Electroplated Ni-P films with a range of P contents were prepared an electroplating method based on a Watt type bath. Vicker's hardness of each specimen was measured after annealing at 200-800°C. The hardening mechanisms for the Ni-5wt%P and Ni-11wt%P films were studied in detail by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Hardness of the Ni-5wt%P film increased gradually at annealing temperatures up to 400°C and then decreased at higher annealing temperatures. The hardness of the Ni-11wt%P films, on the other hand, showed no increase at annealing temperatures up to 300°C, and a sudden increase at 400°C, with no further change at higher annealing temperatures. The mechanisms of hardening were different for the two films. As-plated Ni-11wt%P film had an amorphous structure, while hardened specimens exhibited the two phase structure of Ni3P-Ni with an Ni3P matrix, and its hardening is attributed to the formation of Ni3P. As-plated 5wt%P film had an f. c. c structure, and the specimens annealed at 400°C showed little structural change except for a gradual increase in the lattice parameter as annealing time increased, indicating that oversaturated P is excluded from the Ni lattice. Given that no Ni3P precipitates were observed, it was concluded that segregation of P to the grain boundaries was the mechanism of hardening. The softening of this film at 800°C was considered to be due to the formation of an Ni-rich structure containing the Ni3P phase.
When carbon steels had been were immersed in molten borax baths containing both Fe-V (or Fe-Nb) and metallic Cr at 850-1050°C, two kinds of carbides were observed in the layer formed on the steel surface. One carbide was VC (or NbC) crystal structure containing small amount of Cr element, and the other was M7C3 containing some V (or Nb) and Fe elements. The M7C3 phase increased with the addition ratio of Cr to V (or Nb) in the baths, and was generally formed in the layer contacting the substrate. The activation energy for the growth of the complex carbide layer was smaller than that of VC (or NbC) layer formed in baths containing V (or Nb), and larger than that of Cr7C3+Cr23C6 layer formed in baths containing Cr. The complex carbide layer showed as high hardness and excellent wear resistance as VC layer, and as good oxidation resistance as Cr carbide layer.
Tin-coated phosphor-bronze with good adhesion as deposited sometimes shows an almost complete lack of adhesion at the interface of the tin layer and the phosphor bronze substrate after aging at 100-200°C. In this paper, this adhesion failure is called“thermal peeling”, since peeling accompanies metallurgical changes caused by such thermal processes as diffusion and segregation. Main findings are summarized as follows. 1. Tin-coated phosphor-bronze with no copper undercoat is much less sensitive to thermal peeling than that with a thin (about 0.5μm) copper undercoat. 2. Plastic bending prior to aging increases thermal peeling: thermal peeling appears after aging for 4000 hours at 85°C even for tin-coated phosphor-bronze with no copper undercoat. 3.ε-Cu3Sn and η-Cu6Sn5 are formed between the tin layer and the surface of the phosphor-bronze. 4. Accumulations of phosphorus and the formation of Kirkendall voids having concentrated amounts of phosphorus and oxygen are found along the ε-Cu3Sn/phosphor-bronze interface. 5. It is suggested that surface segregation of phosphorus accelerates the formation of the Kirkendall voids, which in turn causes the thermal peeling.
To clarify the mechanism of thermal peeling, that is to say the thermally accelerated loss of adhesion of coated films, the effects of alloying elements and copper undercoat on the propensity to thermal peeling were examined using tin-coated copper and copper alloys. Main findings are summarized as follows. The presence of tin in the phosphor-bronze promotes thermal peeling when the tin layer has a copper undercoat 0.5μm thick or less, but the aging time necessary for thermal peeling to occur does not depend on the concentration of tin. On the other hand, the propensity to thermal peeling increases with higher phosphorus concentration in the phosphor-bronze regardless of whether there is a copper undercoat or not. Alloying phosphor-bronze with 0.1∼0.3wt% of Zn, Mn and/or Fe increased the resistance to thermal peeling. Within the range of conditions covered by these detailed tests, no thermal peeling from phosphor-bronze takes place when there is a reflowed tin-plated film, provided that either the thickness of the copper undercoat is greater than 3μm, or that without copper undercoat phosphorus concentration is less than 0.3wt%.
Zn-Cr and Zn-Fe-Cr alloys were electrodeposited from sulfate baths. It was found that the Zn-Cr deposits contained extremely small amounts of Cr(III) compound. In Zn-Fe-Cr system, on the other hand, the Cr content in the deposits was considerably higher and the partial current density of Cr increased linearly with that of Fe. Cr(III) compound was present near the surface of the deposits and metallic Cr was co-present with Cr(III) compound in the bulk of the deposits. The increase in the concentration of Cr3+ in the baths brought about an increase in the transition current density or critical current density at which massive Zn deposition began to occur, indicating that Cr3+ behaved as a buffering agent during the deposition of both alloys.
Underpotential deposition of Pb from Pb(ClO4)2 aqueous solutions onto Ag substrates has successfully been detected by photoacoustic (PA) technique. PA measurements were carried out on specimens of single crystalline silver, and the electrolyte used for underpotential deposition was an aqueous solution of NaClO4+Pb(ClO4)2+HClO4. The current-potential curve obtained showed clear peaks of Pb underpotential deposition for each crystal face ((100) or (111)), while the PA signal-potential curve showed a slight increase at the potential of underpotential deposition. This increase is thought to be caused by underpotential deposition of Pb. The slow transformation phenomena of Pb adsorbates on Ag (111) electrode surfaces has been previously reported by T. Vitanov and coworkers, and the present work has succeeded in detecting this phenomenon with respect to the (111) crystal face as a slight fluctuation in the PA signal.