Which of the following hardness scales uses a diamond cone indenter?

Explanation:

In the context of hardness testing, the Rockwell C scale is a widely used method for determining the hardness of materials, particularly metals. The Rockwell C hardness test, also known as the Rockwell Hardness Test, employs a diamond cone indenter, also known as a Brale indenter, to measure the hardness of a material. This scale is particularly suitable for harder materials, such as steels and other hard alloys.

Rockwell C Scale (Correct Option 4):

Definition: The Rockwell hardness test is a method for measuring the hardness of a material by determining the depth of penetration of an indenter under a large load compared to the penetration made by a preload. The Rockwell C scale specifically uses a diamond cone indenter with a 120-degree angle and applies a major load of 150 kgf (kilogram-force).

Working Principle: The Rockwell C hardness test involves two main steps. First, a minor load of 10 kgf is applied to the material using the diamond cone indenter, establishing a zero reference position. Then, a major load of 150 kgf is applied, causing the indenter to penetrate the material further. After holding the major load for a specific dwell time, the major load is removed while maintaining the minor load. The depth of the indentation is measured, and the hardness value is calculated based on the difference in depth between the minor and major loads.

Advantages:

• Quick and straightforward testing process.
• Non-destructive, as the indentation is usually small.
• Suitable for a wide range of materials, including hard metals.
• Provides direct reading of hardness value on the dial or digital display.

Disadvantages:

• Limited to testing materials with flat and smooth surfaces.
• Not suitable for very soft materials, as the diamond cone may cause excessive deformation.

Applications: The Rockwell C scale is widely used in industrial applications to determine the hardness of steel, tool steel, and other hard alloys. It is commonly employed in quality control, material selection, and research and development.

Let's now analyze the other options provided in the question:

Option 1: Mohs Scale

The Mohs scale of mineral hardness is a qualitative ordinal scale that characterizes the scratch resistance of various minerals by comparing their ability to scratch softer materials. It was created by the German geologist and mineralogist Friedrich Mohs in 1812. The Mohs scale ranges from 1 (talc) to 10 (diamond), with diamond being the hardest known natural material. The Mohs scale does not use an indenter; instead, it relies on the ability of a material to scratch another material. Therefore, the Mohs scale is not suitable for determining the hardness of metals and does not use a diamond cone indenter.

Option 2: Brinell Scale

The Brinell hardness test is a method used to determine the hardness of materials, primarily metals and alloys. The test involves pressing a hard steel or carbide ball indenter into the material's surface under a specified load. The diameter of the resulting indentation is measured, and the Brinell hardness number (BHN) is calculated. The Brinell test does not use a diamond cone indenter; instead, it uses a spherical indenter, typically made of hardened steel or tungsten carbide. The Brinell test is more suitable for materials with coarse or uneven surfaces and is often used for softer metals and alloys.

Option 3: Vickers Scale

The Vickers hardness test is another method for measuring the hardness of materials, particularly metals and ceramics. The test involves pressing a diamond pyramid indenter with a square base and an angle of 136 degrees between opposite faces into the material's surface under a specified load. The Vickers hardness number (VHN) is calculated based on the diagonal length of the resulting indentation. While the Vickers test uses a diamond indenter, it is not a cone-shaped indenter like in the Rockwell C scale. The Vickers test is known for its high accuracy and is suitable for a wide range of materials, including very hard and very thin materials.

In conclusion, the correct option for the hardness scale that uses a diamond cone indenter is the Rockwell C scale (Option 4). This scale is widely used for measuring the hardness of metals, particularly harder alloys, and provides quick and straightforward results. The Mohs scale, Brinell scale, and Vickers scale each have their specific applications and indenter types, but none of them employ a diamond cone indenter as used in the Rockwell C scale.