Hardness and Abrasiveness

The abrasiveness of a pigment is not identical to its intrinsic hardness, i.e., the hardness of its primary particles. In practice the Mohs hardness is therefore not a useful indication of the abrasiveness of a pigment. Abrasion rather depends on pigment particle size and shape and is usually caused by the sharp edges of the particles. A standard test procedure for determining abrasiveness does not exist. A method based on the abrasion of steel balls by the pigment is described in Ref [1.17]. 

---

Hardness (qv), which determines the resistance of a material to abrasion and deformation, is affected not only by composition but also by porosity and microstmcture. Higher cobalt content and larger carbide grain size reduce hardness and abrasion resistance but iacrease the toughness of cemented carbides. The trade-off of abrasion resistance and toughness enables the cemented carbide manufacturer to tailor these materials to a wide variety of metal-cutting and nonmetal-cutting appHcations. 

Coke The grinding characteristics of coke vary widely. Byproduct coke is hard and abrasive, while certain foundry and retort coke is extremely hard to grind. For certain purposes it may be necessary to produce a uniform granule with minimum fines. This is best accomplished in rod or bah mihs in closed circuit with screens. Hourly capacity of a 1.2- by 3-m (4- by 10-ft) rod mill with screens, operating on by-product-coke breeze, was 8.1 Mg (9 tons), 100 percent through No. 10 sieve, and 73 percent on No. 200 sieve power requirement, 30kW(40 hp). 

Select a tungsten carbide insert bit with no offset and conical or double cone inserts when drilling hard and abrasive limestone, hard dolomite, chert, pyrite, quartz, basalt, etc. Use bit type 7-4 to 8-3. 

Soft, gummy formations—Congo, cubed shaped Soft formation—large Congo, spherically shaped Abrasive formation—premium West Africa Hard and abrasive—Special premium West Africa... 

PDC Bit Design. Figures 4-152 and 4-153 show typical PDC bits. Figure 4-152 is for soft formation. Figure 4-153 is for hard and abrasive formation [43A]. 

Figure 4-153. PDC bit designed for hard and abrasive formations [43A], (Courtesy Hughes Christensen.)...

The hardness and abrasion resistance of anodic coatings have never been easy properties to measure, but the development of a British Standard on hard anodising has made this essential. Film hardness is best measured by making microhardness indents on a cross-section of a film , but a minimum film thickness of 25 tm is required. For abrasion resistance measurements, a test based on a loaded abrasive wheel , which moves backwards and forwards over the film surface, has improved the sensitivity of such measurements. 

Silicon Monoxide. SiO, mw 44.09, hard and abrasive, black to brn-black amorph or cubic crysts, mp > 1702°, bp 1880°, d 2.13-.20. Sol in coned aq alk and dil HF+HN03. Prepn is by subliming finely divided silicon at 1250° under high vacuum for 4 hrs. It is used as a coating for precision optical lenses... 

In general, minerals which accumulate under placer conditions must be hard and abrasion-resistant to survive. The same properties are important if minerals are to be concentrated by gravity methods of separation, based on the difference in their densities. Although heavy, soft minerals can be concentrated in gravity plants, losses are high. Gold is an exception it is not a hard mineral, but it is cohesive and although battered out of shape, does not slime to a fine mud. 

Hardness. The hardness of the material affects the power consumption and the wear on the machine. With hard and abrasive materials it is necessary to use a low-speed machine and to protect the bearings from the abrasive dusts that are produced. Pressure lubrication is recommended. Materials are arranged in order of increasing hardness in the Mohr scale in which the first four items rank as soft and the remainder as hard. The Mohr Scale of Hardness is ... 

Friction, hardness, and abrasion resistance are related closely to the viscoelastic properties of polymers. 

The reinforcement can also cause increases in hardness and abrasion resistance, along with decreases in creep and dimensional changes with humidity. Undesirable effects brought on by the addition of glass reinforcement include opacity, low surface gloss, some difficulty in electroplating, and loss of mechanical flexibility. 

Fig. 4.5.9. Hardness and abrasiveness of porcelain (1), semi-vitreous china (2) and faience (3) glazes on products collected from industry in 1967.

The Bohme s disc method for petrochemical estimation of rocks now used appears to be equivalent to the dynamic abrasion method only for monolithic, monomineral rocks. For rocks softer than silica or those composed of minerals of distinct hardness, porous or partially weathered, no correlation is found between Bohme s method and Vickers hardness or any other hardness, and abrasive action on a surface 42-49 cm2 in area does not allow estimation of the structural and textural specificity of orientations in the rock under test. 

Fig. 4.5.7. Brittleness of minerals as a function of hardness and abrasiveness. (After Engelhardt and Haussuehl, 1965)...

Fig. 4.5.10. Hardness and abrasiveness of ceramic glazes studied by Wihsmann and Petzold (1960).

Solid lines—region of parallel change in hardness and abrasiveness plotted by Wihsmann and Petzold. Broken lines—region delimited by points of boundary measurements. flVm—hardness after Hanneman, 22—abrasiveness. 

Hardness and abrasiveness test results for porcelain, semivitreous china and faience glazes have been compiled in Fig. 4.5.9. As expected, an uptrend in abrasiveness with increasing hardness of glaze is indicated in the graph. However, variation does not run parallel, as would be expected—from the... 

Stem W., 1952, Directional hardness and abrasion resistance of synthetic corundum, Ind. Diam. Rev., 12, 137-140. 

These modified woods are being made to aTimited extent largely to take advantage of the improved mechanical properties, especially hardness and abrasion resistance (68). Good dimensional stability, 60-70% ASE, is obtained only with acrylonitrile and its combination with other vinyl monomers and then only at high loadings (69). 

Parquet flooring was the primary product and the increased hardness and abrasion resistance were used to advantage in high traffic commercial installations. The long life and ease of maintenance justified the increased cost over normal flooring. 

Fifteen years ago, when wood-plastic composites were first introduced many people predicted that this process would solve the problem of wood dimensional stability and great claims were made for its future use. Now that the physical properties of wood-polymer composites are better understood, specific commercial products are being produced which take advantage of the desirable aesthetic appearance, the high compression strength, increased hardness and abrasion resistance and improved dimensional stability. Future use of wood-polymer composites will depend upon the imagination of the producer and the market place. 

The hardness and abrasive properties of silica gels and powders are of interest in their use as reinforcing agents in composite materials. 

Fumed silica is always found to be amorphous, and therefore does not cause silicosis. The respective AI2O3 is crystalline and consists of the thermodynamically metastable S form instead of the stable a form. It can be transformed to the a-AI203 phase by heating to i200°C. This conversion is associated with a loss of surface area and an increase of hardness and abrasiveness. In the commercial Ti02 obtained by flame hydrolysis, the thermodynamically metastable modification anatase is the main phase, with about 30% rutile. The lattice transformation towards higher amounts of rutile becomes notable at temperatures above 700 °C. It is also associated with a decrease of... 

Among nonoxides, silicon carbide and silicon nitride are two very important ceramics. Both are very hard and abrasive materials, and show excellent resistance to erosion and chemical attack in redudng environments. In oxidizing environments, any free silicon present in a silicon carbide or silicon nitride compact will be oxidized readily. Silicon carbide itself can also be oxidized at very high temperatures, the exact temperatures being a function of purity and... 

---