Boride Coatings on Steel Protecting it Against Corrosion by a Liquid Lead-Free Solder Alloy

The goal of this research is to study the applicability of the diffusion boriding process as a high-temperature thermochemical heat treatment to enhance the lifetime of steel selective soldering tools. The main purpose of the work is to discuss the behavior of double-phase (FeB/Fe2B) iron-boride coating on the surface of different steels (DC04, C45, CK60, and C105U) against the stationary SAC309 lead-free solder liquid alloy. The boride coating was formed on the surface of the steel samples through the powder pack boriding technique. The microstructure of the formed layer was examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The borided samples were first cut in half and then immersed into a stationary SAC309 lead-free solder liquid alloy (Sn–3Ag–0.9Cu) for 40 days. Microstructure examinations were performed by SEM with energy-dispersive spectroscopy and an elemental distribution map after the dissolution test. Excessive dissolution/corrosion of the original steel surface was observed at the steel/SAC interfaces, leading also to the formation of Fe–Sn intermetallic phases. This was found to be the major reason for the failure of selective soldering tools made of steel. On the contrary, no dissolution and no intermetallic compounds were observed at the FeB/SAC and at the Fe2B/SAC interfaces; as a result, the thicknesses of the FeB and Fe2B phases remained the same during the 40-day dissolution tests. Thus, it was concluded that both FeB and Fe2B phases show excellent resistance against the aggressive liquid solder alloy. The results of the dissolution tests show a good agreement with the thermodynamic calculations.

A mesoscopic analysis of a localized shear band propagation effect on the deformation and fracture of coated materials

The numerical simulations of the deformation and fracture in an iron boride coating – steel substrate composition are presented. The dynamic boundary-value problem is solved numerically by the finite-difference method. A complex geometry of the borided coating – steel substrate interface is taken into account explicitly. To simulate the mechanical behavior of the steel substrate, use is made of an isotropic strain hardening model including a relation for shear band propagation. Local regions of bulk tension are shown to arise near the interface even under simple uniaxial compression of the composition and in so doing they determine the mesoscale mechanisms of fracture. The interrelation between plastic deformation in the steel substrate and cracking of the borided coating is studied. Stages of shear band front propagation attributable to the interface complex geometry have been revealed. The coating cracking pattern, location of the fracture onset regions and the total crack length are found to depend on the front velocity in the steel substrate.

Bending Strength and Cutting Performance of WB-Coated-Diamond/Fe-Ni Composites

Diamond particle with tungsten boride (WB) coating was synthesized by the molten salt method. Three different diamond/Fe-Ni composites made from pristine diamond, B4C coated diamond and WB coated diamond with Fe-Ni powders were prepared by powder metallurgy. The composition and microstructure of the tungsten boride coating were investigated. Both bending strength and cutting performance of the composites were investigated. Addition of the WB coating provided an increased bending strength (871.2 MPa) and relative density (93.54%), compared with the composites consist of uncoated diamond and Fe-Ni (746.8 MPa, 92.81%). Three different Fe-Ni-based impregnated diamond drill bits contained 20 vol.% pristine diamond, B4C coated diamond and WB coated diamond were manufactured by powder metallurgy, respectively. Drilling rate of bits was measured by XY-4 geological core drill on granite. The test results show that the drilling rate of bits with WB coated diamond (2.42 m/h) was 40% higher than that with pristine uncoated diamond (1.72 m/h).

Study of structure and properties of steel 38KhN3МFА after low-temperature liquid borating

The effect of low-temperature liquid borating on the structure and hardness of steel 38KhN3МFА is studied. It is found that in the borating process at temperature 600...660 °C and duration 8...32 hours boride coating with thickness of 6...19 μm with surface hardness of 1900...2000 HV is formed on the steel surface. The optimal borating regimes are determined, in which hardened layer with solid core is formed. The presence of two boride phases FeB and Fe2B in the boride layer is established by metallographic, X-ray and electron microscopic analyzes.

ON THE PROBLEM OF STRAIN LOCALIZATION AND FRACTURE SITE PREDICTION IN MATERIALS WITH IRREGULAR GEOMETRY OF INTERFACES

The interfacial mechanisms of the stress-strain localization in non-homogeneous media are investigated, using a steel substrate - iron boride coating composition subjected to tension as an example. A dynamic boundary-value problem in a plane-strain formulation is solved numerically by the finite-difference method. The curvilinear substrate-coating interface geometry is assigned explicitly in calculations and is in agreement with experiment. Constitutive relations accounting for an elastic-plastic response of the isotropically-hardened substrate and for a brittle fracture of the coating are employed. Three stages of the plastic strain localization in the steel substrate are found to occur due to the irregular interface geometry. Distributions of the stress concentration regions in the coating are shown to be different at different stages. The stress concentration in the coating is demonstrated to increase nonlinearly during the third stage. The location of fracture is found to depend on the strength of the coating.