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Imagine this...
What if you could look at a piece of steel and understand how strong it is, how much impact it can withstand, or how it will behave during the production process?
Not just with a glance, of course but with the right tests, it’s absolutely possible!
This is exactly where hardness comes into play.
But wait a second...
What exactly do we mean by "hardness"?
Glass is also hard yet it shatters when dropped.
So why doesn’t steel break?
Does hardness always mean durability?
In this article, you’ll find clear answers to all these questions.
Hardness may sound like a simple, oneword concept.
But once material science gets involved, that one word opens the door to an entire world of knowledge.
So, what really is “hardness”?
In its simplest form:
Hardness is a material’s resistance to plastic deformation in other words, permanent shape change.
But is that all there is to it?
Definitely not.
“Glass is hard. Steel is hard. Yet one breaks, the other bends. Why?”
Because hardness is not a property that fully defines a material's overall behavior.
Hardness Is a Surface Property That Indicates:
• How well a material resists penetration when a sharp object is pressed into it,
• How prone the surface is to scratching or denting,
• How resistant the material is to wear over time.
But pay attention:
Not every hard material is strong.
And not every strong material is hard.
Hardness ≠ Durability
Let’s clarify with an example:
Diamond is the hardest known natural material.
But strike it with a hammer and it can shatter.
Spring steel, on the other hand, is much less hard than diamond,
yet it flexes under high impact, absorbs energy, and does not break.
What does this mean?
While hardness expresses a material's surface resistance,
toughness refers to its ability to withstand impact.
That’s why the Charpy impact test should not be confused with a hardness test.
Reading the Character of a Material Through Hardness
From an engineer’s perspective, hardness is the first test used to determine:
• Whether gears will wear out quickly,
• How long blades will stay sharp,
• How effective surface coatings are.
In short, it’s not just a scientific property it has real, practical applications.
So how did hardness testing emerge, who first used it, and how has it evolved over time?
Today, we measure hardness in modern laboratories with digital screens, sure.
But hundreds of years ago, how did people decide that “this stone is harder than that one”?
What began as intuition in the Stone Age turned into measurement in the Industrial Age.
Hardness has always mattered to humans.
Cutting tools, weapons, armor all needed to be made from harder materials.
People rubbed stones together to see which one was more durable.
But the scientific foundation for this concept was established much later.
In 1812, German mineralogist Friedrich Mohs developed a hardness scale based on how ten different minerals could scratch one another.
On this scale:
• Talc is the softest (1),
• Diamond is the hardest (10).
Just think:
What Mohs did back then laid one of the cornerstones of modern engineering.
The fact that it’s still in use today is the greatest proof of its significance.
However, the Mohs scale is based on scratch resistance.
Over time, there was a need for more precise, quantitative results.
This led to indentationbased methods, which emerged during the Industrial Revolution.
In the early 1900s, Swedish engineer Johan August Brinell developed the first scientific hardness test.
He pressed a steel ball into a metal surface and calculated the hardness based on the diameter of the indentation left behind.
And yes the name may sound familiar: The Brinell Hardness Test.
Then came a domino effect:
• Rockwell (1914): Based on penetration depth; provides fast results.
• Vickers (1920s): Uses a diamond pyramidshaped tip for more precise readings.
• Knoop (1950s): Developed for ultraprecise microscale testing.
Each of these tests met different needs, supporting the growth of industrial technology.
And Today...
We can now measure hardness not only in metals,
but also in plastics, glass, ceramics, and even biomedical materials.
What’s more, we can do it in seconds using digital devices connected to mobile phones.
The history of hardness testing is the story of humanity’s quest to better understand its relationship with materials.
And now, let’s move on to another critical question:
Because hardness defines the character of a material.
Just like a person's threshold for patience, a material's hardness indicates how well it resists external forces.
And this knowledge is not just an academic curiosity it plays a vital role across hundreds of industries, from manufacturing to aerospace, from automotive to medical devices.
For example:
The surface hardness of titanium alloys used in aircraft landing gear is specifically measured and controlled to prevent deformation due to friction during highspeed landings.
Even a millimeterscale error can lead to catastrophic consequences!
Why Do We Measure a Material’s Hardness?
No matter the name of the material steel, aluminum, or plastic when working with it, shaping it, or using it as part of a structure, one of the first things we must know is:
How hard is it?
Let’s explain with reallife examples:
• In the automotive industry, if the hardness of a brake disc isn’t accurately measured, the braking distance may increase after just a few hundred kilometers.
• In screw manufacturing, if the screw tip is too hard, it may break; if it’s too soft, it may fail to penetrate the surface.
• In medical needles, optimal hardness is essential for both comfort and sterility.
And these are just a few examples.
From construction to medical devices, from electronics to space technology nearly all engineering applications rely on knowing the correct hardness values.
“How hard is a material? And how do we find that out?”
Now we’re stepping into the heart of the process.
Hardness is not a property visible to the naked eye.
That’s why scientists and engineers have, over the years, developed various test methods to turn this surface resistance into a measurable value.
But hold on…
Do all tests work the same way?
Of course not!
Hardness tests are based on three fundamental approaches:
• Scratch Hardness
• Indentation Hardness
• Rebound Hardness
Each follows a different principle.
Let’s now look at what they mean and explore the most commonly used methods.
This is the oldest method of measuring hardness.
The basic principle is simple: A harder material will scratch a softer one.
And the foundation of this idea was laid in 1812 by Friedrich Mohs.
Mohs Hardness Scale
Mohs created a scale based on whether one of ten minerals found in nature could scratch another.
At the bottom of the scale is talc (hardness 1) easily scratched,
while at the top is diamond (hardness 10) which can only be scratched by another diamond.
You can easily scratch glass with a diamond.
But you can’t even damage the surface of a diamond with glass.
That’s how simple and clear this concept is.
This method offers a more precise and numerical approach.
A tip (diamond, steel ball, etc.) is pressed into the material’s surface under a certain load,
and the depth or diameter of the resulting indentation is measured.
This method yields highly quantitative and sensitive results.
Most commonly used hardness tests fall into this group:
Rockwell Hardness Test (HR)
Practical, fast, and versatile.
In the Rockwell test, a minor load is applied first, followed by the main load.
The depth of the indentation formed under this force is then measured.
Common indenters:
• 120° conical diamond tip (used for hard materials like in C, A scales)
• 1/16" or 1/8" steel balls (used for softer materials like in B, E scales)
The entire process takes just a few seconds,
which makes it highly preferred in production lines.
Brinell Hardness Test (HB)
This one takes things a bit more “slow and heavy.”
A large steel or tungsten carbide ball is pressed into the surface with a high load.
Instead of depth, the diameter of the indentation is measured.
Ideal for softer and heterogeneous materials,
but be warned:
Brinell marks are large and can permanently damage the surface.
Vickers Hardness Test (HV)
A sharp squarebased diamond pyramid...
This is the indenter used in the Vickers test.
It provides highly accurate results, especially for thin hard coatings and in lab environments.
After applying the load, the two diagonals of the indentation are measured,
and the Vickers value is calculated using the load/area ratio.
Test results are usually expressed like this:
455 HV / 30 / 20 → 30 kg load, 20 seconds duration, 455 Vickers hardness.
Vickers offers high accuracy, international standard compliance (ASTM E92, DIN 50133, etc.),
and is excellent for microhardness testing.
Knoop Hardness Test (HK)
“This surface is very thin, but we need to measure it...”
Enter Knoop testing.
The diamond tip has a very elongated shape,
allowing it to test very small areas.
Ideal for coatings, ceramics, and biomedical applications.
In this method, a weight or hammer is dropped onto the material,
and the height of the rebound is measured.
Durometer (Shore Hardness Tester)
Used especially for soft materials like rubber, elastomers, and plastics.
There are different Shore scales: Shore A, Shore D, etc.
The test is simple but keep in mind:
It only measures elastic behavior.
It cannot be used on rigid materials like metals.
Charpy and Izod Impact Tests
Important!
These are not hardness tests.
They are toughness tests measuring how much energy a material can absorb from sudden impacts.
They calculate the energy absorbed during fracture.
This is critical for parts that will operate under impact conditions.
And yes…
Diamond is very hard, but not tough.
Spring steel can be very tough, but not extremely hard.
Think about it:
You have a steel part in your hand,
a plastic sample on the next table,
a ceramic plate across the room,
and a shiny alloy at the edge of your desk...
Can all of them be tested with the same method?
The answer is short and clear: No!
But why? Let’s explain.
Every material behaves differently.
Every material requires a different testing method.
Hardness tests must be selected based on:
• The structure of the material,
• Its elasticity,
• Its ability for plastic deformation,
• Its surface properties.
Here’s a quick guide by material type:
These are the classic heroes of hardness testing.
Steel, aluminum, copper, titanium all can be tested with:
• Rockwell
• Brinell
• Vickers
• Knoop
Why?
Because they exhibit plastic deformation and are suitable for indentationbased tests.
“These are hard, but brittle.”
Yes, ceramics are highly hard, but they shatter instead of deforming plastically.
So, Rockwell or Brinell are generally not suitable.
Instead, tests like Vickers or Knoop (microhardness) are used.
Interesting fact:
The fate of a dropped porcelain plate and a smartphone screen is nearly the same
hard, but not tough.
Flexible, soft, elastic...
Here, it’s not about indentation, but about rebound and deformation.
The most common method:
Shore Durometer (Shore A, Shore D)
A springloaded needle is pressed into the material,
and the resistance to penetration is measured.
Results are expressed as:
e.g., Shore A 85
“I don’t want to test the material I want to test its surface.”
This is where micro and nanolevel tests come into play:
• Vickers Microhardness
• Knoop Hardness Test
• Nanoindentation (uses very small loads with nanometer precision)
Perfect for evaluating coating quality and the effects of surface treatments.
Titanium implants, dental crowns, prosthetic materials...
Here, both hardness and biocompatibility are critically important.
Standard tests are not directly applied.
Instead, Vickers, Knoop, or special microtests are preferred.
What About Glass?
Glass is quite hard, but also very brittle.
So it is usually tested using microhardness methods.
Vickers or Knoop are the most common choices.
But caution is needed:
The applied load must be carefully selected to avoid damaging the surface.
Every material’s hardness test is like a fingerprint completely unique.
Trying to understand all surfaces with a single test is like driving at night with sunglasses on:
You may see something,
but it won’t be the right thing.
Many of us tend to use these two terms interchangeably, right?
But wait stop right there!
Because in the world of materials science, these are two very different concepts.
First of all, hardness is defined as:
The resistance of a material to plastic deformation when a harder object is pressed against it.
In essence, hardness relates to how durable the surface of a material is.
Simply put, it’s a material’s resistance to indentation, scratching, or wear.
Here’s where chemistry and internal structure come into play.
Hardenability is a particularly important concept in steels and can be explained as follows:
During heat treatment (quenching), hardenability refers to the material’s ability to form a very hard phase known as martensite within its internal structure.
More technically:
• Hardenability describes how deeply into the material martensite can form from the surface toward the core.
• The depth of hardening depends on how extensively martensite spreads within the steel.
• Another critical factor is the critical cooling rate the minimum cooling speed required to obtain 100% martensitic structure.
Why Is This Difference Important?
Because just being hard doesn’t mean a material is hardenable.
For example:
• Hardness is a momentary surface property that can be measured in a single test.
• Hardenability is the material’s ability to permanently harden through heat treatment.
Think of a steel knife:
If its hardness isn’t high, it will lose sharpness quickly.
But if it has high hardenability and is properly quenched, it will remain both sharp and durable.
In short:
• Hardness is a current measure of resistance,
• Hardenability is the material’s potential to gain that resistance in the future.
“Apply too little load, and the result is inaccurate.
Apply too much, and you may damage the material.”
When talking about hardness testing, most people first ask:
Should we use Rockwell, Brinell, or Vickers?
But in reality, what often makes the real difference is the amount of load applied.
Why Is Load So Important?
Because the applied load directly affects:
• The material’s response,
• The size of the indentation,
• And most importantly, the calculated hardness value.
If the load is too low:
The material may not deform enough resulting in inaccurate data.
If the load is too high:
Thin parts, coatings, or small samples may get damaged or permanently deformed.
Think of the load as the “language” of the test it must match the material and test type.
Every hardness testing method has specific standard load ranges.
Let’s explore the typical loads used in common test methods:
Rockwell Hardness Test
There are three main load ranges in the Rockwell test:
• 60 kgf
• 100 kgf
• 150 kgf
The load used depends on the material type and the test scale.
Examples:
• For softer metals → HRB scale (1/16" steel ball, 100 kgf)
• For hardened steels → HRC scale (conical diamond tip, 150 kgf)
That’s why Rockwell doesn’t rely on a single “load” it uses scale-specific load combinations.
Brinell Hardness Test
Brinell tests apply much higher loads typically between 500 and 3000 kgf.
Example:
• A typical Brinell test might use a 10 mm steel ball with a 3000 kgf load.
These high loads provide reliable results for large and heterogeneous materials.
Vickers Hardness Test
The Vickers method is known for its precision.
The applied loads range very widely:
• From 1 gf (gram-force)
• Up to 100 kgf
This allows Vickers to be used for:
• Microhardness testing (e.g., thin coatings)
• Macro-level hardness measurement
The beauty of the Vickers method:
The formula stays the same regardless of load making it extremely versatile.
Knoop Hardness Test
Knoop tests are generally conducted with loads in the 10 – 1000 gf range.
It leaves tiny, almost invisible indentations.
This makes it ideal for extremely thin layers, coatings, and biomaterials.
What Happens If the Wrong Load Is Used?
Using an incorrect load in a hardness test can result in:
• Subsurface effects altering the result,
• Indent size that doesn’t reflect the true microstructure,
• In coated surfaces, the substrate may affect the result,
• The sample may become permanently damaged or deformed.
