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Is the Harder a Material, the More Wear Resistant?

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Is the Harder a Material, the More Wear Resistant?

Issue Time:2019-06-26

Is the harder a material, the morewear resistant?

For pure material, this is indeed the case: the harderthe material, the more resistant it is.

Here we first need to understand how the wearresistance of the material is defined. Typically, the laboratory measures thewear resistance of a material using the following instrument:

The basic principle is also very simple, that is, usea material with a high hardness as a reference material (usually sand), rubagainst the material to be tested, and then measure the amount of material tobe ground. Of course, materials with poor wear resistance will be worn more,and materials with good wear resistance will be worn less.

As for the hardness, the hardness is higher for thepure material, and the higher the strength, the stronger the bond between themolecules (atoms) on the microscopic surface, the lower the possibility thatthe atom is peeled off by the external force, and naturally it is morewear-resistant. Many pure materials have been tested in the literature, andthere is even a perfect linear growth relationship between hardness and wearresistance.

However, there are always exceptions in everything.Especially for our world today, pure material materials are far from meetingthe needs of human society. Most of the materials in our lives are compositematerials, including polymer composites, alloys, etc. By mixing differentsubstances and processing them, humans obtain excellent properties that purematerial materials are far from being able to achieve, but in the process, asmaterials become more complex, from various properties of materialsRelationships are becoming more and more uncertain. To study the relationshipbetween wear resistance and hardness, it is far from enough to start withstrength. Also consider variables with more dimensions such as composition andmicro structure. Here is a very simple and universal example of steel.

First,explain how the hardness of the material is measured, such as the followingfigure:

Usually, we use an indenter to press the surface ofthe material out of an indentation and measure the size of the indentation intoa hardness. Soft material indentation is large, hard material indentation issmall, yes, this is a very reasonable test method.

Above we usually call it micro hardness, but there is aproblem with the measurement here, that is, it does not consider which part ofthe material is contributed by the hardness, such as tool steel and chrome castiron, the hardness is 600 by this method. BHN, but the micro structure of thetwo materials is completely different.

As shown in the above figure, the left picture (a) isa photo micro graph of the tool steel. The whole material is equal in size, andthe hardness of each grain is almost the same, so this 600 BHN is basically anyplace of this material. The hardness of the chrome-plated cast iron in Figure(b) on the right is completely different. It is uniformly distributed with a largeamount of chromium carbide. These small particles are very hard and can reach1200 HBN, while the base of cast iron is soft. Less than 200 HBN, so for chromecast iron, the 600 HBN comes from the weighted average of the matrix andchromium carbide.

Therefore, it is conceivable that even if the hardnessis the same, the wear resistance of the material in (a) can only reach 1/5 ofthe material in (b), because even if the matrix of (b) is very soft, a largeamount of hard carbonization is distributed. Chromium particles are the mainforce of wear resistance.

Therefore, micro hardness is often calledmacro hardness, which reflects the macroscopic properties of the material andignores a lot of micro structure information. This is why we use macro hardnessto measure the performance of different materials.

In addition, for compositematerials, even if hardness can improve wear resistance, the relationshipbetween the two is very complicated, and it is not solved by one or twoformulas. For example, a WC-Co alloy can be processed to obtain different sizesof grains and hardness. Here, four grain sizes are selected as comparisongroups:

Thendo the wear test, and then compare the result with the hardness, you can getthe following results:

It can be seen that the increase in hardness for thewear resistance has been far less linear than the pure material material, andeven has no obvious effect in the low hardness range, and has becomeexponentially increasing in the high hardness range. Moreover, by comparing thetrends of (a), (b), (c), (d), a significant influence on the grain size can beseen.

In addition, as mentioned before, the ASTM standardmethod is usually used to measure the wear resistance, and there are manystandard methods, each with its own focus. Therefore, the wear resistance ofthe materials measured by different methods may be completely different, so becareful when comparing these complicated data.

In summary, for all the materials in our lives, thehardness and wear resistance are theoretically positively correlated, and thereare of course exceptions. Moreover, when comparing the hardness and wearresistance between different materials, it is necessary to consider theinfluence of material composition, micro structure, processing, and even themeasurement method on the results. Otherwise, it is straightforward to concludethat the harder and harder the wearer is, the more bugs will occur.