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Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.
AL-FARABI KAZAKH NATIONAL UNIVERSITY
METHODS OF MICROHARDNESS MEASUREMENTS FOR METALLIC MATERIALS, METHODS OF PHYSICAL - CHEMICAL ANALYSIS, METHODS OF DENSITY MEASUREMENTS.
Second course master degree student of Material Science and Technology of new materials Nakysbekov Zhasulan
Almaty 2012
KNOOP TEST
Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.
THE KNOOP INDENTER
Knoop, Peters, and Emerson described an unusually
sensitive pyramidal-diamond indenter which is known as the microhardness
tester, or Knoop indenter. The Wilson Mechanical Instrument Company
manufactures a machine, the Tukon tester (Fig. 1), which utilizes this
indenter. In measuring the hardness of a specimen, a polished flat surface
is first prepared. The Knoop indenter is then brought into contact with
this surface for 20 seconds (the minimum time found adequate to assure
consistent results), with a known load. The indentation thus produced
is measured with a microscope, and the hardness number I is proportional
to the load - divided by the area of the indentation. For relatively
heavy loads - say 1 to 3 kilograms-the hardness number is essentially
independent of the load. Tate (1944) showed, however, that this is not
strictly true for loads of 100 grams or less; he concluded that the
applied load should always be reported with the hardness number: that
practice is followed here. The Tukon tester is provided with several
weights corresponding to loads from 100 grams up.
FIG. 1. Tukon testing machine, showing Knoop indenter and rising platform
on which specimens are tested.
The latest model of the Tukon testing machine embodies an electromagnetic device for applying the load without overloading by impact between the specimen and the diamond indenter. The Hamilton instrument was rebuilt to afford that protection against shock after about half of the corundum tests reported here had been completed. The error due to impact before installation of the device is believed to be mostly the result of fractures in brittle specimens, although there must have been some decrease of the hardness number due to impact of the unguarded indenter in the old form of the instrument. The tests of a fluorite specimen (Table 2) before and after rebuilding show essentially no change in hardness number due to this modification to the instrument.
The Knoop indenter possesses certain advantages over other similar hardness measuring tools, and these are exactly the advantages that make it suitable for testing minerals. Figure 2 shows the shape of the indenter. An extremely shallow penetration is sufficient to produce an indentation long enough to be measured with a relative accuracy of about 1%. Thus, for an indentation 100 microns (0.1 mm.) long, the penetration is only about 3 microns. The smallness of the penetrtaion was demonstrated by Peters and Knoop (1940) when they showed that a valid reading of the hardness of electrolytic chromium plate can be obtained, regardless of the nature of the base metal upon which the chromium was deposited, if the thickness of the plating is greater than 0.001 inch or 25 microns. The validity of extending this conclusion to cover small grains in a polished section of a mineral assemblage is not debated here, but does not seem unreasonable for roughly equant grains which appear about 100 microns in diameter in the plane of the section, especially if several such grains are tested and found to give consistent results. By reducing the load applied to the indenter, the length of the indentation can always be kept small.
STANDARDS
Knoop test methods are defined in ASTM E384.
KNOOP TEST METHOD
Knoop testing is done with a rhombic-based pyramidal diamond indenter that forms an elongated diamond shaped indent.
The constant is a function of the indenter geometry and the units of force and diagonal. The Knoop number, which normally ranges from HK 60 to HK1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Knoop hardness is specified as follows:
450HK0.5
Where 450 is the calculated hardness and 0.5 is the test force in kg.
CALCULATIONS
The conversion of the measured length of the indentation and the load on the indenter to the hardness number is made by means of the following formula, which may be expressed by a family of parallel straight lines on logarithmic graph paper:
I=W/L2c
I=Knoop hardness number.
W=Load applied to the indenter, in kilograms.
L=Length of the indentation, originally defined as in centimeters; but L may be measured in any length units desired and the conversion factor to centimeters may be included in c.
c=a constant depending upon the shape of the indenter. It may also include conversion factors depending upon the units actually used to measure W and L.
As stated above, the equation may be expressed by straight lines, one for each applicable load, on logarithmic graph paper. The scale of the graph may be made such that there will be no danger of introducing errors that are larger than probable errors inherent in the measurement of the indentation by optical methods.
The form of the above equation shows that to achieve a given relative or percentage accuracy in I, L must be measured with a maximum relative or percentage error one-half as great. For example, if L is measured with an error of 1 part in 100, the resulting error in I would be 2 parts in 100.
APPLICATIONS
Because of the wide test force range, the Knoop test can be used on almost any metallic material. The part size is only limited by the testing instrument's capacity.
STRENGTHS
WEAKNESSES
BRINELL SCALE
Force diagram
The Brinell scale characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.
Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness.
The typical test uses a 10 millimetres (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29 kN; 6,600 lbf) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:
where:
P = applied force (kgf)
D = diameter of indenter (mm)
d = diameter of indentation (mm)
The BHN can be converted into the ultimate tensile strength (UTS), although the relationship is dependent on the material, and therefore determined empirically. The relationship is based on Meyer's index (n) from Meyer's law. If Meyer's index is less than 2.2 then the ratio of UTS to BHN is 0.36. If Meyer's index is greater than 2.2, then the ratio increases.
BHN is designated by the most commonly used test standards (ASTM E10-08 and ISO 6506–1:2005) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters.
HBW is calculated in both standards using the SI units as
where:
F = applied force (N)
D = diameter of indenter (mm)
d = diameter of indentation (mm)
VICKERS HARDNESS TEST
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with a pressure, which also has units of pascals. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not a pressure.
The hardness number is not really a true property of the material and is an empirical value that should be seen in conjunction with the experimental methods and hardness scale used.
IMPLEMENTATION
It was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size; the impression should have well-defined points of measurement; and the indenter should have high resistance to self-deformation. A diamond in the form of a square-based pyramid satisfied these conditions. It had been established that the ideal size of a Brinell impression was 3/8 of the ball diameter. As two tangents to the circle at the ends of a chord 3d/8 long intersect at 136°, it was decided to use this as the included angle of the indenter, giving an angle to the horizontal plane of 22° on each side. The angle was varied experimentally and it was found that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load. Accordingly, loads of various magnitudes are applied to a flat surface, depending on the hardness of the material to be measured. The HV number is then determined by the ratio F/A where F is the force applied to the diamond in kilograms-force and A is the surface area of the resulting indentation in square millimeters. A can be determined by the formula
which can be approximated by evaluating the sine term to give
where d is the average length of the diagonal left by the indenter in millimeters. Hence,
where F is in kgf and d is in millimeters.
The corresponding units of HV are then kilograms-force per square millimeter (kgf/mm²). To calculate Vickers hardness number using SI units one needs to convert the force applied from kilogram-force to newtons by multiplying by 9.806 65 (standard gravity) and convert mm to m. To do the calculation directly, the following equation can be used:
where F is newtons and d is millimeters.
Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or xxxHVyy/zz if duration of force differs from 10 s to 15 s, e.g. 440Hv30/20, where:
Vickers values are generally independent of the test force: they will come out the same for 500 gf and 50 kgf, as long as the force is at least 200 gf.
ROCKWELL SCALE
The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, where A is the scale letter.
When testing metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.
OPERATION
The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load, and then noting the depth of penetration, vis a vis, hardness value directly from a dial, in which a harder material gives a higher number. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques.
It is typically used in engineering and metallurgy. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.
In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. Also, readings should be taken from a flat perpendicular surface, because convex surfaces give lower readings. A correction factor can be used if the hardness of a convex surface is to be measured
METHODS OF PHYSICAL - CHEMICAL
ANALYSIS
Introduction
Scientific and technological disciplines rely on
measurements of physical and chemical properties. Such measurements
are central to, for example, analytical chemistry, that branch of chemistry
concerned with determining the identity of a substance (qualitative
analysis) or with calculating the
amount of a substance whose identity is known (quantitative analysis).
This article includes a description of the techniques and measurements
that are most often used by scientists, engineers, and laboratory technicians
to identify a substance, separate it into its components, remove impurities,
or determine a specific chemical or physical property. Various methods
of analysis are discussed, including classical wet techniques such as
precipitations and titrations and instrumental methods
such as chromatography, mass spectrometry, spectroscopy, and electroanalysis.
Many methods of analysis and measurement involve the interaction of
radiation with matter. Accordingly, the sources,
interactions, detection, and measurement of various types of ionizing
radiations are discussed in some detail, as are significant applications
of radiation detection and measurement in science and industry. The
behaviour of materials--e.g., certain metals, ceramics, and plastics--under
various conditions is an important factor in determining their suitability
for specific applications. Measuring the behaviours and characteristics
of materials is the concern of materials testing. Several test methods,
along with the properties that they measure, are discussed here.
Qualitative Chemical Analysis, branch of chemistry
that deals with the identification of elements or
grouping of elements present in a sample. The techniques employed in
qualitative analysis vary in complexity, depending on the nature of
the sample. In some cases it is necessary only to verify the presence
of certain elements or groups for which specific tests applicable directly
to the sample (e.g., flame tests, spot tests) may be available. More
often the sample is a complex mixture, and a systematic analysis must
be made in order that all the constituents may be identified. It is
customary to classify the methods into two classes: qualitative inorganic
analysis and qualitative organic analysis.
The classical procedure for the complete systematic
analysis of an inorganic sample consists of several parts. First, a
preliminary dry test may be performed, which may consist of heating
the sample to detect the presence of such constituents as carbon (marked
by the appearance of smoke or char) or water (marked by the appearance
of moisture) or introducing the sample into a flame and noting the colour
produced. Certain elements may be identified by means of their characteristic
flame colours. After preliminary tests have been performed, the sample
is commonly dissolved in water for later determination of anionic constituents
(i.e., negatively charged elements or groupings of elements)
and cationic constituents (i.e., positively charged elements or groupings
of elements). The procedure followed is based on the principle of treating
the solution with a succession of reagents so that each reagent separates
a group of constituents. The groups are then treated successively with
reagents that divide a large group into subgroups or separate the constituents
singly. When a constituent has been separated it is further examined
to confirm its presence and to establish the amount present (quantitative
analysis). Portions of the material are dissolved separately, and different
procedures are used for each to detect the cationic and anionic constituents.
A typical analytical scheme for the
separation of the cations into groups is summarized in the table. The
analysis for anions is more difficult and less systematic than that
for cations.
The organic nature of a compound is generally indicated
by its behaviour on being heated in air; solids usually melt, then burn
with either a smoky or nonsmoky flame, in some instances leaving a black
residue of carbon. The elements usually present in these compounds are
carbon, hydrogen,
oxygen, nitrogen, sulfur, and, occasionally, phosphorus, halogens, and
some metals. Specific tests are available for each of the individual
elements.
Quantitative Chemical Analysis, branch of chemistry
that deals with the determination of the amount or percentage of one
or more constituents of a sample. A variety of methods is employed for
quantitative analyses, which for convenience may be broadly classified
as chemical or physical, depending upon which properties are utilized.
Chemical methods depend upon such reactions as
precipitation, neutralization, oxidation, or, in general, the formation
of a new compound. The major types of strictly chemical methods are
known as gravimetric analysis (q.v.) and volumetric, or titrimetric,
analysis (see volumetric analysis). Physical methods involve the measurement
of some
physical property such as density, refractive index, absorption or polarization
of light, electromotive force, magnetic susceptibility, and numerous
others. An analysis will often require a combination of
methods: qualitative for separating desired constituents from a sample
and quantitative for measuring the amounts present.