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Everything You Need to Know About Hardness of Materials

It is a rule of thumb that you consider the functional environment wherein a particular part is going to be used or installed. Choosing the materials that go into the production of a product can surely be tricky, but the end use is your best guide in case of such confusion.

One of the most important aspects that take center stage while making deal-breaking production decisions is material hardness. 

Material hardness, at first glance, may seem like a given concept, but the undercurrent of this aspect runs deep. In this article, we are going to uncover and analyze the importance and types of material hardness, so get ready to be enlightened!

What Is Material Hardness?

As you know, it is important to have material that can withstand deformation, dents, scratches, etc. In simple words, material hardness is the measure of resistance a material has towards the outside forces. Basically, it’s like talking about the toughness of metals, minerals, plastics, etc.

Hardness of Materials

For example, run your fingers over the surface of a diamond and then on a piece of chalk. You will observe that the diamond is smooth, and the chalk is crumbly & uneven. The difference in their response to external forces tells you all about their material hardness.

Steel Hardness in Mold Manufacturing

Steel hardness is an essential factor in mold manufacturing. The reason is that it affects the performance, durability, & lifespan of the mold. Now, you must understand that hardness is an essential factor to consider while picking the perfect steel for molding. 

In mold manufacturing, you can measure hardness by using scales like Rockwell (HRC), Brinell (HB), or Vickers (HV). For steel, higher hardness will give you better wear resistance and a longer life for the tool.

Here are some of the common mold steel and hardness ranges. Firstly, there are pre-hardened steels (HRC 30-40), which are okay for low to medium production volumes. Next, you have hardened steels (HRC 40-50), which are used for general-purpose molding applications. After that, you will find a highly hardened steel (HRC 50-60+), which is used for high-volume production. 

You can choose any one of the above-mentioned categories. However, remember that if you want to go ahead with injection molding & compression molding, pick high-hardened steel & hardened steel, respectively. Medium-hardened steel is good for blow molding. 

In the end, surface treatments can always enhance the hardness and wear & tear quality of the mold.

Types Of Material Hardness

Therefore, it is not wrong to say that material hardness depends on things like flexibility, strength, strain, plasticity, viscosity, etc. And because every material behaves differently, these are the different types of hardness:

Scratch Hardness

Scratches are sharp lines that happen when the hard material brushes against a softer material. This can cause the scratches to appear. Let’s say you are about to unwrap your brand-new phone, and you are worried about the inevitable scratches. Now, if the material has a good scratch-resistant property, your phone will be safe. However, the opposite can be hurtful.

Types Of Material Hardness

There are many ways to check the material’s scratch resistance, like the Mohs Hardness Scale. You can use this for minerals or ceramics. So, a material with high scratch hardness can withstand every scratch under the sun.

The benefit of having scratch hardness is that you will not have to spend money on repairs anymore. You can buy a new-looking product for the rest of your life. If you still have doubts, you can always go for a scratch hardness test and not lose the smooth surface.

Rebound Hardness

Next is rebound hardness. Imagine that you’re dribbling a basketball, and it rebounds back up. Now, this is possible because of the rebound hardness property of the material. So, rebound hardness is a bit like bounce-back quality but for materials. 

A material basically can absorb energy when it’s hit and then bounce back to its original shape. For example, rubber compresses when you squeeze it and then returns to its original shape when you release it. 

You can use Charpy and Izod tests to check the rebound hardness of a material. This way, you can check how well the material can take the hit and come back strong. In simple words, just drop a hammer from some height and see if it returns back to the same level. If it doesn’t, it probably has low rebound hardness. 

Indentation Hardness

The perfect example is trying to press a thumb into a piece of dough. So, the impression it leaves on the dough gives you an idea of how soft or firm the dough is. Just like that, indentation hardness measures how resistant your material is to external force.

There are a few tests for this, like the Brinell and Vickers tests. You just have to press the object hard enough into the material and measure the indentation. As a result, you will have some material in your hand that does not give up under pressure, especially when there is a sudden impact. 

Factors Affecting Material Hardness

It is always important to keep in mind some factors that can affect the material hardness of a material, and they are:

Crystal Structure

Imagine materials like they’re made up of tiny building blocks. Some materials organize these blocks in a neat, ordered way, like a well-arranged stack of bricks (crystalline structure). 

Others are a bit more haphazard, like a bunch of scattered Legos (amorphous structure). Now, when it comes to hardness, the way these blocks are organized matters. Materials with a crystal-clear plan tend to have different hardness characteristics compared to those with a more random structure.

Grain Size

Think of materials as if they’re made up of many small grains, like a scoop of sand. Now, if these grains are big, there will be fewer obstacles in the way. But, if they’re small, there are going to be lots of tiny hurdles. 

Smaller grain sizes generally mean more obstacles that can get in the way. This simply means that there can be tiny defects or irregularities that can affect how materials bend. So, more obstacles often result in higher hardness.

Temperature

Now, let’s talk about temperature. Imagine your material as a group of people at a party. At higher temperatures, it’s similar to those guests mingling freely and moving around with ease, which increases the movement of atoms. 

But, when things cool down, the guests will start to freeze in place. In materials, reduced atomic mobility at low temperatures can make them harder. You can say that they have become more resistant to changes because the atoms can’t move around as much.

How To Test Material Hardness?

Even though you are familiar with the theories of material hardness, it is important to understand how to perform the tests. Let’s talk about those tests now:

Rockwell Hardness Test

Rockwell Hardness Test

The principle behind this test involves checking the strength of the material by measuring how much it can resist when pressed hard. For this, you can use the tool called an indenter. This tool is used to check how far it can push a heavy load into the material.

For starters, use only a lighter load to ensure the indenter is in the right spot. Then comes the main event, where you can use the heavy load. Please observe how deep the indenter goes into the material. 

However, there are different scales to analyze the results. For hard materials, you can use the HRC scale, and for soft materials, the HRB scale. This test works great for metals and alloys. For example, you can make sure the metal in your car engine or the alloy in your bike is up for a challenge. 

Brinell Hardness Test

The Brinell Hardness Test is like giving a material a friendly squeeze and seeing how it responds. It is all about checking out the “dentability” of a material by measuring the size of the dent. You can use a special tool called spherical indenter for this job. 

The procedure is simple; just begin by pressing a small, round ball into a piece of material. In this test, you can apply a known amount of pressure to this spherical indenter. After the press, take a peek at the dent left behind. It’s like looking at the aftermath of a thumb pressing into a playdough. The size of this dent is crucial, and a formula to turn this dent size into a number is always used. 

This test is like the superhero of hardness testing for metals. It’s commonly used to check out how tough and resistant metals are when they get a bit of a squeeze. You can make sure the alloy in your favorite cooking pot can handle the everyday pressures they might face.

Vickers Hardness Test

In this test, you can check how well a material stands up to a tiny diamond point. Picture a tiny diamond in the shape of a pyramid. Now, imagine pressing this diamond into a material to see how it responds.

Picture a tiny diamond in the shape of a pyramid. Now, imagine pressing this diamond into a material to see how it responds. In simpler words, it’s all about measuring the size of the dent left by this little diamond when it meets a material under a load.

Firstly, you have to apply a load to this tiny diamond, kind of like gently pushing it into the material. Think of it as the diamond giving the material a little nudge. When you’re done with this, take a look at the dent created. Now, here’s the cool part – measure the diagonals of the dent, sort of like checking how wide and long it is.

Knoop Hardness Test

Knoop Hardness Test

This is a close cousin of the Vickers test, the Knoop hardness test. It’s like a sibling but with a different-shaped tool. Imagine a diamond, but this time, it’s shaped like a little rhombus, not a pyramid. That’s the Knoop indenter. Now, gently press this unique diamond into a material to see how it reacts.

The Knoop test is all about measuring the dent left by this rhombohedral diamond when you apply a load. In this test, you should measure the long and short diagonals of the dent. Moreover, you can check out the dent from two different angles.

Shore Hardness Test

If you’re curious about how soft or tough your rubber phone case is, then Shore Hardness Test is the guy for you. This test is like a gentle poke to see how much a material, especially those squishy elastomers and polymers (think rubber and plastics), can handle before showing signs of indentation.

It’s similar to checking how much your memory foam mattress squishes when you press on it. Instead of diamonds or pyramids, here you have to use a special tool called a durometer. It’s a friendly probe that gently presses into the material. 

The durometer measures how much the material indents. This way, you can see how deep your finger presses into a piece of clay. Depending on what you’re testing, whether it’s soft rubber or tough plastic, different rulers or scales are used. For softer materials, there’s the Shore A scale, and for harder ones, there’s the Shore D scale. 

Microhardness Test

Imagine those tests, but zooming in on the tiniest details is what Microhardness Testing is all about. Unlike other tests, you can have fine-grained, small parts instead of dealing with large chunks. It’s identical to using a magnifying glass to inspect the microscopic world of materials.

Instead of a regular-sized indenter, use a small one for this test. It is simply similar to pressing a mini diamond or rhombohedral tool into the material. After the press, you have to measure the size of the indentation, but on a much smaller scale. All you have to do is take a close look at the little footprints left by the tiny indenter.

Here’s the interesting part – use the measurements to calculate the Vickers or Knoop Hardness Number. It gives you an idea about how well the material stands up to pressure when you’re dealing with its tiny features.

Scratch Hardness Test

Take a key or a coin & scratch it against a surface. You’ll either find some scratches or see that nothing happens to the surface. Eventually, this test is all about figuring out how well a material can resist scratches when it comes into contact with something harder. 

It’s like testing your phone screen against your car key to see if it leaves any marks. For example, get a tougher material, kind of like an epitome with an unscratchable shield. You can use this tougher material to scratch the sample you’re testing.

As the scratching happens, look very closely at the objects. Any scratches or marks that appear on the sample will tell you how well it stood up to the challenge.

What Is The Measuring Unit Of Material Hardness?

It is quite easy to confuse the measuring units of hardness and pressure. The SI unit of material hardness is Megapascals (MPa). However, there is one difference: some tests are qualitative and do not have specific SI units. Now, each hardness type you read about earlier has different scales of measurement. Here is a summary:

Hardness testMeasuring SI unit
Mohs Hardness ScaleThis test is qualitative
Brinell HardnessMegapascals (MPa)
Vickers HardnessMegapascals (MPa)
Rockwell HardnessNone (Dimensionless)
Microhardness TestingMegapascals (MPa)
Shore HardnessNone (Dimensionless)

Parting Words!

The bottom line is that understanding the hardness of a material is important for you to pick the right material, product design, etc. This way, you can have control over what you are making. Recent advances are improving the ability to be more precise with these tests, and if you want to perform one under proper guidance, contact us!

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