Papers by Keyword: Sintered Materials

Abstract: The present work aimed to contribute to the development of high performance self-lubricating sintered composites, with low friction coefficient and high mechanical strength. Self-lubricating composites presenting embedded solid lubricants in a ferrous matrix were produced. Hexagonal boron nitride (hBN) and graphite were the solid lubricants powders added during the mixing step. The composites were processed by conventional powder metallurgy. The liquid phase sintering, by adding copper, improved the degree of continuity of the matrix by rearranging the solid lubricant particles. With this, besides the hardening effect on the matrix, the mechanical properties of the composites were improved, with tensile strength increasing when compared to the same composite without copper. By using the proposed methodologies, optimized composites presenting friction coefficient of 0.12, tensile strength of 500 MPa and scuffing resistance of 29300 N.m were obtained.

Abstract: When the depth-sensing (nano)indentation is applied to sintered samples, measured properties, which are expected to represent the material of an individual grain, seem to depend on the overall porosity of the macroscopic sample. To understand such a result, it is assumed that while the nanoindenter penetrates into the surface grain and probes the properties of its material, the grain itself serves as another, larger indenter indenting the rest of sample and probing the properties that represent the bulk of material rather than individual grains. Load vs. displacement curve reflects the synergetic response of these two “indenters” and so it contains information about the sample’s mechanical properties at both microscopic and macroscopic scales. Obtained theoretical results agree qualitatively with the experimental data (the dependence of the indentation modulus on the porosity of sample; the indentation size effect).

Abstract: Equations for an estimation of hardness and durability of materials of particles of iron powders in various zones are received. The opportunity of reception of materials with physicomechanical properties above separate zones of particles is shown. By development of technological processes of manufacturing sintered details with demanded physicomechanical properties it is necessary to know the physical characteristics of particles of an initial material, and sintered ensemble with them and their possible limiting values. However, definition of separate mechanical characteristics of particles is impossible because of their small sizes and complex form. Authors attempt to define strength and fluidity of various zones of particles of iron powders and sintered materials on their basis, using a method of microhardness testing.

Abstract: The analysis of a large range of sintered parts shows the fact, they are strained at contact fatigue, in most of the cases. The Hertzian pressure, at which materials in contact are subjected, establishes the appearance of variable stress, which records a maximum at some depth under the contact surface. In the case of sintered materials this stress variation conducts to a faster degradation of strained surface than in the case of compact materials. This is because the stress variation establishes a critical state at the level of stress concentrators, generated by pores, standing in material structure, leading to formation of some cracks which may propagate at the level intergranular necks, producing removal for some particles of material. The accomplished research on sintered materials, having different chemical compositions, revealed the fact, at the beginning, that the materials undergo a cold hardening, indicated by a hardness increasing, followed by some cracks generating, accompanied by their extension and material removal. This process develops in the same mode for all investigated materials, but with different intensity.