About 200 years after Mohs – Nanoscratching LiB₃O₅

Abstract

Though simple scratch hardness tests after Mohs are still used today, the development of diamond nanoscratching equipment offers new possibilities to meet demands of modern nanotechnology. Preceding approaches to assign hardness values to materials are briefly reviewed, and scratch hardness is related to indentation hardness. Taking single-crystalline LiB 3 O 5 as an example, the dependence of scratch morphology on the direction of scratching is demonstrated quantitatively. The coefficient of friction depends on normal load and varies between 0.25 and 0.37. Moreover, it is oscillating during scratching thus reflecting processes at nanoscale. Dislocation etch pits were observed due to scratching. Scratching in the pastAt the end of the 17 th century people were already aware of the varying resistance of minerals and other materials against mechanical loading, in particular against scratching. Huygens (1690) noticed the fascinating fact that the cleavage plane of calcite may be scratched when moving the knife forward and may not when moving it in the opposite direction [1]. In 1722 Réaumur [2] differentiated the quality of steel, instead of testing it with the aid of a file, by scratching with seven different kinds of reference materials. Moreover, he prepared a steel bar with increasing hardness from one end to the other. The degree of hardness was attributed to the specimen according to the position on the bar which the specimen could scratch first. Linné (1768 and 1793) listed a number of terms relevant for an appropriate terminology of the mechanics of solids which has been adopted by other writers [3]. Then Werner [4] introduced in 1774 four categories of hardness (hard, semihard, soft, very soft) defined by the behaviour of the material when scratched by a knife or a file using gauge minerals for comparison. Following Werner, Hauy classified minerals according to their ability to scratch each other, assigning four grades to the materials scratching quartz, glass or calcite and those not scratching calcite, respectively [5].Almost 200 years ago, in 1812 Carl Friedrich Christian Mohs published the first approach to the hardness scale still used today [6]. He considered hardness (combined with several other properties) as a distinguishing feature of solids. Later he introduced, on the basis of the definition of hardness, the ten-stage hardness scale (now named after him) and instructions for its realisation [7] (His definition reads:' Härte ist der Widerstand,welchen die festen Mineralien der Verschiebung ihrer Theile entgegensetzen. Die Größe dieses Widerstandes heißt Grad der Härte.'; "...daß jedes Mineral, welches ein anderes ritzt, von demselben aber nicht geritzt wird, härter als dieses sey..."; For a biography cf. Rösler [18]). Pansner (1813) was seemingly not aware of Mohs' scale when he proposed a subdivision of all minerals into four hardness classes (represented by diamond, steel, copper, and lead etalons) with the additional subsubdivision into mass density subclasses [8]. The anisotropy of the depth...