Medical Device Manufacturing: Pulse Plasma Nitriding Provides Unique Surface Treatment of Implantable Metals

Metallic biomaterials are used for a wide range of 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 that depend on well-engineered biomaterials.

Applications have improved through the broad adoption of materials, coating, processing methods, and designs.  A Mayo Clinic study released in 2014 showed that in 2010 there was a combined total of 7.2 million Americans living with total knee arthroplasty (TKA), and total hip arthroplasty (THA) implants.

According to the 2017 paper, “Bioinert Ceramics: State-of-the-Art” in Key Engineering Materials,” materials for TKA and THA typically include titanium alloys, cobalt-chromium alloys, stainless steel (SS) alloys, polyethylene, and ceramics. Since the 1990s, the dominant design for THA has been a rough or porous coated titanium shell with an alumina [ceramic] liner, according to the paper.

Surface treatments are used in the medical and dental industries for materials such as stainless steel, titanium alloys, cobalt-chrome alloys, and other specialty alloys.  Traditionally, these treatments have included either carburizing, salt-bath nitrocarburizing, or gas nitriding.  Each process has advantages and disadvantages. However, if one is seeking the most precise control of the diffusion layer formation to enhance material properties, advanced pulse plasma nitriding may offer the right solution.

Although pulse plasma nitriding has been utilized for decades, superior controls for the direct current (DC) pulsing signal, along with improved chamber design and construction, allow for precise temperature control and uniform distribution of the heat zone throughout the hot-wall chamber. The result is extremely consistent and uniform nitriding batch-to-batch, with less gas consumption per process when compared to traditional nitriding methods.

“The benefits of pulse plasma nitriding include precise control of the diffusion layers.  Also, 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.

In addition, commercial heat treat shops have a wide variety of processing equipment to choose from as well as a variety of vendors.  High-volume part producers can select from multiple system configurations that offer flexibility, efficiency, repeatability, and throughput optimization.  As a result, manufacturers of medical device products in Europe, North America, and Asia are leveraging these systems to run a cleaner, more efficient operation.

Pulse Plasma Nitriding Advantages

Commonly used alloys for medical applications include a wide variety of SS, commercially pure titanium, β-type titanium alloys, titanium-niobium alloys, and specialty alloys, all of which can have mechanical and electrochemical properties enhanced by carburizing (carburizing of titanium is not usual) and nitriding.

Due to the high temperatures and hold-time-at-temperature associated with carburizing, which can lead to part distortion, carburizing is not the preferred method to treat materials used in medical applications, and is mainly used in the aerospace and automotive industries.  However, a corrosion-resistant, martensitic SS alloy, known as BÖHLER N360, which has been used in medical fastener applications for implantable devices, has been successfully vacuum carburized to a depth of 0.0015 in (0.038 mm) as indicated in the 2005 ASM International document, “Vacuum Carburizing of Aerospace and Automotive Materials.”

An alternative to carburizing is nitriding, a lower-temperature, time-dependent, thermo-chemical process used to diffuse nitrogen into the surface of metal.  One method is salt bath nitriding.  In this process, liquid immersion is required, and is typically conducted at 550 to 570 °C.  The salt bath process uses the principle in which anhydrous ammonia is dissolved in cyanide, forming cyanate, a nitrogen-rich salt, often producing a solution greater than 50% in concentration. Salt bath nitriding, which may have a duration of 24 h, imparts unique improvements in surface roughness, hardness, and wear resistance with the addition of a post process of either air quenching or water quenching.  CP Grade 2 Titanium is the most formable and corrosion- resistant among the pure grades of titanium and grade 5 alloy is biocompatible and exhibits excellent tribological properties, according to the 2017 IOP conference series document, “Materials Science and Engineering.”

However, with salt bath nitriding there is a post-treatment requiring a high washing effort to remove the residual cyanate salt.  In addition, there are disposal costs for salt and washing lye, environmental handling costs, as well as safety and operational liabilities.

Gas nitriding (500 oC) and gas nitrocarburizing (540 to 580 oC) are universally accepted procedures, and typically require a high concentration of ammonia 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 necessary, and therefore the chance for distortion and cracking are lower.  Disadvantages of gas nitriding are that it requires the use of flammable gases like ammonia, high gas consumption, and it is not able to treat nitride rust- and acid-resistant steels.

With recent advancements in pulse plasma nitriding, however, a new level of precision and control is possible that results in uniform and consistent case hardening. Together with the advantages of using environmentally friendly gases only—in contrast to the use of ammonia in gas nitriding—plasma-based nitriding has become a focal point for additional innovations, and broader application in the medical device field, particularly for manufacturers who seek a more environmentally and safe solution.

In pulse plasma nitriding, parts are processed by loading components into a vacuum vessel.  Components are first positioned on a support structure or grate.  Spacing is maintained for treatment uniformity.  Mechanical masking can also be used to isolate sections from treatment.  After parts are positioned, and thermo couples are placed, a bell housing is lowered to enclose the load.  The chamber is evacuated to below 10 Pascals prior to heating and a pulsating DC voltage of several hundred volts is applied between the charge (cathode) and the chamber wall (anode).  A low-flow process gas is introduced and the oscillating electrical pulse ionizes the gas.  For plasma nitriding, a mixture of nitrogen and hydrogen gases are used, to which carbon-containing gas, such as methane, may 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, as well as in the form of included nitrate (metallic nitride or special nitrides) deposition.

Adding further precision, innovators in pulse plasma nitriding have discovered methods to optimize the process through better control of the pulses. In the PulsPlasma® process developed by PVA TePla AG Industrial Vacuum Systems, for example, a precision regulated gas mixture of nitrogen, hydrogen, and carbon-based methane is used. A pulsating DC voltage signal of several hundred volts is delivered in less than 10 microseconds per pulse to ionize the gas. This serves to maximize the time between pulses for superior temperature control throughout the chamber.

“If you have a temperature variance of plus-minus 10 degrees 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), the manufacturer of PulsPlasma nitriding systems. “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.”

A unique feature with this approach is that the system can be switched on to a stable glow discharge at room temperature. Most systems cannot do this because the generators do not supply stable plasma. To compensate, those systems must first be heated to 300 to 350 0C before plasma can be applied, adding time to the process. With PulsPlasma, the heat-up time to temperature is used to prepare the surface by giving it a fine cleaning.

Even the materials of construction used to manufacture the nitriding systems furnace itself have been optimized.  In all systems, PlaTeG uses insulative materials developed in the aerospace industry to create a furnace wall as thin as 40 mm, compared to the industry standard of 150 mm. With less wall mass, the furnace requires less energy and time to heat, while still protecting workers that may accidentally touch the outside of the chamber.

With better overall control, the PulsPlasma nitriding furnaces offer multiple heating and cooling zones with each controlled by its own thermocouple. “This will create a very uniform temperature distribution within plus or minus 5 0C from the bottom to the top of the furnace,” says Voigtländer.

Uniformity of temperature within a chamber pays a dividend beyond the consistency of nitriding results. With an even temperature throughout the chamber, the entire space is available for loading components, which effectively increases the chamber’s capacity.

Stainless Steel

For medical device manufacturing, one of the key advantages of pulse plasma nitriding is that it is more suited to heat treating of high alloy materials such as SS.

When working with steels that have a higher chromium content, liquid nitriding and gas nitriding can react with the elemental content and “rob” the metal of some of its corrosion resistance.

SS has a natural passivation layer of chromium oxide (Cr2O3), which inhibits corrosion. To bring nitrogen into the material, the chromium oxide layer must first be removed. With gas nitriding, removal of the passivation layer requires the application of a special gas chemistry. SS can also be nitrided in salt baths, but it comes with a tradeoff: corrosion resistance is sacrificed at the surface.

With PulsPlasma nitriding, the treatment is applied directly through controlled ionic bombardment of the surface. By choosing a nitriding temperature below 450 °C, and with exact control of the gas mixture, the material surface can be treated without reducing the corrosion resistance of the material.

Increased Production Throughput

Nitriding is a batch process.  Innovation in furnace design, through an optimized mechanical operation, can increase efficiency and increase production capacity.  While the actual time for nitriding does not change, efficient loading and unloading scenarios plays an important part. The PlaTeG plant design can use any one of a Mono, Shuttle, or Tandem footprint 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 are now available to streamline the batch process.

With a shuttle extension, an additional vacuum chamber bottom may be added to a furnace. During a running nitriding process, the unloading of an earlier batch and the loading/preparing of a subsequent batch on the second vacuum chamber is possible.  The cycle time therefore for two consecutive batches is reduced because of the overlapping of the time for unloading/loading of a vacuum chamber with the treatment time of the running process.

A tandem extension has two complete vacuum chambers that are operated alternately by the vacuum pumps, the process gas supply, the plasma generator, and the control unit of the system. In situations such as unmanned weekend operations, an automatic process can be started and controlled for both batches in succession.

Because plasma nitriding uses environmentally friendly nitrogen and hydrogen, the furnaces can be collocated with the machining of components without requiring a separate room.   Moreover, the pulse plasma nitriding systems produce no polluting gases.  This makes nitriding more efficient as part of an overall manufacturing process as an operator can locate the furnaces beside their drilling machines.

Pulse plasma offers significantly more precision in nitriding through the control of the mixture of gases, the controllability of glow discharge intervals, the design of the pulsed signal, and the use of a highly insulated hot wall nitride furnace.  Together with innovations in the design of the furnaces 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.

For more information on pulse plasma nitriding, visit PVA Industry Vacuum Systems at www.pvatepla-ivs.com.