Plasma Nitriding in Modern Cold Wall Units *

Transcription

1 Plasma Nitriding in Modern Cold Wall Units * Moderm Plasma Nitriding Unit Plasma nitriding is set apart from other nitriding processes by a series of distinctive properties, such as effective nitriding at low temperatures, selective layer build-up, or the ability to protect individual sections against nitriding with the help of simple mechanical covers. Furthermore, pulsed plasma nitriding is highly energy-efficient and, in comparison with gas nitriding, consumes a dramatically lower amount of process gas. Over the time, two different technologies have become established on the market: "cold wall units" and "hot wall units". A "cold wall unit" is a plasma nitriding chamber in which the workload is heated up solely by means of the plasma, and which possesses a water-cooled double wall. In "hot wall units" on the other hand, the workload is brought to treatment temperature by a heating system on the wall of the heat-insulated reaction chamber. The plasma generator is active only during the nitriding process. While most manufacturers and service providers eventually adopted the hot-wall technology, a few businesses decided to keep working with the cold wall and have been able to advance this technology considerably over the past 20 years. In order to explain why we consider the cold-wall technology to be the superior one, we have to first examine the history of hot-wall plasma nitriding. * Author: Wolfgang Singewald, Riemerling, Germany, Plasma Nitriding in Modern Cold Wall Units - Page 1 of 8

2 The Development of Pulsed Plasma Nitriding A plasma nitriding chamber during the process In the 1980's, the industry witnessed a significant advancement of plasma nitriding - the step from direct current to pulsed plasma. This new technology made it possible to achieve uniform nitriding results on workpieces of even the most complex and irregular shapes. At the same time, the arcing effect, which can cause severe damage to the surface of a workpiece, could be suppressed much more effectively than in a DC unit, as the new process parameters of pulse on and off time made it possible to maintain a much more stable plasma. However, using the plasma to heat up a workload requires up to to four times as much electrical power as the plasma nitriding process itself. This posed a problem for the development of pulsed plasma nitriding, as the semiconductor technology of the 1980's was not able to switch sufficiently powerful currents fast enough (in a matter of µs) to allow heating up in plasma. In order to overcome this problem, additional heating systems had to be installed which made it possible to bring batches to nitriding temperature within an economically viable amount of time. The solution were common resistance heating systems such as those used in conventional industrial furnaces. This was the beginning of the hot wall plasma nitriding unit. Plasma Nitriding in Modern Cold Wall Units - Page 2 of 8

3 The Hot Wall Chamber Process control in plasma nitriding is a more complex affair than e.g. in gas nitriding. Hot wall units add to this complexity with their two independent sources of heat, i.e. the resistance heating and the plasma. Multiple Heating Zones It is often argued that this very property allows the operator to improve temperature uniformity during the process without regard to the density and design of the workload. However, such manual on-the-fly adjustments go against the principle of process repeatability, a cornerstone of quality management, and may cause new problems even while solving existing ones. Even with three separate heating zones, the resistance heating always radiates heat into areas where this heat is not required, and thus the control loops of heating system and plasma generator will influence each other if the heating system is used to improve temperature uniformity. Cooling Furthermore, there is a minimum power density (ma/cm²), i.e. a minimum amount of energy that must be transferred from the plasma to the workpiece in order to actually cause a nitriding reaction. This means that above a certain packing density, the nitriding process creates more heat than is necessary to keep the workload at nitriding temperature. Every large workload has to be cooled during nitriding. In hot wall chambers, up to three cooling fans circulate air through the chamber wall in order to achieve this - another addition to the complexity of temperature control. Fixed Pulse On-/Off-Time Conventional control systems maintain a fixed pulse on/off ratio while varying the voltage to control how much heat the plasma emits. This has a direct and significant impact on the nitriding result, as the voltage determines the speed at which the nitrogen ions move toward the workload surface. The Plasma Power Another weakness of hot wall units is the power rating of their plasma generators, which merely matches the overall power consumption during the nitriding process. However, for densely loaded batches or workloads with a high surface to mass ratio, extremely short and high-powered pulses are necessary. High-current pulse followed by a long off-time Plasma Nitriding in Modern Cold Wall Units - Page 3 of 8

4 So even though the overall power consumption remains unchanged (higher power but shorter pulse-on time), the power rating of the individual pulses exceeds that which most hot wall plasma generators can provide. In these cases, the user will witness voltage breakdowns in the pulses. These are quite simple to explain with the help of Ohm's law: U=R*I. An increased surface area means a decreased electrical resistance between the anode (inner chamber wall) and the cathode (workload surface). In order to maintain a constant nitriding voltage U (this determines the speed of the nitrogen ions), the pulse current I must be increased. Voltage (black) breakdown when pulse current exceeds maximum If the generator is not powerful enough to achieve this, the process voltage will drop mid-pulse. The result is an uneven electric field, which leads to inadequate nitriding result and the absence of a white layer in those workpieces furthermost from the chamber wall. Virtually every occurrence of these types of quality problems can be attributed to irregularities in the electric field. In other words, the key to a successful nitriding of densely packed workloads is to provide strong pulse currents. Unfortunately, the pulse power of most hot wall units is not sufficient to allow an optimal use of the available packing space. Plasma Nitriding in Modern Cold Wall Units - Page 4 of 8

5 The Cold Wall Chamber Even though the basic concept of the cold wall chamber dates back more than 80 years, it can still provide high quality nitriding results today. As mentioned earlier, the cold wall chamber has a double-walled jacket through which warm water (~50 C) flows during the process, and cold water during cool-down. This type of chamber design is more economic both to acquire and to maintain than hot wall chambers, while the absence of a thermal insulation and a heating system mean shorter cool-down times and a larger packing space at equal outside dimensions. Plasma Power Cold wall: small difference between packing space and outside diameter With the help of today's semiconductor technology, plasma generators have been developed which put out a pulse power of up to 400KVA and pulse currents of up to 2000 Amperes. The speed at which these units can heat up the workload is only limited by the shape of the workpieces (danger of distortion). Due to the high power output of the plasma generator, additional heating systems are not required. Depending on the shape of the workpieces, packing densities can be achieved in cold wall units which almost rival those known in gas nitriding. This holds true not only for mass production batches, but also for mixed loads. Cooling At this point, it is often argued that the continuous flow of cooling water in cold wall recipients, while on the one hand allowing short cooling periods, on the other hand also absorbs vast amounts of energy, thus making cold wall units operate at a much lower efficiency than hot wall units. This problem can be eliminated by employing a non-continuous flow of "warm" cooling water: the water enters the cooling jacket at 45 C and remains there until it has reached a temperature of 55 C. Only then are Plasma Nitriding in Modern Cold Wall Units - Page 5 of 8

6 the valves opened and the cooling water keeps flowing until the temperature has dropped to 45 C again. Densely packed load in a modern cold-wall unit With its low consumption of process gas and its excellent energy efficiency, cold wall plasma nitriding with our modern generators provides a cost-effective alternative to gas nitriding. Plasma Nitriding in Modern Cold Wall Units - Page 6 of 8

7 Temperature curve of a non-continuous cooling system With this procedure, just as much heat is absorbed from the chamber as is necessary to keep the workload at treatment temperature, raising the efficiency of the cold wall unit to the same level as that of the hot wall unit. (Remember that plasma nitriding creates more heat in dense workloads than is required to maintain treatment temperature.) Plasma Nitriding in Modern Cold Wall Units - Page 7 of 8

8 Modular Design Furthermore, cold wall units have the great advantage of being stackable, so that the height of the chamber can be adjusted to meet the requirements for each individual job. Only possible with cold-wall units: stacking of chamber modules We believe that, combined with the right control system and a sufficiently powerful plasma generator, the cold wall is by far the more effective technology for economically efficient pulsed plasma nitriding. Plasma Nitriding in Modern Cold Wall Units - Page 8 of 8