May/June_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 0 3 3 manner to the engineering accomplish- ments of each successive era. Some of the first tests originated on bars that varied in hardness from end to end. The concentration at which the material being tested could form a scratch on the bar was a major factor in determining the specimen’s hard- ness. These rudimentary methods gave a relative and often comparative indica- tion of material strength and were ade- quate for the time. More refined forms of scratch testing were introduced during the 1800s by a German miner- alogist named Friedrich Mohs. In what eventually became known as the Mohs hardness test, the user would scratch an unknown sample with a material of known hardness. Later, this test was improved to a more standardized for- mat and involved scratching material surfaces with a diamond and measur- ing the width of the resultant line. Mohs chose diamond based on its proper- ty as the hardest known natural sub- stance and the fact that a diamond can produce a scratch on virtually all other materials. This refined test uses a scale from 1-10 where the higher the value, the harder the material. For some pro- cesses, the Mohs method is still in use. Various scratch tests continued to be in- troduced over the next hundred years or so, but none gained a hold in mate- rials testing the way the Mohs test did. THE BIRTH OF BRINELL The first transformation to a more systematic testing format came with the introduction of the indentation test. The earliest form arrived in 1859 and was based on the force required to pro- duce a 3.5-mm indent in the test mate- rial. Depth was measured with a vernier scale system and the total weight need- ed to reach 3.5 mmwas indicated as the hardness. The penetrator consisted of a truncated cone that tapered from 5 mm at the top to 1.25 mm at the point. This method was mostly effective with soft materials. The first widely accepted and stan- dardized indentation hardness test was proposed by J.A. Brinell in 1900. His goal was to find a consistent and fast means of determining material hardness. The Brinell hardness test, still in widespread use, consists of indenting the metal surface with a 1-10 mm diameter ball at loads to 3000 kgf. The resultant cir- cular impression was measured with a low power, manual eye microscope at the time. The diameter of the impres- sion was then mathematically calculat- ed to a hardness value. The Brinell test essentially introduced the production phase of indentation hardness testing and paved the way for additional inden- tation tests that were more relevant to different material types. Today, the Brinell test is often used for testing aluminum and copper al- loys at lower forces, and steels and cast irons at higher force ranges. The test method is particularly useful for certain material finishes as it is more tolerant of surface conditions due to the indent- er size and heavy applied force. Brinell testers are often manufactured to ac- commodate large parts, such as engine castings and large-diameter piping. In addition to the still widely used manual microscopes, automatic, digital camera systems are capable of generating rapid and extremely accurate results. While the Brinell test proved to be an effective means of material testing and contributed to ushering in a new- ly standardized era in hardness testing, it had its limitations. For one, the rel- atively large indenter along with high test forces made it inadequate for small precision type testing. The nature of the test also requires a second operation to measure the indent. In addition, the heavy force required by the test leaves an obvious and potentially damaging impression, so finished goods testing is not always practical. For these rea- sons, along with increasing demand for more reliable and efficient testing tech- niques, the hardness testing aspect of materials analysis was ripe for change. VICKERS AND KNOOP DEBUT The race was on to develop ever more effectivemethods. With that came the first acceptable alternative to Bri- nell—the Vickers hardness test—which partially solved the problem by provid- ing a more consistent, lighter load hard- ness test. Developed in the U.K. in 1921, the test used the same principle as Bri- nell, that of a regulated impression, but employed a pyramid-shaped diamond rather than the Brinell ball indenter. The resultant impression is measured using a high power microscope in combina- tion with a filar micrometer eyepiece. In 1939, an alternative to Vickers— the Knoop hardness test—was intro- duced by the U.S. National Bureau of Standards, now NIST. The Knoop test used a shallower, more elongated for- mat of the diamond pyramid and was designed for use under lower forces than the Vickers test, allowing for more accurate testing of brittle or thin ma- terials. These methods continue to be widely used to analyze small test areas, brittle materials, case hardened and steel components, coatings, wire, and precision parts, but often utilize more advanced indentation and measuring procedures than the earlier techniques. EARLY WILSON YEARS, CHANGING OWNERSHIP Despite the progress, a desire and drive toward even more efficient hard- ness testing methods remained. It was this need that led to the success of the Rockwell indentation test. The Rockwell method, originally introduced in a basic form in 1914, essentially revolutionized hardness testing. It used displacement measurement and thereby produced a direct-reading result, eliminating the need for time-consuming secondary measuring operation. With full-cycle test time requiring about 12 seconds, and in some cases as little as three Johan August Brinell.