Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N3/40—Investigating hardness or rebound hardness
  • G01N3/42—Investigating hardness or rebound hardness by performing impressions under a steady load by indentors, e.g. sphere, pyramid
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/02—Details not specific for a particular testing method
  • G01N2203/06—Indicating or recording means; Sensing means
  • G01N2203/0617—Electrical or magnetic indicating, recording or sensing means

Definitions

• the present invention relates to a method for measuring the hardness of materials by measuring the electrical contact resistance between a conducting indenter and a metal object applicable to measuring the hardness of conducting materials such as metals and the hardness of non-conducting materials (such as ceramics and plastics) that have been coated with a thin metal layer.
• a method for determining the hardness of certain materials from measurements of the thermal contact resistance has recently been described in H.J. Golds id and J.N. Johnston.J.Phys. E., Sci Instrum., 1 (1981) 1329, and H.J. Goldsmid and J.N. Johnston.J Materials Science, 1_7 (1982) 1012.
• the method is really satisfactory only when the thermal conductivity of the material under test is very high.
• the present invention thus consists in a method of determining the hardness of certain materials by the measurement of electrical resistance between a conducting indenter and an electrically conducting sample, wherein the indenter is made of a hard semi-conducting material, e.g. silicon carbide. It is preferred that the indenter be of a composition such that its electrical conductivity is almost independent of temperature. It is further preferred that the geometry of the indenter be the same as that of the Vickers pyramid diamond indenter to permit its use in conventional hardness test ng mac nes. T s g ves a major advantage of ready calibration against the Vickers hardness scale.
• a pre-test operation consisting of passing an electrical discharge through the contact between the indenter and the material be carried out to improve the reproducibility of the contact resistance.
• Fig. 1 shows electrical resistivity plotted against temperature for the silicon carbide material used in the indenter. Also shown is the data for two of the samples measured by Burgemeister et al as described in J. Appl. Phys. , _50. (1979) 5790.
• Fig. 4 shows electrical resistance before and after condenser discharge plotted against reciprocal of the observed diagonal of the indentation for a typical steel sample.
• Fig. 5 is a plot of electrical resistance against reciprocal of the diagonal for all the samples studied for loads between 0.2. and 10 kg.
• Fig. 7 shows a variation of relative hardness (proportional to the square of the resistance) with time for indentations on metallised Perspex and CR-39.
• Zwick hardness testing machine Type Z3.2A This machine is normally used for the performance of Vickers hardness tests, using loads of up to 10.kg.
• leads were attached to the indenter and to the metallic test samples, the latter being electrically insulated from the rest of the machine.
• the indenter In the Vickers test, the indenter consists of a diamond pyramid having an angle of 136 between opposite faces. In the present experiment, the diamond indenter was replaced by one of a number made from hot-pressed, high density silicon carbide, originally supplied by the American National Bureau of Standards in bar form. The indenters were either of the standard Vickers shape or were conical with a half-angle of 60°. The results were qualitatively similar for all the indenters but the data that are presented here refer specifically to one of the standard pyramidal shape, with well-polished faces, that was made from a particular bar of SiC. All of the measurements with the indenter were carried out at a temperature of 300°K.
• the electrical resistance between the indenter and the metal sample was determined by observing the potential drop, using a multi-range digital voltmeter, for various values of the current. It was established that the only significant voltage occurred in the region of the contact between the two materials. Provision was later made for the discharge of a condenser through the contact, as will be described shortly.
• Table 1 lists the samples that were studied together with their Vickers hardness numbers, as determined using a diamond indenter. The surfaces of the samples were polished, as is normal for hardness testing, and degreased, but no special cleaning procedure was necessary. As mentioned in the introduction, it is desirable that the electrical resistivity p of the indenter should be much greater than that of any of the metals that might be tested.
• Figure 1 shows the observed values for p, over temperature range 300-450°K, obtained using a four-contact technique on a rectangular bar. The resistivity at 300°K is 0.37 ohm m, whereas metals and metallic alloys have resistivities in the range 10 -8 to
• Fig. 1 also shows the variation of electrical resistivity with temperature for two of the crystalline samples that were studied by Burgeffle et al. referenced aboye. It is noted that whereas the negative temperature coefficient of the resistivity - (dp/dT)/p for the hot-pressed SiC is less than for the crystalline sample No. 4, it still has the relatively high value of 6.9 x 10 " K . However, the crystalline sample No. 1 has almost zero temperature coefficient and its electrical resistivity is still several orders of magnitude greater than that of a metal. Thus, an indenter made from material similar to Burgemeister's sample No. 1 would have the important additional advantage of virtual temperature independence.
• the electrical resistivity of the semiconductor should not be too high or it is likely to encounter barrier problems such as those experienced in semiconductor rectifiers.
• the resistivity of the semiconductor indenter is at least 100 times that of the electrically conducting sample but it is preferred that resistivity of the indenter be less than, say, 100 ohm metre.
• the upper curves of Fig. 2 show typical behaviour of the electrical contact resistance, as measured over a wide range of applied voltage. The features that are apparent are: (i) a small difference in resistance according to whether the indenter is positive or negative with respect to the metal; (ii) an increase or decrease of resistance, according to the polarity, at low voltages; and ( ⁇ i) a substantial decrease at high voltages.
• Fig. 2 applies specifically for a sample of steel, with a load of 0.2 kg applied to the indenter, but it was invariably found that the plateau region included measurements at an applied potential difference of lOmV, whatever the test metal or the load. For this reason, the observations were normally carried out with this value for the applied voltage.
• the behaviour at very low voltages, including the effect of polarity, may be indicative of interfacial barrier effects, while the trend at high temperatures could be due to Joule heating.

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• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

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• 1986
  • 1986-05-02 JP JP61502788A patent/JPS62503052A/ja active Pending
  • 1986-05-02 WO PCT/AU1986/000122 patent/WO1986006833A1/en active Application Filing
  • 1986-05-02 EP EP19860903124 patent/EP0221165A1/en not_active Withdrawn
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