Hardness–Grain‐Size Relations in Ceramics

Abstract

Both Vickers and Knoop hardness ( H ) , measured at two or more loads in the range of 100-2000 g (most commonly 100 and 500 g) for a variety of dense oxide and non-oxide materials, covering a range of grain sizes (G), including single crystals where possible, were shown to generally be consistent with (often more limited) literature data. Apparently, conflicting trends of H (1) showing either no G dependence, (2) decreasing from single-crystal or large G values with decreasing G, or (3) having the generally accepted increase with decreasing G are shown to be due to the combination of the limited extent of data and H generally being determined by two basic trends. These two trends are (a) the normal inverse G (i.e., H-G-"') dependence at finer G, (b) a variable G minimum at intermediate G, and (c) H increasing with increasing G at larger G (to single-crystal values). The H minimum is due to local cracking around the indent (mostly along grain boundaries), generally reaching a maximum effect, e.g., minimum in H , when the indent and grain sizes are similar, and tends to be greater for Vickers vs Knoop indents, higher loads and probably greater grain boundary impurity, additive contents, and stresses.

Characterisation of microstructure and hardness of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3− under different sintering conditions

“…For example, the hardness is an essential prerequisite of evaluating the fracture toughness of ceramic materials [14]. The utility of hardness is further enhanced by the fact that it is obtained by a simple indentation test that can be rapidly performed on relatively small samples at moderate cost [15].…”

“…As for some ceramic materials, grain orientation [16] and chemical decomposition [17] also affect hardness. From the literature, the dependence of hardness on grain size does not always show a single trend that applies to every ceramic material [15], as suggested by Rice et al who have reviewed the relationship between grain size and hardness of a variety of oxides and non-oxide ceramics [15]. For example, studies on ceramic materials including MgO, BeO, Al 2 O 3 , MgAl 2 O 4 and B 4 C [15] as well as hydoxyapatite [18] have shown that an increase in grain size leads to a decrease in hardness.…”

“…From the literature, the dependence of hardness on grain size does not always show a single trend that applies to every ceramic material [15], as suggested by Rice et al who have reviewed the relationship between grain size and hardness of a variety of oxides and non-oxide ceramics [15]. For example, studies on ceramic materials including MgO, BeO, Al 2 O 3 , MgAl 2 O 4 and B 4 C [15] as well as hydoxyapatite [18] have shown that an increase in grain size leads to a decrease in hardness. In contrast, the hardness of some other ceramic materials such as TiB 2 , SiC, TiC and Si 3 N 4 is essentially independent of grain size [15].…”

Wear characterization of nano-hydroxyapatite with addition of titanium (HA-Ti)

“…Use of nanograin ceramics can also improve sinterability due to the higher surface area, and resultant nano scale grains can enhance mechanical reliability by reducing the critical flaw size. The high volume fraction of grain boundaries in nano grain compacts can increase fracture toughness as well as other mechanical properties [10]. The sintering temperature of nano-HA with the addition of Ti was studied by microwaves synthesis [11].…”

Load-dependent indentation behavior of β-SiAlON and α-silicon carbide

“…Hardness of a material depends upon several parameters, including grain size, indenter load and loading rate, indenter geometry, surrounding environment, and accuracy in measurements [1][2][3]. Generally, hardness decreases with the increase of indenter load and indentation size, known as indentation size effect (ISE) [1,[3][4][5][6][7][8].…”