Definition
A micro Wechsler hardness tester is a precision instrument used to measure the ability of a material's surface to resist indentation. The core principle is to use a specific shape of an indenter (usually a diamond prism with a standard angle) to press into the surface of the specimen under the action of a specified test force, and determine the hardness value of the material by measuring the diagonal length of the indentation. This instrument is specifically used for hardness testing of small areas or thin layers of materials, typically less than 9.807 Nm (1 kgf).
How it works:
The micro-Wechsler hardness tester is based on the plastic deformation law of metal materials under static pressing conditions. During the test, a diamond regular tetragram indenter with a relative angle of 136 degrees presses vertically into the surface of the test sample under the action of a preset test force F and maintains it for a specified time (usually 10 to 15 seconds). After removing the test force, a diamond-shaped indentation is left on the surface of the specimen. The length of the two diagonals (d1 and d2) of the indentation is measured by a high-magnification optical measurement microscope (magnification typically 100x to 600x).
According to standards such as GB/T 4340.1 or ISO 6507-1, the formula for calculating the hardness value HV is:
HV = 0.102 × (2F) × sin(α/2) / (d²)
Among them:
HV is the Vickers hardness value, and the dimension is kilogram-force/square millimeter;
F is the test force, unit Newton (N);
α is the angle of the opposite surface of the indenter, 136 degrees;
d is the arithmetic mean of the length of the two diagonals of the indentation, in millimeters (mm).
0.102 is the constant that converts Newtons into kilogram-force (1 kilogram-force≈ 9.807 N).
Measurement method and steps
Micro Wechsler hardness testing follows a standardized process to ensure reproducible results.
First, sample preparation. The surface of the specimen needs to be polished or ground to remove oxide layer, oil stains, etc., and the surface roughness Ra value is usually not greater than 0.32 microns. For thin or coated specimens, ensure the flatness of the back side to avoid bending and deformation affecting indentation measurements.
Second, the selection of experimental power. According to the expected hardness of the material and the thickness of the measured area, the appropriate test force range (e.g., 9.8 mN to 9.8 N) is selected. The standard recommends test forces usually include HV0.01, HV0.025, HV0.05, HV0.1, HV0.2, HV0.3, HV0.5 and HV1. Excessive testing force may cause specimen penetration or indentation too large.
Third, loading and holding. Place the specimen on the stage and adjust the focal length to make the surface image clear. Start the loading mechanism and the indenter approaches the specimen surface at a steady speed, holding it for 10 to 15 seconds after reaching the set force. Unloading should be smooth to avoid vibration.
Fourth, indentation measurement. The length of the two diagonals is measured by a light microscope eyepiece or digital imaging system. Measure both endpoints per diagonal and take the average. Measurement accuracy is usually required to be 0.1 microns. If the edges of the indentation are blurred or irregularly shaped, re-test.
Influencing factors
Several factors can affect the accuracy of microscopic Wechsler hardness testing and need to be controlled in practice.
The accuracy of the test force is a key factor. If the test force deviation exceeds the standard allowable range (e.g., ± 0.5% to 1%), it will directly cause the hardness value deviation. Wear or deformation of the indenter geometry, especially the diamond angle and edge state, can alter the stress distribution. The standard requires that the angle deviation of the relative surface of the indenter should not exceed ±0.1 degrees.
The surface quality of the specimen determines the clarity of the indentation edges. Rough surfaces, scratch residues, or scaling can cause blurred indentation boundaries, resulting in systematic errors in the measurement diagonal. In addition, too fast loading speed may cause uneven plastic deformation; Loading speed that is too slow may introduce creep effects. Specimen temperature fluctuations can lead to variations in indentation size, and the test ambient temperature should generally be kept between 10 and 35 degrees Celsius.
Applications:
Micro Wechsler hardness testers are widely used in materials science research and industrial quality control, especially in the following scenarios:
Hardness differentiation of microscopic components of metal materials. For example, the hardness differences of different phases (such as ferrite, martensitic, and carbide) in steel materials can be identified by microhardness measurement, which can assist in phase diagram analysis and heat treatment process optimization.
Hardness evaluation of coatings and surface modification layers. For example, carbide coating, nitride layer, chrome plating layer, carburizing layer, hardening layer, etc., the thickness is usually micron to millimeter, and the micro Wechsel hardness tester can effectively measure the hardness distribution and gradient changes in these areas, providing a basis for surface engineering.
Quality inspection of precision parts and micro components. For example, gold-plated solder joints on circuit boards, tiny gears in precision machinery, instrument pointers, etc., cannot be used with conventional hardness testers due to their small structure, and micro-Wechsley hardness testers are feasible testing methods.
Hardness assessment of non-metallic hard materials. For some ceramics, glass, hard plastics, etc., when their brittleness is large, choosing a smaller test force can reduce the risk of cracks, but it is still necessary to pay attention to the integrity of the indentation edge.
Key points of selection
When purchasing a micro Wechlet hardness meter, consider the following core elements:
Test force range and accuracy. Determine the minimum and maximum testing force required based on the type and thickness of the material to be tested on a daily basis. For example, a range of 0.01 to 0.2 N is required for inspection of film materials, while 1 to 9.8 N is required for metal blocks. The loading and unloading control methods of the test force include manual, electric or automatic closed-loop control, and the automatic closed-loop system generally has higher repeatability.
Optical measurement system performance. The magnification of the objective lens (commonly 40x, 100x, 200x) and the digital resolution (pixel size) directly affect the measurement accuracy. Instruments with automated image analysis software can reduce human measurement errors and improve testing efficiency, but the software's edge recognition algorithms need to be validated for complex surfaces such as rough or heterogeneous materials.
Specimen fixture and positioning system. For special-shaped or tiny workpieces, rotatable or movable stages, special fixtures, and XY-axis motorized positioning systems can significantly improve ease of operation and positioning accuracy. Some devices are also equipped with CCD cameras and displays for real-time observation.
Environmental adaptability. The instrument should have vibration resistance, dust resistance and temperature compensation capabilities, especially high-precision equipment. For laboratories with poor constant temperature conditions, it is recommended to choose a model with temperature drift compensation.
Standards compliance. The equipment should be able to meet the requirements of GB/T 4340, ISO 6507, ASTM E384 and other standards, and come with a third-party calibration certificate. Prioritize equipment with modular design and easy subsequent upgrades or repairs to reduce long-term usage costs.