CA1161626A - Method of making si.sub.3n.sub.4 based cutting tools - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
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

6~6 METHOD OF MAKING Si 3N4 BASED CUTTING TOOLS

The present invention relates to cutting tools.
In the metal cutting i~dustry 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 ~ear resistant film consisting of carbides, oxides and nitrides of a metal which is typically deposited over tungstencar~ide or other metal carbide base, and (d~
ceramic materials primarily based on alumina (A12O3), but with some recent work using silicon carbide or silicon nitride based materials. The first alumina based ceramic cutting tools were introduced around 1960.
Historically, t~e popularity of commercial use of these lS 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 inter-mediate speeds (~i.e., 5Q-800 sfm for tungsten-earbide and 200-1200 sfm for titanium carbide). Class (c) materials led to the use of tool speeds which ranged up to the beginning threshhold of high speeds. For example, complex carbide coated tungsten carbide had a utility range of 375-1600 sfm and al~minum 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 pene-trated 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 todayls ,................................................... ,,,~
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~2--available commercial cerami~ cutting tools are alumina based, most containing small additions of other oxides such as Zr203, MgO, CaO, SiO2, and TiO2. These additivies are used to aid densification, to control the microstructure, and to 5 improve the mechanical properties. Despite the improvement of ~he strength of alumina at room temperature, its success as a cutting tool has been marginal in the machining industryO
It is generally agreed ~y those in ~he prior art that in order for the alumina based ceramics to be successful, rigid 10 machinery and the absence of chattering are necessary pre-conditions. Alumina based ceramics are not recommencled 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 15 and the absence of chattering is optimized. These pre-conditions 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 2Q material such as grey cast iron.

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In this early work and development of si~icon nitricle as a cutting tool material,it was discovexed how to abricate the material so that it possessed a high ther~al shock ~actor, even thou~h the stren~th of such material at high tool temperatures is only moderately good. The excellent thermal shock factor contributed greatly, in an unprecedented manner, to the long~vity of this tool material in the machjning of cast iron.
Control of the thermal shock factor has been effected by appropriate additions of densifying agents in certain ranges. The densi~ying agents us~d to date by the art have included MgO, Y203, ZrO2, BeO and CeO. The most successful of these densifying agents has been Y203 in the range of 4-12%. These addit;ves and their qualities influence the strength, refractoriness, creep and oxidakion resist~llce of the final prcduct.
In general, the role of these additives is to form a molten flux by reacting with the silicon nitride alon~
the boundary of the silicon nitride grains under pres-sure and temperature. This flux,upon cooling, forms auniform secondary phase ~hich binds tlle primary silicon nitride grains. Without special precautions, such pxocess ~70uld ~orm the secondary phase as an amorphous or glassy material ~hich s undesirable because it is brittle. Improved processing avoids such amorphous material in the final product either by subsequent heat txeatment or by special proce~sing during hot pressing so that the final secondary phase is of a crystalline ~ature~ The secondary phase is believed to consist, particularly in the case wh~re ~23 is used as a pressing aid, of silicates/and more particularly oxynitrides, which form as a result of the ternary system of Si3N4, SiO2(the latter may be present as a coating on the silicon nitride po-~der particles) and the pressin~ a~ent ~23 ~,~6~Z6 Although such mixtures of silicon nitride and the specified pressing aids have demonstr~ted 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.
In accordance wlth the present invention, there is provided a method of making a silicon nitride based cutting tool comprising: (a) preparing a uniform powder mixture consisting of 4-12% by weight HfO ~ up to 3.5~ milling media impurity, and the remainder being of silicon nitride con-sisting essentially of Si3N4 powder having an SiO2 surface coating and up to 1% cation impurities; (b) hot press:ing the mixture at a pressure and temperature and for a period of time to achieve a pressed body having substantially full density; and (c) shaping the pressed body as a cutting tool.
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 ~haracteristics as silicon nitride based materials using known pressing aids. The retained physical characteristics typically co~prise high thermal conductivity, low coefficient of expansion, good thenmal fatigue resistance, and the elimination of any tool failure by thermal cracking or deformation. In addition, HfO2 provides improved chemical sta~ility, 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 averaye particle size of about 1.5 microns, using milling media selected from the group consisting of W, WC, A12O3 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 to 7 ksi (kilo pounds per square inch) -5 ~ 6 , (preferably 6 ksi), a temperature of 1680 -1740 C
~preferably 1700 C), and for a period of 1 to 8 hours (p~eferably 5iX hoursJ.
With respect to machine cutting utilizing such improved material, the present invention also provides a process for interrupted or continuous machine cutting of solid cast iron stock by milling, turning or boring, comprising: moving a shaped cutting tool relative to and in engagement with the stock to remove a cast iron metal chip, the tool being constituted of a substantially fully dense hot pressed ag~lomerated powder mixture, the par~icles of the hot pressed mixture having been uniformly blended and sized by rotation milling with milling media, the hot pressed mixture consisting essentially of 4-12% HfO2, up to 3.5% milling media impurity and the remainder silicon n~tride consisting essentially of Si3N4 powder having an SiO2 coating and up to 1~ cation impurities, excluding free silicon.
The pressed body usually consists essentially of a primary matrix phase of silicon nitride and an intergranular secondary phase consisting of Si3N4, SiO2, HfO2.
A ceramic cutting tool according to this invention is made by preparing selected powder ingredients of Si3N4 and HfO2 to form a uniform mixture, hot pressing the mixture to substantially full densi~y 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 4tol2% by weight of the mixture.
In carrying out the method of making the cutting tool, it is perferable to select a supply of silicon nitride.

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po~er whic~ contains SiO2 as a surface oY.ide coating - (t~pically .75-1. 540) and contains at least 85go alpha silicon nitride. Impurities are controlled to ~e less than the following amounts: .5~ by weight iron, .01% calcium, .4 5 aluminum/ 2% ox.ygen, and 1.5~ free silicon. Thus, the - total cation impurities will be less than 1~, e~cluding free silicon. It is desirable to employ an average particle size diameter of 2-2.5 microns ror such silicon nitride powder~ It is advantageous to employ a supply of stabilized 10 hafnium oxide powder which has a purity of 99~9~ and which is available commercially having a particle size of -325 mesh. Stabilized HfO2 is 88% HfP2 and 12% Y2O3.
Blending and mixing of the powders is carried out by use of a ball mill ~hich not only mixes, but also ~ills lS the mixture to a desired particle size. In ball mulling for this pxocess, the milling media comprises W, WC, A12O3 and SiC, and may consist of 1/2 inch length grinding rods.
The rotation of the ball mill is continued for a period of seYeral days and is calculated to promote an a~erag~ particle size in the resulting blended powder of about 1.5 microns.
With this operation, it is typical to experience a corresponding ball ~ear in the range of L.75-2.5~ by weight of the final bat~h mixture. Such percentage wear represents the milling media impurities present on 25 the powder in the final mixture.
Hot pressing is carried out at a pressure in the range of 4-7 ksi (advantageously at about 6000 psi), the ultimate pressing temperature in the range of 1680-1750C ~preferab'y 1700C), and the time at such 30 temperature and pressure being in the range of 2 8 hours ~preferably about 6 hours).
The hot pressing is pre~erably carried out in a graphite die assem~ly, which graphite serves to act as a protective environment for the materialO Ho~ever, it ~7~
is also ~esirable to employ a protective atmosphere, such a~s N2, in the die asscmbly In the graphiLe die asse~ly, it is advantageous to initially precompact the powder mir.ture at a pressure of about S00 psl to promote a stabilized cor.~pact. A~tel- having achieved precompaction, the pressure of the die assel~bly is increased to about 6000 psi at a rate o about 1000 psi per minute. At the initiation of the increase in pressure a~ove 500 psi, it is desirable to begin the urnace run by increasing the furnace temperature until the u]timate target temperat~re of 1700C is obtained. Hot pressing is prefera~l~ 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 assem~ly is anal~zed. ~7hen such ram experienccs no greater than .00~ inches movement ~uring a 15 minute interval, the. pressure and temperature is relieved and shut off~
The resulting hot pressed body is shape~ as a cutting tool, such as by diamond sawing, to assume a desired geometry tailored for a specific machinin~
application. The shaped tool will have a primary matrix phase of silicon nitride wi~h intergranular secondary phases (acting as a binder for the matrix) which consist of a system of Si3N~.SiO2.HfO2. Sucl secondary phases promote an increased chemical stability for the resulting ceramic and a lower coefficient o~ friction ~or the tool when used for a cutting opera~ion. The modulus cf rupture for such material will be in the range o 60,000-90,000 psi at room temperaturer its coeficient of thermal expansion will b~ no great~r than 1.9~:106in~inF, its thermal conduc~ivity will be no greater than 3.0 BTU/Hr inF, its hardncss will be at least 86 Rockwell ~S-N, with a den~ity of ~bout 3.25-3,5 grams per cm .

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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 iron7 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 nitride/
Y2O3 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 sdgnificant influence on the chemical interaction leading to the wear of the tool tip. Several mechanisms have been suggested to explain the interaction at the inter-face 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 tem-perature 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 illus-trated in Mendeleef'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 1~0C to a -87 at 2000C.

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~^"1 ~ owever, the knowledse tha~ hafnium o~ide will prorote cl-lemieal staDility 2nd lower the coeficient of friction in a silicon nitxide rjase m~t~rial is not -sufficient '~noil2cce to Xrow ~Ihether it will have the other interrelated phvsic21 characteristics so important to a good tool material. The worX of this invention has sho~rn that HfO2 will pxo~ote most of the properties desired. For example, it is Lmportant tha~ ~nen intro-duci~g hafnium oxide, that the mechanical stren~th, coefficient of thermal expansion, and thermal conducti-Yity be e~tremely favorable so ~hat ~he thermal shock characteristics and high temperature strength which is achievable wi~h o~her hot pressing aids in a silicon ~ nitride matrix,can ~e comparably acnieved.
Sa~ple 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 20 ~fOz at a pressure of 6000 psi and a te.~perature of 1700C 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 tes~ results a~e displayed in Tabl 1 for the identified machining op~ration of turning or milling on grey cast iron. In the turning operation, speeds of 1000-2000 ft/min for p2riods of 10 minutes indicated a wear of no greater than .001 witn no observed ther~al cracki~g or deformation. Similarly, at millin~ speeds of 6~00 ft/min.,and a mass re~o~al rate in excess of 10 in3/min.t and for a period of about 10 munutes, a ~ear of no greater than .003 inches was experienced, and even for a period of up to 67 minutes, no greater than .018 inches Oc wear was experiencPd.

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In commercial tools of aluminum o~ide, it is typical to experience a w2ar of about .01 inches at tool feeds of 1000-2000 ft/min for cutting times of about 10 minutes. Such cor~mercial tools are not usable at hi~h speeds of 6000 fpm l,Jithout premature failure. Therefore, it is quite clear that the ceramic system of the present invention, when used as a cutting tool on cast iror"
achieves unprecedented tool life at high cutting speeds and mass removal rates.
Sample 2 The material prepared in accordance with the Sarnple 1 test was also made into a cutting tool which was used for a col~nercial test. An actual production environ~ent ~Tas experienced ~ith actual production machines at ~ord ~lotor Compan~s machinlng plant. The casting to be machined was a difficult production v~hicle casting (stator support for a txansmission). For the stator support, continuous cutting was e~perienced at certain surfaces and inter~mitent cutting was experienced at other suraces. The cutting tool was used in a rough acing operation on the flange of the stator support~
and the nature of the operation encompassed severe interrupted cutting on the as-cas~ 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 acceptecl in the industry) to mean loss of wor}piece tolerance or failure by fracture or chipping.

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--ll--, In this test, using t~e in~.~e~.tive rnaterial, 620 pieces wer~ produc~d ~r corn~r of t~.e cuttir.g tool.
The same operation ~as per~ormed on the samt-~ equivalent stator support u~ing a commercial c~1tting tool comprised of aluminum oxide coated with tungsten carbide. The tool life measured usin~ this tool was 50 pieces ~er corner o~ the cutting tool. Comparison of the number of pieces produced by each tool demonstrat~s a significant quantum jump in tool life for the material d~scribed herein. The tool of this invention showed greater chemical stability and lo~ler coeficient of friction at speeds of 2000 sfm or greater, and mass removal rates in excess of 10 in3/min.

TGRY TEST ~S~J1~TS

~!~EI~ `~EED ~1 - DC~ M,R. ~All~ ~I~I, ~ ~B~.riRr~
IPR) ~IN) (IN3/~ AI~
J ~ .01~ .lGO 26.4 lO ,~ No ~bservecl therm~l crack~
lOOO .~ .lGO 26,4 lO .OO~l ing or deformation, (IPT) MILLIN~ ~000 .0036 .lOO 26 61 .0179 ther~
6a~ .Q21 ,100 15~ 10.L~ ~3 clc~orm t~.n.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making a silicon nitride based cutting tool comprising:
(a) preparing a uniform powder mixture consisting of 4 to 12% by weight HfO2, up to 3.5% milling media impurity, and the remainder being silicon nitride consisting essentially of Si3N4 powder having an SiO2 surface coating and up to 1%
cation impurities;
(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; and (c) shaping said pressed body as a cutting tool.

2. The method of Claim 1 wherein said milling media impurities is from the group consisting of W, WC, Al2O3 and SiC.

3. The method of Claim 1, wherein said mixture is rotation milled for a period of time to provide an average particle size of about 1.5 microns.

4. The method of Claim 1, wherein said hot pressing is carried out at a pressure of 4 to 7 ksi, a temperature of 1680° to 1740°C, and for a time between 1 and 8 hours.

5. The method of Claim 1 wherein HfO2 powder has a purity of 99.9% or more and a particle size of -325 mesh.

6. The method of Claim 1, wherein said HfO2 is stabilized.

7. The method of claim 1 including the further step of moving the shaped cutting tool relative to and in engage-ment with solid cast iron stock to remove a cast iron metal chip.

8. The method of claim 7, wherein said powder mixture comprises a primary matrix phase of Si3N4 and intergranular secondary phases consisting of Si3N4?SiO2?HfO2.