GB1588920A - Joining of metals to ceramics - Google Patents

Description

(54) THE JOINING OF METALS TO CERAMICS (71) We, THE BRITISH CERAMIC RESEARCH ASSOCIATION, a British Company, of Queens Road, Penkhull, Stoke-on-Trent ST4 7LQ, England, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to the joining of metals to ceramics.

One method of making strong joints between ceramics and metals is to shape the ceramic into a cylinder, form a hole in the metal component of slighly smaller diameter than that of the cylinder and then heat the metal component until the metal expands sufficiently for the ceramic cylinder to be inserted into the hole.

Upon cooling, the metal shrinks into the ceramic cylinder and grips it firmly. This method can be referred to as the conventional method of shrink fitting.

A disadvantage of the conventional method of shrink fitting is that the joint is difficult to assemble because of the problem of handling hot components. Also the metal and ceramic components both need to be machined accurately in order to ensure that, on cooling the joint, the metal grips the ceramic sufficiently tightly to avoid the joint being easily sheared but not so tightly as to cause the ceramic to be weakened or even broken.

It is therefore an object of the invention to provide an improved way of joining metals to ceramics.

According to the invention there is provided a method of joining a metal component to a ceramic compondnt, the metal component having been formed by powder metallurgy to a density less than that of the fully dense metal, in which the two components are assembled in juxtaposition such that when the metal component shrinks it will grip the ceramic component, and then the two components are heated so that the metal component sinters and shrinks onto and grips the ceramic component upon cooling.

By following this method it is less difficult to assemble the joint between the two components than in conventional shrink fitting.

Also by varying the degree of sintering (temperature and/or time) it is relatively easy to control the magnitude of the compressive force exerted by the metal so that a satisfactory joint is formed. In particular by not sintering the metal powder compact to full density it is possible to decrease the modulus of elasticity of the metal and to control the magnitude of the compressive force exerted by the metal so that a satisfactory joint is formed. This can be particularly advantageous where a joint has to be formed between components of widely different coefficients of thermal expansion and/or where a relatively weak ceramic component has to be joined.

The metal powder compact component can be fabricated as a separate component or may be formed directly onto the ceramic component by, for example, die pressing, isostatic pressing, electrophoretic deposition or other methods of powder technology. In the case where the metal powder compact is fabricated as a separate component the compact may be pre-sintered to a density less than that of the fully dense metal before being assembled into the joint.

The bond between the sintered metal component and the ceramic material will normally be mainly mechanical but some reaction or diffusion at the joint may occur during the heating.

We have also found that the high temperature adhesion between a sintered metal component and a ceramic component can be improved by glazing the surface of the ceramic before the two components are assembled. The glazing can be effected by coating the surface of the ceramic component with a thin layer of a glass.

Further to improve adherence between the metal component and the ceramic component, the sintered metal blank is preferably one which will contain a liquid phase during sintering. Such sintering and densification is well known under the term 'liquid phase sintering'.

When joining a metal component to a ceramic component the bond, as noted above, will be mainly mechanical. It is therefore usually desirable for ensuring long life for the bond, although not essential if the ceramic has a glazing coating or the metal contains a liquid phase during sintering, that the metal of the sintered metal component be one which has a higher coefficient of thermal expansion than the ceramic.

The ceramic material can be any of the well known ceramic materials such as silicon carbide, silicon nitride, for example hot pressed silicon nitride (h.p.SiN.) or reaction sintered silicon nitride (r.s.SiN.) alumina, electrical porcelain, fused silica, zirconia, magnesia, titania, graphite and 'glassy' forms of carbon, while the powder metal component can be made by conventional powder metallurgy methods from metals such as nickel, copper, aluminium, stainless steel, molybdenum and various alloys.

The completed joint can be hermetically sealed by providing a coating over the joint and at least the areas of the two components near the joint. This sealing coating can be provided by various coating techniques including, for example, electroplating, electroless plating, ion plating, sputtering and other forms of vapour deposition, and coating with molten metal or glass.

The invention will now be illustrated by the following Examples.

Example 1.

Discs 12.7 mm diameter and 4 to 8 mm thick were formed by pressing powdered nickel in a steel die. The forming pressure was chosen such that the compacted nickel was strong enough to allow a hole to be drilled centrally through the disc. The size of the hole which was drilled was slightly larger than the diameter of the ceramic tube or rod to which the metal was to be joined. To bond the metal collar onto the ceramic the metal collar was fitted over the ceramic component and the metal sintered at high temperature in an atmosphere of 90 /O argon and 10% hydrogen. On cooling the metal collars were found to be firmly bonded to the ceramic.

The force required to pulI the metal collar off the ceramic or fracture the ceramic was measured on a Hounsfield tensometer using an arrangement in which the sintered metal collar of the joint according to the invention was gripping in one jaw of the tensometer. To the free end of the ceramic rod or tube was joined a metal bolt by means of a metal sleeve which passed over both the end of the ceramic rod and the shank of the bolt, the sleeve being joined to the rod and shank by Araldite adhesive, and the head of the metal bolt being gripped by the other jaw of the tensometer. (The word "Araldite" is a Registered Trade Mark).

Strength measurements for joints made from Refel (British Nuclear Fuels Limited) silicon carbide tube (6.35 mm outside diameter, 3.15 mm inside diameter, 25 mm long) are shown in Table 1, together with observations on whether reaction occurred between the metal and the ceramic.

TABLE 1 Joining Ni* to Refel Silicon Carbide tube

Sintering Sintered Disc forming Temp/time metal Breaking Shear Tensile Reaction4 pressure density load Type of strength strength and/or MN/m C h g/cm N Failure NM/m MN/m adhesion 102 650 2 5.4 1160 Slip 12 - Nil 102 650 4 5.6 1870 " 17 - Nil 102 650 7 6.1 2400 " 22 - Nil 51 750 1 4.8 1560 " 15 - Nil 256 750 1 6.5 2000 " 23 - Nil 256 860 1 7.1 2980 " 36 - Nil 77 860 5 6.4 2360 " 19 - Nil 51 930 1 not determined 2630 Facture - 110 Nil 51 950 6.3 2180 Slip 23 - Nil 102 970 1 6.8 580 Facture - 24 R.A.

51 1000 6.8 45 " - 1.9 R.A.

256 1025 1 8.1 200 " - 8.5 R.A.

775 975 1 6.0a 780 " - 33 Nil * Nickel type 123 Mond Nickel Co.

Unless otherwise indicated the metal density given is that of a disc sintered at the same time as the test specimen. Densities with the suffix 'a' were obtained from measurements on the collar after failure. It has been found that in general the density of the sintered collar is less than the density of a disc sintered simultaneously.

Breaking load # Surface area of overlap.

Breaking load # Cross sectional area of ceramic.

4Reaction (R) is said to have occurred when there was evidence of the formation of a metal-Si alloy zone at the metal/ceramic interface. Adhesion (A) is said to have occurred if a quadrant of the metal collar remained fixed to the ceramic or broke off a piece of the ceramic after radial saw cuts had been made, at right angles through the metal collar to the ceramic.

5SiC. oxidized at 1250 C for 4h in air before bonding.

The following Table 2 shows similar measurements for joints containing hot pressed silicon nitride and nickel.

TABLE 2 Joining Ni* to hot-pressed silicon nitride rod

Sintering Sintered Disc forming Temp/time metal Breaking Shear Tensile Reaction pressure density load Type of Strength Strength and/or MN/m C h g/cm N failure MN/m MN/m adhesion 77 1000 1 6.4 3340 Slip 21 - Nil 77 1100 1 6.9 3020 " 25 - " 77 1200 1 7.4 3340 " 29 - " 77 1250 1 7.3 2360 " 20 - " 77 1300 1.25 7.9 1900 " 22 - " 77 920 1 5.5a 2800 Slip 23 - Nil 77 1000 1 5.7a 3030 " 23 - " 77 1100 1 - 3560 " 27 - " 87 1200 1 6.9a 4800 " 34 - " 174 1200 1 7.4 4450 " 34 - " 261 1200 1 - 760 Fracture - 24 Trace of reaction 348 1200 1 - 3340 Fracture - 110 " * Nickel type 123 Mond Nickel Co.

** 6.35 mm dia., 25 mm long.

Two batches of hot-pressed Si3N4 were used in this work. The first batch (results in the top section of the Table) had a surface roughness centre line average (CLA) of 0.1 m. The second batch had rougher surface with a CLA value of 1 m.

The following Table 3 shows strengths of joints formed between reaction sintered silicon nitride and nickel.

TABLE 3 Ni*/r.s. Si3N4 joints

Final Sintering density1 Average Average tensile Temperature of metal No. of breaking strength ( C) (g/ml) Samples load (N) (MN/m) 850 5.66 4 1512 72 900 5.97 7 762 45 1200 7.51 6 533 25 * Inco 123 (10- Mm) All of the joints reported in Table 3 above failed by fracture of the silicon nitride rod near the neck of the joint. The joint strength can be seen to decrease as the density of the nickel powder compact is increased and so increases the stress on the silicon nitride.This demonstrates the advantage of being able to control the density of the sintered metal component.

Example 2.

Joints similar to those described in Example l were made using copper, stainless steel (St.St.) or aluminium powders in place of the nickel powder. Results are shown in Tables 4 and 5.

TABLE 4

Disc Sintering Sintered forming temp time metal Breaking Over Shear pressure density load lap Strength Ceramic and Metal MN/m C h g/cm N mm MN/m h.p. Si3N4 with PML. 77 800 1 5.3 530 9.0 3.0 Irregular* 77 800 1 5.4 755 9.5 4.0 77 850 1 5.5 755 8.6 4.4 77 850 1 5.4 755 9.3 4.1 (- 150 m) 77 900 1 5.4 845 8.9 4.7 77 900 1 5.5 935 8.4 5.4 77 950 1 5.8 800 9.1 4.4 77 950 1 5.8 890 8.2 5.4 h.p. Si3N4. (batch 1) 77 800 1 - 980 5.5 8.9 with GFM2N.** 77 800 1 - 1200 5.7 10.5 (- 150 m) 77 850 1 7.5 1020 4.4 11.6 77 900 1 - 1820 6.6 13.8 77 900 1 - 1420 7.2 9.9 128 850 1 7.8 1270 4.2 15.1 180 850 1 8.0 760 2.9 13.1 231 850 1 8.0 710 3.2 11.1 282 850 1 8.1 445 2.5 17.8 h.p. Si3N4 (batch 2) 77 800 1 - 1420 4.2 17.0 with GFM2N** 77 1050 1 8.0 710 5.0 7.1 r.s. Si3N4 with GFM2N* 77 800 1 - 1780 3.8 23.4 see note (1), Table 2 * Copper powder produced by Powder Metallurgy Ltd. (PML), Canning Road Stratford, London.

Lightly sintered after pressing for ease of machining.

** Copper supplied by Goodfellow Metals Ltd.

TABLE 5 Joining of miscellaneous metals to various ceramics

Metal Sintering Sintered forming temp time metal Breaking Shear Tensile * pressure density load strength strength Metal/ceramic MN/m C h g/cm N MN/m MN/m Al h.p. Si3N4 35 610 2 1.7 400 2.9 Al r.s. Si3N4 35 610 2 1.7 760 4.9 Al r.s. Si3N4 35 610 2 1.7 820 5.3 St. st.4 h.p. Si3N4 5 174 1200 1 5.4 3340 22 St. st. h.p. Si3N4 348 1200 1 5.7 2900 19 St. st. h.p. Si3N4 348 1350 1 6.4 4140 - 130 St. st. h.p. Si3N4 348 1350 1 6.4 4100 32 St. st. h.p. Si3N4 348 1350 1 6.4 133 - 5.7 St. st. h.p. Si3N4 348 1350 1 6.4 very small - very small * Neither reaction nor adhesion was observed.

An entry in the shear strength column indicates failure by slipping. An entry in the tensile strength column indicates failure by fracture of the ceramic.

BDH medium Al powder.

1st batch (CLA 0.1 m).

4 316L stainless steel, -45 m dia. powder supplied by Goodfellow Metals Ltd.

5 2nd batch (CLA 1.0 m).

Example 3.

A collar of molybdenum powder was isostatically pressed onto a Fefel silicon carbide tube and the molybdenum then machined down to form a uniformly thick collar (wall thickness 1.5 mm). The sample was then sintered in an atmosphere of 90% argon and 10% hydrogen. The strength measurements are shown in Table 6.

The samples failed by fracture of the molybdenum collar.

TABLE 6 Joining Mo* to Refel silicon carbide tube

Sintering Sintered Metal forming temp time metal Breaking pressure density load MN/m2 OC h g/cm3 N 174 1400 5.4 580 380 1400 6.1 2800 * Powder from Hermann Starck.

Example 4.

A 316L stainless steel collar, approximately l inch long, was produced by isostatically pressing metal powder around a shaped steel mandrel so that the bore of half of the collar was 0.25 in. and the other half was 0.23 in. A EN58A stainless steel tube 0.23 in. diameter was pushed into one half of the metal powder compact and a hot pressed silicon nitride rod 0.25 in. diameter was pushed into the other half of the compact. The sample was then heated for I hour at 12000C in an atmosphere of 90% argon and 10% hydrogen. On cooling the sintered metal collar gripped both the metal tube and the ceramic, thus forming a strong joint. Attempts to break the joint using an assembly such as that shown in the drawing resulted in failure of the grip (stuck on to the silicon nitride with epoxy resin) at a load of 2135N.

Example 5.

Joints similar to those described in Example 3 have also been produced in which the ceramic is shaped in such a way that the metal powder keys to the ceramic after sintering e.g. the ceramic can be grooved or tapered to further decrease the chances of the metal collar being pulled off the ceramic rod, or the section of the ceramic may be non-circular to prevent the metal-ceramic joint failing in torque.

Example 6.

Nickel powder was isostatically compacted at a pressure 275 MNIm2 around a twin-bore alumina thermocouple sheath of approximately rectangular crosssection (8 mm x 5 mm) to form a collar of diameter 15 mm and length 10 mm close to one end of the thermocouple sheath. The collar was sintered at a temperature of 1200"C for l hour in an atmosphere of 90% argon and 10% hydrogen. On cooling, the collar was firmly attached to the thermocouple sheath. The collar was subsequently machined and a screw-thread was formed on its outer surface.

Example 7.

Nickel powder was isostatically compacted at a pressure of 275 MN/m2 around the end of an alumina tube (10 mm o.d. x 6 mm i.d.) to form a closure for the tube.

After sintering at 12000C for 1 hour in an atmosphere of 90% argon and 10% hydrogen the metal cap was firmly attached to the alumina tube.

Example 8.

A collar 15 mm o.d. with a flange of 30 mm o.d. was formed around alumina thermocouple sheath (10 mm o.d. x 500 mm long) by isostatically compacting stainless steel powder around the sheath at a pressure of 175 MN/m2. After sintering the stainless steel flanged collar at 12000C for 1 hour in an atmosphere of 90% argon and 10% hydrogen, the collar was firmly fixed. The metal flange was then brazed to a larger metal plate.

Example 9.

Nickel powder was formed into a disc of diameter 50 mm and thickness 10 mm by pressing in a steel die. The disc was subsequently further densified and the density made substantially uniform by isostatic compaction at a pressure of 200 MN/m2. Prior to sintering at 12000C for l hour in an atmosphere of 90% argon and 10% hydrogen a multiplicity of holes was drilled through the disc, each hole allowing the insertion of a ceramic tube or rod. After sintering, the various ceramic tubes and rods, which included tubes or rods of alumina, titania, fused silica, electrical porcelain and mullite, were found to be firmly secured to the nickel disc.

Example 10.

Discs (12 mm dia. x 6 mm thick) were formed in a steel die by pressing a mixture of metal powders consisting of 80% molybdenum, 12% manganese and 8% nickel. A hole (N6 mm dia.) was drilled centrally through each disc, the hole being slightly larger in diameter than the diameter of a hot-pressed silicon nitride rod which was placed in the hole prior to sintering. After sintering at a temperature of 1150"C for T hour in an atmosphere of 90% argon and 10% hydrogen the metal collar was firmly attached to the ceramic rod. When the joints so formed were tested by applying an axial tensile load to the ceramic and to the metal collar the joints withstood tensile stresses of up to 146 MN/m2.When similarly tested at higher temperatures the joints withstood tensile stresses of up to 70 MN/m2 at 5000C and up to 55 MN/m2 at 7600 C. The Mo-Mn-Ni alloy used was chosen because it is one of a large number of similar alloys which can be densified by the process known as liquid-phase sintering and because the Mn-Ni alloy which forms the liquid phase is one member of a family of similar alloys known to react with hotpressed silicon nitride to form a chemical bond.

Example l l.

Refel silicon carbode tubes (6.4 mm o.d. x 3.15 i.d.) and reaction sintered silicon nitride rods (6.4 mm o.d.) were glazed with a borosilicate glass and a manganese aluminosilicate (MnA1Si) glass. After glazing annuli of powdered nickel were fitted over the glazed rods and tubes and the nickel sintered at various temperatures. The tensile strengths of the joints are shown in the Table. Most of the joints failed by fracture of the ceramic and by sectioning the annulus it was found that the nickel was chemically bonded to the ceramic via the glaze layer.

TABLE 7 Room Temperature tensile strengths of sintered Ni joints using glass coated ceramic components. Sintered for 1 hour in 90:10 Ar/H2

Nickel Mean tensile Sintering Glass Sintering No. of strength Ceramic Glass ( C) Samples (MN/m2) SiC. tube Borosilicate 800 4 59.0 Borosilicate 850 4 55.0 MnA1Si 850 5 83.0 r.s. Si3N4 rod. Borosilicate 850 5 22.6 Borosilicate 900 5 10.5 MnA1Si 850 5 54.2 MnAlSi 1200 3 38.2 WHAT WE CLAIM IS: 1. A method of joining a metal component to a ceramic component, the metal component having been formed by powder metallurgy to a density less than that of the fully dense metal, in which the two components are assembled in juxtaposition such that when the metal component shrinks it will grip the ceramic component, and then the two components are heated so that the metal component sinters and shrinks onto and grips the ceramic component upon cooling.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. Example 9. Nickel powder was formed into a disc of diameter 50 mm and thickness 10 mm by pressing in a steel die. The disc was subsequently further densified and the density made substantially uniform by isostatic compaction at a pressure of 200 MN/m2. Prior to sintering at 12000C for l hour in an atmosphere of 90% argon and 10% hydrogen a multiplicity of holes was drilled through the disc, each hole allowing the insertion of a ceramic tube or rod. After sintering, the various ceramic tubes and rods, which included tubes or rods of alumina, titania, fused silica, electrical porcelain and mullite, were found to be firmly secured to the nickel disc. Example 10. Discs (12 mm dia. x 6 mm thick) were formed in a steel die by pressing a mixture of metal powders consisting of 80% molybdenum, 12% manganese and 8% nickel. A hole (N6 mm dia.) was drilled centrally through each disc, the hole being slightly larger in diameter than the diameter of a hot-pressed silicon nitride rod which was placed in the hole prior to sintering. After sintering at a temperature of 1150"C for T hour in an atmosphere of 90% argon and 10% hydrogen the metal collar was firmly attached to the ceramic rod. When the joints so formed were tested by applying an axial tensile load to the ceramic and to the metal collar the joints withstood tensile stresses of up to 146 MN/m2.When similarly tested at higher temperatures the joints withstood tensile stresses of up to 70 MN/m2 at 5000C and up to 55 MN/m2 at 7600 C. The Mo-Mn-Ni alloy used was chosen because it is one of a large number of similar alloys which can be densified by the process known as liquid-phase sintering and because the Mn-Ni alloy which forms the liquid phase is one member of a family of similar alloys known to react with hotpressed silicon nitride to form a chemical bond. Example l l. Refel silicon carbode tubes (6.4 mm o.d. x 3.15 i.d.) and reaction sintered silicon nitride rods (6.4 mm o.d.) were glazed with a borosilicate glass and a manganese aluminosilicate (MnA1Si) glass. After glazing annuli of powdered nickel were fitted over the glazed rods and tubes and the nickel sintered at various temperatures. The tensile strengths of the joints are shown in the Table. Most of the joints failed by fracture of the ceramic and by sectioning the annulus it was found that the nickel was chemically bonded to the ceramic via the glaze layer. TABLE 7 Room Temperature tensile strengths of sintered Ni joints using glass coated ceramic components. Sintered for 1 hour in 90:10 Ar/H2 Nickel Mean tensile Sintering Glass Sintering No. of strength Ceramic Glass ( C) Samples (MN/m2) SiC. tube Borosilicate 800 4 59.0 Borosilicate 850 4 55.0 MnA1Si 850 5 83.0 r.s. Si3N4 rod. Borosilicate 850 5 22.6 Borosilicate 900 5 10.5 MnA1Si 850 5 54.2 MnAlSi 1200 3 38.2 WHAT WE CLAIM IS:

1. A method of joining a metal component to a ceramic component, the metal component having been formed by powder metallurgy to a density less than that of the fully dense metal, in which the two components are assembled in juxtaposition such that when the metal component shrinks it will grip the ceramic component, and then the two components are heated so that the metal component sinters and shrinks onto and grips the ceramic component upon cooling.

2. A method as claimed in Claim I in which the metal component is sintered to

a density less than its full theoretical density.

3. A method as claimed in Claim l or Claim 2 in which the metal component is formed as a separate powder metal blank.

4. A method as claimed in Claim 3 in which the separate powder metal blank is partially sintered before assembly with the ceramic component.

5. A method as claimed in Claim l or Claim 2 in which the metal component is formed directly onto the ceramic component.

6. A method as claimed in any preceding claim in which the surface of the ceramic component in the region of the joint has been glazed.

7. A method as claimed in any preceding claim in which the metal component is one which will sinter by liquid phase sintering.

8. A method as claimed in any preceding claim in which the joint is hermetically sealed by providing a sealing coating over the joint and at least the regions of the two components adjacent the joint.

9. A method of joining a metal component to a ceramic component substantially as herein described with reference to any test of any of Examples l to 5.

10. A method of joining a metal component to a ceramic. component substantially as herein described with reference to any of Examples 6 to 10 or any test of Example

I I.

l l. A joint between a metal component and a ceramic component when made by a method as claimed in any preceding claim.