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.
• Fig. 3 shows how the resistance of the contact fell, as the voltage on the 15 nF condenser was increased, for a variety of samples. There was little or no change for the gold sample Gl, but in all other cases there was a significant effect. It was apparent that little further reduction of resistance occurred after the condenser voltage reached 100 V. Thus, a discharge from the condenser at this voltage was adopted as a standard pre-test operation. There is, of course, no reason why the capacitance of 15 nF should be better than any other value, and further studies could be made on the optimisation of the discharge.
• Fig. 5 shows R plotted against 1/d for all the samples listed in Table 1, at loads between 0.2. kg and 10 kg.
• a single straight line of slope 0.083 ohm m satisfies all the data with a standard deviation of less than 10%. This slope is called the indenter constant C. It is concluded that the measured contact resistance can be employed to predict the length of the diagonal of the indentation for a given load and, thus, the hardness of the metal.
• R Q is the resistance at zero time. It is seen that, whereas for gold the resistance is time-independent, it takes several minutes with CR-39 and Perspex before constant values are approached. Clearly, important information about the time-dependent mechanical properties of these materials is thus made available by a comparatively simple technique, much less time consuming than an equivalent optical microscope method.
• the indenter constant In practice, we would expect the indenter constant to be somewhat greater than P /2, because a square of diagonal d occupies less area than a circle of diameter d and there is extra resistance associated with the part of the pyramid lying within the indentation.
• the indenter constant would also be made larger by any imperfection in the contact between the surfaces. in fact, the observed value for the indenter constant is no more than 0.083 ohm m whereas p/2 is 0.185 ohm m.
• the resistivity of the SiC near the tip of the indenter must be of lower resistivity than the value measured on the rectangular bar from which the indenter was cut.
• the bar was found to be quite uniform on a microscopic scale but it is possible that it was microscopically inhomogeneous. There is some evidence for this, in that a different indenter constant was obtained when the SiC was reground and repolished. Be that as it may, the indenter constant is obviously of the same order as the predicted value which suggests that the surfaces must be in intimate electrical contact with one another, at least after the pre-test operation.

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• 1986
  • 1986-05-02 JP JP50278886A patent/JPS62503052A/ja active Pending
  • 1986-05-02 DE DE19863690235 patent/DE3690235T1/de not_active Withdrawn
  • 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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