CA1127189A - Si.sub.3n.sub.4 composite cutting tool material and method of making - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A cutting tool for cutting of cast iron is formed by blending a nitride powder and an oxide powder, and heating and shaping the mixture under controlled conditions. The nitride powder consists of substantially alpha phase silicon nitride having a cation impurity content no greater than 1% and the oxide powder is at least one of yttria, magnesium oxide and zirconium oxide and is devoid of spinel. The cutting tool exhibits a failure mode by wear and not by fracture or thermal cracking when used to cut cast iron.

Description

llZ7185~

The present invention relates to cutting tool materials.
Recently, ceramics composed principally of silicon nitride (Si3N4) have found significant use as ceramic components for machines or as vessel coatings. This material is known to have many good characteristics such as high oxidation resistance at high temperatures (1400C), good mechanical strength at high -1400 temperatures, and good hardness at high temperature.
Strength of this material is related to density and it has been bound that the densification property of silicon nitride, sintered under atmospheric pressure, is very inferior.
Therefore, it has been considered important to employ high pressure when a product of good strength is desired. This is routinely referred to as hot pressing of silicon nitride. However, in spite of the use of hot pressing, the bend strength of simple Si3N4 has not been as high as desired at high temperatures.
Accordingly, other avenues of strength improvement have been sought such as through the use of additives which operate as a low temperature liquid phase to ~acilitate densification and not significantly impairing the creep resistance of the ceramic body at high temperatures. These added materials have included relatively large amounts of chromium oxide, zinc oxide, nickel oxide, titanium oxide, cerium oxide, magnesium oxide, yttrium oxide and others, ranging in excess o~ 20% (wt.) of the matrix materials. Silicon nitride with these particular additives tends to form a structure having a strength level which does not usually exceed 50 KSI at high temperatures. In one instance (U.S. Patent 3,830,652 to Gaza) did the prior ar~ obtain strength levels in excess of 50 KSI. In this instance, the concern was for physical characteristics useful for turbine elements: hardness, oxidation resistance (inertness) and - transverse rupture strength. Gaza explored metal oxide additives to a Si3N4 system which ranged in amounts related solely to

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machine element usage. The additions were added in amounts up to 20%.
However, commercial cutting tools today exhibit the same or better physical properties that were the focus of Gaza's work. For example, commercial A12O3 or TiC tools have excellent hardness at high temperatures and have high resistance to oxidation and have transverse rupture strengths at high termperatures which range up to 100,000 psi. Strength is considered the most important feature because of the necessity to withstand forces imposed on the tool material by the tool fixture and by the resistance of the stock material, particularly at heavy depths of cutting. These forces become unusually exaggerated when cutting ferrous material such as cast iron at high speeds and feeds. Without increased strength, it is believed by those skilled in the art that further improvements in tool life cannot be achieved. Since the strength level of Si3N4 is equal to or lower than commercial materials now available, it has been rejected as a tool material candidate with little hope in impr~ving tool life.
In only one known instance has the art attempted to employ Si3N4 directly as a cutting tool material and this was for use only on hypereutectic aluminum alloys. This attempt ir s t forth in a Japanese patent 49-113803 (10-30-1974) by Ka~utaka Ohgo, appearing in Chemical Abstracts, Volume 84, 1976, page 286 (84:21440t). In this work, silicon nitride was sintered (as opposed to hot pressing) and metal oxide spinels were employed in solid solution in the silicon nitride matrix. The spinels were formed by a mixture of divalent and trivalent metal oxides (including magnesium oxide and Y2O3). Only a quaternary system was employed involving Si3N4, SiO2, MgO, and Y2O3. This produced many secondary phases which weakened the physical characteristics, particularly strength, thermal conductivity, and increased the thermal coefficient of expansion.

- 3 llZ71~il9 A loss of these physical characteristics make it most difficult ot obtain even equivalent performance to commercially available tools when applied to a rigorous cutting environment such as interrupted cutting on cast iron. The cu~ting operation was of very short duration (2 minutes) of continuous machining and at low metal removal rates (cutting speeds of 1000 sfm, .012 inches per rev. of feed and .060 inches of depth of cut and metal removal of 8.64 in.3/min.). This type of test information, of course, did not investigate cutting applications where large forces are applied to the tool, did not investigage the elimination of spinel additives, did not investigate heavy cutting against rough surfaces such as cast iron, nor continuous cutting for periods of several hours or greater, nor did it explore intermittent, interrupted high speed cutting at speeds of 4000-5000 sfm at heavy feeds and depths of cutting. The demonstrated wear of .006-.008 inches, in Ohgo's work, for 2 minutes of cutting time is highly excessive when compared to the goals of the present invention. Therefore, this work did not demonstrate that Si3N4 possessed sufficient characteristics to be used as a tool material on ferrous materials which apply large bend forces to the tool.
Moreover, the art has been possessed of sufficient knowledge in the making of Si3N4 with additives for many years; during this long term no effort was made to apply this material as a cutting tool against cast iron. This tends to support the contention of this invention that if tool life is dramatically increased for certain Si3N4 composites when used for machining cast iron, there must be some unobvious characteristics independent of strength that layed undiscovered to promote this new use.
This invention has discovered a correlation between a thermal shock parameter and promotion of prolonged life in Si3N4 materials when used as a cutting tool on cast iron.

;~ - 4 -llZ7J ~39 This parameter consists of KS where K is thermal conductivity ~E
of the material, S is the modulus of rupture, o<the coefficient of thermal expansion, and E is Young's modulus. E can be eliminated from the parameter since it remains substantially constant for the contemplated variation in ceramic chemistry which controls this parameter. This parameter must exceed 26 lbs/in. as minimal if significant improvement in tool life is to be obtained. It has been further discovered that a simple ternary ceramic system (Si3N4 . SiO2 . low temp.
liquid phase) with SiO2 present, not as an additive, but as an inherent reaction product of heating Si3N4, serves as the proper mechanism for achieving the required thermal shock parameter. The low termperature liquid phase must be one which produces a small amount of a highly refractory silicate which will reside totally in the grain boundary of the matrix.
There are many other physical characteristics beyond its thermal shock parameter that should be improved in silicon nitride if it is to be successful as a tool material for cutting cast iron. As indicated earlier, the densification of the material has been a point of concern and has been alleviated by use of hot pressing techniques and oxide additives. This has permitted the density to be elevated close t~ theoretical density, but improving density by itself through increasing amounts of oxide leads to a decrease in several other physical properties.
Investigators have failed to perceive this interplay of physical characteristics.
More importantly, known silicon nitride compositions, when used as a cutting tool against relatively rough surfaces such as cast iron, exhibit a failure mode under such circumstances is typically due to thermal shock as opposed to the more desirable mode by wear. Further the attainable hardness level and general rigidity of the known silicon nitride composites have yet to be comparable to commercial cutting tools.

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In accordance with one aspect of the present invention, there is provided a method of making a cutting tool specifically useful for continuous or interrupted cutting of cast iron, the method comprising: (a) preparing a first powder consisting of substantially alpha phase silicon nitride having a cation impurity content of no greater than 1%, excluding free silicon;
(b) preparing a second powder consisting essentially of at least one member of a metal oxide group comprised of yttria, magnesium oxide and zirconium oxide, the metal oxide being devoid of spinel;
(c) mixing 1-15% of the second powder with the first powder, the total amount of each member in the final mixture being limited to 1-5% MgO, 4-12% Y2O3, and 4-12~ ZrO2; (d) heating the mixture to a temperature level of 1700-1750C for a period of time between 3-8 hours under a pressure of 5,000-6,500 psi, and allowing the hot press material to cool at a rate of 100C/sec.;
and (e) shaping the resulting hot pressed prodcut as a cutting tool, the product being characterized by a density of 95% or more of theoretical density, a thermal shock parameter at 1200 C of at least 26 x 10 BTU-lbs , a transverse rupture Hr (in ) strength of at least 67,000 psi at 1200C, a hardness level of at least 86 Rockwell 45-N, and a density of at least 3.25g/cm3.
Cutting tools produced by th~s procedure are useful in cutting rough ferrous materials, particularly cast iron, and may be used under severe cutting conditions. In accordance with a second aspect of this invention, there is provided a ceramic tool bit composition for continuous or intermittent cutting of cast iron, the composition comprising a ceramic consisting of a solid solution of Si3N4 - SiO2 - low temperature liquid phase metal oxide, the metal oxide consisting of at least one member selected from the group of Y2O3, MgO, and ZrO, the metal oxide being employed in such volume fraction related to Si3N4 so as to be contained in the grain boundaries llZ~189 \

of the silicon nitride in non-spinel form, the composition being characterized by a thermal shack parameter at 1200C of at least 26 x 109 BTU-lbs , density of at least 3.25g/cm a ~ r(in3) bend strength of at least 67,000 psi at 1200C, and a hardness level of at least 86 Rockwell 45-N.
The present invention also includes an improvement in the method of using hot pressed silicon nitride ceramics containing at least one metal oxide mem~er of the group consisting of Y2O3, ZrO2, and MgO, the improvement being characterized by deploying the ceramics as a cutting tool for use in the machining of cast iron.
The tendency toward higher speeds and feeds to achieve lower production costs and higher productivity imposes an ever increasing demand for greater wear life on cutting tools.
The temperature rise at the tool tips at these speeds and feeds is very high. Tool materials have to be inert at such working temperatures and their dynamic properties have to ~e superior than those of existing tool materials.
In accordance with this invention, it has been discovered that Si3N4, when combined with critically controlled amounts of certain metal oxides in a narrow volume fraction range, then hot pressed under controlled temperature, pressure and time conditions, will produce a cutting tool composition for a simple ternary system (Si3N4.SiO2.Y2O3) that can be utilized at high speeds and heavy feeds on cast iron and will exhibit high wear resistance and will fail by a mode of gradual wear rather than thermal fatigue or catas~rophic failure.

The cutting tool formulation meets the needs of a specific set of cutting conditions created by working on cast iron. The tip temperature of the tool will experience a temperature typically in the range of 600-800C. This temperature factor is important since it i5 high enough to create thermal shock ~lZ'71~9 sites in conditions of the tool, but not high enough to create a serious oxidation erosion problem.
When machining cast iron, the chip easily fragments keeping any hot ch~ normally away from the tool tip.
Due to the resistance of cast iron to cutting, large fixture forces must be applied to the tool to move it through the stock material. Moreover, the mass removal rates required in many cutting operations on cast iron is considerably greater than used with other stock materials. This necessitates a strong tool material with respect to transverse rupture strength.
All tool designers consider this latter aspect the most important consideration with respect to evaluating the success of a new tool material.
The three virtues normally recognîzed of Si3N4 would not suggest to one skilled in the art that it would be a successful candidate for machining cast iron. Its transverse rupture strength at high temperature rarely exceeded 50,000 psi (while commercial tools regularly experienced strength levels of 100,000-200,000 psi); its high oxidation resistance was not critically needed; and its good hardness at hightemperature was easily exceeded by the hardness of commercially available silicon carbide tools.
The present inve.ltion recognized for the first time the role played by thermal shock resistance factors, namely, the coefficient of thermal expansion (C~) and thermal conductivity (K) related to the modulus of rupture (S). This is defined herein by the expression KS where E is Young's modulus and c~E
can be eliminated because it remains substantially constant under varying cutting conditions and material variations. By maintaining this thermal shock parameter above 26xlO9 BTV lbs , it has been determined that a significant increase in wear life can be achieved on cast iron. It is difficult to theorize why this phenomenon takes place, but it may be due to the greater structural stability that is achieved by the ceramic at 700 C
;B 8 -llZ71~39 when generated heat is quickly conducted away pre~enting a large temperature gradient in the tool which leads to cracking if the material has an undesirable coefficient of thermal expansion;
this may more readily be experienced when the tool tip is slightly cooled by intermittent or interrupted cutting.
A preferred method for making such a cutting tool is as follows:
(1) A first powder of substantially alpha phase silicon nitride is prepared having less than 1.0% cation impurities ïO texcludin~ free silicon), about 1% free silicon, and about .7%
by weight oxygen.
(2) A second powder of one or more selected metal oxides is prepared. The metal oxides are selected from a group consisting of Y2O3, MgO, and ZrO2. These metal oxides are characterized by their small atom size and their affinity to form a low temperature liquid phase which solidifies as a highly refractory silicate residing solely within the grain boundaries of the matrix. The oxides must be simple; it is important to avoid the formation of spinels which will produce wea~er secondary phases.
(3) The first and second powders are blended and mixed in a prescribed proportion. The second powder should have a weight percent of 1-15%. However, the ingredients of the second powder must fall within the following ranges: .75-5% MgO,

4-12% Y2O3, and 1-13% ZrO2.
(4) The powder mixture is heated to a temperature level of 1700-1750C (operably 1600-1800C) for a period of 3.-6.8 hours (operably 1-8 hours) under a pressure of 5,000-6,500 psi (operably 1,000-8,000 psi.), and allowed to cool at an average rate of 100 C/hr. The hot pressed compact will exhibit substantially complete Beta phase silicon nitride.

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The resulting product must exhibit the following combination of physical characteristics:

(a) 100% theoretical density (zero porosity);
(b' a thermal shock parameter of at least 26x109 BTU lb3s ;
Hr(in ) (c) a transverse rupture strength at 1200C ~in 4-point bending)of at least 67,000 psi;
(d) a hardness level of at least 86 Rockwell 45-N;
(e) a measured density of at least 3.25g/cm3;
(f) a wear life characterized by measured wear of no greater than .010" after one hour of continuous cutting of cast iron at a mass removal rate of at least 25 in.3/min or mechanical failure under cutting conditions prescribed by at least 2000 feet per minute with a depth of cut of at least .06 inches and a rate of feed of at least .012 inches per IPR;
(g) the absence of tool failure by fracture or chipping.
The invention is illustrated further by the following examples. In these examples, reference is made to the accompanying draw1ngs, wherein:
Figure 1 is a perspective view of the work stock employed 2~ in a first set of laboratory cutting operations requirina continuous cutting simulation;
Figure 2 is an end view of a work stock similar to Figure 1 illustrating the mode to simulate interrupted cutting;
Fiqure 3 i9 central sectional view of a stator support casting used the stock material for production machining examples;
Figures4 and 5 are before machining and after machining photographs of the actual ~asting of Figure 3; Figure 4 depicts the front face side and Figure 5 depicts the rear spindle side;
Figure 6 is graphical illustration of wear life to failure vs. thermal shock parameter employing the invention;
and Figure 7 is a graphical illustration of wear life to failure VS- Y203/sio2 ratiO.

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EXAMPLE I

A sample cutting tool material (identified as Sample 1) was prepared employing hot pressed silicon nitride with 7.47 weight percent Y2O3 (8% Y2O3 was added as a powder to a powder having 85% alpha phase Si3N4); the powder mixture was hot pressed at a temperature 1740C under 6500 psi (9.55 x 106 kilogram/cm2) for a period of 6-1/2 hours. The pieces of hot pressed material were ground to a tool geometry of SNG 434 and the edges were prepared with 0.006" x 30K land (chamfer). Each of the cutting tool prepared from this material were subjected to a cutting sequence on a simple cylindrical cast iron casting which varied between continuous, intermittent, or interrupted.
As shown in Figure 1, the continuous cutting consisted of generating cylindrical surface 10 at a prescribed speed, feed and depth of cut. Due to the expanded length of the cylinder 11, the tool tip experiences a relatively constant high temperature for the duration of the pass. Intermittent cutting consisted of withdrawing the tool intermittently along a longitudinal pass. Interrupted cutting consisted of passing the tool circumferentially about the cylinder along a path 12 (as shown in Figure 2) which path encounters the previously cut longitudinal grooves. The latter provides repeated impacts for the tool.
Each type of cutting st~le imposes a different thermal condition on the tool which affects tool life in different ways.
Other tool material samples were similarly prepared with different chemistries, as shown in Table I, along with their resultant physical properties.
The cutting operation for all these samples was carried out in a laboratory environment at a variety of speeds and varying feed rates at a constant depth of cut; the tool geometry was varied only in the corner configuration as indicated. The wear in inches was measured for specific period of time. The results of such cutting are shown in Table II. None of the tools were liZ7~89 used to full life; the cutting time was terminated when a signi-ficant increase in tool life was perceived. Five minutes, under high cutting speeds (3,0Q0-4,000 sfm) was deemed an unusually high increase in life when compared to commercial tools which typically fail after one minute.
It is believed that optimization of the thermal shock parameter of Si3N4 under extreme temperature conditions has led to this increase in tool life. The ability to have a stable structure at 600-800C temperature while under severe stress along with the ability to effectively conduct away heat preventing a loss in high temperature strength provides the basis for this life improvement. These physical characteristics are critically affected by the compositional phase of the Si3N4 composite.
It is most important that the selected additives form a highly stable refractory silicate which resides totally in the grain boundary of the matrix.

~lZ~ 9 EXAMPLE II

Substantially the same cutting tool materials, prepared as indicated from Example I, were prepared for cutting use in a production environment with actual production machines at Ford Motor Company's machining plants. The casting to be machined was a difficult production vehicle casting (stator support) in some cases and an engine block in others; the stator support is shown in Figures 3, 4 and 5. For the stator support, continuous cutting was experienced at surfaces B and D, intermittent cutting at surface A, and interrupted cutting at surfaces C and F (see Fig~re 4).
These sample materials were run under a variety of cutting conditions as set forth in Table III. All tool materials were run to failure which is measured by the number of pieces p~oduced up to that failure event. Failure herein is defined (as regularly as accepted in the industry) to mean loss of work-piece tolerance or failure by fracture or chipping.
From the date in Tables II and III, we have discovered that controlled processing of Si3N4 with Y2O3, MgO, or ZrO
provides the kind of thermal shock parameter that leads to longer tool life when machining cast iron at large mass removal rates or high speed. These metal oxides operate upon the controlled free silica to form a hi~hly stable refractory silicate which resides totally in the grain boundary o~ the Si3N4 matrix.
Table III proceeds from lower cutting speeds to higher speeds with compara~-ive materials grouped adjacent each other.
For each comparison the inventive samples render sigrificant increases in tool life.
The inventive materials perform 3-8 times better than the current commercial tools. In finish machining of the front end of an engine block, the number of blocks milled were 2100 pieces per corner with a depth of cut of .065 inch.

llZ71~9 Whereas using commercially available ceramics with half of the feed rate, th.e number is 600 pieces. The inventive material will provide ta) increased production at current cutting conditions, tb) increased production capacity at higher cutting speeds and feeds, (c) savings in tool material cost, (d) reduction in tool change downtime, and (e) increased production capacity leading to free machine time for preventive maintenance.
To obtain at least a four-fold increase in tool life over commercially availa~le tool materials the Si3N4 . SiO2 . Y2O3 ternary system must be employed with Y2O3 controlled to a limit of 7-9%. In this manner the thermal shock parameters can be optimized at either continuous or interrupted cutting conditions.
Figure 6 and Table IV portrays the role played by the thermal shock parameter.

Additionally, Y2o3/Sio2 ratio must exceed 1.60. The influence of the Y2O3/SiO2 ratio is portrayed in Figure 7 and Table V.

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~ ~lZ71~39 _ _ I~BLE II

No. r ~ion Cutting Condition M~beri 1 ~ime ~inches) Kech~n~al (sfm~ (ipr) (in.) (min-sec) Flar~c Crater Continu~ us Cutting LA 1 4000 ~010 .100 48 5.2 none nc~ne NIL
lB 3 3000 .020 .100 72 5.7 none none NIL
lC 1 3000 .010 .100 36 17.5 nc~e none NIL
LD 1 2000 .0111 .100 26.4 43.7 none nDne NIL
lE 3 1000 .0222 .100 26.4 26.8 none none NIL
LF 2 750 .029 .100 26.4 11.7 none none NI~
lG 1 500 .044 .100 26.4 10.5 none none NIL
lH 2 333 .066 .100 26.4 1.9 none n~ne ~ rac~ re lI 3 2000 .011 .100 _ 21.0 .015" none NIL
Intermit tent Cuttinc LK 2 1000 ,011 ,100 13.2 2.3 nc~e none NIL
lL 5 750 .029 .100 26. d 1 . 1 none none NIL
lM 6 750 .044 .100 39.6 O.6 none none NIL
lN 7 500 .011 .100 6.6 4.1 none none NIL
4 2000 .qll .100 26.4 8.8 none none NIL
lP 8 1000 .022 .100 26.4 7.7 ncne none NIL
Interruc ted Cutting lQ 6 1000 .0111 .100 13.2 3.7 .0016" NIL NIL
lR 8 1000 .0222 .100 26.4 10.0 .0013 NIL NIL
lS 7 2000 .0111 .100 26.4 10.5 ,0021 NIL NIL

Cbntinuc us Cutting 3A l(IriPl 1) 2000 .0111 .100 26.4 2.05 .0055 _ NIL
3B 3(Trial 2) 2000 .0111 .100 26.4 9.95 .0015" _ NIL
3C 4 1000 .0222 .100 26.4 9.92 .0018 _. NIL
Intsrru~ ~ted Cutting 3D 4 1000 .0222 .100 26.4 10.00 _ NIL
abntinuc,us Cutting 4A 1 2000 .0111 .100 26,4 9.65 .002 _ NIL
4B 2 1000 .0222 .100 26.4 10.5 .001 _ NIL
5A2(Trial 1) 2000 .0111 .100 26.4 9.82 .0116 __ NIL
5B3(Trial 2) 2000 .0111 .100 26.4 10.00 .002 _ NIL
5C 1 1000 .0222 .100 26.4 9.88 .0019 _ NIL

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q2~E II (C~ntin~d) ple Corner Cutting Condition Mass ofTilie l~ol ~ar Illerma ~o. Configu- Speed Feed CepthM~terial (I~N) and Nunber cutin3/~nin or Cracks . . (~n) (ipr) ~in.) (n~n-sec) Flank Crater 6A tl~ial 1)2000 .011 .100 26.4 10.00 .005 _ N~
63~ (~rial 2)2000 .011 .100 26.4 5.15 .002 _ NIL.
6C 1000 .022 .100 26.4 10.10 .002 _ NIL
~tin~ as Cutting 7A 2000 .0111 .10 26.4 0.02" FFaral ion by Fractu~e 7B 1000 .0222 .10 26.4 0.15" FFar~ ion by Fracture 7C 1000 .0111 .100 13.2 0.11" Fra~ b~rebY I F3crhaacntucal _ .~
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TAELE III
.i--Cutting Ibol Cutting Conditions Work Pieces Sample ~aterial OFeration Ceo~et~y S~eed Feed Depth Prcduced be-of cut fore Failure . _ _ _ (sfn~ ~ipr) 'inches)_ Hot Pressed Broaching of SNG 636 150 __ .02 1910 3 4 Engine block Y203 Surfa oe , _ . . __ (Prior Art T ol) ,. ll 150 _ .02 800 . _ _ __ Hot Pressed P~ughfacing SNG 434 496 .016 .125- 1200 Si3N4+8% of Surface A .250 Y203 (intermittent . _ cutting) _ Hot Pressed Si3N4+1% " " " " " 800 ._ ~30__ __ _ Hot Pressed Si3N4+5% .. ., ll .. ,. 740 , M~O _ (Prior Act Ibol) SiC
base tool .. .. .. .. .. 150 o~ated with A1203 _ Hot P~essedSemi finish and TPG 322 516 .012- .025 1000 Si3N4+8&finish bore .006 Y O surfa oe E (Con, 2 3tinuous cutting) _ . . _ _ (Prior Art WTCl) . , . . .. 250 _ _ _ Hot Pressed Send-finish andTPG 322 516 .0135 Si3N4+ finish bore sur- -. 006 .025 320 Y203 face F (spline hole)~Inter-rupted cutting) . _ _ Hot Pressed Rough face ofTNC434 965- .012 .093 420 sl.N4~89~ Surface C ~in- 496 Y203 terrupted _. . . __ . .~ . _ _ _ .. _ Hot Pressed MsO_ _ . .. 140 Tcol) SiC
base tcol . - - - - 50 coated with A12O3 __ _ - _ ~ot PressedRough turning ofING 434 998 .014 .0625 420 si N +8% outside dia-3 4 Y20 m~ter on surface 3 B ~oontinuous Cutting) _ _ .

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~! ~ TABLE III (Contin~ d) _ Sanpl ~aterial Cutting Tool Cutting Conditions Work Pieoes OFerationGoolretry SpeedFeed Depth Produced l~e of cutfore Failur~
_ .. __ (s~n) (ipr?(inches) (Prior Art Rough turningTNG 434998 .014 .0625 50 Tool) SiC of outside dia-base tcol mster on sur-coated with fac~ B (con-A123 tinuous cutting) . . _ . _ Hot Pressed Rough boring of TNG 434 1026 .0189 .0625 157 3 4 8 inside diamster Y2O3 on surface D 674 to (Continuous .O 0 39 cutting) _ _ _ _ (Prior Art Tcol) SiC
base tcol ,. .. .. .. .. 50 coated with .. _ . . _ _ ~ot Pressed Finish mill end 3 4 y%o (interr(littent __ (ipt) .265 2100 . _ ~5% TiN .. .. .. .. 400 .

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TABLE IV
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Sanple ¦ ~ernell S Parts Pmdueed No. S}~c Paralreter K (Contin~l~s (Interrupted Room l~p.~igh Z~p. Cutting) Cutting) (1200 C)End Facing Flange 5233~2~ 179 182 O ,_ ~.

N4+1% 334 36 800 140 Si3N4+5% 220 26 740 _ Si3N4+12% ___ A123 16 200 Fthaerm~bly crack ng R = 'rhermal Conductivity, BTU/Hr in F.
S = Modulus of Rupture, KSI, (4-point bending).

O~= Coefficient of Thermal Expansion, X10 6 in/inF.

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TABLE V
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. __ - Cutting' Condition Material Speed Feed Depth of Cut No. of Wor~c Pieoes .. _ .. _ (sfm) ( pr) (in) Prod~ced Hot Pressed Si3N4with 8%
Y2O3; Y2O3 496 .016 125to 1200 2.30 _ . .-yS2aOn3e ~=1.7 " .. .. 1112 ,L .. ,. 620-Sane',-- . . .._ ~=1.18 .. 200 Hot Pressed Si3N4 with 11%Y2O3; ll .. .. 580 ~ ................. ___ !~1 I

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Claims (6)

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

1. A method of making a cutting tool specifically useful for continuous or interrupted cutting of cast iron, the method comprising:
(a) preparing a first powder consisting of substantially alpha phase silicon nitride having a cation impurity content of no greater than 1%, excluding free silicon;
(b) preparing a second powder consisting essentially of at least one member of a metal oxide group comprised of yttria, magnesium oxide and zirconium oxide, said metal oxide being devoid of spinel;
(c) mixing 1-15% of said second powder with said first powder, the total amount of each member in the final mixture being limited to 1-5% MgO, 4-12% Y2O3, and 4-12% ZrO2;
(d) heating the mixture to a temperature level of 1700-1750°C for a period of time between 3-8 hours under a pressure of 5,000-6,500 psi, and allowing said hot press material to cool at a rate of 100°C/sec; and (e) shaping the resulting hot pressed product as a cutting tool, said product being characterized by a density of 95% or more of theoretical density, a thermal shock parameter at 1200°C of at least 26 x 109 , a transverse rupture strength of at least 67,000 psi at 1200°C, a hardness level of at least 86 Rockwell 45-N, and a density of at least 3.25g/cm3.

2. The method as in Claim 1, in which the thermal conductivity of said hot pressed material is at least 3.0 BTU/HR
in °F and the coefficient of thermal expansion is no greater than 1.9 x 10-6 in/in°F.

3. The method as in Claim 1, in which said resulting product is characterized by modulus of elasticity which is no greater than 56 x 106psi.

4. In the method of using hot pressed silicon nitride ceramics containing at least one metal oxide member of the group consisting of Y2O3, ZrO2 and MgO, the improvement being characterized by deploying said ceramics as a cutting tool for use in the machining of cast iron.

5. The method of Claim 4 in which said ceramic is limited to the ternary system of Si3N4. SiO2. metal oxide having a thermal shock parameter exceeding 26 x 109 .

6. A ceramic tool bit composition for continuous or intermittent cutting of cast iron, said composition comprising:
a ceramic consisting of a solid solution of Si3N4 -SiO2 - low temperature liquid phase metal oxide, said metal oxide consisting of at least one member selected from the group of Y2O3, MgO, and ZrO, said metal oxide being employed in such volume fraction related to Si3N4 so as to be contained in the grain boundaries of said silicon nitride in non-spinel form, said composition being characterized by a thermal shock parameter at 1200°C of at least 26 x 109 , density of at least 3.25g/cm3 a bend strength of at least 67,000 psi at 1200°C, and a hardness level of at least 86 Rockwell 45-N.