Introduction: What is Hardness
Hardness is the property of any material to resist deformations to its shape when it is subjected to an external force. Hardness testing is important for technical and engineering applications as it helps to determine the hardness and tensile strength of materials.
It helps to develop a clear understanding of whether the specimen being tested is suitable for specific applications. It helps industries to make safe and high-quality end products which meet the safety regulations and guidelines laid down by the relevant authorities.
The different types of hardness and various methods of hardness testing and determining the hardness values are discussed below:
Scratch hardness is the ability of a material to resist deformation usually by scratches and abrasions. Scratch hardness is one type of hardness which is measured when a specimen surface is scratched by a stylus which is dragged along its surface under a fixed test load.
Scratch hardness method defines the resistance of a material to plowing by a hard stylus. It measures the hardness of the material and its ability to resist scratches and abrasions. Generally, a scratch hardness tester consists of a stylus, portable sample holding stage, load-applying device, and a data processing and display unit.
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The earliest scratch hardness tests were conducted by Friedrich Mohs in 1820 who also developed the Mohs scale. The scratch hardness test measures the resistance of the specimen to fracture or permanent deformation due to friction caused by the stylus along its surface.
This test uses a harder material to scratch the specimen surface which is made of a relatively softer material. When you need to test coatings, then scratch hardness refers to the force which is necessary to cut through the film to the substrate. Mohs scale is used for measuring the scratch resistance of material along with a measuring device known as sclerometer.
Indentation hardness refers to the hardness of a material which is determined by making an indentation on its surface using an appropriate indenter under a test load.
There are many different indentation testing methods which measure the depth of indentation made on the specimen surface for measuring its hardness. Indentation hardness tests can be further subdivided into two categories- macro indentation tests and micro-indentation tests. Macro indentation tests make use of large test loads in excess of 1Kgf and can go up to 3000Kgf in some cases.
The prominent type of macro indentation testing methods includes Rockwell hardness testing method, Brinell method, Vickers test, Knoop hardness testing method, etc. Micro-indentation testing, on the other hand, is used for measuring the hardness of soft metals, thin and small specimen, etc.
The test loads applied for micro-indentation hardness tests are much lower and can range between 1 to 1000gf. Micro-indentation tests can be used for measuring changes in hardness on a microscopic scale. Vickers method and Knoop hardness testing methods are two popular micro-indentation hardness testing methods.
Indentation hardness tests can be prone to errors. The main sources of errors while measuring hardness include poor technique, improper calibration of the testing equipment, improper finishing of the specimen surface, etc. The test surface needs to be free from any dirt, oxidation, or lubrication to get reliable estimates of hardness. The indenter should also be perpendicular to the specimen surface as any inclination will result in faulty hardness measurements.
Rebound hardness which is also known as dynamic hardness measures the hardness of a material by determining the height to which a diamond-tipped hammer bounces when it is dropped from a height on to the specimen. This type of hardness is related to elasticity.
Rebound hardness is generally measured using the Leeb rebound hardness test. This method was developed in 1975 by Leeb and Brandestini by using a portable hardness tester. This tester provided a new alternative to the usually intricate and complex traditional hardness testing equipment.
The Leeb rebound hardness testing method follows standardized procedures. The velocity of the body ranges between 1.4 m/s to 3 m/s. Under this method, the velocity of the body before and after the impact is measured to determine the hardness of the specimen.
The ratio of the impact velocity to the rebound velocity gives the dynamic Leeb hardness of the test specimen. The object used for making the impact on the specimen can be tungsten carbide cobalt, ceramic or diamond, or a ball-shaped indenter with different radius.
The sample hardness determined by this method is represented as (example) 750 HL D in which 750 denotes the hardness value, HL denotes “Hardness according to Leeb” and D represents Leeb method with spherical impact body of tungsten carbide-cobalt with a radius of 1.5 mm and weight of 4.5 grams.
Selecting the Best Hardness Testing Method
The hardness of a material depends on a number of factors like homogeneity of the material, type of material, its size, and condition.
There are different types of methods for hardness testing which needs to be selected carefully for getting the most accurate and reliable hardness measurements.
Factors which need to be considered before selecting the ideal hardness testing methods include- the type of material to be tested, the hardness of the material to be tested, homogeneity of the material, size of the specimen, if mounting is necessary for hardness testing, thickness of the specimen, etc.
Rockwell Hardness Test
Rockwell hardness testing is the most common and fast method for testing hardness. It is generally ideal for testing hardness of larges sized samples.
It can be used either on the assembly line or in laboratories for testing hardness. It uses a steel ball or diamond-tipped cone indenter for measuring hardness depending upon the hardness of the material to be tested.
To start the Rockwell test, a minor load of generally 10Kgf is applied on the indenter. The indenter then moves under the test load on to the specimen surface. While the indenter is still under the influence of the minor test load, an additional major test load is also applied to it. This ensures better indentation on the specimen surface which is clear and measurable.
The Rockwell method of hardness testing is popular as it is not influenced by either the operator’s bias or the roughness of the surface whose hardness is being tested.
It does not make use of any sophisticated or costly optical equipment for measuring the hardness of specimen, which makes it a cost-effective method for hardness testing. It is a non-destructive method for hardness testing which means that the sample specimen being tested is not destroyed and it can be used for other purposes after the testing procedure is completed.
This method has its drawbacks also as it is not very accurate compared to other methods of hardness testing. Small deviation in measuring the indentation depth can throw off the hardness readings considerably.
If the indenter in this method is worn out, then it can provide faulty hardness measurements which are non-reliable.
Rockwell hardness value is calculated using the conversion charts. There are nearly 30 Rockwell scales but most of the materials are covered by Rockwell C and B scales. The hardness values in Rockwell testing are represented as (example) 70 HRB where 60 is the hardness reading on the B scale.
Brinell Hardness Test
The Brinell hardness test is one of the oldest and most widely used methods for hardness testing of materials. This method was developed by JA Brinell in the year 1900. It is ideal for measuring the hardness of specimens which are too rough or course to be measured by other methods.
The Brinell method involves usage of higher test loads which can go up to 3000Kgf and a ball indenter generally 10mm in diameter.
For measuring the hardness of softer metals and alloys, smaller test loads of up to 500Kgf are also used. The predetermined test load is applied to the spherical indenter which is held to the specimen surface for generally for 10-15 seconds and then moved.
The indentation depth made by the indenter on the specimen surface is then measured and studied using advanced optical equipment which ensures better accuracy and reliability.
The Brinell conversion chart is then used to convert the average diameter of the indentation made to the corresponding Brinell hardness value. Using the conversion charts, the Brinell hardness value can also be converted into the corresponding tensile strength.
The Brinell method of hardness testing comes with few drawbacks also. The operator can make mistakes in measuring the indentation depth on the specimen surface which can considerably impact the hardness measurements.
Since the method involves advanced and sophisticated optical equipment for measuring the specimen hardness, it is costlier compared to the Rockwell method. It also takes more time to test hardness as the specimen surface needs to be prepared before testing.
The Brinell method will also not work accurately if the specimen surface is too thin i.e. less than 9.6 mm.
The hardness readings using the Brinell method are represented as 600 HBW where 600 denotes the hardness value and HBW denotes “Brinell Hardness” with tungsten ball indenter. If steel ball indenter is used then the readings will be represented as 600 HBS where HBS denotes “Brinell Hardness” with steel ball indenter.
Vickers Hardness Test
The Vickers test uses the same principle as the Brinell method with the only exception being the type of indenter being used.
The type of indenter needs to be changed under the Brinell method depending on the type of material being tested. However, the same diamond indenter is used in the Vickers method for measuring the hardness of all specimen types.
The indenter used in this method is in the form of a right pyramid. A test load is applied on the indenter which presses itself against the specimen surface thereby leaving an indentation.
The diagonal lengths of these indentation marks are measured using optical systems, resulting in highly accurate hardness readings. The dwell time- the time for which the test force is applied through the indenter on the specimen surface- is generally between 10-15 seconds in this method.
The Vickers test using micro-test loads which are much lesser compared to the Brinell method. It is a microhardness testing method which is best suited for measuring the hardness of materials which are too thin or small for macro hardness testing.
This method is best suited for measuring the hardness of thin sheets of metals, small specimens, etc. The Vickers test is a non-destructive method which ensures that the specimen can be used after the testing is complete. There are a few limitations of the Vickers method.
It requires the specimen surface to be free from any imperfections and thus time is needed for preparing the specimen surface before conducting the test. It takes at least 30-60 seconds for conducting this test and the time is exclusive of the time taken for preparation of the specimen surface.
Vickers testing is not recommended for bulk production in assembly lines and is more suited for laboratory testing. The hardness measurements in the Vickers test are represented as 700 HV/10 where 700 is the Vickers hardness value which has been arrived at by using a 10Kgf test force.
Knoop Hardness Test
Knoop hardness testing method is an alternative to the Vickers method. It is a microhardness measuring method which is suitable for measuring the hardness of fragile and brittle materials like ceramics. It is also useful for hardness testing of small elongated areas like coatings.
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The Knoop method also uses a pyramidal diamond as indenter, but the indenter is elongated instead of being a right pyramid as in the Vickers method. Since the method is used for hardness testing of fragile materials, it uses micro loads of up to 1Kgf.
The indenter used in this method penetrates the specimen surface only half as deep as in the Vickers method, which makes it perfect for hardness testing of the brittle specimen.
Due to the shape of the indenter, the Knoop method is better suited for measuring longer elongated specimens like coatings. It is important to note that the specimen surface should be prepared properly before conducting the Knoop test for ensuring accurate and reliable hardness measurements.