Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which:
In one preferred embodiment of a method for forming a Ni-based layered structure on a Mg alloy substrate according to this invention, the method includes the steps of:
(a) forming a transition layer on the Mg alloy substrate, the transition layer containing nickel crystals and crystals of an M-metal selected from the group consisting of Zn, Co, Cd, and alloys thereof;
(b) forming a first Ni-based layer on the transition layer; and
(c) thermal treating the assembly of the Mg alloy substrate, the transition layer and the first Ni-based layer at a temperature sufficient to permit formation of a liquid phase (i.e., a melt) of Mg and the M-metal at an interface between the transition layer and the Mg alloy substrate, followed by cooling the melt so as to form a boundary layer of a solid solution of Mg and the M-metal at the interface.
Referring to
Referring to
Preferably, the Mg alloy substrate 1 is cleaned prior to the formation of the transition layer 3 on the Mg alloy substrate 1 in such a manner to expose a texture of a hexagonal closed-packed (HCP) crystal structure on an outer surface 13 of the solid solution 11 of the magnesium alloy substrate 1.
More preferably, the cleaning of the magnesium alloy substrate 1 is conducted by applying a cleaning solution to the Mg alloy substrate 1, and the cleaning solution contains an organic acid, an anionic surfactant, and a polar organic solvent. The cleaning solution reacts with the inter-metallic compounds present in the grain boundaries 12 so as to form into residues 2. Most preferably, the cleaning of the Mg alloy substrate 1 further includes a washing step using a washing solvent to remove the residues 2 from the Mg alloy substrate 1 so as to form the recesses 14 in the Mg alloy substrate 1 and so as to form a substantially residue-free surface 15 of the Mg alloy substrate 1.
Non-limiting examples of the Mg alloy substrate 1 suitable to be treated with the method according to this invention include those made from the stabilized solid solutions 11 of Mg and a metal selected from the group consisting of Al, Zn, Zr, Li, Th, manganese (Mn), and alloys thereof. Commercially available examples of the Mg alloy substrate 1 include but are not limited to AZ31B, AZ61A, ZK60A, LA141A, HM21A, HK31A, and EZ33A. In one preferred embodiment, Mg content in the Mg alloy substrate 1 reaches 83 wt % or more.
The organic acid of the cleaning solution is used for dissolving the inter-metallic compounds present in the grain boundaries 12. Non-limiting examples of the organic acid of the cleaning solution are those selected from the group consisting of lactic acid, acetic acid, oxalic acid, succinic acid, adipic acid, citric acid, malic acid, and combinations thereof. Preferably, the organic acid is lactic acid, and the residues 2 thus formed contain magnesium lactate and lactate of the metal that forms the solid solution 11 with Mg.
The anionic surfactant is used for making hydrophobic molecules more hydrophilic. Non-limiting examples of the anionic surfactant are those selected from the group consisting of sodium lauryl sulfate, sodium iso-alkyl sulfate, sodium lauryl polyvinylether sulfate, sodium glycerol monolaurate sulfate, polyglycerol esters of interesterified ricinoleic acid sodium salt, sodium lauryl sulfonate, 1,2-alkyl phosphate, and combinations thereof. Preferably, the anionic surfactant is selected from the group consisting of sodium lauryl sulfonate, 1,2-alkyl phosphate, and combinations thereof.
In another preferred embodiment, the polar organic solvent contained in the cleaning solution serves to reduce the dissolution rate of the residues 2 dissolved by the organic acid. Consequently, the residues 2 can be retained in the grain boundaries 12 for a certain period of time prior to being washed out, thereby permitting controlling of the dissolution rate of the inter-metallic compounds and of the etching depth into the grain boundaries 12. In one preferred embodiment, the etching depth preferably ranges from 5 to 10 μm. Non-limiting examples of the polar organic solvent are those selected from the group consisting of methanol, ethanol, propanol, isopropanol, and combinations thereof.
In yet another preferred embodiment, the magnesium alloy substrate 1 is made from a solid solution of Mg and Al, and Mg17Al12 ultrafine crystals present in the grain boundaries of the solid solution of Mg and Al; the cleaning solution contains lactic acid, isopropanol, and anionic surfactant; and the residues 2 thus formed contain magnesium lactate and aluminum lactate.
In one preferred embodiment, the concentrations of the organic acid and the anionic surfactant in the cleaning solution range from 0.1 to 2 M and 0.001 to 0.01 M, respectively. More preferably, the concentrations of the organic acid and the anionic surfactant in the cleaning solution range from 0.4 to 0.7 M and 0.002 to 0.004 M, respectively. Most preferably, the concentrations of the organic acid and the anionic surfactant in the cleaning solution range from 0.5 to 0.6 M and 0.0025 to 0.0035 M, respectively.
In another preferred embodiment, the cleaning of the magnesium alloy substrate 1 is assisted by applying an ultrasonic frequency ranging from 300 to 360 KHz to the cleaning solution. The application of the ultrasonic frequency may be conducted by harmonic oscillation techniques at a frequency range selected from one of 300 to 360 kHz, 150-180 kHz and 20-45 kHz.
Alternatively, the cleaning of the magnesium alloy substrate 1 is conducted by applying a first cleaning solution containing the anionic surfactant and the polar organic solvent to the Mg alloy substrate 1 so as to remove hydrophobic molecules on the outer surface 13, and then applying a second cleaning solution containing the organic acid and the polar organic solvent so as to dissolve the inter-metallic compounds.
In one preferred embodiment, the washing solvent is selected from the group consisting of water and an alcohol having a carbon number less than 4. More preferably, the washing solvent is water. In another preferred embodiment, removal of the residues 2 is assisted by applying an ultrasonic frequency ranging from 300 to 360 KHz to the washing solvent. The application of the ultrasonic frequency may be conducted by harmonic oscillation techniques at a frequency range selected from one of 300 to 360 kHz, 150-180 kHz and 20-45 kHz.
In order to further strengthen the structural strength of the transition layer 3 during the thermal treating process, the M-metal 32 contained in the transition layer 3 has an atom radius similar to that of nickel atom. More preferably, the M-metal 32 is Zn.
The transition layer 3 functions as a catalyst layer for formation of the first Ni-based layer 4. Hence, a relatively thick transition layer 3 is not required. In one preferred embodiment, the transition layer 3 has a thickness ranging from 20-200 nm, more preferably, 30-100 nm, and most preferably, 40-60 nm.
In one preferred embodiment, the formation of the transition layer 3 is conducted by applying a transition layer composition to the Mg alloy substrate 1. The transition layer composition includes water, fluoride ions, ammonium ions, M-metal ions, and nickel ions.
In another preferred embodiment, when the M-metal ions are zinc ions, the transition layer composition is maintained at a temperature ranging from 0 to 85° C. and a pH value ranging from 0.1 to 2. The concentrations of the fluoride ions, ammonium ions, zinc ions, and nickel ions respectively range from 0.1-5 M, 0.1-5 M, 0.02-2 M, and 0.05-2 M. More preferably, the transition layer composition is maintained at a temperature ranging from 0 to 30° C. and a pH value ranging from 0.2 to 1.5, and the concentrations of the fluoride ions, ammonium ions, zinc ions, and nickel ions respectively range from 0.7-1.4 M, 0.5-0.9 M, 0.12-0.25 M, and 0.2-0.25 M. Most preferably, the transition layer composition is maintained at a temperature ranging from 20 to 25° C. and a pH value ranging from 0.5 to 1, and the concentrations of the fluoride ions, ammonium ions, zinc ions, and nickel ions respectively range from 0.9-1.2 M, 0.65-0.75 M, 0.16-0.2 M, and 0.22-0.24 M.
Referring to
Preferably, the formation of the first Ni-based layer 4 is controlled so as to partially fill the recesses 14 in the Mg alloy substrate 1. In another preferred embodiment, the first Ni-based layer 4 has a thickness ranging from 2-10 μm, more preferably, 3-8 μm, and most preferably, 4-6 μm.
In yet another preferred embodiment, the formation of the first Ni-based layer 4 is conducted through electroless plating techniques. In still another preferred embodiment, the first Ni-based layer 4 contains nickel and the M-metal 32 as major components and phosphorus (P) as a dopant.
In one preferred embodiment, the formation of the first Ni-based layer 4 is conducted by applying a first Ni-based layer composition to the transition layer 3. The first Ni-based layer composition includes water, fluoride ions, ammonium ions, M-metal ions, nickel ions, hypophosphite ions, and a buffer selected from C2-C8 organic acid ions. That is, the first Ni-based composition is prepared by adding hypophosphite ions and the buffer into the transition layer composition.
In another preferred embodiment, when the M-metal ions are zinc ions, the first Ni-based layer composition is maintained at a temperature ranging from 70 to 100° C. and a pH value ranging from 2 to 6.5. The concentrations of the fluoride ions, ammonium ions, zinc ions, nickel ions, hypophosphite ions, and C2-C8 organic acid ions respectively range from 0.1-5M, 0.1-5 M, 0.02-2 M, 0.02-2 M, 0.05-1 M, and 0.02-2 M. More preferably, the first Ni-based layer composition is maintained at a temperature ranging from 80 to 97° C. and a pH value ranging from 3 to 4.5, and the concentrations of the fluoride ions, ammonium ions, zinc ions, nickel ions, hypophosphite ions, and C2-C8 organic acid ions respectively range from 0.35-0.53 M, 0.35-0.53 M, 0.06-0.09 M, 0.127-0.155 M, 0.1-0.2 M, and 0.07-0.1 M. Most preferably, the first Ni-based layer composition is maintained at a temperature ranging from 90 to 95° C. and a pH value ranging from 3.5 to 4.0, and the concentrations of the fluoride ions, ammonium ions, zinc ions, nickel ions, hypophosphite ions, and C2-C8 organic acid ions respectively range from 0.4-0.5 M, 0.4-0.5 M, 0.07-0.08 M, 0.135-0.145 M, 0.14-0.16 M, and 0.08-0.09 M.
In yet another preferred embodiment, the thermal treating of the assembly of the Mg alloy substrate 1, the transition layer 3 and the first Ni-based layer 4 is conducted at a temperature ranging from 140° C. to 250° C. More preferably, the temperature ranges from 170° C. to 190° C. Most preferably, the thermal treating of the assembly of the Mg alloy substrate 1, the transition layer 3 and the first Ni-based layer 4 is conducted by heating the same to about 180° C. at a heating rate of about 150° C./hr, maintaining this temperature for 60 minutes, and then maintaining at a temperature of about 170° C. to 190° C. for 60 minutes, followed by cooling at a cooling rate of about −150° C./hr to room temperature.
Referring to
In another preferred embodiment, the concentration ratio of Ni to the M-metal 32 in the boundary layer 52 along the layer thickness of the boundary layer 52 is gradually increased from the interface between the boundary layer 52 and the Mg alloy substrate 1 to the interface between the boundary layer 52 and the first Ni-based layer 4. More preferably, for the purpose of intimate bonding of the boundary layer 52 to the Mg alloy substrate 1, the boundary layer 52 has a thickness not less than 20 nm.
In yet another preferred embodiment, the M-metal 32 contained in the boundary layer 52 is Zn, and the boundary layer 52 contains a solid solution of Ni51Zn21 which is disposed adjacent to the first Ni-based layer 4.
More preferably, the concentrations of the ions in the first Ni-based layer composition and the ions in the transition layer composition and the thermal treating temperature are suitably controlled in such a manner that the boundary layer 52 thus formed further includes ultrafine crystals of the M-metal 32 having a hcp crystal structure so as to avoid occurrence of dislocation defects.
In one preferred embodiment, when the M-metal 32 contained in the first Ni-based layer 4 is zinc, the first Ni-based layer 4 thus formed is an amorphous Ni—Zn alloy doped with P, and can be directly welded to other articles through soldering techniques. In another preferred embodiment, when the M-metal 32 contained in the first Ni-based layer 4 is cobalt, the first Ni-based layer is an amorphous Ni-cobalt (Co) alloy doped with P. The first Ni-based layer 4 thus formed has good hardness and low internal stress, in addition to corrosion resistance. Similarly, when the M-metal 32 contained in the first Ni-based layer 4 is Cd, the first Ni-based layer 4 is an amorphous Ni-Cd alloy doped with P. The first Ni-based layer 4 thus formed can also be directly welded to an object through soldering techniques.
Referring to
More preferably, the second Ni-based layer 5 contains Ni crystals having a face-centered cubic (FCC) structure, NiP alloy having a texture of a body-centered tetragonal (BCT) structure, amorphous Ni, and P doped in grain boundaries of the FCC and BCT structures and the amorphous Ni. More preferably, the formation of the first and second Ni-based layers 4, 5 is controlled in such a manner that the first and second Ni-based layers 4, 5 both extend into the recesses 14 in the Mg alloy substrate 1. Most preferably, the first Ni-based layer 4 has a surface formed with recesses 16, and the second Ni-based layer 5 extends into the recesses 16 in the surface of the first Ni-based layer 4.
In yet another preferred embodiment, the formation of the second Ni-based layer 5 is conducted by applying a second Ni-based layer composition to the first Ni-based layer 4.
In another preferred embodiment, the formation of the second Ni-based layer 5 on the first Ni-based layer 4 is conducted through electroless plating techniques.
More preferably, the second Ni-based layer composition includes water, fluoride ions, ammonium ions, nickel ions, hypophosphite ions, a chelating agent selected from the group consisting of diethylene amine, ethylene diamine, triethylene tetraamine and combinations thereof, and a buffer selected from C2-C8 organic acid ions. More preferably, the C2-C8 organic acid ions are citrate ions.
In one preferred embodiment, the second Ni-based layer composition is maintained at a temperature ranging from 70 to 100° C. and a pH value ranging from 2 to 6.5. The concentrations of the fluoride ions, ammonium ions, nickel ions, hypophosphite ions, the chelating agent and the buffer respectively range from 0.1-5 M, 0.1-5 M, 0.02-2 M, 0.05-1 M, 0.001-0.1 M, and 0.02-2 M. More preferably, the second Ni-based layer composition is maintained at a temperature ranging from 80 to 97° C. and a pH value ranging from 3 to 5, and the concentrations of the fluoride ions, ammonium ions, nickel ions, hypophosphite ions, the chelating agent and the buffer respectively range from 0.35-0.53 M, 0.35-0.53 M, 0.13-0.15 M, 0.1-0.2 M, 0.005-0.01 M, and 0.07-0.1 M. Most preferably, the second Ni-based layer composition is maintained at a temperature ranging from 90 to 95° C. and a pH value ranging from 3.2 to 4.0, and the concentrations of the fluoride ions, ammonium ions, nickel ions, hypophosphite ions, the chelating agent and the buffer respectively range from 0.4-0.5 M, 0.4-0.5 M, 0.135-0.145 M, 0.14-0.16 M, 0.006-0.008 M, and 0.08-0.09 M.
When the most preferred embodiment of the second Ni-based layer composition is applied, the second Ni-based layer 5 has a relatively high phosphorus content due to the relatively low pH value. The presence of phosphorus doped in the second Ni-based layer 5 will reduce the amount of hydrogen doped in the second Ni-based layer 5. Hence, undesired compressive stress resulting from release of hydrogen free radicals from the second Ni-based layer 5 during thermal treatment can be reduced. In addition, after the formation of the first Ni-based layer 4 through electroless plating techniques, numerous crystalline seeds are formed on the surface of the first Ni-based layer 4, which enhances activity of the surface of the first Ni-based layer 4, and density and strength of the second Ni-based layer 5. During the electroless plating process for forming the second Ni-based layer 5, electrons are released due to reaction of the hypophosphite ions and are attached to the surface of the first Ni-based layer 4, which imparts a negative charge on the surface of the first Ni-based layer 4. The cationic chelating agent, such as small molecular amines, chelate with nickel ions in the second Ni-based layer composition, which enhances the migration rate of the chelated Ni compound toward the negative charged surface of the first Ni-based layer 4. In addition, the high migration rate enhances the strength of an internal tensile stress in the second Ni-based layer 5.
Since the Mg alloy substrate 1 has a thermal expansion coefficient ranging from 25 to 30 μm/(m*° C.), and since the second Ni-based layer 5 has a thermal expansion coefficient ranging from 15 to 15 μm/(m*° C.) peeling of the second Ni-based layer 5 from the Mg alloy substrate 1 can occur. However, the relatively high internal tensile stress in the second Ni-based layer 5 is advantageous in preventing the peeling from occurring.
In another preferred embodiment, for the purpose of enhancing the brightness, corrosion resistance and hardness of the surface-treated Mg alloy substrate 1, a third Ni-based layer is formed on the second Ni-based layer 5 through one of electroplating, electroless plating, brush coating, and powder coating techniques. More preferably, the third Ni-based layer contains Ni crystals having a texture of a FCC structure.
In yet another preferred embodiment, the formation of the third Ni-based layer on the second Ni-based layer 5 is conduced by applying a third Ni-based layer composition to the second Ni-based layer 5. The third Ni-based layer composition includes fluoride ions, ammonium ions, nickel ions, and a buffer selected from C2-C8 organic acid ions. More preferably, the buffer is citrate ions.
In another preferred embodiment, the third Ni-based layer composition is maintained at a temperature ranging from 25 to 70° C. and a pH value ranging from 0.5 to 5.0. The concentrations of the fluoride ions, ammonium ions, nickel ions, and the C2-C8 organic acid ions respectively range from 0.1-5 M, 0.1-5 M, 0.1-2 M, and 0.02-2 M. More preferably, the third Ni-based layer composition is maintained at a temperature ranging from 40 to 60° C. and a pH value ranging from 1.5 to 3, and the concentrations of the fluoride ions, ammonium ions, nickel ions, and the C2-C8 organic acid ions respectively range from 1.75-2.1 M, 1.75-2.1 M, 1-1.3 M, and 0.48-0.72 M. Most preferably, the third Ni-based layer composition is maintained at a temperature ranging from 45 to 55° C. and a pH value ranging from 2 to 3, and the concentrations of the fluoride ions, ammonium ions, nickel ions, and the C2-C8 organic acid ions respectively range from 1.8-2 M, 1.8-2 M, 1.1-1.2 M, and 0.56-0.64 M.
In another preferred embodiment, the third nickel-based layer is formed through electroplating techniques under a current density ranging from 1 to 10 A/dm2. More preferably, the current density ranges from 2 to 3 A/dm2.
In one preferred embodiment, a surface treatment solution according to this invention includes water, fluoride ions, ammonium ions, and nickel ions. Use of the fluoride ions as conductive anions is advantageous in preventing corrosion of the Mg alloy substrate 1. In addition, the fluoride ions have a relatively small ion radius, and relatively high negative electricity and conductivity. The surface treatment solution is suitable for preparing a solution of the transition layer composition, and the first, second and third Ni-based layer compositions. In one preferred embodiment, when the surface treatment solution further contains the M-metal ions selected from the group consisting of zinc ions, cobalt ions, and cadmium ions, the solution thus made is suitable for the solution of the transition layer composition. In another preferred embodiment, when the surface treatment solution further contains hypophosphite ions, and a buffer selected from C2-C8 organic acid ions and the M-metal ions as defined above, the solution thus made is suitable for the solution of the first Ni-based layer composition. In yet another preferred embodiment, when the surface treatment solution further contains hypophosphite ions, a buffer selected from C2-C8 organic acid ions as defined above, the M-metal ions as defined above, and the chelating agent as defined above, the solution thus made is suitable for the solution of the second Ni-based layer composition.
More preferably, the surface treatment solution includes a sulfur-free brightening agent, such as 1,4-butynediol and coumarin, for inhibiting corrosion attributed to sulfur. In addition, the surface treatment solution contains ammonium ions as the chelating agent of the nickel ions so as to enhance the solubility of the nickel fluoride in the surface treatment solution.
The pores in the Mg alloy substrate 1 can be exposed during the cleaning operation of the Mg alloy substrate 1. In one preferred embodiment, the Mg alloy substrate 1 may be chemically polished prior to the formation of the transition layer 3 More preferably, after the chemical polishing operation of the Mg alloy substrate 1, the cleaning operation of the Mg alloy substrate 1 is conducted once again. In another preferred embodiment, the chemical polishing of the Mg alloy substrate 1 is conducted by applying an acidic solution to the magnesium alloy substrate 1. The acidic solution contains fluoride ions, ammonium ions, and nitrate ions. More preferably, the concentrations of the fluoride ions, ammonium ions, and nitrate ions in the acidic solution range from 50-70 cc/L, 30-50 g/L, and 30-50 g/L, respectively. The fluoride ions may be provided by a fluoride source selected from the group consisting of fluoric acid, ammonium fluoride, sodium fluoride, potassium fluoride, and mixtures thereof. Nitrate ions may be provided by a nitrate source selected from the group consisting of nitric acid, ammonium nitrate, sodium nitrate, potassium nitrate, and mixtures thereof. Ammonium ions may be provided by an ammonium source selected from the group consisting of ammonium fluoride, ammonium nitrate, and mixtures thereof. More preferably, the chemical polishing operation of the magnesium alloy substrate 1 is assisted by applying an ultrasonic frequency ranging from 300 to 360 KHz to the cleaning solution. Preferably, the application of the ultrasonic frequency is conducted by harmonic oscillation techniques at a frequency range selected from one of 300 to 360 kHz, 150-180 KHz and 20-45 kHz.
In addition, according to the preferred embodiment of this invention, all the compositions, including the cleaning composition, the chemical polishing composition, the transition layer composition, the first Ni-based layer composition, the second Ni-based layer composition, and the third nickel-based layer composition, used in the preferred embodiment of the method according to this invention include fluoride ions and have similar basic formulations. In the method of this invention, only one washing step is required for the removal of the residues 2. However, numerous washing steps are required by the conventional electroless plating or electroplating process. Hence, the adverse effect on bonding of the magnesium alloy substrate 1 to other articles attributed to the washing steps can be avoided.
Non-limiting examples of the fluoride source for providing fluoride ions in the above compositions according to this invention include fluoric acid, ammonium fluoride, sodium fluoride, potassium fluoride, zinc fluoride, and nickel fluoride, Non-limiting examples of the ammonium source for providing ammonium ions in the above compositions include ammonium fluoride and ammonium hypophosphite. Non-limiting examples of the zinc source for providing the zinc ions in the above compositions include zinc carbonate, zinc hydroxide, zinc fluoride, and zinc hypophosphite. Non-limiting examples of the nickel source for providing the nickel ions in the above compositions include nickel hydroxide, nickel fluoride, nickel citrate arid nickel hypophosphite. Non-limiting examples of the hypophosphite source for providing the hypophosphite ions in the above compositions include hypophosphorous acid, sodium potassium hypophosphite, potassium hypophosphite, and ammonium hypophosphite. Non-limiting examples of the C2-C8 organic acid source for providing C2-C8 organic acid ions include oxalic acid, succinic acid, malic acid, adiapic acid and lactic acid.
It is noted that the source of respective ions is determined according to the effect to which the respective composition is desired to provide. For example, presence of hypophosphite ions, which tend to result in crack down of the electroplating cell, is to be avoided in the transition layer composition Hence, presence of zinc hypophosphite or nickel hypophosphite should be avoided in the transition layer composition. In addition, presence of the M-metal ions such as zinc ions, is to be avoided in the second and third nickel-based layer compositions, and thus, presence of zinc fluoride should be avoided in these compositions.
With respect to the application of the oscillation frequency to the above compositions, it can be conducted through any method known in the art, e.g., applying ultrasounds to a container receiving the above compositions, placing a sonicating probe into the container, or placing the container in an ultrasonator.
Seven LA141A-T7 alloy substrates (made in USA) were respectively designated as Specimens 1 to 7 and surface treated by the method for forming a nickel-based layered structure on a magnesium alloy substrate according to this invention as follows:
According to analysis of X-ray diffraction, before thermal treatment of the above step (7), each of the specimens 1 to 7 has a zinc to nickel ratio of 10:1 at the interface between the boundary layer and the first nickel-based layer and of 1:9 at the interface between the first and second nickel-based layers, while no absorption peak of specific crystal structure was observed at these two layers since the crystals present in the boundary layer are ultrafine crystals. Both the first and second nickel-based layers contain face-centered cubic nickel, amorphous nickel, and the doped phosphorus present at grain boundaries of face-centered cubic nickel and in the amorphous nickel; while the third nickel-based layer contains face-centered cubic nickel.
After thermal treatment according to the above step (7), a liquid phase of magnesium and zinc was formed at the interface between the transition layer and the respective specimen, and zinc present in the transition layer permeated into the specimen. Consequently, the boundary layer formed after thermal treatment contains a solid solution of magnesium and zinc having a texture of HCP crystal structure, HCP zinc ultrafine crystals, and at least one inter-metallic compound composed of at least two of zinc, nickel and phosphorus. In particular, HCP Zn9Ni1 was observed at a bottom portion of the boundary layer adjacent to the respective specimen, and δ phase HCP Zn5Ni21 was observed at a top portion of the boundary layer adjacent to the first nickel-based layer. Such a phenomenon is so called “Martensitic transformation” behavior, which is favorable to bonding of the coating to the respective specimen.
In addition, after thermal treatment according to the above step (7), the first nickel-based layer has a phosphorus doped amorphous structure containing nickel and zinc; and the second nickel-based layer contains fcc nickel, a bat alloy of nickel and phosphorus, amorphous nickel, and phosphorus doped in the amorphous nickel and in grain boundaries of fcc nickel and the bct alloy of nickel and phosphorus.
The specimens 1 to 7 obtained after surface treatment according to the method of this invention were subjected to the following tests: ASTM D3359, CNS 7094 Z8017, internal stress test, and ASTM B368-61T.
Each of the specimens 1 to 7 was forced to bend at an angle of 90 degrees. The adhesion strength of the coating on the respective specimen was tested according to ASTM D3359. The results of the test are shown in Table 1. No peeling or detachment of the coating was found for each specimen during the test. Hence, the coating thus formed on each specimen has an excellent bonding strength on the respective specimen.
A diamond probe was pressed into the coating on each specimen under a load of 100 g for hardness measurement. The results are expressed in the unit “Hv” and are shown in Table 1.
Measurement of the internal stress of the coating on each of the specimen was conducted by allowing the coatings to deform solely by the internal stress, followed by applying a force (in a unit of kgf/mm2) that is sufficient to recover the initial shape thereof. A positive value for the applied force is an indication of having a tensile stress, whereas a negative value for the applied force is an indication of having a compressive stress. Results of the internal stress test of each of the specimens 1 to 7 are shown in Table 1, and show that the coating on each specimen exhibits a tensile stress, which can diminish the peeling problem of the coating during thermal expansion and contraction process of the specimens 1 to 7.
The specimens 1 to 7 surface-treated according to the method of this invention were subjected to the corrosion resistance test according to ASTM B368-61T, Results obtained are classified into 10 levels according to Durbin's standard, The higher the level is, the higher will be the corrosion resistance, and the lower will be the porosity of the coating on each specimen. Results of the corrosion resistance test are shown in Table 1. Most of the surface-treated specimens 1 to 7 have corrosion resistance of level 10, indicating that most of the specimens 1 to 7 are endurable to at least 160 hours during the corrosion resistance test.
Ten LA141A-T7 alloy substrates (made in USA) were respectively designated as Specimens 8 to 17 and were surface treated by the method similar to that of Example 1, except that the third nickel-based layer was formed in hull cell, wherein the high current area has a current density of 5 A/dm2; while the low current area has a current density of 1 A/dm2.
Thickness and appearance of the coating formed on each specimen at the high and low current density areas were determined. The thickness of the coating formed on each specimen was evaluated by using a thickness clamp (available from INOX company, Germany), and appearance of the coating formed on each specimen was evaluated by naked eye. Results of the thickness and appearance of the coating on each specimen are shown in Table 2.
The results shown in Table 2 indicate that the coating on each of the specimens 8 to 17 exhibits bright metal gloss and achieves the required decorating property within a thickness ranging from 20 to 40 μm. In addition, ratio of the coating formed in the high current area to the coating formed in the low current area in layer thickness is relatively small and ranges from 1.4 to 2.2. It indicates that the fluoride ions in the third nickel-based layer composition have excellent conductivity and thus diminish the difference in thickness between the coating formed in the high current area and the coating formed in the low current area.
Specimens of Examples 3 to 8 were prepared. The specification of the specimens is shown in the following Table 3. The specimens were surface treated in a manner similar to that of Example 1. The surface-treated specimens were subjected to the bending-adhesion test and the corrosion test in a manner similar to that of Example 1, and the thickness of the coating formed on the specimen of the respective Examples 3 to 8 was determined. Results of the tests and the thickness measurement are shown in Table 3.
According to the results shown in Table 3, even if the magnesium alloy substrates have different specifications, the coating including the boundary layer, the first nickel-based layer, the second nickel-based layer and the third nickel-based layer formed on the magnesium alloy substrates according to the method of this invention has a relatively large thickness, as high as 40 μm, and a good adhesion strength to the respective magnesium alloy substrate (i.e., no peeling was found). Therefore, the coating formed on the respective magnesium alloy substrate exhibits excellent corrosion resistance and is able to reach level 10 in the corrosion resistance test.
In view of the foregoing, by forming a boundary layer having a crystal structure similar to a magnesium alloy substrate on the magnesium alloy substrate, other functional layers, such as the first, second and third Ni-based layers, can be firmly formed on the magnesium alloy substrate through the boundary layer so as to improve corrosion resistance of the magnesium alloy substrate.
While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.
Number | Date | Country | Kind |
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095117849 | May 2006 | TW | national |