This presentation is tailored toward a general and technical audience in the field of jewelry design and manufacturing to introduce the basic concept of hardness, from the atomistic to the macroscopic point of view, to show how materials behave during hardness testing. Compared to other mechanical testing techniques such as tensile-loading and beam-bending, hardness testing is simple and repeatable as well as time-saving. Because most tested samples can be used afterward, this economical technique has been adopted by jewelry manufacturers and researchers as a way to benchmark and to ensure a material's quality. Currently there are many scales and protocols available for a wide variety of materials covering different hardness ranges from soft polymers to super-hard materials. Some of the hardness numbers and values from different scales may be compared directly, but most comparisons must be done with great caution. The focus here will be on accepted hardness techniques for metals and the jewelry manufacturing industry. The latter part of the presentation will emphasize recent research results showing how to engineer the alloys to increase or decrease hardness. Key mechanical properties such as yield strength, stress-strain relationship, and fracture toughness will also be discussed along with the approximation techniques which can (or cannot) be linked directly to the hardness values.

Author(s): Dr. Boonrat Lohwongwatana, Ekasit Nisaratanaporn, Ekasit