The present invention relates to a method of manufacturing a compound body by providing the body in porous state with a layer by dipping during subpressure in said body in a slurry of components of the layer material whereafter the compound body is heat treated e.g. by sintering.

In order to increase the wear resistance of a body exposed to wear without losing toughness, the body is often coated with a wear resistant layer. Such is the case for inserts for machining where one or more layers are applied using CVD- or PVD-technique. Layers deposited in this way are usually single phase and a suitable thickness is some ten microns. A limitation with said layers is that it is difficult to combine fine grain size, thick layers ^' and good adhesion.

Applying a layer on porous bodies by dipping is a suitable production method. This leads however often to problems with bubbling and blistering and an incompletely covering layer. The bubbling and blistering is due to the pore system and is accentuated by coarse and/or unevenly distributed pores. A particularly unfavourable pore system is found in materials containing fibres and/or needle- and/or platelike single crystals.

When pressing a porous body isostatically the body is usually enclosed in a dense enclosure in order to prevent the pressure medium from penetrating into the pore system of the body. The enclosure is usually a glass or an at heating glass forming material which

is applied in powder form. In order to avoid that glass penetrates into the pores of the body and/or reacts with the same during the isostatic pressing one or several intermediate layers are applied which acts as barrier or protective layers. After completion of the isostatic pressing the glass as well as the intermediate layers are removed. Said intermediate layers are generally applied by dipping in a slurry of components included in the layer material but due to bubbling and blistering during the dipping there are often hollows in the intermediate layer. From Swedish patent application SE 9004134-4 is known an improved method of applying such layers by dipping in a slurry of powdered material included in the layer whereby a subpressure is obtained in the porous body by at least one suction cup. Such layers are not part of this invention.

The invention is illustrated in fig. 1 which is a light optical micrograph (300X) of a ceramic body coated with AI2O3 according to the invention, figs 2 - 3 which illustrate the method according to the invention in which A - layer B - body and C - suction cup.

Fig 4 - 5 show examples of products for the coating of which the invention can be used.

According to the invention there is now provided a method for manufacturing a compound body characterized in that at least one, at least partly covering layer is applied to a porous body. The

invention is characterized in that the porous body is evacuated by means of a subpressure applied on an external surface of the body with the aid of one or more suction cups or similar and a vacuum pump while dipping the body in a slurry of powdered materials included in the layer. If several layers are included the body is, of course, dipped several times with or without intervening drying in order to obtain desired layer combination and/or thickness. The method is particularly suited for coating of porous powdermetallurgically manufactured bodies in compacted, presintered or sintered state particularly with multiphase fine-grained layers. In order to preserve the finegrained structure grain growth inhibitors can easily be added. After the application of the layer the body is densified by sintering, HIP, gas pressure sintering or other heat treatment. The body obtained hereby is either still porous or essentially completely dense. The applied layer can during sintering/heat treatment react with the body and a more or less sharp transition zone is obtained.

The slurry of powdered materials is prepared by dispersing 2 - 50, preferably 5 - 30 % by volume of the material with a grain size <100 μ in a liquid (alcohol, ketone, hydrocarbon or water or a mixture of two or more of these) . For cutting tool applications the grain size should preferably be 0.1 - 5 μ . In order to obtain a good deagglomeration and a stable dispersion suitable organic surface active compounds are added. Organic compounds can also be added in order to give the dispersion optimum viscosity and to give good strength to the applied layer. If water is used as dispersing medium the pH- value has to be adjusted in such a way that it

preferably is well above or well below the zeta- potential of the powdered material. If a coating is desired consisting of several layers with different composition a suspension for each composition is made.

The dipping is repeated until desired thickness and composition is obtained. The thickness of the layer depends on the application. The whole body or only part of it is coated depending application. After the dipping the layer is dried. The drying can be accelerated by e.g. a hot air fan. It is important to adjust the drying rate so that the layer does not crack. Often one or more additional drying steps between the dippings are required. The organic compounds are driven off either in a separate evaporation step or during the subsequent sintering.

The method according to the invention is suitable for coated cutting inserts made of e.g. ceramics, cemented carbides, titanium based carbonitride alloys so called cermets etc. Hereby a more wear resistant layer is applied on top of a more tough core e.g. single phase layers such as Tie, TiN and/or AI2O3 or preferably multiphase layers such as gamma-phase containing cemented carbide layers on a core of only WC-Co-cemented carbide or a layer of finegrained cemented carbide on a more coarse-grained. The wear resistant layer can also contain particles of diamond or other wear resistant materials. Other possibilities are applying layers of WC and/or gamma phase and/or cobalt on ceramic bodies, preferably AI2O3-based. The thickness of the layer should preferably be <50 μm, most preferably 5 - 15 μm.

Another alternative is to apply layers that are chemically more stable than the body such as an AI2O3-layer on a SiC-body.

Yet another alternative is to apply layers that gives the coated body more suitable frictional properties higher as well as lower. For example a porous body of Si3N4 doped with 2O3 can be coated with a layer of the same material including hexagonal BN to obtain lower friction or with a layer of the same material including SiC to obtain higher friction.

Yet another alternative is to apply layers that have other electrical properties, such as other electrical conductivity than the body. For example a porous body of TiB2 and Si3N4 can be coated with a layer of TiN, Si3N4 and Y2O3 to obtain insulating layer on a conductive core.

The layer material shall preferably be chosen with a coefficient of thermal expansion that is lower than the coefficient of the body in order to obtain compressive prestresses in the layer.

The method is particularly suitable for applying layers on materials preferably for cutting tools that are reinforced with whiskers, fibres and/or platelets, e.g. an Al2θ3~layer on an AI2O3-material reinforced with SiC-whiskers. The layer is thereby applied on the material in presintered condition and thereafter sintered, preferably by isostatic sintering. A certain shaping of the presintered material before coating can be needed in order to counteract formchanges during the isostatic sintering. In particular, for tool pressed parts it

can be necessary to remove material which during the compaction has been sheared against the die and has obtained a substantial orientation of the whisker material.

The method according to the invention can also be applied to materials which can not be sintered to closed porosity by conventional sintering and therefore have to be isostatic sintered using a gas pressure transmitting medium. A first layer is then applied by dipping during subpressure. During the densification process this layer at least partly reacts with the material in the porous body or in other ways forms an adhering layer which is not removed after finished densification. Thereafter another at least one intermediate layer is applied by dipping and after that a cover of glass or material which forms a glass when heated is applied in powder form. This enclosure is made impenetrable for the pressure medium by heating. The body is then compacted to an essentially dense body by isostatic pressing and sintering whereafter said intermediate layer and glassy enclosure is removed resulting in a coated body.

According to the method of the invention coated bodies with thicker layers than what is possible and reasonable in known ways can be manufactured. The method is also convenient from a production point of view and it gives possibility to manufacture fine grained layer with good adhesion and multiphase composition. For bodies with unfavourable pore system coatings manufactured according to the invention can be advantageous because the slurry with fine-grained material can penetrate into the agglomerate pores,

see fig 1. In such a way the occurrence of flaws originating from agglomerate pores in the surface region of the sintered material will be reduced.

The method according to the invention for the manufacture of a compound body has also turned out to be a suitable way of producing ceramic fiber composites, preferably inserts for machining with or without preformed chip breaker with high relative density without using sophisticated methods such as hot pressing, (HP) or hot isostatic pressing (HIP) with glass encapsulation. Ceramic fiber composites with high amount of fibres, whiskers and/or platelets can not be sintered dense without the use of high pressure because they effectively counteract densification by forming a network. In the method according to the invention an even, crack-free layer of densely packed particles is made by dipping of compacted or presintered bodies in a particle slurry with water, cyklohexane or other solvent. Thanks to the high density of particles in the surface a dense surface layer is obtained after sintering, preferably under vacuum, for which reasons the sintered body which now only contains closed porosity can be densified afterwards by a hot isostatic pressing so called post-HIP-treatment without having to apply further layers. It has turned out that dipping without subpressure in various particle slurries does not give sufficient quality of the applied layer and insufficient densification is obtained at a subsequent post-HIP-treatment.

Example 1

A ceramic part to an electrical contact used in a tube for corrosive liquids was manufactured in the following way as illustrated in figs 2 - 3. The part consists of a conductive core coated by a non- conductive layer.

A ceramic powder with the following composition 48% by weight iB2 and rest Si3N4 was tool pressed to a body (B) in fig 2. The body was dipped using a suction cup (C) connected to a vacuum pump to the half in a slurry. This consisted of 20 parts by volume of a ceramic powder with the composition 30 % by weight TiN, 67 % by weight Si3N4 and 3 % by weight Y2O3 in isopropanol with 5 parts by volume of an acrylic binder. The dipping was repeated three times with a 1 minute drying in between. The dipped side was allowed to dry and then the opposite side was coated in the corresponding way. The layer (A) obtained had a thickness of about 500 μ . The coated body was encapsulated in a glass powder and the organic binder removed by heating in vacuum to 600 °C. After that the- coated body, fig 3, was densified by hot isostatic pressing at 1650 °C and 150 MPa for 1 h. The glass was removed by blasting.

The insulating layer on the protruding parts of the body was ground on both sides to get contact with the electrically conductive core. The insulating layer was slightly ground on the flat surfaces which are to be used for clamping.

The finished part was mounted into an electrical contact according to fig 5.

Example 2

Three die pressed cemented carbide inserts (SNGN 1204-style) were manufactured in three different ways and compared technologically. The cemented carbide had the composition 5.5 % by weight Co, 8.6 % by weight TiC+TaC+NbC and rest WC.

Insert 1 was gas pressure sintered at 1400 °C and 10 MPa in Ar-atmosphere. The insert was finished by edge treatment to 30 μm edge radius.

Insert 2 was gas pressure sintered in the same way as insert 1 and was then given an edge rounding to 30 μm. It was after that coated by CVD with 1-2 μm TiC and 6 - 7 μm AI2O3.

Insert 3 was sintered to 70 % relative density and was after that given an edge rounding of 30 μ . After that an about 30 μm thick AI2O3-layer on the insert was applied with aid of the dipping technique according to the invention. The insert was held by a suction cup connected to a vacuum pump as illustrated in fig 2 and dipped in a slurry of 20 % by volume AI2O3 with a grain size of about 0.5 μm in butanol with an acrylic binder. The AI2O3-coated insert was gaspressure sintered in the same way as insert 1 and 2. In this way a thicker, about 15 μ , and more finegrained AI2O3-layer was obtained.

The three insert were compared with respect to life in case hardened steel (SS2511) with the following resul :

Insert 1 (no layer) obtained as expected a severe wear already at low cutting speeds and was therefore tested only at 200 m/min. The wear life was 4 min.

Inserts 2 (CVD-coated AI2O3) and 3 (coated according to the invention) were tested at 500 m/min. Hereby insert 2 obtained wear life of 7 min and insert 3 11 min. The results show that the AI2O3-layer applied according to the invention had better adhesion than the AI2O3-layer applied by CVD-technique.

Example 3

Three tool pressed ceramic inserts consisting of 25 weight-% SiC-whiskers with the rest AI2O3 were manufactured in three different ways and compared later in technological testing.

Insert 1 was presintered at 1300 °C in H2-atmosphere and was then ground peripheral in such a way that about 100 μm material was removed around. Protective layers were applied and the insert was densified using glass encapsulated HIP according to previously mentioned patent application. The protective layers and the glass encapsulation were removed and the insert was edgerounded to about 30 μm.

Insert 2 was presintered and peripheral ground in the same way as insert 1 and edge rounded to about 30 μ . Thereafter an about 10 μm thick layer consisting of 25 % TiC and rest AI2O3 was applied by dipping technique. The insert was held by a suction cup connected to a vacuum pump as illustrated in fig 2 and dipped in a slurry of 20 % by volume ceramic powder with the composition of 70% by weight AI2O3

with a grain size of about 0.5 μ and 30% by weight of TiC with a grain size of about 0.8 μm in butanol with an acrylic binder. Application of protective layers, densification and the removal of protective layers and the glass encapsulation were performed in the same way as for insert 1. The finished insert had a TiC + AI2O3-layer with about 5 μm thickness.

Insert 3 was manufactured in the same way as insert 2 but the layer was made twice as thick.

The inserts were compared in a turning test in hardened ball bearing steel. Hereby insert 2 obtained twice as long life as insert 1 and insert 3 obtained an additional increase with 50 % tool life.

Examination of the wear pattern showed that the applied TiC+Al2θ3-layer reduces the crater wear.

Example 4

A cemented carbide blank was manufactured in such a way that a core was compacted from cobaltrich i.e. tough material (10.2 % by weight Co, 5.8 % by weight TiC+TaC+NbC and rest WC) . The blank was presintered to 70 % relative density, whererafter a 150 μ thick layer of a more Co poor material (7.2 % by weight Co, 5.8 % by weight TiC+TaC+NbC) and rest WC was applied by dipping technique similar to previous examples. On top of this layer another 200 μ thick layer with the composition 2.2 % by weight Co, 8.8 % by weight

TiC+TaC+NbC and rest WC was applied. Everything was gas pressure sintered at 1350 °C and 10 MPa to a dense body.

Example 5

I2O3 with 29 % by volume SiC-whiskers was sintered without pressure with and without layer of AI2O3 applied by dipping. The porous body was dipped in a particle slurry with 20 % by weight AI2O3 and cyclohexane as solvent. Dipping was performed both with and without evacuation of the porous body with the aid of a vacuum pump, see fig. 2. Sintering was performed at 1550 °C in Ar at low pressure and the subsequent post-HIP-treatment at 1550 °C and 200 MPa pressure in Ar.

The results below were obtained shown as % of theoretical density:

After After sintering post-HIP Without layer <60 <60

Layer by dipping <60 <60

Layer with dipping and evacuation 58 99

The results show that only the method according to the invention has after sintering resulted in a dense surface layer which allows densification to full density at the post-HIP treatment.

Example 6

Three tool pressed ceramic inserts consisting of 25 weight-% SiC-whiskers with the rest AI2O3 were manufactured in two different ways.

Insert 1 was presintered at 1300 °C. Protective layers were applied by the dipping technique whereafter the insert was densified using glass encapsulated HIP-technique. The HIP-ed insert was ground peripherical and edgerounded to about 30 μ .

Insert 2 was presintered in the same way as insert 1 whereafter an about 30 μ I2O3-layer was applied by dipping technique without subpressure. Application of protective layers, glass encapsulated HIP, peripheral grinding and edge rounding were performed in the same way as for insert 1. The finished insert had a partly flaking coating with craterlike defects.

Insert 3 was manufactured in the same way as insert 2 but the AI2O3-layer was applied with subpressure according to the invention, similar to previous examples. The finished insert had a well adhering 10- 15 μm AI2O3-layer on the rake face as illustrated in fig. 1.

Example 7

The inserts described in example 5 were compared in a turning test in a constructional steel (SS1672, equivalent to AISI 1045) at the following conditions: Speed: 350 m/min Feed: 0.2 mm/rev

Cepth of cut: 1.0 mm

The tool life of insert 1 was 7 minutes, insert 2 was 8 minutes and insert 3 was 12 minutes. The results show that the I2O3-layer applied according to the invention had better wear resistance than the AI2O3- layer applied without subpressure. No flaking could

be observed on insert 3 whereas insert 2 showed such flaking behaviour that tool life was essentially similar to that of the uncoated insert.