High-dazzle coatings at low temperatures

Strut tubes on hydraulic bicycle forks were initially finished by bright anodizing to protect the 7050 T-6 aluminum alloy from wear at the cylinder seal. The anodizing had an attractive gold finish and served to protect the tube fairly well. But, the anodized surface (5-10 mm) chewed through the elastomer seals in short order. Designers now apply a 1.5-mm coating of TiN on top of chrome and nickel undercoatings that boost the surface stiffness of the aluminum. The TiN coating reduced friction by 35% and more than tripled the tube's fatigue life.

Brass and zinc faucets originally coated with lacquer for corrosion protection and a shiny appearance soon lost their luster as the lacquer wore off, exposing the substrate to corrosion. Low temperature arc vapor deposition now lets one faucet OEM extend faucet warranty for the lifetime of the product. To get the new look, a bright nickel coating is deposited to smooth the surface of the brass substrate. Next the part is chrome plated and top coated with ZrN. This not only gives an appearance of brass but also boosts scratch resistance. For zinc-based products, the process is similar except that the first layer of plating is copper which fills pits in the zinc surface and provides improved conductivity for the PVD process.

Process Schematic

Marty Koval
LTAVD Product Director
Vapor Technologies Inc.
Longmont, Colo.

Vapor-deposition processes let designers apply finishes that are both functional and aesthetic. The process deposits metals and refractory compounds such as ZrN, TiN, CrN, TiCN, and TiAlN that can't be applied easily by other means. The low-friction coatings have good wear and corrosion resistance. They are uniform and provide fine metallic finishes that can resemble gold, nickel, and stainless steel. Vapor deposition is also eco-friendly. It has none of the environmental limitations or hydrogen embrittlement associated with platings. Faucet and door hardware, interior automotive parts, cutting tools, bakeware, and surgical implements look and perform better with vapor-deposition coatings.

Vapor deposition generally divides into two broad, sometimes competing, categories. Chemical vapor deposition (CVD) is typically associated with an application process employing extreme-temperatures where hot erosion is a problem. High temperatures (750°C), relegate most commercial CVD processes to coating high-temperature materials such as cemented carbides. In contrast, physical vapor-deposition (PVD) processes can apply decorative and functional coatings on low-temperature materials, including polymers and aluminum alloys. PVD is more commonly used for aesthetics and mechanical components.

PVD can deposit almost any metal or refractory-metal compound. Refractories are the usual choice where a combination of properties such as extreme hardness, corrosion resistance, and aesthetics are important. Coatings are ultrathin, typically ranging from 50 nm (2 µin.) to 5 µm (200 µin.). Many PVD films are biologically compatible with the human body and find use on implants.

Heat ceiling

From a designer's standpoint, however, vapor deposition has had a severe limitation: temperature. In the past, the process had to match the substrate being coated to avoid exceeding the thermal limitations of the substrate. This meant that conventional vapor-deposition techniques were out of the question for many parts that would be dynamically loaded. The high temperatures needed would anneal the hardened substrates. In some cases, relatively high processing temperatures altered critical dimensions of high-precision parts.

Recent developments in PVD now let vapor-deposited coatings go on at low temperatures. The technique, known as low-temperature arc-vapor deposition (LTAVD), can now apply both refractory metals and conventional metal coatings at near ambient temperatures. Parts to be coated go in a chamber and revolve around a cathode that is the metallic source of the coating (often zirconium). A vacuum is drawn on the chamber and a low-voltage arc is established on the metallic source. The arc evaporates the metal from the source temperatures rarely above 100°C.

The chamber gets charged with a mixture of common inert and reactive gasses, such as argon and nitrogen, and an arc-generated plasma surrounds the source. Arc-evaporated metal atoms and reactive-gas molecules ionize in the plasma and accelerate away from the source. Arc-generated plasmas are unique in that they generate a flux of atoms and molecules that have high energies and are mostly (>95%) ionized. The high energy causes hard and adherent coatings to form on parts mounted to fixtures that rotate around the source. A bias power supply can be used to apply a negative charge to the parts which further boosts the energy of the condensing atoms.

Dial-a-finish

A range of forged and tempered parts can now be coated without warping or compromising their hardness or grain orientation. Surface properties can be tailored for appearance, wear resistance, release, corrosion resistance, biomedical compatibility, or friction coefficient. Electroplated plastics, aluminum, titanium, or steel parts can now have the look of brushed nickel, gold, silver, and stainless steel. In addition, coated aluminum, titanium, or high-nickel steel parts can work together without galling. All in all, the process makes it possible for multipart assemblies with components made from different materials to all have the same finish.

COMPARISON OF METAL-COATING PROCESSES
Process	Application Temp. °C	Suitable substrates	Common film materials	Characteristics
PVD sputtering	50 to 500	Stainless steel, glass, plated zinc, plated brass, high-carbon (tool) steel, structural plastics, chrome, ceramics	TiN, CrN	Nonthermal vaporization process in which surface atoms are physically ejected by momentum transfer. Requires finely tuned control of all variables such as gasses used. Deposition rates are low. Deposition is directional.
PVD LTAVD	40 to 180	Aluminum, forged steel, titanium, plated plastics, unplated plastics, zinc, brass, stainless steel, tool steel	TiN, CrN, ZrN, TiCN, TiA1N, pure metals or alloys	Lower processing temperatures enable it to be used with wider array of substrates, particularly those that would be degraded by high temperatures. More easily controlled. Fast. Coating 3D parts.
CVD	350 to 1,500	Superalloys, tool steel	TiN, WC, platinum-aluminide	Penetrates blind holes and channels with uniform coating. Materials
PACVD	Room temp. to 500+	Thermoplastics, structural plastics, metals, glass, ceramics	DLC, a-C:H:Si, TiN, TiC, Ti(C,N), Zr(C,N)	Compatible with thermally sensitive substrates. Film composition and morphology can be tuned easily. Coating of 3D parts. Corrosion-resistant coatings, hard coats, optical coatings, diffusion-barrier coatings, and high/low surface-energy coatings.
Hard-chrome plating	Elevated room temperature	Wrought and forged steel	Cr	Electrochemical in nature. Can expose plating temperature forged steel substrates to hydrogen embrittlement.

Properties of Metal-based Refractory Coatings
Material	Hardness (HV)	Corrosion resistance (hr) on brass/on stainless steel*	Abrasion resistance**
Zirconium nitride (ZrN)	2,600	300/200	9,700
Titanium nitride (TiN)	2,900	2/126	4,000
Titanium carbo-nitride (TiCN)	2,700	-	-
Titanium aluminum nitride (TiAlN)	2,900	1/1	2,600
Chromium nitride (CrN)	2,500	40/320	6,700
Stainless steel 304***	200	775	1,800
* Corrosion Resistance based on CASS test on plated brass and on stainless steel (SS). Abrasion Resistance based on Taber Abraser with CS-10 wheels under 1,000 gm weight (cycles/µm/material removed). ***** Stainless steel is included for comparison.