Ceramics Knoop hardness

ISO 9385, Glass and Glass Ceramics, Knoop Hardness, International Organization for Standards, Geneva, 1990. 

Table 10 Knoop hardness values of some ceramics at 25°C from [2, 22] ...

Quartz is abundant and hence inexpensive, relatively hard and chemically inert. Similar to other ceramics, high hardness is a useful property of quartz. Knoop hardness data for a number of ceramic materials including quartz are given in Table 5 [32], The densities of a number of ceramic materials including quartz are given in Table 6 [32],... 

Table 5 Knoop hardness for quartz and some common ceramic materials [32]...

Another micro-hardness test is the Knoop Hardness Test (henceforth KHT, discussed below). It is worth mentioning that superficial Rockwell tests are also used for ceramics measurements. Hardness tests have been upgraded by the application of instrumented Knoop and Vickers hardness measurements. 

Fig. 1.62 Knoop hardness plotted according to relation (1.128) for SIC ceramics. Note that in the graph P of relation (1.128) is replaced by F/d. [8]. With kind permission of John Wiley and Sons...

The Knoop hardness for several ceramics, as a function of load, is indicated in Fig. 1.65. The pattern of the lines in Fig. 1.65 is similar to those in Figs. 1.56 and 1.59 in the VHT. The plateaus are not well defined, though there is such a tendency. The load variation with d is illustrated for all the materials shown in Figs. 1.65 and 1.66. Experience with a wide range of ceramics has proven that Knoop indentations are far less likely to crack than Vickers indentations. This explains the development of KHTs as an alternative to VHTs. Indeed, for a wide range of ceramics and other brittle materials, this is justified. 

A practical and relatively easy method for obtaining mechanical properties is by indentation tests. For ceramics, the most common methods of performing such tests are the Vickers and the Knoop hardness tests, which are very attractive, practical and relatively less expensive than the other tests discussed in Chap. 1. Assessing the toughness of ceramic materials by means of indentation testing is often done, also due to the ease of performance and low cost of conducting the measurements. Vickers hardness can be expressed [7] as ... 

Table 63. Standard values for the Vickers hardness and Knoop hardness of ceramic materials, according to Morell. HVO.l = Vickers hardness under load of 0.981 N. HKO.l = Knoop hardness under load of 0.981 N...

Which of the materials in this list would you classify as a ceramic What were your criteria for making this classification Does classification as a ceramic correlate with Knoop hardness If you think it does, is hardness alone a sufficient criterion to determine whether a substance is a ceramic Ejqrlain. 

Boron Phosphate. BPO4 vaporizes at 1400 C sp. gr. 2.81 related structurally to high-cristobalite. It has been used as a constituent of a ceramic body that fires to a translucent porcelain at 1000°C. Boron Phosphide. There are two compounds BP, Sp. gr. 2.97 Knoop Hardness (lOOg) 3200 decomposes at 1130 C. B13P2, sp. gr. 2.76 Knoop Hardness (100 g) 3800 melts or sublimes at approx. 2000 C. 

Knoop Hardness. A pyramidal-diamond indentation test. The result is expressed in kg/mm2, the applied load being indicated, eg. Knoop 100 or KlOO a US abbreviation is KHN(Knoop Hardness Number) The test was introduced by F. Knoop, C G Peters and W. B. Emerson (/. Res. Nat. Bur. Stand., 23,39,1939). ASTM C849 describes its application to ceramic whitewares C730 to glass. 

A glass-ceramic material was produced with the formation of the primary crystal phase of cordierite. This glass-ceramic material is distinguished for its high fracture toughness of approximately 2.2 MPa m -, a high degree of hardness (Knoop hardness 700), and thermal conductivity of 37.7 W/(m K). The linear thermal expansion coefficient is 45 x 10" K ... 

Table 4.4. Knoop Hardness of Some Ceramics When Wet and Dry ...

Table 3.6. Knoop Hardness Anisotropy of Some Tetragonal Special Ceramics...

Having prepared the world s first standard reference material for Knoop Hardness of Ceramics, SRM 2831, the author fully appreciates the metrological and practical problems posed by the Knoop hardness method. A broad range of... 

Standard Reference Material 2830, Knoop Hardness of Ceramics, National Institute of Standards and Technology, Gaithersburg, MD 20899. 

DIN 52333, Knoop Hardness Testing, Glass and Glass Ceramic, German Institute for Standards, Berlin, 1987. 

G. D. Quinn, P. Green, and K. Xu, Cracking and the Indentation Size Effect for Knoop Hardness of Glasses, accepted by J. Amer. Ceram. Soc., Nov 2002. 

Ceramics are among the hardest substances known, and their hardness is correspondingly difficult to measure. Most methods rely on the ability of one material to scratch another and the measurement is presented on a relative scale. Of the available methods, the Knoop method is the most frequently used. In this approach, the surface is highly polished and a pointed diamond stylus under a light load is allowed to impact on the material. The depth of the indentation formed by the stylus is measured and converted to a qualitative scale called the Knoop or HK scale. The Knoop hardness of selected ceramics is given in Table 4.8. 

For structural materials, the hardness is coimected with the wear and abrasion resistance, which is why it might be useful for research and lab testing. Rockwell and Brinell hardness values are not used for the characterization of ceramics. The hardness values, determined by Vickers (square pyramid), Knoop (rhombic pyramid), and Berkovich (triangle pyramid) indenters, which have physical values, have a dimension of gigapascals (GPa) and may be used for the characterization of ceramics and refractories. 

Table 12.6 Vickers (and Knoop) Hardnesses for Eight Ceramic Materials...

Quinn G, Green P, Xu K. Cracking and the indentation size effect for Knoop hardness of giasses. J Am Ceram Soc 2003 86 441-48. 

Wilantewicz T, Cannon WR, Quinn G. The indentation size effect (ISE) for Knoop hardness in five ceramic materids. Am Ceram Soc Proc 2013 27 237-50. 

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