Sapphire hardness

Figure 6.16. Effect of surface finish on sapphire hardness (a) (0001) (b) (1120), with GP = gas polish, G = dry grinding, (c) Effect of Ti dose on AI2O3 surface and its hardness. After Becher. " ... 

Aluminum compounds, particularly the hydroxides and oxides are very versatile. Properties range from a hardness iadicative of sapphire and comndum to a softness similar to that of talc [14807-96-6] and from iuertness to marked reactivity. Aluminas that flow and filter like sand may be used for chromatography (qv) others are viscous, thick, unfilterable, and even thixotropic (1). 

Hard Materials (7) Quartz, granite (8) topaz (9) corundum, sapphire, emeiy (10) diamond. 

Cutting, grinding, and shaping stone, and in particular burnishing and polishing the surface of stone as well as metals, requires the use of abrasive materials that are harder than the solids to be cut, ground, burnished, or polished. Sapphire and ruby, two very hard gemstones, for example, can be cut or polished only with the assistance of diamond powder, an abrasive that is harder than sapphire or ruby. Diamond is the hardest material... 

Figure 13.5(d) presents experimental stiffness measurements using differential UFM for three high modulus surfaces sapphire, Si(100) and LiF(lOO) (Dinelli et al. 2000b). The samples were probed with the same silicon tip on a V-shaped cantilever (nominally cantilever stiffness was kc - 2.8 nN nm 1,and radius of curvature R = 10 nm). The surface RMS roughness of the surfaces was less than 0.2 nm over a few square micrometres for all three samples. The relative difference between the three sets of data reveals that the elastic properties of these three materials can be distinguished by differential UFM the relative independence of the applied force may indicate the fact that the tip had been flattened by extended contact with such hard samples. 

Sapphire, like ruby, is made of corundum (A1203) and has a hardness of 9. However, instead of Cr impurities, the crystal contains iron and titanium... 

Spinel is a colorless magnesium aluminate (MgAl204) of cubic structure. It is hard and durable, but, like white sapphire, it is not a good diamond substitute because it has a low refractive index and lacks brilliance. However, it is readily doped to produce other gems of various colors. Artificial ruby, for example, is often natural red spinel, and most synthetic blue sapphires on the market are actually blue spinel. 

The crystallites in alumina ceramics are mainly oc-A1203, mineralogically known as corundum (a synonym for sapphire). Corundum is 9 on the Mohs scale of hardness, i.e. it is next hardest to diamond, at 10. Examples of the micro structures of a high-purity alumina and a debased alumina are shown in Figs 5.20(a) and 5.20(b) respectively. The latter consists of a-Al203 crystallites embedded in a glass-crystalline matrix usually composed of calcium and magnesium silicates. 

The mechanical properties of materials involve various concepts such as hardness, shear and bulk modulus. The group III nitrides are now mostly used as fihns or layers grown by metal organic vapour phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) on sapphire, GaAs or SiC. The lattice parameters of the substrate do not generally match those of the deposited layer, and therefore, stresses appear at the interface and in the layer and modify its physical properties. Hence, it is necessary to have a good knowledge of these properties. 

Because of the expense and small size of ZnO substrates, it seems unclear that ZnO has any outstanding advantages over substrates such as SiC or even sapphire for use with high temperature processes such as MOVPE. Nonetheless, the potential advantages of an isostructural substrate are hard to ignore, and ZnO may prove to be important, particularly for MBE growth. 

Diamonds are the hardest natural substance, but they can cleave easily. They occur in every color, but shades of yellow and brown are most common. Today s faceted diamonds are cut and polished with tools embedded with tiny diamond chips or dust. Before the advent of modern methods, diamonds were used in their natural state, or roughly shaped and polished by hand. The hardness of this mineral led many early jewelers to do a minimum of work before setting diamonds in jewelry or regalia. Common imitations of diamond are colorless spinel, sapphire, zircon, topaz, quartz, and many synthetics. 

Rubies and sapphires are color varieties of the mineral corundum, which is simple aluminum oxide. This mineral is second only to diamond in hardness, but is much tougher and harder to break. Rubies are, by definition, red or purplish red. The term sapphire is used for every other color. 

As a result, cuvettes for Raman spectroscopy should be carefully selected. They may, due to their impurities, add a background to the spectrum of the sample. In addition, all cuvette materials produce their own Raman spectra, which have to be considered, when the Raman spectra of the sample are evaluated. Fig. 3.5-17 a shows a Raman spectrum of a typical optical glass BK7, Fig. 3.5-17 b that of quartz glass suprasil, and Fig. 3.5-17 c of sapphire. Suprasil is a synthetic quartz which does not normally contain impurities. Therefore, Suprasil of ESR quality is highly recommended as Raman cuvette material. Also, sapphire is a good cuvette material, as it is very hard, inert, has a good thermal conductance, and shows only weak but sharp Raman lines (Porto and Krishnan, 1967). It is used for the production of the universal Raman cell (Schrader, 1987). The sharp Raman lines of sapphire observed in the spectra of the sample may be subtracted from the spectrum or used as internal standard for quantitative analyses (Mattioli et al, 1991). 

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