ASTM C1326 Standard Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures

Get Certified ASTM C1326 Testing for Reliable Knoop Hardness Measurement

ASTM C1326 testing is a standard method for determining the Knoop indentation hardness of advanced ceramics. The procedure results in a comparatively small permanent impression, and the long diagonal of the impression is measured with an optical microscope. Thus, the hardness value obtained can be used by engineers to characterize advanced ceramic materials. Knoop indentation can also be helpful when assessing the hardness of coatings, since the indentations are shallower than a Vickers indentation made with the same force.

• Material type: Advanced ceramics.
• Purpose: Determination of Knoop indentation hardness.
• Property measured: Knoop indentation hardness.
• Indenter: Pointed, rhombic-based, pyramidal diamond indenter of prescribed shape.
• Test force: A predetermined force applied to the ceramic surface.
• Indentation: A relatively small, permanent indentation produced by the Knoop indenter.
• Measurement: Surface projection of the permanent indentation’s long diagonal.
• Instrumentation: Light microscope for measuring the long diagonal.
• Calculation: The applied force and the long diagonal are used to calculate Knoop hardness.
• Force units: Newtons (N).
• Length units: Millimeters (mm) or micrometers (μm).
• Result: Knoop hardness is reported in GPa or as a dimensionless Knoop hardness number.
• Application: Characterization of advanced ceramics based on resistance to penetration by the Knoop indenter.

What are the Uses of ASTM C1326 Testing?

The Knoop indentation hardness of advanced ceramics is measured by ASTM C1326 testing. The method can be used to obtain hardness data for the characterization of ceramic materials and to assess the resistance to penetration of the Knoop indenter.  The ASTM C1326 standard:

• Measures the Knoop indentation hardness of advanced ceramics.
• Describes high-performance ceramics by means of indentation hardness.
• Measures the resistance to penetration of the Knoop indenter.
• Measures the long diagonal on the surface of the impression.
• Uses the applied force and projected indentation area to calculate hardness.
• Reports either in GPa or as a dimensionless Knoop hardness number.
• Helps to assess coating hardnesses.
• Supplies a standard method for Knoop indentation hardness measurement.
• Helps in hardness characterization with the required test force.
• Permits the comparison of Knoop indentation hardness under controlled test conditions.

Which materials can be tested under ASTM C1326?

ASTM C1326 applies to advanced ceramics for Knoop indentation hardness testing. The technique involves pressing the prescribed diamond indenter into the surface of a ceramic, and forming a relatively small permanent indentation. Technicians then measure the long diagonal of the indentation with a light microscope and use the measurement with the applied force to calculate Knoop hardness.

Why is ASTM C1326 Important?

ASTM C1326 is important because it provides a standardized method for determining the Knoop indentation hardness of advanced ceramics. Knoop indentation hardness is one of the properties that can be used to characterize advanced ceramics. The Knoop indentation also has a long diagonal, which is about 2.8 times that of the Vickers diagonal under the same applied force, and less cracking is a problem.

ASTM C1326 Testing Process and Data Collection

ASTM C1326 testing starts with the application of a predetermined force to an advanced ceramic specimen with the prescribed Knoop diamond indenter. The indenter makes a relatively small permanent mark in the ceramic surface. Afterwards, technicians use a light microscope to measure the surface projection of the long diagonal. Finally, the analysts use the applied force and the projected area of the indentation to determine the Knoop hardness. The test is conducted in SI units: force is measured in newtons (N) and the indentation length in millimeters (mm) or micrometers (μm). The resulting Knoop hardness is reported by ASTM C1326 either as a Knoop hardness number (dimensionless) or in GPa.

Common Challenges and Troubleshooting

To get reliable ASTM C1326 results, accurately measuring the long diagonal of the Knoop indentation is critical. The elongated, thin end of the Knoop impression may make it harder to identify and measure, especially for ceramics with low optical contrast. Thus, technicians should make sure that the indentation is not so deep as to obscure the measurement and that test conditions do not result in excessive cracking. Another effect, known as the indentation size effect (ISE) by ASTM, is a decrease in Knoop hardness as indentation size or applied force increases. The test force must be sufficient to create an impression that is measurable, but not so large that it causes excessive cracking. The complete ISE characterization needs measurements over a large range of indentation forces; ASTM C1326 measures hardness at a fixed indentation force.

• The laboratory will provide the Knoop hardness in GPa or as a Knoop hardness number (without dimensions).
• The light microscope is used to measure the surface projection of the long diagonal of the indentation.
• Knoop hardness is determined by applying a force to the indenter and relating it to the area of the impression projected on the surface.
• The Knoop indentation hardness is one property of advanced ceramics used by engineers.
• The Knoop hardness, in general, decreases as the size of the indent increases or as the force of the indent increases.
• Engineers should be wary of using conversions from Knoop hardness to other hardness scales, as there are no universally agreed-upon methods of conversion.
• Knoop indentations may also help in comparing coating hardnesses, as the indentations are shallower than the Vickers indentations made at the same force.

Link to ASTM C1326