Hardness
Understanding Hardness in Materials
Definition of Hardness: Hardness in materials refers to their ability to resist plastic deformation, often from forces like penetration. This can also encompass a material's resistance to being bent, scratched, abraded, or cut.
How Hardness is Determined: Hardness isn't a property directly determined by basic units such as mass, length, or time. Instead, a material's hardness value emerges from a specific measurement process.
Historically, the hardness of materials might have been gauged by their resistance to scratching or cutting. For instance, if material B can scratch material C but not material A, it provides insights into their relative hardness. Or, if material A slightly scratches material B but significantly marks material C, it gives a comparative understanding of hardness. The Mohs Scale offers a classic example of this concept, ranking materials based on their scratch resistance. Contemporary tests, such as the file test, employ a similar approach: a file of known hardness is used on a material, and if it glides without leaving a mark, the material is deemed harder than the file.
However, such relative tests are often not suitable for today's materials, especially metals, as they lack precise numerical data. A more common approach to determine hardness is by analyzing the depth or area of an indentation made by a specific indenter shape under a given force over a defined duration. Three primary standard methods—Brinell, Vickers, Schmidt and Rockwell—outline the correlation between hardness and the impression's size. For practicality and calibration, each method comprises various scales, each defined by a specific combination of force applied and indenter shape.
Understanding the Schmidt Rebound Hardness Test
The Schmidt rebound hardness test on rock is a widely recognized method for rapidly gauging the surface hardness of rock formations, offering valuable insights into their mechanical properties. Employing the Schmidt hammer, this technique presents a cost-effective and swift alternative to labor-intensive methods. Nonetheless, to ensure consistent and reliable results, the application of the Schmidt hammer necessitates a thorough understanding of various influencing factors, including the specific type of hammer utilized, rebound value normalization, specimen size, the smoothness of the rock surface, its weathering state, moisture content, and the precise methodologies adopted for testing, data compilation, and analysis. The Schmidt hammer, tailored specifically for rock testing, boasts of several standout features. The rebound value it provides is independent of the angle of impact, making it versatile in varying field conditions. Furthermore, the device is optimally designed for field work, offering protection against external contaminants, a lightweight and ergonomic design, and capabilities to store multiple readings for later transfer to a PC. It also comes with built-in statistical methods aligned with standards set by organizations such as the ISRM and ASTM. One can automatically calculate the rebound number using these methods or opt for a custom statistical method. The RockSchmidt software allows users to establish correlations between the Unconfined Compressive Strength (UCS) and the rebound value, based on the formula UCS = aebR, where R is the rebound value. Similarly, the correlation between the elastic modulus and the rebound value is given by Et = cedR. The tool also aids in determining the weathering grade of the rock by permitting repeated impacts on the same location. With the inclusion of the ISRM-recommended method, the device becomes indispensable in applications that require correlations to both the Unconfined Compressive Strength (UCS) and Young’s Modulus.
Understanding the Rockwell Hardness Test
The Rockwell hardness test is a method where the material in question is indented using either a diamond cone or a hardened steel ball. Initially, a minor load, often 10 kgf, denoted as F0, is applied to the material (refer to Fig. 1A). Once the material has settled under this load, an indicator, which tracks the indenter's movements and thus notes any change in its penetration depth, is reset to a reference point.
While maintaining this initial minor load, an added major load is introduced, leading to further penetration into the material (see Fig. 1B). After the material reaches a stable state under this combined load, only the major load is withdrawn, yet the minor load remains. This action lets the material rebound slightly, causing a decrease in the indenter's depth (illustrated in Fig. 1C). The permanent change in penetration depth, which happens due to the application and subsequent removal of the major load, determines the Rockwell hardness value.
The Rockwell hardness number can be represented as: HR = E - e.
Understanding the Rockwell Superficial Hardness Test
The Rockwell Superficial hardness test method involves making an indentation on the test material using either a diamond cone (referred to as the N scale) or a hardened steel ball. Initially, a minor load, commonly 3 kgf, denoted as F0, is placed on the material (as seen in Fig. 1A). After the material stabilizes under this minor load, an indicator, which tracks the indenter's movements, is adjusted to a reference point. This indicator responds to any variations in the penetration depth of the indenter.
With the minor load still in place, an additional major load is introduced, causing a deeper indentation in the material (illustrated in Fig. 1B). Upon achieving stability under this combined load, the major load is removed, but the minor load continues to be applied. This allows the material to experience a slight recovery, which in turn results in a reduction of the indentation's depth (depicted in Fig. 1C). The Rockwell Superficial hardness value is derived from the lasting change in penetration depth, denoted as e, due to the application and subsequent withdrawal of the major load.
The Rockwell Superficial hardness number is given by: HR = E - e.
Understanding the Brinell Hardness Test
The Brinell hardness test involves making an indentation on the test material using a ball with a 10 mm diameter, made of hardened steel or carbide, under a typical load of 3000 kg. For materials with softer attributes, the load might be adjusted to 1500 kg or even 500 kg to prevent overly deep indentations. For metals like iron and steel, the full load is maintained for a span of 10 to 15 seconds, while for other metals, it's held for at least 30 seconds. A microscope with low magnification is then used to measure the diameter of the resulting indentation. The Brinell hardness number is determined by dividing the applied load by the indentation's surface area.
Brinell Hardness Overview
Brinell Hardness Number (BHN) Calculation
To ascertain the diameter of the resulting impression, the average of two measurements taken perpendicularly is used. A Brinell hardness number table can assist in easily determining the Brinell hardness. A typical Brinell hardness number entry, indicating the test conditions, might appear as "75 HB 10/500/30". This signifies a Brinell Hardness of 75 achieved with a 10mm diameter ball made of hardened steel, using a 500 kilogram load, sustained for 30 seconds. For extremely hard metals, a ball made of tungsten carbide might replace the steel ball.
One distinguishing feature of the Brinell test is the pronounced size and depth of its indentation. This ensures that the hardness measurement encompasses a broader material area, offering a more comprehensive evaluation, especially beneficial for materials with diverse grain structures or inconsistencies. Among various hardness testing methodologies, the Brinell method is optimal for gauging the overall, or macro-hardness, especially in materials that possess diverse structures.
An Overview of the Vickers Hardness Test
The Vickers hardness test involves indenting the test material with a diamond-shaped indenter. This indenter is shaped like a pyramid with a square base and an angle of 136 degrees between its opposite faces. Loads ranging from 1 to 100 kgf are applied, typically for a duration of 10 to 15 seconds. Post indentation, the two diagonals of the mark left on the material are measured with a microscope, and their mean value is determined. Subsequently, the area of the indentation's sloped surface is computed. The Vickers hardness is then determined by dividing the applied load (in kgf) by the indentation's area (in square mm).
After obtaining the mean diagonal of the indentation, the Vickers hardness can be deduced using the formula, though conversion tables are often more convenient. An example of a reported Vickers hardness might look like "800 HV/10", indicating a Vickers hardness of 800 using a force of 10 kgf. Various load settings can yield similar hardness numbers on consistent materials, making it superior in uniformity compared to some other testing methods. A notable advantage of the Vickers test is its high accuracy across different materials and treatments with a single indenter. While versatile and precise, the Vickers machine, typically floor-standing, is more costly than Brinell or Rockwell machines.
Understanding the Microhardness Test
Microhardness testing typically involves creating static indentations with forces less than or equal to 1 kgf. Two common indenters for this method are the Vickers diamond pyramid or the Knoop elongated diamond pyramid. The testing process mirrors the standard Vickers hardness test, but it's executed on a much smaller scale and demands more precision equipment.
For the test to be effective, the target surface typically needs a metallographic polish. Notably, as the applied force decreases, the requirement for a smoother surface finish intensifies. The indentations formed during testing are gauged using specialized microscopes, typically boasting magnifications close to X500. These microscopes can discern details with an accuracy of approximately ±0.5 micrometres. With a consistent observer, even finer distinctions of ±0.2 micrometres can often be discerned. However, it's crucial to note that achieving such precision demands a mix of expertise and meticulousness.
Understanding the Mohs Hardness Scale
The Mohs hardness scale, established in 1822, is a benchmark for gauging the hardness of minerals. This scale features a list of 10 minerals, ordered from the softest to the hardest. At the pinnacle of this scale, diamond stands as the hardest mineral with a rating of 10, while talc, known for its softness, is positioned at the bottom with a rating of 1.