GB2065715A - Hot pressed silicon nitride - Google Patents

Abstract

A method of making Si3N4 based cutting tools is disclosed. A uniform mixture of Si3N4 powder, having an SiO2 surface coating, and 4-12% by weight HfO2 is prepared. The mixture is hot pressed to substantially full density and the pressed body is shaped as a cutting tool.

Description

SPECIFICATION Method of manufacturing silicon nitride articles This invention relates to methods of manufacturing silicon nitride articles, especially cutting tools.

In the metal cutting industry today there are three principal classes of cutting tool materials that are used, (a) ferrous alloy cutting tools which were introduced around 1900, (b) a carbide family of tools with the first commercial use occurring around 1928, (c) coated tools having a thin layer of a wear resistant fiim consisting of carbides, oxides and nitrides of a metal which is typically deposited over tungsten carbide or other metal carbide base, and (d) ceramic materials primarily based on alumina (Al203), but with some recent work using silicon carbide or silicon nitride based materials. The first alumina based ceramic cutting tools were introduced around 1 960.

Historically, the popularity of commercial use of these cutting tool materials has proceeded in sequence in the order listed above. The ferrous alloy cutting tool materials were originally used as cutting tools because they retained enough hardness to cut metal at low ranges of speed. However, such low speeds limited productivity significantly. The (b) class of metallic carbide with binder phases, such as cobalt or nickel, have permitted cutting tools to be used at intermediate speeds (i.e., 50-800 sfm for tungsten carbide and 200-1200 sfm for titanium carbide). Class (c) materials led to the use of tool speeds which ranged up to the beginning threshold of high speeds.For example, complex carbide coated tungsten carbide had a utility range of 375-1 600 sfm and aluminum oxide coated metallic carbides had a utility range of 625-2000 sfm. These Class (c) materials rely on the concept that a thin layer (about .002 inch thick) of wear resistant material can retard the wear of the substrate.

The thin layer materials have included carbides, oxides and nitrides of titanium, hafnium, zirconium or aluminum. Unfortunately, a relatively thin film can be easily penetrated under certain machining conditions and therefore tool life is shortened.

The material made by the method of this invention is related to the class (d) materials. Many of today's available commercial ceramic cutting tools are alumina based, most containing small additions of other oxides such as Zr203, MgO, CaO, SiO2, and TiO2. These additives are used to aid densification, to control the microstructure, and to improve the mechanical properties. Despite the improvement of the strength of alumina at room temperature, its success as a cutting tool has been marginal in the machining industry. It is generally agreed by those in the prior art that in order for the alumina based ceramics to be successful, rigid machinery and the absence of chattering are necessary preconditions.

Alumina based ceramics are not recommended for rough machining or interrupted cuts which can fracture the brittle material. Alumina has been recently restricted to finish machining operations where rigidity of the machinery and the absence of chattering is optimized. These preconditions make the use of such alumina ceramics unattractive.

The use of hot pressed silicon nitride as a base material with certain densification aids has shown to have extremely high utility with respect to machining certain types of material such as grey cast iron.

In this early work and development of silicon nitride as a cutting tool material, it was discovered how to fabricate the material so that it possessed a high thermal shock factor, even though the strength of such material at high tool temperatures is only moderately good. The excellent thermal shock factor contributed greatly, in an unprecedented manner, to the longevity of this tool material in the machining of cast iron.

Control of the thermal shock factor has been effected by appropriate additions of densifying agents in certain ranges. The densifying agents used to date by the art have included MgO, Y203, Zero2, BeO and CeO. The most successful of these densifying agents has been Y203 in the range of 412%.

These additives and their qualities influence the strength, refractoriness, creep and oxidation resistance of the final product. In general, the role of these additives is to form a molten flux by reacting with the silicon nitride along the boundary of the silicon nitride grains under pressure and temperature. This flux, upon cooling, forms a uniform secondary phase which binds the primary silicon nitride grains. Without special precautions, such process would form the secondary phase as an amorphous or glassy material which is undesirable because it is brittle. Improved processing avoids such amorphous material in the final product either by subsequent heat treatment or by special processing during hot pressing so that the final secondary phase is of a crystalline nature.The secondary phase is believed to consist, particularly in the case where Y203 is used as a pressing aid, of silicates, and more particularly oxynitrides, which form as a resuit of the ternary system of Si3N4, SiO2 (the latter may be present as a coating on the silicon nitride powder particles) and the pressing agent Y2O3.

Although such mixtures of silicon nitride and the specified pressing aids have demonstrated an increase in productivity over comparative prior art material such as alumina, there still remains an area for improvement in the chemical stability of the secondary phase and in the use of a pressing aid which facilitates an even greater reduction in the coefficient of friction of the resulting product when used as a cutting tool.

According to the present invention there is provided a method of making a silicon nitride based article comprising the steps of: (a) preparing a uniform mixture of silicon nitride powder having an SiO2 surface coating and 4-10% by weight HfO2 powder; and (b) hot pressing said mixture at a pressure and temperature and for a period of time to achieve a pressed body having substantially full density.

It has been discovered that the use of HfO2 powder in a controlled amount as a hot pressing aid for silicon nitride powder retains at least the same level of excellent cutting tool characteristics as silicon nitride based materials using known pressing aids. The retained physical characteristics typically comprise high thermal conductivity, low coefficient of expansion, good thermal fatigue resistance, and the elimination of any tool failure by thermal cracking or deformation. In addition, HfO2 provides improved chemical stability, decreased coefficient of friction for the tool, and a slightly better high temperature rupture strength level at high machining speeds and mass removal rates.

It is advantageous if the HfO2 powder is 99.9% pure and has a particle size no greater than 325 mesh. The mixture is preferably milled to an average particle size of about 1.5 microns, using milling media selected from the group consisting of W, WC, Awl203 and SiC. The milling is controlled so that the mixture may contain up to 3.5% of milling media impurities. It is also advantageous if the hot pressing is carried out at a pressure of 4-7 ksi (preferably 6 ksi), a temperature of 1680--17400C (preferably 1 7000 C), and for a period of 2-8 hours (preferably six hours).

With respect to machine cutting utilizing such improved material, the machining process comprises continuous or interrupted machine cutting of solid cast iron stock such as by milling, turning or boring, wherein the shaped tool is moved relative to the stock at a predetermined linear surface rate, feed rate and depth of cut to provide a predetermined mass removal rate of cast iron. The pressed body consists essentially of a primary matrix phase of silicon nitride and an intergranular secondary phase consisting of Si3N4- SiO2 - HfO2.

According to a preferred embodiment of this invention, a cutting tool is made by preparing selected powder ingredients of Si3N4 and HfO2 to form a uniform mixture, hot pressing the mixture to substantially full density by applying pressure and heat for a period of time to effect said density and shaping the pressed body as a tool. HfO2 functions as a fluxing agent and is limited to 1 12% by weight of the mixture. In carrying out the method of making the cutting tool, it is preferable to select a supply of silicon nitride powder which contains SiO2 as a surface oxide coating (typically .751.5%) and contains at least 85% alpha silicon nitride.Impurities are controlled to be less than the following amounts: .5% by weight iron, .01% calcium, .4% aluminum, 2% oxygen, and 1.5% free silicon. Thus, the total cation impurities will be less than 1%, excluding free silicon. It is desirable to employ an average particle size diameter of 2-2.5 microns for such silicon nitride powder. It is advantageous to employ a supply of stabilized hafnium oxide powder which ha a purity of 99.9% and which is available commercially having a particle size of -325 mesh. Stabilized HfO2 is 88% HfO2 and 12% Y203.

Blending and mixing of the powders is carried out by use of a ball mill which not only mixes, but also mills the mixture to a desired particle size. In ball milling for this process, the milling media comprises W, WC, Al2O3 and SiC, and may consist of 1/2 inch length grinding rods. The rotation of the ball mill is continued for a period of several days and is calculated to promote an average particle size in the resulting blended powder of about 1.5 microns. With this operation, it is typical to experience a corresponding ball wear in the range of 1.752.5% by weight of the final batch mixture. Such percentage wear represents the milling media impurities present on the powder in the final mixture.

Hot pressing is carried out at a pressure in the range of S7 ksi (advantageously at about 6000 psi), the ultimate pressing temperature in the range of 1680-1 7500C (preferably 17000 C), and the time at such temperature and pressure being in the range of 2-8 hours (preferably about 6 hours).

The hot pressing is preferably carried out in a graphite die assembly, which graphite serves to act as a protective environment for the material. However, it is also desirable to employ a protective atmosphere, such as N2, in the die assembly. In the graphite die assembly, it is advantageous to initially precompact the powder mixture at a pressure of about 500 psi to promote a stabilized compact. After having achieved percompaction, the pressure of the die assembly is increased to about 6000 psi at a rate of about 1000 psi per minute. At the initiation of the increase in pressure above 500 psi, it is desirable to begin the furnace run by increasing the furnace temperature until the ultimate target temperature of 1 7000C is obtained.Hot pressing is preferably carried out at the ultimate temperature and ultimate pressure for a period of time which is governed by at least 99%, advantageously 99.5%, of full theoretical density or greater. To be sure this is reached, the ram movement of the die assembly is analyzed. When such ram experiences no greater than .002 inches movement during a 1 5 minute interval, the pressure and temperature is relieved and shut off.

The resulting hot pressed body is shaped as a cutting tool, such as by diamond sawing, to assume a desired geometry tailored for a specific machining application. The shaped tool will have a primary matrix phase of silicon nitride with intergranular secondary phases (acting as a binder for the matrix) which consist of a system of Si3N4. Ski02. HfO2. Such secondary phases promote an increased chemical stability for the resulting ceramic and a lower coefficient of friction for the tool when used for a cutting operation. The modulus of rupture for such material will be in the range of 60,000-90,000 psi at room temperature, its coefficient of thermal expansion will be no greater than 1.9 x 106 in/inOF, its thermal conductivity will be no greater than 3.0 BTU/Hr in F, its hardness will be at least 86 Rockwell 45-N, with a density of about 3.25-3.5 grams per cm3.

The ceramic system, as demonstrated by the following samples, when used as a shaped cutting tool, has been found to be very successful in the machining of cast iron, particularly grey cast iron, as measured by tool life. The ceramic system not only provides equal performance in high speed and feed capabilities when compared to silicon nitdde/"'2O ceramic systems, but additionally has enhanced chemical stability and a lower coefficient of friction. Chemical stability is important because of the high thermal conditions existing at the tool tip. Thermal conditions at the tool tip have a significant influence on the chemical interaction leading to the wear of the tool tip. Several mechanisms have been suggested to explain the interaction at the interface between the cutting tool and the chip.In the presence of large, normal and shear stresses and temperature at this interface, the tool undergoes adhesive and abrasive wear. There are also instances of diffusion of the components of the tool and the work material, and mutual solubility. Diffusion and solubility can be controlled by the peak temperature at the tool chip interface and, most importantly, by the chemical stability of the tool material. Wear resistance of the tool due to the chemical stability has been discovered by the applicant to be qualitatively related to the free energy of formation of the tool material. It has been observed by applicant that the properties illustrated in Mendelson's Periodic Table and its free energy of formation is lower than that of aluminum oxide, titanium oxide, or any of the carbides and nitrides of aluminum, titanium, tungsten and silicon.The free energy of hafnium oxide varies from a -125 K.cal/gm at c.n.o. at 10O0Ctoa-87 at20000C.

However, the knowledge that hafnium oxide will promote chemical stability and lower the coefficient of friction in a silicon nitride base material is not sufficient knowledge to know whether it will have the other interrelated physical characteristics so important to a good tool material. The work of this invention has shown that HfO2 will promote most of the properties desired. For example, it is important that when introducing hafnium oxide, that the mechanical strength, coefficient of thermal expansion, and thermal conductivity be extremely favorable so that the thermal shock characteristics and high temperature strength which is achievable with other hot pressing aids in a silicon nitride matrix, can be comparably achieved.

SAMPLE 1 A number of small, laboratory hot pressed, Si3N4 based billets were prepared, each a few inches in length and about 2-5/8 inches in diameter. The billets were prepared by hot pressing silicon nitride powder and 8% HfO2 at a pressure of 6000 psi and a temperature of 1700 C for about six hours. The hot pressed material was then shaped as a cutting tool appropriate for the particular type of machining operation that was to be performed. The laboratory test results are displayed in Table 1 for the identified machining operation of turning or milling on grey cast iron. In the turning operation, speeds of 1000--2000 ft/min for periods of 10 minutes indicated a wear of no greater than .001 with no observed thermal cracking or deformation.Similarly, at milling speeds of 6000 ft/min, and a mass removal rate in excess of 10 in3/min, and for a period of about 10 minutes, a wear of no greater than .003 inches was experienced, and even for a period of up to 67 minutes, no greater than .018 inches of wear was experienced.

In commercial tools of aluminum oxide, it is typical to experience a wear of about .01 inches at tool feeds of 1000-2000 ft/min for cutting times of about 10 minutes. Such commercial tools are not usable at high speeds of 6000 fpm without premature failure. Therefore, it is quite clear that the ceramic system of the present invention, when used as a cutting tool on cast iron, achieves unprecedented tool life at high cutting speeds and mass removal rates.

SAMPLE 2 The material prepared in accordance with the Sample 1 test was also made into a cutting tool which was used for a commercial test. An actual production environment was experienced with actual production machines at Ford Motor Company's machining plant. The casting to be machined was a difficult production vehicle casting (stator support for a transmission). For the stator support, continuous cutting was experienced at certain surfaces and intermittent cutting was experienced at other surfaces.

The cutting tool was used in a rough facing operation on the flange of the stator support, and the nature of the operation encompassed severe interrupted cutting on the as-cast surface. The tool material was run to failure which is measured by the number of pieces produced up to that failure event. The failure herein was defined (as regularly accepted in the industry) to mean loss of workpiece tolerance or failure by fracture or chipping.

In this test, using the inventive material, 620 pieces were produced per corner of the cutting tool.

The same operation was performed on the same equivalent stator support using a commercial cutting tool comprised of aluminum oxide coated with tungsten carbide. The tool life measured using this tool was 50 pieces per corner of the cutting tool. Comparison of the number of pieces produced by each tool demonstrates a significant quantum jump in tool life for the material described herein. The tool of this invention showed greater chemical stability and lower coefficient of friction at speeds of 2000 sfm or greater, and mass removal rates in excess of 10 in3/min.

TABLE 1 Laboratory Test Results Operation Speed Feed Doc M.R. Rate Cutting Time 'clear Remarks (fpm) (ipr) (in) (in3/min) (min) (in) Turning 2000 .011 .100 26.4 10 .0006 No observed thermal cracking or deformation 1000 .022 .100 26.4 10 .0011 No observed thermal cracking or deformation (in3/min/ (ipt) tooth) Milling 6000 .0036 .100 26 67 .0179 No observed thermal cracking or deformation 6000 .021 .100 151 10.4 .003 No observed thermal cracking or deformation In the above examples, which are described for the purpose of illustration only, fpm means feet per minute, ipr means inches per revolution, doc means depth of cut, M.R. Rate means mass removal rate, in means inches and ipt means inches per tooth.

Claims (8)

1. A method of making a silicon nitride based article comprising the steps of: (a) preparing a uniform mixture of silicon nitride powder having an SiO2 surface coating and 412% by weight HfO2 powder; and (b) hot pressing said mixture at a pressure and temperature and for a period of time to achieve a pressed body having substantially full density.

2. A method according to Claim 1, in which said mixture additionally contains up to 3.5% milling media impurity resulting from milling said powders to said uniform mixture.

3. A method according to Claim 2, in which said milling media impurity comprises W, WC, Al203 or SiC.

4. A method according to any one of Claims 1 to 3, in which said mixture has an average particle size of about 1.5 microns.

5. A method according to any one of Claims 1 to 4, in which said hot pressing is carried out at a pressure of 4-6.5 ksi, a temperature of 1680-1 7400C, and for a time between 1 and 8 hours.

6. A method according to any one of Claims 1 to 5, in which said HfO2 powder has a purity of 99.9% or more.

7. A method according to any one of Claims 1 to 6 wherein the HfO2 powder has a particle size of -325 mesh.

8. A method according to any one of Claims 1 to 7 wherein the article is a cutting tool.