Knoop Hardness Test
The Knoop Hardness Test provides a microhardness measurement technique for brittle materials and thin surface layers. This method enables precise hardness evaluation where only a small indentation is possible. Researchers and engineers often use it in the material science and engineering fields.
- Common applications include:
- Testing ceramics
- Examining glass
- Analyzing thin coatings
Key Takeaways
- The Knoop Hardness Test measures hardness by making a small, shallow indentation, ideal for brittle materials and thin coatings.
- It uses a special diamond indenter with an elongated shape, measuring only the long diagonal of the indent for precise results.
- This test applies low loads to avoid damaging delicate surfaces, making it perfect for ceramics, glass, and thin layers.
- Compared to other tests, Knoop offers higher accuracy and less damage to fragile samples but requires smooth surfaces and careful preparation.
- Choosing the right hardness test depends on the material and sample size; Knoop excels in micro-scale testing where other methods may fail.
Knoop Hardness Test Basics
Definition of Hardness
Hardness describes a material's resistance to localized plastic deformation, such as indentation or scratching. In the context of the Knoop Hardness Test, hardness receives a precise numerical definition. The Knoop Hardness (HK) is calculated using the formula:
HK = 14.23 F / d² where F represents the applied load in kilogram-force (kgf) and d is the length of the long diagonal of the indentation in millimeters. The result is expressed in kgf/mm², which is are unit of stress. The long diagonal is critical for this calculation, measuring about 30 times the indentation depth and 7 times its width. This approach allows for accurate hardness measurement, especially in thin layers or brittle materials. Researchers often convert HK values to SI units such as MPa or GPa for consistency in scientific reporting.
Note: The test load varies depending on the material and must be specified when reporting hardness values.
Knoop Indenter Shape and Design
The Knoop Hardness Test uses a uniquely shaped indenter. This indenter consists of a rhombic-based pyramidal diamond with a long diagonal approximately 7.114 times the length of the short diagonal. The elongated geometry minimizes elastic recovery and enables precise testing of thin layers and closely spaced indents. Unlike the Vickers test, which uses both diagonals and the true contact area, the Knoop method relies on the projected area determined by the long diagonal.
The following table summarizes key design parameters and calculation aspects:
| Parameter/Aspect | Knoop Indenter Description | Calculated Value / Formula |
|---|---|---|
| Indenter Shape | Rhombohedral diamond with a long diagonal ~7.114 times short diagonal | - |
| Angles between faces | θ = 172°5 (between opposite faces), φ = 130° (between other two faces) | - |
| Hardness Calculation Basis | Uses projected area of indentation (not true contact area) | KHN = 14.229 * P / L² (P in N, L in mm) |
| Measurement | Only the long diagonal (L) of the indent is measured | - |
| Comparison to Vickers | Vickers uses true contact area and both diagonals; Knoop uses projected area and one diagonal | Vickers formula: VHN = 1.8544 * P / d² (d = diagonal length) |
This design allows the Knoop Hardness Test to produce shallow indents, which reduces distortion from hardness gradients and supports accurate measurement in thin or brittle specimens.
Test Procedure Overview
The Knoop Hardness Test follows a systematic procedure to ensure reliable results. The steps include:
- Select a Knoop indenter with an elongated pyramid shape and a length-to-width ratio of about 7:1.
- Apply a known low load, typically not exceeding 1 kg, to press the indenter into the sample surface.
- Maintain the load for a predefined dwell time to create a small, elongated diamond-shaped indentation.
- Measure the length of the indentation using microscopy or in situ visual imaging.
- Use the measured indentation length to calculate the Knoop hardness value.
Researchers have developed advanced statistical methods to correct for initial contact errors and surface roughness effects during nanoindentation tests. For example, studies on sandblasted aluminum alloy specimens demonstrated that these corrections reduce variability in hardness measurements. The results showed macro-hardness values between 1.63 and 1.93 GPa, with improved accuracy and reproducibility. This quantitative relationship between surface roughness and initial contact error highlights the reliability of indentation hardness tests, including the Knoop Hardness Test.
A comparison of Knoop and Vickers indentation on plasma-sprayed alumina coatings further demonstrates the effectiveness of the Knoop method:
| Parameter | Knoop Indentation Result | Vickers Indentation Result |
|---|---|---|
| Hardness (GPa) | 1.75 ± 0.42 | N/A |
| Elastic Modulus (GPa) | 110 ± 40 | N/A |
| Result Dispersion (%) | 15% | 33% |
| Representativeness of Results | More representative | Less representative |
The Knoop Hardness Test provides less variable and more representative hardness measurements, especially for brittle coatings and thin layers.
Knoop Hardness Test Calculation
Knoop Hardness Number (KHN) Formula
The Knoop Hardness Test uses a specific formula to determine the Knoop Hardness Number (KHN). This formula relates the applied load to the size of the indentation left by the diamond indenter. The standard formula is:
KHN = 14.229 × (F / D²) In this equation, F represents the applied load in kilograms-force (kgf), and D is the length of the long diagonal of the indentation in millimeters (mm). The constant 14.229 accounts for the geometry of the Knoop indenter. The test procedure involves applying a controlled load, usually between 10 and 1000 grams-force, for a set dwell time. After removing the load, the operator measures the long diagonal of the indentation using a microscope. The Knoop Hardness Number is then calculated using the measured value and the applied load.
ASTM standards recommend repeating the test at multiple locations to ensure accuracy. The Knoop Hardness Test provides reliable results for brittle materials and thin layers because the indenter creates a shallow, elongated impression. A 1% error in the applied load or indenter angle leads to a 1% error in the KHN, while a 0.5% error in the diagonal measurement causes a 1% error in KHN. This sensitivity highlights the importance of precise measurement.
Tip: Always record the load and dwell time when reporting Knoop hardness values for clarity and repeatability.
Units and Notation
The Knoop Hardness Test follows international standards for units and notation. The table below summarizes the key elements:
| Element | Description |
|---|---|
| Indenter Shape | Rhombohedral diamond; long diagonal ~7 times the short diagonal |
| Calculation Formula | KHN = 14.229 × (F / D²) (F in kgf, D in mm) |
| Load Range | 10 – 1000 grams-force (gf) |
| Measurement | Long diagonal measured in micrometers (µm) or millimeters (mm) |
| Notation Example | 375 HK0.3 (375 Knoop Hardness at 300 gf load) |
| Units | Hardness reported as HK; kgf/mm² unit is no longer standard |
| Official Standards | ASTM E384, ISO 4545 |
| Dwell Time | Typically 2–15 seconds; must be specified |
The notation for Knoop hardness includes the hardness value, the HK symbol, and the applied load. For example, "870 HK 1/30" means a Knoop hardness of 870, measured with a 1 kgf load and a 30-second dwell time. Only the long diagonal is measured, which saves time compared to other microhardness tests.
Advantages of the Knoop Hardness Test
Testing Thin Layers and Brittle Materials
The Knoop Hardness Test offers significant benefits for evaluating thin layers and brittle materials. The unique indenter creates a shallow, elongated impression with a length-to-diameter ratio of about 7:1. The indentation depth measures only about 1/30 of the long diagonal. This geometry reduces the risk of cracking or spalling in fragile materials such as ceramics, minerals, and glass. Researchers can measure hardness on small or anisotropic samples, including thin coatings and laminates, without causing the extensive damage often seen in macrohardness tests.
The test applies micro loads, typically below or equal to 1 kgf. This low force allows precise measurements on delicate surfaces. The Knoop Hardness Test requires indentations with diagonal lengths not less than 20 μm to maintain accuracy. The shallow and narrow indentation profile makes this method ideal for brittle or layered specimens, where traditional hardness tests might fail or cause excessive damage.
High Precision and Minimal Damage
The Knoop Hardness Test provides high precision in hardness measurements. Laboratories often perform multiple tests on the same sample or on samples from the same batch to ensure reproducibility. They calculate statistical parameters such as mean, standard deviation, and confidence intervals to evaluate the variability in hardness values. This statistical approach delivers accurate, reproducible, and detailed hardness profiles at the microscopic level.
- Test results include essential data:
- Applied load
- Indenter type
- Dwell time
- Calculated hardness values
These details support both quality control and research. The Knoop Hardness Test follows standardized procedures, such as ASTM E92, which ensures reliable and comparable results across different laboratories. The shallow indentation minimizes damage to the sample, preserving its integrity for further analysis or testing.
Disadvantages of the Knoop Hardness Test
Limitations Compared to Other Methods
The Knoop Hardness Test offers unique advantages for brittle materials and thin coatings, but it also presents several limitations when compared to other hardness testing methods. This test requires small, delicate specimens, often demanding sectioning before evaluation. Many laboratories find this requirement restrictive, especially when handling larger or irregular samples. The method demands a high-quality surface finish because the optical measurement of the indent relies on clear, well-prepared surfaces. Specimen preparation becomes more time-consuming and critical than in macrohardness tests.
The testing process itself proceeds at a slower pace than methods like Rockwell. Each test cycle, excluding preparation, can take 30 to 60 seconds. Laboratories must also invest in specialized equiPment, including integrated optical systems, which increases costs. The Knoop method remains less versatile than the Vickers test, which adapts more easily to a wide range of metallic materials. Adoption of the Knoop method remains limited, especially in Europe, where other hardness tests are more common.
- Key limitations include:
- High surface quality is required for accurate optical measurement.
- Slower testing process compared to Rockwell.
- Higher equipment costs due to optical measurement needs.
- Less versatility for general metallic materials.
- Limited adoption outside specific research and industrial applications.
Sources of Error
Several sources of error can affect the accuracy and reliability of results in this microhardness test. Differences in indentation methods and challenges in correlating results between different hardness scales introduce complexity. Converting values between scales, such as from Knoop to Vickers, often leads to inaccuracies.
Non-uniform pressure distribution beneath the indenter can distort results. Elastic recovery of the material after indentation may alter the measured area, while sink-in and pileup phenomena around the indent can change the actual contact area. The geometry of the indenter itself influences measurement outcomes, especially in thin or layered specimens.
- Common sources of error include:
- Non-uniform pressure beneath the indenter.
- Elastic recovery affects the measured area.
- Sink-in and pile-up phenomena alter the contact area.
- Limitations in microscope magnification and calibration.
- Variability in calculation methods, such as using projected versus actual contact areas.
- Underestimation or overestimation of hardness due to instrument or method limitations.
Researchers must carefully control these factors to ensure reliable and reproducible hardness measurements.
Knoop Hardness Test vs. Other Hardness Tests
Comparison with Vickers Hardness Test
The Vickers and Knoop methods both serve as microhardness tests, but they differ in several important ways. The Vickers test uses a square-based diamond pyramid indenter, while the Knoop test employs an elongated, rhombohedral diamond indenter. This difference in indenter shape leads to distinct indentation geometries. Vickers produces a more symmetrical, deeper impression, making it suitable for a wider range of materials, including metals and alloys.
Knoop excels when testing brittle materials or thin coatings. Its shallow, elongated indent reduces the risk of cracking and allows for precise measurement in small or layered samples. The Vickers method requires measurement of both diagonals, which can introduce more measurement error, especially on rough or uneven surfaces. In contrast, the Knoop method only requires the long diagonal, simplifying the process.
| Feature | Vickers Test | Knoop Test |
|---|---|---|
| Indenter Shape | Square-based pyramid | Elongated rhombohedral |
| Suitable Materials | Metals, alloys, ceramics | Brittle, thin layers |
| Indentation Depth | Deeper | Shallower |
| Measurement | Two diagonals | One diagonal |
| Risk of Cracking | Higher in brittle samples | Lower |
Note: Both tests provide high precision, but Knoop offers advantages for delicate or thin specimens.
Comparison with Rockwell and Brinell Tests
Rockwell and Brinell tests operate as macrohardness methods. They use larger loads and create bigger indents, making them ideal for bulk metals and components. Rockwell uses either a diamond cone or a hard steel ball as the indenter, while Brinell relies on a large steel or carbide ball. These tests measure hardness by the depth or size of the indentation under heavy loads.
Knoop and Vickers tests, in contrast, use much lower forces and produce smaller indents. This makes them better suited for microstructures, coatings, and brittle materials. The Rockwell and Brinell methods cannot accurately test thin layers or small features because their large indents may penetrate through coatings or cause excessive damage.
- Key differences:
- Rockwell and Brinell provide rapid, straightforward testing for large, homogeneous samples.
- Knoop and Vickers enable detailed analysis of small, thin, or brittle specimens.
- Macrohardness tests lack the sensitivity required for micro-scale features.
Tip: Select the hardness test based on sample size, material type, and the required level of detail.
Researchers value this microhardness method for its precision in testing brittle materials and thin layers. Load selection plays a critical role, as indentation size effects can influence results, especially at low forces. Measurement accuracy depends on factors like optical resolution and operator consistency. For best results, experts recommend using consistent loads, high-resolution imaging, and complementary techniques. Those seeking deeper understanding can consult ASTM E384 or explore advanced methods such as BioDent RPI.
FAQ
What materials are best suited for the Knoop Hardness Test?
Ceramics, glass, and thin coatings respond well to the Knoop Hardness Test. These materials often crack or deform under heavier loads, so the shallow indentation from the Knoop indenter provides accurate results without causing significant damage.
How does the Knoop Hardness Test differ from the Vickers test?
The Knoop test uses a rhombohedral diamond indenter and measures only the long diagonal. The Vickers test uses a square-based pyramid and measures both diagonals. Knoop works better for thin layers and brittle materials, while Vickers suits a wider range of metals.
Why is surface preparation important in the Knoop Hardness Test?
Surface preparation ensures accurate measurement. A smooth, polished surface allows the indenter to create a clear, measurable impression. Poor preparation can lead to inaccurate readings or difficulty in identifying the indentation.
Can the Knoop Hardness Test measure very small features?
Yes. The Knoop Hardness Test can evaluate microstructures, thin films, and small regions. The elongated indent and low applied load make it ideal for analyzing features that other hardness tests cannot assess without causing damage.