Mass Flow Control for New Physical Vapor Deposition Applications

Physical vapor deposition system with custom mass flow controllerYou know one problem with anodizing and bluing? While it prevents corrosion, it can scratch off. Most anodizing doesn’t strengthen a surface much. Chrome plating has been an alternative, but chrome-6 is toxic, and can still wear, pit and spall. Duralar, a vacuum deposition system manufacturer, wanted to develop a fast, diamond-like coating process using physical vapor deposition that’s harder and more durable than either. It would have applications in prevention of corrosion, erosion and wear on metal parts.

Suppose, like Duralar, you’re a physical vapor deposition innovator. And just to make life interesting, let’s pretend that you don’t have unlimited time, and that money doesn’t grow on trees(!). R&D means testing, allowing for new formulations of barrier and reactive gases. You’ll need to change flow ratios and gases—all the while maintaining accurate flow, so success will be reproducible. You can’t keep buying new vacuum flow controllers for every test, and you can’t afford the downtime to have them recalibrated for each new recipe. You get the picture.

Duralar started with a generic vacuum deposition system, but none of the six built-in thermal controllers were calibrated for their special gas, tetramethylsilane (TMS), a vaporized liquid. The fact is, those controllers couldn’t change gases accurately, without downtime for a recalibration. So Duralar called Alicat. Although TMS isn’t on our standard list, with a bit of engineering we provided them with a corrosion resistant flow controller with a custom gas calibration for TMS.

PVD physical vapor deposition parts after vacuum coating process, with a plain steel part.

After the diamond-hard PVD vacuum coating, plain steel shows as black.

They retrofitted their system using the Alicat controller—it’s a drop-in replacement—and they are now assured that the accuracy of their TMS flow will remain high, whether flowing full scale or turned down to 0.5% of full scale. That’s because Alicat’s gas selection isn’t just a k-factor offset for a single point, it’s a complete performance curve based on NIST’s viscosity tables. Going forward, Duralar needn’t worry about inaccuracy due to changing temperatures or pressures in their factory, since the closed-loop sensor in the controller compensates for volumetric changes—unlike those original thermal controllers.

Soon, Duralar was producing coated parts by pumping down the chamber to a vacuum, flowing in just the right amount of gases, adding material vapor and turning it to a plasma which bonds to the parts. In physical vapor deposition, the vacuum is needed because the chemistry won’t work in the presence of atmospheric gases.

Mass flow controllers used to develop a PVD process

A vacuum pump and three mass flow controllers for prototyping.

Next, they wanted to develop a process to deposit this thin coating inside tubes and pipes. With the inside of the pipe coated, you could reduce abrasive wear and corrosion—perfect for flowing abrasive liquids in a pipe, or protecting the inside of a rifle barrel from rust. The technique is clever: seal the ends of the tubes, and the interior of the tube becomes the vacuum chamber. The catch is, you need to be able to vary the location of the plasma-producing electrical discharge inside a narrow tube to get a complete coating. So, more prototyping, more experimentation, and Alicat to the rescue again.

By using an MCS flow controller for the corrosive TMS and just two more Alicat MC mass flow controllers, they were able to change gases at will, selecting from up to 98 built-in types and 20 user-customizable mixes—even more on the corrosion-resistant controller.

laptop with LabVIEW interface

Alicat’s LabVIEW downloads customized to run a prototype system

Then, they used our downloadable LabVIEW drivers to build the vacuum process steps, controlling the MFCs and their shut-off solenoids from a remote laptop. In due time, they had perfected the process and begun building coating systems to order. Now, you can order diamond-like coatings for your metal parts—or the systems to provide the service for others—and the vacuum coating is more durable than anodizing, while quicker and harder than chrome plating. Thanks to Alicat’s flow controller adaptability, physical vapor deposition gets better and better.

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