
The Leeb Hardness Testing
Leeb hardness testing is a rebound method. It employs a “small hammer” equipped with a diamond probe to impact the material surface using spring or electromagnetic force. The material’s hardness value is then calculated based on the loss of kinetic energy.
Leeb hardness testing produces extremely small indentations and offers rapid testing speed. This method is relatively non-destructive. Leeb hardness testing is a truly portable non-destructive inspection method. Its compact equipment can be carried directly to the required site for testing, offering great convenience.
Symbol: HL
The Shore Hardness Testing
Shore hardness testing operates by allowing a fixed indenter (with a diamond-tipped sphere) to fall freely from a fixed height within a glass tube before impacting the specimen surface. The hardness value of the material is then calculated by measuring the rebound height of the indenter. A higher rebound height indicates greater material hardness.
Similar to the Leeb test, this method produces an extremely small indentation, typically negligible. Consequently, it is also an ideal tool for on-site non-destructive testing. However, caution is advised when testing extremely thin surfaces or surfaces with stringent finish requirements.
Symbol: HS
Selecting the Appropriate Hardness Testing Method
Given the multitude of hardness testing methods available, determining the suitable testing method for the material under examination presents a challenge. Below are several key considerations for selecting the appropriate hardness testing method:
Material Type and Properties
This is the primary determining factor for selecting a suitable hardness testing method for the material under examination. For soft materials like copper, aluminium, and cast iron forgings, Brinell hardness testing is the preferred choice. Its larger indentation reflects the material’s average hardness level. For brittle materials, Vickers and Knoop hardness tests are suitable. These methods apply lower loads with shallower indentations, minimising the risk of material cracking.
Sample size and geometry
This is a key constraint when selecting a hardness testing method. For large, heavy workpieces, portable Leeb hardness testing and Shore hardness testing are the only viable options. For extremely thin samples, Knoop hardness testing is the most suitable choice.
Testing Efficiency and Cost
In terms of efficiency, Rockwell hardness testing is the fastest. By comparison, Brinell and Vickers hardness testing are slower. Regarding initial investment, Rockwell and Vickers hardness testing equipment, being bench-top devices, carry higher costs than the portable machines used for Leeb hardness testing.
Surface Damage Requirements
If minimal surface damage to the material is required post-testing, Leeb or Shore hardness testing is recommended. Their indentations are typically invisible to the naked eye, making them the preferred choice for non-destructive on-site testing. Conversely, Brinell, Vickers, Rockwell, and Knoop hardness tests all leave permanent indentations on the material surface.
Applications of Hardness Testing Across Various Fields
Hardness testing is employed in nearly every industry with material performance requirements, making it ubiquitous across all industrial and scientific sectors.
Metals and Metal Processing
Used to verify whether materials meet performance standards after heat treatment or machining, indirectly confirming tensile strength and wear resistance.
Automotive
Tests the hardness and wear resistance of critical engine components such as crankshafts and gears.
Aerospace
Strictly monitors fatigue resistance and other properties of high-strength components like turbine blades.
Construction
Uses rebound hammers to verify whether concrete structures meet strength standards.
Heavy Equipment
Testing the hardness of components that bear weight or are subjected to loads.
Pharmaceutical
Hardness testing plays an indispensable role in tablet pharmaceutical inspection within the
pharmaceutical industry.
Electronics and Semiconductor Industry
Used to test the hardness of silicon wafers, ceramic substrates, and thin-film coatings.
Materials Science
Hardness serves as a fundamental metric for evaluating performance when developing new materials (metals or composites).
Factors Affecting the Accuracy of Hardness Testing
The accuracy of hardness test results is influenced by a series of factors. Beyond the inherent precision limitations of the equipment itself, environmental temperature, the surface finish of the material being tested, sample thickness, and material-specific characteristics all exert a certain degree of influence on the measurement outcomes.
Therefore, precise hardness testing requires execution in a temperature-controlled environment. Simultaneously, the surface of the material being tested typically requires polishing, grinding, and cleaning—particularly for Vickers and Knoop hardness tests, which demand extremely high surface finish standards. Generally, the sample thickness should be at least ten times the depth of the indentation. If the thickness is insufficient, the back of the specimen may generate interference during testing that affects the measured hardness value. Finally, if the material itself exhibits anisotropy, the measured results will naturally vary at different locations.
Hardness Testing Standards
The internationally recognised hardness standards primarily originate from ASTM (American Society for Testing and Materials) and ISO (International Organisation for Standardisation). The following lists the main international standards corresponding to the primary hardness testing methods.
| Hard Testing Method | ASTM | ISO |
| The Rockwell Hardness Testing | ASTM E18 | ISO 6508 |
| The Brinell Hardness Testing | ASTM E10 | ISO 6506 |
| The Vickers Hardness Testing | ASTM E92 | ISO 6507 |
| The Knoop Hardness Testing | ASTM E92 | ISO 4545 |
Conclusion
Hardness testing holds an irreplaceable position in its field as a material testing method that directly reflects material properties. In this article, we comprehensively explore the importance of various hardness testing methods, along with the characteristics and applicable scenarios of each technique, hoping to provide you with some assistance.
Pingheng Machinery is a foundry specialising in precision casting. With our in-house foundry workshop and machining equipment, we offer Rockwell hardness testing services for your castings. Should you require other hardness tests, we have reliable third-party partners to provide satisfactory solutions. Feel free to contact us anytime.
