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Ni-coated ti powders

Ni-coated ti powders description/claims

The present invention relates to coated powder, comprising a Ti-bearing core and a Ni-bearing coating, which can be used for the production of porous Ni—Ti alloys.

Such a porous Ni—Ti alloy article is described in U.S. Pat. No. 5,986,169. The article has a porosity of 8 to 90% and is defined by a network of interconnected passageways extending throughout. The network exhibits an isotropic permeability permitting complete migration of fluids. The material is elastically deformable. These characteristics render it useful in biomedical and other applications. For producing the porous article, the so-called self-propagating high-temperature synthesis (SHS) method is used in which the alloy is produced by means of a layered combustion which exploits exothermic heat emitted during interaction of elemental nickel and titanium powders.

U.S. Pat. No. 2,853,403 describes a method for producing composite metal powders. This method consists of dispersing, in solid form, particles of one or more metals of interest as nuclei in an ammoniated solution in which another metal of interest having different chemical and/or physical properties is present as a soluble salt, and precipitating the latter metal from the solution by gas reduction to form composite metal particles in which the dispersed metal particles are coated by the precipitated metal. This method was however never specifically applied for the manufacture of Ni-coated Ti powder.

The use of elemental Ni and Ti powders renders the production process sensitive to segregation problems, resulting in composition fluctuations and/or non-homogeneous porosity. Composition fluctuations can in turn lead to the formation of unwanted secondary phases.

According to the invention, the above drawbacks can be overcome using coated powder, comprising a metallic Ti-bearing core and a metallic Ni-bearing coating, characterised by a Ni:Ti atomic ratio of more than 0.5, preferably between 0.9 and 1.1, and more preferably between 0.96 and 1.04. Atomic ratios of more than 0.5 to about 1 are preferred as this avoids the formation of secondary phases and yields an alloy with better mechanical properties.

The Ti-bearing core may consist of pure Ti, while the Ni-bearing coating can consist of pure Ni.

It may be useful to mix Ti-bearing powder or Ni-bearing powder with the coated powder so as to obtain a Ni:Ti atomic ratio of the mixture of between 0.9 and 1.1, and preferably between 0.99 and 1.01. This procedure allows for easy adjustments to the Ni:Ti ratio.

The particle size of the powders should preferably be finer than 150 mesh.

Above described powders can be used for the manufacture of sintered objects, possibly using the SHS technique.

Another object of the invention concerns a process of manufacturing a coated powder comprising the steps of: providing for suitable quantities of a Ti-bearing powder and of a Ni salt bearing aqueous solution; feeding said powder and said solution in an autoclave together with a quantity of NH4OH, and, optionally, with a quantity of ammonium salts; precipitating the Ni onto the Ti-bearing powder by hydrogen reduction; washing, filtering and drying the slurry obtained, thereby obtaining a Ni-coated Ti powder.

The Ni is preferably precipitated onto the Ti-bearing powder at a temperature of at least 100° C. and a hydrogen pressure in the autoclave of at least 1.4 MPa.

Powders obtained by this coating process are, as such or after mixing with Ni-bearing or Ti-bearing powders, suitable for SHS sintering of objects.

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