Scratch hardness
Scratch hardness is a measure of a material's resistance to permanent deformation, such as scratching or abrasion, caused by a harder material drawn forcefully across its surface.[1] This property is distinct from other hardness measures like indentation hardness, as it specifically evaluates surface integrity under frictional plowing rather than localized pressure.[2] In mineralogy and materials science, scratch hardness provides insights into a material's durability, wear resistance, and suitability for applications ranging from geological identification to engineering components.[3] The most widely used method for assessing scratch hardness is the Mohs scale of mineral hardness, developed in 1812 by German mineralogist Friedrich Mohs as a qualitative ordinal scale to classify minerals based on their relative resistance to scratching.[4] The scale consists of ten reference minerals, ordered from softest to hardest: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10), where a mineral can scratch all those below it but not those above.[4] Although not linearly proportional to absolute hardness, the Mohs scale remains a practical tool for field identification and preliminary assessments due to its simplicity and use of common reference materials like a fingernail (approximately 2.5) or glass (approximately 5.5).[4] In modern materials engineering, scratch hardness testing has evolved beyond the Mohs scale to include quantitative methods standardized by organizations like ASTM International, such as ASTM G171-24, which uses a diamond stylus to measure deformation under controlled loads for evaluating coatings, polymers, and metals.[5] These tests quantify parameters like groove width and penetration depth to assess tribological performance, fracture toughness, and abrasion resistance in applications including automotive parts, ceramics, and thin films.[2] For ceramics and hard surfaces, ASTM C1895 provides a procedure adapted from the Mohs method to determine scratch hardness values directly.[6] Such standardized approaches ensure reproducible results, aiding in quality control and material selection across industries.[5]Fundamentals
Definition
Scratch hardness is the measure of a material's resistance to fracture or permanent plastic deformation due to friction from a sharper or harder material sliding across its surface.[2] This property quantifies how well a material withstands surface damage from lateral forces, such as those encountered in abrasive wear or sliding contact.[1] Unlike indentation hardness tests, such as Vickers or Brinell, which apply vertical loads to assess resistance to penetration and localized deformation, scratch hardness emphasizes lateral shear forces generated during sliding.[2] This distinction makes scratch hardness particularly suitable for evaluating anisotropic or heterogeneous materials, like composites or coatings, where directional properties influence performance under tribological conditions.[7] The basic mechanism of scratching involves two primary modes of material response: plowing, where the indenter displaces material sideways to form a groove without removal, and cutting, where material is sheared off as chips.[2] These modes depend on factors like the indenter geometry, applied load, and material ductility, leading to either elastic recovery or permanent deformation.[2] Scratch hardness is typically assessed qualitatively using ordinal scales, such as the Mohs scale, but quantitative evaluations involve measuring applied load against resulting scratch width or depth. For instance, hardness can be calculated as the normal load divided by the projected contact area, often yielding values in gigapascals (GPa).Principles of Scratch Testing
Scratch hardness testing relies on the mechanical interaction between a hard indenter and the material surface, where resistance to deformation is determined by key material properties including yield strength, fracture toughness, and work hardening. Yield strength sets the threshold for initiating plastic deformation under the indenter's stress, preventing groove formation at low loads, while fracture toughness governs the material's ability to resist crack initiation and propagation along the scratch path. Work hardening further bolsters resistance by dynamically increasing local hardness through dislocation accumulation during sliding contact.[2] The indenter's geometry—characterized by its sharpness, apex angle, and overall shape (such as conical or pyramidal)—plays a pivotal role in localizing stress and defining the contact zone, with sharper tips promoting brittle fracture in susceptible materials and blunter ones favoring ductile plowing. Applied normal load directly scales the penetration depth and groove width, exceeding yield strength to induce permanent damage, whereas sliding velocity modulates strain rate effects, potentially elevating effective hardness in rate-sensitive materials like metals or polymers through adiabatic heating or reduced recovery time.[8][2] Deformation during scratching manifests in distinct modes: elastic recovery, where surface rebound partially erases the groove post-contact, predominates in compliant or viscoelastic materials, contrasting with irreversible plastic flow that displaces material laterally in ductile solids. The friction coefficient critically influences energy dissipation, with higher values amplifying tangential forces that drive plowing and shear, thereby deepening scratches and elevating wear rates in high-friction regimes.[9][10] Quantitatively, scratch hardness H_s is expressed as the ratio of the applied normal load to the projected contact area, often calculated from measurable groove dimensions:H_s = \frac{F_N}{A}
where F_N is the normal load and A is the projected area (e.g., derived as A = \frac{\pi b^2}{8} for a conical indenter with groove width b). In coated systems, the critical load for delamination— the threshold force inducing interfacial separation—quantifies adhesion, typically identified via abrupt changes in friction or acoustic emission signals.[11][8] Surface conditions exert substantial control over scratch propagation, with microstructure features like grain size and boundaries impeding dislocation glide or crack advance to enhance overall resistance. Finer grains elevate hardness by increasing boundary density, which scatters defects and promotes uniform deformation, while anisotropy in single crystals or textured polycrystals leads to orientation-dependent behavior, such as easier scratching along slip planes versus higher resistance perpendicular to them.[12][13]
Historical Development
Friedrich Mohs and the Original Scale
Friedrich Mohs (1773–1839), a German mineralogist, developed the original Mohs scale of mineral hardness in 1812 while serving as professor of mineralogy and curator of the mineral collection at the Joanneum Museum in Graz, Austria.[14] His earlier studies at the Mining Academy in Freiberg, Saxony, under the influential geologist Abraham Gottlob Werner had shaped his approach to mineral classification, emphasizing observable physical properties.[15] Mohs introduced the scale as part of a broader effort to create an accessible system for mineral identification, drawing on his experience curating extensive collections. The primary purpose of the scale was to offer a straightforward, relative measure for assessing mineral hardness in practical settings, such as fieldwork, by testing resistance to scratching with readily available reference materials.[16] This approach facilitated quick identification of unknown minerals without requiring complex laboratory equipment, making it particularly valuable for geologists and mineralogists in the field.[17] Unlike absolute hardness metrics used in modern engineering, the scale focused on qualitative comparisons suited to mineralogy's needs at the time.[18] Mohs first detailed the scale in his 1812 treatise Versuch einer Elementar-Methode zur naturhistorischen Bestimmung und Erkennung der Fossilien, where it formed a key component of his proposed classification method based on external characteristics like hardness.[14] The innovation lay in its ordinal structure: ten reference minerals ranked such that each could scratch those below it on the scale but not those above, prioritizing relative scratch resistance over precise quantitative values.[19] This relative ranking provided a simple ordinal framework that emphasized practical utility in mineral identification.[16]Modifications and Extensions
In the early 20th century, as synthetic abrasives became prominent in industrial applications, researchers sought to extend the Mohs scale beyond its original 10-point limit to better accommodate materials like garnet and diamond for practical use in manufacturing and grinding processes.[20] One key adaptation was the Ridgway modification, developed in 1933 by Raymond R. Ridgway at the Norton Company, which expanded the scale to 15 units to address the needs of the abrasive industry. In this extension, garnet was assigned a value of 10, surpassing the original topaz at 8; fused zirconia received 11, fused alumina 12, silicon carbide 13, boron carbide 14, and diamond 15, allowing for finer differentiation among high-hardness synthetics used in cutting tools and polishing.[20][21] Building on such efforts, Charles E. Wooddell at the Carborundum Company proposed a quantitative extension in 1935, shifting from purely qualitative scratching to measurements of wear resistance under abrasion by diamond particles. This scale retained quartz at 7 and corundum at 9 but extrapolated nonlinearly, assigning diamond bort (industrial-grade diamond) values up to 42.4 based on relative volume loss in standardized tests, providing a more precise metric for evaluating electric furnace products like silicon carbide against natural minerals.[22] Subsequent extensions incorporated advanced synthetic materials, such as cubic boron nitride (CBN), the second hardest material after diamond (approximately 9.5 on the Mohs scale) with superior thermal stability that enhances its cutting performance in high-speed machining applications where diamond may degrade.[23] Additionally, researchers have pursued linearization of the inherently non-uniform Mohs scale—where intervals between values increase exponentially—to improve resolution for engineering applications, often by correlating scratch resistance with quantitative metrics like indentation load or fracture toughness. These modifications collectively aimed to overcome the original scale's limitations in resolving subtle differences among ultra-hard materials critical for industrial abrasives and tools.[24]The Mohs Scale
Minerals and Assigned Values
The Mohs scale of mineral hardness is defined by ten standard reference minerals, each assigned a value from 1 (softest) to 10 (hardest), selected for their relative scratch resistance and availability. These minerals serve as benchmarks for qualitative comparison, where a mineral of a given hardness can scratch those of lower values but not higher ones.[25][26] The following table lists the ten minerals, their assigned hardness values, and key physical properties related to their scratch behavior:| Hardness | Mineral | Chemical Formula | Brief Properties and Scratch Characteristics |
|---|---|---|---|
| 1 | Talc | Mg₃Si₄O₁₀(OH)₂ | Softest mineral; forms flexible sheets that are easily scratched by a fingernail due to its layered silicate structure; has a greasy feel.[25] |
| 2 | Gypsum | CaSO₄·2H₂O | Soft, can be scratched by a fingernail; occurs as cleavable masses or fibrous crystals, often used in plaster.[25][26] |
| 3 | Calcite | CaCO₃ | Scratched by a copper coin (e.g., penny); effervesces with acid; common in limestone, with perfect cleavage in three directions.[25][26] |
| 4 | Fluorite | CaF₂ | Scratched by a steel nail or knife; colorful cubic crystals; used in flux for metallurgy due to its moderate softness.[25][26] |
| 5 | Apatite | Ca₅(PO₄)₃(F,Cl,OH) | Scratched by a knife with some difficulty; hexagonal prisms; primary component of tooth enamel and bones.[25] |
| 6 | Orthoclase | KAlSi₃O₈ | Cannot be scratched by a knife but scratches glass with difficulty; a common feldspar in granites, with good cleavage.[25][26] |
| 7 | Quartz | SiO₂ | Scratches glass easily and steel file; abundant in sand and many rocks; hexagonal structure provides good resistance.[25][26] |
| 8 | Topaz | Al₂SiO₄(F,OH)₂ | Scratches quartz; yellowish prismatic crystals; valued as a gemstone for its durability.[25][26] |
| 9 | Corundum | Al₂O₃ | Nearly as hard as diamond; used in abrasives and as ruby/sapphire gems; scratches all softer minerals.[25][26] |
| 10 | Diamond | C | Hardest known natural material; its rigid tetrahedral carbon lattice structure, where each carbon atom bonds covalently to four others, enables exceptional scratch resistance against all other substances.[25][26][27] |