SE504434C2 - Nitride based ceramic cutting tool with improved cutting and break strength, useful for machining metals - Google Patents

Abstract

Nitride based ceramic grinder contains in addn. to the nitride matrix 5-50(10-40) vol.% homogeneously divided, whisker-like-fibres of TiCxNyOz, where x and y are between 0 and 1, and z is between 0 and 0.2.

Description

It is obvious that the incorporation of hard particles of a chemically more stable compound significantly improves the wear resistance, but the toughness behavior must be sacrificed to some extent.

Further improvements have been made by using the incorporation of carbides, nitrides and borides of refractory transition elements such as TiC in the form of needle-shaped monocrystals, so-called whiskers, (EP-A-0 255 709). Improved fracture toughness through the use of whiskers but the half strength is not measured. Several types of ceramic whiskers (oxides, carbides, This patent application vinitrides and borides) are known. But for reinforcing ceramic cutting tools, SiC are the only available commercial whiskers, which are very expensive and consequently the use of SiC whisker-reinforced ceramic tools is limited to a few applications where cost and performance can compete with other existing tool materials even though the material is marginal point of view could be used on a wider scale.

No commercial producer exists for TiC and TiN whiskers, although these whiskers have great potential for use in cutting tools. However, these whiskers can be made using CVD (Chemical Vapor Deposition) methods. However, to produce reasonable quantities, the preferred whisker diameter is in the range of several (5-7) microns (US Pat 4,576.79l). will adversely affect the strength of fine-grained cera- It is obvious that such large diameters misk composites for cutting tools.

The object of this invention is to provide a silicon nitride-based material with improved wear resistance without obtaining the negative effects on toughness behavior observed when adding particles, and at the same time an improved fracture toughness without the use of expensive whiskers and also a high strength dependent on the use of high-solids polycrystalline fibers with a small diameter representing a small defect size.

It has, however, now surprisingly been found that in addition to the higher combination of material properties discussed above, i.e. increase in wear resistance and fracture toughness, significant increases in strength can also be obtained using, preferably submicron, polycrystalline fibers of TiN, TiC or Ti ( C, N) with small diameter in a matrix based on silicon nitride. 10 15 20 25 30 35 '3 High quality fibers of TiC, TiN or TiCN 5ÄJ4i Å fi šáp contrast to whiskers of the same compounds, easy to produce in large volumes and at low cost 9400613-7).

The ceramic cutting tool material according to the present (Swedish patent application invention comprises a silicon nitride or silicon-aluminum - preferably 15-40% by volume of homogeneously distributed microcrystalline fibers of TiCxNyO 2 where oxinitride based matrix with 5-50, preferably 10-45, The length of said microfibers should be 2.0-100 μm and the diameter in the range 0.15-5 μm, preferably 0.3-3 μm, characterized in that the length / diameter ratio is 5-50, preferably 10 -25.

According to the invention, the whiskey-like fibers are coated with a thin layer of another refractory material by well-known CVD or PVD methods in order to optimize the bonding between the fibers and the matrix.

Some of the fibers, less than 30%, may be replaced by monocrystalline whiskers and / or particles of various hard carbides and / or nitrides and / or borides of titanium, zirconium, hafnium, niobium, tantalum and mixtures and solid solutions hence.

The matrix material is essentially silicon nitride or aluminum silicon oxynitride. Aluminum-silicon oxynitride or Sialon exist in two different crystalline modifications, namely alpha 'and beta'.

Alpha 'Mx (Si, Al) 12 (O, N) where M = Li, Ca, Y, Beta' is a hexagonal phase of the general formula 516-zAlzOzN8-z where O is a hexagonal phase of the general formula or other lanthanides and where x <2.

The presence of alpha 'phase increases the hardness without reducing the fracture toughness to any measurable extent.

The amount of alpha'-Si-Al-ON is controlled by the relative amounts of different raw materials according to the fact that a smaller amount of alumina and silica and a higher amount of aluminum nitride and yttria or the like gives a higher amount of alpha'-sia- lon in the structure. The matrix further comprises a glassy and / or crystalline 4 phase formed of silica which is always present on the silicon nitride grains and one or more sintering aids. Preferred sintering aids are alumina, silica, yttria, magnesium oxide and zirconia and mixtures thereof.

The use of yttrium oxide as a sintering aid leads to an intergranular phase which is largely glassy but may include other phases ie YAG (a cubic phase of the formula Y3Al5O12), YN-alpha-wollastonite (a monoclinic phase of the formula YSiO2N), YAM (monoclinic of the formula Y4Al2O9) and N-YAM (monoclinic phase of the formula Y4Si2O7N2). The sintering aid comprises 1-20% by weight of the matrix, preferably 1-8% by weight of the silicon nitride matrix.

In the examples, yttrium oxide is used as a sintering aid, but similar results can be obtained with one or more oxides from the group strontium, cerium, lanthanum and the rest of the elements from the lanthanide group.

The cutting material according to the invention is manufactured by wet grinding of a nitride-based powder mixture and sintering aid followed by wet mixing with the microcrystalline fibers. After drying, the mixture is pressed to the desired geometric shape and sintered to more than 99% of theoretical density. Depending on the composition and sintering ability, pressureless sintering, (encapsulated) HIP sintering and hot pressing can be used to consolidate the material. The sintering ability decreases with increasing amount of microcrystalline fibers and with decreasing amount of liquid phase during sintering.

The use of TiCXNyOz fiber-reinforced silicon nitride-based matrix results in a significant increase in wear resistance while increasing both fracture toughness and fracture toughness, which has not been possible with previous particle- and whisker-reinforced tools.

Example 1 TiNO 2, 2 fibers were obtained by heating a mixture of carbon powder and titanium oxide whiskers in an atmosphere of nitrogen at a temperature of about 1300 ° C for 4 hours. The fibers thus obtained had diameters of 0.4-1.5 μm and lengths of 5-15 μm.

TiN whiskers were made in a CVD reactor by coating nickel sponge from a gaseous mixture of TiCl 4, N 2 and H 2 434 at a temperature of about 1200 ° C. The whisker crystals were removed by ultrasonic treatment and mechanical brushing in an acetone bath. The majority of the crystals had a diameter of about 5 microns and a length of 20-100 microns.

A silicon nitride-based material made from a starting powder consisting of 6.5% by weight of yttria powder, 1.8% by weight alumina powder and the remaining silicon nitride powder (oxygen content 1.3% by weight) was ground with Sialon cylpebs to mix the components.

The starting powder was mixed with the above-mentioned TiNO fibers and TiN whiskers. The fibers and whiskers were wet blended with silicon nitride base material.

The powder batches were dried and then hot pressed at 1700-1750 ° C at a pressure of 27 MPa.

A composition without additives was also made for comparison.

Table 1 Composition TiN fiber content, TiN whisker content, volume% volume% 0 0. 0 20 3. 20 0 Example gel 2 Inserts SNGN 120408 were manufactured from compositions 1, 2 and 3 according to Table 1.

The materials were tested in a longitudinal steel turning operation in steel SS 2541-03 and flank and crater wear was registered.

The following cutting data was used: 100 m / min 0.36 mm / rev Speed: Feed rate: Cutting depth: 2.0 mm 10 15 20 25 30 35 504 434 Table 2 Tool wear after 5 min in engagement Composition Crater wear, Flank wear,, um mm 1 90 0.80 2 14 0.17 3 12 0.16 Both TiN whiskers and TiN fibers are very effective in reducing the wear of silicon nitride composites.

Example 3 To measure the hall strength, the inserts were cut and finally polished into thin plates 2x12x12 mm and tested in a biaxial bending test rig.

The following values were obtained Composition Solid strength (MPa) Standard deviation 1 914 in 85 2 757 3 68 3 927 3 91 The introduction of small diameter fibers corresponding to the diameter of the elongated whiskey-like silicon nitride grains in the matrix is not detrimental to the strength while 5 μm whiskers clearly appear defects that reduce the strength.

High strength is critical for performance in many cutting applications as the load bearing capacity of the insert increases and reliability improves.

Theoretically, a small diameter whisker would give similar values of strength as a small diameter fiber, but using CVD methods, the cost is extremely high for small diameter whiskers depending on the legal yield. TiN fibers according to the invention are produced using commercially inexpensive raw materials and consequently from a cost point of view, fiber-reinforced materials are superior.

Claims (5)

1. 504 434 Requirement 1. Nitride-based ceramic insert for chip-forming machining with favorable performance / cost ratio, the insert in addition to the nitride-based matrix comprising 5-50% by volume, preferably 10-40% by volume, of homogeneously distributed, whiskey-like polycrystalline fibers of TiCxNyOZ where Oíxíl, Oíyíl and OízíO.2, the lengths of said fibers being 2-100 μm, the diameters in the range of 0.15-5 μm and the length / diameter ratio 5-50, characterized in that the fibers are coated with a thin layer of another refractory material by CVD or PVD technology. 2. A cut according to claim 1, characterized in that said matrix is essentially silicon nitride or Sialon. Cuts according to one of the preceding claims, characterized in that less than 30% of the whisker-like fibers are replaced by monocrystalline whiskers and / or particles of hard carbides and / or nitrides and / or borides of titanium, zirconium, hafnium , niobium, tantalum and mixtures and solid solutions thereof. Cut according to any one of the preceding claims, characterized in that 0ízí0.l 5.