Microhardness testing is a popular method for evaluating the hardness of materials. It measures the ability of a material to withstand permanent surface deformation when an external force is applied. The test can help manufacturers check the quality of raw materials and finished products by measuring the resistance of surfaces to localized deformation when subjected to certain conditions. Many variables can play a role in the accuracy of your microhardness testing results. Depending on what type of material you’re testing and whether or not your microhardness tester is manual or automated, you may encounter various issues during your test. This article explores some of the common problems associated with microhardness testing.
I. What is Microhardness Testing?
Microhardness testing is a type of hardness test that is used to evaluate the hardness of a given material. This test involves supporting a small diamond indenter on the surface of the material being tested. Once the instrument applies a known force to the diamond indenter, it measures how much it compresses the surface of your sample at that particular point. An optical microscope is used to measure this indentation in the test sample’s surface, and the measurements taken are used to calculate the microhardness of the material.
II. What are the Applications of Microhardness Testing?
Microhardness testing is generally used to evaluate the physical properties of ferrous and non-ferrous materials. Microhardness testing is the most common method of evaluating materials, including metals, ceramics, and polymers. Microhardness testing is also commonly used in industries that manufacture metals, rubber, plastics, and many other products. You can apply the microhardness testing process to raw materials and finished products.
III. Three Basic Problems you may Encounter when Performing a Microhardness Testing
Accuracy
The instrument’s linear reading capability on approved test blocks, which are accepted hardness standards, and its capacity to transmit this accuracy to test materials.
Repeatability
A good microhardness tester should be capable of reproducing duplicate results based on the recognized hardness standards.
Correlation
A good microhardness tester should produce results that correlate with another well-calibrated tester or two different operators to get results that correlate using one microhardness tester.
IV. Five Main Causes for the Common Problems in Microhardness Testing
The Machine Itself
Unlike other test methods, microhardness testing uses dead weights to apply force to the indenter. This removes magnification errors and other errors caused by using screwdrivers to apply force to the indenter. In general, the type of force application that a microhardness test machine uses may cause various problems in the results achieved.
When measuring case depths or simply attempting to correctly place an impression on a certain region, the indenter’s alignment with the test sample is essential. However, suppose the operator is measuring effective case depth. In that case, the distance from the edge of the sample may be incorrect, leading to an inaccurate measurement even though the precision of the hardness value is unaffected by this error.
The Professional Level of the Operator
To successfully perform microhardness testing as a profession, the operator must be knowledgeable and experienced. If your material isn’t properly secured within the test fixture, the results you obtain may be inaccurate. In addition, if the operator isn’t skilled at precisely placing the indenter on the surface of your sample, it will be difficult for them to get reproducible results.
In most cases, errors are caused by an operator’s inability to precisely and consistently resolve the ends of these impressions. It isn’t easy to get two operators to measure the same impression perfectly in agreement with one another. Users frequently complete daily machine verifications, which can hide this issue. In this situation, operators can take their time measuring these impressions on test blocks with known hardness, whose test surfaces are normally in the best condition.
The Surrounding Environment
Vibration may affect the loading accuracy of microhardness tests because of the light loads used. Even if the component is not affected during loading, the indenter may penetrate the part further and produce a softer result due to vibration of the indenter or test specimen. Microhardness testers should always be set up on a specific, level, strong, free-standing table. Machines are frequently put on appropriate tables in the wrong places, like next to a wall or another table or counter. A lab door closing and sending movement down the wall, or someone working on the adjacent table sending movement through the table, can both give erroneous results in this circumstance.
A clean inspection environment is also essential. You should consider that any material on the floor, like dirt or oil, can affect the outcome of your microhardness test due to slippage between the sample and the machine base.
Test Sample
① Size
The material’s thickness and its actual size should be considered. The sample material’s thickness will affect measurements related to effective case depth, such as the surface area of the indent, the case depth determination, and the equivalent circuit diameter determination.
② Material Type
Different materials have different individual physical and mechanical consistencies, differences in corrosion resistance, and differences in plating or coating. Different materials may change due to aging or exposure to radiation or solvents.
The properties of materials that are not completely homogenous may be measured using the microhardness test process by measuring at multiple locations on the test material.
③ Surface
The surface should be clean, dry, and representative of the material. The surface of a test sample should be smooth so that it will not affect the amount of force needed to make an impression on the test surface. In addition, areas with die casting flash or flaws in the surface can cause significant errors in testing results.
④ Orientation
The orientation of the test sample is important and may affect the microhardness test’s outcome. Reversing the indenter can result in a more accurate and consistent measurement. In addition, if the impression is applied to an uneven surface, the actual indentation depth will differ from a smooth surface.
Calibration
Microhardness testing should be calibrated annually using a test material of known hardness. In this test, a sample is measured across the entire surface, and then a value for the microhardness is obtained. The machine’s reading should be recorded in the control book and your operating and calibration records to compare it to future test results. It’s important to check if the machine has been used for other purposes since it last operated with good results. Most microhardness testing machines are used in laboratories that are kept free from dirt and tampering. This ensures consistency in calibration for longer periods. You should ensure that the microhardness machines in use are well calibrated to avoid test results errors and ensure consistent and accurate results.
Conclusion
Microhardness testing is an accurate, reliable, and reproducible process used to measure the hardness of materials. Many factors can affect the results of these tests and affect their repeatability.
After learning about what affects the repeatability of micro-hardness testing, it is clear that a greater understanding of how these errors occur will lead to improved research and more accurate results.