MEASURING HARDNESS BY CONTACT RESISTANCE OF INDENTER MATERIAL

Abstract: A method for measuring the hardness of a material by measuring the electrical contact resistance between a hard semi-conducting indenter and the material which avoids the necessity of knowing the electrical resistivity of the material. The reproducibility of the measurement of the electrical contact resistance being greatly enhanced by applying an electrical discharge across the contact between the material and the indenter prior to carrying out that measurement.

Description: Measuring Hardness by contact resistance of indenter material 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. However, the method is really satisfactory only when the thermal conductivity of the material under test is very high. It becomes insensitive when poor conductors of heat are tested, since thermal losses then tend to predominate over heat transfer across the contact and, even for a metal, it is necessary to apply a correction involving its thermal conductivity, which must, therefore, be known or determined in a separate experiment. It will be appreciated that these difficulties might be overcome if the electrical resistance between a conducting indenter and a metallic sample were measured, instead of the thermal resistance. It would be easy to ensure that all the electrical current passed through the contact region and the electrical resistivity of the metal would not need to be known, if its value were much less than that of the indenter material. When two semi-infinite solids of resistivity p j _ and p make contact over a circular area of radius r, the electrical resistance between them is equal to (Pi + p 2 )/4r [3]. Thus, if p - j^ >> p 2 r the resistance becomes equal to p ] _/4r, so that the resistivity of only one of the two materials need be known. The disadvantage of using an electrical measurement to determine the contact area is that it is much more likely to be affected by oxide layers and other surface contaminants than is a thermal measurement. Also, of course, in neither case is there any assurance that the real area of contact is going to be at all close to the apparent area, the macroscopic indentation impression area, as observed under a microscope. Nevertheless, it does seem that under some circumstances it is possible to achieve intimate electrical contact between two solid conductors as described in F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Part I, Oxford University Press (1950) 28. As a result of an investigation of the electrical contact resistance between an indenter made from a hard, electrically conducting material and a number of metals it has been found that effective hardness measurements can be made by the use of a hard semi-conducting material, such as silicon carbide, as an indenter material. Silicon carbide is second only to diamond in Moh's Scale of Hardness and, being a semi-conductor, its electrical resistivity can be much higher than any metallic value while, at the same time, not being so great that resistance measurements are made difficult. 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. It is further preferred that in carrying out hardness testing 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. In order that the nature of the invention may be better understood the use of the method is hereinafter described with reference to the accompanying drawings, in which:- 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. 2 is a plot of electrical resistance between the indenter and a steel sample against applied voltage, before and after condenser discharge. Load = 0.2 kg. The + ve and - ve signs indicate the polarity of the SiC during the resistance measurement. Fig. 3 shows electrical resistance after a 15nF condenser discharge, as a function of the voltage on the condenser, for various samples. Load = 1.0 kg. 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. 6 is a comparison of hardness determined from resistance measurements with the conventionally measured Vickers hardness for different metals. Load = 1.0 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. Experiments were carried out using a 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. In electrical measurements, leads were attached to the indenter and to the metallic test samples, the latter being electrically insulated from the rest of the machine. 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 10 " ohm m. Thus, the required condition is easily satisfied. Fig. 1 also shows the variation of electrical resistivity with temperature for two of the crystalline samples that were studied by Burgemeister 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. Desirably, 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 (ϋi) 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. The value for the resistance at a given load was not at first found to be particularly reproducible. However, it was observed that less variable results were obtained after the electric current was switched off and on while the load was applied. This led to an investigation of the effect of discharging a condenser through the contact and the development of a beneficial pre-test procedure. The lower curves in Fig. 2 were obtained after the discharge of a 15 nF condenser, that had been charged to 100 V. After the discharge, the resistance was much lower and more reproducible, and it no longer significantly depended on the polarity of the indenter with respect to the sample. It must be emphasised that the energy in the discharge is of the order of only 100 J and there is no question of any adhesion between the surfaces. It is possible that the discharge is effective in removing surface contaminants, since neither the indenter nor the metal had been subjected to elaborate cleaning techniques. 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. We have seen that the resistance for a plane circular contact of radius r should be proportional to 1/r. The contact between the indenter and the sample, however, has a square profile and is non-planar. Nevertheless, we would still expect the resistance to depend inversely on the linear dimensions of the indentation and we select as a parameter the length d of the diagonal, as observed with a microscope. It is, of course, this quantity that is used in definition of the Vickers hardness H v in the relation H v = 1.8544 L (1) d This implies that the Vickers hardness is the ratio of the load L to the pyramidal contact area. Hardness values are 2 usually expressed in kg/mm . If there is a geometric similarity between all the contacts, for different loads and samples, we expect that the resistance R should be proportional to 1/d. R is therefore plotted against 1/d in Fig. 4 for a typical steel sample. Before the pre-test operation the results were rather scattered but afterwards all the results followed a straight line through the origin quite closely. It does seem, then, that the contact resistance is inversely proportional to the length of the diagonal across the indentation provided that a suitable testing procedure is adopted. 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. Knowing the indenter constant C, as defined above, we may specify a hardness value H R from the resistance measurement as HR = 1.8544 R 2 L (1) C ~ There is no reason why the Vickers hardness should be dependent on the load but, in case there should be any load-dependent effect, H R has been plotted against the Vickers hardness in Fig. 6, for the nine samples specified in Table 1, using the particular load of 1 kg. The broken line in Fig. 6 represents the plot H R = H v and it is clear that all the experimental results correspond closely with this condition. The largest discrepancy, for the very soft gold sample, could be due to the fact that the indentation has reached a width that is becoming comparable with the width of the indenter. The method of hardness testing that has been described is, of course, only suitable for good electrical conductors. However, it has been found that the technique can be adapted for use with electrical insulators if they are coated with a conducting mater ial. This is of some sign ificance , since it means that micro-hardness tests on ceramics and glass may also be possible by this method and, in addition, continuous time-dependent hardness measurements under load can be made on viscoelastic materials. Some preliminary tests have been carried out on samples of Perspex (poly ethyl methacrylate) and CR-39 (diallyl diglycol carbonate) coated with gold. Continuous chart recordings, showing the variation with time of the resistance under constant load, were used to calculate 2 relative hardness values (proportional to R for a given load) as shown in Fig. 7. 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. If the contact were plane and circular, the indenter constant C would be equal to p/2, d then being taken to be the diameter of the circle, and the assumption beinq made that the two surfaces touch at all points, see R. Holm, Electric Contacts, Theory and Applications, 4th Ed., Springer-Verlag, New York (1967). 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 only possible explanation that can be seen for this discrepancy is that 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. It appears that a hardness test based on the measurement of electrical resistance is now feasible. Such a test could be performed rapidly either in the laboratory or in the field. It is clearly an advantage that the hardness test is carried out under load, since this obviates difficulties due to the change of shape of the indentation that often accompanies recovery, after the load is removed. Also, it would be a simple matter to perform the test for a specific depth of penetration (i.e. a controlled contact area defined by a pre-set minimum contact resistance) rather than a specific load, and this might well lead to more reproducible results. The technique could be applied by means of an attachment to an existing indentation hardness tester and the retention of the Vickers pyramidal geometry would make calibration very simple. The possibility of obtaining continuous measurements on viscoelastic materials is a most attractive feature of the technique. TABLE 1 SAMPLES USED IN THE MEASUREMENTS SAMPLE NO. MATERIAL VICKERS HARDNESS (kg/mm Gl Gold 63 Cl Copper 88 Bl Brass 131 Al Aluminium 140 SI Steel 148 S2 Steel 330 S3 Steel 594 S4 Steel 654 S5 Steel 718

Claims: Claims:- 1. A method of determining the hardness of a material by measuring the electrical resistance between an indenter and an electrically conducting sample where the sample is said material or is a coating on said material and wherein the indenter is made of a hard semi-conducting material. 2. A method as claimed in claim 1 wherein the electrical resistance between the indenter and the sample is measured after applying at least one electrical discharge through the contact between the indenter and the sample. 3. A method as claimed in claim 2 wherein a plurality of cycles of said electrical discharge are applied before measuring the electrical contact resistance. 4. A method as claimed in any one of the preceding claims wherein the hard semi-conducting material is silicon carbide. 5. A method as claimed in any one of the preceding claims wherein the sample is metallic. 6. A method as claimed in any one of the preceding claims wherein the material is an electrical insulator and said sample is a coating on said material. 7. A method as claimed in any one of the preceding claims wherein the measurement of the electrical contact resistance is made after a predetermined depth of penetration is provided for said indenter in the sample. 8. A method as claimed in any one of claims 1 to 5 wherein the indenter is in contact with the sample under a constant load. 9. A method as claimed in any one of the preceding claims wherein the resistivity of the indenter at or near standard temperature is substantially independent of temperature. 10. A method as claimed in any one of the preceding claims wherein the resistivity of the indenter is at least 100 times that of electrically conducting sample. 11. A method as claimed in claim 10 wherein the resistivity of the indenter is less than 100 ohm metre.

Inventors: GOLDSMID HIROSHI JULIAN, HOWES VICTOR RAYMOND, WIECZOREK LECH, GOLDSMID, HIROSHI, JULIAN, HOWES, VICTOR, RAYMOND, WIECZOREK, LECH
Applicants: UNISEARCH LTD, UNISEARCH LIMITED
Application year: 1986
Main classification: G01N3
IPC codes: G01N3, G01N27