Custom coating systems

Manufacturers’ growing use of lightweight, exotic materials with extreme hardness and withstanding extreme temperatures and highly corrosive environments, is driving the coatings industry to meet rising demand for higher component performance.

Applying such coatings is performed through chemical vapor deposition (CVD) systems depositing exotic materials in gaseous form as thin film layers on substrates. While such systems are widely used for silicon carbide, graphite, and pyrocarbons, industry demand is increasingly embracing more exotic deposited coatings such as iridium, tungsten, tantalum, hafnium carbide, zinc sulfide, boron phosphide, and titanium diboride.

Incorporating new exotic materials requires more than modifying an existing CVD system. Experienced system designers are tasked to provide custom systems resolving challenges based on unique substrates with varying sizes, uniformity, throughput, and material.

Challenges can arise when working with exotic precursor materials in a CVD system, starting with the ability to handle each unique material. The process requires converting solid and liquid materials to the gas form required for film deposition. Venting vapors and off-gasses must satisfy stringent ATEX environmental safety standards, while extremely dirty vacuum conditions within the reaction chamber must be managed.

“For most companies, a zinc sulfide or titanium diboride coating is going to be outside of their scope,” says Sylvain Bentivegna, managing director and founder of MPA Industrie, a physical and chemical engineering provider of CVD coating systems and ceramic and fiber matrix composites. MPA was recently acquired by PVA TePla.

“Most CVD system designers are going to piece together a system from different providers of subsystem components such as reactors, pumps, off-gassing, and scrubbers,” Bentivegna says. “These systems aren’t designed from a global perspective for the deposition of specific exotic materials.

“The starting point for us is to create a highly integrated CVD solution designed and built to meet the specific requirements of the exotic material, whether the precursor is in gas, liquid, or solid form.”

Vapor deposition (CVD/CVI/CMC)

CVD is a vacuum-based deposition method for producing high-quality and high-performance solid materials. By exposing a substrate to one or more precursor materials in gas form within a reaction chamber, the desired materials with their unique properties are decomposed and deposited on the surface of a substrate while in a vapor phase. The process must be carefully controlled to achieve the desired properties including the thickness of the film and to manage the volatile by-products produced, which must be carefully removed from the chamber through gas flow.

Chemical vapor infiltration (CVI) and ceramic matrix composite (CMC) are related processes requiring many of the same system components. CVI creates carbon-carbon composites consisting of carbon fibers embedded in a carbon matrix whereas CMC is used to embed ceramic fibers in a ceramic matrix. Both are prized for being lightweight while providing high tensile strength, resistance to thermal stress and shock, and ability to withstand extremely high temperatures.

Customized CVD solutions

A CVD system is a highly orchestrated process integrating complex component subsystems which need to be tailored to the unique properties of each material selected for vapor deposition. Principally, these include a reactor, vacuum pumps, treatments for the off-gasses generated, and oversight through automation controls.

In-depth knowledge of how to extract and apply the desired properties from a range of exotic materials is first required. MPA uses specially designed gas engineering systems that can convert the exotic precursor materials to produce the gasses needed for vapor deposition whether in solid, gas, or liquid form.

Hafnium and zirconium are among the exotic materials used in solid form. For these materials, MPA designed a metal chlorination process to create hafnium chloride and zirconium chloride tetrachloride. These gasses can combine with a hydrocarbon such as propane to produce hafnium carbide and zirconium carbide.

“When a customer asks for an exotic material and we need a solid source, we have a solution,” Bentivegna says. “If engineering dictates a liquid source or we need an inlet gas or need to adapt the pumping system, we have a solution for this too.”

Even a seemingly straightforward component such as the vacuum pumps can be a challenge as the CVD process is very severe for pump components. While a device between the reactor and the pumping system cleans the byproducts of the vacuum, dust and effluent made of corrosive materials still make the CVD vacuum dirty.

“If you have trouble and lose your pump, you lose your process. So, between reactors and pumps, we install a subsystem for quenching gasses, cropping solid byproducts, or filtering byproducts,” Bentivegna says.

These systems (CVD/CVI/CMC) require stringent commitment to safety because of the presence of hazardous gasses such as hydrogen chloride, hydrogen sulfide, phosphine, asin, and boron chloride. These gasses are often toxic, corrosive, and explosive. MPA supplies the gas cabinet with the required safety devices to adhere to ATEX Directives.

Throughout the CVD process, precise control of gas flow, temperature, and pressure is needed for the chemical deposition to occur, so SCADA systems are integrated for control and safety.

“Each time we develop a specific system for a new exotic material, we approach it as an R&D project,” Bentivegna says. “After each project the knowledge we gain supports future projects and research. The accumulation of specialized knowledge of system design continues to expand our capabilities [to customize systems for new exotic materials] in the future.”

The company has developed processes and equipment for advanced coating materials including TiC, TiN, Ti(CN), PyC, SiC, B4C, TiB2, BN, Al203, AIN,W, Re, ZrC, TaC, HiC, DLC, ZnS, ZnSe, and doped aluminizing.

MPA Industrie https://www.mpa.fr

PVA TePla America https://www.pvateplaamerica.com