How to Test the Hardness of Ceramic Materials?
In many fields, we can see the figure of special ceramic materials. Special ceramic materials
are generally characterized by limited thermal expansion coefficient, low density, low fracture toughness, high hardness, high elastic modulus, etc., so they are often used to make engineering structural materials such as bearings, cutting tools, grinding balls, valves, bearing beams, and hard coatings for various components.
The hardness test is the key to check the mechanical strength of ceramic workpieces which are easily affected by wear. For example, the hardness of bearing parts has a great impact on its performance - because the hardness of the bearing will affect its wear resistance, and the wear caused by the poor wear resistance will lead to the precision of precision machinery greatly reduced.
"Hardness" refers to the resistance of a material to be pressed into by a harder object under another force. Generally, the macro hardness of minerals is determined by Mohs hardness tester according to the grade 10 standard. In Mohs hardness, each mineral with high hardness can use it to scratch the mineral in front. However, because the Mohs hardness is very uneven, it is usually measured not only by Mohs hardness, but also by microhardness tester.
Microhardness test is a kind of micro static test method, the most commonly used are Vickers and Knoop. They are both used to calculate the hardness of the tested object by measuring the diagonal length of the residual indentation of the diamond angle cone indenter after pressing into the tested object under a certain test force through optical amplification. See below for details:
The Vickers hardness test is to press the diamond positive pyramid indenter with an included angle of 136 ° (148 ° 6'42 ") between two opposite sides into the surface of the sample under a certain test force, after maintaining a certain time, remove the test force, measure the diagonal length of the indentation, calculate the surface area of the indentation, and obtain the Vickers hardness value of the material.
The advantages of Vickers method are small load, small sample indentation can be measured, no damage to the sample, the disadvantage is that the measurement accuracy is low and the error is large.
Vickers surface indent 10x microscope lens
Knoop hardness method:
The basic principle of Knoop hardness test is to press the diamond pyramid indenter with two long edges at an angle of 172 ° 30 'and a short edge at an angle of 130 ° into the surface of the sample under a certain test force. After a certain period of time, remove the test force, measure the length of the long diagonal line of the indentation, calculate the projection area, and obtain the Knoop hardness value of the material.
The load of Knoop hardness measurement method is also small, and it can also be used to measure small sample indentation. However, because the indentation is long, it is easier to measure, so the measurement accuracy is high and the error is small.
Knoop surface indented 40 times microscope lens
However, it should be noted that the geometric deviation of material and grain size is an important factor affecting the surface finish. Therefore, in order to obtain the surface quality suitable for hardness test, it is recommended to grind or finely polish the test sample first to ensure that the hardness test dent in the ceramic material is clear and visible.