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New-generation ceramics cut costs.

Silicon nitride, sialon, and the new alumina-based compositions are rapidly shedding the stigma of unreliability associated with solid-ceramic tools of a few years ago. Although application sensitive, this latest generation of cutting-tool materials offers extraordinary production economies. The trick is to specify the right one for the job.

So that you can make an informed decision, here are the advantages, limitations, and best applications of what is available. Silicon nitride

Silicon nitride (Si.sub.3.N.sub.4.) is a new-generation ceramic that has high fracture toughness, hot hardness, and thermal-shock resistance. Its resistance to flank wear, however, is much lower than competitive aluminum-oxide (Al.sub.2.O.sub.3.) materials.

Because of this limitation, current applications are generally restricted to roughing malleable iron, nodular iron, Meehanite, and in some cases, superalloys. Experimental work also is being done on titanium alloys, although no conclusive results are available.

The three primary vendors of Si.sub.3.N.sub.4.-based tool materials are: Walmet Div (GTE Products Corp), Pleasant Ridge, MI; NTK cutting Tools, Southfield, MI; and Iscar Metals Inc, through its subsidiary, Advanced Ceramic Systems Inc, Livonia, MI. (Figure 1 shows performance characteristics of NTK's two grades compared to conventional tool materials.)

According to Iscar's Vice President Gerard Scheyer, "We negotiated a non-exclusive license to manufacture and distribute the silicon-nitride material developed by Ford Motor Co." Ford's S-8 was developed during research on silicon-nitride gas-turbine blades. Iscar renamed it Iscanite.

"We have since refined the ceramic to better suit the needs of metalcutting applications," he continues. "The material is a solid-ceramic formulation of Si.sub.3.N.sub.4., yttrium oxide, and other elements. It's produced by a variation of hot pressing to full density. The important aspect of our manufacturing process is that particiles of nonuniform size are shape packed into a uniform pattern.

"Each particle of Si.sub.3.N.sub.4 has a thin film of silicon oxide on its surface. At a certain temperature, the film combines with yttrium oxide and other elements, fusing into solid solution. The result is a high-temperature glass that transmits cutting pressures from particle to particle. The high temperatures associated with machining can't remelt this solid solution. The end material is a grain-boundary engineered ceramic that has the highest Si.sub.3.N.sub.4 content available to the metalworking industries today."

Scheyer adds, "During machining tests on pearlitic gray iron, we've found that the faster the cutting speed, the longer the tool life with this material."

Jay Maddock, Lscar's field product manager, notes that another peculiar phenomenon occurs when exploiting silicon nitride's preference for high cutting speeds. "Besides obvious gains in productivity, increasing the speed of a metalcutting operation decreases the pressure on the tool and workpiece. Here's why.

"Tangential force at the tool/workpiece interface can be estimated by the formula: Tan force = (33,000 ft-lb/min x hp)sfm

For example, consider taking a 5-hp roughing cut in cast iron with a HSS tool, a C2-carbide insert, and a Si.sub.3.N.sub.4 insert. At a typical speed of 75 sfm, the HSS tool generates 2200 lb of force; running the carbide insert at 600 sfm reduces this to 275 lb.

"Now look at Si.sub.3.N.sub.4.," Maddock continues. "It can perform at 2000 sfm, which generates only 82.5 lb of force. This implies machine-tool and fixture rigidity, two bugaboos for ceramics, become less critical when running at the high speeds possible with the new generation ceramics."

Gene Whitfield, Walmet's manager of applications engineering, cautions that few machine tools in the US are capable of taking full advantage of the high-speed potential of silicon nitride. Most machines (e.g., transfer lines) are designed to handle the last generation of cutting-tool inserts--coated carbides. Also, most workholding devices now in place aren't designed to operate safely at high cutting speeds.

In certain applications, however, Si.sub.3.N.sub.4 offers increased tool life even at moderate speeds, feeds, and depths of cut. For example, the best performance a disc-brake manufacturer could achieve was rough turning with a hot-pressed ceramic (Al.sub.2.O.sub.3.-TiC). Speed was 1400 sfm and feed was 0.020 ipr, but tool breakage was a problem. Using the same speed and feed, and converting to Walmet's Si.sub.3.N.sub.4 insert (called Quantum 5000), doubled tool life.

Of course, if the hardware is available to run at high speeds, the advantage of silicon nitride becomes multidimensional. For instance, an electric-notor manufacturer was turning a cast-iron end bracket. The cut was heavily interrupted.

A coated (TiC + TiN) carbide was used at 600 sfm, 0.012 ipr, and 0.125" depth of cut. Cycle time was 0.6 min, and 25 parts were produced per tool index. Using Iscar's Si.sub.3.N.sub.4 insert allowed tripling the speed to 1800 sfm. Feed and depth of cut were unchanged; however, tool life increased to 50 pcs/index and cycle time decreased to 0.2 min/pc.

In a final example, a major manufacturer of truck brake drums roughed them on a 60-hp turning center with a coated (Al.sub.2.O.sub.3.) carbide insert. Speed was 800 sfm, feed rate 0.020 ipr, and a depth of cut of 0.100". Tool life was 3 pcs/index; cycle time, 3.5 min.

Changing to a silicon-nitride tool permitted increasing speed to 2000 sfm and depth of cut to 0.250". Feed stayed the same, but cycle time fell by 60 percent (1.4 min , and tool life more than tripled (10 pcs/index). Sialon

Sialon (silicon-aluminum-oxygen-nitrogen), a special member of the silicon-nitride-based family of ceramics, was developed by Lucas Industries (United Kingdom). The material, and all its variants, has a lower coefficient of thermal expansion and higher thermal conductivity than conventional alumina-based materials.

In fact, the aluminas have approximately one third the thermal-shock factor of sialon. Sialon's calculated thermal-shock factor even exceeds that of C2 carbide, which explains why it's the material of choice for milling superalloys. In such applications, inserts are exposed to very high, cyclical mechanical and thermal loading.

The major industry players offering tools made of this material are Kennametal Inc, Raleigh, NC (Kyon 2000), and Sandvik Inc, Fair Lawn, NJ (CC-680).

Ron Baker, Kennametal's product manager-new materials, comments on this new-generation-ceramic tool material, "The difference between a sialon and Si.sub.3.N.sub.4 is the addition of Al.sub.2.O.sub.3. The application range for both is more restricted than conventional hot-pressed composite ceramics.

"Even through the application range is smaller, the unique properties of the Si.sub.3.N.sub.4.-based family members offer significant performance advantages in certain instances. A prime example is rough milling and semifinishing nickel-based superalloys will sialon." See Figure 2.

Generally, Si.sub.3.N.sub.4.-based ceramics aren't recommended for machining steel because the work material has an affinity for the tool. The reaction usually promotes severe cratering at the cutting edge. In an unusual application of milling D-6 tool steel (50 Rc), however, sialon outperformed coated carbide.

Gary Stephens, Kennametal's manager of applications engineering, speculates that the work material's hardness retarded any significant reaction at the tool/workpiece interface. Also, the intermittent cutting probably worked to sialon's advantage because tool temperature was kept relatively low.

The machine tool used in this atypical example was a Kuraki 10-hp horizontal boring mill with a 4" spindle. The best performance when using coated carbide was 185 rpm, a 1 ipm feed, and 0.040" depth of cut. Inserts were indexed for each part--four parts were produced before discarding the tools.

Sialon permitted increasing feed to 3 ipm, speed to 600 rpm, and each corner now lasts 5 to 6 pcs.

The user estimates per part costs, for a 700-pc lot run, were $4.50 in machine time and $8.25 for inserts when using coated carbide; $1.50 in machine time and $3.10 for inserts when using sialon.

Baker cites a more typical application for sialon, "The part was a housing made from Inconel 617. A double-negative cutter was used with eight sialon inserts (SNG 434). Speed was 2934 sfm (1400 rpm), depth of cut was 0.100", and feed was 25 ipr. Both rough and finish cuts were completed in 2.5 hr using the original eight sialon inserts.

"This is opposed to a 30-hr cycle time using 64 C2-carbide inserts (SPG 424). A coated carbide did the job in 16 hr using 32 inserts. Depth of cut was 0.020" to 0.030" and speed was 200 sfm."

A key to sialon's success when milling work-hardenable materials (such as Inconel) is that a positive geometry tool can be used to cleanly shear the material. Conventional alumina-based ceramics are prone to fracturing when configured in a positive geometry.

Kennametal currently is working on new sialon compositions for cast iron that promise even better thermal shock, impact, and wear resistance at less cost, albeit slightly higher than the new alumina materials. Comprehensive field tests began last month and should be concluded by the end of summer. We will keep you posted on the results. The new aluminas

Manufacturers of alumina-based ceramics haven't taken the arrival of silicon-nitride tool materials lying down. Both Carboloy Systems (a department of General Electric Co), Detroit, MI, and Sandvik Inc, are offering new Al.sub.2.O.sub.3 grades containing zirconia. Carboloy's product is CerMax 460; Sandvik's is CC 620.

"Wear resistance has never been a problem with alumina-based materials," says Carboloy's manager of engineering, Keith McKee. "Silicon nitrides can't compete with alumina ceramics at high speeds on protracted, uninterrupted cuts on superalloys because the lower wear resistance permits workpieces to grow as the insert wears. This is why Si.sub.3.N.sub.4 is used primarily for roughing.

"Unfortunately," he adds, "when machining superalloys, conventional Al.sub.2.O.sub.3 materials usually develop a problem with fracture toughness. We've been reasonably successful in improving this by adding zirconia."

Zirconia is used to exploit the phase transformation characteristic of zirconium oxide. Transformation is controlled by dispersing zirconia particles in the alumina matrix. The zirconia is constrained from transformation, which introduces compressive forces in the structure. These stresses effectively increase the fracture toughness of the material. The result is called transformation toughening, a form of grain-boundary engineering usually associated with structural ceramics.

Dr Steve Burden, Carboloy's manager of ceramic development, comments, "Because cutting-tool ceramics are good in compression, and poor in tension, anything that keeps them in compression improves tool life. On a micro scale, we've created a material that has built-in compressive stresses that produce a 20 to 25 percent increase in fracture toughness for an alumina-based ceramic."

The 460 insert can be used to rough and finish medium to hard steels (35 Rc to 55 Rc), cast iron, and superalloys, Figure 3. The chart shows typical applications, and compares the material's performance to other Al.sub.2.O.sub.3 inserts. Its fracture toughness makes the grade suitable for interrupted cuts and other difficult machining conditions.

McKee reports that high-temperature superalloys, like Inconel, Hastelloy, and Rene, can be machined at rates four to five times higher than conventional cemented-carbide cutting speeds.

Converting to this new generation ceramic drastically reduced the number of scrap parts for one manufacturer by allowing finish machining of ductile-iron parts after heat treatment. Roundness tolerances had been difficult to meet with heat treating as the final process.

Finish machining the hardened stock with the 460 insert resulted in 48 percent fewer scrap parts. Moreover, net machining cost fell nearly 50 percent.

According to McKee, "Another user saved over $9000 annually in perishable tool cost by switching from a sialon tool to the 460 ceramic. In each case, it took 4 min to cut a depth of 0.100" on an Inconel 718 part, with speed and feed at 800 sfm and 0.007 ipr. The economy was simple to account; sialon inserts cost approximately $15 each, while the Al.sub.2.O.sub.3 inserts were $5 (in quantity)."

Like other ceramics, grade 460 inserts should be run at speeds and feeds determined according to material hardness. "Typically," asserts Dr Burden, "optimum speeds range from 1000 sfm to 1800 sfm. Speeds over 2000 sfm often are excessive, resulting in lower tool life and little increase in productivity."

Generally, they recommend running the grade dry. There are instances, however, particularly with superalloys, when users run with flood coolant. This jsn't necessarily for the inserts, but to control chip and part temperature, and to improve surface finish.

So what will be the next generation of ceramic materials? McKee believes tool manufacturers soon will reach the limit for monolithic ceramics. "The next step is to marry different materials together," he says. "The breakthrough will come as reinforced ceramics--maybe enhanced transformation reinforcement, or possibly even fiber reinforcement."
COPYRIGHT 1984 Nelson Publishing
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Author:Coleman, John R.
Publication:Tooling & Production
Date:May 1, 1984
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