Solid state bonding of ceramic and metal parts

For solid state bonding, the faces of ceramic and metal parts to be bonded re placed together, acted upon by a gage pressure if desired, and are heated to a bonding temperature which is under their melting temperatures so that an inbetween or intermediate layer is formed between the two parts. For increasing the bond strength the metal used is one undergoing a phase transition on cooling down from the bonding temperature, the phase in existence at the bonding temperature having a smaller volume than the phase stable at room temperature, so that the decrease in the volume of the metal as dependent on the coefficient of thermal expansion is balanced at least to some degree by the volume change on phase transition.

FIELD OF THE INVENTION 
The present invention relates to solid state bonded structures made up of a 
metal part and a ceramic part, and to a solid state bonding process, in 
which faces of the metal and ceramic parts are placed together, and if 
desired are acted upon by pressure, and are heated to a bonding 
temperature which is below the melting temperature of the metal and the 
ceramic material so that by diffusion and/or chemical reaction the two 
faces of the two parts are bonded together, an inbetween layer being 
formed between them. 
BACKGROUND OF THE INVENTION 
Metal parts have to be strongly bonded to ceramic parts in a great number 
of different fields of technology, as for example in the construction of 
gas turbines, in solar energy technology, for artificially producing 
teeth, for high temperature electrolysis or for producing ceramic armoring 
structures. While it is true that solid state bonding may be used for 
these purposes and at the bonding temperature a strong bond is produced, 
it has been seen from experience that in systems of solid state bonding 
designed so far, the bond strength is generally low or, putting it 
differently, even small loading forces are responsible for damage to the 
bonded structure. 
SUMMARY OF THE INVENTION 
On the footing of the general idea that such low strength is caused by the 
very different coefficients of thermal expansion of ceramic materials and 
metals, so that high inner stresses are produced at the interface, one 
purpose of the present invention is that of increasing the strength of 
such bonded metal/ceramic structures. 
For effecting this purposes, the metal part is made of a metal which has 
different crystalline phases at different temperatures, that is to say 
there is a temperature-dependent phase transition, a phase which is in 
existence at a temperature higher than room temperature having a smaller 
volume than a further phase which is stable at room temperature, the 
bonding temperature being within the temperature range of the phase with 
the smaller volume so that on cooling down of the bonded structure from 
the bonding temperature to room temperature the thermal decrease in volume 
of the metal is, at least to some degree, balanced by the increase in 
volume due to phase transition. 
In this way the thermal decrease in volume may be balanced to a high degree 
by the increase in volume due to the phase transition so that, on cooling 
down, the changes in size of the metal part and the ceramic part will be 
generally of the same order and for this reason there will be a decrease 
in the internal stresses of the structure, this in turn being responsible 
for a higher bond strength. 
It is best for the material used for making the bonded structure to be such 
that the coefficient of thermal expansion of the ceramic material, the 
coefficient of thermal expansion of the metal and the increase in volume 
of the metal produced on cooling down by the phase transition, of the 
metal, are so matched that the difference between the thermal decrease in 
volume of the metal and the phase transition increase in volume of the 
metal are roughly speaking equal to the thermal decrease in volume of the 
ceramic material. 
Because of different properties of the materials such as the lattice 
structure, the modulus of elasticity etc. of ceramic materials and metals, 
there will be an anisotropic stress condition after bonding and cooling 
down within the bonded structure, such condition being able to be worked 
out by calculation or measured physically, for example using 
photoelasticity. This stress condition is a function of the thickness of 
the metal part at the bond, and of other properties. In this respect, with 
an increase in thickness of the metal part there is a change in the 
position of the greatest inner mechanical strain, which shifts in 
direction from the metal part through the inbetween layer into the ceramic 
part. The strength of the bonded structure of the present invention may be 
further increased by keeping the thickness of the metal part in the region 
of the bond greater than a lower limit, over which the direction in which 
there is the greatest inner mechanical strain is toward the ceramic part, 
such direction being the direction of the greatest main stress of the 
anisotropic stress condition produced after cooling down. On making such a 
selection of the metal thickness, the bonding strength between the metal 
part and the ceramic part will be greater than the strength of the ceramic 
material, that is to say the greatest outer or external load supporting 
capacity of the bonded structure will only be dependent on the ceramic 
material. The reasons for this will now be made clear. 
It has been seen from tests that in the case of solid state bonded (or 
welded) metal-ceramic compound structures, fine hairline or hair cracks 
are formed on cooling down, such cracks decreasing inner stresses at the 
bond. On using a metal which undergoes a phase transition with an increase 
in volume on cooling down, such hair cracks, starting from points inside 
the metal, come to an end short of the bonded face of the ceramic part 
without being dependent on the metal thickness. If now the position at 
which there is the greatest inner mechanical strain is in the metal part, 
when an outside load takes effect such hair cracks will make a large or 
macro crack more likely (which would be the cause of the material being 
fractured) the macro crack, running at approximately a right angle to the 
direction of the greatest main stress. If on the other hand, by changing 
the thickness of the metal part, the direction of the greatest inner (or 
internal) strain is changed in position so as to be into the ceramic part 
(where there are no hair cracks) hair cracks will be of no effect on the 
fracturing properties and the bonded structure may be loaded right up to 
the fracture resistance of the ceramic material. 
On using a metal without any phase transition, no such increase in strength 
was to be noted on moving the direction of the greatest inner strain into 
the ceramic part, because in this case, with an increase in the thickness 
of the metal, hair cracks will be present in the ceramic part as well, 
such cracks making fractures more likely. 
A metal which may be used in the invention is zirconium, which undergoes 
transition on cooling at 862.degree. C. from the cubic space-centered 
beta-phase into the hexagonal alpha-phase which has a greater volume than 
the beta-phase. In this case a bonding or welding temperature between 
1150.degree. and 1200.degree. C. may be used. Furthermore, hot-pressed 
silicon nitride may be used as a ceramic material. In the case of a bonded 
structure made up of hot-pressed silicon nitride and zirconium, whose 
lattice structure on cooling is changed over from the beta phase into the 
alpha phase, it may be seen that the lower thickness limit for the metal 
part at the bond (above which the position of the greatest inner stress is 
in the ceramic part) has a value of greater than about 0.6 mm. 
The process as noted may furthermore be used for producing a ceramic 
material-to-ceramic material bond by bonding the two ceramic parts to a 
metal part therebetween. 
A structure produced by solid state bonding in the invention may be 
characterized in that between room temperature and the bonding temperature 
the metal has a phase transition point and the room temperature phase has 
a greater volume than the phase in existence at the bonding temperature.

DETAILED DESCRIPTION 
The bonded or compound structure to be seen in FIG. 1 is made up of a metal 
part 1 and a ceramic part 2 which are solid state bonded together, the 
bonding or welding temperature producing an inbetween (or intermediate) 
layer 3 which is the result of diffusion and/or chemical reactions and is 
responsive for the joint or bond, such inbetween layer 3 being marked in 
chained lines. Because metals have a higher coefficient of thermal 
expansion than ceramic materials, there is a greater decrease in the size 
of the metal part when the bonded structure is cooled down from the 
bonding or welding temperature to room temperature than is the case with 
the ceramic part, this being made clear in FIG. 1 diagrammatically inasfar 
as the metal part 1 will be seen to have a smaller breadth. The inner (or 
internal) stresses produced by this and which may be responsible for 
cracking or breaking of the bonded structure are to be kept as small as 
possible, this being done in the invention by making the metal part 1 of 
the new bonded structure of a metal which has a phase transition point 
somewhere between room temperature and the bonding temperature, the phase 
which is in existence at room temperature having a greater volume than the 
phase in existence at the bonding temperature. Because of this there is an 
increase in volume of the metal part on being cooled down due to the phase 
transition so that the effect of the different volume decreases due to the 
different coefficients of thermal expansion of metal and ceramic is 
balanced somewhat, that is to say made less important. This is made clear 
diagrammatically by the outwardly pointing arrows in FIG. 1. It will be 
clear that by the use of such a metal in the bonded structure lower inner 
stresses will be produced so that the strength will go up. 
The best effects are produced if the difference between the change in size 
of the metal 1 due to the coefficient of thermal expansion and a change in 
volume taking place due to the phase transition of the metal 1 is 
generally speaking equal to the change in size of the ceramic part 2 as 
dependent on the coefficient of thermal expansion of the ceramic material. 
In the present working example the ceramic part 2 is made of hot-pressed 
silicon nitride and the metal part is made of zirconium, whose hexagonal 
phase with a greater volume is stable below 862.degree. C. and whose cubic 
space centered phase with a smaller volume is stable at a higher 
temperature. On bonding these two materials a bonding temperature of for 
example between 1150.degree. C. and 1200.degree. C. is used, the faces of 
the two parts being first cleaned and then pushed together by a pressure 
of for example 10 N/mm.sup.2, the operation best being undertaken in a 
vacuum. 
It will be seen in FIG. 2 that hairline or hair cracks 4 have been formed 
in the bonded structure running out from the inner part of the metal part 
1 and coming to an end short of the ceramic part 2 at the inbetween or 
bonding layer 3. Because of the coming into existence of such hair cracks 
4, stresses produced on cooling are decreased. In this respect, 
independently of the thickness d of the metal part 1, the hair cracks 4 
are limited to the metal part. 
Furthermore in FIG. 2 arrows 5a, 5b and 5c are used for indicating (for 
different thicknesses of the metal part) the direction of the greatest 
main stress of the anisotropic stress condition produced after cooling 
down, which direction is dependent on the thickness of the layer and may 
be worked out by calculation or measured experimentally. Such direction 
gives in each case the position of the greatest inner mechanical strain, 
which with an increase in the thickness d of the layer-like metal part 1 
undergoes a continuous change in position from being directed into the 
metal part (arrow 5a) through a position directed into the inbetween layer 
3 (arrow 5b) and into a position directed into the ceramic part 2 (arrow 
5c). On the bonded structure being acted upon by a great enough force, for 
example by a pulling force at a right angle to the interface between the 
metal part and the ceramic part, a large or macro crack will be produced 
normal to the greatest or maximum main stress direction. If the position 
of the greatest inner strain is into the ceramic part 1, that is to say if 
the direction of the greatest main stress (arrow 5a) is pointing into the 
ceramic part, hair cracks 4 will have the effect of supporting the growth 
of the macro crack so that there will be a fracture at a generally early 
stage. For this reason it is best for the metal part 1 to have such a 
thickness d in the region of the bond (that is to say along the 
ceramic-metal interface) that the direction, dependent on this thickness, 
of the greatest inner mechanical strain is into ceramic paart 2, which is 
free of hair cracks. In this case the high strength of the ceramic 
material may be fully profited from. 
The bond strength may be measured (see FIG. 3) for example by bonding 
separate ceramic parts 2a and 2b on both sides of a layer-like metal part 
1a and causing forces P.sub.1 and P.sub.2 to take effect on one side of 
the sandwich-like body so produced at its outer ends parallel to 
interfaces 6a and 6b, while on the other side of the sandwich-like 
structure, at the metal layer 1a, force P.sub.3 is caused to take effect 
in the middle. This loading effect is responsible, at the side of forces 
P.sub.1 and P.sub.2, for a pulling force P.sub.5 at a right angle to 
interfaces 6a and 6b. The bond strength measured on these lines has been 
plotted in FIG. 4 as a function of the layer thickness d of the metal 
part, curve I being representative of the bond strength of a bonded 
structure made up of two silicon nitride layers 2a and 2b on the outer 
sides of a zirconium layer 1a therebetween, the bond having been produced 
at a temperature greater than the phase transition temperature of 
zirconium. Up to a layer thickness of the metal of about 0.6 mm there is a 
generally unchanging bond strength of about 100 Joule/m.sup.2, but at 
values greater than this thickness there is a sharp increase in the bond 
strength which goes up to about 330 Joule/m.sup.2 at a layer thickness of 
about 1 mm. This sharp increase may be seen to take place with a 
change-over of the position at which there is the greatest inner strain 
out of the metal and into the ceramic material or, putting it somewhat 
differently, on movement of the direction of the greatest main stress away 
from the hair cracks 4. On the other hand, on producing the bonded 
structure under the same conditions of testing but using a metal (in place 
of zirconium) which on cooling down from the bonding temperature does not 
undergo any phase transition, as for example hafnium, the curve II will be 
produced, from which it will be seen that in this case the bond strength 
hardly gets greater than 100 Joule/m.sup.2, even on using thicker metal 
layers. The reason for this may be seen to be the hair cracks produced 
here in the ceramic material as well.