Preparation of ceramic-metal coatings

A metal substrate, e.g. titanium, having a calcium phosphate coating, particularly hydroxylapatite, and containing a metal such as cobalt, codeposited on the substrate by electrolyzing a cobalt salt, particularly cobalt sulfate, liquid electrolyte having a calcium phosphate material, particularly hydroxylapatite, suspended therein, employing a cobalt anode and the metal substrate as cathode. The particles of cobalt so codeposited with the particles of calcium phosphate material, e.g. hydroxylapatite, hold the latter particles strongly on the substrate metal. If desired, a second coating of the pure calcium phosphate material, e.g. "hydroxylapatite", optionally can be applied over the codeposited hydroxylapatite-cobalt coating. The calcium phosphate coated metal substrate of the invention, particularly the codeposited "hydroxylapatite"-cobalt coating, on a titanium or cobalt-chromium substrate, has particular value for application as medical implants, e.g. as hip prosthetics, and for high temperature high stressed applications.

BACKGROUND OF THE INVENTION 
This invention relates to metal substrates containing a ceramic coating, 
and is particularly directed to metal substrates such as titanium, 
containing an electrodeposited coating of a ceramic material, especially 
hydroxylapatite or similar calcium phosphate material, and cobalt, and 
having particular applicability as medical surgical implants. 
The medical profession has been trying to develop a successful long term 
human implant for many years, and has yet to develop one that will last 
for 7 to 10 years in high stressed locations, e.g. hip prostheses and 
dental implants. The short service time before failure is of special 
concern in the case of younger patients as additional surgical operations 
are necessary. 
Numerous techniques have been tried but none have been proven successful 
due to the loss of structural bonding between the human tissues and the 
metal substrate (implant). Loosening of the implant in the surrounding 
bone, leads to pain, and requires surgical removal in replacement of the 
device. Also, adverse tissue reaction is a problem due to the exposed 
metallic surface, which in time will corrode and release metallic ions 
which cause damage to the surrounding tissue. 
Metal substrates such as titanium alloys have heretofore been coated with 
hydroxylapatite by various processes such as plasma spraying, ion sputter 
deposition and electrophoretic deposition in an attempt to obtain chemical 
bonding to the bone surrounding the implant, thus stabilizing the device. 
A primary drawback of all such ceramic coatings has been the lack of 
strength of the bond between the ceramic, e.g. hydroxylapatite, material 
and the metal substrate. Further, each of the coatings produced by the 
above processes has proven unsatisfactory for various reasons such as low 
bond strength, variation in density, lack of crystallinity, low durability 
and high cost. 
U.S. Pat. No. 3,945,893 to Ishimori et al discloses forming a low-abrasion 
surface on a metal base member by dispersing fine particles of a hard 
material in a metal plating solution, immersing in such solution a metal 
object to be coated as a cathode together with a metal rod as an anode, 
and passing current through the bath to codeposit a coating of the anode 
metal and fine particles of the hard material on the metal base member. 
According to a preferred embodiment, powdered silicon carbide is dispersed 
in a nickel plating solution such as a nickel sulfamate bath, and an 
aluminum alloy base member is immersed as a cathode in the bath and a 
nickel rod as an anode. A current is passed through the bath and a plated 
coating of nickel and silicon carbide is formed on the aluminum alloy base 
member. The metal deposit thus formed on the base metal surface is then 
subjected to polishing to remove a given thickness of the metal coating, 
leaving the hard fine particles partly exposed and protruded from the 
deposited layer. 
One object of the present invention is to provide both smooth and porous 
surfaced metal substrates with ceramic coatings, particularly a calcium 
phosphate coating, so that the resulting ceramic coated substrates have 
good bond strength and the coating is uniform and durable. 
Another object of the invention is the provision of metal substrates such 
as titanium and its alloys, containing a coating including hydroxylapatite 
as ceramic material, and a metal, and having characteristics suitable for 
use as medical implants, including hip prostheses and dental endosseous 
subperiosteal implants, permitting human tissues to attach and grow onto 
the coating and insure a strong structural bond between the human tissue 
and the ceramic coating, free from adverse tissue reaction, and such that 
the implant will be durable and have an indefinite life. 
Still another object is to provide ceramic coated substrates of the above 
type, wherein the ceramic, particularly hydroxylapatite, is co-deposited 
with a metal on the substrate, e.g. titanium substrate, to aid in holding 
the ceramic to the metal. 
Yet another object is the provision of a procedure for depositing a coating 
containing a calcium phosphate ceramic material, e.g. hydroxylapatite, on 
the substrate. 
A still further object is to provide a process for co-depositing the 
ceramic material, particularly hydroxylapatite, and a metal, particularly 
cobalt, on the metal substrate. 
SUMMARY OF THE INVENTION 
According to the invention, the above objects are achieved by providing a 
substrate such as titanium or an alloy thereof, containing an 
electrodeposited coating of a calcium phosphate material such as 
hydroxylapatite and a metal such as cobalt, resulting in a ceramic coated 
metal substrate having a strong interfacial bond between the substrate and 
the coating and having particularly advantageous characteristics for 
application as medical implants. 
The above ceramic coated substrate is provided by a process which includes 
forming a solution of a metal salt such as a cobalt salt, e.g. cobalt 
sulfate, containing particles of a calcium phosphate material such as 
hydroxylapatite. The solution containing suspended particles of such 
calcium phosphate material is electrolyzed, that is subjected to 
electrolysis, using a metal anode such as a cobalt anode and a cathode 
formed of a metal substrate material such as titanium or an alloy thereof. 
A coating is electrodeposited on the metal substrate in the form of a 
co-deposit of the calcium phosphate material, e.g. hydroxylapatite, and a 
metal such as cobalt. The co-deposited metal, e.g. cobalt, functions to 
secure the calcium photphate material to the substrate and to increase the 
bond strength therebetween. 
If desired, a second coating of the pure calcium phosphate material, e.g. 
hydroxylapatite, can be deposited over the interfacial electrodeposited 
co-deposit of calcium phosphate material and metal such as cobalt, to 
create an even thicker calcium phosphate or hydroxylapatite layer for 
increased biocompatibility. Such second coating can be applied by various 
processes such as by plasma spray deposition. The resulting coated 
substrate can then be sintered, if desired. 
In addition to medical implants, the invention concept has numerous 
applications for metals requiring ceramic surfaces, such as for high 
temperature/high stressed applications. More specifically, additional 
applications include the fields of friction reduction, conductivity 
reduction for thermal and electrical applications, metal surface 
protection from high temperature oxidation, and formation of thin ductile 
surface undercoatings.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
In the electrolysis process of the invention, the metal substrate to which 
the codeposit of calcium phosphate material and metal such as cobalt is to 
be applied and which functions as the cathode in the electrolysis cell, can 
be titanium or an alloy thereof, such as Ti-6Al-4V, stainless steel, e.g. 
316 stainless steel, or a cobalt-chromium alloy such as the 
cobalt-chromium alloy also including molybdenum, and marketed as Vitallium 
and Zimaloy. Both commercially pure (CP) titanium, or alloys thereof, and 
cobalt-chromium are approved FDA metals and alloys for medical implants. 
The cobalt-chromium substrate provides an even stronger bond with the 
calcium phosphate material where cobalt is employed as a codeposit with 
the calcium phosphate material according to the invention. 
In the electrolytic process of the invention, the anode can be a metal such 
as cobalt, nickel, chromium or rhodium, the preferred anode material being 
cobalt. 
The ceramic material which is electrolytically deposited with the metal, 
e.g. cobalt, on the metal substrate to form the codeposited interface 
bonding layer, is a calcium phosphate material. A preferred form of the 
calcium phosphate material is apatite, a natural calcium phosphate usually 
containing fluorine, sometimes with chlorine, hydroxyl or carbonate 
substituting for part or all of the fluorine. A particularly preferred 
apatite is hydroxylapatite, having the formula Ca.sub.10 (PO.sub.4).sub.6 
(OH).sub.2. Another suitable calcium phosphate is tricalcium phosphate, 
Ca.sub.3 (PO.sub.4).sub.2. The terms "ceramic" material and "calcium 
phosphate" material as employed herein are intended to denote any of the 
foregoing materials. 
The calcium phosphate material in particulate form, e.g. hydroxylapatite, 
is mixed in suspension in the electrolyte, e.g. cobalt sulfate, and in 
which the anode, e.g. cobalt, and the metal substrate, e.g. titanium, are 
suspended, the metal substrate functioning as cathode and spaced 
appropriately from the anode. The size of the calcium phosphate material 
particles can range from about 0.1 to about 100 microns, and such 
particles are maintained in suspension in the electrolyte by suitable 
stirring. The electrolyte is a solution of a salt of the anode metal, e.g. 
a cobalt salt such as cobalt sulfate, and contains the suspended calcium 
phosphate particles. Other cobalt salts, e.g. cobalt sulfamate or cobalt 
tetrafluoborate can be used where the anode is cobalt. Where the anode 
metal is nickel, chromium or rhodium, the liquid electrolyte can be, for 
example, a solution of nickel sulfate, nickel sulfamate, chromium sulfate 
or rhodium sulfate. 
Aqueous solutions of the anode metal salts, e.g. an aqueous solution of 
cobalt sulfate, can be used as electrolyte. However, solutions of the 
anode metal salt in organic media also can be employed as electrolyte, 
such as a solution of cobalt sulfate in methanol. The concentration of the 
anode metal salt in the electrolyte bath can vary, and generally ranges 
from about 200 to about 500 grams per liter. 
The concentration of suspended ceramic material, e.g. hydroxylapatite, in 
the electrolyte bath can range from about 5 to about 300 grams of ceramic 
material, per liter of electrolyte. 
The electrolysis is carried out at a voltage sufficient to obtain a current 
density ranging from about 5 to about 100 mA/cm.sup.2, depending on the 
deposition conditions, e.g. a voltage ranging from about 0.5 to about 20 
volts. The temperature of the electrolyte bath can be maintained within a 
temperature range of about 20.degree. to about 50.degree. C., and duration 
of the electrolysis operation is from about 30 to about 90 minutes, e.g. 
about 60 minutes. The electrolyte, e.g. cobalt sulfate bath, is maintained 
under acidic conditions at a pH of about 1 to about 5. For example for the 
codeposition of cobalt and hydroxylapatite, the pH of the cobalt sulfate 
electrolyte is maintained at about 3.8. Low speed stirring of the 
electrolyte suspension is maintained during electrolysis to prevent 
agglomeration and sedimentation of ceramic particles in the liquid 
electrolyte. 
As a typical example utilizing a cobalt anode and cobalt sulfate 
electrolyte, during electrolysis, cobalt particles are released from the 
cobalt anode, as positive ions, and such particles travel through the 
cobalt sulfate electrolyte, toward the metal substrate cathode, e.g. 
titanium or titanium alloy. The calcium phosphate material, e.g. 
hydroxylapatite, suspended in the electrolyte, and near the cathode 
surface codeposits with the cobalt on the surface of the metal substrate 
cathode. The particles of cobalt so codeposited with the particles of 
ceramic material, e.g. hydroxylapatite, hold the latter particles strongly 
on the substrate metal. The mixture of the calcium phosphate material, e.g. 
hydroxylapatite, and metal such as cobalt, electrolytically codeposited on 
the metal substrate cathode is generally about a 50--50 mixture by weight 
of the metal, e.g. cobalt, and the calcium phosphate particles, but this 
can vary, depending upon conditions of electrolysis and the concentration 
of the suspended calcium phosphate particles in the cobalt salt 
electrolyte. The thickness of the resulting codeposited calcium phosphate, 
e.g. hydroxylapatite, and metal such as cobalt coating can vary, for 
example, from about 5 to about 50 microns. The coating is highly adherent, 
dense and uniform. 
Although the electrodeposited codeposited, e.g. hydroxylapatite-metal 
coating on the metal, e.g. titanium, substrate is effective per se and has 
the physical characteristics rendering the resulting substrate containing 
the ceramic, particularly hydroxylapatite,-cobalt, deposit suitable for 
application as improved medical implants, particularly in hip prosthetics, 
an outer coating of the substantially pure calcium phosphate material, e.g. 
hydroxylapatite, can be applied optionally over the codeposited 
hydroxylapatite-cobalt coating, to provide a pure layer of hydroxylapatite 
on the surface, if desired. Such outer coating can be applied, for example, 
by plasma spraying, ion sputter deposition, or electrophoresis deposition. 
Employing plasma spraying, the ceramic, i.e., calcium phosphate, e.g. 
hydroxylapatite powder, is propelled through a high temperature arc 
discharge equal to or greater than 10,000.degree. C., forming an 
additional hydroxylapatite coating on the initially electrodeposited 
calcium phosphate-cobalt coating on impact. When employing ion sputter 
deposition, an ion beam is used to sputter off atoms from the calcium 
phosphate material, e.g. hydroxylapatite, "target" in vacuum, and the 
sputtered material slowly forms a coating on the initially 
electrodeposited hydroxylapatite-cobalt, codeposited coating on the 
substrate. The resulting pure, e.g. hydroxylapatite, overcoating is a 
dense porous coating. When employing electrophoretic deposition, the 
hydroxylapatite particles are charged to a potential of about 90 volts in 
isopropyl alcohol. The particles then drift toward the substrate and 
attach to the surface. 
The thickness of the overall coating of electrodeposited codeposit, e.g. of 
hydroxylapatite-cobalt, and the top coating of pure hydroxylapatite can 
range from about 20 to about 2000 microns. Where hydroxylapatite is 
employed as the calcium phosphate material, the total thickness preferably 
is not more than 100 microns, and can range from about 20 to about 100 
microns. The above plasma spraying, ion sputter and electrophoretic 
deposition techniques, as well as chemical vapor deposition, are known in 
the art, and hence further details thereof are not included herein. 
As a further optional step, if desired, the substrate containing the 
codeposited, e.g. hydroxylapatite-cobalt, first layer and the second layer 
coating of pure hydroxylapatite can be sintered by heating to harden the 
overall coating. Thus, for example a metal, e.g. titanium, substrate 
containing an electrodeposited hydroxylapatite-cobalt initial layer and a 
pure hydroxylapatite second layer coating can be sintered at 600.degree. 
C. or higher in a vacuum. 
The following examples 1 to 7 are examples of practice of the invention. 
The data and parameters listed below in each of examples 1 to 7 applies to 
an electrolytic apparatus or cell in the form of a cylindrical container 
containing a metal salt electrolyte, e.g. a cobalt sulfate, bath in the 
form of an aqueous solution containing 285 grams per liter of the metal 
salt, and having particles of hydroxylapatite suspended in the bath, in a 
concentration of about 10 grams of hydroxylapatite per liter of 
electrolyte, e.g. cobalt sulfate. The hydroxylapatite material is 
maintained in suspension in the electrolyte bath by a stirrer mounted in 
the bottom of the container. A metal, e.g. cobalt, tube serving as anode 
is mounted centrally in the container, immersed completely in the metal 
salt suspension of the hydroxylapatite particles. A metal substrate 
serving as cathode is mounted axially within the tubular metal, e.g. 
cobalt, anode and suspended in the electrolyte-hydroxylapatite particles 
suspension, the cathode being spaced from the anode. The metal anode and 
the metal substrate cathode are connected to a suitable power source. 
In each of the examples 1 to 7 below, following electrolysis, a strong 
durable coating of an electrodeposited codeposit of hydroxylapatite and 
metal, e.g. cobalt or other anode metal, is formed on the metal substrate 
cathode, having a thickness between 5 and 50 microns. 
______________________________________ 
EXAMPLE 1 
Substrate Material (cathode): 
Titanium 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 0.9 volts 
Cobalt sulfate bath: 
pH 3.8 
Duration of codeposition: 
60 minutes 
Temperature: 20.degree. C. 
Stirring: low speed 
Anode: Cobalt 
Distance between cathode/anode: 
2 cm 
EXAMPLE 2 
Substrate Material (cathode): 
Cobalt-Chromium-molybdenum 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 0.9 volts 
Cobalt sulfate bath: 
pH 3.8 
Duration of codeposition: 
55 minutes 
Temperature: 24.degree. C. 
Stirring: low speed 
Anode: Cobalt 
Distance between cathode/anode: 
2 cm 
EXAMPLE 3 
Substrate Material (cathode): 
316 Stainless Steel 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 0.85 volts 
Cobalt sulfate bath: 
pH 3.7 
Duration of codeposition: 
45 minutes 
Temperature: 24.degree. C. 
Stirring: low speed 
Anode: Cobalt 
Distance between cathode/anode: 
2 cm 
EXAMPLE 4 
Substrate Material (cathode): 
Titanium 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 0.9 volts 
Cobalt sulfamate bath: 
pH 3.8 
Duration of codeposition: 
60 min 
Temperature: 22.degree. C. 
Stirring: low speed 
Anode: Cobalt 
Distance between cathode/anode: 
2 cm 
EXAMPLE 5 
Substrate Material (cathode): 
Titanium 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 1.5 volts 
Nickel sulfate bath: 
pH 3.8 
Duration of codeposition: 
50 minutes 
Temperature: 21.degree. C. 
Stirring: low speed 
Anode: Nickel 
Distance between cathode/anode: 
1.2 cm 
EXAMPLE 6 
Substrate Material (cathode): 
316 Stainless Steel 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 0.85 volts 
Rhodium sulfate bath: 
pH 3.7 
Duration of codeposition: 
45 minutes 
Temperature: 24.degree. C. 
Stirring: low speed 
Anode: Rhodium 
Distance between cathode/anode: 
2 cm 
EXAMPLE 7 
Substrate Material (cathode): 
Titanium 
Coating Material: Hydroxylapatite 
Current Density: 10 mA/cm.sup.2 
Voltage: 0.9 volts 
Chromium sulfate bath: 
pH 3.8 
Duration of codeposition: 
60 minutes 
Temperature: 22.degree. C. 
Stirring: low speed 
Anode: Chromium 
Distance between cathode/anode: 
2 cm 
______________________________________ 
The following are further examples of practice of the invention. 
EXAMPLE 8 
The titanium substrate containing the electrodeposited 
hydroxylapatite-cobalt coating formed in Example 1 is subjected to plasma 
spraying using hydroxylapatite powder propelled through a high temperature 
arc discharge in excess of 10,000.degree. C. to form a dense and pure 
hydroxylapatite coating over the electrodeposited hydroxylapatite-cobalt 
coating. The overall thickness of the dual coating on the titanium 
substrate is between 20 and 100 microns. 
EXAMPLE 9 
The dual coated titanium substrate of Example 8 is subjected to sintering 
by heating to 600.degree. C. for 2 hours to provide a hardened coating and 
a strengthened structural bond between such coating and the titanium 
substrate. 
From the foregoing, it is seen that the invention provides a novel and 
improved combination of metal substrate, e.g. titanium, and calcium 
phosphate coating, particularly hydroxylapatite, containing certain 
metals, e.g. cobalt, codeposited by an electrolytic procedure that 
consists of electrolyzing a metal salt, particularly a cobalt salt such as 
cobalt sulfate, the electrolyte bath having calcium phosphate material, 
particularly hydroxylapatite, suspended therein, employing a metal, e.g. 
cobalt, anode and the metal substrate as cathode. There is no teaching in 
above U.S. Pat. No. 3,945,893 of the use of particles of a calcium 
phosphate, particularly hydroxylapatite, and electrolytically codepositing 
such phosphate together with a metal, particularly cobalt, on a metal 
substrate, e.g. titanium or a cobalt-chromium alloy, and obtaining a 
coated substrate having the particular advantages and properties noted 
below. 
The coated metal substrate of the invention, particularly the codeposited 
hydroxylapatite-cobalt coating, on a titanium or cobalt-chromium 
substrate, has particular value for application as medical implants, e.g. 
as hip prosthetics, dental implants, and artificial heart surfaces to 
minimize the chance of adverse tissue reactions and in some cases to 
provide chemical bonding to the surrounding bone. Such coated substrates 
also have important utility in high temperature/high stressed applications 
including temperature/high stressed applications including hyper velocity 
projectiles requiring ablating surfaces, reduced friction and 
electroconductivity, and including aerospace engine parts. Such coated 
substrates are also useful for computer hardware where metallic surfaces 
give off damaging electrical charges. 
When employed for medical implants the ceramic calcium phosphate coated 
substrates of the invention, particularly utilizing hydroxylapatite, 
provide the strong chemical bonding and in some cases sufficient porosity 
that will allow the human tissues to attach and grow onto the codeposited 
metal-calcium phosphate, particularly cobalt-hydroxylapatite, coating. 
This tissue attachment and/or ingrowth will insure a strong chemical 
and/or mechanical bond between the human body and such coating so that the 
implant will have an indefinite life. The metal-calcium phosphate, e.g. 
cobalt- hydroxylapatite, coating will also help to solve the problem of 
adverse tissue reaction due to the continuous layer of such coating over 
the metal substrate. 
The formation of codeposits on metal substrates according to the invention 
has surface preparation applications in numerous other areas. These 
include dry lubrication for surfaces subject to frictional loads such as 
dry bearings, prevention of high temperature oxidation in hostile 
environments using surface protective coatings, and providing reduced 
image distortion in reflective surfaces of mirrors using various ceramic 
coatings. 
Since various changes and modifications of the invention will occur to 
those skilled in the art within the spirit of the invention, the invention 
is not to be taken as limited except by the scope of the appended claims.