Mohs scale - Understanding metal hardness and mineral hardness

Introduction to hardness scales in metallurgy

Hardness scales are essential tools in metallurgy. They determine a material's resistance to scratching, indentation, and deformation. Engineers and manufacturers can select suitable materials for various applications by understanding metal hardness. The Mohs hardness scale, and other methods, help classify metals and minerals based on their durability and wear resistance. Accurate hardness measurement ensures that metals meet industry strength, longevity, and performance requirements.

The Mohs scale - Measuring metal and mineral hardness

The Mohs hardness scale, developed by Friedrich Mohs in 1812, ranks materials on a scale from 1 to 10 based on their ability to scratch softer substances. Minerals are typically used as reference points, with talc as the softest (1) and diamond as the hardest (10). Although primarily designed for minerals, the Mohs scale is sometimes applied to metals, albeit with limitations.

Metals do not always fit neatly within this scale due to their ductility and work-hardening properties. However, approximate placements exist; for example, pure gold has a Mohs hardness of around 2.5, while hardened steel can reach up to 8. The Mohs scale provides a basic comparative measure, but is not the most precise tool for evaluating industrial metals.

Common metals on the Mohs scale

Different metals have varying hardness levels, which directly influence their applications in various industries. Below are some common metals and their approximate Mohs hardness ratings, along with how their hardness affects their usage:

• Aluminium – (2.5) – A relatively soft, lightweight metal that is easy to machine and form. Used in aerospace, automotive, and construction projects where weight reduction is crucial.

• Copper – (3) – A soft metal known for its excellent electrical and thermal conductivity. Ideal for electrical wiring, plumbing, and heat exchangers.

• Brass – (3-4) – A copper-zinc alloy that is harder than pure copper. Common in fittings, musical instruments, and decorative applications due to its corrosion resistance and aesthetic appeal.

• Bronze – (3-4) – Harder than brass, with excellent wear resistance. Often used in marine applications, bearings, and sculptures.

• Cast Iron – (4.5)– Brittle but highly wear-resistant, making it suitable for engine blocks, cookware, and heavy-duty industrial components.

• Mild Steel – (4-8 depending on composition and treatment) 

• Stainless Steel –(5-6) – Harder than mild steel and highly resistant to corrosion. Used in medical instruments, kitchenware, and structural applications where strength and durability are required.

• Titanium (6) – Strong, lightweight, and corrosion-resistant. Commonly used in aerospace, medical implants, and high-performance engineering applications.

• Tungsten (7.5) – One of the hardest pure metals, often alloyed into tungsten carbide for industrial cutting tools, mining equipment, and armour-piercing ammunition.

Compared to minerals, metals generally fall within the mid-range of the Mohs scale. While harder than many minerals, they are often softer than gemstones and industrial ceramics. Understanding these hardness ratings helps industries select appropriate materials for wear resistance, tooling, and structural applications, ensuring optimal performance and durability.

Alternative hardness scales for metals

The Mohs scale, while helpful, lacks precision for metals, leading to the development of more detailed hardness measurement methods:

• Rockwell Hardness (HR) – Uses indentation depth under specific loads. The Rockwell scale has different variants, including:

• HRB (Rockwell Hardness B) – Uses a steel ball indenter and is typically used for softer metals such as mild steel (HRB 70-100) and aluminium alloys.

• HRC (Rockwell Hardness C) – Uses a conical diamond indenter and is designed for harder materials such as hardened steel (HRC 55-65) and tool steel.

• Brinell Hardness Number (BHN) – Measures indentation diameter under a steel or tungsten carbide ball. This method is useful for softer metals and materials with rough surfaces. Example: Aluminium (BHN 15-50), stainless steel (BHN 150-200).

• Vickers Hardness Number (VHN) – Uses a diamond pyramid-shaped indenter to measure indentation size under a fixed load. This method is extremely  precise and suitable for very thin materials and microhardness testing. Examples: Brass (VHN 80-100), titanium alloys (VHN 250-400).

These scales provide more precise assessments for engineering and manufacturing purposes.

Challenges in Measuring Metal Hardness

Measuring metal hardness presents several challenges:

• Material variability – Different alloys and treatments can alter hardness.

• Testing limitations – Some methods require specialised equipment and preparation.

• Scale differences – Results may not always be directly comparable across different hardness tests.

Accurate hardness measurement requires careful selection of the appropriate testing method based on the metal and its intended application.

Practical applications of metal hardness in industry

Metal hardness testing is crucial in various industries, ensuring materials meet performance and durability standards. Some key applications include:

• Manufacturing – Selecting the best materials for machinery, tools, and construction.

• Aerospace & Automotive – Ensuring high-strength alloys withstand stress and wear.

• Jewellery & Electronics – Determining scratch resistance and longevity of jewellery components.

• Mining & Heavy Equipment – Assessing material toughness for demanding environments.

Industries can improve safety, efficiency, and product lifespan by understanding and testing metal hardness.