Supplier Profile: Machining Silicon Nitride
The incorporation of lasers can provide multiple benefits in silicon nitride machining.
In 2000, Paul Knowlton, Reliance Tool & Mfg.’s president and CEO, began looking at ceramic die inserts for certain stamping applications to solve problems of wear and maintaining tight tolerances. Ceramic parts were also a possible answer to can tooling problems.
Silicon nitride (Si3N4) could be used in a variety of applications if the cost of machined parts could be reduced. After Reliance ground some silicon nitride die inserts and a ceramic curling ring for a major manufacturer of barbeque grills, Knowlton began to look for a faster way to machine ceramics. The answer appeared to be to heat the ceramics to a very high temperature before machining. The heat source would probably have to be lasers.
Recognizing the potential military applications, Reliance applied for grants to assist in the research and was awarded a contract from Northern Illinois University under a contract from U.S. Army Tank Automotive Research Development and Engineering Center (TARDEC) to build a ceramic machining system that uses lasers to eliminate or minimize grinding. This began a collaboration that continues today, with Reliance and NIU jointly preparing to market ceramic machining systems and consulting services to industries that can use ceramics.
The machining process uses lasers to heat the ceramic to a temperature over 1,000°C (1,800°F), which plasticizes the material but not to the point that it becomes soft. Special diamond-tipped cutting tools developed especially for laser machining cut the ceramic just behind the laser beam. The intense heating of the workpiece raises the temperature of the ceramic’s sintering aids. The chemistry of the sintering aids has an impact on the temperature needed to successfully machine the silicon nitride. Since silicon nitride exhibits minimal expansion and contraction under temperature extremes, tolerances as small as 0.0005 in. can
Reliance uses a Mazak Integrex 200 CNC turning center and has a stable of four lasers that can be used in various configurations to heat the ceramic to the desired temperatures. For the threading of a 2-in. OD part with a 1-in. bore, Reliance used a combined 900 watts of power from three fiber lasers. The cycle time was approximately three minutes.
The machining of silicon nitride also requires special cutting tools. Polycrystalline diamond (PCD) inserts, manufactured by Seco, were found to be most effective. Jeff Staes, Reliance’s director of technical services, worked to establish the correct mix of speed and feed to enhance tool life. Tool life is still an issue, but results are much improved over diamond wheel grinding.
An added benefit is that laser-assisted machining actually strengthens the ceramic part. Measurements conducted by Gateway Materials Technology on test segments show conclusively that laser machining healed surface flaws and increased MPa by 28% on average over the MPa of silicon nitride that had not been machined.
Reliance had made cylinder liners for a race car engine using the traditional diamond wheel grinding method. The rough as-received 6 in. long, 4.375 in. OD cylinder needed to be reduced to 4.1875 in., with comparable amounts to be removed on the inside of the liner—a massive amount of silicon nitride to remove by diamond wheel grinding. Material removal rates were low, and it took roughly 150 hours to finish the job, making the parts very expensive.
While Reliance has not completed another set of cylinder liners using the laser-assisted machining (LAM) system, preliminary tests indicate that eight similar liners would take six hours each, a savings in labor of 68%. Similar savings have been found in manufacturing other parts.
The high cost of finishing silicon nitride to the required tolerances and finish has been a barrier to its widespread use in bearings. In most respects, however, it is a superior material to bearing steel due to its light weight, compressive strength and low coefficient of friction, which means fewer concerns about lubrication.
Tests have shown silicon nitride bearings can run under load for hours without any lubrication. While conventional steel bearings in a military helicopter might last 10 minutes without any lubrication, a ceramic bearing can provide at least 30 hours of operation after loss of lubrication. Thus, in a situation where an aircraft’s lubrication system is compromised, ceramic bearings provide ample time for the craft to fly to a secure area for repairs.
Silicon nitride is extremely hard (roughly 90Rc), weighs 30% of the weight of steel, is non-conductive, will not crush under compression, does not expand or contract with temperature changes and sheds heat very quickly. These attributes make it a prime candidate for use in the electrical industry—especially in wind turbine applications—and large electrical motors. The non-conductivity of silicon nitride minimizes damage to bearings by stray currents. The low coefficient of friction requires less electricity to run electric motors than standard steel bearings.