EXPERIENCE IN THE METROLOGICAL CHARACTERIZATION OF PRIMARY HARDNESS STANDARD MACHINES

Abstract − The Istituto Nazionale di Ricerca Metrologica (INRIM) and Galileo section of LTF S.p.a. have cooperated for many years in the field of hardness for developing and improving Primary Hardness Standards and measuring systems for their laboratories. With this experience, Galileo-LTF has realized many installations for several NMIs in the world. All these Hardness Standards and measuring systems have been metrologically characterised by INRIM. In the paper, experiences made during the metrological characterization will be shown. They include the methods and results of direct verification of influence parameters (force, displacement, time, velocity, angle, etc.) and of indirect verification (hardness scales or indenters comparison) of the systems. Keywords : hardness, primary standards, measuring systems 1. INTRODUCTION The activity of realization and improvement of Primary Hardness Standard Machines (PHSM) and related measuring systems began, at Istituto Nazionale di Ricerca Metrologica (INRIM) (formerly Istituto di Metrologia "G. Colonnetti" - IMGC) in Torino (Italy), in the ’70 years of the past Century. With the cooperation of Galileo section of LTF S.p.a. in Antegnate (Italy), which acquired patents and know-how, this activity continued until the present days [1]. The most important result of this cooperation has been, from one side, the possibility for developing and improving the INRIM hardness standards and measuring systems, as well the LTF hardness standards and measuring systems for it accredited calibration laboratory, and, on the other side, the possibility to develop new standards and measuring systems for other National Metrology Institutes (NMIs). Starting from the first realizations for NMIs, like the PHSM for NIST in USA [2] and the automatic measuring system and the indenter measuring system for NPL in UK [3], in the last two years several primary standards and measuring systems have been supplied to many NMIs in the world. In detail: • three PHSMs for INMETRO (Brazil), NCM (Bulgaria), NPL (India), • one Vickers hardness calibration machine for NIM (China), • five automatic measuring systems for Vickers and Brinell indentations for INEMTRO (Brazil), NCM (Bulgaria), NPL (India), UME (Turkey), NIM (China), • two measuring systems for the geometric characterization of Rockwell and Vickers indenters for INMETRO (Brazil), UME (Turkey). All these standards and measuring systems have been realized by LTF and metrologically characterised by INRIM [4-10]. The reported expanded uncertainties of measurement U are calculated following the relative standards and international documents [11-12] and they are stated as the standard uncertainty of measurement multiplied by the appropriated coverage factor corresponding to a coverage probability of 95%. The results have been compared with technical specifications of measuring systems. 2. PRIMARY ROCKWELL HARDNESS STANDARD MACHINES (PHSMs) The INRIM PHSM was design and realized the end of the ’70 years of the last century [13-14]. It has been improved in electronic, electro-mechanics and software control in occasion of the second realization, at the beginning of ’90 years, for NIST (USA) [2]. New improvement was done during the third realization for the LFT accredited calibration laboratory at the beginning of the new century. In the last two years, three others PHSMs were realized for INMETRO (Brazil) (fig. 1), NCM (Bulgaria), NPL (India). It consists in a dead weight machine for the generation of the test forces and a laser interferometric system for the indentation depth measurements. The main characteristics are also the high stiffness, isostatic design and a very flexible software control that permits to set and measure all the most important parameters involved in the test cycle (times and velocities) The metrological characterization consists in the direct and indirect verification of the machine; in the direct verification, the main verified parameters are the forces generated by the PHSM and the geometry of indenters. The indirect verification consists in the comparison of the performances of indenters with the so called “National Indenters” and in the comparison of the hardness scales