The present invention relates to new PCD (polycrystalline diamond) or PcBN (polycrystalline cubic boron nitride) tools with excellent properties in tools for the wood industry. More particularly, the invention relates to PCD or PcBN in which a corrosion and oxidation resistant phase has been distributed in the skeleton of diamonds or cBN-particles characterized by extensive bonding between the hard particles. This corrosion and oxidation resistant phase is based on cobalt and/or nickel and chrome.

Reconstituted wood products, such as medium density fibreboard and chipboard, are, together with solid wood, the main raw materials in the furniture industry. They are also used in the housing industry to some extent.

These products are machined with a variety of tool materials, from high speed steel to cemented carbide to polycrystalline diamond. A leading role is being played by tools made with cemented carbide and PCD or PcBN- tools.

The composition of PCD or PcBN used for woodworking tools consist generally of diamonds or cBN with cobalt as a intergranular phase. Sometimes small amounts of other carbides, nitrides or carbonitrides are added.

The constitution of the PCD are built up by one or several layers/translayers with different compositions regarding diamond contents and cobalt.

Abrasion has been thought to be the primary mechanism of tool wear when machining reconstituted wood products and solid wood. Recent work has proven that chemical mechanisms such as corrosion and oxidation play a significant role in the degradation of cutting edges, as the temperature increases dramatically during the machining process.

The chemical degradation of existing PCD and PcBN- tools is at least a two stage process when machining wood products.

At first the degradation occurs at a low temperature (300-500°C) , in the early period of cutting. As the tool temperature rises, the wood products decompose and numerous chemicals are introduced in the cutting environment. More than 200 different compounds have been identified upon the destructive distillation of wood. The machining of medium density fibreboard and particle board produces even more decomposition products. These products contain also a binder such urea, formaldehyde, wax and glue fillers and extenders and possibly chemicals added as flame retardants. The decomposition products formed are highly corrosive and attack the cobalt-phase that fills the voids between the hard particles. When this occurs, the Diamond-grains losses the sites of the diamond-diamond contact due to the high mechanical stresses and the cutting edge loses its sharpness and its cutting capability.

As the temperature rises above 500°C, the decomposition products are volatilised and removed, but degradation of the cutting edge continues by oxidation of the cobalt phase in air. The oxides formed are readily removed by mechanical action, resulting in a fast degradation of the sharpness of the cutting edge. The present invention relates to new types of PCD with excellent properties regarding corrosion and oxidation resistance particularly satisfying the different needs of the wood industry.

Resistance to corrosion and oxidation has been achieved by alloying cobalt and/or nickel with chrome and distribute it in the PCD-skeleton.

The material according to the invention comprises 3 to 20 vol-% of Co and/or Ni with additives of nitrides

and/or carbonitrides of Zr, Ti, Cr, Ta and/or V. The amount of said additives should be less than 50 volume- %, preferably 5 to 40% of the binder phase. The mean grain size shall be less than 5 μm, preferably 0,3 to 4 μm with an even distribution in the structure.

In one embodiment when sharp edges are required e.g. for finishing of hard wood as hickory or teak for fine surface smoothness the mean grain size of said diamonds is <5 μm, preferably <2 μm, most preferably <1.5 μm. The content of the binder shall be 5 to 20 volume-%, preferably 6 to 11 volume-%.

In an alternative embodiment when the wear properties should be optimized e.g. for machining medium density fibreboard and chipboard the mean grain size of said diamonds should be between 5 and 20 μm, preferably 5 and 8 μm. The content of the binder shall be 3 to 12 volume-%, preferably 4 to 8 volume-%.

In both embodiments the diamond grain size distribution may be unimodal, bimodal, trimodal a.s.o. The binder phase comprises in solution in weight-%, Co max 95, Ni max 95 and Cr 5-45 and, in addition, W max 30, Mo max 15, Al max 2, Mn max 10, Si max 2, Cu max 10, Fe max 20, Ag max 5 and Au max 10. In a preferred embodiment the binder phase is nickel based and comprises in solution, in weight-%, Co max 30 and Mo 1- 6. In another preferred embodiment the binder phase comprises in solution, in weight-%, Co max 30-70 and Mo 1-6.

The materials according to the invention are manufactured by powder metallurgical methods known per se namely mixing, compaction and sintering in the diamond stable area.

The material according to the invention is particularly useful for machining of particle board, chip board, medium density fibre board and dry woods.

For cutting of particle board, chip board and medium density fibre board the binder phase content shall be 3 to 12 volume-% and for cutting of solid woods the binder phase content shall be 5 to 20 volume-%.

Example 1

PCD-tools according to the invention were manufactured with a diamond composition of: 4 volume-% of diamond grain size between 1 and 3 μm, 18 volume-% of diamond grain size 4-7 μm, 71 volume-% of diamond grain size 8-12 μm. TiN was added in an amount of 1 volume-% of the total mix. The binder content was 6 volume-% with a composition of 66% Co, 25% Ni, 9% Cr. The powder mixture was milled in a ball mill with ethanol and cemented carbide milling balls for four hours. The milled powder was dried in a rotating vessel and granulated in a 200 mesh size sieve. The precompaction of the powder was performed onto a disc of a straight grade cemented carbide (6% Co) . The sintering was conducted under such conditions regarding temperature and pressure where the diamonds are stable. The PCD-s were cut out by laser cutting into a conventional rectangular cutter shape with a cutting angle of 65° and ground to final shape.

Example 2.

A PCD-tool according to prior art was tested against a PCD according to the invention, example 1. The test was performed a a turning test on medium density fiberboard discs.

Rotating speed 550 rpm

Cutting depth 0,13 mm

The rake and clearance angles were 15 and 10 degrees, respectively. Three samples of each tool material were tested.

The edge radius for all tools was 2 μm.

The edge wear and the surface finish of the chipboard were measured after each sample cut, a total of fifteen, 19 mm discs for a total length of cut of approximately 6883 m. The following result was obtained expressed as parallel, F _p , and normal, F _n , tool force components, in Newton, as a function of sample cut in number of discs.

Number of discs 0 5 10 15

According to the invention

F _n (in Newton) 46,7 51,2 53,4 55,6

Fp 89,0 95,6 100,1 104,5

According to prior art Fn 47,6 57,8 62,3 71,2 Fp 93,4 97,9 106,8 113,4

The tool of the material according to the invention gave a better smoothness of the surface in comparison with the prior art tool.

The invention has been described with reference to PCD tools but it is obvious that it can be applied also to tools with PcBN.