Whisker-reinforced ceramic cutting tool material

Abstract: The present invention relates to a ceramic whisker-­ containing cutting insert for chipforming machining of steel having improved mechanical, thermal and wearing properties. Besides an oxide-based matrix the material consists of 5-50 % by volume, preferably 10-40 % by volume of homogenously distributed whiskers of nitrides, carbides and/or borides of metals from group IVB (Ti, Zr or Hf) and/or VB (V, Nb or Ta) in the periodical system (or solid solutions thereof) having a linear expansion coefficient being lower than that of Al₂O₃, preferably at the most 85 % of that for Al₂O₃ measured at 300 - 1300 K. The properties of the composite material can be further modified by coating the whisker material with thin layers. The whisker reinforced cutting tool material shows an improved toughness behaviour.

Description: The present invention relates to ceramic cutting tool materials and particularly such cutting materials in which monocrystalline whiskers (hair crystals) are uniformly distributed in a ceramic matrix containing aluminium oxide which leads to an improved strength and toughness without showing a negative influence upon the wear resistance. Ceramic cutting tools have now been available for several decades but they have not until recent years had any great commercial importance for use in chipforming machining. The main reason for the limited growth of ceramic cutting tools has been sudden and unexpected tool failures because of inadequate strength and toughness. In recent years the properties of ceramic cutting tool materials have been improved in many respects and their relative use in cutting of cast iron and hot-strength (nickel-base) alloys has increased. The proportion of ceramic cutting inserts is still very small, however, where steel is the dominating work piece material being machined, because this material makes great simultaneous demands upon strength, tougness and wear resistance which have not been fulfilled by known ceramic cutting tool materials of to-day. Aluminium oxide based cutting tool materials are very sensitive to thermal crack formation because aluminium oxide in itself has a relatively poor thermal conductivity. This leads to very short tool lives in machining steel particularly under conditions with short operating times and varying cutting depth. The thermal properties have to some extent been improved by additions of titanium carbide and/or titanium nitride which enhance the thermal conductivity of the composite material. The addition of titanium carbide/nitride also increases the hardness of the material. In comparison with pure aluminium oxide materials an increased tool life is therefore obtained in the cutting of harder work piece materials and in operations demanding thermal shock. Said kind of material has, however, too poor toughness behaviour for a general use in cutting of steel. Another later step of development relates to alloying of uniformly dispersed fine-grained zirconium oxide particles in a matrix of aluminium oxide. A transition of the "metastable " zirconium oxide particles during use increases both strength and toughness and will thus give a more predicted tool life. The thermal properties of said type of material are, however, only slightly better than those of pure aluminium oxide materials and therefore initiation and growth of thermally induced cracks is still a great problem in practical cutting operations generating high cutting edge temperatures such as the cutting of steel. It has recently been shown that alloying of SiC-whiskers, - with mono-crystalline hair crystals, in a matrix of aluminium oxide leads to a greatly improved fracture toughness and strength. Ceramic cutting tool materials based upon said concept have shown very good performances in the cutting of hot-strength materials particularly but in the cutting of steel they have shown surprisingly short lives because of preferential attack of the SiC-crystals. This leads to a weakening of the surface zone with accompanying high wear and risks of crack initiation. It is thus an object of the invention to obtain a ceramic material for chipforming machining, particularly of steel in which the above-mentioned weaknesses of to-day's known aluminium oxide based cutting tool materials have been eliminated. Thus in materials according to the invention it has been found a unique combination of strength, toughness, thermal-shock-resistance and wear resistance which has not been possible to obtain with earlier known oxide-based cutting tool materials. The primary aim of the present invention is to make oxide-based ceramic cutting tool materials strengthened by whiskers with low solubility in steel. It results in a cutting tool material with an improved and more predictable toughness, strength and resistance to thermal shocks without deterioration of the wear resistance to any appreciable degree particularly when cutting steel. This has not been possible with earlier known material compositions. Such a material is described in EP-A1-0 283 454 which relates to whiskers based upon carbides, nitrides and borides of Ti or Zr or solid solutions thereof. It has now been surprisingly found that further improvements of fracture toughness and resistance to thermal shocks can be obtained if the differences in thermoelastic properties between the aluminium oxide and the whiskers material are large. According to R.W. Rice, Ceram. Eng. Sci. Pro., 2 (1981) 661. and A.G. Evans, H.K.Bowen, R.W. Rice and K.M. Prevo, Rep. Materials Research Council Summer Conf., La Jolla, Ca., 1983. the most essential criteria in developimg a fiber-strengthened ceramic material are as follows: 1. A high fibre modulus and strength (preferably more than twice that of matrix for an effective load transition to the fibre). 2. A small fibre diameter (of the same range of order as the grain size in matrix) . 3. A homogenous distribution of fibres. 4. None or small chemical bond between fibre and matrix. 5. No chemical reactions which destroy or change the properties of the fibre. 6. A large volume fraction of fibres. 7. Comparable thermal expansion coefficients (alpha), but preferably a greater expansion coefficient for the fibre than for matrix so that compressive strains are obtained in the matrix. Under point 7 it is specially mentioned that the thermal expansion coefficient shall be greater than or equal to that of matrix. This requirement is fulfilled in the above-mentioned Swedish patent application because the difference of the thermal expansion coefficient between TiC, TiN, TiB₂ and ZrC, ZrN, ZrB₂, respectively, and Al₂O₃ is very small. It has now been found particularly favourable to use whiskers with lower thermal expansion coefficient than that of Al₂O₃ which leads to tensile strains in the aluminium oxide and compressive strains in the whisker. The exact mechanisms of the improvements of fracture toughness and resistance to thermal shock are not known but probably there is also a formation of microcracks during use which favourably affects the toughness behaviour and resistance to thermal shock. Another factor which can be important is that tensile strains in the aluminium oxide are oriented parallelly with the longitudinal axis of the whisker and therefore crack propagation occurs perpendicularly to the whisker which favours bridging of the cracks (the whisker bridges over a passing crack and strives to hold it together). The ceramic cutting tool material according to the present invention consists of an aluminium oxide-based, matrix with 5-50, preferably 10-40, often 25-35 % by volume of homogenously dispersed whiskers based upon nitrides, carbides and/or borides of Hf, V, Nb or Ta or solid solutions thereof being characterized by a thermal expansion coefficient which is lower than that of Al₂O₃ and a low solubility in steel. In Table 1 below examples are given on whisker materials which fulfil the criteria on low solubility in steel. Thermal expansion coefficient (10⁻⁶ K⁻¹ 300-1300 K) Al₂O₃ 8.4 circa 8 TiN 7.5-8.9 HfC 6.5 circa 6 HfN 6.3 NbC 5-7 TaC 4-6.5 circa 5 TaN 5.0 TaB₂ 5.2 As evident from the table no or very small self-strains can be expected by the use of TiN-whiskers. Use of HfC, HfN and NbC gives tensional strains in the aluminium oxide. These will be still greater by use of TaC, TaN or TaB₂. The whisker material consists of monocrystals with a diameter of 0.2-10 µm and a length of 2.5-100 µm characterized therefore by a length/diameter ratio that is preferably 5-10. The grain size of the oxide matrix shall be <10 µm preferably < 4 µm. The oxide matrix shall consist of essentially ceramic oxides or of ceramic oxides mixed with hard carbides and/or nitrides and/or borides and/or binder metal. Preferably the ceramic matrix shall contain < 20 % by volume of ZrO₂. The actual cutting tool material is made by wet-milling and mixing of oxide-based powder and monocrystalline whisker crystals. After drying the mixture is pressed to desired geometrical form and sintered without pressure to almost theoretical density. After sintering possible remaining so called closed porosity can be removed by hot-isostatic pressing. If it is not possible during sintering to obtain closed porosity the material can be pressure-sintered by a suitable graphite tool or be hot-isostatically pressed after canning to desired density. The sintering method is dependent upon the whisker material and is chosen so that the cutting tool material reaches a density exceeding 99 % of the theoretical density. The use of whisker-reinforcement in the aluminium oxide-based matrix leads to a significant increase of the fracture toughness. The mechanisms causing said increase can be load transition between whisker and matrix, crack deviation and whisker pull-out. Said mechanisms exploit and depend upon crack-propagation occurring along a sufficiently weak interface between whisker and matrix. The bonding force between whisker and matrix is therefore an important parameter which can be influenced by coating the whisker material with thin layers of for example BN or graphite in order to further improve the fracture strength. In the actual case a contribution from the internal strains being formed by use of whiskers with lower thermal expansion coefficient than that of matrix is also obtained. Exampel 1 30 % by volume of titanium nitride-, hafnium nitride- and tantalum nitride-whiskers, respectively, are wet-mixed with 70 % by volume of a mixture of 95.5 % by weight of Al₂O₃ 4.2 % by weight of ZrO₂ and 0.3 % by weight of MgO. After drying in vacuum the mixtures are dry-mixed and pressed to bodies. The bodies are sintered at 1550°C to 99 % of the theoretical density by isostatic hot-pressing. K IC is measured by means of the so called indentation method. In said method an indentation is made by means of a pyramid diamond tip and K IC is calculated from the size of the cracks produced from the corners of the indentation. At the measurement a reference of Al₂O₃ + 4.2 % by weight of ZrO₂ + 0.3 % by weight of MgO was used. K IC -values for the various material combinations are given in table 2. K IC 1. Al₂O₃ + 4.2 % by weight of ZrO₂ + 0.3 % by weight of MgO + 30 % by volume of TiN(whiskers) 6.1 2. Al₂O₃ + 4.2 % by weight of ZrO₂ + 0.3 % by weight of MgO + 30 % by volume of HfN(whiskers) 7.0 3. Al₂O₃ + 4.2 % by weight of ZrO₂ + 0.3 % by weight of MgO + 30 % by volume of TaN(whiskers) 8.2 4. Al₂O₃ + 4.2 % by weight of ZrO₂ + 0.3 % by weight of MgO 4.0 From the table it is evident that an alloying with TiN-, HfN- and TaN-whiskers, respectively, gives a great increase in fracture toughness. The fracture toughness increases with increasing difference in thermal expansion coefficient. The preferred embodiment is therefore a whisker material having an expansion coefficient of at the most 85 % and preferably at the most 70 % of that of Al₂O₃ measured in the temperature interval 300 - 1300 K. The fracture toughness is a parameter which shows the capacity of a material to stand mechanical stresses without leading to catastrophic failures. In the case of chip-forming machining it means that higher feeds can be permitted i.e. the rate of material removal can be increased for a certain cutting speed. The invention has been described by referring to whisker-reinforced materials. It is obvious to those skilled in the art, however, that similar results can be obtained if the whiskers are wholly or partly substituted by monocrystalline platelets or similar. The diameter of the platelets shall be less than 50 µm, preferably less than 20 µm, and the thickness less than 5 µm, preferably less than 2 µm.

Claims: Al₂O₃-based ceramic material for chipforming machining of steel, characterized in that said material besides the Al₂O₃-based matrix contains 5-50 % by volume, preferably 10-40 % by volume of homogeneously dispersed whiskers of nitrides, carbides and/or borides of Hf, V, Nb or Ta (or solid solutions therof) having a linear expansion coefficient which is lower than that of Al₂O₃ and preferably at the most 85 % of that of Al₂O₃ measured at 300 - 1300 K. Material according to the preceding claim, characterized in that the matrix is based upon Al₂O₃ and < 20 % by volume of ZrO₂.

Inventors: BRANDT GUNNAR, BRANDT, GUNNAR
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1990
Main classification: C04B35
IPC codes: C04B35