Note: Descriptions are shown in the official language in which they were submitted.
953S
TITLE: PRODUCTION OF POROUS COA~ING
ON A PROST~IESIS
Tec'mical Field
This disclosure relates to the coating of preselected metallic surface
areas on a prosthesis for subsequent tissue ingrowth applications. The
coating is in the form of a porous material having interconnected pores
through which hard or soft body tissues can grow.
Background Art
U. S. Patent No. 3,855,638, granted on December 24, 1914 to Robert
M. Pilliar discloses a surgical prosthetic device that consists of a solid
metallic material substrate with a porous coating over at least a part of its
surface. The coating has a thickness between one hundred to one
thousand microns. The coating is formed from metallic powder sized
between -100 to +325 mesh. The patent discloses production of the coating
by using a slurry of metallic powder suspended in an aqueous solution
with organic binders. The particle size of the metallic powder and
conditions of formation of the porous coating are controlled to provide the
desired interstitial pore size, porosity, strength and depth of coating.
Both the substrate and powder are sintered to achieve metallurgical
bonding between engaged metal particles and between the metal particles
and the substrate. The disclosure states it to be essential that the
porosity of the surface coating not exceed about 40~ and be at least about
10%. It states that at porosities above 4û% the overall mechanical ~strength
falls below the required level.
U. S. Patent No. 3,852,045, granted on December 3, 1974 to Kenneth
R. Wheeler, Kenneth R. Sump and M&nual T. Karagianes, discloses a
porous metallic mater~al with ir~terconnected voids, again directed to tissue
ingrowth purposes. Voids or pores are produced in the metallic material
by use of expendable void formers. The composite material is treated by
high energy rate forming pressures to densify its structure prior to
removal of the expendable void former. Substantial thicknesses of the
coating on substrate metallic elements are disclosed.
While the products resulting frorn the systems taught in U. S. Patent
3,852,045 have performed satisfactorily, the practical application of the
system is severely limited by both the expense and availability of
equipment for the required high energy rate forming steps. Fur$hermore,
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such steps are of questionnble value when atter~pting to produce a
relatively thin porous coating on implant elements, since the high
pressures to which the elements would be subjected might result in
structural damage to them.
According to the present invention, relatively thin porous met~llic
coatings are produced about selected surface area configurations on a
prosthesis by forming the coating about the surface, using a blended
mixture of primary and secondary particles. The primary particles are
either made from a material identical to or metallurgically compatible with
the metallic surface being coated. The secondary particles are made of
an expendable material. Both are heated and pressed in place about the
surface to effect metallurgical bonding between engaged primary particles
as well as bet~reer~ the surface area and primary particles in contact with
it. The expendable material is subsequently removed to achieve controlled
porosity throughout the coating.
Disclosure of Invention
The basic method of this invention comprises the steps of covering a
preselected metallic surface area of a prosthesis with a blended mixture of
primary and secondary particles, reducing the dimensional thickness of the
mixture by compressing it against the surface and simultaneously applying
heat to form metallurgical bonds between primary particles, and removing
the expendable secondary particles to present a homogeneously porous
coating on the metallic surface area.
The method of covering the metallic surface area can be accomplished
by first loading a mold by positioning the prosthesis with the preselected
surface area spaced inward from the mold cavity surfaces. The space
between the surface area and the mold cavity surfaces is then filled with a
blended mixture of primary particles identical to or metallurgically
compatible with the metallic surface area of the bone implant element and
secondary particles of expendable material in a packed volume containing
random shaped voids between the particles. These steps can be carried
out in a gravity mold or can be accomplished in the pressure molding
apparatus where subsequent compression of the coating occurs. The blend
3 5 of particles can be applied within a gravity or compression mold in a drycondition or can first be mixed wiih a liquid binder to assist in maintaining
it in a homogeneous condition. The blend of particles containing a binder
can also be applied by other suitable coating processes.
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If a time delay is to occur between the covering of the mctallic
surface and final bonding of the particles, the blend of particles can be
initially adhered to the surface by light sinter bonds between primary
particles or by curing or dr~ing of the binder, when used.
The particles are next heated, while under pressure, to an elevated
temperature at which metallurgical bonding occurs between them due to a
combination of heat, pressure, and mechanical deformation. The coating is
compressed against the prosthesis while at the elevated temperature to
reduce the dimensional thickness of the coating and to reduce the volume
percentage of random shaped voids remaining in the coating.
As a final step, the expendable material is removed from the coating,
which will then have a surface configuration complementary to the mold.
Controlled porosity throughout the coating results from a combination of
the inters~itial spaces between the primary and secondary particles and the
voids that remain after removal of the expendable secondary particles.
The prosthesis produced according to this disclosure includes a
metallic substrate and a coating of randomly dispersed metal particles.
The particles are of substantially uniform si~e and are joined to one
another and to the substrate by metallurgical bonds. The outermost
particles are compressibly deformed and dimensioned for implant usage.
The metal coating particles are separated by a network of interconnected
voids having an average size greater than the average size of the metal
particles to present a homogenously porous coating about the substrate.
It is an object of this method to achieve controlled pore size and
morphology in a porous coating comprising discrete particles joined by
metallurgical bonds.
Another object is to provide porosity in the coating in excess of 40~,
while achieving successful metallurgical bonding between the particles
themselves and between the particles and substrate by a combination of
mechanical compression and application of heat under controlled conditions
such that a prosthesis will be covered by a coating having adequate
strength for practical use in implant applications.
Another object is to combine pressures of a magnitude to effect
mechanical deformation and thermal bonding of particles so as to
successfully coat implant surfaces with minimum temperature elevation,
thereby preserving the metallurgical structure and properties of the
substrate through the coating procedure.
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Another object of this invention is to provide a practical thin porous
coating on metallic surfaces of a prosthesis in a manner which retains the
complex surface configurations often required about the surfaces.
Another object is to provide an effective method of controlling final
5coating density and porosity so as to permit design of the coating
structure for tissue ingrowth re~uirements.
Another object is to produce a coated implant surface by compressive
molding techniques in a manner that assures attainment of rigid exterior
tolerances without requiring machining of the coated surfaces.
10Finally, an object of the invention is to develop a practical process
and product by use of available techniques, equipment and raw materials.
Description of the Drawings
Fig. 1 is a perspective view of an implant element having porous
15coated surfaces;
Fig. 2 is an enlarged fragmentary transverse sectional view taken
substantially along line 2-2 in Fig. 1, with a circled corner area further
enlarged for illustration;
Fig. 3 is an illustrative flow diagram illustrating the steps of the
20present process;
Fig. 4 is a plan photograph showing the outer coating surface after
removal of the expendable material;
Fig. 5 is an enlarged (50X) side view of a cross-sectional slice
through the coating and substrate in Fig. 4;
25Fig. 6 is an enlarged (lOOX) side view taken at the center of Fig. 5
after etching out the grain structure of the coating and substrate;
Fig. 7 is A further enlargement (250X) showing bonding at the
substrate surface;
Fig. 8 is a schematic cross-sectional view illustrating molding of the
3 ocoating;
Fig. 9 is a partial schematic cross-sectional view of the open
compression mold; and
Fig. 10 is a similar view of the closed mold.
35Best Mode for Carrying Out the Invention
Figs. 1 and 2 generally illustrat0 a known configuration of a hip
prosthesis 10. The prosthesis 10, which is a relatively common bone
implant used in surgical repair of hip joints, includes an elongated shank
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11 intended to be inserted nxially within a supporting bone structure. It
is capped by a ball structure generally shown at 9. In most surgical
installations of such a prosthesis, the shank 11 is anchored to the bone
structure by bone cements.
As illustrated in Figs. l and 2, the outer surfaces of the shan}; ll
are covered by a porous metallic coating 12 that surrounds an inner solid
metal substrate 13. The coating 12 is bonded to the substrate 13, and
includes interconnec$ed pores through which soft and hard living tissues
can grow after implantation of the prosthesis 10.
The present process is initiated by cleaning and machining of the
preselected metallic surface areas upon which the coating is to be formed.
Additionally, the particles to be used in the coating must be intimately
blended to produce a relatively homogenous mixture of the primary
particles of metallic material matching or metallurgically compatible with the
surface area and the ~secondary particles of expendable material. It is
preferable, but not required, that the particles of both materials be of
identical size. This will result in substantially constant porosity
throughout the final coating. It also assures that the maximum number of
contacts will occur between each particle and those surrounding it. The
particles within both the primary and secondary materials should be
restricted to a relatively narrow size range within the limits of 35 mesh
~500 microns) and 80 mesh (177 microns); e. g., -60 +70 mesh (210 to 250
microns). These particle sizes have practical application in the
development of porous coatings having a thickness of between 500 to 1,000
microns, which would then have a thickness of from three sphere diameters
to about six sphere diameters. Obviously, coatings of greater thickness
can be produced when desired.
While the use of substantially spherical particles has been found
preferable in carrying out this invention, it is contemplated that particles
3 o of an irregular shape might also be used for both the primary and
secondary materials, so long as their size ranges are generally within the
bounds indicated above.
When utilizing a dry blend of particles, the surfaces of prosthesis 10
which are to be coated are next positioned within a precision mold cavity
14 in a gravity sintering mold 15. The preselected metallic surface areas
to which the coating is to be affixed must be spaced inwardly from the
mold cavity surfaces 14. The spacing between the surfaces will be
dependent upon the desired thickness of the final coating, and the amount
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of co~ting compression which will be nccomplished in subsequent steps
described below.
As indicated by arrow 16 in Fig. 3, the blended mi~ture of primary
and secondary particles is fed into the spaces between the stationary mold
cavity and the prosthesis. The mold 15 is next subjected to heat within a
furnace indicated generally at 17. Furnace 17 is preferably a vacuum
furnace, since vacuum pressure during the heating step inhibits oxidation
of the metallic alloys. Oxidation can also be minimized or prevented by
heating the mold within a suitable inert atmosphere. The temperature of
the particles surrounding the prosthesis should be raised to a level at
which minimal metallurgical bonding between adjacent particles will be
achieved, as well as bonding of the particles to the substrate area. The
temperature should be maintained below that at which any adverse
reactions will occur with the expendable phase. In the case of prosthetic
surfaces and spherical particles made fron~ Ti6A14V alloy (6~ Aluminum, 4%
Vanadium, the balance being Titanium) with copper or iron as the
expendable phase, the bonding temperature will be maintained below the
beta transus temperature for the alloy (for example, 970C).
As indicated at numeral 18 in Fig. 3, the prosthesis 10 can then be
removed from the gravity sintering mold as a "preform" with a coating of
the gravity sintered particles formed about it. Were the expendable phase
material to be removed at this stage of product manufac~ure, the resulting
porous coating pro~ride~ by the remaining spheres would have inadequate
strength for surgical implant purposes.
To assure adequate strength properties, deformation pressures are
applied between the metallic spheres by pressure molding. This can be
achieved by use of any pressure molding apparatus capable of accurately
deforming the complex surface areas under controlled conditions to achieve
reproducible coatings on the manufactured bone implant elements. While a
3 o hot isostatic pressing apparatus might b0 used, the specific illustration
shown in Fig. 3 shows use of mechanical dies having two or more segments
for compression of the coating surface areas.
The preform 18 or coated prosthesis is placed within the
complementary jaws 20 of a pressing die, with the die cavity surfaces
3 5 overlying the preselected surface areas of the prosthesis to be coated .The die jaws 20 and prosthesis are then sub)ected to heat within a furnace
21. Furnace 21 again is either a vacuum furnace or a furnace having a
supply of inert gas to minimize or prevent oxidation of the metallic alloys.
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Th0 temperature of the spherical particles surrounding the prosthesis must
again be raised to one at which some metallurgical bonding between
adjacent primary particles and between the primary particles and
underlying substrate will occur. The prosthesis is simultaneously
subjected to surface compression by movement of the jaws 20 as indicated
as arrows 22. This compression step should typically reduce the volume of
the coating about the prosthesis surfaces by 1096 to 30~. This in turn
reduces the dimensional thickness of the mixture of particles to the desired
final coating thickness. The process is completed by removing the
prosthesis from the pressing die. The prosthesis will have the pressed
coating of desired metal and expendable material formed about it. This
condition of the prosthesis is shown at 23 in Fig. 3. Substantial
metallurgical bonds will now exist between the primary coating particles,
with some surface deformation both between the spheres of the desired
matrix metal and between the preselected metallic surface area of the
prosthesis itself and those particles in engagement with it.
The production of the porous coating is completed by chemically
removing the expendable phase in a liquid bath shown at 24. The result
is the prosthesis shown in Fig. 3 at 25, which now has an accurately
controlled porous coating 26 securely formed about the preselected surface
areas. Such a coating will permit bone ingrowth after surgical
implantation .
The materials used in this process must be selected so as to be
compatible with the materials of the prosthesis (capable of metallurgical
bonding without adverse effect) and the processing requirements during
coating formation. The primary coating material will normally be selected
to match the metallic material cf the preselected surfaces to be coated on
the prosthesis. However, other materials having suitable bonding
compatibility, and which are biologically acceptable, might be used in
certain instances.
It is anticipated that the expendable particles will normally be
metallic. Examples of materials usable for this purpose are particles of
copper, iron, steel, or molybdenum. Metals and alloys must be used which
can be readily removed after the production of the coating has been
3 5 completed . Low temperature eutectic reactions with the prosthesis and
primary metallic particles should be avoided.
The mechanical deformation pressure that occurs between the primary
metal particles hastens development of metallurgical bonds at temperatures
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substantially lower than those thflt would be required in the absence of
compression. While compressive deformation of the primary metal particles
is of primary importance with respect to the strength properties of the
final coating, it is to be understood that mechanical deformation can also
5 occur in the expendable particles and at the surface of the substrate.
A significant advantage of this process is the ability to produce an
effective porous coating at relatively low temperatures in comparison to
usual gravity sintering. This is achieved within reasonably short time
spans by combining both mechanical pressure and thermal deformation of
10 particles and substrate to achieve metallurgical bonds. The use of low
sintering temperatures is of particular importance when bonding a coating
to a substrate which woul~ be modified metallurgically by elevation to
higher temperatures.
As an example o~ a practical blend of particles for coating surfaces
15 made from a titanium alloy (Ti6Al4V), the blend of materials in the gravity
sintering step might comprise 45% by volume Ti6A14V spherical particles
and 20% by volume expendable spherical particles~ The remaining space
between the prosthesis surfaces and the surrounding mold cavity will be
void space between the spheres. These voids will equal 35~6 of the coating
20 volume. Af~er the coating has been compacted, heated and pressed, the
volume percentage of Ti6A14V spheres will be 5~Q6, the volume percentage
of expendable spheres will be 22 . 2%, and the void volume will be reduced
to 27.8~6. Removal of the expendable material from the resulting coating
will produce a final porous coating of approximately 50% density, meaning
25 that the interconnecting pores formed throughout the coating occupy 50% of
the total coating volume.
Varying the size and/or percentage of the primary spherical particles
with respect to the expendable spherical particles ~an be utilized as a
production control to select the structure ar.d density of the resulting
30 coating on the prosthesis.
Spherical particles are preferred as starting materials for production
of the coating, but spherical particle shapes are ~ot essential to practice
of the invention. Spherical powders of the required metallic materials are
readily available in closely deffned size ranges. They are also easily
35 handled and packed with minimum damage and resulting dust. By using
spheres of the same diameter for both the primary and expendable
materials, one can maximize the theoretical number of contact locations to
be expected between adjacent particles. This leads to greater bonding of
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the primary particles and improved strength in the final coating.
However, non-spherical granulur or particulate powders of eit]ler the
primary or expendable material can be utilized when available.
Voids in the final coating are attributable to both the spaces initially
S occupied by the particles of expendable materinl (spherical voids) and the
spaces between the adjacent particles of both materials (random shaped
voids). The number and size of the spherical voids is determined by the
number and size of the expendable particles, which are essentially
unchanged during compression of the coating. The volume percentage of
the random shaped voids is decreased during compression, primarily due to
deformation of the primary material particles against one another, against
the expendable material particles, against the substrate surface, and
against the outer die surfaces.
The method of this invention reduces the void volume in the covering
blend of particles, but does not eliminate it. The void volume can be
either totally empty or can be partially or completely filled by a binder or
other liquid. The maintenance of a void volume assures that the primary
particles can remain as discrete particles after deformation and that
deformation of the secondary particles will not flow into areas of contact
between adjacent primary particles so as to disturb the desired
metallurgical bonds being formed between them.
The spherical voids, which are ultimately integrated with random
shaped voids as well, are important for tissue ingrowth. They guarantee
that the resulting coating will contain interconnected larger irregular voids
between the discrete primary particles. The voids will have an average
size greater than the average size of the particles, assuming the primary
and secondary particles to have initially been essentially identical in size.
The interconnection between such voids presents effective pathways for
tissue growth.
The random shaped voids guarantee a continuous network of voids in
the coating prior to removal of the expendable material. This permits
removal of the expendable material through the void network, whether by
etching, application of heat, or by any other suitable method. In general
at least a 59~ void volume will be required in the compressed coating to
3 5 assure complete removal of the expendable material .
Another important feature of the coating is the fact that its outer
surfaces are mechanically deformed against the outer die surfaces within
which it is compressed. By utilizing compression dies pressed against
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fixed stops which define the limits of compression, one can accurately
manufacture a coated element having the outer dimensional tolerances
required in bone prosthesis applications and similar usages. The
compressed outer coating surfaces are free of burrs, rough edges and
particle remnants which might be presented after surface machining
operations .
In general, a coating having approximately 50% density (50% solid
volume, 50% void volume) is desirable for tissue ingrowth applications.
The percentage of densification can vary between different surfaces on an
element, depending upon their relative orientations and complementary die
opening design. For illustration, Figs. 8-10 schematlcally show
compression molding at a cross-section of shank 11 defined by normal
surfaces 30 which are perpendicular to the directions in which pressure is
applied (arrows 31) by a split d~e 32,33. Surfaces 30 are joined by
angular surfaces 35 along the edges of shank 11 which are arranged at an
angle of 60 relative to the normal. The following illustration shows the
effect of compression. It assumes the use of a split die 32, 33 having a
seam 34 midway between and parallel to the normal surfaces, dimensioned
to produce a compressed constant depth of coating on all surfaces when
fully closed.
The original coating thickness B about the normal surfaces 30 is
0.0533 inches (1.35 mm). When compressed, the final coating thickness D
is 0.040 inches (1.02 mm~. The corresponding origin~l coating thickness A
about the angular surfaces 35 should be 0.0467 inches (1.186 mm) to
achieve the same final coating thickness D in this die configuration. The
die spacing C is 0.0267 inches (0.68 mm) . The coating material on the
normal surfaces 30 will be compressed by 25% and the coating material on
the angular surfaces 35 will be compressed by 14.3%.
The original blend of spherical particles coating all surfaces is 40%
primary metal (Ti6Al4V), expendable metal 20% (Iron) and 40% voids, by
volume. After compression to stops defining the final 0.040 inch coating
thickness, the coating at the normal surfaces will comprise 40% primary
metal, 20% iron and 15% voids, based on the orifinal volume. The coating
at the an~ular surfaces will also be 40% primary metal and 20% iron, but
the voids will be 25.7%, again based on the orifinal volume. After removal
of the expendable metal, the volume of primary metal content of the
coating on the normal and angular surfaces will then comprise 53.5% and
46.7% of total coating volume, respectively.
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. ~
In tabular form, such densification can be illustrated as follows:
Packed Hot Acid
Blend Press Etch
(~ vol)
Normal Surfaces 40 Ti6Al4V _40 ~40 = 53.3%
(25~ Densification 20 Fe 20 _20 Void solids
0.0533 in~0.040 in) 40 Void ~15 Void_15 Void by vol.
Angular Surfaces (60)40 Ti6Al~V ~ 40 _40 = 46.7%
(14 396 Densification 20 Fe ~ 20 ~20 Void solids
0.0461 in~0.040 in) 40 Void ~25.7 Void_25.7 Void by vol.
1 5 In the initial filling of the mold or die cavity, the particles must be
closely packed so as to substantially fill its volume without mechanical
deformation occurring. This assures the greatest number of particle
contacts by each particle in the blend, both primary and expendable.
After compression, the volume of coating is substantially reduced by
partial elimination of the void volume between adjacent particles as a direct
result of the deformation of the primary pflrticles. By compressing the
coating to fixed stops, excess pressure can be used, and accurate coating
thickness and surface control is assured.
It should be noted that the gravity sintering can be accomplished in
the compression die used to ultimately press the coating about the
preselected metallic surfaces. While it is convenient to produce a preform
in a mold having no seams, the separate production of the preform is not
essential to the disclosed process.
As an alternative to initial gravity sintering, either in a separate
mold or in the compression ~e, this step can be totally eliminated by use
of a binder in the ori~inal blend of particles. An important benefit
achieved by using a binder is the fact that the two different types of
particles can be uniformly blended with the binder and will not
subsequently tend to segregste as they are being placed in the mold or die
3 5 cavity . Such segregation must be avoided when the particles are dry,
making storage and handling of the blended particles more difficult.
Binders are preferably water-based and capable of being heated to
approximately 350-500F (180-270C) to remove water content and leave a
11
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, ~
residue layer thnt bonds the particles together. This remaining materinl in
turn must volatilize nt a temperature below the selected sintering
temperature for the coating.
Binder materials tested to date include cellulose gum powders,
polyvinyl pyrrolidone, hydroxypropyl cellulose, polyethylene glycol, and
polyvinyl alcohol . These were used at concentrations between 0 . 5 g to 8 . 0
g in 200 cc of water. Other organic binders can be selected for use in
blending and handling the particles, and they can be mixed with solvents
other than water, so long as the binder is inert relative to the particles.
The amount of binder added to the blend of particles must merely be
sufficient to coat their surfaces and prevent their segregation during
handling and placement on the substrate. No particular volume
relationships are required.
The use of a binder permits one to produce a green, uncompacted
coating without compression or sintering. After curing or drying the
binder, the particles will be adhered to the prosthesis surfaces and the
coated element can be stored or handled as required by production
techniques. The binder also facilitates introduction of the homogeneously
blended particles directly into the compression dies, assuring uniform
distribution of primary and expendable particles about closely restricted
areas separating substrate surface and outer die surfaces.
It is also preferable that the mold cavities and dies be produced with
sufficient precision as to obviate the need for any finish machining of the
coating surfaces. However, should machining steps be required, they
would normally be accomplished prior to removal of the expendable phase.
As an example of the removal of the expendable phase, the
expendable material discussed in the above example can be chemically
removed ~y immersion in a bath of suitable acid. The finished product
would subsequently be treated to a low temperature vacuum outgassing to
assure removal of any contaminants and to further promote metallurgical
bonding .
EX AMPLE
In a specific test of the process which led to this disclosure, a disk
of titanium alloy (Ti6Al4V) was coated with a porous cover of microspheres
of the same alloy . The coating was approximately 1. 25 millimeters thick .
The primary microspheres were between -40 and +80 mesh (177~42û
microns). The expendable copper microspheres were of an identical size
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rangc. A sintered preform ~vas not used. The composite coating was
fabricated by hot pressing at 850C and 2,000 psi in an argon atmosphele.
The copper microspheres were removed in dilute nitric acid. The density
of the coating was 48% by ~olume of the theoretical volume of the coating
5 total. Resulting metallography of the sample showed clcar evidence of
bonding between the titanium alloy microspheres and to the titanium alloy
disk .
EXAMPLE 2
In another experiment designed to demonstrate that gravity sintering
of microspheres would form a suitable preform for subsequent hot
pressing, titanium alloy (Ti6Al4V) microspheres were gravity sintered
about a titanium alloy ~Ti6Al4V) rod in an aluminum oxide crucible.
Sintering was accomplished in vacuum at 1,025C for one hour. The
resulting part evidenced sufficient strength in the coating to serve as a
preform. No further processing of this sample was accomplished.
RXAMPLE 3
Tests have been conducted to demonstrate coating of substrate
material typically used in hip implants as generally shown in Figures 1 and
2. This comprised a 0.078 in (1.98 mm) thick, circular disk of Ti6Alg~T
alloy. It had an alpha structure content.
The titanium alloy (Ti6Al4V) spheres used as primary particles for
the coating were -60+70 mesh spheres. Iron spheres of the same size
range were of C1018 iron, containing 0 . 20% oxygen by weight and 0 . 84%
manganese, the balance being iron.
A cellulose gum binder in water (3g/200ml) and a similar concentration
of polyvin~l alcohol were separately blended with titanium and iron
spherical particles and placed at opposite sides of the titanium disk in a
hot press die. They were first hot pressed at 900C at a pressure of 1000
psi for about 1 hour, but this pressure was inadequate to achieve full
closure of the die. The assembly was subsequently hot pressed at 900C
at a pressure of 2000 psi for approximately another hour. Complete
movement of the dies to a predetermined stop position was accomplished,
resulting in the desired densification of the coating.
The disk of titanium alloy had a~ least two good layers of spherical
particles bonded to it. The disk was cut and two quadrants were placed
in dilute nitric acid to remove the iron spheres. The total coated
13
~ ~ "
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sandwich was 0.167 in (4.24 mm) thick. Evaluation was conducted
primarily by metallograpl-y. Figure 4 shows a top view of one coated
surface nfter removal of the expendable iron spheres. Good distribution of
the light-colored remaining discrete particles is evident.
Fig. 5 shows an enlarged side view along one cut surface or edge of
the substrate. Some of the bonded spherical particles (light-colored
circles) shown in Fig. 5 have a smaller diameter because they were not
sliced through their centers. The deformation and relatively wide
compressed bonds formed between contacting discrete spherical particles is
evident as they form a lattice of connected spheres. The deformation of
the boundary layer of spheres against the substrate shown across the
lower portion of Fig. 5 in this instance was substantial. Substrate
deformation was also evident from the indentations or dimples which remain
after removal of the iron spheres. The even compression of the outermost
layer of spheres across the top of Fig. 5 shows the compressive effect of
the die surface in forming an outer surface of uniform depth and accurate
tolerance along a prescribed plane.
Fig. 5 graphically illustrates the desirable interconnected voids (dark
areas) between the bonded particles. These can be seen to be an
accumulation of the random void spaces or void volume which remain
between adjacent titanium spheres, and the larger spaces which include the
original positions of the iron spheres plus the random voids which were
located about them. It can be readily seen that the result is that the
metal particles in the coating are separated from one another by a network
of interconnected voids having an average size greater than the average
size of the discrete metal particles or spheres in the original blend of
spheres introduced into the die.
Fig. 6 is a further enlargement of the central portion of Fig. 5 after
etching of the surface. It shows the grain structure of the particles along
the dimpled surfaces, and bonding between particles and substrate.
Metallurgical bonding is even more evident in Fig. 7.
While the spheres and substrate shown in Figs. 4-7 are each the same
alloy, Figs. 6 and 7 illustrate their different granular structures. The
substrate remained an alpha worked structure, since bonding of the
spheres occurred without raising its temperature above its beta-transus.
This is an important aspect of the invention as applied to a prosthesis
made from Ti6Al4V alloy. It successfully achieves metallurgical bonding of
thin coatings without the high temperatures typically required to achieve
14
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similar bonding of this alloy by gravity sintering. The process has been
shown to successfully substitute mechanical energy in deforming the
spheres to replace part of the thermal energy required for metallurgical
bonding. This permits coating to be accomplished at reduced temperatures
at which the metallurgical properties of the substrate material are not
altered .
The coated implant is uniquely recognizable from the enlarged views
of the coating material as illustrated in Figs. 5, 6 and 7. The coating,
typically three to six layers thick, includes randomly dispersed discrete
metal particles having a substantially uniform size. They are clearly
joined to one another and to the substrate by metallurgical bonds. The
outermost layer of particles is compressively deformed to present an outer
surface of proper dimensions for implant purposes. The particles in the
coating are separated by a network ~f interconnected voids having an
average size greater than the average size of the discrete metal particles.
The surface areas of the particles which surround the interconnected voids
are dimpled.
In compliance with the statute, the invention has been described in
language more or less specific as to structural features. It is to be
understood, however, that the invention is not limited to the specific
features shown, since the means and construction herein disclosed comprise
a preferred form of putting the invention into effect. The invention is,
therefore, claimed in any of its forms or modifications within the proper
scope of the appended claims, appropriately interpreted in accordance with
the doctrine of equivalents.
