O. N. Grigor'ev

Resistance of covalent crystals to microindentation

ConclusionsA study was made of the resistance of various covalent crystals to microindentation, involving comparison of reduced and unreduced hardness values. A parameter p-“unit” indenter load necessary for the elastic-plastic penetration of an indenter to unit depth — has been introduced.A simple method is proposed for the determination of the elastic and plastic components of the displacement of an indenter during its penetration. By analogy with p, another two parameters, pp and pe — indenter loads necessary for the plastic and elastic penetration of an indenter to unit depth, respectively — have been introduced. It is shown that the ratio of the elastic to the plastic strain component varies from material to material, and is determined by the HV/E (or σs/E) ratio.The relative contributions from the elastic and plastic components to the total strain caused by the penetration of an indenter change with load, resulting in marked deviations from the law of similarity and in increases in hardness at small indenter loads.The elastic resistance of a material to microindentation, as characterized by the parameter pe, enables the elastic modulus of the material to be estimated, and a method of doing this is described. On the other hand, the plastic resistance of a material to microindentation, in particular the parameter pp determines the plastic deformation resistance of the material more accurately than does its hardness. At small indenter penetration depths the contribution from plastic deformation decreases compared with that from elastic deformation, resulting in a marked increase in hardness.

Influence of temperature on the failure of brittle materials in concentrated loading

ConclusionsIn the area of low temperatures (T<Tc) the crack resistance and failure stress of high strength brittle materials decrease linearly with temperature, which is caused by the purely brittle character of the failure processes.Maximums are observed on the temperature relationships of radial and surface crack lengths. The formation of maximums may be explained by the increase with temperature of the “power” of the fields of residual stresses caused by the zones of plastic deformation with a simultaneous drop in crack resistance and failure stress. At increased temperatures with an increase in plasticity and relaxation capacity of the material there is a reduction in the level of residual stresses with a corresponding decrease in crack length until disappearance of them.The position of the maximums in crack length and the temperature of their disappearance depends upon the crack resistance and flow stress (hardness) and for materials with G1C ≈ 2y0 the temperature of disappearance of radial cracks is close to the ductile-plastic transition temperature.

Effect of heat treatment conditions on the mechanical properties of boron nitride polycrystals

ConclusionsIn the initial condition, the boron nitride cakes have sufficiently high hardness HV and cracking resistance K1c values, although these values are slightly lower than those of the specimens of the optimum phase composition and structure.The examined heat and thermomechanical treatment conditions lead to a large reduction, scatter, and stability loss of the mechanical and service properties of the cakes. This in turn increases the number of rejects of completed components in the final stage of manufacture.The method of securing the polycrystals used by the Instrument Plant, Leningrad, cannot be recommended for use in the mass production of cutting tools of hexanite-R because of the large extent of cracking caused by the DHC method taking place when sharpening the tool blanks. This increases the rejection rate to 60%. The reject rate of the cutters produced from the initial materials made by the method used at the Poltava Plant of Synthetic Diamonds and Diamond Tools in which impact effects and overheating of the polycrystals is prevented, does not exceed the permissible limit.

Effect of heat treatment on the structural state and mechanical properties of polycrystals based on the superhard boron nitride modifications

ConclusionsAn investigation was carried out into the effect of thermal cycling on the structural state of polycrystals based on the high-density boron nitride modifications. It is shown that a two-phase material is characterized by a high level of macro- and microstresses, which control its brittle rupture behavior and mechanical properties. In particular, microcracking processes initiated by internal stresses after the application of external load promote stress relaxation at the tip of a propagating macrocrack, thereby imparting high fracture toughness (Kc ∼16–18 MN/m3/2) to the material. During heat treatment (performed in this work at T ⩾ 800°C) internal stress relaxation takes place, which is accompanied by a fall in fracture toughness to the level characteristic of unstressed single-phase sintering (Kc ∼10–14 MN/m3/2) and changes in hardness and rupturing stress.