Ceramic Hardness is one material property which makes advanced ceramics unique. The high performance of fine ceramics is generally attributed to their hardness data. Hardness makes them perform better as body amors, cutting tools, Pump components, valves and automobile parts. Knowing ceramic hardness is therefore crucial for assessing functionality.
Hardness of Material: An Overview
Hardness is a material property that speaks volumes about the behaviour of a material under applied stress. There has been numerous research done in the past to conclude on the hardness of popular materials. One such standard is Mohs hardness scale.
Things to know about Mohs hardness scale
Mohs hardness scale, constitute the information of hardness or scratch resistance of different substances of interest. This is a giant ranking system where most of the common materials are listed out. It has the hardness data defined in the scale of 1 to 10. Mohs hardness scale helps geologists narrow down to the best performing entities.
How to Read Moh’s hardness scale
Reading Mohs hardness scale is relatively simple. Referring to the above image, the material which is the hardest of all is Diamond. Technical ceramic such as Boron Nitride has ceramic hardness of 10 which is equivalent to that of diamond. The hardness of Tungsten carbide and Titanium Carbide are on similar scale as represented in the graph.
Ceramic hardness Vs Steel
Some other Key takeaways of Moh’s hardness scale is the hardness data of steel and the comparison with ceramics. The hardened steel has a Moh’s hardness scale range of 7-8. The Moh’s hardness of ceramic such as Silicon or Boron carbide is superior to that of hardened steel. Regular steel is far more inferior where the hardness range is 4-5, even lesser than that of tooth enamel.
Where is Porcelain on Moh’s scale?
Porcelain is a type of ceramic material that is translucent and constitute fine grain size. The raw materials such as kaolin, quartz, feldspar and mica make it a harder ceramic. The high temperature processing of the above material results in the formation of porcelain, generally known as Chinese clay.
Looking at Moh’s hardness scale shown above, the range of porcelain ceramic is around 7. The material is better in terms of wear resistance when compared with regular steel. The hardness as per Moh’s scale is equal to the hardness of quartz.
Importance of testing Ceramic hardness
Ceramic hardness is generally referred to a ceramic’s ability to resist any plastic deformation. Hardness obviously is not just the single factor that speaks about mechanical strength. Beside hardness, properties such as toughness and wear resistance help us to build a better outlook on a particular ceramic.
The idea about ceramic hardness is very important to define the applicability of a particular ceramic in any desired field. Many demands of today specify the minimum hardness required. For example, surgical implants need a Zirconia hardness of around 11.8 GPa for a load of around 9.8 N.
Given above is a quick comparison chart of different technical ceramic and their hardness values. Similar to Moh’s data, of all the ceramic available Boron Carbide has the highest hardness value followed by Silicon Carbide.
How to test hardness of Ceramic materials?
Hardness of ceramic materials is a function of the inherent properties of the ceramic chosen. Such properties include composition of the ceramic, Size of grains, the porosity and phase at the grain boundaries. Hardness testing of ceramic helps us to characterise a ceramic material against the application overlooked. Let us see in detail the different ceramic hardness tests present globally.
Indentation tests
Indentation technique is relatively simple and straightforward. An indenter is pressed on to the surface of the ceramic material and the indentation formed is gauged. A calculated load is applied on to the material for creating the indentation. Further with the help load data and the deformation extend, the hardness of ceramic is determined.
The indenters used in the test are of different sizes and shape. Some of the shapes are given below for reference. The sizes vary from nano, micro and macro depending upon the type and properties of the ceramic to be tested. The general idea behind hardness testing of ceramic is, the harder the material, the lesser the indentation formed.
Ther are several types of indentation tests such as Vickers, Berkovich, Knoop, Brinell and Rockwell method. These tests are static as a pre-determined load is used to create the indentation. Most of the worldwide ceramic hardness tests are based on Vickers, Knoop and Berkovich procedures. Rockwell and Brinell are less considered for ceramic hardness testing.
Vickers Hardness test
About 60 % of the global engineering and characterization follows Vickers hardness test. Vickers hardness test typically uses load in the range of 9.8 N as recommended by C1327 standard. It also occasionally uses 98 N depending upon the nature of the ceramic. The indenter material made up of diamond is pressed on to the ceramic for measurement.
The indenter used in Vickers hardness test is a square shaped pyramid. An angle of 136 degrees is maintained between the opposite faces of the indenter. The force applied is around 100 Kgf for a duration of about 10 – 15 seconds. The indentation formed is then measured to find the ceramic hardness by the formula given below.
H =1.8544Pd2
Were,
H is the Vickers ceramic hardness
P is applied forced in Kgf
d is the diagonal distance of the indentation
The image above shows a Vickers indentation on Silicon Nitride Ball bearing used in EV’s.
Shortcomings of Vickers hardness tests
Vickers hardness test suffers certain shortcomings. The technique sometimes generates erroneous readings due to the difficulty in measurement when the indentation size is less. At very high loads, general problems such as cracking and spalling also arises for Vickers hardness test. However, glitches aside, the test is surely recommended as per the standard to characterise any advanced ceramic.
Knoop test of Ceramic hardness
Knoop test to determine hardness of ceramic materials uses a rhombic shaped indenter. It is made up of diamond which is basically a lookalike of a long pyramid. Knoop test is used to measure microhardness of brittle materials or thin sheets. The technique is recommended in ASTM E- 34 standard.
The indenter in Knoop’s test has an angle of 172 degrees between the opposite faces. Here is an image of a basic indentation formed during Knoop Ceramic hardness Test. One major advantage of Knoop test over Vickers is related to the functionality of the lengthy indenters. Lengthy indenters avoid cracking of ceramic during test.
The extend of indentation caused by the Knoop test is generally measured using a microscope. The indentation are basically 2.8 times longer and shallow when compared to Vickers indentation. The expression to calculate the Knoop ceramic hardness is given as:
HK= PCpL2
Were,
P stands for the load
Cp is the correction factor of the indenter. The generally used value is around 0.070279
L is the diagonal length of the indentation
Shortcomings of Knoop hardness test
Theoretically the long indentation should make the measurement easier in Knoop test. But the uncertainty due to tapered ends poses a difficulty in the test. The value of uncertainty of the tip related to Knoop ceramic hardness test is of the order 0.5 to 1 mm. Another factor that affects the reading is the resolution of the commercial microscopes used to measure the indentation.
Rockwell test of Ceramic hardness
Rockwell test uses a predetermined load of around 150 Kgf. It employs an indenter made up of diamond or tungsten carbide. The indenter is ball shaped. Once the indentation is made, the depth and the reference point are compared to get the hardness value. ASTM E -18 is the general code of conduct for Rockwell test.
Rockwell ceramic hardness test is easier to perform and is relatively accurate when compared with the other tests. Initially a preload is applied with the help of a ball indenter. Once the preload breaks through the surface finish, the indentation caused is measured.
Berkovich test to determine hardness of Ceramic material
Berkovich uses a flat pyramid with a sharp tip to measure the hardness of material majorly in nanoscales. The ceramic should be minimum 100 mm thick for performing Berkovich test. Berkovich test is usually used in small scale studies and offers greater precision. The indenter used in Berkovich test of ceramic hardness has an angle of 115 degrees.
A representative image of the Berkovich indenter is given below.
The formula for determining ceramic hardness through Berkovich test is :
H=1569.7 P d2
Where,
P stands for the test force in Kgf
D is the diagonal measurement in micrometres of the indentation
Bottom Line
Knowing Hardness of the ceramic material is important to evaluate the functionality of ceramic under contexts. The testing methods such as Vickers, Knoop and Rockwell has been successful since years. As defined by the standards, the right tests have to be performed on the right material to obtain fruitful data.