The present paper describes the synthesis, characterization, and grain growth behavior of nanocrystalline Ni coatings generated using a novel synthesis approach, namely high velocity oxy‐fuel (HVOF) thermal spraying. In the present investigation, the feedstock powders were prepared by mechanical milling in a methanol environment which yielded agglomerates with a flake‐shaped geometry and an average grain size of less than 100 nm. The milled powders were then introduced into the HVOF spray system in order to investigate the feasibility of generating a coating with grain sizes in the nanocrystalline range (e.g., <100 nm). Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the milled powders and the as‐sprayed coatings. Transmission electron microscopy analysis performed on cross sections of the coating revealed a mixture of fine nanocrystalline grains and elongated coarse grains.
The effects of Cu as pad material and of the metallization of pad (with Sn) and component (with Ni) on the evolving microstructure of lead-free solder joints were studied. A solder paste with composition 95.5wt.%Sn-4.0wt.%Ag-0.5wt.%Cu was used. Partial dissolution of the Cu substrate led to a change in the overall composition of the solder, which caused a precipitation morphology different from the one expected regarding the initial composition. Kinetics of growth of the Cu 6 Sn 5 phase, as particles in the bulk of the solder and as a reaction layer adjacent to the Cu pad, was studied in the temperature range 125-175°C.
PurposeThe purpose of this paper is to investigate the microstructural development of SnAgCu solder joints under different loading conditions (isothermal storage, thermal cycling and vibration).Design/methodology/approachThe observed microstructural changes have been studied with respect to grain growth and grain refinement, crack formation and crack growth. The growth kinetics of the intermetallic phases encountered as particles in the bulk as well as a reaction layer on the copper pad, were studied in the temperature range of 125‐175°C.FindingsDynamic recrystallisation of the tin matrix leads to a change in the diffusion controlled growth mechanism, which causes an increase of the particle growth rate compared to isothermal storage. Thus, these grain boundaries are separated forcibly by crack growth during thermal cycling. This stress causes intergranular cracks while vibration stress induces transgranular cracks.Originality/valueThe paper adds insight into microstructural changes of lead‐free solder joints during long‐term ageing, thermal cycling and vibration fatigue.
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