HARDNESS TESTER AND METHOD FOR MEASURING THE HARDNESS OF METALLIC MATERIALS

Abstract: A hardness tester for metals and alloys is capable of providing comparative values of hardness in Vickers or Brinnel scale by direct reading utilizing an indenter (17) of conducting material having a high resistivity. The size of the indentation produced determines a certain electrical contact resistance between the indenting point (20) and the sample (21), which may be measured by an appropriate measuring apparatus, e.g. a Wheatstone bridge (W). By trimming the apparatus using a blank or master sample of known hardness, the indenter (17) is loaded so as to penetrate in the test sample (21) until attaining again the same electrical contact resistance condition as on the master sample. The difference between the applied loads provides a comparative evaluation of the hardness which may be read directly on a suitable scale without the need of microscope measurements of the size of the indentation. The hardness tester is particularly suited for field use and for testing internal surfaces.

Description: HARDNESS TESTER AND METHOD FOR MEASURING THE HARDNESS OF METALLIC MATERIALS The present invention relates to a hardness tester for metals and metal alloys capable of expressing com¬ parative hardness values in Vikers or Brinnel scales, and providing for a direct reading without the need for carrying out microscopic indentation size measurement or detecting indenter*s displacement. Hardness testers are instruments which are well known to the skilled technician. The apparatus, which may take different forms, comprises an indenter consti¬ tuted essentially by a stylus assembly which may assume various forms depending on the type of sample to be tested provided with an indenting tip of a hard and un- deformable material, commonly a diamond or a hard met¬ al, and a loading system for applying a certain load. The hardness assessment is generally a multistep test whereby the size of the indentation produced by the in- denting tip on the surface of the piece to be tested under a certain load applied to the indenter, or the depth of penetration of the indenter is measured. The tip of the indenter may have a conical, a spheri¬ cal (Brinnel) or square piramidal (Vikers) form. The diamond or the hard metal are the materials commonly used for making these tips which must be sufficiently hard so as not to be scratched by the material being tested and undeformable under the test load. A bench-type apparatus comprises usually a support structure, a loading system through which it is possi¬ ble to load and by an inventory of implements for per¬ mitting to select a certain configuration of the in¬ denter, suitable for the piece to be tested, e.g. for effecting tests on external or internal surfaces of a machined piece of machinery. Where the indentation cannot be viewed through on optical microscope, for example in the case of an inter¬ nal surface of a tubular piece, the assessment of the hardness must necessarily be based upon a micrometric determination of the depth of penetration of the in- denter tip in the material being tested, according to the capabilities of the known apparatuses. These mea¬ surements of size and/or depth of an indentation are burdensome and are often a source of evaluation errors. In US Patent No. 4,848,141 and in the Article "Mea- surements of Hardness at Indentation Depths as Low as 20 Nanometers "; by W. C. Oliver, R. Hutchings and J. B. Pethica, pp 90-108, ASTM Special Technical Publication No. 889, 1986, a method of determining hardness, yield strength, and other mechanical parameters of a material are disclosed wherein the load applied to a icroin- denter and the displacement of the indenter after the establishment of contact with the surface of the sample are electronically measured and elaborated to obtain the value of the particular mechanical parameter. On the other hand, the contact area between two bodies is difficult to determine when the area of con¬ tact is less than few square microns. There have been prior attempts to obtain such a measurement by deter¬ mining the electrical resistance at the junction. How- ever, to date, such a technique has never found applica¬ tion in indenter-type hardness testing apparatus which for other reasons must utilize an intrinsically insu¬ lating indenter tip, such as a diamond tip. There is a necessity of an apparatus for measuring the hardness of surfaces different to access to, such as the side of the tooth of a gear or internal surfaces which cannot be examined under a microscope and easy to be used and which positively reduces the probability of a reading error by part of the operator. This objective is attained by means of the apparatus object of the present invention, which is advantageous- ly capable of providing a precise indication of the hardness of a sample in an easy manner while reducing the probability of an error of assessment by the operator. Basically the hardness tester of the present inven¬ tion utilizes an instrumental determination of an elec¬ trical contact resistance for determining the hardness of a metallic sample. The contact resistance which is measured is the electrical contact resistance between the tip of the indenter and the metallic sample being tested. This resistance is a function of the contact surface area, which is in turn a function of the inden¬ tation produced by the tip in the metallic sample being tested, under the applied load. To this end, the in- denter's tip as well as the metallic sample being tested, are connected to a resistance-test circuit and the indenter's tip is made with a substantially conduc¬ tive material having an electrical conductivity which is relatively low and preferably lower or equal to about a tenth of the electrical conductivity of the me¬ tallic sample being tested. This fact greatly increases the sensibility of the measurement system. In other words, a "concentration" of the resistance for the whole electric path of the resistance measuring at the indenter*s tip, establishing the contact with the me¬ tallic sample circuit which inevitably includes a metal¬ lic tip holder and a metallic sample support beside the electrical wire connections to the input terminals of the resistance measuring instruments or resistance mea- suring circuit, determines a greater usable gradient of variation of measured resistance R in function of a given increment of the penetration of the indenting tip in the metallic sample being tested. The actual value of the hardness of the sample is preferably expressed by the hardness tester throuhg a comparison with a similar electrical contact resistance determination performed on the surface of a "master sample" of precisely known hardness. This comparison may be effected by determining the difference between the loads which attain the same contact resistance on the master sample and on the successively tested sam¬ ple. In this way, by exactly knowing the hardness of the "master sample", the hardness of the test sample may be readily read from a standard Brinnel or a stan¬ dard Vikers scale by utilizing the difference between the loads attaining the same contact resistance, i.e. the same area of contact, i.e. the same size of inden¬ tation, to read the correct hardness value of the test sample. The master sample of known hardness which is used for a particular hardness assessment should preferably be a master having a hardness value which is relatively close to the expected hardness of the test sample and a reasonable inventory of master samples of different pre¬ cisely known hardness will readily permit to accomplish such a prefereable condition of comparison. The comparison method of measurement though implying a two step test procedure practically elimintates all problems which would be associated otherwise to the pe¬ riodical trimming of the apparatus response. In prac- tice, the comparison method has been found to be effec¬ tive in balancing out the effects of innumerable causes of variance of the electrical contact resistance which is subject to the influence of the different conditions under which the measurement is carried out. In this con- nection, as it will be further described later in the description, carrying out the test with the surface of both the test sample as well as of the master sample wetted with oil has been found to further improve the repeatability of the contact resistance measurements and consequent hardness assessments. The different aspects and advantages of the inven- tion will be more easily appreciated through the follow¬ ing description of several embodiments of the invention and by reference to the attached drawings, wherein: Figure 1 is a schematic sectional elevation view of a bench-type hardness tester according to the present invention; Figure 2 is a schematic view of a hardness tester of the present invention schematically showing a resist¬ ance testing circuit. For ease of description, the same parts or parts of the measuring apparatuses shown which are functionally equivalent among each other are indicated with the same reference number in the figures. With reference to Fig. 1, the hardness tester de- picted therein comprises a support base 1, a stanchion structure 2 on which a shelf body 3 may be slidibly set and blocked in a fixed position by means of a blocking knob 4. A mobile loading body 9 is assembled on the shelf body 3, and is suspended therefrom by means of two steel leaf-springs 5 and 6 which are fixed at one end to the shelf body 3 by means of the screws 7 and 8. The loading body 9 is fixed to the other end of the two suspending leaf-springs 5 and 6 by means of the screws 10 and 11. The mobile body 9 may be loaded downword by means of the loading screw 12 acting on the mobile body 9 through a compression spring 13 which exerts a force on the mobile body 9 in opposition to the force exerted on the same body 9 by the restraining or suspension spring 14, the pre-compression of which may be adjusted by means of the stopper 15. By interposing a boss 18 of an electrical insulating material, for example of an insulating ceramic, a "U" shaped indenter holder 17, suited for interior sur¬ faces, is attached to the lower end of the load pin 16 which is fixedly connected to the mobile body 9 by means of the set screw 19. The indenter's tip 20 is mounted into a recess at the end of the indeter body 17. The metallic sample to be tested is identified with 21 and the determination of the harness may be carried out on the internal sur- face 22 of the piece. The metallic sample 21 rests on a metal support 23 (anvil) which is electrically isolated from the base 1 by placing therebetween electrically insulating sup¬ ports 24, e.g. of a nonconductic ceramic material. Wire connection 25 and 26 to a resistance measuring apparatus are depicted. The load which is applicable by means of the hand- wheel 12 may be detected by means of strain gages mounted on the block 27 and the terminals of the strain gages are connected though the cable 28 to a suitable measuring instrument, provided with an analog or digi¬ tal display (not visible in Fig. 1) of the load which is applied to indenter. Fig- 2 depicts schematically a basic system for de- termining the hardness by means of the apparatus of the invention. In Fig. 2 the hardness tester is schematically de¬ picted by utilizing the same number for identifying the components analog to those already described in connec- tion with Fig. 1. In this schematic showing of Fig. 2, the load P which is applied to the indenter is di¬ splayed on the scale 29 where an index 30 indicates the value of the load which is applied to the indenter 17, i.e. to the indentation tip 20, by acting on the loading handwheel (12 in Fig. 1) . As shown, the electrical wire connections 25 and 26 for the "electrical series" formed by the conducting metal body 17 of the indenter, the indentation tip 20, the test sample 21 and the anvil 23 and which "series" of electrically conducting elements is isolated by the insulators 24 and 18, are shown coupled to a Wheatstone test-bridge, indicated with W as a whole, which is pow¬ ered by the battery B through the rheostat RO and the switch S, at a voltage which is measured by the volt¬ meter V, while the equilibrium condition of the test- bridge is determined by means of the galvanometer G. The illustration of the electrical contact resist¬ ance measuring apparatus in the form of a classical Wheatstone bridge test circuit, has a clear illustra¬ tive and nonlimitative purpose, being the Wheatstone bridge the classical test circuit for resistance mea¬ surements. Of course any other instrument capable of measuring electrical resistance with sufficient degree of precision can obviosuly be utilized in the hardness tester of the invention, whether or not such an instru- ment incorporates a Wheatstone-bridge test circuit. According to the preferred method of assessing hard¬ ness of a test sample by means of the apparatus of the invention, a reference or blank or master sample having a precisely known hardness, sufficiently close in order of magnitude to the presumed hardness of the surface of the sample to be tested, is placed on the anvil 23. The hardness of the master sample which is exactly known may be conventionally expressed in terms of specific load in a Vikers and/or Brinnel scale, whereby the hard- ness value corresponds to the load expressed in kilo¬ grams per square millimeter of indentation area pro¬ duced on the surface of the material. A certain refer¬ ence load is applied to the indenter by reading it on the scale of the dynamometer 29 and such a reference load may be chosen in function of the type of surface treatment to which the sample to be tested has been sub¬ jected, i.e. to the order of magnitude of the presumed hardness of the sample to be tested. This produces a certain indentation on the surface of the master or ref- erence sample. Under this loading condition, the Wheatstone bridge is balanced by acting on the variable resistance of the bridge, according to well known resistance measuring procedures, or the resistance is accurately determined by any other means. The load is released and the reference sample is sub¬ stituted with the sample to be tested on the anvil 23 of the instrument. The dynamometer is progressively and gradually loaded until the Wheatstone bridge reaches again a bal- ance condition. When the bridge is returned to the balance condition the load corresponding to such a renewed balance condi¬ tion of the contact resistance measuring bridge is read off the scale 29 of the dynamometer. in this way, it is established that the resistance of the electrical series is equal to the resistance at¬ tained when testing the reference sample and because the electrical resistance of the relative branch of the Wheatstone bridge between the wire connection 25 and 26 is substantially represented by the contact resistance between the indenter*s tip 20 in function of the con¬ tact area of the latter with the sample being tested, the equality of resistance is indicative of an equality of indentation, i.e. of contact area between the in- denting tip 20 and the metallic sample being tested. At this point the load which is read off the scale 29 of the dynamometer is in the same ratio with the load which was applied on the reference sample for establish¬ ing such a contact resistance as the respective hard- nesses. For example, if on the reference sample was ap¬ plied a load of 5 kilograms and on the subsequently tested sample the load for balancing the contact resist¬ ance bridge circuit is of 4,5 kilograms, the hardness of the sample being tested may be correctly assessed as being 10% lower than the known hardness of the refer¬ ence sample. Such a comparison condition for relatively assessing the hardness will be as precise as less significant is the electrical resistance contribution of the metallic masses of the parts 17, 21 and 23, electrically con- nected in series of Fig. 2, as well as of the wire con¬ nections 25 and 26, in respect to the electrical con¬ tact resistance between the indenter's tip 20 and the metal sample . An electrical conductivity of the mate¬ rial with which the indenter's tip 20 is made, lower than about 10 times the electrical conductivity of the material of the piece to be tested 21 or more generally of the electrical conductivity of metals and metallic alloys in general, permits to attain a remarkable preci¬ sion in the hardness assessment according to the co - parative method described above. Such an upper limit of electrical conductivity of the indenter's tip, permits a sufficient sensitivity of the measuring system and allows to satisfactorily use it also for determining the hardness of samples made of particularly resistive metal alloys or other materials which are intrinsically very resistive such as tita¬ nium, tungsten, "cermets" and the like. Of course, when the instrument must be used for assessing the hardness on highly conductive metallic materials such as steel, copper, aluminum, etc., the indenter's tip may have a much greater electrical conductivity in absolute terms. The lower limit of an electrical conductivity of the material with which the indenter*s tip is made, will be such as to permit the measurement of an electrical re- sistance which in absolute terms is not eccessively high, which fact would otherwise represent a factor of reduced precision of the resistance measurements be¬ cause of intrinsic sensitivity limitations of the elec¬ trical resistance measuring instrument. Materials which may be used for manufacturing a sui¬ tably conductive indenter's tip of the hardness tester may be of various kind although possessing sufficient hardness and undeformability properties under load and an electrical conductivity fitting the above-mentioned limits. Borides , carbides, nitrides, hydrides, oxy- borides, oxynitrides, oxycarbides, perowskites, delofos- sites, spinels and mixtures of the same materials may suitably form the indenter*s tip of the invention. These materials may be shaped by sintering and diamond ground for producing the desired profile of the tip. Most preferably the tip is a crystal having sufficient electrical conductivity, such as for example a type lib diamond crystal. More preferably the electrical conduc¬ tivity of an indenter's diamond tip is raised by means of an ion implantation treatment. It has been found that an ion implantation of diamond with nitrogen at a level of 10 1- * -fi° ions per centimeter square, produces an adequately conducting "skin" of the tip. The implanta¬ tion is normally carried out on the diamond tip after it has been already set in the indenter*s body and ground to the desired shape, by exposing the indenter assembly to the flux of accelerated nitrogen ions in¬ side an ion implantation vessel. Of course ions other than nitrogen may be satisfactorily implanted in a diamond crystal for imparting a sufficient electrical conductivity to at least a superficial "skin" layer of the tip. When employing an implanted crystal tip set in a me¬ tallic indenter body, the electrical continuity between the tip and the metallic indeter's body in which the tip is set, may be ensured by depositing a conducting metal coating, for example by a chemical vapor deposi¬ tion (CVD) process, after having masked the vertex of the indenting implanted crystal tip. The conductive coating deposited on the unmasked portion of the im- planted crystal tip and on the adjacent surface of the metallic indenter body in which the tip is solidly set, provides a conductive "bridge" between the relatively conductive implanted "skin" portion of the crystal tip and the metallic indenter body. This, beside preventing a criticity of electrical continuity between the crys- tal tip and the metallic indenter body, further "con¬ centrates the resistance signal" in the contact area established by the indenter's tip with the metallic test sample. Of course the metallic coating on the sur¬ face of the indenter's tip is arrested (by the masking) at a sufficient distance from the vertex of the in¬ denter*s tip so as not to come in contact with the me¬ tallic sample. According to a preferred method of the invention, the repeated electrical resistance measurements (on the reference and on the test samples) are performed after having wetted the surface of the metal sample with oil, such as a lubricating oil. To this effect the samples may also be completely immersed in a pool of oil while measuring the comparative contact resistances. It has been found surprisingly that by conducting the measure¬ ments in oil, the repeatability of the contact resist¬ ance measurements under identical load conditions, is enhanced. This may be explained by assuming that the oil dissolves eventual residual traces of grease which would otherwise remain undisplaced under the relatively high specific loads at the indenter's tip-sample inter¬ face; or by a cleansing action of the oil on micro¬ scopic metallic particles whose presence could affect the contact resistance and which may be detached and electrically neutralized (isolated) by the oil; or the action of the oil may be that of increasing the "defini¬ tion" of the contact areas in presence of the plastic flow of the indented metal. An alternative procedure to the wetting the surface with oil prior to indent the surface and measure the electrical resistance is to ac¬ curately clean the surface of the sample to be in- dented. However this latter technique has been found inferior to conducting the test in presence of oil, in terms of repeatability of the electrical resistance mea¬ surements. Naturally the hardness tester of the invention may be realized in different forms adapted to specific uses. The apparatus may be constructed in the form of a bench-type tester or it may be made in a form of a portable instrument for field use. In this latter form the instrument may be readily reduced in size and have a simplified configuration wherein the load can be manu¬ ally applied by a simple pushing-action. It has in fact been found that the measuring system well tolerates slight oscillations of the tip's axis of application of the load which may occur during the electrical resist¬ ance measurements. This may be explained by the fact that the electrical contact surface between the in¬ denter's tip and the sample depends basically from the specific load (i.e. from the hardness of the sample) . For this reason even for small variations of the angle of incidence of the axis of the indeter's tip and the surface being indented, the variations of the contact resistance are negligeable. Naturally the hardness tester of the invention may be easily automatized by providing for a motorized grad¬ ual load application which may be easily controlled. For example when repeating the contact resistance test on a first sample, the load application may be automat¬ ically arrested upon attainment of equilibrium of the resistance measuring bridge circuit. The automation may also provide for calculation and display of the actual hardness of the tested material. Moreover the system may be equipped with a programmable data-recording and processing unit for a complete analysis of the hardness determination. The electrical contact resistance measurements which are used for comparing and determining hardnesses, instead of being end-point type determinations, may be carried out in the form multiple point determinations under progressively increased loads both on the refer- ence sample of known hardness and subsequently on the sample to be tested. The various pair (equal load) of comparable contact resistance measurements may be analysed for providing information on the variation of hardness in function of the depth of penetration of the indeter in the sample being tested. Also in a case such as the above, the provision of automatic data-col¬ lecting and data-processing units will greatly simplify and make a thorough analysis of the properties of the tested sample practically immediate.

Claims: C L A I M S 1. A hardness tester for metals and metal alloys com¬ prising an indenter carrying an indenting tip of hard and undeformable material and means for loading said indenter with a certain load for penetrating said ' tip in the metallic sample to be tested, the depth and/or size of the indentation produced by said tip on the sur¬ face of the piece being tested providing a measure of the hardness of the material, characterized by the fact that said indenting tip is electrically conductive; said conductive indenting tip and said sample are connected to an electric resistance measuring ap- paratus; the hardness of said sample being established by measuring the contact resistance between said tip and the sample as a function of the depth of indentation produced under a certain load. 2. The hardness tester as defined in claim 1, wherein said indenting tip is made of a conductive ma¬ terial belonging to the group composed of borides, carbides, nitrides, hydrides, oxyborides, oxynitrides, oxycarbides, perowskites, defossites, spinels and mix¬ tures thereof. 3. The hardness tester as defined in claim 1, where¬ in said indenting tip is a sintered material. 4. The hardness tester as defined in claim 1, where¬ in said indenting tip is a monocrystal. 5. The hardness tester as defined in claim 1, where- in said indenting tip is an ion implanted diamond. 6. The hardness tester as defined in claim 1, where¬ in the indenting tip is set in a metallic indenter piece which is connected to said electrical resistance measuring apparatus and the electrical continuity be- - tween said indenting tip and the metallic indenter is ensured by a conductive coating extending over a por¬ tion of the surface of said indenting tip and a portion of the surface of said indenter. 7. The hardness tester as defined in claim 1, where¬ in said electrical contact resistance measuring appa¬ ratus utilizes a Wheatstone bridge circuit. 8. The hardness tester as defined in claim 1, where- in the surface of the sample is wetted with oil. 9. A method for determining the hardness of a metal sample by using a hardness tester provided with an in¬ denter having an indenting tip of a hard and undeform- able material and means for loading said indenter with a certain load for producing the penetration of said tip in the metallic sample, the depth of penetration and/or the width of the indentation produced under a given load providing a measure of the hardness of the material, characterized by comprising measuring the electrical contact resistance between said tip and said metallic sample under a given load; the measurement of the electrical contact resist¬ ance providing a measure of the hardness of said metal- lie sample. 10. The method as defined in claim 8, characterized by comprising: utilizing an electrically conductive indenting tip and connecting said indenter and said metallic sam¬ ple to the input terminals of an electric resistance measuring apparatus; loading said indenter with a preset load on a reference metallic sample having a known hardness and measuring the contact resistance under said preset load; functionally placing the metallic sample to be tested in the tester and gradually increasing the load applied to the indenter until the same electrical con¬ tact resistance is measured by said electrical resistance measuring apparatus and reading the load which is actually applied to the indenter; determining the hardness of the tested sample through a proportionality relationship between said pre¬ set load applied to the reference sample and the load applied on the tested sample for attaining the same electrical contact resistance on the reference sample and on the test sample. 11. The method as defined in claim 8, wherein the testing is conducted with the surface of the sample being indented wetted with oil. 12. The method as defined in claim 8, wherein said indenting tip has an electrical conductivity which is not greater than 1/10 of the electrical conductivity of said reference metal sample and of said metal sample to be tested.

Inventors: ERNST ALFRED, ERNST, ALFRED
Applicants: ERNST ALFRED, ERNST, ALFRED
Application year: 1991
Main classification: G01N3
IPC codes: G01N3