Microhardness tests

Apphcations of microhardness testing greatly extend the conventional indentation hardness test to glass and ceramics, metaHographic constituents, and to thin coatings or other surface treatments not otherwise testable. 

The shortcomings of microhardness tests include numerous sources of errors not found in macrohardness tests such as friction, vibration, inertia, windage, and the skiH of the test operator. 

Ultrasonic Microhardness. A new microhardness test using ultrasonic vibrations has been developed and offers some advantages over conventional microhardness tests that rely on physical measurement of the remaining indentation size (6). The ultrasonic method uses the DPH diamond indenter under a constant load of 7.8 N (800 gf) or less. The hardness number is derived from a comparison of the natural frequency of the diamond indenter when free or loaded. Knowledge of the modulus of elasticity of the material under test and a smooth surface finish is required. The technique is fast and direct-reading, making it useful for production testing of similarly shaped parts. 

Scratch Te.st. The scratch microhardness test is a refinement of the Mohs test. The corner of a cubic diamond is drawn across the surface of a metaHographicaHy poHshed sample under a constant load, usuaHy 29.4 N (3 kgf). The width of the resultant Vee groove scratch varies inversely with the hardness of the material displaced where H = scratch hardness number and A = groove width in micrometers. 

Although Vickers and DPH microhardness tests should yield the same numerical results on a given material, such is not always the case. Much of the observed variance may be a function of differences ia the volume of sample material displaced by the macro and micro iadentations. 

The accuracy of microhardness testing has been questioned a wide range of values appears in the Hterature for plated deposits, especially in hardness extremes. ASTM B8.10 is involved in intedaboratory testing to define the precision and bias of the Specification B576 Microhardness of Electroplated Coatings (55,56). 

J. Homer, Microhardness Testing of Plating Coatings Recent Round-Robin Experiences, ia Ref. 47. 

J. Homer, "Microhardness Testing of Plating Coatings Defining Precision and Bias," Inf/Tech Conf. Proc., AESF SUR/FIN, Atianta, Ga., 1992. 

The techniques, instrumentation and underlying theory of optical microscopy for materials scientists have been well surveyed by Telle and Petzow (1992). One of the last published surveys including metallographic techniques of all kinds, optical and electronic microscopy and also techniques such as microhardness testing, was a fine book by Phillips (1971). 

The wide use of microhardness testing recently prompted Oliver (1993) to design a mechanical properties microprobe ( nanoprobe would have been a better name), which generates indentations considerably less than a micrometre in depth. Loads up to 120 mN (one mN 0.1 g weight) can be applied, but a tenth of that amount is commonly used and hardness is estimated by electronically measuring the depth of impression while the indentor is still in contact. This allows, inter alia, measurement... 

Martensitic traasfonnation Master ec[uations Mean field crossover to Ising Mechanical properties Metallic alloys Metallic glasses Metastable alloys Microhardness test Microscopic theory of nucleation... 

Another employed microhardness test uses a rhombic-based pyramidal diamond (Knoop) with included angles of 172° and 130° between opposite longitudinal edges 14). Again, the microhardness value is given by the force in N divided by the projected area of impression in mm2. 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

Since all practical methods of measuring the hardness of mbber involve measuring the resistance to indentation, hardness may be defined simply as resistance to indentation . Hardness is an expression of the elastic modulus of the mbber. More specifically, the load required to press a ball of given diameter to a given depth into the mbber is proportional to its elastic modulus. See Hardness Testing, Pusey and Jones Plastometer, Microhardness Testing. 

The hardness tests are classified according to the magnitude of the load. Macrohardness tests apply major loads that are greater than 1000 g (1 kg). Microhardness tests employ loads less than 1000 g and are commonly used to measure the hardness of a single grain or a very small area. Brinnell, Rockwell, and Vickers are examples of macrohardness tests, and Knoop is an example of a microhardness test. 

The diffusion layers obtained in specimens were investigated by means of metallography, electron microscopy, microhardness test. X-ray diffractometry and Mossbauer spectroscopy. 

The asphalt is distributed as little pellets or stringers. In the same samples one finds pellets of high reflectivity and nearby, others of dull appearance (Figure 3a). The last are elastic whereas the former are brittle towards the microhardness test. 

Boyarskaya Yu. S., 1972, Deformirovanie kristallov pri ispytaniyakh na mikrotverdost (Deformation of Crystals in Microhardness Tests), Izd. ShUNTsA, Kishinev. 

Grigorovich V. K., 1965, Metody ispytaniya na mikrotverdost. Pribory (Microhardness Testing Methods. Equipment), Izd. Nauka, Moskva. 

Grodziiiski P., 1951, Damaged impressions in microhardness testing, Research London),... 

Lysaght V. E., 1960, The how and why of microhardness testing, Metal Progress, August. 

Pelc A., 1965, Zastosowanie badan mikrotwardosci dla kontroli jakosci w przemysle sciernym (Application of microhardness tests to quality inspection in the abrasive industry), Biul. Inf. POiN. IOS (Krakow), 3, 8-10. 

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