Pulse plasma nitriding

Metallic biomaterials support implant applications where high- and low-cyclic stresses occur concomitant with corrosive effects associated with human body chemistry. This makes enhanced surface properties of implantable alloys crucial in medical device applications such as artificial hip and knee implants.

Surface treatments for medical and dental components made of stainless steel, titanium alloys, cobalt-chrome alloys, and other specialty alloys require carburizing, salt-bath nitrocarburizing, or gas nitriding. Each process has advantages and disadvantages, however, more precise control of diffusion layer formation to enhance material properties comes from advanced pulse plasma nitriding.

With pulse plasma nitriding, updated DC pulsing signaling and improved chamber design and construction allow precise temperature control and uniform distribution of the heat zone throughout the hot-wall chamber. This results in uniform nitriding, batch-to-batch, with less gas consumption per process compared to traditional nitriding methods.

“It has a broad appeal to surface treat many metallic materials used in medical applications, such as titanium alloys, as well as ferritic and austenitic stainless steel,” says Thomas Palamides, senior product and sales manager at PVA TePla America.

High-volume part producers can use multiple system options to maximize flexibility, efficiency, repeatability, and throughput. Medical device producers are leveraging these systems to run cleaner, more efficient operations.

Advantages

Commonly used alloys for medical applications include stainless steels, commercially pure (CP) titanium, -type titanium alloys, titanium-niobium alloys, and specialty alloys, all with mechanical and electrochemical properties enhanced by carburizing and nitriding. However, titanium carburizing isn’t the preferred method to treat medical materials.

Nitriding is a lower-temperature, time-dependent, thermo-chemical process that diffuses nitrogen into the metal’s surface. Salt bath nitriding immerses workpieces in liquid, typically between 550°C and 570°C. The salt bath process dissolves anhydrous ammonia in cyanide – forming cyanate – a nitrogen-rich salt, often producing a solution greater than 50% in concentration. Salt bath nitriding can take up to 24 hours to impart unique improvements in surface roughness, hardness, and wear resistance. Post-treatment requires washing to remove the residual cyanate salt. Additional costs include disposal of salt and washing lye, environmental handling costs, and safety and operational liabilities.

Gas nitriding (500°C) and gas nitrocarburizing (540°C to 580°C) typically require a high concentration of ammonia (NH3), and a high amount of carrier gas flow (normal pressure process) compared with pulse plasma nitriding. The elemental nitrogen gas constituent diffuses into iron and forms hard nitrides. Because of the reduced temperature compared to carburizing, no quenching is needed, reducing chance for distortion and cracking. Disadvantages of gas nitriding are the use of flammable gases such as ammonia, high gas consumption, and the inability to nitride treat rust- and acid-resistant steels.

Recent advancements in pulse plasma nitriding deliver a new level of precision and control, resulting in uniform and consistent case hardening. Manufacturers load parts into a vacuum vessel on a support structure or grate. After part positioning and placement of thermocouples, a bell housing encloses the load. The chamber evacuates air to less than 10 Pascals prior to heating, and several hundred volts of pulsating DC power travel between the charge (cathode) and the chamber wall (anode). The system introduces a low-flow process gas, and the oscillating electrical pulse ionizes the gas. Systems use a mixture of nitrogen and hydrogen gases, and carbon containing gasses such as methane can be added.

Depending on treatment time and temperature, nitrogen atoms diffuse into the outer zone of components and form a diffusion zone. This can be atomic nitrogen dissolved in the iron lattice or included nitrate (metallic nitride or special nitrides) deposition.

Better controlling pulses brings additional advantages. The PulsPlasma process developed by PVA TePla AG Industrial Vacuum Systems uses a precision regulated gas mixture of nitrogen, hydrogen, and carbon-based methane. A pulsating DC voltage signal of several hundred volts is delivered in less than 10 microseconds per pulse to ionize the gas. This maximizes the time between pulses to improve temperature control throughout the chamber.

“If you have a temperature variance of ±10° within a batch, you will get completely different treatment results,” says Dietmar Voigtländer, senior sales manager at PlaTeG – Product Group with PVA Industry Vacuum Systems (IVS), Wettenberg, Germany. “However, by controlling the pulse current by means of an exact pulse on and off time management, the overall temperature can be precisely managed with a uniform distribution, from top to bottom, throughout the hot wall chamber.”

Construction materials used for nitriding systems furnaces have been optimized. PlaTeG uses insulative materials developed in the aerospace industry to create a furnace wall as thin as 40mm, compared to the industry standard of 150mm. Lower wall mass reduces furnace energy requirements and time to heat, while still protecting workers who may accidentally touch the outside of the chamber.

PulsPlasma nitriding furnaces offer multiple heating and cooling zones, each controlled by its own thermocouple, creating a temperature distribution within ±5°C from the bottom to the top of the furnace. An even temperature throughout the chamber maximizes available space for loading components, increasing chamber capacity.

Increased throughput

Innovation in furnace design, through an optimized mechanical operation, can increase efficiency and production capacity. Time for nitriding does not change, but efficient loading and unloading play an important part. The PlaTeG plant design can use mono-, shuttle-, or tandem-footprints to manage throughput, resources, and operations costs.

As a batch process, nitriding typically requires waiting for the prior batch to be treated, cooled, and unloaded before a new batch can be started. Shuttle and tandem extensions can streamline the batch process.

Shuttle extensions add an additional vacuum chamber bottom to a furnace, allowing unloading of an earlier batch and loading/preparing a subsequent batch on the second vacuum chamber. Cycle time for two consecutive batches falls by overlapping the time for unloading/loading a vacuum chamber running process treatment time.

Tandem extension uses two complete vacuum chambers operated alternately by vacuum pumps, process gas supply, plasma generator, and system control unit. During unmanned weekend operation, for example, an automatic process can be started and controlled for both batches in succession. With this type of operational structure, “it’s possible to increase overall nitriding capacity by 30% to 60% annually,” Voigtländer says.

Because plasma nitriding uses environmentally friendly nitrogen and hydrogen, the furnaces can be co-located with component machining, and the pulse plasma nitriding systems don’t produce polluting gases.

With innovations in furnace design to streamline batch management in nitriding operations, medical manufacturers who depend on nitriding components can benefit from greater uniformity of results, better-protected materials, and increased throughput.