Mechanochemically synthesized NbC cermets: Part II. Mechanical properties
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Mechanochemically synthesized NbC cermets: Part II. Mechanical properties B.R. Murphy and T.H. Courtney Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, Michigan 49931 (Received 22 July 1998; accepted 20 September 1999)
The mechanical behavior of mechanochemically synthesized NbC cermets was investigated. Material hardnesses range from a high of 19.6 GPa for as-synthesized cermets containing about 4 vol% of Fe to a low of about 4 GPa for heat-treated cermets containing about 34 vol% Cu. Higher hardness generally correlates with lower fracture toughness (about 2 MPa m1/2 for cermets containing the highest percentage of NbC) and vice versa. Highest fracture toughness (about 7.5 MPa m1/2) is found in NbC–18 vol% Fe cermets heat treated extensively following consolidation. Abnormally low fracture toughnesses are found in high-Cu-content cermets in which Cu segregation takes place during heat treatment. Current models of ceramic toughening can be applied to describe the fracture behavior of NbC–Fe cermets.
I. INTRODUCTION
Mechanochemical synthesis (MCS) can be employed to synthesize a number of inorganic compounds. Part I of this paper1 describes MCS of cermet powders based on NbC, the characteristics of the processed powder, and material structural evolution during powder consolidation and subsequent heat treatment. In this paper, we describe the mechanical characteristics of consolidated powders—specifically their hardness and fracture toughness—and relate these properties to microstructure. We also use current models for toughening of ceramics to describe the fracture toughness of one of these cermets.
II. MECHANICAL TESTING PROCEDURES A. Vickers indentation hardness and fracture toughness
Hardness and fracture toughness (KIc) measurements were performed using Vickers indentation techniques. The Vickers hardness numbers (VHN) were determined in the standard way, while fracture toughnesses were approximated using the development of Lawn and coworkers2– 4 in which the fracture toughness obeys the relationship KIc ⳱ ␣o (E/H)1/2(P/D3/2)
(1)
where E is the material modulus and H its hardness, P the indentor load (in N) and D is one-half the surface trace dimension (in m) of a well-developed indentation crack emanating from the hardness impression. Anstis et al.5 J. Mater. Res., Vol. 14, No. 11, Nov 1999
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have shown that the constant ␣o is (within a scatter of about 25%) 0.016 (corresponding to KIc in MPa m1/2) for a wide variety of ceramic materials. We have used this value of ␣o in calculating fracture toughnesses. (Recent work on diamond reinforced zinc sulfide6 indicates that when this value of ␣o is employed, hardness-deduced KIc values are somewhat higher than conventionally determined values.)
B. Plane-strain fracture toughness testing
Vickers indentations on 24XA100, 24CA100, and 24CA80 materials (see Ref. 1 and Table I for material codes) resulted in well-
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