AnIntroductiontoInvar-TheUniversityofArizonaCollege.docVIP

An Introduction to Invar By Scott Gibb Graduate Student College of Optical Sciences University of Arizona Summary This paper is intended to serve as an introduction to Invar for an audience of opto-mechanical engineers. I wrote this paper as part of a class in opto-mechanical engineering taught by Professor James Burge at the University of Arizona. A major motivation for writing this paper was to educate myself on the physical properties and practical aspects of using Invar and SuperInvar in high precision mechanical systems. This paper should interest those engineers eager to learn the basics of Invar. I do not claim to be an expert in Invar; in fact I’ve only worked with Invar on a few occasions and in writing this paper I read a number of papers on the dimensional stability of Invar and have tried to summarize some of the most important information here. The Basics – What is Invar? Invar is a metal used in applications in which a high degree of dimensional stability under changing temperatures is desired. It is used in precision mechanical systems in many different industries and is not limited to opto-mechanical engineering applications. Invar is actually part of a family of low expansion iron-nickel alloys whose most well known members are probably Invar composed of 64% Fe (iron), 36% Ni (nickel) – also known as Invar 36 or FeNi36 SuperInvar composed of 63% Fe, 32% Ni, 5% Co cobalt Kovar composed of 54% Fe, 29% Ni, 17% Co When someone uses the word Invar they most likely mean Invar 36. This paper focuses mainly on Invar 36 as it is the most common iron-nickel alloy used in opto-mechanical engineering. SuperInvar is also discussed, but less so, and Kovar is only mentioned a few times. The most valuable property of Invar is its low coefficient of thermal expansion (CTE). At room temperature it is approximately 1ppm/K, however like most mechanical properties, CTE varies with temperature. Invar’s CTE is the lowest of any metal (Ref 4 and 5). CTE