1. B. Ilschner N. Cherradi Eds. 3rd International Symposium on Structural and Functional Gradient Materials Lausanne Switzerland 1994 (Presses Polytechniques et Universitaires Romandes Lausanne 1995). Several articles in this volume describe examples of graded materials found in nature.
2. S. Suresh A. Mortensen Fundamentals of Functionally Graded Materials (Institute of Materials London 1998) pp. 3–8.
3. Holl D. L., Proc. High Res. Board 20, 709 (1940).
4. K. Hruban in Non-Homogeneity in Elasticity and Plasticity W. Prager Ed. (Permagon New York 1958) pp. 53–61.
5. C. S. Smith A History of Metallography (MIT Press Cambridge MA 1960) pp. 3–5.