Microhardness studies on some alum crystals

Microhardness studies were carried out using four alums. The hardness of these crystals was found to be more than alkali halides but was comparable with alkaline earth nitrates. The load variation of hardness has been discussed. Hardness results on these alums have been analysed taking into account the structure. It was observed that hardness varied witha ⁻⁴, wherea is the lattice constant. Preliminary results on dislocation etching are reported.     

Microhardness studies in mixed alum crystals

The microhardness of mixed alum crystals of various compositions, (NH₄)_1–xK_xAl(SO₄)₂: 12H₂O, has been determined. It is found that the microhardness varies non-linearly with composition, attaining its highest value for a crystal having nearly equimolar concentration. The observed behaviour is explained using the Kataoka and Yamada model.     

Microhardness studies in ammonium halide crystals

Microhardness studies of NH₄Cl (pure and doped), NH₄Br and alkali halide crystals are presented. The hardness of ammonium halides is found to be less as compared to alkali halide crystals. Doping NH₄Cl crystals with copper (Cu²⁺) is found to increase the hardness enormously and the results obtained with various concentrations of copper are presented. The results have been analysed and the various factors contributing to the increase in hardness at lower loads have been discussed.     

Microhardness studies on nonlinear optical L-alanine single crystals

Vickers and Knoop microhardness tests were carried out on grown L-alanine single crystals by slow evaporation technique over a load range of 10–50 g on selected broad (2 0 3) plane. Vickers (H _v ) and Knoop (H _k ) microhardness for the above loads were found to be in the range of 60–71 kg/mm² and 35–47 kg/mm², respectively. Vickers microhardness number (H _v ) and Knoop microhardness number (H _k ) were found to increase with increasing load. Meyer’s index number (n) calculated from H _v shows that the material belongs to the soft material category. Using Wooster’s empirical relation, the elastic stiffness constant (c ₁₁) was calculated from Vickers hardness values. Young’s modulus was calculated using Knoop hardness values. Hardness anisotropy has been observed in accordance with the orientation of the crystal.     

Microhardness studies on calcium hydrogen phosphate (brushite) crystals

Vicker's and Knoop microhardness studies were carried out on grown calcium hydrogen phosphate dihydrate (CaHPO₄·2H₂O) crystals over a load range of 10-50 g. The Vickers (H_V) and Knoop (H_K) microhardness numbers for the above loads were found to be in the range of 94-170 kg/mm² and 28-35 kg/mm² respectively. It was also found that these numbers increased with increase in load. The Mayer's index (n) was found to be greater than 1.6 showing soft-material characteristics. The fracture toughness values (K_c), determined from measurements of crack length, were estimated to be 6 ± 0.5 × 10³ kg m^−3/2 and 4.5 ± 0.5 × 10³ kg m^−3/2 at 25 g and 50 g respectively. The brittleness indices (B_i) were found as 2.3 ± 0.1 × 10⁴ m^−1/2 for 25 g and 3.7 ± 0.1 × 10⁴ m^−1/2 for 50 g. Using Wooster's empirical relation, the elastic stiffness coefficient (c₁₁) has been calculated from Vicker's hardness values as 4.8 ± 0.5 × 10¹⁵ Pa for 10 g, 9.7 ± 0.5 × 10¹⁵ Pa for 25 g and 13.3 ± 0.5 × 10¹⁵ Pa for 50 g. The Young's modulus was calculated as 1.5 ± 0.1 × 10¹⁰ N m⁻² from Knoop microhardness values.     

Microhardness studies in gel-grown ADP and KDP single crystals

Microhardness studies were carried out on (100) faces of gel-grown ADP and KDP single crystals. Slip lines were observed on (100) face of ADP crystal at the corners of the impressions. Microcracks around the indentation were found on (100) face of KDP crystal from 10g load which spread out as the load increased. Vickers hardness numberH _v decreased with increase in load. ΔH _v at 50g load for solution-grown crystals and gel-grown crystals (present case) was determined. Work hardening indexn for both ADP and KDP crystals was less than 2 showing soft-material characteristics. Using Wooster’s empirical relation, values of C₁₁ from hardness were calculated and found to be close to the reported ones. The work was done under a research project sanctioned by the Council of Scientific and Industrial Research (CSIR), New Delhi.     

Microhardness studies of SnI2 and SnI4 single crystals

The variation in the microhardness of tin-di-iodide (SnI₂) and tin-tetra-iodide (SnI₄) crystals has been determined using Vicker’s microhardness indentor. It is observed that the microhardness of the crystals depends on the applied load and is independent of the duration of loading. Vickers Hardness Numerals (vhn) for SnI₂ is found to be greater than that of SnI₄ crystals. Mayer’s equation and implications have been discussed.     

Microhardness studies on as-grown faces of NaClO3 and NaBrO3 crystals

Single crystals of NaClO₃ and NaBrO₃ are grown from their aqueous solutions at a constant temperature of 35°C by slow evaporation by using good quality seed crystals. Systematic microhardness studies are made on as-grown faces of these crystals at various loads. Typical cracks are observed at the corners of the impressions in NaClO₃ whereas in addition to the cracks at the corners microcracks also appeared in NaBrO₃ crystals around the impressions. The impressions formed in NaBrO₃ are not very clear as in NaClO₃, a possible mechanism for it is discussed. The work hardening index number(n) for both these crystals is around 1.6 suggesting that these are moderately harder samples. The hardness studies point out that NaBrO₃ is harder than NaClO₃ (ΔH ≈ 100 kg/mm ²,this could be due to strong inter ionic forces acting between Na-Br in NaBrO₃ crystals. Using Gilman’s empirical relation, hardness values are calculated from the values of elastic constants (C ₄₄) and are found to be close to the experimental results.     

Microhardness of hexaboride single crystals

Measurements of microhardness are carried out on the (100) plane for single crystals of hexaborides, CaB₆, SrB₆, BaB₆, LaB₆, CeB₆, PrB₆, NdB₆, SmB₆ and EuB₆. The values of hardness for these hexaborides were within the range between 1780 – 1980 kg/mm² (Knoop hardness; 200 g load). SmB₆ had the lowest hardness in these hexaborides.     

Microhardness of gallium crystals grown in a centrifuge

Gallium single crystals with identified faces grown from melt under usual conditions are compared for the first time to those obtained under conditions of centrifugation at 15000g. The crystals grown in a centrifuge exhibit a change in the lattice parameters and possess a significantly higher microhardness. Crystals of both types are characterized by a strong polar anisotropy.     

Microhardness of as-grown and annealed lead sulphide crystals

The results of the measurement of Vickers microhardness of {100} faces of synthetic lead sulphide crystals are reported. It was found that, for loads lower than 60g, the microhardness value depends on the applied load, the duration of indentation, and on the density of dislocations in the crystals, but that at higher loads the microhardness is independent of the applied load and is dependent on the dislocation density in the crystals. Furthermore, an influence of the adsorption of water on the microhardness of the crystals was observed. The observations are discussed in terms of the existence of a distorted zone near the crystal-medium interface, a concept originally advanced by Berzinaet al. Finally, this concept of the distorted zone is briefly compared with more recent approaches of microhardness.     

Microhardness studies of renal calculi

The development of acoustic and other methods of kidney stone fragmentation has given importance to the study of the mechanical properties of such stones. Renal calculi based upon calcium oxalate, calcium phosphate, uric acid, cystine, and magnesium ammonium phosphate have been studied using Knoop microhardness indentation methods. The microhardness of dry stones was found to vary with the calculi chemical composition, yet remained consistent within composition groups. The hardest calculi were found to be calcium oxalates with a Knoop microhardness of 68–88 kg/mm². Other compositions have hardness of up to a factor of two lower than this.