SILICON CARBIDE WHISKER REINFORCED CUTTING TOOL MATERIAL

Abstract: According to the present invention there is now provided an oxide based ceramic cutting insert for chipforming machining of heat resistant alloys comprising a ceramic oxide based matrix and 5-50 % by volume of homogeneously dispersed whiskers of silicon carbide. The whiskers have an average length of 4-7 mu m with a standard deviation of 3-5 mu m, preferably about 4 mu m.

Description: Silicon carbide whisker reinforced cutting too] material. The present invention relates to ceramic cutting tool materials particularly useful for machining of heat resistant alloys and particularly to such cutting tool materials in which monocrystalline silicon carbide whiskers are uniformly distributed in an alumina matrix. US patent 4,543,345 (Wei) discloses a material com- prising a matrix selected from the group consisting of alumina, mullite and B4C having homogeneously dispersed therein 5-60 vol % of silicon carbide whiskers said silicon whiskers having a microcrystalline structure and a size range of about 0,6 μm in diameter and a length of 10-80 μm with an average aspect ratio (length/diameter) of 75. Similar values are found in US patent 4,961,757 (Rhodes) . It is generally understood that a high aspect ratio promotes the toughening mechanisms crack bridging and pull-out (Baeck and Kim, The effect of whisker length on the mechanical properties of alumina-SiC-whisker compo¬ sites, J of Mat Sci 24(1989), p 1589-1593 and Wadsworth and Stevens, The influence of whisker dimensions on the mechanical properties of Cordierete/SiC whisker Compo- sites, J of The European Cer Soc 9 (1992), p 153-163. It has now surprisingly been found that substan¬ tially smaller aspect ratios in the order of 10 give ad¬ vantageous behaviour when using the composite material as cutting tools for machining heat resistant alloys. The ceramic cutting tool material according to the present invention comprises an alumina based matrix with 10-50 %, preferably 15-45 % by volume of homogeneously dispersed whiskers based upon silicon carbide. The alu¬ mina grain size is less than 5 μm and typically in the order of 2 μm. The diameter of the whiskers is less than 1 μm, preferably 0,4-0,7 μm and their average length is pre¬ ferably 4-7 μm (average aspect ratio about 10) with a standard deviation of 3 - 5 μm, preferably about 4 μm. Alternatively the alumina matrix further contains up to 20 vol % ZrC>2 • This addition of zirconia may add fur¬ ther toughening to the material as described in U.S. Patent No. 4,657,877. According to the present invention ceramic compo- sites are prepared by mixing alumina powder and conven¬ tional sintering additives and grain growth inhibitors with monocrystalline SiC whiskers with any suitable mix¬ ing technique. After drying the mixture is hot pressed or hot isostatically pressed to a density at least 99 % of theoretical density. The aspect ratio of the whiskers is not very much affected by the mixing technique and the hot pressing operation although it is expected that at least the longer whiskers will be broken why the desired aspect ratio of the whiskers must be adjusted by premilling of the whiskers prior to mixing with alumina. The aspect ratio of commercial SiC whiskers are normally much higher than 10. Example 1 Alumina-30 vol % SiC whisker composites with diffe¬ rent length of the whiskers were produced by the follo¬ wing procedure. The raw materials used are listed in table 1. Table 1 Raw materials used Material Manufacturer Manufacturer's Particle size designation (diameter) , μm A1 2 0 3 Alcoa A16 SG 0,5 SιC w ACMC Silar SC9 0,5 MgO Merck & Co Y 2 0 3 HC Starck 3,5 * The whisker raw material contains 80-90 % whiskers with the rest being formed of mainly small, submicron silicon carbide particulates. The whisker lengths of the raw material and after the different milling steps were measured after disper¬ sion of the whiskers at a low concentration in an ammo¬ nia solution and subsequently drying on a specimen hol¬ der. SEM micrographs were then analyzed manually in a Quantimet 570 picture analyzer from Cambridge Instru¬ ments. The composition was kept constant for all variants at 25 weight% SiC w and 0,1 weight% MgO and Y 2 0 3 respec¬ tively. All compositions were hot pressed at 1825 °C for 100 in at 25 MPa to discs of diameter 80 mm. Table 2 shows the premilling times that were used for the evaluation of varying aspect ratio on cutting performance and corresponding whisker lengths. Table 2 Premilling times Variant Premilling Whisker length Diameter time, Median, Mean, Std dev, Mean, h μm μm μm μm A 0 23,0 28,2 23,1 0,54 B 4 5,8 6,4 4,0 C 18 4,2 5,9 3,8 D 72 2,7 2,9 1,3 The sintered materials were characterized with re¬ spect to density, hardness, fracture toughness and frac¬ ture strength. Hardness and toughness were measured using a Vicker's indentation with a 10 kg load. All measurements were performed on the plane perpendicular to the hot pressing direction. Fracture toughness was calculated according to the formula: K lc -= 0,0824 — c where P = the force in N and c = the crack length after the indentation. The fracture strength was measured in three point bending. Mean fracture stress was calculated from 9 specimens 13,3 x 3,5 x 2,0 mm, span 9 mm, load rate 50 N/s. The measured values can be found in table 3. Table 3 Density, hardness, fracture toughness and fracture strength. Variant Density, Hardness, Fracture Fracture g/cm- HV toughness, strength, MPam 1 / 2 MPa A 3, ,74 1850 4,9 1042 B 3, ,75 1890 4,8 1056 C 3, ,75 1910 4,6 1066 D 3, ,75 1920 3,8 846 Example 2 The materials were tested in metal cutting opera¬ tions . For this purpose inserts with ISO designation RNGN 120400 T01020 were produced from the hot pressed discs. Two kinds of turning tests were performed; one DOC (Depth of cut) notch resistance test and one tough¬ ness test. The DOC notch resistance test was conducted by wet turning on a cylindrical Inconel 718 bar, using the fol¬ lowing*data: Feed rate = 0,15 mm/rev Cutting speed = 250 m/min Depth of cut = 1,5 mm The toughness test was performed dry using a steel (SS 2242) plate workpiece. The shape in combination with an increasing feed rate results in fracture. The time until fracture is a measure of the toughness of the cut¬ ting tool material . The turning data was: Feed rate = 0,1 mm/rev, increased 0,1 mm/rev every minute Cutting speed = 90 m/min Depth of cut = 1,5 mm Results from the cutting tests can be found in table Table 4 DOC notch wear resistance and toughness behaviour in metal cutting Variant DOC notch wear resistance, Toughness, time to 2 mm notch, median time min to fracture, min A 15 0,51 B 17 0,65 CC 1111 0,46 D 6 0,26 From the tests it can be concluded that variant B is slightly more wear resistant and shows a better tough¬ ness behaviour than variant A with unmilled whiskers. To much milling reduces both wear resistance and toughness behaviour.

Claims: Claims 1. Oxide based ceramic cutting insert for chipfor¬ ming machining of heat resistant alloys comprising an alumina based matrix and 5-50 % by volume of homoge- neously dispersed whiskers of silicon carbide c h a r a c t e r i z e d in that the whiskers have an average length of 4-7 μm with a standard deviation of 3 - 5 μm, preferably about 4 μm. 2. Oxide based ceramic cutting insert according to the preceding claim c h a r a c t e r i z e d in that the matrix further contains up 20 vol % Zr0 2 - 3. Method of producing an oxide based ceramic cut¬ ting insert for chipforming machining of heat resistant alloys comprising a alumina based matrix and 5-50 % by volume of homogeneously dispersed whiskers of silicon carbide by forming a mixture comprising alumina, conven¬ tional sintering additives and grain growth inhibitors and 5-50 % by volume of silicon carbide whiskers, press¬ ing and sintering c h a r a c t e r i z e d in that the silicon carbide whiskers in said mixture have an average length of 4-7 μm with a standard deviation of 3 - 5 μm, preferably about 4 μm.

Inventors: BRANDT GUNNAR, BRANDT, GUNNAR
Applicants: SANDVIK AB
Application year: 1995
Main classification: C04B35
IPC codes: C04B35