BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT High temperatures are experienced at a wear interface when a non-oxide ceramic, such as a metal carbide or metal nitride, is used (a) to cut metals in air at high speeds (greater than 400 surface feet per minute) or under rigorous loading, or (b) to rub against a hot, hard metal surface in a bearing or piston/cylinder application. At these high interface temperatures (above

750°C) , the ceramic will form an oxide in the presence of air or similar oxidizing agent. The oxide causes a welding between the ceramic and metal and substantially reduces the wear -life of the ceramic.

It is interesting to note that such oxides have proven to be a help to iron based tool materials, such as high speed tool steels, because the oxide prevents welding at the interface, the opposite of the phenomenon with non-oxide ceramics. But, for non-oxide ceramics, the oxide formation remains a problem. An attempt by the prior art to reduce the interface temperature by quenching with oil or other cooling medium has resulted in cracking of the ceramic tool, the latter being highly sensitive to thermal shock.

An attempt has been made to modify the cutting conditions surrounding the cutting of tough alloys (such as nickel and cobalt based super alloys or work hardenable stainless steel) with oxide or non-oxide ceramic cutting tools. In U.S. patent 3,990,332, the cutting tool was surrounded or bathed in a stream of oxygen to promote the formation of an oxide that reduces the formation of a notch at the depth of cut line where the thermal gradient changes abruptly. No disclosure is made of the effect of oxygen on the total wear of the tool.

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Tungsten carbide and titanium carbide based tool bits have been used during the past 10-15 years for the machining of steel and iron and are considered conventional. Only recently have tool bits been made of 5 hot pressed silicon nitride, and only recently have they been shown to have exceptionally good properties for the machining of gray cast iron under certain conditions (see U.S. patent 4,323,325. These Si3N ₄ tool bits have demonstrated long life and can be used at higher speeds 0 than tool bits made of more conventional materials. However, one attempt by the prior art to machine a metal other than cast iron, namely aluminum, with a spinel modified Si3N ₄ tool bit, demonstrated relatively poor results (see Japanese patent 49-113803). The strength,

15 hardness, thermal conductivity, and thermal shock resistance would suggest to one experienced in the field that Si3N^ should be a superior tool material when used against a wide range of workpiece materials. The art, not understanding the wear problem of such non-oxide ceramics,

20. turned to other types of ceramic based materials (such as aluminum oxide) in order to be able to achieve some degree of success with respect to machining steel (see U.S. patent 4,286,905) .

It would be very useful if cutting conditions

25 could be found which would allow metals such as steel to be machined with or interface with non-oxide ceramics at high temperatures; and, more particularly, it would be useful if cutting conditions could be found which would allow metals to be machined with a silicon nitride based

3Q tool bit, while retaining the superior properties Si3N ₄ exhibits against gray cast iron.

SUMMARY OF THE INVENTION The invention is a method of extending the wear life of a substantially dense, non-oxide ceramic member 35 movingly interfaced at high temperatures with a metal of

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the type that forms a metal oxide containing compound under the temperature conditions prevailing at the interface with the ceramic member. The method comprises removing oxygen from between the ceramic member and the metal during the interfacing.

Preferably, the removal of oxygen is carried out by the use of a directed stream of inert atmosphere at the interface, the stream reducing the presence of oxygen at the interface by at least 60%. Advantageously, the inert atmosphere is comprised of a flow of relatively pure nitrogen gas.

More particularly, the invention is a method of machining a metal with a substantially dense, non-oxide ceramic cutting tool. The method comprises: (a) moving a substantially dense, non-oxide ceramic cutting tool relatively against the metal at a relative speed of at least 400 surface feet per minute under a loading and for a period of time effective to attain a temperature of at least 750°C at a substantial number of micro loci across the cutting tool interface, and (b) while carrying out step (a), excluding oxygen from between said cutting tool and metal.

It is preferable that the non-oxide ceramic be selected from the group consisting of silicon nitride, tungsten carbide, titanium carbide, titanium nitride, mixtures thereof, or coated ceramics of this group. The metal is preferably selected from the group consisting of iron based metals or alloys, aluminum, zinc, titanium, and super alloys. Advantageously, the cutting tool itself is the fabricated product of heat fusing a powder mixture of silicon nitride and a controlled proportion of oxygen carrying agents, preferably in crystalline form. The agents are added either to silicon prior to nitriding or subsequent thereto, but before fusion. For example, the

ixture may advantageously contain .2-1.0% AI2O3 and 2-14% oxynitrides (which may be added directly to Si3N ₄ powder or to silicon powder which is then subjected to a nitriding operation) , or may contain yttrium oxide added in an amount of 4-12% to the silicon powder or Si3N ₄ and then subjected to a heating treatment to form the oxynitrides.

Preferably, the loading (pressing of the tool against the metal) for the cutting step is carried out in a range of about 25,000-52,000 psi and the density of the cutting tool is preferably at least 98% of full theoretical.

BEST MODE FOR CARRYING OUT THE INVENTION A preferred method for carrying out the invention herein is as follows.

An Si3N ₄ based cutting tool is fabricated and moved relatively against a metal in a cutting or shearing manner. The metal is of the type that forms a metal oxide containing compound on the cutting surface of the Si3N ₄ cutting tool under the heated conditions prevailing at the cutting tool interface with the metal. The compound typically forms as a liquid at the high interface temperatures, but the liquid species can be a solid solution, an oxynitride, or silicate. The cutting tool is moved at a relative speed of at least 400 surface feet per minute under a loading and for a period of time effective to attain a cutting tool temperature of at least 7500°C. While carrying out the cutting operation, oxygen is removed from the interface between the cutting tool and metal.

Fabricating Tool

The Si3N based cutting tool may be fabricated by a method described in U.S. patent 4,323,325 and in U.S. patent application Serial No. 444,251, the disclosures of

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which are incorporated herein by reference. In such disclosure, a powder mixture of silicon nitride is mixed with oxygen carrying agents (preferably 2-14% by weight silicon oxynitride, .2-1.0% by weight aluminum oxide) and 5 either hot pressed or nitrided to form a heat fused product having sufficient density of at least 98% of full theoretical or 3.2-3.3 gm/cm3. Such process may be modified by adding the oxygen carrying agents (preferably 4-12% 2°3 and .2-1.0% AI2O3) to raw silicon metal powder Q and the mixture nitrided, after cold compaction, to form the oxynitride cyrstallites prior to hot pressing and/or eventual sintering.

Although the preferred method employs a cutting tool fabricated of silicon nitride, the invention applies 5 to other non-oxide ceramics, preferably selected from the group consisting of silicon nitride, tungsten carbide, titanium carbide, titanium nitride, mixtures thereof,, and coatings for the ceramics of this group such as cubic boron nitride. 0 Metal To Be Cut

The metal to be cut in accordance with this invention is of the type that forms a metal oxide containing compound on the cutting surface of the tool. Such metals preferably are selected from the group 5 consisting essentially of iron based metals or alloys, aluminum, zinc, titanium, and super alloys. The super alloys are best represented by nickel or cobalt based alloys (which can be wrought, cast, or isostatically pressed); they are characterized by their high strength at Q elevated temperatures. Their compositions include a major amount of nickel or cobalt with significant amounts of alloying elements. One example is 28% Co, 25% Cr, 25% Fe, 5% Ti, 7% Mo, 6% W, 2% tantalum plus niobium, and 2% other alloying ingredients.

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Removing Oxygen

Non-oxide ceramics, when subjected to high temperature wear, are not helped by the presence of an oxide that may be formed during hot cutting tool operation against a metal that facilitates the formation of an oxide. As the cutting tool is pressed against the metal at pressures of 25,000-52,000 psi and at surface speeds of at least 400 surface feet per minute, -the surface will get hot and achieve a temperature of at least 750°C. The metal may melt at the interface and form a liquid such as iron oxide due to the presence of oxygen in the atmosphere surrounding the cutting operation and the presence of iron. The metal may also melt at micro loci; that is, a number of minute high points at the interface surface, due to the high temperature experienced by these points as a result of friction. The bulk temperature of the metal may not reach oxide promoting temperature, but the micro loci do.

Thus, the oxide is formed in separated amounts as the micro loci spread across the interface; this may occur with cast iron particularly. Even ^' though Si3N ₄ machine cuts gray cast iron with effectiveness, the use of exclusion of oxygen can add additional improvements due to the possibility of micro loci oxide formation. It is important that at least one of the oxygen or the metal that forms the oxide be eliminated from the interface so that such metal oxide does not form, and thus facilitate increased wear.

Removal is preferably carried out by directing a flowing stream of inert gas, preferably nitrogen, at the interface during cutting. The stream should be directed to reduce the presence of oxygen at the interface by at least 60% and optimally 100%.

The invention is applicable generally to high temperature wear of non-oxide ceramics, which may include

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bearings in a ceramic engine component, or a piston and cylinder engine application where one of the members carries a non-oxide ceramic. The wear life of the ceramic in these applications can be considerably extended by use of this invention. Examples

A series of experiments were performed to stimulate extended machine cutting conditions by use of a standardized pin and disc test. This test eliminates the time consuming necessity for removal of large amounts of metal to determine if a tool wears. In this test, the chemical aspect is isolated; the test correlates well with actual machining tests. Si3N (containing 8% 2O3 n 1% AI2O3 in a powder mixture prior to hot pressing) was hot pressed into pins according to the fabrication technique discussed above. Pins were then placed in a modified lubrication testing apparatus and moved into contact with a rotating disc or plate of metal (which normally would be cut in a machining operation) . The pins were pressed at loads of up to 18 Kg (5000 psi). The pins were pressed against metal plates for about one minute, the discs rotating at about 1000 surface feet per minute. The loads were estimated to vary between 30,000 psi and 50,000 psi, and pin pressing was carried out under ambient conditions (in air). The metal plates were comprised of 1045 steel or gray cast iron. The steel had a composition of .46% C, .75% Mn, .04% P, .65% S, and the remainder Fe. The steel had a tensile strength of 98,000 psi, a hardness of 94 R _c "B" or 191/205 Brinell. The gray cast iron had a composition of .07% Sn, .069% S, .7% Mn, 2.16% Si, 3.19% C, and the balance Fe. The gray cast iron had a tensile strength of 34,000 psi and a hardness of 94 R _c "B" or 205/216 Brinell. The test was run to deeply cut the plates in such, a manner that the material of the plates would be transferred to the surface of the Si3N ₄ . When

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the metal disc was constituted of 1045 steel, the Si3 pins were badly worn (see Plot A of Figure 1). When the metal was constituted of gray cast iron, little wear was detected (see Plot A of Figure 2). 5 The material transferred to the surface of the

Si3N pins was found to be rich in iron and manganese, regardless of whether the metal was gray cast iron or 1045 steel. Manganese was found in both cast iron and steel in concentrations less than 1%. The temperature of the Si3N

10 pin when working against steel was found to be approximately 800-1200°C, while the temperature of the pin when working against cast iron was estimated to be about 600-900°C.

In order to investigate the reaction of iron,

15 manganese, and Si3 , powders of these respective materials were pressed into pellets and heated in air at different temperatures over the range of 600-1250°C. The course of the reactions was followed by x-ray diffraction. At temperatures of 600-800°C, the iron and manganese each

20. were converted to iron oxide and manganous oxide, respectively, with traces of Siθ2 forming above 800°C. At temperatures above 900°C, the n2θ3 concentration decreased with the formation of increasing quantities of Siθ2 and the formation of MnSiN2« This chemical change

25 was accompanied by gross melting of the samples at 900-1000°C. This is a significant factor contributing to the wear of Si3 ₄ based cutting tools against steel.

Since it is not practical to eliminate manganese from steel, Si3N tool performance was improved by 0: removing oxygen from the system. Pins made of hot pressed Si3N ₄ were again pressed against plates of 1045 steel and gray cast iron, rotating at a speed of 1000 surface feet per minute in the manner previously described, with one notable exception: the experiment was carried out in an 5 atmosphere of flowing nitrogen. This atmosphere was

created by directing a flow of nitrogen gas about the interface. Different rates of flow and different flow directions resulted in varying degrees of oxygen exclusion or presence. Little or no deterioration was found on the pin tested under nitrogen flows against either of the plates (see Plots B-C-D of Figure 1 and Plot B of Figure 2). Noticeable deterioration was found on the pin tested under similar conditions in air against steel and some measurable wear in air against cast iron. With the Si3N ₄ based tool material, the presence of I2O3 in the material affects the degree of wear improvement. As shown in Figure 3, pin on disc tests were performed using Si3N ₄ materials of varying AI2O3 content and under both air and N2 atmospheres. Decreasing amounts of AI2O3 from 1.5 down to .4% by weight of the Si3N ₄ reduced wear in nitrogen. However, the presence of increasing amounts of AI2O3 from .4 to 1.5% reduced wear in air. The AI2O3 is a glass former in the making of SΪ3N ₄ and does assist hot pressing or sintering. However, the amount of AI2O3 must be strictly controlled if the Si3N ₄ tool is to have extended wear life at high temperatures. Working at higher temperatures, the presence of the glass accelerates wear.

Other non-oxide ceramic materials, such as tungsten carbide, were also tested with the pin on disc test in the presence of a nitrogen flow, thereby excluding oxygen from the interface. The WC pin showed increased life (see Figure 4), but not as remarkable as the increase in life with Si3N based pins, the latter being on the order of a factor of 3.

A series of actual machining tests were undertaken which corroborated the pin and disc simulation tests. Cutting tools were fabricated of Si3N ₄ (from a mixture of 8% Y2°3 and 1% AI2O3) and were used in a lathe

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machining operation against cast iron or 1045 steel with a tool speed varying up to 1500 surface feet per minute, total depth of cut of .06 inches, and a feed rate of about .011 inch revolution. A significant difference in wear was experienced between machining in air and a reduced oxygen atmosphere (.6% or 5.0% O2 remaining), see Figures 7 and 8. Air normally contains about 21% O2.